...

ȱ U B FacultatȱdeȱQuímicaȱ

by user

on
Category: Documents
1

views

Report

Comments

Transcript

ȱ U B FacultatȱdeȱQuímicaȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
UNIVERSITATȱDEȱBARCELONAȱ
FacultatȱdeȱQuímicaȱ
ȱ
DEPARTAMENTȱDEȱQUÍMICAȱFÍSICAȱ
Laboratoriȱd’ElectroquímicaȱdelsȱMaterialsȱiȱdelȱMediȱAmbientȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ELECTROCHEMICALȱADVANCEDȱOXIDATIONȱPROCESSESȱȱ
FORȱTHEȱREMOVALȱOFȱTHEȱDRUGSȱȱ
PARACETAMOL,ȱCLOFIBRICȱACIDȱANDȱCHLOROPHENEȱ
FROMȱWATERSȱ
ȱ
ȱ
DOCTORALȱTHESISȱ
ȱ
ȱ
ȱ
IgnacioȱSIRÉSȱSADORNILȱ
ȱ
ȱ
ȱ
ȱ
Barcelona,ȱnovemberȱ2006ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
PARTȱB
RESULTATSȱIȱDISCUSSIÓ
RESULTSȱANDȱDISCUSSION
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
7.ȱȱ DESTRUCCIÓȱ
D’UNȱ
FÀRMACȱ
ANTIINFLAMATORIȱ
NOȱ
ESTEROÍDIC:ȱPARACETAMOLȱȱȱȱȱ
ȱȱȱȱȱȱȱ/ȱ DESTRUCTIONȱ OFȱ Aȱ NONȬSTEROIDALȱ ANTIINFLAMMATORYȱ
DRUG:ȱPARACETAMOLȱ
ȱ
ȱ
ȱ
ȱ
Thisȱ chapterȱ isȱ devotedȱ toȱ theȱ studyȱ ofȱ theȱ degradationȱ ofȱ theȱ nonȬsteroidalȱ
antiinflammatoryȱ drugȱ (NSAID)ȱ paracetamol.ȱ Itȱ isȱ dividedȱ intoȱ threeȱ parts:ȱ (i)ȱ anȱ
introductionȱ givingȱ anȱ overviewȱ onȱ theȱ characteristicsȱ ofȱ paracetamol,ȱ itsȱ
environmentalȱrelevanceȱandȱsomeȱresultsȱpublishedȱinȱliteratureȱonȱitsȱdestruction,ȱ
(ii)ȱ theȱ resultsȱ obtainedȱ forȱ theȱ destructionȱ ofȱ thisȱ drugȱ byȱ electroȬFentonȱ andȱ
photoelectroȬFentonȱprocesses,ȱandȱ(iii)ȱtheȱresultsȱobtainedȱbyȱanodicȱoxidation.ȱ
ȱ
ȱ
ȱ
151
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
PART B –Results and Discussion7. Paracetamol
7.1.
CARACTERÍSTIQUESȱDELȱPARACETAMOLȱ
/ȱCHARACTERISTICSȱOFȱPARACETAMOLȱ
ȱ
Paracetamolȱ (Figureȱ 7.Ȭ1),ȱ knownȱ asȱ acetaminophenȱ inȱ theȱ Unitedȱ States,ȱ isȱ aȱȱȱȱȱȱȱȱȱȱȱ
nonȬsteroidalȱ antiȬinflammatoryȱ drugȱ (NSAID)ȱ belongingȱ toȱ theȱ chemicalȱ familyȱ ofȱ
aromaticȱ amides.ȱ Itȱ isȱ classifiedȱ asȱ aȱ commonȱ analgesicȱ andȱ antipyreticȱ drug,ȱ
analogousȱtoȱacetylsalicylicȱacid.ȱInȱfact,ȱitȱisȱtheȱmostȱwidelyȱusedȱoverȬtheȬcounterȱ
analgesicȱ inȱ USA,ȱ withȱ productionȱ ofȱ 3600ȱ tonsȱ inȱ 2002ȱ [345].ȱ Paracetamolȱ isȱ aȱ
metaboliteȱofȱphenacetine,ȱaȱveryȱcommonlyȱusedȱanalgesicȱinȱpastȱyears.ȱDueȱtoȱtheȱ
factȱthatȱphenacetineȱisȱreallyȱtoxicȱatȱtherapeuticalȱdosageȱandȱsinceȱitȱisȱmetabolizedȱ
toȱparacetamol,ȱphenacetineȱisȱnoȱlongerȱusedȱatȱpresent.ȱ
ȱ
ȱ
ȱ
ȱ
HO
NH
O
H C
3
Figure 7.-1 Paracetamol.
ȱ
Paracetamolȱ isȱ usuallyȱ aȱ whiteȱ crystallineȱ powder,ȱ odourlessȱ andȱ bitterȬtasted.ȱ
Saturatedȱsolutionsȱareȱslightlyȱacid.ȱItȱisȱsolubleȱinȱacetone,ȱhardlyȱsolubleȱinȱetherȱ
orȱ benzeneȱ andȱ highlyȱ solubleȱ inȱ waterȱ asȱ shownȱ inȱ Tableȱ 7.Ȭ1.ȱ Someȱ ofȱ theȱ mostȱ
remarkableȱpropertiesȱofȱparacetamolȱareȱalsoȱgivenȱinȱTableȱ7.Ȭ1.ȱ
ȱ
Itȱ isȱ worthȱ notingȱ thatȱ someȱ chemicalsȱ serveȱ doubleȱ dutyȱ asȱ bothȱ drugsȱ andȱ pestȬ
controlȱ agents.ȱ Forȱ example,ȱ warfarinȱ canȱ actȱ asȱ aȱ ratȱ poisonȱ asȱ wellȱ asȱ anȱ
anticoagulant,ȱ andȱ triclosanȱ isȱ aȱ generalȱ biocideȱ andȱ aȱ gingivitisȱ agentȱ usedȱ inȱ
toothpaste.ȱ Similarly,ȱ paracetamolȱ isȱ frequentlyȱ usedȱ forȱ controlȱ ofȱ Brownȱ Treeȱ
snakes:ȱBoigaȱirregularis,ȱnativeȱtoȱeasternȱIndonesia,ȱbecameȱinvasiveȱpestsȱonȱGuamȱ
startingȱ inȱ theȱ 1940’s/1950’s.ȱ Withoutȱ naturalȱ predators,ȱ theȱ Brownȱ Treeȱ snake’sȱ
populationȱ inȱ Guamȱ isȱ estimatedȱ atȱ upwardsȱ ofȱ 15.000ȱ perȱ squareȱ mile,ȱ causingȱ
153
PART B –Results and Discussion7. Paracetamol
extensiveȱ economicȱ losses.ȱ Noȱ effectiveȱ controlȱ wasȱ achievedȱ untilȱ discoveringȱ thatȱ
paracetamolȱ canȱ killȱ themȱ withinȱ threeȱ daysȱ [346].ȱ Otherȱ usesȱ includeȱ theȱ
manufacturingȱ wastesȱ ofȱ azoicȱ dyesȱ andȱ chemicalȱ productsȱ forȱ photographicȱ
purposesȱ[347].ȱ
Table 7.-1 Paracetamol data [348].
ȱ
CAS number
ȱ
103-90-2
4-Hydroxyacetanilide
ȱ
Generic names
ȱ
4-Acetamidophenol
Acetaminophenol
ȱ
Trade names
ȱ
Molecular formula
Molecular mass (g mol-1)
ȱ
ȱ
APAP, Disprol, Panadol, Tylenol
C8H9NO2
151.17
Melting point (ºC)
169-171
Boiling point (ºC)
> 500
ȱ
Solubility in H2O (g L-1)20 ºC
ȱ
Density (g cm-3)21 ºC
ȱ
pKa
14
1.293
9.71-9.84
ȱ
Paracetamolȱ isȱ aȱ safeȱ drugȱ whenȱ consumedȱ atȱ therapeuticȱ dosages,ȱ sinceȱ itȱ isȱ
metabolizedȱ toȱ labileȱ sulphateȱ andȱ glucuronideȱ conjugatesȱ forȱ excretionȱ (55Ȭ60%ȱ
administeredȱparacetamolȱisȱexcretedȱasȱaȱconjugatedȱspecies).ȱItsȱactionȱmechanismȱ
impliesȱ theȱ inhibitionȱ ofȱ cyclooxygenasesȱ atȱ centralȱ nervousȱ system,ȱ whatȱ highensȱ
theȱ painȱ threshold.ȱ However,ȱ atȱ certainȱ concentrationsȱ paracetamolȱ canȱ beȱ
bioactivatedȱbyȱcytochromesȱP450,ȱwhichȱareȱaȱsuperfamilyȱofȱmonooxygenasesȱthatȱ
areȱ responsibleȱ forȱ theȱ metabolismȱ ofȱ variousȱ endogenousȱ andȱ exogenousȱ
compounds,ȱthenȱcausingȱsevereȱhepatotoxicityȱ[349,ȱ350],ȱasȱwellȱasȱotherȱadditionalȱ
effects.ȱAtȱthisȱpoint,ȱParacelsus’ȱtheoremȱcanȱbeȱreminded:ȱ
ȱ
“Allȱ substancesȱ areȱ poisons,ȱ thereȱ isȱ noneȱ thatȱ isȱ notȱ aȱ poison.ȱ Theȱ rightȱ doseȱ
differentiatesȱaȱpoisonȱandȱaȱremedy.”ȱ
154
PART B –Results and Discussion7. Paracetamol
Paracetamolȱ isȱ aȱ wellȬknownȱ painȱ reliever,ȱ butȱ atȱ theȱ sameȱ timeȱ whenȱ reachingȱ aȱ
certainȱdosageȱitȱcanȱeffectivelyȱcontrolȱBrownȱTreeȱSnakesȱorȱcauseȱhepatotoxicityȱinȱ
humanȱ beings.ȱ Consideringȱ thisȱ theoremȱ inȱ combinationȱ withȱ theȱ factȱ thatȱ theȱ
environmentȱ containsȱ countlessȱ organismsȱ withȱ differentȱ sensitivitiesȱ leadsȱ toȱ theȱ
hypothesisȱthatȱmedicinesȱmayȱalsoȱposeȱaȱriskȱforȱtheȱenvironment.ȱ
ȱ
Inȱ1994,ȱ153.9Ȭmillionȱparacetamolȱdosesȱwereȱprescribed.ȱItȱhasȱbeenȱpreviouslyȱsaidȱ
thatȱ theȱ keyȱ forȱ consideringȱ PPCPsȱ asȱ aȱ matterȱ ofȱ ecologicalȱ concernȱ isȱ theirȱ
continousȱ introductionȱ inȱ theȱ environmentȱ dueȱ toȱ theirȱ widespreadȱ hugeȱ usage.ȱ
Undoubtedly,ȱ inȱ spiteȱ ofȱ beingȱ consideredȱ asȱ readilyȱ degradableȱ (t1/2ȱ <ȱ 1ȱ day),ȱ theȱ
enormousȱ amountȱ ofȱ paracetamolȱ whichȱ isȱ manufacturedȱ andȱ releasedȱ toȱ theȱ
environmentȱcanȱposeȱaȱrisk,ȱyetȱunknownȱatȱpresent,ȱtoȱbothȱhumansȱandȱanimals.ȱ
Indeed,ȱ paracetamolȱ occupiesȱ almostȱ 50%ȱ ofȱ marketȱ sharesȱ inȱ analgesicȬantipyreticȱ
fieldȱofȱtheȱworldȱandȱitsȱdemandȱcanȱbeȱupȱtoȱ70000ȱtonsȱannually.ȱAtȱpresent,ȱtheȱ
internationalȱdemandȱisȱgrowingȱatȱ15%ȱofȱannualȱincreasingȱrate.ȱItȱisȱforecastȱthatȱ
inȱ 2010ȱ theȱ annualȱ consumptionȱ allȱ overȱ theȱ worldȱ willȱ beȱ overȱ 100000ȱ tons.ȱ Moreȱ
informationȱonȱparacetamolȱsalesȱdataȱisȱavailableȱthroughȱseveralȱbooksȱ[351,ȱ352].ȱ
ȱ
Someȱenvironmentalȱstudiesȱhaveȱreportedȱtheȱpresenceȱofȱparacetamolȱupȱtoȱ6ȱPgȱLȬ1ȱ
inȱ Europeanȱ STPsȱ effluents,ȱ whileȱ itsȱ presenceȱ inȱ surfaceȱ watersȱ hasȱ notȱ beenȱ
documentedȱ[353].ȱInȱUSAȱitȱhasȱbeenȱfoundȱatȱPgȱLȬ1Ȭlevelȱinȱ17%ȱofȱstudiedȱstreamsȱ
andȱ atȱ ngȱ LȬ1Ȭlevelȱ inȱ untreatedȱ sewageȱ waters,ȱ withȱ aȱ maximumȱ ofȱ 10ȱ Pgȱ LȬ1ȱ inȱ
naturalȱ watersȱ [27].ȱ Amongȱ 139ȱ surveyedȱ streamsȱ inȱ USA,ȱ paracetamolȱ hasȱ beenȱ
identifiedȱasȱoneȱofȱtheȱmostȱfrequentlyȱdetectedȱanthropogenicȱcompounds.ȱInȱUK,ȱ
paracetamolȱwasȱincludedȱinȱaȱtopȱ10ȱlistȱaccordingȱtoȱitsȱriskȱcharacterisationȱratio,ȱ
obtainedȱ byȱ usingȱ itsȱ Predictedȱ Environmentalȱ Concentrationȱ (PEC)ȱ andȱ Predictedȱ
NoȬEffectȱConcentrationȱ(PNEC)ȱ[354].ȱ
ȱ
ȱ
155
PART B –Results and Discussion7. Paracetamol
Beforeȱcarryingȱoutȱtheȱpresentȱthesis,ȱsomeȱpreviousȱstudiesȱonȱtheȱdegradationȱofȱ
paracetamolȱhadȱbeenȱperformedȱbyȱVognaȱetȱal.ȱ[355]ȱandȱAndreozziȱetȱal.ȱ[356]ȱbyȱ
meansȱ ofȱ ozonationȱ andȱ H2O2/UVȱ inȱ theȱ pHȱ rangeȱ 2.0Ȭ5.5.ȱ Aȱ detailedȱ discussionȱ
aboutȱtheȱintermediatesȱformedȱinȱbothȱcasesȱisȱcarriedȱoutȱbyȱtheseȱauthors.ȱDespiteȱ
theȱ factȱ thatȱ theseȱ proceduresȱ canȱ beȱ appliedȱ toȱ destroyȱ theȱ parentȱ molecule,ȱ theȱ
maximumȱ mineralizationȱ achievedȱ isȱ aroundȱ 30Ȭ40%,ȱ soȱ moreȱ effectiveȱ methodsȱ
mustȱbeȱtestedȱtoȱavoidȱwidespreadȱcontamination.ȱInȱthisȱsense,ȱtheȱelectrochemicalȱ
processesȱcanȱbeȱanȱenvironmentallyȱfriendlyȱalternative,ȱasȱshownȱlater.ȱ
ȱ
Onȱ theȱ otherȱ hand,ȱ someȱ papersȱ haveȱ appearedȱ simultaneouslyȱ asȱ wellȱ asȱ afterȱ
publishingȱ theȱ resultsȱ gotȱ inȱ thisȱ thesis.ȱ Thisȱ factȱ clearlyȱ reflectsȱ theȱ greatȱ interestȱ
aboutȱtheȱroleȱofȱPPCPsȱinȱtheȱenvironment,ȱandȱaboutȱparacetamolȱinȱparticular.ȱ
Bobuȱ etȱ al.ȱ [357]ȱ haveȱ reportedȱ theȱ percentageȱ ofȱ paracetamolȱ conversionȱ byȱ
applyingȱseveralȱAOPsȱforȱ30ȱmin:ȱfotoȬFentonȱ(99.55%),ȱUV/O3ȱ(52.54%),ȱO3ȱ(42.67%)ȱ
andȱ H2O2/UVȱ (11.96%).ȱ However,ȱ theȱ maximumȱ mineralizationȱ degreeȱ achievedȱ isȱ
aroundȱ50%ȱcorrespondingȱtoȱozonationȱprocesses.ȱAgain,ȱmoreȱpowerfulȱprocessesȱ
areȱneededȱifȱcompleteȱconversionȱintoȱCO2,ȱH2Oȱandȱinorganicȱionsȱisȱdesired.ȱ
ȱ
Inȱ ourȱ laboratory,ȱ theȱ limitationsȱ ofȱ simpleȱ ozonationȱ andȱ photolyticȱ ozonationȱ
(O3/UV)ȱ haveȱ beenȱ overcomeȱ byȱ meansȱ ofȱ catalyzedȱ ozonationȱ withȱ Fe2+,ȱ Cu2+ȱ andȱ
UVAȱ lightȱ [194].ȱ Moreȱ thanȱ 83%ȱ ofȱ mineralizationȱ isȱ attainedȱ withȱ theȱ catalyzedȱ
methods.ȱAsȱprovedȱforȱelectroȬFentonȱandȱphotoelectroȬFentonȱprocessesȱshownȱinȱ
sectionȱ7.2ȱofȱthisȱthesis,ȱtheȱhighestȱoxidizingȱpowerȱisȱjustȱachievedȱbyȱcombiningȱ
Fe2+,ȱCu2+ȱandȱUVAȱlight.ȱ
ȱ
TransformationȱofȱparacetamolȱbyȱchlorinationȱhasȱbeenȱstudiedȱbyȱBednerȱetȱal.ȱ[345]ȱ
toȱ simulateȱ wastewaterȱ disinfectionȱ andȱ understandȱ theȱ toxicologicalȱ natureȱ ofȱ theȱ
chlorineȬtransformationȱ products.ȱ Worrisomeȱ chlorinationȱ productsȱ suchȱ asȱȱȱȱȱȱȱȱȱȱȱȱȱ
NȬacetylȬpȬbenzoquinoneȱ imine,ȱ whichȱ isȱ theȱ toxicantȱ associatedȱ withȱ lethalityȱ inȱ
156
PART B –Results and Discussion7. Paracetamol
paracetamolȱoverdoses,ȱhaveȱbeenȱcharacterized.ȱDueȱtoȱitsȱlackȱofȱstability,ȱtheȱimineȱ
readilyȱhydrolyzesȱtoȱtheȱtoxicantȱ1,4Ȭbenzoquinoneȱinȱaqueousȱsolution.ȱ
ȱ
Finally,ȱ Bunceȱ etȱ al.ȱ [358]ȱ haveȱ comparedȱ theȱ electroȬoxidationȱ processȱ (i.e.,ȱ anodicȱ
oxidationȱ process)ȱ ofȱ paracetamolȱ byȱ usingȱ BDD,ȱ Ti/SnO2ȱ andȱ Ti/IrO2ȱ anodes,ȱ
workingȱ inȱ anȱ electrochemicalȱ reactor.ȱ Theȱ formerȱ twoȱ onesȱ ledȱ toȱ electrochemicalȱ
combustion,ȱwhereasȱinȱtheȱlatterȱpȬbenzoquinoneȱwasȱtheȱexclusiveȱproductȱexceptȱ
atȱ veryȱ longȱ electrolysisȱ times.ȱ Asȱ itȱ hasȱ beenȱ alreadyȱ arguedȱ inȱ thisȱ thesis,ȱ theȱ
differenceȱ canȱ beȱ explainedȱ inȱ termsȱ ofȱ theȱ differentȱ mechanismsȱ ofȱ oxidation:ȱ
selectiveȱ conversionȱ atȱ Ti/IrO2ȱ anodeȱ throughȱ theȱ actionȱ ofȱ hydroxylȱ radicalsȱ inȱ theȱ
formȱ ofȱ ‘superoxides’ȱ suchȱ asȱ Ti/IrOx,ȱ vs.ȱ nonȬselectiveȱ combustionȱ involvingȱ
physisorbedȱhydroxylȱradicalsȱatȱBDDȱandȱTi/SnO2.ȱ
ȱ
Inȱ thisȱ work,ȱ paracetamolȱ decayȱ andȱ mineralizationȱ haveȱ beenȱ studiedȱ byȱ differentȱ
EAOPsȱsuchȱ asȱ electroȬFentonȱ (EF)ȱ andȱ photoelectroȬFentonȱ (PEF)ȱ withȱ aȱ Ptȱ anodeȱ
andȱanȱO2Ȭdiffusionȱcathode,ȱandȱanodicȱoxidationȱwithȱbothȱPtȱandȱBDDȱanodes.ȱ
ȱ
ȱ
ȱ
ȱ
157
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
PART B –Results and Discussion7. Paracetamol
7.2.
TRACTAMENTȱMITJANÇANTȱELECTROȬFENTONȱIȱFOTOELECTROȬFENTONȱ
/ȱTREATMENTȱBYȱELECTROȬFENTONȱANDȱPHOTOELECTROȬFENTONȱ
ȱ
7.2.1.ȱȱFinalitatȱdelȱtreballȱ/ȱAimȱofȱtheȱworkȱ
ȱ
Aȱ fundamentalȱ taskȱ (notȱ shownȱ inȱ theȱ relatedȱ papers)ȱ mustȱ beȱ carriedȱ outȱ beforeȱ
startingȱ withȱ theȱ studyȱ onȱ theȱ degradationȱ andȱ mineralizationȱ ofȱ paracetamolȱ byȱ
electroȬFentonȱandȱphotoelectroȬFentonȱprocessesȱusingȱanȱO2Ȭdiffusionȱcathode.ȱ
InȱorderȱtoȱassessȱtheȱabilityȱofȱtheȱCblackȬPTFEȱO2Ȭdiffusionȱcathodeȱtoȱelectrogenerateȱ
hydrogenȱperoxide,ȱseveralȱsolutionsȱcontainingȱ100ȱmLȱofȱ0.05ȱMȱNa2SO4ȱatȱpHȱ3.0ȱ
andȱatȱ35ȱºCȱhaveȱbeenȱelectrolyzedȱbyȱapplyingȱaȱconstantȱcurrent,ȱinȱtheȱpresenceȱ
andȱabsenceȱofȱcatalystsȱ(Fe2+,ȱCu2+ȱandȱUVAȱlight).ȱTheȱexperimentalȱsetupȱconsistsȱ
ofȱanȱopenȱundividedȱthermostatedȱconicȱelectrolyticȱcell,ȱwhereȱaȱPtȱanodeȱandȱanȱ
O2Ȭdiffusionȱ cathodeȱ areȱ placedȱ asȱ shownȱ inȱ Figureȱ 6.Ȭ3.ȱ Theȱ H2O2ȱ concentrationȱ
accumulatedȱ inȱ eachȱ solutionȱ duringȱ theȱ electrolysisȱ hasȱ beenȱ determinedȱ byȱ theȱ
spectrophotometricȱ measurementȱ ofȱ theȱ absorbanceȱ ofȱ theȱ coloredȱ complexȱ formedȱ
betweenȱTi(IV)ȱandȱH2O2ȱatȱOȱ=ȱ408ȱnmȱ(moreȱdetailedȱinȱsectionȱ6.3).ȱ
ȱ
TheȱresultsȱinȱFigureȱ7.Ȭ2ȱshowȱsomeȱtrendsȱthatȱcanȱbeȱaccountedȱforȱby:ȱ
ȱ
(i)
Theȱ cathodeȱ generatesȱ H2O2ȱ throughȱ Reactionȱ 5.Ȭ47,ȱ andȱ theȱ amountȱ ofȱ
accumulatedȱ H2O2ȱ isȱ higherȱ whenȱ theȱ appliedȱ currentȱ intensityȱ rises.ȱ Afterȱ aȱ
while,ȱ aȱ steadyȱ stateȱ isȱ reachedȱ becauseȱ H2O2ȱ formationȱ rateȱ atȱ theȱ cathodeȱ
(Reactionȱ 5.Ȭ47)ȱ andȱ H2O2ȱ destructionȱ rateȱ atȱ theȱ anodeȱ (Reactionȱ 5.Ȭ48ȱ andȱ
Reactionȱ 5.Ȭ49)ȱ becomeȱ equal.ȱ Thisȱ steadyȱ concentrationȱ inȱ theȱ absenceȱ ofȱ
catalystsȱ isȱ aboutȱ 13,ȱ 40ȱ andȱ 60ȱ mM,ȱ atȱ 100,ȱ 300ȱ andȱ 450ȱ mA,ȱ respectively.ȱ
That’sȱ toȱ say,ȱ theȱ maximumȱ H2O2ȱ concentrationȱ achievedȱ isȱ approximatelyȱ
proportionalȱ toȱ theȱ appliedȱ currentȱ intensity.ȱ Thisȱ behaviorȱ agreesȱ withȱ theȱ
factȱthatȱbothȱReactionȱ5.Ȭ47ȱandȱReactionȱ5.Ȭ48ȱverifyȱaȱfirstȬorderȱkinetics.ȱ
159
PART B –Results and Discussion7. Paracetamol
(ii)
Asȱ Fe2+,ȱ Cu2+ȱ orȱ UVAȱ lightȱ areȱ beingȱ usedȱ asȱ catalystȱ atȱ aȱ certainȱ currentȱ
intensityȱ (300ȱ mAȱ inȱ Figureȱ 7.Ȭ2),ȱ accumulatedȱ H2O2ȱ concentrationȱ decreasesȱ
dueȱtoȱitsȱgrowingȱdisappearanceȱcausedȱbyȱitsȱdestructionȱthroughȱFenton’sȱ
reactionsȱ (Reactionȱ 5.Ȭ3ȱ andȱ Reactionȱ 5.Ȭ4),ȱ photoȬFentonȱ reactionȱ (Reactionȱȱȱ
5.Ȭ23),ȱ coȬcatalyzedȱ Fentonȱ reactionsȱ (Reactionsȱ 5.Ȭ28ȱ toȱ 5.Ȭ31)ȱ andȱ H2O2ȱ
photolysisȱ (Reactionȱ 5.Ȭ25ȱ andȱ Reactionȱ 5.Ȭ26),ȱ beingȱ theȱ latterȱ twoȱ reactionsȱ
givenȱtoȱaȱveryȱlowȱextent.ȱTheseȱareȱtheȱmainȱreactions,ȱwhichȱinȱtheȱpresenceȱ
ofȱanȱorganicȱpollutantȱRȱmakeȱitȱpossibleȱtheȱmineralizationȱprocessȱthanksȱtoȱ
theȱ electrogeneratedȱ hydroxylȱ radicalsȱ (•OH),ȱ andȱ toȱ theȱ lessȱ powerfulȱ agentȱ
hydroperoxylȱradicalȱ(HO2•).ȱ
70
2
2
[H O ] / mM
60
50
40
30
20
10
0
0
60
120
180
240
300
time / min
360
420
ȱ
Figure 7.-2 Accumulated H2O2 concentration vs. electrolysis time, for the system
Pt/O2-diffusion cathode. The initial 100-mL solution contained 0.05 M Na2SO4 at
pH = 3.0 and at 35 ºC.
(Ƈ, Ŷ, Ɣ) without catalyst, (¸, Ƒ) 1.0 mM Fe2+ (EF), (×) 1.0 mM Fe2+ under UVA
irradiation (PEF), (¨) 1.0 mM Fe2+ and 0.25 mM Cu2+ (co-catalyzed EF) and (+) latter
solution under UVA irradiation (co-catalyzed PEF).
I applied: (Ƈ, ¸) 450 mA, (Ŷ, Ƒ, ×, ¨, +) 300 mA and (Ɣ) 100 mA.
ȱ
ȱ
Onceȱtheȱproperȱperformanceȱofȱtheȱcathodeȱwasȱassured,ȱtheȱEFȱandȱPEFȱprocessesȱ
usingȱ aȱ 3Ȭcm2ȱ Ptȱ anodeȱ andȱ aȱ 3Ȭcm2ȱ O2Ȭdiffusionȱ cathodeȱ wereȱ appliedȱ inȱ orderȱ toȱ
removeȱparacetamolȱfromȱtheȱinitialȱsolutions.ȱ
160
PART B –Results and Discussion7. Paracetamol
Atȱ theȱ beginning,ȱ theȱ aimȱ wasȱ justȱ assuringȱ thatȱ theseȱ EAOPsȱ wereȱ ableȱ toȱ faceȱ aȱ
largelyȱ discussedȱ problemȱ suchȱ asȱ pharmaceuticalsȱ inȱ theȱ environment.ȱ Inȱ orderȱ toȱ
assessȱ theȱ performanceȱ ofȱ EFȱ andȱ PEFȱ processesȱ whenȱ dealingȱ withȱ paracetamol,ȱȱ
100ȬmLȱsolutionsȱcontainingȱ157ȱmgȱLȬ1ȱparacetamolȱ(i.e.,ȱ100ȱmgȱLȬ1ȱTOC)ȱandȱ0.05ȱMȱ
Na2SO4ȱasȱsupportingȱelectrolyte,ȱatȱpHȱ3.0ȱandȱatȱ35ȱºC,ȱwereȱelectrolyzedȱforȱ6ȱhȱatȱ
100ȱmA.ȱTOCȱabatementȱanalysesȱwereȱdoneȱinȱtheȱabsenceȱandȱpresenceȱofȱcatalystsȱ
(1.0ȱ mMȱ Fe2+ȱ and/orȱ 0.25ȱ mMȱ Cu2+ȱ and/orȱ UVAȱ light).ȱ Inȱ additon,ȱ paracetamolȱ
kineticsȱasȱwellȱasȱfinalȱcarboxylicȱacidsȱevolutionȱwereȱstudied.ȱ
ȱ
Afterȱtheȱconsiderationsȱcarriedȱoutȱinȱtheȱpreviousȱparagraph,ȱtheȱoxidationȱabilityȱ
ofȱdifferentȱsystemsȱwasȱtestedȱthroughȱtheȱTOCȱabatementȱofȱparacetamolȱsolutionsȱ
underȱ theȱ sameȱ conditions,ȱ butȱ applyingȱ 300ȱ mA.ȱ Inȱ theȱ absenceȱ ofȱ catalystsȱ theȱ
processȱisȱcalledȱanodicȱoxidationȱ(AO)ȱwithȱH2O2ȱelectrogeneration.ȱAfterwardsȱtheȱ
sameȱexperimentȱwasȱdoneȱinȱpresenceȱofȱcatalysts:ȱUVAȱlightȱirradiationȱ(AOȱwithȱ
UVAȱ light),ȱ Cu2+ȱ withȱ orȱ withoutȱ UVAȱ lightȱ (FentonȬlikeȱ processes),ȱ Fe2+ȱ withȱ orȱ
withoutȱUVAȱlightȱ(PEFȱandȱEF,ȱrespectively),ȱandȱFe2+ȱ+ȱCu2+ȱwithȱorȱwithoutȱUVAȱ
lightȱ(coȬcatalyzedȱPEFȱandȱcoȬcatalyzedȱEF,ȱrepectively).ȱ
ȱ
Then,ȱtheȱinfluenceȱofȱtheȱvariationȱofȱseveralȱexperimentalȱparametersȱwasȱstudied.ȱ
Firstly,ȱ theȱ effectȱ ofȱ currentȱ onȱ theȱ oxidationȱ abilityȱ ofȱ eachȱ catalyzedȱ methodȱ
(containingȱ 1.0ȱ mMȱ Fe2+ȱ and/orȱ 1.0ȱ mMȱ Cu2+ȱ and/orȱ UVAȱ light)ȱ wasȱ examinedȱ byȱ
electrolyzingȱ solutionsȱ underȱ theȱ experimentalȱ conditionsȱ alreadyȱ described,ȱ atȱ 33,ȱ
100ȱ andȱ 150ȱ mAȱ cmȬ2.ȱ Secondly,ȱ theȱ effectȱ ofȱ pHȱ wasȱ clarifiedȱ byȱ treatingȱ solutionsȱ
containingȱ 157ȱ mgȱ LȬ1ȱ ofȱ drugȱ solutionsȱ atȱ initialȱ pHȱ betweenȱ 2.0ȱ andȱ 6.0,ȱ forȱ theȱ
systemȱ1.0ȱmMȱFe2+ȱ+ȱ1.0ȱmMȱCu2+ȱ+ȱUVAȱlight.ȱThirdly,ȱtheȱinfluenceȱofȱFe2+ȱandȱCu2+ȱ
concentrationsȱ wasȱ testedȱ byȱ electrolyzingȱ 157ȱ mgȱ LȬ1ȱ ofȱ drugȱ solutionsȱ ofȱ pHȱ 3.0ȱ
containingȱbothȱionsȱinȱtheȱrangeȱ0.25Ȭ1.0ȱmMȱatȱ100ȱmAȱcmȬ2ȱunderȱUVAȱirradiation.ȱ
Andȱlastly,ȱtheȱoxidationȱabilityȱofȱtheȱsystemȱ1.0ȱmMȱFe2+ȱ+ȱ1.0ȱmMȱCu2+ȱ+ȱUVAȱlightȱ
toȱdegradeȱdrugȱsolutionsȱofȱpHȱ3.0ȱatȱ100ȱmAȱcmȬ2ȱupȱtoȱnearlyȱ1ȱgȱLȬ1ȱwasȱexamined.ȱ
161
PART B –Results and Discussion7. Paracetamol
Onceȱ theȱ optimalȱ conditionsȱ wereȱ definedȱ throughȱ theȱ TOCȱ decayȱ analysis,ȱ theȱ
evolutionȱ ofȱ inorganicȱ ionsȱ wasȱ studiedȱ toȱ determineȱ theȱ lossȱ ofȱ initialȱ nitrogenȱ ofȱ
paracetamolȱ inȱ theȱ formȱ ofȱ NH4+ȱ andȱ NO3ȱ ions.ȱ Inȱ thisȱ sense,ȱ theȱ aforementionedȱ
catalyzedȱ solutionsȱ (withȱ 1.0ȱ mMȱ Fe2+ȱ and/orȱ 1.0ȱ mMȱ Cu2+ȱ and/orȱ UVAȱ light)ȱ wereȱ
electrolyzedȱforȱ6ȱhȱatȱ300ȱmAȱunderȱtheȱexperimentalȱconditionsȱpointedȱoutȱabove.ȱ
ȱ
Havingȱ concludedȱ withȱ theȱ TOCȱ decayȱ analysis,ȱ chromatographicȱ techniquesȱ wereȱ
usedȱtoȱidentifyȱtheȱstableȱintermediatesȱformedȱduringȱparacetamolȱmineralization.ȱ
GCȬMSȱallowedȱtheȱdetectionȱofȱsomeȱofȱtheȱintermediates,ȱandȱbothȱreversedȬphaseȱ
chromatographyȱ andȱ ionȬexclusionȱ chromatographyȱ wereȱ usedȱ inȱ orderȱ toȱ identifyȱ
theȱ aromaticsȱ andȱ theȱ aliphaticȱ carboxylics,ȱ respectively.ȱ Severalȱ experimentsȱ
involvingȱtheȱdegradationȱofȱintermediatesȱwereȱalsoȱcarriedȱoutȱtoȱclearlyȱestablishȱ
theȱdegradationȱpathway:ȱsolutionsȱcontainingȱ50ȱmgȱLȬ1ȱofȱketomalonic,ȱmaleicȱandȱ
fumaricȱacids,ȱwithȱ1.0ȱmMȱFe2+ȱ+ȱ1.0ȱmMȱCu2+ȱ+ȱUVAȱlight,ȱwereȱelectrolyzedȱatȱpHȱ
3.0ȱandȱatȱ300ȱmA.ȱInȱanȱanalogousȱway,ȱ50ȱmgȱLȬ1ȱofȱacetamideȱwereȱtreated.ȱ
ȱ
Onceȱtheȱidentificationȱofȱpeaksȱwasȱmade,ȱaȱ157ȱmgȱLȬ1ȱparacetamolȱsolutionȱofȱpHȱ
3.0ȱ atȱ 35ȱ ºCȱ wasȱ degradedȱ byȱ allȱ catalyzedȱ andȱ uncatalyzedȱ systemsȱ previouslyȱ
described,ȱ atȱ 100ȱ mAȱ cmȬ2,ȱ andȱ theȱ evolutionȱ ofȱ theȱ drugȱ concentrationȱ andȱ itsȱ
oxidationȱintermediatesȱwasȱdeterminedȱasȱaȱfunctionȱofȱtheȱelectrolysisȱtime.ȱ
ȱ
Finally,ȱconsideringȱallȱtheȱintermediatesȱthatȱwereȱfound,ȱaȱgeneralȱreactionȱschemeȱ
forȱtheȱmineralizationȱofȱparacetamolȱinȱacidȱmediaȱbyȱallȱindirectȱelectroȬoxidationȱ
methodsȱ withȱ H2O2ȱ electrogenerationȱ underȱ actionȱ ofȱ Fe2+,ȱ Cu2+ȱ andȱ UVAȱ lightȱ asȱ
catalystsȱcouldȱbeȱproposed.ȱ
ȱ
ȱ
ȱ
ȱ
162
PART B –Results and Discussion7. Paracetamol
Theȱthoroughȱresultsȱofȱthisȱsectionȱareȱincludedȱinȱtheȱfollowingȱpapersȱ(Paperȱ1Ȭ2):ȱ
ȱ
1.ȱSirés,ȱI.,ȱArias,ȱC.,ȱCabot,ȱP.L.,ȱCentellas,ȱF.,ȱRodríguez,ȱR.M.,ȱGarrido,ȱJ.A.,ȱBrillas,ȱ
E.,ȱ Paracetamolȱ mineralizationȱ byȱ advancedȱ electrochemicalȱ oxidationȱ processesȱ
forȱwastewaterȱtreatment.ȱEnviron.ȱChem.ȱ1ȱ(2004)ȱ26Ȭ28.ȱ
ȱ
2.ȱSirés,ȱI.,ȱGarrido,ȱJ.A.,ȱRodríguez,ȱR.M.,ȱCabot,ȱP.L.,ȱCentellas,ȱF.,ȱArias,ȱC.,ȱBrillas,ȱ
E.,ȱ Electrochemicalȱ degradationȱ ofȱ paracetamolȱ fromȱ waterȱ byȱ catalyticȱ actionȱ ofȱ
Fe2+,ȱ Cu2+,ȱ andȱ UVAȱ lightȱ onȱ electrogeneratedȱ hydrogenȱ peroxide.ȱ J.ȱElectrochem.ȱ
Soc.ȱ153ȱ(2006)ȱD1ȬD9.ȱ
ȱ
Theȱfollowingȱpresentationsȱinȱaȱcongressȱareȱrelatedȱtoȱthisȱwork:ȱ
ȱ
A.ȱBrillas,ȱE.,ȱSirés,ȱI.,ȱArias,ȱC.,ȱCabot,ȱP.L.,ȱCentellas,ȱF.,ȱRodríguez,ȱR.M.,ȱGarrido,ȱ
J.A.,ȱMineralizationȱofȱParacetamolȱbyȱPhotoelectroȬFenton,ȱVol.ȱ1,ȱpagesȱ101Ȭ102,ȱ
3rdȱ Europeanȱ Meetingȱ onȱ Solarȱ chemistryȱ andȱ Photocatalysis:ȱ Environmentalȱ
Applicationsȱ(SPEAȬ3),ȱUniversitatȱdeȱBarcelona,ȱBarcelona,ȱSpain,ȱ30ȱJuneȱȬȱ2ȱJulyȱ
2004.ȱ(Posterȱpresentation)ȱ
ȱ
B.ȱSirés,ȱI.,ȱGarrido,ȱJ.A.,ȱRodríguez,ȱR.M.,ȱCabot,ȱP.L.,ȱCentellas,ȱF.,ȱArias,ȱC.,ȱBrillas,ȱ
E.,ȱMineralizaciónȱdelȱparacetamolȱenȱmedioȱácidoȱusandoȱcátodosȱdeȱdifusiónȱdeȱ
oxígeno:ȱacciónȱcatalíticaȱdeȱFe2+,ȱCu2+ȱyȱluzȱUVAȱsobreȱelȱperóxidoȱdeȱhidrógenoȱ
electrogenerado,ȱVol.ȱ1,ȱpageȱ515,ȱXXXȱReuniónȱBienalȱdeȱlaȱRSEQȱ(XXVIIȱReuniónȱ
delȱ Grupoȱ Especializadoȱ deȱ Electroquímicaȱ deȱ laȱ RSEQ),ȱ Lugo,ȱ Spain,ȱ 19Ȭ23ȱ
Septemberȱ2005.ȱ(Posterȱpresentation)ȱ
ȱ
ȱ
163
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ARTICLEȱ1ȱ/ȱPAPERȱ1
ȱ
ȱ
Paracetamolȱmineralizationȱbyȱadvancedȱelectrochemicalȱȱ
ȱ oxidationȱprocessesȱforȱwastewaterȱtreatmentȱ
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
165
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
166
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
167
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ARTICLEȱ2ȱ/ȱPAPERȱ2
Electrochemicalȱdegradationȱofȱparacetamolȱfromȱwaterȱbyȱȱ
ȱcatalyticȱ actionȱ ofȱ Fe2+,ȱ Cu2+,ȱ andȱ UVAȱ lightȱ onȱ
electrogeneratedȱhydrogenȱperoxideȱ
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
Journal of The Electrochemical Society, 153 共1兲 D1-D9 共2006兲
D1
0013-4651/2005/153共1兲/D1/9/$15.00 © The Electrochemical Society, Inc.
ȱ
Electrochemical Degradation of Paracetamol from Water
by Catalytic Action of Fe2+, Cu2+, and UVA Light
on Electrogenerated Hydrogen Peroxide
ȱ
ȱ
Ignasi Sirés, José Antonio Garrido, Rosa María Rodríguez, Pere l.luís Cabot,*
Francesc Centellas, Conchita Arias, and Enric Brillasz
ȱ
Laboratori de Ciència i Tecnologia Electroquímica de Materials, Departament de Química Física,
Facultat de Química, Universitat de Barcelona, 08028 Barcelona, Spain
ȱ
Acidic aqueous solutions of the drug paracetamol have been degraded by anodic oxidation and indirect electro-oxidation methods
using an undivided electrolytic cell with a Pt anode and an O2-diffusion cathode for H2O2 electrogeneration. Anodic oxidation
yields low mineralization due to the limited production of oxidant hydroxyl radical 共 ·OH兲 from water oxidation at Pt. The presence
of Cu2+ as catalyst, with and without 共ultraviolet A, UVA兲 irradiation, slightly enhances the degradation process. In electro-Fenton,
much more ·OH is produced from Fenton’s reaction between added Fe2+ and electrogenerated H2O2, but stable Fe3+ complexes are
formed. These species are partially photodecomposed in photoelectro-Fenton under UVA irradiation. The use of Fe2+ and Cu2+
yields fast decontamination because Cu2+ complexes are destroyed. Total mineralization of paracetamol is achieved when Fe2+,
Cu2+, and UVA light are combined. The influence of current, pH, and drug concentration upon the efficiency of catalyzed methods
is studied. Hydroquinone, p-benzoquinone, and carboxylic acids, such as ketomalonic, maleic, fumaric, oxalic, and oxamic, are
detected as intermediates. The positive synergetic effect of all catalysts is explained by the oxidation of Cu2+-oxalato and
Cu2+-oxamato complexes with ·OH, along with the photodecarboxylation of Fe3+-oxalato and Fe3+-oxamato complexes by UVA
light. NH+4 and NO−3 are released during drug mineralization.
© 2005 The Electrochemical Society. 关DOI: 10.1149/1.2130568兴 All rights reserved.
ȱ
ȱ
ȱ
ȱ
Manuscript submitted June 6, 2005; revised manuscript received September 2, 2005. Available electronically November 16, 2005.
ȱ
In recent years indirect electro-oxidation methods with
peroxide electrogeneration, such as electro-Fenton and
photoelectron-Fenton reactions, are being developed for the treatment of toxic organic pollutants in waters.1-18 These environmenȱ tally
clean electrochemical techniques are carried out in an electrolytic cell where H2O2 is continuously generated in the contaminated
the two-electron reduction of O2 at reticulated vitreȱ solution from1,3,6,7
graphite,2 mercury pool,8,13 carbon-felt,10,11,16,18
ous carbon,
and O2-diffusion4,5,9,12,14,15,17 cathodes
ȱ hydrogen
ȱ
O2 + 2H+ + 2e− → H2O2
关1兴
Hydrogen peroxide thus produced is a weak oxidant of organics. In
ȱ the electro-Fenton reaction, the oxidizing power of this species is
enhanced by addition of small amounts of Fe2+ as catalyst to the
acidic treated solution. Hydroxyl radical 共 ·OH兲 and Fe3+ are then
ȱ generated
from the classical Fenton’s reaction between Fe2+ and
H2O2 with a second-order rate constant k2 of 53 dm3 mol−1 s−119,20
ȱ
Fe2+ + H2O2 → Fe3+ + ·OH + OH−
关2兴
An advantage of this method is that Reaction 2 is propagated from
ȱ Fe2+ regeneration, which mainly occurs by reduction of Fe3+ species
at the cathode or in the medium with H2O2. Hydroxyl radical acts as
a nonselective, strong oxidant because it is able to react with organȱ ics, yielding dehydrogenated or hydroxylated derivatives, until their
overall mineralization 共conversion into CO2 and inorganic ions兲 is
ȱ achieved.
The photoelectro-Fenton method also involves the irradiation of
the solution with 共ultraviolet A, UVA兲 light to favor the regeneration
photoreduction of Fe共OH兲2+, which is the
ȱ of Fe2+ from additional
predominant Fe3+ species in acid medium19,20
Fe共OH兲2+ + h␯ → Fe2+ + ·OH
ȱ
关3兴
·
Reaction 3 accelerates the production of OH and, hence, the minof organics. In addition, UVA light can photodecompose
ȱ eralization
complexes of Fe3+ with some oxidation products, for example, with
oxalic acid.9,12,14,15,17,21
attempts have also been made to show the possible cataȱ lyticSome
effect of Cu2+, alone or combined with Fe2+, on the above
ȱ
* Electrochemical Society Active Member.
z
ȱ
E-mail: [email protected]
procedures. Gözmen et al.16 have found that bisphenol A in 0.01 M
HCl is more rapidly degraded by the electro-Fenton reaction with
Fe2+ than when electrogenerated H2O2 and Cu2+ are used. In previous work17 we have described that addition of Cu2+ to nitrobenzene
solutions of pH 3.0 accelerates their electro-Fenton and
photoelectro-Fenton processes. A positive synergetic effect of Fe2+
and Cu2+ could then be expected for the degradation of other aromatic compounds in waters using these indirect electro-oxidation
systems. At the end of such treatments, the resulting acid effluent
should be neutralized up to pH 7–9 for complete decontamination by
precipitation of metallic ions in the form of Fe共OH兲2, Fe共OH兲3, and
Cu共OH兲2 before disposal. The collected precipitate could even be
reused as a catalyst in further processes.
Recently, there is great interest in the environmental relevance of
pharmaceutical drugs in waters. This pollution can be due to emission from production sites, direct disposal of overplus drugs in
households, excretion after drug administration to humans and animals, and treatments throughout the water in fish farms.22 A large
number of pharmaceutical drugs such as antiinflammatories, analgesics, betablockers, lipid regulators, antibiotics, antiepileptics, and
estrogens have been detected as minor pollutants with concentrations ⬍10 ␮g L−1 in sewage treatment plant 共STP兲 effluents, surface and ground waters, and even in drinking water.22-26 Paracetamol 关N-共4-hydroxyphenyl兲acetamide兴, a common analgesic and
antiinflammatory for humans and animals, has been found with concentrations up to 6 ␮g L−1 in European STP effluents25 and up to
10 ␮g L−1 in USA natural waters.26
To avoid the potential dangerous accumulation of drugs in the
aquatic environment, research efforts are underway to develop powerful oxidation techniques for achieving their destruction. Several
works have reported the successful use of ozonation and advanced
oxidation processes 共AOPs兲 such as O3 /H2O2, H2O2 /UV, and
H2O2 /Fe2+ /UV, with production of ·OH as the main oxidant, for the
degradation of pharmaceuticals and their metabolites in water.25,27-30
For paracetamol, a poor mineralization of 30 and 40% is found from
O3 and H2O2 /UV methods, respectively, in the pH range
2.0–5.5.25 In both procedures, hydroquinone, 2-hydroxy-4共N-acetyl兲aminophenol, 1,2,4-trihydroxybenzene, maleic acid, and
oxalic acid are detected as intermediates. In previous work,31 we
have studied the direct anodic oxidation of solutions containing
paracetamol concentrations up to 1 g L−1 in the pH range 2.0–12.0
using an electrolytic cell with a Pt or a boron-doped diamond 共BDD兲
ȱ
169
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
Journal of The Electrochemical Society, 153 共1兲 D1-D9 共2006兲
D2
ȱ anode and a graphite cathode. Complete mineralization of the drug
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
with release of NH+4 and NO−3 ions was always obtained with the
BDD anode due to the great production of oxidant ·OH on its surface from water oxidation.32-35 Under these conditions, the mineralization rate was pH-independent, the paracetamol decay followed a
complex kinetics, and only oxalic and oxamic acids were identified
as intermediates in all media. Comparative treatment of the same
solutions with the Pt anode yielded a quite poor mineralization,
because this electrode produces much lower ·OH concentration.
However, a slow but complete destruction of paracetamol was
achieved using the Pt anode following its kinetics as a pseudo firstorder reaction with a constant rate independent of pH.
To clarify the possible application of indirect electro-oxidation
methods with H2O2 electrogeneration to the removal of aromatic
drugs from waters, we have carried out a study on the mineralization
of paracetamol using Fe2+, Cu2+, and/or UVA light as catalysts.
Higher drug concentrations than those found in STP and natural
effluents were chosen to better analyze the oxidation ability of these
methods. In this paper, we report the degradation of 157 mg L−1
paracetamol solutions of pH 3.0 using an undivided cell with a Pt
anode and an O2-diffusion cathode able to generate H2O2. A Pt
electrode, instead of a BDD one, was preferred as anode because its
very low oxidation allows showing an easier and clearer synergetic
effect of catalysts on the degradation process. Comparative electrolyses were then performed with this system 共anodic oxidation
with H2O2 electrogeneration兲 and with UVA light, 1 mM Cu2+,
1 mM Cu2+ + UVA light, 1 mM Fe2+ 共electro-Fenton process兲,
1 mM Fe2+ + UVA light 共photoelectro-Fenton process兲, 1 mM
Fe2+ + 1 mM Cu2+, and 1 mM Fe2+ + 1 mM Cu2+ + UVA light.
The influence of applied current density, solution pH, and drug concentration upon the behavior of the catalytic methods was also explored. For each method, the drug decay was followed and its stable
intermediates were identified and quantified. A reaction scheme for
paracetamol mineralization involving the detected by-products is
proposed.
ȱ
Reagents.— Paracetamol, hydroquinone, p-benzoquinone, aceta-
ȱ
ȱ
oxamic acid were reagent grade from Merck, Sigma-Aldrich, and
Panreac. Anhydrous sodium sulfate, heptahydrated ferrous sulfate,
and pentahydrated cupric sulfate were analytical grade from Fluka.
Analytical grade sulfuric acid was purchased from Merck. All solutions were prepared with pure water obtained from a Millipore
Milli-Q system with resistivity ⬎18 M⍀ cm at 25°C. Organic solvents and other chemicals employed were either high-pressure liquid
chromatography 共HPLC兲 or analytical grade from Panreac.
Apparatus.— Electrolyses were performed with an Amel 2053
The mineralization of paracetamol soluȱ potentiostat-galvanostat.
tions was determined from the abatement of their total organic car-
bon 共TOC兲, monitored on a Shimadzu VCSN TOC analyzer. Arointermediates were separated and identified by gas
chromatography mass spectroscopy 共GC-MS兲 with a HewlettPackard system consisting of a HP 5890 Series II gas chromatograph fitted with an HP-5 0.25-␮m, 30-m ⫻ 0.25-mm column, and
coupled to an HP 5989A mass spectrometer operating in EI mode at
70 eV and at 300°C. The paracetamol decay and the evolution of its
aromatic intermediates were followed by reversed-phase chromatography using a system composed of a Waters 600 HPLC liquid chromatograph fitted with a Spherisorb ODS2 5 ␮m, 150 ⫻ 4.6 mm
column at room temperature, coupled with a Waters 996 photodiode
array detector selected at ␭ = 280 nm and controlled through a
Millennium-32 program. Generated carboxylic acids were detected
by ion-exclusion chromatography using the above HPLC chromatograph fitted with an Aminex HPX 87H, 300 ⫻ 7.8 mm column at
35°C from Bio-Rad and the photodiode array detector selected at
␭ = 210 nm. NH+4 concentration in treated solutions was determined
from the standard colorimetric method with Nessler’s reagent, using
ȱ matic
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Electrolytic system.— All electrolyses were conducted in an
open, undivided, and thermostated glass-cylindrical cell containing
100 mL of solution stirred with a magnetic bar. A 3-cm2 Pt sheet
of 99.99% purity from SEMPSA and a 3-cm2 carbonpoly共tetrafluoroethylene兲 共PTFE兲 electrode from E-TEK were used
as the anode and cathode, respectively. The last electrode was fed
with pure O2 at 20 mL min−1 to generate continuously H2O2 from
Reaction 1. The electrolytic setup and the preparation of the
O2-diffusion cathode have been described.9 For the trials with UVA
irradiation, a Philips 6 W fluorescent black light blue tube was
placed at the top of the open cell at 7 cm above the solution. The
tube emitted UVA light in the wavelength region between 300 and
420 nm, with ␭max = 360 nm, supplying a photoionization energy
input to the solution of 140 ␮W cm−2, detected with a NRC 820
laser power meter working at 514 nm.
Comparative degradation of solutions containing 157 mg L−1 of
paracetamol and 0.05 M Na2SO4 of pH 3.0 adjusted with H2SO4
was carried out at a constant current density 共 j兲 of 33, 100, and
150 mA cm−2, applying an average cell voltage of 5.5, 13.0, and
17.5 V, respectively. The catalytic effect of Fe2+ and/or Cu2+ was
studied by adding 1 mM of each ion, because this Fe2+ content was
very efficient in the electro-Fenton treatment of other
aromatics.9,12,14,15,17 For the electrolyses starting from pH 4.0 and
6.0, the solution pH was regulated within a range of ±0.3 units by
adding small volumes of 0.5 M NaOH each 20 min. All trials were
carried out at 35°C, which is the maximum temperature to work
with the open electrolytic system without significant water evaporation from solution.12
Experimental
ȱ mide, ketomalonic acid, maleic acid, fumaric acid, oxalic acid, and
ȱ
a Unicam UV/vis UV4 spectrophotometer thermostated at 25°C.
NO−3 concentration in the same solutions was obtained by ion chromatography with a Shimadzu LC-10AT共VP兲 liquid chromatograph
coupled with a Metrohm 690 ion chromatograph, fitted with a
Hamilton PRP-X 100 10 ␮m, 150 ⫻ 4.1 mm anion column at room
temperature and controlled with an HP 35900E interface.
ȱ
170
Product analysis procedures.— Before analysis, the samples extracted were filtered with 0.45-␮m PTFE filters from Whatman. Reproducible TOC values were obtained from analysis of 100-␮L aliquots using the standard nonpurgeable organic carbon method. In
reversed-phase chromatography, 70:30 共v/v兲 acetonitrile/water and
95:5 共v/v兲 0.1 M HCOOH + NaOH 共pH 3.0兲/acetonitrile mixtures
were employed as mobile phases at 1.2 mL min−1, whereas in ionexclusion chromatography, the mobile phase was 4 mM H2SO4 at
0.6 mL min−1.1 In both HPLC techniques, 20-␮L samples were injected into the chromatograph. NO−3 concentration was determined
using a 90:10 共v/v兲 2 mM phthalate buffer 共pH 5.0兲/acetone mixture
as mobile phase at 2 mL min−1. To identify the aromatic products,
several paracetamol solutions were electrolyzed during short times
and their organic components were extracted three times with
25 mL of CH2Cl2. Each collected organic solution was then dried
with anhydrous Na2SO4, and once filtered, its volume was reduced
to about 5 mL to concentrate the remaining products for further
analysis by GC-MS.
Results and Discussion
Comparative degradation of paracetamol.— The oxidation ability of the different indirect electro-oxidation treatments was tested
by electrolyzing 157 mg L−1 paracetamol solutions 共equivalent to
100 mg L−1 of TOC兲 of pH 3.0 at 100 mA cm−2 and at 35°C for
6 h. In all experiments the solution pH always remained practically
constant, reaching a final value between 2.8 and 3.0. The comparative TOC abatement for the above trials is depicted in Fig. 1.
In the electrolytic system, hydrogen peroxide is continuously injected into the solution from Reaction 1, whereas adsorbed ·OH is
formed on the Pt surface from water oxidation32-35
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
Journal of The Electrochemical Society, 153 共1兲 D1-D9 共2006兲
D3
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
1. TOC decay vs electrolysis time for the degradation of 100 mL of
ȱ Figure
157 mg L
paracetamol solutions in 0.05 M Na SO of pH 3.0 at
−1
2
−2
4
2
100 mA cm and at 35°C, using a cell with a 3-cm Pt anode and a
3-cm2 O2-diffusion cathode for H2O2 electrogeneration. Catalyst: 共a, 䊊兲
2 2 electrogeneration兲, 共b, 쎲兲 UVA light with
␭max = 360 nm, 共c, 䊐兲 1 mM Cu2+, 共d, 䊏兲 1 mM Cu2+ + UVA light, 共e, 䉭兲
1 mM Fe2+ 共electro-Fenton process兲, 共f, 䉱兲 1 mM Fe2+ + UVA light
共photoelectro-Fenton process兲, 共g, 〫兲 1 mM Fe2+ + 1 mM Cu2+, and 共h, ⽧兲
1 mM Fe2+ + 1 mM Cu2+ + UVA light.
ȱ None 共anodic oxidation with H O
ȱ
ȱ
ȱ
H2O → ·OHads + H+ + e−
ȱ
In addition, part of the electrogenerated H2O2 is also oxidized to O2
at the anode via the hydroperoxyl radical 共HO·2兲, a weaker oxidant
than ·OH9,17
ȱ
关4兴
H2O2 → HO·2 + H+ + e−
关5兴
HO·2 → O2 + H+ + e−
关6兴
ȱ The use of the electrolytic system without any catalyst corresponds
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
to the method of anodic oxidation with H2O2 electrogeneration. As
can be seen in curve a of Fig. 1, this treatment leads to a quite slow
TOC decay, only attaining 15% of mineralization at 6 h. This can be
explained by the low concentration of ·OH formed on the Pt anode
surface from Reaction 4, which is the main oxidant of paracetamol
and its products. When the solution is exposed to UVA light 共see
curve b of Fig. 1兲, the degradation process is slightly enhanced to
give 19% of TOC removal. This behavior suggests a photodecomposition of several intermediates that accelerates the mineralization
process, because UVA light does not photolyze H2O2 to ·OH. Curve
c of Fig. 1 shows that the presence of 1 mM Cu2+ causes a faster
degradation rate to reach 28% of decontamination. This enhancement can be accounted for by 共i兲 the oxidation of complexes of Cu2+
with intermediates17 and 共ii兲 the production of small amounts of ·OH
in the medium from the Cu2+ /Cu+ catalytic system,36,37 involving
the reduction of Cu2+ to Cu+ with HO·2 by Reaction 7 with
k2 = 5 ⫻ 107 dm3 mol−1 s−138 and/or with organic radicals R· by
Reaction 8
Cu2+ + HO·2 → Cu+ + H+ + O2
关7兴
Cu2+ + R· → Cu+ + R+
关8兴
followed by regeneration of Cu2+ by oxidation of Cu+ with H2O2
from the Fenton-like Reaction 9 with k2 = 1 ⫻ 104 dm3 mol−1 s−139
Cu+ + H2O2 → Cu2+ + ·OH + OH−
关9兴
The slightly greater degradation observed in curve d of Fig. 1 under
UVA illumination of the 1 mM Cu2+ solution also suggests additional photolysis of some oxidation products.
A much higher TOC removal is achieved when Fe2+ is added as
catalyst. For the electro-Fenton process with 1 mM Fe2+ 共see curve
e of Fig. 1兲, TOC is rapidly reduced by 66% at 6 h, which can be
Figure 2. Concentration of ammonium ion accumulated during the treatment
of 100 mL of 157 mg L−1 paracetamol solutions of pH 3.0 by the catalyzed
experiments reported in Fig. 1. Catalyst: 共䊐兲 1 mM Cu2+, 共䊏兲 1 mM Cu2+
+ UVA light, 共䉭兲 1 mM Fe2+, 共䉱兲 1 mM Fe2+ + UVA light, 共〫兲 1 mM
Fe2+ + 1 mM Cu2+, and 共⽧兲 1 mM Fe2+ + 1 mM Cu2+ + UVA light.
related to the fast reaction of organics with the great amounts of ·OH
produced from Fenton’s Reaction 2. However, the electro-Fenton
reaction does not yield total mineralization due to the formation of
products that do not react with ·OH, e.g., complexes of short carboxylic acids with Fe3+.9,14,15,17 Combination of UVA light with
1 mM Fe2+ in the photoelectro-Fenton process 共see curve f of Fig.
1兲 already leads to 87% mineralization. This trend can be related to
共i兲 the quick photodecomposition of some stable Fe3+ complexes
under electro-Fenton conditions and/or 共ii兲 the faster generation of
·
OH from additional photoreduction of Fe共OH兲2+ from Reaction 3.
Curve g of Fig. 1, obtained with 1 mM Fe2+ and 1 mM Cu2+ as
catalysts, shows a similar TOC decay to that of the photoelectroFenton reaction, attaining 90% mineralization. This suggests that
·
OH can easily oxidize some complexes of intermediates with Cu2+,
competitively formed with those of Fe3+. As can be seen in curve h
of Fig. 1, all complexes of Cu2+ and Fe3+ are completely destroyed
when 1 mM Fe2+, 1 mM Cu2+, and UVA light are combined, since
overall mineralization 共⬎98% TOC decay兲 is reached at the end of
electrolysis.
The above results indicate that the oxidation ability
of the catalyzed methods increases in the order 1 mM Cu2⫹⬍1 mM
Cu2⫹⫹UVA lightⰆ1 mM Fe2⫹⫹UVA light艋1 mM Fe2⫹⫹1 mM
Cu2⫹⬍1 mM Fe2⫹⫹1 mM Cu2⫹⫹UVA light. However, only the
last method is potent enough to destroy paracetamol completely.
Evolution of inorganic ions.— The possible loss of the initial nitrogen of paracetamol in the form of inorganic ions such as NH+4 and
NO−3 during its mineralization was investigated. No nitrite ions were
detected in electrolyzed solutions. Figure 2 shows a rapid accumulation of NH+4 during the early stages of the above catalyzed treatments and a slow release of this ion from 2 h. The percentage of
initial N converted into NH+4 is 48% for 1 mM Cu2+, 51% for 1 mM
Cu2+ + UVA light, 53% for 1 mM Fe2+, 66% for 1 mM Fe2+
+ UVA light, 75% for 1 mM Fe2+ + 1 mM Cu2+, and 93% for
1 mM Fe2+ + 1 mM Cu2+ + UVA light. In contrast, quite low NO−3
concentrations were found in the same final electrolyzed solutions:
for example, 6.35 mg L−1 共10% of initial N兲 for 1 mM Fe2+
+ 1 mM Cu2+ and a much lower value of 0.7 mg L−1 共1% of initial
N兲 for the same catalysts under UVA irradiation. These results indicate that the nitrogen of paracetamol is mainly lost as NH+4 , whereas
only a minor portion of it is oxidized to NO−3 . More NH+4 is formed
with rising oxidation ability of the methods due to the faster mineralization of some nitrogen-containing intermediates produced at the
early stages of treatments. UVA irradiation also favors the release of
NH+4 , instead of NO−3 , probably by photolysis of such by-products.
ȱ
171
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ D4
Journal of The Electrochemical Society, 153 共1兲 D1-D9 共2006兲
ȱ
contrast, all the other methods involving the Fe3+ /Fe2+ system are
much more efficient because they have much higher oxidation ability. The MCE values at 20 min are close to 13% for the
electro-Fenton reaction, 19% for the photoelectron-Fenton reaction,
and 21% for 1 mM Fe2+ + 1 mM Cu2+ with and without UVA
light. At longer times, however, they undergo a dramatic
drop
toward
the
end
of
electrolysis,
increasing
in
the
order
1 mM Fe2+ ⬍ 1 mM Fe2+ + UVA light
艋 1 mM Cu2+ + 1 mM Fe2+ ⬍ 1 mM Cu2+ +1 mM Fe2+ + UVA
light. The gradual decay in efficiency with time can be related to the
concomitant fall in pollutant content with formation of more stable
by-products, thus favoring the loss of ·OH by parallel nonoxidizing
reactions, e.g., its reaction with Fe2+ and/or Cu+ and its recombination into H2O2.19,36 This trend is not so clear for 1 mM Cu2+ because organics are much more slowly degraded.
ȱ
ȱ
ȱ
ȱ
ȱ
Figure 3. Dependence of mineralization current efficiency calculated from
ȱ Eq. 11 on electrolysis time for the catalyzed experiments given in Fig. 1.
ȱ
ȱ
ȱ
ȱ
Catalyst: 共䊐兲 1 mM Cu2+, 共䊏兲 1 mM Cu2+ + UVA light, 共䉭兲 1 mM Fe2+,
共䉱兲 1 mM Fe2+ + UVA light, 共〫兲 1 mM Fe2+ + 1 mM Cu2+, and 共⽧兲
1 mM Fe2+ + 1 mM Cu2+ + UVA light.
Mineralization current efficiency.— The electrochemical destruction of paracetamol involves its transformation into CO2 and
mainly NH+4 as inorganic ion. The overall reaction can be written as
follows
HO–C6H4–NH–CO–CH3 + 14H2O → 8CO2 + NH+4 + 33H+
关10兴
+ 33e−
ȱ Reaction 10 presupposes the consumption of 33 F per mole of comȱ
ȱ
ȱ
ȱ
pound. The mineralization current efficiency 共MCE兲 for each experiment was then determined from the following expression
MCE = 关⌬共TOC兲exper /⌬共TOC兲theor兴 ⫻ 100
关11兴
where ⌬共TOC兲exper is the experimental TOC removal in the solution
at a given time and ⌬共TOC兲theor is its theoretical TOC decay assuming that the applied electrical charge 共=current ⫻ time兲 is only consumed to mineralize paracetamol by Reaction 10.
Figure 3 shows the evolution of the efficiency calculated from
Eq. 11 for the catalyzed experiments depicted in Fig. 1. For 1 mM
Cu2+ in the absence and presence of UVA light, this parameter is as
low as 1.0–1.4%, slightly increasing at longer electrolysis times. In
Effect of experimental parameters.— The influence of current
on the oxidation ability of each catalyzed method was examined by
electrolyzing 157 mg L−1 paracetamol solutions of pH 3.0 at 33,
100, and 150 mA cm−2. Selected results after 1 and 4 h of such
trials are collected in Table I. In all systems the percentage of TOC
removal increases with increasing j. This enhancement in degradation power can be ascribed to a greater production of ·OH at the Pt
anode from Reaction 4 and of H2O2 by the O2-diffusion cathode
from Reaction 1.9,17 The larger accumulation of H2O2 causes the
acceleration of Reactions 2 and/or 9, yielding more ·OH concentration that favors the oxidation of pollutants. Results of Table I indicate that even at 150 mA cm−2 the action of Cu2+ with and without
UVA light is notably poor, leading to a maximum TOC removal of
21% at 4 h. Under these conditions, the electro-Fenton reaction with
Fe2+ is much more effective with 60% TOC decay, because of the
much faster generation of ·OH by Reaction 2 than by Reaction 9.
The use of either Fe2+ + UVA light or Fe2+ + Cu2+ yields a similar
TOC reduction of 80–81% after 4 h at 150 mA cm−2, indicating that
different stable species under electro-Fenton conditions are mineralized in each one of these systems. These products are totally destroyed by the combined action of Fe2+, Cu2+, and UVA light, reaching about 95–96% mineralization after 4 h at both 100 and
150 mA cm−2. Table I also shows that at a given time the efficiency
of each method always drops with increasing j, i.e., when more ·OH
is produced, as stated above. This apparent contradictory behavior
ȱ
ȱ
Table I. Effect of applied current on the percentage of TOC removal and MCE for the degradation of 157 mg L−1 paracetamol solutions of pH
3.0 at 35°C by indirect electro-oxidation methods with H2O2 electrogeneration using different catalysts under selected experimental conditions.
After 1 h of treatment
ȱ
Catalyst
1 mM Cu2+
ȱ
ȱ
1 mM Cu2+ + UVA light
ȱ
1 mM Fe2+
共electro-Fenton process兲
ȱ
1 mM Fe2+ + UVA light
共photoelectro-Fenton process兲
ȱ
1 mM Fe2+ + 1 mM Cu2+
ȱ
1 mM Fe2+ + 1 mM Cu2+
+UVA light
ȱ
ȱ
172
j 共mA cm−2兲
33
100
150
33
100
150
33
100
150
33
100
150
33
100
150
33
100
150
% TOC removal
0.3
3.2
3.9
0.4
3.4
4.7
24
33
39
39
44
49
21
47
48
28
53
61
After 4 h of treatment
MCE
2.8
0.9
0.8
3.6
0.9
0.9
22
10
8.0
36
13
9.4
19
14
9.4
25
16
12
% TOC removal
16
17
19
19
20
21
55
59
60
75
79
80
53
80
81
65
95
96
MCE
3.7
1.3
1.3
4.3
1.5
0.9
12
4.5
3.0
17
6.0
4.0
12
6.1
4.4
15
7.3
4.9
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
Journal of The Electrochemical Society, 153 共1兲 D1-D9 共2006兲
D5
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Figure 4. Effect of pH on TOC removal of 100 mL of 157 mg L
ȱ etamol
solutions treated with 1 mM Fe + 1 mM Cu + UVA
100 mA cm
ȱ 共〫兲 6.0.
−1
paraclight at
and at 35°C. Initial solution pH: 共䊊兲 2.0, 共䊐兲 3.0, 共䉭兲 4.0, and
2+
−2
2+
ȱ can be related to the oxidation of a larger proportion of this radical
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
to O2 at the anode and the acceleration of its nonoxidizing reactions
in the medium.
The effect of pH was clarified by treating solutions containing
157 mg L−1 of drug and initial pH between 2.0 and 6.0 with 1 mM
2+
Fe + 1 mM Cu2+ + UVA light. As an example, Fig. 4 shows the
TOC–time plots obtained at 100 mA cm−2, where the quickest TOC
decay can be observed at pH 3.0. The same trend was found for this
method at 33 and 150 mA cm−2, as well as for similar treatments
using the Fe2+, Fe2+ + UVA light, and Fe2+ + Cu2+ systems. This
behavior can be related to the highest generation rate of their main
oxidant ·OH from Reaction 2, because its optimum pH is 2.8,19 very
close to pH 3.0 where paracetamol and its oxidation products are
more rapidly destroyed.
The possible influence of Fe2+ and Cu2+ concentrations was
tested by electrolyzing 157 mg L−1 drug solutions of pH 3.0 containing between 0.25 and 1 mM of both ions at 100 mA cm−2 under
UVA illumination. As can be seen in Fig. 5, all solutions are mineralized with similar rate up to 95–98% of TOC reduction at 6 h,
indicating that such ions act in catalytic amounts to destroy paracetamol.
The oxidation ability of the system with 1 mM Fe2+ + 1 mM
Cu2+ + UVA light to degrade drug concentrations ⬍1 g L−1 of pH
3.0 at 100 mA cm−2 was also examined. Figure 6a shows that total
mineralization is attained for up to 313 mg L−1 of paracetamol,
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Figure 5. TOC removal with electrolysis time for the treatment of 100-mL
solutions of pH 3.0 containing 157 mg L−1 paracetamol and different Fe2+
and Cu2+ concentrations under UVA irradiation, at 100 mA cm−2 and at
35°C: 共䊊兲 0.25 mM Fe2+ + 0.25 mM Cu2+, 共䊐兲 1 mM Fe2+ + 0.25 mM
Cu2+, 共䉭兲 0.25 mM Fe2+ + 1 mM Cu2+, and 共〫兲 1 mM Fe2+ + 1 mM Cu2+.
Figure 6. 共a兲 TOC abatement with electrolysis time for the degradation of
100-mL solutions of pH 3.0 containing paracetamol concentrations of 共䊊兲
940, 共䊐兲 625, 共䉭兲 313, 共〫兲 157, and 共䉮兲 78 mg L−1 using 1 mM Fe2+
+ 1 mM Cu2+ + UVA light at 100 mA cm−2 and at 35°C. 共b兲 Change of the
mineralization current efficiency calculated from Eq. 11 with time for the
same experiments.
whereas 6 and 10% TOC remain in solution from 625 and
940 mg L−1, respectively, after prolonged electrolysis. The method
is then able to destroy up to ca. 0.4 g L−1 of drug under the present
experimental conditions. Figure 6b presents the MCE–time plots for
the experiments of Fig. 6a. As can be seen, the efficiency increases
with rising drug concentration, indicating a faster removal of larger
amounts of organics. Because the same production of ·OH is expected from Reactions 2, 3, 4, and 9 in all trials, it seems plausible
to consider that its competitive nonoxidizing reactions become
slower and more ·OH concentration can then react with pollutants.
From 313 mg L−1 of paracetamol, MCE progressively rises during
longer times at early stages of the treatment, reaching a maximum
value of 36% for 940 mg L−1 at 2 h. This suggests an increasing
formation of products that react more easily with ·OH than the drug
at early stages of electrolysis.
From the above findings, one can conclude that indirect electrooxidation methods with H2O2 electrogeneration using at least Fe2+
as catalyst are more effective for paracetamol degradation from water than classical ozonation and H2O2 /UV.25 For these electrochemical techniques, the optimum operative pH is 3.0. When Fe2+, Cu2+,
and UVA light are combined, small quantities 共up to 1 mM兲 of both
ions are needed for achieving total mineralization of solutions containing up to about 0.4 g L−1 of drug at low current.
Identification of intermediates.— An attempt was made to identify the stable aromatic intermediates formed during paracetamol
mineralization by means of GC-MS. To do this, solutions with 157
and 313 mg L−1 of this compound at pH 3.0 were electrolyzed at
100 mA cm−2 and at 35°C by anodic oxidation for 20 min
and using 1 mM Fe2+ + 1 mM Cu2+ + UVA light for 5 min. All MS
ȱ
173
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
Journal of The Electrochemical Society, 153 共1兲 D1-D9 共2006兲
D6
ȱ spectra
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
displayed the peak of the remaining paracetamol
关m/z = 151 共21, M+兲兴 at tr 共retention time兲 = 22.5 min, along with
two other peaks associated with the primary product hydroquinone
关m/z = 110 共100, M+兲兴 at tr = 15.2 min, and its oxidation product
p-benzoquinone 关m/z = 108 共51, M+兲兴 at tr = 9.6 min. No other
products were detected after derivatization of the organics contained
in the same treated solutions with bis共trimethylsilyl兲trifluoroacetamide.
Reversed-phase chromatograms of electrolyzed solutions with a
70:30 共v/v兲 acetonitrile/water mixture as mobile phase exhibited the
peaks of paracetamol 共tr = 1.20 min兲 and p-benzoquinone
共tr = 1.49 min兲, whereas the use of a 95:5 共v/v兲 0.1 M HCOOH
+ NaOH 共pH 3.0兲/acetonitrile mixture allowed the detection of hydroquinone 共tr = 3.50 min兲. These products were unequivocally
identified from comparison of their tr values and uv-visible 共UV-vis兲
spectra, measured on the photodiode array detector, with those of
pure compounds. The ion-exclusion chromatograms of treated solutions displayed peaks associated with generated carboxylic acids
such as oxalic 共tr = 6.7 min兲, ketomalonic 共tr = 6.8 min兲, maleic
共tr = 8.1 min兲, oxamic 共tr = 9.4 min兲, and fumaric 共tr = 15.8 min兲
acids.
Ketomalonic, maleic, and fumaric acids come from the oxidation
of the aryl moiety of paracetamol, as reported for other
aromatics.5,7,12,14,17,33-35 The treatment of solutions containing
50 mg L−1 of each one of these acids of pH 3.0 with 1 mM Fe2+
+ 1 mM Cu2+ + UVA light showed that they are only oxidized to
oxalic acid. Oxamic acid could be produced from ·OH attack on
acetamide, released when paracetamol gives hydroquinone. This
was confirmed by treating 50 mg L−1 of acetamide with the above
system at pH 3 and at 100 mA cm−2, because only oxamic acid was
detected as product. Electrolyses of solutions with 50 mg L−1 of
oxalic or oxamic acid of pH 3.0 at 100 mA cm−2 revealed that both
acids remain stable in the presence of 1 mM Fe2+, whereas they are
slowly degraded using 1 mM Cu2+. When such solutions were exposed to UVA light without applying current, it was found that both
acids are not photolyzed with 1 mM Cu2+, but the presence of
1 mM Fe2+ causes a quick and overall transformation of oxalic acid
into CO2 and a very slow mineralization of oxamic acid. It was also
confirmed that only NH+4 is released when oxamic acid is mineralized.
Paracetamol decay and evolution of intermediates.— Once the
identity of chromatographic peaks was made, a 157 mg L−1 paracetamol solution of pH 3.0 at 35°C was degraded by all treatments at
100 mA cm−2, and the concentration of the drug and its products
was determined as a function of electrolysis time via external calibration by using standard compounds.
Figure 7 shows that paracetamol undergoes a slow and similar
decay for anodic oxidation and in the presence of Cu2+, both with
and without UVA irradiation, disappearing from the medium in
75 min. These findings indicate that the main oxidant in these methods is ·OH formed in small amount on the anode from Reaction 4. In
contrast, the drug is rapidly removed in 6 min with a similar rate for
the four treatments involving Fe2+, alone or combined with Cu2+
and/or UVA light, thus confirming that it is mainly destroyed by the
large amounts of ·OH generated from Fenton’s Reaction 2, with
little contribution of Reaction 4. Note that the decay of paracetamol
does not follow kinetic equations related to simple reaction orders.
This suggests the existence of a complex ·OH attack on this compound, leading to different primary products such as hydroquinone
and 2-hydroxy-4-共N-acetyl兲aminophenol, identified during its treatment with O3 and H2O2 /UV.25 Under our experimental conditions,
however, the second species is undetected, probably because it is
rapidly destroyed by ·OH.
The evolution of hydroquinone and p-benzoquinone for the catalyzed methods is shown in Fig. 8a and b, respectively. In all cases
these products are present in the medium while the initial drug persists in it. A small concentration of about 0.6 mg L−1 is achieved as
ȱ
174
Figure 7. Paracetamol concentration decay for the experiments reported in
Fig. 1. Catalyst: 共䊊兲 none, 共쎲兲 UVA light, 共䊐兲 1 mM Cu2+, 共䊏兲 1 mM
Cu2+ + UVA light, 共䉭兲 1 mM Fe2+, 共䉱兲 1 mM Fe2+ + UVA light, 共〫兲
1 mM Fe2+ + 1 mM Cu2+, and 共⽧兲 1 mM Fe2+ + 1 mM Cu2+ + UVA light.
maximum for hydroquinone using 1 mM Cu2+ in the presence and
absence of UVA irradiation, as expected if it is rapidly oxidized to
p-benzoquinone. In both procedures the latter species is more slowly
degraded and can reach maximum concentrations of 19–23 mg L−1
at 20 min. However, both products are quickly formed and destroyed at a similar rate in the methods catalyzed at least with Fe2+,
attaining their maximum concentrations at 2–3 min. These results
corroborate that these aromatic products are mainly oxidized by
·
OH, not being photodegraded by UVA light.
A very different behavior was found for generated carboxylic
acids. As can be seen in Fig. 8c, ketomalonic acid is not completely
removed after 6 h of electrolysis using both Cu2+ systems, but it is
quickly oxidized to oxalic acid in 40 min by the other methods with
Fe2+. Figure 8d shows that maleic acid, similarly to its trans-isomer
fumaric acid, is completely converted into oxalic acid in all cases.
The slow accumulation of ketomalonic and maleic acids in the presence of Cu2+ can then be related to the slow oxidation of aromatic
intermediates, whereas their fast degradation by the other methods
with Fe2+ indicates that they are mainly oxidized by the action of
·
OH formed from Fenton’s Reaction 2. In contrast, Fig. 8e and f
shows that the evolution of oxalic and oxamic acids depends on the
catalyst used. For both Cu2+ systems, small concentrations between
2 and 4 mg L−1 of both acids remain in solution. When only Fe2+ is
used, 90 mg L−1 of oxalic acid and 31 mg L−1 of oxamic acid are
accumulated without practical destruction. The use of Fe2+ + UVA
light causes a fast removal of oxalic acid up to a final value of
8 mg L−1, while 42 mg L−1 of oxamic acid is reached at 2 h, further
being slowly reduced to 29 mg L−1. By combining Fe2+ and Cu2+,
27 mg L−1 of oxalic acid and 2 mg L−1 of oxamic acid persist at
6 h. For the Fe2+ + Cu2+ + UVA light system, both acids are totally
mineralized, in agreement with the total decontamination found for
the paracetamol solution 共see curve h of Fig. 1兲.
The degradation behavior of oxalic and oxamic acids can be
related to the destruction of their complexes with Fe3+ and Cu2+.17,21
When paracetamol is treated with only Fe2+, Fe3+-oxalate and
Fe3+-oxamato complexes are competitively formed by the efficient
generation of Fe3+ from Fenton’s Reaction 2, but they cannot be
mineralized by ·OH limiting the oxidation ability of the electroFenton reaction. The final solution of this treatment is composed of
a mixture of stable Fe3+ complexes of both acids, because their
concentrations in Fig. 8e and f are equivalent to 33 mg L−1 TOC,
the same value attained by the paracetamol solution at 6 h 共see
curve e of Fig. 1兲. The remaining steady oxamic acid contains 33%
initial nitrogen, indicating that NH+4 present in such a solution 共see
Fig. 2兲 is produced by the degradation of products different from
acetamide. The efficient photodecomposition of Fe3+-oxalato complexes, along with a slower photolysis of Fe3+-oxamato complexes,
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
Journal of The Electrochemical Society, 153 共1兲 D1-D9 共2006兲
D7
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Figure 8. Time-course of the concentration of intermediates detected during the degradation of 157 mg L−1 paracetamol solutions of pH 3.0 at 100 mA cm−2
at 35°C using the following catalysts: 共䊐兲 1 mM Cu , 共䊏兲 1 mM Cu + UVA light, 共䉭兲 1 mM Fe , 共䉱兲 1 mM Fe + UVA light, 共〫兲 1 mM Fe
ȱ and
+ 1 mM Cu , and 共⽧兲 1 mM Fe + 1 mM Cu + UVA light. Plot: 共a兲 hydroquinone, 共b兲 p-benzoquinone, 共c兲 ketomalonic acid, 共d兲 maleic acid, 共e兲 oxalic
2+
2+
2+
2+
2+
2+
2+
2+
acid, and 共f兲 oxamic acid.
ȱ
can account for the faster mineralization of the drug by the
reaction 共see curve f of Fig. 1兲. When only the
ȱ photoelectro-Fenton
2+
2+
2+
ȱ
ȱ
ȱ
ȱ
ȱ
Cu system is used, Cu -oxalato and Cu -oxamato complexes are
formed to a small extent in the medium, being destroyed by ·OH but
not photolyzed by UVA light. The mineralization of these complexes
can also explain the highest decontamination of paracetamol found
in the presence of 1 mM Cu2+ than by anodic oxidation 共see Fig. 1兲.
By combining Fe2+ and Cu2+ as catalysts, Fe3+-oxalato,
Fe3+-oxamato, Cu2+-oxalato, and Cu2+-oxamato complexes are produced, but only the Cu2+ complexes are destroyed, leading to a rapid
TOC decay of paracetamol 共see curve g of Fig. 1兲. The quickest and
total mineralization of the drug with Fe2+ + Cu2+ + UVA light can
thus be related to the oxidation of Cu2+-oxalato and Cu2+-oxamato
complexes with ·OH in parallel with the photodecomposition of
their Fe3+ complexes by UVA light.
Proposed degradation pathway.— A general reaction scheme
for the mineralization of paracetamol in acid media by all indirect
electro-oxidation methods with H2O2 electrogeneration under the
action of Fe2+, Cu2+, and/or UVA light as catalysts is proposed in
Fig. 9. The pathway involves all intermediates detected in this work
and only shows the main oxidant ·OH for sake of simplicity, although parallel reactions with other weaker oxidizing agents 共H2O2,
HO·2, Cu2+, Fe3+, etc.兲 are also possible. The process is initiated by
·
OH attack at the C共4兲-position of paracetamol, breaking its N-bond
to yield hydroquinone and acetamide. Further oxidation of hydroquinone gives p-benzoquinone, which is degraded to a mixture of
ketomalonic, maleic, and fumaric acids. These acids are subsequently transformed into oxalic acid. Parallel oxidation of acetamide
leads to oxamic acid. Oxalic and oxamic acids are slowly converted
ȱ
175
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
Journal of The Electrochemical Society, 153 共1兲 D1-D9 共2006兲
D8
ȱ
ȱ
ȱ
ȱ
ȱ
Figure 9. Proposed reaction sequence for
paracetamol degradation in acid aqueous
medium by the catalytic action of Fe2+,
Cu2+, and/or UVA light on electrogenerated H2O2.
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ into2+CO2 by ·OH, although they form complexes with Fe3+ and/or
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Cu when one or both ions are present in the medium. Although
Cu2+-oxalato and Cu2+-oxamato complexes are mineralized with
·
OH, Fe3+-oxalato and Fe3+-oxamato complexes are very stable under electro-Fenton conditions. Both Fe3+ complexes can be photodecarboxylated with loss of Fe2+ under the action of UVA light, as
proposed by Zuo and Hoigné.21 The mineralization of oxamic acid is
accompanied by the loss of NH+4 . This inorganic ion, along with
small amounts of NO−3 , is also released during the degradation of
undetected products, probably coming from unstable 2-hydroxy-4共N-acetyl兲aminophenol formed from direct hydroxylation at the
C共2兲-position of paracetamol.25
is further oxidized to oxamic acid. Degradation of p-benzoquinone
leads to a mixture of ketomalonic, maleic, and fumaric acids, which
are subsequently converted into oxalic acid.
Acknowledgments
The authors thank AGAUR 共Agència de Gestió d’Ajuts Universitaris i de Recerca, Generalitat de Catalunya兲 for the grant given to
I. Sirés to do this work, and MEC 共Ministerio de Educación y Ciencia, Spain兲 for financial support under project CTQ2004-01954/
BQU.
Universitat de Barcelona assisted in meeting the publication costs of this
article.
References
Conclusions
It has been demonstrated that acidic aqueous solutions of paracetamol can be rapidly degraded using an undivided electrolytic cell
with a Pt anode and an O2-diffusion cathode able to electrogenerate
H2O2 under the combined catalytic action of 1 mM Fe2+, 1 mM
Cu2+, and UVA light. This indirect electro-oxidation method allows
complete mineralization for drug concentrations ⬍0.4 g L−1, because of the high amounts of ·OH produced from Fenton’s reaction
that oxidize the complexes of oxalic and oxamic acids with Cu2+,
along with the parallel photolysis of their complexes with Fe3+. This
treatment is more efficient than one involving 1 mM Fe2+ and 1 mM
Cu2+, or a photoelectro-Fenton system with 1 mM Fe2+ and UVA
light, where the complexes of oxalic and oxamic acids with Fe3+
and/or Cu2+ are more slowly destroyed. For an electro-Fenton system with 1 mM Fe2+, a lower decontamination is reached because
Fe3+-oxalato and Fe3+-oxamato complexes are not destroyed by
·
OH. The optimum pH for all these electro-oxidation methods is 3.0.
In contrast, the presence of 1 mM Cu2+ as catalyst or the use of
direct anodic oxidation, both with and without UVA light, leads to
slow destruction of pollutants due to the formation of small amounts
of ·OH from water oxidation at the Pt anode, which are not significantly enhanced by reaction of Cu+ with H2O2. The percentage of
TOC removal in all treatments increases with increasing applied
current due to the greater production of ·OH. The original nitrogen
of the drug is mainly lost as NH+4 ion, along with a very small
proportion of NO−3 ion. In all cases, hydroquinone and
p-benzoquinone are identified as aromatic products. The formation
of hydroquinone is accompanied by the release of acetamide, which
ȱ
176
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
Y. L. Hsiao and K. Nobe, J. Appl. Electrochem., 23, 943 共1993兲.
J. S. Do and C. P. Chen, J. Electrochem. Soc., 140, 1632 共1993兲.
C. Ponce de Leon and D. Pletcher, J. Appl. Electrochem., 25, 307 共1995兲.
E. Brillas, E. Mur, and J. Casado, J. Electrochem. Soc., 143, L49 共1996兲.
E. Brillas, R. Sauleda, and J. Casado, J. Electrochem. Soc., 144, 2374 共1997兲; E.
Brillas, R. Sauleda, and J. Casado, J. Electrochem. Soc., 145, 759 共1998兲.
A. Alvarez-Gallegos and D. Pletcher, Electrochim. Acta, 44, 2483 共1999兲.
T. Harrington and D. Pletcher, J. Electrochem. Soc., 146, 2983 共1999兲.
M. A. Oturan, J. Pinson, N. Oturan, and D. Deprez, New J. Chem., 23, 793 共1999兲.
E. Brillas, J. C. Calpe, and J. Casado, Water Res., 34, 2253 共2000兲.
M. A. Oturan, J. Appl. Electrochem., 30, 475 共2000兲.
J. J. Aaron and M. A. Oturan, Turk. J. Chem., 25, 509 共2001兲.
B. Boye, M. M. Dieng, and E. Brillas, Environ. Sci. Technol., 36, 3030 共2002兲.
A. Ventura, G. Jacquet, A. Bermond, and V. Camel, Water Res., 36, 3517 共2002兲.
E. Brillas, B. Boye, and M. M. Dieng, J. Electrochem. Soc., 150, E148 共2003兲.
B. Boye, E. Brillas, and M. M. Dieng, J. Electroanal. Chem., 540, 25 共2003兲.
B. Gözmen, M. A. Oturan, N. Oturan, and O. Erbatur, Environ. Sci. Technol., 37,
3716 共2003兲.
