Impregnated Membranes for Water Purification Using Forward Osmosis

Article pubs.acs.org/IECR

Impregnated Membranes for Water Purification Using Forward Osmosis Shizhong Zhao, Kaipin Huang, and Haiqing Lin* Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States S Supporting Information *

ABSTRACT: The wide use of forward osmosis (FO) for water purification has been restrained by the lack of membranes with high water flux. Current FO membranes consist of a thick microporous support and a paper layer, which creates internal concentration polarization and presents major resistance for water transport across such membranes. This study investigates novel FO membranes consisting of a thin porous structure fully impregnated with a hydrophilic polymer. The elimination of the open-pore structures in these impregnated membranes (IMs) avoids the internal concentration polarization. More specifically, IMs consisting of hydrophilic cross-linked poly(ethylene glycol) diacrylate (PEGDA) in a porous hydrophobic Solupor support were prepared and thoroughly characterized for water and salt transport properties using a dead-end filtration system, salt kinetic desorption experiments, and an FO system. The IMs showed performance ratios (defined as the water permeance in FO mode to that in the dead-end filtration system) 50% higher than those of state-of-the-art commercial FO membranes, demonstrating the promise of the use of IMs to mitigate internal concentration polarization. This work provides a new route to the design of FO membranes with potentially high performances.

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INTRODUCTION

costs, thus enabling the FO process to provide a feasible route for water purification. Various approaches have been explored to minimize the water transport resistance in the support and paper layer.20,21 For example, the support and paper layer have been coated with hydrophilic polydopamine to enhance their hydrophilicity.19 Membranes based on hydrophilic cellulose ester were shown to exhibit extremely high water permeances in FO mode.21 Membranes were prepared directly on an electrospun nanofiber support, eliminating the thick microporous membranes.22,23 These approaches have improved the water flux, confirming the critical need to reduce the transport resistance in the open pores of the membranes. Another approach, examined in this study, is to design impregnated membranes, completely eliminating the open-pore structures in the membranes. These membranes consist of a thin, highly porous support matrix (such as 84% porosity) impregnated with a hydrophilic polymer, as shown in Figure 1b. The porous support (10−40 μm) can be made considerably thinner than a conventional ultrafiltration membrane (100−150 μm), while still providing sufficient mechanical integrity for membrane handling and operation. These impregnated membranes (IMs) were originally developed for fuel-cell membrane applications24−31 and spacers for battery applications.32,33 Recently, these IMs have also been explored as membrane-exchange humidifiers for fuel cells34,35 and energyrecovery ventilators for buildings.36

Polymeric membranes have been widely used in seawater desalination and wastewater treatment to meet the increasing demand for clean water, because of their high energy efficiency and low cost.1−6 Forward osmosis (FO) has recently emerged as an attractive technology for water purification, partially because of its low-pressure operation and, thus, high energy efficiency.7−11 The core of the FO process is a highperformance membrane that is highly permeable to water while rejecting solutes. During FO operation, water selectively permeates through the membrane from a feed solution with low osmotic pressure to a draw solution with high osmotic pressure. FO membranes need to have high rejection rates for solutes to maintain high differences in osmotic pressure across the membrane and high water permeance to reduce the membrane area required for the separation. Figure 1a shows a schematic of a typical thin-film composite (TFC) membrane for FO water purification.10,12−14 TFC membranes were originally developed for reverse-osmosis desalination.4,15 The thin, dense skin layer (∼0.2 μm) performs molecular separation, and the hydrophobic porous bulk of the membrane (100−150 μm) provides mechanical strength but offers no resistance to water transport. However, the thick porous bulk presents major resistance to water transport during FO operation.16,17 Both the microporous support and the paper layer act as diffusion barriers between the selective layer and the draw solution in the bulk, generating concentration gradients. This effect is known as concentration polarization, and it reduces the effectiveness of the draw solution and countercurrent operation.7,10,12,13,16−19 There is a critical need to mitigate the internal concentration polarization in the porous structure to increase the water flux and reduce the operating © XXXX American Chemical Society

Received: September 1, 2015 Revised: November 23, 2015 Accepted: November 24, 2015

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DOI: 10.1021/acs.iecr.5b03241 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Schematic illustrations of (a) conventional asymmetric thin-film composite membranes used for FO water purification10,12−14 and (b) thin impregnated membranes (IMs).

Figure 2. Effects of concentration polarization on the driving force for water permeation in FO mode characterized by the osmotic pressure in (a) a conventional asymmetric membrane with the selective layer facing the feed solution and (b) a symmetric impregnated membrane.

The objective of this study was to investigate impregnated membranes consisting of a mechanically strong porous support filled with hydrophilic polymers for water purification using forward osmosis. First, a series of IMs were prepared and characterized for solubility, diffusivity, and permeability of water and salt. Second, the IMs were tested in the laboratory in FO mode to understand the effects of operating parameters (including the salt contents in the feed and draw solutions and the flow rates of the feed and draw solutions) on separation performance. Finally, the effect of concentration polarization on the water permeance in these IMs is discussed. The separation performance of the FO membranes prepared in this work is compared with those of commercial membranes and others reported in the literature, and the potential of IMs is evaluated.

