电絮凝Usingelectrocoagulationformetaland

JournalofHazardousMaterials174(2010)670–678ContentslistsavailableatScienceDirectJournalofHazardousMaterialsjournalhomepage:∗AgronomyDepartment,BiotechnicalFaculty,UniversityofLjubljana,Jamnikarjeva101,1000Ljubljana,SloveniaarticleinfoArticlehistory:Received30March2009Receivedinrevisedform18September2009Accepted18September2009Availableonline25September2009Keywords:HeavymetalsSoilremediationSoilwashingEDTAElectrocoagulationabstractElectrocoagulationwithanAlsacrificialanodewastestedfortheseparationofchelantandheavymetalsfromawashingsolutionobtainedafterleachingPb(3200mgkg−1),Zn(1100mgkg−1),andCd(21mgkg−1)contaminatedsoilwithEDTA.Intheelectrochemicalprocess,thesacrificialanodecorrodedtoreleaseAl3+whichservedascoagulantforprecipitationofchelantandmetals.Aconstantcurrentdensityof16–128mAcm−2appliedbetweentheAlanodeandthestainless-steelcathoderemovedupto95%Pb,68%Znand66%Cdfromthesoilwashingsolution.ApproximatelyhalfoftheinitialEDTAremainedinthewashingsolutionaftertreatment,upto16.3%oftheEDTAwasadsorbedonAlcoagulantandprecipitated,therestoftheEDTAwasdegradedbyanodicoxidation.Inaseparatelaboratory-scaleremediationexperiment,weleachedasoilwith40mmolEDTAperkgofsoilandreusedthewashingsolution(afterelectrocoagulation)inaclosedloop.Itremoved53%ofPb,26%ofZnand52%ofCdfromthesoil.Thedischargesolutionwasclearandcolourless,withpH7.52and170mgL−1Pb,50mgL−1Zn,1.5mgL−1Cdand11mMEDTA.©2009ElsevierB.V.Allrightsreserved.1.IntroductionHeavymetalcontaminatedsoilisoneofthecommonestenvi-ronmentalengineeringproblems.Removalofmetalsfromsoilbymeansofsoilwashingusingchelantsisapotentialremediationoption,especiallyforcontaminatedcalcareoussoils,wheretheextractionofmetalswithacidsisimpededbythehighsoilpHbuffercapacity.Chelantsformcoordinatechemicalbondswithmetals(complexes)andfacilitatetheirsolubilizationfromthesoilintothewashingsolution.Ethylenediaminetetraaceticacid(EDTA)hasbeenthechelantmostoftentestedforsoilwashing,sinceitformsstrongcomplexes(witharatio1:1)withmostpollutingheavymetals[1]andisrela-tivelyinexpensivecomparedtootherchelants(inEurope,itcostsabout1.3Dperkgforthetechnical-gradechemicalfromamajorEuropeanmanufacturer).Themainobstacletofullscaleapplicationandcommercializa-tionofsoilwashingtechnologieswithEDTAremains,however,thesubsequenttreatmentofthewashingsolution(richwithmetal–EDTAcomplexes)beforethewastesolutioncanbesafelydischarged.Variousstrategieshavebeenproposedforthis.KimandOng[2]usedtrans-complexation:thePbintheEDTAcomplex∗Correspondingauthorat:CentreforSoilandEnvironmentalScience,Biotech-nicalFaculty,UniversityofLjubljana,Jamnikarjeva101,1000Ljubljana,Slovenia.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](D.Lestan).wasreplacedwithFe3+atlowpHandthePbwassubsequentlyremovedusingNaOHasaprecipitatingagent.TheprocessproveddifficultiftheEDTAwascomplexedwithmorethanonemetalandthelargeconsumptionofchemicalsmeansrelativelyhighoperatingcosts.AgerandMarshall[3]investigatedtheabilityofzero-valentMgandPdtosubstitutemetalsinEDTAcomplexes.Zengetal.[4]proposedtheprecipitationofheavymetalsfromthesoilwashingsolutionasinsolublesulphides.Thesodiumsul-phideconsumedinthisprocess,however,ismoreexpensivethantheEDTA.JuangandWang[5]proposedelectrolyticrecoveryofPbandCufromasolutioncontainingEDTA.Thisprocessusessensi-tiveandexpensivecation-exchangemembranesandistechnicallyquitecomplicated.DiPalmaetal.[6]proposedreverseosmosisfortheseparationofEDTAcomplexesfromthewashingsolution.Thesoilcolloidalparticlestendtoclogthemembranes.Tejowu-lanandHendershot[7]separatedEDTAusingananionexchangeresin.Theseareexpensive($100for500g).FinzgarandLestan[8]proposedoxidativedecompositionofEDTAcomplexesinwashingsolutionusingadvancedoxidationprocesses(AOP).Thetreatmentwasmuchlesseffectivewhenthewashingsolutionwaseithertur-bidorcoloured.Electrochemical(E)AOP[9,10]wasmorerobustbutconsumedasignificantamountofelectricityandusesanexpensiveboron-dopeddiamondanode.MetalswereremovedfromthesoilwashingsolutionbyabsorptioninAOPandmainlybyfiltrationinEAOP.Inthecurrentstudy,weevaluatedelectrocoagulationwithanAlsacrificialanodefortheseparationofPb,ZnandCdfromawashingsolutionobtainedafterleachingcontaminatedsoilwithEDTA.Elec-0304-3894/$–seefrontmatter©2009ElsevierB.V.Allrightsreserved.doi:10.1016/j.jhazmat.2009.09.103M.Pociecha,D.Lestan/JournalofHazardousMaterials174(2010)670–678671trocoagulationisasimpleandefficientelectrochemicalmethodforthepurificationofwastewaterscontainingorganicormetallicpol-lutants[11].WhenapotentialdifferenceisappliedbetweentheAl(orFe)anodeandacathode,Al(orFe)ionsaregeneratedfromtheanodeandhydroxylionsfromthecathode.ThereactionsfortheelectrochemicalsystemwithanAlanodeareasfollows:Attheanode,Eq.(1):Al→Al3++3e−(1)Atthecathode,Eq.(2):2H2O+2e−→H2+2OH−(2)Al3+andOH−ionsreactfurthertoformvariousmonomericAlhydroxidessuchasAl(OH)2+,Al(OH)2+orAl2(OH)24+andpoly-mericAlhydroxidessuc