Hydrophilic Nanofibers as New Supports for Thin Film Composite Membranes for Engineered Osmosis

Hydrophilic Nanofibers as New Supports for Thin Film CompositeMembranes for Engineered OsmosisNhu-Ngoc Bui and Jeffrey R. McCutcheon*

Department of Chemical and Biomolecular Engineering and Center for Environmental Sciences and Engineering, University ofConnecticut, 191 Auditorium Road, Unit 3222, Storrs, Connecticut, 06269-3222, United States

ABSTRACT: Engineered osmosis (e.g., forward osmosis,pressure-retarded osmosis, direct osmosis) has emerged as anew platform for applications to water production, sustainableenergy, and resource recovery. The lack of an adequatelydesigned membrane has been the major challenge that hindersengineered osmosis (EO) development. In this study,nanotechnology has been integrated with membrane scienceto build a next generation membrane for engineered osmosis.Specifically, hydrophilic nanofiber, fabricated from differentblends of polyacrylonitrile and cellulose acetate via electro-spinning, was found to be an effective support for EO thin filmcomposite membranes due to its intrinsically wetted open porestructure with superior interconnectivity. The resulting composite membrane exhibits excellent permselectivity while alsoshowing a reduced resistance to mass transfer that commonly impacts EO processes due to its thin, highly porous nanofibersupport layer. Our best membrane exhibited a two to three times enhanced water flux and 90% reduction in salt passage whencompared to a standard commercial FO membrane. Furthermore, our membrane exhibited one of the lowest structuralparameters reported in the open literature. These results indicate that hydrophilic nanofiber supported thin film compositemembranes have the potential to be a next generation membrane for engineered osmosis.

■ INTRODUCTIONEngineered osmosis (EO) is a state-of-the-art technology whichharnesses the natural phenomenon of osmosis to address globalissues related to water and energy.1−3 In this process, anosmotic pressure difference drives water across a semi-permeable membrane from a dilute feed solution to aconcentrated draw solution. EO has the potential to sustainablyproduce fresh water at low energy consumption,4−8 generateelectricity,9−13 and recover high-value dissolved solids.14−16

However, EO has not progressed beyond conceptualization andlab scale studies due to obstacles in membrane design, drawsolution recovery, system integration, scale-up, and definitiveprocess economics. This study focuses on membrane design, asit has been considered the primary obstacle to EO develop-ment.17−25

It has been shown that conventional thin film composite(TFC) membranes typically used in reverse osmosis (RO),while exhibiting excellent permselectivity, are not suitable forEO. The supporting layers of TFC RO membranes arecomprised of both a cast polysulfone layer and a nonwovenfabric. These layers cause severe mass transfer resistance nearthe interface of the selective thin film layer, which gives rise tointernal concentration polarization (ICP).18 ICP is the primaryphenomenon that reduces effective osmotic driving force andresults in poor water flux performance.26−30

A TFC membrane tailored for EO should produce highosmotic water fluxes combined with a high selectivity that bothrejects solutes from the feed solution and prevents solutes from

the draw solution from diffusing into the feed solution. Itshould also be robust, chemically stable, thermally stable, andeasy to fabricate at a large scale. Most importantly, the supportlayer must be designed to have a low structural parameter (S)to minimize ICP. The effective structural parameter Seff isdetermined in the following equation:31

τε

=St

effs eff

eff

where εeff is an effective porosity, τeff is an effective tortuosity,and ts is the thickness of the support layer.

18 Effective porosityand tortuosity refer to the interconnected region of the porousstructure that can be saturated with water and is, hence,available for transport of solutes.Electrospun nanofibers are a class of material that exhibits an

intrinsically high porosity and low tortuosity. These propertieshave led to their investigation for liquid filtration applicationssuch as water treatment32−35 and biopharmaceutical pro-cesses.36 These same properties make nanofiber mats promisingcandidates for TFC EO membrane supports.18 In our previousstudy, polysulfone (PSf) nanofiber was used to make a newgeneration of TFC EO membrane and yielded water fluxes up

Received: October 15, 2012Revised: December 11, 2012Accepted: December 12, 2012Published: December 12, 2012

Article

pubs.acs.org/est

© 2012 American Chemical Society 1761 dx.doi.org/10.1021/es304215g | Environ. Sci. Technol. 2013, 47, 1761−1769

pubs.acs.org/est

to three times higher than commercial FO membranes. PSf waschosen due to its chemical resistance and thermal stability aswell as its common use in TFC membranes. However, as withother TFC membranes, its hydrophobicity was found to beproblematic.18 In early work on TFC membranes use in FO,support layer wetting was found to be essential for osmoticflow. This is due to the fact that solutes can only diffusethrough the wetted porosity of the support.37 Any unsaturatedportion of the support layer does not contribute to the effectiveporosity and increase Seff. We therefore hypothesize that Seff canbe reduced by using an intrinsically hydrophilic nanofiber,which will fully wet and decrease τeff and increase εeff. The fullywetted and interconnected porous network will yield a supportmaterial that will create a membrane with one of the lowestpossible structural parameters to date and maximize osmoticwater flux performance.In this study, two common hydrophilic polymers, poly-

