差分光谱法原位检测质子化作用过程

InSituExaminationoftheProtonationBehaviorofFulvicAcidsUsingDifferentialAbsorbanceSpectroscopyDEBORAHJ.DRYER,†GREGORYV.KORSHIN,*,†ANDMASSIMILIANOFABBRICINO‡DepartmentofCivilandEnvironmentalEngineering,UniversityofWashington,Seattle,Washington98195-2700UnitedStates,[e-mail],andDipartimentodiIngegneriaIdraulicaedAmbientale”GirolamoIppolito”,Universit´adegliStudidiNapoliFedericoIIViaClaudio21,80125NaplesItalyReceivedMarch14,2008.RevisedmanuscriptreceivedJune23,2008.AcceptedJune30,2008.TheeffectsofpHchangesontheabsorbancespectraoffulvicacids,whichconstituteamajorpartofnaturalorganicmatter(NOM)wereexaminedbasedprimarilyontheexampleofstandardSuwanneeRiverfulvicacid(SRFA).DifferentialabsorbancespectraofSRFAgeneratedatvaryingpHvaluesweredeterminedtoberichinfeaturesthatareabsentintheconventionalspectra.AnalysisofthedifferentialabsorbancerevealedtheexistenceoftwopredominanttypesofpH-activechromophoreswhichproducedfeaturesthatemergedinthepHrangesassociatedwithcarboxylicandphenolicfunctionalgroupsoffulvicacids.ComparisonofthedifferentialabsorbancespectraofSRFAandthoseofthefractionofhydrophobicacidsofSouthPlatteRiverfulvicacidshowtheexistenceofdifferencesindicativeoffulvicacids’ssite-specificity.TheevolutionofthepH-differentialspectraoffulvicacidswasinaccordwithpredictionsmadebasedontheNICA-Donnantheory.TheseresultsshowthatpH-differentialabsorbancespectroscopycanbeusedtoquantifyinsitutheprotonationandsite-specificityoffulvicacids.IntroductionNaturalorganicmatter(NOM)isanimportantcomponentofpristineandengineeredenvironmentalsystems.Itexertsastrongcontrolonthespeciationandtransportofheavymetalsandotherpollutantsinaquaticsystems.Inordertounderstandandmodeltheseprocesses,precisemeasure-mentsofNOMpropertieslikebindingcapacitiesandstabilityconstantsforinteractionswithprotonandmetalcationsareneeded(1-4).ThisisadifficulttasksinceNOMisacomplexentity,withpropertiesthatstronglydependonlocalbio-geochemicalandtreatmentconditions(5,6).ThepresenceofdiversefunctionalgroupsandmolecularsizesinNOMcomplicatestheperformanceofanalyticaltechniquesusedforNOMcharacterization.ToobtainlesscomplexNOMsamples,techniquestoseparateNOMintooperationallydefinedfractions,mostlybasedongradationsofhydrophi-licityandacidityofNOMmolecules,havebeendeveloped(5-8).WhilethesemethodshavebeeninstrumentalforthesuccessfulexplorationofmanyaspectsofNOMchemistry,theirutilityremainslimitedbythenecessitytopreconcentrateNOMandthepossibilitythatpropertiesoftheNOMfractionsmaynotnecessarilyberepresentativeofthoseoftheunalteredNOMsample.Asaresult,thereremainsavitalneedforanefficientmethodtoquantifyNOMproperties,notablyitsprotonationbehavior,insitu.TheprotonationofhumicandfulvicacidsthatconstituteapredominantpartofNOMhasbeenstudiedbycalorimetricandespeciallypotentiometricpHtitrationtechniques,butthesemethodsarelimitedbytheneedforhighDOCconcentrations(500mg/L)(3,9-15).ThepolydispersenatureoffulvicacidsresultsintitrationcurvesthatindicatethepresenceofcomplexdistributionsofacidicfunctionalgroupswithvaryingpKavaluesthatmostlyfallinthecarboxylicandphenolicpHranges(9,13,14).Poten-tiometricdataforavarietyoffulvicacidsindicatethattheapparentmedianpKvaluesofthecarboxylicandphenolicgroupsrangefrom2to3.8and7.2to10.9,respectively(16).BecauseNOMisthemajorlightabsorbingcomponentofnaturalwatersinthe200-800nmrange(17),opticalspectroscopyhasshownpromiseforelucidatingimportantaspectsofNOMchemistrywithoutrequiringanyprecon-centration.However,thewidevarietyoflight-absorbingfunctionalgroups(chromophores)inNOMandpossiblepresenceofcomplexintramolecularinteractionsresultinabsorbanceandfluorescencespectrathatarebroadandfeatureless(18,19).Asaresult,interpretationofthesespectrahasfrequentlybeenlimitedtousingparticularspectralparameters(e.g.,absorbanceataparticularwavelength,absorbanceratios,SUVA254,andwavelengthsofmaximumandhalf-maximumfluorescencepeakheight)assurrogatesforNOMpropertiessuchasitsconcentration,molecularsize,andaromaticity(17,20-25).AnalternativeinsitumethodwhichhasprovenusefulforelucidatingNOMpropertiesisdifferentialabsorbancespec-troscopy(DAS).Thismethodexamineschangesinabsor-bancespectraassociatedwithevolvingreactionconditionswhichaffectNOMchemistry.DASiscapableofdetectingverysubtlechangesinNOMchemistryandhashighsensitivitywhicheliminatestheneedforsamplepretreat-ment.Inpriorresearch,thistechniquehassuccessfullybeenusedtoquantifyNOMoxidationandchlorinationreactions(26-28).Inthisstudy,DASisusedtoexploreinsitutheprotonationof“standard”SuwanneeRiverfulvicacid(SRFA)andanotherrepresentativefulvicacid(thefractionofhydrophobicacidsofSouthPlatteRiverNOM);protonationpropertiesofbothsampleshavebeenascertainedinpriorresearch(3,5,16,29).MaterialsandMethodsASRFAsamplewasobtainedfromtheInternationalHumicSubstanceSociety(standardnumber1S101F).AsampleofSouthPlattehydrophobicacid(HPOA)fractionwasisolatedandprovidedforthisstudybyDr.J.A.Leenheer.Itwasobtainedbypassinganacidified(pH2)sampleofSouthPlatteNOMthroughacolumncontainingAmberliteXAD-8resinandthenelutingtheHPOAfractionwithsodiumhydroxide(7).S