Graphene oxide–polysulfone filters for tap water purification,obtained by fast microwave oven treatment

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Citation for the original published paper (version of record):Kovtun, A., Zambianchi, M., Bettini, C. et al (2019)Graphene oxide–polysulfone filters for tap water purification, obtained by fast microwave oventreatmentNanoscale(11): 22780-22787http://dx.doi.org/10.1039/c9nr06897j

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Grapheneoxide-polysulfonefiltersfortapwaterpurification,obtainedbyfastmicrowaveoventreatment.AlessandroKovtun,a†MassimoZambianchi,a†CristianBettini,aAndreaLiscio,bMassimoGazzano,a,FrancoCorticelli,bEmanueleTreossi,aMariaLuisaNavacchia,aVincenzoPalermo,a,c,*ManuelaMeluccia,*Theavailabilityofclean,purewaterisamajorchallengeforthefutureofoursociety.2-dimensionalnanosheetsofGOseempromisingasnanoporousadsorbentorfiltersforwaterpurification;however,theirprocessinginmacroscopicfiltersischallenging,andtheircostvs.standardpolymerfiltersistoohigh.Herewedescribeanovelapproachtocombinegrapheneoxide(GO)sheetswithcommercialpolysulfone(PSPSU)granulesforimprovedremovaloforganiccontaminantsfromwater.TheadsorptionphysicphysicsofcontaminantsonthePSPSU-GOcompositefollowstheLangmuirandBrunauer-Emmet-Teller(BET)modelmodels,withpartialswellingandintercalationofthemoleculesinbetweenGOlayers.Suchmechanism,well-knowninlayeredclays,hasnotbeenreportedpreviouslyforgrapheneorGO.OurapproachrequiresminimalamountsofGO,depositeddirectlyonthesurfaceofthepolymer,followedbystabilizationusingmicrowavesorheat.ThepurificationefficiencyofthePSU-GO-PScompositesissignificantlyimprovedvs.benchmarkcommercialPSPSU,asdemonstratedbyremovaloftwomodelcontaminants,RhodamineBandOfloxacin.Theexcellentstabilityofthecompositeisconfirmedbyextensive(100hours)filtrationtests(100h)incommercialwatercartridges.Theoutstandingadsorptionefficiencyofgrapheneoxide(GO)towardorganicmoleculeshasbeenwidelyprovedinthelastfewyears.1-7Thehighsurfaceareaandthemultiplesurfacechemicalgroupsenablestronginterfaceinteractionsbyp-pstackingthroughthesp2region,byHbondthroughthecarboxyl,hydroxylorepoxygroupsaswellasvanderWaalsandhydrophobicinteractions..3,8,9Suchuniquefeatureshavemotivatedtheextensiveuseofgraphene(bothGOandreducedGO,rGO)andpolymer-GO/rGO3Dcompositessuchashydrogel/aerogel,10,11sponges,12-14membranes15-20asadsorbentforwaterremediation.TheplethoralargeamountofreporteddataishighlightinghighlightsgrapheneandGOasthemostpromisingnanomaterialsforthedevelopmentofadvancedwatertreatmenttechnologies,bothdrinkingandwastewater,technologies.21-23KeyadvantagesofhybridGObased3Dstructuresare:i)theeasyrecoveringoftheadsorbentafteruse,ii)thesignificantenhancementofadsorptionperformancebyadditionofGOatevenatlowamount,iii)thepossiblemodificationofGOforpromotingselectivebindingprocesses.24-30

Figure1.a)RepresentativechemicalstructureofGOandPSPSU.b)SEMandc)opticalimageofthePSPSU-GOcompositefilter.TheGOsheetshavebeenhighlightedinblueinb).SEMunfilteredimagesareavailableinSI.Werecentlydemonstratedthesuperiorperformanceofpolysulfone-GO(PSPSU-GO)compositesfortheremovalofseveralclassesofemergingorganiccontaminantsfromtapwater(figure1).).31,32Thesecompoundsarecurrentlycauseofmajorenvironmentalconcernduetotheirincreasingoccurrenceinsurfacewaterbodiesandevendrinkingwaterandtotheirpotentialeffectonhumanhealthandecosystem.33-35ThecompositewasobtainedbyphaseinversionofaPSPSU-GOmixture(5%w/wofGO)usingwaterandN-methyl-2-pyrrolidinone(NMP)asorthogonalsolvents,i.e.thestandardindustrial