JW = AW (pF − pP ) =

(2)

During FO operation, there is usually no pressure difference across the membrane (i.e., pF = pP), and thus, the water flux is given by18 JW = AW (πD − πF) =

PW VW ̅ (πD − πF) l R gT

(3)

where πD, instead of πP, is used to characterize the permeate osmotic pressure, because the permeate stream is often called the draw solution during FO operation. The salt flux, JS (grams per m2 per hour or gMH), is often expressed by37,38

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BACKGROUND Theory of Water Transport in Membranes. Water flux, JW (liters per m2 per hour or LMH), through a dense polymer film follows the solution−diffusion mechanism and can be described by37−39 JW = AW (ΔP − Δπ ) =

PW VW ̅ (p − pP ) l R gT F

JS = B(CS,F − CS,P) =

PS (CS,F − CS,P) l

(4)

where B (cm/s) is the salt permeance in the membrane; PS (cm2/s) is the salt permeability; and CS,P and CS,F are the salt concentrations (g/cm3) in the permeate and feed solutions, respectively. Within the context of the solution−diffusion model, the salt permeability is expressed as37

PW VW ̅ [(p − pP ) − (πF − πP)] l R gT F (1)

PS = DSKS

where AW is the water permeance (LMH/bar), ΔP is the pressure difference across the membrane (bar), and Δπ is the osmotic pressure difference (bar) across the membrane. PW (cm2/s) is the water permeability, l (cm) is the film thickness at equilibrium with the feed solution, V̅ W is the molar volume of water (18 cm3/mol), Rg is the gas constant [83.1 cm3 bar/(mol K)], and T is the temperature (K). The subscripts F and P indicate properties on the membrane feed side and the permeate side, respectively. For pure-water permeation, there is no osmotic pressure difference across the membrane (i.e., πF = πP), and thus, the water flux is given by

(5) 2

where DS is the average salt diffusivity (cm /s) in the polymer and KS [(grams of salt per cm3 of swollen polymer)/(grams of salt per cm3 of solution)] is the salt solubility in the polymer. The salt solubility is given by39

KS =

m CS,F

CS,F

Cm S,F

(6) 3

where (grams of salt per cm of swollen polymer) is the equilibrium salt concentration in the membrane surface in contact with the feed solution. B


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were purchased from Sigma-Aldrich Chemical Co. (Milwaukee, WI) and used as received. Ethanol as the solvent was purchased from Fisher Scientific Co. (Waltham, MA). A porous support of Solupor 7P07C (with a pore size of 0.7 μm, a porosity of 84%, and a thickness of 45 μm) was purchased from Lydall Performance Materials, Inc. (Rochester, NH). An HTI OsMem TFC-ES membrane was purchased from Hydration Technology Innovations (Albany, OR). A Filmtec SW30XLE reverse-osmosis membrane was purchased from Dow Water & Process Solutions (Minneapolis, MN). Deionized water was produced using a Milli-Q water purification system (EMD Millipore, Billerica, MA). Preparation of Free-Standing Polymer Films. Freestanding polymer films were prepared using PEGDA by freeradical UV photopolymerization following procedures described in the literature.48,49 First, a solution containing PEGDA, solvent (deionized water or ethanol), and HCPK was prepared and stirred for 5 h. The HCPK content was 0.1 wt % with respect to PEGDA, and the solvent content was varied from 0 to 80 wt %. Second, the solution was sandwiched between a quartz plate and a glass plate separated by spacers with known thicknesses and exposed to UV light with a wavelength of 254 nm in an ultraviolet cross-linker (CX-2000, Ultra-Violet Products Ltd., Upland, CA) for 120 s at 3.0 mW/ cm2. The thickness of the polymer film was controlled by the thickness of the spacers. Finally, the film was removed from the plates and soaked in deionized water for 24 h, before being allowed to dry in air for 24 h and then in a vacuum oven overnight. Figure 3 shows the chemical structure of PEGDA and a schematic of cross-linked PEGDA (XLPEGDA).48

The osmotic pressure (π) is often related to the total ion concentration. In this study, NaCl solutions were used, and the osmotic pressure was estimated using the van’t Hoff equation40,41 π = 2CSR gT /MW,S

(7)

where MW,S is the molecular weight of the salt. The efficiency of salt separation by membranes is described using the salt rejection, RS37,38 ⎛ CS,P ⎞ ⎟⎟ × 100% R S ≡ ⎜⎜1 − CS,F ⎠ ⎝

(8)

Concentration Polarization. The adverse effect of concentration polarization on membrane performance has been well-documented in many applications, such as ultrafiltration,13,42 reverse osmosis,13 pervaporation,43 gas and vapor separation,12,44 and FO.16,17,19 Figure 2 presents the concentration polarization phenomena in FO for a conventional asymmetric membrane and a symmetric impregnated membrane. Figure 2a shows the concentration polarization in a typical FO membrane comprised of a selective layer on top of a porous support layer.7,16 The selective layer faces the feed solution, and the porous support faces the draw solution. As the water selectively diffuses through the selective layer, the solute content in the feed solution adjacent to the membrane increases (i.e., external concentration polarization), whereas the salt content on the draw side adjacent to the membrane in the porous support decreases (i.e., internal concentration polarization).45 The actual driving force (Δπ = πD,m − πF,m) is much less than the apparent driving force (Δπ = πD,b − πF,b), where the subscripts m and b indicate properties on the membrane surface and in the bulk solution, respectively. Figure 2b illustrates water diffusion in a thin dense membrane such as an impregnated membrane. The membrane does not have a porous support, and therefore, there is no internal concentration polarization. Considering external concentration polarization on both sides of the membrane, the following equation was derived41,46 JW = AW

Figure 3. (a) Structure of poly(ethylene glycol) diacrylate (PEGDA). (b) Schematic of cross-linked PEGDA (XLPEGDA), where R is COO(CH2CH2O)13OCO from the PEGDA.