E. Brillas, M. A. Baños, S. Camps, C. Arias, P. L. Cabot, J. A. Garrido, and R. M.
Rodríguez, New J. Chem., 28, 314 共2004兲.
A. Wang, J. Qu, J. Ru, H. Liu, and J. Ge, Dyes Pigm., 65, 225 共2005兲.
J. J. Pignatello, Environ. Sci. Technol., 26, 944 共1992兲.
Y. Sun and J. J. Pignatello, Environ. Sci. Technol., 27, 304 共1993兲.
Y. Zuo and J. Hoigné, Environ. Sci. Technol., 26, 1014 共1992兲.
C. Zwiener and F. H. Frimmel, Water Res., 34, 1881 共2000兲.
Pharmaceuticals in the Environment: Sources, Fate and Risks, K. Kümmerer, Editor, Springer, Berlin 共2001兲.
D. W. Kolpin, E. T. Furlong, M. T. Meyer, E. M. Thurman, S. D. Zaugg, and L. B.
Barber, Environ. Sci. Technol., 36, 1202 共2002兲.
R. Andreozzi, V. Caprio, R. Marotta, and D. Vogna, Water Res., 37, 992 共2003兲.
J. P. Bound and N. Vaulvaulis, Chemosphere, 56, 1143 共2004兲.
M. Ravina, L. Campanella, and J. Kiwi, Water Res., 36, 3553 共2002兲.
T. A. Ternes, J. Stüber, N. Herrmann, D. McDowell, A. Ried, M. Kampmann, and
B. Teiser, Water Res., 37, 1976 共2003兲.
M. M. Huber, S. Canonica, G. Y. Park, and U. Von Gunten, Environ. Sci. Technol.,
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
ȱ
ȱ
ȱ
Journal of The Electrochemical Society, 153 共1兲 D1-D9 共2006兲
37, 1016 共2003兲.
30. D. Vogna, R. Marotta, A. Napolitano, R. Andreozzi and M. d’Ischia, Water Res.,
38, 414 共2004兲.
31. E. Brillas, I. Sirés, C. Arias, P. L. Cabot, F. Centellas, R. M. Rodríguez, and J. A.
Garrido, Chemosphere, 58, 399 共2005兲.
32. C. Comninellis and A. De Battisti, J. Chim. Phys. Phys.-Chim. Biol., 93, 673
共1996兲.
33. M. A. Rodrigo, P. A. Michaud, I. Duo, M. Panizza, G. Cerisola, and C. Comninellis, J. Electrochem. Soc., 148, D60 共2001兲.
D9
34. J. Iniesta, P. A. Michaud, M. Panizza, G. Cerisola, A. Aldaz, and C. Comninellis,
Electrochim. Acta, 46, 3573 共2001兲.
35. E. Brillas, B. Boye, I. Sirés, J. A. Garrido, R. M. Rodríguez, C. Arias, P. L. Cabot,
and C. Comninellis, Electrochim. Acta, 49, 4487 共2004兲.
36. H. Gallard, J. De Laat, and B. Legube, Rev. Sci. Eau, 12, 715 共1999兲.
37. J. De Laat and H. Gallard, Environ. Sci. Technol., 33, 2726 共1999兲.
38. B. H. J. Bielski, D. E. Cabelli, R. L. Arudi, and A. B. Ross, J. Phys. Chem. Ref.
Data, 14, 1041 共1985兲.
39. V. K. Sharma and F. J. Millero, Environ. Sci. Technol., 22, 768 共1988兲.
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
177
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
PART B –Results and Discussion7. Paracetamol
7.2.2.ȱȱResultatsȱiȱDiscussióȱ/ȱResultsȱandȱDiscussionȱ
ȱ
Theȱoptimumȱcatalystsȱconcentrationsȱ areȱ 1.0ȱ mMȱFe2+ȱandȱ1.0ȱ mMȱCu2+,ȱsinceȱtheyȱ
provideȱ aȱ slightlyȱ quickerȱ mineralizationȱ byȱ EFȱ andȱ PEF.ȱ Accordingȱ toȱ TOCȱ decayȱ
plots,ȱtheȱoxidationȱabilityȱofȱtheȱmethodsȱtestedȱincreasesȱinȱtheȱfollowingȱorder:ȱAOȱ
<ȱAOȱ+ȱUVAȱlightȱ<ȱ1.0ȱmMȱCu2+ȱ<ȱ1.0ȱmMȱCu2+ȱ+ȱUVAȱlightȱ«ȱ1.0ȱmMȱFe2+ȱ«ȱ1.0ȱmMȱ
Fe2+ȱ+ȱUVAȱlightȱ <ȱ1.0ȱmMȱFe2+ȱ+ȱ1.0ȱmMȱCu2+ȱ <ȱ1.0ȱmMȱFe2+ȱ+ȱ1.0ȱmMȱCu2+ȱ+ȱUVAȱ
light.ȱ
ȱ
Uncatalyzedȱ processesȱ (AO)ȱ leadȱ toȱ aȱ quiteȱ slowȱ mineralizationȱ dueȱ toȱ theȱ lowȱ
concentrationȱofȱtheȱmainȱoxidant,ȱ •OHads,ȱformedȱfromȱH2Oȱoxidationȱatȱtheȱanode.ȱ
WhenȱFe2+,ȱCu2+ȱandȱUVAȱlightȱcatalystsȱareȱusedȱseparatelyȱanȱimprovedȱbutȱpartialȱ
mineralizationȱisȱachievedȱdueȱtoȱtheȱlackȱofȱenoughȱ •OHȱinȱtheȱmediumȱand/orȱtheȱ
stabilityȱ ofȱ hardlyȱ oxidizableȱ Fe3+ȱ complexesȱ andȱ Cu2+ȱ complexes.ȱ Finally,ȱ overallȱ
mineralizationȱisȱreachedȱafterȱ5Ȭ6ȱhȱwhenȱ1.0ȱmMȱFe2+,ȱ1.0ȱmMȱCu2+ȱandȱUVAȱlightȱ
areȱcombinedȱasȱcatalystsȱ(coȬcatalyzedȱPEF),ȱevenȱatȱlowȱcurrentȱdensities.ȱThisȱfactȱ
canȱbeȱexplainedȱbyȱtheȱoxidationȱofȱCu2+ȬoxalatoȱandȱCu2+Ȭoxamatoȱcomplexesȱwithȱ
•
OHȱ inȱ parallelȱ withȱ theȱ photodecompositionȱ ofȱ theirȱ Fe3+ȱ complexesȱ byȱ UVAȱ lightȱ
(Reactionȱ 5.Ȭ24).ȱ Inȱ addition,ȱ irradiationȱ withȱ UVAȱ lightȱ causesȱ photoreductionȱ ofȱ
uncomplexedȱFe3+,ȱi.e.ȱFe(OH)2+,ȱthusȱregeneratingȱFe2+ȱ(whichȱcanȱproduceȱmoreȱ•OHȱ
fromȱ Fentonȱ reaction)ȱ andȱ enhancingȱ theȱ productionȱ ofȱ •OHȱ andȱ hence,ȱ theȱ
mineralizationȱofȱorganicsȱ(Reactionȱ5.Ȭ23).ȱ
ȱ
Asȱ apparentȱ currentȱ densityȱ (japp)ȱ increases,ȱ aȱ higherȱ TOCȱ removalȱ isȱ achievedȱ atȱ aȱ
givenȱtimeȱbecauseȱtheȱproductionȱofȱ •OHadsȱatȱtheȱPtȱandȱtheȱH2O2ȱelectrogeneratedȱ
atȱ theȱ cathodeȱ (and,ȱ consequently,ȱ theȱ •OHȱ inȱ theȱ medium)ȱ isȱ enhanced.ȱ Atȱ longerȱ
electrolysisȱ timeȱ someȱ hardlyȱ oxidizableȱ intermediates,ȱ suchȱ asȱ aliphaticȱ carboxylicȱ
acids,ȱareȱformed,ȱsoȱsignificantȱdifferencesȱasȱaȱfunctionȱofȱtheȱcurrentȱdensityȱtendȱ
toȱdisappear.ȱAȱcurrentȱofȱ300ȱmAȱisȱselectedȱasȱtheȱmostȱsuitableȱcurrentȱintensity,ȱ
179
PART B –Results and Discussion7. Paracetamol
sinceȱ100ȱmAȱleadsȱtoȱaȱsignificantlyȱslowerȱdegradationȱofȱparacetamol.ȱ
ȱ
Electrolysesȱ atȱ differentȱ initialȱ pHȱ valuesȱ revealȱ thatȱ theȱ quickestȱ TOCȱ decayȱ isȱ
observedȱ atȱ pHȱ 3.0,ȱ dueȱ toȱ theȱ highestȱ generationȱ rateȱ ofȱ •OH.ȱ Thisȱ factȱ completelyȱ
agreesȱwithȱtheȱoptimumȱpHȱforȱFentonȱreactionȱ(pHȱ=ȱ2.8).ȱ
ȱ
Totalȱmineralizationȱisȱattainedȱforȱupȱtoȱ315ȱmgȱLȬ1ȱ(200ȱmgȱLȬ1ȱTOC)ȱofȱparacetamol,ȱ
whereasȱ10%ȱofȱTOCȱremainsȱinȱsolutionȱwhenȱinitialȱconcentrationȱisȱ940ȱmgȱLȬ1ȱ(600ȱ
mgȱLȬ1ȱTOC).ȱTheȱmethodȱisȱthenȱableȱtoȱdestroyȱupȱtoȱca.ȱ0.4ȱgȱLȬ1ȱofȱdrug.ȱ
ȱ
Noȱ nitriteȱ ionsȱ wereȱ detectedȱ inȱ theȱ electrolyzedȱ solutions.ȱ NH4+ȱ ionȱ isȱ quicklyȱ
accumulatedȱ duringȱ theȱ earlyȱ stagesȱ ofȱ theȱ describedȱ catalyzedȱ treatments,ȱ andȱ
furtherȱitȱisȱslowlyȱreleased.ȱTheseȱresultsȱindicateȱthatȱtheȱinitialȱNȱisȱmainlyȱlostȱasȱ
NH4+:ȱ forȱ theȱ coȬcatalyzedȱ PEFȱ system,ȱ 93%ȱ ofȱ initialȱ Nȱ isȱ convertedȱ intoȱ NH4+,ȱ
whereasȱonlyȱ1%ȱisȱtransformedȱintoȱNO3.ȱMoreover,ȱtheȱaccumulatedȱNH4+ȱamountȱ
isȱhigherȱasȱoxidationȱabilityȱofȱtheȱmethodȱrises,ȱdueȱtoȱtheȱfasterȱmineralizationȱofȱ
NȬcontainingȱintermediates.ȱ
ȱ
Theȱ overallȱ mineralizationȱ reactionȱ involvesȱ 34ȱ Fȱ forȱ eachȱ molȱ ofȱ paracetamolȱ
(Reactionȱ6.Ȭ2).ȱMineralizationȱCurrentȱEfficiencyȱ(MCE)ȱhasȱbeenȱdeterminedȱforȱtheȱ
catalyzedȱ systemsȱ byȱ usingȱ Equationȱ 6.Ȭ1,ȱ andȱ theȱ resultsȱ showȱ thatȱ theȱ efficiencyȱ
increasesȱinȱtheȱfollowingȱorder:ȱ1.0ȱmMȱCu2+ȱwithȱorȱwithoutȱUVAȱlightȱ «ȱ1.0ȱmMȱ
Fe2+ȱ «ȱ1.0ȱmMȱFe2+ȱ+ȱUVAȱlightȱ <ȱ1.0ȱmMȱFe2+ȱ+ȱ1.0ȱmMȱCu2+ȱ <ȱ1.0ȱmMȱFe2+ȱ+ȱ1.0ȱmMȱ
Cu2+ȱ +ȱ UVAȱ light.ȱ Theȱ efficiencyȱ forȱ theȱ latterȱ processȱ atȱ 20ȱ minȱ isȱ 21%,ȱ furtherȱ
undergoingȱ aȱ gradualȱ decayȱ withȱ timeȱ dueȱ toȱ theȱ concomitantȱ fallȱ inȱ pollutantȱ
contentȱ andȱ theȱ formationȱ ofȱ hardlyȱ oxidizableȱ intermediates,ȱ thusȱ favoringȱ theȱ
parasiteȱ nonoxidizingȱ reactionsȱ ofȱ •OH.ȱ Itȱ mustȱ beȱ notedȱ thatȱ whileȱ theȱ systemsȱ
withoutȱ Fe2+ȱ exhibitȱ MCEȱ valuesȱ aboutȱ 1.5%,ȱ theȱ methodsȱ involvingȱ theȱ Fe3+/Fe2+ȱ
systemȱ areȱ muchȱ moreȱ efficientȱ becauseȱ theyȱ haveȱ muchȱ higherȱ oxidationȱ ability.ȱȱȱȱ
180
PART B –Results and Discussion7. Paracetamol
Atȱaȱgivenȱtime,ȱtheȱefficiencyȱofȱeachȱmethodȱalwaysȱdropsȱwithȱincreasingȱjappȱdueȱ
toȱaȱlargerȱproportionȱofȱ •OHȱoxidizedȱtoȱO2ȱatȱtheȱanode,ȱandȱtheȱaccelerationȱofȱitsȱ
parasiteȱ reactionsȱ inȱ theȱ medium.ȱ Inȱ addition,ȱ MCEȱ increasesȱ withȱ risingȱ drugȱ
concentration,ȱ indicatingȱ aȱ fasterȱ removalȱ ofȱ largerȱ amountȱ ofȱ organicsȱ becauseȱ theȱ
parasiteȱ reactionsȱ inȱ whichȱ •OHȱ isȱ involvedȱ becomeȱ slowerȱ andȱ moreȱ amountȱ ofȱ
hydroxylȱradicalsȱcanȱreactȱwithȱorganicȱcompounds.ȱFromȱ315ȱmgȱLȬ1ȱofȱparacetamol,ȱ
MCEȱ progressivelyȱ risesȱ duringȱ longerȱ timesȱ atȱ earlyȱ stagesȱ ofȱ theȱ treatment,ȱ
reachingȱaȱmaximumȱvalueȱofȱ36%ȱforȱ940ȱmgȱLȬ1ȱatȱ2ȱh,ȱthanksȱtoȱtheȱformationȱofȱ
moreȱeasilyȱoxidizableȱintermediatesȱatȱtheȱbeginningȱofȱtheȱoxidationȱprocess.ȱ
ȱ
Hydroquinoneȱ andȱ pȬbenzoquinoneȱ areȱ identifiedȱ byȱ GCȬMSȱ andȱ reversedȬphaseȱ
chromatography,ȱ andȱ thenȱ quantifiedȱ byȱ theȱ latterȱ technique.ȱ Acetamideȱ comingȱ
fromȱtheȱattackȱofȱ •OHȱonȱtheȱCȬNȱbondȱhasȱnotȱbeenȱidentified,ȱbutȱneverthelessȱitsȱ
presenceȱ canȱ beȱ assumedȱ asȱ reportedȱ byȱ Andreozziȱ etȱ al.ȱ [356]ȱ andȱ Skoumalȱ etȱ al.ȱ
[194],ȱ andȱ fromȱ interpretationȱ ofȱ theȱ surroundingȱ data.ȱ Byȱ meansȱ ofȱ ionȬexclusionȱ
chromatographyȱ severalȱ aliphaticȱ carboxylicȱ acidsȱ areȱ identifiedȱ andȱ quantified:ȱ
ketomalonic,ȱ maleicȱ andȱ fumaricȱ acidsȱ (comingȱ fromȱ theȱ oxidationȱ ofȱ theȱ arylȱ
moiety),ȱ oxalicȱ acidȱ (HOOCȬCOOH,ȱ comingȱ fromȱ theȱ oxidationȱ ofȱ theȱ formerȱ threeȱ
acids),ȱandȱoxamicȱacidȱ(HOOCȬCONH2,ȱprobablyȱgeneratedȱfromȱtheȱattackȱofȱ •OHȱ
onȱacetamide).ȱ
ȱ
Paracetamolȱ isȱ notȱ photolyzedȱ underȱ UVAȱ irradiation.ȱ Thisȱ pharmaceuticalȱ
undergoesȱ aȱ slowȱ andȱ similarȱ decayȱ forȱ AOȱ andȱ inȱ thoseȱ processesȱ withȱ Cu2+ȱ butȱ
withoutȱFe2+,ȱdisappearingȱfromȱtheȱmediumȱafterȱ75ȱminȱatȱ300ȱmA.ȱInȱcontrast,ȱtheȱ
drugȱisȱquicklyȱremovedȱinȱ6ȱminȱ(25ȱminȱifȱapplyingȱ100ȱmA)ȱwithȱaȱsimilarȱrateȱforȱ
theȱfourȱtreatmentsȱinvolvingȱFe2+,ȱdueȱtoȱtheȱgreatȱamountȱofȱ •OHȱinȱtheȱmedium.ȱItȱ
isȱworthȱremarkingȱthatȱtheȱdecayȱofȱparacetamolȱdoesȱnotȱfollowȱkineticȱequationsȱ
relatedȱ toȱ simpleȱ reactionȱ orders,ȱ justȱ suggestingȱ theȱ existenceȱ ofȱ aȱ complexȱ •OHȱ
attackȱonȱparacetamol.ȱ
181
PART B –Results and Discussion7. Paracetamol
HydroquinoneȱandȱpȬbenzoquinoneȱareȱpresentȱinȱtheȱmediumȱwhileȱtheȱinitialȱdrugȱ
persistsȱ inȱ it.ȱ Bothȱ compoundsȱ areȱ quicklyȱ formedȱ andȱ destroyedȱ atȱ similarȱ rateȱ inȱ
theȱmethodsȱcatalyzedȱbyȱFe2+,ȱattainingȱtheirȱmaximumȱconcentrationsȱatȱ2Ȭ3ȱmin.ȱ
ȱ
Asȱ forȱ theȱ carboxylicȱ acids,ȱ ketomalonicȱ acidȱ isȱ quicklyȱ oxidizedȱ toȱ oxalicȱ acidȱ inȱȱȱȱȱȱ
40ȱ minȱ byȱ theȱ methodsȱ withȱ Fe2+.ȱ Maleicȱ acid,ȱ similarlyȱ toȱ itsȱ transȬisomerȱ fumaricȱ
acid,ȱisȱremovedȱinȱallȱcasesȱandȱitȱisȱtransformedȱintoȱoxalicȱacid.ȱOnȱtheȱcontrary,ȱ
theȱevolutionȱofȱoxalicȱandȱoxamicȱacidsȱdependsȱonȱtheȱcatalystȱused,ȱandȱonlyȱtheȱ
systemȱ withȱ 1.0ȱ mMȱ Fe2+ȱ +ȱ 1.0ȱ mMȱ Cu2+ȱ +ȱ UVAȱ lightȱ isȱ ableȱ toȱ reachȱ theirȱ totalȱ
removal.ȱTheȱdegradationȱbehaviourȱofȱbothȱacidsȱcanȱbeȱrelatedȱtoȱtheȱdestructionȱofȱ
theirȱ complexesȱ withȱ Fe3+ȱ andȱ Cu2+:ȱ Fe3+Ȭoxalato,ȱ Fe3+Ȭoxamato,ȱ Cu2+Ȭoxalatoȱ andȱȱȱ
Cu2+Ȭoxamatoȱ complexes.ȱ Fe3+ȱ complexes,ȱ whichȱ areȱ formedȱ byȱ theȱ efficientȱ
generationȱofȱFe3+ȱfromȱFenton’sȱreaction,ȱcanȱnotȱbeȱdestroyedȱbyȱ •OH,ȱthusȱlimitingȱ
theȱ oxidationȱ abilityȱ ofȱ theȱ EFȱ reaction,ȱ forȱ example.ȱ Inȱ contrast,ȱ sinceȱ Fe3+Ȭoxalatoȱ
complexesȱ canȱ beȱ efficientlyȱ photodecomposedȱ andȱ Fe3+Ȭoxamatoȱ complexesȱ areȱ
slowlyȱphotolyzed,ȱPEFȱyieldȱaȱhigherȱTOCȱremovalȱcomparedȱtoȱEF.ȱByȱcombiningȱ
Fe2+ȱ withȱ Cu2+ȱ (coȬcatalyzedȱ EF),ȱ allȱ fourȱ complexesȱ pointedȱ outȱ aboveȱ areȱ formed,ȱ
butȱonlyȱCu2+ȱcomplexesȱcanȱbeȱoxidizedȱbyȱ •OH,ȱsoȱcompleteȱmineralizationȱcanȱnotȱ
beȱ achievedȱ yet.ȱ Itȱ isȱ necessaryȱ toȱ combineȱ Fe2+,ȱ Cu2+ȱ andȱ UVAȱ lightȱ toȱ completelyȱ
decontaminateȱ theȱsolutions,ȱ sinceȱaȱ synergisticȱeffectȱcanȱbeȱachieved:ȱoxidationȱ ofȱ
Cu2+ȱcomplexesȱbyȱ•OHȱandȱphotodecompositionȱofȱFe3+ȱcomplexesȱbyȱUVAȱlight.ȱ
ȱ
Finally,ȱ theȱ reactionȱ pathwayȱ forȱ theȱ mineralizationȱ ofȱ paracetamolȱ withȱ •OHȱ
involvesȱ allȱ intermediatesȱ detected:ȱ •OHȱ firstlyȱ attacksȱ atȱ theȱ CȬNȱ bondȱ ofȱ
paracetamol,ȱ yieldingȱ hydroquinoneȱ andȱ acetamide.ȱ Furtherȱ oxidationȱ ofȱ
hydroquinoneȱgivesȱpȬbenzoquinone,ȱwhichȱisȱdegradedȱtoȱaȱmixtureȱofȱketomalonic,ȱ
fumaricȱ andȱ maleicȱ acids.ȱ Theseȱ acidsȱ areȱ subsequentlyȱ transformedȱ intoȱ oxalic.ȱ
Parallelȱoxidationȱofȱacetamideȱleadsȱtoȱoxamicȱacid.ȱFinally,ȱtheȱcomplexesȱofȱoxalicȱ
andȱoxamicȱacidsȱwithȱFe3+ȱandȱCu2+ȱareȱconvertedȱintoȱCO2,ȱreleasingȱFe2+,ȱCu2+ȱandȱ
182
PART B –Results and Discussion7. Paracetamol
NH4+ȱ ions.ȱ Inȱ contrast,ȱ Vognaȱ etȱ al.ȱ [355,ȱ 356]ȱ areȱ ableȱ toȱ identifyȱ aȱ greatȱ dealȱ ofȱ
intermediates,ȱ mainlyȱ carboxylicȱ acidsȱ (malonic,ȱ glyoxylic,ȱ glycolic,ȱ DȬcetoglutaricȱ
andȱotherȱones),ȱbutȱtwoȱcommentsȱmustȱbeȱdone:ȱfirstly,ȱtheirȱworkȱisȱbasedȱonȱlessȱ
oxidizingȱ methodsȱ (30Ȭ40%ȱ mineralization),ȱ soȱ reactionsȱ takeȱ placeȱ slowlierȱ andȱ
intermediatesȱ canȱ haveȱ aȱ longerȱ residenceȱ time,ȱ andȱ secondly,ȱ theirȱ analysesȱ areȱ
mainlyȱcarriedȱoutȱbyȱGCȬMSȱandȱNMRȱinsteadȱofȱHPLC,ȱsoȱtheȱimportanceȱofȱallȱofȱ
theseȱ intermediatesȱ inȱ theȱ reactionȱ pathwayȱ isȱ relativeȱ becauseȱ identificationȱ ofȱ
compoundsȱdoesȱnotȱimplyȱtheirȱsignificantȱaccumulationȱinȱtheȱbulkȱsolution.ȱ
ȱ
ȱ
ȱ
ȱ
183
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
PART B –Results and Discussion7. Paracetamol
7.3.
TRACTAMENTȱMITJANÇANTȱOXIDACIÓȱANÒDICAȱȱ
/ȱTREATMENTȱBYȱANODICȱOXIDATIONȱ
ȱ
7.3.1.ȱȱFinalitatȱdelȱtreballȱ/ȱAimȱofȱtheȱworkȱ
ȱ
Onceȱ theȱ effectivityȱ ofȱ EAOPsȱ inȱ theȱ Pt/O2ȱ diffusionȱ cellȱ toȱ removeȱ andȱ mineralizeȱ
paracetamolȱ fromȱ aqueousȱ solutionsȱ atȱ acidȱ pHȱ wasȱ confirmed,ȱ theȱ aimȱ wasȱ
comparingȱ itsȱ oxidationȱ abilityȱ withȱ thatȱ ofȱ anodicȱ oxidationȱ (AO)ȱ processes,ȱ sinceȱ
amongȱ theȱ electrochemicalȱ treatmentsȱ forȱ theȱ destructionȱ ofȱ organicȱ pollutantsȱ inȱ
waters,ȱ AOȱ isȱ definitelyȱ theȱ mostȱ usualȱ technique.ȱ Asȱ previouslyȱ explainedȱȱȱȱȱȱȱȱȱȱȱȱ
(seeȱ sectionȱ 5.3.3.2),ȱ AOȱ treatmentsȱ areȱ basedȱ onȱ theȱ decontaminationȱ byȱ directȱ
reactionȱofȱpollutantsȱwithȱadsorbedȱhydroxylȱradicals,ȱ •OHads,ȱformedȱatȱtheȱanodeȱ
surface.ȱTherefore,ȱhereinȱitȱisȱreportedȱtheȱstudyȱofȱtheȱeffectivityȱofȱaȱBDDȱanodeȱtoȱ
bothȱ degradeȱ andȱ mineralizeȱ paracetamolȱ aqueousȱ solutionsȱ inȱ aȱ wideȱ rangeȱ ofȱ
experimentalȱ conditions.ȱ Comparativeȱ treatmentsȱ usingȱ aȱ Ptȱ anodeȱ wereȱ madeȱ toȱ
underlineȱ theȱ highȱ oxidizingȱ powerȱ ofȱ BDD.ȱ Graphiteȱ wasȱ usedȱ asȱ cathodeȱ inȱ allȱ
cases,ȱandȱlikeȱPtȱandȱBDDȱelectrodesȱitsȱareaȱwasȱ3ȱcm2.ȱ
ȱ
Theȱ firstȱ objectiveȱ wasȱ comparingȱ theȱ oxidationȱ abilityȱ ofȱ Ptȱ andȱ BDDȱ anodesȱ toȱ
degradeȱ paracetamol.ȱ Inȱ thisȱ sense,ȱ 100ȬmLȱ paracetamolȱ solutionsȱ containingȱ 157ȱȱȱ
mgȱLȬ1ȱparacetamolȱ(i.e.,ȱ100ȱmgȱLȬ1ȱTOC)ȱandȱ0.05ȱMȱNa2SO4,ȱatȱpHȱ3.0ȱandȱatȱ35ȱºC,ȱ
wereȱelectrolyzedȱforȱ6ȱhȱatȱ300ȱmAȱusingȱtheȱPt/graphiteȱandȱBDD/graphiteȱsystems.ȱ
Thisȱ workȱ wasȱ completedȱ byȱ carryingȱ outȱ theȱ sameȱexperimentsȱ atȱ pHȱ 2.0,ȱ 4.0,ȱ 8.0,ȱ
10.0ȱ andȱ 12.0ȱ toȱ clarifyȱ ifȱ theȱ observedȱ behaviorȱ couldȱ beȱ generalizedȱ toȱ differentȱ
aqueousȱmedia.ȱTOCȱabatementȱanalysesȱwereȱdoneȱinȱallȱcases.ȱ
ȱ
Onceȱ theȱ greatȱ oxidizingȱ powerȱ ofȱ BDDȱ wasȱ confirmed,ȱ theȱ possibleȱ effectȱ ofȱ theȱ
variationȱ ofȱ otherȱ experimentalȱ parametersȱ onȱ TOCȱ decayȱ wasȱ assessedȱ inȱ orderȱ toȱ
optimizeȱ theȱ AOȱ processȱ forȱ theȱ BDD/graphiteȱ systemȱ atȱ laboratoryȱ scale.ȱȱȱȱȱȱȱȱȱȱ
185
PART B –Results and Discussion7. Paracetamol
Firstly,ȱparacetamolȱsolutionsȱofȱpHȱ3.0ȱupȱtoȱ948ȱmgȱLȬ1ȱ(i.e.,ȱ600ȱmgȱLȬ1ȱTOC)ȱwereȱ
electrolyzedȱatȱ300ȱmAȱandȱatȱ35ȱºC.ȱTheȱPt/graphiteȱsystemȱwasȱalsoȱstudiedȱunderȱ
theȱsameȱconditions.ȱSecondly,ȱTOCȱabatementȱanalysesȱwereȱcarriedȱoutȱbyȱvaryingȱ
theȱ currentȱ appliedȱ toȱ aȱ 157ȱ mgȱ LȬ1ȱ paracetamolȱ solutionȱ ofȱ pHȱ 3.0ȱ atȱ 35ȱ ºC.ȱ Aȱ
constantȱcurrentȱofȱ100,ȱ300ȱandȱ450ȱmAȱwasȱapplied.ȱAndȱlastly,ȱtemperatureȱofȱtheȱ
aboveȱsolutionȱtreatedȱatȱ100ȱmAȱwasȱvariedȱfromȱ25ȱtoȱ45ȱºCȱ(higherȱtemperaturesȱ
canȱnotȱbeȱusedȱdueȱtoȱfastȱevaporationȱofȱwaterȱfromȱtheȱopenȱcellȱused).ȱ
ȱ
Followingȱ theȱ experimentalȱ sequenceȱ reportedȱ forȱ EFȱ andȱ PEF,ȱ afterȱ theȱ detailedȱ
studyȱofȱTOCȱdecayȱunderȱmanyȱexperimentalȱconditions,ȱtheȱevolutionȱofȱinorganicȱ
ionsȱ wasȱ examinedȱ byȱ ionȱ chromatographyȱ toȱ determineȱ theȱ possibleȱ lossȱ ofȱ initialȱ
nitrogenȱ ofȱ paracetamol.ȱ Inȱ thisȱ sense,ȱ aȱ 157ȱ mgȱ LȬ1ȱ paracetamolȱ solutionȱ ofȱ pHȱ 3.0ȱ
wasȱelectrolyzedȱwithȱBDDȱatȱ300ȱmAȱandȱatȱ35ȱºC.ȱMoreover,ȱtoȱclarifyȱtheȱbehaviorȱ
ofȱNH4+ȱionȱinȱBDDȱsystems,ȱaȱ100ȬmLȱsolutionȱcontainingȱ100ȱmgȱLȬ1ȱofȱ(NH4)2SO4ȱatȱ
pHȱ3.0ȱwasȱelectrolyzedȱunderȱsimilarȱconditionsȱforȱ6ȱh.ȱ
ȱ
Afterwards,ȱ theȱ kineticsȱ forȱ theȱ reactionȱ betweenȱ paracetamolȱ andȱ •OHadsȱ wasȱ
studiedȱ forȱ bothȱ Ptȱ andȱ BDDȱ systems.ȱ Severalȱ solutionsȱ ofȱ pHȱ 3.0ȱ andȱ 12.0ȱ wereȱ
electrolyzedȱ atȱ 300ȱ mAȱ andȱ atȱ 35ȱ ºC,ȱ andȱ paracetamolȱ decayȱ wasȱ thenȱ followedȱ byȱ
reversedȬphaseȱ chromatography.ȱ Simultaneously,ȱ aromaticȱ intermediatesȱ wereȱ
identifiedȱandȱquantified,ȱwithȱtheȱhelpȱofȱinterpretationȱofȱmassȱspectraȱobtainedȱbyȱ
GCȬMS.ȱFinally,ȱparacetamolȱsolutionsȱunderȱtheȱconditionsȱpreviouslyȱreferredȱwereȱ
alsoȱ electrolyzedȱ byȱ AOȱ withȱ BDDȱ toȱ obtainȱ theȱ ionȬexclusionȱ chromatogramsȱ
reflectingȱtheȱcarboxylicȱacidsȱaccumulated.ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
186
PART B –Results and Discussion7. Paracetamol
Theȱthoroughȱresultsȱofȱthisȱsectionȱareȱincludedȱinȱtheȱfollowingȱpaperȱ(Paperȱ3):ȱ
ȱ
3.ȱBrillas,ȱE.,ȱSirés,ȱ I.,ȱArias,ȱC.,ȱCabot,ȱP.L.,ȱCentellas,ȱF.,ȱRodríguez,ȱR.M.,ȱGarrido,ȱ
J.A.,ȱMineralizationȱofȱparacetamolȱinȱaqueousȱmediumȱbyȱanodicȱoxidationȱwithȱ
aȱboronȬdopedȱdiamondȱelectrode.ȱChemosphereȱ58ȱ(2005)ȱ399Ȭ406.ȱ
ȱ
Theȱfollowingȱpresentationȱinȱcongressȱareȱrelatedȱtoȱthisȱwork:ȱ
ȱ
C.ȱSirés,ȱI.,ȱSkoumal,ȱM.,ȱArias,ȱC.,ȱCabot,ȱP.L.,ȱCentellas,ȱF.,ȱGarrido,ȱJ.A.,ȱRodríguez,ȱ
R.M.,ȱ Brillas,ȱ E.,ȱ Mineralizaciónȱ delȱ paracetamolȱ enȱ medioȱ ácidoȱ medianteȱ
procesosȱ electroquímicosȱ yȱ químicosȱ deȱ oxidaciónȱ avanzada,ȱ Vol.ȱ 1,ȱ pageȱ C47,ȱ
XXVIȱReuniónȱdelȱGrupoȱEspecializadoȱdeȱElectroquímicaȱdeȱlaȱRSEQȱ(VIIȱIbericȱ
Meetingȱ ofȱ Electrochemistry),ȱ Córdoba,ȱ Spain,ȱ 12Ȭ15ȱ Aprilȱ 2004.ȱ (Oralȱ
presentation)ȱ
ȱ
ȱ
ȱ
ȱ
187
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ARTICLEȱ3ȱ/ȱPAPERȱ3
ȱ
Mineralizationȱofȱparacetamolȱinȱaqueousȱmediumȱbyȱȱ
anodicȱoxidationȱwithȱaȱboronȬdopedȱdiamondȱelectrodeȱ
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱMineralization
ȱ
Enric Brillas *, Ignasi Sirés, Conchita Arias, Pere Lluı́s Cabot,
Francesc Centellas, Rosa Marı́a Rodrı́guez, José Antonio Garrido
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Laboratori de Ciència i Tecnologia Electroquı́mica de Materials (LCTEM), Departament de Quı́mica Fı́sica,
Facultat de Quı́mica, Universitat de Barcelona, Martı́ i Franquès 1-11, 08028 Barcelona, Spain
ȱ
ȱ
ȱ
www.elsevier.com/locate/chemosphere
of paracetamol in aqueous medium by
anodic
oxidation with a boron-doped diamond electrode
ȱ
ȱ
ȱ
Chemosphere 58 (2005) 399–406
Received 30 March 2004; received in revised form 6 September 2004; accepted 20 September 2004
ȱ
Abstract
ȱ
The degradation of 100 ml of solutions with paracetamol (N-(4-hydroxyphenyl)acetamide) up to 1 g l1 in the pH
range 2.0–12.0 has been studied by anodic oxidation in a cell with a boron-doped diamond (BDD) anode and a graphite
cathode,ȱ both of 3-cm2 area, by applying a current of 100, 300 and 450 mA between 25 and 45 C. Complete mineralization is always achieved due to the great concentration of hydroxyl radical (OH) generated at the BDD surface, with
release of
ȱ NHþ4 and NO3 ions. The mineralization rate is pH-independent, increases with increasing applied current and
temperature, but decreases when drug concentration raises from 315 mg l1. Reversed-phase chromatography revealed a
similar ȱcomplex paracetamol decay in acid and alkaline media. Ion-exclusion chromatography allowed the detection of
oxalic and oxamic acids as ultimate carboxylic acids. When the same solutions have been comparatively treated with a
Pt anode, a quite poor mineralization is found because of the production of much lower OH concentration. Under
ȱ
these conditions, the degradation rate is enhanced in alkaline medium and polymerization of intermediates is favored
in concentrated solutions. Paracetamol can be completely destroyed with Pt and its kinetics follows a pseudo-first-order
reactionȱ with a constant rate independent of pH.
2004 Elsevier Ltd. All rights reserved.
ȱ
Keywords: Paracetamol; Anodic oxidation; Boron-doped diamond; Water treatment; Decay kinetics
ȱ
ȱ
ȱ
ȱ 1. Introduction
ȱ
ȱ
ȱ
There is growing interest in the environmental relevance of pharmaceutical drugs in waters. This pollution
ȱ to emission from production sites, direct discan be due
posal of overplus drugs in households, excretion after
ȱ
ȱ
*
ȱ
Corresponding
author. Tel.: +34 93 4021223; fax: +34 93
ȱ 4021231.
ȱ
ȱ
E-mail
ȱ addresses: [email protected], [email protected] (E.
Brillas).
drug administration to humans and animals and treatments throughout the water in fish farms (Zwiener and
Frimmel, 2000). Since thousands of tons per year of
drugs are consumed worldwide, a high number of antiinflammatory, analgesics, betablockers, lipid regulators,
antibiotics, antiepileptics and estrogens has been found
as minor pollutants, usually with concentrations
<10 lg l1, in sewage treatment plant (STP) effluents,
surface and ground waters and even in drinking waters
(Daughton and Jones-Lepp, 2001; Kümmerer, 2001;
Ternes et al., 2002). For paracetamol (N-(4-hydroxyphenyl)acetamide), concentrations up to 6 lg l1 have been
ȱ
0045-6535/$ - see front matter 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.chemosphere.2004.09.028
ȱ
189
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
ȱ
ȱ
400
ȱ
detected in STP effluents. Although there has been no
proof that very low amounts of pharmaceuticals in natural waters have any adverse health effects, they can produce toxic effects to aquatic organisms and in the case of
antimicrobials, the development of multi-resistant
strains of bacteria (Balcioǧlu and Ötker, 2003). To avoid
the dangerous accumulation of drugs in the aquatic
environment, research efforts are underway to develop
more powerful oxidation methods than those currently
applied in wastewater treatments for achieving their
complete destruction.
Ozonation and some advanced oxidation processes
(AOPs), such as O3/H2O2, H2O2/UV and H2O2/Fe2+/
UV, have been successfully used to remove several common pharmaceuticals in aqueous media (Zwiener and
Frimmel, 2000; Ravina et al., 2002; Ternes et al., 2002,
2003; Vogna et al., 2002; Andreozzi et al., 2003;
Balcioǧlu and Ötker, 2003; Huber et al., 2003). The great
effectiveness of AOPs is due to the production of hydroxyl radical (OH), which is a non-selective, very powerful oxidizing agent able to react with organics giving
dehydrogenated or hydroxylated derivatives, up to their
complete mineralization is reached (conversion into
CO2, water and inorganic ions). For paracetamol, however, only partial mineralization of 30% and 40% has
been found from ozonation and H2O2/UV, respectively,
in the pH range 2.0–5.5 (Andreozzi et al., 2003), indicating that more potent methods have to be applied to be
completely mineralized.
In the last years, effective electrochemical treatments
for the destruction of biorefractory organics in waters
are being developed. The most usual technique is anodic
oxidation, where solutions are decontaminated during
electrolysis by the direct reaction of pollutants with adsorbed OH formed at the anode surface from oxidation
either of water in acid and neutral media or hydroxide
ion at pH P 10 (Brillas et al., 2003; Marselli et al.,
2003; Torres et al., 2003):
H O ! OH þ Hþ þ e
ð1Þ
acetic, malic, formic and oxalic (Gandini et al., 2000),
4-chlorophenol (Rodrigo et al., 2001), phenol (Iniesta
et al., 2001) and herbicide 4-chlorophenoxyacetic acid
(Boye et al., 2002), as well as for malic acid at pH 2.7
and ethylenediaminetetraacetic acid at pH 9.2 (Kraft
et al., 2003) and for amarantha dyestuff in Na2SO4 solutions (Hattori et al., 2003).
This paper reports a study on the anodic oxidation
with BDD of solutions containing paracetamol concentrations lower than 1 g l1 and a low salt content of
0.05 M Na2SO4 to operate under similar conditions to
those of aquatic environment. Higher drug concentrations than those found in STP and natural effluents were
chosen to analyze better the oxidation ability of this
method. The effect of pH in the range 2.0–12.0, applied
current and temperature on the mineralization rate of
this compound was examined. Comparative treatments
using Pt as anode were made to confirm the high oxidizing power of BDD. The drug decay and the evolution of
generated carboxylic acids were determined by chromatographic techniques.
OH ! OHads þ e
All electrolyses were performed with an Amel 2053
potentiostat-galvanostat. The solution pH was measured with a Crison 2000 pH-meter. Samples extracted
from electrolyzed solutions were filtered with 0.45 lm
PTFE filters from Whatman before analysis. The degradation of paracetamol solutions was monitored by the
removal of their Total Organic Carbon (TOC), determined on a Shimadzu VCSN TOC analyzer. Reproducible TOC values were always obtained using the
standard non-purgeable organic carbon method. The
paracetamol decay was followed by reversed-phase
chromatography with a Waters system composed of a
Waters 600 HPLC liquid chromatograph fitted with a
Spherisorb ODS2 5 lm, 150 · 4.6 mm, column at room
temperature, and coupled with a Waters 996 photodiode
array detector selected at 246 nm, controlled through a
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
E. Brillas et al. / Chemosphere 58 (2005) 399–406
2
2. Experimental
2.1. Chemicals
Paracetamol, oxalic acid and oxamic acids were
reagent grade from Merck and Avocado. Anhydrous
sodium sulfate used as background electrolyte was analytical grade from Fluka. All solutions were prepared
with water from a Millipore Milli-Q system (conductivity <6 · 108 S cm1). The solution pH was adjusted
with sulfuric acid or sodium hydroxide, both of analytical grade, from Merck. Organic solvents and the other
chemicals used were either HPLC or analytical grade
from Panreac and Aldrich.
2.2. Apparatus and analysis procedures
ads
ð2Þ
However, most aromatics in acid and alkaline media
treated by anodic oxidation with conventional anodes
such as Pt, PbO2, doped PbO2, doped SnO2 and IrO2,
are slowly depolluted due to the generation of difficulty
oxidizable carboxylic acids (Brillas et al., 1998, 2003;
Bonfatti et al., 1999; Rodgers et al., 1999; Torres
et al., 2003). The recent use of a boron-doped diamond
(BDD) thin film anode has shown that it has much
larger O2 overvoltage than the above anodes, giving a
much higher concentration of adsorbed OH and a
quicker oxidation of pollutants. Anodic oxidation with
BDD then seems a suitable method for degrading organics up to their total mineralization, as found for HClO4
aqueous solutions containing carboxylic acids such as
ȱ
190
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
E. Brillas et al. / Chemosphere 58 (2005) 399–406
Millennium-32 program. These analyses were made by
injecting 20-ll aliquots into the chromatograph and circulating a 70:30 (v/v) acetonitrile/water mixture at
1.2 ml min1 as mobile phase. Generated carboxylic
acids were followed by ion-exclusion chromatography
by injecting 20-ll samples into the above HPLC system
with an Aminex HPX 87H, 300 · 7.8 mm column, at
35 C from Bio-Rad. For these measurements, the photodiode detector was selected at 210 nm and the mobile
phase was 4 mM H2SO4 at 0.6 ml min1. NHþ
4 concentration was determined following the standard colorimetric phenate method by flow injection analysis with
an Alpkem Flow Solution IV system. NO
3 concentration was obtained by ion chromatography, using a Kontron 600 HPLC fitted with a Waters IC-Pak anion
column at 35 C and coupled with a Waters spectrophotometric detector. These analyses were carried out with
100-ll aliquots after being ultramicrofiltrated by Ultrafree filters 10 000 dalton cutoff and a borate-gluconate
buffer of pH 8.5 as mobile phase.
2.3. Electrolytic system
All electrolyses were conducted in an open, one-compartment and thermostated cylindrical cell containing a
100-ml solution stirred with a magnetic bar. The anode
was a 3-cm2 BDD thin film deposited on a conductive
Si sheet purchased from CSEM. For comparative purposes, a 3-cm2 Pt sheet of 99.99% purity from SEMP
was also employed as anode. The cathode was always
a 3-cm2 graphite bar from Sofacel. The interelectrode
gap was about 3 cm.
Solutions containing less than 1 g l1 of paracetamol
and 0.05 M Na2SO4 of initial pH between 2.0 and 12.0
were comparatively degraded using a Pt or a BDD
anode at constant current (I) of 100, 300 and 450 mA
and at 35 C. The effect of temperature (T) in the range
25–45 C was also studied. During electrolyses for initial
pH values P4.0, the solution pH was continuously regulated within a range of ±0.03 units by adding small volumes of 0.5 M NaOH each 20 min.
401
produced. The small pH decay for the Pt anode can then
be related to the parallel oxidation of organic pollutants
on its surface that releases an excess of H+ from their
hydrogen atoms (see, for example, Eq. (3)), thus slightly
raising the medium acidity. The opposite trend found
for the BDD anode can be due to the existence of a small
proportion of oxidations of other species present in its
surface without H+ liberation, giving rise to the accumulation of an excess of OH in the medium that slightly
increases its pH. The starting colorless solution always
became clear yellow from 10–20 min of treatment due
to the formation of some soluble aromatic products,
although for the BDD anode, it was turned colorless
again after 90 min because of the overall destruction of
such species by OH adsorbed on its surface. The change
in solution TOC with applied specific charge Q (in
A h l1) for such trials is depicted in Fig. 1. A quite slow
mineralization can be observed for Pt, only attaining
19% of TOC removal at 6 h (Q = 18 A h l1). In contrast,
TOC very rapidly falls using BDD, so that paracetamol
is completely mineralized (>98% of TOC decay) at the
same time. These results indicate that anodic oxidation
with BDD is a useful method for the fast and total
destruction of paracetamol and its oxidation products,
whereas the Pt anode has much smaller oxidizing power
(with lower production of adsorbed OH) and leads to
poor mineralization.
To clarify if the above behavior can be generalized to
different aqueous media, comparative electrolyses at
300 mA were also carried out with 157 mg l1 of paracetamol at pH 2.0 and in the pH range 4.0–12.0. While
the solution pH did not vary along the experiments
starting from pH 2.0, gradual pH decay with time
was found at pH P 4.0, this being the reason why the
solution pH was continuously regulated to its initial
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
3. Results and discussion
3.1. Comparative degradation behavior
A solution of 157 mg l1 of paracetamol (corresponding to 100 mg l1 of TOC) of pH 3.0 was initially electrolyzed at 300 mA and at 35 C for 6 h to test its
comparative degradation using a Pt or a BDD anode.
In both cases, the solution pH remained practically constant up to final values of 2.8 or 3.3, respectively. Note
that this parameter does not change when the same
amounts of H+ in the anode and OH in the cathode
from water oxidation and reduction, respectively, are
Fig. 1. TOC removal vs. specific charge for the anodic
oxidation of 100 ml of a 157 mg l1 paracetamol solution in
0.05 M Na2SO4 at 300 mA and at 35 C using a cell with a
(d,j,m,r,., ) Pt or (s,h,n,,,,”) BDD anode and a
graphite cathode, all them of 3-cm2 area. Initial pH: (d,s) 2.0,
(j,h) 3.0; (m,n) 4.0; (r,) 8.0; (.,,) 10.0; and ( ,”) 12.0.
ȱ
191
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
ȱ
ȱ 402
E. Brillas et al. / Chemosphere 58 (2005) 399–406
ȱ value. Under the latter conditions, the anode feature afȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
fected the color of treated solutions. For Pt, dark-orange
solutions were always obtained at 1–2 h, becoming yellow at 6 h. This suggests the generation of soluble polyaromatic compounds on Pt, which are further slowly
oxidized. A very different behavior was observed for
anodic oxidation with BDD where clear yellow solutions
were obtained for 2 h as maximum, being colorless at
longer times, as expected if small amounts of soluble
aromatic products are formed and quickly destroyed
on this anode in all media. Fig. 1 also shows the TOCQ plots obtained for these experiments. A very poor
depollution can be seen for all pH values when Pt is
used, since at 18 A h l1 TOC is only reduced by 17%
at pH 2.0 and 4.0, 27% at pH 8.0 and about 35% at
pH 10.0 and 12.0. From these data, one can conclude
the existence of a larger mineralization of paracetamol
in alkaline than in acid solutions. This tendency can be
accounted for: (i) the generation of a greater concentration of oxidizing OH at the Pt surface by reaction (2)
than by reaction (1); and/or (ii) the faster destruction
of more easily oxidizable compounds present in alkaline
medium, such as the anionic forms of possible phenolic
derivatives (Torres et al., 2003). In contrast, a similar
TOC decay can be observed in Fig. 1 in all media under
treatment by anodic oxidation with BDD, reaching
overall mineralization at Q values between 15 and
18 A h l1. This brings to consider that the concentration
of generated OH at the BDD surface is always so high
that all oxidation products have similar degradation rate
within the pH range 2.0–12.0.
The above results confirm the great oxidizing power
of BDD for an efficient and complete mineralization of
paracetamol. The possible effect of other experimental
parameters on TOC decay, as drug concentration, applied current and temperature, was then studied to know
the best operative conditions for this method.
Effect of experimental parameters on paracetamol
ȱ 3.2.
mineralization
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
A series of experiments was performed by electrolyzing solutions of paracetamol with concentration lower
than 1 g l1 of pH 3.0 at 300 mA and at 35 C up to total
mineralization by anodic oxidation with BDD. In all
cases, the solution pH practically did not change with
electrolysis time and a clear yellow color solution was
observed for 90–120 min as maximum, as expected if soluble aromatic products are formed in low extent and
rapidly oxidized. As can be seen in Fig. 2a, all solutions
are completely mineralized, being required the consumption of a specific charge of about 36 A h l1 (12 h),
30 A h l1 (10 h), 24 A h l1 (8 h), 15 A h l1 (5 h) and
12 A h l1 (4 h) for 948, 677, 315, 157 and 78 mg l1 of
initial compound. The increase in Q with increasing
drug concentration can be simply related to the existence
ȱ
192
Fig. 2. (a) TOC abatement with specific charge and (b) current
efficiency calculated from Eq. (4) vs. specific charge for the
anodic oxidation with BDD of 100-ml paracetamol solutions in
0.05 M Na2SO4 + H2SO4 of pH 3.0 at 35 C. Initial drug
concentration: (s) 948; (h) 677; (n) 315; (,d,j) 157; and (,)
78 mg l1. Applied current: (d) 100; (s,h,n,,,) 300; and (j)
450 mA.
of more pollutants. This behavior can be better analyzed
from the percentages of TOC removal after 1 and 4 h of
treatment collected in Table 1. These data reveal a similar TOC decay of 27–30% at 1 h and 86–91% at 4 h up to
315 mg l1, whereas at higher concentration, the percentage of TOC removal undergoes a gradual fall attaining
values of 15% at 1 h and 70% at 4 h for 948 mg l1. All
these results allow establishing that the quickest mineralization rate for paracetamol by anodic oxidation with
BDD is achieved working up to 315 mg l1, although
the method is effective enough to depollute more concentrated solutions. Note that when the same experiments
were comparatively made with Pt, a dark-orange precipitate was formed from 315 mg l1 of paracetamol after
6 h, indicating the generation of insoluble polyaromatic
compounds by slow reactions between products present
in great concentration in solution. Anodic oxidation
with Pt then favors polymerization of intermediates in
concentrated solutions, thus limiting their mineralization process.
A more significant effect was found by varying the
applied current. Fig. 2a shows that Q values of 9 A h l1
(9 h), 15 A h l1 (5 h) and 18 A h l1 (4 h) are needed to
depollute a 157 mg l1 paracetamol solution of pH 3.0
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
ȱ
ȱ
E. Brillas et al. / Chemosphere 58 (2005) 399–406
403
Table 1
ȱ Percentage of TOC removal and current efficiency obtained for selected degradations of 100-ml paracetamol solutions in 0.05 M
Na2SO4 of pH 3.0 using anodic oxidation with a 3-cm2 BDD electrode
ȱ c0a (mg l1)
ȱ 78
157
ȱ
ȱ
ȱ
315
677
948
a
ȱ
ȱ
b
T (C)
35
25
35
35
35
45
35
35
35
I (mA)
300
100
100
300
450
100
300
300
300
After 1 h of treatment
After 4 h of treatment
% TOC removal
CEb
% TOC removal
CEb
30
13
20
27
36
26
28
19
15
4.7
12
18
8.6
7.6
25
18
25
28
87
41
64
91
98
74
86
72
70
3.4
9.7
15
7.2
5.2
18
14
24
33
Initial concentration.
Current efficiency calculated from Eq. (4).
at 100, 300 and 450 mA, respectively, and at 35 C. Since
the specific charge increases as current rises, less electroȱ lysis time is needed for total mineralization. This presupposes an increase in removed TOC with increasing I at
ȱ constant time, as can be easily deduced from results
given in Table 1, where at 1 h of treatment, for example,
the
ȱ percentage of TOC removal gradually raises from
20% to 36% between 100 and 450 mA. The fact that
increasing current causes faster depollution can be reȱ lated to the concomitant generation of more OH on
the BDD surface. On the other hand, a similar behavior
ȱ was found by varying the temperature from 25 to 45 C.
Note that higher temperatures cannot be used in our sysdue to fast evaporation of water. Results of Table 1
ȱ tem
show that at 100 mA, the TOC of the above solution is
reduced by 13%, 20% and 26% for 1 h and by 41%,
ȱ 64% and 74% for 4 h at 25, 35 and 45 C, respectively.
The reaction of pollutants with OH is then accelerated
ȱ with raising temperature, enhancing drug mineralization. Since the increase in temperature causes a greater
mass transfer to the anode due to the decrease of
ȱ medium viscosity, it can be inferred that the oxidation
process under such experimental conditions is limited,
ȱ at least partially, by the mass transfer of organics to
the BDD surface.
inorganic ions released after total mineralization
ȱ of aThe
157 mg l1 paracetamol solution of pH 3.0 by anodic
oxidation with BDD at 300 mA and at 35 C were deterȱ mined. Concentrations for NHþ4 and NO3 of 12.2 and
22.0 mg l1, respectively, corresponding to 65% and
ȱ 35% of initial nitrogen content, were found. No nitrite
ions were detected by ion chromatography. To ascertain
the nature of NO
3 generation, a 100-ml solution with
ȱ 100 mg l1 of ammonium
ion (as (NH4)2SO4) of pH 3.0
was electrolyzed under similar conditions for 6 h and
ȱ 20 mg l1 of NO3 , due to the oxidation of 5.8% of initial
NHþ
4 , were determined in the final solution. These re
sults
indicate the main release of NHþ
4 during the OH
ȱ
attack on N-derivatives of paracetamol, with a small
proportion of it transformed into NO
3 at the BDD
anode. However, the major part of NO
3 accumulated
in the mineralization process comes from the degradation of N-intermediates.
3.3. Current efficiency for the mineralization process
The above considerations allow concluding that mineralization of paracetamol involves its conversion into
carbon dioxide and mainly NHþ
4 . The overall reaction
can be written as follows:
HO–C6 H4 –NH–CO–CH3 þ 14H2 O
þ
! 8CO2 þ NHþ
4 þ 33H þ 34e
ð3Þ
The current efficiency (CE) at a given time t for the
above solutions was then comparatively estimated from
the following equation:
current efficiency ¼ ½DðTOCÞexper =DðTOCÞtheor 100
ð4Þ
where D(TOC)exper is the experimental solution TOC removal at time t and D(TOC)theor is the theoretically calculated TOC removal assuming that the applied
electrical charge (=current · time) is only consumed to
yield Eq. (3) as mineralization process.
Fig. 2b shows the efficiencies determined from Eq. (4)
for the trials depicted in Fig. 2a. Several selected CE values are also collected in Table 1. As can be seen in Fig.
2b, in most cases the efficiency increases at the first treatment stages, up to 2 h for 677 and 948 mg l1 of paracetamol, suggesting the initial generation of more easily
oxidizable products than the initial compound. Results
of Fig. 2b and Table 1 show a continuous drop in CE
with time once passed its maximum value, indicating a
concomitant fall in oxidizing ability of the electrolytic
system. This behavior can be ascribed to the gradual
ȱ
193
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
404
E. Brillas et al. / Chemosphere 58 (2005) 399–406
decay in pollutant concentration favoring that OH can
be wasted by other parallel non-oxidizing reactions,
e.g. its recombination into H2O2; that is, the remaining
products are more slowly degraded with decreasing
OH concentration at the BDD surface. The drop in efficiency with decreasing drug concentration also supports
this interpretation (see Fig. 2b and Table 1). For example, CE values of 33%, 24%, 14%, 7.2% and 3.4% for
948, 677, 315, 157 and 78 mg l1 of this compound are
found after 4 h of treatment at 300 mA and at 35 C. This
trend can be related again with a slower degradation of
organics by an enhancement of parallel non-oxidizing
reactions of generated OH. The same explanation can
justify the decreasing efficiencies found with increasing
I for 157 mg l1 of paracetamol at 35 C (see Fig. 2b
and Table 1). Although the increase in current yields
more TOC removal of this solution because of the generation of more OH at the BDD, a larger proportion of
this radical is gradually wasted, making less efficient its
reaction with organics. Results of Table 1 also confirm
that the process becomes much more efficient as temperature raises, as expected if reactions of organics with
OH are progressively enhanced in front of non-oxidizing reactions of this radical.
The above findings allow concluding that concentrated paracetamol solutions can be efficiently mineralized by anodic oxidation with BDD, even at low
currents. This method can then be useful in practice,
increasing its efficiency as temperature raises.
ȱ
Fig. 3. Decay of paracetamol concentration with electrolysis
time during the treatment of 100-ml solutions in 0.05 M Na2SO4
at 300 mA and at 35 C by anodic oxidation. Plot (a): Pt anode.
Plot (b): BDD anode. Initial drug concentration: (s,d) 315;
(h,j) 157; and (n,m) 78 mg l1. Initial pH: (s,h,n) 3.0; and
(d, j,m) 12.0. The inset in plot (a) gives the kinetic analysis for
the corresponding experiments assuming a pseudo-first-order
reaction for paracetamol.
3.4. Paracetamol decay and time-course of products
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
The kinetics for the reaction between paracetamol
and OH generated at the Pt or BDD anode was studied for several solutions of pH 3.0 and 12.0 at
300 mA and at 35 C. The decay of its concentration
was followed by reversed-phase chromatography, where
it displayed a well-defined peak at a retention time
tr = 1.56 min.
As can be seen in Fig. 3a, the drug is slowly destroyed
with similar rate at a given concentration for both pH
values when Pt is used, disappearing from the medium
in 300–360 min for 315 mg l1 and 240 min for 157
and 78 mg l1. Kinetic analysis of these results only gave
good linear plots, with regression coefficients >0.996,
when they were fitted to a pseudo-first-order reaction.
These correlations are presented in the inset of Fig. 3a,
yielding the same pseudo-first-order rate constant of
0.013 ± 0.002 min1. This suggests that paracetamol reacts with a practically constant concentration of OH at
the Pt surface, discarding its higher generation in alkaline
medium by the participation of reaction (2), proposed
above as a possibility to explain the greater TOC removal
found under these conditions than in acid medium (see
Fig. 1). The pH-independence of the pseudo-first-order
rate constant for paracetamol (pKa = 9.5) indicates the
ȱ
194
presence of the same electroactive species in all media,
probably its neutral (not charged) form. It seems then
plausible to associate the quicker mineralization of the
drug in alkaline medium with the production of more
easily oxidizable products than those formed in acid
solutions, as also stated in Section 3.1.
Fig. 3b shows a complex fall of paracetamol concentration with time using BDD, without following any kinetic equation related to simple reaction orders. A quite
similar degradation rate for this drug can be observed
for pH 3.0 and 12.0 at each initial concentration tested,
as also obtained for Pt (see Fig. 3a), corroborating the
oxidation of the same electroactive form of paracetamol
in both media. However, comparison of Fig. 3a and b
allows establishing that it is more rapidly destroyed with
BDD, requiring lower times close to 240, 150 and
120 min for 315, 157 and 78 mg l1, respectively, to yield
its complete removal, in agreement with the higher oxidizing power of this anode. The complex kinetics depicted in Fig. 3b can then be related to the competitive
consumption of OH at the BDD surface by parallel fast
reactions with products. This causes the attack of a variable OH concentration on the initial compound giving
an undefined kinetics.
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
E. Brillas et al. / Chemosphere 58 (2005) 399–406
Reversed-phase chromatograms of solutions electrolyzed with Pt also exhibited two small peaks associated
with primary product hydroquinone at tr = 1.62 min
and its oxidation derivative p-benzoquinone at tr =
1.95 min. Note that both aromatic products have been
detected during the oxidation of paracetamol by ozonation and H2O2/UV in the pH range 2.0–5.5 (Vogna et al.,
2002; Andreozzi et al., 2003). Hydroquinone can be produced from OH attack on the C(1) position of aromatic
ring of paracetamol, causing the breaking of the C(1)–N
bond with the release of acetamide. However, the above
aromatics were not detected in the reversed-phase chromatograms of solutions treated with BDD, as expected
if they are so rapidly destroyed on the anode that are
not accumulated in the medium. This supports the existence of competitive OH consumption by parallel oxidizing reactions of all pollutants leading to a complex
kinetics for paracetamol, as shown in Fig. 3b.
The above solutions treated with BDD were also analyzed by ion-exclusion chromatography to try to detect
generated carboxylic acids. In all cases, only two ultimate acids, oxalic (HOOC–COOH) at tr = 6.75 min
and oxamic (HOOC–CONH2) at tr = 9.29 min, were
found. Oxalic acid come from the destruction of the
benzenic ring of aromatic pollutants by OH (Brillas
et al., 1998, 2003; Boye et al., 2002), while oxamic acid
is expected to be produced from oxidation of acetamide
released during the primary OH attack on the C(1) position of paracetamol. The evolution of such carboxylic
acids during the treatments of solutions with 157 mg l1
of this compound of pH 3.0 and 12.0 at 300 mA and at
35 C is presented in Fig. 4. A small, but similar, accumulation of both acids can be observed in the two
media, reaching their maximum concentrations between
60 and 90 min and disappearing in 240 min, a time lower
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
405
than 300–360 min needed for total mineralization (see
Fig. 1). These results, along with those found for paracetamol in Fig. 3b, corroborate the fast and similar
destruction rate of all organics on BDD in acid and
alkaline solutions, thus justifying the pH-independence
for their TOC decay (see Fig. 1).
4. Conclusions
It has been demonstrated that anodic oxidation with
BDD is a very effective method for the complete mineralization of paracetamol up to 1 g l1 in aqueous medium within the pH range 2.0–12.0. The TOC removal
is pH-independent since the destruction of all organics
by reaction with the great concentration of OH at the
BDD surface has a similar rate in all media tested. This
has been confirmed by the similar decay found for its
concentration at pH 3.0 and 12.0 by reversed-phase
chromatography, as well as by the analogous evolution
of oxalic and oxamic acids detected as ultimate products
in the same solutions by ion-exclusion chromatography. The mineralization process is accompanied by the
release of NHþ
4 and NO3 ions. Its current efficiency increases with raising drug concentration and temperature, because of the gradual enhancement of OH
concentration to oxidize pollutants in front its participation in non-oxidizing reactions. The increase in current
causes generation of more OH, and hence, more TOC
removal, but efficiency drops since a larger proportion
of this radical is wasted. Paracetamol decay follows a
complex kinetics, ascribed to its reaction with a variable
OH concentration caused by the parallel consumption
of this oxidant in fast reactions with products. In contrast, anodic oxidation with Pt has much lower oxidizing
power and yields poor mineralization, although a higher
TOC removal is reached in alkaline than in acid medium. This fact is related to the production of more easily
oxidizable products in alkaline media. The treatment of
concentrated solutions favors polymerization of intermediates, limiting the mineralization process. Paracetamol can be completely removed with Pt and its
kinetics follows a pseudo-first-order reaction with a constant rate independent of pH, as expected if the same
electroactive species of this compound is oxidized in
all media tested.
Acknowledgments
Fig. 4. Time-course of the concentration of (s,d) oxalic acid;
and (h,j) oxamic acid, determined during the mineralization
of 157 mg l1 of paracetamol by anodic oxidation with BDD
under the same conditions as in Fig. 3(b). Initial pH: (s,h) 3.0;
and (d,j) 12.0.
Financial support received from MCYT (Ministerio
de Ciencia y Tecnologı́a, Spain) under project
BQU2001-3712 is acknowledged. The authors also
thank AGAUR (Agència de Gestió dÕAjuts Universitaris i de Recerca, Generalitat de Catalunya) for the grant
awarded to I. S. to do this work.
ȱ
195
PART B –Results and Discussion7. Paracetamol
ȱ
ȱ
ȱ
ȱ
406
ȱ
References
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
E. Brillas et al. / Chemosphere 58 (2005) 399–406
Andreozzi, R., Caprio, V., Marotta, R., Vogna, D., 2003.
Paracetamol oxidation from aqueous solution by means of
ozonation and H2O2/UV system. Wat. Res. 37, 992–1004.
Balcioǧlu, I.A., Ötker, M., 2003. Treatment of pharmaceutical
wastewater containing antibiotics by O3 and O3/H2O2
processes. Chemosphere 50, 85–95.
Bonfatti, F., Ferro, S., Lavezzo, M., Malacarne, M., Lodi, G.,
De Battisti, A., 1999. Electrochemical incineration of
glucose as a model organic substrate. I. Role of the
electrode material. J. Electrochem. Soc. 146, 2175–
2179.
Boye, B., Michaud, P.A., Marselli, B., Dieng, M.M., Brillas, E.,
Comninellis, C., 2002. Anodic oxidation of 4-chlorophenoxyacetic acid on synthetic boron-doped diamond electrodes. New Diamond Front. Carbon Technol. 12, 63–72.
Brillas, E., Sauleda, R., Casado, J., 1998. Degradation of
4-chlorophenol by anodic oxidation, electro-Fenton, photoelectro-Fenton and peroxi-coagulation processes. J. Electrochem. Soc. 145, 759–765.
Brillas, E., Cabot, P.L., Casado, J., 2003. Electrochemical
methods for degradation of organic pollutants in aqueous
media. In: Tarr, M. (Ed.), Chemical Degradation methods
for Wastes and Pollutants. Environmental and Industrial
Applications. Marcel Dekker, New York, USA, pp. 235–
304.
Daughton, C.G., Jones-Lepp, T.L. (Eds.), 2001. Pharmaceuticals and Personal Care Products in the Environment.
Scientific and Regulatory Issues. ACS Symposium Series,
Washington, USA.
Gandini, D., Mahé, E., Michaud, P.A., Haenni, W., Perret, A.,
Comninellis, C., 2000. Oxidation of carboxylic acids at
boron-doped diamond electrodes for wastewater treatment.
J. Appl. Electrochem. 30, 1345–1350.
Hattori, S., Doi, M., Takahashi, E., Kurosu, T., Nara, M.,
Nakamatsu, S., Nishiki, Y., Furuta, T., Iida, M., 2003.
Electrolytic decomposition of amarantha dyestuff using
diamond electrodes. J. Appl. Electrochem. 33, 85–91.
Huber, M.M., Canonica, S., Park, G.Y., Von Gunten, U., 2003.
Oxidation of pharmaceuticals during ozonation and
advanced oxidation processes. Environ. Sci. Technol. 37,
1016–1024.
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
196
Iniesta, J., Michaud, P.A., Panizza, M., Cerisola, G., Aldaz, A.,
Comninellis, C., 2001. Electrochemical oxidation of phenol
at boron-doped diamond electrode. Electrochim. Acta 46,
3573–3578.
Kraft, A., Stadelmann, M., Blaschke, M., 2003. Anodic
oxidation with doped diamond electrodes: a new advanced
oxidation process. J. Hazard. Mat. 103, 247–261.
Kümmerer, K. (Ed.), 2001. Pharmaceuticals in the Environment. Sources, Fate and Risks. Springer, Berlin, Germany.
Marselli, B., Garcia-Gomez, J., Michaud, P.A., Rodrigo, M.A.,
Comninellis, C., 2003. Electrogeneration of hydroxyl radicals on boron-doped diamond electrodes. J. Electrochem.
Soc. 150, D79–D83.
Ravina, M., Campanella, L., Kiwi, J., 2002. Accelerated
mineralization of the drug diclofenac via Fenton reactions
in a concentric photo-reactor. Wat. Res. 36, 3553–3560.
Rodgers, J.D., Jedral, W., Bunce, N.J., 1999. Electrochemical
oxidation of chlorinated phenols. Environ. Sci. Technol. 33,
1453–1457.
Rodrigo, M.A., Michaud, P.A., Duo, I., Panizza, M., Cerisola,
G., Comninellis, C., 2001. Oxidation of 4-chlorophenol at
boron-doped diamond electrode for wastewater treatment.
J. Electrochem. Soc. 148, D60–D64.
Ternes, T.A., Meisenheimer, M., McDowell, D., Sacher, F.,
Brauch, H.J., Haist-Gulde, B., Preuss, G., Wilme, U., ZuleiSeibert, N., 2002. Removal of pharmaceuticals during
drinking water treatment. Environ. Sci. Technol. 36, 3855–
3863.
Ternes, T.A., Stüber, J., Herrmann, N., McDowell, D., Ried,
A., Kampmann, M., Teiser, B., 2003. Ozonation: a tool for
removal of pharmaceuticals, contrast media and musk
fragrances from wastewater?. Wat. Res. 37, 1976–1982.
Torres, R.A., Torres, W., Peringer, P., Pulgarin, C., 2003.
Electrochemical degradation of p-substituted phenols of
industrial interest on Pt electrodes. Attempt of a structure–
reactivity relationship assessment. Chemosphere 50, 97–104.
Vogna, D., Marotta, R., Napolitano, A., dÕIschia, M., 2002.
Advanced oxidation chemistry of paracetamol. UV/H2O2induced hydroxylation/degradation and 15N-aided inventory of nitrogenous breakdown products. J. Org. Chem. 67,
6143–6151.
Zwiener, C., Frimmel, F.H., 2000. Oxidative treatment of
pharmaceuticals in waters. Wat. Res. 34, 1881–1885.
PART B –Results and Discussion7. Paracetamol
7.3.2.ȱȱResultatsȱiȱDiscussióȱ/ȱResultsȱandȱDiscussionȱ
ȱ
TOCȱdecayȱasȱfunctionȱofȱappliedȱspecificȱchargeȱQȱ(inȱAȱhȱLȬ1)ȱclearlyȱshowsȱthatȱaȱ
slowȱ mineralizationȱ isȱ achievedȱ withȱ Ptȱ inȱ comparisonȱ withȱ BDD:ȱ afterȱ 6ȱ hȱ (Qȱ =ȱ 18ȱȱȱȱ
AȱhȱLȬ1)ȱatȱ300ȱmA,ȱ19%ȱofȱTOCȱremovalȱisȱattainedȱatȱpHȱ3.0ȱandȱ35%ȱatȱpHȱ10.0ȱandȱ
12.0ȱusingȱPt,ȱwhereasȱcompleteȱmineralizationȱforȱallȱpHȱvaluesȱevenȱatȱ5ȱhȱ(Qȱ=ȱ15ȱ
Aȱ hȱ LȬ1)ȱ isȱ reachedȱ usingȱ BDD.ȱ Theȱ enhancedȱ mineralizationȱ ofȱ paracetamolȱ inȱ
alkalineȱ mediaȱ usingȱ Ptȱ canȱ notȱ beȱ accountedȱ forȱ byȱ theȱ generationȱ ofȱ aȱ greaterȱ
amountȱofȱ •OHadsȱ(Reactionȱ5.Ȭ46),ȱbecauseȱkineticȱanalysesȱsuggestȱaȱconstantȱ •OHadsȱ
concentrationȱ inȱ allȱ media,ȱ soȱ theȱ observedȱ trendȱ mustȱ beȱ attributedȱ toȱ theȱ fasterȱ
destructionȱ ofȱ moreȱ easilyȱ oxidizableȱ anionicȱ formsȱ ofȱ possibleȱ phenolicȱ
intermediatesȱ presentȱ inȱ alkalineȱ mediaȱ (itȱ canȱ notȱ beȱ attributedȱ toȱ differencesȱ inȱ
paracetamolȱelectroactivity,ȱsinceȱtheȱsameȱrateȱconstantȱisȱobtainedȱinȱallȱmedia).ȱInȱ
contrast,ȱ theȱ amountȱ ofȱ effectiveȱ •OHadsȱ generatedȱ atȱ theȱ BDDȱ surfaceȱ isȱ alwaysȱ soȱ
highȱthatȱallȱoxidationȱproductsȱhaveȱaȱsimilarȱdegradationȱrateȱwithinȱtheȱpHȱrangeȱ
2.0Ȭ12.0,ȱsoȱAOȱwithȱBDDȱisȱanȱeffectiveȱpHȬindependentȱmineralizationȱprocess.ȱAtȱ
thisȱ pointȱ itȱ isȱ interestingȱ toȱ establishȱ aȱ comparisonȱ withȱ PEFȱ withȱ 1.0ȱ mMȱ Fe2+ȱȱȱȱȱȱȱȱȱȱȱȱ
+ȱ 1.0ȱ mMȱ Cu2+ȱ +ȱ UVAȱ light,ȱ becauseȱ thisȱ isȱ theȱ processȱ thatȱ allowsȱ completesȱ
mineralizationȱusingȱtheȱO2Ȭdiffusionȱcathode.ȱFigureȱ7.Ȭ3ȱshownȱbelowȱindicatesȱthat:ȱȱȱ
(i)ȱAOȱwithȱPtȱleadsȱtoȱaȱveryȱlowȱTOCȱremoval,ȱandȱ(ii)ȱAOȱwithȱBDDȱandȱPEFȱwithȱ
1.0ȱmMȱFe2+ȱ+ȱ1.0ȱmMȱCu2+ȱ+ȱUVAȱlightȱleadȱtoȱcompleteȱmineralizationȱafterȱ5Ȭ6ȱh,ȱ
butȱ PEFȱ exhibitsȱ aȱ greaterȱ mineralizationȱ rateȱ atȱ nearlyȱ allȱ ofȱ theȱ stagesȱ dueȱ toȱ theȱ
highȱamountȱofȱ•OHȱgeneratedȱinȱtheȱmediumȱfromȱFenton’sȱreaction.ȱ
ȱ
Allȱsolutionsȱofȱ pHȱ3.0ȱ upȱ toȱ 948ȱ mgȱ LȬ1ȱparacetamolȱ areȱcompletelyȱmineralizedȱatȱ
300ȱ mAȱ andȱ atȱ 35ȱ ºCȱ withȱ BDD,ȱ requiringȱ betweenȱ 4ȱ hȱ (Qȱ =ȱ 12ȱ Aȱ hȱ LȬ1)ȱ andȱ 12ȱ hȱȱȱȱȱȱȱȱ
(Qȱ=ȱ36ȱAȱhȱLȬ1)ȱforȱ78ȱandȱ948ȱmgȱLȬ1,ȱrespectively.ȱTheȱincreaseȱinȱQȱwithȱincreasingȱ
initialȱdrugȱconcentrationȱcanȱbeȱexplainedȱbyȱtheȱpresenceȱofȱmoreȱpollutantsȱinȱtheȱ
medium.ȱ Aȱ relevantȱ trendȱ canȱ beȱ detectedȱ fromȱ theȱ percentagesȱ ofȱ TOCȱ removalȱ
197
PART B –Results and Discussion7. Paracetamol
afterȱ1ȱhȱandȱ4ȱhȱofȱtreatment:ȱ86Ȭ91%ȱofȱTOCȱremovalȱisȱattainedȱupȱtoȱ315ȱmgȱLȬ1ȱ
paracetamolȱatȱ4ȱh,ȱwhereasȱ70%ȱisȱreachedȱforȱ948ȱmgȱLȬ1.ȱInȱconclusion,ȱtheȱquickestȱ
mineralizationȱrateȱisȱachievedȱupȱtoȱ315ȱmgȱLȬ1ȱparacetamol.ȱItȱisȱworthȱremindingȱ
thatȱPaperȱ2ȱshowsȱthatȱPEFȱwithȱ1.0ȱmMȱFe2+ȱ+ȱ1.0ȱmMȱCu2+ȱ+ȱUVAȱlightȱisȱableȱtoȱ
destroyȱ uniquelyȱ upȱ toȱ ca.ȱ 0.4ȱ gȱ LȬ1ȱ ofȱ drug.ȱ Onȱ theȱ otherȱ hand,ȱ theȱ Pt/graphiteȱ
systemȱ favorsȱ polymerizationȱ ofȱ intermediatesȱ inȱ concentratedȱ solutions,ȱ thusȱ
limitingȱtheȱmineralizationȱprocess.ȱ
TOC / mg L
-1
120
100
80
60
40
20
0
0
60
120
180
240
time / min
300
360
420
ȱ
Figure 7.-3 TOC decay vs. electrolysis time for the degradation of 100 mL of 157
mg L-1 paracetamol solutions in 0.05 M Na2SO4 of pH 3.0 at 300 mA and at 35 ºC.
Process: (Ɣ) PEF with 1.0 mM Fe2+ + 1.0 mM Cu2+ + UVA light (co-catalyzed PEF), using a
3-cm2 Pt anode and a 3-cm2 O2-diffusion cathode, (Ŷ) AO with a 3-cm2 BDD anode and a
3-cm2 graphite cathode, and (Ƒ) AO with a 3-cm2 Pt anode and a 3-cm2 graphite cathode.
ȱ
Qȱvaluesȱofȱ9,ȱ15ȱandȱ18ȱAȱhȱLȬ1ȱ(9,ȱ5ȱandȱ4ȱh,ȱrespectively)ȱareȱneededȱtoȱdepolluteȱaȱ
157ȱ mgȱ LȬ1ȱ paracetamolȱ solutionȱ ofȱ pHȱ 3.0ȱ atȱ 35ȱ ºCȱ whenȱ 100,ȱ 300ȱ andȱ 450ȱ mAȱ areȱ
applied,ȱ respectively.ȱ Thatȱ isȱ toȱ say,ȱ asȱ currentȱ intensityȱ (I)ȱ increasesȱ theȱ timeȱ
requiredȱtoȱreachȱaȱcertainȱTOCȱvalueȱdecreasesȱbecauseȱaȱhigherȱamountȱofȱ •OHadsȱ
areȱformedȱatȱtheȱanodeȱsurface,ȱbutȱsimultaneouslyȱaȱproportionallyȱgreaterȱamountȱ
ofȱ •OHadsȱ isȱ alsoȱ destroyed,ȱ thusȱ requiringȱ aȱ greaterȱ Qȱ (i.e.,ȱ higherȱ electricalȱ
consumption).ȱInȱotherȱwords,ȱaȱsimilarȱconclusionȱtoȱthatȱofȱEFȱandȱPEFȱprocessesȱ
canȱ beȱ obtained:ȱ asȱ Iȱ increases,ȱ aȱ higherȱ TOCȱ removalȱ isȱ achievedȱ atȱ aȱ givenȱ time.ȱȱȱȱȱ
198
PART B –Results and Discussion7. Paracetamol
InȱtheȱcaseȱofȱEFȱandȱPEFȱthisȱtrendȱisȱattributedȱtoȱtheȱenhancedȱproductionȱofȱ •OHȱ
inȱtheȱmedium,ȱwhereasȱhereȱitȱisȱdueȱtoȱtheȱhigherȱamountȱofȱ•OHads.ȱToȱtellȱtheȱtruth,ȱ
theȱsituationȱisȱaȱbitȱmoreȱcomplex,ȱsinceȱtheȱproductionȱofȱweakȱoxidantsȱsuchȱasȱO3,ȱ
H2O2ȱandȱS2O82ȬȱionsȱisȱalsoȱenhancedȱwithȱincreasingȱI,ȱthusȱindicatingȱtheȱexistenceȱ
ofȱ aȱmassȬtransportȱ controlledȱ process:ȱ aȱ gradualȱ accumulationȱ ofȱ theseȱspeciesȱ canȱ
beȱ observed,ȱ attainingȱ aȱ quasiȬsteadyȱ concentrationȱ fromȱ 4ȱ hȱ ofȱ electrolysis,ȱ justȱ
whenȱtheyȱareȱgeneratedȱandȱdestroyedȱatȱtheȱsameȱrate.ȱWhenȱPtȱisȱusedȱasȱanode,ȱ
noȱ weakȱ oxidantsȱ areȱ detected,ȱ soȱ theȱ lowȱ amountȱ ofȱ effectiveȱ •OHadsȱ isȱ theȱ uniqueȱ
sourceȱofȱoxidizingȱspeciesȱtoȱremoveȱtheȱpollutants.ȱInȱaddition,ȱasȱTȱincreasesȱfromȱ
25ȱtoȱ45ȱºC,ȱaȱpartȱofȱtheȱaforementionedȱoxidizingȱagentsȱ(•OHads,ȱO3,ȱH2O2ȱandȱS2O82Ȭ)ȱ
isȱconsumedȱdueȱtoȱtheirȱdecompositionȱand/orȱtheirȱreactionȱwithȱgreaterȱamountȱofȱ
organics.ȱ Theȱ effectȱ ofȱ temperatureȱ variationȱ canȱ thenȱ beȱ summarizedȱ inȱ theȱ
followingȱway:ȱtheȱreactionȱofȱpollutantsȱisȱacceleratedȱwhenȱTȱraises,ȱthusȱenhancingȱ
drugȱ mineralization.ȱ Moreover,ȱ sinceȱ theȱ increaseȱ inȱ temperatureȱ causesȱ aȱ greaterȱ
massȱ transferȱ toȱ theȱ anodeȱ dueȱ toȱ theȱ decreaseȱ ofȱ mediumȱ viscosity,ȱ itȱ canȱ beȱ
concludedȱagainȱthatȱtheȱoxidationȱprocessȱisȱlimitedȱbyȱtheȱmassȱtransferȱofȱorganicsȱ
towardsȱtheȱBDDȱsurface.ȱ
ȱ
Asȱ pointedȱ outȱ forȱ EFȱ andȱ PEF,ȱ theȱ determinationȱ ofȱ NȬcontainingȱ inorganicȱ ionsȱ
revealsȱthatȱtheȱinitialȱNȱisȱmainlyȱlostȱasȱNH4+.ȱNevertheless,ȱforȱsuchȱanȱoxidizingȱ
methodȱ asȱ theȱ PEFȱ processȱ withȱ 1.0ȱ mMȱ Fe2+ȱ +ȱ 1.0ȱ mMȱ Cu2+ȱ +ȱ UVAȱ light,ȱ 93%ȱ ofȱ
initialȱ Nȱ isȱ convertedȱ intoȱ NH4+ȱ andȱ onlyȱ 1%ȱ isȱ transformedȱ intoȱ NO3,ȱ whereasȱ inȱ
AOȱ withȱ BDDȱ 65%ȱ ofȱ initialȱ Nȱ contentȱ isȱ convertedȱ intoȱ NH4+ȱ andȱ 35%ȱ intoȱ NO3.ȱ
AfterȱelectrolyzingȱanȱammoniumȱsaltȱwithȱtheȱsameȱBDDȱsystem,ȱitȱcanȱbeȱsaidȱthatȱ
onlyȱaȱsmallȱproportionȱofȱNO3ȱcomesȱfromȱNH4+ȱoxidationȱatȱtheȱBDDȱsurface.ȱThen,ȱ
NO3ȱshouldȱmainlyȱcomeȱfromȱtheȱoxidationȱofȱNȬintermediatesȱatȱtheȱBDD.ȱSomeȱofȱ
theseȱ intermediatesȱ couldȱ resembleȱ toȱ theȱ onesȱ proposedȱ byȱ Vognaȱ etȱ al.ȱ [355,ȱ 356]ȱ
thanksȱ toȱ 15NȬlabeling:ȱ NȬacetylaminocatecholȱ andȱ NȬacetylaminoresorcinolȱ comingȱ
fromȱparacetamolȱhydroxylation,ȱandȱsomeȱNȬcontainingȱcarboxylicȱacids.ȱ
199
PART B –Results and Discussion7. Paracetamol
Inȱ sectionȱ 7.2.2ȱ itȱ hasȱ beenȱ commentedȱ thatȱ theȱ overallȱ mineralizationȱ reactionȱ
involvesȱ 34ȱ Fȱ forȱ eachȱ molȱ ofȱ paracetamolȱ (Reactionȱ 6.Ȭ2).ȱ MCEȱ valuesȱ forȱ severalȱ
intensitiesȱ andȱ initialȱ paracetamolȱ concentrationsȱ canȱ beȱ calculatedȱ byȱ usingȱ
Equationȱ 6.Ȭ1.ȱ Asȱ inȱ sectionȱ 7.2,ȱ resultsȱ showȱ aȱ continuousȱ dropȱ inȱ efficiencyȱ withȱ
timeȱ (i.e.,ȱ withȱ Q)ȱ afterȱ goingȱ throughȱ theȱ maximumȱ value,ȱ thusȱ indicatingȱ aȱ
concomitantȱ decreaseȱ inȱ theȱ oxidizingȱ abilityȱ ofȱ theȱ electrolyticȱ system.ȱ Thisȱ trendȱ
canȱbeȱascribedȱagainȱtoȱtheȱlargerȱproportionȱofȱ •OHadsȱoxidizedȱtoȱO2ȱatȱtheȱanode,ȱ
andȱ itsȱ recombinationȱ toȱ H2O2.ȱ Similarly,ȱ higherȱ MCEȱ valuesȱ areȱ obtainedȱ asȱ initialȱ
concentrationȱ ofȱ pollutantȱ rises.ȱ Moreover,ȱ decreasingȱ efficienciesȱ canȱ beȱ observedȱ
whenȱ Iȱ increases,ȱ asȱ couldȱ beȱ inferredȱ fromȱ theȱ aforementionedȱ comment:ȱ asȱ Iȱ
increases,ȱaȱgreaterȱelectricalȱconsumptionȱ(i.e.,ȱaȱgreaterȱQ)ȱisȱrequiredȱtoȱmineralizeȱ
becauseȱ aȱ largerȱ proportionȱ ofȱ hydroxylȱ radicalȱ isȱ wastedȱ inȱ parasiteȱ reactions.ȱ
Finally,ȱtheȱprocessȱisȱmoreȱefficientȱwithȱrisingȱT,ȱasȱisȱexpectedȱifȱreactionsȱbetweenȱ
organicsȱandȱallȱtheȱoxidizingȱagentsȱ(•OHads,ȱO3,ȱH2O2ȱandȱS2O82Ȭ)ȱareȱenhancedȱdueȱ
toȱtheȱincreaseȱinȱmassȱtransferȱofȱpollutants.ȱAȱreallyȱforcefulȱconclusionȱcanȱthenȱbeȱ
presentedȱ afterȱ comparingȱ theȱ dataȱ inȱ Paperȱ 2ȱ andȱ Paperȱ 3:ȱ atȱ lowȱ paracetamolȱ
concentrations,ȱ PEFȱ withȱ 1.0ȱ mMȱ Fe2+ȱ +ȱ 1.0ȱ mMȱ Cu2+ȱ +ȱ UVAȱ lightȱ isȱ muchȱ moreȱ
efficientȱthanȱAOȱwithȱBDDȱatȱnearlyȱallȱofȱtheȱstages,ȱwhereasȱatȱhighȱconcentrationsȱ
bothȱprocessesȱexhibitȱsimilarȱefficiencies.ȱThisȱexplanationȱcanȱbeȱquantifiedȱatȱ300ȱ
mA,ȱpHȱ3.0ȱandȱ35ȱºC:ȱ53%ȱofȱTOCȱremovalȱ(16%ȱMCE)ȱandȱ95%ȱ(7%)ȱisȱachievedȱatȱ
1ȱandȱ4ȱhȱbyȱPEFȱwithȱ1.0ȱmMȱFe2+ȱ+ȱ1.0ȱmMȱCu2+ȱ+ȱUVAȱlight,ȱwhereasȱ27%ȱ(9%)ȱandȱ
91%ȱ(7%)ȱisȱreachedȱbyȱAOȱwithȱBDD.ȱInȱcontrast,ȱwhenȱ948ȱmgȱLȬ1ȱofȱparacetamolȱ
areȱ electrolyzed,ȱ theȱ TOCȱ removalȱ atȱ 4ȱ hȱ isȱ aroundȱ 70%ȱ (32%ȱ MCE)ȱ forȱ bothȱ
processes.ȱTheseȱvaluesȱcorroborateȱtheȱideaȱgivenȱbeforeȱandȱconfirmedȱbyȱseveralȱ
authors,ȱthatȱAOȱwithȱBDDȱisȱaȱmassȬtransferȱcontrolledȱprocess,ȱthereforeȱincreasingȱ
itsȱ efficiencyȱ asȱ pollutantȱ concentrationȱ rises.ȱ Figureȱ 7.Ȭ4ȱ givenȱ belowȱ gathersȱ theseȱ
tendenciesȱinȱaȱveryȱclearȱway.ȱ
200
PART B –Results and Discussion7. Paracetamol
40
35
MCE / %
30
25
20
15
10
5
0
0
200
400
time / min
600
800
ȱ
Figure 7.-4 Change of MCE with electrolysis time for the degradation of 100 mL of
paracetamol solutions in 0.05 M Na2SO4 of pH 3.0 at 300 mA and at 35 ºC. Initial drug
concentration: (Ɣ,Ŷ) 157 mg L-1, (ż,Ƒ) 948 mg L-1. Process: (Ɣ,ż) PEF with 1.0 mM Fe2+
+ 1.0 mM Cu2+ + UVA light (co-catalyzed PEF), using a 3-cm2 Pt anode and a 3-cm2
O2-diffusion cathode, (Ŷ,Ƒ) AO with a 3-cm2 BDD anode and a 3-cm2 graphite cathode.
