Asymmetric polyelectrolyte multilayer membranes with ultrathin separation layers for highly efficient micropollutant removal

layers

for

highly

efficient

micropollutant

removal

Esra

te

Brinke,

Dennis

M.

Reurink,

Iske

Achterhuis,

Joris

de

Grooth,

Wiebe

M.

de

Vos

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received29July2019 Receivedinrevisedform 13September2019 Accepted16September2019 Keywords: Polyelectrolytemultilayers Nanofiltration Micropollutants Waterpurification Chimeramembrane

a

b

s

t

r

a

c

t

Newmembranematerials areurgentlyneededtoaddresstheincreasingconcentrationsofharmful organicmicropollutants(e.g.pharmaceuticals,pesticidesandplasticizers)inoursurfaceanddrinking water.Currently,thedensestavailablemembranescanremovemicropollutantssufficiently,butonlyat verylowpermeabilitiesandbyproducingahighlysaline,difficulttotreatwastestream.Weimprove permeability5–10foldbyproducinganasymmetricpolyelectrolytemultilayer(PEM)onaporous mem-brane,withaseparationlayerthicknessofonly4nm.Thisisachievedbyfirstcoatinganopenmultilayer topreventdefects,andsubsequentlyathinanddensemultilayer.Thisnovelmembraneshowsavery high(98%)retentiontowardamixofcommonmicropollutants.Moreover,itonlyretains10–15%ofNaCl, preventingtheformationofasalinewastestream.Adetailedliteraturestudyshowsthatthe asymmet-ricPEMmembrane,withthisuniquecombinationofproperties,significantlyoutperformscommercial membranesformicropollutantremovalapplications.

©2019TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Theemergenceofsmallorganicmolecules(100–1000Da)from medicines,pesticidesandplasticizers(micropollutants)in wastew-aterandsurfacewaterisasignificantchallengethatdemandsfor newandspeciallydesignedmembranematerials[1–3].Growing welfareand world-populationleadtoincreasingmicropollutant concentrations in surface waters and drinking water sources, threateningboththeaquaticenvironmentanddrinkingwater qual-ity.

Current technologiesare unable toefficiently remove these micropollutants from wastewater and surface water streams. Adsorption,forexample,isnotsufficienttoremove micropollu-tantsfor more than 90% [4]. Membrane technologiesare more promising,asmanycommercialreverseosmosis(RO)membranes do remove >90% ofmicropollutants, but operating these mem-branes is very energy-consuming because of their low water permeabilities (1–2Lm−2h−1bar−1 for modern RO membranes) [5].Commercialnanofiltration(NF)membranesaremoreopen,but withincreasingpermeability comeslowerretentions,especially towardsmallandunchargedmicropollutantssuchasbisphenolA [6,7].Theeffectivenessofcurrentlyavailablemembranesis

there-∗ Correspondingauthor.

E-mailaddress:w.m.devos@utwente.nl(W.M.deVos).

forelimited.Moreover,ROandtightNFmembranesweredesigned for desalination and can only remove micropollutants together withions[8].Thiscanleadtoaproductstreamwheresaltsneedto bere-addedtomakethewatersuitableforagricultureorhuman consumption. Even more problematic is the creation of highly salinemicropollutantwastestreams,thatarecomplexand expen-sivetotreat[9,10].