πD,b exp( − JW /kF) − πF,b exp(JW /kD) 1 + (B /JW )[exp(JW /kD) − exp(−JW /kF)]

Preparation of Impregnated Membranes. In the preparation of the impregnated membranes, the first step was to prepare prepolymer solutions containing PEGDA and HCPK (0.1 wt %). Ethanol was used as the solvent because the Solupor support is hydrophobic and ethanol can easily wet the support. Second, a sheet of Solupor support was taped onto a Teflon plate. The solution was coated using a foam brush two times on each side of the Solupor support. Third, after the liquid had been removed from the support surface using paper, the support was exposed to UV light for 72 s for the PEGDA to polymerize. Finally, the impregnated membrane was soaked in deionized water, and the water was changed twice to remove the ethanol. Characterization of Polymer Physical Properties. The conversion of the acrylate groups in PEGDA was monitored by attenuated-total-reflection Fourier transform infrared (ATRFTIR) spectroscopy (Vertex 70, Bruker, Billerica, MA). The resolution of the measurement was 4 cm−1. The sol and gel fractions in the prepared free-standing films and IMs were determined.48 After polymerization, the polymer sample was dried and weighed (with the mass defined as m0).

(9)

where kD (cm/s) and kF (cm/s) are the mass-transfer coefficients in the draw solution and feed solution, respectively, adjacent to the membrane. Figure 2 clearly illustrates the advantage of symmetric membranes in having a greater practical driving force, compared to asymmetric membranes. However, current symmetric membranes require large thicknesses to obtain good mechanical properties, and therefore, they have not been extensively investigated for FO applications.9,16,47 Nevertheless, internal concentration polarization is regarded as one of the major obstacles to the implementation of FO processes for practical applications.7,9 The goal of this study was to explore the use of thin symmetric membranes for FO applications by preparing impregnated membranes (as shown in Figure 1b) providing good mechanical properties and high water fluxes.

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MATERIALS AND METHODS Materials. Poly(ethylene glycol) diacrylate (PEGDA, Mn = 700 g/mol) and 1-hydroxycyclohexyl phenyl ketone (HCPK) C


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Figure 4. Photographs of (a) the logo only, (b) a Solupor support on top of the logo, and (c) an impregnated membrane on top of the logo.

pure nitrogen gas. The permeated water was collected using a beaker, and the weight was monitored with time. Before the measurements, the film was equilibrated with water at 20 psig overnight. The water hydraulic permeance and permeability across each membrane were calculated using eq 2. Characterization of Salt Transport Properties. The salt diffusivity and solubility were determined using kinetic desorption experiments.52 The impregnated membrane was first equilibrated with deionized water and then cut into a disk shape with a diameter of 1 in. Second, the sample was soaked in 15 mL of 0.5 M NaCl (29.2 g/L) solution for 48 h at room temperature (∼23 °C) for the sample to equilibrate with the NaCl solution. Third, after the sample had been taken out of the NaCl solution and the liquid on the surface had been removed, the sample was immersed in deionized water (referred to as extraction solution). The salt content in the extraction solution was monitored as a function of time using a conductivity meter (Oakton CON 11 m, Oakton Instruments, Vernon Hills, IL). The solution was continuously stirred to ensure that the extraction solution was well-mixed. The NaCl diffusivity in the polymer sample, DS, was estimated using the Fickian diffusion model53−56

The sample was then immersed in deionized water for 24 h to remove the sol, before being dried and weighed again (with the mass designated as mgel). The gel fraction (Wgel) was calculated as mgel wgel = × 100% m0 (10) The film thickness was measured using a digital micrometer (Starrett 2900, The L.S. Starrett Co., Athol, MA). The films had thicknesses ranging from 30 to 500 μm. The sample density at room temperature was determined using a balance equipped with a density kit (XS 64, MettlerToledo Inc., Columbus, OH). Iso-octane was used as an auxiliary liquid. The following equation was used to calculate the polymer density (ρp)48 mair ρp = (ρ − ρair ) + ρair mair − mliquid liquid (11) where mair and mliquid are the weights of the sample in air and iso-octane, respectively, and ρair and ρliquid are the densities of air and iso-octane, respectively. The hydrophilicity of the polymer surface was determined using a contact angle goniometer (model 190, Ramé-Hart Instrument Co., Succasunna, NJ) with deionized water as the probing liquid. Characterization of Water Transport Properties. The water absorption in the polymer samples was measured at room temperature (22 °C). After being immersed in deionized water to remove the sol, the sample was dried and weighed, with the mass designated as mdry. The sample was then immersed in deionized water for 24 h to reach equilibrium. The weight of the wet sample was measured to be mwet. The percentage of water absorption (wW) was calculated as m wet − mdry wW = × 100% m wet (12)

