TiO2有机无机杂化膜

Environ.Sci.Technol.2001,35,2388-2394

HybridOrganic/InorganicReverseOsmosis(RO)MembraneforBactericidalAnti-Fouling.1.

PreparationandCharacterizationofTiO2NanoparticleSelf-AssembledAromaticPolyamide

Thin-Film-Composite(TFC)Membrane

SEUNG-YEOPKWAK*ANDSUNGHOKIMHyperstructuredOrganicMaterialsResearchCenter(HOMRC)andSchoolofMaterialsScienceandEngineering,SeoulNationalUniversity,San56-1,Shinlim-dong,Kwanak-ku,Seoul151-744,KoreaSOONSIKKIM

SaehanIndustriesIncorporated,#14,Nongseo-Ri,

Kiheung-Eub,Yongin-City,Kyunggi-Do449-900,Korea

membraneunderUVillumination,comparedtothatofthesamemembraneindarkness,aswellasthosefortheneatmembranesundereitherlightcondition.

Introduction

Oneofthegoalsofresearchandindustryinthereverseosmosis(RO)membranefieldshasbeentoenhance,oratleastmaintain,waterfluxwithoutsacrificingsaltrejectionoveralongperiod,inordertoincreaseefficiencyandreducethecostofoperation.Nevertheless,themaindifficultyinaccomplishingthisgoalisfouling(1,2),whereaseriousfluxdeclineoccursastheactualoperationtimeelapses.AdetailedassessmentofthecostsofbiofoulingwasmadefortheROplantatWaterFactory21inOrangeCounty,CA(3).Accordingtothereport,themembranes,owingtotheadditionalhydraulicresistanceofthebiofoulinglayer,operateatabout150%oftheirinitialoperatingpressureover80%oftheirlife.Aregularamountofchlorineisaddedcontinuouslytothefeedwaterandmembranesarecleanedperiodically.Thebottomlineis$727,816spenteachyeartocontrolmembranebiofouling.Thatrepresentsabout30%ofthetotaloperatingcostsforthefacility.Theprincipaltypesoffoulingarecrystallinefouling(mineralscaling,ordepositofmineralsduetoanexcessofthesolutionproduct),organicfouling(depositionofdissolvedhumicacids,oil,grease,etc.),particleandcolloidfouling(depositionofclay,silt,particulatehumicsubstances,debris,andsilica),andmicrobialfouling(bio-fouling,adhesionandaccumulationofmicroorganisms,andformingbiofilms)(3).Variousapproachestoreducingfoulinghavebeenused,whichgenerallyinvolvepretreatmentofthefeedsolution,modificationofthemembranesurfaceprop-erties(suchashydrophobicorhydrophilicandelectro-negativeorelectropositive),optimizationofmodulear-rangementandprocessconditions,andperiodiccleaning(4).However,thesemethodsvarywidelyinapplicabilityandefficiency,therebyrequiringabreakthroughtosolvefoulingproblems.

ThemostcommonROmembraneusedforwatertreat-mentisthethin-film-composite(TFC)typecomposedofaromaticpolyamide.ParticularlyforsucharomaticpolyamideTFCmembranesfoulingfromtheformationofbiofilmonthesurfacecausedbymicroorganismshasbeenregardedasoftheuppermostimportance(5).Microorganismssuchasbacteriaandvirusesinwateradheretomembranesurfacesandgrowattheexpenseofnutrientsaccumulatedfromthewaterphase.Theattachedmicroorganismsexcreteextra-cellularpolymericsubstances(EPS)and,thus,formbiofilms(6).Ithasbeenreportedthatbiofilmformationwasrelatedtothedepletionofresidualdisinfectantconcentration,andthatnobiofilmwasformedfromdisinfectant-treatedwater,suchaschlorinatedwatercontainingaresidualof0.04-0.05mg/Lfreechlorine(7,8).However,itisnotedthatchlorina-tion,althougheffectiveforthedestructionofmicroorganisms,generatesharmfulbyproductssuchastrihalomethanesandothercarcinogens.Motivatedbytheseresults,thepresentpaperisaimedatdevelopingandcharacterizingahybridmembranepossessinginorganicnanoparticlescapableofkillingthemicrobeswithoutformingunwantedbyproductsasameansofprecludingtheformationofbiofilmsand,hence,reducingfouling.

Titaniumdioxide(TiO2)hasbeenthefocusofnumerousinvestigationsinrecentyears,particularlybecauseofits

*Correspondingauthorphone:+82-2880-6082;fax:+82-2876-6086;e-mail:sykwak-p@gong.snu.ac.kr.

