分子量和化学性质对NOM光谱的的影响

EffectsofMolecularWeightDistributionandChemicalPropertiesofNaturalOrganicMatteronGoldNanoparticleAggregationStaceyM.Louie,†,‡RobertD.Tilton,†,§,⊥andGregoryV.Lowry*,†,‡,§†CenterfortheEnvironmentalImplicationsofNanoTechnology(CEINT),‡DepartmentofCivilandEnvironmentalEngineering,§DepartmentofChemicalEngineering,and⊥DepartmentofBiomedicalEngineering,CarnegieMellonUniversity,Pittsburgh,Pennsylvania15213,UnitedStates*SSupportingInformationABSTRACT:Thecomplexityofnaturalorganicmatter(NOM)motivatesdeterminationofhowspecificcomponentsinaNOMmixtureinteractwithandaffectnanoparticle(NP)behavior.TheeffectsoftwoSuwanneeRiverNOMfractions(separatedbya100,000g/molultrafiltrationmembrane)ongoldNPaggregationarecompared.Theweight-averagemolecularweight,Mw,fortheunfractionatedNOMwas23,300g/mol,determinedbysizeexclusionchromatographywithmultianglelightscattering.TheNOMwascomprisedof∼1.8wt%of100,000g/molretentate(NOMr,Mw=691,000g/mol)and98wt%offiltrate(NOMf,Mw=12,800g/mol).TenppmofNOMrprovidedsignificantlybetterNPstabilityagainstaggregationthan10ppmofNOMfin100mMNaClduetostericeffects.IntheunfractionatedNOM,therelativeimportanceofthetwocomponentswasconcentration-dependent.ForalowconcentrationofunfractionatedNOM(10ppm),bothfractionscontributedtotheNOMeffects;forahighconcentration(560ppm),NPstabilitywascontrolledbythesmallamount(10ppm)ofNOMrpresent,ratherthanthehigheramount(550ppm)ofNOMf.Therefore,largehumicaggregatesinaheterogeneousNOMsamplecanhavedisproportionatelystrongeffects,andcharacterizationofMwdistributions(ratherthanaverageMw)mayberequiredtoexplainNOMeffectsonNPbehavior.■INTRODUCTIONTheincreasingmanufactureofengineerednanoparticles(NPs)andtheiruseinconsumerproductshasraisedconcernsovertheirpotentialriskstohumanhealthandtheenvironment.Nanomaterialsofinterestincludemetalormetaloxidenanoparticles,fullerenes,andcarbonnanotubesaswellasmoresophisticated“active”nanostructures.1Nanoparticlesarelikelytobecoatedwithorganicmacromolecules,eitherintentionallyaddedbymanufacturersfordesiredproperties(e.g.,polymersforimprovedstabilityordrugdelivery)orincidentallyobtainedduringtheNPlifecycle(e.g.,naturalorganicmatterorproteins).MacromolecularcoatingscansignificantlychangeNPfateandtransport,bioavailability,andtoxicity.2−4Naturalorganicmatter(NOM)isofparticularinterestbecauseitislikelytobeencountereduponreleaseofNPsintotheenvironment.NOMcomprisesahighlyheterogeneousmixtureofcomponentswithvariousmolecularweightsandchemicalproperties.5,6ThesourceoftheNOMsampleandcollectionorfractionationmethoddetermineitscompositionandeffectsoncolloidorNPbehavior.NOMtypesaretypicallydefinedoperationally−forexample,theInternationalHumicSubstancesSociety(IHSS)collects“NOM”byreverseosmosis,whereas“humicacid”(HA)and“fulvicacid”(FA)areextractedonanXADresinanddistinguishedbysolubilityatacidicpH.Otherdescriptorsandacriticalreviewofvariabilityincollectionandpreparationmethodshavebeenpublished.5ThephysicalstructureofNOMisalsocomplex.Asupramolecularmodelhasbeenproposed,inwhichsmallercomponentsformimperma-nentaggregatesheldtogetherbyhydrogenbonding,cationbridging,dipole−dipole,andhydrophobicinteractions.7NOMstructure(e.g.,aggregationstate7−10andfractaldimen-sion11,12),andhenceapparentsizeormolecularweight,canchangewithconcentrationandsolutionchemistry(pH,ionicstrength,divalentcations).GiventhecomplexityinherentinNOMsamplesandvariabilityamongexperimentalprotocols,itisdesirabletounderstandhowspecificcomponentsinaNOMmixtureinteractwithNPsandaffectNPbehavior.Theimportanceofspecificchemicalmoietiesorsizefractionshasbeendemonstratedinsoilscience(adsorptionontosoilormineralsurfaces13−15)andotherenvironmentalandengineeredprocesses,includingcolloidorNOM-mediatedpollutanttransport(bindingofdissolvedmetalsororganicpollu-tants16,17),drinkingwatertreatment(e.g.,disinfectionby-productformation18),andmembranefouling.19ForNPfateandtransport,specificpropertiesofadsorbedmacromoleculesReceived:January9,2013Revised:April2,2013Accepted:April3,2013Published:April3,2013Articlepubs.acs.org/est©2013AmericanChemicalSociety4245dx.doi.org/10.1021/es400137x|Environ.Sci.Technol.2013,47,4245−4254(layerthicknessanddensity,charge,solvency,molecularweight)areexpectedtodetermineimportantstericorelectrostericforcesaffectingaggregationanddeposition,asdescribedinextendedDLVOtheory.20−22However,environ-mentalnanotechnologystudiesoftenavoidexplicitconsid-erationoftheheterogeneityoftheNOMused.BulkNOMtypicallyreducesNPaggregationordepositionontomineralsurfaces.6,23,24UsingvarioustypesofNOM,thestabilityofZnSNPswasshowntocorrelatewitharomaticcontentandaveragemolecularweight.25EnhancedNPaggregationbyNOMisalsopossibleviachargeneutralization,26−28bridging(especiallyinthepresenceofdivalentcations),29−31orhydrophobicinteractions.32ChangesintheaggregationstateandfractalcharacteristicsofNOM-NPaggregatesuponremoval/read-ditionofNOMhavealsobeenshown.33,34However,quantitativeinterstudycomparisonisconfoundedbythevariabilityacrossNOMsamplesandlackofextensivecharacterizationofheterogeneousNOM,5forwhichaverageorbulkcharacteristicsmaybeinadequate