Polyelectrolytemultilayers(PEMs)arepromisingmaterialsto helpsolvethisproblem.PEMscanbecoatedonsurfaces,including porousmembranesupports,undermildandaqueousconditions byalternatinglyexposingthesubstratetopolycationandpolyanion solutions.Duringeachofthesesteps,electrostaticinteractionslead totheformationofathin(0.5–5nm)polyelectrolytelayerontopof theprevious,oppositelychargedlayer[11–13].Previousresearch showsthatsuchmultilayersshowahighstabilitywhenusedas denseseparationlayers ona chargedmembranesupport,being resistantagainstrepetitivebackwashing[14].Thestrengthof coat-ingaPEMasaseparationlayeristhatthepropertiesofthePEMcan befinelytuned,forexamplebythenumberoflayersandthesalt concentrationandpHduringthecoatingprocess[13,15]. Addition-ally,alargevarietyofpolyelectrolytescanbeusedtodeterminethe propertiesofthelayer[15,16].Hence,PEMbasedmembranescan beoptimizedtowardspecificapplicationsincludingdesalination [17]andionseparation[18].Moreover,PEMcoatingscanbeeasily appliedonhollowfibermembranes,thathavesignificant

advan-https://doi.org/10.1016/j.apmt.2019.100471

2352-9407/©2019TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4. 0/).

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Fig.1.ConceptofasymmetricPEMmembranes.a:Coatingathindenselayeronaporoussupportwillleadtodefects;b:ifthesupportporesarefilledwithadensePEMwith goodseparationproperties,waterneedstotravelalongpathwaythroughthePEMandthewaterpermeabilitywillbelow;c:ifthesupportporesarefilledwithanopen PEM,andathinlayerofdensePEMwiththedesiredseparationpropertiesiscoatedontop,acombinationbetweenahighselectivityandpermeabilitycanbeachieved. tagesoverothermembranegeometriessuchasreducedfouling,

whicheliminatestheneedforpretreatment[19].

Theirlargestadvantage,however,istheirpotentialtoformvery thinseparationlayers.Sincetheearlydaysofmembraneusage, theactiveseparationlayersofmembraneshavebecomethinner and thinnertoincrease permeability and thus decrease energy demand.Oneofthemostimportantbreakthroughsinthisregard wastheintroductionofmembraneswithanasymmetricpore dis-tribution[20,21],consistingofathinactiveseparationlayerwith asmallporesizeandasupportinglayerwithalargerporesizeto providemechanicalstabilitywithoutunnecessarilyreducingwater permeability.

Unfortunately,whencoatingafewnanometerthinPEMona poroussupport,itwillnotcompletelyclosethepores(Fig.1a).A muchthickerlayerisrequiredtoproduceadefect-freemembrane. Moreover,oncetheporesarefilled,theeffectivethicknessofthe PEMlayerismuchhigherthanthecoatedthicknessbecause perme-atingwaterhastotravelalongpathwaythroughthefilledpores (Fig.1b).Asaconsequence,thepotentialofPEMstoform ultra-thinseparationlayerscannotbefullyexploitedandthepurewater permeabilityofPEMbasedmembranesisstillrelativelylow.

Hereweintroduceauniqueapproachthatimprovesseparation propertiesbytakingmembranestothenextlevelofasymmetry. Tomaintainhighwaterpermeability,weclosethesupportpores witharelativelyopen,permeablePEM,andsubsequentlycoata thin,densePEMontopofthefirstlayertogeneratehighselectivity (Fig.1c).Wewilldemonstratethatthisapproachallowsustomake membraneswithseparationlayersasthinas4nm.

2. Resultsanddiscussion

Anessentialpart inthedesign of anasymmetric PEM coat-ingis the choiceof polyelectrolytes.From previousstudies we knowthatpoly(acrylicacid)/poly(allylaminehydrochloride) lay-ers(PAA/PAH),coatedatpH6andterminatedwithaPAAlayer, formdensePEMswith60–80%retentionofbothchargedand neu-tralmicropollutants[22].However,theseretentionswereachieved atthelowwaterpermeabilityof1.8Lm−2h−1bar−1,inlinewith expectations(Fig.1b).PEMmembranespreparedwithother poly-electrolytescanhave much higherpermeabilities. For example, membranesbased onpoly(styrene sulfonate) (PSS)/PAH, termi-natedwithPSS,haveapermeabilityof13–16Lm−2h−1bar−1after poreclosure(SupplementaryFig.S1).Moreover,PSSandPAHform verystablemultilayerswithalowpolyelectrolytemobility[23].A goodmicropollutantremovalmembranerequiresthe permeabil-ityandstabilityofPSS/PAHlayers,buttheselectivityofPAA/PAH layers.

