Prototype composite membranes of partially reduced graphene oxide/TiO2 for photocatalytic ultrafiltration water treatment under visible light

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ContentslistsavailableatScienceDirect

Applied

Catalysis

B:

Environmental

jo u r n al ho me p a g e :w w w . e l s e v i e r . c o m / l o c a t e / a p c a t b

Prototype

composite

membranes

of

partially

reduced

graphene

oxide/TiO

2

for

photocatalytic

ultraﬁltration

water

treatment

under

visible

light

Chrysoula

P. Athanasekou

a

_,

_Sergio

_{Morales-Torres}

b

_,

_Vlassis

_Likodimos

a,c

_,

George

Em.

Romanos

a,∗

,

Luisa

M. Pastrana-Martinez

b

_,

_Polycarpos

_Falaras

a

_,

Dionysios

D. Dionysiou

d

_,

_Joaquim

_L.

_Faria

b

_,

_José

_L.

_Figueiredo

b

_,

_Adrián

_M.T.

_Silva

b,∗∗

a_Division_of_Physical_Chemistry,_Institute_of_Advanced_Materials,_{Physicochemical}_Processes,_{Nanotechnology}_and_Microsystems_(IAMPPNM),

NCSRDemokritos,AghiaParaskevi,Attikis,15310Athens,Greece

b_{LCM—Laboratory}_of_Catalysis_and_{Materials—Associate}_Laboratory_LSRE/LCM,_Faculdade_de_Engenharia,_Universidade_do_Porto,

RuaDr.RobertoFrias,4200-465Porto,Portugal

c_Department_of_Solid_State_Physics,_University_of_Athens,_{Panepistimioupolis,}_GR-157₈₄_Athens,_Greece d_{Environmental}_Engineering_and_Science_Program,_University_of_Cincinnati,_Cincinnati,_Ohio_45221-0012,_USA

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received16January2014

Receivedinrevisedform4April2014 Accepted7April2014

Availableonline19April2014 Keywords: Ultraﬁltrationmembranes Titaniumdioxide Grapheneoxide Photocatalysis Cleanwater

a

b

s

t

r

a

c

t

Ahighlyefficienthybridphotocatalytic/ultrafiltrationprocessisdemonstratedforwaterpurification usingvisiblelight.Theprocessreliesonthedevelopmentofpartiallyreducedgrapheneoxide/TiO2

com-positemembranesandtheirincorporationintoaninnovativewaterpurificationdevicethatcombines membranefiltrationwithsemiconductorphotocatalysis.Compositesconsistingofgrapheneoxidesheets decoratedwithTiO2nanoparticlesweredepositedandstabilizedintotheporesofultrafiltration

mono-channelmonolithsusingthedip-coatingtechnique.Cross-flowanddead-endfiltrationexperimentswere sequentiallyconductedindark,underUVandvisiblelight.Themembranesurfacewasirradiatedforthe eliminationoftwosyntheticazo-dyes,methylorangeandmethyleneblue,fromwatersolutions.The synergeticeffectsofgrapheneoxideonpollutantadsorptionandphotocatalyticdegradationcapacity ofTiO2werethoroughlystudied,whiletheinfluenceoftheporesizeofthemonolithicsubstrateonthe

depositionmorphologieswasalsoelucidated.Moreover,theperformanceofthenovelhybridprocesswas comparedwiththatofstandardnanofiltrationwithrespecttopollutantremovalefficiencyandenergy consumption,providingfirmevidenceforitseconomicfeasibilityandefficiency.

1. Introduction

Photocatalyticmembranesexhibitingthesimultaneousaction ofpollutantrejectionandphotocatalyticdegradationhavereceived muchattention[1–7],duetothesimplicityoftheoverallwater treatmentprocessandthebeneficialeffectsarisingfromthe pres-enceof the photocatalyst onthe membrane surface and pores (anti-biofouling, cleaner permeate, higher flux) and vice versa (increasedpollutantconcentrationinthevicinityofthe photocat-alyst,turbulentflowandefficientmixingduetotheasymmetric

∗ Correspondingauthorat:NCSRDemokritos,AghiaParaskeviAttikis,15310 Athens,Greece.Tel.:+302106503972-3.

∗∗ Correspondingauthorat:UniversidadedoPorto,RuaDr.RobertoFrias,4200-465 Porto,Portugal.Tel:351220414908.

E-mailaddresses:groman@chem.demokritos.gr(G.Em.Romanos),

adrian@fe.up.pt(A.M.T.Silva).

pore structure of the membrane). Moreover, hybrid photocat-alyticmembraneprocesseshavethepotentialtoeliminateamajor drawbackofmembraneseparationtechnology,whichisthe gen-erationoftoxiccondensates.SincethepioneerworkofBarnietal., whodescribedthePHOTOPERM®_process_[8],_and_the_succeeding synthesisofcompositeTiO2/Al2O3 membraneswithhierarchical mesoporousmultilayerstructure[9,10],there havebeen persis-tent efforts toward the development of efficientphotocatalytic membraneswithconcurrent water filtrationandphotocatalytic degradationproperties.Polymerbased[11],free-standing[12]or ceramic[13]basedphotocatalyticmembraneshavealreadybeen successfullypreparedandtheiranti-fouling,highflux, photodegra-dationandfiltrationefficiencyhasalreadybeenproved.Despite theirrelativelyhighcostcomparedtothepolymericcandidates, ceramicporousfiltersarethesubstratesofchoicefordeveloping photocatalyticmembranes.Ceramicfiltersexhibitexcellent ther-mal,chemical, and mechanicalproperties, whileretaining their http://dx.doi.org/10.1016/j.apcatb.2014.04.012

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capacitytobereusedaftercalcination.Thesecharacteristicsensure theintegrityofthesubstrateduringthedeposition,stabilization andactivationofthephotocatalyst[14,15]aswellasduringthe watertreatmentprocessunderUVirradiationandtheconcomitant attackbythehydroxylradicalsgeneratedonthephotocatalyst.The mostcommonwaytodevelopthephotocatalyticallyactive mem-branelayerisbymeansofsequentialdip-coatingofthesubstrate intoappropriate solscomposedofdifferentprecursormaterials, thatgenerateultrathinlayersconsistingofnanocrystalliteswith decreasingsize, aswemoveaway fromthesubstrate’ssurface. Thepurposeofthismulti-coatingprocedureistomasksupport’s defects[16,17]thatunderminetheintegrityoftheactivetoplayer. Asamoreversatiletechnique,CVDofvariousmetal–organic pre-cursorscaneffectivelycontroltheporesizebydepositingeither activemetal–oxidenanocrystalline layersonthepore mouthof theroughsubstrateoruniformnanoparticles,whichcanbesmaller thanthoseusuallyformedbythesol–gelroutes.

