NicaDonna模型

MetalIonBindingbyHumicAcid:ApplicationoftheNICA-DonnanModelDAVIDG.KINNIBURGH,*,†,‡CHRISTOPHERJ.MILNE,§MARCF.BENEDETTI,|JOSEP.PINHEIRO,⊥JEROENFILIUS,†LUUKK.KOOPAL,4ANDWILLEMH.VANRIEMSDIJK*,†DepartmentofSoilScienceandPlantNutritionandDepartmentofPhysicalandColloidChemistry,WageningenAgriculturalUniversity,Dreijenplein10,6703HBWageningen,TheNetherlands,BritishGeologicalSurvey,Wallingford,Oxon,OX108BB,U.K.,LaboratoiredeGe´ochimieetMe´talloge´nie,Universite´P&MCurie,4PlaceJussieu,75252ParisCedex05,France,andUniverAlgarveUnidadedeCienciasExactaseHumana,CampusdeGambelas,8000Faro,PortugalADonnan-typemodelfornonspecificbindingofelectrolyteionshasbeencombinedwiththenon-idealcompetitiveadsorption(NICA)modelforspecificbindingtoproduceamodelforionbindingtohumicsubstances.Themodelconsiderssiteheterogeneity,non-ideality,multicomponentcompetition,andelectrostaticinteractions.TheNICA-DonnanmodelwasfittedtodataforH,Ca,Cd,Cu,andPbbindingbyapurifiedpeathumicacid.ThemodelfitsweregoodandcoveredawiderangeofpHandfreemetalconcentrations.TheparametersfromthesesinglemetaldatasetswerethenusedtopredictthecompetitiveeffectofCaonCdandCubindingatvariouspHs.Thesepredictionsagreedwellwiththeexperimentaldataalthoughthereweresomesmallbutsystematicdifferences.ThenewNICA-DonnanmodelalsopredictedreasonablywelltheincreaseinCdandCubindingonchangingfroma0.1MKNO3backgroundelectrolyteto0.01MKNO3.Ashortcom-ingofthemodelisthatinsomecasesitsignificantlyunderestimatedtheH+/M2+exchangeratio,especiallyathighpHandforCubinding.IntroductionHumicsubstances(humics)arepresentinmostnaturalwatersatconcentrationsof0.1-200mgL-1dissolvedorganiccarbon(DOC).Solidorganicmatterisalsofoundinsoilsandsedimentsatconcentrationsof0.1-10%byweight.Thesehumicsubstancesbindavarietyofmetalionsandtypicallycontain2-10equivkg-1ofionizablegroups.Thebindingofprotonsandmetalionstothesesubstancesisimportantforthespeciation,transport,andtoxicityofmanytracemetals,butithasprovendifficulttofindequationsthatdescribethisbindingoverawiderangeofconditions.Factorsthatneedtobeconsideredareasfollows:(i)individualhumicsshowintrinsicchemicalheterogeneityonamolecularscale;(ii)theboundionscompeteforbindingsites;(iii)thereareelectrostaticinteractionsbetweentheboundionsandthebindingsitesandamongtheboundions;and(iv)humicsfromdiversesourceshavecommonpropertiesandbehaviorbutalsoshowimportantdifferencesthatneedtobecharacterizedandspecified.Itislikelythattheabilitytodescribecompetitivemetalbindingwillprovetobeaparticularlyimportanttestofthevariousmodels(1-3).Alargenumberofmodelsforionbindingbyhumicsexist,butmanyofthesehavelimitedscopeorareundulysensitivetomoderateextrapolation.Intermsofscope,ModelVofTippingandHurley(4)probablycomesclosesttobeingacomprehensivemodelthatcandescribeionbindingbyarangeofhumics.Itallowsforeightdifferenttypesofsites,someofwhicharecloseenoughtogethertobehaveasbidentatesites,andusesasimpleelectrostaticmodeltodifferentiatebetweengenericCoulombicbindingandspecificbinding.ModelVhasalsobeenusedtodescribemetal-metalcompetition(2).ModelVassumesthatthemetalionaffinitiesaredirectlyrelatedtotheprotonaffinities,i.e.,theshapesoftheimpliedaffinitydistributionsforthemetalionsarethesameasfortheprotonexceptthattheyare“shifted”byanamountspecifiedbyeachofthemetal-bindingparameters.Wehaverecentlydemonstratedthatthis“fullycoupled”assumptionisnotverysuccessfulwhenappliedtocalciumandcadmiumbindingoverawiderangeofconcentrations(5).ModelValsocontainsaninconsistencyinthedevelopmentoftheelectrostaticmodel.Thisisdiscussedlater.Thenon-idealcompetitiveadsorption(NICA)model(6,7)attemptstoseparatethoseeffectsduetothegenericheterogeneityexperiencedbyallionicspeciesfromthoseeffectsarisingfromeffectsspecifictoeachparticularion.WehaverecentlyappliedtheNICAmodeltocationbindingbyafulvicacid(6),toapurifiedpeathumicacid(7),tosoilorganicmatter(8),andtometalionbindingbysoilsandlakewater(9).Ourprimarydataareforapurifiedpeathumicacid(PPHA)withthebindingexperimentscarriedoutinahighandconstantconcentrationofbackgroundelectrolyte(0.1MKNO3).Ourearlieranalysisdidnotconsidertheeffectthatanychangeinionicstrengthwouldhave.Theeffectofchangesinbackgroundelectrolytecanbedescribedintermsofelectrostaticmodels.Variousap-proachestotheelectrostaticshavebeenadopted(Table1).TheserangefromnumericalsolutionsofthePoisson-Boltzmannequationforspherical-andcylindrical-shapedparticles(1,10,11)toDonnanmodelsinwhichthehumicmaterialistreatedasifitisadistinct,electricallyneutral*Correspondingauthors;e-mailaddresses:[e-mail]@bodsch.benp.wau.nl.†DepartmentofSoilScienceandPlantNutrition,WageningenAgriculturalUniversity.‡OnleavefromtheBritishGeologicalSurvey.§BritishGeologicalSurvey.|Universite´P&MCurie.⊥UniverAlgarveUnidadedeCienciasExactaseHumana.4DepartmentofPhysicalandColloidChemistry,WageningenAgriculturalUniversity.Environ.Sci.Technol.1996,30,1687-16980013-936X/96/0930-1687$12.00/01996AmericanChemicalSocietyVOL.30,NO.5,1996/ENVIRONMENTALSCIENCE&TECHNOLOGY91687phase(12).Allapproachesarenecessarilyrathersimplistic,butbearinginmi