⎡ d(M /M ) ⎤2 t ∞ ⎥ DS = 0.196l ⎢ 1/2 ⎣ d(t ) ⎦ 2

where Mt (g) is the mass of NaCl in the extraction solution at time t and M∞ (g) is the mass of NaCl in the extraction solution at equilibrium. Equation 14 is valid only when the value of Mt/M∞ is less than 0.6. The NaCl solubility, KS, was calculated as the ratio of the concentration of NaCl in the polymer at equilibrium to that in the salt solution (29.2 g/L NaCl in this study). The NaCl sorbed in the membrane was assumed to be the same as the total salt in the extraction solution at equilibrium (i.e., M∞).51,52 Characterization of Membrane Performance in FO Mode. The IMs were evaluated for FO applications using a custom-built system similar to those reported in the literature.16 The permeation cell (SEPA CF II, Sterlitech Corporation, Kent, WA) was modified to have countercurrent flow for the feed and draw solutions. Permeate carriers or spacers with a thickness of 272 μm (part number 1142817, GE Infrastructure Water & Process Technologies, Minnetonka, MN) were used for both the feed and draw solution sides to provide flow channels, mimicking the assemblies in industrial spiral-wound or plate-and-frame modules.12,13 The effective membrane area was 140 cm2. Two peristaltic pumps (model 913, MityFlex, Anko Products, Inc., Bradenton, FL) were used to circulate the solutions into the permeation cell in a countercurrent manner. In FO operation, the feed solution is deionized water, and the draw solution is salty water containing various amounts of

The water absorbed in the polymer was assumed to have the density the same as the liquid (ρW, g/cm3). Assuming the additive mixing of the polymer and water, the volume fraction of water in the hydrated polymer sample, vW, was calculated as48 vW =

(m wet − mdry )/ρW (m wet − mdry )/ρW + mdry /ρp

(14)

(13)

The water permeabilities in the free-standing films and IMs were determined using a dead-end filtration system.50,51 The permeation cell (model UHP-43, Advantec MFS, Inc., Dublin, CA) had a magnetic stir bar that could be tuned for operation at 300−1200 rpm. After the membrane sample had been mounted, the cell was filled with water and pressurized with D


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PEGDA has been shown to be easily photopolymerized with complete conversion.48,50,54 The spectrum of the Solupor support is also shown in Figure 5; it exhibits no interference with the characteristic peaks of the acrylate groups. Table 1 lists the physical properties of the impregnated membranes. The membranes had gel fractions of nearly 1,

NaCl. The draw-solution container was large enough to retain approximately the same draw-solution concentration during the experiments.57 In typical operation, the flow superficial velocity in the spacer channels (v∞) was kept at 38 cm/s, and the Reynolds number (Re) was 210 (Re = 2Hv∞/ν, where H is the height of the flow channel or the thickness of the permeate spacer and v is the solution kinetic viscosity).58 The weight change of the draw solution was monitored as a function of time to determine the water flux using eq 3. The salt flux (JS) was determined by monitoring the change in the salt concentration in the feed solution as a function of time, and the salt permeance was calculated using eq 4.22

Table 1. Physical Properties of Impregnated Membranes Prepared from PEGDA and Ethanol (EtOH)

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RESULTS Preparation and Characterization of Impregnated Membranes (IMs). To prepare impregnated membranes, the prepolymer solution needs to be compatible with the Solupor support so that it can easily penetrate into the pores. Because Solupor is made of hydrophobic polyethylene, ethanol, instead of water, was used as the solvent for PEGDA in the prepolymer solution. Figure 4 compares the appearances of the Solupor support and the prepared impregnated membrane. As shown in Figure 4b, the logo at the bottom is not clear because the support itself is opaque due to its porous structure. On the other hand, the impregnated membrane is transparent (Figure 4c), presumably because the polymer completely fills the pores, resulting in a relatively constant refractive index throughout the films. ATR-FTIR spectroscopy was used to characterize the conversion of the acrylate groups of the monomer, PEGDA. Figure 5 compares typical spectra of a polymer and an

a

prepolymer composition PEGDA/EtOH

gel fraction (%)

100:0 80:20 60:40 40:60 20:80

99 99 99 99 97

impregnated membrane density (g/cm3)

water sorption wW (%)

contact anglea

± ± ± ± ±

32.1 39.7 55.7 72.4 N/Ab

47.0 ± 0.1 42.1 ± 0.1 41.1 ± 0.2 N/Ab N/Ab

1.145 1.151 1.159 1.152 1.154

0.005 0.005 0.005 0.005 0.005

For free-standing films. bNot available.

independent of the prepolymer solutions, confirming the almost complete conversion of acrylate. These results are consistent with those of earlier work on free-standing films from PEGDA and H2O.48,53,54 Table 1 also reports the densities of IMs prepared from PEGDA and ethanol. The density values are almost independent of the prepolymer solutions. The Solupor support has a density of 0.156 g/cm3 and a porosity of 0.84. Assuming that the pores were completely filled with XLPEGDA (with a density of 1.182 g/cm3), the density of the IMs would be 1.149 g/cm3, which is very similar to the values measured (cf. Table 1). In this analysis, it was assumed that the XLPEGDA inside the pores had a density equal to that of free-standing films. The consistency between the predicted and measured densities of the IMs provides strong evidence that there are essentially no voids between the XLPEGDA and the Solupor support. The contact angle results for the free-standing films are also included in Table 1. The XLPEGDAs were found to be more hydrophilic than the Solupor support with a contact angle of 92.2 ± 0.1 using water as the probing liquid. The contact angles for the films with prepolymer solutions containing 60% and 80% EtOH could not be reproducibly determined, because the water droplet were rapidly absorbed, leading to curled surfaces. Pure-Water Sorption and Permeation in Impregnated Membranes. Figure 6 compares the water sorption behaviors of IMs prepared from PEGDA/EtOH with those of freestanding polymer films prepared from PEGDA/EtOH and PEGDA/H2O. Increasing the solvent (water or ethanol) content in the prepolymer solutions increased the water sorption in all samples, which can be ascribed to a lower cross-linking density with lower monomer concentration.48,53 Figure 6 shows that the use of ethanol as the solvent yielded polymers with higher water uptakes than that from a prepolymer solution containing water. The IMs showed water uptakes similar to those of the corresponding free-standing polymer samples, indicating that the Solupor support had minimal effect on the water sorption in the polymers. This result provides additional evidence of the absence of pores in the IMs. If there were voids between the XLPEGDA and the Solupor walls, additional water pockets would have formed inside the IMs, leading to water sorption values higher than those of the free-standing films.