ProducingasymmetricPEMmembraneswithaPSS/PAHbottom layerandathinPAA/PAHtoplayeristhereforeapromisingoption

tofullybenefitfromthehighstabilityandpermeabilityofthefirst polyelectrolytepairandtheselectivepropertiesofthelatter.

Indeed, we find that building such layers on model sur-faces is a straightforward process (Supplementary Fig. S2). To demonstratethepotentialofsuchasymmetriclayers,fivetypes of hollow fiber membranes were prepared, each with 10.5 bilayers in total. This includes two control membrane types that are coated completely with either PSS/PAH or PAA/PAH ([PSS/PAH]10.5 and [PAA/PAH]10.5), and threeasymmetric

mem-brane types [PSS/PAH]8.5+[PAA/PAH]2, [PSS/PAH]7.5+[PAA/PAH]3

and[PSS/PAH]6.5+[PAA/PAH]4.Allmembranesareterminatedwith

thepolyanion,becausewepreviouslyfoundthatPAA-terminated PAA/PAHlayersshowhighermicropollutantretentionsthan PAH-terminatedones[22].

Ofthefivedifferentmembranetypes,wedeterminedpurewater permeabilityandmolecularweightcutoff,MWCO(Fig.2a).The per-meabilityofthe[PSS/PAH]10.5membranesis15.2Lm−2h−1bar−1,

whilefor[PAA/PAH]10.5itisonly1.4Lm−2h−1bar−1,showingthe

clear difference in permeability between a dense and a more openPEMcoating.Theasymmetricmembraneshavesignificantly higherpermeabilitiesthan the[PAA/PAH]10.5 membranes,upto

12.8Lm−2h−1bar−1forthe[PSS/PAH]8.5+[PAA/PAH]2membranes.

While thepermeabilitykeeps ondecreasingwithanincreasing numberofPAA/PAHlayers,the90%MWCOplateausaround240Da fortheasymmetricmembranes.Theseresultsalreadydemonstrate thepromiseoftheseasymmetricmembranes,showingthatwecan decreasetheMWCObycoatingPAA/PAHontopofPSS/PAH lay-ers,whileretainingapermeabilityclosetothemuchmoreopen PAH/PSSmultilayer.Surprisinglywefoundaveryhigh90%MWCO forthe10.5 bilayerPAA/PAHmembranes.Forabetter compari-son,SupplementaryFig.S3shows thefullsievingcurvesof our membranes,wherewecanseethatthe[PAA/PAH]10.5membranes

actuallyperformbetterthantheothersinthelowmolecularweight range.Thepoor90%MWCOperformanceofthesemembranes indi-catesthattheysufferfromsmalldefects;weelaborateonthisin S3.

AlthoughtheobtainedMWCOresultsgivea goodindication thatourasymmetricmembranescouldretainmicropollutants,itis criticaltoprovethisbymeasuringrealmicropollutantretentions. For this purposewe use a micropollutant mix with8 microp-ollutants,containingpositivelycharged,negatively chargedand neutralorganicmolecules,withsizevaryingfrom216to624Da. Amongthemareseveralpharmaceuticals(atenolol, sulfamethox-azole, naproxen and bezafibrate), a pesticide (atrazine) and a plasticizer(bisphenolA).Structures,molecularweightandpKa

val-uesofthemicropollutantscanbefoundinSupplementaryTable S4.