In former studies of our group, CVD derived double sided activephotocatalyticceramicultrafiltration(UF)membraneswere employedfororganicdyeremovalfromwastewater[18–21].The innovativehybridphotocatalytic/UFprocessinvolvedthefiltration ofpollutedwaterinthecross-flowordead-endmembrane con-figuration,withUVirradiationappliedonbothphotocatalytically activesurfacesofthemembranes.Theoverallconceptandmethod ofimplementationintoanefficientwaterpurificationdeviceare alreadyprotectedbyaEuropeanpatent[22].Whencomparedto thestandard nanofiltration (NF) with ceramic membranes, the hybridphotocatalytic/UFprocessachievedfivetimeshigher pol-lutantremovalefficiencywhileperformingwiththesamewater recovery[21].Mostimportant,due tothelowtrans-membrane pressure,theenergyexpense(kWh/m3₎_of_the_pump_feeding_the waterstreaminthemembranemodulewasalmosthalftheenergy usuallyspentinNF.However,properevaluationoftheeconomic feasibilityoftheproposedhybridphotocatalytic/UFwater treat-mentrequirestakingalsointoaccounttheenergyconsumedfor poweringtheUVsources.TheconcernisclearlydepictedinFig.1, wheretheenergyconsumptionofa CVDderived photocatalytic ceramicmembrane [21]is comparedwiththestandard NF and reverseosmosis(RO)processes.WhenapplyingthreeormoreUVA lightsourcesof8Weach,withthepurposetoachievesufficient irradiationdensityonthephotocatalyticallyactivemembrane sur-face,thetotalenergyconsumedfortreatingthesameamountof waterexceedsthatrequiredinatypicalNFprocess,andinsome casesitisequaltothatofthemostenergydemandingRO.

Inthiscontext,theexploitationofcost-freesolarenergyinstead ofUVlightsourcesishighlyappealing.Followingthespreadingof visible-light-activematerialsandtechnologiesusingsolar radia-tion,likephotovoltaicsandopticalfibers,attemptstoextendthe photoresponseofTiO2intothevisibleregionandexploitationof solarlightdrivenapplicationshasbecomeatopicofgreat inter-est.SurfacemodificationofTiO2byanionormetaldoping[23–26], orthedevelopmentofcompositeswithcarbonmaterialslike car-bonnanotubes(CNTs),fullerenes,graphene,nanodiamonds and othernanocarbonsmainlyofgraphitictype,haveledtosignificant enhancementof theirphotocatalyticactivity undervisible light [27–29].

EspeciallythetopicofCNTs-TiO2andgrapheneoxide–TiO2 com-positeshaslatelyattractedmuchattentionbyourgroup[30,31] duetochargetransfereffectsforeseenwhenthegraphiticsurface iscoupledwiththesurfaceofthephotocatalyst.

In thepresent study,partially reduced grapheneoxide/TiO2 composite membranes were developed for a photocatalytic/UF water treatmentprocessactivated byvisible light. Tothis aim, ceramicmembraneswerecoatedwithpartiallyreducedgraphene oxide/TiO2 composites (GOT) and tested in the photocatalytic removal of organic dyes from synthetic wastewater, under

continuous flow conditions. Nanofiltration and ultrafiltration ceramicmonochannelmonolithswereusedassubstrateswiththe purposetounveiltheeffectoftheporesizeonthestabilityand pollutantremovalefficiencyofthedevelopedGOTphotocatalytic membranes.Inaddition,aTiO2membranehadbeendevelopedby usingthesamedippingsolbutwithoutdispersingGO,inorderto haveareferencematerialtocomparewiththeGOTmembranesand toelucidatetheeffectofpartiallyreducedgrapheneoxide(GO)on thephotodegradationandadsorptioncapacity.

2. Experimental

2.1. Materialsandreagents

Ammoniumhexaﬂuorotitanate(IV)(>99.99%),boricacid(>99%), sulfuric acid(>95%),methylorange - MO(99%) and methylene blue-MB(99%)wereobtainedfromSigma-Aldrich.Thesubstrates appliedfor thedevelopmentoftheGOTmembranesweresilica NFsinglechannelmonolithswithnominalporesizeof1nmand molecularweightcutoff(MWCOT)of0.45kDand␥-aluminaUF singlechannelmonolithswithnominalporesizeof5and10nm andrespectiveMWCOTof7.5kDand50kD.TheNForUF sepa-rationlayer(1.5␮minthickness)waslocatedontheshellsideof themonolith,whichhad alengthof15cm,IDandODof7and 10mm,respectively,andglazedendsof1.5cmlength.Therestof thetubularmembranefromtheshelltothelumensideconsisted oftwointermediate␥-aluminalayersandaroughmacroporous ␣-aluminasupport.Detailsontheporesizeandvolumeoftheseveral layerscanbefoundelsewhere[32].

Naturalgraphite(99.9995%purity,20␮m,fromSigma-Aldrich) wasusedasprecursorofGO.First,graphiteoxidewasprepared throughthemodiﬁedHummersmethod[33,34].Then,the result-ingmaterialwasdispersedinagivenvolumeofwaterandsonicated withanultrasonicprocessor(UP400S,24kHz)for1h.The result-ingsonicateddispersionwascentrifugedfor20minat3000rpmto obtainasuspensionofGO.

Then, the GOT composite was synthesized with these dis-persions by theliquid phase deposition(LPD) method atroom temperature,asdescribedelsewhere[33].Briefly,ammonium hex-afluorotitanate(IV),(NH4)2TiF6(0.1mol/L),andboricacid,H3BO3 (0.3mol/L),wereaddedtoacertainamountoftheGOdispersion heatedat60◦Cfor2hundervigorousstirring.Thematerialwas sep-aratedbyfiltration,washedwithwateranddriedat100◦Cunder vacuumfor2h.Thecarbonloading(4wt.%)wasselectedtaking intoaccountthehighestphotocatalyticactivityobtainedwiththis GOTcompositeinourpreviouswork[33].BareTiO2wasalso pre-paredandtreatedbythesamemethod,withouttheadditionofany carbonmaterial.

2.2. Membranedevelopment

Anaqueousdispersion(50g/L) containingthecorresponding photocatalyst(GOTorbareTiO2)wasusedtodepositthe mate-rialonthedifferentceramicmembranesbydip-coating(down/up velocityof 50mm/minand diptimeof 30s).Threelayerswere appliedonthemembranesandaftereachcoatinglayerthe mem-branewasdriedat120◦C inanovenfor30min.Afterthat,the coated membranes weretreated at 200◦C, witha heating and cooling rate of 1◦C/min and under a N2 atmosphere. The lat-ter treatment resulted in the enhancement of the composite’s photocatalyticactivitybytheimprovementofTiO2 nanoparticle crystallinityandthepartialreductionoftheGOsheetsthat fur-therenhanceschargetransferbetweenthecompositeconstituents [33].Finally,theceramicmembranesweresoftlyﬂushedwith com-pressedairinordertoremovetheparticlesnotwell-adheredbefore beingusedinreaction.Themembranesdevelopedonthemonoliths

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0 20 40 60 80 100 120 NF 1 nm without irradiation RO CVD photocatalytic membrane UVA 40W CVD photocatalytic membrane UVA 36W CVD photocatalytic membrane UVA 30W CVD photocatalytic membrane UVA 24W CVD photocatalytic membrane UVA 20W E ne rg y con s umpt ion ( kW h/ 1 00 m 3)

Energy (light source)

Energy (pump)

Fig.1. Barchartpresentingacomparisonoftheenergyconsumptionbetweenastandardmembranetechnologyandtheproposedhybridphotocatalytic/ultrafiltration process.Thebarswithtwocolorsareforthephotocatalytic/ultrafiltration.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtotheweb versionofthisarticle.)

withporesizeof1,5,and10nmusingtheGOTaqueous suspen-sionarereferredasGOT-1,GOT-5andGOT-10,respectively,while themembranedevelopedonthemonolithwithporesizeof10nm usingtheTiO2aqueoussuspensionisreferredasreference mem-brane.