Figure 5. Comparison of typical ATR-FTIR spectra of a polymer film and an impregnated membrane (IM) prepared from 80% PEGDA and 20% ethanol (XLPEGDA80) with those of liquid PEGDA and the Solupor support.

impregnated membrane prepared from 80% PEGDA and 20% ethanol (XLPEGDA80) with that of the monomer PEGDA. The polymer is labeled as XLPEGDAxx, where xx represents the weight percentage of PEGDA in the prepolymer solution. The acrylate groups have characteristic peaks at 810, 1190, and 1410 cm−1 that practically disappeared in the spectra of the polymer and impregnated membrane, indicating the almost complete conversion of PEGDA.48 The IR spectra of other polymers and IMs are similar to those of XLPEGDA80 and are not shown here for brevity. The results are expected because E


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the prepolymer solution decreased the cross-linking density48 and significantly increased the water permeability. Figure 7a compares the pure-water permeability in the IMs with that in the free-standing films. All of the free-standing samples had thicknesses of about 250 μm. When the prepolymer solutions contained less than 20% EtOH, these two types of samples exhibited very similar water permeabilities. However, when the prepolymer solutions contained more than 20% EtOH, the water permeability started to deviate. The freestanding films exhibited strong degrees of swelling and even phase separation during polymerization, leading to extraordinarily high water permeabilities.53 On the other hand, the hydrophobic Solupor support restricted the swelling of the hydrophilic XLPEGDA, avoiding phase separation during the polymerization of the IMs. For example, the free-standing film of XLPEGDA40 became opaque after polymerization, indicating that it underwent a polymerization-induced phaseseparation process, whereas no visible phase separation was observed for the IMs, even when the EtOH content was increased to 80% in the prepolymer solution. Figure 7a also compares the water permeabilities in the freestanding films prepared from PEGDA/EtOH and those prepared from PEGDA/H2O. The polymers from PEGDA/ EtOH showed higher permeabilities than those from PEGDA/ H2O.53 Interestingly, with the prepolymer solutions containing 80% solvent, the IM showed a lower permeability than the freestanding film of PEGDA/H2O, presumably because of the restriction of the swelling in the IMs and the lack of visible phase separation. The water permeabilities of most of the polymers seemed to exceed the self-diffusive permeability of water at 25 °C (2.8 × 10−5 cm2/s),51 presumably because these polymers were highly swollen and significant convection flow occurred under the hydraulic pressure. The following equation was derived to account for convection flow, which relates the diffusive permeability, PDW (cm2/s), to the hydraulic permeability (PW, determined from the dead-end filtration system)59

Figure 6. Comparison of the water sorption of impregnated membranes (IMs) prepared from PEGDA/EtOH with that of freestanding polymer films prepared from PEGDA/EtOH and PEGDA/ H2O.48

The water-swollen IMs consisted of two phases: a hydrophobic polyethylene (PE) phase with essentially zero water sorption and a hydrophilic XLPEGDA phase with high water sorption. Assuming that the water sorption was only in the XLPEGDA phase, the volume fraction of PE in the swollen IMs was only 0.10 for the IMs prepared from PEGDA without any ethanol, and 0.04 for the IMs prepared from PEGDA/EtOH 40:60. Therefore, to simplify the data analysis, the IMs were treated as one phase in this study. The water and salt transport properties are regarded as average properties. The use of different porous supports might lead to different physical properties, even if the same prepolymer solutions were used. Figure 7a shows the effects of the prepolymer compositions on the pure-water permeability in the IMs and free-standing films prepared from mixtures of PEGDA and EtOH. The IMs had thicknesses between 43 and 55 μm after swelling in deionized water overnight, consistent with the Solupor support thickness of about 45 μm. The deviations in the thicknesses were caused by the different degrees of water sorption in the IMs (as shown in Figure 6). Increasing the solvent content in

D PW = PW(1 − v W )2 (1 − 2χv W )

(15)

where χ is the Flory−Huggins interaction parameter between XLPEGDA and water, which was taken as 0.426.48,51

Figure 7. Comparison of the (a) water permeability and (b) water diffusivity of impregnated membranes (IMs) prepared from PEGDA/EtOH with those of free-standing polymer films prepared from PEGDA/EtOH and PEGDA/H2O53 at a feed pressure of 60 psig and 23 °C. Note: 1 × 10−6 cm2/s = 0.263 L μm/(m2 h bar). F


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Figure 8. Effects of water volume fraction (vW) in the impregnated membranes at equilibrium on (a) NaCl diffusivity and (b) NaCl solubility at room temperature. The results are also compared with those in free-standing films prepared from PEGDA and H2O reported by Ju et al.51