methodforthepreparationofPSUultrafiltrationmembranes.Thecontaminantscapturedcouldthenberemovedbysimplewashingofthecompositeinethanol,31allowinginthiswaytorecyclereusethefiltersmaterial.Amajorchallengeindevelopingnewmaterialsforfiltersforwaterfilterremediationistoensuretheirsafety,i.e.utilizecheapandenvironmentfriendlymethodstoproducethem,andstabilizethematerialsothatnopoisonousdebrisormoleculesarereleased,evenafterextensiveuse.Buildingonourpreviousresults,herewedescribeanewmethodtostabilizeGOnanosheetsonPSusingmicrowavesorheath.Theprocedureisentirelyperformedinwater.ThemethodisbasedondirectlycoatingGOoncoatthesurfaceofcheapindustrialscrapsofPS.PSUhollowfiberswithGO32Insteadofusingphaseinversion,wefixtheGOnanosheetsonthepolymerbyuseofmicrowavesorthermalconventionalheattreatments.Theprocedureisentirelyperformedinwater.SuchapproachallowstoproducegramsscaleofPSPSU-GOcompositepowdersmaterialwithnoneedoftoxicsolvents.,onPSUfiltersalreadycommercialized.Wethentesttestedtheabilityofsuchcompositestocapturetwoimportantorganiccontaminants(atextiledyeandamedicaldrug)comparingthemincomparisonwithstandard,commercialPSunmodifiedPSUgranules,unravellingthisallowingtounravelthephysicsoftheadsorptionprocess.WefinallytestFinally,wetestedthestabilityofthecompositeuponprolongedwaterflowoperation,byextensivetestinwaterpurificationfilters.CoatingofGOonPSPSUrecycledgranulesWeusedasstartingsubstratethescrapsofantheindustrialproductionofpolysulfonefiltrationhollowfibersultrafiltrationmembranes;madeofPSU(representingabout10%ofthetotalyearlyproduction);thissubstrate,evenifmorechallengingandcomplexthanpolymericsubstratesweusedinpreviousworks,36,37hasthekeyadvantage,asindustrialwaste,ofhavingnegativecostsandrepresentsanenvironmentaladdedvalueofthefinalcomposite.32Polysulfonehollowfiber(MediSulfone®)scrapsweremechanicallygroundedtoobtainpolysulfonehollowporousgranules(figureS1a-c,ESI).Polysulfonehollowfiberscraps(MediSulfone)weremechanicallygroundtoobtainpolysulfonehollowporousgranules(figureS1c).ThegranuleswerethendispersedinasolutionofGOinDIwater;GOconcentrationwastunedtohave5wt%GOcontentinthefinalcomposite.Waterwasremovedbyheatingat50°Conarotaryevaporator.ThegranuleswerethendispersedinasolutionofGOinDIwater,preparedasdescribedinESI;GOconcentrationwastunedtohave5wt%GOcontentinthefinalcompositeinordertoallowdirectcomparisonwithwhatalreadydescribedinpreviouswork.31Waterwasremovedbyheatingat50°Conarotaryevaporator.StabilizationofGOcoatingsonPSPSUAmainchallengeingraphene-polymercompositesistoensureagoodinteractionbetweenthenanosheetsandthepolymer,typicallytoenhancemechanicalproperties.38Incompositesforwaterfilterfilters,agoodinteractionisrequiredtoensurealong-lastingadhesionofthenano-additivestothematrix,toavoidtheirreleaseinthetreatedwater.GrapheneandGOareexcellentadsorbersofmicrowaves,andweexploitedthisopportunityinthepasttoachievefast,environmentallyfreefunctionalizationofGO.39Here,wedonotusemicrowavesforaspecificfunctionalizationoftofunctionalizeGOinsolution,buttoensurefixGOnanosheetstothePSUscraps,thusensuringagoodcohesionofperformanceandstabilizingthecomposite.ThePSstabilizationwasperformedonthePSU-GOpowderwasirradiatedwithbymicrowavesatatmosphericpressureinaCEMDiscoverSPapparatus(f=2.45GHz)whichhasinsitumagneticvariable

speed,irradiationmonitoredbyPCcomputer,infraredtemperaturemeasurementandcontinuousfeedbacktemperaturecontrol.Sampleswereirradiatedfor45minat100W(fixedpower)..DuringMWirradiationthetemperaturewasmeasured,andneverexceeded70°C.ThePSPSU-GOwasfinallywashedina1:1mixtureofwater/ethanoltoremoveanytracesofunreactedGOandfinallylefttodryatroomtemperatureuntilweightstabilized.(PSU-GO-MW,Figure1).Inanalternativeapproach,PSPSU-GOwasalsotreatedinastandardovenfor2hoursat120°C.Thematerialwasallowedtocooltoroomtemperature,thenwashedanddriedasdescribedabove.(figureS1d,ESI).Remarkably,theThepresenceofGOcoatingwasstableonlyaftertheMWorovenannealingtreatmentconfirmedbyhighresolutionScanningElectronMicroscope(SEM)images(figureS2).,ESI).Nochangesinsurfaceporosityandcross-sectionwereobservedincomparisontopristinePSgranulesaftercoating,asmeasuredbystandardtechniques(N2adsorption).Remarkably,bothtreatmentsenhancedgreatlythestabilityofGOonPSU,asshownbyextendedimmersioninwater(figureS3,ESI).ThepristinePSPSU-GOcomposite,thecompositetreatedwithmicrowavesandtheonetreatedinoven(namedrespectivelyBlank,PSGOPSU,PSU-GO-MWandPSGOPSU-GO-OV)werethencharacterizedbyXPSXRDandXRDXPS.