ȱ
ReversedȬphaseȱ chromatogramsȱ forȱ solutionsȱ electrolyzedȱ atȱ 300ȱ mAȱ showȱ aȱ slowȱ
degradationȱ ofȱ paracetamolȱ inȱ comparisonȱ withȱ EFȱ andȱ PEFȱ processes,ȱ whereȱ
paracetamolȱ canȱ beȱ degradedȱ inȱ 6ȱ min.ȱ Whenȱ AOȱ withȱ Ptȱ isȱ used,ȱ paracetamolȱ
disappearsȱ afterȱ nearlyȱ 240ȱ minȱ forȱ 157ȱ mgȱ LȬ1,ȱ andȱ theȱ degradationȱ isȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱ
pHȬindependentȱ forȱ allȱ theȱ initialȱ concentrationsȱ tested.ȱ Kineticȱ analysesȱ giveȱ goodȱ
linearȱ plotsȱ whenȱ fittedȱ toȱ aȱ pseudoȬfirstȬorderȱ reaction,ȱ yieldingȱ theȱ sameȱȱȱȱȱȱ
pseudoȬfirstȬorderȱ rateȱ constantȱ ofȱ k1ȱ =ȱ 0.013±0.002ȱ minȬ1ȱ upȱ toȱ 315ȱ mgȱ LȬ1.ȱ Thisȱ
suggestsȱ thatȱ •OHadsȱ atȱ Ptȱ surfaceȱ isȱ practicallyȱ constant.ȱ Onȱ theȱ otherȱ hand,ȱ aȱ
complexȱ decayȱ ofȱ paracetamolȱ withȱ timeȱ isȱ observedȱ inȱ AOȱ withȱ BDD,ȱ withoutȱ
followingȱanyȱkineticȱequationȱrelatedȱtoȱsimpleȱreactionȱorders,ȱsimilarlyȱtoȱwhatȱisȱ
observedȱ inȱ EFȱ andȱ PEFȱ processes.ȱ Thisȱ complexȱ kineticsȱ canȱ beȱ relatedȱ toȱ theȱ
competitiveȱ consumptionȱ ofȱ •OHadsȱ byȱ parallelȱ fastȱ reactionsȱ withȱ someȱ products,ȱ
yieldingȱ anȱ undefinedȱ kineticsȱ dueȱ toȱ aȱ variableȱ •OHadsȱ concentrationȱ attackingȱ
paracetamol.ȱ Aȱ pHȬindependentȱ decayȱ isȱ observedȱ forȱ AOȱ withȱ BDDȱ asȱ well.ȱ
Nevertheless,ȱ theȱ higherȱ oxidizingȱ powerȱ ofȱ BDDȱ inȱ comparisonȱ withȱ Ptȱ isȱ evidentȱ
201
PART B –Results and Discussion7. Paracetamol
becauseȱ paracetamolȱ isȱ destroyedȱ inȱ 150ȱ minȱ (stillȱ reallyȱ slowȱ inȱ comparisonȱ toȱ EFȱ
andȱPEF).ȱFigureȱ7.Ȭ5ȱcomparesȱparacetamolȱdegradationȱcurvesȱforȱPEFȱwithȱ1.0ȱmMȱ
Fe2+ȱ +ȱ 1.0ȱ mMȱ Cu2+ȱ +ȱ UVAȱ light,ȱ AOȱ withȱ Ptȱ andȱ AOȱ withȱ BDD,ȱ andȱ theȱ obviousȱ
differencesȱcanȱbeȱobservedȱatȱfirstȱsight:ȱ
[paracetamol] / mg L
-1
200
150
100
50
0
0
60
120
180
time / min
240
300
ȱ
Figure 7.-5 Decay of paracetamol concentration with electrolysis time for the
degradation of 100 mL of 157 mg L-1 paracetamol solutions in 0.05 M Na2SO4 of pH 3.0
at 300 mA and at 35 ºC. Process: (Ɣ) PEF with 1.0 mM Fe2+ + 1.0 mM Cu2+ + UVA light
(co-catalyzed PEF), using a 3-cm2 Pt anode and a 3-cm2 O2-diffusion cathode,
(Ŷ) AO with a 3-cm2 BDD anode and a 3-cm2 graphite cathode, and (Ƒ) AO with a 3-cm2
Pt anode and a 3-cm2 graphite cathode.
ȱ
ReversedȬphaseȱ chromatogramsȱ fromȱ electrolysesȱ withȱ Ptȱ exhibitȱ theȱ peaksȱ ofȱ
hydroquinoneȱ andȱ pȬbenzoquinoneȱ asȱ aromaticȱ intermediates.ȱ Theseȱ productsȱ areȱ
alsoȱobservedȱbyȱ EFȱandȱPEF.ȱHowever,ȱtheyȱareȱnotȱ detectedȱ inȱAOȱ withȱ BDD,ȱasȱ
expectedȱifȱtheyȱcanȱnotȱbeȱaccumulatedȱinȱtheȱmediumȱbecauseȱparacetamolȱandȱitsȱ
intermediatesȱ areȱ simultaneouslyȱ destroyedȱ withȱ •OHadsȱ (andȱ otherȱ weakȱ oxidantsȱ
reportedȱelsewhere),ȱthusȱyieldingȱaȱcomplexȱkinetics.ȱ
ȱ
Finally,ȱ ionȬexclusionȱ chromatogramsȱ forȱ AOȱ withȱ BDDȱ showȱ theȱ accumulationȱ ofȱ
oxalicȱandȱoxamicȱacidsȱasȱtheȱultimateȱintermediates.ȱNotȱketomalonicȱnorȱmaleicȱorȱ
fumaricȱ acidsȱ areȱ accumulated.ȱ Theȱ evolutionȱ ofȱ oxalicȱ andȱ oxamicȱ acidsȱ showsȱ noȱ
significantȱ differencesȱ atȱ pHȱ 3.0ȱ andȱ 12.0.ȱ Itȱ mustȱ beȱ highlightedȱ thatȱ maximumȱ
202
PART B –Results and Discussion7. Paracetamol
concentrationsȱofȱoxalicȱandȱoxamicȱacidsȱareȱaboutȱ10ȱandȱ4ȱmgȱLȬ1,ȱrespectively,ȱandȱ
theyȱdisappearȱafterȱ240ȱmin,ȱwhereasȱinȱPEFȱwithȱ1.0ȱmMȱFe2+ȱ+ȱ1.0ȱmMȱCu2+ȱ+ȱUVAȱ
lightȱupȱtoȱ80ȱandȱ15ȱmgȱLȬ1ȱareȱaccumulated,ȱandȱtheyȱremainȱinȱtheȱsolutionȱduringȱ
allȱtheȱmineralizationȱprocessȱ(i.e.,ȱ360ȱmin).ȱInȱotherȱwords,ȱbothȱparacetamolȱandȱitsȱ
intermediatesȱareȱoxidizedȱatȱfastȱandȱsimilarȱdestructionȱrateȱonȱBDDȱinȱacidicȱandȱ
alkalineȱ media,ȱ thusȱ justifyingȱ theȱ lowȱ accumulationȱ ofȱ theȱ productsȱ andȱ theȱȱȱȱȱȱȱȱȱȱ
pHȬindependenceȱforȱtheirȱTOCȱdecay,ȱandȱyieldingȱaȱcomplexȱkinetics.ȱ
ȱ
Afterȱtheȱanalysesȱcarriedȱoutȱinȱorderȱtoȱestablishȱtheȱintermediatesȱinvolvedȱinȱtheȱ
reactionȱ pathway,ȱ itȱ canȱ beȱ concludedȱ thatȱ theȱ schemeȱ proposedȱ inȱ sectionȱ 7.2.2ȱ isȱ
definitelyȱ aȱ goodȱ electrochemicalȱ sequenceȱ forȱ paracetamol,ȱ sinceȱ noȱ otherȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱ
byȬproductsȱ haveȱ beenȱ identifiedȱ byȱ AOȱ comparedȱ toȱ EFȱ andȱ PEF.ȱ Hydroquinoneȱ
andȱ acetamideȱ areȱ theȱ primaryȱ products,ȱ beingȱ subsequentlyȱ oxidizedȱ toȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱ
pȬbenzoquinoneȱ andȱ oxamicȱ acid,ȱ respectively.ȱ Ringȱ openingȱ leadsȱ toȱ oxalicȱ acid,ȱ
whileȱoxamicȱacidȱoxidationȱreleasesȱNH4+ȱions.ȱAtȱtheȱend,ȱallȱtheȱorganicȱCȱcontentȱ
canȱbeȱtransformedȱintoȱCO2ȱusingȱBDD,ȱwhereasȱwithȱPtȱtheȱmineralizationȱisȱveryȱ
poor.ȱ
ȱ
Whenȱ BDDȱ isȱ usedȱ theȱ solutionsȱ areȱ turnedȱ colorlessȱ becauseȱ ofȱ theȱ overallȱ
destructionȱofȱsolubleȱaromaticȱproducts,ȱresponsibleȱforȱtheȱcolorationȱ observedȱ inȱ
Ptȱsystems.ȱItȱmustȱbeȱnotedȱthatȱinȱtheȱpreviouslyȱcommentedȱEFȱandȱPEFȱprocessesȱ
theȱinitialȱsolutionsȱturnedȱintoȱdarkȱyellowȱdueȱtoȱtheȱcomplexesȱbetweenȱFe3+ȱandȱ
H2O2.ȱTheȱsolutionȱpHȱalwaysȱremainsȱpracticallyȱconstantȱwhenȱtheȱinitialȱpHȱisȱ3.0,ȱ
asȱinȱtheȱcaseȱofȱEFȱandȱPEFȱsystems.ȱOnȱtheȱcontrary,ȱaȱgradualȱpHȱdecayȱisȱfoundȱatȱ
pHȱǃȱ4.0ȱdueȱtoȱtheȱformationȱofȱcarboxylicȱacids.ȱ
ȱ
ȱ
203
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
8.ȱȱ DESTRUCCIÓȱ
D’UNȱ
REGULADORSȱ
DEȱ
METABÒLITȱ
LÍPIDSȱ
ENȱ
ACTIUȱ
SANG:ȱ
DEȱ
ÀCIDȱ
FÀRMACSȱ
CLOFÍBRICȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱ ȱȱȱȱȱ
/ȱ DESTRUCTIONȱ OFȱ Aȱ BLOODȱ LIPIDȱ REGULATORȱ AGENT:ȱ
CLOFIBRICȱACIDȱ
ȱ
ȱ
ȱ
ȱ
Thisȱchapterȱisȱdevotedȱtoȱtheȱstudyȱofȱtheȱdegradationȱofȱtheȱbloodȱlipidȱ regulatorȱ
agentȱclofibricȱacid.ȱItȱisȱdividedȱintoȱthreeȱmainȱparts:ȱ(i)ȱanȱintroductionȱgivingȱanȱ
overviewȱ onȱ theȱ characteristicsȱ ofȱ clofibricȱ acid,ȱ itsȱ environmentalȱ dataȱ andȱ someȱ
resultsȱ publishedȱ inȱ literatureȱ onȱ itsȱ destruction,ȱ (ii)ȱ theȱ resultsȱ obtainedȱ forȱ theȱ
treatmentȱofȱthisȱdrugȱbyȱanodicȱoxidation,ȱandȱ(iii)ȱtheȱresultsȱobtainedȱbyȱelectroȬ
FentonȱandȱphotoelectroȬFentonȱprocesses.ȱ
ȱ
ȱ
205
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
PART B –Results and Discussion8. Clofibric Acid
8.1.
CARACTERÍSTIQUESȱDEȱL’ÀCIDȱCLOFÍBRICȱ
/ȱCHARACTERISTICSȱOFȱCLOFIBRICȱACIDȱ
ȱ
Clofibricȱacidȱ(2Ȭ(4Ȭchlorophenoxy)Ȭ2Ȭmethylpropionicȱacid,ȱFigureȱ8.Ȭ1)ȱisȱtheȱactiveȱ
metaboliteȱofȱtheȱdrugsȱclofibrate,ȱetofibrateȱandȱetofyllineclofibrate,ȱwidelyȱusedȱasȱ
bloodȱ lipidȱ regulatorsȱ atȱ highȱ therapeuticalȱ dosesȱ (upȱ toȱ 1Ȭ2ȱ gramsȱ perȱ dayȱ andȱ
person).ȱ Theseȱ substancesȱ areȱ usedȱ toȱ decreaseȱ theȱ plasmaticȱ concentrationȱ ofȱ
cholesterolȱ andȱ triglycerides.ȱ Moreover,ȱ clofibricȱ acidȱ isȱ aȱ structuralȱ isomerȱ ofȱ theȱ
phenoxyalkanoicȱ acidȱ herbicideȱ mecopropȱ (2Ȭ(4ȬchloroȬ2Ȭmethylphenoxy)propionicȱ
acid).ȱ
ȱ
Fibrates,ȱ aȱ groupȱ ofȱ drugsȱ marketedȱ sinceȱ 1963,ȱ areȱ classifiedȱ asȱ peroxisomalȱ
proliferatorsȱ (PPs)ȱ becauseȱ theyȱ haveȱ demonstratedȱ affinityȱ forȱ theȱ activatedȱ
receptors,ȱ thenȱ increasingȱ theȱ numberȱ andȱ sizeȱ ofȱ cellularȱ peroxisomesȱ notȱ onlyȱ inȱ
theȱ liverȱ butȱ alsoȱ inȱ manyȱ otherȱ tissues.ȱ Peroxisomesȱ areȱ singleȱ membraneȱ boundȱ
organellesȱ foundȱ inȱ mostȱ plantȱ andȱ animalȱ cellsȱ performingȱ variousȱ metabolicȱ
functions,ȱ andȱ segregateȱ harmfulȱ products,ȱ suchȱ asȱ H2O2,ȱ fromȱ theȱ restȱ ofȱ theȱ cellȱ
whileȱ carryingȱ onȱ EȬoxidationȱ ofȱveryȱlongȬchainȱ fattyȱ acids.ȱ Effectsȱonȱfishȱ suchȱasȱ
rainbowȱ troutsȱ orȱ Atlanticȱ salmonȱ haveȱ beenȱ reportedȱ byȱ Trudeauȱ etȱ al.ȱ [359].ȱ
Clofibricȱacidȱtoxicityȱhasȱbeenȱrecentlyȱassessedȱusingȱthreeȱestuarineȱorganismsȱ(anȱ
alga,ȱ aȱ crustaceanȱ andȱ aȱ fish)ȱ [360]:ȱ noȱ adverseȱ effectsȱ onȱ theȱ basisȱ ofȱ parametersȱ
examinedȱ areȱ expectedȱ toȱ occurȱ atȱ clofibricȱ acidȱ environmentalȱ levels,ȱ butȱ theȱ
potentialȱ forȱ longerȱ termȱ effectsȱ canȱ notȱ beȱ ruledȱ out.ȱ Inȱ addition,ȱ possibleȱ mixtureȱ
toxicity,ȱ bioaccumulation,ȱ andȱ trophicȱ transferȱ ofȱ thisȱ contaminantȱ shouldȱ beȱ
considered.ȱ
ȱ
Moreover,ȱclofibricȱacidȱisȱknownȱtoȱpossessȱantiauxinȱactivity.ȱAuxinȱisȱoneȱofȱmajorȱ
plantȱ hormonesȱ whichȱ affectȱ numerousȱ plantȱ growthȱ processesȱ functionsȱ includingȱ
cellȱ divisionȱ andȱ elongation,ȱ autumnalȱ lossȱ ofȱ leaves,ȱ andȱ theȱ formationȱ ofȱ buds,ȱ
207
PART B –Results and Discussion8. Clofibric Acid
roots,ȱ flowersȱ andȱ fruits.ȱ Auxinsȱ usuallyȱ haveȱ aȱ ringȱ systemȱ withȱ anȱ attachedȱ sideȬ
chainȱthatȱterminatesȱinȱaȱcarboxylȱgroup,ȱsoȱitȱcanȱbeȱconsequentlyȱunderstoodȱthatȱ
clofibricȱacidȱaffectsȱseveralȱplantȱhormonalȱfunctions.ȱInȱfact,ȱthisȱacidȱhasȱbeenȱusedȱ
asȱanȱefficientȱantiauxinȱforȱmoreȱthanȱfiveȱdecades.ȱ
ȱ
Clofibricȱ acidȱ isȱ usuallyȱ foundȱ asȱ paleȱ yellowȱ crystals,ȱ withȱ aȱ veryȱ characteristicȱ
odour.ȱSomeȱofȱitsȱmostȱremarkableȱpropertiesȱareȱsummarizedȱinȱTableȱ8.Ȭ1.ȱ
ȱ
Cl
ȱ
O
C
ȱ
H C
3
CH
3
C OOH
ȱ
Figure 8.-1 Clofibric acid.
ȱ
Table 8.-1 Clofibric acid data (several sources).
ȱ
CAS number
ȱ
882-09-7
Clofibrate
ȱ
Parent lipid-lowering agents
ȱ
Etofibrate
Etofyllineclofibrate
ȱ
Trade names
ȱ
Molecular formula
ȱ
ȱ
Atromid-S (clofibrate)
C10H11ClO3
Molecular mass (g mol-1)
214.66
Melting point (ºC)
118-119
Boiling point (ºC)
-
ȱ
Solubility in H2O (mg L-1)20 ºC
ȱ
Density (g cm-3)21 ºC
ȱ
pKa
582.5 [360]
3.18
ȱ
Clofibricȱacidȱisȱaȱhighȱvolumeȱchemicalȱwithȱanȱestimatedȱannualȱproductionȱinȱtheȱ
lowȱ kilotonȱ range.ȱ Itȱ isȱ mainlyȱ usedȱ inȱ theȱ formȱ ofȱ theȱ ethylȱ esterȱ (clofibrate)ȱ inȱ
humanȱmedicalȱcare.ȱ
ȱ
208
PART B –Results and Discussion8. Clofibric Acid
Twoȱmainȱcharacteristicsȱcanȱdefineȱclofibricȱacid:ȱubiquityȱandȱpersistence.ȱPrecisely,ȱ
itsȱenvironmentalȱinterestȱisȱdueȱtoȱitsȱwidespreadȱoccurrenceȱasȱwellȱasȱitsȱlongȬtimeȱ
exposureȱ(itsȱestimatedȱenvironmentalȱpersistenceȱisȱaboutȱ21ȱdays)ȱ[361].ȱHebererȱetȱ
al.ȱshowedȱnoȱremovalȱrateȱinȱdifferentȱSTPsȱofȱBerlinȱ[362],ȱbutȱtheȱresultsȱofȱStumpfȱ
etȱ al.ȱ pointȱ outȱ aȱ relativelyȱ highȱ removalȱ rateȱ ofȱ 15%ȱ (biologicalȱ filtration),ȱ 34%ȱ
(activatedȱ sludge)ȱ andȱ 51%ȱ (activatedȱ sludgeȱ andȱ FeCl3)ȱ [363].ȱ Inȱ otherȱ words,ȱ
clofibricȱacidȱisȱgenerallyȱnotȱeliminatedȱfromȱwastewater,ȱbutȱaȱremovalȱcanȱappearȱ
inȱ someȱ sewageȱ treatmentȱ plants.ȱ Sinceȱ itsȱ logKOWȱ isȱ 3.1,ȱ clofibricȱ acidȱ isȱ liableȱ toȱ
persistȱinȱtheȱaquaticȱenvironment.ȱProvidedȱthatȱitȱisȱnotȱbiotransformedȱintoȱaȱmoreȱ
hydrophilicȱderivative,ȱitȱisȱnotȱlikelyȱtoȱbioaccumulateȱ[361].ȱ
ȱ
Clofibricȱ acidȱ wasȱ oneȱ ofȱ theȱ firstȱ drugs/metabolitesȱ everȱ reportedȱ inȱ sewageȱ
influent/effluent.ȱInȱ1976ȱitȱwasȱdetectedȱinȱrawȱsewageȱandȱactivatedȱsludgeȱeffluentȱ
withȱ valuesȱ fromȱ 0.8ȱ toȱ 2.0ȱ Pgȱ LȬ1ȱ [44].ȱ Inȱ 1977ȱ itȱ wasȱ measuredȱ inȱ Missouriȱ STPȱ
effluentsȱ withȱ anȱ averageȱ concentrationȱ equalȱ toȱ 2.1ȱ kg/dayȱ [364].ȱ Later,ȱ inȱ 1992,ȱ
researchersȱ lookingȱ forȱ phenoxyalkanoicȱ herbicidesȱ inȱ waterȱ foundȱ clofibricȱ acidȱ
aroundȱ theȱ cityȱ ofȱ Berlin.ȱ Sinceȱ then,ȱ clofibricȱ acidȱ hasȱ beenȱ widelyȱ reportedȱ asȱ
occurringȱ inȱ STPsȱ influentsȱ andȱ effluents,ȱ inȱ Germanȱ rivers,ȱ inȱ Swissȱ lakes,ȱ groundȱ
watersȱ andȱ evenȱ drinkingȱ waterȱ inȱ studiesȱ carriedȱ outȱ acrossȱ Europeȱ andȱ USAȱȱȱȱȱȱ
[365,ȱ 366].ȱ Itȱ hasȱ alsoȱ beenȱ describedȱ asȱ anȱ ubiquitousȱ contaminantȱ inȱ theȱ marineȱ
environment,ȱ presentȱ atȱ concentrationsȱ betweenȱ 0.03ȱ andȱ 19ȱ ngȱ LȬ1ȱ inȱ samplesȱ
collectedȱ fromȱ theȱ Northȱ Seaȱ [367,ȱ 368].ȱ Researchersȱ estimateȱ thatȱ theȱ Northȱ Seaȱ
containsȱ48ȱtoȱ96ȱtonsȱofȱclofibricȱacid,ȱwithȱ50ȱtoȱ100ȱtonsȱenteringȱtheȱSeaȱeachȱyear.ȱ
Theȱ Danubeȱ inȱ Germany,ȱ andȱ theȱ Poȱ inȱ Italy,ȱ alsoȱ containȱ mesurableȱ quantitiesȱ ofȱ
clofibricȱ acid.ȱ Twoȱ ofȱ theȱ samplesȱ collectedȱ fromȱ UKȱ estuariesȱ containedȱ
concentrationsȱ ofȱ approximatelyȱ 100ȱ ngȱ LȬ1ȱ [369].ȱ Influentȱ concentrationȱ ofȱ 1ȱ Pgȱ LȬ1ȱ
hasȱbeenȱfoundȱinȱSTPsȱinȱBrazilȱ[370].ȱFinally,ȱaȱmoreȱimmediateȱconcernȱtoȱhumansȱ
isȱtheȱfindingȱthatȱupȱtoȱ270ȱngȱLȬ1ȱhaveȱbeenȱmeasuredȱinȱGermanȱtapȱwaterȱ[366].ȱ
Theseȱ findingsȱ suggestȱ thisȱ contaminationȱ toȱ beȱ notȱ onlyȱ aȱ localȱ problemȱ fromȱ
209
PART B –Results and Discussion8. Clofibric Acid
improperȱwasteȱdisposalȱbutȱalsoȱlikelyȱtoȱbeȱaȱgeneralȱenvironmentalȱproblem.ȱ
ȱ
Severalȱworksȱdescribeȱthatȱclofibricȱacidȱisȱeasilyȱdegradedȱbutȱpoorlyȱmineralizedȱ
byȱ ozoneȱ [32,ȱ 371],ȱ sunlightȱ andȱ UVȱ photolysisȱ usingȱ XeȬarcȱ lampȱ solarȱ simulatorsȱ
[372,ȱ373],ȱandȱAOPsȱsuchȱasȱH2O2/UVȱ[371].ȱOturanȱetȱal.ȱ[286]ȱhaveȱalsoȱshownȱtheȱ
completeȱ degradationȱ ofȱ clofibricȱ acid,ȱ byȱ meansȱ ofȱ electroȬFentonȱ processȱ usingȱ aȱ
mercuryȱ poolȱ asȱ theȱ workingȱ electrode,ȱ butȱ thisȱ canȱ notȱ beȱ consideredȱ asȱ anȱ
environmentallyȱfriendlyȱprocedureȱinȱwaterȱremediation.ȱ
ȱ
Zwienerȱ etȱ al.ȱ [374]ȱ haveȱ reportedȱ theȱ poorȱ mineralizationȱ ofȱ clofibricȱ acidȱ byȱ
applicationȱofȱbiologicalȱtreatments.ȱTheȱcompoundȱreachedȱaȱlevelȱofȱapproximatelyȱ
95%ȱ ofȱ itsȱ initialȱ concentration,ȱ soȱ itȱ isȱ definedȱ asȱ nonȬbiodegradableȱ underȱ theȱ
experimentalȱconditionsȱapplied.ȱ
ȱ
Upȱ toȱ theȱ presentȱ thesisȱ onlyȱ TiO2/UVȱ [375]ȱ hasȱ beenȱ presentedȱ asȱ anȱ effectiveȱ
methodȱ forȱ clofibricȱ acidȱ mineralization.ȱ Thisȱ latterȱ paperȱ proposesȱ aȱ multiȬstepȱ
schemeȱforȱtheȱdegradationȱofȱclofibricȱacidȱbyȱphotocatalysis.ȱMoreover,ȱaȱreactionȱ
mechanismȱisȱdiscussed.ȱInȱaȱrecentȱpaper,ȱCanterinoȱetȱal.ȱ[376]ȱpayȱattentionȱtoȱtheȱ
presenceȱ ofȱ clofibricȱ acidȱ inȱ slurries,ȱ ratherȱ thanȱ inȱ theȱ aquaticȱ environment.ȱ
Ozonationȱ thenȱ seemsȱ toȱ beȱ aȱ particularlyȱ suitedȱ processȱ forȱ theȱ treatmentȱ ofȱ
recalcitrantȱsoilȱcontaminantsȱsuchȱasȱclofibricȱacid.ȱ
ȱ
Inȱconclusion,ȱmoreȱpotentȱoxidationȱproceduresȱareȱneededȱtoȱbeȱappliedȱtoȱdestroyȱ
thisȱcompoundȱinȱwastewaters.ȱ
ȱ
Inȱthisȱthesis,ȱclofibricȱacidȱdegradationȱandȱmineralizationȱhaveȱbeenȱstudiedȱunderȱ
differentȱEAOPsȱapplied:ȱanodicȱoxidationȱwithȱbothȱPtȱandȱBDDȱanodes,ȱasȱwellȱasȱ
electroȬFentonȱ andȱ photoelectroȬFentonȱ processesȱ withȱ bothȱ anodesȱ andȱ anȱȱȱȱȱȱȱȱȱȱȱȱȱ
O2Ȭdiffusionȱcathode.ȱ
210
PART B –Results and Discussion8. Clofibric Acid
8.2.
TRACTAMENTȱMITJANÇANTȱOXIDACIÓȱANÒDICAȱȱ
/ȱTREATMENTȱBYȱANODICȱOXIDATIONȱ
ȱ
8.2.1.ȱȱFinalitatȱdelȱtreballȱ/ȱAimȱofȱtheȱworkȱ
ȱ
Inȱsectionȱ7.3ȱtheȱeffectivityȱofȱAOȱprocessȱusingȱaȱBDDȱanodeȱforȱtheȱremovalȱandȱ
mineralizationȱ ofȱ paracetamolȱ inȱ watersȱ underȱ severalȱ experimentalȱ conditionsȱ atȱ
laboratoryȱscaleȱhasȱbeenȱshown.ȱInȱorderȱtoȱverifyȱallȱtheȱconclusionsȱdrawnȱforȱthatȱ
process,ȱ mainlyȱ asȱ forȱ theȱ relevanceȱ ofȱ theȱ oxidizingȱ speciesȱ pointedȱ outȱ beforeȱ
(•OHads,ȱ O3,ȱ H2O2ȱ andȱ S2O82Ȭ),ȱ theȱ destructionȱ ofȱ theȱ bloodȱ lipidȱ regulatorsȱ bioactiveȱ
metaboliteȱ calledȱ clofibricȱ acidȱ hasȱ beenȱ thoroughlyȱ studied.ȱ Nextlyȱ theȱ resultsȱ
obtainedȱ withȱ BDDȱ areȱ reported,ȱ asȱ wellȱ asȱ theȱ onesȱ usingȱ Ptȱ becauseȱ itsȱ
comparativelyȱ lowerȱ oxidizingȱ powerȱ allowsȱ aȱ greaterȱ andȱ largerȱ accumulationȱ ofȱ
intermediates,ȱ thusȱ makingȱ itȱ easierȱ toȱ proposeȱ aȱ possibleȱ electrochemicalȱ
degradationȱ pathwayȱ forȱ thisȱ pharmaceutical.ȱ Aȱ stainlessȱ steelȱ sheetȱ wasȱ usedȱ asȱ
cathodeȱinȱallȱcases,ȱandȱlikeȱPtȱandȱBDDȱelectrodesȱitsȱareaȱwasȱ3ȱcm2.ȱ
ȱ
Firstȱ ofȱ all,ȱ theȱ anodicȱ oxidationȱ ofȱ 100ȱ mLȱ ofȱ 179ȱ mgȱ LȬ1ȱ clofibricȱ acidȱ solutionsȱȱȱȱȱȱ
(i.e.,ȱ 100ȱ mgȱ LȬ1ȱ TOC)ȱ containingȱ 0.05ȱ Mȱ Na2SO4ȱ wasȱ performedȱ byȱ carryingȱ outȱ aȱ
seriesȱ ofȱ electrolysesȱ inȱ theȱ pHȱ rangeȱ 2.0Ȭ12.0ȱ atȱ 100ȱ mAȱ cmȬ2ȱ andȱ atȱ 35ȱ ºC.ȱ Theȱ
oxidationȱ abilityȱ ofȱ bothȱ Pt/steelȱ andȱ BDD/steelȱ systemsȱ toȱ mineralizeȱ clofibricȱ acidȱ
wasȱstudiedȱforȱ7ȱh.ȱTOCȱdecayȱwasȱanalyzedȱinȱallȱcases.ȱ
ȱ
OnceȱtheȱgreatȱoxidizingȱpowerȱofȱBDDȱwasȱconfirmedȱwithinȱaȱwideȱpHȱrange,ȱtheȱ
possibleȱ effectȱ ofȱ theȱ variationȱ ofȱ apparentȱ currentȱ densityȱ (jappȱ =ȱ 33,ȱ 100ȱ andȱ 150ȱȱȱȱ
mAȱcmȬ2)ȱandȱtemperatureȱ(25,ȱ35ȱandȱ45ȱºC)ȱonȱTOCȱdecayȱandȱMCEȱwasȱassessedȱ
inȱ orderȱ toȱ optimizeȱ theȱ AOȱ processȱ withȱ BDDȱ atȱ laboratoryȱ scale.ȱ Withȱ thisȱ aim,ȱ
severalȱ electrolysesȱ ofȱ 179ȱ mgȱ LȬ1ȱ clofibricȱ acidȱ solutionsȱ wereȱ performedȱ atȱ pHȱ 3.0ȱ
andȱ 12.0.ȱ Inȱ addition,ȱ toȱ knowȱ theȱ influenceȱ ofȱ electrogeneratedȱ speciesȱȱȱȱȱȱȱȱȱȱȱȱȱȱ
211
PART B –Results and Discussion8. Clofibric Acid
(•OHads,ȱ O3,ȱ H2O2ȱ andȱ S2O82Ȭȱ ions)ȱ onȱ theȱ mineralizationȱ ofȱ clofibricȱ acid,ȱ theȱ
concentrationsȱ ofȱ H2O2ȱ andȱ totalȱ oxidizingȱ agentsȱ wereȱ determinedȱ duringȱ theȱ
treatmentȱofȱ100ȱmLȱofȱ179ȱmgȱLȬ1ȱclofibricȱacidȱsolutionsȱofȱpHȱ3.0ȱatȱseveralȱjappȱandȱ
temperatures.ȱAsȱreportedȱinȱsectionȱ6.3,ȱH2O2ȱwasȱanalyzedȱspectrophotometricallyȱ
atȱOȱ=ȱ408ȱnmȱ(Reactionȱ6.Ȭ6)ȱ[339,ȱ340],ȱandȱtheȱamountȱofȱtotalȱoxidizingȱspeciesȱwasȱ
determinedȱbyȱiodometricȱtitrationȱ[341].ȱ
ȱ
TheȱgreatȱoxidizingȱpowerȱofȱAOȱwithȱBDDȱanodeȱwasȱthenȱstudiedȱbyȱelectrolyzingȱ
solutionsȱ ofȱ pHȱ 12.0ȱ withȱ metaboliteȱ contentsȱ upȱ toȱ closeȱ toȱ saturationȱ (45,ȱ 89,ȱ 179,ȱ
268,ȱ358,ȱ447ȱandȱ557ȱmgȱLȬ1ȱclofibricȱacid,ȱcorrespondingȱtoȱ25,ȱ50,ȱ100,ȱ150,ȱ200,ȱ250ȱ
andȱ313ȱmgȱLȬ1ȱTOC,ȱrespectively)ȱatȱ100ȱmAȱcmȬ2ȱandȱatȱ35ȱºC.ȱ
ȱ
AfterȱtheȱdetailedȱstudyȱofȱTOCȱabatementȱunderȱmanyȱexperimentalȱconditions,ȱtheȱ
evolutionȱ ofȱ inorganicȱ ionsȱ releasedȱ fromȱ initialȱ chlorineȱ ofȱ theȱ metaboliteȱ wasȱ
examinedȱbyȱionȱchromatography.ȱInȱthisȱsense,ȱ100ȱmLȱofȱ179ȱmgȱLȬ1ȱclofibricȱacidȱ
solutionsȱ ofȱ pHȱ 3.0ȱ andȱ 12.0ȱ wereȱ electrolyzedȱ usingȱ aȱ BDDȱ anodeȱ atȱ 100ȱ mAȱ cmȬ2ȱ
andȱatȱ35ȱºC.ȱSeveralȱelectrolysesȱusingȱaȱPtȱanodeȱwereȱcomparativelyȱdone.ȱȱ
ȱ
Afterwards,ȱ theȱ kineticsȱ forȱ theȱ reactionȱ betweenȱ clofibricȱ acidȱ andȱ •OHadsȱ wasȱ
studiedȱ forȱ bothȱ Ptȱ andȱ BDDȱ systems.ȱ Firstly,ȱ itȱ wasȱ necessaryȱ toȱ clarifyȱ whetherȱ
clofibricȱacidȱcanȱalsoȱbeȱoxidizedȱwithȱH2O2ȱand/orȱS2O82Ȭ,ȱtwoȱofȱtheȱweakȱoxidantsȱ
havingȱnotoriousȱroleȱinȱAOȱwithȱaȱBDDȱanode.ȱChemicalȱtestsȱwereȱcarriedȱoutȱbyȱ
preparingȱ 100ȱ mLȱ solutionsȱ ofȱ pHȱ 3.0ȱ andȱ 12.0ȱ containingȱ 179ȱ mgȱ LȬ1ȱ ofȱ theȱ
pharmaceutical,ȱ 20ȱ mMȱ ofȱ bothȱ oxidizingȱ speciesȱ andȱ 0.05ȱ Mȱ Na2SO4.ȱ Severalȱ
solutionsȱofȱ179ȱmgȱLȬ1ȱclofibricȱacidȱofȱpHȱ3.0ȱandȱ12.0,ȱatȱ35ȱºCȱandȱatȱdifferentȱjappȱ
valuesȱ wereȱ furtherȱ electrolyzed.ȱ Inȱ allȱ cases,ȱ clofibricȱ acidȱ decayȱ wasȱ followedȱ byȱ
reversedȬphaseȱ chromatography.ȱ Theȱ possibleȱ influenceȱ ofȱ clofibricȱ acidȱ
concentrationȱ onȱ itsȱ decayȱ kineticsȱ wasȱ clarifiedȱ fromȱ electrolysesȱ ofȱ differentȱ
solutionsȱofȱpHȱ12.0ȱatȱ100ȱmAȱcmȬ2ȱandȱatȱ35ȱºC.ȱ
212
PART B –Results and Discussion8. Clofibric Acid
Simultaneously,ȱ aromaticȱ intermediatesȱ wereȱ unequivocallyȱ identifiedȱ andȱ
quantifiedȱ byȱ reversedȬphaseȱ chromatographyȱ ofȱ aliquotsȱ withdrawnȱ fromȱ
electrolyzedȱ179ȱmgȱLȬ1ȱclofibricȱacidȱsolutionsȱofȱpHȱ3.0ȱandȱpHȱ12.0ȱatȱ100ȱmAȱcmȬ2ȱ
andȱatȱ35ȱºCȱusingȱPtȱandȱBDD.ȱClofibricȱacidȱsolutionsȱunderȱtheȱconditionsȱreferredȱ
wereȱ electrolyzedȱ byȱ AOȱ withȱ Ptȱ andȱ BDDȱ toȱ obtainȱ theȱ ionȬexclusionȱ
chromatogramsȱreflectingȱtheȱcarboxylicȱacidsȱaccumulated.ȱDueȱtoȱtheȱformationȱofȱ
2Ȭhydroxyisobutyricȱ acid,ȱ aȱ particularȱ carboxylicȱ acidȱ notȱ deeplyȱ studiedȱ byȱ otherȱ
authors,ȱ duringȱ theȱ degradationȱ ofȱ theȱ pharmaceutical,ȱ anȱ electrolysisȱ ofȱ thisȱ acidȱ
withȱ BDDȱ wasȱ carriedȱ outȱ atȱ pHȱ 3.0,ȱ atȱ 100ȱ mAȱ cmȬ2ȱ andȱ atȱ 35ȱ ºCȱ toȱ clarifyȱ itsȱ
oxidationȱpathway.ȱ
ȱ
Toȱ helpȱ productȱ identification,ȱ severalȱ 179ȱ mgȱ LȬ1ȱ clofibricȱ acidȱ solutionsȱ ofȱ pHȱ 3.0ȱ
andȱ12.0ȱwereȱelectrolyzedȱwithȱaȱPtȱanodeȱatȱ100ȱmAȱcmȬ2ȱandȱatȱ35ȱºCȱforȱ60ȱandȱ40ȱ
min,ȱ respectively,ȱ andȱ afterȱ someȱ preparativesȱ (seeȱ sectionȱ 6.3)ȱ theȱ remainingȱ
intermediatesȱwereȱanalyzedȱbyȱGCȬMS.ȱ
ȱ
Finally,ȱ consideringȱ allȱ theȱ intermediatesȱ thatȱ wereȱ identified,ȱ aȱ plausibleȱ reactionȱ
sequenceȱ forȱ theȱ anodicȱ oxidationȱ ofȱ clofibricȱ acidȱ inȱ aqueousȱ mediumȱ couldȱ beȱ
proposed.ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
213
PART B –Results and Discussion8. Clofibric Acid
Theȱthoroughȱresultsȱofȱthisȱsectionȱareȱincludedȱinȱtheȱfollowingȱpaperȱ(Paperȱ4):ȱ
ȱ
4.ȱSirés,ȱI.,ȱCabot,ȱP.L.,ȱCentellas,ȱF.,ȱGarrido,ȱJ.A.,ȱRodríguez,ȱR.M.,ȱArias,ȱC.,ȱBrillas,ȱ
E.,ȱ Electrochemicalȱ degradationȱ ofȱ clofibricȱ acidȱ inȱ waterȱ byȱ anodicȱ oxidation.ȱ
Comparativeȱ studyȱ withȱ platinumȱ andȱ boronȬdopedȱ diamondȱ electrodes.ȱ
Electrochim.ȱActaȱ52ȱ(2006)ȱ75Ȭ85.ȱ
ȱ
Theȱfollowingȱpresentationsȱinȱaȱcongressȱareȱrelatedȱtoȱthisȱwork:ȱ
ȱ
D.ȱ Sirés,ȱ I.,ȱ Cabot,ȱ P.L.,ȱ Centellas,ȱ J.A.,ȱ Garrido,ȱ J.A.,ȱ Rodríguez,ȱ R.M.,ȱ Arias,ȱ C.,ȱ
Brillas,ȱE.,ȱDestructionȱofȱclofibricȱacidȱinȱaqueousȱmediumȱusingȱbothȱplatinumȱ
andȱboronȬdopedȱdiamondȱelectrodes,ȱVol.ȱ1,ȱpageȱ59,ȱ7thȱElectrochemistryȱDaysȱ
(7.ȱ Elektrokimiyaȱ Günleri),ȱ Hacettepeȱ Üniversitesi,ȱ Ankara,ȱ Turkey,ȱ 28Ȭ30ȱ Juneȱ
2006.ȱ(Posterȱpresentation)ȱ
ȱ
E.ȱSirés,ȱI.,ȱCabot,ȱP.L.,ȱCentellas,ȱF.,ȱGarrido,ȱJ.A.,ȱRodríguez,ȱR.M.,ȱArias,ȱC.,ȱBrillas,ȱ
E.,ȱ Unȱesquemaȱdegradativoȱ paraȱlaȱmineralizaciónȱcompletaȱdelȱ ácidoȱclofíbricoȱ
medianteȱ oxidaciónȱ anódica,ȱ Vol.ȱ 1,ȱ pageȱ 55,ȱ XXVIIIȱ Reuniónȱ delȱ Grupoȱ
Especializadoȱ deȱ Electroquímicaȱ deȱ laȱ RSEQȱ (IXȱ Ibericȱ Meetingȱ ofȱ
Electrochemistry),ȱAȱCoruña,ȱSpain,ȱ10Ȭ13ȱJulyȱ2006.ȱ(Oralȱpresentation)ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
214
ARTICLEȱ4ȱ/ȱPAPERȱ4
ȱ
ȱ
ȱ
ȱ
Electrochemicalȱdegradationȱofȱclofibricȱacidȱinȱwaterȱbyȱȱ
ȱ anodicȱoxidation:ȱComparativeȱstudyȱwithȱplatinumȱandȱȱ
ȱ boronȬdopedȱdiamondȱelectrodesȱ
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱȱ
ȱȱ
ȱȱ
Electrochimica Acta 52 (2006) 75–85
ȱȱ
ȱȱ Electrochemical degradation of clofibric acid in water by anodic oxidation
ȱȱ Comparative study with platinum and boron-doped diamond electrodes
Ignasi Sirés, Pere Lluı́s Cabot 1 , Francesc Centellas, José Antonio Garrido,
Rosa Marı́a Rodrı́guez, Conchita Arias, Enric Brillas ∗,1
ȱȱ
ȱȱ
Laboratori de Ciència i Tecnologia Electroquı́mica de Materials, Departament de Quı́mica Fı́sica, Facultat de Quı́mica,
Universitat de Barcelona, Martı́ i Franquès 1-11, 08028 Barcelona, Spain
ȱȱ
Received 10 February 2006; received in revised form 27 March 2006; accepted 28 March 2006
Available online 27 April 2006
ȱȱ
Abstract
ȱȱ
Aqueous solutions containing the metabolite clofibric acid (2-(4-chlorophenoxy)-2-methylpropionic acid) up to close to saturation in the pH
range 2.0–12.0 have been degraded by anodic oxidation with Pt and boron-doped diamond (BDD) as anodes. The use of BDD leads to total
mineralization in all media due to the efficient production of oxidant hydroxyl radical (• OH). This procedure is then viable for the treatment of
ȱȱ wastewaters containing this compound. The effect of pH, apparent current density, temperature and metabolite concentration on the degradation rate,
consumed specific charge and mineralization current efficiency has been investigated. Comparative treatment with Pt yields poor decontamination
ȱȱ with complete release of stable chloride ion. When BDD is used, this ion is oxidized to Cl2 . Clofibric •acid is more rapidly destroyed on Pt than
on BDD, indicating that it is more strongly adsorbed on the Pt surface enhancing its reaction with OH. Its decay kinetics always follows a
pseudo-first-order reaction and the rate constant for each anode increases with increasing apparent current density, being practically independent of
ȱȱ pH and metabolite concentration. Aromatic products such as 4-chlorophenol, 4-chlorocatechol, 4-chlororesorcinol, hydroquinone, p-benzoquinone
and 1,2,4-benzenetriol are detected by gas chromatography–mass spectrometry (GC–MS) and reversed-phase chromatography. Tartronic, maleic,
ȱȱ fumaric, formic, 2-hydroxyisobutyric, pyruvic and oxalic acids are identified as generated carboxylic acids by ion-exclusion chromatography. These
acids remain stable in solution using Pt, but they are completely converted into CO2 with BDD. A reaction pathway for clofibric acid degradation
involving all these intermediates is proposed.
ȱȱ © 2006 Elsevier Ltd. All rights reserved.
Keywords: Clofibric acid; Anodic oxidation; Platinum anode; Boron-doped diamond anode; Oxidation products
ȱȱ
ȱȱ 1.
Introduction
The presence of drugs and their metabolites as emerging
ȱȱ pollutants
in the aquatic environment has been recently documented [1–13]. A fairly large number of these compounds such
ȱȱ as anti-inflammatories, analgesics, betablockers, lipid regulators, antibiotics, antiepileptics and estrogens has been detected
ȱȱ in sewage treatment plant effluents, surface and ground waters
and even in drinking water at concentration usually ranging from
to micrograms per liter. This pollution can be due
ȱȱ nanograms
to different sources involving emission from production sites,
direct disposal of overplus drugs in households, excretion after
ȱȱ
ȱȱ
ȱȱ
∗
1
Corresponding author. Tel.: +34 93 4021223; fax: +34 93 4021231.
E-mail address: [email protected] (E. Brillas).
ISE member.
drug administration to humans and animals, treatments throughout the water in fish and other animal farms and inadequate
treatment of manufacturing waste [10]. Widespread contamination is produced when drugs are recalcitrant towards degradation
and can only be partially removed in sewage treatment plants.
Powerful oxidation methods are then needed to be applied to
ensure the complete degradation of drugs and their metabolites
in waters to avoid their potential adverse health effects on both,
humans and animals.
Anodic oxidation is one of the most promising electrochemical technologies for the treatment of wastewaters with low
contents of organic pollutants. In this method contaminants are
destroyed by reaction with adsorbed hydroxyl radical (• OH)
generated at the surface of a high O2 -overvoltage anode from
oxidation of water in acid and neutral media [14–18]:
H2 O → • OHads + H+ + e−
(1)
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2006.03.075
ȱȱ
ȱ
215
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
76
I. Sirés et al. / Electrochimica Acta 52 (2006) 75–85
or hydroxide ion at pH ≥ 10:
ȱ OH− →
• OH
ȱ
ads + e
−
(2)
Hydroxyl radical is a non-selective, very powerful oxidizing agent able to react with organics giving dehydrogenated or
ȱ hydroxylated by-products until their total mineralization, i.e.,
their overall transformation into CO2 , water and inorganic ions.
The recent use of a boron-doped diamond (BDD) thin-film
ȱ electrode in anodic oxidation has shown that it possesses technologically important characteristics such as an inert surface with
ȱ low adsorption properties, remarkable corrosion stability and
an extremely wide potential window in aqueous medium [17].
electrode has much greater O2 -overvoltage than a Pt anode,
ȱ This
yielding quicker oxidation of most organics. This has been confirmed from recent studies showing the total mineralization of
ȱ several aromatics and short carboxylic acids in waters from electrolysis with a BDD anode [15–36]. The use of anodic oxidation
ȱ with BDD then seems an adequate technique for the treatment
of wastewaters containing toxic and biorefractory organic pollutants.
ȱ In previous work we have reported that anodic oxidation with BDD can destroy toxic chlorophenoxy acids
ȱ such as 4-chlorophenoxyacetic [23] and 4-chloro-2methylphenoxyacetic, 2-(4-chloro-2-methylphenoxy)propionic
2-(4-chlorophenoxy)-2-methylpropionic (clofibric acid)
ȱ and
[36] in 1 M HClO4 . The degradation of 4-chlorophenoxyacetic,
2,4-dichlorophenoxyacetic and 2,4,6-trichlorophenoxyacetic
ȱ acids in acid aqueous solutions of pH 2.0–6.0 using this technique has also been investigated [28]. Among these compounds,
ȱ clofibric acid has relevant environmental interest due to its
significant accumulation and large persistence in waters. It is
the active metabolite of clofibrate, etofibrate and etofyllineclofiȱ brate, which are drugs widely used as blood lipid regulators
with therapeutic doses of about 1–2 g day−1 per person, since
ȱ they decrease the plasmatic concentration of cholesterol
and triglycerides [3,37]. Clofibric acid concentrations up
to 10 ␮g l−1 have been detected in sewage treatment plant
ȱ influents and effluents and in rivers, lakes, North Sea, ground
and drinking waters [1–3,9]. Several papers have described that
ȱ this compound is poorly mineralized in aqueous medium by
different oxidation methods such as electrogenerated Fenton’s
ȱ reagent [38], ozone [8,39], H2 O2 /UV [39], sunlight and UV
photolysis [40] and TiO2 /UV [41], as well as after application
of different biological and physico-chemical methods in sewage
ȱ treatment plants [37,42].
This paper reports a detailed study on the electrochemical
ȱ degradation of aqueous solutions of clofibric acid, as a model of
chlorophenoxy compounds, in the pH range 2.0–12.0 to know
ȱ the characteristics of its anodic oxidation with BDD for the possible application of this method to the treatment of wastewaters
containing such kind of compounds. The effect of pH, apparȱ ent current density, temperature and clofibric acid concentration
up to close to saturation on the degradation rate, consumed speȱ cific charge and mineralization current efficiency was examined.
The metabolite decay and the evolution of its by-products were
followed by chromatographic techniques. Comparative experi-
ȱ
ȱ
216
ments with a Pt anode were also made, since the metabolite is
mineralized in smaller extent than with the BDD one, allowing a better interpretation of its kinetic behavior and a clearer
detection of by-products produced during its degradation.
2. Experimental
2.1. Chemicals
Clofibric acid, 4-chlorophenol, 4-chlororesorcinol, hydroquinone, p-benzoquinone and 1,2,4-benzenetriol were reagent
grade, with purity >97%, supplied by Sigma–Aldrich, Merck,
Panreac and Avocado. 4-Chlorocatechol was synthesized by
chlorination of pyrocatechol with SO2 Cl2 at room temperature following a standard method reported in the literature
[43]. 2-Hydroxyisobutyric, maleic, fumaric, pyruvic, tartronic,
formic and oxalic acids were either reagent or analytical grade
from Sigma–Aldrich, Panreac and Avocado. Anhydrous sodium
sulfate used as background electrolyte was analytical grade
from Fluka. All solutions were prepared with high-purity water
obtained from a Millipore Milli-Q system with resistivity
>18 M cm at 25 ◦ C. The initial solution pH was adjusted with
sulfuric acid or sodium hydroxide, both of analytical grade,
purchased from Merck or Fluka, respectively. Organic solvents
and the other chemicals utilized were either HPLC or analytical
grade from Merck and Fluka.
2.2. Instruments
Electrolyses were carried out with an Amel 2053 potentiostatgalvanostat. The solution pH was measured with a Crison
2000 pH-meter. Aromatic intermediates were identified by gas
chromatography–mass spectrometry (GC–MS) with a HewlettPackard system composed of a HP 5890 Series II gas chromatograph fitted with a HP-5 0.25 ␮m, 30 m × 0.25 mm, column and coupled with a HP 5989A mass spectrometer operating in EI mode at 70 eV. The temperature ramp was 35 ◦ C
for 2 min, 10 ◦ C min−1 up to 320 ◦ C and hold time 5 min,
and the temperatures of the inlet, transfer line and detector
were 250, 250 and 290 ◦ C, respectively. The degradation of
clofibric acid solutions was monitored by the removal of their
total organic carbon (TOC), determined on a Shimadzu VCSN
TOC analyzer. The metabolite decay and the evolution of its
aromatic intermediates were followed by reversed-phase chromatography using a Waters 600 HPLC liquid chromatograph
fitted with a Spherisorb ODS2 5 ␮m, 15 cm × 4.6 mm, column
at room temperature, coupled with a Waters 996 photodiode
array detector and controlled through a Millennium-32® program. For each compound, this detector was selected at the
maximum wavelength of its UV-absorption band. Generated
carboxylic acids were followed by ion-exclusion chromatography with the above chromatograph fitted with a Bio-Rad
Aminex HPX 87H, 30 cm × 7.8 mm, column at 35 ◦ C and with
the photodiode detector selected at λ = 210 nm. Cl− concentration in treated solutions was determined by ion chromatography using a Shimadzu 10Avp HPLC chromatograph fitted
with a Shim-Pack IC-A1S, 10 cm × 4.6 mm, anion column at
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
I. Sirés et al. / Electrochimica Acta 52 (2006) 75–85
40 ◦ C and coupled with a Shimadzu CDD 10Avp conductivity
detector.
2.3. Electrolytic system
All electrolyses were conducted in a one-compartment and
thermostated cylindrical cell of 100 ml capacity. The anode was
either a Pt sheet of 99.99% purity from SEMP or a BDD thin-film
deposited on conductive single crystal p-type Si (1 0 0) wafers
from CSEM, both of 3 cm2 of apparent area. The cathode was
always a 3 cm2 stainless steel (AISI 304) sheet. The interelectrode gap was about 1 cm.
Solutions of 100 ml containing up to 0.56 g l−1 of clofibric
acid (close to saturation) and 0.05 M Na2 SO4 in the pH range
2.0–12.0 were comparatively degraded using a Pt or a BDD
anode at a constant apparent current density (japp ) of 33, 100 and
150 mA cm−2 and at 35.0 ◦ C. The effect of temperature between
25.0 and 45.0 ◦ C was also studied. All solutions were vigorously
stirred with a magnetic bar during treatment. The pH value of
solutions starting from initial pH between 3.0 and 10.0 decreased
with electrolysis time and then, it was continuously regulated
within a range of ±0.03 units by adding small volumes of 0.1 M
NaOH.
2.4. Analytical procedures
To identify the aromatic products, several 179 mg l−1 clofibric acid solutions of pH 3.0 and 12.0 were electrolyzed with
a Pt anode at 100 mA cm−2 and at 35.0 ◦ C for 60 and 40 min,
respectively, and their organic components were extracted with
45 ml of CH2 Cl2 in three times. Each collected organic solution
was then dried with anhydrous Na2 SO4 , filtered and its volume
reduced to about 5 ml to concentrate the remaining products for
further analysis by GC–MS.
Before TOC and chromatographic analyses, all samples withdrawn from electrolyzed solutions were filtered with 0.45 ␮m
Whatman PTFE filters. Reproducible TOC values were always
obtained by injecting 100 ␮l aliquots into the TOC analyzer,
using the standard non-purgeable organic carbon method. From
these data, the mineralization current efficiency (MCE) for
treated solutions at a given time t was calculated from the following equation:
MCE =
(TOC)exp
× 100
(TOC)theor
77
and 2.4 mM tris(hydroxymethyl)aminomethane solution of pH
4.0 as mobile phase at 1.5 ml min−1 . The concentration of H2 O2
accumulated in electrolyzed solutions was obtained from the
light absorption of the titanic-hydrogen peroxide colored complex at λ = 408 nm [44], measured with a Unicam UV4 Prisma
double-beam spectrometer thermostated at 25.0 ◦ C. The concentration of total oxidants generated in the same solutions was
determined by iodometric titration [21].
3. Results and discussion
3.1. TOC removal using a BDD or Pt anode
A series of electrolyses was carried out with 179 mg l−1
clofibric acid solutions (corresponding to 100 mg l−1 of TOC) in
the pH range 2.0–12.0 at 100 mA cm−2 and at 35.0 ◦ C for 7 h to
test their comparative degradation by anodic oxidation with Pt
and BDD. In the experiments performed with initial pH between
3.0 and 10.0, the solution pH underwent a progressive decay to
lower values with time due to the formation of acidic products
and hence, it was continuously regulated to its initial pH with
0.1 M NaOH. All solutions treated with the BDD anode always
remained colorless, but their degradation with the Pt one caused
a change in color, being pale pink at 5 min, orange at about 1 h,
dark-brown at ca. 2 h and yellow at approximately 4 h, further
being slowly decolorized up to become colorless again after 6 h
of treatment. The coloration of solutions degraded with Pt can be
related to the generation of soluble polyaromatics in large extent,
which can be totally destroyed by oxidant • OH produced by
reaction (1) or (2). These colored products are not accumulated
in the medium using BDD, probably due to the faster destruction
of aromatic intermediates.
The variation of solution TOC with applied specific charge
(Q, in A h l−1 ) for the above trials is depicted in Fig. 1. A continuous and quick TOC abatement can be observed for all solutions
treated with BDD, being reduced by more than 91% at 7 h
(3)
ȱ where (TOC)exp is the experimental solution TOC removal at
time t and (TOC)theor is the theoretically calculated TOC decay
ȱ considering that the applied electrical charge (=current × time)
ȱ
ȱ
ȱ
ȱ
is only consumed in the mineralization process of clofibric acid.
Reversed-phase chromatography of initial and treated
solutions was made by injecting 20 ␮l samples into the
HPLC chromatograph under circulation of a 50:47:3 (v/v/v)
methanol/0.01 M phosphate buffer (pH 2.5)/pentanol mixture at
1.0 ml min−1 as mobile phase. For ion-exclusion chromatography, 20 ␮l samples were also injected into the HPLC chromatograph and the mobile phase was 4 mM H2 SO4 at 0.6 ml min−1 .
Cl− measurements were carried out with a 2.5 mM phtalic acid
Fig. 1. TOC removal versus specific charge for the anodic oxidation of 100 ml
of 179 mg l−1 clofibric acid solutions in 0.05 M Na2 SO4 at 100 mA cm−2 and at
35.0 ◦ C using a one-compartment cell with a (,) Pt, (,䊉,,, ,) BDD
anode and a stainless steel cathode, all them of 3 cm2 area. Initial pH: () 2.0,
(,䊉) 3.0, () 6.0, () 8.0, ( ) 10.0, and (,) 12.0.
ȱ
217
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
78
I. Sirés et al. / Electrochimica Acta 52 (2006) 75–85
(Q = 21 A h l−1 ) in all cases. These results indicate that clofibric
acid and its by-products are destroyed at similar rate in the pH
range 2.0–12.0 by anodic oxidation with BDD. This behavior
can be explained by the generation of similar concentration of
• OH from reaction (1) or (2). Note that overall mineralization
(>97% TOC decay) is achieved starting from pH 3.0 and from
pH 12.0, where the degradation rate is slightly faster. In contrast, Fig. 1 also shows that both solutions are slowly degraded
with Pt up to reach near 30% of mineralization at 4 h, whereas
at longer time they are not practically decontaminated. Consequently, under comparable conditions the use of a Pt anode yields
quite poor mineralization of clofibric acid solutions. That means
that Pt is unable to destroy some products that are hardly oxidizable with • OH, as short carboxylic acids, as will be discussed
below.
The possible effect of apparent current density and temperature on the degradation rate of the above clofibric acid solutions
taking place in anodic oxidation with BDD was studied to clarify
the oxidation ability of this anode. As an example, Fig. 2 presents
the TOC–Q plots obtained for pH 3.0 and 12.0 at 33, 100 and
150 mA cm−2 and at 35.0 ◦ C. The TOC of both solutions decays
at similar rate for each japp , confirming that the degradation of
the metabolite and its by-products is practically pH-independent.
However, increasing japp causes faster TOC removal with time
and more consumption of specific charge for total mineralization, which varies from 10 A h l−1 at 33 mA cm−2 to 27 A h l−1
at 150 mA cm−2 . This corresponds to a drop in time required
for overall decontamination from 10 to 5.5 h since TOC is more
rapidly removed with time. Fig. 2 also shows that an increase
in temperature from 25.0 to 45.0 ◦ C working at pH 12.0 and at
100 mA cm−2 also accelerates the degradation process, decreasing the time for total mineralization from 10 to 6 h, with a
concomitant fall in Q from 30 to 18 A h l−1 .
The change in TOC abatement with varying japp and temperature can be explained taking into account that in Na2 SO4
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Fig. 2. Effect of experimental parameters on the variation of TOC with specific
charge for the treatment of 100 ml of 179 mg l−1 clofibric acid solutions by
anodic oxidation with BDD electrode. Initial pH: (,,) 3.0 and (䊉,,,,)
12.0. Apparent current density: (,䊉) 33 mA cm−2 , (,,,) 100 mA cm−2 ,
and (,) 150 mA cm−2 . Temperature: () 25.0 ◦ C, (,,,䊉,,) 35.0 ◦ C,
and () 45.0 ◦ C.
ȱ
218
Fig. 3. Time-course of the concentration of: (,,) total oxidants, (䊉,,)
hydrogen peroxide generated during the anodic oxidation of 100 ml of
179 mg l−1 clofibric acid solutions of pH 3.0 using a BDD electrode. Apparent
current density: (,,,) 33 mA cm−2 and (,䊉) 100 mA cm−2 . Temperature: (,,䊉,) 25.0 ◦ C and (,) 45.0 ◦ C.
medium other weaker oxidants such as ozone, peroxodisulfate
ion and H2 O2 can be competitively formed with • OH at the BDD
anode from the following reactions [17]:
3H2 O → O3 + 6H+ + 6e−
−
2HSO4 → S2 O8
2−
(4)
+
+ 2H + 2e
+
2H2 O → H2 O2 + 2H + 2e
−
−
(5)
(6)
• OH
where H2 O2 generation in reaction (6) can proceed via
recombination. To know the influence of such electrogenerated
species on the mineralization of clofibric acid, the concentrations of H2 O2 and total oxidants (ozone, S2 O8 2− and H2 O2 )
were determined during the treatment of 100 ml of 179 mg l−1
clofibric acid solutions of pH 3.0 at several apparent current
densities and temperatures. The results obtained are shown in
Fig. 3, where the difference between the concentration of total
oxidants and H2 O2 mainly corresponds to that of S2 O8 2− . A
gradual accumulation of all species can be always observed,
attaining a quasi-steady concentration usually from 4 h of electrolysis, just when they are generated and destroyed at the same
rate. An increase in japp from 33 to 100 mA cm−2 at 25.0 ◦ C leads
to the formation of more weak oxidants due to the acceleration
of reactions (4)–(6), indicating the existence of a mass-transport
controlled process. However, part of these species is consumed
when the temperature increases to 45.0 ◦ C at 33 mA cm−2 , indicating that increasing temperature enhances their decomposition
and/or their reaction with greater amount of organics accelerating their oxidation. Note that no electrogenerated oxidants were
detected when comparable solutions were degraded with Pt.
From the above findings, the fact that TOC is more rapidly
removed with time when japp raises for the 179 mg l−1 clofibric
acid solutions of pH 3.0 and 12.0 can be ascribed to the higher
production of • OH from reaction (1) or (2) and weaker oxidants
(ozone, S2 O8 2− and H2 O2 ) from reactions (4)–(6), thus favoring
the degradation of organics. The increase in Q for total mineralization under these conditions is indicative of a lower relative
generation of the main oxidant • OH due to its quicker oxida-
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
I. Sirés et al. / Electrochimica Acta 52 (2006) 75–85
ȱ
79
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Fig. 4. TOC abatement with specific charge for the degradation of 100 ml of solu−1
−1
−1
with: (䊉) 557 mg l (close to saturation), () 447 mg l , () 358 mg l ,
ȱ tions
() 268 mg l−1 , () 179 mg l−1 , ( ) 89 mg l−1 , and ( ) 45 mg l−1 of clofibric
acid at pH 12.0, at 100 mA cm−2 and at 35.0 ◦ C by anodic oxidation with BDD.
Fig. 5. Concentration of chloride ion accumulated during the treatment of 100 ml
of 179 mg l−1 clofibric acid solutions of pH: (䊉,) 3.0 and (,) 12.0, at
100 mA cm−2 and at 35.0 ◦ C by anodic oxidation with a: (䊉,) Pt and (,)
BDD electrode.
ȱ
tion to O2 . On the other hand, the faster TOC decay found when
ȱ temperature increases, as exemplified in Fig. 2 for the metabolite
solution of pH 12.0 at
100 mA cm−2 ,
can be explained by the
increase in mass transfer of pollutants towards the
ȱ concomitant
BDD anode due to the decrease of medium viscosity. This causes
the acceleration of their reaction with • OH and weaker oxidants
ȱ such as ozone, S2 O8 2− and H2 O2 , enhancing metabolite min-
eralization. It can then be inferred that the oxidation process
ȱ is limited, at least partially, by the mass transfer of organics to
BDD surface.
These results indicate that anodic oxidation with BDD is
ȱ potent enough to decontaminate wastewaters of clofibric acid
in a large variety of experimental conditions. The great oxiȱ dizing power of this method was confirmed by electrolyzing
solutions with metabolite contents up to close to saturation
100 mA cm−2 and at 35.0 ◦ C. Fig. 4 illustrates the fast and
ȱ at
total TOC abatement found for 45, 89, 179, 268, 358, 447 and
557 mg l−1 of this compound at pH 12.0. Similar TOC–Q plots
ȱ were obtained for the same solutions at pH 3.0. As can be seen,
the specific charge consumed for overall mineralization graduȱ ally increases from 15 to 30 A h l−1 as initial concentration rises.
This trend can be, in principle, related to the existence of more
organic matter in solution. However, increasing metabolite conȱ centration causes a quicker TOC abatement. For example, at
2 h of electrolysis (Q = 6 A h l−1 ) 15, 28, 56, 81, 95, 101 and
ȱ 121 mg l−1 of TOC are removed starting from 45, 89, 179, 268,
358, 447 and 557 mg l−1 of clofibric acid, respectively. This
ȱ gradual enhancement in oxidizing power of•the BDD anode
can be accounted for by the reaction of more OH with greater
amount of pollutants, and hence, this radical is wasted in smaller
ȱ extent by other nonoxidizing reactions such as its decomposition
to O2 and its recombination to H2 O2 .
ȱ
3.2. Mineralization current efficiency
ȱ
Generation of inorganic ions from the initial chlorine of the
ȱ metabolite is expected during its mineralization process. Ionic
chromatographic analysis of all electrolyzed solutions revealed
the formation of chloride ion. However, the presence of other
chlorine–oxygen ions such as chlorite, chlorate and perchlorate
in treated solutions was not detected by this technique. As can
be seen in Fig. 5, Cl− is rapidly accumulated for 180–240 min
of anodic oxidation with Pt of 179 mg l−1 clofibric acid solutions at both pH 3.0 and 12.0, operating at 100 mA cm−2 and at
35.0 ◦ C. At longer time than 360 min, this ion reaches a quasisteady concentration of about 29 mg l−1 in both media, a value
practically equal to 29.5 mg l−1 corresponding to the initial chlorine contained in solution. All chloro-organics are then mainly
destroyed for 5–6 h of electrolysis, with the release of chloride
ion. A very different behavior can be observed in Fig. 5 for the
evolution of Cl− in the same solutions comparatively degraded
with BDD. In both media this ion attains a maximum concentration of ca. 7 mg l−1 at 120 min and further, it is slowly destroyed
until disappearing at 420 min. The instability of Cl− under these
conditions can be explained by its oxidation to Cl2 gas on BDD,
as reported for the electrolysis of NaCl aqueous solutions with
this anode [45].
These findings allow establishing that the mineralization of
clofibric acid by anodic oxidation with BDD involves its conversion into CO2 and chloride ion as primary inorganic ion. This
overall reaction can be written as follows:
C10 H11 ClO3 + 17H2 O → 10CO2 + Cl− + 45H+ + 44e−
(7)
where the destruction of each mole of the metabolite needs the
consumption of 44 F.
Taking into account reaction (7), the mineralization current efficiency of solutions treated with BDD was determined
from Eq. (3). This parameter was found to be practically pHindependent, although it strongly increased with increasing initial metabolite concentration and temperature, as well as with
decreasing apparent current density. To illustrate these trends,
Fig. 6 presents the MCE–Q plots found for different clofibric
ȱ
219
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
80
I. Sirés et al. / Electrochimica Acta 52 (2006) 75–85
3.3. Kinetics of clofibric acid decay
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Fig. 6. Dependence of mineralization current efficiency calculated from Eq. (3)
on specific charge for the anodic oxidation with BDD of 100 ml of clofibric acid
solutions of pH 12.0 at 35.0 ◦ C. Initial metabolite concentration: (䊉) 557 mg l−1
(close to saturation), () 447 mg l−1 , () 358 mg l−1 , () 268 mg l−1 , (,,)
179 mg l−1 , ( ) 89 mg l−1 , and ( ) 45 mg l−1 . Apparent current density: ()
33 mA cm−2 , (䊉,,,,, , ) 100 mA cm−2 , and () 150 mA cm−2 .
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
acid solutions of pH 12.0 at several japp values and at 35.0 ◦ C. A
slight increase in efficiency can be observed at the early stages
of most treatments, as expected if a higher amount of pollutants
is converted more quickly into CO2 . This enhancement in MCE
can be accounted for by the faster degradation of some products
that are able to react simultaneously with • OH and with weaker
oxidants as ozone, S2 O8 2− and H2 O2 produced from reactions
(4)–(6). When electrolyses are prolonged, the efficiency always
undergoes a slow, but continuous, decay, as expected if products
that are more difficultly oxidizable with • OH than the initial
compound, such as short carboxylic acids, are progressively
formed. The slower production of such hardly oxidizable species
with raising initial metabolite content can then justify the concomitant increase in efficiency at constant japp . This behavior
can be easily deduced from results of Fig. 6, where, for example,
after 2 h (Q = 6 A h l−1 ) of electrolysis at 100 mA cm−2 , increasing MCE values of 2.5, 4.5, 8.9, 13, 16, 17 and 20% are obtained
for increasing clofibric acid concentrations of 45, 89, 179, 268,
358, 447 and 557 mg l−1 , respectively. This tendency also confirms the gradual reaction of higher amount of • OH with more
pollutants, indicating that this radical is lost in smaller extent in
other nonoxidizing reactions. Fig. 6 also shows a dramatic fall
in efficiency as higher japp is applied, confirming the existence
of a mass-transport controlled process. For example, the MCE
values at 1 h of treatment of 179 mg l−1 of the metabolite are 18,
9.7 and 6.7% at 33, 100 and 150 mA cm−2 , respectively. This
trend corroborates the progressive faster production of O2 and
other weak oxidants (ozone, S2 O8 2− and H2 O2 ), to the detriment of the main oxidant • OH with increasing apparent current
density.
All these results allow concluding that concentrated clofibric acid solutions can be efficiently and totally mineralized by
anodic oxidation with BDD, even at low current, increasing its
efficiency as temperature rises. This method is then viable for
treating wastewaters containing this metabolite.
ȱ
220
To clarify whether clofibric acid can be oxidized with H2 O2
and S2 O8 2− , chemical tests were carried out by preparing 100 ml
solutions with 179 mg l−1 of this metabolite, 20 mM of each
one of these oxidants and 0.05 M Na2 SO4 at pH 3.0 and 12.0.
The concentration of clofibric acid in these experiments was
determined by reversed-phase chromatography, where it exhibits
a well-defined absorption peak with a retention time (tr ) of
7.6 min. However, no change in the metabolite content of solutions was found for 3 h at 35.0 ◦ C, indicating that it does not react
with such weak oxidants. That means that in anodic oxidation
this compound can only react with • OH.
The kinetics for the reaction between clofibric acid and • OH
formed from reaction (1) or (2) at the Pt and BDD anodes was
comparatively studied with 179 mg l−1 metabolite solutions of
pH 3.0 and 12.0 at different japp values and at 35.0 ◦ C. The
change of its concentration with time is presented in Fig. 7(a) and
(b). As can be seen in Fig. 7(a), it disappears from the medium
at pH 12.0 after 420, 360 and 240 min of anodic oxidation with
Pt at 33, 100 and 150 mA cm−2 , respectively. However, longer
Fig. 7. Clofibric acid concentration decay with electrolysis time for the treatment
of 100 ml of 179 mg l−1 metabolite solutions at 35.0 ◦ C by anodic oxidation with
a: (a) Pt and (b) BDD electrode. Initial pH: () 3.0 and (䊉,,) 12.0. Apparent
current density: (䊉) 33 mA cm−2 , (,) 100 mA cm−2 , and () 150 mA cm−2 .
The inset panel presents the kinetic analysis assuming a pseudo-first-order reaction for clofibric acid.
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
I. Sirés et al. / Electrochimica Acta 52 (2006) 75–85
electrolyses time of about 540, 420 and 360 min is needed to
be removed under comparable conditions using BDD, as it is
shown in Fig. 7(b). This is surprising if one takes into account
that the BDD anode produces much more reactive • OH able to
mineralize the metabolite and all intermediates, while the Pt one
is unable to destroy some by-products giving poor mineralization (see Fig. 1). The greater oxidation ability of clofibric acid
on Pt can then be ascribed to its higher adsorption on its surface
favoring its reaction with more amount of • OH. Note that the
time required for total destruction of this compound on BDD at
each applied japp is practically equal to that needed for its overall
mineralization (see Figs. 2 and 7(b)), indicating that it persists
up to the end of its degradation process. Results of Fig. 7(a) and
(b) also show a similar destruction rate for the metabolite at pH
3.0 and 12.0 and at 100 mA cm−2 on each electrode. This brings
to consider that the same electroactive species of the metabolite
is oxidized in the pH range tested, probably its anionic (unprotonated) form since its pKa = 3.18 [40].
The above clofibric acid concentration decays were well fitted
to a pseudo-first-order kinetic equation. The excellent linear correlations obtained are depicted in the panel inset of Fig. 7(a) and
(b). This behavior suggests the production of a constant concentration of • OH from reaction (1) or (2) at each anode during
electrolysis, which is much greater than that of the metabolite adsorbed on its surface. From this analysis, an increasing
pseudo-first-order rate constant (k) of 2.4 × 10−4 s−1 (square
regression coefficient, R2 = 0.994), 4.0 × 10−4 s−1 (R2 = 0.993)
and 5.4 × 10−4 s−1 (R2 = 0.998) for Pt and of 7.2 × 10−5 s−1
(R2 = 0.995), 1.3 × 10−4 s−1 (R2 = 0.991) and 1.8 × 10−4 s−1
(R2 = 0.994) for BDD is found at increasing japp of 33, 100
and 150 mA cm−2 , respectively. Note that k does not vary
proportionally with japp , confirming that a smaller proportion
of this oxidant reacts with pollutants when apparent current
density rises, since it is more quickly oxidized to O2 in both
anodes.
The possible influence of clofibric acid concentration on
its decay kinetics was clarified from electrolyses of different solutions of pH 12.0 at 100 mA cm−2 and at 35.0 ◦ C. The
concentration–time plots obtained with Pt and BDD are shown in
Fig. 8(a) and (b), respectively. Comparison of these data allows
concluding that the metabolite is always more quickly removed
with Pt, confirming the existence of a greater adsorption of this
compound on this anode that accelerates its reaction with • OH.
Note that for all solutions treated with BDD, a similar time
is required for clofibric acid disappearance and for its overall
mineralization (see Figs. 4 and 8(b)). This evidences the existence of simultaneous degradation of the initial pollutant and
its intermediates under such conditions. Assuming a pseudofirst-order reaction kinetics for clofibric acid, good straight lines
with R2 > 0.991 were found in all cases, as can be seen in the
inset of Fig. 8(a) and (b). This analysis shows a rather similar
constant rate for all initial concentrations tested, corresponding to an average k-value of (4.0 ± 0.6) × 10−4 s−1 for Pt and
(1.3 ± 0.1) × 10−4 s−1 for BDD. This behavior corroborates the
existence of a much greater amount of • OH than metabolite
adsorbed on each electrode surface, even operating with a concentration close to saturation.
81
Fig. 8. Time-course of clofibric acid concentration for the anodic oxidation of
100 ml of: (䊉) 557 mg l−1 (close to saturation), () 447 mg l−1 , () 268 mg l−1 ,
() 179 mg l−1 , ( ) 89 mg l−1 , and ( ) 45 mg l−1 metabolite solutions of pH
12.0 at 100 mA cm−2 and at 35.0 ◦ C using a: (a) Pt and (b) BDD electrode. The
corresponding kinetic analysis assuming a pseudo-first-order reaction for the
metabolite is shown in the inset panel.
3.4. Identification and time-course of intermediates
The MS spectra obtained in the GC–MS analyses of organics
extracted after short-time electrolyses of 179 mg l−1 metabolite solutions of pH 3.0 and 12.0 with Pt at 100 mA cm−2 and
at 35.0 ◦ C, displayed three peaks associated with stable aromatic products such as 4-chlorophenol (m/z = 128 (100, M+ ),
130 (33 (M + 2)+ )) at tr = 11.5 min, hydroquinone (m/z = 108
(100, M+ )) at tr = 13.2 min and p-benzoquinone (m/z = 110 (45,
M+ )) at tr = 6.5 min. Reversed-phase chromatograms of the
same degraded solutions exhibited peaks related to these compounds at retention time of 5.0, 1.7 and 2.0 min, respectively,
along with other additional peaks ascribed to 4-chlorocatechol
at tr = 3.1 min, 4-chlororesorcinol at tr = 2.3 min and 1,2,4benzenetriol at tr = 1.8 min. All these compounds were unequivocally identified by comparing their retention times and UV–vis
spectra, measured on the photodiode array, with those of pure
products.
The evolution of aromatic intermediates in the above solutions treated with Pt is shown in Fig. 9(a). A much greater
accumulation of these products can be observed at pH 3.0 than
at pH 12.0, where only 1,2,4-benzenetriol and 4-chlorophenol
ȱ
221
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
82
I. Sirés et al. / Electrochimica Acta 52 (2006) 75–85
from the oxidative breaking of the benzenic moiety of aromatic
intermediates [16–18,25,28], while 2-hydroxyisobutyric acid is
expected to be released when 4-chlorophenol is formed from
clofibric acid. The pathway of the last product was clarified from
the anodic oxidation with BDD of a solution with 50 mg l−1
of 2-hydroxyisobutyric acid of pH 3.0 at 100 mA cm−2 and at
35.0 ◦ C. Under these conditions, this compound gives pyruvic
acid, which is further oxidized to oxalic acid. This acid can also
be generated from the independent degradation of longer chain
acids as tartronic, maleic and fumaric [16,18,28]. Oxalic [30]
and formic acids are finally converted into CO2 .
The production and destruction rate of generated carboxylic
acids depends on both, pH and anode tested. Fig. 10(a) shows
that large amounts of these products are slowly accumulated
using Pt, without apparent degradation, as expected from the
quite low mineralization achieved in these conditions (see
Fig. 1). At pH 3.0 tartronic, 2-hydroxyisobutyric and oxalic
acids are the main products, whereas the two latter acids are also
largely formed in the solution of pH 12.0. A different behavior
can be seen in Fig. 10(b) for BDD, where all carboxylic acids
are destroyed at 420 min when total mineralization of starting
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
9. Evolution of the concentration of aromatic intermediates detected during
ȱ Fig.
the degradation of 100 ml of 179 mg l−1 clofibric acid solutions at 100 mA cm−2
ȱ
and at 35.0 ◦ C by anodic oxidation with a: (a) Pt and (b) BDD electrode.
Initial pH: (䊉,, , ,,) 3.0, and (,) 12.0. Compound: (䊉,) 4chlorophenol, () 4-chlorocatechol, ( ) 4-chlororesorcinol, ( ) hydroquinone,
() p-benzoquinone, and (,) 1,2,4-benzenetriol.
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
are detected up to 20 and 120 min, respectively. At pH 3.0,
1,2,4-benzenetriol, 4-chlorocatechol and p-benzoquinone persist to 420 min after reaching maximum contents of 4.5, 13.0
and 13.2 mg l−1 at about 60 min, whereas 4-chlorophenol, 4chlororesorcinol and hydroquinone are more quickly destroyed,
disappearing in 240 min. In contrast, reversed-phase chromatograms of the same solutions degraded under the same conditions with BDD only allowed the detection of 4-chlorophenol
and p-benzoquinone. As can be seen in Fig. 9(b), both compounds are present in the medium of pH 3.0 up to 420 min,
i.e., a time similar to that of clofibric acid disappearance (see
Fig. 7(b)), but only the first product is accumulated up to 240 min
at pH 12.0. These findings indicate that the degradation of all
aromatics, except the initial pollutant, is more rapid on BDD,
although they are completely degraded with both anodes.
Ion-exclusion chromatography of the above electrolyzed
solutions revealed the generation of carboxylic acids such
as oxalic at tr = 6.6 min, tartronic at tr = 7.7 min, maleic at
tr = 8.1 min, pyruvic at tr = 9.2 min, 2-hydroxyisobutyric at
tr = 12.6 min, formic at tr = 14.0 min and fumaric at tr = 16.1 min.
Tartronic, maleic, fumaric and formic acids can be produced
ȱ
222
Fig. 10. Time-course of the concentration of carboxylic acids generated during
the anodic oxidation of 179 mg l−1 clofibric acid solutions under the same conditions as in Fig. 9. Anode: (a) Pt and (b) BDD. Initial pH: (䊉,,,,, , )
3.0 and (,,,♦,,,) 12.0. Compound: (䊉,) 2-hydroxyisobutyric acid,
(,) maleic acid, (,) fumaric acid, (,♦) pyruvic acid, (,) tartronic acid,
( ,) formic acid, and ( ,) oxalic acid.
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
I. Sirés et al. / Electrochimica Acta 52 (2006) 75–85
83
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Fig. 11. Proposed reaction sequence for clofibric acid degradation in aqueous medium by anodic oxidation with a Pt or BDD electrode.
ȱ
solutions is reached (see Fig. 2). Oxalic acid is the most largely
ȱ accumulated product in both media, although great amounts of
ȱ
ȱ
ȱ
ȱ
pyruvic acid at pH 3.0 and formic and maleic acids at pH 12.0
are also produced.
The balance of carbon content from detected pollutants
shown in Figs. 7, 9 and 10 was analyzed and compared with TOC
data given in Fig. 1 for 179 mg l−1 metabolite solutions of pH
3.0 and 12.0 at 100 mA cm−2 . This study allows concluding that
the carbon content of solutions degraded with the BDD anode
practically corresponds to the remaining initial compound. For
example, after 60 min of both treatments, the resulting solu-
tions contain about 73 mg l−1 of TOC (see Fig. 1), which can
be ascribed to the main presence of ca. 70 mg l−1 of carbon
coming from clofibric acid, along with minor contribution of
2 and 0.1 mg l−1 from 4-chlorophenol and 0.6 and 1.5 mg l−1
from oxalic acid at pH 3.0 and 12.0, respectively. A different
behavior is found for the Pt anode, where detected aromatics
and carboxylic acids lead to high carbon contents while clofibric acid persists. Thus, after 60 min of degradation, the solution
TOC is close to 86 mg l−1 for both pH 3.0 and 12.0 (see Fig. 1),
but only ca. 23 mg l−1 of carbon come from clofibric acid and
some products significantly contribute to the carbon content,
ȱ
223
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
84
I. Sirés et al. / Electrochimica Acta 52 (2006) 75–85
for example, 4-chlorocatechol (7 mg l−1 ) and p-benzoquinone
(11 mg l−1 ) at pH 3.0 and 2-hydroxyisobutyric acid (15 mg l−1 )
ȱ at
pH 12.0. When clofibric acid disappears, 2-hydroxyisobutyric,
tartronic and oxalic acids always remain in both media, but at
ȱ 420 min all carboxylic acids only yield 48 and 25 mg l−1 of total
carbon at pH 3.0 and 12.0, respectively, values much lower than
ȱ about 63 mg l−1 of TOC found for the resulting solutions (see
Fig. 1), thus indicating the formation of large amounts of other
products, mainly at pH 12.0.
ȱ undetected
The above results evidence that different carboxylic acids,
mainly oxalic acid (see Fig. 10(b)), and some aromatic interȱ mediates (see Fig. 9(b)) are oxidized on BDD while clofibric
acid is destroyed (see Fig. 7(b)). These species are then present
ȱ in the medium during the degradation process of this metabolite. In contrast, generated carboxylic acids cannot be removed
with Pt (see Fig. 10(a)) and then, they remain in the final treated
ȱ solutions.
ȱ 3.5.
Proposed mineralization pathway
ȱ
A plausible reaction sequence for the anodic oxidation of
clofibric acid in aqueous medium is proposed in Fig. 11. This
includes all products detected using Pt and BDD as
ȱ pathway
anodes, but carboxylic acids can only be destroyed on the last
electrode, as pointed out above. Electrogenerated • OH is stated
ȱ as the main oxidant, although the reaction of some organics with
other weaker oxidants (ozone, S2 O8 2− and H2 O2 ) on BDD is
ȱ also possible.
The process starts with the breaking of the C(1)–O bond of
the metabolite to yield 4-chlorophenol and 2-hydroxyisobutyric
ȱ acid. 4-Chlorophenol can then undergo a parallel attack of • OH
on its C(2)-, C(3)- and C(4)-positions giving 4-chlorocatechol,
ȱ 4-chlororesorcinol and hydroquinone with loss of chloride ion,
respectively. The subsequent hydroxylation with dechlorinaof 4-chlorocatechol and 4-chlororesorcinol leads to 1,2,4ȱ tion
benzenetriol. The last product can also be formed from • OH
attack on hydroquinone, which is oxidized in parallel to pȱ benzoquinone. Further degradation of 1,2,4-benzenetriol and
p-benzoquinone yields a mixture of tartronic, maleic, fumaric
ȱ and formic acids. The three former acids are independently
transformed into oxalic acid, which is also generated from the
oxidation of the initially electrogenerated 2-hydroxyisobutyric
ȱ acid via pyruvic acid. The ultimate carboxylic acids, oxalic and
formic, are finally converted into CO2 .
ȱ
ȱ
4. Conclusions
It has been demonstrated that aqueous solutions of clofib-
ȱ ric acid up to close to saturation can be completely mineralized
in the pH range 2.0–12.0 by anodic oxidation with BDD in a
large variety of experimental conditions due to the efficient proȱ duction of oxidant • OH by reaction (1) or (2). The removal
of solution TOC is practically pH-independent and becomes
ȱ faster with increasing j, although with consumption of more
specific charge, because of the higher production of this radical
and other weaker oxidants such as ozone, S2 O8 2− and H2 O2 .
ȱ
ȱ
224
The increase in temperature favors the degradation rate, indicating that the process is limited, at least partially, by the mass
transfer of organics to the BDD surface. Increasing metabolite concentration also enhances the oxidizing power of this
anode, since more • OH is able to react with greater amount of
pollutants, decreasing the rate of other nonoxidizing reactions
of this oxidant. The mineralization current efficiency increases
with decreasing japp and with increasing initial metabolite concentration and temperature. Comparative treatment of the same
solutions with a Pt anode leads to poor mineralization, although
all chloro-organics are destroyed with release of chloride ion,
which remains stable in solution. In contrast, this ion is completely oxidized to Cl2 on BDD. The clofibric acid decay always
follows a pseudo-first-order kinetics, being quicker for Pt than
for BDD. This evidences a stronger adsorption of the metabolite
on the Pt surface that enhances its reaction with electrogenerated • OH. The pseudo-rate constant calculated for each anode
increases with increasing japp and it is practically independent of
pH and initial metabolite concentration. Aromatic products such
as 4-chlorophenol, 4-chlorocatechol, 4-chlororesorcinol, hydroquinone, p-benzoquinone and 1,2,4-benzenetriol are detected by
GC–MS and reversed-phase chromatography. All these intermediates are destroyed with both anodes, although they are more
rapidly degraded on BDD. Generated carboxylic acids such as
tartronic, maleic, fumaric, formic, 2-hydroxyisobutyric, pyruvic and oxalic are identified by ion-exclusion chromatography.
While these acids remain stable in solution using a Pt anode,
they are completely mineralized with the BDD one. Most of
these species and some aromatic intermediates are simultaneously oxidized with clofibric acid on BDD up to the end of its
degradation process.
Acknowledgments
Financial support from MEC (Ministerio de Educación
y Ciencia, Spain) under project CTQ2004-01954/BQU is
acknowledged. The authors thank DURSI (Departament
d’Universitats, Recerca i Societat de la Informació, Generalitat de Catalunya) for the grant given to I. Sirés to do this work.
References
[1] Th. Heberer, H.J. Stan, Int. J. Environ. Anal. Chem. 77 (1997) 113.
[2] T.A. Ternes, Water Res. 32 (1998) 3245.
[3] H.R. Buser, M.D. Muller, N. Theobald, Environ. Sci. Technol. 32 (1998)
188.
[4] C. Zwiener, F.H. Frimmel, Water Res. 34 (2000) 1881.
[5] C.G. Daughton, T.L. Jones-Lepp (Eds.), Pharmaceuticals and Personal Care
Products in the Environment, Scientific and Regulatory Issues, Symposium
Series 791, American Chemical Society, Washington, 2001.
[6] K. Kümmerer (Ed.), Pharmaceuticals in the Environment. Sources, Fate
and Risks, Springer, Berlin, 2001.
[7] D.W. Kolpin, E.T. Furlong, M.T. Meyer, E.M. Thurman, S.D. Zaugg, L.B.
Barber, Environ. Sci. Technol. 36 (2002) 1202.
[8] T.A. Ternes, M. Meisenheimer, D. McDowell, F. Sacher, H.J. Brauch, B.
Haist-Gulde, G. Preuss, U. Wilme, N. Zulei-Seibert, Environ. Sci. Technol.
36 (2002) 3855.
[9] C. Tixier, H.P. Singer, S. Oellers, S.R. Müller, Environ. Sci. Technol. 37
(2003) 1061.
[10] I.A. Balcioglu, M. Ötker, Chemosphere 50 (2003) 85.
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱȱ
ȱȱ
I. Sirés et al. / Electrochimica Acta 52 (2006) 75–85
[11] P.E. Stackelberg, E.T. Furlong, M.T. Meyer, S.D. Zaugg, A.K. Henderson,
D.B. Reissman, Sci. Total Environ. 329 (2004) 99.
[12] J.P. Bound, N. Vaulvaulis, Chemosphere 56 (2004) 1143.
[13] M. Carballa, F. Omil, J.M. Lema, M. Llompart, C. Garcia-Jares, I.
Rodriguez, M. Gomez, T. Ternes, Water Res. 38 (2004) 2918.