The water diffusivity was calculated as the ratio of PDW to vW.59 As shown in Figure 7b, the IMs showed higher water diffusivities than the free-standing films of XLPEGDA prepared from PEGDA/H2O, which is consistent to the behavior of water sorption in these polymers. On the other hand, the IMs showed unexpected water diffusivity behaviors, namely, water diffusivity initially increased and then decreased as the solvent content exceeded 20% in the prepolymer solution. The water diffusivity of IM-XLPEGDA80 even exceeded the water selfdiffusion coefficient (2.8 × 10−5 cm2/s).51 These behaviors might be caused by the nature of the phase separation in the IMs. Equation 15 is valid only if the polymer is homogeneous without phase separation. The dispersed water phase leads to more significant convection, which could be underestimated by eq 15, causing the overestimation of self-diffusivity. A model considering phase separation might be needed, as well as a thorough understanding of the morphology of the swollen polymers.53 However, these issues are beyond the scope of this study. Salt Permeation Properties Characterized Using Kinetic Desorption Experiments. The salt permeation properties of the IMs were determined by kinetic sorption experiments.51,52,55,56,60,61 Figure 8 presents the NaCl diffusivity and solubility as functions of the equilibrium water volume fraction in the IMs. The salt diffusivity increased exponentially with increasing reciprocal free volume,39,62 with the free volume equal to the water volume fraction in the membranes.51,55,56,63 On the other hand, the IMs consisting of XLPEGDA40 did not follow this trend, presumably because of the phase separation induced during polymerization, leading to a liquid water phase dispersed in the polymer/water phase.50,64,65 The water phase might not be interconnected, in which case it would not contribute to the diffusivity as effectively as in a homogeneous polymer/water phase. Figure 8a also compares the salt diffusivity in the IMs with that in the free-standing films prepared from PEGDA and water.51 Both series of polymers showed the same trend, and the diffusivity was lower than expected based on the water volume fraction when the polymers underwent phase separation. The NaCl diffusivity in these polymers approached that in pure water (1.6 × 10−5 cm2/s),39,66 as the free volume or vW increased toward 1. Figure 8b shows the correlation between NaCl solubility and water volume fraction in the IMs. The NaCl solubility increased

with increasing equilibrium water content of the membranes. Figure 8b also shows a parity line, indicating equal salt sorption in the water present in the membranes and pure water. The amount of NaCl sorption in the swollen polymers was lower than that in the pure water, and the difference became more significant for the polymers with lower water sorption (vW), which was similar to that in the free-standing polymer films prepared from PEGDA and water.51 This phenomenon is ascribed to the effect of the polymers on the salt sorption in water. The polymer network itself exhibits negligible salt sorption,55 and the presence of polymer chains decreased the water activity and, thus, decreased the salt sorption. The NaCl permeability was calculated from the solubility and diffusivity, and the results are shown in Figure 9. The NaCl

Figure 9. NaCl permeability in impregnated membranes (IMs) as a function of 1/vW, which is also compared with that in free-standing films prepared from PEGDA and H2O reported by Ju et al.51

permeability decreased exponentially with the reciprocal water volume fraction (i.e., 1/vW), which is consistent with the salt diffusion behavior. This behavior was modeled theoretically by Yasuda and co-workers.55,63 The NaCl permeability approached that in pure water (1.6 × 10−5 cm2/s),39,66 as the free volume or vW increased toward 1. The dependence of the NaCl permeability on water volume fraction was also similar to that of free-standing films prepared from PEGDA/H2O.51 G


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Table 2. Water and Salt Permeances in the Membranes for FO Applications,a Compared with Those from a Dead-End Filtration System FO operation membrane

water permeance (LMH/bar)


performance ratio

salt permeance (gMH)

salt rejection (%)

HTI SW30-XLE IM-XLPEGDA80 IM-XLPEGDA60 IM-XLPEGDA40

1.5 ± 0.1b 1.4 ± 0.1b 0.23 ± 0.02c 0.31 ± 0.03c 0.53 ± 0.03c

0.23 ± 0.01 0.046 ± 0.005 0.052 ± 0.005 0.077 ± 0.005 0.10 ± 0.01

0.15 ± 0.01 0.032 ± 0.003 0.22 ± 0.02 0.25 ± 0.02 0.19 ± 0.02

5.9 1.3 7.0 7.1 100

98.9 99.0 95.3 96.8 65.5

a

FO was operated with a feed solution of Milli-Q water and a draw solution of 1.0 M NaCl solution at room temperature. The Reynolds number in both channels was 210. bPure-water permeance. cWater permeance determined with a feed containing 2 g/L NaCl.

