acsest5b00601

EffectsofIonicStrengthontheChromophoresofDissolvedOrganicMatterYuanGao,*,†MingquanYan,‡andGregoryV.Korshin††DepartmentofCivilandEnvironmentalEngineering,UniversityofWashington,Seattle,Washington98195,UnitedStates‡DepartmentofEnvironmentalEngineering,CollegeofEnvironmentalSciencesandEngineering,PekingUniversity,theKeyLaboratoryofWaterandSedimentSciences,MinistryofEducation,Beijing100871,China*SSupportingInformationABSTRACT:Thisstudyexaminedeffectsofvariationsoftheionicstrength(IS)ontheabsorbanceofdissolvedorganicmatter(DOM).ThemeasurementsperformedforDOMofallochthonous(SuwanneeRiverhumicandfulvicacids,SRHAandSRFA)andautochthonous(PonyLakefulvicacid,PLFA)originshowedthatincreasesofIS(whichwascontrolledbyadditionsofsodiumperchlorate)from0.001to0.3mol/LwereaccompaniedbyincreasesoftheabsorbanceofDOM.TheextentoftheincreaseofDOMabsorbanceobservedatincreasingISwasconsistentlygreaterathigherpHvalues,anditfollowedtheorderofPLFASRFASRHA.Theabsolutevaluesofthespectralslopesofthelog-processedabsorbancespectraofDOMcalculatedfora350to400nmwavelengthrangedecreasedproportionallytothelogarithmofISvalues.ThisresultwashypothesizedtobeindicativeofthedeprotonationoftheDOMchromophoresatincreasingISvalues,whichwassupportedbymodelcalculationsshowingthatvaluesofthespectralslopeswerenearlylinearlycorrelatedwiththeextentofIS-induceddeprotonationoftheoperationallydefinedphenolicgroupsinDOM.■INTRODUCTIONDissolvedorganicmatter(DOM)isoneofthemostimportantcomponentsofsoils,sedimentsandaquaticenvironments.1Oneofthewell-documentedeffectsofDOMisitsinfluenceonconcentrationsandmobilityofmetalsintheenvironment.PriorresearchhasshownthatmetalconcentrationstendtoincreaseinthepresenceofDOMduetotheformationofmetal−DOMcomplexesandmetal/DOMcolloidalparticles,2−6althoughmetal/DOMcomplexestendtohavelowertoxicitycomparedtothefreemetalions.7TheextentofDOM−metalbindingandthenatureofDOMfunctionalitiesengagedininteractionswithmetalcationsaresite-specificbecauseDOMcanbeformedautochthonouslyfromitsprecursorsgeneratedinthewaterbodyperse(e.g.,algae)oritcanbeproducedviaallochthonousprocessesinvolvingthedegradationofterrestrialbiota.8ThediversityofDOMsourcesandgenerationpathwaysresultinahighlycomplexnatureofDOMwhosemoleculeshavevaryingmolecularweights,conformations,protonaffinitiesandconcentrationsofcarboxylic,phenolicandothermetalcation-bindingfunctionalgroups,9,10andotherpropertiesthat,forengineeredsystems,arealsoaffectedbyspecificsofwatertreatmentprocesses.11Priorstudieshaveshownthatthemodelingofmetal−DOMsystemsmusttakeintoaccountbothspecificinteractionsbetweenMen+ionsandDOMfunctionalities,aswellastheelectrostaticallycontrolledinteractionsaffectedbyvariationsofionicstrength(IS).PriorstudieshavealsoprovidedevidencethatconcentrationsofbackgroundcationssuchasNa+areimportantforsuchaspectsofDOMchemistryduetothesupramolecularpropertiesofDOMmoleculesandtheiraggregation,9,12−15DOMadsorptiononthesurfacesofmineralparticlesandtheircolloidalbehavior.16−18Becauseenviron-mentalconcentrationsofsodiumandotherbackgroundcations(e.g.,K+,Ca2+,Mg2+)tendtobeseveralordersofmagnitudehigherthanthoseoftracelevelmetals(e.g.,Pb2+andCu2+),thiscouldresultinastrongcompetitionbetweenthebackgroundandtrace-levelcationsforbindingsitesinDOM.19Severaltheoreticalapproaches,forinstancetheNon-IdealCompetitiveAdsorption(NICA)modelcoupledwiththeDonnanelectrostaticsubmodelhavebeendevelopedtodescribethebindingofmanycationstoDOMoverawiderangeofconditions.20−22IntheNICA−Donnanandrelatedmodels,metal-bindingDOMfunctionalitiesareseparatedintooperationallydefinedcarboxylicandphenolicmoieties,eachwithacontinuousGaussian-typedistributionofpKvalues.23,24TheDonnanmodelcanbeusedtoquantifyconcentrationsofcounterions(e.g.,Na+)accumulatingtobalancethechargeofReceived:August28,2014Revised:April17,2015Accepted:April21,2015Published:April21,2015Articlepubs.acs.org/est©2015AmericanChemicalSociety5905DOI:10.1021/acs.est.5b00601Environ.Sci.Technol.2015,49,5905−5912DownloadedviaPEKINGUNIVonAugust17,2018at06:28:58(UTC).SeeficbindingofcounterionssuchasNa+orhardnesscationsthataccumulateintheDonnanvolume(VD).TheDonnanvolume,animportantparameterintheNICA−Donnanmodel,isdeemedtoincludeahumicmoleculealongwiththediffusepartoftheelectricdoublelayersurroundingit.LogarithmsofVDvalueshavebeenhypothesizedtodecreaseasalinearfunctionofthelogarithmoftheISvaluesofthesolution7Thefundamentalconceptsandformalapparatusincorpo-ratedintheadvancedmodelsofDOM−metalcationinteractionshavebeendevelopedbasedonthedataofdiverseexperimentaltechniques,notablypotentiometricmethodsthatuseion-selectiveelectrodes,ISE,26,27X-rayabsorptionfinestructurespectroscopy(EXAFS),28−31infrared(IR)spectros-copyandnuclearmagneticresonance(NMR).Thesemethodsa