10.1021/es0017099CCC:$20.00

?2001AmericanChemicalSociety

PublishedonWeb05/04/2001

Hybridorganic/inorganicreverseosmosis(RO)membranescomposedofaromaticpolyamidethinfilmsunderneathtitaniumdioxide(TiO2)nanosizedparticleshavebeen

fabricatedbyaself-assemblyprocess,aimingatbreakthroughofbiofoulingproblems.First,positivelychargedparticlesofthecolloidalTiO2weresynthesizedbyasol-gelprocess,andthediameteroftheresultingparticlesinacidic

aqueoussolutionwasestimatedtobe≈2nmbyanalyzingtheUV-visibleabsorptioncharacteristicswithaquantummechanicalmodeldevelopedbyBrus.Transmission

electronmicroscopy(TEM)furtherconfirmedtheformationofthequantum-sizedTiO2particles(～10nmorless).

TheTiO2particlesappearedtoexistinthecrystallographicformofanataseasobservedwiththeX-raydiffraction(XRD)patternincomparisonwiththoseofcommercial100%rutileandcommercial70:30%anatase-to-rutilemixture.Thehybridthin-film-composite(TFC)aromaticpolyamidemembraneswerepreparedbyself-assemblyoftheTiO2nanoparticlesonthepolymerchainswithCOOHgroupsalongthesurface.TheyshowedimprovedRO

performanceinwhichthewaterfluxevenincreased,thoughslightly.Field-emissionscanningelectronmicroscopy(FESEM)exhibitedtheTiO2nanoparticleswelladsorbedontothesurface.X-rayphotoelectronspectroscopy(XPS)demonstratedquantitativelythataconsiderableamountoftheadsorbedparticlesweretightlyself-assembledattheexpenseoftheinitiallossofthosethatwerelooselybound,andbecamestabilizedevenafterexposuretothevariouswashingandharshROoperatingconditions.TheantibacterialfoulingpotentialoftheTiO2hybridmembranewasexaminedandverifiedbymeasuringtheviablenumbersanddeterminingthesurvivalratiosoftheEscherichiacoli(E.coli)asamodelbacterium,bothwithandwithoutUVlightillumination.ThephotocatalyticbactericidalefficiencywasremarkablyhigherfortheTiO2hybrid

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FIGURE1.Schematicofthethin-film-composite(TFC)reverseosmosis(RO)membraneandthechemicalstructureofthearomaticpolyamidethin-filmlayer.

photocatalyticeffectsthatdecomposeorganicchemicalsandkillbacteria(9).TiO2photocatalysisisknowntogeneratevariousactiveoxygenspecies,suchashydroxylradical,hydrogenperoxide,etc.,byreductivereactionsoroxidativereactionsunderlight(10,11).Theseactiveoxygenspeciesfurtherdestroytheoutermembraneofthebacteriumcellsanddecomposetheendotoxinfromthem.Nanoscaletech-nologymanipulatesthingsonthenanoscale(generallyregardedas1-100nm)whichmakesitpossibletoarriveatfundamentallynewtypesofdeviceswithmuchimprovedpropertiesand/ornovelfunctionality(12).NanosizedTiO2particles,fromtheviewpointoftheirphotocatalyticcapabilitytobreakdownbacteriaandorganisms,willalsobeveryusefulbecauseoftheirhighsurfaceareaperunitvolumeandhighabrasiveresistancewhencoatedonthetargetmaterials(9).SeveraldifferentstrategiestointegrateTiO2withtargetmaterialshavebeenreported,whichincludeself-assemblymonolayeradsorptiononfunctionalizedsurfaces,sol-gelsynthesis,vacuumvaporization,sputtering,metalorganicchemicalvapordeposition(MOCVD),chemicalvapordepo-sition(CVD),andtheLangmuir-Blodgett(LB)method.Amongthem,themethodofself-assemblyofTiO2onsurfaces(forexample,single-crystalsilicon,quartz,andglasssub-strates),employingpolymerswith-CO2Hor-SO3Hfunc-tionalgroups,canbeperformedtofabricatemultilayerultrathinfilms,overcomingthelimitations(suchashightemperature,solventinvolvement,costlyfabrication,andcomplexprocesscontrol)inherentlyassociatedwithothermethods(13-15).Itisworthnotingthatthethin-filmactivelayerofaromaticpolyamideTFCROmembranesiscomposedofthecross-linkedformofthreeamidelinkagesandthelinearformwithpendantfreecarboxylicacidasshowninFigure1.Thefractionofthelinearcarboxylicacidformhasbeenestimatedtobe30%to50%,dependingontheinvestigators(16-18).Thus,itisprobabletoself-assembletheTiO2nanoparticlesonthearomaticpolyamideTFCmembranesurface,thereafterexpectingtheappearanceofanovelorganic/inorganichybridTFCmembraneforthephotocatalyticbactericidalanti-fouling.

