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102AENVIRONMENTALSCIENCE&TECHNOLOGY/MARCH1,20032003AmericanChemicalSocietyEnvironmentalTechnologiesattheNanoscaleNanotechnologycouldsubstantiallyenhanceenvironmentalqualityandsustainabilitythroughpollutionprevention,treatment,andremediation.
TINAMASCIANGIOLIAAASENVIRONMENTALSCIENCE&TECHNOLOGYPOLICYFELLOWWEI-XIANZHANGLEHIGHUNIVERSITYMARCH1,2003/ENVIRONMENTALSCIENCE&TECHNOLOGY103AMaintainingandimprovingsoil,water,andairqualityrepresentsomeofthemostformidablechallengesfacingglobalsocietyinthe21stcen-tury.
Pollutantsfromsuchdiversesourcesasoilandchemicalspills,pesticideandfertilizerrunoff,abandonedindustrialandminingsites,andair-bornegaseousandparticulatematterfromautomobilesexacerbatethesituationonadailybasis.
Detectingandtreatingexistingcontaminantsandpreventingnewpollutionareamongthechallenges.
Theaggregatefinancialburdenforim-provingair,water,andsoilqualityistrulystaggering(1).
Inlightoftheseenor-mousandcomplexchallenges,itisperhapsironicthatoneprospectivesolutionisdiminutiveinsizebutimmenselypowerfulincapacity.
DEEPAKSRIVASTAVA/NASAEnternanotechnologytherevolutionaryscienceandartofmanipulatingmatterattheatomicormolecularscalethathascutacrosssuchdisci-plinesaschemistry,physics,biology,andengi-neering(2).
Despitealargelyunproventrackrecordintheenvironmentalarena,nan-otechnologyoffersgreatpromisefordeliv-eringnewandimprovedenvironmentaltechnologies.
However,proliferationofnanotechnologycouldalsoleadtonewenvironmentalproblems,suchasnewclass-esoftoxinsorrelatedenvironmentalhazards.
NanotechnologyoverviewNanotechnologyrefersbroadlytousingmaterialsandstructureswithnanoscaledimensions,usuallyrang-ingfrom1to100nanometers(nm).
Perhaps,withoutrealizingit,wealreadyencountersomelikenessofnanotechnologyindailylife.
Forexample,proteina-ceousmoleculesinlivingorganismsfrombacteriatobeetlestohumansserveas"molecularmotors"todriveeverythingfromflagellarmotiontomuscleflex-ion.
Nanometer-sizedparticleshavebeendevelopedtoimprovethemechanicalpropertiesoftires,initi-atephotographicfilmdevelopment,andserveasvitalcatalystsinthepetrochemicalindustry.
Tosomeextent,environmentalscientistsanden-gineersarealreadyworkingwithnanoscalestruc-tures(3).
Naturalweatheringofminerals,suchasironoxidesandsilicates,andmicroorganisms,suchasbacteriaandalgae,producenanoscalecolloids,whichincludedispersionsofnanosized-particlesinmediawithspecialpropertiesthatcanbeimportantinthefate,transport,transformation,andbioavail-abilityofenvironmentallyharmfulsubstances.
Anthropogenicandnaturalcolloidsofsolids,liquids,andgasesinliquids(sols,emulsions,andfoams,re-spectively),andsolidsandliquidsingases(smokesandfogs,respectively)arecommonlyencounteredintheenvironment.
However,nanotechnologyisnotjustaboutthesizeofverysmallthings.
Moreimportant,itisaboutstruc-tureandtheabilitytowork—observe,manipulate,andbuild—attheatomicormolecularlevel.
Thisre-sultsinmaterialsandsystemsthatoftenexhibitnovelandsignificantlychangedphysical,chemical,andbi-ologicalpropertiesduetotheirsizeandstructure.
Thesenewproperties,whichwewillelaborateonlaterinthisarticle,includeimprovedcatalysis(4),tunablewavelengthsensingability(5),andincreasedmechanicalstrength(6).
Thebasicstructuresofnanotechnologyincludenanoparticlesornanocrystals,nanolayers,andnano-tubes.
Thesenanostructuresdifferinhowtheyaremadeandhowtheiratomsandmoleculesareor-dered.
Ananoparticle—acollectionoftenstothou-sandsofatomsmeasuringabout1–100nminaggregatediameter—isthemostbasicstructureinnanotechnology.