Themicropollutantretentionsshowaclearoptimumforthe asymmetricmembranes(Fig.2b),whichisinlinewiththeMWCO

Fig.2. Performanceoftheasymmetricandcontrolmembranes.a:Purewaterpermeabilityandmolecularweightcutoff;b:micropollutantandNaClretention.Errorbars: standarderror;n=4.

results.Moreover,allmicropollutantretentionsareabove90%for theasymmetricmembranes,showingthattheasymmetricconcept providesanexcellentselectivityforwaterover micropollutants. ThedensePAA/PAH top layers rejectthemicropollutantsmore effectively than PSS/PAH,but asthe PAA/PAHtop layeris very thinthewaterpermeabilityremainshigh.Theasymmetric mem-branesareevenshowntobemoreselectivethanasymmetricdense PAA/PAHlayer.WeexpectthatthestablePAH/PSSbottomlayer helpstopreventdefectsthatwouldotherwiselowerthe selectiv-ity.Moreover,innanofiltrationahigherwaterfluxtendstoleadto ahighersoluteretentionaccordingtothesolution-diffusionmodel [24],whichalsoexplainstheoptimuminmicropollutantretention fortheasymmetricmembranes.

Thedifferencesinretentionbetweenthemicropollutantsseem tobelargelydependentonmicropollutantsize.Bromothymolblue, thelargestmoleculeinthemix,isnotdetectedinanyofthe perme-ates.Theotherlargemicropollutantsbezafibrate,phenolphthalein and atenolol are retained well by the asymmetric membranes without a clear charge dependency. The smallest micropollu-tants(atrazine,bisphenolA,sulfamethoxazoleandnaproxen)are retainedslightlyless.Differencesinretentionbetweenthosedonot correlatetosizeorcharge,butcouldbeexplainedaccordingtothe solution-diffusionmodelbydifferencesinaffinityforthePEM.

Thelowsodiumchlorideretentionsoftheasymmetric mem-branes, only 11–14%, demonstrate even more clearly that size exclusionisthepredominantseparationmechanism.Sodium chlo-rideions,withadiameterof0.2nmforbothNa+_andCl−_[25],are

muchsmallerthanthemicropollutantsthattypicallyhave dimen-sionsaround1nm[26]andarethereforeretainedless.Ifacharge basedexclusionmechanismwouldbedominant,nolarge differ-encewouldbeexpectedbetweentheretentionofsodiumchloride ionsandothermonovalentspeciessuchasthecharged microp-ollutants.Interestingly,thelackof chargeexclusion shows that ourmembranescontainaverylownetcharge,eventhoughtheir separationlayersconsistofpolyelectrolytes.

Theaveragemicropollutantretentions ofalltheasymmetric membranes are around 98%. In this region of highretentions, a smallchangein retention requiresa very largeimprovement in membrane selectivity. From standard PAH/PSS membranes (93% retention) to the asymmetric membranes we observe a 3.5 fold increase in membrane selectivity.Specificallywiththe

[PSS/PAH]8.5+[PAA/PAH]2 membranes,wecanthereforeproduce

waterwith3.5timeslessmicropollutantsatonlyslightlylower fluxescomparedwithourPSS/PAHcontrolmembrane.The asym-metriccoatingapproachclearlyleadstomuchhigherselectivities, withjustasmallreductioninflux.Becauseofthiscombinationof highselectivityandpermeability,weexpectthattheasymmetric membraneshaveverythinseparationlayers.Indeed,by ellipsome-tryweestimatedthe2,3and4bilayerPAA/PAHseparationlayersto beonly4,9and16nminthickness,respectively,intheirhydrated state(SupplementaryfileS5).The[PSS/PAH]8.5+[PAA/PAH]2

lay-ers were also visualized by field emission scanning electron microscopy(SupplementaryFig.S6),wherethetotalthicknessof thedryPEMwasfoundtobearound30nm,inlinewiththe ellip-sometryresults.

