电絮凝ElectrochemistryBehindElectrocoagulationUsi

doi:10.1149/1.27903972007,Volume6,Issue9,Pages1-15.ECSTrans.  Parga,EricPetersonandCGarciaHectorA.MorenoCasillas,DavidL.Cocke,JewelA.Gomes,PaulMorkovsky,JoseR. ElectrodesElectrochemistryBehindElectrocoagulationUsingIronserviceEmailalerting clickhereboxatthetoprightcornerofthearticleorReceivefreeemailalertswhennewarticlescitethisarticle-signupinthe:ECSTransactionsTosubscribeto©2007ECS-TheElectrochemicalSocietyógicodeSaltillo,Saltillo,Coahuila25280,MéxicodChemicalEngineering,InstitutoTecnológicodeLalaguna,Torreon,Coahuila27000,MéxicoElectrocoagulation(EC)hasbeenknownformorethanacentury.Applicationsinindustriesasawaterandwastewatertreatmentprocesshavebeenadaptedfortheremovalofmetals,nonmetals,suspendedsolids,organiccompounds,CODandBOD.Ironelectrodeshavebeenpreferredoveraluminumduetotheirdurabilityandcost.However,theelectrochemicalreactionsoccurringwithECusingironelectrodeshavenotbeensystematicallystudied.ForabetterunderstandingofthemechanismandreactionsforECusingironelectrodes,wepresentareviewoftheconceptofgreenrust(GR)anditsrelationshiptothecurrenttheoryofEC.ExperimentalresultsobtainedbymeasuringpHatdifferentzonesneartheironelectrodesduringtheECprocessaredetailed,andareusedtoillustratethemechanismandreactionsthatoccuratboth,theanodeandcathode.MechanismandreactionspresentedexplainphenomenaassociatedwithECandarecongruentwithsolubilityandPourbaixdiagrams.IntroductionElectrocoagulation(EC)isanelectrochemicalwaterandwastewatertreatmenttechnologythatinitssimplestformusesanelectrochemicalcellwhereadcvoltageisappliedtoelectrodes,usuallymadeofironoraluminum,andwaterorwastewateristheelectrolyte.ECinvolvesthegenerationofcoagulantinsitubydissolutionofmetalfromtheanodewithsimultaneousformationofhydroxylionsandhydrogengasatthecathode.Thisprocessproducesthecorrespondingaluminumorironhydroxidesand/orpolyhydroxides.Thegeneratedgasalsohelpstofloattheflocculatedparticlesatthewatersurface.ECSTransactions,6(9)1-15(2007)10.1149/1.2790397,©TheElectrochemicalSociety1(1).Mattesonetal.describedadeviceofthe1940’s,the“electroniccoagulator”whichelectrochemicallydissolvedaluminum(fromtheanode)intosolution,reactingthiswiththehydroxylion(fromthecathode)toformaluminumhydroxide(2).Thehydroxideflocculatesandcoagulatesthesuspendedsolidsandtherebypurifieswater.AsimilarprocesswasusedinBritainin1956inwhichironelectrodeswereusedtotreatriverwater(3,4).TreatmentofwastewaterbyECwaspracticedformostofthe20thCenturywithlimitedsuccessandpopularity.However,inthelateryearsitsusagehasincreasedasevidencedwithalargernumberofcompaniessellingECunits.Thisisbecausethetechnologyhasbeenoptimizedtominimizeelectricalpowerconsumptionandmaximizeeffluentthroughputrates.IthasalsoproventobecompetitiveandeffectiveinthetreatmentofwaterandwastewatertoremoveheavymetalssuchasPb,Cd,Cr,andAs,metalssuchasMn,Cu,Zn,Ni,Al,Fe,Co,Sn,Mg,Se,Mo,Ca,andPt.IthasalsoseenapplicationsinremovinganionssuchasCN-,PO4-3,SO4-2,NO3-,F-andCl-,nonmetalssuchasP,organiccompoundssuchastotalpetroleumhydrocarbons(TPH),toluene,benzeneandxylenes(TBX),methyltert-butylether(MTBE),COD,BOD,suspendedsolids,clayminerals,organicdyes,andoilandgreasesfromavarietyofindustrialeffluents(1,2,5,6,7,8,9).Greenrust¤ttheoryofECRustRustisareddish-browncorrosionproductofironideallyconsistingofthreelayersofironoxideindifferentstatesofoxidation:•HydrousferrousoxideFeO.nH2O(orferroushydroxideFe(OH)2)2Fe+O2+H2O→2Fe(OH)2[1]Fe(OH)2+(n-1)H2O→FeO*nH2O[2]Fe(OH)2whosecolorisnormallygreenturnstogreenish-blackbecauseofincipientoxidationbyair.•HydrousferricoxideorferrichydroxideAttheoutersurfaceofthecorrosionlayer,accesstodissolvedoxygenconvertsferrousoxidetohydrousferricoxideorferrichydroxideaccordingtothefollowingreaction:4Fe(OH)2+O2→2Fe2O3*H2O+2H2O[3]ECSTransactions,6(9)1-15(2007)α-Fe2O3(hematite)ormagneticδ-Fe2O3(maghemite).Theα-formpossessesgreaternegativefreeenergyofformation(10).•MagnetichydrousferrousferriteAmagnetichydrousfer