Inthisstudy,quantum-sizedTiO2particlesarepreparedfromthecontrolledhydrolysisoftitaniumtetraisopropoxide(19)andthencharacterized.TheparticlesizeandcrystalstructureoftheresultingTiO2nanoparticlesarecharacterizedbyUV-visibleabsorptionspectroscopy,transmissionelec-tronmicroscopy(TEM),andX-raydiffraction(XRD).Intro-ductionoftheTiO2nanoparticleswithinthearomaticpolyamideTFCmembraneisperformedbywayofself-assemblyofthenanoparticlesthroughionicinteractionandH-bondingforcewith-COOHfunctionalgroupsofthearomaticpolyamide.ThemorphologicalstructuresoftheresultingTiO2self-assembledTFCmembraneareinvestigatedbyfield-emissionscanningelectronmicroscopy(FE-SEM),aswellasbyX-rayphotoelectronspectroscopy(XPS),whichanalyzestheatomicconcentrationsoftitanium.TheROperformancetestisthencarriedouttoseewhetheranyvariationofwaterfluxandsaltrejectionoccursinthepresenceoftheTiO2nanoparticlesontopoftheTFCmembranesurface.Thereafter,XPSanalysisofatomicconcentrationsofTiisalsoperformedwiththeRO-testedmembranetoevaluatethebindingdurabilityoftheself-assembledTiO2nanoparticles,evenaftertheharsh,actualoperationcondi-tions.Finally,toverifythephotocatalyticbactericidalca-pabilityofthisTiO2nanoparticleself-assembledTFCmem-brane,themembranesurfaceiscoveredbyamodelsuspensionofEscherichiacoli(E.coli)bacteriumcellsgrownaerobicallyinthenutrientbroth,whichisinturnilluminatedbyUVradiation.Then,thebactericidaleffectisobservedandconfirmedbycountingtheviablenumberofE.colicells.

ExperimentalSection

SynthesisandCharacterizationoftheNanosizedTiO2Particles.NanosizedTiO2colloidswerepreparedfromthecontrolledhydrolysisoftitaniumtetraisopropoxide,Ti(OCH-(CH3)2)4,byfollowingproceduresintheliterature(19).A1.25-mLsampleofTi(OCH(CH3)2)4(Aldrich,97%)dissolvedin25mLofabsoluteethanol(J.T.Baker)byinjectionwasaddeddropbydropundervigorousstirringto250mLofdistilledwater(4°C)adjustedtopH1.5withnitricacid.Thismixturewasstirredovernightuntilitwasclearandthetransparentcolloidalsuspension(1.34g/L)resulted.TiO2surfacesdissociativelyabsorbwatertoformsurfacehydroxylgroups,whicharebelievedtobetheactivesitesfortheadsorptionofreactants.ThesurfacepropertiesofTiO2particlesaredescribedbyacid-baseequilibriainvolvingsurfacehydroxylgroups.AstheisoelectricpointoftitaniumdioxidecorrespondedtopH)4.5-6.8(20),theresultingcolloidswouldtaketheshapeofstablecationicTiO2complexatpH1.5.

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TheUV-visiblespectrumofthetransparentTiO2colloidalsuspensionwasrecordedwithaHewlett-PackardHP8452diodearrayspectrophotometertoanalyzetheopticalab-sorptioncharacteristicsofTiO2and,thus,todeterminetheparticlesizestheoretically.TheparticlesizeswerealsodeterminedbyaJEOLJEM-200CXtransmissionelectronmicroscope(TEM)at120kV.FortheTEMobservation,theTiO2colloidalsuspensionwasdroppedonacarbon-coatedgridandthendriedatroomtemperature.ThecrystalstructureofTiO2wascharacterizedbyX-raydiffraction(XRD).XRDanalysiswasperformedonTiO2powdersampleswithaMACScienceX-raydiffractometer(MXP18X-MF22-SRA),operatinginthetheta-thetageometryusing18kWCuKR(λ)1.5418?)radiation.Forcomparisonpurposes,othercommercialTiO2particlessuchasSigma-AldrichrutileTiO2andDegussa-Hu¨lsP25TiO2werealsoanalyzedbyXRD.