Impregnated Membranes for FO Applications. Before testing the IMs for FO applications, commercial membranes including HTI OsMem TFC-ES (abbreviated as HTI) and Dow Filmtec SW30-XLE (abbreviated as SW30-XLE) were tested to establish a baseline, because these two membranes have been widely evaluated for FO applications.19,67 Table 2 reports the water permeances for HTI, SW30-XLE, and IMs operating in FO mode. The feed solution was pure water, and the draw solution was 1.0 M NaCl solution with an osmotic pressure of 49.2 bar (cf. eq 7). The superficial velocity in the feed and draw flow channel was 38 cm/s, resulting in a Reynolds number of 210. The HTI membrane showed a water flux of 11 LMH, a salt flux of 7.0 gMH, and a salt rejection of 98.9%, which are comparable to the literature values of 14 LMH, 5.0 gMH, and 99.4%, respectively.68 The discrepancies can be ascribed to the differences in the testing conditions. In this study, the permeation cell was operated at Re = 210 and with spacers on both sides. However, in the literature, the permeation cell was often operated at Re = 1125 without spacers.68 Both factors might contribute to higher water fluxes than those obtained in this study. Increasing the EtOH content in the prepolymer solution for the IMs increased the FO water permeance, which is consistent with the pure-water permeation results from the dead-end cell systems. Table 2 also compares the water permeances in FO mode with the pure- and salty-water permeances from the dead-end filtration systems. The performance ratio is defined as the water permeance ratio of FO to the dead-end filtration system.16,45 The ratio is also equal to the percentage of driving force effectively inducing the water transport across the membrane during FO.45 In general, lower values of the performance ratio indicate more severe concentration polarization during FO testing. The SW30-XLE membranes had low performance ratio values (as low as 0.032), because of internal and external concentration polarization. The IMs had higher performance ratio values (about 0.22) than the SW30-XLE membrane. These results indicate that eliminating the open porous structure in FO membranes significantly reduces the concentration polarization. The performance ratio of 0.22 for the IMs suggests that there exists significant external concentration polarization, because the IMs do not have the porous supports and thus there is no internal concentration polarization. It should also be pointed out that the water permeances of these IMs were still lower than that of HTI membranes. The reverse salt flux (gMH) of the IMs in IM-XLPEGDA80 and XLPEGDA60 was 7 gMH, which is comparable to that of HTI membranes and much higher than that of SW30-XLE. Increasing the EtOH content in the prepolymer solution seemed to have a negligible effect on the salt flux for the IMs of

XLPEGDA80 and XLPEGDA60. On the other hand, the IM consisting of XLPEGDA40 had a salt flux of 100 gMH, because of its much higher water sorption (cf. Table 1) and thus higher free volume than for the other IMs (as shown in Figure 8a). In general, the salt permeance in FO mode was less than that from the kinetic sorption experiments, as shown in the Supporting Information. The external concentration polarization for the IMs was investigated by varying the flow rate and, thus, the Reynolds number in the permeation cell. The typical operating conditions corresponded to a Reynolds number of 210 for the feed- and draw-solution flow channels (10 cm3/s or 38 cm/ s). The flow rate was also increased to 90% of the maximal flow rate provided by the pump with a flow velocity of 68 cm/s and a Reynolds number of 380. Table 3 summarizes the water Table 3. Water Permeance (AW, LMH/bar) for Various Membranes and Flow Rates of the Feed and Draw Solutiona Reynolds number


feed solution

draw solution

HTI TFC-ES

Filmtec SW30-XLE

IM: XLPEGDA80

IM: XLPEGDA60

210 210 380

210 380 210

0.22 0.22 0.23

0.046 0.047 0.053

0.052 0.051 0.052

0.077 0.076 0.078

a

FO testing conditions: 1 M NaCl solution as the draw solution, pure water as the feed solution, T = 23 °C.

permeances at various flow rates for the IMs compared with those of HTI and SW30-XLE membranes. An increase in the flow rate from 38 to 68 cm/s had a negligible effect on the water permeance. The membranes also showed similar NaCl fluxes at various feed- and draw-solution flow rates. It should be noted that the Reynolds numbers investigated here are still much lower than that (1125) used in most of the studies reported.57 There is a need to study the effect of external concentration polarization at higher flow rates, which, however, could not be achieved using the current apparatus and will be pursued in our future studies. Figure 10a shows the effects of the NaCl concentration of the draw solution on the salt permeability in various IMs. The feed solution was pure water. Increasing the salt concentration of the draw solution had minimal effect on the NaCl permeability, whereas increasing the EtOH content in the prepolymer solution increased the NaCl permeability in the resulting IMs. Figure 10b shows the effect of the NaCl content in the draw solution on the water permeance. As the NaCl content in the draw solution increased, the overall water flux (JW) across the membrane increased because of the increase in the driving H


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Figure 10. Effects of the NaCl concentration in the draw solution on the (a) salt permeability and (b) water permeance in the three impregnated membranes based on PEGDA/EtOH.

force, leading to an increase in concentration polarization and, thus, a decrease in water permeance, as suggested by eq 9. To elucidate the effects of the feed-side concentration polarization on water permeance, the IM consisting of XLPEGDA60 was tested with a feed solution containing 0.1 M NaCl and a draw solution containing 1.0 M NaCl. As the feed NaCl content increased from 0 to 0.1 M, the water permeance decreased from 0.077 to 0.062 LMH/bar, and the salt rejection decreased from 96.8% to 93.9%. This result suggests the existence of external concentration polarization on the feed side of the membrane, presumably because the two membrane sides use the same spacer with the same Reynolds number (210). Future work is needed to understand the effects of the feed and permeate spacer structures on the membrane performance in FO systems to reduce the adverse effects of concentration polarization.69

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Figure 11. Correlation of water flux (JW) with mass-transfer coefficient (kD) in the impregnated membranes for FO applications. The feed was pure water, and the draw solution contained 0.5, 1.0, or 1.5 mol/L at 23 °C. The fitting line is based on eq 16 and has a slope of 0.23 LMH−1.