[14] B. Marselli, J. Garcı́a-Gomez, P.A. Michaud, M.A. Rodrigo, Ch. Comninellis, J. Electrochem. Soc. 150 (2003) D79.
[15] A. Kraft, M. Stadelmann, M. Blaschke, J. Hazard. Mater. B 103 (2003)
247.
[16] E. Brillas, I. Sirés, C. Arias, P.L. Cabot, F. Centellas, R.M. Rodrı́guez, J.A.
Garrido, Chemosphere 58 (2005) 399.
[17] M. Panizza, G. Cerisola, Electrochim. Acta 51 (2005) 191.
[18] C. Flox, J.A. Garrido, R.M. Rodrı́guez, F. Centellas, P.L. Cabot, C. Arias,
E. Brillas, Electrochim. Acta 50 (2005) 3685.
[19] M.A. Rodrigo, P.A. Michaud, I. Duo, M. Panizza, G. Cerisola, Ch.
Comninellis, J. Electrochem. Soc. 148 (2001) D60.
[20] J. Iniesta, P.A. Michaud, M. Panizza, G. Cerisola, A. Aldaz, Ch. Comninellis, Electrochim. Acta 46 (2001) 3573.
[21] M. Panizza, P.A. Michaud, G. Cerisola, Ch. Comninellis, J. Electroanal.
Chem. 507 (2001) 206.
[22] F. Montilla, P.A. Michaud, E. Morallón, J.L. Vázquez, Ch. Comninellis,
Electrochim. Acta 47 (2002) 3509.
[23] B. Boye, P.A. Michaud, B. Marselli, M.M. Dieng, E. Brillas, Ch. Comninellis, New Diamond Front. Carbon Technol. 12 (2002) 63.
[24] S. Hattori, M. Doi, E. Takahashi, T. Kurosu, M. Nara, S. Nakamatsu, Y. Nishiki, T. Furuta, M. Iida, J. Appl. Electrochem. 33 (2003)
85.
[25] P. Cañizares, J. Garcı́a-Gómez, C. Sáez, M.A. Rodrigo, Ind. Eng. Chem.
Res. 42 (2003) 956.
[26] G. Lissens, J. Peters, M. Verhaege, L. Pinoy, W. Verstraete, Electrochim.
Acta 48 (2003) 1655.
[27] M. Panizza, G. Cerisola, Electrochim. Acta 49 (2004) 3221.
ȱȱ
ȱȱ
ȱȱ
ȱȱ
ȱȱ
ȱȱ
ȱȱ
ȱȱ
ȱȱ
ȱȱ
ȱȱ
85
[28] E. Brillas, B. Boye, I. Sirés, J.A. Garrido, R.M. Rodrı́guez, C. Arias, P.L.
Cabot, Ch. Comninellis, Electrochim. Acta 49 (2004) 4487.
[29] P. Cañizares, C. Sáez, J. Lobato, M.A. Rodrigo, Ind. Eng. Chem. Res. 43
(2004) 1944.
[30] C.A. Martinez-Huitle, S. Ferro, A. De Battisti, Electrochim. Acta 49 (2004)
4027.
[31] P. Cañizares, C. Sáez, J. Lobato, M.A. Rodrigo, Electrochim. Acta 49
(2004) 4641.
[32] M. Mitadera, N. Spataru, A. Fujishima, J. Appl. Electrochem. 34 (2004)
249.
[33] P. Cañizares, J. Lobato, R. Paz, M.A. Rodrigo, C. Sáez, Water Res. 39
(2005) 2687.
[34] B. Nasr, G. Abdellatif, P. Cañizares, C. Sáez, J. Lobato, M.A. Rodrigo,
Environ. Sci. Technol. 39 (2005) 7234.
[35] X. Chen, G. Chen, Sep. Purif. Technol. 48 (2006) 45.
[36] B. Boye, E. Brillas, B. Marselli, P.A. Michaud, Ch. Comninellis, G. Farnia,
G. Sandonà, Electrochim. Acta 51 (2006) 2872.
[37] A. Tauxe-Wuersch, L.F. De Alencastro, D. Grandjean, J. Tarradellas, Water
Res. 39 (2005) 1761.
[38] M.A. Oturan, J.J. Aaron, N. Oturan, J. Pinson, Pestic. Sci. 55 (1999) 558.
[39] R. Andreozzi, V. Caprio, R. Marotta, A. Radovnikovic, J. Hazard. Mater.
B 103 (2003) 233.
[40] J.L. Packer, J.J. Werner, D.E. Latch, K. McNeill, W.A. Arnold, Aquat. Sci.
65 (2003) 342.
[41] T. Doll, F.H. Frimmel, Water Res. 38 (2004) 955.
[42] C. Zwiener, F.H. Frimmel, Sci. Total Environ. 309 (2003) 201.
[43] J. Frejka, B. Sefránek, J. Zika, Collect. Czech. Chem. Commun. 9 (1937)
238.
[44] F.J. Welcher (Ed.), Standard Methods of Chemical Analysis, vol. 2, 6th ed.,
RE Krieger Publ. Co., Huntington, New York, 1975, p. 1827 (Part B).
[45] S. Ferro, A. De Battisti, I. Duo, Ch. Comninellis, W. Haenni, A. Perret, J.
Electrochem. Soc. 147 (2000) 2614.
ȱȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
225
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
PART B –Results and Discussion8. Clofibric Acid
8.2.2.ȱȱResultatsȱiȱDiscussióȱ/ȱResultsȱandȱDiscussionȱ
ȱ
Theȱ variationȱ ofȱ TOCȱ withȱ appliedȱ specificȱ chargeȱ (Q)ȱ forȱ theȱ AOȱ treatmentȱ ofȱ 100ȱ
mLȱofȱ179ȱmgȱLȬ1ȱclofibricȱacidȱsolutionsȱinȱtheȱpHȱrangeȱ2.0Ȭ12.0ȱatȱ100ȱmAȱcmȬ2ȱandȱ
atȱ 35ȱ ºCȱ isȱ analogousȱ toȱ theȱ oneȱ describedȱ inȱ sectionȱ 7.3.2ȱ forȱ paracetamol:ȱ aȱ quickȱ
andȱ continuousȱ TOCȱ decayȱ canȱ beȱ observedȱ forȱ allȱ experimentsȱ usingȱ BDD,ȱ
achievingȱ aȱ reductionȱ >ȱ 97%ȱ atȱ 7ȱ hȱ (i.e.,ȱ 21ȱ Aȱ hȱ LȬ1)ȱ inȱ mostȱ media,ȱ whereasȱ aȱ slowȱ
degradationȱyieldingȱaȱmaximumȱmineralizationȱofȱ30%ȱatȱ4ȱhȱisȱobservedȱusingȱPtȱ
dueȱ toȱ theȱ formationȱ ofȱ hardlyȱ oxidizableȱ intermediates.ȱ Again,ȱ theȱ almostȱȱȱȱȱȱȱȱȱȱȱȱȱ
pHȬindependenceȱcanȱbeȱexplainedȱbyȱtheȱgenerationȱofȱsimilarȱ •OHadsȱconcentrationȱ
fromȱ Reactionsȱ 5.Ȭ44ȱ andȱ 5.Ȭ46.ȱ Itȱ isȱ worthȱ notingȱ thatȱ TOCȱ abatementȱ isȱ slightlyȱ
fasterȱ atȱ pHȱ 12.0,ȱ andȱ thatȱ isȱ theȱ reasonȱ whyȱ theȱ possibleȱ influenceȱ ofȱ otherȱ
experimentalȱparametersȱhasȱbeenȱstudiedȱatȱthisȱinitialȱpHȱvalue.ȱ
ȱ
Theȱ electrolysesȱ ofȱ severalȱ 100ȬmLȱ clofibricȱ acidȱ solutionsȱ ofȱ pHȱ 3.0ȱ andȱ 12.0ȱ
containingȱ100ȱmgȱLȬ1ȱinitialȱTOC,ȱatȱ33,ȱ100ȱandȱ150ȱmAȱcmȬ2ȱandȱatȱ35ȱºC,ȱshowȱaȱ
similarȱTOCȱabatementȱrateȱforȱbothȱpHȱvalues,ȱthusȱconfirmingȱtheȱaforementionedȱ
trendȱ thatȱ theȱ degradationȱ ofȱ thisȱ pharmaceuticalȱ andȱ itsȱ byȬproductsȱ isȱ practicallyȱ
pHȬindependentȱ usingȱ aȱ BDDȱ anode.ȱ However,ȱ asȱ describedȱ inȱ sectionȱ 7.3.2,ȱ
increasingȱ jappȱ causesȱ fasterȱ TOCȱ removalȱ withȱ timeȱ andȱ moreȱ consumptionȱ ofȱ
specificȱ chargeȱ forȱ totalȱ mineralization,ȱ varyingȱ fromȱ 10ȱ Aȱ hȱ LȬ1ȱ (i.e.,ȱ 10ȱ h)ȱ atȱ 33ȱȱȱȱȱȱ
mAȱ cmȬ2ȱ toȱ 27ȱ Aȱ hȱ LȬ1ȱ (i.e.,ȱ 6ȱ h)ȱ atȱ 150ȱ mAȱ cmȬ2.ȱ Inȱ addition,ȱ anȱ increaseȱ inȱ
temperatureȱfromȱ25ȱtoȱ45ȱºC,ȱworkingȱatȱpHȱ12.0ȱandȱatȱ100ȱmAȱcmȬ2,ȱenhancesȱtheȱ
degradationȱprocess,ȱthusȱdecreasingȱtheȱtimeȱrequiredȱforȱtotalȱmineralizationȱfromȱ
10ȱtoȱ6ȱhȱ(thatȱisȱtoȱsay,ȱtheȱconsumptionȱofȱspecificȱchargeȱfallsȱfromȱ30ȱtoȱ18ȱAȱhȱLȬ1).ȱ
Thisȱtrendȱagreesȱwithȱtheȱdataȱsummarizedȱforȱparacetamolȱinȱsectionȱ7.3.ȱ
ȱ
TheȱchangeȱinȱTOCȱabatementȱwhenȱvaryingȱjappȱandȱtemperatureȱcanȱbeȱexplainedȱ
inȱ termsȱ ofȱ theȱ multipleȱ oxidizingȱ agentsȱ electrogeneratedȱ inȱ theseȱ systems,ȱ takingȱ
227
PART B –Results and Discussion8. Clofibric Acid
intoȱaccountȱthatȱinȱNa2SO4ȱmediumȱseveralȱweakȱoxidantȱspeciesȱsuchȱasȱO3,ȱS2O82Ȭȱ
ionsȱ andȱ H2O2ȱ canȱ beȱ competitivelyȱ formedȱ (reactionsȱ 4,ȱ 5ȱ andȱ 6ȱ inȱ pageȱ 218).ȱ
Remindingȱtheȱexplanationȱintroducedȱinȱsectionȱ7.3.2,ȱitȱhasȱbeenȱdemonstratedȱthatȱ
theȱ amountȱ ofȱ O3,ȱ H2O2ȱ andȱ S2O82Ȭȱ ionsȱ isȱ higherȱ asȱ jappȱ increases,ȱ attainingȱ aȱ quasiȬ
steadyȱconcentrationȱfromȱ4ȱhȱofȱelectrolysis.ȱInȱthisȱstudy,ȱtheȱdifferenceȱbetweenȱtheȱ
concentrationȱ ofȱ totalȱ oxidizingȱ agentsȱ andȱ theȱ concentrationȱ ofȱ H2O2ȱ mainlyȱ
correspondsȱtoȱthatȱofȱS2O82Ȭȱions,ȱbecauseȱO3ȱisȱreallyȱunstableȱandȱ•OHȱareȱadsorbedȱ
onȱ theȱ BDDȱ surfaceȱ andȱ areȱ highlyȱ reactiveȱ too.ȱ Theȱ fasterȱ TOCȱ removalȱ whenȱ
increasingȱjappȱisȱthenȱdueȱtoȱtheȱgreaterȱamountȱofȱ •OHadsȱ(fromȱReactionsȱ5.Ȭ44ȱandȱ
5.Ȭ46),ȱ O3,ȱ H2O2ȱ andȱ S2O82Ȭȱ ionsȱ generatedȱ inȱ theȱ BDDȱ system.ȱ Whenȱ Ptȱ isȱ usedȱ asȱ
anodeȱnoȱweakȱoxidizingȱagentsȱareȱdetected,ȱsoȱtheȱlowȱamountȱofȱeffectiveȱ•OHadsȱisȱ
theȱuniqueȱsourceȱofȱoxidantȱspeciesȱtoȱremoveȱtheȱpollutants,ȱthusȱyieldingȱtheȱpoorȱ
mineralizationȱalreadyȱdiscussed.ȱ Inȱ addition,ȱwhenȱTȱ increasesȱ fromȱ 25ȱtoȱ45ȱºC,ȱ aȱ
partȱofȱtheȱaforementionedȱagentsȱ(•OHads,ȱO3,ȱH2O2ȱandȱS2O82Ȭȱions)ȱisȱconsumedȱdueȱ
toȱ theirȱ decompositionȱ and/orȱ theirȱ reactionȱ withȱ greaterȱ amountȱ ofȱ organics.ȱ Thisȱ
factȱ canȱ beȱ relatedȱ toȱ theȱ improvedȱ mineralizationȱ processȱ withȱ risingȱ temperatureȱ
fromȱ25ȱtoȱ45ȱºC.ȱItȱmustȱbeȱhighlightedȱthatȱtheȱoxidationȱprocessȱisȱlimited,ȱatȱleastȱ
partially,ȱbyȱtheȱmassȱtransferȱofȱorganicsȱtoȱtheȱBDDȱsurfaceȱ(seeȱsectionȱ7.3.2).ȱ
ȱ
Clofibricȱ acidȱ solutionsȱ ofȱ pHȱ 12.0ȱ upȱ toȱ closeȱ toȱ saturationȱ canȱ beȱ completelyȱ
mineralizedȱ withȱ aȱ BDDȱ anodeȱ atȱ 100ȱ mAȱ cmȬ2ȱ andȱ atȱ 35ȱ ºC.ȱ Aȱ quickȱ andȱ totalȱ
abatementȱisȱreachedȱworkingȱinȱtheȱconcentrationȱrangeȱ45Ȭ557ȱmgȱLȬ1ȱclofibricȱacidȱ
(i.e.,ȱ 25Ȭ313ȱ mgȱ LȬ1ȱ TOC).ȱ Theȱ Qȱ consumedȱ forȱ overallȱ mineralizationȱ graduallyȱ
increasesȱfromȱ15ȱtoȱ30ȱAȱhȱLȬ1ȱ(i.e.,ȱfromȱ5ȱtoȱ10ȱh)ȱasȱinitialȱTOCȱrisesȱfromȱ25ȱtoȱ313ȱ
mgȱLȬ1.ȱThisȱtrendȱcanȱbeȱrelatedȱtoȱtheȱexistenceȱofȱmoreȱorganicȱmatterȱinȱsolution.ȱ
ItȱisȱworthȱremarkingȱthatȱincreasingȱmetaboliteȱconcentrationȱcausesȱaȱquickerȱTOCȱ
abatement:ȱforȱexample,ȱafterȱ2ȱhȱofȱelectrolysisȱ(6ȱAȱhȱLȬ1)ȱ15,ȱ28,ȱ56,ȱ81,ȱ95,ȱ101ȱandȱ
121ȱ mgȱ LȬ1ȱ ofȱ TOCȱ canȱ beȱ removedȱ startingȱ fromȱ 25,ȱ 50,ȱ 100,ȱ 150,ȱ 200,ȱ 250ȱ andȱ 313ȱȱȱ
mgȱLȬ1ȱofȱTOC,ȱrespectively.ȱThisȱgradualȱenhancementȱinȱtheȱoxidizingȱpowerȱofȱtheȱ
228
PART B –Results and Discussion8. Clofibric Acid
BDDȱ anodeȱ canȱ beȱ basicallyȱ accountedȱ forȱ byȱ theȱ reactionȱ ofȱ moreȱ •OHadsȱ withȱ aȱ
greaterȱ amountȱ ofȱ pollutants,ȱ decreasingȱ theȱ amountȱ ofȱ thisȱ radicalȱ wastedȱ inȱ
nonoxidizingȱ reactions,ȱ suchȱ asȱ itsȱdecompositionȱ toȱ O2ȱand/orȱ itsȱrecombinationȱ toȱ
H2O2.ȱSimilarȱTOCȬQȱplotsȱareȱobtainedȱforȱtheȱsameȱsolutionsȱatȱpHȱ3.0.ȱ
ȱ
Theȱ generationȱ ofȱ inorganicȱ ionsȱ studiedȱ byȱ ionȱ chromatographyȱ revealsȱ theȱ
formationȱofȱchlorideȱion,ȱwhileȱotherȱchlorineȬoxygenȱionsȱsuchȱasȱchlorite,ȱchlorateȱ
andȱperchlorateȱinȱtreatedȱsolutionsȱwereȱnotȱdetected.ȱChlorideȱionȱevolutionȱforȱtheȱ
electrolysesȱofȱ100ȱmLȱofȱ179ȱmgȱLȬ1ȱclofibricȱacidȱsolutionsȱofȱpHȱ3.0ȱandȱ12.0,ȱatȱ100ȱ
mAȱcmȬ2ȱ andȱatȱ 35ȱºC,ȱallowsȱconcludingȱ thatȱ inȱAOȱ withȱaȱ Ptȱanodeȱ Clȱisȱquicklyȱ
accumulatedȱ atȱ bothȱ pHȱ valuesȱ forȱ 180Ȭ240ȱ min,ȱ furtherȱ reachingȱ aȱ quasiȬsteadyȱ
concentrationȱ ofȱ aboutȱ 29ȱ mgȱ LȬ1,ȱ whichȱ isȱ aȱ valueȱ practicallyȱ equalȱ toȱ 29.5ȱ mgȱ LȬ1ȱ
correspondingȱ toȱ theȱ chlorineȱ containedȱ inȱ theȱ initialȱ solution.ȱ Thisȱ meansȱ thatȱ allȱ
chloroȬorganicsȱ areȱ definitelyȱ destroyedȱ afterȱ 5Ȭ6ȱ hȱ ofȱ electrolysisȱ withȱ Pt,ȱ withȱ theȱ
releaseȱ ofȱ chlorideȱ ion.ȱ Inȱ contrast,ȱ inȱ AOȱ withȱ aȱ BDDȱ anodeȱ aȱ maximumȱ
concentrationȱofȱ7ȱmgȱLȬ1ȱatȱ120ȱminȱisȱattainedȱinȱbothȱmedia,ȱfurtherȱbeingȱslowlyȱ
destroyedȱtoȱdisappearȱatȱ420ȱmin.ȱThisȱlackȱofȱstabilityȱofȱClȱcanȱbeȱexplainedȱbyȱitsȱ
oxidationȱ toȱ Cl2ȱ gasȱ onȱ BDD,ȱ asȱ reportedȱ forȱ theȱ electrolysisȱ ofȱ NaClȱ aqueousȱ
solutionsȱwithȱthisȱanodeȱ[377].ȱ
ȱ
Allȱthoseȱpreviousȱfindingsȱallowȱestablishingȱthatȱtheȱoverallȱmineralizationȱreactionȱ
byȱAOȱwithȱaȱBDDȱanodeȱinvolvesȱtheȱconsumptionȱofȱ44ȱFȱforȱeachȱmolȱofȱclofibricȱ
acid,ȱ withȱ chlorideȱ ionȱ asȱ primaryȱ inorganicȱ ionȱ (Reactionȱ 6.Ȭ3).ȱ Fromȱ theseȱ
considerations,ȱ Equationȱ 6.Ȭ1ȱ canȱ beȱ appliedȱ toȱ determineȱ theȱ MCEȱ values.ȱ Theȱ
efficiencyȱ isȱ almostȱ pHȬindependentȱ (thisȱ factȱ agreesȱ withȱ theȱ pHȬindepenenceȱ
observedȱinȱtheȱTOCȬQȱplotsȱpreviouslyȱcommented),ȱbutȱitȱstronglyȱincreasesȱwhenȱ
initialȱ clofibricȱ acidȱ andȱ temperatureȱ rise,ȱ asȱ wellȱ asȱ whenȱ jappȱ decreases.ȱ Inȱ fact,ȱ
MCEȬQȱtrendsȱareȱsimilarȱtoȱtheȱonesȱofȱparacetamol,ȱalthoughȱtheȱmaximumȱMCEȱ
valueȱ forȱ clofibricȱ acidȱ isȱ 22%ȱ whereasȱ forȱ paracetamolȱ itȱ isȱ 35%.ȱ Thisȱ differenceȱ
229
PART B –Results and Discussion8. Clofibric Acid
clearlyȱ explainsȱ theȱ interestȱ ofȱ studyingȱ severalȱ moleculesȱ toȱ correctlyȱ assessȱ theȱ
oxidizingȱ abilityȱ ofȱ BDDȱ anodes,ȱ becauseȱ theȱ natureȱ ofȱ eachȱ compoundȱ andȱ itsȱ
intermediatesȱmodifiesȱtheȱactivityȱofȱtheȱelectrodeȱtowardsȱtheȱtotalȱmineralization,ȱ
andȱthisȱisȱ undoubtedlyȱanȱinterestingȱfactorȱregardingȱaȱpossibleȱfutureȱscalingȬupȱ
ofȱtheȱprocess.ȱResultsȱshowȱaȱslightȱincreaseȱinȱefficiencyȱatȱtheȱearlyȱstagesȱofȱmostȱ
treatments,ȱwhichȱmeansȱthatȱaȱhigherȱamountȱofȱpollutantsȱisȱmoreȱeasilyȱconvertedȱ
intoȱ CO2.ȱ Thisȱ enhancementȱ inȱ MCEȱ isȱ dueȱ toȱ theȱ fasterȱ degradationȱ ofȱ someȱȱȱȱȱȱȱȱȱȱȱ
byȬproductsȱ thatȱ areȱ ableȱ toȱ reactȱ simultaneouslyȱ withȱ •OHads,ȱ O3,ȱ H2O2ȱ andȱ S2O82Ȭȱ
ions.ȱAȱcontinuousȱdropȱinȱtheȱefficiencyȱwithȱtimeȱ(i.e.,ȱwithȱQ)ȱafterȱgoingȱthroughȱ
theȱ maximumȱ valueȱ isȱ observed,ȱ indicatingȱ aȱ concomitantȱ decreaseȱ inȱ oxidizingȱ
abilityȱofȱtheȱelectrolyticȱsystem.ȱThisȱtrendȱcanȱbeȱascribedȱtoȱtheȱlargerȱproportionȱ
ofȱ •OHadsȱoxidizedȱtoȱO2ȱatȱtheȱanodeȱand/orȱitsȱrecombinationȱtoȱH2O2,ȱasȱwellȱasȱtoȱ
theȱ continuousȱ formationȱ ofȱ moreȱ difficultlyȱ oxidizableȱ intermediates.ȱ Similarly,ȱ atȱ
constantȱjappȱhigherȱefficienciesȱareȱobtainedȱasȱinitialȱconcentrationȱofȱpollutantȱrises,ȱ
becauseȱ ofȱ theȱ slowerȱ productionȱ ofȱ suchȱ hardlyȱ oxidizableȱ intermediates.ȱ Forȱ
example,ȱafterȱ2ȱhȱ(6ȱAȱhȱLȬ1)ȱofȱelectrolysisȱatȱ100ȱmAȱcmȬ2,ȱincreasingȱMCEȱvaluesȱofȱ
2.5%,ȱ4.5%,ȱ8.9%,ȱ13.0%,ȱ16.0%,ȱ17.0%ȱandȱ20.0%ȱareȱobtainedȱforȱ45,ȱ89,ȱ179,ȱ268,ȱ358,ȱ
447ȱ andȱ 557ȱ mgȱ LȬ1ȱ clofibricȱ acid,ȱ respectively.ȱ Thisȱ tendencyȱ alsoȱ confirmsȱ theȱ
gradualȱreactionȱofȱhigherȱamountȱofȱ•OHadsȱwithȱmoreȱpollutants,ȱindicatingȱthatȱthisȱ
hydroxylȱradicalȱisȱwastedȱtoȱaȱsmallerȱextent.ȱFinally,ȱdecreasingȱefficienciesȱcanȱbeȱ
observedȱ asȱ jappȱ increases.ȱ Thatȱ meansȱ thatȱ theȱ mineralizationȱ requiresȱ aȱ greaterȱ
electricalȱ consumptionȱ (i.e.,ȱ greaterȱ Q)ȱ becauseȱ aȱ largerȱ proportionȱ ofȱ hydroxylȱ
radicalȱisȱwastedȱandȱfurthermore,ȱotherȱweakȱagentsȱ(O3,ȱH2O2ȱandȱS2O82Ȭȱions)ȱareȱ
formedȱtoȱtheȱdetrimentȱofȱtheȱmainȱoxidantȱagentȱ•OHadsȱthusȱcorroboratingȱtheȱideaȱ
thatȱAOȱwithȱBDDȱisȱaȱmassȬtrasportȱcontrolledȱprocess,ȱasȱexplainedȱinȱsectionȱ7.3.2.ȱ
Forȱ example,ȱ theȱ MCEȱ valuesȱ atȱ 1ȱ hȱ areȱ 18%,ȱ 9.7%ȱ andȱ 6.7%ȱ atȱ 33,ȱ 100ȱ andȱ 150ȱȱȱȱȱȱȱȱ
mAȱcmȬ2,ȱrespectively.ȱ
ȱ
ȱ
230
PART B –Results and Discussion8. Clofibric Acid
Regardingȱtheȱkineticsȱofȱclofibricȱacidȱdecay,ȱtheȱroleȱofȱtheȱweakȱoxidizingȱspeciesȱ
hasȱ beenȱ assessed.ȱ ReversedȬphaseȱ chromatogramsȱ forȱ solutionsȱ ofȱ pHȱ 3.0ȱ andȱ pHȱ
12.0ȱ containingȱ clofibricȱ acid,ȱ Na2SO4,ȱ H2O2ȱ andȱ S2O82Ȭȱ showȱ noȱ changeȱ inȱ theȱ
pharmaceuticalȱ contentȱ atȱ 35ȱ ºCȱ afterȱ 3ȱ h,ȱ indicatingȱ thatȱ thisȱ compoundȱ doesȱ notȱ
reactȱwithȱH2O2ȱandȱS2O82Ȭȱ(theyȱcouldȱplayȱaȱrelevantȱroleȱinȱtheȱoxidationȱofȱsomeȱ
intermediates),ȱsoȱtheȱkineticsȱcanȱbeȱestablishedȱonȱtheȱbasisȱofȱtheȱreactionȱbetweenȱ
clofibricȱ acidȱ andȱ •OHads.ȱ Thisȱ reactionȱ hasȱ beenȱ studiedȱ byȱ AOȱ withȱ Ptȱ andȱ BDDȱ
anodes,ȱ byȱ electrolyzingȱ 179ȱ mgȱ LȬ1ȱ clofibricȱ acidȱ solutionsȱ ofȱ pHȱ 3.0ȱ andȱ 12.0ȱ atȱ
differentȱ jappȱ valuesȱ andȱ atȱ 35ȱ ºC.ȱ Atȱ pHȱ 12.0ȱ clofibricȱ acidȱ disappearsȱ fromȱ theȱ
mediumȱ afterȱ 420,ȱ 360ȱ andȱ 240ȱ minȱ inȱ AOȱ withȱ Ptȱ atȱ 33,ȱ 100ȱ andȱ 150ȱ mAȱ cmȬ2,ȱ
respectively.ȱHowever,ȱ540,ȱ420ȱ andȱ360ȱminȱareȱneededȱtoȱremoveȱclofibricȱacidȱinȱ
AOȱ withȱ BDDȱ underȱ comparableȱ experimentalȱ conditions.ȱ Thisȱ meansȱ thatȱ despiteȱ
theȱ factȱ thatȱ clofibricȱ acidȱ isȱ moreȱ slowlyȱ mineralizedȱ withȱ Ptȱ thanȱ withȱ BDD,ȱ itȱ isȱ
moreȱ quicklyȱ destroyedȱ andȱ transformedȱ intoȱ itsȱ intermediates.ȱ Thisȱ isȱ surprisingȱ
takingȱintoȱaccountȱthatȱBDDȱanodeȱproducesȱmuchȱmoreȱreactiveȱ •OHads.ȱThen,ȱtheȱ
greaterȱ oxidationȱ abilityȱ ofȱ clofibricȱ acidȱ onȱ Ptȱ canȱ beȱ ascribedȱ toȱ itsȱ higherȱ
adsorptionȱonȱitsȱsurface,ȱfavoringȱitsȱreactionȱwithȱaȱgreaterȱamountȱofȱ•OHads.ȱTheseȱ
resultsȱ areȱ oppositeȱ toȱ theȱ onesȱ obtainedȱ forȱ paracetamolȱ (seeȱ Figureȱ 7.Ȭ5ȱ inȱsectionȱ
7.3.2),ȱwhichȱisȱmoreȱquicklyȱdestroyedȱusingȱaȱBDDȱanode.ȱTheseȱdifferencesȱreflectȱ
theȱimportanceȱofȱtheȱparticularitiesȱofȱeachȱcompoundȱandȱitsȱinteractionsȱwithȱtheȱ
electrodeȱ surface.ȱ Itȱ canȱ beȱ notedȱ thatȱ theȱ timeȱ requiredȱ forȱ totalȱ destructionȱ ofȱ
clofibricȱ acidȱ onȱ BDDȱ atȱ eachȱ jappȱ isȱ veryȱ similarȱ toȱ theȱ timeȱ neededȱ forȱ itsȱ overallȱ
mineralizationȱ (10,ȱ 7ȱ andȱ 6ȱ hȱ atȱ 33,ȱ 100ȱ andȱ 150ȱ mAȱ cmȬ2),ȱ confirmingȱ theȱ trendȱ
alreadyȱobservedȱforȱparacetamolȱthatȱtheȱinitialȱcontaminantȱpersistsȱinȱtheȱsolutionȱ
upȱtoȱtheȱendȱofȱtheȱdegradationȱprocessȱwhenȱaȱBDDȱanodeȱisȱused.ȱThisȱisȱdueȱtoȱaȱ
simultaneousȱdegradationȱofȱinitialȱcompoundȱandȱitsȱintermediates.ȱAOȱwithȱPtȱandȱ
BDDȱ alsoȱ showȱ aȱ similarȱ destructionȱ rateȱ atȱ bothȱ pHȱ values,ȱ andȱ thisȱ bringsȱ toȱ
considerȱthatȱtheȱsameȱelectroactiveȱclofibricȱacidȱspeciesȱisȱoxidizedȱinȱtheȱpHȱrangeȱ
tested,ȱ probablyȱ itsȱ unprotonatedȱ formȱ sinceȱ itȱ hasȱ aȱ pKaȱ =ȱ 3.18.ȱ Theȱ concentrationȱ
231
PART B –Results and Discussion8. Clofibric Acid
decaysȱ areȱ wellȱ fittedȱ toȱ aȱ pseudoȬfirstȱ orderȱ kineticȱ equation,ȱ exhibitingȱ excellentȱ
linearȱcorrelations.ȱThisȱsuggestsȱthatȱaȱconstantȱ •OHadsȱconcentration,ȱwhichȱisȱmuchȱ
greaterȱthanȱthatȱofȱtheȱpharmaceuticalȱadsorbedȱonȱtheirȱsurface,ȱisȱproducedȱatȱeachȱ
anodeȱ duringȱ theȱ electrolysis.ȱ Fromȱ thisȱ analysis,ȱ anȱ increasingȱ pseudoȬfirstȱ orderȱ
rateȱ constantȱ (k1)ȱ ofȱ 2.4ȱ xȱ 10Ȭ4,ȱ 4.0ȱ xȱ 10Ȭ4ȱ andȱ 5.4ȱ xȱ 10Ȭ4ȱ sȬ1ȱ forȱ Pt,ȱ andȱ ofȱ 7.2ȱ xȱ 10Ȭ5,ȱȱȱȱȱȱȱȱȱȱ
1.3ȱxȱ10Ȭ4ȱandȱ1.8ȱxȱ10Ȭ4ȱsȬ1ȱforȱBDDȱisȱfoundȱatȱ33,ȱ100ȱandȱ150ȱmAȱcmȬ2,ȱrespectively.ȱ
Theseȱ valuesȱ doȱ notȱ varyȱ proportionallyȱ withȱ japp,ȱ indicatingȱ thatȱ aȱ smallerȱ
proportionȱ ofȱ hydroxylȱ radicalȱ reactsȱ withȱ pollutantsȱ whenȱ jappȱ rises,ȱ sinceȱ itȱ isȱ
progressivelyȱ moreȱ quicklyȱ wasted.ȱ Andȱ finally,ȱ theȱ possibleȱ influenceȱ ofȱ initialȱ
clofibricȱ acidȱ concentrationȱ onȱ itsȱ decayȱ kineticsȱ wasȱ clarifiedȱ fromȱ electrolysesȱ ofȱ
clofibricȱacidȱsolutionsȱofȱpHȱ12.0ȱupȱtoȱcloseȱtoȱsaturation,ȱatȱ35ȱºCȱandȱ100ȱmAȱcmȬ2,ȱ
usingȱPtȱandȱBDD.ȱAgain,ȱtheȱpharmaceuticalȱisȱmoreȱquicklyȱremovedȱwithȱPtȱinȱallȱ
cases,ȱ confirmingȱ theȱ existenceȱ ofȱ aȱ greaterȱ adsorptionȱ onȱ Pt.ȱ Inȱ addition,ȱ theȱ timeȱ
requiredȱforȱclofibricȱacidȱdisappearanceȱinȱAOȱwithȱBDDȱisȱquiteȱcloseȱtoȱtheȱtimeȱ
neededȱ forȱ totalȱ mineralization.ȱ Goodȱ linearȱ correlationsȱ areȱ obtainedȱ forȱ allȱ initialȱ
concentrationsȱ tested,ȱ assumingȱ aȱ pseudoȬfirstȱ orderȱ reactionȱ kinetics,ȱ andȱ givingȱ
averageȱ k1Ȭvaluesȱ ofȱ (4.0±0.6)ȱ xȱ 10Ȭ4ȱ sȬ1ȱ forȱ Ptȱ andȱ (1.3±0.1)ȱ xȱ 10Ȭ4ȱ sȬ1ȱ forȱ BDD.ȱ Thisȱ
kineticȱ behaviorȱ corroboratesȱ theȱ existenceȱ ofȱ aȱ muchȱ greaterȱ amountȱ ofȱ •OHadsȱ inȱ
comparisonȱ withȱ theȱ amountȱ ofȱ clofibricȱ acidȱ adsorbedȱ onȱ eachȱ electrodeȱ surface,ȱ
evenȱworkingȱcloseȱtoȱsaturation.ȱ
ȱ
GCȬMSȱ spectraȱ obtainedȱ fromȱ electrolysesȱ describedȱ inȱ sectionȱ 8.2.1ȱ displayȱ threeȱ
peaksȱ associatedȱ withȱ stableȱ aromaticȱ intermediatesȱ suchȱ asȱ 4Ȭchlorophenol,ȱ
hydroquinoneȱ andȱ pȬbenzoquinone.ȱ ReversedȬphaseȱ chromatogramsȱ ofȱ theȱ sameȱ
electrolyzedȱsolutions,ȱusingȱPtȱandȱBDD,ȱallowȱdefiningȱtheȱevolutionȱofȱtheseȱthreeȱ
compounds,ȱ asȱ wellȱ asȱ theȱ evolutionȱ ofȱ 4Ȭchlorocatechol,ȱ 4Ȭchlororesorcinolȱ andȱȱ
1,2,4Ȭbenzenetriol.ȱForȱAOȱwithȱaȱPtȱanode,ȱaȱmuchȱgreaterȱaccumulationȱofȱaromaticȱ
intermediatesȱ canȱ beȱ observedȱ atȱ pHȱ 3.0ȱ (atȱ pHȱ 12.0ȱ onlyȱ aȱ reducedȱ numberȱ ofȱ
intermediatesȱ areȱ detected,ȱ exhibitingȱ lowerȱ amountsȱ andȱ shorterȱ times).ȱȱȱȱȱȱȱȱȱȱȱȱȱȱ
232
PART B –Results and Discussion8. Clofibric Acid
1,2,4ȬBenzenetriol,ȱ 4Ȭchlorocatecholȱ andȱ pȬbenzoquinoneȱ persistȱ forȱ 420ȱ minȱ afterȱ
reachingȱmaximumȱcontentsȱequalȱtoȱ4.5,ȱ13.0ȱandȱ13.2ȱmgȱLȬ1ȱatȱaboutȱ60ȱmin,ȱandȱȱȱ
4Ȭchlorophenol,ȱ 4Ȭchlororesorcinolȱ andȱ hydroquinoneȱ disappearȱ afterȱ 240ȱ min.ȱ Theȱ
resultsȱ forȱ AOȱ withȱ aȱ BDDȱ anodeȱ showȱ thatȱ onlyȱ 4Ȭchlorophenolȱ andȱȱȱȱȱȱȱȱ ȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱ
pȬbenzoquinoneȱareȱaccumulatedȱtoȱaȱcertainȱextent,ȱbeingȱpresentȱinȱtheȱmediumȱupȱ
toȱ420ȱminȱatȱpHȱ3.0ȱ(aȱtimeȱsimilarȱtoȱthatȱofȱclofibricȱacidȱdisappearance),ȱandȱupȱtoȱ
240ȱminȱatȱpHȱ12.0,ȱbutȱtheyȱreachȱlowerȱconcentrationsȱthanȱusingȱPt.ȱInȱconclusion,ȱ
allȱ theȱ aromaticsȱ generatedȱ duringȱ theȱ electrolysisȱ canȱ beȱ destroyedȱ usingȱ bothȱ
anodes,ȱ althoughȱ theyȱ areȱ moreȱ quicklyȱ transformedȱ usingȱ aȱ BDDȱ anode,ȱ thusȱ
confirmingȱ theȱ ‘simultaneousȱ degradation’ȱ abilityȱ ofȱ thisȱ anodeȱ towardsȱ allȱ theȱ
compoundsȱpresentȱinȱtheȱtreatedȱsolution,ȱasȱpointedȱoutȱinȱsectionȱ7.3.2.ȱ
ȱ
IonȬexclusionȱ chromatogramsȱ ofȱ theȱ aboveȱ electrolyzedȱ solutionsȱ revealȱ theȱ
accumulationȱofȱseveralȱcarboxylicȱacids:ȱtartronic,ȱmaleic,ȱfumaricȱandȱformicȱacids,ȱ
thatȱ canȱ beȱ formedȱ fromȱ theȱ oxidativeȱ breakingȱ ofȱ theȱ benzenicȱ moietyȱ ofȱ aromaticȱ
intermediates,ȱalongȱwithȱ2Ȭhydroxyisobutyricȱandȱoxalicȱacids.ȱ2Ȭhydroxyisobutyricȱ
acidȱisȱexpectedȱtoȱbeȱreleasedȱwhenȱ4Ȭchlorophenolȱisȱgeneratedȱfromȱclofibricȱacid,ȱ
andȱanȱelectrolysisȱwithȱBDDȱatȱ100ȱmAȱcmȬ2ȱshowsȱthatȱitȱisȱdegradedȱtoȱoxalicȱacid.ȱ
Thisȱ latterȱ acidȱ canȱ alsoȱ beȱ formedȱ fromȱ theȱ degradationȱ ofȱ tartronic,ȱ maleicȱ andȱ
fumaricȱ acidsȱ previouslyȱ identified.ȱ Oxalicȱ andȱ formicȱ acidsȱ areȱ finallyȱ convertedȱ
intoȱ CO2.ȱ Theȱ evolutionȱ curvesȱ ofȱ carboxylicȱ acidsȱ dependȱ onȱ bothȱ pHȱ andȱ anodeȱ
tested.ȱ Largeȱ amountsȱ ofȱ carboxylicsȱ areȱ slowlyȱ accumulatedȱ usingȱ Pt,ȱ withoutȱ
apparentȱdegradation,ȱasȱexpectedȱfromȱtheȱquiteȱlowȱmineralizationȱachievedȱwithȱ
suchȱ anȱ anode.ȱ Tartronic,ȱ 2Ȭhydroxyisobutyricȱ andȱ oxalicȱ acidsȱ areȱ theȱ mainȱ
carboxylicȱacidsȱinȱbothȱmediaȱforȱAOȱwithȱaȱPtȱanode.ȱInȱcontrast,ȱallȱtheȱcarboxylicȱ
acidsȱ haveȱ disappearedȱ afterȱ 420ȱ minȱ whenȱ BDDȱ isȱ used,ȱ accordingȱ toȱ theȱ timeȱ
neededȱ forȱ theȱ overallȱ mineralizationȱ withȱ thisȱ anode.ȱ Oxalicȱ acidȱ isȱ theȱ mainȱ
carboxylicȱacidȱinȱbothȱmedia.ȱ
ȱ
233
PART B –Results and Discussion8. Clofibric Acid
Onceȱtheȱproductȱanalysisȱhasȱbeenȱfinished,ȱitȱisȱinterestingȱtoȱcheckȱtheȱbalanceȱofȱ
carbonȱ contentȱ fromȱ initialȱ compoundȱ andȱ detectedȱ intermediates,ȱ andȱ compareȱ itȱ
withȱ TOCȱ valuesȱ atȱ pHȱ 3.0ȱ andȱ 12.0.ȱ Itȱ isȱ obviousȱ thatȱ theȱ TOCȱ measuredȱ duringȱ
electrolysesȱ withȱ BDDȱ mainlyȱ correspondsȱ toȱ theȱ remainingȱ clofibricȱ acid,ȱ becauseȱ
veryȱ smallȱ amountsȱ ofȱ aromaticȱ andȱ carboxylicȱ intermediatesȱ areȱ foundȱ usingȱ thisȱ
anode.ȱ Onlyȱ minorȱ contributionȱ ofȱ 4Ȭchlorophenolȱ andȱ oxalicȱ acidȱ areȱ worthȱ
mentioning,ȱ andȱ theyȱ areȱ presentȱ uniquelyȱ duringȱ theȱ degradationȱ processȱ ofȱ theȱ
pharmaceutical.ȱ Ptȱ showsȱ aȱ differentȱ behavior,ȱ becauseȱ intermediatesȱ leadȱ toȱ highȱ
carbonȱ contentsȱ whileȱ clofibricȱ acidȱ persists,ȱ andȱ furtherȱ on.ȱ Forȱ example,ȱ afterȱ 60ȱ
minȱ theȱ solutionȱ TOCȱ isȱ closeȱ toȱ 86ȱ mgȱ LȬ1ȱ inȱ bothȱ media,ȱ butȱ onlyȱ ca.ȱ 23ȱ mgȱ LȬ1ȱ
carbonȱ comeȱ fromȱ clofibricȱ acidȱ becauseȱ 4Ȭchlorocatecholȱ andȱ pȬbenzoquinoneȱ
amountsȱareȱequivalentȱtoȱ7ȱmgȱLȬ1ȱandȱ11ȱmgȱLȬ1ȱcarbonȱatȱpHȱ3.0,ȱrespectively,ȱandȱ
2Ȭhydroxyisobutyricȱacidȱisȱequivalentȱtoȱ15ȱmgȱLȬ1ȱatȱpHȱ12.0.ȱHowever,ȱatȱ420ȱminȱ
allȱcarboxylicȱacidsȱinȱtheȱsolutionȱonlyȱyieldȱ48ȱandȱ25ȱmgȱLȬ1ȱofȱtotalȱcarbonȱatȱpHȱ
3.0ȱandȱ12.0,ȱrespectively,ȱvaluesȱmuchȱlowerȱthanȱ63ȱmgȱLȬ1ȱofȱTOCȱgivenȱinȱTOCȬQȱ
plots,ȱmeaningȱthatȱlargeȱamountsȱofȱotherȱproductsȱareȱformed,ȱmainlyȱatȱpHȱ12.0.ȱ
ȱ
Consideringȱallȱtheȱintermediatesȱreportedȱabove,ȱaȱplausibleȱreactionȱschemeȱforȱtheȱ
AOȱofȱclofibricȱacidȱinȱaqueousȱmediumȱisȱproposed,ȱremindingȱthatȱcarboxylicȱacidsȱ
canȱonlyȱbeȱdestroyedȱusingȱBDD,ȱandȱacceptingȱ •OHadsȱasȱtheȱmainȱoxidizingȱagent.ȱ
ThisȱradicalȱfirstlyȱbreaksȱtheȱC(1)ȬOȱbondȱofȱclofibricȱacid,ȱyieldingȱ4Ȭchlorophenolȱ
andȱ 2Ȭhydroxyisobutyricȱ acid.ȱ Subsequentȱ attackȱ ofȱ •OHadsȱ onȱ ortoȬ,ȱ metaȬȱ andȱ paraȬ
positionȱ
ofȱ
4Ȭchlorophenolȱ
(regardingȱ
–OH)ȱ
releasesȱ
4Ȭchlorocatechol,ȱ ȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱ
4Ȭchlororesorcinolȱ andȱ hydroquinone,ȱ respectively.ȱ Hydroquinoneȱ formationȱ isȱ
simultaneousȱ toȱ chlorideȱ ionȱ release.ȱ Hydroxylationȱ ofȱ 4Ȭchlorocatecholȱ andȱȱȱȱȱȱȱȱȱȱȱȱȱȱ
4Ȭchlororesorcinolȱ givesȱ 1,2,4Ȭbenzenetriolȱ withȱ lossȱ ofȱ chlorideȱ ion,ȱ whereasȱ
hydroquinoneȱcanȱbeȱhydroxylatedȱtoȱformȱ1,2,4Ȭbenzenetriolȱtoo,ȱorȱcanȱbeȱoxidizedȱ
toȱ pȬbenzoquinone.ȱ Furtherȱ degradationȱ ofȱ theȱ latterȱ twoȱ compoundsȱ yieldsȱ aȱ
mixtureȱ ofȱ tartronic,ȱ maleic,ȱ fumaricȱ andȱ formicȱ acids.ȱ Theȱ formerȱ threeȱ acidsȱ areȱ
234
PART B –Results and Discussion8. Clofibric Acid
transformedȱ intoȱ oxalicȱ acid,ȱ whichȱ isȱ alsoȱ generatedȱ fromȱ theȱ oxidationȱ ofȱ theȱ
initiallyȱ electrogeneratedȱ 2Ȭhydroxyisobutyricȱ acid.ȱ Inȱ AOȱ withȱ BDDȱ anode,ȱ theȱ
ultimateȱcarboxylicȱacids,ȱoxalicȱandȱformicȱacids,ȱareȱfinallyȱconvertedȱintoȱCO2,ȱandȱ
ClȱisȱoxidizedȱtoȱCl2.ȱ
ȱ
Theȱ colorationȱ observedȱ forȱ Pt/steelȱ andȱ BDD/steelȱ systemsȱ isȱ analogousȱ toȱ theȱ oneȱ
commentedȱ inȱ sectionȱ 7.3.2ȱ forȱ theȱ AOȱ ofȱ paracetamol.ȱ Allȱ solutionsȱ treatedȱ withȱ
BDDȱ anodeȱ alwaysȱ remainȱ colorlessȱ becauseȱ ofȱ theȱ overallȱ destructionȱ ofȱ solubleȱ
polyaromaticȱproducts.ȱInȱcontrast,ȱtheȱdegradationȱwithȱPtȱcausesȱaȱchangeȱinȱcolor,ȱ
beingȱpaleȱpinkȱafterȱ5ȱmin,ȱorangeȱatȱaboutȱ1ȱh,ȱdarkȬbrownȱatȱca.ȱ2ȱhȱandȱyellowȱatȱ
approximatelyȱ4ȱh,ȱfurtherȱbeingȱslowlyȱdecolorizedȱupȱtoȱbecomeȱcolorlessȱafterȱ6ȱhȱ
ofȱ treatment.ȱ Aȱ gradualȱ pHȱ decayȱ withȱ electrolysisȱ timeȱ isȱ foundȱ forȱ solutionsȱ
startingȱatȱpHȱǃȱ4.0ȱdueȱtoȱtheȱformationȱofȱcarboxylicȱacids,ȱsoȱcontinuousȱregulationȱ
withinȱaȱrangeȱofȱ±0.03ȱunitsȱisȱcarriedȱoutȱbyȱaddingȱsmallȱvolumesȱofȱ0.1ȱMȱNaOH.ȱ
ȱ
ȱ
235
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
PART B –Results and Discussion8. Clofibric Acid
8.3.
TRACTAMENTȱMITJANÇANTȱELECTROȬFENTONȱIȱFOTOELECTROȬFENTONȱ
/ȱTREATMENTȱBYȱELECTROȬFENTONȱANDȱPHOTOELECTROȬFENTONȱ
ȱ
8.3.1.ȱȱFinalitatȱdelȱtreballȱ/ȱAimȱofȱtheȱworkȱ
ȱ
Theȱ completeȱ mineralizationȱ ofȱ aqueousȱ solutionsȱ ofȱ clofibricȱ acidȱ upȱ toȱ closeȱ toȱ
saturationȱ inȱ theȱ pHȱ rangeȱ 2.0Ȭ12.0ȱ hasȱ beenȱ justȱ describedȱ byȱ AOȱ withȱ BDDȱ inȱ aȱ
largeȱvarietyȱofȱexperimentalȱconditions,ȱsoȱthisȱisȱcertainlyȱanȱinterestingȱ optionȱtoȱ
beȱ takenȱ intoȱ accountȱ regardingȱ wastewatersȱ whereȱ theȱ presenceȱ ofȱ suchȱ aȱ
pharmaceuticalȱ isȱ described.ȱ Immediately,ȱ aȱ questionȱ canȱ beȱ posed:ȱ wouldȱ EFȱ andȱ
PEFȱprocessesȱbeȱaȱfeasibleȱalternativeȱcomparedȱtoȱAOȱwithȱBDD?ȱCouldȱcouplingȱ
betweenȱBDDȱanodeȱandȱO2Ȭdiffusionȱcathodeȱevenȱenhanceȱtheȱoxidizingȱabilityȱofȱ
AOȱ withȱ BDDȱ andȱ PEFȱ withȱ Pt?ȱ Inȱ orderȱ toȱ analyzeȱ criticallyȱ theȱ situationȱ onȱ theȱ
basisȱofȱwellȱverifiedȱdata,ȱEFȱandȱPEFȱprocessesȱwereȱappliedȱusingȱanȱO2Ȭdiffusionȱ
cathodeȱandȱPtȱorȱBDDȱasȱanode,ȱallȱofȱthemȱwithȱanȱareaȱofȱ3ȱcm2.ȱBothȱEFȱandȱPEFȱ
treatmentsȱ wereȱ alwaysȱ performedȱ byȱ addingȱ 1.0ȱ mMȱ Fe2+ȱ (chosenȱ asȱ theȱ optimalȱ
concentration,ȱ asȱ pointedȱ outȱ inȱ sectionȱ 7.2.2)ȱ andȱ 0.05ȱ Mȱ Na2SO4ȱ toȱ theȱ solutionȱ atȱ
theȱ beginningȱ ofȱ theȱ electrolyses.ȱ Allȱ trialsȱ wereȱ carriedȱ outȱ atȱ 35ȱ ºC,ȱ whichȱ isȱ theȱ
maximumȱtemperatureȱallowedȱwithoutȱsignificantȱwaterȱevaporationȱfromȱsolution.ȱ
Similarly,ȱ treatmentsȱ byȱ AOȱ withȱ electrogeneratedȱ H2O2ȱ butȱ inȱ theȱ absenceȱ ofȱ Fe2+ȱ
wereȱ carriedȱ outȱ toȱ underlineȱ theȱ effectivityȱ ofȱ theȱ •OHȱ producedȱ fromȱ Fenton’sȱ
reactionȱinȱEFȱandȱPEF.ȱ
ȱ
Comparativeȱ electrolysesȱ wereȱ initiallyȱ madeȱ forȱ 100ȬmLȱ solutionsȱ ofȱ pHȱ 3.0ȱ
containingȱ179ȱmgȱLȬ1ȱclofibricȱacidȱ(i.e.,ȱ100ȱmgȱLȬ1ȱTOC)ȱatȱ100ȱmAȱcmȬ2,ȱusingȱPtȱorȱ
BDDȱ asȱ anode.ȱ Noteȱ thatȱ theȱ electrolyticȱ systemȱ continuouslyȱ producesȱ H2O2ȱ fromȱ
bielectronicȱ reductionȱ ofȱ O2ȱ atȱ theȱ O2Ȭdiffusionȱ cathode.ȱ Theȱ useȱ ofȱ thisȱ systemȱ
withoutȱcatalystȱcorrespondsȱtoȱtheȱmethodȱofȱAOȱwithȱelectrogeneratedȱH2O2.ȱThen,ȱ
theȱsameȱexperimentsȱwereȱdoneȱwithȱUVAȱirradiation.ȱLater,ȱ1.0ȱmMȱFe2+ȱwasȱusedȱ
237
PART B –Results and Discussion8. Clofibric Acid
asȱcatalystȱwithoutȱ(EF)ȱorȱwithȱ(PEF)ȱUVAȱilluminationȱtoȱtestȱtheȱoxidationȱabilityȱ
ofȱallȱtheseȱprocesses.ȱ
ȱ
Ionȱ chromatogramsȱ forȱ theȱ aboveȱ treatedȱ solutionsȱ byȱ AOȱ withoutȱ H2O2ȱ
electrogeneration,ȱEFȱandȱPEFȱwereȱrecordedȱtoȱstudyȱtheȱevolutionȱofȱinorganicȱionsȱ
releasedȱfromȱinitialȱchlorineȱofȱtheȱpharmaceutical.ȱ
ȱ
Afterwards,ȱ theȱ effectȱ ofȱ pH,ȱ currentȱ densityȱ andȱ metaboliteȱ concentrationȱ onȱ theȱ
oxidizingȱpowerȱofȱPEFȱwithȱPtȱandȱEFȱwithȱBDDȱwasȱinvestigatedȱfromȱTOCȱdecayȱ
andȱMCEȱcalculationȱinȱorderȱtoȱclarifyȱtheȱoptimumȱoperativeȱconditions.ȱTheseȱtwoȱ
processesȱ wereȱ selectedȱ becauseȱ theyȱ provideȱ overallȱ mineralization,ȱ andȱ EFȱ withȱ
BDDȱ inȱ particularȱ wasȱ consideredȱ toȱ beȱ moreȱ suitableȱ thanȱ PEFȱ becauseȱ itsȱ slowerȱ
TOCȱabatementȱallowsȱaȱbetterȱunderstandingȱandȱcriticalȱanalysisȱofȱtheȱdifferencesȱ
observed.ȱFirstly,ȱtheȱinfluenceȱofȱpHȱwasȱstudiedȱinȱtheȱpHȱrangeȱ2.0Ȭ6.0ȱunderȱtheȱ
experimentalȱ conditionsȱ pointedȱ outȱ above.ȱ Secondly,ȱ severalȱ solutionsȱ ofȱ pHȱ 3.0ȱ
withȱ 179ȱ mgȱ LȬ1ȱ ofȱ theȱ metaboliteȱ wereȱ electrolyzedȱ atȱ 33,ȱ 100ȱ andȱ 150ȱ mAȱ cmȬ2ȱ toȱ
assessȱ theȱ effectȱ ofȱ currentȱ density.ȱ Andȱ finally,ȱ theȱ greatȱ oxidizingȱ powerȱ ofȱ theseȱ
twoȱ methodsȱ wasȱ studiedȱ byȱ degradingȱ solutionsȱ containingȱ 89,ȱ 179,ȱ 358ȱ andȱ 557ȱ
(closeȱtoȱsaturation)ȱmgȱLȬ1ȱofȱclofibricȱacidȱatȱpHȱ3.0ȱandȱatȱ100ȱmAȱcmȬ2.ȱ
ȱ
Theȱ decayȱ ofȱ theȱ pharmaceuticalȱ inȱ theȱ fourȱ electrochemicalȱ methodsȱ (AOȱ withoutȱ
andȱ withȱ UVAȱ irradiation,ȱ EFȱ andȱ PEF)ȱ wasȱ followedȱ byȱ reversedȬphaseȱ HPLCȱ
chromatographyȱusingȱaȱPtȱanodeȱorȱaȱBDDȱanode.ȱAsȱstatedȱinȱsectionȱ8.2.1,ȱfirstȱofȱ
allȱ itȱ wasȱ necessaryȱ toȱ clarifyȱ whetherȱ clofibricȱ acidȱ canȱ beȱ oxidizedȱ withȱ otherȱ
weakerȱ oxidizingȱ speciesȱ generatedȱ alongȱ theȱ electrolysis.ȱ Aȱ previousȱ chemicalȱ testȱ
wasȱcarriedȱoutȱbyȱaddingȱ20ȱmMȱH2O2ȱandȱ0.05ȱMȱNa2SO4ȱtoȱaȱ100ȬmLȱsolutionȱofȱ
pHȱ 3.0ȱ containingȱ 179ȱ mgȱ LȬ1ȱ clofibricȱ acid,ȱ becauseȱ H2O2ȱ isȱ certainlyȱ theȱ mostȱ
concentratedȱ ofȱ theȱ weakerȱ oxidizingȱ agentsȱ formedȱ dueȱ toȱ theȱ notoriousȱ
accumulationȱ alreadyȱ discussedȱ inȱ sectionȱ 7.2.1ȱ (seeȱ Figureȱ 7.Ȭ2).ȱ Afterȱ this,ȱ theȱ
238
PART B –Results and Discussion8. Clofibric Acid
comparativeȱ kineticsȱ ofȱ clofibricȱ acidȱ decayȱ byȱ itsȱ reactionȱ withȱ •OHȱ inȱ theȱ bulkȱ
solutionȱandȱ •OHadsȱinȱtheȱelectrodeȱsurfaceȱwasȱdeterminedȱbyȱelectrolyzingȱ179ȱmgȱ
LȬ1ȱofȱtheȱpharmaceuticalȱwithȱ1.0ȱmMȱFe2+ȱatȱpHȱ3.0ȱandȱatȱ100ȱmAȱcmȬ2ȱforȱtheȱfourȱ
processesȱ mentionedȱ usingȱ eachȱ anode.ȱ Theȱ influenceȱ ofȱ jappȱ onȱ clofibricȱ acidȱ decayȱ
wasȱ furtherȱ studiedȱ forȱ PEFȱ withȱ Ptȱ andȱ EFȱ withȱ BDD,ȱ byȱ electrolyzingȱ 100ȬmLȱ
solutionsȱofȱpHȱ3.0ȱwithȱ179ȱmgȱLȬ1ȱclofibricȱacid,ȱatȱ33,ȱ100ȱandȱ150ȱmAȱcmȬ2.ȱLastly,ȱ
theȱ roleȱ ofȱ initialȱ clofibricȱ acidȱ concentrationȱ onȱ itsȱ decayȱ kineticsȱ wasȱ clarifiedȱ forȱ
PEFȱwithȱPtȱaloneȱfromȱelectrolysesȱofȱdifferentȱsolutionsȱofȱpHȱ3.0ȱcontainingȱ89,ȱ179,ȱ
358ȱandȱ557ȱmgȱLȬ1ȱofȱpharmaceuticalȱatȱ100ȱmAȱcmȬ2.ȱ
ȱ
Toȱ helpȱ identifyingȱ aromaticȱ intermediates,ȱ aȱ 100ȬmLȱsolutionȱ ofȱ pHȱ 3.0ȱ containingȱ
179ȱ mgȱ LȬ1ȱ ofȱ clofibricȱ acidȱ wasȱelectrolyzedȱ byȱ EFȱ usingȱ Ptȱ andȱ BDDȱ withȱ 1.0ȱ mMȱ
Fe2+ȱatȱ100ȱmAȱcmȬ2ȱandȱatȱ35ȱºCȱforȱ2ȱmin,ȱandȱafterȱsomeȱpreparativesȱ(seeȱsectionȱ
6.3)ȱtheȱremainingȱintermediatesȱwereȱanalyzedȱbyȱGCȬMS.ȱSimilarly,ȱtoȱidentifyȱtheȱ
finalȱcarboxylicȱacids,ȱtheȱaboveȱsolutionȱwasȱtreatedȱunderȱtheȱsameȱEFȱconditionsȱ
forȱ6ȱh,ȱfurtherȱevaporatingȱitȱatȱlowȱpressureȱandȱdissolvingȱtheȱremainingȱsolidȱinȱ
ethanolȱ toȱ characterizeȱ theȱ esterifiedȱ acidsȱ byȱ GCȬMS.ȱ ReversedȬphaseȱ
chromatogramsȱ andȱ ionȬexclusionȱ chromatogramsȱ wereȱ obtainedȱ toȱ followȱ theȱ
evolutionȱ ofȱ theȱ correspondingȱ aromaticȱ andȱ carboxylicȱ acidȱ intermediatesȱ byȱ AO,ȱ
EFȱandȱPEFȱwithȱPtȱorȱBDDȱunderȱtheȱconditionsȱalreadyȱdescribedȱaboveȱforȱGCȬMS.ȱ
ȱ
Consideringȱ allȱ theȱ intermediatesȱ thatȱ wereȱ identified,ȱ plausibleȱ reactionȱ sequencesȱ
forȱ theȱ degradationȱ ofȱ clofibricȱ acidȱ byȱ EFȱ andȱ PEFȱ withȱ 1.0ȱ mMȱ Fe2+ȱ and/orȱ UVAȱ
lightȱasȱcatalystsȱwereȱproposed.ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
239
PART B –Results and Discussion8. Clofibric Acid
Theȱthoroughȱresultsȱofȱthisȱsectionȱareȱincludedȱinȱtheȱfollowingȱpapersȱ(Paperȱ5Ȭ6):ȱ
ȱ
5.ȱSirés,ȱI.,ȱArias,ȱC.,ȱCabot,ȱP.L.,ȱCentellas,ȱF.,ȱGarrido,ȱJ.A.,ȱRodríguez,ȱR.M.,ȱBrillas,ȱ
E.,ȱDegradationȱofȱclofibricȱacidȱinȱacidicȱaqueousȱmediumȱbyȱelectroȬFentonȱandȱ
photoelectroȬFenton.ȱChemosphere,ȱdoi:10.1016/j.chemosphere.2006.07.039.ȱ
ȱ
6.ȱSirés,ȱI.,ȱGarrido,ȱJ.A.,ȱCentellas,ȱF.,ȱRodríguez,ȱR.M.,ȱCabot,ȱP.L.,ȱArias,ȱC.,ȱBrillas,ȱ
E.,ȱ Mineralizationȱ ofȱ clofibricȱ acidȱ byȱ electrochemicalȱ advancedȱ oxidationȱ
processesȱ usingȱ aȱ boronȬdopedȱ diamondȱ anodeȱ andȱ Fe2+ȱ andȱ UVAȱ lightȱ asȱ
catalysts.ȱAppl.ȱCatal.ȱB:ȱEnviron.ȱ(submitted)ȱ
ȱ
Theȱfollowingȱpresentationȱinȱaȱcongressȱisȱrelatedȱtoȱthisȱwork:ȱ
ȱ
F.ȱSirés,ȱI.,ȱGarrido,ȱJ.A.,ȱCentellas,ȱF.,ȱRodríguez,ȱR.M.,ȱCabot,ȱP.L.,ȱArias,ȱC.,ȱBrillas,ȱ
E.,ȱClofibricȱacidȱmineralizationȱbyȱelectrochemicalȱadvancedȱoxidationȱprocessesȱ
usingȱ aȱ boronȬdopedȱ diamondȱ anodeȱ andȱ cathodicallyȱ generatedȱ H2O2ȱ withȱ Fe2+ȱ
andȱ UVAȱ lightȱ asȱ catalysts,ȱ Vol.ȱ 1,ȱ pageȱ 152,ȱ EAAOPȬ1:ȱ Environmentalȱ
ApplicationsȱofȱAdvancedȱOxidationȱProcesses.ȱTechnicalȱUniversityȱofȱCreteȱandȱ
Aristotleȱ Universityȱ ofȱ Thessaloniki,ȱ Chania,ȱ Greece,ȱ 7Ȭ9ȱ Septemberȱ 2006.ȱ (Posterȱ
presentation)ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
240
ARTICLEȱ5ȱ/ȱPAPERȱ5
ȱ
ȱ
Degradationȱofȱclofibricȱacidȱinȱacidicȱaqueousȱmediumȱbyȱȱ
ȱ electroȬFentonȱandȱphotoelectroȬFentonȱ
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ARTICLE IN PRESS
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Chemosphere xxx (2006) xxx–xxx
www.elsevier.com/locate/chemosphere
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Degradation of clofibric acid in acidic aqueous medium
by electro-Fenton and photoelectro-Fenton
ȱ
ȱ Ignasi
Sirés, Conchita Arias, Pere Lluı́s Cabot, Francesc Centellas, José Antonio Garrido,
Rosa Marı́a Rodrı́guez, Enric Brillas
ȱ
ȱ
ȱ
ȱ
*
ȱ
Laboratori d’Electroquı́mica dels Materials i del Medi Ambient, Departament de Quı́mica Fı́sica, Facultat de Quı́mica,
Universitat de Barcelona, Martı́ i Franquès 1-11, 08028 Barcelona, Spain
ȱ
Received 28 April 2006; received in revised form 17 July 2006; accepted 18 July 2006
ȱ
ȱ Abstractȱ
Acidic
ȱ aqueous solutions of clofibric acid (2-(4-chlorophenoxy)-2-methylpropionic acid), the bioactive metabolite of various lipid-regȱ ulating
drugs, have been degraded by indirect electrooxidation methods such as electro-Fenton and photoelectro-Fenton with Fe2+ as
catalyst using an undivided electrolytic cell with a Pt anode and an O2-diffusion cathode able to electrogenerate H2O2. At pH 3.0 about
ȱ
with the electro-Fenton method due to the efficient production of oxidant hydroxyl radical from
ȱ 80% of mineralization is achieved
2+
3+
Fenton’s reaction between Fe and H2O2, but stable Fe complexes are formed. The photoelectro-Fenton method favors the photoȱ
decomposition
of these species under UVA irradiation, reaching more than 96% of decontamination. The mineralization current effiȱ ciency increases with rising metabolite concentration up to saturation and with decreasing current density. The photoelectro-Fenton
methodȱ is then viable for treating acidic wastewaters containing this pollutant. Comparative degradation by anodic oxidation (without
2+
) yields poor decontamination. Chloride ion is released during all degradation processes. The decay kinetics of clofibric acid always
ȱ Fe
follows ȱa pseudo-first-order reaction, with a similar rate constant in electro-Fenton and photoelectro-Fenton that increases with rising
current density, but decreases at greater metabolite concentration. 4-Chlorophenol, 4-chlorocatechol, 4-chlororesorcinol, hydroquinone,
ȱ p-benzoquinone
and 1,2,4-benzenetriol, along with carboxylic acids such as 2-hydroxyisobutyric, tartronic, maleic, fumaric, formic and
ȱ
oxalic, are detected as intermediates. The ultimate product is oxalic acid, which forms very stable Fe3+-oxalato complexes under electroconditions. These complexes are efficiently photodecarboxylated in photoelectro-Fenton under the action of UVA light.
ȱ Fenton
2006 ȱElsevier Ltd. All rights reserved.
ȱ Keywords:ȱ Drug mineralization; Electro-Fenton; Photoelectro-Fenton; Catalysis; Water treatment
ȱ
ȱ
ȱ
ȱ
1. Introduction
ȱ is great interest in the environmental relevance of
There
pharmaceutical
drugs and their metabolites as emerging polȱ
lutantsȱ in waters (Daughton and Jones-Lepp, 2001; Kümmerer, 2001; Heberer, 2002; Kolpin et al., 2002; Heberer
ȱ and Adam,
ȱ
2004; Weigel et al., 2004; Tauxe-Wuersch
et al., 2005). Different anti-inflammatories, analgesics,
lipid regulators, antimicrobials, antiepileptics
ȱ
ȱ betablockers,
and estrogens have been detected in sewage treatment plant
ȱ
ȱ
ȱ
ȱ
ȱ
*
Corresponding author. Tel.: +34 93 4021223; fax: +34 93 4021231.
E-mail
ȱ address: [email protected] (E. Brillas).
effluents, surface and ground waters and even in drinking
water at concentrations usually ranging from ng l1 to
lg l1. The sources of this pollution involve emission from
production sites, direct disposal of overplus drugs in households, excretion after drug administration to humans and
animals, treatments throughout the water in fish and other
animal farms and inadequate treatment of manufacturing
waste. Among these compounds, clofibric acid (2-(4-chlorophenoxy)-2-methylpropionic acid, 1) has long term persistence in the environment. It is the bioactive metabolite of
clofibrate, etofibrate and etofyllineclofibrate, which are
drugs widely used as blood lipid regulators with therapeutic
doses of about 1–2 g d1 per person, since they decrease the
0045-6535/$ - see front matter 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.chemosphere.2006.07.039
Please cite this article as: Ignasi Sirés et al., Degradation of clofibric acid in acidic aqueous medium ..., Chemosphere (2006),
doi:10.1016/j.chemosphere.2006.07.039.
241
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ARTICLE IN PRESS
ȱ
2
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
I. Sirés et al. / Chemosphere xxx (2006) xxx–xxx
plasmatic concentration of cholesterol and triglycerides
(Buser et al., 1998; Tauxe-Wuersch et al., 2005). Concentrations of 1 up to 10 lg l1 have been detected in sewage treatment plant influents and effluents and in rivers, lakes, North
Sea, ground and drinking waters (Heberer and Stan, 1997;
Buser et al., 1998; Ternes, 1998; Tixier et al., 2003).
To avoid the potential adverse health effects of drugs
and their metabolites as water pollutants on both humans
and animals, research efforts are underway to develop efficient techniques for achieving their total destruction (Zwiener and Frimmel, 2000; Doll and Frimmel, 2004).
However, 1 is poorly degraded by oxidation methods such
as ozonolysis (Ternes et al., 2002; Andreozzi et al., 2003),
H2O2/UV (Andreozzi et al., 2003), sunlight and UV photolysis (Packer et al., 2003) and TiO2/UV (Doll and
Frimmel, 2004), as well as after application of different biological and physico-chemical methods in sewage treatment
plants (Tauxe-Wuersch et al., 2005). More potent oxidation procedures are then needed to be applied to destroy
this compound in wastewaters.
Recently, indirect electrooxidation methods such as electro-Fenton and photoelectro-Fenton are being developed
for wastewater remediation. In these environmentally clean
electrochemical techniques, hydrogen peroxide is continuously generated in an acidic contaminated solution from
the two-electron reduction of O2 at reticulated vitreous carbon (Xie and Li, 2006), mercury pool (Ventura et al.,
2002), carbon-felt (Oturan et al., 1999; Gözmen et al.,
2003; Hanna et al., 2005; Irmak et al., 2006) and O2-diffusion (Boye et al., 2002; Brillas et al., 2004a,b; Sirés et al.,
2006) cathodes:
O2 + 2Hþ + 2e ! H2 O2
ð1Þ
The oxidizing power of H2O2 is enhanced in the electroFenton method by adding small amounts of Fe2+ as catalyst to the acidic solution. Hydroxyl radical (OH) and Fe3+
are then generated from the classical Fenton’s reaction
between Fe2+ and H2O2 (Sun and Pignatello, 1993):
Fe2þ + H2 O2 ! Fe3þ + OH + OH
ð2Þ
Reaction (2) is propagated from Fe2+ regeneration,
which mainly occurs by reduction of Fe3+ species at the
cathode (Oturan et al., 1999). OH acts as a non-selective,
strong oxidant, with ability to react with organics yielding
dehydrogenated or hydroxylated derivatives until their
overall mineralization (conversion into CO2, water and
inorganic ions). In the photoelectro-Fenton process the
solution is irradiated with UVA light to favor: (i) the photodecomposition of complexes of Fe3+ with generated carboxylic acids (Zuo and Hoigné, 1992; Brillas et al., 2004a,b;
Sirés et al., 2006) and (ii) the regeneration of Fe2+ from
additional photoreduction of Fe(OH)2+, which is the predominant Fe3+ species in acid medium (Sun and Pignatello,
1993):
Fe(OH)2þ + hm ! Fe2þ + OH
ð3Þ
Reaction (3) also enhances the production of OH and
hence, the mineralization of organics.
The electro-Fenton treatment of 100-ml solutions with
1 mM of 1 and 2 mM Fe2+ in 0.01 M HCl has been previously reported by Oturan et al. (1999), but these authors
only described its decay kinetics and the detection of some
initial aromatic products. To show the possible viability of
the electro-Fenton and photoelectro-Fenton methods to
remove this metabolite in wastewaters, we have carried
out a detailed study on the degradation of acidic aqueous
solutions of 1 in the pH range 2.0–6.0 using 1.0 mM Fe2+
in both procedures. Comparative treatments in the absence
of this catalyst were also made to demonstrate the positive
oxidation action of OH formed from reaction (2). The
effect of current density and clofibric acid concentration
on the degradation process and current efficiency was
explored. Aromatic products were identified by gas chromatography–mass spectrometry (GC–MS). The decay of
1 and the evolution of its by-products were followed by
chromatographic techniques. The results obtained in this
study are reported herein.
2. Experimental
2.1. Chemicals
Clofibric acid (1), 4-chlorophenol (2), hydroquinone (3),
4-chlororesorcinol (4), p-benzoquinone (6), 1,2,4-benzenetriol (7), 2-hydroxyisobutyric acid (9), tartronic acid (10),
maleic acid (11), fumaric acid (12), formic acid (13) and
oxalic acid (14) were either reagent or analytical grade from
Sigma-Aldrich, Merck, Panreac and Avocado. 4-Chlorocatechol (5) was synthesized by chlorination of pyrocatechol with SO2Cl2 at room temperature, as reported
elsewhere (Boye et al., 2002). Analytical grade sulfuric acid
was purchased from Merck. Anhydrous sodium sulfate
and heptahydrated ferrous sulfate were analytical grade
from Fluka. All solutions were prepared with pure water
obtained from a Millipore Milli-Q system with resistivity
>18 MX cm at 25 C. Organic solvents and other chemicals
employed were either HPLC or analytical grade from
Panreac.
2.2. Apparatus and analysis procedures
The solution pH was measured with a Crison 2000 pHmeter. Electrolyses were carried out at a constant current
density (j) of 33, 100 and 150 mA cm2 with an Amel
2053 potentiostat–galvanostat. All samples withdrawn
from treated solutions were filtered with Whatman
0.45 lm PTFE filters before analysis. The mineralization
of each solution of 1 was monitored by the abatement of
its total organic carbon (TOC), determined on a Shimadzu
VCSN TOC analyzer. Reproducible TOC values were
obtained from analysis of 100-ll aliquots using the standard non-purgeable organic carbon method. From these
data, the mineralization current efficiency (MCE) for elec-
Please cite this article as: Ignasi Sirés et al., Degradation of clofibric acid in acidic aqueous medium ..., Chemosphere (2006),
doi:10.1016/j.chemosphere.2006.07.039.
242
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ARTICLE IN PRESS
ȱ
I. Sirés et al. / Chemosphere xxx (2006) xxx–xxx
ȱ
ȱ
ȱ
trolyzed solutions at a given time t was calculated as
follows:
MCE ¼
DðTOCÞexp
100
DðTOCÞtheor
ð4Þ
where D(TOC)exp is the experimental TOC removal and
D(TOC)theor is the theoretically calculated TOC decay
ȱ assuming that the applied electrical charge (=current ·
time) is only consumed in the mineralization process of 1.
ȱ The decay of 1 and the evolution of aromatic intermediates were followed by reversed-phase chromatography
ȱ using a Waters 600 HPLC liquid chromatograph fitted with
a Spherisorb ODS2 5 lm, 150 · 4.6 mm (i.d.), column at
room temperature, and coupled with a Waters 996 photoȱ diode array detector, controlled through a Millennium-32
program. For each compound, this detector was selected at
ȱ the maximum wavelength of its UV-absorption band.
These analyses were made by injecting 20-ll aliquots into
ȱ the chromatograph and circulating a 50:47:3 (v/v/v) methanol/phosphate buffer (pH = 2.5)/pentanol mixture at
1.0 ml min1 as mobile phase. Generated carboxylic acids
ȱ were followed by ion-exclusion chromatography by injecting 20-ll samples into the above HPLC chromatograph
ȱ fitted with a Bio-Rad Aminex HPX 87 H, 300 · 7.8 mm
(i.d.), column at 35 C. For these measurements, the photodiode detector was selected at 210 nm and the mobile phase
ȱ was 4 mM H2SO4 at 0.6 ml min1. Cl concentration in
electrolyzed solutions was determined by ion chromatograȱ phy using a Shimadzu 10Avp HPLC chromatograph fitted
with a Shim-Pack IC-A1S, 100 · 4.6 mm (i.d.), anion colȱ umn at 40 C and coupled with a Shimadzu CDD 10Avp
conductivity detector. These measurements were carried
out with a 2.5 mM phtalic acid and 2.4 mM tris(hydroxyȱ methyl)aminomethane solution of pH 4.0 as mobile phase
at 1.5 ml min1.
ȱ A 100 ml-solution with2179 mg l1 of 1 of pH 3.0 was
electrolyzed at 100 mA cm and at 35.0 C by electro-Fenton with 1.0 mM Fe2+ for 2 min. The resulting organics
ȱ were extracted with 45 ml of CH Cl in three times. The
2 2
collected organic solution was dried with anhydrous
ȱ Na2SO4, filtered and evaporated to about 2 ml. The
remaining products were separated and identified by
ȱ GC–MS using a Hewlett-Packard 5890 Series II gas chromatograph fitted with a HP-5 0.25 lm, 30 m · 0.25 mm
(i.d.), column, and a Hewlett-Packard 5989 A mass
ȱ spectrophotometer operating in EI mode at 70 eV. The
temperature ramp for this column was 35 C for 2 min,
ȱ 10 C min1 up to 320 C and hold time 5 min, and the
temperature of the inlet, transfer line and detector was
250 C, 250 C and 290 C, respectively. To identify the
ȱ final
carboxylic acids, the above solution was treated under
the same electro-Fenton conditions for 6 h. The resulting
ȱ solution was evaporated at low pressure and the remaining
solid was dissolved in 2 ml of ethanol. The esterified acids
ȱ were further analyzed by GC–MS using the gas chromatograph fitted with a HP-INNOWax 0.25 lm, 30 m ·
0.25 mm (i.d.), column. In this case the temperature ramp
ȱ
ȱ
3
was 35 C for 2 min, 10 C min1 up to 250 C and hold
time 15 min, and the temperature of the inlet, transfer line
and detector was always 250 C.
2.3. Electrolytic system
All electrolyses were conducted in an open, undivided
and thermostated cylindrical cell containing 100 ml of solution stirred with a magnetic bar. The anode was a 3-cm2 Pt
sheet of 99.99% purity from SEMPSA and the cathode was
a 3-cm2 carbon-PTFE electrode from E-TEK, which was
fed with pure O2 at 12 ml min1 to generate continuously H2O2 from reaction (1). The electrolytic setup and
the preparation of the O2-diffusion cathode have been
described (Boye et al., 2002). For the experiments with
UVA irradiation, a Philips 6 W fluorescent black light blue
tube was placed at 7 cm above the solution. The tube emitted UVA light in the wavelength region between 300 and
420 nm, with kmax = 360 nm, supplying a photoionization
energy input to the solution of 140 lW cm2, detected with
a NRC 820 laser power meter working at 514 nm. Both
electro-Fenton and photoelectro-Fenton treatments were
performed after addition of 1.0 mM Fe2+ to the solution.
All trials were carried out at 35.0 C, which is the maximum temperature to work with the open electrolytic cell
without significant water evaporation from solution (Boye
et al., 2002).
3. Results and discussion
3.1. Comparative degradation
Comparative electrolyses at 100 mA cm2 for 6 h were
initially made for solutions containing 179 mg l1 of 1
(equivalent to 100 mg l1 of TOC) and 0.05 M Na2SO4
regulated with H2SO4 at pH 3.0 and at 35.0 C. In these
experiments the solution pH remained practically constant,
reaching final values between 2.8 and 3.0. The change in
solution TOC with consumed specific charge (Q, in A h l1)
for such trials is depicted in Fig. 1a. Note that the electrolytic system produces continuously hydrogen peroxide
from reaction (1), whereas adsorbed OH is formed at the
Pt anode from water oxidation (Boye et al., 2002; Brillas
et al., 2004a,b; Sirés et al., 2006):
H2 O ! OHads + Hþ + e
ð5Þ
The use of this system without catalyst corresponds to
the method of anodic oxidation with electrogenerated
H2O2. Fig. 1a shows that this procedure gives a quite slow
TOC removal, attaining 41% of mineralization at 6 h
(Q = 18 A h l1). This behavior can be accounted for by
the low concentration of OH formed at the Pt surface from
reaction (5), which is the main oxidant of 1 and its by-products. A similar degradation rate can be seen in Fig. 1a when
the solution without catalyst is illuminated with UVA light,
yielding 39% of TOC decay at the end of electrolysis. This
brings to consider that organics are not directly photode-
Please cite this article as: Ignasi Sirés et al., Degradation of clofibric acid in acidic aqueous medium ..., Chemosphere (2006),
doi:10.1016/j.chemosphere.2006.07.039.
243
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ARTICLE IN PRESS
ȱ
4
I. Sirés et al. / Chemosphere xxx (2006) xxx–xxx
ȱ
ȱ
-1
ȱ
a
100
TOC / mg l
ȱ
80
60
40
20
ȱ
0
b
[Cl– ] / mg l
ȱ
-1
25
ȱ
(Q = 2 A h l1), whereupon it undergoes a slight drop due
to its oxidation to Cl2 at the Pt anode. These findings indicate that chloro-organics are always degraded with release
of Cl, although they are much more rapidly destroyed in
the two last methods. However, all procedures only lead
to the release of 78–85% of the initial chlorine content of
1 (29.5 mg l1), suggesting that stable colored polyaromatics formed during degradation contain the remaining
chlorine.
These results indicate that electro-Fenton only yields
partial decontamination of 1, whereas this pollutant can
be almost completely mineralized by photoelectro-Fenton.
For this last technique, the effect of pH, current density and
metabolite concentration on its oxidizing power was investigated to clarify its optimum operative conditions.
120
ȱ
20
15
10
3.2. Effect of experimental parameters on the photoelectroFenton process
5
0
ȱ
0
3
6
9
12
-1
Q/Ahl
15
18
21
Fig. 1. (a) Total organic carbon and (b) concentration of accumulated
ȱ chloride ion vs. specific charge for the degradation of 100-ml solutions
ȱ
ȱ
with 179 mg l1 clofibric acid (1) and 0.05 M Na2SO4 of pH 3.0 at
100 mA cm2 and at 35.0 C using an undivided cell with a 3-cm2 Pt anode
and a 3-cm2 carbon-PTFE O2-diffusion cathode. Method: (d) anodic
oxidation with electrogenerated H2O2; (j) anodic oxidation with electrogenerated H2O2 under a 6-W UVA irradiation with kmax = 360 nm; (m)
electro-Fenton with 1.0 mM Fe2+ in the solution; () photoelectro-Fenton
with 1.0 mM Fe2+ and UVA light.
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
composed by UVA light. A different behavior can be
observed in Fig. 1a when 1.0 mM Fe2+ is added as catalyst.
For the electro-Fenton process, TOC is rapidly reduced by
79% at 6 h due to the fast reaction of organics with the great
amounts of OH produced from Fenton’s reaction (2). In
contrast, the photoelectro-Fenton process leads to quicker
TOC decay with almost overall mineralization (>96%
TOC removal) at the end of electrolysis. This trend can be
related to: (i) the rapid photolysis of some stable complexes
of Fe3+ with generated carboxylic acids under electro-Fenton conditions (Zuo and Hoigné, 1992; Brillas et al., 2004a;
Sirés et al., 2006) and/or (ii) the enhanced generation of OH
from additional photoreduction of Fe(OH)2+ from reaction
(3). Note that the starting pale yellow solution changed to
pale orange color at the end of both electro-Fenton and
photoelectro-Fenton degradations. This is indicative of
the formation of soluble colored polyaromatics in small
extent, which can be not be destroyed by oxidant OH produced by reactions (2), (3) and (5).
Mineralization of 1 is accompanied by its overall dechlorination. Ion chromatograms for the above treated solutions only displayed a defined peak related to Cl ion. No
other chlorine–oxygen ions such as ClO
3 and ClO4 were
detected by this technique. Fig. 1b shows a gradual accumulation of Cl up to 23 mg l1 for 4 h (Q = 12 A h l1) by
anodic oxidation with electrogenerated H2O2, whereas for
electro-Fenton and photoelectro-Fenton, a Cl concentration of about 25 mg l1 is already attained at 40 min
The TOC–Q plots obtained for solutions of 179 mg l1
of 1 in the pH range 2.0–6.0 degraded by photoelectroFenton at 100 mA cm2 are depicted in Fig. 2a. The pH
of solutions with initial pH 4.0 and 6.0 underwent a progressive decrease with time, mainly during the first hour
of electrolysis, due to the generation of acid products and
for this reason, it was continuously regulated within a range
of ±0.3 units by adding 1 M NaOH. Fig. 2a shows that the
quickest TOC decay takes place starting from pH 3.0,
whereas for the other solutions, the degradation rate falls
in the order pH 2.0 > pH 4.0 pH 6.0. This behavior can
be associated with the highest generation rate of the main
oxidant OH from Fenton’s reaction (2), since its optimum
pH is 2.8 (Sun and Pignatello, 1993), very close to pH 3.0
where 1 and its by-products are more rapidly destroyed.
The influence of current density on the oxidation
ability of this method was examined by electrolyzing solutions with 179 mg l1 of 1 of pH 3.0 at 33, 100 and
150 mA cm2. As can be seen in Fig. 2b, a progressive
increase in Q from 7 to 27 A h l1 for achieving total
decontamination takes place when j increases. However,
the time needed for overall mineralization drops from 7 h
at 33 mA cm2 to 5.5 h at 150 mA cm2. The faster mineralization rate with time when j raises can be ascribed to a
greater production of OH at the Pt anode from reaction
(5) and in the medium from reaction (2) due to the electrogeneration of more H2O2 by the O2-diffusion cathode from
reaction (1) (Brillas et al., 2004a). The increase in Q for
total decontamination under these conditions is indicative
of a slower relative generation of oxidant OH due to the
acceleration of non-oxidizing reactions of this radical, for
example, its oxidation to O2 at the Pt anode and its recombination into H2O2.
The great oxidizing power of the photoelectro-Fenton
method was confirmed by degrading up to 0.56 g l1 (close
to saturation) of 1 at pH 3.0 and at 100 mA cm2. The
TOC–Q plots thus obtained are shown in Fig. 2c. As can
be seen, more than 96% of mineralization is achieved after
Please cite this article as: Ignasi Sirés et al., Degradation of clofibric acid in acidic aqueous medium ..., Chemosphere (2006),
doi:10.1016/j.chemosphere.2006.07.039.
244
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ARTICLE IN PRESS
ȱ
I. Sirés et al. / Chemosphere xxx (2006) xxx–xxx
ȱ
a
100
80
ȱ
60
40
ȱ
20
ȱ
0
b
ȱ
TOC / mg l-1
100
ȱ
80
60
40
ȱ
20
ȱ
300
0
c
250
ȱ
200
150
ȱ
100
50
ȱ
0
0
3
6
9 12 15 18 21 24 27 30 33
-1
Q /A hl
ȱ Fig. 2. Effect of experimental parameters on TOC abatement vs. specific
ȱ
ȱ
ȱ
charge for the treatment of 100 ml of different clofibric acid (1) solutions
with 1.0 mM Fe2+ at 35.0 C by photoelectro-Fenton. In plot (a),
concentration of 1: 179 mg l1; initial solution pH: (d) 2.0; () 3.0; (j)
4.0; (m) 6.0; current density: 100 mA cm2. In plot (b), concentration of 1:
179 mg l1; solution pH: 3.0; current density: (d) 33; () 100; (j)
150 mA cm2. In plot (c), initial concentration of 1: (d) 557 (close to
saturation); (j) 358; () 179; (m) 89 mg l1; solution pH: 3.0; current
density: 100 mA cm2.
1
89 mg l1 of 1, respectively. The drop in Q with decreasing
ȱ clofibric acid concentration could be simply associated with
the presence of lower amount of organics. However, results
ȱ of Fig. 2c evidence the removal of more TOC at a given
time with increasing initial pollutant content. For example,
after 2 h of electrolysis, TOC is reduced by 33, 81, 143 and
ȱ 218 mg l1 starting from 89, 179, 358 and 557 mg l1 of 1,
respectively. Since the same production of OH is expected
ȱ from reactions (2), (3) and (5) in these trials, it seems plausible to consider that its competitive non-oxidizing reactions become slower and more OH concentration can
ȱ then
react with pollutants.
ȱ 3.3. Mineralization current efficiency
ȱ
ȱ
The mineralization of 1 yields carbon dioxide and Cl as
final products. The overall reaction can be written as
follows:
ð6Þ
Taking into account reaction (6) to calculate the theoretical TOC removal, the mineralization current efficiency of
electrolyzed solutions was determined from Eq. (4). The
MCE values thus obtained for the different treatments
reported in Fig. 1a are depicted in Fig. 3a. The efficiency
for both anodic oxidation procedures is very small, reaching a maximum value of 3.3–3.8% at 2 h (Q = 6 A h l1), as
expected from their low oxidation ability. In contrast, this
parameter attains a value of 25% and 23% at the early
stages (20 min) of electro-Fenton and photoelectro-Fenton
processes, respectively. When electrolysis is prolonged, a
dramatic drop in MCE can be observed in Fig. 3a for such
treatments, indicating the generation of products that are
more difficultly oxidized with OH than the initial pollutant. The efficiency for the photoelectro-Fenton method
is clearly higher from 1 h of electrolysis (Q = 3 A h l1),
because it is able to destroy most products, including complexes of Fe3+ with generated carboxylic acids that are stable under electro-Fenton conditions.