Theestimated4nmthicknessofthe[PAA/PAH]2 top layeris

incrediblythin,giventhatcommoncommercialseparationlayer thicknessesare 0.1–1␮m [24]. Via someproceduresitis possi-bletoobtainmorehomogeneousandthinnerlayersviainterfacial polymerization, downto 25nm [27] oreven 8nm[28]. To our knowledge,however,our4nmseparationlayeristhethinnest sep-arationlayerreportedsofarbyusingscalabletechnology.Besides beingreadilyscalable,asymmetricPEMcoatingalsoprovidesavery easyandenvironmentallyfriendlywaytoproducetheseextremely thin,defect-freeseparationlayers.Moreover,theasymmetric mul-tilayersarephysicallystableasincubatingtheminanacidicsalt bathhardlyinfluencespermeabilityandMWCO(Supplementary Fig.S7).ThisdemonstratesthepotentialofasymmetricPEMsfor large-scaleapplications.

To validate the outstanding properties of our asymmetric membranes, in particular the [PSS/PAH]8.5+[PAA/PAH]2

mem-branes, we compared theperformance ofourmembranes with theperformanceof commerciallyavailablereverseosmosisand nanofiltration membranes asdescribed in literature.A detailed overviewcanbefoundinsupplementaryTableS8.InFig.3a, per-meabilityand micropollutantretentionareplotted againsteach otherforourasymmetricPEMmembranes(darksymbols), sym-metricPEMmembranes(greysymbols)andcommerciallyavailable membranes(whitesymbols).WheretheasymmetricPEM mem-braneshaveaconsistentlyhighretentionforallmicropollutants, thecommercialmembranestendtoworkwellforonlysomeofthe investigatedmicropollutants,whileshowinglowretentionfor

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Fig.3.AsymmetricandsymmetricPEMmembranesincomparisonwithcommercialnanofiltrationandreverseosmosismembranes.a:Permeabilityversusmicropollutant retention;b:sodiumchlorideretentionversusmicropollutantretention.Whitesymbols:commercialmembranes;greysymbols:symmetricPEMmembranesinthisstudy; darksymbols:asymmetricmembranesinthisstudy.Dashedlines:bestsingleresultsobtainedwithcommercialmembranes;dottedlines:bestsingleresultsobtainedwith commercialmembranesforbisphenolA.Ifforasinglemicropollutantdifferentdatawereobtainedinliteraturewiththesamecommercialmembrane,resultswereaveraged. DetailedinformationcanbefoundinSupplementaryTableS8.

ers.Thedashedlinesshowtheupperlimitforsinglemicropollutant resultsobtainedwithcommercialmembranes.Theretentionsof our[PSS/PAH]8.5+[PAA/PAH]2membranesareabovetheselinesfor

allmicropollutants,whichshowsthattheiraverageperformanceis evenbetterthanthebestresultsforcommercialmembranes. Dot-tedlinesshowtheupperlimitforbisphenolAasobtainedwith commercialmembranes.BisphenolAisthemostdifficult microp-ollutanttoretain,asforthecommercialmembranes,onlytheRO membranesretainitformorethan95%.Thehighretentionofour membranestowardallmicropollutants,includingbisphenolA,thus demonstrates that ourasymmetric membranes have a microp-ollutant retention comparable with or betterthan the bestRO membranes.Atthesametimetheyhaveapurewaterpermeability intherangetypicalforNFmembranes.

Finally,asshownin Fig.3b themembraneshave avery low sodiumchlorideretentioncomparedwithcommercialmembranes, intherangeoftightultrafiltration(UF)membranes.The asymmet-ricPEMcoatingthusallowsustodecoupletheusualcorrelation betweenpermeability,saltretentionandmicropollutantretention, andimprovesmembraneefficiencybeyondthetraditionallimits. ThismeansthatcategorizationofournewasymmetricPEMbased membraneasanNFmembraneisnotsuitablebecausetypically, NFmembranesareconsideredtoshowaverageseparation proper-ties,inbetweenthoseofROandUFmembranes.Incontrast,our membranedecouplesandcombinessingledesirablepropertiesof RO,NFandUFtypemembranes:highwaterpermeability(NF),high micropollutantretention(RO)andlowsaltretention(UF).Wethus proposethatthemembraneshouldbeconsideredthefirstinanew category,theChimeramembrane,namedafterthemythical crea-turethatalsocombinedaspectsofvariousdifferentanimalsinto one.