Figure2.(a).XRDpatternofGO(bluline),PSGOPSU-GO-MW(redline)andPSGOPSU-GO-OV(blackline).The(001)peakoftheGOstructureiswellappreciatedinbothPSGOPSU-GOsamples,at10-12°,togetherwiththeamorphousbell-shapedbandduetoPS.PSUat15-20°.XPSC1sspectraofb)pristineGO,c)PSGOPSU-GO-MWsamplesafterpolymerdissolutionwithDCMandd)PSGOPSU-GO-OVsamplesafterpolymerdissolutionwithDCM.

StructuralcharacterizationofthePSGOPSU-GOcompositesX-raydiffractionpatternswerecollectedinBragg-Brentanogeometry(CuKradiation,0.15418nm).AveragethicknessofGOcrystalstackswascalculatedfrompeakwidthbyusingtheScherrerequation.ThenumberofGOlayerswasestimatedastheratioofthesizeandtheinterlayerdistance,obtainedfrompositionof(001)GOpeak.Figure2acomparestheXRDofpureGOwiththePSGOPSU-GO-MWandPSGOPSU-GO-OVcomposites.PureGOshowedasharp,intensepeak,asexpectedforabulk2Dmaterial.Anaveragespacingof7.8±0.3Åwasobserved,withaverageof14±2GOlayersstackedontopofeachother.PSGOPSU-GO-MWandPSGOPSU-GO-OVsamplesshowedtheamorphousbell-shapedbandduetoPSPSU,butalsoaclear(001)peakoftheGOstructure,indicatingthepresenceofstackedGOmultilayersonthesurfaceofthePSPSUgranules.Thestackingdistancewas8.3±0.3ÅforPSGOPSU-GO-MWand7.9±0.3ÅforPSGOPSU-GO-OV;theestimatednumberoflayerswasca.10layers,slightlylowerthanwhatobservedinblanksample(seeTableS2S1,ESIforfulldetailsandexp.errors).Thefastmicrowaveoroventreatmentsdidnotchangethemesoscalestructureofthemembranes,withnodeformationordestructionofthePSUmicrochannelsasobservedbySEM.Nochangesinsurfaceporosityandcross-sectionwereobservedincomparisontopristinePSUgranulesaftercoating,asmeasuredbystandardtechniques(N2adsorption).ThedifferencesbetweenthetwomethodswereinsteadobservedclearlybyXPS.ChemicalcharacterizationofthePSGOPSU-GOcompositesXPSisapowerfultechnique,abletoprobetheoutersurfaceofthegranules,thusallowingtogetinformationonthepossiblepresenceoftheGOcoating,itsuniformity,itsoxygen/carbonratioandtheabundanceofdifferentchemicalgroups.WeperformedXPSonallthesamples,andanalysedtheresultswithanoveldeconvolutionprocedurewhichallowtoobtainprecisechemicalanalysisandoxygencontentinGOalsoinpresenceofotheroxygen-containingmaterials(figureFigure2b,c,dandTable1S2).40Suchprocedureisbasedonquantitativeline-shapeanalysisofC1ssignalswithasymmetricpseudo-Voigtline-shapes(APV),incontrasttoGaussian-basedapproachesconventionallyused(seeESIandref.40fordetails).TheXPSofuntreated,bulkPSPSUgranulesyieldedcarbon/oxygenandsulphur/carbonO/C=0.13±0.01andS/C=0.04±0.01ingoodagreementwithPSPSUchemicalcomposition(O/C=0.15andS/C=0.04,respectively).XPSofpristineGO(Figure2b)showedthepresenceofsp2areasandseveralchemicalgroupstypicalofGO,withanaverageoxygen/carbonratioO/C=0.39±0.01.(seealsofigs.S4,5andtableS2,ESI).XPStechniqueprobesonlytheoutersurface(2-3nm)ofthepowders;thus,fromthecontributiontoXPSdataofGOandPSPSUwecouldestimatetheamountofsurfacecomposedofuncoatedPSPSUandtheonecoatedfromtheGOnanosheets.TheGOcoatingcovered>50%ofthesurfaceofthegranules;however,itwasnotpossibletoobtainamorepreciseestimateestimation.ToovercomethisissueandhaveadetailedanalysisoftheGOcoating,wethushadtoremovecompletelythepolymerphase.Thiswasachievedbyimmersionindichloromethane(DCM).This);PSUissolubleinDCMwhileGOisstableinsuchsolventcompletelyremoves,allowingamoreprecisecharacterizationofthePS,whileleavingunalteredtheoxidationstateofGO.remainingGObyXPS.Forcomparisonsake,pureGOwasalsoimmersedinDCMandanalyzedanalysedbyXPS.Table1S2showstheO/Cratiomeasuredforallchemicalcompositionofthedifferentsamples,includingtheirO/Cratio.Weseethat,whiletreatmentwithMWdoesnotaltersignificantlytheGOchemistry(O/C=0.38±0.01),

classicalheatinginoven,evenifatarelativelylowtemperatureof120°C,givespartialreductionoftheGO,withtheO/Cratiodecreasingto0.30±0.01.Table1:Oxygen/CarbonratiomeasuredbyfittingtheXPSC1speakondifferentsamples.Exp.erroris±0.01inallsamples.Material O/CratioBlankPS* 0.13GO-pristine 0.39GOwashedinDCM 0.40GOMWafterPSdissolutioninDCM