For the photoelectro-Fenton treatment at the optimum
pH 3.0, the MCE value always decays with rising current density, as can be seen in Fig. 3b. For example, after
1 h of electrolysis of 179 mg l1 of 1, decreasing efficiencies
of 46% (Q = 1 A h l1), 20% (Q = 3 A h l1) and 14%
(Q = 4.5 A h l1) are found at increasing j values of 33,
100 and 150 mA cm2, respectively. This tendency corroborates the enhancement of parallel non-oxidizing reactions
of OH (e.g., its anodic oxidation to O2 and its recombina30
a
25
20
15
10
1
of 30 A h l
(10 h), 24 A h l
(8 h),
ȱ consumption
18 A h l1 (6 h) and 15 A h l1 (5 h) for 557, 358, 179 and
ȱ
5
C10 H11 ClO3 + 17H2 O ! 10CO2 + Cl + 45Hþ + 44e
120
MCE / %
ȱ
5
0
b
50
40
30
20
10
0
0
3
6
9 12 15 18 21 24 27 30 33
-1
Q /A hl
Fig. 3. Dependence of mineralization current efficiency calculated from
Eq. (4) on specific charge for the degradation of 100 ml of clofibric acid (1)
solutions of pH 3.0 at 35.0 C. Plot (a) corresponds to the different
treatments shown in Fig. 1a at 100 mA cm2. Plot (b) corresponds to the
photoelectro-Fenton treatment of: (d) 557; (j) 358; (s, , n) 179; (m)
89 mg l1 of 1 with 1.0 mM Fe2+ at (s) 33; (d, j, , m) 100; (n)
150 mA cm2.
Please cite this article as: Ignasi Sirés et al., Degradation of clofibric acid in acidic aqueous medium ..., Chemosphere (2006),
doi:10.1016/j.chemosphere.2006.07.039.
245
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ARTICLE IN PRESS
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
200
120
The kinetics of the reaction between 1 and OH generated in the different methods tested was comparatively
studied by electrolyzing 179 mg l1 of this compound at
pH 3.0, at 100 mA cm2 and at 35.0 C. Its concentration
was determined by reversed-phase chromatography, where
it exhibits a well-defined absorption peak with a retention
time (tr) of 7.9 min. As can be seen in Fig. 4a, the concentration of 1 undergoes a similar fall by anodic oxidation
without and with UVA irradiation, disappearing from
the medium in 240 min in both cases. This confirms that
1 is not directly photolyzed by UVA light. Good straight
lines were obtained when the above concentration decays
were fitted to a pseudo-first-order kinetic equation, as
depicted in the inset of Fig. 4a. From this analysis, an average pseudo-first-order rate constant (k) of (4.7 ± 0.1) ·
104 s1 (square regression coefficient, R2 = 0.991) is found
for both anodic oxidation treatments. This behavior suggests the production of a constant concentration of OH
from reaction (5) at the Pt anode during electrolysis.
Fig. 4b shows a much quicker decay of 1 under comparable electro-Fenton and photoelectro-Fenton treatments
of 179 mg l1 of 1 at 100 mA cm2, as expected if the production of oxidant OH from Fenton’s reaction (2) is much
greater than that of reaction (5) at the Pt anode. In both
cases 1 is destroyed at a similar rate, being completely
removed in approximately 7 min. Kinetic analysis of these
data also agrees with a pseudo-first-order reaction of the
metabolite (see inset of Fig. 4b), leading to an average kvalue of (1.35 ± 0.10) · 102 s1 (R2 = 0.996). This behavior indicates a very low generation of OH by reaction (3)
under the action of UVA light.
The concentration–time plots obtained for the photoelectro-Fenton treatment of different metabolite contents
and current densities at pH 3.0 are also presented in
Fig. 4b. As can be seen, the complete removal of 1 at
3
2
1
80
0
40
0
0
400
300
30
120
60
90 120
time / min
180
150
240
300
4
b
500
0
60
600
3
2
1
200
0
100
0
3.4. Clofibric acid decay and kinetic analysis
4
a
160
ln (c0 / c)
ȱ
tion into H2O2) when j raises, yielding a smaller proportion
of this oxidant with ability to destroy pollutants. Fig. 3b
also illustrates that the efficiency for the photoelectroFenton degradation at pH 3.0 and at 100 mA cm2
undergoes a progressive increase with rising metabolite
concentration, indicating the removal of larger amounts
of organics with OH because its non-oxidizing reactions
become slower. Thus, the MCE values at 1 h
(Q = 3 A h l1) are 8.2%, 20%, 32% and 45% for 89, 179,
358 and 557 mg l1 of 1, respectively. Under these conditions, the highest efficiency of 50% is obtained at the beginning (20 min) of the degradation of the more concentrated
solution.
The above results allow concluding that the photoelectro-Fenton method is viable for treating acidic wastewaters
containing clofibric acid up to close saturation at optimum
pH 3.0. This technique becomes more efficient when the
content of this pollutant increases and current density
decreases.
ln (c0 / c)
ȱ
I. Sirés et al. / Chemosphere xxx (2006) xxx–xxx
[clofibric acid] / mg l-1
6
ȱ
0
4
0
2
4
6
8
time / min
8
12
time / min
10
12
16
20
Fig. 4. Clofibric acid (1) decay with electrolysis time at pH 3.0 and at
35.0 C. Plot (a) presents the degradation of 100 ml of a solution with
179 mg l1 of 1 at 100 mA cm2 by (d) anodic oxidation with electrogenerated H2O2; (j) anodic oxidation with electrogenerated H2O2 and
UVA light. Plot (b) shows the treatments by: () electro-Fenton of the
same solution with 1.0 mM Fe2+ at 100 mA cm2; photoelectro-Fenton
of: (d) 557; (j) 358; (s, , n) 179; (m) 89 mg l1 of 1 with 1.0 mM Fe2+
at (s) 33; (d, j, , m) 100; (n) 150 mA cm2. The corresponding kinetic
analysis assuming a pseudo-first-order reaction for 1 is given in the inset
panels.
100 mA cm2 is achieved at longer time when its initial
concentration rises. Thus, it disappears after 3, 7, 12 and
18 min for 89, 179, 358 and 557 mg l1, respectively. Their
kinetics analysis (see the inset of Fig. 4b) gives decreasing
k-values of 3.88 · 102 s1 (R2 = 0.992), 1.26 · 102 s1
(R2 = 0.997), 5.6 · 103 s1 (R2 = 0.996) and 4.3 ·
103 s1 (R2 = 0.995). The decay in k with raising the content of 1 indicates the gradual acceleration of competitive
reactions between OH and by-products, thus enhancing
TOC removal and MEC values as experimentally found
(see Figs. 2c and 3b). Fig. 4b evidences a more rapid decay
of 179 mg l1 of 1 with rising j and their kinetic analysis
shows greater k-values of 6.5 · 103 s1 (R2 = 0.9996),
1.26 · 102 s1 (R2 = 0.997) and 1.81 · 102 s1 (R2 =
0.990) at higher current densities of 33, 100 and
150 mA cm2, respectively. Note that k does not vary
proportionally with j, confirming the reaction of a smaller
proportion of OH with pollutants when j rises, since it is
more quickly wasted by parallel non-oxidizing reactions.
3.5. Identification and evolution of intermediates
A solution of 179 mg l1 of 1 of pH 3.0 was treated by
electro-Fenton at 100 mA cm2 and at 35.0 C for 2 min
Please cite this article as: Ignasi Sirés et al., Degradation of clofibric acid in acidic aqueous medium ..., Chemosphere (2006),
doi:10.1016/j.chemosphere.2006.07.039.
246
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ARTICLE IN PRESS
I. Sirés et al. / Chemosphere xxx (2006) xxx–xxx
ȱ
ȱ
ȱ
5
a
4
3
2
concentration / mg l-1
and the remaining organics were extracted and analyzed by
GC–MS. The MS spectrum displayed peaks related to staȱ ble aromatics such as 4-chlorophenol (2) (m/z = 128 (100,
M+), 130 (33, (M + 2)+)) at tr = 17.0 min, hydroquinone
ȱ (3) (m/z = 108 (100, M+)) at t+r = 21.5 min, 4-chlorocatechol (5) (m/z = 144 (100, M ), 146 (33, (M+2)+)) at
min and p-benzoquinone (6) (m/z = 110 (53,
ȱ tMr =+))18.2
at tr = 4.1 min. In addition, an intense peak ascribed
to a chloro-derivative, with m/z = 214 (12, (M+2)+), 212
ȱ (36, M+), 184 (22), 169 (100) and 144 (49) as main fragmentation, was detected at tr = 14.2 min. Although this
ȱ product was not identified by pure standards, it can be reasonably assigned to a dehydrated species of 2-(4-chloro-2hydroxyphenoxy)-2-methylpropionic acid (8), a hydroxylȱ ated product of 1 that can be transformed into 5 (molecular
peak = 144). The silylated derivatives of 2, 3, 5 and 8 were
ȱ also detected by GC–MS after derivatization with N,O-bis(trimetylsilyl)acetamide.
ȱ Reversed-phase chromatograms of treated solutions
exhibited peaks related to the products 2 at tr = 5.0 min,
5 at tr = 3.1 min and 6 at tr = 2.0 min, along with other
ȱ additional peaks associated with 4-chlororesorcinol (4) at
tr = 2.8 min and 1,2,4-benzenetriol (7) at tr = 1.8 min. All
ȱ these aromatics were unequivocally identified by comparing their tr-values and UV–vis spectra, measured on the
photodiode detector, with those of pure products. Note
ȱ that only 2, 5 and 6 have been previously reported as products of 1 during its electro-Fenton degradation in 0.01 M
ȱ HCl (Oturan et al., 1999).
The evolution of aromatic intermediates during the
treatment
of 179 mg l1 of 1 at pH 3.0 and at 100 mA cm2
ȱ
by anodic oxidation with electrogenerated H2O2 is shown
in Fig. 5a. Under these conditions, all products are poorly
ȱ accumulated and persist during long time, as expected from
the slow removal of 1 in 240 min (see Fig. 4a). Compounds
min, respecȱ 5, 6 and 7 are detected up to 300, 360 and 240
tively, after reaching 4.2, 2.1 and 2.8 mg l1 as maximum
at 30–40 min, whereas 2 and 4 attain ca. 2.5 mg l1 at
ȱ 30
min and disappear after 180 min. In contrast, the same
products are much more quickly formed and destroyed
ȱ under comparable electro-Fenton and photoelectro-Fenton
degradations due to the greater generation of OH from
both cases the prodȱ reaction (2). Fig. 5b shows that in 1
uct 2 is accumulated up to 7.3 mg l at 1 min and persists to 10–12 min, whereas 5 and 6 are formed in smaller
ȱ extent and destroyed in 7 and 10 min, respectively. The
fact that all products show a similar evolution in both
ȱ electro-Fenton and photoelectro-Fenton processes confirms that they are not photolyzed under UVA illumiȱ nation.
Ion-exclusion chromatograms of electrolyzed solutions
showed well-defined peaks ascribed to carboxylic acids
ȱ such as 2-hydroxyisobutyric (9) at tr = 12.6 min, tartronic
(10) at tr = 7.7 min, maleic (11) at tr = 8.1 min, fumaric
ȱ (12) at tr = 16.1 min, formic (13) at tr = 14.0 min and oxalic (14) at tr = 6.6 min. Acids 10–13 come from the oxidation of the aryl moiety of aromatics (Boye et al., 2002;
7
1
0
0
60
120
180
240
300
360
420
8
b
7
6
5
4
3
2
1
0
0
2
4
6
8
time / min
10
12
14
Fig. 5. Evolution of the concentration of aromatic intermediates detected
during the degradation of 100 ml of 179 mg l1 clofibric acid (1) solutions
of pH 3.0 at 100 mA cm2 and at 35.0 C. In plot (a), anodic oxidation
with electrogenerated H2O2. In plot (b), electro-Fenton (hollow symbols)
and photoelectro-Fenton (solid symbols), both with 1.0 mM Fe2+.
Compound: (d, s) 4-chlorophenol (2); (m) 4-chlororesorcinol (4);
(j, h) 4-chlorocatechol (5); (, ) p-benzoquinone (6); (.) 1,2,4-benzenetriol (7).
Brillas et al., 2004a; Sirés et al., 2006), whereas 9 is expected
to be released in the first degradation stages of 1. This was
confirmed from the GC–MS analysis of organics produced
after 2 min of the electro-Fenton treatment of 179 mg l1
of 1 at pH 3.0 and at 100 mA cm2, since the MS spectrum
after derivatization exhibited a peak of the disylilated
derivative of 9 (m/z = 248 (10, M+)) at tr = 10.2 min. The
photoelectro-Fenton treatment of 50 mg l1 of 9 at pH
3.0, at 100 mA cm2 and at 35.0 C corroborated its oxidation to acid 14. This acid can also be generated from the
independent degradation of 10–12 (Sirés et al., 2006). The
production of 13 and 14 as ultimate carboxylic acids was
confirmed by electrolyzing 179 mg l1 of 1 at pH 3.0 and
at 100 mA cm2 under electro-Fenton conditions for 6 h.
The GC–MS analysis after esterification of the remaining
acids with ethanol revealed the presence of an intense peak
corresponding to diethyl oxalate (m/z = 146 (2, M+)) at
tr = 7.9 min, and a very weak peak related to ethyl formate
(m/z = 74 (10, M+)) at tr = 10.5 min.
As can be seen in Fig. 6a for the anodic oxidation treatment with electrogenerated H2O2 of 179 mg l1 of 1 at pH
3.0 and at 100 mA cm2, large amounts of acids 9–14 are
slowly accumulated without apparent degradation, except
for acid 10 that reaches a maximum content of 57 mg l1
at 180 min. After 360 min of electrolysis, 23.8, 25.1,
3.5, 1.7, 11.0 and 14.1 mg l1 of 9, 10, 11, 12, 13 and 14,
Please cite this article as: Ignasi Sirés et al., Degradation of clofibric acid in acidic aqueous medium ..., Chemosphere (2006),
doi:10.1016/j.chemosphere.2006.07.039.
247
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ARTICLE IN PRESS
ȱ
8
I. Sirés et al. / Chemosphere xxx (2006) xxx–xxx
ȱ
that all detected carboxylic acids give 27 mg l1 of soluble
TOC, a value much lower than 59 mg l1 determined for
the final degraded solution (see Fig. 1a). That means that
this solution contains high contents of undetected products, probably hardly oxidizable aromatics.
A very different behavior is found for carboxylic acids in
the electro-Fenton and photoelectro-Fenton processes of
the above solution of 1. These products are rapidly
degraded by both treatments at 100 mA cm2, so that only
the ultimate acid 14 is largely accumulated (see Fig. 6b),
although less than 0.1 mg l1 of acid 13 is also detected
at the end of electro-Fenton. Fig. 6b illustrates that 9
and 13 persist in large extent to ca. 60 min when smaller
amount of OH is produced by photoelectro-Fenton at
33 mA cm2. In both methods complexes of acid 14 with
Fe3+ generated from reaction (2) are expected to be formed
(Zuo and Hoigné, 1992). These Fe3+-oxalato complexes are
difficulty oxidized with OH in electro-Fenton, remaining
ca. 60 mg l1 of 14, corresponding to 16 mg l1 of TOC,
at 360 min (see Fig. 6b). Since the resulting solution contains 21 mg l1 of TOC (see Fig. 1a), one can conclude that
the stable colored chlorinated polyaromatics formed yield
about 5 mg l1 of TOC. In contrast, Fig. 6b shows the
complete mineralization of acid 14 in photoelectro-Fenton,
because Fe3+-oxalato complexes are efficiently photodecarboxylated under the action of UVA light (Zuo and
Hoigné, 1992). The remaining solution TOC (<4 mg l1,
Fig. 1a) can then be ascribed to the stable colored
chlorinated polyaromatics generated during degradation
of 1.
60
ȱ
a
50
40
ȱ
ȱ
ȱ
concentration / mg l-1
ȱ
30
ȱ
ȱ
20
10
0
80
70
60
50
40
30
20
10
0
ȱ
b
0
60
120
180 240
time / min
300
360
420
Fig. 6. Time-course of the concentration of carboxylic acids detected
ȱ under the same conditions as in Fig. 5. In plot (a), anodic oxidation with
ȱ
ȱ
electrogenerated H2O2. In plot (b), electro-Fenton (hollow symbols) and
photoelectro-Fenton (solid symbols), both with 1.0 mM Fe2+. Compound:
() 2-hydroxyisobutyric acid (9); ( ) tartronic acid (10); (m) maleic acid
(11); (j) fumaric acid (12); (.) formic acid (13); (d, s) oxalic acid (14).
Concentrations of 9 and 13 in photoelectro-Fenton were determined at
33 mA cm2.
ȱ respectively,
corresponding to 11.0, 7.5, 1.5, 0.7, 2.9 and
3.8 mg l1 of TOC, are found. This balance indicates
ȱ
OH
O
ȱ
OH
COOH
CHOH
OH
3
OH
ȱ
- Cl–
OH
6
OH
ȱ
COOH
10
O
OH
OH
+
OH
OH
OH
OH
COOH
OH
- Cl–
COOH
11
OH
ȱ
Cl
2
ȱ
CH3
H3 C C COOH
H3 C C COOH
ȱ
Cl
1
ȱ
COOH
OH
ȱ
+
Cl
5
HCOOH
13
Cl
8
OH
OH
9
OH
COOH
OH
CO2
COOH
14
hν
Fe3+
-Fe2+
Fe3+-oxalato
complexes
ȱ
ȱ
12
OH
H3 C C COOH
ȱ
HOOC
OH
CH3
ȱ
+
- Cl–
OH
O
OH
OH
7
OH
OH
CH3
O
Cl
4
OH
Fig. 7. Proposed reaction pathway for clofibric acid (1) degradation in acidic aqueous medium by electro-Fenton and photoelectro-Fenton processes with
Fe2+ as catalyst.
Please cite this article as: Ignasi Sirés et al., Degradation of clofibric acid in acidic aqueous medium ..., Chemosphere (2006),
doi:10.1016/j.chemosphere.2006.07.039.
248
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ARTICLE IN PRESS
ȱ
I. Sirés et al. / Chemosphere xxx (2006) xxx–xxx
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
3.6. Proposed degradation pathway
Fig. 7 presents a plausible pathway for the degradation
of 1 by the electro-Fenton and photoelectro-Fenton processes with Fe2+ as catalyst. The sequence involves all
intermediates detected in this work, including those that
are only identified in anodic oxidation with electrogenerated H2O2 since they are quickly destroyed, without accumulation, in the above methods. Electrogenerated OH is
stated as the main oxidant and the possible parallel oxidation of complexes of Fe3+ with other products containing
OH groups, different from acid 14, is not indicated for sake
of simplicity.
The process is initiated either by the breaking of the
C(1)–O bond of clofibric acid to form the phenol 2 with
loss of acid 9, or the direct hydroxylation on its C(2)-position to give 8. Further parallel attack of OH on the C(4)-,
C(3)- and C(2)-positions of 2 yields the benzenediols 3,
with release of Cl ion, 4 and 5, respectively. Product 5
is also formed from the oxidation of 8 with loss of acid
9. The subsequent hydroxylation with dechlorination of 4
and 5 leads to the benzenetriol 7. This product is also
formed from OH attack on 3, which is oxidized in parallel
to 6. Further degradation of 6 and 7 leads to a mixture of
acids 10, 11, 12 and 13. The latter acid is directly mineralized to CO2, whereas the three former ones are independently transformed into acid 14, which is also generated
from the oxidation of 9. The ultimate carboxylic acid 14
is very slowly converted into CO2 by OH since it forms
very stable Fe3+-oxalato complexes under electro-Fenton
conditions. These species can be photodecarboxylated with
loss of Fe2+ under the action of UVA light (Zuo and
Hoigné, 1992).
4. Conclusions
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
It has been demonstrated that the photoelectro-Fenton
method with Fe2+ and UVA light as catalysts is able to
mineralize more than 96% of 1 in aqueous medium of pH
3.0. Its efficiency rises with increasing metabolite content
and with decreasing j. This procedure is then viable for
treating acidic wastewaters containing this pollutant. In
contrast, the electro-Fenton method only yields about
80% of decontamination due to the formation of products
hardly oxidizable with OH, which is mainly formed from
reaction (2). Comparative treatment by anodic oxidation
with electrogenerated H2O2 confirms that OH is produced
in much smaller extent at the Pt anode from water oxidation. Cl ion is released during mineralization. The decay
of 1 always follows a pseudo-first-order kinetics with similar rate constant for electro-Fenton and photoelectro-Fenton. Aromatic products and generated carboxylic acids
have been identified by GC–MS. From their quantification
by HPLC chromatography, the different oxidation ability
of both methods can be explained from the behavior of
acid 14. This acid forms very stable Fe3+-oxalato complexes under electro-Fenton conditions, which can be effi-
9
ciently photolyzed to CO2 in photoelectro-Fenton under
the action of UVA light.
Acknowledgements
The authors thank the financial support from MEC
(Ministerio de Educación y Ciencia, Spain) under project
CTQ2004-01954/BQU and the grant given to I. Sirés by
DURSI (Departament d’Universitats, Recerca i Societat
de la Informació, Generalitat de Catalunya) to do this
work.
References
Andreozzi, R., Caprio, V., Marotta, R., Radovnikovic, A., 2003.
Ozonation and H2O2/UV treatment of clofibric acid in water: a kinetic
investigation. J. Hazard. Mater. B 103, 233–246.
Boye, B., Dieng, M.M., Brillas, E., 2002. Degradation of herbicide 4chlorophenoxyacetic acid by advanced electrochemical oxidation
methods. Environ. Sci. Technol. 36, 3030–3035.
Brillas, E., Baños, M.A., Camps, S., Arias, C., Cabot, P.L., Garrido, J.A.,
Rodrı́guez, R.M., 2004a. Catalytic effect of Fe2+, Cu2+ and UVA light
on the electrochemical degradation of nitrobenzene using an oxygendiffusion cathode. New J. Chem. 28, 314–322.
Brillas, E., Boye, B., Sirés, I., Garrido, J.A., Rodrı́guez, R.M., Arias, C.,
Cabot, P.L., Comninellis, C., 2004b. Electrochemical destruction of
chlorophenoxy herbicides by anodic oxidation and electro-Fenton
using a boron-doped diamond electrode. Electrochim. Acta 49, 4487–
4496.
Buser, H.R., Muller, M.D., Theobald, N., 1998. Occurrence of the
pharmaceutical drug clofibric acid and the herbicide mecoprop in
various Swiss lakes and in the North Sea. Environ. Sci. Technol. 32,
188–192.
Daughton, C.G., Jones-Lepp, T.L., (Eds.), 2001. Pharmaceuticals and
Personal Care Products in the Environment. Scientific and Regulatory
Issues. ACS Symposium Series, Washington.
Doll, T., Frimmel, F.H., 2004. Kinetic study of photocatalytic degradation of carbamazepine, clofibric acid, iomeprol and iopromide assisted
by different TiO2 materials—determination of intermediates and
reaction pathways. Water Res. 38, 955–964.
Gözmen, B., Oturan, M.A., Oturan, N., Erbatur, O., 2003. Indirect
electrochemical treatment of bisphenol A in water via electrochemically generated Fenton’s reagent. Environ. Sci. Technol. 37, 3716–
3723.
Hanna, K., Chiron, S., Oturan, M.A., 2005. Coupling enhanced water
solubilization with cyclodextrin to indirect electrochemical treatment
for pentachlorophenol contaminated soil remediation. Water Res. 39,
2763–2773.
Heberer, T., 2002. Tracking persistent pharmaceutical residues from
municipal sewage to drinking water. J. Hydrol. 266, 175–189.
Heberer, T., Adam, M., 2004. Transport and attenuation of pharmaceutical residues during artificial groundwater replenishment. Environ.
Chem. 1, 22–25.
Heberer, T., Stan, H.J., 1997. Determination of clofibric acid and N(phenylsulfonyl)-sarcosine in sewage, river, and drinking water. Int. J.
Environ. Anal. Chem. 67, 113–124.
Irmak, S., Yavuz, H.I., Erbatur, O., 2006. Degradation of 4-chloro-2methylphenol in aqueous solution by electro-Fenton and photoelectroFenton processes. Appl. Catal. B: Environ. 63, 243–248.
Kolpin, D.W., Furlong, E.T., Meyer, M.T., Thurman, E.M., Zaugg, S.D.,
Barber, L.B., 2002. Pharmaceuticals, hormones, and other organic
wastewater contaminants in U.S. Streams, 1999–2000: A national
reconnaissance. Environ. Sci. Technol. 36, 1202–1211.
Kümmerer, K. (Ed.), 2001. Pharmaceuticals in the Environment. Sources,
Fate and Risks. Springer, Berlin.
Please cite this article as: Ignasi Sirés et al., Degradation of clofibric acid in acidic aqueous medium ..., Chemosphere (2006),
doi:10.1016/j.chemosphere.2006.07.039.
249
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ARTICLE IN PRESS
ȱ
10
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
I. Sirés et al. / Chemosphere xxx (2006) xxx–xxx
Oturan, M.A., Aaron, J.J., Oturan, N., Pinson, J., 1999. Degradation of
chlorophenoxyacid herbicides in aqueous media, using a novel
electrochemical method. Pestic. Sci. 55, 558–562.
Packer, J.L., Werner, J.J., Latch, D.E., McNeill, K., Arnold, W.A., 2003.
Photochemical fate of pharmaceuticals in the environment: Naproxen,
diclofenac, clofibric acid, and ibuprofen. Aquat. Sci. 65, 342–351.
Sirés, I., Garrido, J.A., Rodrı́guez, R.M., Cabot, P.L., Centellas, F.,
Arias, C., Brillas, E., 2006. Electrochemical degradation of paracetamol from water by catalytic action of Fe2+, Cu2+, and UVA light on
electrogenerated hydrogen peroxide. J. Electrochem. Soc. 153, D1–D9.
Sun, Y., Pignatello, J.J., 1993. Photochemical reactions involved in the
total mineralization of 2,4-D by iron(3+)/hydrogen peroxide/UV.
Environ. Sci. Technol. 27, 304–310.
Tauxe-Wuersch, A., De Alencastro, L.F., Grandjean, D., Tarradellas, J.,
2005. Occurrence of several acidic drugs in sewage treatment plants in
Switzerland and risk assessment. Water Res. 39, 1761–1772.
Ternes, T.A., 1998. Occurrence of drugs in German sewage treatment
plants and rivers. Water Res. 32, 3245–3260.
Ternes, T.A., Meisenheimer, M., McDowell, D., Sacher, F., Brauch, H.J.,
Haist-Gulde, B., Preuss, G., Wilme, U., Zulei-Seibert, N., 2002.
Removal of pharmaceuticals during drinking water treatment. Environ. Sci. Technol. 36, 3855–3863.
Tixier, C., Singer, H.P., Oellers, S., Müller, S.R., 2003. Occurrence and fate
of carbamazepine, clofibric acid, diclofenac, ibuprofen, ketoprofen, and
naproxen in surface waters. Environ. Sci. Technol. 37, 1061–1068.
Ventura, A., Jacquet, G., Bermond, A., Camel, V., 2002. Electrochemical
generation of the Fenton’s reagent: application to atrazine degradation. Water Res. 36, 3517–3522.
Weigel, S., Berger, U., Jensen, E., Kallenborn, R., Thoresen, H.,
Hühnerfuss, H., 2004. Determination of selected pharmaceuticals
and caffeine in sewage and seawater from Tromsø/Norway with
emphasis on ibuprofen and its metabolites. Chemosphere 56, 583–592.
Xie, Y.B., Li, X.Z., 2006. Interactive oxidation of photoelectrocatalysis
and electro-Fenton for azo dye degradation using TiO2–Ti mesh and
reticulated vitreous carbon electrodes. Mater. Chem. Phys. 95, 39–50.
Zuo, Y., Hoigné, J., 1992. Formation of hydrogen peroxide and depletion
of oxalic acid in atmospheric water by photolysis of iron(III)-oxalato
complexes. Environ. Sci. Technol. 26, 1014–1022.
Zwiener, C., Frimmel, F.H., 2000. Oxidative treatment of pharmaceuticals
in water. Water Res. 34, 1881–1885.
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Please cite this article as: Ignasi Sirés et al., Degradation of clofibric acid in acidic aqueous medium ..., Chemosphere (2006),
doi:10.1016/j.chemosphere.2006.07.039.
250
ARTICLEȱ6ȱ/ȱPAPERȱ6
ȱ
Mineralizationȱ ofȱ clofibricȱ acidȱ byȱ electrochemicalȱ advancedȱ
oxidationȱprocessesȱusingȱaȱboronȬdopedȱdiamondȱanodeȱandȱ
Fe2+ȱandȱUVAȱlightȱasȱcatalystsȱ
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱȱ
ȱȱ
Elsevier Editorial System(tm) for Applied Catalysis B:
Environmental
Manuscript Draft
ȱȱ
ȱȱ
ȱȱ
ȱȱ
ȱȱ
Manuscript Number:
Title: Mineralization of clofibric acid by electrochemical advanced oxidation
processes using a boron-doped diamond anode and Fe2+ and UVA light as catalysts
Article Type:
Full Length Article
Keywords: Boron-doped diamond anode; Catalysis; Electro-Fenton; PhotoelectroFenton; Drug mineralization
ȱȱ
Corresponding Author:
ȱȱ
Corresponding Author's Institution:
ȱȱ
First Author:
ȱȱ
ȱȱ
Prof. Enric Brillas, PhD
Universitat de Barcelona
Enric Brillas
Order of Authors: Enric Brillas; Ignasi Sir«s, PhD; Fancesc Centellas, PhD;
Jose A Garrido, PhD; Rosa M Rodriguez, PhD; Conchita Arias, PhD; Pere L Cabot,
PhD
ȱȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
251
PART B –Results and Discussion8. Clofibric Acid
ȱ Cover Letter
ȱ
ȱ
Dear Prof. X. Verykios,
I am sending you our paper entitled: “Mineralization of clofibric acid by electrochemical
ȱ
advanced oxidation processes using a boron-doped diamond anode and Fe2+ and UVA light as
ȱ
catalysts”, co-authored by E. Brillas, I. Sirés, F. Centellas, J.A. Garrido, R.M. Rodríguez, C.
Arias and P.L. Cabot, for its publication in Applied Catalysis B: Environmental.
ȱ
This original paper deals with the degradation of a widely used drug as clofibric acid, a well-
ȱ
known pollutant of the aquatic environment, by electrochemical advanced oxidation processes
ȱ
(EAOPs) such as electro-Fenton and photoelectro-Fenton. In these environmentally friendly
techniques hydrogen peroxide is electrogenerated from an oxygen-diffusion cathode and its
ȱ
reaction with catalytic Fe 2+ produces hydroxyl radical (•OH) as strong oxidant of organic
ȱ
pollutants. The degradation is made using an undivided electrolytic cell with a boron-doped
diamond (BDD) anode that also yields adsorbed hydroxyl radical (BDD( •OH)). In the work the
ȱ
oxidizing ability of both kinds of hydroxyl radicals (•OH and (BDD(•OH)) are compared. Thus,
ȱ
electro-Fenton with 1.0 mM Fe 2+ as catalyst is found as a very efficient method to mineralize
ȱ
rapidly and completely this compound. Nevertheless, the overall mineralization is strongly
enhanced in the photoelectro-Fenton method with UVA irradiation since it photodecomposes
ȱ
Fe 3+ complexes of some products. It should be noted that the use of photoelectro-Fenton with a
BDD anode and Fe2+ and UVA light as catalysts has not been reported previously in the
ȱ
literature. The effect of applied current and clofibric acid concentration on the degradation rate
ȱ
and mineralization current efficiency of such EAOPs is examined to clarify their oxidation
ȱ
power. The kinetics of clofibric acid decay is followed by reversed-phase HPLC
chromatography. Aromatic products are detected by GC-MS and also followed by this technique
ȱ
to discuss the initial reaction pathway of this compound. The quantification of final generated
ȱ
carboxylic acids by ion-exclusion chromatography shows that in photoelectro-Fenton UVA light
enhances the photodegradation of Fe 3+-oxalato complexes, which are also oxidized with
ȱ
BDD(•OH), but not by •OH. Our results show clearly that the photoelectro-Fenton method is the
ȱ
most adequate EAOP for the remediation of wastewaters containing clofibric acid. From these
considerations, we believe that this paper is of general and great interest for researchers in
ȱ
catalytic chemistry and electrochemical treatment of organic pollutants in waters and
ȱ
consequently, it can be published in Applied Catalysis B: Environmental.
ȱ
Sincerely yours,
Prof. E. Brillas
ȱ
ȱ
ȱ
ȱ
ȱ
252
PART B –Results and Discussion8. Clofibric Acid
*ȱ List of Three (3) Potential Reviewers
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Three potential reviewers, excellent specialists in the electrochemical treatment of
wastewaters, are:
- Prof. Cesar Pulgarin, Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of
Chemical Science and Engineering, GGEC, Station 6, CH-1015 Lausanne,
Switzerland, e-mail: [email protected]
- Prof. Oktay Erbatur, Department of Chemistry, Çukurova University, 01330 Balcalı,
Adana, Turkey, e-mail: [email protected]
- Dr. Birame Boye, Istituto di Chimica Fisica ed Elettrochimica, Università degli
Study di Padova, Via Marzolo 14, 35131 Padova, Italy, e-mail: [email protected]
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
253
PART B –Results and Discussion8. Clofibric Acid
*ȱ Manuscript
ȱ
ȱ
ȱ
ȱ
1
Mineralization of clofibric acid
2
advanced
3
diamond anode and Fe2+ and UVA light as catalysts
ȱ
ȱ
oxidation processes
by electrochemical
using a boron-doped
ȱ
4
ȱ
5
Enric Brillas*, Ignasi Sirés, Francesc Centellas, José Antonio Garrido, Rosa María
ȱ
6
Rodríguez, Conchita Arias, Pere-Lluís Cabot
ȱ
7
Laboratori d’Electroquímica dels Materials i del Medi Ambient, Departament de Química Física,
8
Facultat de Química, Universitat de Barcelona, Martí I Franquès 1-11, 08028 Barcelona (Spain)
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
9
10
11
12
13
14
15
ȱ
16
ȱ
17
ȱ
18
ȱ
19
ȱ
ȱ
Paper submitted to be published in Applied Catalysis B: Environmental
20
21
*Corresponding author: Tel.: +34 93 4021223; Fax: +34 93 4021231; e-mail: [email protected]
ȱ
ȱ
ȱ
1
ȱ
ȱ
254
PART B –Results and Discussion8. Clofibric Acid
ȱ 1
Abstract
ȱ 2
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
3
This work shows that aqueous solutions of clofibric acid (2-(4-chlorophenoxy)-2-
4
methylpropionic acid), the bioactive metabolite of various lipid-regulating drugs, up to saturation at
5
pH 3.0 are efficiently and completely degraded by electrochemical advanced oxidation processes
6
such as electro-Fenton and photoelectro-Fenton with Fe2+ and UVA as catalysts using an undivided
7
electrolytic cell with a boron-doped diamond (BDD) anode and an O2-diffusion cathode able to
8
electrogenerate H2O 2. This is feasible in these environmentally friendly methods by the production
9
of oxidant hydroxyl radical at the BDD surface from water oxidation and in the medium from
10
Fenton's reaction between Fe2+ and electrogenerated H2O 2. The degradation process is accelerated
11
in photoelectro-Fenton by additional photolysis of Fe3+ complexes under UVA irradiation.
12
Comparative treatments by anodic oxidation with electrogenerated H2O2, but without Fe 2+, yield
13
much slower decontamination. Chloride ion is released and totally oxidized to chlorine at the BDD
14
surface in all treatments. The decay kinetics of clofibric acid always follows a pseudo-first-order
15
reaction.
16
hydroxyisobutyric, tartronic, maleic, fumaric, formic and oxalic acids, are detected as intermediates.
17
The ultimate product is oxalic acid, which is slowly but progressively oxidized on BDD in anodic
18
oxidation. In electro-Fenton this acid forms Fe3+-oxalato complexes that can also be totally
19
destroyed at the BDD anode, whereas in photoelectro-Fenton the mineralization rate of these
20
complexes is enhanced by its parallel photodecarboxylation with UVA light.
ȱ
4-Chlorophenol,
4-chlorocatechol,
hydroquinone,
p-benzoquinone
and
2-
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
21
22
23
24
25
Keywords: Boron-doped diamond anode; Catalysis; Electro-Fenton; Photoelectro-Fenton; Drug
26
mineralization
ȱ
ȱ
2
ȱ
ȱ
255
PART B –Results and Discussion8. Clofibric Acid
ȱ1
1. Introduction
ȱ2
3
The detection of a large variety of pharmaceutical drugs and metabolites including analgesics,
4
anti-inflammatories, antimicrobials, antiepileptics, beta-blockers, estrogens and lipid regulators as
5
emerging pollutants in waters at concentrations from nanograms to micrograms per litre has been
6
recently documented [1-10]. The main sources of this contamination include emission from
7
production sites, direct disposal of overplus drugs in households, excretion after drug administration
8
to humans and animals, treatments throughout the water in fish and other animal farms and
9
inadequate treatment of manufacturing waste [8]. To avoid the potential adverse health effects of
10
these pollutants on living beings, research efforts are underway to develop efficient oxidation
11
techniques for achieving their total mineralization, i.e. their complete conversion into CO2.
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
12
Clofibric acid (2-(4-chlorophenoxy)-2-methylpropionic acid) is the bioactive metabolite of
13
drugs such as clofibrate, etofibrate and etofyllineclofibrate, widely used as blood lipid regulators
14
because they decrease the plasmatic content of cholesterol and triglycerides [9]. This compound has
15
an estimated environmental persistence of 21 days [10] and has been found up to 10 µg l-1 in
16
sewage treatment plant effluents, rivers, lakes, North Sea, ground waters and drinking waters
17
[1,2,6]. However, it is poorly degraded by ozonation [5,11], H2O 2/UV [11], sunlight and UV
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
18
ȱ
19
ȱ
20
ȱ
21
ȱ
22
ȱ
23
ȱ
24
ȱ
25
photolysis [7] and TiO 2/UV [12], as well as after application of biological and physico-chemical
methods in sewage treatment plants [9]. In previous work [13] we have explored the
electrochemical degradation of clofibric acid solutions in the pH range 2.0-12.0 by means of the
classical method of anodic oxidation with a cell containing either a Pt or boron-doped diamond
(BDD) anode and a stainless steel cathode. Under these conditions, the metabolite solutions were
poorly decontaminated with a Pt anode, whereas the alternative use of BDD yielded their complete
mineralization, but with very low degradation rate and current efficiency. The greater oxidizing
power of BDD compared to Pt is ascribed to its higher O 2-overpotential, which allows the
ȱ
ȱ
ȱ
3
ȱ
ȱ
256
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ 1
ȱ
ȱ 2
ȱ
ȱ 3
ȱ
ȱ 4
ȱ5
ȱ
ȱ6
ȱ
ȱ7
ȱ
ȱ8
ȱ
ȱ9
ȱ
10
ȱ
ȱ
11
ȱ
ȱ
12
ȱ
ȱ
13
ȱ
ȱ
14
ȱ
ȱ
15
ȱ
ȱ 16
ȱ
ȱ 17
ȱ
18
ȱ
ȱ
19
ȱ
ȱ
20
ȱ
ȱ
21
ȱ
ȱ
22
ȱ
ȱ
23
ȱ
24
ȱ
ȱ 25
ȱ
ȱ 26
ȱ
ȱ 27
ȱ
ȱ
ȱ
ȱ
generation of more amount of the strong oxidant hydroxyl radical (BDD(•OH)) adsorbed on its
surface from water oxidation [14-18]:
BDD(H2O) → BDD( •OH) + H+ + e −
(1)
Under these conditions, other weaker oxidants such as peroxodisulfate ion, H2O 2 and O3 at the BDD
anode are also produced [18]. Anodic oxidation with a BDD anode seems a viable technique to
mineralize clofibric acid, but its very low oxidation power prevents its possible application to the
treatment of industrial wastewaters containing this compound. This makes necessary the search of
other potent technologies with higher ability to remove this pollutant from waters.
Recently, powerful indirect electrooxidation methods such as electro-Fenton and photoelectroFenton are being developed for water remediation [19-31]. These electrochemical advanced
oxidation processes (EAOPs) are environmentally friendly technologies based on the continuous
supply of H2O 2 to an acidic contaminated solution from the two-electron reduction of injected O2:
O 2 + 2 H+ + 2 e− → H2O 2
(2)
Reticulated vitreous carbon [19,20], carbon-felt [21,22,24,27,30], activated carbon fibre [28] and
O 2-diffusion [23,25,26,29,31] cathodes are usually employed to reduce efficiently O2 from reaction
(2). In the electro-Fenton process the oxidizing ability of electrogenerated H2O 2 is strongly
enhanced by adding to the solution a small quantity of Fe 2+ to produce hydroxyl radical (•OH) and
Fe3+ from the classical Fenton’s reaction [32]:
Fe 2+ + H2O 2 → Fe3+ + •OH + OH−
(3)
An advantage of this method is that the Fe3+/Fe2+ system is catalytic and reaction (3) is propagated
from Fe 2+ regeneration, mainly by reduction of Fe3+ at the cathode [21]. However, a part of
4
ȱ
ȱ
257
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
1
generated •OH is wasted by non-oxidizing reactions, for example, with Fe 2+ and H2O 2 or its direct
2
recombination to hydrogen peroxide [32,33]:
3
ȱ 4
Fe 2+ + •OH → Fe3+ + OH−
(4)
ȱ 5
H2O 2 + •OH → HO 2• + H2O
(5)
ȱ 6
2 •OH → H2O 2
(6)
ȱ 7
ȱ 8
In the photoelectro-Fenton process, the treated solution is illuminated with UV light, which can also
ȱ 9
act as catalyst to favor: (i) the photodecomposition of complexes of Fe3+ with generated carboxylic
ȱ 10
acids [23,25,30,34] and/or (ii) the regeneration of more Fe2+ with additional production of •OH from
ȱ 11
photoreduction of Fe(OH)2+, the predominant Fe 3+ species in acid medium [32]:
ȱ 12
ȱ
Fe(OH) 2+ + hν → Fe2+ + •OH
13
(7)
14
ȱ 15
This paper reports a comparative study on the degradation of clofibric acid by electro-Fenton
ȱ 16
and photoelectro-Fenton using an undivided electrolytic cell with a BDD anode and an O2-diffusion
ȱ 17
cathode to electrogenerate continuously H2O 2 from reaction (2). Both EAOPs were tested with
ȱ 18
metabolite solutions containing a low content of 0.05 M Na 2SO 4 as background electrolyte and 1.0
ȱ 19
mM Fe 2+ as catalyst at pH 3.0, near the optimum pH of 2.8 for Fenton’s reaction (3) [32]. For these
ȱ 20
methods, organic pollutants are expected to be mainly oxidized by BDD( •OH) and •OH formed
ȱ 21
from reactions (1) and (3), respectively, although parallel reactions with weaker oxidants such as
ȱ 22
electrogenerated H2O 2, as well as peroxodisulfate ion [18], ozone [18] and ferrate ion [35] also
ȱ 23
produced at the BDD anode, are possible in much less extent. Photoelectro-Fenton was performed
ȱ 24
by irradiating the solution with UVA light. Comparative treatments by anodic oxidation without and
ȱ 25
with UVA irradiation were also made to assess the higher oxidation power of electro-Fenton and
ȱ 26
photoelectro-Fenton. The influence of current density and metabolite concentration on the
ȱ
ȱ
5
ȱ
ȱ
258
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
ȱ
1
degradation rate and mineralization current efficiency of these EAOPs was investigated. The decay
2
kinetics of clofibric acid in each method was determined. The evolution of identified aromatic
3
products and carboxylic acids was followed by chromatographic techniques to clarify their
4
pathways in the different oxidation processes.
5
ȱ 6
ȱ 7
2. Experimental
8
ȱ 9
Clofibric acid, 4-chlorophenol, hydroquinone, p-benzoquinone, 2-hydroxyisobutyric acid,
ȱ 10
tartronic acid, maleic acid, fumaric acid, formic acid and oxalic acid were either reagent or
ȱ 11
analytical grade from Sigma-Aldrich, Merck, Panreac and Avocado. 4-Chlorocatechol was
ȱ 12
synthesized by chlorination of pyrocatechol with SO2Cl2 [23]. Anhydrous sodium sulfate and
ȱ 13
heptahydrated ferrous sulfate were analytical grade from Fluka. Solutions were prepared with high-
ȱ 14
purity water obtained from a Millipore Milli-Q system (resistivity > 18 MΩ cm at 25 ºC) and their
ȱ
pH was adjusted to 3.0 with analytical grade sulfuric acid from Merck. Other chemicals and organic
15
ȱ 16
solvents were either HPLC or analytical grade from Panreac.
ȱ 17
The solution pH was determined with a Crison 2000 pH-meter. Aliquots withdrawn from treated
ȱ 18
solutions were filtered with Whatman 0.45 µm PTFE filters before analysis. The degradation of
ȱ 19
clofibric acid solutions was monitored from the removal of their total organic carbon (TOC),
ȱ 20
measured on a Shimadzu VCSN TOC analyzer. Reproducible values were obtained using the
ȱ 21
standard non-purgeable organic carbon method. From these results, the mineralization current
ȱ 22
efficiency (MCE) for each treated solution at a given electrolysis time was calculated from the
ȱ 23
following equation:
ȱ 24
ȱ 25
ȱ
26
MCE =
∆(TOC) exp
∆(TOC) theor
x 100
(8)
ȱ
ȱ
6
ȱ
ȱ
259
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
1
where ∆(TOC) exp is the experimental TOC decay and ∆(TOC) theor is the theoretically calculated
2
TOC removal assuming that the applied electrical charge (= current x time) is only consumed in the
3
mineralization process of clofibric acid.
4
The concentration of chloride ion in treated solutions was determined by ion chromatography
5
with a Shimadzu 10Avp HPLC chromatograph fitted with a Shim-Pack IC-A1S, 100 mm x 4.6 mm
6
(i. d.), anion column at 40 ºC and coupled with a Shimadzu CDD 10Avp conductivity detector. A
7
mixture of 2.5 mM phtalic acid and 2.4 mM tris(hydroxymethyl)aminomethane) of pH 4.0 at 1.5 ml
8
min -1 was used as mobile phase for this analysis. Aromatic products were identified by gas
ȱ 9
chromatography-mass spectrometry (GC-MS) using a Hewlett-Packard 5890 Series II gas
ȱ 10
chromatograph fitted with a HP-5 0.25 µm, 30 m x 0.25 mm (i. d.), column, and a Hewlett-Packard
ȱ 11
5989A mass spectrophotometer operating in EI mode at 70 eV and 290 ºC. The metabolite decay
ȱ 12
and the time-course of its aromatic products were followed by reversed-phase HPLC
ȱ 13
chromatography using a Waters 600 high-performance liquid chromatograph fitted with a
ȱ 14
Spherisorb ODS2 5 µm, 150 mm x 4.6 mm (i. d.), column at room temperature, coupled with a
ȱ 15
Waters 996 photodiode array detector, and circulating a 50:47:3 (v/v/v) methanol/phosphate buffer
ȱ 16
(pH = 2.5)/pentanol mixture at 1.0 ml min -1 as mobile phase. For each product, this detector was
ȱ 17
selected at the maximum wavelength of its UV-absorption band. Carboxylic acids were identified
ȱ 18
by ion-exclusion chromatography using the above HPLC chromatograph fitted with a Bio-Rad
ȱ 19
Aminex HPX 87H, 300 mm x 7.8 mm (i. d.), column at 35 ºC. For these measurements, the
ȱ 20
photodiode detector was selected at 210 nm and the mobile phase was 4 mM H2SO 4 at 0.6 ml min -1.
ȱ 21
All electrolyses were conducted in an open, cylindrical, undivided and thermostated cell
ȱ 22
containing 100 ml of solution vigorously stirred with a magnetic bar. The anode was a 3-cm2 BDD
ȱ 23
thin-film deposited on conductive single crystal p-type Si (100) wafers from CSEM and the cathode
ȱ 24
was a 3-cm2 carbon-PTFE electrode from E-TEK, which was fed with pure O2 at 12 ml min -1 to
ȱ 25
generate continuously H2O 2 from reaction (2). The setup of the electrolytic system and the
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
7
ȱ
ȱ
260
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
1
characteristics of the O 2-diffusion cathode have been described elsewhere [23,25]. Experiments
2
were made at a constant current density (j) of 33, 100 and 150 mA cm-2, supplied by an Amel 2053
3
potentiostat-galvanostat. Electro-Fenton and photoelectro-Fenton treatments were carried out with
4
solutions containing 0.05 M Na 2SO 4 as background electrolyte and 1.0 mM Fe 2+ as catalyst of pH
5
3.0 at 35.0 ºC, which were found as optimum conditions for the degradation of other aromatics in
6
the cell used [23,25]. The latter method became operative when the solution was irradiated with
7
UVA light of λ max = 360 nm emitted by a Philips 6-W fluorescent black light blue tube, yielding a
8
photoionization energy input to the solution of 140 µW cm-2, as detected with a NRC 820 laser
9
power meter working at 514 nm. Comparative anodic oxidation treatments without catalyst Fe 2+
10
were performed in the absence and presence of UVA irradiation at 100 mA cm-2.
11
ȱ 12
ȱ 13
3. Results and discussion
14
ȱ
15
3.1. Comparative degradation of clofibric acid
ȱ 16
Comparative treatments were made for solutions containing 179 mg l-1 clofibric acid (equivalent
ȱ 17
to 100 mg l -1 TOC) of pH 3.0 at 100 mA cm-2. In these trials the solution pH did not practically
ȱ 18
vary, reaching final values of 2.8-2.9. The change in solution TOC with applied specific charge (Q,
ȱ 19
in A h l-1) for anodic oxidation without and with UVA irradiation, electro-Fenton and photoelectro-
ȱ 20
Fenton is depicted in Fig. 1. As can be seen, total degradation (> 97% TOC removal) is attained in
ȱ 21
all cases, although the time required for overall mineralization depends on the method tested. Both
ȱ 22
anodic oxidation methods lead to a slow, but similar, TOC decay up to yield total mineralization at
ȱ 23
Q = 18 A h l -1, i.e., after 6 h of both treatments. This behavior indicates that all organics are
ȱ 24
destroyed by the oxidant BDD(•OH) formed at the anode surface from reaction (1), without
ȱ 25
significant photodecomposition by UVA light, at least of final products. Fig. 1 evidences that the
ȱ 26
degradation rate (the change of TOC with time) is strongly enhanced using both EAOPs due to the
ȱ
ȱ
8
ȱ
ȱ
261
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
1
catalytic action of the Fe 3+/Fe 2+ system combined with UVA light when the solution is
2
simultaneously irradiated. The significant acceleration of the destruction of organic pollutants in the
3
early stages of the electro-Fenton process can be explained by their quicker reaction with the great
4
amount of •OH formed from Fenton’s reaction (3). For this EAOP, however, the rate in TOC decay
5
gradually falls at longer electrolysis time, probably due to the formation of complexes of Fe3+ with
6
final carboxylic acids that are hardly oxidized, and the solution is decontaminated after about 6 h of
7
electrolysis, that is, at similar time to that needed for both anodic oxidation treatments. In contrast,
8
TOC is much more rapidly removed by photoelectro-Fenton, where total mineralization is achieved
9
at Q = 12 A h l-1 (4 h). The increase in mineralization rate in photoelectro-Fenton can be related to:
10
(i) the parallel photodegradation of complexes of Fe3+ with final carboxylic acids and/or (ii) the
11
enhanced generation of •OH due to additional photoreduction of Fe(OH)2+ from reaction (7).
12
The above comparative study shows that photoelectro-Fenton is the method with highest
13
oxidation power, then being the best EAOP for the treatment of wastewaters containing clofibric
14
acid. Electro-Fenton also yields much faster degradation than anodic oxidation, but its oxidation
15
power drops significantly at the end of electrolysis due to the very slow destruction of final
16
products, which is strongly enhanced by UVA light in photoelectro-Fenton.
17
The influence of current density and clofibric acid concentration on the oxidizing ability of the
18
above electro-Fenton and photoelectro-Fenton processes was explored. It was found that these
19
experimental parameters showed the same trends in both EAOPs, as expected if they mainly affect
20
the behavior of the electrolytic system. These effects are depicted in Figs. 2a and 2b for electro-
21
Fenton in which they were more clearly observed due to its lower oxidation power. Thus, Fig. 2a
ȱ 22
shows that when j increases from 33 to 150 mA cm-2, the specific charge for total decontamination
ȱ 23
of 179 mg l-1 of clofibric acid rises from 12 to 22 A h l-1, but the time needed for overall
ȱ 24
mineralization drops from 12 to about 5 h since the degradation rate is strongly enhanced. This
ȱ 25
latter tendency can be accounted for by the faster destruction of all pollutants due to the greater
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
9
ȱ
ȱ
262
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
1
production of BDD(•OH) from reaction (1) and of •OH from Fenton’s reaction (3) as j increases,
2
because more H2O 2 is generated at the O 2-diffusion cathode from reaction (1) [25] and then, its
3
reaction with Fe 2+ becomes faster. However, the increase in Q for total decontamination when j
4
rises suggests a lower amount of reactive BDD( •OH) and •OH. That means that a higher proportion
5
of both oxidants is progressively wasted by their non-oxidizing reactions since they take place in
6
larger extent. These reactions involve, for example, the anodic oxidation of BDD(•OH) to O 2 and
7
reactions (4)-(6) for •OH. Moreover, increasing j can accelerate the formation of weaker oxidants
8
such as peroxodisulfate ion and ozone [18] that also reduces the relative proportion of BDD( •OH)
9
adsorbed at the anode. On the other hand, Fig. 2b shows that at 100 mA cm-2 overall mineralization
10
is achieved with decreasing consumption of 24 A h l-1 (8 h), 21 A h l-1 (7 h), 18 A h l-1 (6 h) and 12
11
A h l-1 (4 h) starting from 557 (close to saturation), 358, 179 and 89 mg l-1 of the metabolite,
12
respectively, as expected if lower amount of organic matter is destroyed in solution. These results
13
also evidence the removal of more TOC at a given time with rising initial pollutant content. As an
14
example, at 2 h of electrolysis (Q = 6 A h l-1) the TOC of the above solutions is reduced by 231,
15
150, 70 and 39 mg l-1. Since the same quantity of BDD(•OH) and •OH is expected to be produced
16
from reactions (1) and (3) in these trials carried out at 100 mA cm-2, it can be assumed that their
17
parallel non-oxidizing reactions occur in less proportion with rising metabolite concentration. This
18
favors the reaction of more amounts of both kinds of hydroxyl radicals with organics, thus raising
19
the degradation rate of the process. All these findings allow establishing that the oxidation power of
20
EAOPs, corresponding to their degradation rate, increases with increasing current density and initial
21
substrate concentration.
22
ȱ 23
3.2. Mineralization current efficiency
ȱ 24
It is well-known that reaction of chloroaromatics with hydroxyl radical leads to the release of
ȱ 25
chloride ion [16,23,26]. This point was confirmed for clofibric acid by recording the ion
ȱ 26
chromatograms of all treated solutions, which only displayed a defined peak at a retention time (tr)
ȱ
10
ȱ
ȱ
263
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
1
of 2.3 min related to Cl− ion. The formation of ClO 3− and ClO 4− ions was discarded since they were
2
not detected in these chromatograms. The evolution of Cl− concentration during the degradation of
3
179 mg l-1 of metabolite by anodic oxidation with electrogenerated H2O 2 and the two EAOPs at 100
4
mA cm-2 is presented in Fig. 3. As can be seen, this ion is accumulated and completely removed in
5
300-360 min in all cases, after reaching a maximum concentration of about 8 mg l-1 at 180 min of
6
anodic oxidation, 23 mg l-1 at 20 min of electro-Fenton and 19 mg l-1 at 40 min of photoelectro-
7
Fenton, corresponding to 27%, 78% and 64% of the initial chlorine content in solution (29.5 mg l-1).
8
The slow accumulation of Cl− in the former method confirms the slow reaction of chloro-organics
9
with BDD(•OH), whereas its much faster release at the early stages of both EAOPs corroborates the
10
quick destruction of these pollutants with •OH. The gradual destruction of this ion when electrolysis
11
is prolonged can be explained by its slow oxidation to Cl2 on BDD, as reported by Kraft et al. [16].
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
12
13
The above findings allow concluding that the mineralization of clofibric acid involves its
conversion into CO 2 and Cl− as primary ion. This reaction can be written as follows:
14
C10H11ClO 3 + 17 H2O → 10 CO 2 + Cl− + 45 H+ + 44 e−
15
(9)
ȱ 16
ȱ 17
Reaction (9) was then used to calculate the value of ∆(TOC)theor for each treated solution at chosen
ȱ 18
electrolysis times and from these data, the corresponding efficiency by means of Eq. 8.
ȱ 19
Fig. 4a presents the MCE values determined for the trials reported in Fig. 1. An increase in
ȱ 20
efficiency with increasing the oxidation power of the method can be observed. Thus, the two less
ȱ 21
potent anodic oxidation processes possess a similar, small and practically constant MCE value of
ȱ 22
about 7%, suggesting that most organics are mineralized at the same rate by BDD( •OH) along
ȱ 23
electrolysis without significant role of UVA light. In contrast, this parameter attains a much higher
ȱ 24
value for electro-Fenton and photoelectro-Fenton, although the latter procedure with highest
ȱ 25
oxidation power is the most efficient because of the parallel photodecomposition of some final
ȱ 26
products. Note that 33% and 35% efficiencies are found after 20 min of these EAOPs, respectively,
ȱ
11
ȱ
ȱ
264
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
1
confirming that organics are more quickly mineralized with •OH than with BDD( •OH). At longer
2
time, a dramatic drop in MCE takes place in both cases due to the generation of final products such
3
as complexes of Fe 3+ with carboxylic acids that are more difficultly destroyed by both oxidants
4
and/or UVA light.
5
Fig. 4b illustrates the effect of current density and clofibric acid concentration reported in Figs.
6
2a and 2b on the efficiency of EAOPs as a function of specific charge. A gradual drop in MCE, at
7
least up to 6 A h l-1, can be observed when j increases from 33 to 150 mA cm-2. This trend could
8
seem contradictory to the fact that rising j causes the increase in degradation rate due to the
9
production of more amounts of reactive BDD( •OH) and •OH, as pointed out above. The
10
concomitant loss in efficiency under these conditions can be associated with the larger waste of
11
both oxidants in faster parallel non-oxidizing reactions giving rise to lower amounts of them with
12
ability to destroy organics and hence, favoring the consumption of more ineffective specific charge.
13
Fig. 4b also evidences a gradual increase in efficiency of EAOPs with rising metabolite
14
concentration, in agreement with the higher degradation rate found in these trials. This confirms the
15
removal of greater amounts of pollutants with BDD( •OH) and •OH, because their competitive non-
16
oxidizing reactions become less significant.
17
ȱ 18
3.3. Kinetics of clofibric acid decay
ȱ 19
The decay of the metabolite in the different electrochemical methods was followed by reversed-
ȱ 20
phase HPLC chromatography, where it exhibited a well-defined peak at tr = 7.9 min. A previous
ȱ 21
experiment carried out by adding 20 mM H2O 2 to a 179 mg l-1 clofibric acid solution of pH 3.0
ȱ 22
showed that the content of this compound remained unchanged, indicating that it can not react
ȱ 23
directly with electrogenerated H2O 2 in the electrolytic systems.
ȱ 24
The comparative kinetics of the removal of clofibric acid with generated strong oxidizing agents
ȱ 25
(mainly BDD( •OH) and/or •OH) was determined from the treatment of 179 mg l-1 metabolite
ȱ
ȱ
12
ȱ
ȱ
265
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
1
solutions at 100 mA cm-2. Fig. 5a shows that clofibric acid concentration undergoes a similar fall by
2
anodic oxidation without and with UVA illumination, disappearing in 360 min in both cases, a time
3
similar to that needed for its total mineralization (see Fig. 1). This confirms that this compound is
4
mainly oxidized by BDD( •OH) from reaction (1), without direct photolysis by UVA light. The
5
above concentration decays were well-fitted to a pseudo-first-order equation, as can be seen in the
6
inset panel of Fig. 5a. From this kinetic analysis, an average pseudo-first-order rate constant (k) of
7
(1.70±0.13)x10 -4 s -1 (square regression coefficient (R2) = 0.992) is found for both anodic oxidation
8
treatments. This behavior suggests that a steady BDD(•OH) concentration reacts with the metabolite
9
along electrolysis.
10
On the other hand, Fig. 5b evidences a much quicker and similar abatement of the metabolite
11
under comparable electro-Fenton and photoelectro-Fenton treatments, being completely removed in
12
7 min, as expected if it reacts with a much greater amount of oxidant •OH formed from Fenton’s
13
reaction (3). The inset panel of Fig. 5b shows that the kinetic analysis of these data also agrees with
14
a pseudo-first-order reaction, giving the same k-value of 1.35x10-2 s-1 (R2 = 0.993). This allows
15
concluding that •OH is produced in insignificant quantity by reaction (7) under UVA irradiation.
16
The effect of current density on the decay kinetics of this compound was further explored for
17
the electro-Fenton treatment. As can be seen in Fig. 5b, increasing k-values of 5.10x10-3 s-1 (R 2 =
18
0.991), 1.35x10-2 s-1 (R2 = 0.993) and 2.04x10-2 s-1 (R2 = 0.992) are found for j values of 33, 100
19
and 150 mA cm-2, respectively. This trend confirms a higher •OH production in the medium from
20
Fenton’s reaction (3) when j rises, due to the concomitant accumulation of more electrogenerated
21
H2O 2 from reaction (2) [25].
22
ȱ 23
3.4. Identification and evolution of intermediates
ȱ 24
A 179 mg l-1 clofibric acid solution of pH 3.0 was treated by electro-Fenton at 100 mA cm-2 for
ȱ 25
2 min and its organic components were extracted with 45 ml of CH2Cl 2 in three times. The collected
ȱ
ȱ
13
ȱ
ȱ
266
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
1
organic solution was dried with Na 2SO 4, filtered and its volume reduced to 2 ml to concentrate the
2
remaining aromatics to be analyzed by GC-MS. The MS spectrum showed peaks related to stable
3
aromatics such as 4-chlorophenol (m/z = 128 (100, M+), 130 (33, (M+2) +)) at tr = 17.0 min,
4
hydroquinone (m/z = 108 (100, M+)) at t r = 21.5 min, 4-chlorocatechol (m/z = 144 (100, M+), 146
5
(33, (M+2)+)) at t r = 18.2 min and p-benzoquinone (m/z = 110 (53, M+)) at tr = 4.1 min. These
6
products were confirmed in the reversed-phase HPLC chromatograms of electrolyzed solutions,
7
which exhibited well-defined peaks corresponding to 4-chlorophenol at t r = 5.0 min, 4-
8
chlorocatecol at tr = 3.1 min and p-benzoquinone at tr = 2.0 min. These peaks were unequivocally
9
identified by comparing their t r-values and UV-Vis spectra, measured on the photodiode detector,
10
with those of pure compounds. However, only traces of hydroquinone were detected by this
11
technique in all cases, as expected if it is very quickly converted into p-benzoquinone by all
12
oxidizing agents.
13
The evolution of aromatic intermediates during the different treatments of 179 mg l-1 metabolite
14
solutions at 100 mA cm-2 is presented in Fig. 6. As can be seen in Fig. 6a, 4-chlorophenol is largely
15
produced in all cases and persists long time, up to 360 min, in both anodic oxidation processes, but
16
it is removed very rapidly, for 7-8 min, in electro-Fenton and photoelectro-Fenton. Comparison of
17
results of Figs. 6a and 5 evidences that in each method this primary product disappears at the same
18
time as the initial pollutant. In contrast, Fig. 6b shows that 4-chlorocatechol is accumulated in much
19
smaller extent in the two latter EAOPs, disappearing in 7 min. The same removal time is found for
20
p-benzoquinone in electro-Fenton and photoelectro-Fenton, although it persists for 60 and 360 min
21
in anodic oxidation with and without UVA irradiation, respectively (see Fig. 6c). These findings
22
suggest the parallel quick photolysis of p-benzoquinone by UVA light, which it is not observed in
23
photoelectro-Fenton because it reacts much more quickly with •OH.
24
From the above results, a general reaction sequence for the initial degradation of clofibric acid is
25
proposed in Fig. 7, where pollutants can react with BDD(•OH) formed at the anode surface from
ȱ
ȱ
14
ȱ
ȱ
267
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
1
reaction (1) and/or with •OH produced from Fenton’s reaction (3) in the medium. The process is
2
initiated by the breaking of the C(1)-O bond of clofibric acid by both oxidants to yield 4-
3
chlorophenol and 2-hydroxyisobutyric acid as primary products. Further attack of BDD( •OH) and
4
•
5
oxidized to p-benzoquinone. Parallel hydroxylation of 4-chlorophenol only by attack of •OH on its
6
C(2)-position leads to 4-chlorocatechol. The subsequent oxidation of the latter product, with release
7
of Cl−, and p-benzoquinone (not shown in Fig. 7) can cause the opening of their benzenic rings to
8
yield different carboxylic acids. The formation of such products was confirmed from analysis of
9
degraded solutions by ion-exclusion HPLC chromatography.
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
OH on the C(4)-position of 4-chlorophenol gives hydroquinone, with loss of Cl− ion, which is then
10
Ion-exclusion chromatograms of solutions treated by the two anodic oxidation methods
11
displayed peaks ascribed to small contents of generated carboxylic acids such as 2-
12
hydroxyisobutyric at tr = 12.6 min, tartronic at tr = 7.7 min, maleic at tr = 8.1 min, fumaric at tr =
13
16.1 min, formic at tr = 14.0 min and oxalic at tr = 6.6 min. Tartronic, maleic, fumaric and formic
14
acids come from the oxidation of the aryl moiety of aromatics [26,29,31], whereas 2-
15
hydroxyisobutyric acid is expected to be released in the early stages of the degradation process
16
when 4-chlorophenol is formed (see Fig. 7). All these acids, except oxalic acid, were undetected or
17
detected as traces for short time in electro-Fenton and photoelectro-Fenton. Oxalic acid was
18
accumulated in large extent and persisted up to the end of the mineralization in both processes. This
19
ultimate acid formed from the independent oxidation of the precedent longer-chain carboxylic
20
acids, as well as formic acid, are directly converted into CO2 [17,29,31].
21
Fig. 6d presents the time-course of oxalic acid concentration during all treatments. In both
22
anodic oxidation methods this acid is formed and destroyed at similar rate, reaching 5-6 mg l-1 as
23
maximum at 180 min and disappearing in 360 min, just when the initial substrate is completely
24
removed (see Fig. 5a) and the solution is totally decontaminated (see Fig. 1). This confirms the
25
simultaneous destruction of clofibric acid and most of its products with BDD( •OH) in these
ȱ
15
ȱ
ȱ
268
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
1
processes, in agreement with the constant efficiency found during degradation (see Fig. 4a). In
2
contrast, oxalic acid reaches high contents of 68 and 59 mg l-1 after 40 min of electro-Fenton and
3
photoelectro-Fenton, respectively, due to the very quick oxidation of precedent organics with •OH
4
formed from Fenton’s reaction (3). Nevertheless, it is completely removed in 240 min by
5
photoelectro-Fenton, just when the solution is totally decontaminated (see Fig. 1), still remaining
6
about 6 mg l-1 (less than 1.5 mg l-1 of TOC) in solution after 360 min of electro-Fenton. Since in
7
these EAOPs a large amount of Fe3+ is formed in the medium from reactions (3) and (4), oxalic acid
8
is really expected to be present in the form of Fe3+-oxalato complexes, which can not be oxidized by
9
•
OH in the medium [23,29,33]. Our results indicate that these complexes are slowly mineralized in
10
electro-Fenton with a BDD anode and even more quickly photodecomposed by UVA irradiation in
11
photoelectro-Fenton.
12
According to these considerations, Fig. 8 shows a proposed degradation pathway for oxalic acid
13
under the present experimental conditions. This acid is oxidized to CO2 with BDD(•OH) at the
14
anode surface either directly in both anodic oxidation treatments or as Fe3+-oxalato complexes in
15
electro-Fenton. The latter complexes also undergo a parallel quick photodecarboxylation under the
16
action of UVA light in photoelectro-Fenton, with regeneration of Fe 2+ as proposed by Zuo and
17
Hoigné [34]. This photolytic reaction explains the fastest degradation rate and highest efficiency of
18
photoelectro-Fenton. The fact that oxalic acid is still detected after 6 h of electro-Fenton, while it is
19
removed at the same time for anodic oxidation, suggests a slower reaction of BDD(•OH) with its
20
Fe3+ complexes that causes the decay in oxidation power of this EAOP at long electrolysis time.
21
ȱ 22
ȱ 23
ȱ
ȱ
ȱ
4. Conclusions
24
25
It is demonstrated that EAOPs such as electro-Fenton with Fe2+ and photoelectro-Fenton with
26
Fe2+ and UVA light, both with a BDD anode, yield an efficient and complete degradation of
ȱ
16
ȱ
ȱ
269
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
1
aqueous solutions of clofibric acid up to saturation at pH 3.0. The efficiency of both methods
2
increases with rising metabolite concentration and with decreasing current density. Comparative
3
treatments by anodic oxidation are much slower, confirming the high production of •OH from
4
Fenton’s reaction (3) in the above EAOPs. In all methods Cl− is released and totally oxidized to Cl2
5
on BDD. The clofibric acid decay always follows a pseudo-first-order kinetics. This compound is
6
hydroxylated to yield 4-chlorophenol, which is further oxidized either to p-benzoquinone via
7
hydroquinone or to 4-chlorocatechol. These products are subsequently degraded to tartronic, maleic
8
and fumaric acids, which are quickly converted into oxalic acid. The latter acid is also obtained
9
from the oxidation of 2-hydroxyisobutyric acid, initially generated when 4-chlorophenol is formed.
10
Formic acid also generated in the degradation path is rapidly converted into CO2. The ultimate
11
product oxalic acid is then transformed into CO 2 on BDD either directly in anodic oxidation or as
12
Fe3+-oxalato complexes in electro-Fenton. The parallel quick photolysis of these complexes by
13
UVA light in photoelectro-Fenton explains the fastest degradation rate and highest efficiency of this
14
method, which appears to be the best EAOP for the treatment of wastewaters containing clofibric
15
acid.
16
ȱ 17
ȱ 18
ȱ
ȱ
ȱ
Acknowledgements
19
20
Financial support from MEC (Ministerio de Educación y Ciencia, Spain) under project
21
CTQ2004-01954/BQU and the grant awarded to I. Sirés from DURSI (Departament d’Universitats,
22
Recerca i Societat de la Informació, Generalitat de Catalunya) to do this work are acknowledged.
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
17
ȱ
ȱ
270
PART B –Results and Discussion8. Clofibric Acid
ȱ
1
References
ȱ 2
ȱ 3
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
[1] T. Heberer, H.J. Stan, Int. J. Environ. Anal. Chem. 67 (1997) 113-124.
4
[2] H.R. Buser, M.D. Muller, N. Theobald, Environ. Sci. Technol. 32 (1998) 188-192.
5
[3] K.Kümmerer, (Ed.), Pharmaceuticals in the Environment. Sources, Fate and Risks, Springer,
6
7
8
9
10
Berlin, 2001.
[4] D.W. Kolpin, E.T. Furlong, M.T. Meyer, E.M. Thurman, S.D. Zaugg, L.B. Barber, Environ.
Sci. Technol. 36 (2002) 1202-1211.
[5] T.A. Ternes, M. Meisenheimer, D. McDowell, F. Sacher, H.J. Brauch, B. Haist-Gulde, G.
Preuss, U. Wilme, N. Zulei-Seibert, Environ. Sci. Technol. 36 (2002) 3855-3863.
11
[6] C. Tixier, H.P. Singer, S. Oellers, S.R. Müller, Environ. Sci. Technol. 37 (2003) 1061-1068.
12
[7] J.L. Packer, J.J. Werner, D.E. Latch, K. McNeill, W.A. Arnold, Aq. Sci. 65 (2003) 342-351.
13
[8] J.P. Bound, N. Vaulvaulis, Chemosphere 56 (2004) 1143-1155.
14
[9] A. Tauxe-Wuersch, L.F. De Alencastro, D. Grandjean, J. Tarradellas, Water Res. 39 (2005)
15
1761-1772.
16
[10] J.P. Emblidge, M.E. DeLorenzo, Environ. Res. 100 (2006) 216-226.
17
[11] R. Andreozzi, V. Caprio, R. Marotta, A. Radovnikovic, J. Hazard. Mat. B103 (2003) 233-246.
18
[12] T. Doll, F.H. Frimmel, Water Res. 38 (2004) 955-964.
19
[13] I. Sirés, P.L. Cabot, F. Centellas, J.A. Garrido, R.M. Rodríguez, C. Arias, E. Brillas,
20
21
22
23
24
Electrochim. Acta 52 (2006) 75-85.
[14] M. Panizza, P.A. Michaud, G. Cerisola, Ch. Comninellis, J. Electroanal. Chem. 507 (2001)
206-214.
[15] B. Marselli, J. García-Gomez, P.A. Michaud, M.A. Rodrigo, Ch. Comninellis, J. Electrochem.
Soc. 150 (2003) D79-D83.
25
[16] A. Kraft, M. Stadelmann, M. Blaschke, J. Hazard. Mat. B 103 (2003) 247-261.
26
[17] C.A. Martinez-Huitle, S. Ferro, A. De Battisti, Electrochim. Acta 49 (2004) 4027-4034.
ȱ
ȱ
18
ȱ
ȱ
271
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
1
[18] M. Panizza, G. Cerisola, Electrochim. Acta 51 (2005) 191-199.
2
[19] A. Alverez-Gallegos, D. Pletcher, Electrochim. Acta 44 (1999) 2483-2492.
3
[20] T. Harrington, D. Pletcher, J. Electrochem. Soc. 146 (1999) 2983-2989.
4
[21] M.A. Oturan, J.J. Aaron, N. Oturan, J. Pinson, Pestic. Sci. 55 (1999) 558-562.
5
[22] M. A. Oturan, J. Appl. Electrochem. 30 (2000) 475-482.
6
[23] B. Boye, M.M. Dieng, E. Brillas, Environ. Sci. Technol. 36 (2002) 3030-3035.
7
[24] B. Gözmen, M.A. Oturan, N. Oturan, O. Erbatur, Environ. Sci. Technol. 37 (2003) 3716-3723.
8
[25] E. Brillas, M.A. Baños, S. Camps, C. Arias, P.L. Cabot, J.A. Garrido, R.M. Rodríguez, New J.
9
10
ȱ
11
ȱ
12
Chem. 28 (2004) 314-322.
[26] E. Brillas, B. Boye, I. Sirés, J.A. Garrido, R.M. Rodríguez, C. Arias, P.L. Cabot, Ch.
Comninellis, Electrochim. Acta 49 (2004) 4487-4496.
[27] K. Hanna, S. Chiron, M.A. Oturan, Water Res. 39 (2005) 2763-2773.
ȱ 13
[28] A. Wang, J. Qu, J. Ru, H. Liu, J. Ge, Dyes Pigments 65 (2005) 227-233.
ȱ 14
[29] I. Sirés, J.A. Garrido, R.M. Rodríguez, P.L. Cabot, F. Centellas, C. Arias, E. Brillas, J.
ȱ 15
Electrochem. Soc. 153 (2006) D1-D9.
ȱ 16
[30] S. Irmak, H.I. Yavuz, O. Erbatur, Appl. Catal. B: Environ. 63 (2006) 243-248.
ȱ 17
[31] C. Flox, S. Ammar, C. Arias, E. Brillas, A.V. Vargas-Zavala, R. Abdelhedi, Appl. Catal. B:
ȱ 18
Environ. 67 (2006) 93-104.
ȱ 19
[32] Y. Sun, J.J. Pignatello, Environ. Sci. Technol. 27 (1993) 304-310.
ȱ 20
[33] G.U. Buxton, C.L. Greenstock, W.P. Helman, A.B. Ross, J. Phys. Chem. Data Ref. 17 (1988)
ȱ 21
513-886.
ȱ 22
[34] Y. Zuo, J. Hoigné, Environ. Sci. Technol. 26 (1992) 1014-1022.
ȱ 23
[35] J. Lee, D.A. Tryk, A. Fujishima, S.M. Park, Chem. Commun. (2002) 486-487.
ȱ
ȱ
ȱ
ȱ
19
ȱ
ȱ
272
PART B –Results and Discussion8. Clofibric Acid
ȱ
1
Figure captions
ȱ 2
ȱ 3
Fig. 1. TOC removal with specific charge for the degradation of 100-ml solutions containing 179
ȱ 4
mg l-1 clofibric acid and 0.05 M Na 2SO 4 of pH 3.0 at 100 mA cm-2 and at 35.0 ºC using an
ȱ 5
undivided cell with a 3-cm2 BDD anode and a 3-cm2 carbon-PTFE cathode fed with pure O 2 at 12
6
ml min -1. Method: ({) anodic oxidation with electrogenerated H2O 2, ( z) anodic oxidation with
ȱ 7
electrogenerated H2O 2 under a 6-W UVA irradiation with λ max = 360 nm, ( „) electro-Fenton with
ȱ 8
1.0 mM Fe2+ and (S) photoelectro-Fenton with 1.0 mM Fe 2+ and UVA light.
ȱ
ȱ 9
ȱ
ȱ
ȱ
ȱ
ȱ
10
Fig. 2. Effect of experimental parameters on TOC abatement vs. specific charge for the treatment of
11
100 ml of clofibric acid solutions of pH 3.0 at 35.0 ºC by electro-Fenton with a BDD anode and 1.0
12
mM Fe 2+. In plot (a), metabolite concentration: 179 mg l-1; current density: (‹) 33, („) 100 and (T)
13
150 mA cm-2. In plot (b), metabolite concentration: ( †) 557 (close to saturation), ( ‘) 358, („) 179
14
and (∆) 89 mg l-1; current density: 100 mA cm-2.
15
ȱ 16
Fig. 3. Concentration of chloride ion accumulated during the treatment of 100 ml of 179 mg l-1
ȱ 17
clofibric acid solutions of pH 3.0 at 100 mA cm-2 and at 35.0 ºC using a BDD anode and
ȱ 18
electrogenerated H2O 2 by: ({) anodic oxidation, („) electro-Fenton and (S) photoelectro-Fenton.
ȱ 19
ȱ 20
Fig. 4. Mineralization current efficiency calculated from Eq. 8 vs. specific charge. Plot(a)
ȱ 21
corresponds to the experiments shown in Fig. 1 and plot (b) to those reported in Figs. 2a and 2b.
ȱ 22
ȱ
ȱ
ȱ
ȱ
23
Fig. 5. Time-course of clofibric acid concentration during the degradation of 100 ml of 179 mg l-1
24
metabolite solutions of pH 3.0 at 35.0 ºC with a BDD anode and electrogenerated H2O 2. Plot (a):
25
({) anodic oxidation and (z) anodic oxidation with UVA light at 100 mA cm-2. Plot (b): electro-
26
Fenton at (‹) 33, („) 100 and (T) 150 mA cm-2 and (S) photoelectro-Fenton at 100 mA cm-2. The
ȱ
ȱ
20
ȱ
ȱ
273
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
1
inset panels show the corresponding kinetic analysis assuming a pseudo first-order reaction for
2
clofibric acid.
3
ȱ 4
Fig. 6. Evolution of the concentration of selected intermediates during the mineralization of 100 ml
ȱ 5
of 179 mg l-1 clofibric acid solutions of pH 3.0 at 100 mA cm-2 and at 35.0 ºC with a BDD anode
ȱ 6
and electrogenerated H2O 2. Plots correspond to: (a) 4-chlorophenol, (b) 4-chlorocatechol, (c) p-
ȱ 7
benzoquinone and (d) oxalic acid. Method: ({) anodic oxidation, ( z) anodic oxidation with UVA
ȱ 8
light, („) electro-Fenton and ( S) photoelectro-Fenton.
ȱ 9
ȱ 10
Fig. 7. Proposed reaction sequence for the initial degradation of clofibric acid with a BDD anode
ȱ 11
and electrogenerated H2O 2 by anodic oxidation, electro-Fenton with Fe2+ and photoelectro-Fenton
12
with Fe2+ and UVA light. The oxidant hydroxyl radical is denoted as BDD( •OH) or •OH when it is
13
formed at the BDD anode surface or from Fenton’s reaction, respectively.
ȱ
ȱ
ȱ
ȱ
ȱ
14
15
Fig. 8. Proposed reaction pathways for oxalic acid mineralization with a BDD anode and
16
electrogenerated H2O 2 by anodic oxidation, electro-Fenton with Fe2+ and photoelectro-Fenton with
17
Fe2+ and UVA light.
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
21
ȱ
ȱ
274
PART B –Results and Discussion8. Clofibric Acid
ȱ Figure(s)
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
120
ȱ
100
ȱ
ȱ
ȱ
TOC / mg l
ȱ
-1
ȱ
80
60
40
20
ȱ
ȱ
0
0
3
6
9
12
15
18
21
-1
ȱ
Q/Ahl
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Fig. 1
ȱ
ȱ
275
PART B –Results and Discussion8. Clofibric Acid
ȱ Figure(s)
ȱ
ȱ
ȱ
ȱ
120
ȱ
(a)
100
ȱ
TOC / mg l
-1
ȱ
ȱ
ȱ
ȱ
80
60
40
20
ȱ
0
0
ȱ
3
6
9
12
15
18
21
24
27
-1
Q/Ahl
ȱ
ȱ
350
ȱ
(b)
300
ȱ
250
TOC / mg l
-1
ȱ
ȱ
ȱ
200
150
100
ȱ
50
ȱ
0
0
ȱ
3
6
9
12
15
Q / A h l-1
ȱ
ȱ
ȱ
ȱ
Fig. 2
ȱ
ȱ
276
18
21
24
27
PART B –Results and Discussion8. Clofibric Acid
ȱ Figure(s)
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
25
ȱ
20
ȱ
ȱ
15
−
ȱ
[Cl ] / mg l
ȱ
-1
ȱ
10
5
ȱ
ȱ
ȱ
0
0
60
120
180
240
300
360
420
time / min
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Fig. 3
ȱ
ȱ
277
PART B –Results and Discussion8. Clofibric Acid
ȱ Figure(s)
ȱ
ȱ
ȱ
ȱ
40
ȱ
35
30
ȱ
25
MCE / %
ȱ
ȱ
ȱ
(a)
20
15
10
ȱ
5
ȱ
0
0
ȱ
3
6
9
12
15
18
21
-1
Q/Ahl
ȱ
ȱ
60
ȱ
(b)
50
ȱ
40
MCE / %
ȱ
ȱ
ȱ
30
20
ȱ
10
ȱ
0
0
ȱ
3
6
9
12
15
Q / A h l-1
ȱ
ȱ
ȱ
ȱ
Fig. 4
ȱ
ȱ
278
18
21
24
27
PART B –Results and Discussion8. Clofibric Acid
ȱ Figure(s)
ȱ
ȱ
ȱ
ȱ
200
ȱ
ȱ
ȱ
ȱ
0
-1
150
ln (c / c)
ȱ
2.0
[clofibric acid] / mg l
ȱ
2.5
(a)
100
1.5
1.0
0.5
0.0
0
60
120
50
180
240
time / min
ȱ
0
0
ȱ
60
120
180
ȱ
200
ȱ
ȱ
ȱ
ȱ
360
420
5
(b)
3
0
150
ln (c / c)
-1
4
[clofibric acid] / mg l
ȱ
300
time / min
ȱ
ȱ
240
100
2
1
0
50
0
4
8
12
time / min
ȱ
ȱ
ȱ
0
0
5
10
15
20
time / min
ȱ
ȱ
ȱ
Fig. 5
ȱ
ȱ
279
PART B –Results and Discussion8. Clofibric Acid
ȱ Figure(s)
ȱ
ȱ
ȱ
ȱ
10
ȱ
[4-chlorophenol] / mg l-1
ȱ
ȱ
ȱ
ȱ
ȱ
(a)
8
6
4
2
ȱ
0
ȱ
0
60
120
180
240
300
360
420
time / min
ȱ
ȱ
2.5
[4-chlorocatechol] / mg l-1
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
(b)
2.0
1.5
1.0
0.5
ȱ
0.0
0
ȱ
2
4
time / min
ȱ
ȱ
ȱ
ȱ
Fig. 6
ȱ
ȱ
280
6
8
PART B –Results and Discussion8. Clofibric Acid
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
1.2
[p-benzoquinone] / mg l-1
ȱ
(c)
0.8
0.4
0.0
0
60
120
180
240
300
360
420
time / min
80
(d)
[oxalic acid] / mg l-1
ȱ
60
40
20
0
0
60
120
180
240
300
360
420
time / min
Fig. 6
ȱ ȱ
281
PART B –Results and Discussion8. Clofibric Acid
ȱ Figure(s)
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
BDD( •OH)
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
•OH
•OH
CH3
BDD( •OH)
H3C C COOH
Cl
CH3
ȱ
OH
ȱ
ȱ
ȱ
ȱ
Cl
•OH
OH
OH
Cl
ȱ
ȱ
ȱ
ȱ
ȱ
Fig. 7
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
282
Cl−
BDD( •OH), •OH
H3 C C COOH
ȱ
O
OH
OH
O
ȱ
ȱ
O
OH
ȱ
PART B –Results and Discussion8. Clofibric Acid
ȱ Figure(s)
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Fe3+-oxalato
complexes
ȱ
Fe3+
hν
ȱ
BDD( •OH)
-Fe2+
ȱ
COOH
ȱ
COOH
BDD( •OH)
CO2
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Fig. 8
ȱ
ȱ
ȱ
283
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
PART B –Results and Discussion8. Clofibric Acid
8.3.2.ȱȱResultatsȱiȱDiscussióȱ/ȱResultsȱandȱDiscussionȱ
ȱ
TheȱoxidizingȱabilityȱofȱtheȱAO,ȱEFȱandȱPEFȱprocessesȱhasȱbeenȱasessedȱthroughȱtheȱ
variationȱofȱTOCȱwithȱspecificȱchargeȱ(Q,ȱinȱAȱhȱLȬ1)ȱforȱtheȱtreatmentȱofȱ100ȱmLȱofȱ
179ȱmgȱLȬ1ȱclofibricȱacidȱsolutionsȱatȱpHȱ3.0ȱandȱatȱ100ȱmAȱcmȬ2ȱusingȱaȱPtȱanode.ȱAsȱ
expected,ȱAOȱwithȱelectrogeneratedȱH2O2ȱgivesȱaȱquiteȱslowȱTOCȱremoval,ȱattainingȱ
41%ȱofȱmineralizationȱatȱ18ȱAȱhȱLȬ1ȱ(6ȱh).ȱThisȱbehaviorȱcanȱbeȱaccountedȱforȱbyȱtheȱ
lowȱconcentrationȱofȱeffectiveȱ•OHadsȱformedȱatȱtheȱPtȱsurfaceȱfromȱReactionȱ5.Ȭ44.ȱAnȱ
analogousȱdegradationȱrateȱisȱobservedȱbyȱAOȱwithȱUVAȱirradiation,ȱyieldingȱ39%ȱofȱ
TOCȱabatementȱatȱ18ȱAȱhȱLȬ1.ȱThatȱmeansȱthatȱclofibricȱacidȱandȱitsȱintermediatesȱareȱ
notȱ directlyȱ photodegradedȱ byȱ UVAȱ light.ȱ Whenȱ 1.0ȱ mMȱ Fe2+ȱ isȱ presentȱ inȱ theȱ
solution,ȱTOCȱdecayȱatȱ6ȱhȱisȱ79%ȱdueȱtoȱtheȱfastȱreactionȱofȱorganicsȱwithȱtheȱgreatȱ
amountȱ ofȱ •OHȱ producedȱ fromȱ Fenton’sȱ reactionȱ (Reactionȱ 5.Ȭ3),ȱ butȱ someȱ hardlyȱ
oxidizableȱproductsȱremainȱstillȱstable.ȱPEFȱleadsȱtoȱquickerȱTOCȱdecayȱandȱalmostȱ
overallȱmineralizationȱ(>ȱ96%)ȱatȱtheȱendȱofȱelectrolysis.ȱThisȱcanȱbeȱreachedȱbecauseȱ
UVAȱ illuminationȱ favors:ȱ (i)ȱ theȱ photodecompositionȱ ofȱ complexesȱ ofȱ Fe3+ȱ withȱ
generatedȱ carboxylicȱ acidsȱ (Reactionȱ 5.Ȭ24),ȱ andȱ (ii)ȱ theȱ regenerationȱ ofȱ Fe2+ȱ fromȱ
additionalȱphotoreductionȱofȱFe(OH)2+,ȱwithȱaȱsimultanousȱproductionȱofȱadditionalȱ
•
OHȱ(Reactionȱ5.Ȭ23).ȱ
ȱ
Inȱ contrast,ȱ aȱ continuousȱ TOCȱ decayȱ untilȱ attainingȱ totalȱ mineralizationȱ canȱ beȱ
observedȱforȱallȱexperimentsȱusingȱBDD.ȱThisȱpeculiarȱbehaviorȱhasȱbeenȱpreviouslyȱ
shownȱ forȱ AOȱ withȱ BDDȱ inȱ theȱ treatmentȱ ofȱ paracetamolȱ (sectionȱ 7.3)ȱ andȱ clofibricȱ
acidȱ (sectionȱ 8.2),ȱ andȱ againȱ itȱ isȱ anȱ evidenceȱ ofȱ theȱ greatȱ oxidizingȱ powerȱ ofȱ thisȱ
anode.ȱ Butȱ inȱ thisȱ case,ȱ theȱ timeȱ requiredȱ forȱ totalȱ mineralizationȱ dependsȱ onȱ theȱ
methodȱtested.ȱThus,ȱAOȱwithoutȱorȱwithȱUVAȱilluminationȱleadsȱtoȱaȱsimilarȱslowȱ
TOCȱabatementȱupȱtoȱcompleteȱmineralizationȱforȱ18ȱAȱhȱLȬ1ȱ(6ȱh),ȱindicatingȱthatȱallȱ
organicsȱ areȱ destroyedȱ byȱ theȱ highȱ amountȱ ofȱ BDD(•OH),ȱ i.e.ȱ •OHads,ȱ formedȱ atȱ theȱ
BDDȱsurface.ȱTheȱdegradationȱrateȱisȱstronglyȱenhancedȱinȱtheȱEFȱprocessȱdueȱtoȱ•OHȱ
285
PART B –Results and Discussion8. Clofibric Acid
formedȱfromȱFenton’sȱreactionȱ(Reactionȱ5.Ȭ3),ȱbutȱcomplexesȱofȱFe3+ȱwithȱcarboxylicȱ
acidsȱ canȱ onlyȱ beȱ oxidizedȱ byȱ BDD(•OH),ȱ soȱ totalȱ mineralizationȱ isȱ slowlyȱ attainedȱ
forȱ 15Ȭ18ȱ Aȱ hȱ LȬ1ȱ (5Ȭ6ȱ h).ȱ However,ȱ TOCȱ isȱ quicklyȱ removedȱ byȱ PEFȱ asȱ pointedȱ outȱ
aboveȱ(Reactionsȱ5.Ȭ23ȱandȱ5.Ȭ24),ȱandȱoverallȱmineralizationȱisȱattainedȱforȱ12ȱAȱhȱLȬ1ȱ
(4ȱh).ȱ
ȱ
Inȱ Figureȱ 8.Ȭ2ȱ aȱ compendiumȱ ofȱ allȱ theȱ electrochemicalȱ processesȱ proposedȱ inȱ thisȱ
thesisȱ forȱ theȱ overallȱ mineralizationȱ ofȱ clofibricȱ acidȱ isȱ presented.ȱ Sixȱ methodsȱ areȱ
ableȱtoȱdegradeȱtotallyȱ100ȬmLȱsolutionsȱofȱpHȱ3.0ȱcontainingȱ179ȱmgȱLȬ1ȱclofibricȱacidȱ
atȱ100ȱmAȱcmȬ2.ȱAOȱwithȱaȱBDDȱanodeȱandȱaȱstainlessȱsteelȱcathodeȱleadsȱtoȱcompleteȱ
mineralizationȱafterȱtheȱconsumptionȱofȱ21ȱAȱhȱLȬ1ȱ(7ȱh).ȱAOȱwithȱaȱBDDȱanodeȱandȱ
H2O2ȱelectrogenerationȱslightlyȱacceleratesȱtheȱprocessȱ(6ȱh)ȱdueȱtoȱtheȱcontributionȱofȱ
H2O2,ȱ butȱ TOCȱ evolutionȱ isȱ practicallyȱ analogousȱ becauseȱ itȱ canȱ beȱ consideredȱ thatȱ
theȱsameȱamountȱofȱeffectiveȱBDD(•OH)ȱisȱformedȱatȱtheȱBDDȱsurface.ȱItȱisȱclearȱthatȱ
UVAȱirradiationȱdoesȱnotȱaffectȱsignificantlyȱtoȱclofibricȱacidȱandȱitsȱintermediates.ȱAȱ
greatȱenhancementȱofȱtheȱmineralizationȱrateȱisȱachievedȱwithȱ1.0ȱmMȱFe2+ȱasȱcatalystȱ
dueȱtoȱtheȱgenerationȱofȱhighȱamountsȱofȱ•OHȱinȱtheȱbulkȱsolution.ȱTOCȱabatementȱisȱ
quiteȱ similarȱ inȱ EFȱ withȱ BDDȱ andȱ PEFȱ withȱ Pt,ȱ butȱ anywayȱ totalȱ mineralizationȱ isȱ
attainedȱ atȱ theȱ sameȱ timeȱ asȱ thatȱ describedȱ forȱ AOȱ becauseȱ Fe3+ȱ complexesȱ withȱ
carboxylicȱacidsȱareȱveryȱslowlyȱoxidizedȱbyȱBDD(•OH)ȱinȱEFȱwithȱBDDȱandȱbyȱUVAȱ
lightȱinȱPEFȱwithȱPt.ȱPEFȱwithȱaȱBDDȱanodeȱcombinesȱtheȱoxidizingȱpowerȱofȱ •OH,ȱ
BDD(•OH)ȱandȱUVAȱlight,ȱandȱasȱaȱresultȱTOCȱisȱcompletelyȱremovedȱatȱ4ȱh.ȱ
ȱ
Atȱthisȱpoint,ȱitȱisȱworthȱmentioningȱthatȱparallelȱoxidationȱofȱorganicsȱwithȱweakerȱ
oxidizingȱspeciesȱformedȱinȱtheȱbulkȱsolution,ȱsuchȱasȱHO2•,ȱH2O2,ȱSO4•-ȱ[182],ȱferrateȱ
ionsȱ[92]ȱandȱotherȱhypervalentȱironȱspeciesȱ[148],ȱasȱwellȱasȱatȱtheȱBDDȱsurface,ȱasȱ
forȱ exampleȱ O3,ȱ H2O2ȱ andȱ S2O82Ȭȱ ions,ȱ isȱ alsoȱ possible.ȱ Inȱ addition,ȱ wheneverȱ BDDȱ
anodeȱisȱusedȱandȱchlorinatedȱcompoundsȱareȱtreated,ȱtheȱoxidizingȱsubstanceȱCl2ȱisȱ
formedȱ inȱ theȱ mediumȱ [377].ȱ Thereȱ isȱ noȱ doubtȱ aboutȱ theȱ factȱ thatȱ allȱ theseȱ speciesȱ
286
PART B –Results and Discussion8. Clofibric Acid
canȱplayȱaȱsignificantȱroleȱregardingȱtheȱdestructionȱofȱpollutantsȱinȱtheȱsolution,ȱbutȱ
atȱ theȱ sameȱ timeȱ itȱ isȱ necessaryȱ toȱ realizeȱ thatȱ consideringȱ hydroxylȱ radicalsȱ asȱ theȱ
mainȱ oxidizingȱ agentsȱ constitutesȱ moreȱ thanȱ anȱ acceptableȱ approachȱ thatȱ helpsȱ
simplifyingȱ theseȱ systemsȱ andȱ theȱ reactionsȱ involved,ȱ soȱ theȱ conclusionsȱ canȱ beȱ
presentedȱonȱtheȱbasisȱofȱthisȱprevailingȱoxidizingȱagent.ȱ
TOC / mg L
-1
120
100
80
60
40
20
0
0
3
6
9
12
15
18
-1
Q /AhL
21
24
ȱ
Figure 8.-2 TOC vs. specific charge for the degradation of 100 mL of 179 mg L-1
clofibric acid solutions in 0.05 M Na2SO4 of pH 3.0 at 100 mA cm-2 and at 35 ºC, using
an undivided cell with 3-cm2 electrodes.
