电絮凝RemovalofZnIICuIINiIIAgIand

Availableonlineat(2008)934–941RemovalofZn(II),Cu(II),Ni(II),Ag(I)andCr(VI)presentinaqueoussolutionsbyaluminiumelectrocoagulationIlonaHeidmann∗,WolfgangCalmanoInstituteofEnvironmentalTechnologyandEnergyEconomics,HamburgUniversityofTechnology,EissendorferStr.40,21073Hamburg,GermanyReceived31October2006;receivedinrevisedform4June2007;accepted24July2007Availableonline27July2007AbstractTheperformanceofanelectrocoagulationsystemwithaluminiumelectrodesforremovingheavymetalions(Zn2+,Cu2+,Ni2+,Ag+,Cr2O72−)onlaboratoryscalewasstudiedsystematically.Severalparameters–suchasinitialmetalconcentration,numbersofmetalspresent,chargeloadingandcurrentdensity–andtheirinfluenceontheelectrocoagulationprocesswereinvestigated.Initialconcentrationsfrom50to5000mgL−1Zn,Cu,NiandAgdidnotinfluencetheremovalrates,whereashigherinitialconcentrationscausedhigherremovalratesofCr.Increasingthecurrentdensityacceleratedtheelectrocoagulationprocessbutmadeitlessefficient.Zn,CuandNishowedsimilarremovalratesindicatingauniformelectrochemicalbehavior.Thestudygaveindicationsontheremovalmechanismsoftheinvestigatedmetals.Zn,Cu,NiandAgionsarehydrolyzedandco-precipitatedashydroxides.Cr(VI)wasproposedtobereducedfirsttoCr(III)atthecathodebeforeprecipitatingashydroxide.©2007ElsevierB.V.Allrightsreserved.Keywords:Electrocoagulation;Heavymetals;Aluminiumelectrodes;Industrialwastewater1.IntroductionIndustrialwastewaterslikeelectroplatingoracidminewastewaterscontainvariouskindsoftoxicsubstancessuchascyanides,alkalinecleaningagents,degreasingsolvents,oil,fatandmetals[1,2].Mostofthemetalssuchascopper,nickel,chromium,silverandzincareharmfulwhentheyaredischargedwithouttreatment[1–3].Duetotheirhightoxicity,industrialwastewatersarestrictlyregulatedandhavetobetreatedbeforebeingdischarged[1,3].ThemostwidelyusedmethodforthetreatmentofmetalpollutedwastewaterisprecipitationwithNaOHandcoagulationwithFeSO4orAl2(SO4)3withsub-sequenttime-consumingsedimentation[2,3].Othermethodsincludeadsorption,ionexchangeandreverseosmosis[3,4].Althoughprecipitationisshowntobequiteefficientintreat-ingindustrialeffluents,thechemicalcoagulationmayinducesecondarypollutioncausedbyaddedchemicalsubstances[3].Thesedisadvantagesencouragedmanystudiesontheuseofelec-∗Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](I.Heidmann).trocoagulationforthetreatmentofseveralindustrialeffluents[3].Theelectrocoagulationprocessisbasedonthecontinuousin-situproductionofacoagulantinthecontaminatedwater.Ithadbeenshownthatelectrocoagulationisabletoeliminateavari-etyofpollutantsfromwastewaters,asforexamplemetalsandarsenic[1–6],clayminerals[7,8],aswellasoil[9,10],chem-icaloxygendemand[9–12],color[13]andorganicsubstances[13,14].Thistechniquedoesnotrequiresupplementaryadditionofchemicals,reducesthevolumeofproducedsludge[14–17]andfirsteconomicalstudiesindicatealsoafinancialadvantagecomparedtotheconventionalmethods[17].Electrocoagulationhasthepotentialtoextensivelyeliminatethedisadvantagesoftheclassicaltreatmenttechniquestoachieveasustainableandeconomictreatmentofpollutedwastewater[14,15,17,18].Sincetheturnofthe19thcentury,electrocoagulationhasbeenappliedforwastewatertreatment[7]andmanystudiesattendedtooptimizetheprocessforspecificproblems.Typically,empir-icalstudiesweredone[3,9–11,13,14].Thesestudiesshowthesuccessfultreatmentofthewastewaters,however,theyprovidelittleinsightintofundamentalchemicalandphysicalmecha-nisms[15,19].Therefore,themechanismsinvolvedareyetnot0304-3894/$–seefrontmatter©2007ElsevierB.V.Allrightsreserved.doi:10.1016/j.jhazmat.2007.07.068I.Heidmann,W.Calmano/JournalofHazardousMaterials152(2008)934–941935clearlyunderstood[15,20].Butexactlythesephysico-chemicalmechanismshavetobeunderstoodtooptimizeandcontroltheprocess,toallowmodelingofthemethodandtoimprovethedesignofthesystem.Themainobjectivesofthepresentworkweretogaininsightintosomefundamentalmechanismsandpossibleinteractionsinfluencingtheremovalprocessofheavymetalsbyelectro-coagulation.Therefore,westudied(i)theinfluenceofinitialmetalconcentration,chargeloadingandcurrentdensityontheremovalofZn,Cu,Ni,CrandAgionsbyelectrocoagulation.Wedetermined(ii)theremovalratesofthesemetalsandexamined(iii)possibleinteractionsbetweenthemetalsduringthecoag-ulationprocess.Theseresultsgaveindicationsontheremovalmechanismsbyelectrocoagulation.2.ElectrocoagulationprocessTheelectrocoagulation(EC)processinvolvesmanychemi-calandphysicalmechanisms[15].Generally,aluminiumorironisdissolvedbyanodicdissolution.Arangeofcoagulantspeciesandhydroxidesareformedwhichdestabilizeandcoagulatethesuspendedparticlesorprecipitateandadsorbdissolvedcontam-inants[13].ItisgenerallyacceptedthattheECprocessinvolvesthreesuccessivestages[15].(i)FormationofcoagulantsbyelectrolyticoxidationofthesacrificialanodeThemainreactionoccurringatthealuminiumanodeisdissolution:Al(s)→Al3++3e−(1)Additionally,waterelectrolysisoccursatthecathodeandanode:2H2O+2e−→H2(g)+2OH−(cathodicreaction)(2)2H2O→4H++O2(g)+4e−(anodicreaction)(3)(ii)Des