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S1SupportingInformationSection12EffectsofIonicStrengthontheChromophoresofDissolvedOrganic3Matter4*YuanGao,†MingquanYan‡andGregoryKorshin,†5†DepartmentofCivilandEnvironmentalEngineering,UniversityofWashington,6Seattle,WA7‡DepartmentofEnvironmentalEngineering,CollegeofEnvironmentalSciencesand8Engineering,PekingUniversity,theKeyLaboratoryofWaterandSedimentSciences,9MinistryofEducation,Beijing100871,China1011Numberofpages:812Numberoffigure:413Numberoftables:214151617S2181920S321FigureS1(a)OriginalspectraofSRFArecordedatincreasingionicstrengths(DOC=5mg/L,22cuvettepathlength5cm,pH7);(b)Differentialabsorbancespectrapriortotheirnormalization23bycelllengthandDOCconcentration.pH5,DOC5mg/L,cuvettepathlength5cmand(c)24DifferentialabsorbancespectrapriortotheirnormalizationbycelllengthandDOC25concentration.pH7,DOC5mg/L,cuvettepathlength5cm262728S429FigureS2Log-transformedspectraofSRFArecordedatincreasingionicstrengths(DOC=530mg/L,celllength=5cm,pH=7)313233343536373839404142434445S54647FigureS3Log-transformeddifferentialabsorbancespectraofPLFAandSRHAcalculatedfor48increasingionicstrengths.49505152S6TableS1Totalmetalconcentration(ppb)inDOM(5mg/lasDOC)5354SRHASRFAPLFACu2.500.296.81ZnN.A.N.A.0.02Cd0.000.000.06Pb0.250.600.93Ba2.710.000.46Al6.351.053.09Fe35.47N.A.N.A.MgN.A.3.0958.66Ca54.121.0910.48555657S7DatainterpretationbasedontheNICA-Donnanmodel.Inaccordwith58theapproachpresentedinpriorresearch(Yanetal.,2013a;Yanetal.,2013b;Yanet59al.,2014),weusedthedifferentialspectralslopeintherangeofwavelengths350to60400nm(DSlope350-400)measuredatvaryingpHstoexamineeffectsofpHonthe61protonationofSRHAandSRHA.Theresultswereinterpretedbasedonthe62NICA-Donnantheoryandexpressionspresentedinpriorliteratures(Kinniburghetal.,631999;RitchieandPerdue,2003;Dryeretal.,2008;Janotetal.,2010;Yanetal.,642014):6566(S1)6768Intheaboveequation,()λLASDSλopeand()λHASDSλopecorrespondtothemaximum69changeofabsorbanceassociatedwiththedeprotonationoftheLAS(mostly70carboxylic)andHAS(mostlyphenolic)groups,respectively,()λLASDSλopeand71()λHASDSλopereferredtoDSlopevalueintherangeofwavelengths325to375nm.72andarethemedianvaluesoftheprotonsaffinitydistributionsforthese73groups,mLASandmHASdefinethewidthofthesedistributionsandaremeasuresofthe74heterogeneityofDOM(Milneetal.,2001;Dryeretal.,2008).75ThebehaviorofDSlope350-400valuesforSRHAandSRFAvs.pHandparameters76usedfortheirfittingareshowninFigureS4andTableS2,respectively.Thedata77S8demonstratethatexcellentagreementcouldbereachedbetweentheDSlope350-400data78forbothSRHAandSRFAsamplesandthepredictionsmadebasedontherevised79NICA-Donnanmodel(R20.99).808182FigureS4EffectsofpHonthedifferentialspectralslopedeterminedforthe83rangeofwavelengths350to400nm(DSlope350-400)anditsNICA-basedfittingfor84SRHA(a)andSRFA(b).85S9TableS2PotentiometricandspectrophotometricNICA-Donnanparametersfor86protonation-activegroupsinSRHAandSRFA.87SRHASRFAPotentiometricparametersfromMINTEQSofterwaredatabase2.932.348.008.60QLAS,meq/g3.155.88QHAS,meq/g2.551.86mLAS0.810.66mHAS0.630.76b0.490.57SpectrophotometricparametersdeducedfromFigureS44.504.609.859.03DSlopeLAS(350-400nm)0.0009360.00356DSlopeHAS(350-400nm)0.007740.00592mLAS0.300.20mHAS0.200.39S10References88Dryer,D.J.,Korshin,G.V.,Fabbricino,M.,2008.Insituexaminationofthe89protonationbehavioroffulvicacidsusingdifferentialabsorbancespectroscopy.90EnvironmentalScience&Technology42,6644-6649.91Janot,N.,Reiller,P.E.,Korshin,G.V.,Benedetti,M.F.,2010.Using92spectrophotometrictitrationstocharacterizehumicacidreactivityatenvironmental93concentrations.EnvironmentalScience&Technology44,6782-6788.94Kinniburgh,D.G.,vanRiemsdijk,W.H.,Koopal,L.K.,Borkovec,M.,Benedetti,M.F.,95Avena,M.J.,1999.Ionbindingtonaturalorganicmatter:competition,heterogeneity,96stoichiometryandthermodynamicconsistency.ColloidsandSurfacesA:97PhysicochemicalandEngineeringAspects151,147-166.98Milne,C.J.,Kinniburgh,D.G.,Tipping,E.,2001.GenericNICA-Donnanmodel99parametersforprotonbindingbyhumicsubstances.EnvironmentalScience&100Technology35,2049-2059.101Ritchie,J.D.,Perdue,E.M.,2003.Proton-bindingstudyofstandardandreference102fulvicacids,humicacids,andnaturalorganicmatter.GeochimCosmochimAc67,10385-96.104Yan,M.,Benedetti,M.F.,Korshin,G.V.,2013a.Studyofironandaluminumbinding105toSuwanneeRiverfulvicacidusingabsorbanceandfluorescencespectroscopy:106ComparisonofdatainterpretationbasedonNICA-DonnanandStockholmhumic107models.WaterRes47,5439-5446.108Yan,M.,Korshin,G.V.,Claret,F.,Croué,J.-P.,Fabbricino,M.,Gallard,H.,Schäfer,109T.,Benedetti,M.F.,2014.Effectsofchargingonthechromophoresofdissolved110organicmatterfromtheRioNegrobasin.WaterRes59,154-164.111Yan,M.,Wang,D.,Korshin,G.V.,Benedetti,M.F.,2013b.Quantifyingmetalions112bindingontodissolvedorganicmatterusinglog-transformedabsorbancespectra.113WaterRes47,2603-2611.114115116