2.3. Physicochemicalandperformancecharacterization 2.3.1. Physicochemicalcharacterization

Micro-Ramanspectroscopymeasurementswereperformedin backscatteringconfigurationonaRenishawinViaReflex spectrom-eterusinganAr+_ion_laser₍₌_514.5_nm,_2.41_eV)_and_a_high_power nearinfrared(NIR)diodelaser(=785nm,1.58eV)asexcitation sources.Thelaserbeamwasfocusedontothespecimensbymeans ofa50×(NA=0.75)objective,whileanalysisofthescatteredbeam wasperformedona250mmfocallengthspectrometeralongwitha 1800lines/mmdiffractiongratingandahigh-sensitivityCCD detec-tor.Ramanmappingonthemembranesurfaceswasimplemented onamotorizedfeedback-controlledXYZmappingspecimenstage. The morphology of the GOT composite was determined by scanning electron microscopy (SEM) in a FEI Quanta 400FEG ESEM/EDAXGenesis X4M instrument.The point zeroof charge (pHPZC)ofthepowdermaterialswasdeterminedfollowingapH drifttestdescribedelsewhere[35,36].Solutionswithvaryinginitial pH(2–12)werepreparedusingHCl(0.1mol/L)orNaOH(0.1mol/L) and50mLofNaCl(0.01mol/L) aselectrolyte.Eachsolutionwas contactedwith0.15gofthematerial(GOTorTiO2)andthefinalpH wasmeasuredafter24hofcontinuousstirringatroom tempera-ture.ThePZCvalueofthematerialwasdeterminedbyintercepting theobtainedfinal-pHvs. initial-pHcurvewiththestraight line final-pH=initial-pH.

2.3.2. Performanceevaluation

Thehybridphotocatalytic/membranefiltrationprocessestook place in a photocatalytic purification device, which has been describedindetailelsewhere[21].Inbrief,thepurificationdevice allowedtodeterminetheperformanceofphotocatalytictestsinthe cross-flowanddead-endfiltrationmodewithirradiationapplied

ontheshellsidesurfaceofthesinglechannelmonolith.Near-UV radiation(315–380nm)withapeakat365nmwasappliedatalight intensityof2.1mW/cm2_._This_irradiation_density_was_achieved_by fourUVlampsof9W(Phillips-UVA(PUVA)PL-S/PL-L)placedata distanceof3cmfromtheouterPlexiglascellofthemembrane reac-torandsleevedbyacylinderofthickaluminumfoil.Fourvislamps of9W(OsramDULUXS9W/21-840G23LUMILUXCoolWhite), placedatthesamedistanceof3cmfromthePlexiglascell,were usedtoachieveavislightirradiationdensityof7.2mW/cm2_.

The total flow of polluted water feeding the reactor was 1.5mL/minfortheGOT-5,GOT-10andreferencemembranes,and thefiltrationwasperformedinthedead-end mode.The GOT-1 membranewasexaminedinthetangentialflow(cross-flow)mode withatotalpollutedwaterfeedof0.3mL/min.Theconcentrations of MOand MBin thefeedsolutionwere4.4×10−3mg/mL and 1.8×10−3mg/mL,respectively.

The concentrations (C) of MO and MB collected from the retentateandpermeatesideofthemembrane,andthe concentra-tion(C0)ofthefeed,weredeterminedbymeasuringthevisible light absorbance at 466nm and 664nm,respectively. The total amountsofremovedpollutantwereobtainedfromthemass bal-ancebetweenthefeed,retentateandpermeatesideoftheGOT-1 membraneandbetweenthefeedandpermeatesideoftheGOT-5, GOT-10andreferencemembranes.

3. Resultsanddiscussion

3.1. TheeffectofGOontheadsorptionandphotocatalytic performanceofthemembranes

Comparative performance evaluation of the GOT modiﬁed photocatalytic membrane(GOT-10) and that modiﬁed withthe respectiveTiO2phase(reference)wasperformedinordertounveil theoccurrenceornotofsynergeticchargetransferandadsorption effectsofGOonthephotocatalyticactivityofTiO2.

ThecapacityofthetwomembranestoremoveMOandMBfrom theiraqueoussolutionswasexaminedinthedead-endﬁltration modewithafeedﬂowof1.5mL/min.Filtrationexperimentswere

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0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030 0.0035 0 0.5 1 1.5 2 mas s of po llutant re mo ve d ( mg /c m 2)

mass of pollutant introduced in the

reactor (mg) GOT-10 MO reference MO GOT-10 MB reference MB

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0 5 10 15 20 25 30 35 40 0 125 250 375 Pe rme a n ce (L/m 2/h/ ba r)

Filtraon me (min) GOT-10 MO reference MO GOT-10 MB reference MB

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Fig.2.(a)Pollutantadsorptionand/orrejectionefﬁciencyoftheGOT-10andthereferencemembranesforMBandMOinthedark.(b)Therespectiveevolutionofpermeance duringtheﬁltrationexperimentsindark.

sequentiallyperformed,startingwithoutirradiation(indark)and continuingwithUVandvisirradiationappliedontheshellside sur-faceofeachmembrane.Theﬂowdirectionoftheaqueoussolution wasfromtheshelltothelumenside,andthephotocatalytically activesurfaceofeachmembranewasapproximately25cm2_.

Fig.2a presents theretainedamount ofpollutant (adsorbed and/orrejected)persquarecentimeterofmembranesurfacevs.the totalamountfedinthereactorunderdarkconditions.Theretained amountwascalculatedfromthemassbalancebetweenthefeed andpermeatesideofthemembrane,andnormalizedovertheactive membrane’ssurface.

No signiﬁcanteffects due toenhanced adsorption of MB or MOontheGOsurfacecouldbeidentiﬁed(Fig.2a).Inaprevious studyoftheauthors[36],ithasbeenshownthatthetotalpore volume(Vp)andBET surfacearea(SBET)are comparableforthe GOTcomposite(0.17cm3_/g_and₁₁₀_m2_/g,_{respectively)}_and_TiO

2 (0.11cm3_/g_and₁₂₀_m2_/g,_{respectively),}_both_being_higher_than_for GO(0.0027cm3_/g_and₂₁_m2_/g,_{respectively)}_because_aggregation ofGOsheetsoccurswhenthesuspensionofGOisdriedtoperform N2adsorptionanalysis.Becauseofthehigherporosity,a benefi-cialeffectonthephotocatalyticactivityoftheGOT-10membrane couldbeexpectedduetoincreasedconcentrationofthepollutant moleculesinthevicinityofthephotocatalyst.However,therewas nodifferenceontheadsorptionperformanceofGOT-10compared tothereferencemembrane(Fig.2a)whichmightjustifyapossible significantenhancementofthephotodegradationefficiency.

