Mg和NOM络合的原位研究

In-SituInvestigationofInteractionsbetweenMagnesiumIonandNaturalOrganicMatterMingquanYan,*,†YujuanLu,‡YuanGao,§MarcF.Benedetti,∥andGregoryV.Korshin§†DepartmentofEnvironmentalEngineering,TheKeyLaboratoryofWaterandSedimentSciences,MinistryofEducation,PekingUniversity,Beijing100871,China‡CollegeofChemistryandChemicalEngineering,ShenzhenUniversity,Shenzhen518060,China§DepartmentofCivilandEnvironmentalEngineering,UniversityofWashington,Box352700,Seattle,Washington98195-2700,UnitedStates∥InstitutdePhysiqueduGlobedeParisSorbonneParisCitéUniversitéParisDiderot,UMRCNRS7154,Paris,France*SSupportingInformationABSTRACT:Naturalorganicmatter(NOM)generatedinallnichesoftheenvironmentconstitutesalargefractionoftheglobalpooloforganiccarbonwhilemagnesiumisoneofthemostabundantelementsthathasmultiplerolesinbothbioticandabioticprocesses.AlthoughinteractionsbetweenMg2+andNOMhavebeenrecognizedtoaffectmanyenvironmentalprocesses,littleisunderstoodaboutrelevantmechanismsandequilibria.ThisstudyaddressedthisdeficiencyandquantifiedMg2+−NOMinteractionsusingdifferentialabsorbancespec-troscopy(DAS)incombinationwiththeNICA−Donnanspeciationmodel.DASdatawereobtainedforvaryingtotalMgconcentrations,pHsfrom5.0to11.0andionicstrengthsfrom0.001to0.3molL−1.DASresultsdemonstratedtheexistenceofstronginteractionsbetweenmagnesiumandNOMatallexaminedconditionsanddemonstratedthatthebindingofMg2+byNOMwasaccompaniedbythereplacementofprotonsintheprotonation-activephenolicandcarboxylicgroups.Theslopeofthelog-transformedabsorbancespectraofNOMintherangeofwavelength350−400nmwasfoundtobeindicativeoftheextentofMg2+−NOMbinding.ThedifferentialandabsolutevaluesofthespectralslopeswerestronglycorrelatedwiththeamountofNOM-boundMg2+ionsandwiththeconcentrationsofNOM-boundprotons.■INTRODUCTIONMagnesiumhasmanyimportantrolesinabioticandbiochemicalenvironmentalprocessesandisanessentialnutrientaffectingmanyphysiologicalfunctions.1−3Intheenvironment,interactionsbetweenmagnesium,otherhardnesscations,notablycalciumand,ontheotherhand,dissolvedorsoilorganicmatterandmineralsurfacesareaccompaniedbytheformationofsolubleandsorbedcomplexesandavarietyofsolidphases.Theseinteractionsalsoaffectthespeciationofmanyheavymetalsandotherprocesses,forinstancetheformationoffoulingscalesonmembranesurfaces,removalofnaturalorganicmatter(NOM)inwatersoftening.4−11Duetoitsimportanceinbiologicalprocesses,interactionsofMg2+withmacromoleculesinvolvedinbioticprocesseshavebeenexaminedinconsiderabledetail.3,12−15Elucidationofthemicroscopicaspectsofsuchinteractionscanbeachievedusingthestructure-sensitivemethodsofNMRorX-rayabsorbancespectroscopy(XAS)thatcanprovideimportantdetailsconcerningthenumberandchemicalnatureoftheatomssurroundingamagnesiumatom.16−20However,thesemethodsrequireusingrelativelyhighconcentrationsofthetargetmetalandtheinterpretationoftheirresultsmaydependontheselectionofrepresentativemodelcompounds.8,21−23Inves-tigationofinteractionsbetweenMg2+andNOMisalsocomplicatedbythefactthatNOMisasite-specificpolydisperseentity10,11thathasmultipletypesofproton-andmetal-bindinggroups.MechanismsofbindingofMg2+byNOMhavenotbeenunambiguouslyascertained.SelectedpriorstudieshavesuggestedthatCa2+andMg2+arelikelytobecoordinatedprimarilythecarboxylgroupsofNOMandcontributionsofthephenolicgroupsmaynotbeasimportant.8,11Ca2+hasbeenpositedtointeractmorestronglywiththenegativelychargedcarboxylgroupsofNOMthanMg2+.24BecausethebindingofMg2+byNOMwasdeemedtobemediatedprimarilybynonspecificDonnanelectrostaticinteractions,theconcentra-tionofMg2+boundontoNOMwasexpectedtobenegligiblecomparedwiththatofCa2+.8,24RecentdevelopmentsinMgisotopegeochemistrydemonstratetheoccurrenceofpro-Received:January1,2015Revised:April22,2015Accepted:June19,2015Published:June19,2015Articlepubs.acs.org/est©2015AmericanChemicalSociety8323DOI:10.1021/acs.est.5b00003Environ.Sci.Technol.2015,49,8323−8329nouncedfractionationamongitisotopesandtheseobservationsrequirethatamoredetailedunderstandingofMginteractionswithNOMandsuspendedmaterialsbeavailabletointerpretthefieldfractionationdata.6SomeofthechallengesinelucidationthenatureandextentofthebindingofMg2+byNOMareassociatedwiththelimitationsofmethodsavailableforprobingtheseinteractions,preferablyatenvironmentallyrelevantconcentrationsofMg2+andNOM.AsopposedtotheextensivedataobtainedusingCa2+-selectiveelectrodes(ISE),10Mg2+−specificISEhavenotbeensuccessfullyusedtoexamineNOM−Mg2+interac-tions,25−27apparentlyduetothelimitationsoftheirperform-ance.StabilityconstantsderivedtoaccountforMg2+-complex-ationingeochemicalmodels,forexample,NICA−DonnanModelandModelVhavebeenderivedviatheanalysisoftheacid−basetitrationcurvesgeneratedintheabsenceandpresenceofMgCl2.9−11,28−30TheseandrelatedexperimentswerecarriedoutusingsolutionsinwhichtheconcentrationsofMg2+andhumicsubstanceswereseveralordersofmagnitudehigherthanthoseinenvironmentalsystems.28−30ThedataobtainedinthesestudiesforMg2+−NOMinteractionsarelimited,andthemodelparameterssuggestedinpriorresearchremainlargelyuntested.9−11ThisissueofthenatureofMg2+−NOMinteractionscanbeexploredinmored