电絮凝Removalofhexavalentchromiumfromelectropla

GlobalNESTJournal,Vol13,No4,pp412-418,2011Copyright©2011GlobalNESTPrintedinGreece.AllrightsreservedREMOVALOFHEXAVALENTCHROMIUMFROMELECTROPLATINGWASTEWATERBYELECTROCOAGULATIONWITHIRONELECTRODESK.DERMENTZIS1,*1DepartmentofScienceA.CHRISTOFORIDIS2LaboratoryofChemicalTechnologyandElectrochemistryE.VALSAMIDOU2TechnologicalEducationInstitute(TEI)ofKavalaA.LAZARIDOU265404AgiosLoucas,Kavala,GreeceN.KOKKINOS22DepartmentofPetroleum&NaturalGasTechnologyLaboratoryofEnvironmentProtectionTechnologicalEducationInstitute(TEI)ofKavala65404AgiosLoucas,Kavala,GreeceReceived:15/11/10*towhomallcorrespondenceshouldbeaddressed:Accepted:02/12/11e-mail:[e-mail]ofelectrocoagulationusingironelectrodesfortheremovalofhexavalentchromiumfromsyntheticaqueoussolutionsandactualindustrialelectroplatingwastewaterwasstudied.Parametersaffectingtheelectrocoagulationprocess,suchasinitialpH,appliedcurrentdensity,initialmetalionconcentration,CODandtimeofelectroprocessingwereinvestigated.TheoptimumpHwasfoundtobeintherange4-8.Initialchromiumconcentrationsof200–800mgL-1didnotinfluenceitsremovalrate.Higherconcentrationswerereducedsignificantlyinrelativelylesstimethanlowerconcentrations.Increasedcurrentdensityacceleratedtheelectrocoagulationprocess,however,oncostofhigherenergyconsumption.Resultsrevealedthatbestremovalwasachievedatacurrentdensity40mAcm-2.Theelectrocoagulationprocesswassuccessfullyappliedtothetreatmentofanelectroplatingwastewatersample.ItsCr(VI)ionconcentrationandCODwereeffectivelyreducedundertheadmissiblelimitsin50minutesofelectroprocessing.KEYWORDS:electrochemicalcoagulation,electroplatingeffluent,hexavalentchromium,ironelectrode.1.INTRODUCTIONHeavymetals,suchasmercury,lead,chromium,nickel,copper,cadmiumandzinc,areofconsiderableconcernbecausetheyarenon-biodegradable,highlytoxicandprobablycarcinogen.Severaltreatmentprocesseshavebeensuggestedfortheremovalofheavymetalsfromaqueouswastestreams:adsorption(Lazaridisetal.,2005),biosorption(Senthikumaretal.,2010),ionexchange(Inglezakisetal.,2003),chemicalprecipitation(Kurniawanetal.,2006)andelectrochemicalmethods:electrowinning(BolgerandSzlag,2004),electrodialysis(Marderetal.,2004),electrodeionization(Yeonetal.,2003;Luetal.,2010),membrane-lesselectrostaticshieldingelectrodialysis/electrodeionizationDermentzisetal.,2010)andelectrocoagulation.Chromiumisreleasedintotheenvironmentfromelectroplating,anodizing,chromating,metalfinishing,tannery,dyingandfertilizerindustries(Liuetal.,2011;Vasudevanetal.,2010).Wastestreamsfromelectroplatingunitsmaycontainupto2500mgL-1Cr(VI),whichaccordingtoenvironmentalregulationsworldwidemustbecontrolledtoanacceptablelevelbeforebeingdischargedtotheenvironment.ThepermissiblelimitsforCr(VI)is0.5mgL-1ineffluentdischargetosewagesystemsand0.1mgL-1indrinkingwater.Themostcommonandeconomicalmethodforhexavalentchromiumremovalinvolvesreductiontoitstrivalentstateandsubsequentprecipitationwithalkali.Asreducingagentsareusedferrousions,sulfurdioxide,sodiumbisulfiteorhydraziniumsalts.Thereducedtrivalentchromiumisthenprecipitatedbyadditionofanalkalie.g.limeorcausticsodaaccordingtoreactions(1),(2)and(3):REMOVALOFHEXAVALENTCHROMIUMFROMELECTROPLATINGWASTEWATER413Cr2O72-+6Fe2++14H+→2Cr3++6Fe3++7H2O(1)Cr3++3OH-→Cr(OH)3(2)Fe3++3OH-→Fe(OH)3(3)Theproducedsludgeamountintheaboveprocessisverylarge,sincefor1kgofchromiumremoved32kgofsludgeisgenerated,whichisdifficulttohandle(Reddithotaetal.,2007).Chemicalcoagulationisaquiteeffectivemethodfortreatingheavymetalbearingwastewatersbutmayinducesecondarypollutionbyaddingcoagulants,suchasaluminumorironsaltsororganicpolyelectrolytestoremovecolloidalmatterasgelatinoushydroxides.Alsothiswastewatertreatmentprocessproducesalargeamountofsludge.Electrochemicalcoagulationorotherwisecalledelectrocoagulationusesnochemicalsascoagulatingagents.Thesearegeneratedduringtheelectrolysisprocessbyelectrodissolutionofasacrificialanodemadeofaluminumoriron.Electrocoagulationhasbeensuccessfullyperformedfortreatmentandremediationoftextilewastewaters(Dermentzisetal.,2011;Kobyaetal.,2003;Rajuetal.,2008),oilwastes(Abdelwahabetal.,2009;Unetal.,2009),diaryeffluents(Tchamangoetal.,2010),dieselandbio-dieselwastewaters(Chavalparit&Ongwandee,2009;El-Naasetal.,2009),laundrywastewaters(Wangetal.,2009),slaughterhouseeffluents(Asselinetal.,2008),arsenicorfluoridecontainingwaters(Hansenetal.,2007;Huetal.,2008)andheavymetalbearingeffluents(Adhoumetal.,2004;Heidmann&Calmano2008;Kongsricharoern&Polprasert,1996;Nourietal.,2010).Theobjectiveofthispaperwastoinvestigatetheefficiencyofelectrocoagulationinremovinghexavalentchromiumfromsyntheticaqueoussolutionsandactualindustrialwastewater.Inaddition,theeffectofpH,currentdensity,initialmetalconcentration,timeofelectroprocessingandCODwasinvestigated.2.METHODS2.1ChemicalsKCl,K2Cr2O7andNaOHwereofanalyticalgrade(Merck).pHwasadjustedby0.1MsolutionsofHClandNaOHasrequired.TheactualwastewaterwasobtainedfromanelectroplatingunitlocatednearThessaloniki,northernGreece.2.2ApparatusAlaboratorymodelDCpowersupplyapparatus(PHYWESystemeGmbH&C