Processes without an O2-diffusion cathode: (×) AO with a BDD anode and a stainless
steel cathode. Processes with an O2-diffusion cathode: (¸) PEF with a Pt anode and 1.0
mM Fe2+ + UVA light, (Ɣ) AO with a BDD anode and electrogenerated H2O2, (Ŷ) latter
AO under UVA irradiation, (Ÿ) EF with a BDD anode and 1.0 mM Fe2+, (Ƈ) PEF with a
BDD anode and 1.0 mM Fe2+.
ȱ
Mineralizationȱ ofȱ clofibricȱ acidȱ isȱ accompaniedȱ byȱ itsȱ overallȱ dechlorination.ȱ Ionȱ
chromatogramsȱ onlyȱ displayȱ aȱ definedȱ peakȱ relatedȱ toȱ Clȱ ion.ȱ Noȱ otherȱ chlorineȬ
oxygenȱionsȱsuchȱasȱClO2,ȱClO3ȱandȱClO4ȱwereȱdetectedȱbyȱthisȱtechnique.ȱChlorideȱ
ionȱevolutionȱforȱtheȱelectrolysesȱofȱ179ȱmgȱLȬ1ȱclofibricȱacidȱsolutionsȱofȱpHȱ3.0ȱatȱ100ȱ
mAȱ cmȬ2ȱ usingȱ aȱ Ptȱ anodeȱ showsȱ thatȱ inȱ AOȱ withȱ electrogeneratedȱ H2O2,ȱ Clȱ isȱ
graduallyȱ accumulatedȱ upȱ toȱ 23ȱ mgȱ LȬ1ȱ forȱ 12ȱ Aȱ hȱ LȬ1ȱ (4ȱ h)ȱ andȱ furtherȱ onȱ itȱ keepsȱ
stable,ȱ whereasȱ inȱ EFȱ andȱ PEFȱ aȱ quasiȬsteadyȱ concentrationȱ ofȱ aboutȱ 25ȱ mgȱ LȬ1ȱ isȱ
alreadyȱ attainedȱ atȱ 2ȱ Aȱ hȱ LȬ1ȱ (40ȱ min),ȱ justȱ undergoingȱ aȱ slightȱ dropȱ dueȱ toȱ itsȱ
287
PART B –Results and Discussion8. Clofibric Acid
oxidationȱtoȱCl2ȱatȱtheȱPtȱanode.ȱTheseȱfindingsȱallowȱconcludingȱthatȱchloroȬorganicsȱ
areȱalwaysȱdegradedȱwithȱreleaseȱofȱCl,ȱbeingȱmuchȱmoreȱquicklyȱdestroyedȱbyȱEFȱ
andȱ PEF.ȱ Theseȱ threeȱ methodsȱ onlyȱ leadȱ toȱ theȱ releaseȱ ofȱ 78Ȭ85%ȱ ofȱ theȱ chlorineȱ
containedȱinȱtheȱinitialȱsolutionȱofȱ179ȱmgȱLȬ1ȱclofibricȱacidȱ(29.5ȱmgȱLȬ1),ȱsuggestingȱ
thatȱstableȱcoloredȱpolyaromaticsȱformedȱduringȱtheȱdegradationȱprocessȱcontainȱtheȱ
remainingȱ chlorine.ȱ Onȱ theȱ otherȱ hand,ȱ theȱ resultsȱ forȱ theȱ sameȱ processesȱ usingȱ aȱ
BDDȱanodeȱshowȱthatȱClȱisȱacumulatedȱandȱcompletelyȱremovedȱafterȱ300Ȭ360ȱminȱ
inȱallȱcases,ȱafterȱreachingȱaȱmaximumȱconcentrationȱofȱaboutȱ8ȱmgȱLȬ1ȱatȱ180ȱminȱinȱ
AO,ȱ23ȱmgȱLȬ1ȱatȱ20ȱminȱinȱEFȱandȱ19ȱmgȱLȬ1ȱatȱ40ȱminȱinȱPEF,ȱcorrespondingȱtoȱ27%,ȱ
78%ȱandȱ64%ȱofȱtheȱinitialȱchlorineȱcontent.ȱTheȱslowȱaccumulationȱinȱAOȱconfirmsȱ
theȱslowȱreactionȱofȱchloroȬorganicsȱwithȱBDD(•OH),ȱwhereasȱitsȱmuchȱfasterȱreleaseȱ
atȱ earlyȱ stagesȱ ofȱ EFȱ andȱ PEFȱ corroboratesȱ theȱ quickȱ destructionȱ ofȱ pollutantsȱ withȱ
•
OHȱ inȱ theȱ bulkȱ solution.ȱ Theȱ progressiveȱ destructionȱ ofȱ Clȱ whenȱ electrolysisȱ isȱ
prolongedȱcanȱbeȱexplainedȱbyȱitsȱslowȱoxidationȱtoȱCl2ȱonȱBDD.ȱInȱcomparisonȱwithȱ
theȱ chlorideȱ ionȱ evolutionȱ depictedȱ inȱ sectionȱ 8.2,ȱ theȱ AOȱ behaviorȱ isȱ veryȱ similar,ȱ
butȱEFȱandȱPEFȱexhibitȱsuchȱanȱoxidizingȱabilityȱthatȱaȱfastȱgreatȱaccumulationȱofȱClȱ
canȱ beȱ attainedȱ atȱ theirȱ earlyȱ stages,ȱ furtherȱ beingȱ quasiȬstabilizedȱ whenȱ Ptȱ isȱ usedȱ
andȱgraduallyȱoxidizedȱwhenȱBDDȱisȱused.ȱ
ȱ
Theȱ studyȱ ofȱ theȱ influenceȱ ofȱ initialȱ pHȱ inȱ PEFȱ withȱ Ptȱ andȱ EFȱ withȱ BDD,ȱ atȱ 100ȱȱȱȱȱ
mAȱ cmȬ2,ȱ confirmsȱ inȱ bothȱ casesȱ thatȱ theȱ quickestȱ TOCȱ decayȱ takesȱ placeȱ forȱ initialȱ
pHȱ 3.0,ȱ asȱ canȱ beȱ seenȱ forȱ exampleȱ inȱ Figureȱ 8.Ȭ3ȱ givenȱ below.ȱ Forȱ theȱ restȱ ofȱ
electrolysesȱtested,ȱtheȱmineralizationȱrateȱfallsȱinȱtheȱorderȱpHȱ2.0ȱ>ȱpHȱ4.0ȱ>>ȱpHȱ6.0.ȱ
Thisȱfactȱcanȱbeȱeasilyȱassociatedȱtoȱtheȱhighestȱgenerationȱrateȱofȱ •OHȱfromȱFenton’sȱ
reaction.ȱ Itȱ isȱ interestingȱ toȱ noteȱ thatȱ inȱ PEFȱ withȱ Ptȱ theȱ totalȱ mineralizationȱ canȱ beȱ
attainedȱ uniquelyȱ atȱ pHȱ 3.0,ȱ whereasȱ inȱ EFȱ withȱ BDDȱ itȱ canȱ beȱ achievedȱ atȱ allȱ pHȱ
valuesȱ studied.ȱ Thisȱ isȱ coherentȱ becauseȱ theȱ systemsȱ withȱ Ptȱ areȱ basedȱ onȱ Fenton’sȱ
reactionȱwithȱ •OHȱformation,ȱandȱthusȱanȱincreaseȱinȱpHȱisȱrelatedȱtoȱaȱtotalȱlossȱofȱ
theȱ oxidizingȱ ability,ȱ whereasȱ inȱ theȱ systemsȱ withȱ BDDȱ aȱ significantȱ parallelȱ
288
PART B –Results and Discussion8. Clofibric Acid
oxidizingȱ routeȱ withȱ BDD(•OH)ȱ andȱ otherȱ weakerȱ speciesȱ continuesȱ actingȱ atȱ highȱ
pHȱvalues,ȱmakingȱtheseȱprocessesȱviableȱinȱaȱwiderȱvarietyȱofȱconditions.ȱ
TOC / mg L
-1
120
100
80
60
40
20
0
0
3
6
9
12
15
-1
Q /AhL
18
21
ȱ
Figure 8.-3 Effect of pH on TOC removal vs. specific charge for the degradation of
100 mL of 179 mg L-1 clofibric acid solutions by EF with 1.0 mM Fe2+ in 0.05 M Na2SO4
of pH 3.0 at 100 mA cm-2 and at 35 ºC, using an undivided cell with a 3-cm2 BDD anode
and a 3-cm2 O2-diffusion cathode.
Initial solution pH: (Ɣ) 2.0, (Ÿ) 3.0, (Ŷ) 4.0, (Ƈ) 6.0.
ȱ
Theȱ effectȱ ofȱ currentȱ densityȱ onȱ theȱ oxidationȱ abilityȱ ofȱ theȱ twoȱ processesȱ
aforementionedȱhasȱbeenȱstudiedȱforȱ100ȬmLȱsolutionsȱwithȱ179ȱmgȱLȬ1ȱclofibricȱacidȱ
atȱpHȱ3.0ȱandȱatȱ33,ȱ100ȱandȱ150ȱmAȱcmȬ2.ȱAȱprogressiveȱincreaseȱinȱQȱfromȱ7ȱtoȱ27ȱȱȱȱ
AȱhȱLȬ1ȱ(i.e.,ȱaȱdecreaseȱinȱtimeȱfromȱ7ȱtoȱ5.5ȱh)ȱandȱfromȱ12ȱtoȱ22ȱAȱhȱLȬ1ȱ(12ȱtoȱ5ȱh)ȱforȱ
PEFȱ withȱ Ptȱ andȱ EFȱ withȱ BDD,ȱ respectively,ȱ isȱ requiredȱ forȱ overallȱ mineralizationȱ
whenȱ jappȱ increasesȱ fromȱ 33ȱ toȱ 150ȱ mAȱ cmȬ2.ȱ Thisȱ increaseȱ isȱ indicativeȱ ofȱ anȱ
accelerationȱ ofȱ parallelȱ nonȬoxidizingȱ reactionsȱ involvingȱ •OHȱ asȱ wellȱ asȱ aȱ higherȱ
productionȱ ofȱ otherȱ weakerȱ oxidantsȱ describedȱ elsewhere,ȱ whereasȱ theȱ decreaseȱ inȱ
timeȱrequiredȱcanȱbeȱmainlyȱascribedȱtoȱaȱgreaterȱproductionȱofȱ •OHadsȱatȱtheȱanodeȱ
surfaceȱdueȱtoȱlargerȱH2Oȱoxidation,ȱandȱ •OHȱinȱtheȱmediumȱbecauseȱofȱtheȱgreaterȱ
electrogenerationȱ ofȱ H2O2ȱ atȱ theȱ cathodeȱ (and,ȱ toȱ aȱ certainȱ extent,ȱ theȱ actionȱ ofȱ theȱ
weakerȱoxidizingȱspeciesȱmentioned).ȱ
ȱ
289
PART B –Results and Discussion8. Clofibric Acid
Theȱ greatȱ oxidizingȱ powerȱ ofȱ theseȱ twoȱ methodsȱ hasȱ beenȱ confirmedȱ byȱ degradingȱ
upȱ toȱ 0.56ȱ gȱ LȬ1ȱ (closeȱ toȱ saturation)ȱ ofȱ clofibricȱ acidȱ atȱ pHȱ 3.0ȱ andȱ atȱ 100ȱ mAȱ cmȬ2.ȱ
TOCȬQȱ plotsȱ forȱ PEFȱ withȱ Ptȱ showȱ thatȱ overallȱ mineralizationȱ isȱ achievedȱ afterȱ
consumptionȱ ofȱ 30,ȱ 24ȱ andȱ18ȱ Aȱ hȱLȬ1ȱ(10,ȱ8ȱandȱ 6ȱh)ȱ forȱ557,ȱ 358ȱandȱ 179ȱ mgȱLȬ1ȱofȱ
clofibricȱacid,ȱrespectively.ȱTheȱsameȱinitialȱconcentrationsȱofȱclofibricȱacidȱrequireȱ24,ȱ
21ȱandȱ15ȱAȱhȱLȬ1ȱ(8,ȱ7ȱandȱ5ȱh),ȱrespectively,ȱforȱEFȱwithȱBDD.ȱItȱisȱclearȱthatȱsystemsȱ
usingȱ anȱ anodeȱ suchȱ asȱ BDD,ȱ whichȱ hasȱ aȱ greaterȱ oxidizingȱ ability,ȱ requireȱ lessȱ
energyȱ consumptionȱ toȱ attainȱ overallȱ mineralizationȱ (i.e.,ȱ BDDȱ systemsȱ canȱ attainȱ
overallȱ mineralizationȱ moreȱ quicklyȱ thanȱ Ptȱ systems).ȱ Inȱ bothȱ cases,ȱ asȱ initialȱ
pollutantȱconcentrationȱdecreases,ȱaȱlowerȱQȱisȱrequiredȱdueȱtoȱtheȱpresenceȱofȱlowerȱ
amountȱofȱorganics.ȱMoreover,ȱaȱhigherȱTOCȱremovalȱisȱattainedȱatȱaȱgivenȱtimeȱwithȱ
increasingȱinitialȱpollutantȱcontent,ȱbecauseȱtheȱcompetitiveȱnonȬoxidizingȱreactionsȱ
ofȱ •OHȱ andȱ •OHadsȱ becomeȱ slowerȱ andȱ theseȱ radicalsȱ canȱ reactȱ withȱ pollutantsȱ toȱ aȱ
largerȱextent.ȱ
ȱ
Allȱtheseȱfindings,ȱalongȱwithȱtheȱresultsȱdiscussedȱinȱsectionȱ8.2.2,ȱallowȱconcludingȱ
thatȱ overallȱ mineralizationȱ reactionȱ forȱ clofibricȱ acidȱ involvesȱ 44ȱ Fȱ forȱ eachȱ molȱ ofȱ
pharmaceutical,ȱ withȱ chlorideȱ ionȱ asȱ primaryȱ inorganicȱ ionȱ (Reactionȱ 6.Ȭ3).ȱ Theȱ
efficiencyȱcanȱthenȱbeȱdeterminedȱfromȱEquationȱ6.Ȭ1ȱforȱtheȱfourȱprocessesȱtestedȱatȱ
theȱbeginningȱofȱthisȱsectionȱusingȱPtȱandȱBDD.ȱItȱcanȱbeȱobservedȱthatȱinȱtheȱsystemsȱ
withȱ Ptȱ theȱ efficiencyȱ forȱ bothȱ AOȱ proceduresȱ isȱ veryȱ small,ȱ reachingȱ aȱ maximumȱ
valueȱofȱ3.3%Ȭ3.8%ȱatȱ6ȱAȱhȱLȬ1ȱ(2ȱh),ȱasȱexpectedȱfromȱtheirȱlowȱoxidationȱability.ȱInȱ
contrast,ȱ MCEȱ reachesȱ aȱ valueȱ ofȱ 25%ȱ andȱ 23%ȱ atȱ theȱ earlyȱ stagesȱ inȱ EFȱ andȱ PEF,ȱ
respectively,ȱbutȱtheȱefficiencyȱofȱPEFȱisȱclearlyȱhigherȱfromȱ3ȱAȱhȱLȬ1ȱ(1ȱh)ȱbecauseȱitȱ
isȱ ableȱ toȱ destroyȱ allȱ products.ȱ Onȱ theȱ otherȱ hand,ȱ inȱ theȱ systemsȱ withȱ BDDȱ aȱ
constantȱefficiencyȱofȱaboutȱ7%ȱisȱfoundȱforȱbothȱAOȱmethods,ȱsuggestingȱaȱconstantȱ
slowȱ mineralizationȱ rateȱ ofȱ allȱ organicsȱ withȱ BDD(•OH).ȱ MCEȱ valuesȱ areȱ muchȱ
higherȱinȱEFȱandȱPEF,ȱattainingȱ33%ȱandȱ31%ȱatȱ20ȱmin,ȱandȱagainȱPEFȱisȱtheȱmostȱ
efficientȱmethodȱfromȱ3ȱAȱhȱLȬ1ȱ(1ȱh).ȱInȱFigureȱ8.Ȭ4ȱgivenȱbelowȱtheȱMCEȬQȱplotsȱforȱ
290
PART B –Results and Discussion8. Clofibric Acid
theȱsixȱexperimentsȱleadingȱtoȱtotalȱmineralizationȱshownȱinȱtheȱaboveȱFigureȱ8.Ȭ2ȱareȱ
represented.ȱ Allȱ AOȱ processesȱ haveȱ aȱ lowȱ constantȱ MCEȱ alongȱ theȱ electrolysis,ȱ
whereasȱEFȱandȱPEFȱexhibitȱmuchȱhigherȱMCEȱvaluesȱatȱtheirȱearlyȱstagesȱthanksȱtoȱ
theȱgreatȱproductionȱofȱ •OHȱfromȱFenton’sȱreaction.ȱSystemsȱwithȱBDDȱinȱparticularȱ
showȱ theȱ highestȱ efficienciesȱ becauseȱ ofȱ theȱ actionȱ ofȱ additionalȱ weakerȱ oxidizingȱ
species.ȱInȱEFȱandȱPEF,ȱMCEȱalwaysȱundergoesȱaȱdramaticȱdropȱwithȱtimeȱ(i.e.,ȱwithȱ
Q),ȱ dueȱ toȱ theȱ hardlyȱ oxidizableȱ productsȱ generatedȱ and/orȱ theȱ increaseȱ ofȱ parallelȱ
nonȬoxidizingȱ reactionsȱ becauseȱ lowerȱ amountsȱ ofȱ organicsȱ areȱ presentȱ inȱ theȱ
medium.ȱ
35
MCE / %
30
25
20
15
10
5
0
0
3
6
9
12
15
Q / A h L-1
18
21
24
ȱ
Figure 8.-4 Change of MCE with specific charge for the experiments shown in Fig.8.-2.
ȱ
Onȱ theȱ otherȱ hand,ȱ fromȱ theȱ MCEȱ valuesȱ atȱ theȱ differentȱ jappȱ andȱ initialȱ
concentrationsȱ forȱ PEFȱ withȱ Ptȱ andȱ EFȱ withȱ BDD,ȱ itȱ canȱ beȱ concludedȱ thatȱ theȱ
efficiencyȱ stronglyȱ increasesȱ withȱ risingȱ initialȱ clofibricȱ acidȱ andȱ decreasingȱ japp.ȱ
Resultsȱshowȱaȱslightȱincreaseȱinȱtheȱefficiencyȱatȱtheȱearlyȱstagesȱofȱmostȱtreatments,ȱ
asȱ expectedȱ ifȱ higherȱ amountȱ ofȱ pollutantsȱ isȱ moreȱ easilyȱ convertedȱ intoȱ CO2.ȱ
Afterwards,ȱaȱcontinuousȱdropȱinȱMCEȱisȱobservedȱinȱallȱtreatments.ȱElectrolysesȱofȱ
179ȱmgȱLȬ1ȱclofibricȱacidȱatȱpHȱ3.0ȱshowȱdecreasingȱefficienciesȱasȱjappȱincreases.ȱThisȱ
tendencyȱ couldȱ seemȱ contradictoryȱ toȱ theȱ factȱ thatȱ risingȱ jappȱ causesȱ theȱ increaseȱ inȱ
degradationȱ rateȱ dueȱ toȱ theȱ productionȱ ofȱ moreȱ amountsȱ ofȱ •OHȱ andȱ •OHadsȱ inȱ theȱ
291
PART B –Results and Discussion8. Clofibric Acid
mediumȱ andȱ atȱ theȱ anodeȱ surface,ȱ respectively.ȱ Butȱ certainly,ȱ aȱ greaterȱ electricalȱ
consumptionȱ(i.e.,ȱaȱgreaterȱQ)ȱisȱrequiredȱtoȱmineralizeȱbecauseȱaȱlargerȱproportionȱ
ofȱ bothȱkindȱofȱhydroxylȱradicalsȱisȱwastedȱ inȱ parasiteȱ reactions,ȱ yieldingȱaȱ smallerȱ
proportionȱ ofȱ thisȱ oxidizingȱ agentȱ withȱ enoughȱ abilityȱ toȱ destroyȱ organics.ȱ Forȱ
example,ȱ afterȱ 1ȱ hȱ ofȱ PEFȱ withȱ Ptȱ decreasingȱ MCEȱ valuesȱ ofȱ 46%ȱ (1ȱ Aȱ hȱ LȬ1),ȱ 20%ȱȱȱȱȱȱȱ
(3ȱAȱhȱLȬ1)ȱandȱ14%ȱ(4.5ȱAȱhȱLȬ1)ȱareȱfoundȱatȱincreasingȱjappȱvaluesȱofȱ33,ȱ100ȱandȱ150ȱ
mAȱ cmȬ2,ȱ respectively.ȱ Atȱ constantȱ jappȱ ofȱ 100ȱ mAȱ cmȬ2ȱ andȱ atȱ pHȱ 3.0ȱ higherȱ MCEȱ
valuesȱ areȱ obtainedȱ whenȱ initialȱ concentrationȱ ofȱ pollutantȱ rises,ȱ becauseȱ ofȱ theȱ
slowerȱproductionȱofȱhardlyȱoxidizableȱintermediates.ȱForȱexample,ȱatȱ1ȱhȱ(3ȱAȱhȱLȬ1)ȱ
ofȱPEFȱwithȱPt,ȱincreasingȱMCEȱvaluesȱofȱ8.2%,ȱ20%,ȱ32%ȱandȱ45%ȱareȱobtainedȱforȱ89,ȱ
179,ȱ358ȱ andȱ 557ȱ mgȱLȬ1ȱclofibricȱ acid,ȱrespectively.ȱThisȱtendencyȱ alsoȱconfirmsȱtheȱ
gradualȱ reactionȱ ofȱ higherȱ amountȱ ofȱ •OHȱ andȱ •OHadsȱ withȱ moreȱ pollutants,ȱ
indicatingȱ thatȱ thisȱ hydroxylȱ radicalȱ isȱ wastedȱ toȱ aȱ smallerȱ extent.ȱ Overallȱ
mineralizationȱ isȱ achievedȱ byȱ PEFȱ withȱ Ptȱ andȱ byȱ both,ȱ EFȱ andȱ PEFȱ withȱ BDD.ȱ Itȱ
mustȱ beȱ notedȱ thatȱ theȱ greatestȱ maximumȱMCEȱ valuesȱ amongȱ theȱ differentȱstudiesȱ
carriedȱoutȱinȱthisȱthesisȱareȱfoundȱforȱEFȱandȱPEFȱofȱclofibricȱacid.ȱThus,ȱefficienciesȱ
ofȱ50%ȱandȱ57%ȱareȱobtainedȱatȱ20ȱminȱwhenȱsaturatedȱsolutionsȱofȱthisȱcompoundȱatȱ
pHȱ3.0ȱareȱelectrolyzedȱatȱ100ȱmAȱcmȬ2ȱbyȱPEFȱwithȱPtȱandȱEFȱwithȱBDD,ȱrespectively.ȱ
ȱ
Regardingȱtheȱkineticsȱofȱclofibricȱacidȱdecay,ȱfirstȱofȱallȱtheȱroleȱofȱweakȱoxidantsȱhasȱ
beenȱ assessed.ȱ ReversedȬphaseȱ chromatogramsȱ forȱ 100ȬmLȱ solutionsȱ ofȱ pHȱ 3.0ȱ
containingȱ 179ȱ mgȱ LȬ1ȱ clofibricȱ acid,ȱ 20ȱ mMȱ H2O2ȱ andȱ 0.05ȱ Mȱ Na2SO4ȱ showȱ noȱ
alterationȱ inȱ theȱ pharmaceuticalȱ content,ȱ thusȱ assuringȱ thatȱ itȱ canȱ notȱ reactȱ withȱ
electrogeneratedȱ H2O2.ȱ Inȱ addition,ȱ itȱ mustȱ beȱ remindedȱ thatȱ inȱ sectionȱ 8.2.2ȱ itȱ wasȱ
alsoȱdemonstratedȱthatȱtheȱconcentrationȱofȱclofibricȱacidȱremainsȱunalteredȱtowardsȱ
chemicalȱ oxidationȱ byȱ S2O82Ȭ,ȱ oneȱ ofȱ theȱ oxidizingȱ speciesȱ producedȱ inȱ theȱ systemsȱ
withȱ BDD.ȱ Asȱ aȱ whole,ȱ itȱ meansȱ thatȱ theȱ comparativeȱ kineticsȱ ofȱ theȱ removalȱ ofȱ
clofibricȱ acidȱ canȱ beȱ discussedȱ onȱ theȱ basisȱ ofȱ itsȱ reactionȱ withȱ generatedȱ strongȱ
oxidizingȱ agentsȱ suchȱ asȱ •OHȱ andȱ •OHads.ȱ Therefore,ȱ theȱ kineticsȱ ofȱ clofibricȱ acidȱ
292
PART B –Results and Discussion8. Clofibric Acid
destructionȱbyȱbothȱkindȱofȱhydroxylȱradicalsȱhasȱbeenȱstudiedȱforȱtheȱfourȱmethodsȱ
pointedȱ outȱ aboveȱ usingȱ Ptȱ andȱ BDD,ȱ byȱ electrolyzingȱ 179ȱ mgȱ LȬ1ȱ clofibricȱ acidȱ
solutionsȱofȱpHȱ3.0ȱatȱ100ȱmAȱcmȬ2.ȱOnȱtheȱoneȱhand,ȱclofibricȱacidȱdecaysȱwithȱtimeȱ
byȱAOȱwithȱelectrogeneratedȱH2O2ȱusingȱPtȱandȱBDDȱhaveȱbeenȱstudied.ȱForȱaȱbetterȱ
comparisonȱ betweenȱ allȱ theȱ AOȱ processesȱ appliedȱ toȱ theȱ clofibricȱ acidȱ destruction,ȱ
theȱ decaysȱ forȱ theseȱ fourȱ AOȱ processesȱ withȱ electrogeneratedȱ H2O2,ȱ alongȱ withȱ theȱ
decaysȱ forȱ theȱ twoȱ AOȱ processesȱ usingȱ aȱ stainlessȱ steelȱ cathodeȱ inȱ sectionȱ 8.2,ȱ areȱ
gatheredȱ inȱ Figureȱ 8.Ȭ5ȱ shownȱ below.ȱ Fromȱ theseȱ results,ȱ itȱ isȱ possibleȱ toȱ comeȱ toȱ
threeȱ mainȱ conclusions:ȱ (i)ȱ clofibricȱ acidȱ concentrationȱ undergoesȱ aȱ similarȱ fallȱ
withoutȱandȱwithȱUVAȱillumination,ȱthusȱconfirmingȱthatȱthisȱpharmaceuticalȱisȱnotȱ
directlyȱ photolyzedȱ byȱ UVAȱ light,ȱ (ii)ȱ inȱ allȱ casesȱ clofibricȱ acidȱ isȱ moreȱ quicklyȱ
destroyedȱ andȱ transformedȱ intoȱ itsȱ intermediatesȱ usingȱ Pt,ȱ despiteȱ theȱ factȱ thatȱ
clofibricȱacidȱisȱmoreȱslowlyȱmineralizedȱwithȱthisȱanodeȱthanȱwithȱBDD,ȱandȱthen,ȱaȱ
higherȱadsorptionȱofȱclofibricȱacidȱonȱPtȱsurface,ȱfavoringȱitsȱreactionȱwithȱtheȱmainȱ
oxidizingȱagentȱinȱAOȱ(i.e.,ȱ•OHads),ȱcanȱbeȱhypothesized,ȱandȱ(iii)ȱwhenȱBDDȱisȱused,ȱ
clofibricȱacidȱdisappearsȱatȱaȱtimeȱsimilarȱtoȱthatȱneededȱforȱitsȱtotalȱmineralization,ȱ
thusȱconfirmingȱthatȱtheȱinitialȱpollutantȱpersistsȱinȱtheȱsolutionȱupȱtoȱtheȱendȱofȱtheȱ
degradationȱ processȱ dueȱ toȱ itsȱ simultaneousȱ degradationȱ alongȱ withȱ allȱ
intermediates.ȱ Asȱ anȱ exampleȱ toȱ corroborateȱ theseȱ threeȱ trends,ȱ itȱ canȱ beȱ notedȱ inȱ
Figureȱ8.Ȭ5ȱthatȱclofibricȱacidȱdisappearsȱafterȱ240ȱminȱbyȱAOȱprocessesȱwithȱPt,ȱandȱ
afterȱ360ȱminȱbyȱAOȱwithȱBDD.ȱTheȱlatterȱdataȱisȱsimilarȱtoȱtheȱoneȱobservedȱforȱtotalȱ
mineralizationȱ withȱ BDDȱ depictedȱ inȱ Figureȱ 8.Ȭ2.ȱ Goodȱ straightȱ linesȱ areȱ obtainedȱ
whenȱ theȱ concentrationsȱ decaysȱ inȱ Figureȱ 8.Ȭ5ȱ areȱ fittedȱ toȱ aȱ pseudoȬfirstȬorderȱ
kineticȱequation,ȱasȱshownȱinȱtheȱrespectiveȱinsetȱpanel.ȱThisȱbehaviourȱsuggestsȱthatȱ
aȱ steadyȱ •OHadsȱ concentrationȱ reactsȱ withȱ theȱ drugȱ alongȱ theȱ electrolysis,ȱ givingȱ anȱ
averageȱpseudoȬfirstȬorderȱrateȱconstantȱ(k1)ȱofȱ(4.70±0.10)ȱxȱ10Ȭ4ȱsȬ1ȱandȱ(1.70±0.13)ȱxȱ
10Ȭ4ȱ sȬ1ȱ forȱ AOȱ withȱ H2O2ȱ electrogenerationȱ usingȱ Ptȱ andȱ BDD,ȱ respectively.ȱ Theseȱ
valuesȱareȱveryȱcloseȱtoȱ4.0ȱxȱ10Ȭ4ȱandȱ1.3ȱxȱ10Ȭ4ȱsȬ1ȱforȱPtȱandȱBDD,ȱrespectively,ȱfoundȱ
inȱAOȱwithȱaȱstainlessȱsteelȱcathode.ȱ
293
PART B –Results and Discussion8. Clofibric Acid
200
4
ln (c0 / c)
[clofibric acid] / mg L
-1
3
150
100
2
1
0
50
0
0
60
120
180
240
300
420
480
time / min
0
60
120
180
240
300
time / min
360
ȱ
Figure 8.-5 Clofibric acid decay with electrolysis time for the AO process of 100 mL
of 179 mg L-1 clofibric acid solutions in 0.05 M Na2SO4 of pH 3.0 at 100 mA cm-2 and at
35 ºC, using an undivided cell with 3-cm2 electrodes.
Process: (+) AO with a Pt anode and a stainless steel cathode, (×) AO with a BDD anode
and a stainless steel cathode, (ż) AO with a Pt anode and electrogenerated H2O2,
(Ƒ) Latter AO under UVA irradiation, (Ɣ) AO with a BDD anode and electrogenerated
H2O2, (Ŷ) Latter AO under UVA irradiation.
The corresponding kinetic analysis assuming a pseudo-first-order reaction for clofibric
acid is given in the inset panel.
ȱ
Similarly,ȱclofibricȱacidȱdecaysȱwithȱ timeȱbyȱ bothȱEFȱandȱPEFȱwithȱPtȱandȱBDDȱcanȱ
beȱcompared.ȱAȱcomparisonȱbetweenȱtheȱdecaysȱforȱtheseȱfourȱprocessesȱduringȱtheȱ
electrolysisȱ ofȱ 179ȱ mgȱ LȬ1ȱ clofibricȱ acidȱ atȱ pHȱ 3.0ȱ andȱ atȱ 100ȱ mAȱ cmȬ2ȱ isȱ depictedȱ inȱȱ
Figureȱ 8.Ȭ6ȱ shownȱ below.ȱ Aȱ muchȱ quickerȱ andȱ similarȱ decayȱ ofȱ clofibricȱ acidȱ isȱ
achievedȱcomparedȱtoȱAOȱtreatmentsȱpointedȱoutȱabove,ȱasȱexpectedȱfromȱtheȱgreatȱ
productionȱofȱ •OHȱfromȱFenton’sȱreaction.ȱInȱallȱcasesȱthisȱpollutantȱisȱdestroyedȱatȱaȱ
similarȱrate,ȱbeingȱcompletelyȱremovedȱafterȱca.ȱ7ȱmin.ȱAgain,ȱkineticȱanalysisȱinȱtheȱ
insetȱ panelȱ inȱ Figureȱ 8.Ȭ6ȱ agreesȱ withȱ aȱ pseudoȬfirstȬorderȱ reaction,ȱ leadingȱ toȱ anȱ
averageȱk1Ȭvalueȱofȱ(1.35±0.10)ȱxȱ10Ȭ2ȱsȬ1ȱforȱtheȱfourȱexperiments,ȱthusȱconfirmingȱtheȱ
prevailingȱ roleȱ ofȱ •OHȱ comparedȱ toȱ •OHads.ȱ Moreover,ȱ theȱ almostȱ coincidenceȱ
betweenȱEFȱandȱPEFȱindicatesȱaȱveryȱlowȱgenerationȱofȱ•OHȱfromȱReactionȱ5.Ȭ23ȱwithȱ
UVAȱirradiation.ȱ
294
PART B –Results and Discussion8. Clofibric Acid
200
5
150
ln (c0 / c)
[clofibric acid] / mg L
-1
4
100
2
1
0
0
50
0
3
2
4
6
time / min
0
2
4
time / min
6
8
ȱ
Figure 8.-6 Clofibric acid decay with electrolysis time for the degradation of 100 mL
of 179 mg L-1 clofibric acid solutions in 0.05 M Na2SO4 with 1.0 mM Fe2+ of pH 3.0 at
100 mA cm-2 and at 35 ºC, using an undivided cell with a 3-cm2 anode and a 3-cm2
O2-diffusion cathode.
Process: (¨) EF with a Pt anode, (¸) PEF with a Pt anode, (Ÿ) EF with a BDD anode,
(Ƈ) PEF with a BDD anode.
The corresponding kinetic analysis assuming a pseudo-first-order reaction for clofibric
acid is given in the inset panel.
ȱ
TheȱconcentrationȬtimeȱplotsȱobtainedȱforȱtheȱtreatmentsȱofȱ179ȱmgȱLȬ1ȱclofibricȱacidȱ
atȱdifferentȱcurrentȱdensitiesȱandȱatȱpHȱ 3.0ȱforȱPEFȱwithȱ PtȱandȱEFȱwithȱBDD,ȱshowȱ
thatȱaȱmoreȱ rapidȱdecayȱofȱclofibricȱacidȱisȱachievedȱwhenȱjappȱrises,ȱwithȱincreasingȱ
k1Ȭvaluesȱofȱ6.50ȱxȱ10Ȭ3,ȱ1.26ȱxȱ10Ȭ2ȱandȱ1.81ȱxȱ10Ȭ2ȱsȬ1ȱforȱPEFȱwithȱPt,ȱandȱ5.10ȱxȱ10Ȭ3,ȱ
1.35ȱxȱ10Ȭ2ȱandȱ2,04ȱxȱ10Ȭ2ȱsȬ1ȱforȱEFȱwithȱBDDȱatȱhigherȱjappȱofȱ33,ȱ100ȱandȱ150ȱmAȱcmȬ2,ȱ
respectively.ȱ Thisȱ trendȱ confirmsȱ aȱ largerȱ •OHȱ andȱ •OHadsȱ productionȱ whenȱ jappȱ
increases.ȱ Itȱ isȱ interestingȱ toȱ sayȱ thatȱ k1ȱ doesȱ notȱ varyȱ proportionallyȱ withȱ thisȱ
parameter,ȱ indicatingȱ aȱ progressiveȱ risingȱ wasteȱ ofȱ hydroxylȱ radicalsȱ byȱ parasiteȱ
reactions.ȱFinally,ȱtheȱpossibleȱinfluenceȱofȱinitialȱpollutantȱconcentrationȱinȱPEFȱwithȱ
PtȱhasȱbeenȱclarifiedȱfromȱelectrolysesȱofȱclofibricȱacidȱsolutionsȱofȱpHȱ3.0ȱupȱtoȱcloseȱ
toȱsaturationȱatȱ100ȱmAȱcmȬ2.ȱAȱcompleteȱremovalȱofȱtheȱpharmaceuticalȱisȱreachedȱinȱ
allȱcases.ȱThus,ȱitȱdisappearsȱafterȱ3,ȱ7,ȱ12ȱandȱ18ȱminȱforȱ89,ȱ179,ȱ358ȱandȱ557ȱmgȱLȬ1,ȱ
respectively.ȱ Goodȱ linearȱ correlationsȱ areȱ obtainedȱ forȱ allȱ concentrationsȱ tested,ȱ
295
PART B –Results and Discussion8. Clofibric Acid
assumingȱ aȱ pseudoȬfirstȱ orderȱ reactionȱ kinetics,ȱ andȱ thusȱ decreasingȱ k1Ȭvaluesȱ ofȱȱȱ
3.88ȱ xȱ 10Ȭ2,ȱ 1.26ȱ xȱ 10Ȭ2,ȱ 5.60ȱ xȱ 10Ȭ3ȱ andȱ 4.30ȱ xȱ 10Ȭ3ȱ sȬ1ȱ areȱ found.ȱ Thisȱ kineticȱ behaviorȱ
confirmsȱagainȱtheȱexistenceȱofȱaȱmuchȱgreaterȱandȱconstantȱamountȱofȱreactantȱ •OHȱ
inȱcomparisonȱwithȱtheȱamountȱofȱclofibricȱacid,ȱevenȱworkingȱcloseȱtoȱsaturation.ȱInȱ
addition,ȱ theȱ decayȱ inȱ k1ȱ withȱ risingȱ pollutantȱ concentrationȱ indicatesȱ theȱ gradualȱ
accelerationȱ ofȱ competitiveȱ reactionsȱ betweenȱ hydroxylȱ radicalsȱ andȱ intermediates,ȱ
thusȱ enhancingȱ theȱ TOCȱ removalȱ andȱ theȱ MCEȱ values,ȱ asȱ previouslyȱ discussedȱ inȱ
thisȱsection.ȱ
ȱ
Simultaneouslyȱ toȱ theȱ clofibricȱ acidȱ decayȱ study,ȱ theȱ evolutionȱ ofȱ aromaticȱ
intermediatesȱ hasȱ beenȱ carriedȱ out.ȱ GCȬMSȱ spectraȱ forȱ theȱ experimentsȱ reportedȱ inȱ
sectionȱ 8.3.1ȱ showȱ peaksȱ relatedȱ toȱ stableȱ aromaticsȱ suchȱ asȱ 4Ȭchlorophenol,ȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱ
4Ȭchlorocatechol,ȱhydroquinoneȱandȱpȬbenzoquinone.ȱInȱaddition,ȱinȱtheȱelectrolysesȱ
withȱaȱPtȱanodeȱanȱintenseȱpeakȱascribedȱtoȱaȱchloroȬderivativeȱisȱdetected.ȱAlthoughȱ
thisȱproductȱcanȱnotȱbeȱidentifiedȱbyȱpureȱstandards,ȱitȱcanȱbeȱreasonablyȱassignedȱtoȱ
aȱ dehydratedȱ speciesȱ ofȱ 2Ȭ(4ȬchloroȬ2Ȭhydroxyphenoxy)Ȭ2Ȭmethylpropionicȱ acid,ȱ
whichȱisȱaȱhydroxylatedȱproductȱofȱclofibricȱacid.ȱThisȱcompoundȱisȱdetectedȱneitherȱ
usingȱ aȱ BDDȱ anodeȱ becauseȱ itȱ isȱ quicklyȱ oxidized,ȱ norȱ inȱ AOȱ withȱ aȱ Ptȱ anodeȱ andȱ
stainlessȱ steelȱ cathodeȱ becauseȱ itȱ isȱ aȱ lowȱ oxidizingȱ method.ȱ ReversedȬphaseȱ
chromatographyȱ forȱ electrolyzedȱ solutionsȱ ofȱ 179ȱ mgȱ LȬ1ȱ clofibricȱ acidȱ ofȱ pHȱ 3.0ȱ atȱ
100ȱ mAȱ cmȬ2ȱ hasȱ beenȱ carriedȱ outȱ forȱ AO,ȱ EFȱ andȱ PEFȱ usingȱ bothȱ Ptȱ andȱ BDDȱ toȱ
knowȱ theȱ differentȱ evolutionȱ ofȱ eachȱ aromatic.ȱ 4ȬChlorophenol,ȱ 4Ȭchlororesorcinol,ȱȱ
4Ȭchlorocatechol,ȱ pȬbenzoquinoneȱ andȱ 1,2,4Ȭbenzenetriolȱ areȱ identifiedȱ andȱ
quantifiedȱ usingȱ Pt,ȱ whereasȱ onlyȱ 4Ȭchlorophenol,ȱ 4Ȭchlorocatecholȱ andȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱ
pȬbenzoquinoneȱareȱfoundȱwhenȱBDDȱisȱused.ȱHydroquinoneȱisȱnotȱdetectedȱinȱanyȱ
caseȱbecauseȱitȱisȱquicklyȱconvertedȱintoȱpȬbenzoquinone.ȱInȱAOȱwithȱPtȱorȱBDDȱallȱ
productsȱ areȱ poorlyȱ accumulatedȱ andȱ persistȱ longȱ time,ȱ asȱ expectedȱ fromȱ theȱ slowȱ
removalȱ ofȱ clofibricȱ acid.ȱ Inȱ contrast,ȱ theyȱ areȱ muchȱ moreȱ quicklyȱ formedȱ andȱ
destroyedȱ underȱ comparableȱ EFȱ andȱ PEFȱ degradationsȱ dueȱ toȱ theȱ greaterȱ •OHȱ
296
PART B –Results and Discussion8. Clofibric Acid
production.ȱ 4ȬChlorophenolȱ isȱ theȱ aromaticȱ intermediateȱ thatȱ showsȱ theȱ highestȱ
accumulationȱ inȱ EFȱ andȱ PEFȱ withȱ Ptȱ andȱ BDD,ȱ beingȱ upȱ toȱ 7.3ȱ andȱ 9.0ȱ mgȱ LȬ1ȱ
quantifiedȱ atȱ 1ȱ minȱ forȱ Ptȱ andȱ BDD,ȱ respectively,ȱ butȱ itȱ isȱ removedȱ inȱ lessȱ thanȱ 10ȱ
min.ȱAllȱtheȱrestȱofȱtheȱaromaticsȱareȱalsoȱremovedȱinȱlessȱthanȱ10Ȭ12ȱminȱbyȱEFȱandȱ
PEF,ȱthusȱconfirmingȱtheȱoxidizingȱabilityȱofȱtheseȱprocesses.ȱMoreover,ȱtheȱfactȱthatȱ
allȱ productsȱshowȱ aȱsimilarȱ evolutionȱinȱ bothȱ treatmentsȱconfirmsȱthatȱtheyȱareȱnotȱ
photolyzedȱ underȱ UVAȱ illumination.ȱ Onlyȱ pȬbenzoquinoneȱ seemsȱ toȱ beȱ influencedȱ
byȱ UVAȱ light,ȱ becauseȱ itȱ persistsȱ forȱ 360ȱ andȱ 60ȱ minȱ withoutȱ andȱ withȱ UVAȱ
irradiation,ȱrespectively.ȱ
ȱ
IonȬexclusionȱ chromatographyȱ analysesȱ forȱ theȱ aboveȱ AO,ȱ EFȱ andȱ PEFȱ processesȱ
usingȱPtȱorȱBDDȱallowȱcomparingȱtheȱevolutionȱofȱeachȱcarboxylicȱacid.ȱAcidsȱsuchȱ
asȱ tartronic,ȱ 2Ȭhydroxyisobutyric,ȱ maleic,ȱ fumaric,ȱ formicȱ andȱ oxalicȱ areȱ identified.ȱ
Tartronic,ȱ fumaric,ȱ maleicȱ andȱ formicȱ acidsȱ comeȱ fromȱ theȱ oxidationȱ ofȱ theȱ arylȱ
moietyȱofȱaromatics,ȱwhereasȱ2Ȭhydroxyisobutyricȱacidȱisȱreleasedȱinȱtheȱearlyȱstagesȱ
ofȱtheȱdegradationȱprocessȱwhenȱ4Ȭchlorophenolȱisȱformed.ȱInȱAOȱlargeȱamountsȱofȱ
theseȱcarboxylicȱacidsȱareȱslowlyȱaccumulated,ȱbutȱtheyȱareȱundetectedȱorȱdetectedȱasȱ
tracesȱforȱshortȱ timeȱ inȱ EFȱandȱ PEFȱ becauseȱtheyȱareȱquicklyȱdegraded.ȱ Inȱcontrast,ȱ
oxalicȱacidȱisȱalwaysȱaccumulatedȱtoȱaȱlargeȱextentȱandȱpersistsȱupȱtoȱtheȱendȱofȱtheȱ
mineralizationȱprocesses.ȱThisȱultimateȱacid,ȱformedȱfromȱtheȱindependentȱoxidationȱ
ofȱ theȱ precedentȱ longerȬchainȱ carboxylicȱ acids,ȱ asȱ wellȱ asȱ formicȱ acidȱ areȱ directlyȱ
covertedȱ intoȱ CO2.ȱ Theȱ productionȱ ofȱ theȱ latterȱ twoȱ acidsȱ isȱ confirmedȱ byȱ GCȬMSȱ
spectraȱ afterȱ esterificationȱ withȱ ethanol.ȱ Figureȱ 8.Ȭ7ȱ givenȱ belowȱ presentsȱ theȱ
evolutionȱofȱoxalicȱacid,ȱwhichȱisȱtheyȱkeyȱtoȱunderstandȱtheȱmineralizationȱabilityȱofȱ
EFȱandȱPEFȱprocesses.ȱFe3+ȱcomplexesȱofȱcarboxylicȱacidsȱareȱformedȱinȱbothȱcases.ȱInȱ
particular,ȱ Fe3+Ȭoxalatoȱ complexesȱ areȱ hardlyȱ oxidizableȱ withȱ •OH,ȱ andȱ thatȱ isȱ theȱ
reasonȱ whyȱ oxalicȱ acidȱ remainsȱ stableȱ inȱ EFȱ usingȱ Pt,ȱ thusȱ makingȱ itȱ impossibleȱ toȱ
completelyȱmineralizeȱtheȱtreatedȱsolution.ȱAboutȱ60ȱmgȱLȬ1ȱofȱthisȱacidȱremainȱinȱtheȱ
mediumȱatȱ360ȱmin,ȱcorrespondingȱtoȱ16ȱmgȱLȬ1ȱTOC,ȱwhereasȱtheȱresultingȱsolutionȱ
297
PART B –Results and Discussion8. Clofibric Acid
containsȱaboutȱ21ȱmgȱLȬ1,ȱsoȱoneȱcanȱconcludeȱthatȱstableȱpolyaromaticsȱareȱformed.ȱ
Inȱcontrast,ȱcompleteȱoxalicȱacidȱdegradationȱisȱachievedȱafterȱ360ȱminȱinȱPEFȱusingȱ
Ptȱ thanksȱ toȱ theȱ actionȱ ofȱ Reactionsȱ 5.Ȭ23ȱ andȱ 5.Ȭ24,ȱ thusȱ leadingȱ toȱ overallȱ
mineralization.ȱWhenȱBDDȱisȱused,ȱoxalicȱacidȱcanȱbeȱalwaysȱdestroyed,ȱbothȱinȱEFȱ
andȱPEF.ȱThisȱacidȱreachesȱhighȱcontentsȱofȱ68ȱandȱ59ȱmgȱLȬ1ȱafterȱ40ȱminȱofȱEFȱandȱ
PEF,ȱ respectively,ȱ dueȱ toȱ theȱ quickȱ oxidationȱ ofȱ organicsȱ withȱ •OH.ȱ Atȱ longerȱ time,ȱ
thisȱacidȱisȱgraduallyȱdestroyed,ȱuntilȱcompleteȱremovalȱatȱ360ȱandȱ240ȱminȱinȱEFȱandȱ
PEF.ȱItȱhasȱbeenȱsaidȱthatȱFe3+Ȭoxalatoȱcomplexesȱcanȱnotȱbeȱoxidizedȱwithȱ•OHȱinȱEF,ȱ
soȱtheyȱareȱslowlyȱmineralizedȱwithȱBDD(•OH).ȱFinally,ȱPEFȱusingȱBDDȱisȱtheȱmostȱ
potentȱ method,ȱ becauseȱ hardlyȱ oxidizableȱ complexesȱ ofȱ oxalicȱ acidȱ canȱ beȱ
simultaneouslyȱdestroyedȱbyȱBDD(•OH)ȱandȱUVAȱlight.ȱ
[oxalic acid] / mg L
-1
100
80
60
40
20
0
0
60
120
180
240
300
360
420
time / min
ȱ
Figure 8.-7 Time-course of the amount of oxalic acid for the experiments in Fig.8.-6.
ȱ
Consideringȱallȱtheȱintermediatesȱreportedȱaboveȱandȱacceptingȱthatȱhydroxylȱradicalȱ
isȱ theȱ mainȱ oxidizingȱ species,ȱ plausibleȱ reactionȱ schemesȱ forȱ theȱ degradationȱ ofȱ
clofibricȱ acidȱ inȱ acidicȱ aqueousȱ mediumȱ byȱ EFȱ andȱ PEFȱ withȱ 1.0ȱ mMȱ Fe2+ȱ areȱ
proposed.ȱTheȱpathwayȱforȱtheȱ systemsȱwithȱPtȱisȱveryȱ similarȱtoȱthatȱofȱAOȱwithȱaȱ
stainlessȱ steelȱ cathodeȱ alreadyȱ explainedȱ inȱ sectionȱ 8.2.2.ȱ Nevertheless,ȱ itȱ includesȱ
somethingȱworthȱmentioning:ȱ•OHȱcanȱhydroxylateȱclofibricȱacidȱonȱitsȱC(2)Ȭposition,ȱ
yieldingȱ aȱ ‘hydroxyȬclofibricȱ acid’.ȱ Subsequentȱ attackȱ ofȱ
298
•
OHȱ releasesȱ ȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱ
PART B –Results and Discussion8. Clofibric Acid
4Ȭchlorocatecholȱ andȱ 2Ȭhydroxyisobutyricȱ acid.ȱ Forȱ theȱ systemsȱ withȱ BDD,ȱ theȱ
reactionȱ pathwayȱ isȱ aȱ bitȱ different.ȱ Here,ȱ theȱ twoȱ kindȱ ofȱ hydroxylȱ radicalsȱ widelyȱ
discussedȱ throughoutȱ thisȱ sectionȱ areȱ ableȱ toȱ oxidizeȱ organics.ȱ Dueȱ toȱ theȱ greaterȱ
oxidizingȱabilityȱofȱBDD,ȱaȱlowerȱaccumulationȱofȱintermediatesȱisȱobserved,ȱsoȱtheȱ
sequenceȱproposedȱisȱlikeȱaȱreducedȱversionȱofȱtheȱoneȱwithȱPt:ȱonlyȱ4Ȭchlorophenol,ȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱ
4Ȭchlorocatechol,ȱ hydroquinoneȱ andȱ pȬbenzoquinoneȱ areȱ identifiedȱ asȱ aromaticȱ
intermediates.ȱ Alsoȱ 2Ȭhydroxyisobutyricȱ isȱ aȱ primaryȱ product,ȱ generatedȱ whenȱȱȱȱȱȱȱȱȱ
4Ȭchlorophenolȱ isȱ formed.ȱ Then,ȱ theȱ oxidationȱ ofȱ pȬbenzoquinoneȱ andȱ ȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱ
4Ȭchlorocatecholȱ canȱ causeȱ theȱ openingȱ ofȱ theirȱ benzenicȱ ringsȱ toȱ yieldȱ differentȱ
carboxylicȱacidsȱand,ȱatȱtheȱend,ȱoxalicȱacid.ȱ
ȱ
Figureȱ8.Ȭ8ȱgivenȱbelowȱshowsȱaȱproposedȱdegradationȱpathwayȱforȱoxalicȱacid.ȱThisȱ
acidȱisȱoxidizedȱtoȱCO2ȱwithȱBDD(•OH)ȱatȱtheȱanodeȱsurfaceȱeitherȱdirectlyȱinȱAOȱorȱ
asȱFe3+ȬoxalatoȱcomplexesȱinȱEFȱ(becauseȱtheseȱcomplexesȱareȱnotȱoxidizedȱwithȱ•OH).ȱ
Itȱ mustȱ beȱnotedȱ thatȱ BDD(•OH)ȱoxidizesȱmoreȱquicklyȱfreeȱoxalicȱ acidȱthanȱ itsȱ Fe3+ȱ
complexes.ȱ Theȱ latterȱ speciesȱ alsoȱ undegoȱ aȱ parallelȱ quickȱ photodecarboxylationȱ
underȱ theȱ irradiationȱ ofȱ UVAȱ lightȱ inȱ PEF,ȱ withȱ regenerationȱ ofȱ Fe2+.ȱ Theȱ actionȱ ofȱ
UVAȱ lightȱ justifiesȱ theȱ fastestȱ degradationȱ rateȱ andȱ highestȱ efficiencyȱ ofȱ PEFȱ withȱ
BDD.ȱ
ȱ
Fe3+Ȭoxalato
ȱȱcomplexes
ȱ
Fe3+
ȱ
hQ
BDD( xOH)
ȬFe2+
ȱ
COOH
ȱ
COOH
BDD(x OH)
CO2
Figure 8.-8 Proposed reaction pathways for oxalic acid mineralization
with a BDD anode and electrogenerated H2O2 by AO, EF and PEF.
ȱ
ȱ
ȱ
299
PART B –Results and Discussion8. Clofibric Acid
Theȱ solutionȱ pHȱ forȱ theȱ electrolysesȱ atȱ pHȱ 3.0ȱ remainsȱ practicallyȱ constantȱ
throughoutȱallȱtheȱexperiments,ȱreachingȱfinalȱvaluesȱbetweenȱ2.8ȱandȱ3.0.ȱMoreover,ȱ
theȱ startingȱ paleȱ yellowȱ solutionȱ changesȱ toȱ paleȱ orangeȱ colorȱ atȱ theȱ endȱ ofȱ EFȱ andȱ
PEFȱ degradationsȱ usingȱ aȱ Ptȱ anodeȱ dueȱ toȱ theȱ formationȱ ofȱ solubleȱ coloredȱ
polyaromaticsȱtoȱaȱsmallȱextent,ȱwhichȱcanȱnotȱbeȱdestroyedȱbyȱ •OH.ȱInȱcontrast,ȱinȱ
EFȱandȱPEFȱusingȱaȱBDDȱanodeȱtheȱinitialȱcolorȱchangesȱtoȱdarkȱyellow,ȱtypicalȱofȱtheȱ
complexesȱ betweenȱ Fe3+ȱ andȱ H2O2,ȱ butȱ orangeȱ colorȱ fromȱ polyaromaticsȱ isȱ notȱ
observedȱbecauseȱBDD(•OH)ȱisȱableȱtoȱdestroyȱthem.ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
300
PART B –Results and Discussion9. Chlorophene
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
9.ȱȱ DESTRUCCIÓȱ
D’UNȱ
FÀRMACȱ
ANTIMICROBIAL:ȱ
CLOROFÈȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱ
/ȱDESTRUCTIONȱOFȱANȱANTIMICROBIALȱDRUG:ȱCHLOROPHENEȱ
ȱ
ȱ
ȱ
ȱ
Thisȱ chapterȱ isȱ devotedȱ toȱ theȱ studyȱ ofȱ theȱ degradationȱ ofȱ theȱ antimicrobialȱ drugȱ
chlorophene.ȱ Inȱ thisȱ caseȱ itȱ isȱ dividedȱ intoȱ twoȱ parts:ȱ (i)ȱ anȱ introductionȱ givingȱ anȱ
overviewȱ onȱ theȱ characteristicsȱ ofȱ chlorophene,ȱ itsȱ environmentalȱ dataȱ andȱ someȱ
resultsȱ publishedȱ inȱ literatureȱ onȱ itsȱ destruction,ȱ (ii)ȱ theȱ resultsȱ obtainedȱ forȱ theȱ
destructionȱ ofȱ thisȱ drugȱ byȱ electroȬFentonȱ process,ȱ consideringȱ theȱ useȱ ofȱ twoȱ
differentȱCȬbasedȱmaterialsȱactingȱasȱtheȱcathode,ȱtheȱO2ȬdiffusionȱandȱtheȱcarbonȬfeltȱ
electrodes.ȱ
ȱ
Thisȱ workȱ hasȱ beenȱ carriedȱ outȱ duringȱ aȱ fourȬmonthsȱ stageȱ inȱ theȱ researchȱ teamȱ
Chimieȱdeȱl’Environnementȱ(LaboratoireȱdesȱGéomaériauxȱetȱGéologieȱdeȱl’Ingénieur,ȱInstitutȱ
FrancilienȱdesȱSciencsȱAppliquées,ȱUniversitéȱdeȱMarneȱlaȱVallée,ȱParis,ȱFrance)ȱunderȱtheȱ
supervisionȱofȱProfessorȱMehmetȱAliȱOturan.ȱ
ȱ
ȱ
301
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
PART B –Results and Discussion9. Chlorophene
9.1.
CARACTERÍSTIQUESȱDELȱCLOROFÈȱ
/ȱCHARACTERISTICSȱOFȱCHLOROPHENEȱ
ȱ
Theȱconcernsȱregardingȱantimicrobials,ȱespeciallyȱasȱforȱtheȱpromotionȱofȱpathogenicȱ
resistance,ȱ andȱ hormonesȱ inȱ theȱ environmentȱ areȱ betterȱ establishedȱ thanȱ forȱ otherȱ
PPCPsȱ[378].ȱAntimicrobials,ȱmainlyȱatȱlowȱconcentrations,ȱimposeȱselectiveȱpressureȱ
forȱ resistanceȱ (unabatedȱ growth),ȱ orȱ moreȱ commonly,ȱ toleranceȱ (temporaryȱ growthȱ
stoppage,ȱ butȱ continuedȱ viability)ȱ amongȱ potentiallyȱ pathogenicȱ microorganisms.ȱ
Promotionȱofȱantimicrobialȱresistanceȱisȱatȱleastȱpartlyȱcausedȱbyȱtheȱproliferationȱorȱ
overȬexpressionȱ ofȱ cellularȱ ‘multidrugȱ resistance’ȱ systemsȱ (effluxȱ pumpȬmediatedȱ
drugȱ resistance),ȱ whichȱ serveȱ toȱ minimizeȱ theȱ intracellularȱ concentrationsȱ ofȱ
contaminants.ȱ Theȱ acquiredȱ resistanceȱ orȱ toleranceȱ canȱ beȱ evenȱ permanentȱ andȱ
geneticallyȱ conserved,ȱ persistingȱ inȱ theȱ absenceȱ ofȱ continuedȱ selectiveȱ pressureȱ byȱ
theȱ antimicrobial.ȱ Aȱ straightforwardȱ evidenceȱ ofȱ thisȱ overuseȱ andȱ misuseȱ isȱ thatȱ
diseasesȱthatȱonceȱwereȱeasilyȱcuredȱbyȱantimicrobialsȱareȱbecomingȱmoreȱdifficultȱtoȱ
treat,ȱ andȱ theȱ reasonȱ isȱ simple:ȱ evolution.ȱ Overȱ time,ȱ theseȱ hardȱ strainsȱ comeȱ toȱ
predominateȱ inȱ theȱ populationȱ andȱ theȱ drugsȱ areȱ noȱ longerȱ effectiveȱ againstȱ them.ȱ
Antimicrobialsȱalsoȱhaveȱtheȱpotentialȱtoȱalterȱmicrobialȱspeciesȱdiversity,ȱleadingȱtoȱ
alteredȱsuccessionalȱconsequences.ȱ
ȱ
Chloropheneȱ (oȬbenzylȬpȬchlorophenol,ȱ Figureȱ 9.Ȭ1)ȱ isȱ anȱ arylȱ halideȱ biocideȱ
belongingȱ toȱ theȱ therapeuticalȱ classȱ knownȱ asȱ antimicrobialsȱ (alsoȱ referredȱ asȱ
antibacterialsȱorȱantiseptics),ȱandȱwasȱfirstȱregisteredȱinȱUSAȱinȱ1948ȱasȱdisinfectant.ȱ
Itȱ isȱ aȱ whiteȱ flouryȱ powder,ȱ withȱ aȱ slightlyȱ phenolicȱ odourȱ butȱ withȱ aȱ clearȱ
specificationȱ ofȱ ‘carcinogenicȱ agent’ȱ stuckȱ inȱ theȱ commercialȱ can.ȱ Itȱ isȱ aȱ cutaneousȱ
irritantȱandȱitȱhasȱbeenȱrecognizedȱtoȱpossessȱaȱweakȱskinȱtumorȱpromotingȱactivityȱ
inȱhumanȱbeings.ȱChloropheneȱalsoȱappearsȱtoȱbeȱnephrotoxicȱforȱratsȱandȱmice.ȱ
ȱ
SomeȱofȱtheȱmostȱremarkableȱpropertiesȱofȱchloropheneȱareȱsummarizedȱinȱTableȱ9.Ȭ1.ȱ
303
PART B –Results and Discussion9. Chlorophene
OH
ȱ
ȱ
ȱ
ȱ
Cl
Figure 9.-1 Chlorophene.
ȱ
Table 9.-1 Chlorophene data [379].
ȱ
ȱ
CAS number
120-32-1
4-Chloro-2-(phenylmethyl)phenol
ȱ
Generic names
ȱ
Trade names
ȱ
Molecular formula
ȱ
Molecular mass (g mol-1)
ȱ
Melting point (ºC)
48.5
Boiling point (ºC)
175
Solubility in H2O (mg L-1)25 ºC
149
ȱ
ȱ
ȱ
o-benzyl-p-chlorophenol
Santophen 1, Preventol BP, Nipacide
C13H11ClO
218.68
Density (g cm-3)20 ºC
1.188
pKa
10.8
ȱ
ChloropheneȱhasȱbeenȱchosenȱsinceȱitȱisȱaȱwidespreadȱbroadȬspectrumȱantimicrobialȱ
pharmaceutical,ȱ commonlyȱ usedȱ inȱ hospitalsȱ andȱ householdsȱ forȱ generalȱ cleaningȱ
andȱdisinfecting,ȱasȱwellȱasȱinȱindustrialȱandȱfarmingȱenvironmentsȱasȱanȱactiveȱagentȱ
inȱ disinfectantȱ formulationsȱ [380Ȭ382].ȱ Itȱ isȱ alsoȱ usedȱ asȱ anȱ algaecide,ȱ fungicide,ȱ
microbicide/microbistatȱandȱvirucide.ȱThereȱareȱcurrentlyȱ143ȱproductsȱregisteredȱbyȱ
theȱEPAȱcontainingȱchloropheneȱactiveȱingredients.ȱ
ȱ
Althoughȱchloropheneȱisȱexpectedȱtoȱposeȱaȱlowȱtoxicityȱforȱhumans,ȱevidenceȱofȱitsȱ
carcinogenicȱ andȱ mutagenicȱ activityȱ inȱ animalsȱ isȱ documented,ȱ soȱ certainȱ attentionȱ
mustȱbeȱdevotedȱtoȱitsȱbehavior.ȱ
ȱ
304
PART B –Results and Discussion9. Chlorophene
Thereȱ areȱ notȱ availableȱ dataȱ regardingȱ theȱ usageȱ ofȱ chloropheneȱ andȱ itsȱ salts,ȱ butȱ
theseȱ productsȱ accountȱ forȱ aȱ substantialȱ shareȱ ofȱ householdȱ disinfectantȱ productsȱ
usedȱ inȱ theȱ midȬ1980s.ȱ Industryȱ hasȱ longȱ reliedȱ onȱ theȱ lackȱ ofȱ suchȱ dataȱ toȱ justifyȱ
inactionȱ onȱ theȱ basisȱ ofȱ itsȱ beliefȱ thatȱ theȱ linkȱ betweenȱ useȱ ofȱ antimicrobialsȱ inȱ
animalsȱ andȱ humanȱ healthȱ consequencesȱ wasȱ unproved.ȱ Thisȱ hasȱ alwaysȱ beenȱ aȱ
cynicalȱ position,ȱ sinceȱ industryȱ possessesȱ theȱ dataȱ thatȱ wouldȱ makeȱ theȱ linkȱ moreȱ
apparent.ȱAntimicrobialȱagentsȱcanȱbeȱfoundȱinȱsewageȱeffluents,ȱespeciallyȱinȱplacesȱ
whereȱtheyȱareȱusedȱextensively,ȱsuchȱasȱhospitals,ȱpharmaceuticalȱproductionȱplants,ȱ
andȱnearȱfarmsȱwhereȱanimalȱfeedȱcontainingȱantimicrobialȱagentsȱisȱused.ȱ
ȱ
Occurrenceȱofȱchloropheneȱinȱtheȱaquaticȱenvironmentȱisȱnotȱveryȱwellȱdocumented.ȱ
Thomasȱetȱat.ȱ[383]ȱhaveȱreportedȱitsȱdetectionȱinȱsedimentsȱcollectedȱfromȱestuariesȱ
inȱtheȱUnitedȱKingdom.ȱChloropheneȱhasȱbeenȱroutinelyȱfoundȱinȱbothȱinfluentsȱ(upȱ
toȱ 0.71ȱ Pgȱ LȬ1)ȱ andȱ effluentsȱ ofȱ STPsȱ [384],ȱ butȱ itȱ hasȱ beenȱ detectedȱ evenȱ atȱ
concentrationsȱ upȱ toȱ 50ȱ mgȱ LȬ1ȱ inȱ activatedȱ sludgeȱ sewageȱ systems,ȱ andȱ upȱ toȱ 10ȱȱȱȱȱȱ
ΐgȱLȬ1ȱinȱsewageȱtreatmentȱplantȱeffluentsȱandȱrivers.ȱItȱisȱknownȱthatȱitsȱremovalȱisȱ
notȱasȱextensiveȱasȱforȱbiphenylol,ȱanotherȱcommonȱantimicrobialȱagent.ȱ
ȱ
Thereȱ isȱ aȱ greatȱ scarcityȱ ofȱ informationȱ aboutȱ theȱ waysȱ toȱ avoidȱ theȱ dangerousȱ
accumulationȱofȱchloropheneȱinȱsoilsȱandȱtheȱaquaticȱenvironment.ȱArnoldȱetȱal.ȱ[381]ȱ
haveȱ reportedȱ theȱ photodegradationȱ ofȱ itsȱ deprotonatedȱ phenolateȱ form,ȱ asȱ wellȱ asȱ
theȱ reactionȱ ofȱ chloropheneȱ withȱ hydroxylȱ radicalsȱ (showingȱ aȱ secondȬorderȱ rateȱ
constant,ȱk2ȱ=ȱ7.1ȱxȱ109ȱMȬ1ȱsȬ1).ȱZhangȱetȱal.ȱhaveȱdescribedȱitsȱoxidativeȱdegradationȱ
withȱ MnO2ȱ andȱ theyȱ haveȱ reportedȱ theȱ conversionȱ ofȱ chloropheneȱ intoȱ polymericȱ
intermediates.ȱ Precisely,ȱ Zhangȱ hasȱ presentedȱ inȱ hisȱ doctoralȱ thesisȱ theȱ mostȱ
extensiveȱinvestigationȱonȱtheȱdegradationȱofȱantibacterialȱagentsȱusingȱmetalȱoxidesȱ
[385].ȱ Anȱ interestingȱ traitȱ ofȱ thisȱ compoundȱ isȱ thatȱ noȱ previousȱ worksȱ areȱ foundȱ inȱ
literatureȱonȱitsȱremovalȱfromȱwaterȱbyȱmeansȱofȱpotentȱoxidationȱproceduresȱsuchȱasȱ
AOPs,ȱtoȱmineralizeȱchloropheneȱratherȱthanȱtransformȱit.ȱ
305
PART B –Results and Discussion9. Chlorophene
Asȱ inȱ theȱ caseȱ ofȱ paracetamolȱ andȱ clofibricȱ acid,ȱ anȱ importantȱ goalȱ inȱ theȱ studyȱ ofȱ
chloropheneȱ isȱ toȱ designȱ effectiveȱ andȱ optimizedȱ processesȱ toȱ removeȱ itȱ fromȱ
wastewaters.ȱ Therefore,ȱ theȱ presentȱ workȱ dealsȱ withȱ someȱ ofȱ theȱ fundamentalȱ
aspectsȱofȱtheȱEFȱprocess:ȱ(i)ȱtheȱactualȱreductionȱabilityȱofȱtheȱcathodeȱtoȱregenerateȱ
Fe2+ȱfromȱdirectȱreductionȱofȱFe3+,ȱ(ii)ȱtheȱoxidationȱabilityȱofȱPtȱandȱBDDȱanodesȱtoȱ
convertȱFe2+ȱintoȱFe3+,ȱ(iii)ȱtheȱactionȱofȱweakȱoxidantsȱformedȱatȱtheȱanodeȱonȱtheȱFe2+ȱ
contentȱ inȱ solutionȱ andȱ (iv)ȱ theȱ roleȱ ofȱ Fe2+/Fe3+ȱ complexesȱ withȱ carboxylicȱ acidsȱ toȱ
analyzeȱtheȱcomparativeȱrateȱremovalȱofȱtheȱpollutant.ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
306
PART B –Results and Discussion9. Chlorophene
9.2.
TRACTAMENTȱMITJANÇANTȱELECTROȬFENTONȱȱ
/ȱTREATMENTȱBYȱELECTROȬFENTONȱ
ȱ
9.2.1.ȱȱFinalitatȱdelȱtreballȱ/ȱAimȱofȱtheȱworkȱ
ȱ
Allȱ treatmentsȱ wereȱ conductedȱ atȱ roomȱ temperatureȱ inȱ anȱ undividedȱ glassȱ cellȱ byȱ
electrolyzingȱ200ȬmLȱchloropheneȱsolutionsȱofȱpHȱ3.0ȱcontainingȱ0.05ȱMȱNa2SO4ȱandȱ
differentȱ concentrationsȱ ofȱ Fe3+ȱ asȱ catalyst.ȱ Fourȱ EFȱ systemsȱ wereȱ tested:ȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱ
Pt/O2ȱ diffusion,ȱ BDD/O2ȱ diffusion,ȱ Pt/carbonȱ feltȱ andȱ BDD/carbonȱ felt.ȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱȱ
Theȱ O2Ȭdiffusionȱ cathodeȱ wasȱ directlyȱ fedȱ withȱ pureȱ O2ȱ atȱ 20ȱ mLȱ minȬ1ȱ toȱ generateȱ
continuouslyȱ H2O2,ȱ whereasȱ theȱ cellsȱ withȱ aȱ carbonȬfeltȱ cathodeȱ wereȱ saturatedȱ ofȱ
thisȱgasȱbyȱbubblingȱcompressedȱairȱatȱ1ȱLȱminȬ1,ȱstartingȱ15ȱminȱbeforeȱelectrolysis.ȱ
ȱ
TheȱfirstȱgoalȱofȱthisȱstudyȱwasȱtoȱclarifyȱtheȱcatalyticȱbehaviorȱofȱtheȱFe3+/Fe2+ȱsystemȱ
inȱ theȱ EFȱ processȱ inȱ theȱ absenceȱ ofȱ chlorophene.ȱ Withȱ thisȱ aim,ȱ firstȱ ofȱ allȱ theȱ
oxidationȱ abilityȱ ofȱ theȱ Ptȱ andȱ BDDȱ anodesȱ toȱ transformȱ directlyȱ Fe2+ȱ intoȱ Fe3+ȱ wasȱ
tested.ȱSeveralȱelectrolysesȱwithȱ4.0ȱmMȱFe2+ȱwereȱmadeȱatȱ300ȱmAȱwithȱPtȱorȱBDD,ȱ
usingȱaȱstainlessȱsteelȱcathodeȱtoȱfocusȱtheȱstudyȱonȱtheȱactivityȱofȱtheȱanode.ȱThen,ȱ
severalȱexperimentsȱwereȱperformedȱtoȱanalyzeȱtheȱevolutionȱofȱFe2+ȱandȱFe3+ȱions,ȱasȱ
wellȱ asȱ theȱ accumulationȱ ofȱ H2O2,ȱ inȱ theȱ fourȱ EFȱ systemsȱ pointedȱ outȱ above.ȱ
Solutionsȱwithȱ4.0ȱmMȱFe3+ȱwereȱelectrolyzedȱatȱ300ȱmAȱforȱ60ȱminȱusingȱPtȱandȱBDDȱ
anodesȱwithȱanȱO2Ȭdiffusionȱcathode,ȱandȱsolutionsȱwithȱ0.2ȱmMȱFe3+ȱunderȱtheȱsameȱ
conditionsȱwereȱtreatedȱwithȱaȱcarbonȬfeltȱcathode.ȱ
ȱ
ToȱconfirmȱtheȱbehaviorȱofȱtheȱFe3+/Fe2+ȱsystemȱseveralȱdegradationsȱinȱtheȱpresenceȱ
ofȱ chloropheneȱ wereȱ carriedȱ outȱ byȱ theȱ fourȱ EFȱ processesȱ aforementioned.ȱ
Chloropheneȱ destructionȱ wasȱ followedȱ byȱ reversedȬphaseȱ HPLCȱ chromatography.ȱ
Firstȱ ofȱ all,ȱ theȱ chloropheneȱ decayȱ wasȱ studiedȱ byȱ electrolyzingȱ 50ȱ mgȱ LȬ1ȱ
chloropheneȱ solutions,ȱ withȱ Fe3+ȱ initialȱ contentȱ betweenȱ 0.2ȱ andȱ 8.0ȱ mM,ȱ atȱ pHȱ 3.0ȱ
307
PART B –Results and Discussion9. Chlorophene
andȱ atȱ 300ȱ mAȱ usingȱ theȱ Pt/O2ȱ diffusionȱ cell.ȱ Aȱ chemicalȱ testȱ usingȱ 50ȱ mgȱ LȬ1ȱ
chloropheneȱandȱ20ȱmMȱH2O2ȱwasȱalsoȱperformedȱtoȱassessȱtheȱoxidizingȱpowerȱofȱ
H2O2.ȱTheȱPt/O2ȱdiffusionȱcellȱwasȱthenȱusedȱtoȱelectrolyzeȱaȱsolutionȱunderȱtheȱsameȱ
conditionsȱ butȱ inȱ theȱ absenceȱ ofȱ Fe3+ȱ toȱ observeȱ theȱ oxidationȱ abilityȱ ofȱ AOȱ withȱ
electrogeneratedȱ H2O2.ȱ Aȱ parallelȱ studyȱ wasȱ carriedȱ outȱ withȱ theȱ BDD/O2ȱ diffusionȱ
cellȱunderȱtheȱpreviousȱconditionsȱtoȱdiscussȱtheȱinfluenceȱofȱtheȱanode.ȱAfterȱusingȱ
theȱ O2Ȭdiffusionȱ cathode,ȱ theȱ carbonȬfeltȱ cathodeȱ wasȱ testedȱ withȱ bothȱ anodesȱ andȱ
Fe3+ȱconcentrationȱbetweenȱ0.1ȱandȱ2.0ȱmMȱatȱ60ȱandȱ300ȱmA.ȱKineticȱanalysisȱofȱtheȱ
aboveȱchloropheneȱdecaysȱwasȱsimultaneouslyȱdone.ȱInȱaddition,ȱtheȱsecondȬorderȬ
rateȱ constantȱ forȱ theȱ reactionȱ betweenȱ chloropheneȱ andȱ hydroxylȱ radicalȱ wasȱ
determinedȱthroughȱtheȱmethodȱofȱcompetitiveȱkinetics.ȱToȱdoȱthis,ȱ200ȬmLȱsolutionsȱ
ofȱ pHȱ 3.0ȱ containingȱ50ȱmgȱLȬ1ȱchlorophene,ȱ 122ȱmgȱLȬ1ȱbenzoicȱacidȱ(asȱ aȱstandardȱ
competitionȱ substrate)ȱ andȱ 0.2ȱ mMȱ Fe3+ȱ wereȱ electrolyzedȱ atȱ 60ȱ mAȱ usingȱ theȱ
Pt/carbonȱfeltȱandȱBDD/carbonȱfeltȱcells.ȱ
ȱ
Onceȱ theȱ abilityȱ ofȱ theȱ fourȱ EFȱ methodsȱ toȱ destroyȱ chloropheneȱ wasȱ assessed,ȱ itsȱ
mineralizationȱpowerȱhadȱtoȱbeȱdemonstratedȱfromȱtheȱcorrespondingȱTOCȱdecayȱtoȱ
assureȱtheirȱcompleteȱefficacy.ȱThisȱstudyȱwasȱcarriedȱoutȱbyȱelectrolyzingȱ84ȬmgȱLȬ1ȱ
chloropheneȱsolutionsȱ(i.e.,ȱ60ȱmgȱLȬ1ȱTOC)ȱatȱpHȱ3.0ȱandȱatȱ60,ȱ100,ȱ200ȱandȱ300ȱmA,ȱ
usingȱtheȱfourȱEFȱcellsȱpointedȱoutȱ above.ȱ EfficientȱFe3+ȱcontentsȱ ofȱ4.0ȱandȱ 0.2ȱmMȱ
wereȱusedȱforȱtheȱcellsȱwithȱtheȱO2ȬdiffusionȱandȱtheȱcarbonȬfeltȱcathode,ȱrespectively.ȱ
ȱ
Chlorideȱ ionȱ evolutionȱ wasȱ followedȱ byȱ recordingȱ theȱ ionȱ chromatogramsȱ
correspondingȱ toȱ theȱ treatmentȱ ofȱ 84Ȭmgȱ LȬ1ȱ chloropheneȱ solutionsȱ withȱ 0.015ȱ Mȱ
Na2SO4ȱandȱ0.2ȱmMȱFe3+ȱatȱpHȱ3.0ȱandȱatȱ150ȱmAȱforȱtheȱfourȱEFȱcells.ȱ
ȱ
TheȱevolutionȱofȱintermediatesȱwasȱfollowedȱbyȱreversedȬphaseȱchromatographyȱandȱ
ionȬexclusionȱchromatography.ȱToȱidentifyȱtheȱaromatics,ȱseveralȱtrialsȱwereȱmadeȱbyȱ
applyingȱ lowȱ currentsȱ andȱ usingȱ theȱ Pt/O2ȱ diffusionȱ systemȱ withȱ lowȱ oxidizingȱ
308
PART B –Results and Discussion9. Chlorophene
power.ȱ Carboxylicȱ acidsȱ wereȱ identifiedȱ andȱ quantifiedȱ inȱ theȱ fourȱ EFȱ systemsȱ byȱ
treatingȱ84ȬmgȱLȬ1ȱchloropheneȱsolutionsȱofȱpHȱ3.0ȱatȱ60ȱandȱ300ȱmA,ȱwithȱ4.0ȱandȱ0.2ȱ
mMȱFe3+ȱforȱtheȱcellsȱusingȱtheȱO2ȬdiffusionȱandȱtheȱcarbonȬfeltȱcathode,ȱrespectively.ȱ
GCȬMSȱwasȱalsoȱusedȱtoȱdetectȱtheȱaromaticȱintermediatesȱduringȱtheȱdegradationȱofȱ
50ȱ mgȱ LȬ1ȱ chloropheneȱ inȱ theȱ Pt/O2ȱ diffusionȱ cellȱ atȱ 60ȱ mAȱ forȱ 30ȱ min.ȱ Forȱ theȱ
identificationȱofȱcarboxylicȱacids,ȱtheȱsameȱtreatmentȱwasȱperformedȱandȱprolongedȱ
forȱ 2ȱ h.ȱ Priorȱ toȱ injectionȱ ofȱ theȱ samples,ȱ differentȱ preparativeȱ sequencesȱ wereȱ
appliedȱtoȱtheȱsolutionsȱobtained.ȱ
ȱ
Finally,ȱ theȱ possibleȱ reactionȱ pathsȱ ofȱ oxalicȱ acid,ȱ whichȱ isȱ theȱ ultimateȱ byȬproductȱ
formedȱduringȱtheȱmineralizationȱprocessȱbeforeȱtheȱtotalȱconversionȱofȱallȱtheȱinitialȱ
organicȱcarbonȱintoȱCO2,ȱcouldȱbeȱschematizedȱforȱtheȱEFȱsystemsȱused.ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
309
PART B –Results and Discussion9. Chlorophene
Theȱthoroughȱresultsȱofȱthisȱsectionȱareȱincludedȱinȱtheȱfollowingȱpaperȱ(Paperȱ7):ȱ
ȱ
7.ȱ Sirés,ȱ I.,ȱ Garrido,ȱ J.A.,ȱ Rodríguez,ȱ R.M.,ȱ Brillas,ȱ E.,ȱ Oturan,ȱ N.,ȱ Oturan,ȱ M.A.,ȱ
Catalyticȱ behaviourȱ ofȱ theȱ Fe3+/Fe2+ȱ systemȱ inȱ theȱ electroȬFentonȱ degradationȱ ofȱ
theȱantimicrobialȱchlorophene.ȱAppl.ȱCatal.ȱB:ȱEnviron.ȱ(acceptedȱforȱpublication)ȱ
ȱ
Theȱfollowingȱpresentationȱinȱcongressȱisȱrelatedȱtoȱthisȱwork:ȱ
ȱ
G.ȱ Sirés,ȱ I.,ȱ Oturan,ȱ N.,ȱ Brillas,ȱ E.,ȱ Oturan,ȱ M.A.,ȱ Electrochemicalȱ degradationȱ ofȱ
antimicrobialsȱbyȱelectroȬFentonȱprocess:ȱComparativeȱperformanceȱofȱcarbonȱfeltȱ
cathodeȱ versusȱ oxygenȱ diffusionȱ cathode,ȱ Vol.ȱ 1,ȱ pageȱ 28,ȱ 7thȱ Electrochemistryȱ
Daysȱ (7.ȱ Elektrokimiyaȱ Günleri),ȱ Hacettepeȱ Üniversitesi,ȱ Ankara,ȱ Turkey,ȱ 28Ȭ30ȱ
Juneȱ2006.ȱ(Oralȱpresentation)ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
310
ARTICLEȱ7ȱ/ȱPAPERȱ7
ȱ
ȱ
CatalyticȱbehaviourȱofȱtheȱFe3+/Fe2+ȱsystemȱinȱtheȱelectroȬFentonȱ
ȱ degradationȱofȱtheȱantimicrobialȱchloropheneȱ
PART B –Results and Discussion9. Chlorophene
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Elsevier
Editorial System(tm) for Applied Catalysis B: Environmental
ȱ
ȱ
Manuscript Draft
ȱ
ȱ
Manuscript Number:
ȱ
Title:
ȱ
Catalytic behavior of the Fe3+/Fe2+ system in the electro-Fenton degradation of the antimicrobial
ȱ
chlorophene
ȱ
Article Type: Full Length Article
ȱ
ȱ
Keywords:
Antimicrobials;
Electro-Fenton method; Advanced oxidation processes; Degradation; Water treatment
ȱ
ȱ
Corresponding Author: Prof. Mehmet A. Oturan,
ȱ
ȱ
Corresponding Author's Institution: Universit« de Marne la Vall«e
ȱ
ȱ Author: Ignacio Sir«s
First
ȱ
ȱ
ȱ
ȱ
ȱ
Order of Authors: Ignacio Sir«s; Jos« A. Garrido; Rosa M. Rodr¯guez; Enric Brillas; Nihal Oturan; Mehmet
ȱ
A. Oturan
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
311
PART B –Results and Discussion9. Chlorophene
ȱ Cover Letter
ȱ
ȱ
Please find attached a copy of the manuscript entitled “Catalytic behavior of the Fe3+/Fe 2+
ȱ
system in the electro-Fenton degradation of the antimicrobial chlorophene" by Ignasi Sirés,
José Antonio Garrido, Rosa María Rodríguez, Enric Brillas, Nihal Oturan and myself, which
ȱ
we submit to your consideration in order to be published in "Applied Catalysis B:
ȱ
Environmental". The corresponding author will be myself. I am available at [email protected]; mailing address: Laboratoire des Géomatériaux, Université de Marne la Vallée, 5
ȱ
Boulevard Descartes, Champs-sur-Marne, 77454 Marne-la-Vallée Cedex 2 - France; phone:
ȱ
+33 1 49 32 90 65 ; fax: +33 1 49 32 91 37.