PreparationofTiO2NanoparticleSelf-AssembledThin-Film-Composite(TFC)MembranesandMeasurementofTransportCharacteristics.Thethin-film-composite(TFC)membraneswerepreparedviainterfacialpolymerizationofm-phenylenediamine(MPD)intheaqueousphase(2wt%)andtrimesoylchloride(TMC)intheorganicphase(0.1wt%)onthepolysulfonesupportsreinforcedbythenonwovenfabricasschematicallydepictedinFigure1.ThepolysulfonelayeractsasthesupporttogivemembranesthemechanicalstrengthtoresistROpressure.Thethin-filmlayergovernstheactualseparationofthesoluteandthepassageofthesolvent.TheresultingaromaticpolyamideTFCmembranewasrinsedinasodiumcarbonatesolution(0.2wt%)andthenwashedwithdistilledwater.Thefinalmembrane,withanareaofca.50.0cm2,wasdippedinthetransparentTiO2colloidalsolutionfor1htodepositTiO2nanoparticlesonthemembranesurface,thenrinsedextensivelywithwater.Reverseosmosis(RO)performancetestswereconductedat225psiusing2000ppmNaClsolutionat25°Cwiththeapparatusofacontinuous-flowtype.Thewaterfluxwasdeterminedbydirectmeasurementofthepermeateflow:

FIGURE2.UV-visibleabsorptionspectrumofthedilutenanosizedTiO2colloidalsuspension.

Theelementalcompositionanalysiswasperformedoncarbon,nitrogen,oxygen,andtitanium,whichconstitutedthehybridmembraneaswellastheneatmembrane.Thesensitivityfactorsofindividualelementsweretakenwiththevaluesfromthestandardvisionlibraryprovidedbythemanufacturer,whichwerebasedonacombinationofphotoelectriccross-section,transmissionfunction,andinelasticmeanfreepath.

EvaluationofPhotocatalyticBactericidalEffectofTiO2Self-AssembledTFCMembrane.Escherichiacoli(E.coli)bacteriumcells(DH5Rstrain)weregrownaerobicallyin10mLofnutrientbroth(Luria-Bertanimedium)at37°Cfor12-16h.TheLuria-Bertani(LB)mediumwaspreparedwith1wtocto-tryptone,0.5wt%yeast,and1wt%NaCl.Thegrowncellswerecentrifugedat10000rpmfor1minanddilutedtoanappropriateconcentrationwithsterilizedwater.TheE.colicelldilution(150μL,total1.0×104cells)waspipettedontoeitheraTiO2hybridTFCmembraneoraneatmembrane,whichwereplacedinanincubatorataconstanttemperatureof37°C.Someoftheindividualmembraneswereilluminatedwithan8-Wblacklight(VWRUVLS-28)andsomewerenot.Thelightintensityatthepeakof365nmwas500μWcm-2at3in.,whichwasdeterminedbytheprocedureprovidedbythemanufacturer.Afterilluminationuptotheintendedexposuretime,thecellswerepipettedoutandcollectedin1.0wt%aqueoussodiumchloridesolution.ThecollectedsolutionswerespreadontoaLBagarplateandincubatedfor12-16htodeterminethenumberofviablecellsintermsofcolony-formingunits(CFU)asafunctionoftime.Theinitialcellnumberwasdeterminedtobe9420in150μLofcelldilutionsuspensionspreadontoaLBplatewithoutillumination.

Flux(gfd))

permeate(gallon)

membranearea(ft2)?time(day)

(1)

Thesaltrejectionwasmeasuredbythesaltconcentrationinthepermeateobtainedthroughmeasurementsoftheelectricalconductanceofthepermeateandthefeedusingaconductancemeter(Orionmodel162):

Rejection(%))1-

(permeateconductance

×100(2)

feedconductance)CharacterizationofMorphologicalandChemicalStruc-turesofMembraneSurface.ThesurfacetopologiesoftheTiO2nanoparticle-introducedaromatic-polyamide-TFCmem-branewereinvestigatedwithaPhilipsXL30FEGfieldemissionscanningelectronmicroscope(FESEM).Thesurfacemorphologyoftheneataromatic-polyamide-TFCmembranewasalsoexaminedandwascomparedwiththatoftheTiO2self-assembledversion.FortheFESEMobservation,themembranesampleswerecutintoappropriatesizesandthesurfaceswerecoatedwithplatinumorgoldbyasputter-coatingmachine.

X-rayphotoelectronspectroscopy(XPS)experimentswerecarriedoutwithaKratosAXISHSspectrometerusingaMgKRX-raysource(1253.6eV).TheX-raygunwasoperatedat10kVand1mA,andthechargeneutralizationsystemwasusedtoobtainhigh-resolutionspectrafortheinsulatingmaterials,suchaspolymers,byreducingthesurfacecharge.Thespectrumwasobtainedatthephotoelectrontakeoffangles(definedastheanglebetweenthedetectedphoto-electronbeamandthemembranesurfaces)of30°and90°togivesamplingdepthsofca.23?andca.45?,respectively.