Suchnanoparticlesarecreatedatombyatom,sothesize,andoftentheshape,ofaparti-cleiscontrolledbyexperimentalconditions.
Theseparticlescanalsobedescribedasnanocrystalsbe-causetheatomswithintheparticlearehighlyor-dered,orcrystalline.
Nanostructures,suchasthoseshowninFigure1,areoftenarrangedorself-assembledintohighlyorderedlayersarisingfromhydrogenbonding,dipo-larforces,hydrophilicorhydrophobicinteractions,gravity,andotherforces(7).
Manynaturallyoccur-ringbiologicalstructures,likemembranes,vesicles,andDNA,formbecauseofsuchself-assembly.
Repeatingstructureswithatailoredperiodicityareessentialinmanyapplicationsofnanotechnology,suchasphotonics,catalysts,andmembranes.
Understandingandbuildingnanostructuresthroughself-assemblyisthecoreofthenanotechnologycre-ationprocess(8).
Theextraordinarymechanical,electronic,andchemicalcharacteristicsofnanostructurescalledcar-bonnanotubes,whichwerediscoveredbySumioIijimain1991,havecapturedtheimaginationofsci-entistsandengineers(9).
AsshowninFigure2,nano-tubesgenerallyconsistofhexagonallatticesofcarbonatomsarrangedspirallytoformconcentriccylinders(10).
Thetubesarenearlyperfectcrystalsandthin-nerthangraphitewhiskers.
Single-wallnanotubes(SWNTs)haveadiameterofapproximately1.
4nm,andmultiwallnanotubes(MWNTs)consistofbe-tween2and30concentrictubesthatformanouterdiameterof30–50nm.
Nanotubesrangeinlengthfromafewtensofnanometerstoseveralmicrometers.
Nanotubeshavecharacteristicsthatleadtovari-ousapplications(11).
Forexample,nanotubesarestrongerthansteelbutveryflexibleandlightweight,whichmakesthemasuitablematerialforbothsuper-strongcableandtipsforscanningprobemicro-scopes.
Inadditiontotheremarkablemechanicalproperties,nanotubescouldpotentiallyreplacecop-perasanelectricalconductoror,withasmallchangeinstructure,replacesiliconasasemiconductor.
Nanotubesalsotransferheatbetterthananyotherknownmaterial.
Researchersareexploringtheuseofnanotubesindrugdeliveryandmedicaldiagnosticapplications.
SeveraldifferentcompaniesintheUnitedStates,Japan,andEuropearetestingproductionofnano-tubesonapilotscale.
However,atseveralhundreddollarspergram,nanotubesarestilltooexpensiveformostapplications.
PollutionpreventionPollutionpreventionrefersto"sourcereduction"andotherpracticesthatefficientlyuserawmaterials,en-ergy,water,orotherresourcestoreduceoreliminatecreationofwaste.
Thisstrategyalsoincludesusinglesstoxicandrenewablereagentsandprocessingma-terials,wherepossible,andtheproductionofmoreenvironmentallybenignmanufacturedproducts.
Nanotechnologycouldplayakeyroleinpollutionpreventiontechnologies.
Forexample,nanotechnol-ogy-basedhomelightingcouldreduceenergycon-sumptionbyanestimated10%intheUnitedStates,saving$100billionannuallyandreducingcarbonemissionsby200milliontonsperyear(12).
FIGURE1Nanostructures(a)SiIiconnanowiresthatdetectpH,(b)carbonnanotubes,(c)smallorganicmolecules,and(d)biomoleculesareexamplesofnanoscalematerials,devices,andcircuitsthatcouldbeusedforpollutantsensinng,prevention,andtreatment.
NNNNZnSSSSSSSSSSSiO2SourceDrainSiOSiOOHOSiSiSiSiNH2HOSiNWSiOSiOOSiSiSiSiNH2SiNWSiOSiOOHOSiSiSiSiNH3HOSiNW+–O–O–H++H+–H++H+Nanowire(a)(b)(c)(d)Source:Figure1awasadaptedwithpermissionfromReference(28),andFigures1b–dwereadaptedwithpermissionfromReference(37).
104AENVIRONMENTALSCIENCE&TECHNOLOGY/MARCH1,2003Nanostructuredcatalysts,forexample,canmakechemicalmanufacturingmoreefficientbyprovidinghigherselectivityfordesiredreactionproducts(13).