0.38

GOOVafterPSdissolutioninDCM

0.30

*O/CofblankPSwasobtainedfromthearearatioofO1sandC1sduetothesignificantpresenceofS-OgroupsinPSstructure.ContaminantsadsorptiontestsTheadsorptioncapacitiesofblankPS,PSGOPSU,PSU-GO-MWandPSGOPSU-GO-OVsampleswerestudiedbyexposingthemtotapwatercontaminatedwithdifferentemergingorganiccontaminants(EOC).TheremovalperformanceofPSU-GOtowardsmanywidelyusedcontaminants(i.e.drugs,pigments,surfactantsetc.)waspreviouslydescribedinref.31;herewefocusedmoreonthepreparationtechniqueofthecompositeandontheirapplicationinwatercartridges.Astestcontaminants,wechoseRhodamineB(RhB,adyelargelyusedintextileandpharmaceuticindustries),andofloxacine(OFLOX,aquinolonicantibiotic).StructureofbothmoleculesisshowninFigure3.Bothmoleculesareofsignificantconcernforpollutionofsurfacewaterbodies.,41Structureofbothmoleculesisshowninfigure3.andadsorptioncapacityofOFLOXandRhBhasbeenalreadyreportedforGOsheets.42,43Using100%GOinthecartridgestoremovesuchmoleculeswouldbeeffective,butbulkGOinpowdercannotbeextrudedorprocessedinpelletsorfibers,itcanbevolatileandalsoburnorexplodeinsuitableconditions.HighadsorptioncapacitypriceofOFLOXandRhBwasthefiltrationcartridgeandproblemsofmaterialaggregationormechanicalstabilitywouldalsohinderstraightforwardapplicationofGO,makingitincompatiblewithactualcartridgeproductiontechnique,whichisbasedonPSUfibers.Conversely,theadvantageofourmethodisthatcanbeappliednotonlyoncommercialmembranesalreadyreportedforGOsheets.42,43Wethusproducedonlargescale,butalsoonscrapsderivingfromtheirpreparation,asdemonstratedintheprevioussections.WetestedtheadsorptionperformanceofpureGO,blankPS,PSGOPSU,PSU-GO-MWandPSGOPSU-GO-OVcompositesbydedicatedexperimentsperformedintapwatercontaminatedonpurposespikedwiththetargetmolecules(Figs.S10,ESI)..ExperimentaldatawerethenfittedusingBrunauer–Emmett–Teller(BET,eq.1),Langmuir(eq.2)andFreundlichmodels,seetablesS5-S10.DetailsofHPLCmethodusedforanalyticaldeterminationarereportedinESI.

TheadsorptionisothermofGOwasperformedatfixedconcentrationofRhBandOFLOXbyvaryingtheamountofGO.Thetwocontaminants(inpowder,asreceived)wereaddedtoGOsuspensions(5ml)atdifferentconcentrations.ThedetailsofsamplepreparationandalistofsamplespreparedarereportedintablesS3andS4.Theisothermcurveswereobtainedequilibratingthesolutionsfor24hrsatroomtemperature.ThesolutionswereanalysedbyHPLC.TheadsorptionisothermsofblankPS,PSGO-MWandPSGO-OVwerefittedwiththeBrunauer–Emmett–Teller(BET)adsorptionmodel(eq.1,figure3).TheBETmodeldescribesamultilayeradsorptionmechanisminthegas–solidandliquid-solidequilibriumsystems.describedbytheformula:44Qe=(Qm·CBET·x)/((1-x)·(1+CBET·x-x)) (1)HereQe(mg/g)isthequantityamountofmoleculesadsorbedwhentheequilibriumconcentrationisCe(mg/mL);Qmisthe,thequantityamountofmoleculesneededtocovertheentireadsorbentsurfacewithamonolayer(i.e.themonolayersaturationcapacity,mg/g).ThethermodynamicequilibriumBETconstantis𝐶"#$ = exp ∆𝐸 𝐾,𝑇 ,where∆Eisthedifferencebetweentheenergyofadsorptionoffirstandsecondlayersofmolecules,𝐾,𝑇isthetermalthermalenergy;x=Ce/Cs,whereCs(mg/mL)istheadsorbatesaturationconcentrationinBETadsorptionprocess(seeref.45).Thelinearfitwasperformedplottingy=1/(Qe·(1⁄x-1))vs.xandtheresultsareshowninfigure3andintableS3x.Langmuirmodeldescribesamonolayeradsorptionprocess,whichdependsontheequilibriumconcentrationCe(mg/mL);theQm(mg/g)andthethermodynamicequilibriumconstantKL(mL/mg).Qe=Qm·KL·Ce/(1+KL·Ce) (2)Remarkably,Eq.1canfitverywellalladsorptionisothermstestedwithofOfloxacineonthe3threedifferentmaterialsand2targetmolecules..ThisindicatesthattheadsorptionfollowsinallthesecasestheBETphysicalmodel,i.e.canformmultilayersonthesubstrate,fillingsmallporesfirst.OnceassessingthatOnthesamephysicalotherhand,theadsorptionofRhBonthe3differentmaterialscanbebetterdescribedbyLangmuirmodelapplies(eq.2).ItshouldbenotedanyhowthatbothmoleculesshowagoodfittobothBETandLangmuirmodels(tablesS8-S10,ESI).Freundlichisothermmodelgaveinsteadapoorfitforallthesurfacesundertest,wecomparedtheirperformanceincapturingRhBandOFLOXinsolution.samples.Figure3showsthatbothPSGOPSU-GOcompositesoutperformthestandardPSpolymerPSUmaterial.Inparticular,PSGOPSU-GO-MWsamplescapturedca.3timesmoreRhBthanblank,and>ca.2timesmoreOFLOXthanPSU.WealsocomparedItisinterestingtocomparetheadsorptionofRhBandOFLOXinsolutionwithadsorptionofN2moleculesfromgas,whichistheconventionaltechniqueusedtomeasurethespecificsurfacearea(SSA)ofnanomaterials(tableS11,ESI.