Inorder toelucidatethereasons behindthesimilar adsorp-tioncapacityofGOT-10andthereferencemembranes,thepHPZC ofthe GOTcomposite and thepossiblecharge of thepollutant moleculeswereexaminedinrelationtothepHof theinvolved solutions.ThepHPZC≈3.2ofGOTtreatedat200◦Crevealsastrongly acidiccharacter.Thus,atthepHconditionsoftheusedMOsolution (pH=6.0ataconcentrationof4.4mg/L)thesurfaceofthe compos-iteisnegativelychargedandthesameholdsforMO,whichhas lostitsamphotericcharacterduetodeprotonationatthe N N bridgebetweentherings.Thesameeffectoccursinthecaseofthe referencemembrane,becauseTiO2 alsopresentsanacidic char-acter(pHPZC≈3.5)due totheprecursorusedinthepreparation

ofthisTiO2materialbytheLPDmethod.Concerningthecationic MB dye,both membranes exhibited a 4-foldhigher adsorption capacity compared toMO, asexpected due tothe electrostatic attractionbetweentheirnegativelychargedsurfaces(thepHofthe MBsolutionis7.2)andpositivelychargedions(Fig.2a).Likewise, theGOT-10membraneexhibitssimilaradsorptioncapacityasthe reference,sincetheemployedGOTandTiO2materialshavequite similarSBETandpHPZC.Ontheotherhand,thedepositionoftheGOT compositeexertedanegativeeffectontheﬂuxcapacityofthe GOT-10 membrane.Fig.2b showsthat GOT-10’spermeancereached steadystateafter4honstreamandwasconsiderablylowerthan thatofthereferencemembrane.ThelowerpermeanceofGOT-10 couldbeattributedtopore-blockingeffectsinducedbythe intro-ductionorattachmentoftheGOTcompositeintothe10nmpores ofthesupport.Theporesizeof10nmwassufﬁcienttohostthe GO–TiO2composites,ensuringthestabilityofthederivedGOT-10 membrane,incontrasttothesupportsofsmallerporesize(see Section3.2).

Althoughhighﬂuxisadesirablepropertyofmembrane tech-nology,itshouldbenotedthatthe30–35%reductioninthewater permeanceofGOT-10comparedtothereferencemembranewas notsosevereandcouldbeeffectivelycompensatedbythe advan-tageofhigherstability.

Itisalsoimportanttonotethecontinuousdeclineofthe per-meanceforbothmembranesduringﬁltrationindark(Fig.2b).This istheresultoflimitationstothepassageofwater,posedbythe adsorption(stacking)ofMOorMBmoleculesontheactivesites ofthemembrane’ssurface.Thesameeffectoccurswiththe ref-erence membrane as evidenced in Fig.3a and b where optical imagesoftheshellsidesurfaceofthereferencemembraneafterthe MOﬁltrationexperimentsindarkandunderUV,respectively,are compared.

Moreover,theobservedpermeancedeclinecannotberelated tothemembranes’fouling,whichusuallyresultsfromthe adsorp-tionoflargerorganicmoleculessuchashumicandfulvicacidsthat blocktheporeentrances.Thiseffectcanberatherrelatedtochanges ofthemembrane’ssurfacechemistryandchargeuponadsorption ofMOandMB.Thepermeancelevelsoffwhenallactivesitesare

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Fig.3.Photosshowingtheshellsidesurfaceofthereferencemembraneafterthe ﬁltrationexperimentswithMO.(a)Filtrationexperimentindark.(b)Filtration experimentunderUV.

occupied,andfromthispointonwardtheﬂuxpropertiesofthe membranesreachasteadystate(Fig.2b).

Uponcompletionoftheﬁltrationexperimentindark,theUV sourceslocatedaroundthereactorcellwereswitchedonandthe experimentunderUVirradiationcommenced.

Experimentswereperformedinthissequenceinordertoenable thedistinction betweentheeffectofphotocatalyticdegradation andthetransientadsorptiononafullyfreshsurface.Asconcluded from theresults presented in Fig.2a, the amountof pollutant adsorbedontheunsaturatedmembranesurfaceishighandcould haveasigniﬁcantadditiveeffectontheestimatedphotocatalytic membraneperformance.Inthiscontext,ourprimarypurposewas toallowadsorptionequilibrium tobereachedandthentostart theirradiationexperimentsinordertofocusourstudysolelyon thecomparisonofthephotocatalyticefﬁciencybetweenthetwo membranes.

The results obtained under UV irradiation are presented in Fig.4aandb.Thefirstsignificantaspectisthetemporalevolution ofthemembranespermeanceunderUV(Fig.4b).Atthebeginning oftheirradiationexperiment,thepermeanceincreasesasa con-sequenceofthephotodegradationofthepre-adsorbedpollutant molecules.InthecaseofGOT-10,thepermeancelevelsoffafter 2.0–2.5honstreamandremainsstableforthenext3.0hofthe fil-trationexperimentunderUV.Thestabilityisduetothecontinuous degradationofthepollutantmoleculesthatarenewlyadsorbedon thephotocatalyst.

Ontheotherhand,thepermeanceofthereferencemembrane levelsoffmuchlaterandmoreimportantly,itssteadystatevalue is10–13%higherthanthatobtainedattheinitialstagesofthe fil-trationexperimentindark(Fig.2b).Thisfactrevealsenhancement ofthehydrophiliccharacterofthedepositedTiO2 during irradi-ation.Thebeneficialeffectsofphotoinducedhydrophilicityhave alreadybeenmentionedinpreviousstudiesoftheauthors con-cerningTiO2 photocatalysts[37]andphotocatalyticmembranes. Veryrecently,athoroughinvestigationwasconductedinorderto corroboratetheeffectofUVirradiationonthefluxpropertiesof photocatalyticceramicmembranes.Thisstudyconfirmedthatthe observedenhancementofthepermeanceisnotduetowarmingup ofthefiltrationfluidbutratherduetotheinherentpropertyofTiO2 toincrease thepopulationofsurfacehydroxylswhenirradiated withUV[23].InthecaseoftheGOT-10membranesuchaneffect wasnotevident,indicatingthatGOpreventsthehighhydrophilic characterofirradiatedTiO2.

Furthermore,asuddendropinthepermeanceofboth mem-braneswasobservedatabout1hafterswitchingfromUVtovislight irradiation(Fig.4d).DuringMBfiltration,thepermeanceofthe ref-erencemembranedroppedwellbelowthevalueof30L/m2_/h/bar (theinitialpermeancevalue),indicating thatthereferenceTiO2 materialexhibitsmoderateefficiencyforthephotocatalytic degra-dationof MB undervisible light. Asa result, adsorptionof MB moleculescausesasignificantdecreaseofthewaterflux.Onthe contrary,thesteadystatepermeancevalueoftheGOT-10 mem-braneunder visible light wasalmostidentical tothat obtained duringtheinitialstageoffiltrationindark(Fig.2b),afactunveiling continuousdegradationofthenewlyadsorbedpollutantmolecules

undervislightirradiation.TheplotsofMOandMBremovalvs.the amountfed(Fig.4aandc)revealthehigherphotocatalytic activ-ityofGOT-10comparedtothereferencemembrane.Theenhancing effectofGOissignificantandbycomparingtheslopesoftheplots,a 2-foldhigherpollutantremovalefficiencyisconcludedforGOT-10. Asalready shown,theadsorptioncapacity ofthetwo mem-branes was similar. Therefore, electron sinking effects are concludedbythefactthatGOT-10ismuchmoreefficientthanthe referencemembraneinthedegradationofMBunderUV.

