manta ray gill inspired radially distributed nanofibrous

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Manta ray gill inspired radially distributednanofibrous membrane for efficient andcontinuous oil–water separation

Li, Zhengtao; Tan, Carl M.; Tio, Wee; Ang, Jeremy; Sun, Darren Delai

2018

Li, Z., Tan, C. M., Tio, W., Ang, J., & Sun, D. D. (2018). Manta ray gill inspired radiallydistributed nanofibrous membrane for efficient and continuous oil–water separation.Environmental Science: Nano, 5(6), 1466‑1472. doi:10.1039/C8EN00258D

https://hdl.handle.net/10356/107522

https://doi.org/10.1039/C8EN00258D

© 2018 The Royal Society of Chemistry. All rights reserved. This paper was published inEnvironmental Science: Nano and is made available with permission of The Royal Societyof Chemistry.

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Thisjournalis©TheRoyalSocietyofChemistry20xx J.Name.,2013,00,1-3|1

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Received00thJanuary20xx,Accepted00thJanuary20xx

DOI:10.1039/x0xx00000x

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MantaRayGillsInspiredRadiallyDistributedNano-fibrousMembraneforEfficientandContinuousOil-WaterSeparationZhengtaoLia,CarlM.Tana,WeeTioa,JeremyAngaandDarrenD.Sun*a

Tableofcontentsentry

Manta ray gills inspired super-hydrophilic membrane for efficient and continuous oil-water separation

Suspension feeding animals such as manta rays can separatewater and food particles quickly and continuously through theirspecialstructuredgillrakersviacrossflowfiltration.Inspiredbythisecosystemengineeringfromnature,afacilemembranebasedset-upforoil-waterseparationmimickingthemantaraygillrakerswasproposedinthispaper.Thiscrossflowprocesswasfabricatedusingaligned electrospun nano-fibrous silk fibroin membrane. In thisprocess,astheoil-watermixturetravel inparallelmanneracrossthemembranesurface,waterpermeatedthroughthemembranewhileoilwasrejectedbythemembraneandcollectedinthemiddlepipe. Compared to traditional super-hydrophilic membraneseparation conducted by gravity-driven dead-end approach, thismethod can avoid fouling issues and function continuously.Therefore, this nature-inspiredmethod creates new opportunityfor efficient oil-water separation.As an environmentally friendly

material, silk fibroin is naturally super-hydrophilic whichdemonstrateditspotentialinwatertreatment.

EnvironmentalSignificanceStatementOil spill and oily wastewater have become great threats toecosystem and human beings. Bio-inspired super-wetting surfaceshave shown great potential in oil-water separation. Super-hydrophilic and underwater super-oleophobic membrane isdevelopedwidely foroil removal fromwater.However, traditionaldead-endfiltrationset-upforoil-waterseparationissufferingfrompore-blockingduetotheaccumulationofoil.Inspiredbysuspensionfeedinganimalslikemantaray,afacilecrossflowmembraneset-upisdesignedforefficientandcontinuousfiltration.Withthisset-up,waterisabletopassthroughthemembrane,andoilwillbecollected.Silk fibroin as natural super-hydrophilic material is utilized formembranefabrication.

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1.Introduction

Mantaraysarelargeseacreatureswhichfeedonsuspendedfood particles in the ocean by filtering water rapidly andcontinuously[1]. Manta rays have never suffered from thecloggingoffoodparticles,evenalthoughtheyhavetofilterlargeamountofwaterallthetime,andthisphenomenonisowingtotheir special structured gill rakers[2]. As illustrated in Figure1(a),mantaraysseparatethefoodparticlesfromseawaterviaa crossflow filtration instead of dead-end filtration whichpreventsthecloggingoftheirgillrakers[3-5].Asseawaterandthe suspended food particles enter its mouth, the streamtravelsacrossgill rakers. These rakersallowseawater topassthrough but retains the suspended food particles. Thesesuspendedfoodparticlesarewashedoffthegillrakerstoavoidclogging. Thereafter, the stream of seawater becomes moreconcentratedwithsuspendedfoodparticlesandtravelsintotheesophagusofmantarays.Thiscrossflowfiltrationisbeneficialastheparticlesandcontaminantsarelesslikelytobeattachedto thegill rakersofmanta rays, comparedwith thedead-endfiltration[6]. This unique filtering structure from nature isinspiringforwatertreatment,especiallyinoil-waterseparation.