acrylonitrile (PAN) and cellulose acetate (CA) were blended atdifferent weight ratios in dimethylformamide (DMF) to formnanofiber mats by electrospinning. In a study on the glasstransition temperatures of PAN/CA blends, Barani andBahrami reported that these two polymers are compatibleand partially miscible at the molecular level in the amorphousregion and incompatible in crystal regions.38 It is believed thatblends generated from molecular mixtures of miscible polymersor highly dispersed mixtures of immiscible polymers maycombine properties of the miscible components to obtainsuperior mechanical properties to component polymers.39,40 Inother words, mixtures of polymers can be effectively used tomodify the properties of high molecular weight materials.39 Wehypothesize that by combining the relatively high hydro-philicity, flexibility, and spinnability of PAN with the toughnessand lower hydrolyzability of CA, we can tailor a robust blendednanofiber nonwoven for supporting a TFC membrane forengineered osmosis. The membranes produced in thisinvestigation exhibit a low Seff, a high osmotic water flux, alow reverse solute flux, and a remarkable mechanical integrity towithstand the stresses applied during operation and fabrication.

■ MATERIALS AND METHODSMaterials and Chemicals. Eastman cellulose acetate (CA-

398-3, acetyl content = 39.8%, Mw = 24 000 g/mol41) wasprovided by Eastman Chemical Co. Polyacrylonitrile (Mw= 150000 g/mol), m-phenylene diamine (MPD, >99%), 1,3,5-benzenetricarbonyl trichloride (TMC, 98%) and N,N-dime-thylformamide (DMF, anhydrous, 99.8%) were purchased fromSigma Aldrich (St. Louis, MO). Sodium chloride (NaCl,crystalline, certified ACS) was obtained from Fisher Scientific(Pittsburgh, PA). Sodium bicarbonate (NaHCO3) waspurchased from J.T. Baker (Phillipsburg, NJ). ISOPAR-G,referred to hereafter as “isopar”, was supplied by GalladeChemical, Inc. (Santa Ana, CA). Deionized water was obtainedfrom a Millipore Integral 10 water system (Millipore, Billerica,MA). Commercial asymmetric cellulose triacetate forwardosmosis membranes (CA) were provided by HydrationTechnology Innovation (HTI, Albany, OR) and used ascontrols. Polyester nonwoven fabric sheet (PET, Novatexx2442) was supplied by Freudenberg (Weinheim, Germany).This PET was removed using tweezers for some osmotic fluxtests. These samples were designated as TFC-no-PET hereafter.Electrospinning for Nanofiber Formation. Blends of

PAN and CA at different weight ratios were dissolved in DMFat 60 °C for 16 h to obtain homogeneous solutions of 16 wt %

of polymers. The solutions were then continuously stirred atroom temperature overnight. The ratios of PAN to CA variedfrom 0/10 to 2/8, 5/5, 8/2, and 10/0. TFC membranes formedon these supports were designated as 100CA, 80CA, 50CA,20CA, and PAN, respectively. A volume of 3 mL of as-preparedpolymeric solutions was electrospun onto the PET backinglayer under a potential field of 28.5 kV to form a nanofibrousmat. The flow rate was 1.0 mL/h, and the tip-to-collectordistance was 16 cm. The experiments were conducted in a 50%relative humidity atmosphere at ambient temperature.

Interfacial Polymerization for Polyamide Formation.Polyamide (PA) was formed on the PAN/CA nanofibroussupports by interfacial polymerization between m-phenylenediamine (MPD) and 1,3,5-benzenetricarbonyl trichloride(TMC), as described in our previous study.18 The as-preparedTFC membrane was then dried in the ambient atmosphere for4 min, dry-cured in the oven at 90−95 °C for 8 min, and rinsedwith NaHCO3 and deionized (DI) water before storing in DIwater at 4 °C.

Membrane Characterization. A cold cathode JSM-6335Ffield emission scanning electron microscope (FESEM) wasused to observe the surface morphology and cross-sectionalstructure of the nanofibrous support and the TFC-no-PETmembranes. Samples were first sputter coated with a thin layerof gold (Au) and platinum (Pt) before imaging to obtain bettercontrast and to avoid charge accumulation. For cross-sectionalimaging, TFC-no-PET membranes were freeze-fractured usingliquid nitrogen to achieve a clean edge with preserved porousstructure.The average equilibrium sessile drop contact angles of

deionized water on the nanofibrous support surfaces weremeasured by a CAM 101 series contact angle goniometer (KSVCompany Linthicum Heights, MD) at room temperature inambient atmosphere. Nanofibrous mats were first dried invacuum at 35 °C until obtaining constant mass and stored atroom temperature before testing. The values are the average of12 measurements from multiple samples. The volume of thesessile drops was adjusted at 10 ± 1 μL. The contact angle wasmeasured within seconds of the water drop being deposited.The mechanical properties of as-spun nanofibrous mats and

the TFC-no-PET membranes were obtained from the tensiletests in air at 25 °C using an Instron microforce tester. Adynamic mechanical analysis (DMA) controlled force modulewas selected and the loading rate was 0.5 N/min. As a control,the as-spun fiber mats were exposed to each step of thefabrication conditions but no TMC and MPD monomers wereused. Each measurement represents an average of at least 6samples.