Moreover,themuchhigheractivityofGOT-10undervislightcan beattributedtothefactthatGO(4wt.%)actsassensitizer(electron donor)ofTiO2 undervislight,asalreadyprovedinourprevious publicationwherethismaterial,preparedbytheLPDmethod,was testedintheformofpowder[33,38].Infact,thisGOTcomposite, independentlyofitsmodeofapplication,eitherfreelydispersed inwater[33],immobilizedintohollowﬁbers[36]orstabilizedinto theporesofUFmembranes(asshowninthepresentwork),exhibits remarkablephotocatalyticactivityundervis-light.

Finally,weshouldnotethatwhenthetargetistocomparethe photocatalyticefficiencyofdifferentmembranes,afiltration exper-imentofprolongedperiodunderdarkmustprecedethefiltration under irradiation. Indeed, when examining a freshly prepared sample,unoccupiedadsorptionsitesareavailablebothonthe pho-tocatalyticandnon-photocatalyticpartoftheentiremembrane. Inthatcase,differencesintheadsorptioncapacitycanleadto mis-leadingconclusionsrelevanttotherankingofthephotocatalytic efficiency, since adsorption on photocatalytically inactive sites andphotocatalyticdegradationmayoccursimultaneously.Thisis clearlydepictedinFig.5,wherewepresentthepollutantremoval efficiencyofthemembranesforthecaseswhenfiltrationindark precededornottheirradiationexperiments.Itcanbeseenthatthe referencemembranepresentedsimilar(forMB)orslightlybetter (forMO)performancecomparedtoGOT-10whentheirradiation wasappliedfromthebeginningofthefiltrationexperiment.

However,astheﬁltrationproceedsanduponoccupationofall activesiteswithadsorbedmoleculestheslopesoftheplots(Fig.5) decline,especiallythosecorrespondingtothereferencemembrane (comparebetweenFig.5aandb)andthesteadystateperformance highlightsGOT-10asthemostefﬁcientphotocatalyst.

3.2. EffectofthesubstrateporesizeontheGOTmembranes morphologyandperformance

Todeterminetheoptimumporesizeoftheceramicsupportfor thedevelopmentofanefficientandstablemembranehosting par-tiallyreducedgrapheneoxide/TiO2composites,GOT-10,GOT-5and GOT-1membraneswerecomparativelyevaluatedfortheremoval ofbothMOandMBunderdarkandUVlightconditions.BoththeUV photocatalyticefficiencyandadsorptioncapacityindarkofGOT-5 differedmarkedlyfromthoseofGOT-10andthereference mem-branes (Fig.6a andb),indicativeof significantvariationsin the depositionefficiencyofGOTonmembranesofdifferentporesizes bydipcoating.

ThephotocatalyticefﬁciencyofGOT-5wasmarkedlyreduced forboth MOandMB pollutants,implyingadrasticreductionof theamountofphotocatalystdepositedonthe5nmsubstrate.As evidencedbythecorrespondingmicro-Ramananalysis(described below), a relatively smaller fraction of the GOT composite is introducedandstabilized intothe5nm pores,whilea substan-tialamountofGOTisrejectedandleachedoutofthemembrane’s surface.

ThephotocatalyticefﬁciencyofGOT-5ismuchlowerthanthat ofGOT-10andreferencemembranes(Fig.6b),conﬁrmingthatless GOTwasstabilizedintotheporesandsurfaceofGOT-5in compar-isontotheGOT-10membrane.

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0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0 1 2 3 m ass of p o ll u ta nt re m ov ed ( m g / cm 2)

mass of pollutant introduced in the

reactor (mg)

UV

GOT-10 MO reference MO GOT-10 MB reference MB

(a)

0 5 10 15 20 25 30 35 40 45 0 125 250 375 Pe rm e an ce ( L / m 2/h / b a r)

Filtraon me (min)

UV

(b)

0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030 0.0035 0 1 2 ma ss of p ol lu ta nt r e mo ved (m g/ cm 2)

mass of pollutant introduced in the reactor (mg)

Vis

(c)

0 5 10 15 20 25 30 35 40 0 125 250 375 Pe rm e an ce ( L / m 2/h / b a r)

Vis

(d)

Fig.4.(a)PollutantremovalefficiencyoftheGOT-10andreferencemembranesduringfiltrationunderUV.(b)EvolutionoftheGOT-10andreferencemembranepermeance duringfiltrationoftheMBandMOsolutionswiththeapplicationofUV.(c)PollutantremovalefficiencyoftheGOT-10andreferencemembranesduringfiltrationundervis. (d)EvolutionoftheGOT-10andreferencemembranepermeanceduringfiltrationoftheMOandMBsolutionswiththeapplicationofvis.

Onthecontrary,GOT-5exhibitedsignificantlyhigher adsorp-tionefficiencyin darkcompared toboth thereferenceand the GOT-10membranes.Thiscanbeexplainedintermsofthelarger fractionofthe_␥-aluminasurfaceindirectcontactwiththesolution. AtthepHconditionsinvolved,␥-alumina,withapHPZCofabout9.1, ispositivelychargedandcanstronglyattracttheMOmolecules. Indeed,asshowninFig.6a,GOT-5presentedabout1.5foldhigher adsorptionefficiencyforMBand3foldhigheradsorptionforMO comparedtothereferenceandGOT-10membranes.

Besidesthephotocatalyticefﬁciencyandadsorptioncapacity, thepermeancepropertiesofGOT-5reﬂectedtheloweramountof thephotocatalyststabilizedintothemembraneporescomparedto GOT-10.

DuringﬁltrationindarkandsubsequentlyunderUV,the perme-anceofGOT-5(Fig.6candd)waslowerthanthatofthereference andhigherthanthatofGOT-10indicatingthatthephasestabilized intothe5nmporesdidnotprovokeintenseporeblockingaswas thecasefortheGOT-10membrane.Therefore,thestabilizedphase wasinferredtobelessbulkyand/or,morelikely,theporesofthe ␥-aluminalayerfailedtohostmostoftheGOTcompositestructures intheGOT-5membrane.

Moreover,theGOT-5membraneperformstheﬁltration exper-imentwithoutanysigniﬁcantchangeinitspermeanceuponUV irradiation(Fig.6d),incontrasttotheothertwomembranes,where thepermeanceincreasescontinuouslyduetothehigheramountof TiO2thatphotodegradesthepre-adsorbedpollutants.

