电絮凝RemovalofHeavyMetalIonsbyElectrocoagulat

974Bull.KoreanChem.Soc.2008,Vol.29,No.5YounSuJungandMyounghoPyoRemovalofHeavyMetalIonsbyElectrocoagulationforContinuousUseofFe2+/Fe3+-MediatedElectrochemicalOxidationSolutionsYounSuJungandMyounghoPyo*DepartmentofChemistry,SunchonNationalUniversity,Sunchon,Chonnam540-742,Korea.*E-mail:[e-mail]15,2008Removalofheavymetalions(Cd2+andZn2+)byelectrocoagulation(ECG)wasinvestigatedinanacidiccondition,whichisnecessaryforre-usingordischargingthemediatedelectrochemicaloxidation(MEO)media.Effectsofvariousparameterssuchaselectrolytes,currentdensities,andelectrodematerialswereexaminedforametal-contaminatedMEOsystemusingFe2+/Fe3+pairsasamediator.ItwasfoundthatECGwithAlelectrodesisgreatlyaffectedbythepresenceofFe2+.[Cd2+]and[Zn2+]remainconstantuntil[Fe2+]reachesacertainconcentrationlevel(ca.10mM).ThispreferentialremovalofFe2+duringECGwithAlelectrodesisnotalleviatedbycontrollingcurrentdensities,potentialprograms,andsolutionmixing.ECGwithFeelectrodes,ontheotherhand,resultedinrelativelyfastremovalofCd2+andZn2+undercoexistenceofFe2+,indicativeofthedifferentrolebetweenFen+generatedfromanelectrodeandFe2+initiallypresentinasolution.WhenECGwasperformedwithFeelectrodesuntil[Fen+]becamethesameastheconcentrationofinitiallypresentFe2+,[Cd2+]and[Zn2+]werereducedtoone-tenthoftheinitialconcentrations,suggestingthepossibilityofacontinuoususeofthemediumforasubsequentMEOprocess.KeyWords:Electrocoagulation,MEO,Metalions,Feelectrode,AlelectrodeIntroductionSincetheelectrochemicaltreatmentofwastewaterisenvironmentallyfriendlyandappropriateforapplyingtoaneffluentsystem,ithasbeenwidelystudiedduringalastfewdecadesinspiteoflargecapitalinvestmentandenergyconsumption.1Electrochemicalmethodsincludeelectro-flotation,2electrodeposition,3direct/indirectelectrochemicaloxidation,4-6andECG..7,8Amongthese,ECGisprobablythemostversatilemethodinthatvariouspollutingmaterials,includingawiderangeofionicspecies9-15aswellasorganicmaterialssuchastextiledyes,16,17phenoliccompounds,18andpolymerlatex19canbesuccessfullytreated.ThisispossiblebecauseECGutilizesvariousformsofmonomericandpolymericmetalhydroxidecomplexeswhichcaninteractwithpollutingmaterialsviaseveralroutes(electro-staticforce,surfaceadsorption,physicalentrapment,andcoprecipitation).7,8Ingeneral,anECGcellisequippedwithFeorAlelectrodes.Uponapplyingapotentialbetweentwoelectrodes,metaldissolutionattheanodeandcathodicreactionofelectroactivespeciesinsolutiontakeplacetoproducecoagulants.ECG,therefore,usuallyrequiresfre-quentchangeofsacrificialanodesandcleaningofcathodesformaintenance.Inthisregard,whenlimitedtothedecompositionoforganicwastes,4-6indirectelectrochemicaloxidation(MEO)hasadvantagesoverECGsincemediatorscancompletelydecomposeorganichydrocarbonstoCO2andH2O.5,6,20-25Reactiveandhigh-valentmetalionswhichareelectro-chemicallygeneratedfromstableandlowvalentstateareutilizedasoxidants.VariousredoxcouplessuchasAg+/Ag2+,20Co2+/Co3+,21Fe2+/Fe3+,22andCe3+/Ce4+23-25havebeenexaminedtodegradeorganicpollutants.Foreconomicalandenvironmentalreason,theoverallMEOsystemisoperatedasaclosedloop.Themediator-containinganolytesarecirculatedthroughanelectrochemi-calcell,inwhichhigh-valentmetalionsareelectrochemi-callyproduced,andmixedwithorganicwastes.Thecon-centrationofmediatorsismaintainedviaappropriatede-hydrationandgasevolutionprocesses,inprinciplemakingrepeateduseofthesolutionpossible.25Repeatedtreatmentsofwastewater,however,essentiallyresultintheaccumu-lationofheavymetalionswhichcanbeproblematictocontinuousoperation.Inthispaper,wedescribetheremovalofCd2+andZn2+fromFe2+/Fe3+-MEOmediabyECG.Cd2+andZn2+,whicharecommonlyfoundinvariouswastewatersources,weretestedasheavymetalionsaccumulatedinaMEOsolution.AFe2+/Fe3+-MEOsystemwaschosensinceweconsideredthatitshalide-toleranceandfacilechangeofredoxstateswouldmakeitbeneficialtospecificMEOsystems.Ithasbeenaddressedthat,despitetherelativelylowstandardreductionpotential(E0),Fe3+isanefficientoxidantforthecompletedestructionoforganiccompoundssuchasethyleneglycol.22MEOprocess10Fe2+→10Fe3++10e−[generationofmediatorinanolyte]10H++10e−→5H2[reductionincatholyte]10Fe3++(CH2OH)2inwastewater+2H2O→10Fe2++2CO2+10H++(concentratedMn+)[destructionoforganics]RemovalofHeavyMetalIonsbyElectrocoagulationBull.KoreanChem.Soc.2008,Vol.29,No.5975SubsequentECGprocess4Fe(s)→4Fe2++8e−[dissolutionofanode]4Fe2++O2+10H2O→4Fe(OH)3(flocs)+8H+[insolution]flocs+concentratedMn+→Mn+adsorbedflocs[precipitation]ExperimentalHNO3,FeCl2,Fe(NO3)3·9H2O(fromJunsei,Japan),Zn(SO4)·7H2O(fromDaejung,Korea),Cd(NO3)2·4H2O,NaCl,NaNO3,Alplates(99.9+),andFeplates(99.9+)(fromAldrich)wereusedasreceived.WaterwaspurifiedthroughaMilli-QPlussystem(MilliporeCo.)untilitsresistivitywasover18MΩcm.ECGwascarriedoutusing2parallelAlorFeelectrodesseparatedby0.8cm.Theelectrolyticcellwasfilledwithsolutionsof3mLandvariousconstantcurrentswereappliedbetweentheelectrodesthroughanECEpsilonelectro-chemicalanalyzer.Duringelectrolysis,samplesof10μLwereperiodicallytakenfromtheECGsolution.Afterfiltration,sampleswerediluted100timeswith0.1MHNO3andth