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ComparativeExaminationofEffectsofBindingofDifferentMetalsonChromophoresofDissolvedOrganicMatterMingquanYan*,†andGregoryV.Korshin‡†TheKeyLaboratoryofWaterandSedimentSciences,MinistryofEducation,DepartmentofEnvironmentalEngineering,PekingUniversity,Beijing100871,People’sRepublicofChina‡DepartmentofCivilandEnvironmentalEngineering,UniversityofWashington,Box352700,Seattle,Washington98195-2700,UnitedStates*SSupportingInformationABSTRACT:Thisstudyquantifiedthebindingofdissolvedorganicmatter(DOM)fromSuwanneeRiverwithninemetals,Ca(II),Mg(II),Fe(III),Al(III),Cu(II),Cd(II),Cr(III),Eu(III),andTh(IV),usingadifferentialabsorbanceapproach.ThedifferentialspectraofDOMwerecloselyfittedwithsixGaussianbandsthatwerepresentforallofthemetalsatvaryingpHvalues.Theirmaximawerelocatedatca.200,240,276,316,385,and547nm(denotedasA0,A1,A2,A3,A4,andA5,respectively).TherelativecontributionsandsignsoftheGaussianbandsweremetal-specificandcorrelatedtosomedegreewiththecovalent-bondingindexoftheionsandapplicablecomplexationconstantsoftheNICA-Donnanmodel.TheintensityofbandA4waslinearlyproportionaltotheconcentrationofDOM-complexedmetal,althoughthesecorrelationsformedtwogroupswithdifferentslopes,reflectingthenatureofDOM−metalinteractions.TheresultsdemonstratethatdifferentialspectrayieldresultsindicativeofthenatureandextentofmetalandDOMinteractions.■INTRODUCTIONRecentresearchhasshownthatdifferentialabsorbancespectraofdissolvedorganicmatter(DOM)measuredatenvironmentallyrelevantconcentrationsofdissolvedorganiccarbon(DOC)haveconsistentlyobservedfeaturesassociatedwithvariousprocesses,suchasdeprotonationofDOMmolecules1−5andtheirinteractionswithsurfaces,6,7halogenspeciesandoxidants,8−10andmetalcations(e.g.,Cu2+,Cd2+,Fe3+,andAl3+).3,11−16Theintensityandshapeofthelinearandlog-transformeddifferentialspectra[e.g.,differentialabsorbanceatwavelength400nm(DA400),differentiallogarithmofDOMabsorbanceat350nm(DLnA350),andchangeofthespectralslopeintherangeofwavelengths325−375nm(DSlope325−375)]havebeenshowntobecorrelatedstronglywiththeamountofmetalsboundtoDOMestimatedfromthedataofNICA-Donnanmodeling.11−13PriorresearchhasalsoshownthatthedifferentialspectraofstandardSuwanneeRiverfulvicacid(SRFA)recordedatdifferentpHvaluesandcopperconcentrationscanberesolvedintodiscreteGaussianbands,whosepositionsandwidthshavecommonalitiesforthecasesofSRFAdeprotonationanditsinteractionwithcopper(II)ions.12ThesebandsmaybemanifestationsofinteractionsbetweenH+orCu2+withdiscretegroups(e.g.,thecarboxylicand/orphenolicmoietiesortheirsubgroups),ortheymaybeindicativeofcharge-transfereffectivenessofinteractionsthatcannotbeascribedtodiscretefunctionalities(e.g.,viaelectrostaticDonnangelbinding).5However,onlyaverylimitedsimilaritywasshowntoexistbetweendifferentialspectroscopicresponsesofselectedmodelcompounds(e.g.,tannicacidandsulfosalicylicacid)andSRFA,andtheissueofpossiblesimilaritiesbetweendiscretefunctiongroupsinDOMandthoseinmodelcompoundsneedstobeelucidatedfurther.Accordingly,thisstudycomparedmajorfeaturesofthedifferentialspectraofSRFAandotherselectedDOMscausedbytheirinteractionswiththeprotonandawiderangeofmetalions,suchasalkalineearthmetalscalcium(II)andmagnesium(II),trivalentionsiron(III)andaluminum(III),heavymetalscopper(II),cadmium(II),andchromium(III),andtherareearthionsthelanthanideeuropium(III)andtheactinidethorium(IV).Theaimsofthisstudyinclude(1)toexaminetheutilityofthedifferentialspectraapproachtoawiderangeofmetalcationsand(2)toexplorethenatureoftheengagedSFRAfunctionalitiesthroughthecomparisonofresponsesofthisstandardDOMtointeractionswithawiderangeofmetalcationsforwhichextensivemodelingandotherrelevantinformationisavailable.■MATERIALSANDMETHODSReagentsandChemicals.Allchemicalswereanalyticalreagent(AR)grade,unlessotherwisementioned.AllsolutionswerepreparedusingMilli-Qwater(18.2MΩcm−1,MilliporeCorp.,MA).SuwanneeRiverhumicacid(SRHA)(1R101H)Received:October10,2013Revised:February15,2014Accepted:February18,2014Published:February18,2014Articlepubs.acs.org/est©2014AmericanChemicalSociety3177dx.doi.org/10.1021/es4045314|Environ.Sci.Technol.2014,48,3177−3185andSRFA(1S101F)wereobtainedfromtheInternationalHumicSubstancesSociety(IHSS).TheconcentrationsofSRHAandSRFAwere5.0mgL−1[asdissolvedorganiccarbon(DOC)].Titrations.Ca(II),Mg(II),Fe(III),Al(III),Cu(II),Cd(II),Cr(III),Eu(III),andTh(IV)titrationswerecarriedoutbyaddingrequisitevolumesofmetalionstockinto100mLaliquots.ThepHwasmaintainedconstantduringmetaladditionsandcontrolledbyaddingsmallamountsofdiluteHClO4orNaOH.Ithasbeendescribedinpreviousliteratureindetail.11−13TheabsorbancespectrawererecordedusingaPerkin-ElmerLambda18UV/visspectrophotometer(witha5cmcell)atwavelengthsfrom200to600nm.StocksolutionsofthemetalswerepreparedusingCa(ClO4)2,Mg(ClO4)2,Fe(ClO4)3,Al(ClO4)3,Cu-(ClO4)2,Cd(ClO4)2,Cr(ClO4)3,Eu(ClO4)3,Th(ClO4)4,HClO4,andNaOH.Th(ClO4)4wasobtainedviatheadjustmentofpHof1000mgL−1thoriumnitrateinductivelycoupledplasma/massspectrometry(ICP/MS)standard,separationoftheprecipitatedthoriumhydroxideona0.45μmfilter,anddissolutionofthesolidphaseinhigh-purityHClO4.DOM−metali