ȱ
We present in this work a comparative study on oxidizing power and mineralization
ȱ
efficiency of four variants of electro-Fenton process which is developed by Brillas's and
Oturan's team during the last decade. We show that the efficiency of different systems under
ȱ
study is determined by the nature of cathode/anode materials used and the catalytic behaviour
of the Fe 3+/Fe 2+ redox couple as catalyst. In this paper we demonstrate that the four electro-
ȱ
Fenton system studied can be successfully applied to the treatment of an antimicrobial
ȱ
(chlorophene) aqueous solution, which is an emergent environmental pollutant.
ȱ
We think this manuscript is appropriate for publication in Applied Catalysis B: Environmental
ȱ
on account of the growing importance of advanced electrochemical oxidation process
(AEOPs) in treatment of persistent organic pollutants (POPs). This innovative and
ȱ
environmentally friendly technology can have high potential impact on this field due to its
ȱ
very high mineralization efficiency.
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
312
PART B –Results and Discussion9. Chlorophene
*ȱ Manuscript
ȱ
ȱ
1
Catalytic behavior of the Fe3+/Fe2+ system in the electro-
ȱ
2
Fenton degradation of the antimicrobial chlorophene
ȱ
3
ȱ
4
Ignasi Sirés a, José Antonio Garrido a, Rosa María Rodríguez a, Enric Brillas a,
ȱ
5
Nihal Oturan b, Mehmet Ali Oturan b*
ȱ
6
ȱ
7
a
ȱ
8
Física, Facultat de Química, Universitat de Barcelona, Martí i Franquès 1-11, 08028
9
Barcelona, Spain
ȱ
ȱ
ȱ
ȱ
Laboratori d’Electroquímica dels Materials i del Medi Ambient, Departament de Química
10
b
11
Boulevard Descartes, Champs-sur-Marne, 77454 Marne-la-Vallée Cedex 2, France
Université de Marne la Vallée, Laboratoire des Géomatériaux et Géologie de l'Ingénieur, 5
12
ȱ
ȱ
ȱ
ȱ
13
14
15
Paper submitted to be published in Applied Catalysis B:Environmental
16
ȱ
17
ȱ
18
ȱ
19
ȱ
20
*Corresponding author: Tel.: + 33 149 32 90 65,
ȱ
21
E-mail address: [email protected] (M.A. Oturan)
ȱ
ȱ
ȱ
ȱ
1
ȱ
ȱ
313
PART B –Results and Discussion9. Chlorophene
ȱ
ȱ
22
23
Abstract
24
Solutions of the antimicrobial chlorophene with 0.05 M Na 2SO 4 and Fe3+ as catalyst of
25
pH 3.0 have been comparatively degraded by the electro-Fenton method using four undivided
26
electrolytic cells containing a Pt or boron-doped diamond (BDD) anode and a carbon-felt or
ȱ 27
O 2-diffusion cathode at constant current. Under these environmentally friendly conditions,
ȱ 28
pollutants are oxidized with hydroxyl radical (•OH) formed at the anode from water oxidation
29
and in the medium from Fenton’s reaction between electrogenerated Fe2+ and H2O 2 at the
30
cathode. The catalytic behavior of the Fe3+/Fe2+ system mainly depends on the cathode tested.
31
In the cells with an O 2-diffusion cathode, H2O 2 is largely accumulated and their Fe3+ content
ȱ 32
remains practically unchanged, while the chlorophene decay is enhanced when Fe3+
ȱ 33
concentration rises due to the greater •OH production from the higher quantity of Fe2+
ȱ 34
regenerated at the cathode. When a carbon-felt cathode is used, H2O 2 is electrogenerated in
35
small extent with large accumulation of Fe2+, because this ion is more rapidly regenerated at
36
the cathode than oxidized to Fe3+ at the Pt or BDD anode, only being required the presence of
37
0.2 mM Fe3+ to obtain the maximum •OH generation with the quickest chlorophene removal.
ȱ 38
Chlorophene is poorly mineralized in the Pt/O 2 diffusion cell due to the difficult oxidation of
ȱ 39
final Fe3+-oxalate complexes with •OH. These species are completely destroyed using a BDD
ȱ 40
anode at high current due to the great •OH generation on its surface. Total mineralization is
41
also achieved in the Pt/carbon felt and BDD/carbon felt cells with 0.2 mM Fe3+, where oxalic
42
acid and its Fe2+ complexes are directly oxidized with •OH in the medium. The highest
43
oxidizing power for total mineralization at high current is attained for the BDD/carbon felt
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ 44
ȱ
ȱ
ȱ
cell, because oxalic acid can be simultaneously destroyed on BDD.
45
46
47
ȱ 48
Keywords: Antimicrobials; Electro-Fenton
ȱ 49
Degradation; Water treatment
method; Advanced
oxidation
processes;
ȱ
ȱ
2
ȱ
ȱ
ȱ
314
PART B –Results and Discussion9. Chlorophene
ȱ
ȱ
50
51
1. Introduction
52
A large variety of advanced oxidation processes (AOP’s) have been recently proposed for
53
the degradation of toxic and biorefractory organics in wastewaters [1-3]. They are chemical,
54
photochemical, photocatalytic and electrochemical procedures characterized by the in situ
ȱ
55
generation of hydroxyl radical (•OH) as the main oxidizing agent of pollutants. This radical
ȱ
56
has a high standard potential (Eº(•OH/H2O) = 2.80 V vs. NHE) and it is the second most
ȱ
57
strong oxidizing species known, after fluorine. •OH has enough ability to react non-
58
selectively with organics yielding dehydrogenated or hydroxylated derivatives up to their
59
final mineralization, i.e., their total conversion into CO2, water and inorganic ions. One of the
60
most popular AOP’s for the treatment of acidic waters is the Fenton’s reagent [2-4],
ȱ
61
composed of a mixture of Fe 2+ and hydrogen peroxide that is added to the contaminated water
ȱ
62
to produce •OH and Fe3+ according to the classical Fenton’s reaction (1) with a second-order
ȱ
63
rate constant (k) of 63 M-1 s -1 [4]:
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
64
65
Fe2+ + H2O 2 → Fe3+ + •OH + OH−
(1)
66
ȱ
67
This method becomes effective because reactions between •OH and organics are usually very
ȱ
68
fast, with k-values of 108-1010 M-1 s-1. However, a part of the generated radical is lost due to
69
its direct reaction with Fe2+ (k = 3.2x10 8 M-1 s-1 [5]) and H2O 2 (k = 2.7 x107 M-1 s-1 [6]), as
70
shown in reaction (2) and reaction (3), respectively:
ȱ
ȱ
71
ȱ
72
Fe2+ + •OH → Fe3+ + OH−
(2)
ȱ
73
H2O 2 + •OH → HO 2• + H2O
(3)
ȱ
74
ȱ
ȱ
ȱ
75
An advantage of the use of the Fe3+/Fe 2+ system is its catalytic behavior, since Fe 2+ is not
76
completely removed by reactions (1) and (2) because it can be regenerated in small extent
77
from the reduction of Fe3+ by H2O 2 from reaction (4) with k = 8.4 x 10-6 M-1 s-1 [7], by
78
hydroperoxyl radical (HO 2•) from reaction (5) with k = 2 x 103 M-1 s -1 [8] and/or by organic
ȱ
ȱ
3
ȱ
ȱ
ȱ
315
PART B –Results and Discussion9. Chlorophene
ȱ
79
radical intermediates R• from reaction (6). HO 2• is an oxidant much weaker than •OH and can
ȱ
80
also oxidize Fe2+ from reaction (7) with k = 1.2x10 6 M-1 s -1 [8].
ȱ
81
ȱ
82
ȱ
ȱ
Fe3+ + H2O 2 → Fe2+ + H+ + HO 2•
83
84
85
Fe
3+
Fe
3+
Fe
2+
+
HO 2•
→ Fe
•
+ R → Fe
+
HO 2•
2+
→ Fe
2+
+
+ H + O2
+
+ R
3+
+
(4)
(5)
(6)
HO 2−
(7)
ȱ
86
ȱ
87
Reactions (4)-(6) propagate Fenton’s reaction (1) with the continuous production of •OH for
88
the destruction of organic pollutants.
ȱ
ȱ
89
Electro-oxidation methods such as anodic oxidation and electro-Fenton are also being
90
developed for water remediation [9-45]. These environmentally friendly electrochemical
ȱ
91
techniques are more potent than chemical AOP’s because they can produce greater amount of
ȱ
92
oxidant •OH under control of the applied current. In anodic oxidation contaminants are
ȱ
93
destroyed by reaction with adsorbed hydroxyl radical generated at the surface of a high O2-
94
overvoltage anode from water oxidation [9-12], according to reaction (8):
ȱ
ȱ
ȱ
95
H2O → ƔOHads + H+ + e−
96
(8)
97
ȱ
98
Conventional anodes such as Pt, PbO 2, IrO 2, etc., lead to poor degradation of aromatics due to
ȱ
99
the formation of carboxylic acids that are difficultly oxidizable by ƔOH. These products can
ȱ 100
be destroyed using a boron-doped diamond (BDD) thin-film anode, which possesses much
101
greater O 2-overvoltage and produces higher amount of effective ƔOH from reaction (8) than
102
the above anodes, thus leading to a quicker oxidation of organics [12]. Recent studies have
103
confirmed the total mineralization of several aromatics and short carboxylic acids in waters
ȱ
ȱ
ȱ 104
by anodic oxidation with a BDD anode [9-22].
ȱ 105
Electro-Fenton is an indirect electro-oxidation treatment based on the combined use of
ȱ 106
cathodically generated hydrogen peroxide and iron ions as catalyst [23-45]. The method
107
consists in the continuous supply of H2O 2 to the acidic contaminated solution from the two-
108
electron reduction of oxygen gas given by reaction (9):
ȱ
ȱ
4
ȱ
ȱ
ȱ
316
PART B –Results and Discussion9. Chlorophene
ȱ
109
ȱ 110
O 2(g) + 2 H+ + 2 e− → H2O 2
(9)
ȱ 111
ȱ
ȱ
112
Reaction (9) can take place at reticulated vitreous carbon [23,24,26,27,44], carbon-felt [28-
113
30,33-38,40,43], activated carbon fiber [41] and carbon-polytetrafluoroethylene (PTFE) O2-
114
diffusion [17,25,31,32,39,42,45] cathodes. Fe2+ or Fe3+ is then added to the solution to
ȱ 115
generate the oxidizing agent •OH from Fenton’s reaction (1).
ȱ 116
In our laboratories we have previously studied the electro-Fenton degradation of some
ȱ 117
aromatic compounds, mainly pesticides, such as chlorophenoxy acids [17,28-31,32,39] and
118
organophosphorus [34], dyes [37], industrial pollutants [33,38,40,45] and analgesic
119
pharmaceuticals as emerging pollutants [42] using different undivided electrolytic cells. The
120
outstanding oxidizing power of these electro-Fenton systems has been explained by the fast
ȱ 121
reaction of organics with •OH formed in the medium from reaction (1) and in some cases, at
ȱ 122
the anode from reaction (8), being the former reaction enhanced by the additional
ȱ 123
regeneration of Fe2+ from cathodic reduction of Fe 3+:
ȱ
ȱ
ȱ
ȱ
124
125
Fe3+ + e− → Fe2+
(10)
126
ȱ 127
However, these previous works have not yet examined extensively some fundamental aspects
ȱ 128
of the Fe3+/Fe2+ catalytic system involved in the electrolytic cell such as: (i) the actual
ȱ 129
reduction ability of the cathode to regenerate Fe2+ from reaction (10), (ii) the oxidation ability
130
of the anode to convert Fe2+ into Fe3+ from reaction (11), (iii) the action of weak oxidants
131
formed at the anode on the Fe 2+ content in solution and (iv) the comparative removal rate of
132
generated carboxylic acids and their complexes with Fe 2+ and/or Fe 3+ with regard to the
ȱ
ȱ
ȱ 133
oxidizing power of the system to achieve total mineralization.
ȱ 134
ȱ
ȱ
135
Fe2+ → Fe 3+ + e−
(11)
136
137
The understanding of these effects for different electro-Fenton systems is needed to establish
ȱ 138
their optimum operational conditions for the treatment of agricultural, industrial and urban
ȱ
5
ȱ
ȱ
ȱ
317
PART B –Results and Discussion9. Chlorophene
ȱ
139
wastewaters containing aromatics. To clarify them, we have undertaken a comparative study
ȱ 140
on the behavior of the Fe3+/Fe2+ system using four undivided electrolytic cells containing a Pt
ȱ 141
or BDD anode and a carbon-felt or O 2-diffusion cathode. The comparative oxidizing power of
142
these cells was tested from the degradation of chlorophene (o-benzyl-p-chlorophenol, see
143
chemical structure in Fig. 1). Pharmaceuticals belonging to several therapeutical classes are
144
being continuously detected in the environment, but their effects on humans and aquatic fauna
ȱ 145
are not well known for the moment. Chlorophene was chosen since it is a widespread broad-
ȱ 146
spectrum antimicrobial pharmaceutical, commonly used in hospitals and households for
ȱ 147
general cleaning and disinfecting, as well as in industrial and farming environments as an
148
active agent in disinfectant formulations [46-48]. It has been detected at concentrations up to
149
50 mg l-1 in activated sludge sewage systems and up to 10 µg l -1 in sewage treatment plant
150
effluents and rivers. Although chlorophene is expected to pose a low toxicity for humans,
ȱ 151
evidence of its carcinogenic and mutagenic activity in animals is documented [49]. To avoid
ȱ 152
its dangerous accumulation in soils and the aquatic environment, this compound and its by-
ȱ 153
products need to be removed from wastewaters by potent and viable oxidation methods. In
ȱ
ȱ
ȱ
ȱ
ȱ
154
this sense, only its oxidative degradation with MnO 2 has been described [46].
155
This paper reports a detailed investigation on the catalytic behavior of the Fe3+/Fe2+
156
system in the electro-Fenton degradation of chlorophene using undivided Pt/O2 diffusion,
ȱ 157
BDD/O 2 diffusion, Pt/carbon felt and BDD/carbon felt cells. Comparative experiments were
ȱ 158
carried out with solutions containing 0.05 M Na2SO 4 as background electrolyte and Fe3+ at pH
ȱ 159
3.0, near the optimum pH of 2.8 for Fenton’s reaction (1) [5]. The evolution of Fe2+, Fe3+ and
160
H2O 2 in each cell was examined in the absence of pollutants to know the extent of reactions
161
(10) and (11). Concentrated solutions of chlorophene (solubility in water 145 mg l-1) were
162
degraded to clarify better the effects of the Fe 3+/Fe 2+ system. In each electro-Fenton system
ȱ 163
the influence of Fe 3+ content and applied current on its degradation rate and oxidizing power
ȱ 164
for total mineralization was also explored. The kinetics of chlorophene decay and the
ȱ
ȱ
ȱ
165
ȱ 166
evolution of its generated carboxylic acids were followed by chromatographic techniques.
ȱ 167
ȱ
168
169
ȱ
6
ȱ
ȱ
ȱ
318
PART B –Results and Discussion9. Chlorophene
ȱ
170
2. Experimental
ȱ 171
ȱ 172
2.1. Chemicals
173
Chlorophene was reagent grade from Sigma-Aldrich, being used in the electrolytic
174
experiments as received. Benzoic, maleic, fumaric, malonic, glycolic, glyoxylic, formic and
175
oxalic acids were either reagent or analytical grade supplied by Sigma-Aldrich, Fluka and
ȱ 176
Acros Organics. Sulfuric acid, anhydrous sodium sulfate, heptahydrated ferrous sulfate and
ȱ 177
ferric sulfate were analytical grade purchased from Fluka and Acros Organics. All solutions
178
were prepared with ultra-pure water obtained from a Millipore Milli–Q system with resistivity
179
> 18 MΩ cm at room temperature. Organic solvents and the other chemicals used were either
ȱ
ȱ
ȱ
ȱ
180
ȱ 181
ȱ
182
HPLC or analytical grade from Fluka, Panreac and Acros Organics.
2.2. Instruments
183
Electrolyses were performed either with a Hameg HM8040 triple power supply or a
184
Micronics-Systems MX 30 V-10 A microlab power supply. The solution pH was measured
ȱ 185
with a Eutech Instruments CyberScan pH1500 pH-meter. The mineralization of chlorophene
ȱ 186
solutions was monitored by the abatement of their total organic carbon (TOC), determined on
ȱ 187
a Shimadzu VCSH TOC analyzer. The decays of chlorophene and benzoic acid were followed
188
by reversed-phase HPLC chromatography using a Merck Lachrom liquid chromatograph
189
equipped with a L-7100 pump, fitted with a Purospher RP-18 5 µm, 25 cm x 4.6 mm, column
190
at 40 ºC, and coupled with a L-7455 photodiode array detector selected at λ = 280 nm.
ȱ 191
Generated carboxylic acids were identified and quantified by ion-exclusion HPLC
ȱ 192
chromatography with a Merck Lachrom liquid chromatograph equipped with a L-2130 pump,
ȱ 193
fitted with a Supelco Supelcogel H 9 µm, 25 cm x 4.6 mm, column at 40 ºC, and coupled with
194
a L-2400 UV detector selected at λ = 210 nm. In both HPLC techniques 20 µl samples were
195
injected into the liquid chromatograph and measurements were controlled through an
196
EZChrom Elite 3.1 program. Cl− concentration in treated solutions was determined by ion
ȱ 197
chromatography just injecting 25 µl aliquots into a Dionex ICS-1000 Basic Ion
ȱ 198
Chromatography System fitted with an IonPac AS4A-SC, 25 cm x 4 mm, anion-exchange
199
column, linked to an IonPac AG4A-SC, 5 cm x 4 mm, column guard, and coupled with a DS6
200
conductivity detector containing a cell heated at 35 ºC under control through a Chromeleon
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
7
ȱ
ȱ
319
PART B –Results and Discussion9. Chlorophene
ȱ
201
SE software. The sensitivity of this detector was improved from electrolyte suppression using
ȱ 202
a SRS-ULTRA II self-regenerating suppressor. Colorimetric measurements were made with a
ȱ 203
Unicam UV4 Prisma double-beam spectrometer thermostated at 25.0 ºC.
ȱ 204
Oxidation products were detected by gas chromatography-mass spectrometry (GC-MS)
205
using a Hewlett-Packard system composed of a HP 5890 Series II gas chromatograph fitted
206
either with a HP-5 0.25 µm or a HP-Innowax 0.25 µm, both of 30 m x 0.25 mm, column and
207
coupled with a HP 5989A mass spectrometer operating in EI mode at 70 eV. The temperature
ȱ 208
ramp for the HP-5 column was 35 ºC for 2 min, 10 ºC min -1 up to 320 ºC and hold time 5 min,
ȱ 209
and the temperatures of the inlet, transfer line and detector were 250 ºC, 250 ºC and 290 ºC,
ȱ 210
respectively. The temperature ramp for the HP-Innowax column was 35 ºC for 2 min, 10 ºC
211
min -1 up to 250 ºC and hold time 15 min, and the temperature of the inlet, transfer line and
212
detector was 250 ºC.
ȱ
ȱ
ȱ
ȱ 213
ȱ 214
2.3. Electrolytic systems
215
All electrolyses were conducted in an undivided glass cell of 6 cm diameter and 250 ml
216
capacity. Four different electro-Fenton systems were tested: (i) a Pt/O2 diffusion cell, with a 3
217
cm2 Pt sheet from SEMP as anode and a 3 cm2 carbon-PTFE O 2-diffusion electrode from E-
ȱ 218
TEK as cathode, (ii) a BDD/O 2 diffusion cell, containing a 3 cm2 BDD thin-film deposited on
ȱ 219
conductive single crystal p-type Si (100) wafers from CSEM as anode and the above O2-
220
diffusion cathode, (iii) a Pt/carbon felt cell, with a 4.5 cm2 Pt cylindrical mesh as anode and a
221
70 cm2 (17 cm x 4,1 cm) carbon felt from Carbone-Lorraine as cathode and (iv) a
222
BDD/carbon felt cell, containing the above BDD anode and carbon-felt cathode. The
ȱ 223
preparation of the O 2-diffusion cathode has been reported elsewhere [25,31]. In the Pt/O2
ȱ 224
diffusion and BDD/O 2 diffusion cells, the interelectrode gap was about 1 cm and the cathode
ȱ 225
was directly fed with pure O 2 at 20 ml min -1 to generate continuously H2O 2 from reaction (9).
226
In the Pt/carbon felt and BDD/carbon felt cells, the corresponding anode was centered in them
227
and each cathode covered their inner wall, where H2O2 was produced from reduction of O 2
228
dissolved in the solution, also from reaction (9). Continuous saturation of this gas at
ȱ 229
atmospheric pressure was ensured by bubbling compressed air at 1 l min -1, starting 15 min
ȱ 230
before electrolysis.
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
8
ȱ
ȱ
ȱ
320
PART B –Results and Discussion9. Chlorophene
ȱ
231
Solutions of 200 ml containing up to 84 mg l-1 chlorophene with 0.05 M Na 2SO 4 and
ȱ 232
different concentrations of Fe3+ at pH 3.0 adjusted with H2SO 4 were comparatively degraded
ȱ 233
in the above four electro-Fenton systems at constant current between 60 and 300 mA and at
ȱ 234
room temperature (20±1 ºC). All solutions were vigorously stirred with a magnetic bar during
235
ȱ 236
ȱ 237
ȱ
ȱ
treatment.
2.4. Analytical procedures
238
Reproducible TOC values were always obtained by injecting 100 µl aliquots into the
239
TOC analyzer using the non-purgeable organic carbon method. The mineralization current
240
efficiency (MCE) for treated solutions at a given time was then calculated from the following
ȱ 241
equation:
ȱ 242
ȱ 243
ȱ
ȱ
MCE =
∆(TOC)exp
x 100
∆(TOC)theor
(12)
244
245
where ∆(TOC) exp is the experimental TOC removal and ∆(TOC)theor is the theoretically
246
calculated TOC decay at a given time considering that the applied electrical charge (= current
ȱ 247
x time) is only consumed in the mineralization process of chlorophene.
ȱ 248
Reversed-phase chromatography analyses were carried out was made under circulation of
ȱ 249
a 50:50 (v/v) acetonitrile/water mixture at 0.8 ml min -1 as mobile phase. For ion-exclusion
250
chromatography, the mobile phase was 4 mM H2SO4 at 0.2 ml min -1. Cl− measurements were
251
conducted with a solution of 1.8 mM Na 2CO 3 and 1.7 mM NaHCO 3 circulating at 1.0 ml
252
min -1 as mobile phase. The concentration of H2O 2 in electrolyzed solutions was determined
ȱ 253
from the light absorption of the titanic-hydrogen peroxide colored complex at λ = 408 nm
ȱ 254
[50]. The Fe2+ and Fe 3+ contents in the same solutions were obtained by measuring the light
ȱ 255
absorption of their corresponding colored complexes with 1,10-phenantroline at λ = 508 nm
ȱ
ȱ
ȱ
256
[51] and with SCN− at λ = 466 nm [52], respectively.
257
To detect the aromatic intermediates, 50 mg l-1 of chlorophene were degraded in the Pt/O 2
ȱ 258
diffusion cell at low current for 30 min. The remaining organics were withdrawn with 45 ml
ȱ 259
of CH2Cl 2 in three times. This solution was then dried with anhydrous Na 2SO 4, filtered and its
ȱ 260
volume reduced to 2 ml to concentrate the aromatics for their analysis by GC-MS using the
ȱ
9
ȱ
ȱ
ȱ
321
PART B –Results and Discussion9. Chlorophene
ȱ
261
HP-5 column. For the identification of final carboxylic acids, the treatment of 50 mg l-1 of
ȱ 262
chlorophene in the same cell was prolonged for 2 h. The resulting solution was evaporated at
ȱ 263
low pressure and the remaining solid was dissolved in 2 ml of ethanol. The esterified acids
ȱ 264
were further analyzed by GC-MS using the HP-Innowax column.
265
ȱ 266
ȱ 267
ȱ
3. Results and Discussion
268
269
3.1. Behavior of the Fe3 +/Fe2 + system without pollutants
ȱ 270
A preliminary study was carried out to test the oxidation ability of the Pt and BDD
ȱ 271
anodes to transform Fe2+ into Fe3+ via reaction (11). Several electrolyses of 200 ml of a 0.05
ȱ 272
M Na 2SO 4 solution with 4.0 mM Fe 2+ at pH 3.0 and at 300 mA were made using undivided
273
cells containing one of the above anodes and a 3 cm2 stainless steel (AISI 304) sheet as
274
cathode. A quick decay of Fe 2+ concentration, along with the simultaneous increase in Fe3+
275
content, was found in both cases, indicating a very poor regeneration of Fe2+ at the stainless
ȱ 276
steel cathode from reaction (10) compared to its fast oxidation at each anode from reaction
ȱ 277
(11). Fe2+ was completely removed in 45 min using BDD (current efficiency 95%), whereas
ȱ 278
for Pt, a longer time of 64 min was required (current efficiency 67%). Kinetic analysis of
279
these data showed a pseudo-first-order decay for Fe 2+ during the initial 20-30 min of both
280
electrolyses, with a pseudo-rate constant (k1) of 9.05x10-4 s -1 (square linear regression
281
coefficient R2 = 0.995) for BDD and 5.87x10-4 s-1 (R2 = 0.991) for Pt. The faster Fe 2+ removal
ȱ 282
with BDD can be accounted for by its greater O 2-overpotential [12] that favors reaction (11)
ȱ 283
instead of O 2 evolution from water oxidation, a process taking place in larger extent in Pt. The
ȱ 284
greater O2-overpotential of BDD causes an average cell voltage equal to 9.8 V, a value much
ȱ
ȱ
ȱ
ȱ
ȱ
285
higher than 5.7 V when it is replaced by Pt.
286
Several experiments were further performed to clarify the evolution of Fe 3+, Fe2+ and
287
accumulated H2O 2 in the four electro-Fenton systems with H2O 2 electrogeneration considered
ȱ 288
in the present work. The time-course of these species during the electrolysis of 200 ml of a
ȱ 289
0.05 M Na 2SO 4 solution with 4.0 mM Fe3+ at pH 3.0 and at 300 mA for 60 min using the
290
Pt/O2 diffusion and BDD/O 2 diffusion cells, is depicted in Fig. 2a. As can be seen, the Fe3+
291
concentration remains practically unchanged in both trials. In addition, no significant quantity
ȱ
ȱ
ȱ
10
ȱ
ȱ
ȱ
322
PART B –Results and Discussion9. Chlorophene
ȱ
292
of Fe2+ was detected in the electrolyzed solutions, as expected if reaction (10) occurs in such a
293
small extent at the O2-diffusion cathode that the generated Fe 2+ is rapidly converted into Fe3+
ȱ 294
from reactions (1), (2) and (7) and mainly at the anode from reaction (11), thus preventing its
ȱ 295
accumulation in the medium. The predominant reaction in this cathode is the reduction of
ȱ 296
injected O 2 to electrogenerate H2O 2 from reaction (9). This can be easily deduced from Fig. 2a
297
because this species is continuously accumulated up to reach a steady concentration of about
298
9 mM after 45 min in both electrolyses, just when its electrogeneration and decomposition
299
rates become equal. Under these conditions, H2O 2 can be slowly decomposed with Fe 2+ by
ȱ 300
reaction (1) and with Fe 3+ by reaction (4), but it is much more rapidly oxidized to O2 via
ȱ 301
formation of HO 2• as intermediate at the anode surface [31,39]:
ȱ
ȱ
ȱ
ȱ 302
ȱ
ȱ
ȱ
ȱ
303
H2O 2 → HO 2• + H+ + e−
(13)
304
HO 2• → O 2 + H+ + e−
(14)
305
306
Our results indicate that the rates of reactions (13) and (14) are practically independent of the
307
anode used, despite the average cell voltage raises from 12.0 V for the Pt/O2 diffusion cell to
ȱ 308
19.0 V for the BDD/O 2 diffusion one.
ȱ 309
A very different behavior of these species was found using the Pt/carbon felt and
310
BDD/carbon felt cells. These trials were also performed at 300 mA, but with a smaller content
311
of Fe3+ (0.20 mM) in the 0.05 M Na 2SO4 solution of pH 3.0 to try to clarify better the extent
312
of reactions (10) and (11). As can be seen in Fig. 2b, the use of the Pt/carbon felt cell gives
ȱ 313
rise to the reduction of all Fe3+ to Fe 2+ in 20 min, indicating that reaction (10) is very fast at
ȱ 314
the carbon felt cathode. Its kinetic analysis allows determining a k 1-value of 4.05x10-3 s -1 for
ȱ 315
overall Fe2+ regeneration, corresponding to a reaction rate of 8.10x10 -7 M s-1 for 0.20 mM
316
Fe 2+. This value is much higher than 1.17x10 -7 M s -1 expected from its oxidation at the Pt
317
anode from reaction (11) with k1 = 5.87x10-4 s-1, as determined above. The parallel generation
318
of H2O 2 from reaction (9) in the carbon-felt cathode is rather slow, being detected at a
ȱ 319
concentration as low as 0.23 mM after 60 min of electrolysis in the Pt/carbon felt cell (not
ȱ 320
shown in Fig. 2b). This confirms the consumption of Fe 2+ by reactions (1), (2) and (7) in
ȱ 321
parallel to reaction (11), although its regeneration from reaction (10) is so fast that only Fe2+
ȱ
ȱ
ȱ
ȱ
ȱ
11
ȱ
ȱ
323
PART B –Results and Discussion9. Chlorophene
ȱ
322
is detected for electrolysis times longer than 20 min. In contrast, Figure 2b shows that Fe3+ is
ȱ 323
not totally converted into Fe 2+ in the BDD/carbon felt cell under comparable conditions. The
ȱ 324
Fe 2+ content in this cell immediately rises up to 0.058 mM in 2 min and thereafter, it is slowly
ȱ 325
removed to disappear in 60 min, when 0.20 mM of H2O2 is accumulated. This anomalous
326
trend can be explained by the additional destruction of Fe2+ with weak oxidant species
327
produced at the BDD anode, since its great O 2-overpotential causes an average voltage
328
applied to the BDD/carbon felt cell at 300 mA equal to 12.5 V, a value much higher than 2.2
ȱ 329
V using a Pt anode. Under these conditions, it is well-known that peroxodisulfate is
ȱ 330
competitively formed with •OH at the BDD anode from the reaction (15) [12,14]:
ȱ
ȱ
ȱ 331
ȱ
ȱ
2 HSO 4− → S2O 82− + 2 H+ + 2 e −
332
(15)
333
334
ȱ 335
and this ion can further react with Fe2+ to yield sulfate and Fe3+ from reaction (16), with k =
23 M-1 s-1 [53]:
ȱ 336
S2O82− + 2 Fe2+ → 2 SO 42− + 2 Fe3+
ȱ 337
ȱ
(16)
338
339
The slow disappearance of Fe2+ generated from reaction (10) in the BDD/carbon felt cell can
340
then be accounted for by the increase in rate of reaction (16) due to the continuous
ȱ 341
accumulation of S2O 82− in the medium. However, Fe3+ does not seem to be completely
ȱ 342
regenerated in this cell, since its concentration undergoes a progressive abatement with
343
electrolysis time (see Fig. 2b). An inspection of the carbon felt cathode after this trial revealed
344
the presence of a yellow precipitate of Fe(OH) 3 on its large and porous surface (geometric
345
area 70 cm2). The decay in Fe3+ content can be related to the gradual formation of such
ȱ 346
precipitate due to the high OH− concentration present in the vicinity of the cathode coming
ȱ 347
from the simultaneous water reduction to hydrogen. Note that no precipitation of Fe(OH) 3
ȱ 348
was observed neither in the systems containing an O 2-diffusion cathode with much smaller
349
geometric surface (3 cm2), nor in the Pt/carbon felt cell where Fe3+ is completely reduced to
350
Fe 2+ at the cathode.
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
12
ȱ
ȱ
ȱ
324
PART B –Results and Discussion9. Chlorophene
ȱ
351
The above results are indicative of a main dependence of the Fe 3+/Fe 2+ catalytic system
ȱ 352
on the cathode used. The fast generation of H2O 2 from reaction (9) at the O 2-diffusion cathode
ȱ 353
favors the accumulation of large amounts of this species in the Pt/O 2 diffusion and BDD/O 2
ȱ 354
diffusion cells. The regenerated Fe2+ by reaction (10) is then rapidly oxidized to Fe 3+ by
355
reactions (1), (2), (7) and (11), so Fe3+ concentration does not practically varies along
356
electrolysis. In contrast, the carbon felt cathode yields a much smaller quantity of H2O 2, but
357
reaction (10) becomes so fast on its surface that Fe2+ is accumulated in large extent in the
ȱ 358
Pt/carbon felt cell. Under these conditions, a maximum production of •OH from Fenton’s
ȱ 359
reaction (1) is expected. For the BDD/carbon felt cell, however, regenerated Fe 2+ is slowly
ȱ 360
removed by the weak oxidant peroxodisulfate formed at the BDD anode. This causes two
361
negative effects: (i) a decay in rate of reaction (1) due to the accumulation of a lower quantity
362
of Fe2+ and (ii) the enhancement of Fe(OH)3 precipitation on the large surface of the carbon
363
felt cathode with loss of soluble Fe3+. To confirm the catalytic behavior of the Fe3+/Fe 2+
ȱ 364
system in these four systems, their comparative oxidizing power under electro-Fenton
ȱ 365
conditions was evaluated by studying the degradation of chlorophene.
ȱ
ȱ
ȱ
ȱ
366
ȱ
367
3.2. Chlorophene decay under electro-Fenton conditions
ȱ 368
A series of electrolysis was carried out with 50 mg l-1 chlorophene solutions of pH 3.0 at
ȱ 369
300 mA to determine the influence of Fe 3+ concentration on its destruction rate in the Pt/O2
370
diffusion cell. The kinetics for the reaction of the antimicrobial with generated oxidants was
371
followed by reversed-phase HPLC chromatography, where it exhibits a well-defined peak
372
with a retention time (t r) of 16.5 min. The change of its concentration with time for an initial
ȱ 373
Fe 3+ content between 0.2 and 8.0 mM is depicted in Fig. 3. A fast and complete removal of
ȱ 374
chlorophene can be observed in all cases. Its decay rate undergoes a gradual acceleration
ȱ 375
when Fe3+ concentration increases, disappearing from the medium in 20 min for 0.2 mM Fe3+,
376
but in only 3 min for 8.0 mM Fe3+. This effect can be related to an increasing quantity of Fe2+
377
regenerated from reaction (10) that enhances the production of strong oxidant •OH by
378
Fenton’s reaction (1) and hence, its reaction with chlorophene. However, in this system this
ȱ 379
compound could also react with •OH produced at the anode surface from reaction (8) and
ȱ 380
other weaker oxidizing agents such as H2O 2 and HO 2•. Note that greater amounts of HO 2• are
ȱ
ȱ
ȱ
ȱ
ȱ
13
ȱ
ȱ
ȱ
325
PART B –Results and Discussion9. Chlorophene
ȱ
381
formed from Fenton-like reaction (4) when Fe3+ content rises, although this species is also
382
generated from H2O 2 oxidation by reaction (13). The possible action of H2O2 as oxidant was
ȱ 383
discarded by confirming that the antimicrobial concentration does not vary in 200 ml of a
ȱ 384
solution of pH 3.0 prepared with 50 mg l-1 of chlorophene and 20 mM H2O 2. To clarify the
ȱ 385
influence of •OH and HO 2• produced at the Pt anode, a 50 mg l-1 chlorophene solution of pH
386
3.0 was electrolyzed in the Pt/O 2 diffusion cell, but without Fe 3+, i.e., using anodic oxidation
387
in the presence of electrogenerated H2O 2. Figure 4 shows that this method yields a much
388
slower removal of this compound, disappearing in 300 min, a time much longer than that
ȱ 389
needed in the presence of Fe3+ (see Fig. 3). Since in anodic oxidation organics are oxidized by
ȱ 390
•
ȱ
ȱ
ȱ
ȱ
ȱ
OH formed from reaction (8) and in smaller extent by HO2• largely produced from reaction
391
(13) [39], one can infer that the much faster destruction of the antimicrobial under the electro-
392
Fenton conditions shown in Fig. 3 is due to its reaction with •OH formed from Fenton’s
393
reaction (1), which is enhanced with raising Fe2+ regeneration when more Fe 3+ is present in
ȱ 394
the solution.
ȱ 395
A similar influence of Fe 3+ concentration on chlorophene destruction was observed by
ȱ 396
electrolyzing 50 mg l-1 antimicrobial solutions of pH 3.0 at 300 mA in the BDD/O 2 diffusion
397
cell when the Fe3+ content increased from 0.2 to 8.0 mM. However, its decay rate was
398
significantly reduced in comparison to that found for the Pt/O2 diffusion cell. Figure 4
399
illustrates that the time required for overall removal of chlorophene increases from 7 min for
ȱ 400
the Pt/O 2 diffusion cell to 90 min for the BDD/O 2 diffusion one using 4.0 mM Fe3+. This trend
ȱ 401
seems surprising at first sight, since an acceleration of the destruction of this compound using
ȱ 402
BDD could be expected due to its much greater oxidizing power [12], as was corroborated by
403
treating comparatively the antimicrobial solution by anodic oxidation in the presence of
404
electrogenerated H2O 2 either with a BDD or Pt anode. As can be seen in Fig. 4, anodic
405
oxidation with BDD gives faster chlorophene decay, in agreement with the production of
ȱ 406
more reactive •OH from reaction (8). The opposite effect found under electro-Fenton
ȱ 407
conditions can then be accounted for by the quicker oxidation of Fe2+ to Fe3+ at the BDD
ȱ 408
anode than at the Pt one from reaction (11), along with its additional destruction by reaction
409
(16) with S2O 82− generated at the former anode. This causes a drop in the regeneration rate of
410
Fe 2+ from reaction (10) and consequently, in •OH produced from Fenton’s reaction (1),
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
14
ȱ
ȱ
326
PART B –Results and Discussion9. Chlorophene
ȱ
ȱ
411
reducing its reaction rate with chlorophene. These results also evidence that this compound is
412
not significantly oxidized by S2O 82− .
ȱ 413
When a carbon-felt cathode was used in the electrolytic cell, a more positive action of the
ȱ 414
Fe /Fe 2+ system was found due to the much faster regeneration of Fe2+ at its surface (see Fig.
ȱ 415
2b) leading to much greater •OH production. Figure 5a shows the chlorophene abatement for
416
50 mg l-1 of the antimicrobial solutions of pH 3.0 with Fe3+ contents between 0.1 and 2.0 mM
417
treated in the Pt/carbon felt cell at 60 mA. The quickest antimicrobial decay in this system is
418
achieved either with 0.1 or 0.2 mM Fe3+, disappearing in 20 min in both cases, since its
ȱ 419
removal rate undergoes a gradual drop at higher Fe3+ concentration up to needing a time as
ȱ 420
long as 60 min for its destruction with 2.0 mM Fe3+. This tendency can be associated with a
421
progressive fall of •OH content in solution, mainly due to the participation of non-oxidizing
422
reaction (2), which is strongly accelerated when much larger amounts of Fe 2+ are formed from
423
reaction (10) as more Fe 3+ is added to the starting solution. That means that the maximum
ȱ 424
concentration of reactive •OH in the Pt/carbon felt cell at 60 mA is attained using 0.1-0.2 mM
ȱ 425
Fe 3+, that is, when this radical formed from Fenton’s reaction (1) is wasted in smaller extent
ȱ 426
with regenerated Fe2+. This effect was also found by applying higher currents. As an example,
427
Figure 5b presents the chlorophene removal in the same conditions as in Fig. 5a, but at 300
428
mA. By comparing both figures one can easily deduce that the destruction of this compound
429
is enhanced with raising current, because more amount of oxidant •OH is produced due to the
ȱ 430
quicker H2O 2 formation and Fe2+ regeneration at the cathode. At 300 mA, however, the
ȱ 431
antimicrobial disappears in 5 min using 0.2 mM Fe3+, whereas it undergoes a slower removal
ȱ 432
for the other Fe 3+ contents. These findings allow concluding that a small concentration equal
433
to 0.2 mM of Fe3+ in the starting solution is optimal for this electro-Fenton system. Note that
434
an increase in current also causes the production of more •OH at the Pt anode surface from
435
reaction (8) [20,39,42]. However, the reaction of this species with chlorophene on Pt is
ȱ 436
insignificant in comparison to that of •OH formed from Fenton’s reaction (1), as deduced
ȱ 437
from Fig. 4. Similar results were obtained by electrolyzing the same solutions with the
ȱ 438
BDD/carbon felt cell, although the antimicrobial was more slowly removed under comparable
439
conditions, as expected from the negative effect of reaction (16) on Fe2+ regeneration, as
440
discussed in section 3.1.
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
3+
ȱ
ȱ
15
ȱ
ȱ
327
PART B –Results and Discussion9. Chlorophene
ȱ
441
Kinetic analysis of the above chlorophene concentration decays only fit to a pseudo-first-
442
order equation for the cells containing a carbon-felt cathode, where this compound and •OH
ȱ 443
react very rapidly in solution. The second-order rate constant or absolute rate constant for the
ȱ 444
aforementioned reaction was then determined using these electro-Fenton systems from the
ȱ 445
proposed method of competitive kinetics [40], taking benzoic acid as standard competition
446
substrate. Following this method, the pseudo-first-order rate constants for chlorophene (k1,CP)
447
and benzoic acid (k1,BA) were simultaneously obtained in such systems for short electrolysis
448
times, then calculating the second-order rate constant for chlorophene (kCP) from that of
ȱ
ȱ
ȱ
ȱ 449
benzoic acid (kBA = 4.30x109 M-1 s-1 [55]) as follows:
ȱ 450
ȱ
ȱ
kCP =
451
452
k1,CP
k1,BA
x kBA
(17)
453
Figure 6 shows the excellent linear correlations found for the pseudo-fist-order kinetic
ȱ 454
analysis of the decay of both compounds followed by reversed-phase HPLC chromatography
ȱ 455
(tr,CP = 15.4 min, t r,BA = 3.4 min) during the electrolysis of 50 mg l-1 of chlorophene and 122
ȱ 456
mg l-1 of benzoic acid with 0.2 mM Fe 3+ in the Pt/carbon felt and BDD/carbon felt cells at 60
457
mA. From these plots, values of k 1,CP = 1.15x 10-3 s-1 (R2 = 0.997) and k1,BA = 4.82x10-4 s -1 (R2
458
= 0.995) for the first cell and k1,CP = 1.03x 10 -3 s -1 (R2 = 0.996) and k1,BA = 4.47x10 -4 s -1 (R2 =
459
0.997) for the second one were determined. Taking these data in Eq. 16, one obtains an
ȱ 460
average value for kCP of (1.00±0.01)x1010 M-1 s-1, close to 7.1x109 M-1 s-1 reported by Arnold
ȱ 461
et al. for chlorophene degradation with Fenton’s reagent [47]. This kCP-value allows
ȱ 462
calculating a steady •OH concentration in solution of about 10-13 M at 60 mA.
ȱ 464
3.3. TOC removal and mineralization current efficiency
ȱ
ȱ
463
ȱ 465
The oxidizing power of the four electro-Fenton systems to mineralize chlorophene
ȱ 466
solutions was evaluated from their TOC decay. The change of this parameter with time
467
represents the degradation rate of all pollutants. This study was carried out with solutions of
468
pH 3.0 containing 84 mg l-1 of antimicrobial (corresponding to 60 mg l-1 of TOC) and an
469
efficient Fe3+ content by applying different currents for 11 h as maximum. In all cases the
ȱ
ȱ
ȱ
ȱ
ȱ
16
ȱ
ȱ
328
PART B –Results and Discussion9. Chlorophene
ȱ
ȱ
470
solution pH slightly decreased during electrolysis due to the formation of acidic products, up
471
to a final value of 2.7-2.8.
ȱ 472
Figure 7a shows selected TOC-time plots for the degradation of the above chlorophene
ȱ 473
solution with 4.0 mM Fe3+ using an O 2-diffusion cathode. A continuous, but slow, TOC
ȱ 474
abatement can be observed in the Pt/O2 diffusion cell, only attaining 52% of mineralization
475
after 660 min of electrolysis at 300 mA. This poor decontamination can be related to the
476
formation of products, as short carboxylic acids and their complexes with iron ions, that are
477
difficultly oxidizable with ƔOH produced in the medium from Fenton’s reaction (1) and at the
ȱ 478
Pt anode surface from reaction (8) [31,32,39,42]. In contrast, these species can be completely
ȱ 479
removed in the BDD/O 2 diffusion cell at 300 mA (see Fig. 7a), indicating that they are
480
oxidized by Ɣ OH on BDD. This agrees with the greater oxidation ability of this anode than
481
that of Pt [12]. As can be seen in Fig. 7a, the degradation rate of the chlorophene solution in
ȱ
ȱ
ȱ
ȱ
482
the BDD/O 2 diffusion cell rapidly increases with raising current from 60 to 300 mA, as
ȱ 483
expected from the concomitant production of more amount of ƔOH on the anode. After 11 h
ȱ 484
of treatment, its TOC is reduced by 33%, 45%, 85% and 97% at 60, 100, 200 and 300 mA,
ȱ 485
respectively. This electro-Fenton system then yields a rapid and total mineralization by
ȱ
486
applying high currents, when BDD has great oxidizing power.
487
A much faster degradation of the antimicrobial solution was found using the Pt/carbon
488
felt cell. Figure 7b illustrates that this system is able to decontaminate completely the solution
ȱ 489
with an optimum concentration 0.2 mM of Fe3+ for all currents between 60 and 300 mA. An
ȱ 490
enhancement of the degradation rate with raising current can be observed, mainly for the first
ȱ 491
60 min when aromatic intermediates are expected to be more easily destroyed. In all cases and
492
after 540 min of treatment, more than 95% of mineralization is achieved. The great oxidizing
493
power of this system can be ascribed to the rapid oxidation of all products (aromatics and
494
carboxylic acids) with the high amounts of ƔOH formed from Fenton’s reaction (1) due to the
ȱ 495
fast regeneration of Fe2+ at the carbon-felt cathode by reaction (10), without significant
ȱ 496
participation of ƔOH generated at the Pt anode. Figure 7b also shows that the BDD/carbon felt
ȱ 497
cell has even higher oxidizing power to decontaminate completely the same solution in a
498
shorter time of 6 h at 300 mA, although TOC is more hardly reduced for the first 2 h and
499
further, much more rapidly removed. The different degradation rate of the BDD/carbon felt
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
17
ȱ
ȱ
329
PART B –Results and Discussion9. Chlorophene
ȱ
ȱ
500
501
cell compared to that of the Pt/carbon felt one can be accounted for: (i) the slower oxidation
rate of chlorophene and its aromatics products in the medium due to the lower production of
ȱ 502
Ɣ
ȱ 503
the BDD anode, and (ii) the parallel quicker destruction of all pollutants with ƔOH at the BDD
ȱ 504
anode, enhancing the mineralization of more persistent products such as final carboxylic
505
acids. The much quicker TOC abatement in the BDD/carbon felt cell than in the BDD/O2
506
diffusion one at 300 mA (see Figs. 7a and 7b) corroborates the oxidation path via ƔOH in the
507
medium.
ȱ
ȱ
OH, since Fe2+ is accumulated in much less extent by its reaction with S2O 82− generated at
ȱ 508
The mineralization of chlorophene is expected to be accompanied by the loss of its
ȱ 509
chlorine atom in the form of inorganic ions. This was confirmed by treating 84 mg l-1
510
antimicrobial solutions with 0.015 M Na 2SO 4 and 0.2 mM Fe 3+ of pH 3.0 in the four cells at
511
150 mA. Ion chromatograms of all electrolyzed solutions only displayed one peak related to
ȱ
ȱ
512
chloride ion, discarding the formation of other ions such as chlorate and perchlorate. As can
ȱ 513
be seen in Fig. 8, Cl− is rapidly accumulated for 120 min in the cells with a Pt anode and at
ȱ 514
longer time, it reaches a quasi-steady concentration of about 13 mg l-1, a value very close to
ȱ 515
13.6 mg l-1 corresponding to the initial chlorine contained in solution. This evidences that all
516
chloro-organics are destroyed with the release of stable chloride ion. A very different
517
behavior can be observed in Fig. 8 for the evolution of Cl− in the cells with a BDD anode,
518
where this ion reaches a maximum content between 5 and 9 mg l-1 at 180 min, further being
ȱ 519
slowly destroyed until disappearing at 540 min. The instability of Cl− under these conditions
ȱ 520
is due to its oxidation to Cl2 gas on BDD, as reported for the electrolysis of NaCl aqueous
ȱ 521
solutions with this anode [10].
ȱ
ȱ
ȱ
ȱ
522
The above results allow establishing that the mineralization of chlorophene by electro-
523
Fenton involves its conversion into CO 2 and chloride ion as primary inorganic ion. Its overall
524
reaction can be written as follows:
ȱ 525
C13H11ClO + 25 H2O ĺ 13 CO 2 + Cl− + 61 H+ + 60 e−
ȱ 526
(18)
ȱ 527
ȱ
528
where 60 electrons are involved in the destruction of each molecule of the antimicrobial.
ȱ
ȱ
18
ȱ
ȱ
330
PART B –Results and Discussion9. Chlorophene
ȱ
529
The mineralization current efficiencies for the experiments given in Figs. 7a and 7b were
ȱ 530
then calculated from Eq. 12, considering reaction (18) to evaluate ∆(TOC)theor. The
ȱ 531
corresponding MCE-time plots thus obtained are presented in Figs. 9a and 9b. In all cases this
ȱ 532
parameter undergoes a dramatic fall with electrolysis time, as expected if products that are
533
more difficultly oxidizable with •OH than the initial compound, such as short carboxylic
534
acids, are progressively formed. In contrast, all electro-Fenton treatments become much more
535
efficient when current drops. For example, Figure 9a shows that after 60 min of electrolysis in
ȱ 536
the BDD/O 2 diffusion cell, increasing MCE values of 10%, 12%, 14% and 19% are obtained
ȱ 537
for decreasing currents of 300, 200, 100 and 60 mA, respectively. Under these same
ȱ 538
conditions, Figure 9b also shows a gradual rise in efficiency of 30%, 41%, 71% and ca. 100%
539
for the Pt/carbon felt cell. This trend is contrary to the concomitant fall in TOC removal found
540
in these systems due to the smaller production of Ɣ OH from reactions (1) and (8) (see Figs. 7a
541
and 7b). The increase in efficiency with decreasing current can then be ascribed to the higher
ȱ 542
decay in rate of non-oxidizing reactions of this radical, such as reaction (2), giving rise to a
ȱ 543
larger relative proportion of ƔOH with ability to react with pollutants. On the other hand,
ȱ 544
comparison of Figs. 9a and 9b for the trials at 300 mA confirms that the efficiency for
545
chlorophene degradation in the cells, at least at the early stages of treatment, increases in the
546
order: Pt/O 2 diffusion < BDD/O 2 diffusion < BDD/carbon felt < Pt/carbon felt. This can be
547
easily deduced taking into account that after 60 min of electrolysis, for example, the
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ 548
corresponding MCE values are 8.3%, 10%, 17% and 30%.
ȱ 549
All these findings show that chlorophene can be totally mineralized in the Pt/carbon felt,
550
BDD/carbon felt and BDD/O 2 diffusion cells under electro-Fenton conditions, raising their
551
efficiency as current decreases. The action of the Fe3+/Fe2+ catalytic system is optimal in the
552
Pt/carbon felt cell, which yields the highest degradation rate at the beginning of electrolysis
ȱ 553
with efficiency close to 100% at low current. However, the BDD/carbon felt cell has the
ȱ 554
highest oxidizing power for overall mineralization at high current due to the greater oxidation
ȱ 555
ability of BDD than Pt. These systems are then viable for the treatment of wastewaters
ȱ
ȱ
556
ȱ 557
containing this antimicrobial.
558
ȱ 559
ȱ
19
ȱ
ȱ
ȱ
331
PART B –Results and Discussion9. Chlorophene
ȱ
560
3.4. Identification and time-course of intermediates
ȱ 561
Reversed-phase chromatograms of solutions treated by all electro-Fenton systems did not
ȱ 562
display any identified peak related to aromatic intermediates coming from chlorophene
ȱ 563
oxidation. Several attempts were then made to try to identify some of such products by
564
electrolyzing 50 mg l-1 of the antimicrobial with either 4 mM or 1 mM Fe3+ at 60 mA for 30
565
min using the Pt/O 2 diffusion cell with the lowest oxidizing power. However, the GC-MS
566
analyses of collected organics only showed the presence of the remaining chlorophene. These
ȱ 567
findings evidence that its aromatic products are always oxidized at the same rate as formed,
ȱ 568
without being accumulated in the solution.
ȱ
ȱ
ȱ 569
Generated carboxylic acids were identified by analyzing the treated solutions by ion-
570
exclusion chromatography. These chromatograms displayed well-defined peaks ascribed to
571
oxalic acid at tr = 7.8 min, maleic acid at t r = 9.4 min, glyoxylic acid at tr = 11.4 min, malonic
572
acid at tr = 11.7 min, glycolic acid at t r = 14.5 min, formic acid at t r = 16.0 min and fumaric
ȱ 573
acid at tr = 17.0 min. Maleic, glycolic, malonic and fumaric acids come from the oxidation of
ȱ 574
the aryl moiety of aromatic products, whereas glyoxylic acid is formed from the degradation
ȱ 575
of glycolic acid [11,20,31,42]. Further oxidation of these products yields formic and oxalic
576
acids that are transformed into CO 2. The production of oxalic acid was corroborated by
577
treating 50 mg l-1 of chlorophene with 4 mM Fe3+ at 60 mA for 120 min in the Pt/O 2 diffusion
ȱ 578
cell. The GC-MS analysis after esterification of the remaining acids with ethanol revealed the
ȱ 579
presence of an intense peak related to diethyl oxalate (m/z = 146 (2, M+)) at t r = 7.9 min.
ȱ
ȱ
ȱ
ȱ 580
Once the identification of chromatographic peaks was made, the concentration of the
581
different carboxylic acids during the treatment of 84 mg l-1 chlorophene solutions in the four
582
electro-Fenton systems at 60 and 300 mA was determined as a function of electrolysis time
583
via external calibration by using standard compounds. The evolution of formic and oxalic
ȱ
ȱ
ȱ 584
acids for these trials is presented in Figs. 10a and 10b, respectively.
ȱ 585
For the cells containing an O 2-diffusion cathode and 4.0 mM Fe 3+, maleic, malonic and
ȱ 586
fumaric acids were detected at concentrations < 3 mg l-1 only operating at 60 mA for 60 min
587
as maximum, similarly to formic acid (see Fig. 10a). In contrast, oxalic acid is largely
588
accumulated at 60 and 300 mA, remaining in solution up to the end of both treatments. Figure
589
10b shows that in the Pt/O 2 diffusion cell this acid reaches 88 mg l-1 after 240 min of
ȱ
ȱ
ȱ
20
ȱ
ȱ
ȱ
332
PART B –Results and Discussion9. Chlorophene
ȱ
590
electrolysis at 300 mA, whereupon its concentration drops slightly to 68 mg l-1 at 540 min,
591
corresponding to 18 mg l-1 of TOC, a value much lower than 30 mg l-1 found for the
ȱ 592
remaining solution (see Fig. 7a). This indicates that the solution also contains other
ȱ 593
undetected products that are hardly oxidized by •OH produced in the medium by Fenton’s
594
reaction (1) and at the Pt anode by reaction (8). In this system all iron ions are accumulated as
595
Fe 3+ (see Fig. 2a) and hence, the majority of oxalic acid is expected to be in the form of Fe3+-
596
oxalate complexes, which can not be oxidized by •OH in solution [5,54]. The quite slow
ȱ 597
destruction of these species in the Pt/O 2 diffusion cell can then be ascribed to their hard
ȱ 598
mineralization to CO 2 with •OH at the Pt surface, thus confirming the low oxidation ability of
ȱ 599
this anode. A similar behavior can be seen in Fig. 10b for the BDD/O 2 diffusion cell at 60
600
mA, where oxalic acid attains a quasi steady-concentration of 45 mg l-1 at times longer than
601
180 min, corresponding to 12mg l-1 of TOC, a value very far from 40 mg l-1 of TOC
602
determined for the final electrolyzed solution, as can be seen in Fig. 7a. At this low current,
ȱ 603
this system is unable to destroy Fe3+-oxalate complexes and other undetected products.
ȱ 604
However, when the current rises to 300 mA, oxalic acid (see Fig. 10b) and the solution TOC
ȱ 605
(see Fig. 7a) are completely removed at 660 min. The overall mineralization of chlorophene
606
in the BDD/O 2 diffusion cell at 300 mA can be explained by the efficient oxidation of final
607
Fe 3+-oxalate complexes with •OH on BDD, as expected from its great oxidizing power at high
608
current [12-22].
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ 609
Maleic, malonic, fumaric, glycolic, glyoxylic and formic acids were found in larger
ȱ 610
extent and during longer time in the Pt/carbon felt cell than in the BDD/carbon felt one using
ȱ 611
0.2 mM Fe3+ at 60 mA, but were not detected at 300 mA due to their faster destruction. For
612
example, Figure 10a shows a great accumulation of formic acid up to 17 mg l-1 at 120 min of
613
electrolysis in the first system at 60 mA and its complete mineralization in ca. 360 min,
614
whereas for the second system, this acid only persists for 120 min, reaching 1.5 mg l-1 as
ȱ 615
maximum. These results indicate that in the Pt/carbon felt cell all aromatic intermediates are
ȱ 616
transformed into carboxylic acids, because they react rapidly with the large amounts of •OH
617
formed from Fenton’s reaction (1), which is enhanced by the fast regeneration of Fe2+ at the
618
cathode from reaction (10). Even the persistent final oxalic acid can be totally converted into
619
CO 2 under these conditions at ca. 540 min (see Fig. 10b), when all solution TOC is removed
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
21
ȱ
ȱ
333
PART B –Results and Discussion9. Chlorophene
ȱ
620
(see Fig. 7b). The mineralization of this acid is accelerated with increasing current due to the
ȱ 621
quicker production of oxidant •OH, as can be observed in Fig. 10b, attaining a maximum
ȱ 622
content of 33 and 21 mg l-1 after 120 min of electrolysis at 60 and 300 mA, respectively. The
ȱ 623
low concentration of iron ions, practically all of them as Fe 2+ (see Fig. 2b), in the solution
624
treated in the Pt/carbon felt cell suggests the formation of quite a small proportion of Fe2+-
625
oxalato complexes, and hence, this acid and its Fe2+ complexes are directly oxidized with •OH
626
in the medium, since they can not be destroyed by this radical on Pt [12].
ȱ
ȱ
ȱ 627
The lower accumulation of carboxylic acids in the BDD/carbon felt cell can be related to
ȱ 628
a slower destruction of aromatics since •OH is generated to a lesser extent from Fenton’s
ȱ 629
reaction (1), because of the parallel oxidation of Fe2+ with S2O 82− from reaction (16). This
630
agrees with the fact that the efficiency found in this cell is lower than in the Pt/carbon felt one
631
during the early stages for both treatments at 300 mA (see Fig. 9). However, in the
632
BDD/carbon felt cell the degradation of carboxylic acids is enhanced by their simultaneous
ȱ 633
oxidation at the BDD surface, mainly at high current when this anode produces a large
ȱ 634
amount of •OH from reaction (8). This behavior can be confirmed in Fig. 10b from the
ȱ 635
corresponding evolution of oxalic acid, since it disappears in only 360 min at 300 mA, but in
636
contrast it needs more than 540 min to be removed at 60 mA, when it is mainly destroyed by
637
•
638
to the other electro-Fenton systems at high current accounts for its highest oxidizing power
ȱ
ȱ
ȱ
ȱ
ȱ 639
OH in solution. The faster destruction of this acid in the BDD/carbon felt cell in comparison
for total mineralization.
ȱ 640
The possible reaction paths of oxalic acid, that is the ultimate by-product generated along
641
the mineralization before the conversion of all the initial organic carbon into CO2, in the
642
electro-Fenton systems are schematized in Fig. 11. In the cells with an O2-diffusion cathode
643
Fe 3+-oxalate complexes are accumulated in large extent, so the overall mineralization is
ȱ 644
uniquely achieved by •OH on a BDD anode at high current. In the cells with a carbon-felt
ȱ 645
cathode, oxalic acid can be directly transformed into CO2 by •OH formed in solution. When a
ȱ 646
Pt anode is used under these conditions, this oxidant can also destroy Fe2+-oxalate complexes
647
present in small proportion, whereas for BDD, oxalic acid and its Fe2+ and Fe3+ complexes
ȱ
ȱ
ȱ
648
ȱ 649
can also be oxidized by the efficient •OH formed at the anode surface at high current.
ȱ
22
ȱ
ȱ
ȱ
334
PART B –Results and Discussion9. Chlorophene
ȱ
650
4. Conclusions
ȱ 651
ȱ 652
The catalytic behavior of the Fe3+/Fe2+ system in the electro-Fenton degradation of
653
chlorophene solutions with 0.05 M Na2SO 4 and different Fe3+ concentrations of pH 3.0
654
mainly depends on the cathode tested. The cells with either a Pt or BDD anode and an O2-
655
diffusion cathode yield a large accumulation of electrogenerated H2O 2 while Fe 3+ content
ȱ 656
remains practically constant. Chlorophene falls more rapidly with raising Fe3+ concentration
ȱ 657
up to 8.0 mM, since more amount of oxidant •OH is formed from Fenton’s reaction (1) due to
ȱ 658
the higher amount of Fe2+ regenerated at the O 2-diffusion cathode from reaction (10). In
659
contrast, the latter reaction is so fast at a carbon-felt cathode that Fe2+ is largely accumulated,
660
but H2O 2 is electrogenerated in small extent. This is feasible by the much slower oxidation of
661
Fe 2+ at Pt and BDD from reaction (10), as explained by the pseudo-first-order rate constants
ȱ 662
determined. In these systems the antimicrobial decay is enhanced with raising current thanks
ȱ 663
to the higher generation of H2O2 and Fe2+ leading to greater amount of •OH from Fenton’s
664
reaction (1), only being required 0.2 mM Fe3+ to obtain its maximum production under all
665
applied currents. The removal rate of chlorophene is always lower in the cells with BDD than
666
with Pt, because Fe2+ is less accumulated since it is also oxidized with peroxodisulfate
ȱ 667
generated at the BDD anode. A second-order rate constant of (1.00±0.01)x1010 M-1 s-1 is
ȱ 668
determined for the reaction between chlorophene and •OH in solution from the method of
ȱ 669
competitive kinetics with benzoic acid. Concentrated solutions of the antimicrobial are poorly
670
decontaminated in the Pt/O2 diffusion cell with 4.0 mM Fe3+, whereas total mineralization is
671
achieved using the BDD/O 2 diffusion cell with 4.0 mM Fe 3+ at high current and the Pt/carbon
672
felt and BDD/carbon felt cells with 0.2 mM Fe3+. The initial chlorine is completely released
ȱ 673
as chloride ion, which remains stable in solution using a Pt anode, but it is oxidized to Cl2 on
ȱ 674
BDD. At the early stages of treatment, the efficiency for the degradation process in the cells
ȱ 675
increases in the order: Pt/O 2 diffusion < BDD/O 2 diffusion < BDD/carbon felt < Pt/carbon
676
felt, although it always rises with decreasing current. The hard oxidation of final Fe3+-oxalate
677
complexes and other undetected products with •OH in the medium and at the Pt surface
678
accounts for the poor degradation in the Pt/O2 diffusion cell. These species are completely
ȱ 679
mineralized at a BDD anode at high current due to the great production of reactive •OH on its
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
23
ȱ
ȱ
335
PART B –Results and Discussion9. Chlorophene
ȱ
680
surface. In the cells with a carbon-felt cathode oxalic acid and its Fe 2+ complexes are directly
ȱ 681
oxidized with •OH in the medium. The highest oxidizing power for total mineralization at
ȱ 682
high current is attained for the BDD/carbon felt cell, when this acid can be simultaneously
ȱ 683
destroyed on BDD. These results show that electro-Fenton is a viable environmentally
ȱ
684
friendly technology for the remediation of wastewaters containing chlorophene.
685
ȱ
686
ȱ 687
ȱ
Acknowledgments
688
689
Financial support from MEC (Ministerio de Educación y Ciencia, Spain) under project
690
CTQ2004-01954/BQU and from MJENR (Ministère de la Jeunesse, de l’Education Nationale
ȱ 691
et de la Recherche, France) under decision number 03V398 is acknowledged. The authors
ȱ 692
thank DURSI (Departament d’Universitats, Recerca i Societat de la Informació, Generalitat
ȱ 693
de Catalunya) for the grant given to I. Sirés to do this work.
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
24
ȱ
ȱ
ȱ
336
PART B –Results and Discussion9. Chlorophene
ȱ
694
References
ȱ 695
ȱ 696
[1] R. Andreozzi, V. Caprio, A. Insola, R. Marotta, Catal. Today 53 (1999) 51-59.
ȱ 697
[2] M. Tarr (Ed.), Chemical Degradation Methods for Wastes and Pollutants.
698
Environmental and Industrial Applications, Marcel Dekker, Inc., New York, 2003.
699
[3] M. Pera-Titus, V. García-Molina, M.A. Baños, J. Giménez, S. Esplugas, Appl. Catal. B:
ȱ
ȱ
700
Environ. 47 (2004) 219-256.
ȱ 701
[4] H. Gallard, J. De Laat, B. Legube, New J. Chem. 22 (1998) 263-268.
ȱ 702
[5] Y. Sun, J.J. Pignatello, Environ. Sci. Technol. 27 (1993) 304-310.
ȱ 703
[6] G.U. Buxton, C.L. Greenstock, W.P. Helman, A.B. Ross, J. Phys. Chem. Data Ref. 17
ȱ
ȱ
704
705
[7] J. De Laat, H. Gallard, Environ. Sci. Technol. 33 (1999) 2726-2732.
706
[8] J.D. Rush, B.H.J. Bielski, J. Phys. Chem. 89 (1985) 5062-5066.
ȱ 707
ȱ 708
ȱ 709
ȱ
ȱ
710
711
712
ȱ 713
ȱ 714
ȱ
ȱ
(1988) 513-886.
715
716
717
ȱ 718
[9] B. Marselli, J. García-Gomez, P.A. Michaud, M.A. Rodrigo, Ch. Comninellis, J.
Electrochem. Soc. 150 (2003) D79-D83.
[10] A. Kraft, M. Stadelmann, M. Blaschke, J. Hazard. Mat. B 103 (2003) 247-261.
[11] E. Brillas, I. Sirés, C. Arias, P.L. Cabot, F. Centellas, R.M. Rodríguez, J.A. Garrido,
Chemosphere 58 (2005) 399-406.
[12] M. Panizza, G. Cerisola, Electrochim. Acta 51 (2005) 191-199.
[13] J. Iniesta, P.A. Michaud, M. Panizza, G. Cerisola, A. Aldaz, Ch. Comninellis,
Electrochim. Acta 46 (2001) 3573-3578.
[14] M. Panizza, P.A. Michaud, G. Cerisola, Ch. Comninellis, J. Electroanal. Chem. 507
(2001) 206-214.
[15] S. Hattori, M. Doi, E. Takahashi, T. Kurosu, M. Nara, S. Nakamatsu, Y. Nishiki, T.
Furuta, M. Iida, J. Appl. Electrochem. 33 (2003) 85-91.
ȱ 719
[16] M. Panizza, G. Cerisola, Electrochim. Acta 49 (2004) 3221-3226.
ȱ 720
[17] E. Brillas, B. Boye, I. Sirés, J.A. Garrido, R.M. Rodríguez, C. Arias, P.L. Cabot, Ch.
ȱ
ȱ
721
722
723
Comninellis, Electrochim. Acta 49 (2004) 4487-4496.
[18] P. Cañizares, C. Sáez, J. Lobato, M.A. Rodrigo, Ind. Eng. Chem. Res. 43 (2004) 19441951.
ȱ
25
ȱ
ȱ
ȱ
337
PART B –Results and Discussion9. Chlorophene
ȱ
724
[19] C.A. Martinez-Huitle, S. Ferro, A. De Battisti, Electrochim. Acta 49 (2004) 4027-4034.
ȱ 725
[20] C. Flox, J.A. Garrido, R.M. Rodríguez, F. Centellas, P.L. Cabot, C. Arias, E. Brillas,
ȱ 726
ȱ 727
ȱ
ȱ
Electrochim. Acta 50 (2005) 3685-3692.
[21] B. Nasr, G. Abdellatif, P. Cañizares, C. Sáez, J. Lobato, M.A. Rodrigo, Environ. Sci.
728
Technol. 39 (2005) 7234-7239.
729
[22] X. Chen, G. Chen, Sep. Purif. Technol. 48 (2006) 45-59.
730
[23] Y.L. Hsiao, K. Nobe, J. Appl. Electrochem. 23 (1993) 943-946.
ȱ 731
[24] C. Ponce de Leon, D. Pletcher, J. Appl. Electrochem. 25 (1995) 307-314.
ȱ 732
[25] E. Brillas, E. Mur, R. Sauleda, L. Sánchez, J. Peral, X. Domènech, J. Casado, Appl.
ȱ 733
ȱ
ȱ
Catal. B: Environ. 16 (1998) 31-42.
734
[26] A. Alverez-Gallegos, D. Pletcher, Electrochim. Acta 44 (1999) 2483-2492.
735
[27] T. Harrington, D. Pletcher, J. Electrochem. Soc. 146 (1999) 2983-2989.
736
[28] M.A. Oturan, J.J. Aaron, N. Oturan, J. Pinson, Pestic. Sci. 55 (1999) 558-562.
ȱ 737
[29] M. A. Oturan, J. Appl. Electrochem. 30 (2000) 475-482.
ȱ 738
[30] J.J. Aaron, M.A. Oturan, Turk. J. Chem. 25 (2001) 509-520.
ȱ 739
[31] B. Boye, M.M. Dieng, E. Brillas, Environ. Sci. Technol. 36 (2002) 3030-3035.
740
[32] E. Brillas, M.A. Baños, J.A. Garrido, Electrochim. Acta 48 (2003) 1697-1705.
741
[33] B. Gözmen, M.A. Oturan, N. Oturan, O. Erbatur, Environ. Sci. Technol. 37 (2003)
ȱ
ȱ
742
3716-3723.
ȱ 743
[34] E. Guivarch, N. Oturan, M.A. Oturan, Environ. Chem. Lett. 1 (2003) 165-168.
ȱ 744
[35] G. Kaichouh, N. Oturan, M.A. Oturan, K. El Kacemi, A. El Hourch, Environ. Chem.
ȱ 745
ȱ
ȱ
746
Lett. 2 (2004) 31-33.
[36] M.C. Edelahi, N. Oturan, M.A. Oturan, Y. Padellec, A. Bermond, K. El Kacemi,
747
748
ȱ 749
Environ. Chem. Lett. 1 (2004) 233-236.
[37] E. Guivarch, T. Trévin, C. Lahitte, M.A. Oturan, Environ. Chem. Lett. 1 (2003) 39-44.
[38] M.A. Oturan, N. Oturan, C. Lahitte, S. Trévin, J. Electroanal. Chem. 507 (2001) 96-
ȱ 750
ȱ 751
ȱ
102.
[39] E. Brillas, M.A. Baños, S. Camps, C. Arias, P.L. Cabot, J.A. Garrido, R.M. Rodríguez,
752
753
New J. Chem. 28 (2004) 314-322.
[40] K. Hanna, S. Chiron, M.A. Oturan, Water Res. 39 (2005) 2763-2773.
ȱ
26
ȱ
ȱ
ȱ
338
PART B –Results and Discussion9. Chlorophene
ȱ
ȱ
754
[41] A. Wang, J. Qu, J. Ru, H. Liu, J. Ge, Dyes Pigments 65 (2005) 227-233.
755
[42] I. Sirés, J.A. Garrido, R.M. Rodríguez, P.L. Cabot, F. Centellas, C. Arias, E. Brillas, J.
ȱ 756
Electrochem. Soc. 153 (2006) D1-D9.
ȱ 757
[43] S. Irmak, H.I. Yavuz, O. Erbatur, Appl. Catal. B: Environ. 63 (2006) 243-248.
ȱ 758
[44] Y.B. Xie, X.Z. Li, Mat. Chem. Phys. 95 (2006) 39-50.
ȱ
ȱ
759
760
761
ȱ 762
ȱ 763
ȱ
ȱ
764
765
766
ȱ 767
ȱ 768
ȱ 769
ȱ
ȱ
770
771
772
[45] C. Flox, S. Ammar, C. Arias, E. Brillas, A.V. Vargas-Zavala, R. Abdelhedi, Appl.
Catal. B: Environ. 67 (2006) 93-104.
[46] H. Zhang, C.H. Huang, Environ. Sci. Technol. 37 (2003) 2421-2430.
[47] W.A. Arnold, K. McNeil, J.L. Packer, D.E. Latch, A.L. Boreen, Report of the USGSWRRI 104G National Grants Competition, 2003, p. 18.
[48] W. Boehmer, H. Ruedel, A. Wenzel, C. Schroeter-Kermani, Organohalogen Comp. 66
(2004) 1489-1494.
[49] T.A. Yamarik, Int. J. Toxicol. 23 (2004) 1-27.
[50] F.J. Welcher, (Ed.), Standard Methods of Chemical Analysis, R.E. Krieger Pub. Co.,
Huntington, New York, 1975, Vol. 2 (Part B), 6th ed., p. 1827.
[51] N.H. Furman, (Ed.), Standard Methods of Chemical Analysis, R.E. Krieger Pub. Co.,
Huntington, New York, 1975, Vol. 1, 6th ed., p. 553.
[52] E.B. Sandell, in: B.L. Clarke, P.J. Elving, I.M. Kolthoff (Eds.), Chemical Analysis,
Interscience Publishers, Inc., New York, 1959, Vol. III, 3rd ed., p.522.
ȱ 773
[53] S.S. Gupta, Y.K. Gupta, Inorg. Chem. 20 (1981) 454-457.
ȱ 774
[54] Y. Zuo,J. Hoigné, Environ. Sci. Technol. 26 (1992) 1014-1022.
ȱ 775
[55] M.A. Oturan, J. Pinson, J. Phys. Chem. 99 (1995) 13948-13954.
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
27
ȱ
ȱ
339
PART B –Results and Discussion9. Chlorophene
ȱ
776
Figure captions
ȱ 777
ȱ 778
Fig. 1. Chemical structure of chlorophene.
ȱ 779
780
Fig. 2. Time-course of Fe 3+, Fe2+ and H2O 2 concentrations during the electrolysis of 200 ml of
781
0.05 M Na 2SO 4 solutions with different Fe3+ contents at pH 3.0, 300 mA and room
782
temperature in a one-compartment cell. Plots: (a) 4.0 mM Fe3+ and a 3 cm2 O 2-diffusion
ȱ 783
cathode; (b) 0.20 mM Fe3+, air saturated solutions and a 70 cm2 carbon-felt cathode. Species:
ȱ 784
( {) Fe3+, ( …) Fe2+ and (∆) H2O 2 using a 3 cm2 Pt anode; (z) Fe3+, („) Fe 2+ and (Ÿ) H2O 2
ȱ 785
using a 3 cm2 BDD anode.
ȱ
ȱ
ȱ
786
787
Fig. 3. Effect of Fe3+ concentration on chlorophene concentration decay during the electro-
788
Fenton treatment of 200 ml of 50 mg l-1 antimicrobial solutions in 0.05 M Na 2SO 4 of pH 3.0
ȱ 789
at 300 mA and room temperature using a Pt/O2 diffusion cell. [Fe3+] 0: (z) 0.2 mM, („) 1.0
ȱ 790
mM, (Ƈ) 2.0 mM, ( Ÿ) 4.0 mM, (d) 6.0 mM, ([) 8.0 mM.
ȱ
ȱ 791
792
Fig. 4. Chlorophene abatement with electrolysis time for 200 ml of 50 mg l-1 antimicrobial
793
solutions in 0.05 M Na 2SO 4 of pH 3.0 degraded at 300 mA and room temperature by: anodic
ȱ 794
oxidation (without Fe3+) in the presence of electrogenerated H2O2 with a (z) Pt/O 2 diffusion
ȱ 795
and („) BDD/O 2 diffusion cell; electro-Fenton with 4.0 mM Fe 3+ using a ( Ÿ) Pt/O 2 diffusion
ȱ 796
and (Ƈ) BDD/O 2 diffusion cell.
ȱ
ȱ
797
798
Fig. 5. Influence of current and Fe 3+ content on chlorophene concentration decay during the
799
electro-Fenton treatment of 200 ml of 50 mg l-1 of this antimicrobial and 0.05 M Na2SO 4 at
ȱ 800
pH 3.0 and room temperature using a Pt/carbon felt cell. Each solution was previously
ȱ 801
saturated with air. Current: (a) 60 mA, (b) 300 mA. [Fe 3+]0: ({) 0.1 mM, („) 0.2 mM, (Ƈ) 0.5
ȱ 802
mM, (Ÿ) 1.0 mM, (d) 2.0 mM.
ȱ
ȱ
ȱ
803
804
Fig. 6. Kinetic analysis for the pseudo first-order reaction of ({,…) chlorophene and (z,„)
805
benzoic acid with hydroxyl radical. Electro-Fenton experiments were carried out with 200 ml
ȱ
28
ȱ
ȱ
ȱ
340
PART B –Results and Discussion9. Chlorophene
ȱ
ȱ
806
of an air saturated solution containing 50 mg l-1 chlorophene, 122 mg l-1 benzoic acid, 0.05 M
807
Na2SO 4 and 0.2 mM Fe3+ at pH 3.0, 60 mA and room temperature using a ( {,z) Pt/carbon
ȱ 808
felt and (…,„) BDD/carbon felt cell.
ȱ 809
ȱ 810
Fig. 7. TOC removal vs. electrolysis time for 200 ml of 84 mg l-1 chlorophene solutions in
811
0.05 M Na2SO 4 with different Fe3+ contents of pH 3.0 treated by electro-Fenton at room
812
temperature. In plot (a), solutions with 4.0 mM Fe3+ in a (¡) Pt/O 2 diffusion and (z,„,Ÿ,Ƈ)
813
BDD/O 2 diffusion cell. In plot (b), air saturated solutions with 0.2 mM Fe3+ in a (z,„,Ÿ,Ƈ)
ȱ 814
Pt/carbon felt and (¡) BDD/carbon felt cell. Current: (z) 60 mA, („) 100 mA, (Ÿ) 200 mA,
ȱ 815
( Ƈ,¡) 300 mA.
ȱ
ȱ
ȱ 816
817
Fig. 8. Concentration of chloride ion accumulated during the electro-Fenton treatment of 200
818
ml of an 84 mg l-1 chlorophene solution with 0.015 M Na 2SO 4 and 0.2 mM Fe3+ of pH 3.0 at
ȱ 819
150 mA and at room temperature using a (z) Pt/O 2 diffusion, („) Pt/carbon felt, (Ƈ) BDD/O 2
ȱ 820
diffusion and (Ÿ) BDD/carbon felt cell.
ȱ
ȱ 821
ȱ
ȱ
822
Fig. 9. Dependence of mineralization current efficiency calculated from Eq. 11 on electrolysis
823
time for the experiments reported in: (a) Fig. 7a, (b) Fig. 7b.
824
ȱ 825
Fig. 10. Evolution of the concentration of (a) formic and (b) oxalic acids detected as final
ȱ 826
carboxylic acids during the electro-Fenton degradation of 200 ml of 84 mg l-1 chlorophene
ȱ 827
solutions in 0.05 M Na2SO 4 of pH 3.0 at room temperature. System: Pt/O2 diffusion cell with
828
4.0 mM Fe3+ at (z) 60 mA and ({) 300 mA; BDD/O 2 diffusion cell with 4.0 mM Fe3+ at ( Ƈ)
829
60 mA and (¡) 300 mA; Pt/carbon felt cell with 0.2 mM Fe3+ at („) 60 mA and ( …) 300 mA;
830
BDD/carbon felt cell with 0.2 mM Fe 3+ at (Ÿ) 60 mA and (∆) 300 mA.
ȱ
ȱ
ȱ 831
ȱ 832
Fig. 11. Reaction paths of oxalic acid in the electro-Fenton systems. •OH denotes the
ȱ 833
hydroxyl radical generated in solution and BDD(•OH) represents the hydroxyl radical formed
834
ȱ 835
on the BDD anode surface.
ȱ
ȱ
29
ȱ
ȱ
341
PART B –Results and Discussion9. Chlorophene
*ȱ List of Three (3) Potential Reviewers
ȱ
ȱ
Possible reviewers
ȱ
ȱ
Prof. André Savall : [email protected]
Laboratoire de Génie Chimique, Université Paul Sabatier, 118 route de Narbonne, 31062
Toulouse, France
ȱ
ȱ
Dr. Birame Boye : [email protected]
Istituto di Chimica Fisica ed Elettrochimica, Università degli Study di Padova, Via Marzolo
14, 35131 Padova, Italy
ȱ
Prof. Dr. Christos Comninellis : [email protected]
Groupe de génie Electrochimique, Ecole Polytechnique Fédérale de Lausanne,
SB-ISP-UGEC, 1015 Lausanne - SWITZERLAND.
Fax : +41-21 693 3190
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
342
PART B –Results and Discussion9. Chlorophene
ȱ Figure(s)
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
OH
ȱ
ȱ
ȱ
Cl
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Fig. 1
ȱ
ȱ
ȱ
ȱ
343
PART B –Results and Discussion9. Chlorophene
ȱ Figure(s)
ȱ
ȱ
ȱ
ȱ
ȱ
10
ȱ
(a)
8
ȱ
ȱ
6
ȱ
4
concentration / mM
ȱ
ȱ
ȱ
ȱ
2
0
(b)
0.20
ȱ
0.15
ȱ
ȱ
0.10
ȱ
0.05
ȱ
0.00
ȱ
0
10
20
30
40
time / min
ȱ
ȱ
ȱ
ȱ
ȱ
Fig. 2
ȱ
ȱ
ȱ
ȱ
344
50
60
70
PART B –Results and Discussion9. Chlorophene
ȱ Figure(s)
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
60
ȱ
50
ȱ
ȱ
ȱ
ȱ
[chlorophene] / mg l
-1
ȱ
40
30
20
10
ȱ
0
ȱ
0
5
10
15
20
25
time / min
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Fig. 3
ȱ
ȱ
ȱ
ȱ
345
PART B –Results and Discussion9. Chlorophene
ȱ Figure(s)
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
60
ȱ
50
[chlorophene] / mg l
-1
ȱ
ȱ
ȱ
ȱ
ȱ
40
30
20
10
ȱ
0
ȱ
0
60
120
180
time / min
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Fig. 4
ȱ
ȱ
ȱ
ȱ
346
240
300
360
PART B –Results and Discussion9. Chlorophene
ȱ Figure(s)
ȱ
ȱ
ȱ
ȱ
60
ȱ
(a)
50
ȱ
40
ȱ
30
ȱ
20
ȱ
ȱ
ȱ
ȱ
[chlorophene] / mg l-1
ȱ
10
0
ȱ
30
ȱ
ȱ
20
30
40
50
60
70
(b)
50
40
ȱ
10
60
ȱ
ȱ
0
20
10
0
0
5
10
15
20
25
time / min
ȱ
ȱ
ȱ
ȱ
ȱ
Fig. 5
ȱ
ȱ
ȱ
ȱ
347
PART B –Results and Discussion9. Chlorophene
ȱ
Figure(s)
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
1.5
ȱ
ȱ
ln (c / c)
1.0
0
ȱ
ȱ
0.5
ȱ
ȱ
0.0
ȱ
0
5
10
15
time / min
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Fig. 6
ȱ
ȱ
ȱ
ȱ
348
20
25
PART B –Results and Discussion9. Chlorophene
ȱ Figure(s)
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
60
ȱ
40
ȱ
30
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
20
10
0
60
ȱ
50
ȱ
ȱ
40
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
120
240
360
480
600
720
(b)
30
20
10
ȱ
ȱ
ȱ
0
70
ȱ
ȱ
(a)
50
ȱ
ȱ
ȱ
70
TOC / mg l-1
ȱ
ȱ
0
0
120
240
360
time / min
480
600
Fig. 7
ȱ
ȱ
ȱ
ȱ
349
PART B –Results and Discussion9. Chlorophene
ȱ Figure(s)
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
15
ȱ
12
[Cl ] / mg l
-1
ȱ
9
−
ȱ
ȱ
ȱ
6
3
ȱ
0
ȱ
0
120
240
360
time / min
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Fig. 8
ȱ
ȱ
ȱ
ȱ
350
480
600
PART B –Results and Discussion9. Chlorophene
ȱ Figure(s)
ȱ
ȱ
ȱ
ȱ
ȱ
35
30
ȱ
25
ȱ
20
ȱ
15
ȱ
10
ȱ
5
ȱ
ȱ
MCE / %
ȱ
0
100
ȱ
80
ȱ
60
ȱ
40
ȱ
ȱ
0
120
240
360
480
600
720
120
ȱ
ȱ
(a)
(b)
20
0
0
120
240
360
480
600
time /min
ȱ
ȱ
ȱ
ȱ
Fig. 9
ȱ
ȱ
ȱ
ȱ
351
PART B –Results and Discussion9. Chlorophene
ȱ Figure(s)
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
20
(a)
-1
ȱ
[formic acid] / mg l
ȱ
15
10
5
ȱ
ȱ
ȱ
ȱ
0
ȱ
ȱ
100
0
60
120
180
240
300
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
-1
ȱ
ȱ
60
40
20
0
0
120
240
360
time / min
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
352
420
(b)
80
[oxalic acid] / mg l
ȱ
360
Fig. 10
480
600
720
PART B –Results and Discussion9. Chlorophene
ȱ Figure(s)
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
COOH
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
COOH
Fe2+
Fe2+ -oxalato
complexes
.OH
BDD(.OH)
− Fe3+
Fe3+
.OH
BDD(.OH)
Fe3+-oxalato
complexes
.