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0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0 1 2 3 ma ss of po ll utant rem ov ed (mg /c m 2)

mass of pollutant introduced in the reactor (mg) UV GOT-10 MO reference MO GOT-10 MB reference MB

(a)

0 0.002 0.004 0.006 0.008 0.01 0.012 0 0.5 1 1.5 2 mass of po ll utant r emo v ed (mg /c m 2)

mass of pollutant introduced in the reactor (mg) UV GOT-10 MO reference MO GOT-10 MB reference MB

(b)

0.000 0.001 0.002 0.003 0.004 0 0.2 0.4 0.6 0.8 m ass of po ll utant r em ov ed (mg /cm 2)

mass of pollutant introduced in the reactor (mg) Vis GOT-10 MB reference MB GOT-10 MB reference MB

(c)

Fig.5.(a)ExperimentsperformedwiththeUVirradiationappliedafterhavingreachedpollutantadsorptionequilibriumindark.(b)ExperimentsperformedwiththeUV irradiationappliedatthebeginningoftheﬁltrationexperiment.(c)MBdegradationundervislight.Circlesymbols:irradiationappliedafteradsorptionequilibrium.Diamond symbols:Irradiationappliedatthebeginningoftheﬁltrationexperiment.

0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0 1 2 3 4 m ass of poll ut a nt rem ov ed (m g/ cm 2)

mass of pollutant introduced in the reactor (mg)

dark

GOT-5 MB GOT-10 MB reference MB GOT-5 MO GOT-10 MO reference MO

(a)

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0 1 2 3 mass of p oll ut a nt rem o ved ( mg /c m 2)

UV

(b)

0 5 10 15 20 25 30 35 40 0 100 200 300 400 P erme an ce (L /m 2/h/b a r)

dark

(c)

0 5 10 15 20 25 30 35 40 45 0 200 400 600 P erme an ce (L /m 2/h /ba r)

UV

(d)

Fig.6.PollutantsremovalefficiencyandpermeanceevolutionofthethreemembranesdevelopedonUFsubstrates.(a)AdsorptionduringfiltrationwithMBandMOindark. (b)MOandMBremovalefficiencyduringfiltrationunderUV.(c)Permeanceevolutionduringfiltrationindark.(d)PermeanceevolutionduringfiltrationunderUV.

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Fig.7.Micro-RamanspectraonthesurfaceoftheGOTandreferencemembranesincomparisonwiththeGOTpowderat(a)514.5and(b)785nmlaserexcitations.Optical imagesandcorrespondingRamanspectraonthesurfaceofGOT-5membraneat(c)514.5and(d)785nm.

ThedegreeofsurfacemodiﬁcationfortheGOTmembraneswas directlyinvestigatedbymicro-Ramanspectroscopyunder514.5 and785nmlaserexcitations(Fig.7aandb).

TheGOT-10membraneexhibitedastableRamanspectrumover itssurface,comprisingtherelativelybroadanataseTiO2 Raman modesatlowerfrequencies(100–700cm−1)arisingfromthesmall TiO2nanoparticlesgrownbytheLPDmethod(ca.4nm).In addi-tion,thebroadgraphiticGandthedefectactivatedDbandswere observedathigherfrequencies(1300–1700cm−1),characteristicof theGOflakes[33].InthecaseofGOT-10,therelativeintensityof themostintenseTiO2modeat∼156cm−1withrespecttothatof theGO-derivedGorDbandswassimilartothoseobtainedforGOT inpowderformatboth514.5and785nmexcitationwavelengths (Fig.7aandb).ThisconfirmedthattheGOTsheetswere homoge-neouslydepositedandstabilizedinthe10nmporesoftheUFlayer ofthemembranesupport,retainingtheirstructuralintegrity. Like-wise,thebroadanataseTiO2modesweredetectedonthesurface ofthereferencemembrane withsimilarspectralcharacteristics tothoseforGOT[33],confirmingthehomogeneousdepositionof small(4–5nm)anatasenanoparticlesinthe10nmUFmembrane layer.

Ontheotherhand,astheporesizeofthemembrane’sﬁltration layerdecreasedforGOT-5andGOT-1,asystematicincreaseofthe TiO2modeRamanintensityrelativetothatoftheGand/orDbands wasobservedatbothlaserexcitations,indicatinginhomogeneous depositionandlossoftheGOTcomposite’sintegrityonthe mem-branesurface.ThiswasmostprominentinthecaseofGOT-5,where thepresenceofdarkcoloredspatialregionswasadditionally iden-tiﬁedonthemembranesurface(Fig.7candd).Inthesedarkregions, onlyaweakRamanspectrumfromanatasenanoparticlescouldbe tracedtogetherwithtwoweakmodesat379and418cm−1 due

to␣-Al2O3.ThissuggestsalowamountofTiO2depositionmaking visibletheinner␣-Al2O3membranelayers[39].Ontheotherhand, theGandDbandsweresuppressedinthedarkregions,conﬁrming theabsenceofGOsheets.Areductionoftheoverallphotocatalyst loadingontheGOT-5membraneaswellasleachingofa substan-tialfractionofGOTsheetsleadingtoaninhomogeneouscoverage ofthemembranesurfacecouldbethusevidenced.ForGOT-1a spa-tiallyconstantRamansignaloveritssurfaceisobtained,comprising arelativelyweakerGORamanspectrumwithrelativelyenhanced TiO2 modes,mostlikelyarisingfromTiO2 nanoparticlesthatare notjointedwithGOsheets.Thisindicatesahomogeneousthough sparsesurfacedepositionofthephotocatalystontheNFﬁltration layer.

AcomparisonoftheperformancebetweenGOT-1andGOT-10 membranesispresentedinFig.8.Whentheefficiencyisevaluated intermsoftheeffectivereductionofthepollutantconcentrationat thepermeateeffluent(Fig.8a),thenGOT-1canbeclassifiedasthe bestamongstallthedevelopedmembranes.Indeed,GOT-1 mem-branehasthecapacitytohinderthepassageofthedyemolecules tothepermeatesiderejectingthemtotheretentateeffluent.

On theotherhand,whenthe targetis todegrade(e.g. pho-tocatalytically)thepollutantfrombothefﬂuents(permeate and retentate),thenGOT-1andGOT-10membranesdisplaysimilar per-formance(Fig.8c).

Itis importanttonotethat thereweremajordifferences on theimplementationofthephotocatalyticﬁltrationexperiments betweenGOT-1andGOT-10thatmaycreatetheimpressionof sim-ilarperformance.Inparticular,duetolimitationsatthemaximum operatingpressureofthephotocatalyticmembranereactor(a Plex-iglascellwasinvolvedtomakepossibletheirradiationontheshell surfaceofthemembrane),trans-membranepressurescouldnot

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0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 500 1000 1500 C/ C o me (min)

Experiment under UV

perme

ate si

de

GOT-10 MB GOT-1 MB GOT-10 MO GOT-1 MO

(a)

0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 500 1000 1500 C/ Co me (min)

Experiment under UV

feed side

GOT-1 MB GOT-1 MO

(b)

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 ma ss of po ll u ta nt r em o ved (m g/c m 2)

UV

(c)

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0 0.5 1 1.5 2 m ass of po llut a nt re mo ve d ( mg /cm 2)

dark

(d)

Fig.8. (a)Concentrationofthepollutantinthepermeateefﬂuent(C)normalizedbytheconcentrationinthefeedstream(C0).(b)Concentrationofthepollutantintheretentate

effluent(C)normalizedbytheconcentrationinthefeedstream(C0).(c)PollutantsremovalefficiencyofGOT-1andGOT-10duringfiltrationunderUV.(d)Pollutantsremoval

efﬁciencyofGOT-1andGOT-10duringﬁltrationindark.