Forexample,aluminosilicatemolecularsieves(zeo-lites)areporouscrystallinesolidswithwell-definedstructureswidelyusedforseparationsandcatalysis.
Nanometer-sizedzeolites(10–100nm)arebeingde-velopedtoselectivelyoxidizehydrocarbons,suchastoluenetobenzaldehyde(14).
Usingnanostructuredzeolitesmakesthisexamplemoreenvironmentallybenignfortworeasons.
First,theoxidationreactionisinitiatedbyvisiblelight,whichreducesenergycon-sumption.
Second,usingvisiblelightaccesseslow-en-ergyreactionpathwaysthathelpeliminatewastefulsecondaryphotoreactionsandincreasetheyieldofthedesiredproduct.
Inthisstudy,selectivityforbenz-aldehydeusingthenanostructureswas87%,com-paredtolessthan35%forthesamereactionwithconventionalzeolitematerial.
Assemblingnanostructuresfrombiopolymersorbio-inspiredmaterialsisanexampleofanenviron-mentallybenignapproachtofabricatingmicroelec-tronics.
Accordingtorecentcalculations,1.
7kgoffossilfuelandchemicalsand32kgofwaterarerequiredtoproduceasingle2-gram,32-megabytemicrochip(15).
Biomolecularnanolithographyrep-resentsa"bottom-up"approachtoreplacecurrentsemiconductorchipproductionmethods(16).
Nanoscalemetalparticlesonabiopolymer(poly-L-lysine)templateorscaffoldarestretchedoutonasurfaceintowell-definedchiparchitectures,suchaslinesandgrids,atroomtemperature.
Biodegradablematerialslikepoly-L-lysineareusedforconstructionofthesestructures.
Nanotechnologyapplicationscouldalsohelpcre-atebenignsubstancesthatreplacecurrentlyusedtoxicmaterials.
Forexample,nontoxic,energy-effi-cientcomputermonitorsarereplacingthosemadeofcathoderaytubes(CRT),whichcontainmanytoxicmaterials(17).
Newerliquidcrystallinedisplaysaresmaller,donotcontainlead,andconsumelesspowerthanCRTcomputermonitors(17).
Usingcarbonnan-otubesincomputerdisplaysmayfurtherdiminishtheenvironmentalimpactsbyeliminatingtoxicheavymetalsanddrasticallyreducingmaterialandenergyuserequirements,whileprovidingenhancedperfor-manceforconsumerneeds.
Fieldemissiondisplays(FEDs)thatusecarbonnanotubesarethelatestde-velopmentindisplaytechnologyandmaybecom-merciallyavailablefromseveralmanufacturerswithinthenextyear(18).
Althoughthetoxicityofcarbonnanotubesislargelyunknown,theamountofcarbonnanotubesusedinFEDsisquitesmall(0.
5g/moni-tor)comparedtothekilogramsofleadinCRTs(17).
TreatmentandremediationTheearlyimpactofnanotechnologyresearchhasbeenmostlyinremediationandend-of-pipetreatmenttechnologies.
Variousreportshaveappearedontheuseofavarietyofnanoparticlesfortreatmentandre-mediationofpollutantsintheenvironment.
Forex-ample,nanoparticlesofvariousoxidants,reductants,andnutrientshavebeensuggestedasusefulforpro-motingcontaminanttransformationandstimulatingmicrobialgrowth,becausetheirsmallsizeandlargersurfaceareamakethemmorereactiveandmoreflex-ibleintermsofdeployment.
Nanoparticlescanalsoexhibituniquechemicalreactivitynotobservedforlargerparticlesbecauseoftheirunusualcrystalshapesandlatticeorder.
Forexample,conventionalmethods(e.
g.
,bioremediationandzero-valentiron)forinsituremediationofchlorinatedorganicsolvents,suchastrichlorethylene,tendtoproduceundesirablebyprod-MARCH1,2003/ENVIRONMENTALSCIENCE&TECHNOLOGY105AFIGURE2Carbonnanotubescanbespecialized(a)Nanotubesarestrongerthansteelbutveryflexibleandlightweight.
Theseexceptionalmechanicalcharacteristicsmakethemsuitableas"tips"forscanningprobemicroscopesorenforcingfibersfornanotube–polymercompositematerials.
(b)Dependingontheconstruction,nanotubescanbehavelikeelectricalconductorsorsemiconductors,whichmakesthemextremelyusefulfornanoscaleelectronicsapplications.