Membrane Performance Tests. Reverse Osmosis Teststo Determine Membrane Permeability Coefficients. A bench-scale crossflow RO testing unit was used to evaluate theintrinsic pure water permeability coefficient, A, and solutepermeability, B, of the TFC membranes at 25 ± 0.5 °C. A andB were derived from elsewhere.19 The system was operated at100 psi with a fixed crossflow velocity of 26.36 cm/s (Re ∼1312) using a 2000 ppm NaCl feed solution to determine B.No spacer was used.

Osmotic Flux Tests and Determination of TFC MembraneStructural Parameters. Osmotic water flux and reverse saltleakage through TFC and TFC-no-PET membranes werecharacterized using a lab-scale cross-flow forward osmosissystem. The experimental setup was described elsewhere.18,29

The fluxes were measured in forward osmosis (FO) and

Environmental Science & Technology Article

dx.doi.org/10.1021/es304215g | Environ. Sci. Technol. 2013, 47, 1761−17691762

pressure-retarded osmosis (PRO) modes at 25 ± 0.5 °C usingDI water as the feed solution and 1.5 M NaCl as the drawsolution. The hydraulic pressure was equal (1 psi) on both sidesof the membrane. Note that there was no pressure difference atboth sides of the membrane in both FO and PRO modes. Thecrossflow velocity was maintained at 15.82 cm/s (Re ∼ 757) forboth the feed and draw solution. A polypropylene mesh with anopening size of 0.080 × 0.055 was used on both sides of themembrane as spacers. The effective structural parameter, Seff,was derived from empirical values A, B, and Jw obtained fromRO and FO tests with the following equation:

ππ

=+

+ +S

DJ

B A

B J Alneff

w

D,b

w F,m

where D is the diffusion coefficient of the draw solute at 25 °C,Jw is the experimental water flux, πD,b is the bulk osmoticpressure of the draw solution, and πF,m is the osmotic pressureat the membrane surface on the feed side.42

■ RESULTS AND DISCUSSION

Nanofibers Morphology and Hydrophilicity. Thesurface morphology of PAN/CA nanofibrous supports wereshown in Figure 1. Holding the total polymer concentrationconstant at 16% by mass, pure PAN, pure CA, and mixtures ofthe two polymers were electrospun into a nonwoven mat. Asshown in Figure 1, pure CA at this concentration did not formfibers and instead formed droplets indicating an electrosprayingprocess. When PAN was blended with CA, smooth anduniform fibers were obtained. The fibers containing higherpercentages of PAN exhibited larger fiber diameters. This wasattributed in part to the higher viscosity of these solutions(Figure 1f), which has been known to increase fibers diametersand pore sizes of the nanofibrous mats.43−46 The viscosity ofthese solutions was measured using a Brookfield Viscometer at50 rpm at room temperature using spindle 64. The contactangles of the PAN/CA nanofibrous supports were tabulated inTable 1. While it is noted that the roughness and size of the

Figure 1. FE-SEM images (1000× magnification) of electrospun polyacrylonitrile (PAN)/cellulose acetate (CA) blended nanofibers prepared inDMF at different weight ratios. The bottom right image is a plot of the viscosity of polymer solutions at different PAN/CA ratios, standard deviation0.5−0.8% that of the average values.

Table 1. Properties of TFC Membranes

membranescontact angles of nanofibrous supports

(deg.)pure water permeability, Aa

(Lm−2 h−1 bar−1)salt permeability, Ba

(Lm−2 h−1)avg. effective structural param., Seff

b

(μm)

BW30 4.074 ± 0.218 2.016 ± 0.149HTI-CTA 0.683 ± 0.025 0.340 ± 0.039 578.0 ± 16.280CA 104.2 ± 5.03 1.169 ± 0.660 2.252 ± 0.356 693.2 ± 180.7 (with PET)50CA 99.44 ± 4.00 1.288 ± 0.265 0.555 ± 0.327 624.0 ± 61.9 (with PET)20CA 89.28 ± 4.52 1.799 ± 1.137 0.577 ± 0.218 311.1 ± 62.6 (with PET)PAN 69.86 ± 15.83 2.036 ± 0.949 1.572 ± 1.161 290.7 ± 53.1 (with PET)

109.1 ± 4.6 (no PET)aA and B were obtained from a bench-scale RO crossflow unit at 25 ± 0.5 °C. A was measured at 50, 75, 100, and 125 psi. B was determined at afixed crossflow velocity of 26.36 cm/s (100 psi, Re ∼ 1312) using a 2000 ppm NaCl feed solution. bAssumed that the external concentrationpolarization occurring on the porous side of the membrane during forward osmosis test was insignificant. A large standard deviation of Seff for thesamples 80CA is likely due to the fact that the brittleness of these samples made them susceptible to be broken in testing conditions.



