Chemical Equilibrium Modeling ofOrganic Acids, pH, Aluminum, andIron in Swedish

Environ.Sci.Technol.2010,44,8587–8593

ChemicalEquilibriumModelingofOrganicAcids,pH,Aluminum,andIroninSwedishSurfaceWaters

CARINS.SJ?STEDT,?

JONPETTERGUSTAFSSON,?ANDSTEPHANJ.K?HLER*,?

DepartmentofLandandWaterResourcesEngineering,KTH(RoyalInstituteofTechnology),Teknikringen76,SE-10044Stockholm,Sweden,andDepartmentofAquaticSciencesandAssessment,SwedishUniversityofAgriculturalSciences,P.O.Box7050,SLUUppsala,Sweden

ReceivedJuly16,2010.RevisedmanuscriptreceivedSeptember20,2010.AcceptedSeptember20,2010.

AconsistentchemicalequilibriummodelthatcalculatespHfromchargebalanceconstraintsandaluminumandironspeciationinthepresenceofnaturalorganicmatterispresented.Themodelrequiresinputdatafortotalaluminum,iron,organiccarbon,?uoride,sulfate,andchargebalanceANC.ThemodeliscalibratedtopHmeasurements(n)322)byadjustingthefractionofactiveorganicmatteronly,whichresultsinanerrorofpHpredictiononaveragebelow0.2pHunits.Thesmallsystematicdiscrepancybetweentheanalyticalresultsforthemonomericaluminumfractionationandthemodelresultsiscorrectedforseparatelyfortwodifferentfractionationtechniques(n)499)andvalidatedonalargenumber(n)3419)ofgeographicallywidelyspreadsamplesalloverSweden.Theresultingaverageerrorforinorganicmonomericaluminumisaround1μM.Initspresentformthemodelisthe?rstinternallyconsistentmodelingapproachforSwedenandmaynowbeusedasatoolforenvironmentalqualitymanagement.Soilgibbsitewithalog*Ksof8.29at25°CtogetherwithapHdependentloadingfunctionthatusesmolarAl/CratiosdescribestheamountofaluminuminsolutioninthepresenceoforganicmatterifthepHisroughlyabove6.0.

acids8,9)tomoreadvancedmodelssuchastheStockholmHumicModel(SHM)(10)orModelVI(6).

Whenequilibriummodelsarecalibrated,attentionisoftenpaidtoonlyoneparametersuchasmajorionchargebalance(11),orinorganicaluminumfractions(12),oralimitedrangeoflandscapeelements(13).Thisapproachmayresultinmodelsthatmaybeelement-orsite-speci?c.

ThesetofequationsusedtoaccountfororganicacidswhenmodelingpHorinorganicaluminumspeciationvariesfromsimpletriproticorganicacidapproaches(8,13)tocomplexdistributionmodelssuchasWHAM(e.g.ref12).Previously,calibratedvaluesoforganicsitedensityoforganicacidsinSwedishsurfacewatersdifferedwhenmodelingpHoraluminumfractions(12,13).ThiswillleadtoinconsistentmodelingifboththepHandaluminumdataaretobemodeledsimultaneously.Simultaneousmodelingofalumi-numchemistryandequilibriumpHforlargerareashasbeenstudiedonlyveryrarely(8).RecentlytheinterestinacidraineffectsinbothNorthAmerica(2,3)andScandinavia(14,15)hasbeengrowingowingtothefactthatchemicalchangesinsurfacewatersareduetoacombinationofchangingsulfateandnitrateconcentrationsandchangingDOCconcentrationsbothofwhichaffectpHandmetalmobility.AssessmentofenvironmentalqualitystandardsinSwedenneedtobebasedonaquantitative,internallyconsistentmodelthathasbeenvalidatedacrossthewholecountry.

Ouraimwastotesttheperformanceofaprocess-basedgeochemicalmodel(VisualMINTEQusingtheSHMfororganiccomplexation)thatcansimulatebothpH,inorganicmonomericaluminum,andtheparticlefractionofAlandFeinaconsistentmanner.Animportantcriterionforsuchamodelisthatitshouldbebothinternallyconsistent(i.e.sameparametersettingsforallsamplesconsidered)andgeochemicallyconsistentwithresultsobtainedearlier;therefore,onlythegenericconstantsintheVisualMinteqwereused(see,e.g.refs16and17).ExceptforFe(III)complexationtoDOM,theonlyparameterthatwasoptimizedwasthefractionofactivedissolvedorganicmatter,theso-calledADOM/DOCratio.Thisapproachisbasedonprior?ndingsthatorganicacidsitedensitymaybeconsideredtobeconstantinSwedenwhenmodelingorganicacidchargedensityandpH(18,9).

Methods

DataSets.AlargenumberofsurfacewatersweremadeavailableeitherfromwithintheSwedishenvironmentalmonitoringprogramorfromsynopticstudiesthroughoutthecountry.Intotalthedifferentdatasetsconsistofaround5200samplesforpHand4000samplesforaluminumfractionation.FirstweusedonedatasetforoptimizationoftheADOM/DOCratiobasedonpHsimulations,withsamplesfromacountrywidelakesampling(“M?lsjo¨ar”,thisstudyn)322).Subsequentlythesamedatawasusedtoanalyzetheusefulnessofthedefaultbindingconstantforaluminumtopredictthemeasuredconcentrationofinorganicaluminumandtostudytheoutcomeofvaryingthatconstantbyafactorof2.

Whenevaluatingthemodelingoftheinorganicmono-mericaluminum,weusedthedatasetsM?lsjo¨arand“Foreststreams”(14)(Table1)ascalibrationofacorrectionfunctionforthetwodifferentAlfractionationmethodsused(seesectionlinearcorrectionfunction).

ForvalidationofboththepHcalculationsandtheinorganicmonomericaluminumcalculations,sampleswereusedfromlakesandstreamsfromtwocountrywidelakesurveys(“Lake1995”,“Lake2000”,and“Stream2000”(19)),

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Introduction

Geochemicalmodelsareimportanttoolsforquantifyingtemporalchangesinchemicalconditionsinsoilsandsurfacewatersaffectedbyhumanimpact.Examplesareriskassess-mentofmetaltoxicity,estimationofeffectsofacidrainonaluminummobility(1-4)butpotentiallyevenwhenpre-dictingtheanticipatedchangesinsulfateandDOCcon-centrationsinlargeregionsoverNorthernEurope.

Severalprogramshavebeenpresentedthataccountforgeochemicalequilibriumconditions,suchasALCHEMI(5),WHAM(6),andVisualMinteq(7).Theconstantsforinorganiccomplexesareusuallywellestablishedinthoseprograms,butreactionsinvolvingdissolvedorganicmatter(DOM)aremoredif?culttoquantify.Severalmodelsexistforthedissociationandmetalcomplexationreactionsofmacro-molecularorganicacids,rangingfromsimple(e.g.,1-3-protic

*Correspondingauthorphone:+4618673826;fax:+4618673156;e-mail:Stephan.Kohler@vatten.slu.se.?

KTH(RoyalInstituteofTechnology).?

SwedishUniversityofAgriculturalSciences.

10.1021/es102415r

?2010AmericanChemicalSociety

8587

PublishedonWeb10/19/2010

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