The increasing consumption of fossil fuels and frequent oilspillproducedoilywastewaterhasbecomeagreatthreattotheecosystemandsustainabledevelopmentofhumanbeings[7-9].Traditional oil-water separation technologies, such asskimmers[10], absorbents[11], controlled burning[12] andbiodegradation[13, 14], are suffering from low removalefficiencyorsecondarypollution[15].Thus,facilematerialswithspecial wettability, especially material which enables theeffectiveseparationofwaterandoil,arewidelystudied[16-19].Inspired from natural surfaces with special wetting behavior,materials are synthesized by manipulating the micro-/nano-structure or surface composition with the development ofcolloid and interface science[20-24]. Among them, super-hydrophilicandunderwatersuper-oleophobicmembraneshavebeen widely investigated and effectively employed for thegravity-drivendead-endoil-waterseparation[25-28].However,thismethodhasintrinsiclimitations,thesemembranescanbeeasily clogged by oil and the water flux will decreasesignificantly. Furthermore, the accumulation of oil on themembrane surface renders the separation process cannot beconductedcontinuously.

Inspiredbythecrossflowfiltrationofmantarays,wecansolvethis problem by combining membrane technology and thestructureofmantaraygillrakers.Bymimickingmantaraygills,a membrane based oil-water separation set-up was created.The membrane with aligned nanofiber was fabricated byelectrospinningona speciallydesignedcollector, as shown inFigure 1(c-d). During the oil-water separation process, waterpermeatesthroughthemembranerapidly,whileoilisrepelledandcollectedintheinnerironpipe.Silkfibroinwaschosenastherawmaterialtofabricatethenano-fibrousmembranedueto its naturally super-hydrophilicity and underwater super-oleophobicity.

The laboratory scale experiments have verified that themembrane exhibits the ability to separate oil and watercontinuouslyandefficiently.Moreover,silkfibroinisanaturalmaterial which exhibits no hazard to the environment. Thiswork provides a new perspective to achieve efficient andcontinuous oil-water separation. Additionally, silk fibroin hasexhibited great potential as a novel material for watertreatment.

2.MaterialsandMethods

2.1Materials

Iron pipe (diameter 5 mm) were used as the electrodeconnecting to earth for electrospinning. Chemicals usedincludes sodiumcarbonate (Na2CO3), calciumchloride (CaCl2),formicacid(FA)werepurchasedfromSigma-Aldrich.Deionizedwater (DI water, 18.4 MΩ·cm-1 resistivity), ethanol (Chem-Supply Pty. Ltd., absolute) were used for posttreatment andcleaning.Oliveoil,cookingoil,andsiliconoilwereusedastestliquids.

2.2PreparationofSilkFibroin

Afterdegummingin5%Na2CO3aqueoussolutionat100°Cfor1 hour, silk was dissolved in formic acid-CaCl2 solution.Subsequently,thedissolvedmixtureisdriedandwashedbyDIwatertoobtaintheregeneratedsilkfibroinfilm.