fibers can impact contact angle measurement, the smallercontact angles are indicative of the greater relative hydro-philicity of PAN.TFC Surface Morphology and Microstructure. Uniform

and defect-free thin polyamide selective layers were successfullyformed onto each of the porous PAN/CA nanofibroussupports. Figure 2 shows the surface morphology of polyamideformed on fibers containing 20% PAN (80CA), 50% PAN(50CA), 80% PAN (20CA), and 100% PAN (PAN). The insertsection in this figure displayed the polyamide surfaces at highermagnification (10 000×). The polyamide surface appeared tobe rougher when a higher percentage of PAN was used to makenanofiber. This may be due to the enhanced surface diffusion ofMPD molecules in along the more hydrophilic nanofibersurfaces to the interface with TMC. It resulted in a morevigorous interaction between MPD and TMC to generaterougher polyamide topography.In Figure 3, the cross-sectional structure of the TFC-no-PET

membrane (Figure 3a and b) and the underside of thepolyamide layer (Figure 3c and d) are imaged by SEM. Thenanofibrous support was carefully removed from the polyamidelayer after the FO test. The total thickness of the compositemembrane without the PET was between 10−15 micrometers,as measured using several techniques including SEM and amicrometer. It can be seen in Figure 3c that fibers in the firstlayers of the nanofibrous mat integrate directly into the

polyamide layer. This results in a strong bond between thepolyamide and its nanofiber support.Furthermore, nanometer scale pores were observed on the

underside of the polyamide (Figure 3d) where a few fibersdetached. We hypothesized that these “pores” might in fact bethe hollow “peaks” observed on these and other conventionalTFC polyamide membranes. We found evidence for this bycomparing the top surface morphology of a random PA coatednanofiber to the pore structure SEM images of a fiber coatedwith PA selected at random (Figure 3e, left) and the porestructure of the underside of the PA layer where a fiber ofequivalent size detached (Figure 3e, right). Comparison of thescaled images provides convincing evidence that the pores areactually the hollow peaks visible on the top of the PA layer.This finding provides insight into the elusive structure of thePA layer in TFC membranes. Furthermore, the rest of the PAlayer facing the support appeared to be relatively smooth. Thissuggests that the polyamide layer is anisotropic as has beenhypothesized in other studies.47,48 While further evaluation isbeyond the scope of this study, these images represent the firsttime the underside of the polyamide layer has been clearlyobservable after interfacial polymerization (without a complexsupport layer removal process) and suggest that a nanofiberplatform may serve as a tool for understanding thefundamentals of this widely used membrane fabricationtechnique.

Figure 2. FE-SEM images (1500× magnification, 10 000× inset) of polyamide thin film composite membrane supported on electrospunpolyacrylonitrile (PAN)/cellulose acetate (CA) blended nanofibers prepared in DMF at different weight ratios: (a) PAN/CA 2/8, (b) PAN/CA 5/5,(c) PAN/CA 8/2, (d) pure PAN.



Mechanical Properties of Membranes. Electrospunmaterials are often criticized for their lack of strength. In pastinvestigations, poor strength has been attributed to low fiberstrength and poor bonding between the fibers.49 Figure 4 showsthe mechanical properties of the as-spun fiber, the compositemembrane, and the as-spun fiber undergoing the fabricationprocedures without actually casting the films (treated spunmats). Treatment with isopar solvent and high temperatureslightly increased the strength and modulus of the mats whilesignificantly decreasing their elongation. As shown in Figure 4b,the decrease in elongation of the treated nanofiber was lesspronounced for pure PAN compared to nanofibrous matscontaining CA. It can be seen that, after treatment, PANnanofiber was able to maintain its flexibility better than CA or,in other words, CA fibers were more brittle than PAN fibers.

However, TFC-no-PET membranes had remarkably higherstrength and modulus yet lower elongation than as-spunnanofiber. Strength-at-break and Young’s modulus of TFC-no-PET membranes were increased by a factor of 5 to 8 whencompared to as-spun nanofibers. We attribute this dramaticincrease in strength to the polyamide (PA) layer acting as abinder of the fibers. Placing PA on top of the nanofiber matcreates a “composite polyamide” from the integration of thenanofibers directly into the PA matrix, as shown in Figure 3cabove. This is a remarkable finding since for conventional TFCmembranes, the PA layer sits mostly on top of the support layerand is considered exceedingly fragile. In our case, the relativelytough polyamide layer, though thin, lends strength and rigidityto the nanofiber “skeleton”. With our membranes, the

Figure 3. (a and b) Cross-sectional FE-SEM images of TFC membranes supported on PAN fibers: (a) 1500×, (b) 2500×. (c and d) Bottom view ofthe back side of polyamide selective layer: (c) 10 000×, (d) 20 000×. (e) Zoomed-in image showing the pores that were hypothesized to be formedon the bottom side of the PA selective layer when a fiber was removed off this layer.



composite formed between the PA and nanofiber is strongerthan either material on its own.The PA layer alone did not dictate the mechanical properties

of the membrane. Higher PAN percentages resulted inincreased elongation yet decreased strength and modulus ofthe TFC-no-PET membrane. However, both strength andflexibility are desirable properties of our membrane. Specifically,20CA obtained relatively high strength-at-break, elongation-at-break, and Young’s modulus compared to the other samples.This blend is stronger than PAN and more flexible than 80CAor 50CA. The TFC membrane generated from 20CA supportwas therefore anticipated to better withstand testing conditions.Performance of TFC Membranes. Permeability and

