Electrocoagulation-mechanism-for-COD-removal

SeparationandPurificationTechnology56(2007)204–211ElectrocoagulationmechanismforCODremovalHectorA.Moreno-Casillasa,DavidL.Cockea,∗,JewelA.G.Gomesa,PaulMorkovskyb,J.R.Pargac,EricPetersonaaGillChairofChemistryandChemicalEngineering,LamarUniversity,P.O.Box10022,Beaumont,TX77710,USAbKasparElectroplatingCorporation,Shiner,TX77984,USAcInstitutoTecnol´ogicodeSaltillo,DepartmentofMetallurgyandMaterialsScience,V.Carranza2400,C.P.25280,Saltillo,Coahuila,MexicoReceived25August2006;receivedinrevisedform22January2007;accepted31January2007AbstractThecurrenttheoryofelectrocoagulationhasnotbeenabletoexplainallthephenomenaassociatedwiththeprocess,amongothersthedifferencesinCODremovalefficiencyindifferingwastewaters.Inthispaper,wediscussthefactsassociatedwithCODremovalusingECanddevelopamechanismtoexplainthem.Abriefdescriptionofthetechnologyisdoneandtheconceptsrelatedtochemicaloxygendemand(COD)arediscussedpriortoshowingtheresultsfromapilotplantrelativetoCODremovalefficiency.Twosetofexperiments,andamechanismforCODremovalarepresented.Thismechanismfitsboththedataandobservations.ItfollowsironPourbaixdiagramandsolubilitydiagrams,whileagreeingwiththecharacterizationofECby-products.Finally,thefactorsaffectingCODremovalarementioned.©2007PublishedbyElsevierB.V.Keywords:Electrocoagulation(EC);Chemicaloxygendemand(COD);Pourbaixdiagram;Hydrolysis;Acidity1.Introduction1.1.ElectrocoagulationElectrocoagulation(EC)usedasanelectrochemicalwaterandwastewatertreatmenttechnologyinitssimplestformusesanelectrochemicalcellwhereaDCvoltageisappliedtotheelectrodes,usuallymadeofironoraluminum,andtheelectrolyteiswaterorwastewater.ECinvolvesthegenerationofcoagulantinsitubythedis-solutionofmetalfromtheanodewithsimultaneousformationofhydroxylionsandhydrogengasatthecathode.Thispro-cessproducesthecorrespondingaluminumorironhydroxidesand/orpolyhydroxides.Thegeneratedgasalsohelpstofloattheflocculatedparticlesatthewatersurface.ECwasproposedbeforetheturnofthe20thcentury.AplantinLondonwasbuiltin1889forthetreatmentofsewagebymix-ingitwithseawaterandelectrolyzingit[1].In1906,ECwaspatentedbyA.E.Dietrichandwasusedtotreatbilgewaterfromships.IntheUnitedStates,in1909,J.T.Harries[1]receivedapatentforwastewatertreatmentbyelectrolysiswithsacrifi-∗Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](D.L.Cocke).cialaluminumandironanodes.Mattesonetal.[2]describedadeviceinthe1940s,referredtoasan“electroniccoagulator”whichelectrochemicallydissolvedaluminum(fromtheanode)intosolution,reactingthiswiththehydroxylion(fromthecath-ode)toformaluminumhydroxide.Thehydroxideflocculatesandcoagulatesthesuspendedsolidsandtherebypurifieswater.AsimilarprocesswasusedinBritainin1956inwhichironelectrodeswereusedtotreatriverwater[3,4].TreatmentofwastewaterbyECwaspracticedformostofthepastcenturywithlimitedsuccessandpopularity.However,inthelastfewyears,itsusagehasbeenincreasedasthetechnologyhasbeenimprovedtominimizeelectricalpowerconsumptionandmaximizeeffluentthroughputrates.Ithasproventobecom-petitiveandeffectiveinthetreatmentofwaterandwastewatertoremovemetals,anions,dyes,organicmatter(BOD,COD),suspendedsolids,colloidsandevenarsenic[5–12].1.1.1.CommentsaboutECmechanismsThemechanismsofECareyettobeclearlyunderstood.Therehasbeenlittleconsiderationofthefactorsthatinflu-encetheeffectiveremovalofionicspeciesandcompoundsfromwastewaters.ECisacomplexprocesswithamultitudeofmechanismsoperatingsynergisticallytoremovepollutantsfromwater.Awidevarietyofopinionsexistintheliteratureforkeymechanismsandreactorconfigurations.Asystematic,1383-5866/$–seefrontmatter©2007PublishedbyElsevierB.V.doi:10.1016/j.seppur.2007.01.031H.A.Moreno-Casillasetal./SeparationandPurificationTechnology56(2007)204–211205holisticapproachisrequiredtounderstandECanditscontrollingparameters[5].Moststudieshavefocusedontheefficiencyofaspecificpol-lutantremoval,manipulatingparameters,suchasconductivity,pH,currentdensity,electrodematerials,etc.,withoutexploringthefundamentalmechanismsinvolvedintheECprocess[13],particularlythosethatcouldprovidedesignparameterstoopti-mizetheperformanceofthisrelativelysimpleandinexpensivetechnique[14].GiventhetimescaleoverwhichECtechnologyhasbeenuseditissurprisingthattheavailableliteraturedoesnotrevealanysystematicapproachtoECreactordesignandoperation.ThereasonforthisfailuretodrivetowardsomeagreedbestsolutionseemstobethelackofanyquantitativeunderstandingofthemanyinteractionsthatoccurwithinanECreactor,andinparticulartheabilitytopredicttherelativeimportanceoftheseinteractionsforagivensituation.Consequently,despitemorethanacenturyofapplication,thescienceandengineeringbehindECreactordesignisstilllargelyempiricalandheuristic.ThestudiesmadeforCODremovaloftenprovetheviabilityofthetechnology,butsingularlyfailtocapitalizeonitspotentialbybeingincorporatedwithinabroad-basedunderstandingofECtechnology[15].1.1.2.ECchallengesElectrocoagulationhasbeenusedfortreatmentofwaterandwastewateratlaboratoryandindustriallevels.Unfortunately,theresultsobtainedinlaboratoriesundercontrolledconditionscannotalwaysbeextendedtoi