exceed20bar.Thepressurelimitationincombinationwiththevery lowpermeanceoftheGOT-1membrane(NFregime),necessitated itsexaminationinthecross-flow(tangentialflow)filtrationmode ataverylowtotalflowrateof0.3mL/min.Undertheseconditions thewaterrecoveryachievedwas27and32%fortheMBandMO experiments,respectively.Onthecontrary,theGOT-10membrane wastestedinthedead-endfiltrationmodeatatotalflowrateof 1.5mL/minandwaterrecoveryof100%.Itcanbeconcludedthatin thecaseofGOT-1,thecontacttimeofthesolutionwiththe pho-tocatalyticallyactivesurfaceswasaboutfivetimeshigherthanthe respectiveonewithGOT-10.Notably,thelargedifferenceinthe contacttimeonlypartiallyexplainswhyGOT-1,whilebearingless amountofphotocatalyst,exhibitssimilarpollutantremoval effi-ciencytoGOT-10.Thetruereasonwasthefailuretofullysaturate theactivesitesofGOT-1duringthefiltrationexperimentindark, which precededtheexperiment under UV.Indeed, asobserved inFig.8d,theMBadsorptioncurvewasstilllinearwhentheUV experimentstarted.

Itis alsoimportanttoexaminetheevolutionof the concen-trationofpollutantsattheretentateefﬂuentduringtheﬁltration

experimentsunderUV(Fig.8b).Asmentionedabove,atthe begin-ningoftheUVfiltrationwiththeMBsolution,theshellsidesurface ofGOT-1stillpossessedahighpopulationofactivesites,where MBmolecules couldbeeasilyadsorbed.Thus,contrarytowhat wouldbeexpectedduetosizeexclusion,adsorptiononthe sur-faceledtoasignificantattenuationoftheMBconcentrationinthe retentateeffluent.Thereductionoftheconcentrationcouldnotbe attributedtotheenhancedphotodegradationefficiencyoftheshell sidesurfaceofthemembrane,which,asconcludedfromtheRaman analysis,bearsarelativelylowerquantityofthephotocatalyst.

InthecaseofMO,andjustbeforestartingtheirradiation exper-imentwithUV,theGOT-1surfacehadalreadybeensaturatedwith thepollutantmolecules(Fig.8d).WiththeapplicationofUV,the smallamountofthestabilizedphotocatalystwasnotsufﬁcientto photodegradetheMOmoleculesandmoreover,due tothesize exclusioneffectfromthe1nmpores,theretentateefﬂuentbecame progressivelymoreconcentratedthanthefeedstream.

Anotherinterestingfeature(Fig.8d)isthehigherMOandMB adsorptioncapacityofGOT-1comparedtotheGOT-10membrane. ThehigheradsorptioncapacitywasduetothesilicaNFlayerof

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0.0E+00 5.0E-06 1.0E-05 1.5E-05 2.0E-05 2.5E-05 0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030 0.0035 En e rg y s p e nt (k W h )

amount of pollutant removed (mg/cm2₎

GOT-1

0

MB- dark MB-UV MB-Vis MO-dark MO-UV MO-Vis

(a)

0.0000001 0.000001 0.00001 0.0001 0.001 0.01 0.1 1 0 0.001 0.002 0.003 0.004 0.005 En er gy sp e n t (kW h)

amount of pollutant removed (mg/cm2₎

MB-UV GOT-10 pump energy MO-UV GOT-10 pump energy MB-UV GOT-1 pump energy MO-UV GOT-1 pump energy MO-UV GOT-10 pump+UV energy MB-UV GOT-10 pump+UV energy

(b)

Fig.9. (a)EffectofirradiationontheenergyexpenseandefﬁciencyoftheGOT-10membrane.(b)ComparisonbetweentheperformanceoftheGOT-10andGOT-1membranes.

themonoliththatwasusedassubstrateforthedevelopmentof GOT-1.AsconcludedfromtheRamananalysis,photocatalyst par-ticleswereonlysparselydepositedontheexternalsilicasurfaceof GOT-1,allowingforthedirectinteractionofmostofthesilicalayer withthepollutantmolecules.Inthiscontext,theGOT-1membrane

mainlyconsistsofthesilicaNFmonolith(pHPZC≈2.5)and,thus,at thepHoftheexperiments(6.0–7.2)themembraneisnegatively charged,whileMBispositivelycharged(pKalessthan1andinthe cationicformabovethisvalue[40]andMOisnegativelycharged above4.2[41]).Evenso,forlowmassofpollutantsloadedintothe

Table1

Flowandpressureconditionsappliedduringthephotocatalyticfiltrationexperimentsandperformancecharacteristicsofthedevelopedmembranes.Inallcasesthefeed flowratewas1.5mL/minexceptfromthemembranedevelopedonthesupportwiththeporesizeof1nm(GOT-1)wheretheflowratewas0.3mL/min.

Pressure(bar) Membranesurface area(m2₎

Removalefﬁciency 104_×_(mg/min)

C/C0permeate Energyconsumptionbythe

pump(kWh/100m3₎

ReferenceMBUV 1.34 25.1 2.2 0.86 3.7

ReferenceMBvis 1.42 27.2 1.5 0.88 3.9

ReferenceMOUV 1.15 24.7 4.6 0.95 3.2

ReferenceMOvis 1.14 26.5 1.9 0.98 3.2

GOT-10MBdark 1.70 29.6 N/A 4.7

GOT-10MBUV 1.52 30.4 7.9 0.65 4.2

GOT-10MBvis 1.50 31.5 4.8 0.77 4.2

GOT-10MOdark 2.30 29.3 N/A 6.3

GOT-10MOUV 1.55 32.8 4.9 0.92 4.3 GOT-10MOvis 1.83 31.5 3.5 0.94 5.1 GOT-5MBUV 1.32 20.4 2.7 0.91 3.6 GOT-5MOUV 1.63 22.9 1.1 0.99 4.5 GOT-1MBUV 18.5 33.0 0.9 0.20 51 GOT-1MOUV 19.1 33.0 1.4 0.48 53

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reactor,Fig.8dshowsthattheremovalofthesepollutants(indark) isquitesimilarwhentheGOT-1membraneisemployed.Inthecase oftheGOT-10membrane,itconsistsofa␥-aluminaUFmonolith (pHPZC≈9.1)[42].AluminaispositivelychargedattheoperatingpH conditionsand,thus,theadsorptionofMB(positive)isexpectedto bequitelowandthatofMO(negative)quitehighinabarealumina membrane.However,theadsorptionofMBissigniﬁcantlyhigher thanthatofMO,demonstratingthatGOTwasreallydepositedon thismembrane;i.e.sincethepHPZCoftheGOTcompositeisnear3.2, theGOT-10membraneisnegativelychargedatthepHofthe exper-iments(6.0–7.2)andMBispreferentiallyadsorbedincomparison withMO.

3.3. Comparisonoftheperformanceintermsofpollutant removalefﬁciencyandenergyefﬁciencyoftheprocess

Table1presentstheoperationparametersandefﬁciencyofthe photocatalyticﬁltrationprocessforeachofthemembranes devel-opedinthiswork.TheperformanceresultsindicatethatGOT-10 wasthebestamongstthedevelopedmembranes.