Selectivity of Polyamide Selective Layer. Our TFC mem-branes were tested in reverse osmosis conditions against a

BW30 RO and a HTI FO membrane as controls. Ourmembranes were found to be less permeable and more selectivethan the BW30 membrane. Also, these TFC membranes showhigher permeability with lower selectivity than HTI celluloseacetate membrane. Pure water permeability coefficient, A, andsalt permeability, B, are shown in Table 1. Water permeabilitywas slightly higher when more PAN was blended with CA inthe support. This may be attributed to the rougher PA skinlayer formed on top of PAN/CA supports having larger poresizes and greater hydrophilicity. Roughness has been found tobe proportional to water permeability of TFC membrane.50 It isalso worth noting that the nanofiber-supported TFCmembranes were able to withstand an applied hydraulicpressure of 150 psi in an RO cross-flow unit. This suggeststhat these TFC membranes could be developed for PROapplications.The B value was relatively high for PAN and 80CA samples

when compared to the 50CA and 20CA supported membranes.For PAN-supported TFC, larger pore sizes may result in PAlayer defects, since the layer must bridge between two largerfibers over a larger gap. This may leave the PA layer moresusceptible to breakage, especially if those fibers are swelling.This explains the higher B values for the pure PAN nanofibersupported membranes.Brittleness likely also plays an important role in membrane

performance. The 80CA sample, which shows the highest saltpermeability of all samples, also exhibits the most rigidproperties. If the structure is too brittle, the membrane cannotdeform under flow and fibers may break under the stressesassociated with the test. These fibers may subsequentlyperforate the PA layer either during the fabrication process orduring RO tests. These defects can be mitigated by using morePAN in the blends to increase the flexibility of the nanofibers.The B values of 80CA were 5 times higher than the 50CA and20CA. Therefore, at this polymer concentration, either toomuch PAN (i.e., pure PAN sample) or CA (i.e., 80CA sample)can lead to lower selectivity.

Osmotic Flux Performance of TFC and TFC-no-PETMembranes. In general, TFC membranes supported onnanofibers having higher PAN percentage exhibited higherosmotic water flux in both FO (draw solution on the supportlayer) and PRO (draw solution on the selective layer) modes.The osmotic water fluxes of the membranes are presented inFigure 5a. All TFC membranes achieved higher water flux thanthe HTI's CA membrane. Water fluxes increased considerablywith the degree of hydrophilicity and pore sizes (i.e., fiber size).The 20CA and PAN samples exhibited fluxes more than twicethat of HTI membranes in both FO and PRO modes.Interestingly, removing the PET only improved the waterfluxes slightly regardless of the orientation. Removing the PETlayer was expected to greatly reduce the structural parametergiven its 70 μm thickness. We anticipated that this would resultin substantial increases in water flux. However, removing thePET may also damage the polyamide layer and allow more saltto pass from the draw solution and cause concentrationpolarization (PRO mode) or a reduced osmotic driving force(PRO mode and FO mode). This is likely why the water fluxonly increased marginally after removal of the PET.While all of the TFC membranes exhibited higher water

fluxes than the HTI membrane, each membrane had an equalor lower reverse salt flux. This is not entirely surprising giventhat polyamide is inherently more selective than celluloseacetate. The 20CA membranes exhibited the lowest reverse salt

Figure 4. Mechanical properties of as-spun nanofibers mats, treatednanofibers mats and TFC-no-PET membranes: (a) tensile strength(MPa), (b) elongation-at-break (%), and (c) Young’s modulus (MPa).



flux, which is consistent with its low B values measured in ROtests. This is attributed to the good mechanical properties ofthis support. Meanwhile, the reverse salt flux of the PANsupported TFC was comparable with the HTI membrane.Again, this is likely due to the larger pore sizes of the fiber matsas well as swelling of the fibers, both of which could causedefects. After removing the PET, Js increased for both 20CAand PAN. While these tests indicated that the TFC membraneswere robust enough to undergo a stand-alone EO test withoutthe need of PET backing layer, the increase in the amount of

salt crossing over the membrane noticeably inhibited theosmotic water flux due to creating a more severe ICP.Nearly all of the nanofiber-supported TFC membranes

showed a lower Js in PRO mode versus FO mode, as shown inFigure 5b. While this is not commonly seen, we attributed thisto the extremely high water flux in PRO mode. Water fluxnegatively couples with reverse salt flux, thus leading to lower Jsin PRO mode. There is more work to be done in this area infuture studies on membrane transport.Specific salt flux, Js/Jw, is a metric that is used to determine

the amount of draw solute lost per unit of water that crosses themembrane. Lower Js/Jw is desirable to prevent the loss ofsolutes and, in certain cases, ICP.42,51,52 TFC membranes withand without PET generally had lower specific salt fluxes whencompared to HTI membrane in both FO and PRO modes. TheJs/Jw of the TFC membrane was remarkably lower than that ofthe HTI membrane in PRO mode. Specifically, the 20CAsample achieved a specific salt flux 10 times lower than HTI.Due to its very low specific salt flux, the 20CA TFC isconsidered an excellent candidate for further exploration as amembrane for FO and perhaps PRO. While the PANmembrane had a very high water flux, its higher salt flux mayprevent its use in certain applications (such as those requirehigh selectivity). However, in spite of having relatively highreverse salt flux, PAN membrane still obtained a low specificsalt flux.