Theuniquepropertiesofourmembranesarefurtherhighlighted bytheirexceptionalpermselectivityintermsofsaltpermeation overmicropollutantpermeation,thatisatleastoneorderof mag-nitudehigherthanforcommercialmembranes,asshowninFig.4

(calculationmethodscanbefoundinSupplementaryfileS9).Some commercialmembranesshowapermselectivitybelow1, which meansthattheirsaltretentionishigherthantheirmicropollutant retention, in strong contrast to the highsodium chloride per-meationoftheasymmetricPEMmembranes.Saltpermeationis verybeneficialasitwillpreventtheformationofahighlysaline wastestreamthatistypicalforstandardROandNFmembranes. Moreover,itavoidsbuildupofahighosmoticpressureoverthe membrane,thatwoulddemandahighertransmembranepressure toachievethesamewaterflux.

Fig.4. Purewater permeabilityversuspermselectivity of PEMand commer-cialmembranes.Darksymbols:asymmetricPEMmembranesinthisstudy;grey symbols:symmetricPEMmembranesinthisstudy;whitesymbols:commercial membranes.Ifforasinglemicropollutantdifferentdatawereobtainedin litera-turewiththesamecommercialmembrane,resultswereaveraged.Calculationsare explainedinSupplementaryfileS9.

3. Conclusion

Insummary,ournewconceptofasymmetricPEMmembranes allowsustocreateadvancedmembranematerialswithultrathin separationlayersandauniquecombinationofdesirableseparation properties,allowing itsclassification ina new membrane cate-gory:theChimeramembrane.Wehavesuccessfullyappliedthis newmembranetoachieveanoutstandingmicropollutant reten-tion(98%)andwaterpermeability(12.8Lm−2h−1bar−1),thereby outperformingtheselectivityofcommercialmembranestoalarge extent.With asymmetric PSS/PAH+PAA/PAH coatingswe were abletoproduceselectivelayersasthinas4nm,usingahighly scal-ablemembranefabricationapproach.Anadditionalbenefitofour PEMmembranesistheirlowsaltretention,suchthatNaCldoes notaccumulateinthemicropollutantwastestream.Clearly,our approachtoproduceasymmetricPEMmembranesishighly bene-ficialformicropollutantremovalprocesses.Thesemembranesare very suitablefor theproduction of safedrinkingwater and for micropollutantremovalfromwastewater,butalsoforother appli-cationssuchastheseparationofsaltsandorganiccompoundsin biorefinery.Finally,thisworkprovidesfundamentallynew mem-branedesignprinciplesforimprovedselectivitiesthatcanbetuned toother important applications,including desalination and ion recovery.

Funding:thisworkwassupportedbytheresearchprogramme InnovatiefondsChemie(LIFT)withprojectnumber 731.016.404, whichisfinancedbytheNetherlandsOrganisation forScientific Research(NWO),NXFiltration(Enschede,TheNetherlands)and Oasen(Gouda,The Netherlands).Thisprojecthasalso received fundingfromtheEuropeanResearchCouncil(ERC)underthe Euro-peanUnion’sHorizon2020researchandinnovationprogramme [grantnumberERCStG714744SAMBA].

WethankJurjenRegenspurgandOlafvanGinkelforthe permis-siontousetheirresultsonphysicalmembranestability.

AppendixA. Supplementarydata

Supplementarymaterial relatedto thisarticle canbe found, intheonlineversion,atdoi:https://doi.org/10.1016/j.apmt.2019.

100471.

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