Figure3.ChemicalformulasstructuresofRhB(a)andOFLOX(b),andthecorrespondingadsorptionisotherms.BlacklinesrepresentLangmuirmodelforRhB(a)andBETmodelforOFLOX(b).Thetwoprocessesareverydifferent;N2adsorbsfromgasphase,withweakinteractionbetweenN2moleculesandthesubstrate,mainlyduetoVanvanderWaalsinteractions.Insolution,instead,moleculesadsorbbydisplacingothersolventmoleculesalreadypresentonthesurface,46ofteninteractingstronglywiththesubstrateandthesolventduetoelectrostaticforces.That’sThisexplainswhythesurfaceareameasuredbygasadsorptionofGO,47andmoregenerallyoflayeredmaterials,48issystematicallysmallerthanwhatmeasuredbyadsorptionofmoleculesinsolution.Tocomparetheadsorptioncapabilityofdifferentmolecules,weconvertedthequantityofmoleculesadsorbedQm(measuredaboveinmg/g)intheSSAideallyoccupiedbythosemoleculesusing:SSA=QmMw

-1NAAmol (23)whereMwisthemolecularweight,NAistheAvogadronumberandAmolistheestimated“footprint”ofthemolecule,i.e.theamountofsubstrateideallyoccupiedbyasingleadsorbedmolecule.WecouldestimateAmol»1.81nm2forRhBandAmol»1.30nm2forOFLOX,basedonSTM49orandXRDmeasurements50respectively.Thisisjustanapproximation,andtheactualfootprintofthemoleculesonthesurfacewilllikelyvaryduetospecificinteractionsbetweenthemolecule,thesolventandGO.51Weexpectthoughtthoughthatthefootprintwillremainofthesameorderofmagnitudeofwhatwaspreviouslyreported.4950Figure4acomparestheestimatedSSAofblankPS,PSGOPSU,PSU-GO-MWandPSGOPSU-GO-OV,estimatedbymolecularadsorptionofN2fromgas,RhBandOFLOXfromwatersolutions.WeunderlinethattheSSAreportedshouldbeconsideredasspecifictothematerialstudied,tobeusedonlyfordirectcomparisonamongsamples,becauseitdependsnotonlyonthematerial,butalsoonitsinteractionwiththemoleculeconsidered.

–

Figure4.a)Comparisonoftheareameasuredineachspecificmolecule/substratecombinationtested.Conversionfrommg/gtom2/ghasbeenperformedestimatingmolecularfootprint,asdescribedinmaintext.b,c)SchematiccartoonshowingthedifferentpossiblecapturemechanismofN2ingasandoforganicmoleculesinsolutions(seealsoref.48)Incaseofnitrogenadsorptionfromgas,allthreematerialsgivecomparableSSAvalues,»23-2526m2/g,confirmingthattheGOcoatingandfixationdoesnotchangetheoverallporosityofthematerial,andthatN2adsorptiondoesnotdependsignificantlyonthesurfacechemistryofthedifferentsamples.OFLOXadsorptiononpristine,blankPSPSUgivesSSA=20±2m2/g,avaluecomparabletowhatmeasuredwithnitrogen;conversely,uponGOcoatingthenumberofmoleculeswhichcanbecapturedbythematerialincreasessignificantly.TheestimatedSSAincreasesto3339±2m2/gfortheoven-treatedPSU-GO,andto67±4m2/gfortheMWtreatedPSU-GO,ca.threetimestheoriginalvalue.RhBgivesevenmoreextremechanges,likelyduetoitselectriccharges(beingasalt)..TheoriginalSSAmeasuredonblank(48±5m2/g)increasesto95105±7m2/gforoventreatedPSU-GO,andto135143±9m2/gforthemicrowavetreatedsample,correspondingtoanadsorptioncapacityof6063mgofRhodaminepergramofsamplecomposite.SuchSSAismorethanfivetimeslargerthantheSSAmeasuredwithnitrogen(24m2/g).Suchlargedifferencecannotbeexplaineduniquelybytheuncertaintyinthedetailedmoleculararrangementofthemoleculeonthesubstrates;furthermore,bothRhBandOFLOXgivesignificantlydifferentSSAondifferentsubstrates,whileadsorptionofN2fromgasgivessimilarvaluesforPSGO,PSGO-MWandPSGO-OV.allsamples.