2.3PreparationofElectrospunSilkFibroinMembrane

According to the results of our preliminary experiments, asshowninFigureS1,lowconcentrationofsilkfibroin(<10%)leadto the“beads-on-string” structureofelectrospunnano-fibers,which influenced the quality of membrane. When theconcentrationreached10%,uniformno-beadnano-fiberswereobtained successfully, and therefore 10%was chosen for thefabrication our membrane in this research. 10% silk fibroinsolutionwasproducedbyadding1.0gofsolidsilkfibroininto7.54mLofformicacidandwasstirredfor6hoursuntilthesolidsilkfibroinwasfullydissolved.Theelectrospinningwascarriedoutatroomtemperatureinaclosedchamber.A5mmironpipeisusedastheelectricallyconductedcollector,withaplasticpipesurrounding the iron pipe for the silk fibroin fibers to attachitself. The silk fibroin fibers were then electrospun. The tipcollectordistancewaskeptat20cmandavoltageof24kVwassupplied.Thesilkfibroinsolutionwasplacedinasyringewithablunt-tip needle (nozzle diameter of 0.7mm).Using amicro-syringe pump, the spinning solutionwas pushed through theneedleatafeedingrateof0.2mL/h,6hourswererequiredtoproducethemembranebyelectrospinning.

Thereafter,theelectrospunmembranewasdippedinethanolfor1htoensurethatthefibersdonotclumptogetherwhenitcomeincontactwithwater.AsshowninFigureS2,withoutanytreatment,nano-fiberstructureofthemembranewasdamaged

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afterimmersinginwaterfor3hrs,whileafterethanoltreatment,nano-fiber structure was not influenced even immersing inwaterfor1week.

2.4Characterizations

Themicrostructuresofpreparedsilk fibroinmembranewerecharacterizedbyfieldemissionscanningelectronicmicroscopy(FESEM, JEOL JSM-7600F). The surface wettability of themembrane was tested by Video Contact Angle (VCA) device(ASTProduct Inc.Optimaseries).Avideocamerawasusedtodetermine the contact angle of each test. Thermogravimetricanalyzer (TGA, Perkin Elmer TGA-7) measurement wasperformedfrom30°Cto700°C,ataheatingrateof10°Cmin-1 inN2atmosphere.Fourier transforminfrared(FTIR)spectraof the sampleswas recordedon a Perkin Elmer spectrumGXFTIRspectrometer.

2.4Oil-waterseparation

Inorder to investigate theoil-waterseparationabilityof themembrane, several typesof oil-watermixturewerepreparedusingvegetableoil,oliveoilandsiliconoil.Theoil/watervolumeratiowas1:9.Totalorganiccarbon(TOC,ShimadzuVCSH)wasused to analyze oil concentration. The membrane was pre-wettedbywateratthebeginning.Thereafter,oil-watermixturewas continuously discharged into the filtration setup.Subsequently, water permeate through the membrane bygravitywhileconcentratedoil-watermixturewascollectedviatheironpipeinthecenter.

The separation efficiency can be calculated by oil rejectionratio(R%):

R = 1 −C&C'

×100%

whereC'andC&aretheoilconcentrationofthefeedoil-watermixtureandthepermeatewater.

3ResultsandDiscussions

3.1SurfaceMorphology

By specially designing the collector of electrospinning asillustratedabove, thenano-fibrousmembranesetupcouldbeobtained easily. Field emission scanning microscopy (FESEM)wasutilizedforthecharacterizationofthemicrostructureoftheas-preparedmembrane.Theradiatedistributionofnano-fiberswasprovedbytheFESEMimageasshowninFigure2(a).Thisfiber-radiatemembranewasdesignedtomimicthemantaraygills for continuous oil-water separation. Figure 2(b) is theenlargedimageofthenanofibers.Itisclearlyobservedthatthediameter of the fibers was around 100 nm. The averagediameter of the fiberswas 106 nm. The largestwas 198 nm,whilethesmallestwas40.3nm.Thefiberdiameterdistribution

isshowninFigureS4.Therefore,amantaraygillsinspirednano-fibrousmembranewasobtainedbysilkfibroinelectrospinning.