Theelectroniccharacteristicsofcarbonnanotubesarestronglycoupledtothemechanicalpropertiesandprovidegreatopportunitytodevelopnovelelectromechanicalnanodevices(11).
(c)Variationsinnanotubecompositionallowfortuningelectrontransportpropertiesoverawiderange.
Nanotubescanalsobeconstructedfrommaterialssuchasboron,carbon,andnitrogen(Bx-Cy-Nz)thatarestructurallysimilartothoseconsistingonlyofcarbon.
Theseheteroatomicnanotubesareexpectedtobeinsulators,unlikecarbonnanotubes(38).
Source:DeepakSrivastava/NASA.
(a)(b)(c)ucts,suchasdichloroethylenesandvinylchloride.
Usingnanoscalebimetallicparticlesessentiallyelim-inatesalltheundesirablebyproducts(19,20).
Nanoparticlesthatareactivatedbylight,suchasthelargeband-gapsemiconductorstitaniumdioxide(TiO2)andzincoxide(ZnO),continuetobestudiedfortheirabilitytoremoveorganiccontaminantsfromvariousmedia.
Theseparticlesarereadilyavailable,inexpensive,andhavelowtoxicity(21,22).
Recently,ZnOnanoparticleswereshowntoactasbothasen-sorandphotocatalystfortreatmentofchlorinatedphenols(22).
Researchersarealsoquiteinterestedinmanipulatingthesurfaceofnanoparticleswithor-ganicorinorganicdyestoextendtheirphotoresponsefromUVtovisiblelight,makingthemevenmoreefficientasphotocatalystsforthetransformationofenvironmentalcontaminantsbecauseUVlightrep-resentsonly5%ofthesolarspectrum(23).
Nanoparticlescouldprovideenormousflexibilityforinsituremediationaswell.
Forexample,nanopar-ticlesareeasilydeployedinexsituslurryreactorsforthetreatmentofcontaminatedsoils,sediments,andsolidwastes.
Alternatively,theycanbeanchoredontoasolidmatrixsuchascarbon,zeolite,ormembraneforenhancedtreatmentofwater,wastewater,orgaseousprocessstreams(24).
Directsubsurfacein-jectionofnanoscaleironparticles,whetherundergravity-feedorpressurizedconditions,hasalreadybeenshowntoeffectivelydegradechlorinatedor-ganics,suchastrichloroethylene,toenvironmental-lybenigncompounds(19).
Thetechnologyalsoholdsgreatpromiseforimmobilizingheavymetalsandra-dionuclides.
Researchhasdemonstratedthatnanoscale,bimetallicparticles,suchasiron/palladium,iron/sil-ver,orzinc/palladium,canserveaspotentreductantsandcatalystsforalargevarietyofcommonenviron-mentalcontaminantssuchasPCBs,organochlorinepesticides,andhalogenatedorganicsolvents(4).
Nanoscalemetallicparticlesreducedvirtuallyallchlo-rinatedhydrocarbonstestedtobenignhydrocarbons.
Additionally,ampleevidenceindicatesthatiron-basednanoparticlescanbeusedtoreducemanyotherrecalcitrantcontaminants,includinganions(perchlorate,nitrate,anddichromate),heavymetals(nickelandmercury),andradionuclides(uraniumdioxide).
Nanotubeshavebeensuggestedas"asu-periorsorbent"fordioxins(25).
Thesorp-tionenergyofdioxinoncarbonnanotubesisal-most3timesthatofac-tivatedcarbon.
TheLangmuircon-stant(B),apara-metercharacterizingthesorptionaffinity,ismanyordersofmagnitudehigherfornanotubesthanforactivatedcarbon.
Thus,theprospectofusingcarbonnanotub-esforairandwaterpollutioncontrolappearstobeveryfavorable,butlarge-scaleapplicationsofnano-tubesinthenearfuturearelimitedbycostandavailability.
Anotherexampleofenvironmentaltreatmentandremediation-relatedapplicationofnanomaterialsin-cludesusingdendriticnanoscalechelatingagentsforpolymer-supportedultrafiltration(PSUF)(26).
Den-drimersarehighlybranchedpolymerswithcontrolledcompositionandanarchitecturethatconsistsofnanoscalefeatures.
Thesenanostructurescanbede-signedtoencapsulatemetalionsandzero-valentmet-als,enablingthemtodissolveinsuitablemediaorbindtoappropriatesurfaces.