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ResultsandDiscussion

ParticleSizeandCrystalStructureofSynthesizedTiO2.Figure2showstheUV-visiblespectrumoftheTiO2colloidalsolutionobtainedviasol-gelsynthesisoftitaniumtetra-isoproxide.Theonsetofabsorption(λos)andthecorre-spondingbandgapenergy(Eg)ofthebulkTiO2havebeendeterminedtobeλos)385nmandEg)3.2eVforanatase(21),andλos)415nmandEg)3.0eVforrutile(22),respectively.ThebandgapoftheTiO2colloidalsolutionismeasuredtobe3.44eV(361nm)accordingtothespectralanalysisinthefigure,whichisinagreementwithotherresearch.Thiscorrespondstothe0.24eVblueshiftfromthebulk-phasebandgapofanatase(3.2eV),indicatingthattheultra-smallTiO2particlesareformed.ParticlesizecanbefurtherestimatedaccordingtoatheoreticalpredictionproposedbyBrus(23):

ENVIRONMENTALSCIENCE&TECHNOLOGY/VOL.35,NO.11,2001

h21.8e2

?Eg)2-??R8Rμ

(3)

where?Egisthebandgapshift,Ristheradiusoftheparticle,

μisthereducedmassoftheexciton(μ)1.63me;meistheelectronrestmass)forTiO2,and??isthedielectricconstantofthesemiconductor(??)184)forTiO2(21).Thebandgapshiftof0.24eVfortheTiO2colloidscorrespondstoaparticlediameterofca.2.0nm,whichimpliesso-calledquantum-(Q)sizedTiO2particles.Theparticlesizeisalsoinvestigatedbytransmissionelectronmicroscopy(TEM)asshowninFigure3,whereblackspotsarethesynthesizedTiO2particlesandallofthemmeasurelessthan10nm.ThediscrepancybetweentheparticlesizesdeterminedbyTEMandUV-visiblespectroscopymightascribetotheagglomerationoftheQ-sizedTiO2particlesduringthedryingproceduretoprepareTEMsamples.

X-raydiffraction(XRD)analysisisemployedtocharac-terizethecrystalstructureoftheTiO2nanoparticles.ItisknownthatTiO2particlesareintwodifferentcrystalforms,i.e.,anataseandrutile(24).Intheanatase(Tio[O2]c)structure,theoxygensformacubicclosestpacking,andthetitaniumatomslieinoctahedralvoids.Intherutile(Tio[O2]h)form,theoxygensarearrangedapproximatelyinahexagonalclosestpacking,andthetitaniumatomsoccupyarowpattern.Manyresearchersclaimthattheanataseappearstobethemostphotoactiveandstablenanoparticlesforwidespreadpractical

applications(9,25,26),whereastherutileisphotocatallyti-callyinactive(27-29)ormuchlessactive(30-33),althoughitshowsstrongphotoactivityselectivelytowardsomecases(9,34).Figure4comparestheX-raydiffractionpatternsofthreetypesofTiO2particlesasdesignatedinthefigure.The100%rutileTiO2,Figure4(b),showsthecharacteristicpeakslocatedat2θof27.45°.AsforthediffractionpatternofDegussaP25TiO2,Figure4(c),whichisanonporous7:3anatase-to-rutilemixtureandisoneofthemostoftenusedphotocatalysts(9),the2θofeminentpeaksare25.24°foranataseand27.46°forrutile.Comparingdiffractionpatternsof(a)with(b)and(c),itisconfirmedthatourTiO2nanoparticlesarecomposedentirelyofanatase,whichpromisesthehighestphotoreac-tivityandthebestefficiencyfordestroyingthemicroorgan-isms.

ReverseOsmosisPerformanceandSurfaceCharacter-izationofTiO2Self-AssembledandNeatMembranes.Asdescribedearlier,thefabricationofTiO2self-assembledthin-film-composite(TFC)membraneswascarriedoutbydippingtheneataromaticpolyamidemembraneintothesolutionofcolloidalTiO2particlesfollowedbywashingwithwater.AccordingtotherecentinvestigationoftheadsorptionbehaviorofcarboxylicacidonTiO2byvirtueofdiffusereflectanceinfraredFouriertransform(DRIFT),theprocessofself-assemblybetweencarboxylicacidandTiO2wasexplainedbytwodifferentadsorptionschemes(35).OneschemewasthatTiO2wasboundwithtwooxygenatomsof

FIGURE3.TEMmicrographoftheTiO2nanoparticles.

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