3.2FTIRandTGA

Figure 2(c) shows the FTIR spectrum of the silk fibroinmembrane. Peaks were observed at 3300 cm-1, 1630 cm-1,1530cm-1and1265cm-1.Thepointwherethevibrationis3300cm-1depictsadsorbedwaterinthesilkfibroinmembrane.Theother three notable peaks or troughs are consistentwith silkfibroin fibers thathaveundergoneethanol treatment[29,30].The thermogravimetric curve of as-prepared membrane isshowninFigure2(d).Thefirststageofweightlossbelow150˚Cisduetotheevaporationofwater,whilethesecondstagefrom300˚Cisassociatedwiththedegradationofsilkfibroin[31,32].Silk fibroin fiberswith theseprofilesexhibitbetter stability inwater and better mechanical properties as compared to silkfibroinfibersthathavenotundergoneethanoltreatment[33].

3.3WettingBehavior

WettingBehaviorofthemembraneplaysanimportantroleinoil-waterseparation.AsshowninFigure3(a),duringthecontactangletestingprocess,whenawaterdropletcontactedwiththemembrane surface, it quickly spread, and the water contactangleinairwas0°,whichillustratedthesuper-hydrophilicityofthe membrane. When the membrane was underwater, asshown in Figure 3(b), the oil contact angle was 154° , whichillustrated the underwater super-oleophobicity of themembrane.

This wettability of the membrane is naturally from theabundant hydroxyl and amino groups on the surface of silkfibroin fiber. According to Wenzel model and Cassie-Baxtermodel,ahydrophilic surfacecanbecomemorehydrophilicorsuper-hydrophilicwiththe increaseofroughness[34,35].Dueto themicro-/nano-structure of electrospun nano-fibers, thismembrane possesses great roughness and thus super-hydrophilicity.Meanwhile,withtheaccordanceofthewettingtheories, the in-air-hydrophilic surface always showsoleophobicity underwater[36, 37], which is the same as theresult of our test.With super-hydrophilicity in air and super-oleophobicity underwater, this membrane exhibits greatpotentialinoil-waterseparation.

3.4SeparationofOilandWaterMixtures

Continuous oil-water separation was conducted to test theefficiency of the membrane. The separation efficiency wasevaluatedusingthreetypesofoil,andtheoilconcentrationwasmeasuredbasedonthetotalorganiccarbon.Acertainvolumeof oil-water mixture was discharged into the set-upcontinuously.Waterpassedthroughthemembranequicklybygravityandwascollectedbytheplastictube,whilerejectedoilwascollectedthroughtheinnerpipebyabeakerasshowninFigure4(a).Figure4(d)illustratedthattheseparationefficiency





for three types of oil was at least 99.9%, showing efficientseparationbehavior(99.96%foroliveoil-watermixture,99.92%forsiliconeoil-watermixture,99.96%forhexane-watermixture,99.97%forvegetableoil-watermixture,99.91fordiesel-watermixtureand99.91%forengineoil-watermixture).

Due to the super-hydrophilicity and underwater super-oleophobicityof themembrane, therewillbeawater filmonthemembrane surface, which is a stable underwater Cassie-Baxter wetting mode and can help achieve a robust watercushionbetweenoildropletsandthemembranetopreventoilfrompermeatingthrough[37-40].Meanwhile,theradialarraysofnanofibersandthecrossflowofoil-watermixturecreatesacontinuouslydecreasingoil-watercontactfriction[41-43].Asaresult,whenanoildropletisdepositedonthesurface,itwillbedriventothecenterbyboththewettabilitygradientandgravityasshowninFigure5(a),ratherthanattachingonthemembranesurface and blocking the pores. At the same time, when oildroplets are accumulated to the center, small droplets willconvergetoformbigonesandfinallybecollectedinthecentralpipe,justlikethefeedingprocessofmantarays.Therefore,thismembranesetupisabletoachieveefficientandcontinuousoil-waterseparation.

3.5Longtermfiltrationtests

Inordertoquantifythesuitabilityofthemembranefor longtermfiltration,separationtestswerealsoconductedovertimeandpermeatevolume.