Membrane Structural Parameters. The effective structuralparameter, S, was obtained from empirical data of themembrane support layer resistance to solute diffusion, K, andthe diffusion coefficient of the draw solute, D, at 25 °C.25

Results show that S values of TFC membranes decreased withincreasing PAN in the blends. As tabulated in Table 1, theaverage S decreased from 693.2 to 290.7 μm from samples80CA to PAN, respectively. This finding was in agreement withthe 2−3 fold increase in osmotic water flux results when morePAN was used. Nanofibrous supports containing more PAN aremore hydrophilic, which improves wetting and increases theeffective porosity. Also, the larger pore sizes with higherpercentages of PAN contributed to the reduction of S becauseof a lower tortuosity and higher porosity. Upon removal of thePET support, the structural parameter is further reduced. Forthe PAN supported sample, the empirically calculated value of Swas reduced to 109.1 μm after removal of the PET. This isamong the first reported structural parameters reported near100 μm.53 However, these S values were measured usingcurrent technique in which the external concentration polar-ization occurring at the porous support of the membrane wasassumed to be negligible. This assumption is likely not valid asmembrane design for FO and PRO improves.By electrospinning blends of the highly spinnable, flexible,

and hydrophilic PAN with the less hydrolyzable CA, weobtained hydrophilic nanofibrous supports that had an openand interconnected pore structure with reasonable mechanicalproperties. These materials served as excellent supports forpolyamide TFC membranes with excellent permselectivity, andwith further improvement, they will exhibit higher pressuretolerance and selectivity. These novel membranes exhibitedsome of the lowest structural parameters reported in literaturewith the best membranes having water fluxes that outperformedHTI membranes by a factor of 2−3 while equaling or reducingthe reverse salt flux from the draw solution. In all, hydrophilicnanofiber supported TFC membranes have the potential to bea next generation membrane platform for engineered osmosis.

Figure 5. Membrane performance in osmotic fluxes tests: (a) osmoticwater fluxes, (b) reverse solute fluxes, and (c) specific solute fluxes Js/Jw (mM) across the membranes. Experimental conditions: 25 ± 0.5°C, 1.5 M NaCl as the draw solution, DI water as the feed solution,crossflow velocities of 15.82 cm/s on both sides of the membrane (Re∼ 757). Data was obtained from five tests on independent samples.



■ AUTHOR INFORMATION

Corresponding Author*Phone. +1 (860) 486-4601. Fax: +1 (860) 486-2859. E-mail:[email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

We acknowledge funding from the National ScienceFoundation (CBET No. 1067564), Oasys Water, the Depart-ment of Energy, and the Environmental Protection AgencySTAR Program. We also acknowledge Eastman Chemical,Hydration Technologies Innovations for providing stockpolymers and commercial forward osmosis membrane for thisstudy, respectively.

■ REFERENCES(1) Cath, T. Y.; Childress, A. E.; Elimelech, M. Forward osmosis:Principles, applications, and recent developments. J. Membr. Sci. 2006,281, 70−87.(2) Elimelech, M.; Phillip, W. A. The future of seawater desalination:Energy, technology, and the environment. Science 2011, 333 (6043),712−717.(3) McGinnis, R. L.; Elimelech, M. Global challenges in energy andwater supply: The promise of engineered osmosis. Environ. Sci.Technol. 2008, 42, 8625−8629.(4) Kessler, J. O.; Moody, C. D. Drinking water from sea water byforward osmosis. Desalination 1976, 18, 297−306.(5) McCutcheon, J. R.; McGinnis, R. L.; Elimelech, M. A novelammonia-carbon dioxide forward (direct) osmosis desalinationprocess. Desalination 2005, 174, 1−11.(6) Martinetti, C. R.; Childress, A. E.; Cath, T. Y. High recovery ofconcentrated RO brines using forward osmosis and membranedistillation. J. Membr. Sci. 2009, 331, 31−39.(7) Cath, T. Y.; Hancock, N. T.; Lundin, C. D.; Hoppe-Jones, C.;Drewes, J. E. A multi-barrier osmotic dilution process for simultaneousdesalination and purification of impaired water. J. Membr. Sci. 2010,362, 417−426.(8) Choi, Y.-J.; Choi, J.-S.; Oh, H.-J.; Lee, S.; Yang, D. R.; Kim, J. H.Toward a combined system of forward osmosis and reverse osmosisfor seawater desalination. Desalination 2009, 247, 239−246.(9) Lee, K. L.; Baker, R. W.; Lonsdale, H. K. Membranes for powergeneration by pressure-retarded osmosis. J. Membr. Sci. 1981, 8, 141−171.(10) Seppal̈a,̈ A.; Lampinen, M. J. Thermodynamic optimizing ofpressure-retarded osmosis power generation systems. J. Membr. Sci.1999, 161, 115−138.(11) Skilhagen, S. E.; Dugstad, J. E.; Aaberg, R. J. Osmotic power-power production based on the osmotic pressure difference betweenwaters with varying salt gradients. Desalination 2008, 220, 476−482.(12) Achilli, A.; Cath, T. Y.; Childress, A. E. Power generation withpressure retarded osmosis: An experimental and theoretical inves-tigation. J. Membr. Sci. 2009, 343, 42−52.(13) Achilli, A.; Childress, A. E. Pressure retarded osmosis: From thevision of Sidney Loeb to the first prototype installationReview.Desalination 2010, 261 (3), 205−211.(14) Nayak, C. A.; Rastogi, N. K. Forward osmosis for theconcentration of anthocyanin from Garcinia indica Choisy. Sep. Purif.Technol. 2010, 71, 144−151.(15) Garcia-Castello, E. M.; McCutcheon, J. R. Dewatering pressliquor derived from orange production by forward osmosis. J. Membr.Sci. 2011, 372, 97−101.(16) Jin, X.; Shan, J.; Wang, C.; Wei, J.; Tang, C. Y. Rejection ofpharmaceuticals by forward osmosis membranes. J. Hazard. Mater.2012, 227−228, 55−61.