Thisapproachmaypre-sentameanstoproduceafunctional,cost-effectiveand,dependingonthecompound,environmentallysoundmaterialforPSUF.
PollutionsensinganddetectionRapidandprecisesensorscapableofdetectingpol-lutantsatthemolecularlevelcouldgreatlyenhanceourabilitytoprotecthumanhealthandtheenviron-ment.
Manufacturingprocesscontrol,compliance,ecosystemmonitoring,andenvironmentaldecisionmakingwouldbesignificantlyimprovedifmoresen-sitiveandlesscostlytechniquesforcontaminantde-tectionwereavailable.
Ofparticularinterestareremote,insitu,andcontinuousmonitoringdevicescapableofyieldingreal-timeinformation,andalsothosethatcandetectpollutantsatverylowconcen-trationlevels.
Recently,chemicalsensorsbasedonindividualSWNTshavebeendemonstrated(27).
Alteredelec-tricalresistanceofsemiconductingSWNTsthroughexposuretogaseousmolecules,suchasnitrogendioxideorammonia,formsthebasisforthesesen-sors.
Thenanotubesensorsalsoexhibitedfastre-sponsesatroomtemperaturetothegasesandasubstantiallyhighersensitivitythanexistingsolid-statesensors(27).
Boron-dopedsiliconnanowires(SiNWs)havebeenusedtocreatehighlysensitive,real-timeelec-tricallybasedsensorsforbiologicalandchemicalspecies(28).
TheSiNWscouldbemodifiedinvariouswaystoprovidedifferentfunctionalities.
Specifically,theamine-andoxide-functionalizednanowireswereusedtodetectchangesinpH,andbiotin-modifiedSiNWswereusedtodetectstreptavidininthepicomolarcon-centrationrange.
Inaddition,anti-gen-functionalizedSiNWsshowedreversibleantibodybind-ingandconcentration-dependentdetectioninrealtime.
Detectionofthereversiblebindingofthemetabolicindi-catorcalciumwasalsodemonstrated.
Thesemechanismscouldapplytode-106AENVIRONMENTALSCIENCE&TECHNOLOGY/MARCH1,2003MARCH1,2003/ENVIRONMENTALSCIENCE&TECHNOLOGY107Atectingpathogensandchemicalandbiologicalagentsinwater,air,andfood.
Humanhealthandecologicaldiagnosiscouldalsobenefitfromsensors.
Forinstance,nanobarcodes—highlyencodableandchemicallydiversenanoparti-cles—mayallowforhigh-levelmultiplexingofsmallsamplevolumes(29).
Evaluatingtheexposureofcom-plexsystemsintheenvironmenttotoxicpollutantsoftenhassuchrequirementsforsampleanalysis.
Nanobarcodescangreatlyenhanceourabilitytoiden-tifythesourceandstrengthofcontaminants,deter-minetherouteandmechanismofenvironmentalfateandbioavailability,andassesstheeffectivenessoftreatmentandremediationtechniques.
NanotechnologypitfallsNanotechnologyisarevolutionaryscientificandengineeringventurethatwillinvariablyimpacttheexistinginfrastructureofconsumergoods,manufac-turingmethods,andmaterialsusage.
Notsurpris-ingly,thepotentialbenefitshavedominatedscientificandmassmediacoverageofnanotechnology.
Butanytechnologycanbeadouble-edgedsword.
Environ-mentalandsafetyconcernsaboutnanotechnologyonlyrecentlyhavebeendiscussedinthemainstreammedia(30).
Wearealreadywitnessingsomeprecursorsofnanotechnology-associatedpollution:toxicgalliumarsenideusedinmicrochipsenterslandfillsinin-creasingquantitiesasmillionsofcomputersandcel-lularphonesaredisposedofeveryyear.
Potentiallyharmfuleffectsofnanotechnologymightariseasaresultofthenatureofnanomaterialsthemselves,thecharacteristicsoftheproductsmadefromthem,ortheaspectsofthemanufacturingprocessinvolved.
Thelargesurfacearea,crystallinestructure,andre-activityofsomenanoparticles,forinstance,mayfacilitatetransportoftoxicmaterialsintheenviron-ment,orthesizeandchemicalcompositionofnano-structuresmayleadtobiologicalharmbecauseofthewaytheyinteractwithcellularmaterials(31)(seeFigure3).