Firstly,totesttheeffectivenessofthemembraneperformanceover a long period of time, the electrospun silk fibroinmembrane subjected to constant filtration of a 1:9 oil-watermixturebyvolume.Samplesaretakeneveryhouroveraperiodof 8 hours. These samples are then tested for its TOCconcentrationtocalculateitsseparationefficiencyover8hours.AsseenfromFigure5(b),themembranerejectionperformanceremains high over a period of eight hours. This showed theeffectivenessofthecrossflowfiltrationthatwasadoptedinthemembranesetup.Thisensuresthemembraneperformanceforoil water separation can be maintained at a high level afterextendedperiodsoftime.

Secondly,themembranewastestedfor itspermeatequalityafter producing certain amountsof permeatewater. Samplesaretakenfrom50mLto700mLofpermeatewaterproduced.AsshowninFigure5(c),thepermeatequalityremainshighevenafter producing 700mL of permeate. This demonstrated theeffectivenessofthecrossflowfiltrationthatwasutilizedinthemembranesetup.Thisensuresthatpermeatequality fromoilwater separation can be maintained at a high level afterproducingasubstantialamountofpermeate.

ConclusionsInspired by the manta ray gill rakers, a novel silk fibroinmembrane was successfully fabricated for the efficient oil-

water separation. By specially designing the collector ofelectrospinning, the crossflow nano-fibrous membrane setupcould be obtained easily,whichwas able to achieve efficientandcontinuousoil-waterseparation.Waterwascollectedintheoutercylinder,whileoilwascollectedviathemiddleironpipe.Themembranepossessedsuper-hydrophilicityandunderwatersuper-oleophobicity, which enabled the efficient oil-waterseparation, and the cross-flow design of the membrane alsoallowed continuous separation process. This investigation ofbio-mimicdesigningandfabricatingofnano-fibrousmembraneforcontinuousandefficientoil-waterseparationprovidedgreatinsightsforwaterpurificationandwatercollectionindifferentareas.

AcknowledgementTheauthorwouldliketothankthestaffofEnvironmentalLabandCentralEnvironmentalScienceandEngineeringLaboratory(CESEL)atSchoolofCivilandEnvironmentalEngineering(CEE),NanyangTechnologicalUniversity(NTU)fortheirassistanceintheuseofequipment.Inaddition,thescholarshipprovidedbyNTUisalsoappreciated.

ConflictsofinterestTherearenoconflictstodeclare.

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Figure1Crossflowfiltrationinmantaraygillsandbioinspiredsilkfibroinmembrane.(a)Illustrationofcrossflowfiltrationinfishgills.(b)Gillstructureofthemantarays.(c)Bioinspiredelectrospunsilkfibroinmembrane.(d)Illustraitonofbioinspiredcrossflowoil-waterseparation.





Figure2Characterizationofelectrospunsilkfibroinmembrane.(a)(b)FESEMimagesofnano-fibroussilkfibroinmembrane.(c)FTIRimageofthemembrane.(d)TGAimageofthemembrane.

Figure3Contactangleofnano-fibroussilkfibroinmembrane.(a)Watercontactangleinair.(b)Oilcontactangleinwater.





Figure4Oil-water separation test of nano-fibrous silk fibroinmembrane. (a) Separation setup. (b) Feedoil-watermixture. (c)Permeatewater. (d) Separation efficiency of six types of oil (99.96% for olive oil-watermixture, 99.92% for siliconeoil-watermixture,99.96%forhexane-watermixture,99.97%forvegetableoil-watermixture,99.91fordiesel-watermixtureand99.91%forengineoil-watermixture).

Figure5(a)Schematicoftheradiallyarrayednanofibersforoildropletself-transportation.(b)Separationefficiencyovertime.(c)Separationefficiencyoverpermeatevolume.

manta ray gill inspired radially distributed nanofibrous

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