BDD( OH)
CO2
− Fe
3+
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Fig. 11
ȱ
ȱ
ȱ
ȱ
ȱ
353
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
PART B –Results and Discussion9. Chlorophene
9.2.2.ȱȱResultatsȱiȱDiscussióȱ/ȱResultsȱandȱDiscussionȱ
ȱ
Firstȱ ofȱ all,ȱ theȱ catalyticȱ behaviorȱ ofȱ theȱ Fe3+/Fe2+ȱ systemȱ inȱ theȱ EFȱ processȱ hasȱ beenȱ
testedȱ inȱ theȱ absenceȱ ofȱ chlorophene.ȱ Theȱ oxidationȱ abilityȱ ofȱ Ptȱ andȱ BDDȱ toȱ
transformȱdirectlyȱFe2+ȱintoȱFe3+ȱhasȱbeenȱexaminedȱbyȱelectrolyzingȱ200ȬmLȱsolutionsȱ
ofȱ0.05ȱMȱNa2SO4ȱwithȱ4.0ȱmMȱFe2+,ȱatȱpHȱ3.0ȱandȱatȱ300ȱmA,ȱusingȱaȱstainlessȱsteelȱ
cathode.ȱ Aȱ quickȱ decayȱ ofȱ Fe2+ȱ concentrationȱ andȱ aȱ simultanousȱ increaseȱ ofȱ Fe3+ȱ isȱ
foundȱusingȱbothȱanodes,ȱthusȱindicatingȱaȱfastȱdirectȱoxidationȱatȱeachȱanode.ȱFe2+ȱisȱ
completelyȱ removedȱ afterȱ 45ȱ andȱ 64ȱ minȱ usingȱ BDDȱ andȱ Pt,ȱ respectively.ȱ Thisȱ factȱ
canȱbeȱexplainedȱconsideringȱthatȱBDDȱfavorsȱtheȱdirectȱoxidationȱofȱFe2+ȱinsteadȱofȱ
O2ȱevolution.ȱAȱpseudoȬfirstȬorderȱdecayȱisȱfoundȱinȱbothȱcasesȱatȱtheirȱearlyȱstages.ȱ
ȱ
Severalȱelectrolysesȱofȱ0.05ȱMȱNa2SO4ȱsolutionsȱatȱpHȱ3.0ȱhaveȱbeenȱperformedȱforȱ60ȱ
minȱatȱ300ȱmAȱusingȱPtȱandȱBDDȱanodes,ȱwithȱ4.0ȱandȱ0.2ȱmMȱFe3+ȱinȱO2Ȭdiffusionȱ
andȱ carbonȱ feltȱ cells,ȱ respectively,ȱ inȱ orderȱ toȱ assessȱ theȱ evolutionȱ ofȱ Fe2+,ȱ Fe3+ȱ andȱ
H2O2.ȱ Theȱ resultsȱ forȱ theȱ O2ȱ diffusionȱ cellsȱallowȱ concludingȱ thatȱ Fe3+ȱ concentrationȱ
remainsȱ unaltered,ȱ soȱ theȱ amountȱ ofȱ Fe2+ȱ accumulatedȱ isȱ insignificant.ȱ Thisȱ meansȱ
thatȱdirectȱreductionȱofȱFe3+ȱoccursȱtoȱaȱveryȱsmallȱextentȱatȱtheȱO2Ȭdiffusionȱcathodeȱ
andȱtheȱregeneratedȱFe2+ȱisȱquicklyȱconvertedȱintoȱFe3+ȱfromȱReactionsȱ5.Ȭ3,ȱ5.Ȭ5ȱandȱ
5.Ȭ14,ȱasȱwellȱasȱfromȱdirectȱoxidationȱatȱtheȱanode.ȱInȱaddition,ȱH2O2ȱisȱcontinuouslyȱ
accumulatedȱ upȱ toȱ reachȱ aȱ steadyȱ concentrationȱ ofȱ aboutȱ 9ȱ mMȱ afterȱ 45ȱ minȱ usingȱ
bothȱanodes.ȱTheȱlatterȱbehaviorȱwasȱwidelyȱdescribedȱinȱsectionȱ7.2.1.ȱTheȱresultsȱforȱ
theȱcarbonȱfeltȱcellsȱpresentȱsignificantȱdifferencesȱdependingȱonȱtheȱanodeȱused.ȱTheȱ
useȱofȱPt/carbonȱfeltȱcellȱcausesȱtheȱreductionȱofȱallȱtheȱinitialȱFe3+ȱtoȱFe2+ȱinȱ20ȱmin,ȱ
thusȱindicatingȱthatȱdirectȱreductionȱatȱtheȱcarbonȬfeltȱcathodeȱisȱconsiderablyȱfast:ȱaȱ
reductionȱ rateȱ ofȱ 8.10ȱ xȱ 10Ȭ7ȱ Mȱ sȬ1ȱ isȱ obtained,ȱ aȱ valueȱ muchȱ higherȱ thanȱ theȱ directȱ
oxidationȱrateȱ ofȱ 1.17ȱxȱ10Ȭ7ȱMȱ sȬ1ȱcalculatedȱforȱ Fe2+ȱatȱtheȱPtȱanode.ȱTheȱamountȱofȱ
H2O2ȱ accumulatedȱ isȱratherȱlowȱ (0.23ȱ mMȱ afterȱ 60ȱ min).ȱ Onȱtheȱ otherȱhand,ȱ overallȱ
Fe3+ȱreductionȱisȱnotȱachievedȱinȱtheȱBDD/carbonȱfeltȱcell,ȱsinceȱFe2+ȱimmediatelyȱrisesȱ
355
PART B –Results and Discussion9. Chlorophene
upȱ toȱ 0.058ȱ mMȱ afterȱ 2ȱ min,ȱ furtherȱ beingȱ slowlyȱ removedȱ untilȱ itsȱ disappearanceȱ
afterȱ60ȱmin.ȱTheȱconcentrationȱofȱH2O2ȱafterȱ60ȱminȱisȱagainȱveryȱlow,ȱca.ȱ0.20ȱmM.ȱ
Thisȱsurprisingȱtrendȱcanȱbeȱexplainedȱbyȱtheȱimportantȱroleȱofȱtheȱweakerȱoxidizingȱ
speciesȱproducedȱatȱtheȱBDDȱanodeȱ(seeȱsectionȱ8.2.2),ȱwhichȱareȱableȱtoȱoxidizeȱFe2+ȱ
significantly.ȱ Forȱ example,ȱ S2O82Ȭȱ ionsȱ canȱ oxidizeȱ Fe2+ȱ toȱ Fe3+ȱ withȱ kȱ =ȱ 23ȱ MȬ1ȱ sȬ1.ȱ
However,ȱFe3+ȱisȱnotȱcompletelyȱregeneratedȱfromȱtheȱoxidizingȱreactionsȱbecauseȱitȱ
undergoesȱaȱgradualȱabatement,ȱwhichȱcanȱbeȱaccountedȱforȱbyȱtheȱprecipitationȱofȱ
Fe(OH)3ȱonȱtheȱlargeȱsurfaceȱofȱtheȱcarbonȬfeltȱcathode.ȱThisȱprecipitateȱisȱfavoredȱbyȱ
theȱOHȬȱformedȱfromȱwaterȱreductionȱnearȱtheȱcathodeȱinȱtheȱBDD/carbonȱfeltȱsystem.ȱ
ȱ
Inȱ conclusion,ȱ theȱ Pt/carbonȱ feltȱ systemȱ accumulatesȱ Fe2+ȱ ableȱ toȱ reactȱ largelyȱ withȱ
H2O2ȱ toȱ produceȱ highȱ amountsȱ ofȱ •OHȱ fromȱ Fenton’sȱ reaction,ȱ whereasȱ inȱ theȱ
BDD/carbonȱfeltȱsystemȱFe2+ȱisȱslowlyȱdestroyed,ȱthusȱcausingȱaȱlowerȱproductionȱofȱ
•
OHȱasȱwellȱasȱaȱdecreaseȱofȱsolubleȱFe3+ȱconcentrationȱdueȱtoȱFe(OH)3ȱprecipitation.ȱ
Inȱcontrast,ȱtheȱO2ȬdiffusionȱsystemsȱaccumulateȱH2O2ȱandȱlowerȱamountsȱofȱFe2+.ȱ
ȱ
Toȱ confirmȱ theȱ aboveȱ comments,ȱ theȱ degradationȱ ofȱ chloropheneȱ byȱ theȱ fourȱ EFȱ
processesȱ hasȱ beenȱ thoroughlyȱ studied.ȱ Theȱ influenceȱ ofȱ Fe3+ȱ concentrationȱ onȱ
chloropheneȱ decayȱ hasȱ beenȱ studiedȱ byȱ reversedȬphaseȱ HPLCȱ chromatography.ȱ
Firstly,ȱseveralȱ 50ȬmgȱLȬ1ȱchloropheneȱ solutions,ȱ withȱFe3+ȱcontentsȱ betweenȱ 0.2ȱandȱ
8.0ȱmMȱhaveȱbeenȱelectrolyzedȱatȱpHȱ3.0ȱandȱatȱ300ȱmAȱusingȱtheȱPt/O2ȱdiffusionȱcell.ȱ
Resultsȱshowȱaȱfastȱandȱcompleteȱremovalȱofȱtheȱpharmaceuticalȱwhateverȱtheȱinitialȱ
Fe3+ȱamountȱmayȱbe.ȱItsȱdestructionȱrateȱundergoesȱaȱprogressiveȱaccelerationȱasȱFe3+ȱ
concentrationȱ increases,ȱ disappearingȱ afterȱ 20ȱ andȱ 3ȱ minȱ forȱ 0.2ȱ andȱ 8.0ȱ mM,ȱ
respectively.ȱ Thisȱ trendȱ canȱ beȱ relatedȱ toȱ anȱ increasingȱ amountȱ ofȱ Fe2+ȱ regeneratedȱ
fromȱdirectȱcathodicȱreductionȱofȱFe3+,ȱthusȱenhancingȱtheȱoxidationȱofȱchloropheneȱ
withȱ •OHȱ producedȱ fromȱ Fenton’sȱ reactionȱ (Reactionȱ 5.Ȭ3).ȱ •OHadsȱ formedȱ atȱ theȱ Ptȱ
surfaceȱ(Reactionȱ5.Ȭ44)ȱandȱaȱweakerȱoxidizingȱspeciesȱsuchȱasȱHO2•ȱ(Reactionsȱ5.Ȭ4ȱ
andȱ 5.Ȭ48)ȱ couldȱ contributeȱ toȱ theȱ destructionȱ ofȱ thisȱ pharmaceuticalȱ asȱ well.ȱȱȱȱȱȱȱȱȱȱȱȱȱȱ
356
PART B –Results and Discussion9. Chlorophene
Inȱ contrast,ȱ theȱ oxidizingȱ powerȱ ofȱ H2O2ȱ canȱ beȱ discardedȱ becauseȱ aȱ chemicalȱ testȱ
carriedȱoutȱwithȱ20ȱmMȱH2O2ȱreflectsȱnoȱalterationȱofȱchloropheneȱconcentration.ȱTheȱ
roleȱofȱ •OHadsȱandȱHO2•ȱproducedȱatȱtheȱanodeȱhasȱbeenȱclarifiedȱbyȱelectrolyzingȱaȱ
chloropheneȱ solutionȱ underȱ theȱ conditionsȱ givenȱ above,ȱ butȱ withoutȱ Fe3+,ȱ usingȱ theȱ
Pt/O2ȱ diffusionȱ cell.ȱ Theȱ resultsȱ ofȱ thisȱ degradationȱ byȱ AOȱ withȱ electrogeneratedȱ
H2O2ȱ indicateȱ thatȱ theȱ initialȱ compoundȱ disappearsȱ muchȱ moreȱ slowly,ȱ afterȱ aboutȱ
300ȱmin.ȱItȱcanȱbeȱconcludedȱthatȱtheȱmainȱoxidizingȱagentȱinȱthisȱelectrolyticȱsystemȱ
isȱ •OHȱ producedȱ fromȱ Fenton’sȱ reaction.ȱ Theȱ greaterȱ oxidizingȱ powerȱ ofȱ BDD,ȱ
previouslyȱ discussedȱ forȱ paracetamolȱ andȱ clofibricȱ acidȱ mineralization,ȱ isȱ alsoȱ
demonstratedȱ forȱ chlorophene,ȱ whereȱ AOȱ withȱ electrogeneratedȱ H2O2ȱ usingȱ theȱ
BDD/O2ȱ diffusionȱ cellȱ causesȱ theȱ totalȱ chloropheneȱ destructionȱ afterȱ 180ȱ min.ȱ Theȱ
effectȱofȱFe3+ȱconcentrationȱisȱsimilarȱtoȱthatȱdescribedȱaboveȱusingȱPt,ȱbutȱitȱisȱworthȱ
notingȱ thatȱ theȱ destructionȱ rateȱ ofȱ chloropheneȱ isȱ significantlyȱ reduced.ȱ Aȱ
chloropheneȱremovalȱtimeȱofȱ7ȱandȱ90ȱminȱisȱobtainedȱforȱtheȱPtȱandȱBDDȱsystems,ȱ
respectively,ȱusingȱ4.0ȱmMȱFe3+.ȱThisȱeffectȱcanȱbeȱexplainedȱbyȱtheȱquickerȱoxidationȱ
ofȱ Fe2+ȱ toȱ Fe3+,ȱ andȱ evenȱ toȱ ferrateȱ ionsȱ (seeȱ sectionȱ 8.3.2),ȱ atȱ theȱ BDDȱ anodeȱ inȱ
comparisonȱtoȱthatȱtakingȱplaceȱatȱtheȱPtȱanode,ȱalongȱwithȱitsȱadditionalȱdestructionȱ
byȱ reactionȱ withȱ S2O82Ȭȱ ions,ȱ involvingȱ SO4•-ȱ (seeȱ sectionȱ 8.3.2).ȱ Fe2+ȱ disappearanceȱ
leadsȱ toȱ aȱ dropȱ inȱ theȱ productionȱ ofȱ •OHȱ fromȱ Fenton’sȱ reaction,ȱ thusȱ makingȱ theȱ
chloropheneȱ decayȱ muchȱ slower.ȱ Atȱ thisȱ pointȱ itȱ mustȱ beȱ mentionedȱ theȱ hugeȱ
influenceȱofȱtheȱironȱsourceȱusedȱinȱEFȱonȱtheȱ •OHȱproduction:ȱFigureȱ8.Ȭ6ȱinȱsectionȱ
8.3.2ȱshowsȱaȱveryȱsimilarȱdecayȱofȱclofibricȱacidȱinȱtheȱPt/O2ȱdiffusionȱandȱBDD/O2ȱ
diffusionȱcellsȱwhenȱFe2+ȱisȱused,ȱbecauseȱinȱbothȱcasesȱtheȱamountȱofȱ •OHȱgeneratedȱ
fromȱ Fenton’sȱ reactionȱ isȱ highȱ enoughȱ toȱ yieldȱ anȱ analogousȱ oxidationȱ duringȱ theȱ
earlyȱ stagesȱ (i.e.,ȱ clofibricȱ acidȱ isȱ destroyedȱ afterȱ 7ȱ min).ȱ Onȱ theȱ contrary,ȱ Fe2+ȱ
regenerationȱ isȱ soȱ difficultȱ inȱ suchȱ anȱ oxidizingȱ systemȱ asȱ BDD/O2ȱ diffusionȱ whenȱ
Fe3+ȱisȱused,ȱthatȱtheȱdestructionȱrateȱforȱchloropheneȱisȱsignificantlyȱlowerȱcomparedȱ
toȱthatȱofȱtheȱPt/O2ȱdiffusionȱcell.ȱTheȱuseȱofȱtheȱcarbonȬfeltȱcathodeȱleadsȱtoȱaȱgreaterȱ
productionȱ ofȱ •OHȱ dueȱ toȱ theȱ fastȱ Fe2+ȱ regeneration,ȱ soȱ chloropheneȱ destructionȱ
357
PART B –Results and Discussion9. Chlorophene
shouldȱbeȱaccelerated.ȱThus,ȱchloropheneȱisȱremovedȱafterȱ5ȱminȱatȱ300ȱmAȱwithȱ0.2ȱ
mMȱFe3+ȱusingȱtheȱPt/carbonȱfeltȱcell,ȱandȱitsȱdecayȱbecomesȱslowerȱwithȱrisingȱFe3+ȱ
concentration.ȱ Thisȱ isȱ dueȱ toȱ theȱ actionȱ ofȱ theȱ nonȬoxidizingȱ reactionsȱ involvingȱ anȱ
everȱ increasingȱ Fe2+ȱ concentrationȱ regeneratedȱ atȱ theȱ cathode.ȱ Withȱ 0.1Ȭ0.2ȱ mMȱ Fe3+ȱ
theȱ hydroxylȱ radicalȱ formedȱ fromȱ Fenton’sȱ reactionȱ isȱ wastedȱ toȱ aȱ smallȱ extentȱ inȱ
thoseȱ parasiteȱ reactions.ȱ Inȱ addition,ȱ anȱ increaseȱ inȱ currentȱ causesȱ aȱ quickerȱ decayȱ
dueȱ toȱ theȱ productionȱ ofȱ moreȱ oxidizingȱ species.ȱ Theȱ sameȱ trendsȱ areȱ foundȱ byȱ
electrolyzingȱtheȱaboveȱsolutionsȱwithȱtheȱBDD/carbonȱfeltȱcell,ȱandȱagainȱaȱnegativeȱ
effectȱ onȱ chloropheneȱ removalȱ isȱ observedȱ becauseȱ theȱ oxidizingȱ powerȱ ofȱ BDDȱ
hindersȱtheȱFe2+ȱregeneration.ȱ
ȱ
TheȱaboveȱconcentrationȱdecaysȱcanȱbeȱfittedȱtoȱaȱpseudoȬfirstȬorderȱkineticȱequation.ȱ
Theȱreactionȱbetweenȱchloropheneȱandȱ•OHȱisȱespeciallyȱfastȱforȱtheȱcellsȱcontainingȱaȱ
carbonȬfeltȱ cathodeȱ dueȱ toȱ theȱ highȱ constantȱ concentrationȱ ofȱ thisȱ radicalȱ inȱ theȱ
medium,ȱ soȱ theȱ absoluteȱ rateȱ constantȱ (k2)ȱ forȱ thisȱ reactionȱ hasȱ beenȱ determinedȱ inȱ
thoseȱ cellsȱ usingȱ Ptȱ andȱ BDDȱ anodes.ȱ Takingȱ benzoicȱ acidȱ asȱ standardȱ competitionȱ
substrate,ȱ pseudoȬfirstȬorderȱ rateȱ constantsȱ (k1)ȱ areȱ obtainedȱ forȱ chloropheneȱ andȱ
benzoicȱacid,ȱsoȱconsideringȱk2ȱ=ȱ4.30ȱxȱ109ȱMȬ1ȱsȬ1ȱforȱbenzoicȱacid,ȱanȱaverageȱvalueȱofȱ
k2ȱ =ȱ (1.00±0.10)ȱ xȱ 1010ȱ MȬ1ȱ sȬ1ȱ isȱ obtainedȱ forȱ chlorophene.ȱ Thisȱ valueȱ isȱ veryȱ closeȱ toȱȱ
7.1ȱ xȱ 109ȱ MȬ1ȱ sȬ1ȱ reportedȱ byȱ Arnoldȱ etȱ al.ȱ [381]ȱ forȱ chloropheneȱ degradationȱ withȱ
Fenton’sȱreagent.ȱFromȱthisȱk2Ȭvalue,ȱaȱsteadyȱreactiveȱ •OHȱconcentrationȱca.ȱ10Ȭ13ȱMȱ
atȱ60ȱmAȱcanȱbeȱcalculated.ȱ
ȱ
Theȱ abilityȱ ofȱ theȱ fourȱ EFȱ cellsȱ toȱ mineralizeȱ 84Ȭmgȱ LȬ1ȱ chloropheneȱ solutionsȱ hasȱ
beenȱassessedȱfromȱtheirȱTOCȱdecayȱinȱtheȱrangeȱ60Ȭ300ȱmAȱwithȱanȱefficientȱcontentȱ
ofȱ 4.0ȱ andȱ 0.2ȱ mMȱ Fe3+,ȱ deducedȱ fromȱ theȱ previousȱ studyȱ byȱ HPLC,ȱ usingȱ theȱȱȱȱȱȱȱȱȱȱ
O2ȬdiffusionȱandȱcarbonȬfeltȱcathode,ȱrespectively.ȱAȱcontinuousȱslowȱTOCȱremovalȱ
isȱfoundȱinȱtheȱPt/O2ȱdiffusionȱcell,ȱonlyȱattainingȱ52%ȱmineralizationȱafterȱ660ȱminȱofȱ
electrolysisȱatȱ300ȱmA.ȱThisȱlowȱoxidizingȱpowerȱcanȱbeȱexplainedȱbyȱtheȱformationȱ
358
PART B –Results and Discussion9. Chlorophene
ofȱproductsȱhardlyȱoxidizableȱwithȱ•OH,ȱasȱdiscussedȱforȱEFȱtreatmentȱofȱparacetamolȱ
andȱ clofibricȱ acid.ȱ Inȱ contrast,ȱ theseȱ Fe3+Ȭcarboxylicsȱ complexesȱ canȱ beȱ slowlyȱ butȱ
completelyȱdestroyedȱwithȱBDD(•OH)ȱinȱtheȱBDD/O2ȱdiffusionȱcellȱatȱ300ȱmA,ȱasȱalsoȱ
foundȱ forȱ clofibricȱ acidȱ (seeȱ sectionȱ 8.3.2).ȱ Itȱ canȱ alsoȱ beȱ observedȱ thatȱ inȱ theȱ latterȱ
systemȱtheȱmineralizationȱrateȱincreasesȱasȱcurrentȱrisesȱfromȱ60ȱtoȱ300ȱmA,ȱandȱ33%,ȱ
45%,ȱ85%ȱandȱ97%ȱTOCȱabatementȱisȱachievedȱatȱ60,ȱ100,ȱ200ȱandȱ300ȱmAȱafterȱ11ȱh,ȱ
respectively,ȱ dueȱ toȱ theȱ higherȱ productionȱ ofȱ •OHȱ andȱ BDD(•OH).ȱ Totalȱ
mineralizationȱ canȱ notȱ beȱ attainedȱ atȱ allȱ currentsȱ afterȱ 11ȱ hȱ becauseȱ •OHȱ fromȱ
Fenton’sȱreactionȱareȱslowlyȱproducedȱfromȱtheȱdifficultȱFe2+ȱregeneration.ȱUsingȱtheȱ
carbonȬfeltȱ cathode,ȱ however,ȱ aȱ fastȱ andȱ completeȱ degradationȱ (>ȱ 95%)ȱ isȱ alwaysȱ
reachedȱ afterȱ 540ȱ minȱ ofȱ electrolysis.ȱ Theȱ processȱ isȱ acceleratedȱ asȱ currentȱ rises,ȱ
mainlyȱforȱtheȱfirstȱ60ȱmin,ȱbecauseȱaromaticȱintermediatesȱcanȱbeȱquicklyȱdestroyed.ȱ
Theȱ greaterȱ oxidizingȱ abilityȱ ofȱ theȱ systemsȱ withȱ theȱ carbonȬfeltȱ cathodeȱ inȱ
comparisonȱ toȱ thoseȱ withȱ theȱ O2Ȭdiffusionȱ cathodeȱ canȱ beȱ ascribedȱ toȱ theȱ greatȱ
amountsȱ ofȱ •OHȱ formedȱ fromȱ Fenton’sȱ reactionȱ dueȱ toȱ theȱ fastȱ Fe2+ȱ regenerationȱ atȱ
theȱcathode.ȱAppartȱfromȱthis,ȱwhenȱtheȱtwoȱcarbonȱfeltȱcellsȱareȱcompared,ȱitȱcanȱbeȱ
seenȱthatȱtheȱuseȱofȱBDDȱdoesȱnotȱleadȱtoȱaȱmuchȱmoreȱrelevantȱTOCȱabatement.ȱThisȱ
happensȱbecauseȱregardlessȱtheȱimportantȱcontributionȱofȱBDD(•OH),ȱtheȱproductionȱ
ofȱ•OHȱisȱmoreȱdifficultȱthanȱusingȱPt.ȱ
ȱ
Ionȱ chromatographyȱ displaysȱ aȱ uniqueȱ peakȱ correspondingȱ toȱ chlorideȱ ion.ȱ Theȱ
resultsȱ confirmȱ thatȱ Clȱ isȱ quicklyȱ accumulatedȱ inȱ theȱ cellsȱ withȱ Pt,ȱ reachingȱ aȱȱȱȱ
quasiȬsteadyȱconcentrationȱ ofȱ aboutȱ13ȱmgȱLȬ1ȱ(veryȱ closeȱtoȱtheȱ maximumȱvalueȱ ofȱ
13.6ȱmgȱLȬ1ȱcorrespondingȱtoȱ84ȱmgȱLȬ1ȱchlorophene),ȱwhereasȱinȱtheȱcellsȱwithȱBDDȱ
thisȱ ionȱ onlyȱ attainsȱ betweenȱ 5ȱ andȱ 9ȱ mgȱ LȬ1ȱ andȱ furtherȱ itȱ isȱ destroyedȱ untilȱ
disappearing.ȱ Thisȱ behaviorȱ agreesȱ withȱ thatȱ observedȱ forȱ theȱ EFȱ degradationȱ ofȱ
clofibricȱacid.ȱ
ȱ
ȱ
359
PART B –Results and Discussion9. Chlorophene
Theȱ aboveȱ resultsȱ allowȱ concludingȱ thatȱ theȱ overallȱ mineralizationȱ ofȱ thisȱ
pharmaceuticalȱbyȱEFȱinvolvesȱ60ȱFȱforȱeachȱmolȱofȱchlorophene,ȱwithȱchlorideȱionȱasȱ
primaryȱinorganicȱionȱ(Reactionȱ6.Ȭ4).ȱTherefore,ȱMCEȱcanȱthenȱbeȱdeterminedȱusingȱ
Equationȱ6.Ȭ1.ȱAsȱusual,ȱplotsȱshowȱaȱdramaticȱfallȱofȱefficiencyȱwithȱelectrolysisȱtimeȱ
dueȱ toȱ theȱ formationȱ ofȱ hardlyȱ oxidizableȱ products.ȱ Inȱ addition,ȱ theȱ efficiencyȱ risesȱ
whenȱcurrentȱdecreases.ȱForȱexample,ȱefficienciesȱofȱ10ȱandȱ19%ȱvaluesȱareȱobtainedȱ
atȱ 300ȱ andȱ 60ȱ mA,ȱ respectively,ȱ afterȱ 60ȱ minȱ usingȱ theȱ BDD/O2ȱ diffusionȱ cell.ȱ
ComparisonȱbetweenȱtheȱfourȱEFȱsystemsȱatȱ300ȱmAȱallowsȱorderingȱtheirȱefficiencyȱ
onȱ theȱ basisȱ ofȱ increasingȱ degradationȱ ability:ȱ Pt/O2ȱ diffusionȱ <ȱ BDD/O2ȱ diffusionȱȱȱȱȱȱȱ
<ȱ BDD/carbonȱ feltȱ <ȱ Pt/carbonȱ felt.ȱ Inȱ fact,ȱ theȱ latterȱ cellȱ presentsȱ theȱ greatestȱ MCEȱ
valuesȱamongȱallȱtheȱsystemsȱtestedȱduringȱthisȱthesis,ȱreachingȱanȱefficiencyȱcloseȱtoȱ
100%ȱatȱtheȱearlyȱstagesȱwithȱaȱlowȱcurrentȱapplied.ȱ
ȱ
Theȱclarificationȱofȱtheȱreactionȱpathwayȱturnsȱoutȱtoȱbeȱaȱbitȱmoreȱcomplicatedȱforȱ
chloropheneȱthanȱforȱtheȱotherȱpharmaceuticalsȱstudiedȱinȱthisȱthesis.ȱAnyȱaromaticȱ
intermediateȱhasȱbeenȱidentifiedȱbyȱchromatographicȱtechniques,ȱnotȱevenȱunderȱtheȱ
mildestȱ experimentalȱ conditions.ȱ Theȱ remainingȱ chloropheneȱ isȱ theȱ onlyȱ benzenicȱ
compoundȱdetectedȱinȱtheȱchromatograms.ȱThisȱcanȱbeȱaccountedȱforȱby:ȱ(i)ȱtheȱlackȱ
ofȱ stabiltyȱ ofȱ theseȱ aromaticsȱ inȱ itsȱ oxidizingȱ surroundings,ȱ whichȱ preventsȱ theirȱ
accumulationȱinȱtheȱsolution,ȱand/orȱ(ii)ȱtheȱformationȱofȱsolubleȱpolyaromaticsȱthatȱ
areȱ difficultȱ toȱ characterizeȱ dueȱ toȱ theȱ lackȱ ofȱ standardsȱ andȱ theȱ complexityȱ ofȱ theȱ
mixture.ȱZhangȱetȱal.ȱ[385]ȱhaveȱreportedȱthatȱchloropheneȱtendsȱtoȱformȱthisȱkindȱofȱ
polymersȱthroughȱaȱradicalȱmechanism.ȱFortunately,ȱitȱhasȱbeenȱpossibleȱtoȱidentifyȱ
andȱ quantifyȱ someȱ shortȬchainȱ carboxylicȱ acidsȱ suchȱ asȱ glycolicȱ (HOH2CȬHOOC),ȱ
fumaricȱ (trans,ȱ H2CȬCH=CHȬCH2),ȱ maleicȱ (cis,ȱ H2CȬCH=CHȬCH2),ȱ malonicȱȱȱȱȱȱ
(HOOCȬCH2ȬCOOH),ȱ glyoxylicȱ (CHOȬCOOH),ȱ formicȱ (HCOOH)ȱ andȱ oxalicȱȱ
(HOOCȬCOOH)ȱacids.ȱTheȱformerȱfourȱcomeȱfromȱtheȱoxidationȱofȱtheȱarylȱmoietyȱofȱ
aromaticȱ products,ȱ whereasȱ glyoxylicȱ acidȱ isȱ formedȱ fromȱ theȱ oxidationȱ ofȱ glycolicȱ
acid.ȱAllȱofȱthemȱleadȱtoȱformicȱandȱoxalicȱacidsȱthatȱcanȱbeȱtransformedȱintoȱCO2ȱbyȱ
360
PART B –Results and Discussion9. Chlorophene
theȱmostȱoxidizingȱEFȱsystems.ȱElectrolysesȱusingȱtheȱO2Ȭdiffusionȱcathodeȱevidenceȱ
anȱ accumulationȱ ofȱ maleic,ȱ fumaric,ȱ malonicȱ andȱ formicȱ acidsȱ atȱ concentrationsȱȱȱȱȱȱȱȱȱȱ
<ȱ 3ȱ mgȱ LȬ1,ȱ andȱ uniquelyȱ forȱ 60ȱ minȱ atȱ 60ȱ mA.ȱ Inȱ contrast,ȱ oxalicȱ acidȱ isȱ largelyȱ
accumulatedȱupȱtoȱtheȱendȱofȱtheȱelectrolysisȱwithȱPtȱandȱBDDȱanodes.ȱSinceȱFe3+ȱisȱ
largelyȱ accumulatedȱ inȱ bothȱ systems,ȱ thisȱ acidȱ isȱ expectedȱ toȱ beȱ inȱ theȱ formȱ ofȱȱȱȱȱȱȱ
Fe3+Ȭoxalatoȱcomplexes,ȱwhichȱcanȱnotȱbeȱoxidizedȱbyȱȱ •OHȱinȱtheȱbulkȱsolution,ȱthusȱ
confirmingȱ theȱ mineralizationȱ behaviorȱ explainedȱ above.ȱ Theȱ slowȱ decayȱ observedȱ
usingȱPtȱcanȱbeȱascribedȱtoȱtheȱroleȱofȱ •OHadsȱatȱtheȱanodeȱsurface.ȱSimilarly,ȱBDDȱisȱ
notȱ ableȱ toȱ destroyȱ someȱ ofȱ theȱ Fe3+ȱ complexesȱ atȱ 60ȱ mA,ȱ butȱ atȱ 300ȱ mA,ȱ theseȱ
complexesȱandȱsolutionȱTOCȱareȱcompletelyȱremovedȱatȱ660ȱminȱthroughȱtheȱactionȱ
ofȱ BDD(•OH).ȱ Higherȱ amountsȱ ofȱ carboxylicsȱ canȱ beȱ foundȱ atȱ 60ȱ mAȱ usingȱ theȱ
carbonȬfeltȱ cathode.ȱ Forȱ example,ȱ formicȱ acidȱ attainsȱ 17ȱ mgȱ LȬ1ȱ atȱ 120ȱ minȱ inȱ theȱ
Pt/carbonȱfeltȱcell.ȱThisȱtrendȱconfirmsȱtheȱoxidizingȱpowerȱofȱtheȱsystemsȱwithȱthisȱ
cathode,ȱ whichȱ produceȱ suchȱ aȱ highȱ concentrationȱ ofȱ •OHȱ thatȱ theȱ aromaticsȱ areȱ
quicklyȱ convertedȱ intoȱ significantȱ amountsȱ ofȱ carboxylics.ȱ Inȱ addition,ȱ theȱ
productionȱ ofȱ thisȱ oxidizingȱ speciesȱ isȱ soȱ highȱ thatȱ oxalicȱ acidȱ canȱ beȱ totallyȱ
destroyedȱevenȱinȱtheȱsystemȱwithȱPt.ȱAȱworthȱremarkingȱaspectȱofȱtheȱsystemsȱwithȱ
theȱcarbonȬfeltȱcathodeȱisȱthatȱaȱhighȱconcentrationȱofȱironȱionsȱareȱinȱtheȱformȱofȱFe2+,ȱ
thusȱ generatingȱ Fe2+Ȭoxalatoȱ complexesȱ whichȱ areȱ liableȱ toȱ beȱ destroyedȱ byȱ •OH.ȱ
Whenȱ BDDȱ isȱ coupledȱ toȱ theȱ carbonȬfeltȱ cathode,ȱ Fe2+Ȭcarboxylicȱ complexesȱ canȱ beȱ
simultaneouslyȱoxidizedȱbyȱBDD(•OH),ȱleadingȱtoȱaȱslightlyȱfasterȱTOCȱremoval.ȱ
ȱ
Figureȱ9.Ȭ2ȱgivenȱbelowȱshowsȱtheȱproposedȱdegradationȱpathwayȱforȱoxalicȱacid.ȱInȱ
theȱcellsȱwithȱtheȱO2Ȭdiffusionȱcathode,ȱthisȱacidȱmainlyȱyieldsȱFe3+Ȭoxalatoȱcomplexes,ȱ
whichȱ canȱ notȱ beȱ destroyedȱ byȱ •OHȱ inȱ theȱ solution,ȱ soȱ uniquelyȱ BDD(•OH)ȱ atȱ highȱ
currentȱisȱableȱtoȱdestroyȱthem.ȱInȱcontrast,ȱinȱtheȱcellsȱwithȱaȱcarbonȬfeltȱcathodeȱthisȱ
acidȱisȱfreeȱorȱformingȱFe2+Ȭoxalatoȱcomplexes.ȱBothȱofȱthemȱcanȱbeȱdestroyedȱbyȱ•OHȱ
evenȱwhenȱPtȱisȱused.ȱCouplingȱwithȱBDDȱleadsȱtoȱsimultaneousȱdestructionȱofȱfreeȱ
oxalicȱacidȱandȱitsȱironȱcomplexesȱbyȱtheȱcombinedȱactionȱofȱ•OHȱandȱBDD(•OH).ȱ
361
PART B –Results and Discussion9. Chlorophene
ȱ
COOH
COOH
Fe2+
Fe2+Ȭoxalatoȱ
ȱȱcomplexes
.OH
.
BDD(ȱȱOH)
ȬȱFe3+
Fe3+
.OH
Fe3+Ȭoxalato
BDD(ȱȱOH) ȱcomplexes
.
.
BDD(ȱȱOH)
ȬȱFe
CO2
3+
ȱ
Figure 9.-2 Proposed reaction pathways for oxalic acid in the
EF systems. OH is produced in the bulk solution from Fenton’s
•
reaction and BDD( OH) is adsorbed on the anode surface.
•
ȱ
ȱ
ȱ
ȱ
362
PART B –Results and Discussion10. Summary and General conclusions
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
10.
SUMMARYȱANDȱGENERALȱCONCLUSIONSȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Paracetamol,ȱ clofibricȱ acidȱ andȱ chloropheneȱ areȱ paradigmsȱ ofȱ NSAIDs,ȱ bloodȱ lipidȱ
regulatorsȱ andȱ antimicrobials,ȱ respectively,ȱ whichȱ areȱ threeȱ ofȱ theȱ topȱ salesȱ PPCPsȱ
therapeuticalȱgroupsȱallȱthroughoutȱtheȱworld.ȱ
ȱ
Duringȱ theȱ lastȱ decades,ȱ theȱ impactȱ ofȱ chemicalȱ pollutionȱ hasȱ focusedȱ almostȱ
exclusivelyȱonȱtheȱconventionalȱ‘priority’ȱpollutants,ȱmainlyȱpesticidesȱandȱindustrialȱ
intermediatesȱ exhibitingȱ persistenceȱ inȱ theȱ environment.ȱ Anotherȱ groupȱ thatȱ hasȱ
receivedȱ comparativelyȱ littleȱ attentionȱ includesȱ bothȱ humanȱ andȱ veterinaryȱ
pharmaceuticalȱ compoundsȱ andȱ personalȱ careȱ productsȱ (PPCPs).ȱ Nowadays,ȱ theseȱ
compoundsȱareȱalsoȱconsideredȱasȱpersistentȱpollutantsȱbecauseȱtheyȱareȱcontinuallyȱ
introducedȱinȱtheȱenvironmentȱatȱngȬPgȱLȬ1ȱlevelȱthroughȱseveralȱroutesȱdueȱtoȱtheirȱ
highȱ worldwideȱ consumption.ȱ Inȱ contrastȱ toȱ agrochemicals,ȱ mostȱ ofȱ theseȱ productsȱ
areȱ disposedȱ orȱ dischargedȱ intoȱ theȱ environmentȱ viaȱ domestic/industrialȱ sewageȱ
systems,ȱ beingȱ theȱ mainȱ soucesȱ theȱ metabolismȱ andȱ theȱ treatmentȱ inȱ theȱ STPs.ȱ
363
PART B –Results and Discussion10. Summary and General conclusions
Aquaticȱ pollutionȱ isȱ particularlyȱ troublesomeȱ consideringȱ thatȱ survivalȱ ofȱ livingȱ
organisms,ȱ includingȱ humanȱ beings,ȱ isȱ basedȱ onȱ theȱ waterȬcycle.ȱ PPCPsȱ canȱ poseȱ aȱ
hugeȱrisk,ȱunderȱassessmentȱatȱpresent,ȱbecauseȱlongȱexposureȱtoȱtraceȱlevelsȱleadsȱtoȱ
unpredictedȱ andȱ unknownȱ subtleȱ effectsȱ thatȱ canȱ accumulateȱ soȱ slowlyȱ thatȱ theȱ
changesȱcanȱbecomeȱirreversibleȱandȱbeȱevenȱattributedȱtoȱnaturalȱevolution.ȱ
ȱ
Theȱenormousȱdiversityȱofȱchemicalȱcompositionȱofȱpollutantsȱinȱwatersȱexcludesȱtheȱ
possibilityȱofȱusingȱanȱuniversalȱtreatmentȱmethodȱandȱsuggestsȱtheȱrequirementȱofȱ
specialȱtreatmentȱtechnologiesȱforȱwaterȱdecontamination.ȱManyȱtreatmentȱprocessesȱ
includingȱ severalȱ AOPsȱ reportedȱ inȱ literatureȱ haveȱ beenȱ shownȱ toȱ beȱ inefficientȱ
towardsȱ totalȱ mineralizationȱ ofȱ theȱ pharmaceuticalsȱ pointedȱ outȱ above.ȱ Therefore,ȱ
moreȱeffectiveȱprocessesȱmustȱbeȱdevelopedȱasȱaȱplausibleȱalternative.ȱInȱthisȱsense,ȱ
electrochemicalȱ processesȱ suchȱ asȱ EAOPsȱ andȱ AOȱ usingȱ effectiveȱ anodesȱ appearȱ toȱ
beȱ anȱ appealingȱ environmentallyȱ friendlyȱ choice,ȱ sinceȱ theȱ mainȱ oxidantȱ speciesȱ isȱ
thoughtȱ toȱ beȱ hydroxylȱ radical.ȱ EFȱ andȱ PEFȱ processesȱ usingȱ anȱ O2Ȭdiffusionȱ orȱ aȱ
carbonȬfeltȱcathodeȱareȱableȱtoȱelectrogenerateȱhydroxylȱradicalsȱinȱtheȱbulkȱsolutionȱ
throughȱ Fenton’sȱ reaction,ȱ whereasȱ inȱ AOȱ usingȱ aȱ Ptȱ orȱ aȱ BDDȱ anodeȱ theȱ sameȱ
oxidizingȱagentȱisȱchemisorbedȱorȱphysisorbed,ȱrespectively,ȱatȱtheȱelectrodeȱsurface.ȱ
ȱ
Severalȱ experimentalȱ systemsȱ haveȱ beenȱ studiedȱ byȱ combiningȱ differentȱ cathodesȱ
andȱ anodesȱ andȱ byȱ usingȱ severalȱ catalysts.ȱ Forȱ eachȱ pharmaceutical,ȱ optimumȱ
conditionsȱ forȱ theȱ mineralizationȱ processȱ atȱ laboratoryȱ scaleȱ haveȱ beenȱ establishedȱ
fromȱ theȱ analysisȱ ofȱ theȱ TOCȱ abatementȱ andȱ theȱ correspondingȱ MCEȱ values.ȱ
Subsequently,ȱ theȱ degradationȱ kineticsȱ forȱ theȱ reactionȱ betweenȱ eachȱ drugȱ andȱ
hydroxylȱradicalsȱhasȱbeenȱreported.ȱLastly,ȱtheȱaromatics,ȱcarboxylicsȱandȱinorganicȱ
ionsȱ haveȱ beenȱ identifiedȱ andȱ quantifiedȱ inȱ orderȱ toȱ revealȱ theirȱ trendsȱ alongȱ theȱ
mineralizationȱ processȱ and,ȱ consequently,ȱ theȱ possibleȱ pathwaysȱ forȱ theȱ
electrochemicalȱ degradationȱ ofȱ paracetamol,ȱ clofibricȱ acidȱ andȱ chlorophene.ȱ Inȱ
addition,ȱsomeȱparticularitiesȱofȱtheȱEFȱprocessȱhaveȱbeenȱclarified.ȱ
364
PART B –Results and Discussion10. Summary and General conclusions
Toȱsumȱup,ȱtheȱmainȱconclusionsȱofȱthisȱthesisȱare:ȱ
ȱ
1.ȱȱ Theȱ oxidationȱ abilityȱ ofȱ theȱ systemsȱ underȱ studyȱ dependsȱ onȱ theȱ kindȱ ofȱ
oxidizingȱagentsȱformedȱinȱeachȱone:ȱinȱAOȱwithȱPtȱonlyȱlowȱamountsȱofȱ •OHadsȱ
areȱ involved,ȱ whereasȱ inȱ AOȱ withȱ BDDȱ aȱ highȱ effectiveȱ concentrationȱ ofȱ •OHadsȱ
(alsoȱ notedȱ asȱ BDD(•OH))ȱ isȱ reachedȱ andȱ weakerȱ oxidizingȱ speciesȱ areȱ alsoȱ
identifiedȱ (O3,ȱ H2O2ȱ andȱ S2O82Ȭȱ ions).ȱ Inȱ EFȱ andȱ PEFȱ usingȱ aȱ Ptȱ anodeȱ theȱ mainȱ
oxidizingȱagentȱisȱ•OHȱgeneratedȱinȱtheȱmediumȱfromȱFenton’sȱreaction,ȱalthoughȱ
hypervalentȱironȱspeciesȱasȱwellȱasȱlessȱpowerfulȱoxidizingȱspeciesȱsuchȱasȱHO2•ȱ
andȱH2O2ȱareȱalsoȱpresentȱinȱtheȱbulkȱsolution.ȱCouplingȱbetweenȱaȱBDDȱanodeȱ
andȱH2O2ȱelectrogenerationȱinȱtheȱpresenceȱofȱFe2+ȱionsȱandȱUVAȱlightȱleadsȱtoȱaȱ
multiȬoxidizingȬspeciesȱ mixtureȱ responsibleȱ forȱ theȱ bestȱ performanceȱ ofȱ suchȱ aȱ
processȱtowardsȱtotalȱmineralization:ȱ •OHȱinȱtheȱbulkȱsolutionȱandȱ •OHadsȱatȱtheȱ
BDDȱ surfaceȱ areȱ theȱ mainȱ agents,ȱ butȱ parallelȱ oxidationȱ ofȱ pollutantsȱ withȱ
weakerȱ oxidizingȱ speciesȱ formedȱ inȱ theȱ bulkȱ solutionȱ suchȱ asȱ HO2•,ȱ H2O2,ȱ SO4•-,ȱ
ferrateȱionsȱandȱotherȱhypervalentȱironȱspecies,ȱasȱwellȱasȱatȱtheȱBDDȱsurface,ȱasȱ
forȱexampleȱO3,ȱH2O2ȱandȱS2O82Ȭȱions,ȱisȱalsoȱpossible.ȱInȱaddition,ȱwheneverȱBDDȱ
anodeȱ isȱ usedȱ andȱ chlorinatedȱ compoundsȱ areȱ treated,ȱ theȱ oxidizingȱ substanceȱ
Cl2ȱ isȱ formedȱ inȱ theȱ medium.ȱ Suchȱ anȱ ‘oxidizingȱ cocktail’ȱ shapesȱ aȱ waterȱ
treatmentȱ processȱ withȱ theȱ bestȱ performanceȱ amongȱ allȱ theȱ electrochemicalȱ
proceduresȱ studied.ȱ Theȱ effectȱ ofȱ allȱ thoseȱ oxidizingȱ speciesȱ differentȱ fromȱ •OHȱ
producedȱfromȱFenton’sȱreactionȱisȱlessȱsignificantȱwhenȱtheȱcarbonȬfeltȱcathodeȱ
isȱ used,ȱ becauseȱ thisȱ reactionȱ hasȱ aȱ prevailingȱ roleȱ dueȱ toȱ theȱ highȱ Fe2+ȱ
accumulationȱinȱtheȱmedium.ȱ
ȱ
2.ȱȱ Aȱ synergisticȱ combinationȱ ofȱ Fe2+,ȱ Cu2+ȱ andȱ UVAȱ lightȱ isȱ theȱ keyȱ toȱ theȱ
degradationȱ behaviorȱ ofȱ complexesȱ ofȱ oxalicȱ andȱ oxamicȱ acidsȱ duringȱ theȱ totalȱ
mineralizationȱofȱparacetamol:ȱCu2+ȬoxalatoȱandȱCu2+Ȭoxamatoȱcomplexesȱcanȱbeȱ
efficientlyȱ oxidizedȱ byȱ •OH,ȱ whereasȱ Fe3+ȱ complexesȱ canȱ beȱ destroyedȱ uniquelyȱ
365
PART B –Results and Discussion10. Summary and General conclusions
byȱ photodecompositionȱ withȱ UVAȱ light.ȱ Theȱ optimalȱ conditionsȱ toȱ mineralizeȱ
100ȬmLȱsolutionsȱcontainingȱupȱtoȱ400ȱmgȱLȬ1ȱparacetamolȱbyȱEFȱandȱPEFȱareȱ300ȱ
mA,ȱ35ȱºCȱandȱpHȱ=ȱ3.0.ȱ
ȱ
3.ȱȱ Theȱ formationȱ ofȱ Fe3+Ȭoxalatoȱ complexesȱ isȱ theȱ limitingȱ stepȱ regardingȱ overallȱ
mineralizationȱ wheneverȱ anȱ O2Ȭdiffusionȱ cathodeȱ andȱ Fe2+ȱ ionsȱ areȱ usedȱ toȱ
degradeȱ clofibricȱ acid,ȱ becauseȱ theyȱ canȱ notȱ beȱ oxidizedȱ withȱ •OHȱ inȱ theȱ bulkȱ
solution.ȱ BDD(•OH)ȱ isȱ ableȱ toȱ slowlyȱ destroyȱ theseȱ complexes,ȱ whichȱ areȱ evenȱ
moreȱquicklyȱoxidizedȱunderȱUVAȱirradiationȱinȱtheȱPEFȱprocessȱmainlyȱdueȱto:ȱ
(i)ȱ theȱ photodecompositionȱ ofȱ Fe3+ȱ complexesȱ withȱ carboxylicȱ acids,ȱ andȱ (ii)ȱ theȱ
regenerationȱ ofȱ Fe2+ȱ fromȱ photoreductionȱ ofȱ Fe(OH)2+.ȱ Theȱ actionȱ ofȱ UVAȱ lightȱ
justifiesȱtheȱgreatestȱdegradationȱrateȱandȱhighestȱefficiencyȱofȱPEFȱusingȱBDD.ȱ
ȱ
4.ȱȱ Aȱ poorȱ mineralizationȱ isȱ achievedȱ byȱ AOȱ withȱ aȱ Ptȱ anode,ȱ whereasȱ theȱ
alternativeȱuseȱofȱaȱBDDȱanodeȱleadsȱtoȱtotalȱmineralizationȱofȱparacetamolȱandȱ
clofibricȱacidȱupȱtoȱcloseȱtoȱsaturationȱinȱallȱmediaȱdueȱtoȱtheȱefficientȱproductionȱ
ofȱ •OHads.ȱ Theȱ mineralizationȱ rateȱ isȱ pHȬindependent,ȱ increasingȱ whenȱ bothȱ
temperatureȱ andȱ appliedȱ currentȱ rise,ȱ butȱ decreasingȱ whenȱ drugȱ concentrationȱ
rises.ȱ Couplingȱ betweenȱ BDDȱ andȱ O2Ȭdiffusionȱ cathodeȱ enhancesȱ theȱ
mineralizationȱrateȱandȱincreasesȱtheȱefficiency.ȱ
ȱ
5.ȱȱ Paracetamolȱ andȱ clofibricȱ acidȱ areȱ destroyedȱ afterȱ 6Ȭ7ȱ min,ȱ exhibitingȱ similarȱ
pseudoȬfirstȬorderȱorȱcomplexȱkineticsȱbyȱEFȱandȱPEFȱdueȱtoȱtheȱgreatȱamountȱofȱ
•
OHȱfromȱFenton’sȱreaction,ȱwhereasȱtheyȱremainȱinȱtheȱsolutionȱforȱ150Ȭ240ȱminȱ
byȱ AO.ȱ Parentȱ compoundsȱ andȱ theirȱ intermediatesȱ areȱ oxidizedȱ atȱ similarȱ
destructionȱrateȱbyȱAOȱwithȱBDDȱinȱacidicȱandȱalkalineȱmedia,ȱthusȱjustifyingȱtheȱ
lowȱaccumulationȱofȱtheȱproductsȱandȱtheȱpHȬindependenceȱforȱtheirȱTOCȱdecay.ȱ
Theȱ differentȱ adsorptionȱ ofȱ eachȱ pollutantȱ atȱ theȱ electrodeȱ surfaceȱ justifiesȱ theȱ
decayȱkineticsȱinȱAOȱusingȱPtȱandȱBDDȱanodes.ȱ
366
PART B –Results and Discussion10. Summary and General conclusions
6.ȱȱ Theȱ comparativeȱ performanceȱ ofȱ O2Ȭdiffusionȱ andȱ carbonȬfeltȱ cathodeȱ inȱ EFȱ
showsȱ thatȱ anȱ increasingȱ Fe3+ȱ initialȱ contentȱ causesȱ aȱ slowerȱ destructionȱ ofȱ
chloropheneȱ whenȱ theȱ carbonȬfeltȱ cathodeȱ isȱ used,ȱ becauseȱ nonȬoxidizingȱ
reactionsȱ areȱ graduallyȱ enhanced,ȱ butȱ highȱ Fe3+ȱ amountsȱ areȱ requiredȱ withȱ theȱ
O2Ȭdiffusionȱ cathodeȱ toȱ increaseȱ theȱ amountȱ ofȱ Fe2+ȱ regeneratedȱ atȱ theȱ cathode.ȱ
OverallȱmineralizationȱofȱchloropheneȱsolutionsȱatȱpHȱ3.0ȱcanȱalwaysȱbeȱattainedȱ
usingȱ aȱ carbonȬfeltȱ cathode,ȱ whereasȱ aȱ BDDȱ anodeȱ mustȱ beȱ usedȱ whenȱ theȱ O2Ȭ
diffusionȱ cathodeȱ isȱ tested.ȱ Thisȱ canȱ beȱ relatedȱ toȱ theȱ formationȱ ofȱ Fe3+Ȭoxalatoȱ
complexesȱ thatȱ areȱ hardlyȱ oxidizedȱ withȱ •OHȱ inȱ theȱ Pt/O2Ȭdiffusionȱ system,ȱ
whereasȱtheyȱcanȱbeȱslowlyȱbutȱcompletelyȱdestroyedȱwithȱBDD(•OH).ȱInȱcontrast,ȱ
inȱtheȱsystemsȱwithȱaȱcarbonȬfeltȱcathodeȱFe2+Ȭoxalatoȱcomplexesȱareȱformedȱandȱ
directlyȱoxidizedȱinȱtheȱmediumȱwithȱ •OH,ȱandȱitsȱcouplingȱwithȱBDDȱleadsȱtoȱaȱ
slightȱ increaseȱ inȱ theȱ oxidationȱ abilityȱ atȱ theȱ endȱ ofȱ theȱ treatment.ȱ Onȱ theȱ otherȱ
hand,ȱ theȱ efficiencyȱ sequenceȱ duringȱ theȱ earlyȱ stagesȱ inȱ theȱ fourȱ cellsȱ usedȱ
increasesȱinȱtheȱorder:ȱ
Pt/O2ȱdiffusionȱ<ȱBDD/O2ȱdiffusionȱ<ȱBDD/carbonȱfeltȱ<ȱPt/carbonȱfelt.ȱ
ȱ
7.ȱȱ MCEȱ calculatedȱ onȱ theȱ basisȱ ofȱ theȱ primaryȱ inorganicȱ ionsȱ releasedȱ fromȱ theȱ
initialȱpollutantȱ(ClȱforȱclofibricȱacidȱandȱcloropheneȱandȱNH4+ȱforȱparacetamol)ȱ
alwaysȱrisesȱwithȱincreasingȱtemperatureȱandȱinitialȱpollutantȱconcentration,ȱandȱ
withȱdecreasingȱworkingȱcurrentȱdensity.ȱTheȱMCEȱvaluesȱforȱAOȱwithȱBDDȱareȱ
comparableȱtoȱthoseȱofȱEFȱandȱPEFȱatȱhighȱinitialȱpollutantȱconcentrationȱbecauseȱ
theȱprocessȱisȱmassȬtransferȱcontrolled.ȱTheȱhighestȱMCEȱvaluesȱareȱobtainedȱinȱ
theȱ cellsȱ withȱ aȱ theȱ carbonȬfeltȱ cathodeȱ dueȱ toȱ theȱ efficientȱ productionȱ ofȱ •OHȱ
fromȱ Fenton’sȱ reaction.ȱ Inȱ allȱ casesȱ theȱ efficiencyȱ decreasesȱ atȱ longȱ electrolysisȱ
timeȱ dueȱ toȱ bothȱ parallelȱ nonȬoxidizingȱ reactionsȱ andȱ theȱ generationȱ ofȱ hardlyȱ
oxidizableȱproducts.ȱ
ȱ
ȱ
367
PART B –Results and Discussion10. Summary and General conclusions
8.ȱȱ Generalȱ reactionȱ pathwaysȱ forȱ theȱ mineralizationȱ ofȱ paracetamolȱ andȱ clofibricȱ
acidȱ byȱ electroȬoxidationȱ methods,ȱ includingȱ theȱ aromatic,ȱ carboxylicȱ andȱ ionicȱ
intermediatesȱ detected,ȱ haveȱ beenȱ proposed.ȱ Forȱ chlorophene,ȱ carboxylicȱ
intermediatesȱformedȱasȱaȱresultȱofȱtheȱcleavageȱofȱtheȱbenzenicȱringsȱhaveȱbeenȱ
identified,ȱandȱtheirȱtotalȱdestructionȱhasȱbeenȱdemonstrated.ȱ
ȱ
9.ȱȱ Theȱ electrochemicalȱ technologiesȱ discussedȱ inȱ thisȱ thesisȱ areȱ potentȱ enoughȱ toȱ
decontaminateȱ wastewatersȱ containingȱ paracetamol,ȱ clofibricȱ acidȱ andȱ
chloropheneȱinȱaȱwideȱrangeȱofȱexperimentalȱconditions.ȱAllȱofȱthemȱareȱsuitableȱ
toȱ destroyȱ theȱ initialȱ pollutant,ȱ andȱ mostȱ ofȱ themȱ areȱ evenȱ ableȱ toȱ completelyȱ
mineralizeȱ theȱ solutionsȱ treated.ȱ Therefore,ȱ directȱ andȱ indirectȱ electroȬoxidationȱ
processesȱcanȱbeȱanȱeffective,ȱsimpleȱandȱversatileȱalternativeȱcomparedȱtoȱotherȱ
lessȱoxidizingȱmethodsȱreportedȱinȱliteratureȱtoȱremoveȱtheseȱpharmaceuticals.ȱEFȱ
andȱPEFȱprocessesȱareȱcomplex,ȱsinceȱpHȱmustȱbeȱadjustedȱatȱca.ȱ3.0ȱandȱO2ȱmustȱ
beȱ suppliedȱ continuously.ȱ However,ȱ wastewatersȱ usuallyȱ containȱ theȱ requiredȱ
amountsȱofȱFe2+ȱandȱCu2+ȱionsȱandȱthenȱEFȱandȱPEFȱwithȱacceptableȱefficiencyȱareȱ
suitableȱ methodsȱ toȱ degradeȱ bothȱ theȱ parentȱ compoundsȱ andȱ theirȱ aromaticȱ
intermediatesȱinȱaȱfewȱminutesȱjustȱreleasingȱcarboxylicȱacidsȱandȱinorganicȱions.ȱ
Couplingȱ withȱ biologicalȱ treatmentsȱ couldȱ beȱ easilyȱ andȱ quicklyȱ carriedȱ outȱ ifȱ
totalȱmineralizationȱwasȱnotȱrequiredȱbyȱmeansȱofȱEFȱandȱPEF.ȱOnȱtheȱotherȱhand,ȱ
AOȱ withȱaȱBDDȱanodeȱcanȱbeȱ appliedȱinȱaȱlargerȱvarietyȱofȱ conditionsȱ thanȱtheȱ
aboveȱ methodsȱ andȱ notwithstandingȱ theȱ comparativelyȱ lowerȱ MCEȱ valuesȱ atȱ
earlyȱ stagesȱ forȱ lowȬloadedȱ wastes,ȱ itȱ givesȱ anȱ insignificantȱ accumulationȱ ofȱ
reactionȱ intermediatesȱ thatȱ couldȱ beȱ evenȱ moreȱ dangerousȱ thanȱ theȱ
pharmaceuticalȱtreated.ȱCombinationȱwithȱanȱO2Ȭdiffusionȱcathodeȱenhancesȱbothȱ
theȱmineralizationȱrateȱandȱefficiency.ȱUnfortunately,ȱtheȱcostȱofȱBDDȱelectrodesȱ
isȱatȱpresentȱaȱmajorȱdrawbackȱofȱthisȱtechnology.ȱ
ȱ
ȱ
368
PART B –Results and Discussion11. Resum i Conclusions Generals
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
ȱ
11.
RESUMȱIȱCONCLUSIONSȱGENERALSȱ
ȱ
ȱ
ȱ
ȱ
ȱ
Elȱ paracetamol,ȱ l’àcidȱ clofíbricȱ iȱ elȱ clorofèȱ sónȱ exemplesȱ representatiusȱ deȱ tresȱ delsȱ
grupsȱ terapèuticsȱ mésȱ comercialitzatsȱ deȱ PPCPsȱ enȱ totȱ elȱ món:ȱ fàrmacsȱ
antiinflamatorisȱ noȱ esteroídics,ȱ fàrmacsȱ reguladorsȱ deȱ lípidsȱ enȱ sangȱ iȱ fàrmacsȱ
antimicrobials,ȱrespectivament.ȱ
ȱ
Enȱ elsȱ últimsȱ temps,ȱ l’impacteȱ deȱ laȱ polȉlucióȱ químicaȱ enȱ elȱ mediȱ s’haȱ centratȱ deȱ
maneraȱ quasiȱ exclusivaȱ enȱ elsȱ anomenatsȱ contaminantsȱ convencionalsȱ ‘prioritaris’,ȱ
principalmentȱpesticidesȱiȱintermedisȱindustrialsȱmoltȱpersistentsȱunȱcopȱalliberatsȱenȱ
elsȱ diversosȱ ecosistemes.ȱ Unȱ grupȱ deȱ substànciesȱ tambéȱ presentsȱ enȱ lesȱ aigües,ȱ iȱ alȱ
qualȱ s’haȱdedicatȱ pocaȱatencióȱfinsȱfaȱ pocȱtemps,ȱinclouȱelsȱ compostosȱfarmacèuticsȱ
d’úsȱ humàȱ iȱ veterinariȱ iȱ totȱ unȱ conjuntȱ deȱ productesȱ deȱ curaȱ personalȱ (PPCPs).ȱ
Actualmentȱaquestsȱcompostosȱtambéȱesȱcataloguenȱcomȱaȱcontaminantsȱpersistents,ȱ
donatȱ queȱ sónȱ introduïtsȱ enȱ elȱ mediȱ deȱ maneraȱ continuadaȱ aȱ nivellȱ deȱ ngȬPgȱ LȬ1ȱ aȱ
travésȱ deȱ vàriesȱ rutesȱ mercèsȱ alȱ seuȱ consumȱ àmpliamentȱ extèsȱ aȱ nivellȱ mundial.ȱȱȱȱȱȱ
369
PART B –Results and Discussion11. Resum i Conclusions Generals
Enȱ moltesȱ ocasions,ȱ aquestesȱ substànciesȱ entrenȱ enȱ elȱ mediȱ provinentsȱ d’aigüesȱ
residualsȱdomèstiquesȱiȱindustrials,ȱtrobantȬseȱl’origenȱprimariȱenȱelȱmetabolismeȱperȱ
partȱ delsȱ éssersȱ viusȱ iȱ enȱ elsȱ tractamentsȱ enȱ lesȱ STPs.ȱ Laȱ contaminacióȱ delȱ mediȱ
aquàticȱésȱespecialmentȱpreocupantȱsiȱhomȱtéȱenȱcompteȱlaȱimportànciaȱdelȱcicleȱdeȱ
l’aiguaȱenȱlaȱconservacióȱdelȱplanetaȱiȱdelsȱéssersȱqueȱhiȱhabiten.ȱEnȱdarrerȱterme,ȱelsȱ
PPCPsȱ podenȱimplicarȱ unȱ riscȱ enorme,ȱ queȱavuiȱ diaȱ esȱtrobaȱenȱ faseȱ d’avaluació,ȱjaȱ
queȱ unȱ tempsȱ d’exposicióȱ prolongatȱ aȱ tracesȱ deȱ compostosȱ exògensȱ d’aquestȱ tipusȱ
podriaȱ conduirȱ aȱ efectesȱ moltȱ subtilsȱ iȱ difícilsȱ deȱ predirȱ sobreȱ elsȱ éssersȱ vius,ȱ totȱ
acumulantȬseȱlentamentȱiȱprovocantȱalteracionsȱbiològiquesȱdeȱdiversaȱmagnitud.ȱȱ
ȱ
Avuiȱdiaȱl’estratègiaȱaplicadaȱenȱl’àmbitȱdelsȱtractamentsȱd’aigüesȱresidualsȱambȱunȱ
ampliȱ ventallȱ deȱ contaminantsȱ presentsȱ enȱ ellesȱ esȱ fonamentaȱ enȱ laȱ combinacióȱ deȱ
procedimentsȱsuccessius,ȱincloentȬhiȱtecnologiesȱespecialsȱqueȱsiguinȱefectivesȱcontraȱ
compostosȱmoltȱparticulars.ȱDiversosȱautorsȱhanȱdutȱaȱtermeȱestudisȱd’eliminacióȱdeȱ
fàrmacsȱmitjançantȱvarisȱdelsȱAOPsȱcomentatsȱàmpliamentȱenȱelȱcapítolȱ5ȱd’aquestaȱ
tesi,ȱ peròȱ habitualmentȱ aquestsȱ mètodesȱ noȱ sónȱ capaçosȱ deȱ mineralitzarȱ
completamentȱ lesȱ dissolucionsȱ tractades.ȱ Perȱ tant,ȱ calȱ desenvoluparȱ processosȱ mésȱ
potentsȱ iȱ efectius.ȱ Ambȱ aquestaȱ intenció,ȱ diversosȱ procedimentsȱ electroquímicsȱ queȱ
inclouenȱlaȱAOȱiȱelsȱEAOPsȱesȱpresentenȱcomȱaȱunaȱatractivaȱalternativaȱcompatibleȱ
mediambientalment,ȱ jaȱ queȱ laȱ principalȱ espècieȱ oxidantȱ queȱ intervéȱ ésȱ elȱ radicalȱ
hidroxil.ȱS’hanȱrealitzatȱvarisȱexperimentsȱtotȱcombinantȱdiversosȱcàtodesȱiȱànodes,ȱiȱ
utilitzantȱdiferentsȱcatalitzadors.ȱAȱtravésȱdeȱlaȱreaccióȱdeȱFenton,ȱelsȱprocessosȱEFȱiȱ
PEFȱ emprantȱ càtodesȱ deȱ difusióȱ d’oxigenȱ oȱ deȱ feltreȱ deȱ carbóȱ permetenȱ produirȱ
radicalsȱhidroxilȱenȱelȱsiȱdeȱlaȱdissolucióȱtractada.ȱEnȱlaȱAOȱambȱànodesȱdeȱPtȱoȱBDDȱ
l’agentȱoxidantȱésȱelȱmateix,ȱsiȱbéȱesȱtrobaȱquimisorbitȱoȱfisisorbit,ȱrespectivament,ȱenȱ
laȱsuperfícieȱdeȱl’elèctrode.ȱ
ȱ
Perȱalȱparacetamolȱs’haȱfetȱunȱestudiȱdelsȱprocessosȱEFȱiȱPEFȱambȱunȱànodeȱdeȱPtȱiȱunȱ
càtodeȱdeȱdifusióȱd’oxigen,ȱiȱs’haȱconstatatȱelȱpaperȱrellevantȱqueȱtenenȱelsȱdiferentsȱ
370
PART B –Results and Discussion11. Resum i Conclusions Generals
complexosȱ formatsȱ entreȱ elsȱ catalitzadorsȱ metàlȉlicsȱ empratsȱ iȱ elsȱ àcidsȱ carboxílicsȱ
generatsȱ durantȱ laȱ degradació.ȱ Tambéȱ s’haȱ aplicatȱ elȱ procèsȱ deȱ AOȱ ambȱ dosȱ tipusȱ
d’ànodes,ȱPtȱiȱBDD,ȱiȱemprantȱunȱcàtodeȱdeȱgrafit.ȱ
ȱ
Enȱelȱcasȱdeȱl’àcidȱclofíbricȱs’haȱseguitȱunȱesquemaȱanàleg,ȱperòȱaȱmésȱs’haȱintroduïtȱ
laȱ combinacióȱ deȱ l’ànodeȱ deȱ BDDȱ ambȱ elȱ càtodeȱ deȱ difusióȱ d’oxigen,ȱ fetȱ queȱ haȱ
conduïtȱ aȱ unaȱ milloraȱ significativaȱ delsȱ resultats.ȱ Enȱ l’estudiȱ d’aquestȱ compostȱ s’haȱ
posatȱdeȱmanifestȱlaȱimportànciaȱd’espèciesȱoxidantsȱdiferentsȱdelȱradicalȱhidroxilȱdeȱ
caraȱaȱlaȱdestruccióȱdelsȱcontaminantsȱorgànics.ȱ
ȱ
Iȱ quantȱ alȱ clorofè,ȱ s’haȱ dutȱ aȱ termeȱ unaȱ anàlisiȱ profundaȱ delȱ procèsȱ EF,ȱ
desenvolupadaȱprincipalmentȱenȱelȱLaboratoriȱd’ElectroquímicaȱdelsȱMaterialsȱiȱdelȱ
MediȱAmbientȱiȱenȱelȱlaboratoriȱdelȱprofessorȱOturanȱdurantȱl’estadaȱaȱlaȱUniversitatȱ
deȱMarneȱlaȱValléeȱ(París).ȱFruitȱdeȱlaȱcolȉlaboracióȱentreȱambduesȱpartsȱs’haȱpogutȱ
explicarȱ l’efecteȱ delȱ sistemaȱ catalíticȱ Fe3+/Fe2+ȱ sobreȱ l’efectivitatȱ deȱ lesȱ celȉlesȱ ambȱ
ànodesȱdeȱPtȱiȱBDDȱiȱcàtodesȱdeȱdifusióȱd’oxigenȱiȱfeltreȱdeȱcarbó.ȱ
ȱ
Perȱ aȱ cadascunȱ delsȱ tresȱ fàrmacsȱ estudiatsȱ s’hanȱ definitȱ elsȱ processosȱ òptimsȱ deȱ
mineralitzacióȱaȱescalaȱdeȱlaboratoriȱaȱpartirȱdeȱl’evolucióȱambȱelȱtempsȱd’electròlisiȱ
delȱTOCȱdeȱlaȱdissolucióȱiȱdelsȱvalorsȱdeȱMCEȱcorresponents.ȱEnȱaquestȱsentit,ȱs’haȱ
estudiatȱ laȱ influènciaȱ deȱ lesȱ variablesȱ experimentalsȱ (intensitatȱ deȱ corrent,ȱ pH,ȱ T,ȱ
concentracióȱ deȱ catalitzadors).ȱ Mitjançantȱ l’HPLCȱ enȱ faseȱ inversaȱ s’hanȱ analitzatȱ iȱ
comparatȱ enȱ profunditatȱ lesȱ cinètiquesȱ deȱ degradacióȱ perȱ aȱ laȱ reaccióȱ entreȱ cadaȱ
fàrmacȱ iȱ elsȱ radicalsȱ hidroxil.ȱ Iȱ perȱ últim,ȱ ambȱ l’HPLCȱ enȱ faseȱ inversaȱ iȱ d’exlusióȱ
iònica,ȱlaȱcromatografiaȱiònicaȱiȱlaȱGCȬMSȱs’hanȱidentificatȱiȱquantificatȱelsȱintermedisȱ
aromàticsȱiȱcarboxílicsȱiȱelsȱionsȱinorgànicsȱalliberats,ȱiȱd’aquestaȱmaneraȱs’hanȱpogutȱ
discutirȱ lesȱ evolucionsȱ observadesȱ i,ȱ finalment,ȱ s’hanȱ proposatȱ elsȱ possiblesȱ caminsȱ
deȱreaccióȱperȱaȱlaȱmineralitzacióȱelectroquímicaȱdelȱparacetamol,ȱl’àcidȱclofíbricȱiȱelȱ
clorofè.ȱAȱmés,ȱs’hanȱaclaritȱdeȱformaȱdetalladaȱalgunesȱparticularitatsȱdelȱprocèsȱEF.ȱ
371
PART B –Results and Discussion11. Resum i Conclusions Generals
Lesȱprincipalsȱconclusionsȱdeȱlaȱtesiȱesȱpresentenȱaȱcontinuació:ȱ
ȱ
1.ȱȱ Laȱ capacitatȱ oxidativaȱ deȱ cadascunȱ delsȱ sistemesȱ estudiatsȱ depènȱ delsȱ diversosȱ
agentsȱoxidantsȱqueȱesȱformenȱenȱcadaȱcas:ȱenȱAOȱambȱPtȱesȱgeneraȱunaȱquantitatȱ
moltȱ baixaȱ deȱ •OHads,ȱ mentreȱ queȱ enȱ AOȱ ambȱ BDDȱ s’assoleixȱ unaȱ concentracióȱ
efectivaȱ deȱ •OHadsȱ (altramentȱ identificatsȱ comȱ BDD(•OH))ȱ elevadaȱ iȱ esȱ detectaȱ
tambéȱlaȱpresènciaȱd’espèciesȱoxidantsȱmésȱfeblesȱ(O3,ȱH2O2ȱiȱionsȱS2O82).ȱEnȱEFȱiȱ
PEFȱ ambȱ Pt,ȱ elȱ principalȱ agentȱ oxidantȱ ésȱ elȱ radicalȱ •OHȱ generatȱ enȱ elȱ mediȱ aȱ
partirȱdeȱlaȱreaccióȱdeȱFenton,ȱtotȱiȱqueȱtambéȱcalȱtenirȱenȱcompteȱlaȱpresènciaȱenȱ
elȱ siȱ deȱ laȱ dissolucióȱ d’espèciesȱ hipervalentsȱ deȱ ferroȱ iȱ d’agentsȱ menysȱ potents,ȱ
comȱperȱexempleȱelȱH2O2ȱiȱelȱradicalȱHO2•.ȱL’acoblamentȱentreȱl’ànodeȱdeȱBDDȱiȱ
l’electrogeneracióȱdeȱH2O2ȱambȱpresènciaȱd’ionsȱFe2+ȱiȱllumȱUVAȱcondueixȱaȱunaȱ
mesclaȱ deȱ multiȬoxidantsȱ queȱ ésȱ laȱ responsableȱ deȱ laȱ majorȱ capacitatȱ deȱ
mineralitzacióȱ d’aquestȱ procediment:ȱ elsȱ principalsȱ agentsȱ oxidantsȱ sónȱ elsȱ
radicalsȱ•OHȱenȱelȱsiȱdeȱlaȱdissolucióȱiȱelsȱ•OHadsȱenȱlaȱsuperfícieȱdelȱBDD,ȱsiȱbéȱnoȱ
calȱ oblidarȱ laȱ possibleȱ oxidacióȱ paralȉlelaȱ delsȱ contaminantsȱ mitjançantȱ espèciesȱ
menysȱpoderosesȱformadesȱenȱelȱmedi,ȱcomȱperȱexempleȱHO2•,ȱH2O2,ȱSO4•-,ȱionsȱ
ferratȱ iȱ altresȱ espèciesȱ hipervalentsȱ deȱ ferro,ȱ aixíȱ comȱ enȱ laȱ superfícieȱ delȱ BDD,ȱ
comȱelsȱesmentatsȱO3,ȱH2O2ȱiȱionsȱS2O82Ȭ.ȱAȱmés,ȱsempreȱqueȱs’utilitzaȱl’ànodeȱdeȱ
BDDȱ perȱ aȱ tractarȱ compostosȱ clorats,ȱ esȱ generaȱ enȱ elȱ mediȱ unaȱ altraȱ espècieȱ
oxidantȱ comȱ ésȱ elȱ Cl2.ȱ Enȱ definitiva,ȱ mercèsȱ aȱ aquestȱ ‘còctelȱ oxidant’ȱ
s’aconsegueixȱunȱprocèsȱdeȱtractamentȱd’aigüesȱambȱelȱqualȱs’obtenenȱelsȱmillorsȱ
resultatsȱd’entreȱtotsȱelsȱprocedimentsȱelectroquímicsȱestudiats.ȱL’efecteȱdeȱtotesȱ
aquestesȱespèciesȱoxidantsȱdiferentsȱdelȱradicalȱ•OHȱproduïtȱaȱpartirȱdeȱlaȱreaccióȱ
deȱFentonȱésȱmenysȱsignificatiuȱquanȱs’empraȱelȱcàtodeȱdeȱfeltreȱdeȱcarbó,ȱjaȱque,ȱ
aȱ causaȱ deȱ l’acumulacióȱ d’unaȱ granȱ quantitatȱ d’ionsȱ Fe2+ȱ enȱ elȱ medi,ȱ aquestaȱ
reaccióȱjugaȱunȱpaperȱpreponderant.ȱ
ȱ
ȱ
372
PART B –Results and Discussion11. Resum i Conclusions Generals
2.ȱȱ Laȱ combinacióȱ sinèrgicaȱ deȱ Fe2+,ȱ Cu2+ȱ iȱ llumȱ UVAȱ ésȱ laȱ clauȱ delȱ comportamentȱ
degradatiuȱ delsȱ complexosȱ delsȱ àcidsȱ oxàlicȱ iȱ oxàmicȱ generatsȱ durantȱ laȱ
mineralitzacióȱcompletaȱdelȱparacetamol:ȱtantsȱelsȱcomplexosȱCu2+Ȭoxalatȱcomȱelsȱ
Cu2+Ȭoxamatȱ sónȱ oxidatsȱ eficientmentȱ perȱ partȱ delȱ radicalȱ •OH,ȱ mentreȱ queȱ elsȱ
complexosȱ deȱ Fe3+ȱ únicamentȱ podenȱ serȱ destruïtsȱ perȱ fotodescomposicióȱ ambȱ
llumȱ UVA.ȱ Lesȱ condicionsȱ òptimesȱ perȱ aȱ mineralitzarȱ dissolucionsȱ deȱ 100ȱ mLȱ
ambȱ concentracionsȱ deȱ finsȱ 400ȱ mgȱ LȬ1ȱ deȱ paracetamolȱ mitjançantȱ EFȱ iȱ PEFȱ sónȱ
300ȱmA,ȱ35ȱºCȱiȱpHȱ=ȱ3,0.ȱ
ȱ
3.ȱȱ Laȱ formacióȱ delsȱ complexosȱ Fe3+Ȭoxalatȱ ésȱ l’etapaȱ limitantȱ quantȱ aȱ laȱ
mineralitzacióȱ totalȱ deȱ l’àcidȱ clofíbricȱ sempreȱ queȱ s’usenȱ elȱ càtodeȱ deȱ difusióȱ
d’oxigenȱiȱionsȱFe2+,ȱjaȱqueȱnoȱpodenȱserȱoxidatsȱperȱpartȱdelȱradicalȱ •OHȱenȱelȱsiȱ
deȱ laȱ dissolució.ȱ L’agentȱ BDD(•OH)ȱ ésȱ capaçȱ deȱ destruirȱ lentamentȱ aquestsȱ
complexos,ȱqueȱencaraȱsónȱmésȱràpidamentȱeliminatsȱenȱirradiarȱambȱllumȱUVAȱ
enȱ l’anomenatȱ procèsȱ PEF.ȱ Aquestȱ darrerȱ comportamentȱ esȱ potȱ atribuirȱ
principalmentȱ a:ȱ (i)ȱ laȱ fotodescomposicióȱ delsȱ complexosȱ formatsȱ pelsȱ àcidsȱ
carboxílicsȱ ambȱ Fe3+,ȱ iȱ (ii)ȱ laȱ regeneracióȱ deȱ Fe2+ȱ aȱ partirȱ deȱ laȱ fotorreduccióȱ deȱ
Fe(OH)2+.ȱ L’accióȱ deȱ laȱ llumȱ UVAȱ justificaȱ queȱ laȱ velocitatȱ deȱ degradacióȱ iȱ
l’eficiènciaȱmésȱelevadesȱs’obtinguinȱambȱelȱprocèsȱPEFȱambȱBDD.ȱ
ȱ
4.ȱȱ LaȱmineralitzacióȱassolidaȱmitjançantȱAOȱambȱànodeȱdeȱPtȱésȱpobra,ȱmentreȱqueȱ
l’úsȱdeȱl’ànodeȱdeȱBDDȱcondueixȱaȱlaȱmineralitzacióȱtotalȱdeȱdissolucionsȱfinsȱiȱtotȱ
saturadesȱ deȱ paracetamolȱ iȱ àcidȱ clofíbricȱ enȱ unȱ ampliȱ rangȱ deȱ pH,ȱ gràciesȱ aȱ laȱ
produccióȱ eficientȱ deȱ radicalsȱ •OHads.ȱ Laȱ velocitatȱ deȱ mineralitzacióȱ ésȱ
independentȱ delȱ pHȱ inicial,ȱ totȱ augmentantȱ quanȱ s’apliquenȱ intensitatsȱ iȱ
temperaturesȱmésȱelevades,ȱperòȱdisminuintȱaȱmesuraȱqueȱlaȱconcentracióȱinicialȱ
deȱ fàrmacȱ ésȱ major.ȱ L’acoblamentȱ entreȱ elȱ BDDȱ iȱ elȱ càtodeȱ deȱ difusióȱ d’oxigenȱ
provocaȱunȱincrementȱtantȱdeȱlaȱvelocitatȱdeȱmineralitzacióȱcomȱdeȱl’eficiènciaȱdelȱ
procès.ȱ
373
PART B –Results and Discussion11. Resum i Conclusions Generals
5.ȱȱ Elȱparacetamolȱiȱl’àcidȱclofíbricȱsónȱdestruïtsȱenȱ6Ȭ7ȱminȱmitjançantȱEFȱiȱPEF,ȱambȱ
cinètiquesȱdeȱpseudoȬprimerȱordreȱoȱbéȱcomplexesȱqueȱsónȱsimilarsȱaȱcausaȱdeȱlaȱ
granȱquantitatȱdeȱradicalsȱ•OHȱgeneratsȱperȱlaȱreaccióȱdeȱFentonȱenȱambdósȱcasos.ȱ
Enȱcanvi,ȱmitjançantȱAOȱromanenȱenȱdissolucióȱdurantȱ150Ȭ240ȱmin.ȱElȱcompostȱ
inicialȱ iȱ elsȱ seusȱ intermedisȱ deȱ reaccióȱ sónȱ oxidatsȱ aȱ velocitatsȱ deȱ destruccióȱ
semblantsȱ enȱ AOȱ ambȱ BDD,ȱ enȱ mediȱ àcidȱ iȱ alcalí,ȱ totȱ justificantȱ laȱ baixaȱ
acumulacióȱ d’intermedisȱ iȱ laȱ independènciaȱ delȱ descensȱ deȱ TOCȱ respecteȱ elȱ pHȱ
inicial.ȱ Laȱ diferentȱ cinèticaȱ observadaȱ enȱ AOȱ ambȱ Ptȱ iȱ BDDȱ esȱ potȱ atribuirȱ aȱ laȱ
diferènciaȱd’adsorcióȱdeȱcadaȱfàrmacȱenȱlaȱsuperfícieȱdeȱcadascunȱdelsȱànodes.ȱ
ȱ
6.ȱȱ Laȱ comparacióȱ entreȱ l’actuacióȱ delsȱ càtodesȱ deȱ difusióȱ d’oxigenȱ iȱ deȱ feltreȱ deȱ
carbóȱenȱelȱprocèsȱEFȱrevelȉlaȱqueȱunȱaugmentȱdelȱcontingutȱinicialȱdeȱFe3+ȱcausaȱ
unȱ alentimentȱ deȱ laȱ destruccióȱ delȱ clorofèȱ quanȱ s’empraȱ elȱ càtodeȱ deȱ feltreȱ deȱ
carbóȱ perquèȱ lesȱ reaccionsȱ noȱ oxidantsȱ prenenȱ progressivamentȱ mésȱ notorietat.ȱ
Enȱ canvi,ȱ esȱ requereixenȱ quantitatsȱ elevadesȱ deȱ Fe3+ȱ quanȱ s’empraȱ elȱ càtodeȱ deȱ
difusióȱd’oxigen,ȱperȱtalȱd’incrementarȱlaȱconcentracióȱFe2+ȱregeneratȱalȱcàtode.ȱEnȱ
utilitzarȱelȱcàtodeȱdeȱfeltreȱdeȱcarbó,ȱsempreȱs’assoleixȱlaȱmineralitzacióȱcompletaȱ
deȱlesȱdissolucionsȱdeȱclorofèȱaȱpHȱ3,0,ȱmentreȱqueȱenȱusarȱelȱcàtodeȱdeȱdifusióȱ
d’oxigenȱcalȱrecórrerȱaȱlaȱcombinacióȱambȱl’ànodeȱdeȱBDD.ȱAquestaȱdiferènciaȱesȱ
potȱrelacionarȱambȱlaȱformacióȱdeȱcomplexosȱFe3+Ȭoxalat,ȱqueȱsónȱdifícilsȱd’oxidarȱ
ambȱelȱradicalȱ •OHȱenȱelȱsistemaȱPt/difusió,ȱiȱqueȱpodenȱserȱdestruïtsȱdeȱmaneraȱ
completaȱ peròȱ lentaȱ ambȱ elȱ BDD(•OH).ȱ D’altraȱ banda,ȱ enȱ elsȱ sistemesȱ ambȱ elȱ
càtodeȱ deȱ feltreȱ deȱ carbóȱ esȱ formenȱ complexosȱ Fe2+Ȭoxalatȱ queȱ sónȱ directamentȱ
oxidatsȱ pelȱ radicalȱ •OHȱ enȱ elȱ siȱ deȱ laȱ dissolució,ȱ iȱ l’acoblamentȱ ambȱ l’ànodeȱ deȱ
BDDȱdónaȱcomȱ aȱresultatȱunȱlleugerȱincrementȱ enȱlaȱcapacitatȱ oxidativaȱalȱ finalȱ
delȱ tractament.ȱ Perȱ altraȱ part,ȱ l’eficiènciaȱ delȱ procèsȱ enȱ lesȱ primeresȱ etapesȱ delȱ
tractamentȱenȱlesȱquatreȱcelȉlesȱutilitzadesȱaugmentaȱenȱl’ordre:ȱȱ
Pt/difusióȱ<ȱBDD/difusióȱ<ȱBDD/feltreȱ<ȱPt/feltre.ȱ
ȱ
374
PART B –Results and Discussion11. Resum i Conclusions Generals
7.ȱȱ Laȱ MCEȱ calculadaȱ enȱ baseȱ alsȱ ionsȱ inorgànicsȱ primarisȱ provinentsȱ delȱ
contaminantȱinicialȱ(Clȱperȱaȱl’àcidȱclofíbricȱiȱelȱclorofèȱiȱNH4+ȱperȱalȱparacetamol)ȱ
sempreȱ augmentaȱ enȱ incrementarȱ laȱ temperaturaȱ iȱ laȱ concentracióȱ inicialȱ deȱ
fàrmac,ȱiȱenȱdisminuirȱlaȱdensitatȱdeȱcorrentȱdeȱtreball.ȱAȱconcentracionsȱinicialsȱ
deȱfàrmacȱelevades,ȱelsȱvalorsȱdeȱMCEȱperȱaȱAOȱambȱBDDȱsónȱcomparablesȱalsȱ
obtingutsȱenȱEFȱiȱPEFȱperquèȱelȱprocèsȱestàȱcontrolatȱperȱtransferènciaȱdeȱmatèria.ȱ
Elsȱ valorsȱ mésȱ altsȱ deȱ MCEȱ s’obtenenȱ enȱ lesȱ celȉlesȱ ambȱ elȱ càtodeȱ deȱ feltreȱ deȱ
carbóȱ mercèsȱ aȱ l’eficientȱ produccióȱ deȱ radicalsȱ •OHȱ aȱ partirȱ deȱ laȱ reaccióȱ deȱ
Fenton.ȱEnȱtotsȱelsȱcasos,ȱl’eficiènciaȱdisminueixȱaȱtempsȱd’electròlisiȱllargsȱdegutȱ
aȱ lesȱ reaccionsȱ paralȉlelesȱ noȱ oxidantsȱ iȱ aȱ laȱ generacióȱ d’intermedisȱ difícilsȱ
d’oxidar.ȱ
ȱ
8.ȱȱ S’hanȱproposatȱcaminsȱdeȱreaccióȱgeneralsȱperȱaȱlaȱmineralitzacióȱdelȱparacetamolȱ
iȱ l’àcidȱ clofíbricȱ mitjançantȱ mètodesȱ d’electroȬoxidació,ȱ totȱ incloentȬhiȱ elsȱ
intermedisȱ aromàtics,ȱ carboxílicsȱ iȱ iònicsȱ detectats.ȱ Enȱ elȱ casȱ delȱ clorofèȱ s’hanȱ
identificatȱ elsȱ intermedisȱ carboxílicsȱ generatsȱ aȱ partirȱ deȱ laȱ rupturaȱ delsȱ anellsȱ
benzènics,ȱiȱs’haȱdemostratȱlaȱsevaȱdestruccióȱcompleta.ȱ
ȱ
9.ȱȱ Lesȱtecnologiesȱelectroquímiquesȱdiscutidesȱenȱaquestaȱtesiȱsónȱprouȱpotentsȱcomȱ
perȱ descontaminarȱ aigüesȱ residualsȱ queȱ continguinȱ paracetamol,ȱ àcidȱ clofíbricȱ iȱ
clorofèȱ enȱ unȱ rangȱ ampliȱ deȱ condicionsȱ experimentals.ȱ Totsȱ elsȱ processosȱ
estudiatsȱ sónȱ adientsȱ perȱ aȱ destruirȱ elȱ fàrmacȱ inicial,ȱ iȱ moltsȱ d’ellsȱ aȱ mésȱ sónȱ
capaçosȱ deȱ mineralitzarȱ totalmentȱ lesȱ dissolucionsȱ tractades.ȱ Perȱ tant,ȱ aquestsȱ
procedimentsȱ d’electroȬoxidacióȱ directaȱ oȱ indirectaȱ podenȱ suposarȱ unaȱ
alternativaȱefectiva,ȱsimpleȱiȱversàtilȱenȱcomparacióȱambȱaltresȱmètodesȱqueȱtenenȱ
unaȱ capacitatȱ oxidativaȱ menorȱ enȱ tractarȱ aquestsȱ tipusȱ deȱ compostos.ȱ Elsȱ
processosȱEFȱiȱPEFȱsónȱcomplexos,ȱjaȱqueȱcalȱajustarȱelȱpHȱinicialȱaȱunȱvalorȱdeȱ3,0ȱ
aproximadament,ȱ iȱ aȱ mésȱ calȱ sumministrarȱ O2ȱ continuamentȱ alȱ càtodeȱ oȱ aȱ laȱ
dissolució.ȱ Deȱ totaȱ manera,ȱ lesȱ aigüesȱ residualsȱ contenenȱ habitualmentȱ unaȱ
375
PART B –Results and Discussion11. Resum i Conclusions Generals
quantitatȱ suficientȱ d’ionsȱ Fe2+ȱ andȱ Cu2+,ȱ iȱ perȱ tantȱ aquestsȱ mètodesȱ acostumenȱ aȱ
serȱ adientsȱ iȱ presentenȱ unaȱ eficiènciaȱ acceptableȱ pelȱ queȱ faȱ aȱ laȱ degradacióȱ delsȱ
contaminantsȱ inicialsȱ iȱ delsȱ seusȱ intermedisȱ aromàticsȱ deȱ reaccióȱ enȱ unsȱ pocsȱ
minuts,ȱ totȱ alliberantȱ àcidsȱ carboxílicsȱ iȱ ionsȱ inorgànics.ȱ Enȱ casȱ queȱ noȱ esȱ
requereixiȱlaȱmineralitzacióȱtotalȱmitjançantȱEFȱiȱPEF,ȱaquestsȱprocessosȱesȱpodenȱ
plantejarȱcomȱaȱmètodesȱdeȱpretractamentȱperȱaȱunȱposteriorȱtrasvasamentȱfàcilȱiȱ
ràpidȱ capȱ aȱ reactorsȱ biològics.ȱ D’unaȱ altraȱ banda,ȱ elȱ procèsȱ AOȱ ambȱ ànodeȱ deȱ
BDDȱesȱpotȱaplicarȱenȱunaȱvarietatȱdeȱcondicionsȱmésȱàmpliaȱqueȱelsȱmètodesȱEFȱ
iȱ PEFȱ i,ȱ malgratȱ presentarȱ unsȱ valorsȱ deȱ MCEȱ comparativamentȱ menorsȱ durantȱ
lesȱetapesȱinicialsȱquanȱesȱtractenȱefluentsȱpocȱ carregats,ȱdónaȱllocȱaȱunaȱmenorȱ
acumulacióȱd’intermedisȱdeȱreaccióȱqueȱpodrienȱserȱinclúsȱmésȱperillososȱqueȱelȱ
fàrmacȱ estudiat.ȱ Laȱ combinacióȱ ambȱ unȱ càtodeȱ deȱ difusióȱ d’oxigenȱ faȱ queȱ
s’incrementinȱ laȱ velocitatȱ deȱ mineralitzacióȱ iȱ l’eficiènciaȱ delȱ procès.ȱ
Malauradament,ȱ elȱ costȱ delsȱ elèctrodesȱ deȱ BDDȱ ésȱ encaraȱ avuiȱ diaȱ unȱ
inconvenientȱimportantȱd’aquestaȱtecnologia.ȱ
ȱ
ȱ
376
Fly UP