(17) Saren, Q.; Qiu, C. Q.; Tang, C. Y. Synthesis and characterizationof novel forward osmosis membranes based on layer-by-layerassembly. Environ. Sci. Technol. 2011, 45 (12), 5201−5208.(18) Bui, N.-N.; Lind, M. L.; Hoek, E. M. V.; McCutcheon, J. R.Electrospun nanofiber supported thin film composite membranes forengineered osmosis. J. Membr. Sci. 2011, 385−386, 10−19.(19) Yip, N. Y.; Tiraferri, A.; Phillip, W. A.; Schiffman, J. D.;Elimelech, M. High performance thin-film composite forward osmosismembrane. Environ. Sci. Technol. 2010, 44, 3812−3818.(20) Wang, R.; Shi, L.; Tang, C. Y.; Chou, S.; Qiu, C.; Fane, A. G.Characterization of novel forward osmosis hollow fiber membranes. J.Membr. Sci. 2010, 355, 158−167.(21) Chou, S.; Shi, L.; Wang, R.; Tang, C. Y.; Qiu, C.; Fane, A. G.Characteristics and potential applications of a novel forward osmosishollow fiber membrane. Desalination 2010, 261, 365−372.(22) Qiu, C.; Qi, S.; Tang, C. Y. Synthesis of high flux forwardosmosis membranes by chemically crosslinked layer-by-layer poly-electrolytes. J. Membr. Sci. 2011, 381, 74−80.(23) Setiawan, L.; Wang, R.; Li, K.; Fane, A. G. Fabrication of novelpoly(amide-imide) forward osmosis hollow fiber membranes with apositively charged nanofiltration-like selective layer. J. Membr. Sci.2011, 369, 196−205.(24) Shi, L.; Chou, S. R.; Wang, R.; Fang, W. X.; Tang, C. Y.; Fane,A. G. Effect of substrate structure on the performance of thin-filmcomposite forward osmosis hollow fiber membranes. J. Membr. Sci.2011, 382, 116−123.(25) Tiraferri, A.; Yip, N. Y.; Phillip, W. A.; Schiffman, J. D.;Elimelech, M. Relating performance of thin-film composite forwardosmosis membranes to support layer formation and structure. J.Membr. Sci. 2011, 367, 340−352.(26) McCutcheon, J. R.; Elimelech, M. Influence of concentrativeand dilutive internal concentration polarization on flux behavior inforward osmosis. J. Membr. Sci. 2006, 284, 237−247.(27) Gray, G. T.; McCutcheon, J. R.; Elimelech, M. Internalconcentration polarization in forward osmosis: role of membraneorientation. Desalination 2006, 197, 1−8.(28) Tang, C. Y.; She, Q.; Lay, W. C. L.; Wang, R.; Fane, A. G.Coupled effects of internal concentration polarization and fouling onflux behavior of forward osmosis membranes during humic acidfiltration. J. Membr. Sci. 2010, 354, 123−133.(29) Arena, J. T.; McCloskey, B.; Freeman, B. D.; McCutcheon, J. R.Surface modification of thin film composite membrane support layerswith polydopamine: Enabling use of reverse osmosis membranes inpressure retarded osmosis. J. Membr. Sci. 2011, 375, 55−62.(30) Li, W.; Gao, Y.; Tang, C. Y. Network modeling for studying theeffect of support structure on internal concentration polarizationduring forward osmosis: Model development and theoretical analysiswith FEM. J. Membr. Sci. 2011, 379, 307−321.(31) Park, M.; Lee, J. J.; Lee, S.; Kim, J. H. Determination of aconstant membrane structure parameter in forward osmosis processes.J. Membr. Sci. 2011, 375, 241−248.(32) Ma, H.; Burger, C.; Hsiao, B. S.; Chu, B. Ultra-fine cellulosenanofibers: New nano-scale materials for water purification. J. Mater.Chem. 2011, 21, 7507.(33) Wang, X.; Chen, X.; Yoon, K.; Fang, D.; Hsiao, B. S.; Chu, B.High flux filtration medium based on nanofibrous substrate withhydrophilic nanocomposite coating. Environ. Sci. Technol. 2005, 39,7684−7691.(34) Wang, X.; Fang, D.; Yoon, K.; Hsiao, B. S.; Chu, B. Highperformance ultrafiltration composite membranes based on poly(vinylalcohol) hydrogel coating on crosslinked nanofibrous poly(vinylalcohol) scaffold. J. Membr. Sci. 2006, 278, 261−268.(35) Yoon, K.; Hsiao, B. S.; Chu, B. High flux nanofiltrationmembranes based on interfacially polymerized polyamide barrier layeron polyacrylonitrile nanofibrous scaffolds. J. Membr. Sci. 2009, 326,484−492.(36) Zong, X.; Ran, S.; Kim, K.; Fang, D.; Hsiao, B. S.; Chu, B.Structure and morphology changes during in vitro degradation of



