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GenericNICA-DonnanModelParametersforProtonBindingbyHumicSubstancesCHRISTOPHERJ.MILNE,*,†DAVIDG.KINNIBURGH,†ANDEDWARDTIPPING‡BritishGeologicalSurvey,Wallingford,Oxfordshire,OX108BB,U.K.,andCentreforEcologyandHydrology,WindermereLaboratory,Ambleside,Cumbria,LA220LP,U.K.Forty-ninedatasetsconsistingofliteratureandexperimentaldataforprotonbindingbyfulvicandhumicacidshavebeenanalyzedusingtheNICA-Donnanmodel.Themodelsuccessfullydescribedthebehavioroftheindividualdatasetswithahighdegreeofaccuracyandhighlightedthedifferencesinsitedensityandbindingaffinitybetweenfulvicacids(FA)andhumicacids(HA)whiledemonstratingtheirstrongsimilarities.ThedatahavealsobeenusedtoderivegenericmodeldescriptionsofprotonbindingbyFAandHAthatcanbeusedformodelingintheabsenceofspecificparametersetsfortheparticularhumicsubstanceofinterest.Thesegenericparameterscanprovideestimatesoftheamountofprotonbindingbyawidevarietyofhumicsubstancestowithinapproximately(20%underanygivenconditions.Themaximumsitedensityforprotonswas7.74and5.70equivkg-1foragenericFAandHA,respectively.TherecommendedgenericNICA-DonnanparametervaluesforFAareb)0.57,Qmax1,H)5.88,logK÷H1)2.34,mH1)0.38,Qmax2,H)1.86,logK÷H2)8.6,andmH2)0.53;forHAthevaluesareb)0.49,Qmax1,H)3.15,logK÷H1)2.93,mH1)0.50,Qmax2,H)2.55,logK÷H2)8.0,andmH2)0.26.IntroductionTheinfluentialroleofhumicsubstancesincontrollingthemobilityandbioavailabilityofmetalionsinaqueousenvironmentalsystemsiswell-knownandhasbeenexten-sivelystudied.Despitethisattention,theion-bindingbe-haviorofthehumicsremainsdifficulttodescribeandmodelbecauseoftheheterogeneityofthehumicmaterialsandbecausetheionbindingisdependentonmanyenvironmentalvariables(pH,ionicstrength,metal-ionconcentrationandspeciation,presenceofcompetingions).Overthepastfewyears,twomodelshaveemergedwhichshowencouragingsuccessatdescribinghumicion-bindingbehavioroverawiderangeofconditionsandmetals:ModelVIandNICA-Donnan.ModelVIisadiscretesitemodel,whiletheNICA-Donnanmodelisacontinuousdistributionmodel(1).ModelVI(2)buildsonaseriesofprogressivelymoresophisticatedmodels,eachwithawiderscopeandsubjectedtomoreextensivecalibrationagainstliteraturedata(3-5).ModelVIhasbeenappliedtothelargestrangeofdataofanyhumicion-bindingmodeltodate.However,suchistherateofworkinthesubjectthatsignificantnewdatasetshavebecomeavailableevensinceModelVIwaspublished.DevelopmentoftheNICA(non-idealcompetitiveadsorption)familyofmodelshasincludedextensivetestingandcalibra-tionagainstalargebodyofexperimentaldatacollectedspecificallyforthepurposeusingapurifiedpeathumicacid(PPHA)(6-8).TheConsistentNICA-Donnanmodel(orNICCA-Donnanmodel)isthelatestandmostrigorousversion.ItcombinesthermodynamicconsistencyinitstreatmentofcompetitiveadsorptionwithaDonnandescrip-tionoftheelectrostaticbehaviorofthehumicparticle.Theconsistencyaffectsonlythedescriptionofcompetitiveormulticomponentbinding.Forprotonbindingintheabsenceofspecificmetal-ionbinding,theNICCAandoriginalNICAmodelsareidentical(8).AlthoughtheConsistentNICA-Donnanmodelhasshownitselftobecapableinseveralstudies(9-11),ithassofarnotbeenappliedtothefullrangeofliteraturedata.Atpresent,usableparametervaluesareonlyavailableforthelimitedrangeofmaterialsagainstwhichthemodelhasalreadybeentested.Oneoftheproblemsfacinganyonewhowishestoapplythesemodelstoarealsysteminvolvinghumicsisthatcharacterizingthematerialsisoftendifficult,time-consum-ing,andexpensive.Inmostcasesthecostsofthisareprohibitive.Thereisthereforeaneedforan‘off-the-shelf’setofparametersthatcanbeusedinmodelingprotonandmetal-ioninteractionsofthehumicmaterialofconcern.Thedescriptionshouldbesufficientlyaccurateandversatiletoenableusablemodelingwithoutdetailedcharacterization;yetitshouldalsobeflexibleenoughtoaccommodateasmuchaspossibleoftheavailableinformationaboutthespecifichumicmaterial.TippinghasproducedsuchasetofparametersforbothhumicandfulvicacidsforModelsVandVI(2,5).HerewederiveasimilarsetforprotonbindingbasedontheNICA-Donnanmodel.UsingthedatasetsanalyzedbyTippingasastartingpoint,wehavecompiledacomprehensivedatabaseofhumicion-bindingdatafromtheliterature.Inthispaperwepresentanddiscusstheavailabledataforproton-bindingbyhumicandfulvicacids.WethenapplytheNICA-Donnanmodeltotheproton-bindingdataandsoobtainageneral-purposedescriptionofprotonbinding.Wealsodiscussthelimitationsofsuchanapproach.Inaddition,bymakingthedatabaseopenlyavailable,weaimtoprovideausefulresourceforothers.NICA-DonnanModelThetheoryoftheNICA-Donnanmodelhasbeenpresentedindetailelsewhere(1,7,8).HereitissufficienttostatethatusingtheNICAequation,theamountofprotonsbound,QH,isgivenbywherethesuffixes1and2denotethatadoublesitedistributionisused.Qmax,Histhetotalnumberofavailableprotonbindingsiteswithineachdistribution,K÷Histhemedianvalueoftheaffinitydistributionforprotons,[HS]istheconcentrationofprotonsatthesurfaceofthehumicparticle,andmdefinesthewidthofthedistribution.Themiareameasureoftheapparentheterogeneityofthehumicsub-*Correspondingauthor:phone:[phone];fax:[phone];e-mail:[e-mail].†BritishGeologicalSurvey.‡CentreforEcologyandHydrology.Q