Similarly,testsperformedon100%GO,withnoPS,gaveextremelyhighvaluesofSSA,reportedintableS6(thoughiftheuseofpureGOwouldnotbeeconomicallyviableforfilters).Thedifferencesobservedinfigure4suggestthatGOcanactasaneffectiveadsorbentfororganicmoleculesthankstoitslayeredstructure,abletocapturethetargetpollutantsformorethan6%ofitsweightinthebestscenario(RhBonmicrowavetreatedsamples).AdsorptiontestperformedonpureGOalsogaveveryhighvaluesofSSA(tableS5).ItisknownfromcharacterizationoflayeredmineralslikeclaythatthepresenceofswellingmineralsshouldbesuspectedwhentheSSAmeasurementsinliquidaresignificantlyhigherthangasadsorptionmeasurements.48GOisauniquelayeredmaterial,whichcaneasilybeexfoliatedinwater;52watermoleculescanintercalateandtravelefficientlyamongstackedGOnanosheets.53WatertrappedinbetweentheGOnanosheetscanbehaveasbulkwater.54Todemonstratethishypothesis,weperformedXRDscansonPSGOPSU-GO-OVcompositesexposedtoalargeexcessofRhBcontaminants(figureS9inS8,ESI).Intheexposedsamplesthe(001)peakofGOalmostdisappeared,aclearindicationthattheorganicmoleculesweredisruptingtheperiodicstackingofthenanosheets.WhileintercalationofionsormoleculescangiveXRDpeaksingraphiteintercalationcompounds,noperiodicitycouldbeobservedinthePSGOPSU-GOsamplesexposedtoRhB,likelyduetothesmallnumberofnanosheetsinvolvedandtheunevenintercalationofRhBwithintheGOlayers.GOstability,measuredbyreleasetestsAnymaterialusedforwaterpurificationshouldbesafe,i.e.shouldnotreleaseadditionalcontaminantsintheoutgoingwater.Acompletestudyofadsorbentmaterialsshouldnotonlymeasurehowmuchcontaminantscantransferfromthewatertothefilter;itshouldalsodemonstratethatthereisnotransferofcontaminantsfromthefiltertothewater.Weobservedintargetexperiments(figureS6S10,ESI)thatourGOis“intrinsicallysafe”,,whilebeinghighlysolubleindistilledwater,butisinsolubleintapwater,thusbeing“intrinsicallysafe”forthisspecificapplication.Inadditiontothisevidence,wedecidedtotestanypossiblereleaseofGObyinsertingthecompositesamplesinacommercialwater-filtercartridge55andflowingcirculatingwater(bothmQandtapwater)throughthecartridgeat2L/hfor100hours.

Figure5.a)PhotoofmQwaterfilteredfor100hthroughthePSU-GOfiltercomparedtostandardsolutionofGOinmQwateratdifferentconcentration.b)UV-Visabsorptionspectraofwaterfilteredfor100hthroughthePSU-GOfilter,comparedtocalibrationsamplesofwatercontaminatedwithdifferentamountsofGO.

ThefilteredwaterwasthenanalyzedtodetectpossibleGOtracesbeyondsuchlimit.Forbettersafetyassessment,weusedtwoparallelGOdetectiontechniques:UV-visabsorptionanddynamiclightscattering.WecomparedthefilteredwatertostandardsamplesconsistingofmQwatercontaminatedwithknownamountsofGO.WeperformedUV-visspectroscopyonwaterre-circulatedfor100hoursthroughfilterscontainingPSU-GO-MWandPSU-GO-OVpowders(FigureS11,ESI).Detectionlimit,estimatedwithcalibratedGOsolutions,wasabout1ppm.Figure5showstheimagesoftherecirculatedwater,aswellastheUV-visspectraofthefilteredwater(redline)andofcalibrationsolutionshavingaconcentrationrange0.25-10ppm.ThecomparisonindicatesthatanyGOpossiblyreleasedinfilteredwaterwasbelow1ppm.RecentworkindicatethatsafelimitsofGOconcentrationtoavoidtoxiceffectsarebetweenfewtensandfewhundredsof10-1000ppm(foracompletereviewonthisimportanttopicseeref.56.GOlimitsforaquaticorganismsareintherange40-2000ppm.57Experimentsintapwaterwerealsoperformedshowingtransparentsolutionevenafterconcentrationofthesample(FigureS15,ESI).