mailto:[email protected]

electrospun poly(glycolide-co-lactide) nanofiber membrane. Biomacro-molecules 2003, 4, 416−423.(37) McCutcheon, J. R.; Elimelech, M. Influence of membranesupport layer hydrophobicity on water flux in osmotically drivenmembrane processes. J. Membr. Sci. 2008, 318, 458−466.(38) Barani, H.; Bahrami, S. H. Preparation of polyacrylonitrile andcellulose acetate blend fibers through wet-spinning. J. Appl. Polym. Sci.2006, 103 (3), 2000−2005.(39) Oh, Y. S.; Kim, B. K. Aminated polyacrylonitrile blends withcellulose acetate. J. Macromol. Sci., Part B: Phys. 1997, B36 (5), 667−679.(40) Paul, D. R. and Newman, S. Polymer Blend; Academic Press:New York, 1978; Vol. 2.(41) Manjikian, S.; Loeb, S. ; McCutchan, J. W. Improvement infabrication techniques for reverse osmosis desalination membranes. InThe First International Symposium on Water Desalination, Oct. 3−9,1965; U.S. Department of the Interior, Office of Saline Water:Washington, DC, 1965.(42) Phillip, W. A.; Yong, J. S.; Elimelech, M. Reverse draw solutepermeation in forward osmosis: Modeling and experiments. Environ.Sci. Technol. 2010, 44 (13), 5170−5176.(43) Fong, H.; Chun, I.; Reneker, D. H. Beaded nanofibers formedduring electrospinning. Polymer 1999, 40 (16), 4585−4592.(44) Deitzel, J. M.; Kleinmeyer, J.; Harris, D.; Tan, N. C. B. Theeffect of processing variables on the morphology of electrospunnanofibers and textiles. Polymer 2001, 42, 261−272.(45) Huang, Z.-M.; Zhang, Y.-Z.; Kotaki, M.; Ramakrishna, S. Areview on polymer nanofibers by electrospinning and theirapplications in nanocomposites. Compos. Sci. Technol. 2003, 63,2223−2253.(46) Doshi, J.; Reneker, D. H. Electrospinning process andapplications of electrospun fibers. J. Electrostatics 1995, 35 (2−3),151−160.(47) Freger, V. Swelling and morphology of the skin layer ofpolyamide composite membranes: An atomic force microscopy study.Environ. Sci. Technol. 2004, 38, 3168−3175.(48) Freger, V. Nanoscale heterogeneity of polyamide membranesformed by interfacial polymerization. Langmuir 2003, 19, 4791−4797.(49) Huang, L.; Bui, N.-N.; Manickam, S. S.; McCutcheon, J. R.Controlling electrospun nanofiber morphology and mechanicalproperties using humidity. J. Polym. Sci., Part B: Polym. Phys. 2011,49 (24), 1734−1744.(50) Ghosh, A. K.; Jeong, B.-H.; Huang, X.; Hoek, E. M. V. Impactsof reaction and curing conditions on polyamide composite reverseosmosis membrane properties. J. Membr. Sci. 2008, 311, 34−45.(51) Hancock, N. T.; Cath, T. Y. Solute Coupled Diffusion inOsmotically Driven Membrane Processes. Environ. Sci. Technol. 2009,43 (17), 6769−6775.(52) She, Q.; Jin, X.; Tang, C. Y. Osmotic power production fromsalinity gradient resource by pressure retarded osmosis: Effects ofoperating conditions and reverse solute diffusion. J. Membr. Sci. 2012,401−402, 262−273.(53) Song, X.; Liu, Z.; Sun, D. D. Nano gives the answer: Breakingthe bottleneck of internal concentration polarization with a nanofibercomposite forward osmosis membrane for a high water productionrate. Adv. Mater. 2011, 23, 3256−3260.



Documents

Hydrophilic Nanofibers as New Supports for Thin Film Composite Membranes for Engineered Osmosis