WiththeGOT-10membrane, thehybridprocessexhibited a 4–8fold higherpollutantremovalefficiency(4-foldforMOand 8-foldfor MB) compared to GOT-1, while simultaneously con-sumingabout 13 times less energy for thepump that delivers pollutedwatertothereactor.Havingevidenced thattheGOT-1 membranefailedtoretainsignificantamountsoftheGOT compos-ite,therespectiveperformanceresultscanberegardedasthose ofastandardNFmembranewith1nmnominalporesize.Inthis context,iftheefficiencyofthemembranesisevaluatedin accor-dancetotheircapacitytoreducethedyesconcentration(C/C0)at thepermeateeffluent,thenGOT-1is2–4timesmoreefficientthan GOT-10.Despitethisdifference,itcanbeestimatedthatthe GOT-10membranecouldachieveidenticalperformancetoGOT-1under vis-light(performanceexpressedintermsofpollutant concentra-tionreductionatthepermeate)viarecycling5and10timesofthe MBandMOpermeateeffluents,respectively.Thisistranslatedto 5and10timeshigherenergyconsumption,which,nevertheless,is stilllowerthantheenergyconsumptionofGOT-1.

Fig.9ademonstratesthebeneﬁcialeffectofirradiationonthe energyexpenseofthephotocatalyticultraﬁltrationprocess focus-ingontheresultsobtainedforGOT-10.

Theordinateintheplotcorrespondstotheenergyconsumedby thepumpthatdeliveredtheﬂuidtothereactorwhiletheabscissais theamountofpollutantremovedbyGOT-10uptoacertainenergy expense.Itcanbeseenthatduringﬁltrationindark,theenergy consumedincreasesexponentially,atrendthatisconsistentwith thecontinuousdeclineofthemembrane’spermeancecausedby theadsorptionofthepollutantmoleculesonitssurface.

Ontheotherhand,thedataofthefiltrationexperimentsunder UVfitwelltoalogarithmicfunction,abehaviorthatcanberelated tothesuddenincreaseofthepermeanceat theinitialstagesof theexperimentunderUV(photocatalyticdegradationofthe pre-adsorbedpollutantmolecules,Fig.4b).Furthermore,theplotsof energyexpensevs.theamountofpollutantremovedundervis irra-diationarealmostlinear.Theunderlyingreasonisthatthevis-light experiment started immediately after thefiltration experiment underUV.Thus,duetothevis-lightphotocatalyticactivityofthe GOT-10membrane,thepopulationofsurfaceadsorbedmolecules remainedunchangedandasaconsequencetherewasnot signifi-cantalterationonthefluxcapacityofthemembrane.

Fig.9bcomparestheefﬁciencyandenergyconsumptionof GOT-10 and GOT-1membranes,taking alsointoaccount theenergy expenseoftheUVsourcethatirradiatedtheGOT-10surface.In particular,theenergyconsumedbyfourUVsourcesof9Weach wasaddedtothatconsumedbythepumpduringtheentire exper-imental period. Thepresented plots are very illustrative ofthe

importancetoachievethedevelopmentofvis-lightactive photo-catalyticceramicmembranesandthechallengetodesignahybrid photocatalytic/ultraﬁltrationprocess,wheresolarirradiationwill beexploitedatthemaximumpossiblelevel.

Althoughthepresentedresultswereobtainedinalaboratory scalereactor,wherethesmallsizeoftheinvolvedmembrane sur-face(onemonolithof30cm2₎_does_not_provide_the_flexibility_for appropriateprocessdesignandoptimizationoftheirradiation den-sity,itcanbeconcludedthatinahybridphotocatalysis/UFprocess themostenergyconsumingcomponentisthelightsource.Thus, besidesthemanyotherbenefitsofferedbyphotocatalytic/UF(less fouling ofthe membranes,highflux,notoxic condensates) the economicsustainabilityandpotentialindustrialapplicationofthe processwilldependontheefficientdevelopmentofvis-lighthighly activemembranesthatusesolarirradiation.

4. Conclusions

Partiallyreducedgrapheneoxide/TiO2membraneswere devel-opedbythedepositionofGOTcompositesonceramicUFandNF monolithsviadipcoating.Theporesizeofthemonolithwas cru-cialfortheamountoftheGOTcompositestabilizedonthesubstrate andforthevis-lightphotocatalyticefﬁciencyofthedevelopedGOT membranes.Ithasbeenconcludedthatonlythemembrane devel-opedontheUFmonolithwith10nmporesizeexhibitedenhanced photocatalyticperformanceundervisiblelight.Thiswasattributed tothecapacity ofthe10nmporestohosttheGOTcomposites, whereasasparsedepositionofTiO2,theactivephotocatalyst,was achievedinthemembranesdevelopedonthemonolithswithpore sizesof1and5nm.

Theperformanceofahybridphotocatalysis/UFprocess involv-ingtheGOT-10membranewasimprovedcomparedtothatofa standardNFprocessinregardtotheoveralldyeremovalcapacity andtothedyeconcentrationreductioninthepermeateefﬂuent.

TheliquidphasedepositionmethodinvolvedtodeveloptheGOT compositesandtheirfurtherstabilizationontoceramicmonoliths viadipcoatingareeasilyupscalableandthehybrid photocatal-ysis/UF process can be easily transferredto an industrial scale application for theremoval of azo-dyesfrom water, utilizing a multitude ofmonoliths withhigherlength.It canbeestimated thataphotocatalyticmembranereactormoduleaccommodating 30monolithsof1mlengthissufﬁcienttoproduce1m3_of_water perdayatanenergyexpenseof4–5kWh/100m3_which_is_about 20timeslowercomparedtoastandardNF process.Prerequisite forthis isthatsolarlightisusedastheonlyirradiationsource, somethingthatcouldbeachievedwiththegrapheneoxide/TiO2 membranesdevelopedinthiswork.

Acknowledgments

Financial support for this work was provided by project PTDC/AAC-AMB/122312/2010,co-financedbyFCT(Fundaçãopara aCiênciaeaTecnologia)andFEDERthroughProgrammeCOMPETE (FCOMP-01-0124-FEDER-019503).Thisworkwasalsoco-financed by FCT and FEDER through project PEst-C/EQB/LA0020/2013 (COMPETE),andbyQREN,ON2andFEDERthroughproject NORTE-07-0162-FEDER-000050. SMT and LMPM acknowledgefinancial supportfromSFRH/BPD/74239/2010andSFRH/BPD/88964/2012, respectively.A.M.T.SilvaacknowledgestheFCTInvestigator2013 Programme (IF/01501/2013),withfinancing fromthe European Social Fund and the Human Potential OperationalProgramme. TheauthorsalsoacknowledgefinancialsupportbytheEuropean Commission(CleanWater—GrantAgreementno.227017).Partof thisresearchhasbeenco-financedbytheEuropeanUnion (Euro-peanSocialFundA-ESF)andGreeknationalfundsthroughthe

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Operational Program “Education and Lifelong Learning” of the NationalStrategicReferenceFramework(NSRF)-ResearchFunding Program:Thales“AOP-NanoMat”(MIS379409).

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