Figure5.a)Photooftapwaterfilteredfor100hthroughthePSGOfiltercomparedtowatercontaminatedwith10ppmofGO.b)UV-Visabsorptionspectraofwaterfilteredfor100hthroughthePSGOfilter,comparedtocalibrationsamplesofwatercontaminatedwithdifferentamountsofGO.ThefilteredwaterwasthenanalyzedtodetectpossibleGOtracesbeyondsuchlimits.Forbettersafetyassessment,weusedtwoparallelGOdetectiontechniques:UV-visabsorptionanddynamiclightscattering.WecomparedthefilteredwatertostandardsamplesconsistingofmQwatercontaminatedwithknownamountsofGO.WeperformedUV-visspectroscopyonwaterre-circulatedfor50hoursthroughfilterscontainingPSGO-MWandPSGO-OVpowders.Detectionlimit,estimatedwithcalibratedGOsolutions,was0,5-1ppm.Figure5showstheimagesoftherecirculatedwater,aswellastheUV-visspectraofthefilteredwater(redline)andofcontaminatedcalibrationsolutionshavingaconcentrationrange0,25-10ppm.ThecomparisonindicatesthatanyGOpossiblyreleasedinfilteredwaterwasbelow1ppm.DynamicLightScattering(DLS)measurestheauto-correlationfunctionofphotonsscatteredbynanoscopicobjectsinsolution,andweusedittodetectthepossiblepresenceofsmallparticlesornanosheetsreleasedinwater(seedetailsinESI,Figs.S4-5S13-14).Itsapplicationtonon-spherical

objectsisnotstraightforward,butithasbeensuccessfullyusedforthecharacterizationof2-dimensionalmaterialssuchasgrapheneandBN.58-60DLSdetectionlimitforGOnanosheetswas5ppm,asdeterminedwithcalibrationsolutionsofGOwithknownconcentration(seesection13,ESIfortheDLSspectrogramsandmeasurementdetails).).DLSmeasurementsperformedontapwaterre-circulatedfor100hoursthroughfilterscontainingPSGOPSU-GO-MWandPSGOPSU-GO-OVpowdersdidnotshowanypresenceofcontaminantsinthesizerange1nm-10mmforconcentrations>5ppm,confirmingthatnosignificantamountsofGOnanosheetswerereleasedbythecomposite.withinthelimitofdetectionrange.Forcomparison,blankPSPSU-GOcompositeswithnoMWorOVstabilizationtreatmentwouldinsteadreleaselargeamountsofdebrisandcontaminantswhensuspendedinwater,visiblebynakedeye(FigureS2inS3,ESI).Thisdemonstratedemonstratestheefficacyofthemicrowaveprocessingtostabilize,atlowtemperature,thecompositematerial,whilekeepingitsadsorptionperformancehigh.Attemptstoconcentratethesampletofurtherenhancethedetectionofcontaminants,bothbyultracentrifugationandvacuumevaporationfailed(FiguresS16-S17,ESI).ConclusionsTheprevioussectionsdatadescribedabovedemonstratethattheproposedapproachiseffectivelyastableandcost-effectivePSU-GOcompositecouldbeobtainedbyfixingathinlayerlayers(ca.10sheets)ofGOonthePSPSUgranules.Theprocessiseasilyup-scalable,relyingonlyonwaterprocessingandmicrowavesorconventionalheatingactivationofalreadyproducedmembranes,withouttheneedofadaptingalreadyexistingproductiontechnologiesandplants.Inparticular,microwavetreatmentsarealreadyextensivelyusedonlargescaleinindustry.Thestabilizationeffectofthetreatmentappearsveryeffective,yieldingnoareleaseofGObelowdetectionlimituponextensivetesting(2L/hfor100h)asdemonstratedbytwodifferentindependenttechniques.ThesynergiceffectofGOandPSUcanbedemonstratedbycomparingtheadsorptioncapabilityofthePSU-GOcompositevs.theperformanceofbulkGO(seealsotableS5andS11inESI).ThefinaleffectivesurfaceareameasuredforRhodamineremovalbyPSU-GO-MWis143m2/g,morethan10%oftheareameasuredinpureGO,evenifonly5%ofGOispresentinthecomposite.AsimilarestimatecanbedoneforOFL,wheretheperformanceisca.9%ofbulkGO,forthe5%loading.Theexcellentperformanceobtainedinremovingthetargetcontaminantsfromtapwatercanbeattributedtotheuniquepropertiesof2Dmaterials.BothGOandPSUcancapturewaterorganiccontaminants;however,theirmeso-structureandcapturemechanismisdifferent.WhilePSUfeaturesa3Dnetworkofmeso-andmicro-channels,GOfeaturesastackedstructurewithnanometricspacing,whichchangesuponmoleculeintercalation(seeXRDdatainESI).Toworkatbest,suchGOlayeredstructureshouldbefullyaccessiblefromsolution,i.e.arrangedasthinlayers,avoidingstrongaggregationorbulkclustersascouldhappeninbulkgraphiteoxidepowders.Thestructuredescribedhere,madeofthinGO2Dcoatingsonamechanicallystable3DstructureofPSUmicrochannels(figures1andS2)isthusidealtoensurethatallGOcanactivelycontributetothecontaminantremoval.Remarkably,theGOcoatingdoesnotmodifysignificantlytheporousstructureofthePSPSU.Thestandardmeasurementsofsurfacearea,basedonweakphysisorptionofN2moleculesingas,givesasimilarSSAbeforeandafterthecoating.However,theinteractionofthecompositewithorganicdyesmoleculesinsolutiongivesamuchhighereffectiveareaofinteraction,whichvarieswithsurfacechemistryandtargetmoleculestructure.QuantitativeanalysisindicatesthattheadsorptionprocessfollowsaBETandLangmuirmodel,andthatthereasonofimprovedperformanceisduetothefew-layersstructureoftheGOnanosheet,whichgivesahighereffectiveareaavailableforadsorptionvsstandardbulkpolymer,duetointercalationofthecontaminantsinbetweenthesheets.Suchprocessis

