电絮凝PerformanceEvaluationofElectrocoagulationP

TurkishJ.Eng.Env.Sci.32(2008),59–66.cT¨UB˙ITAKPerformanceEvaluationofElectrocoagulationProcessforRemovalofChromium(VI)fromSyntheticChromiumSolutionsUsingIronandAluminumElectrodesEdrisBAZRAFSHANDepartmentofEnvironmentalHealth,SchoolofPublicHealth,ZahedanUniversityofMedicalSciences,Zahedan-IRANAmirHosseinMAHVI,SiminNASERI,AliRezaMESDAGHINIACenterforEnvironmentalResearch,TehranUniversityofMedicalSciences,Tehran-IRANe-mail:[e-mail]26.12.2006AbstractTheperformanceofelectrocoagulationwithironandaluminumsacrificialanodeforremovalofCr(VI)wasinvestigated.Severalworkingparameters,suchaspollutantconcentration,pH,electricalpotential,COD,turbidity,andcontacttimewerestudiedinanattempttoachieveahigherremovalcapacity.Solutionsofvaryingchromiumconcentrations(5-50-500ppm)wereprepared.Tofollowtheprogressofthetreatment,samplesof25mlweretakenat20minintervalsforupto1handthenfiltered(0.45μ)toeliminatesludgeformedduringelectrolysis.ThepHoftheinitialsolutionwasalsovariedtostudytheireffectsonthechromiumremovalefficiency.ResultsobtainedwithsyntheticwastewaterrevealedthatthemosteffectiveremovalcapacitiesofthestudiedmetalscouldbeachievedwhenthepHwaskeptat3.Inaddition,theincreaseofelectricalpotential,intherangeof20-40V,enhancedthetreatmentratewithoutaffectingthechargeloadingrequiredtoreducemetalionconcentrationsunderadmissiblelegallevels.TheprocesswassuccessfullyappliedtothetreatmentofelectroplatingwastewaterwhereaneffectivereductionofCr(VI)concentrationunderlegallimitswasobtainedjustafter20-60min.Theresultsofthisstudyshowedthattheremovalefficiencyofchromiumwithironelectrodesishighercomparedtoaluminumelectrodes.Also,itcanbeconcludedthattheelectrocoagulationprocesshasthepotentialtobeutilizedforcost-effectiveremovalofheavymetalsfromwaterandwastewater.Keywords:Electrocoagulation,chromiumremoval,aluminumelectrodes,ironelectrodesIntroductionInrecentyears,therehasbeenanincreasingpublicawarenessofthelong-termeffectofwatercontain-ingdissolvedheavymetalions(Sonetal.,2004).Heavymetals,suchaschromium,cadmium,copper,zinc,andnickel,arecontainedinindustrialwastewa-ter,whichpollutestheenvironmentandhasnegativeeffectsonhumanhealth;thepreventionofenviron-mentalpollutionhasbeenstudiedactively(ChoiandKim,2003).Toxicheavymetalsarereleasedintotheenvi-ronmentfromanumberofindustriessuchasmin-ing,plating,dyeing,automobilemanufacturing,andmetalprocessing.Thepresenceofheavymetalsintheenvironmenthasledtoanumberofenvironmen-talproblems.Inordertomeetthewaterqualitystandardsformostcountries,theconcentrationofheavymetalsinwastewatermustbecontrolled(Kimetal.,2005).Separationtechniquesofheavymet-als,suchaschromium,cadmium,copper,zinc,andnickel,fromindustrialwastewaterincludeprecipita-tion,ionexchange,adsorption,electro-dialysis,andfiltration,butthesetechniqueshavelimitationsin59BAZRAFSHAN,MAHVI,NASERI,MESDAGHINIAselectiveseparationandtheyhaveproblemsassoci-atedwithhighinvestmentandoperationcosts(ChoiandKim,2005).Chromium(VI)isreleasedintoaquaticenviron-mentfromelectroplating,metalfinishing,chromatepreparation,tannery,andfertilizerindustries,andfromindustriesthatemployCr6+compoundsascor-rosioninhibitors(Gaoetal.,2005;Adhoumetal.,2004).Itisapotentialcarcinogenanditsdele-teriouseffectsarewelldocumented(Browning,E.,1969).ThepermissiblelimitofCr(VI)forindus-trialwastewatertobedischargedtosurfacewateris0.1mgl−1(Ranganathan,2000).HenceitbecomesimperativetoremoveCr(VI)fromwastewatersbe-foredischargingthemintoaquaticsystemsorontoland.Differentmethods,suchasreductionandpre-cipitation,ionexchange,electrodialysis,reverseos-mosis,solventextraction,electrochemicalprecipita-tion,andactivatedcarbonadsorptionhavebeensug-gestedfortheremovalofhexavelantchromium(Gaoetal.,2005).Mostofthem,however,arenotef-ficientorcosteffective.Electrocoagulationmethodusingaluminumelectrodehasattractedsignificantattentionforchromiumremovalprocessduetoitsoperationalsimplicity.Electrocoagulationisasim-pleandefficientmethodwheretheflocculatingagentisgeneratedbyelectro-oxidationofasacrificialan-ode,generallymadeofironoraluminum.Inthisprocess,thetreatmentisperformedwithoutaddinganychemicalcoagulantorflocculants,thusreducingtheamountofsludgethatmustbedisposed(CenkinandBelevstev,1985).Ontheotherhand,electro-coagulationisbasedontheinsituformationofthecoagulantasthesacrificialanodecorrodesduetoanappliedcurrent,whilethesimultaneousevolutionofhydrogenatthecathodeallowsforpollutantremovalbyflotation.Thistechniquecombines3maininter-dependentprocesses,operatingsynergisticallytore-movepollutants:electrochemistry,coagulation,andhydrodynamics.Anexaminationofthechemicalre-actionsoccurringintheelectrocoagulationprocessshowsthatthemainreactionsoccurringattheelec-trodes(aluminumelectrodes)are:Al↔Al3++3e(anode)(1)3H2O+3e↔3/2H2+3OH−(cathode)(2)Inaddition,Al3+andOH−ionsgeneratedatelec-trodesurfacesreactinthebulkwastewatertoformaluminumhydroxide:Al3++3OH−↔Al(OH)3(3)Iftheanodepotentialissufficientlyhigh,secondaryreactionsmayoccurattheanode,suchasdirectox-idationoforganiccompoundsandofH2OorCl−presentinwastewater:2Cl−↔Cl2+2e(4)2H2O↔O2+4H++4e(5)