Forexample,ifnanostructurescanself-as-sembleinthelaboratory,cantheyreplicateintheenvironmentIfso,whatwillbethefateofthosenanostructuresandtheirenvironmentalandhealthimpactsInaddition,itispossiblethatnanotech-nologycouldleadtosocietalchangesthatinfluencetransportation,urbandevelopment,informationmanagement,andotheractivitiesofoursocietythatdirectlyorindirectlyaffectthequalityoftheenvi-ronment.
Becausenanotechnologyisunlikelytobethefirstentirelybenigntechnologyadvance,thereisanurgentneedtoevaluatetheeffectivenessofcur-rentwaterandairtreatmenttechniquesforthere-movalandcontrolofpotentialnanoscalepollution.
TheU.
S.
NationalNanotechnologyInitiativehasalreadybegunexploringthepotentialenvironmen-talandsocietalimplications(2,32).
Inthefallof2001,RiceUniversityopenedtheCenterforBiologicalandEnvironmentalNanotechnology(CBEN),whichisFIGURE3NanotechnologycouldpollutemanymediaBecausenanoscalematerialsareinthesamesizerangeashemoglobinandvirusesandareevensmallerthancommonirritants,suchasparticulatematter(2m)Microbialcells(~1m)Virus(10–100nm)Hemoglobin(7nm)H2O(0.
2nm)100m10m1m100nm10nm1nm0.
1nmPollens(10–100m)PM2.
5(aerosols)Bacteriophage(80nm)Source:KenRaniere,LehighUniversity.
sponsoredbytheNationalScienceFoundation.
OneofthepriorityareasofCBENistoprobethebehav-iorofnanomaterialsintheenvironment(33).
In2002,theU.
S.
EPAannounceditsfundingplanforexter-nalresearchthatspecificallyaddressesthepotentialbeneficialapplicationsandenvironmentalimplica-tionsofnanotechnology(34).
However,currentfederalfundingforresearchanddevelopmentinthisareaisinadequate.
Theproposedfundingforstudyingnanoscaleprocessesintheen-vironmentaspartoftheNationalNanotechnologyInitiativeinthefiscalyear2003budgetrequestbytheBushadministration,forexample,isaround$20mil-lionoutofatotalinvestmentofapproximately$710million—amere2.
9%ofthetotalinitiative(35).
Theresponsibilitytorecognizeandaddressmanyofthepotentiallyadverserepercussionsisinthehandsofresearchersandenvironmentalprofessionals.
Asnewnanoscalestructuresandtechnologiesareex-ploredanddeveloped,areresearchersconsideringallofthepossibleenvironmentaloutcomesForexample,aretheremorebenignprecur-sormaterialsorsyntheticmethodstouseCanwerecoverthenano-particlesornanotubesforreuseorwilltheydegradeintoenvi-ronmentallybenignproductsNewnanoscalestructuresandotherpreviouslyde-scribedenvironmentalappli-cationsallsupportthepredictionthatnanoscalescienceandengineeringwilllikelyhaveaprofoundim-pactonscienceandtechnol-ogy(36).
However,wemustremainmindfulofthepoten-tialramificationsofthistech-nology,includingthefactthatnanoscalematerialscanenterthefoodchainandbeabsorbedortrans-portedbywaterandfood.
Bioavailabilityandtoxicityofnewlycreatednanoscalematerialsarelargelyunknown.
Nanotechnologyishigh-lyinterdisciplinaryandmaypresentfurtherchallengesforenvironmentalscientistsandengineers.
Fewstudentsarebeingtrainedinenvironmentalnanotechnology,andfewlaboratoriesarecurrentlyequippedtoconductre-searchinthisarea,detectnanopollution,anddevelopef-fectivetreatmentandremediationtechnologies.
AcknowledgmentW.
Z.
thanksU.
S.
EPAandtheNationalScienceFoundation(NSF)fortheirsupport.
However,thisarticlehasnotbeenreviewedbyeithertheEPAorNSFanddoesnotnecessarilyreflecttheirviews.
TinaMasciangioliisanAmericanAssociationfortheAdvancementofScienceEnvironmentalScienceandTechnologyPolicyFellowattheNationalCenterforEnvironmentalResearch,U.
S.
EPA,Washington,D.
C.
Wei-XianZhangisanassociateprofessorofenviron-mentalengineeringatLehighUniversity.
Addresscor-respondencetoZhangatwez3@lehigh.
edu.
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