notobservedinstandardsurfaceareameasurementusingadsorptionofnitrogengasandissimilartowhatwaspreviouslyobservedinlayeredmineralslikeclays.Respecttootherlayeredmaterials,theGOnanosheetshavebetterprocessability,beingsolubleinwater,andaversatile,tunablesurfacechemistrywhichcanattractcontaminantswithp-p,VanvanderWaalsorhydrogenbondinginteractions.SuchmaterialWhilemicrowaveshavebeenusedextensivelytoexfoliateorfunctionalizegraphene,theiruseto“fix”GOonapolymersubstratehasnotbeenreportedbefore(patentsubmitted).Suchmethodcanbeappliedonmicroandultrafiltrationmembranesalreadycommerciallyavailable,aswellasscrapsderivingfromtheirpreparation,withouttheneedofadaptingindustrialproductionsystems.ThePSU-GOcompositeexhibitsthusanoriginaladsorptionmechanism,interestingforfundamentalscience,aswellaspromisingpotentialapplicationincommercialfiltersforwaterpurification,atopicoftimelyimportance.FurtherworkisongoingtobetterevaluatetheindustrialfeasibilityofPS-GObasedfilters(patentsubmitted).AcknowledgementsTheresearchleadingtotheseresultshasreceivedfundingfromtheEuropeanUnion'sHorizon2020researchandinnovationprogrammeunderGrapheneCore2785219–GrapheneFlagshipandtheSwedishResearchCouncil(projectJanus2017-04456).ConflictsofinterestTheauthorsdeclarenoconflictsofinterest.References1. Y.Zhou,O.G.ApulandT.Karanfil,WaterRes.,2015,79,57-67.2. Y.Shen,Q.L.FangandB.L.Chen,EnvironmentalScience&Technology,2015,49,67-84.3. G.Ersan,O.G.Apul,F.PerreaultandT.Karanfil,WaterRes.,2017,126,385-398.4. O.G.Apul,Q.L.Wang,Y.ZhouandT.Karanfil,WaterRes.,2013,47,1648-1654.5. Z.Q.Niu,L.L.Liu,L.ZhangandX.D.Chen,Small,2014,10,3434-3441.6. F.Perreault,A.F.deFariaandM.Elimelech,Chem.Soc.Rev.,2015,44,5861-5896.7. B.Beless,H.S.RifaiandD.F.Rodrigues,EnvironmentalScience&Technology,2014,48,10372-10379.8. L.H.Jiang,Y.G.Liu,S.B.Liu,G.M.Zeng,X.J.Hu,X.Hu,Z.Guo,X.F.Tan,L.L.WangandZ.B.Wu,EnvironmentalScience&Technology,2017,51,6352-6359.9. F.F.Liu,J.Zhao,S.G.Wang,P.DuandB.S.Xing,EnvironmentalScience&Technology,2014,48,13197-13206.10. F.Wang,Y.Wang,W.W.Zhan,S.R.Yu,W.H.Zhong,G.SuiandX.P.Yang,Chem.Eng.J.,2017,320,539-548.11. L.Gan,H.Li,L.W.Chen,L.J.Xu,J.Liu,A.B.Geng,C.T.MeiandS.M.Shang,Colloid.Polym.Sci.,2018,296,607-615.12. H.C.Bi,X.Xie,K.B.Yin,Y.L.Zhou,S.Wan,L.B.He,F.Xu,F.Banhart,L.T.SunandR.S.Ruoff,Adv.Funct.Mater.,2012,22,4421-4425.13. Y.Q.Chen,L.B.Chen,H.BaiandL.Li,JournalofMaterialsChemistryA,2014,2,13744-13744.14. J.P.Zhao,W.C.RenandH.M.Cheng,J.Mater.Chem.,2012,22,20197-20202.15. R.Rezaee,S.Nasseri,A.H.Mahvi,R.Nabizadeh,S.A.Mousavi,A.Rashidi,A.JafariandS.Nazmara,JournalofEnvironmentalHealthScienceandEngineering,2015,13.

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