DISCUSSION Modeling of Concentration Polarization. For FO operation with pure water as the feed solution in this study, the reverse salt flux to the feed solution only slightly increased the feed salt concentration, because of the large volume of the feed solution. Therefore, the osmotic pressure of the feed solution was negligible compared to that of the draw solution, and the concentration polarization on the feed side of the membrane was neglected. Equation 9 can be simplified to47,58,70 ⎛ J +B ⎞ J ⎟⎟ = − W ln⎜⎜ W kD ⎝ AW πD,b + B ⎠

⎛ D 2v ⎞1/3 kD,ideal = 1.62⎜ S ∞ ⎟ ⎝ L 2W ⎠

(17)

where L (14.6 cm) and W (9.5 cm) are the length and width of the flow channel in the permeation cell. The diffusion coefficient of NaCl in solutions of 0.5, 1.0, and 1.5 M is a constant of 1.5 × 10−5 cm2/s.66 The calculated kD,ideal value is 7.7 × 10−4 cm/s, which is higher than the modeled kD value from the experiments as shown in Figure 11. This result might not be surprising, considering the effect of spacers on the flow patterns. Within the framework of film theory, the kD,ideal value can also be used to derive the thickness of the film as δ = DS/ kD,ideal.16,58 The δ value obtained was 190 μm, which is comparable to the spacer thickness (272 μm). The use of permeate carriers with low porosity might have an impact on the mass-transfer coefficient.69 Hypothetically, increasing Re from 210 to 1125 would increase kD,ideal from 7.7 × 10−4 to 1.4 × 10−3 cm/s. For IMXLPEGDA60 with a water permeance of 0.31 LMH/bar (cf. Table 2) and a NaCl permeance of 1.2 × 10−6 cm2/s (cf. Figure

(16)

Equation 16 was used to provide a best fit for the data in Figure 10, and the results are shown in Figure 11. The B values from the kinetic desorption experiments were used here, because these experiments had minimal concentration polarization and yielded intrinsic salt permeance. The model fitting is reasonably good and yields a kD value of 4.3 LMH or 1.2 × 10−4 cm/s. The mass-transfer coefficient (kD) in the solution adjacent to the membrane surface was estimated using film theory.16,58 The draw solution in the spacers has a Reynolds number of 210, and thus, the flow is laminar. The ideal mass-transfer coefficient (kD,ideal) can be expressed as58 I


Article

Industrial & Engineering Chemistry Research 9), the water flux (JW) in FO operation was estimated to be 11.9 LMH or 0.24 LMH/bar using eq 16. A water flux of 0.24 LMH/bar is comparable to that of state-of-the-art commercial HTI membranes, as shown in Table 2. Potential of IMs for FO Applications. Figure 12 compares the water/NaCl permeation properties of the IMs

glycol) (XLPEGDA), were utilized to test this concept. The impregnated membranes were thoroughly evaluated for water and salt transport properties using three tests: hydraulic deadend permeation, salt kinetic desorption, and forward osmosis. Increasing the solvent content in the prepolymer solution increased the water and salt solubilities, diffusivities, and permeabilities and decreased the salt rejection, because of the increasingly open structure. In FO operation, these firstgeneration IMs showed lower water permeances and higher reverse salt fluxes than HTI TFC, the state-of-art commercial FO membrane. However, these IMs exhibited greater performance ratios than the current commercial membranes examined, such as Filmtec SW30 and HTI TFC, indicating the promise of this new type of membrane for FO applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b03241. Pure-water permeation in impregnated membranes, water permeation and salt rejection determined using a dead-end filtration system, and comparison of NaCl permeabilities from different experiments (PDF)

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Figure 12. Upper-bound plot of water permeability (PW, cm2/s) and water/NaCl permeability selectivity (PW/PS).39 The upper-bound line is empirically drawn: PW/PS = 1.4 × 10−7/PW2.39

AUTHOR INFORMATION

Corresponding Author

*Tel.: +1-716-645-1856. E-mail: haiqingl@buffalo.edu.

containing XLPEGDA with those of other polymers considered for FO applications. The upper bound is empirically drawn and indicates the highest water/NaCl selectivity achievable for every possible pure-water permeability.39 In general, the IMs prepared in this work shows high water permeabilities and moderate water/NaCl selectivities. The current state-of-the-art commercial FO membranes exhibit water permeances of 22.9 LMH and reverse salt fluxes of 6.4 gMH, when tested with pure water as the feed and 1 M NaCl solution as the draw solution.9,68 Using the same feed and draw solutions, new FO membranes based on cellulose ester substrates at the laboratory stage have been reported to exhibit water permeances as high as 56.9 LMH with reverse salt fluxes of 7.8 gMH.21 Assuming that IMs can be made with a thickness of 10 μm and that the external concentration polarization can be eliminated, the IMs should have a water permeability of 6.4 × 10−7 cm2/s and a water/salt selectivity of 210 to match the commercial FO membranes (labeled as HTI TFC in Figure 12) and a water permeability of 1.6 × 10−6 cm2/s and water/NaCl selectivity of 430 to match the reported thin-film composite (TFC) membranes based on cellulose ester substrates (labeled as NUS TFC). As shown in Figure 12, these properties are below the upper bound, and they can be achieved using currently available polymers.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge partial financial support of this work from the Korean Carbon Capture and Sequestration R&D Center, the New York State Center of Excellence in Materials Informatics at University at Buffalo (State University of New York), and a RENEW seed grant from University at Buffalo.

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CONCLUSIONS This study reports a new route to the design of FO membranes with potentially high performance, namely, nonporous impregnated membranes consisting of a hydrophobic porous support filled with a highly hydrophilic polymer. Unlike conventional FO membranes consisting of a porous support and a paper layer (which leads to significant internal concentration polarization and low water permeance), these IMs show no internal concentration polarization because the open pores in the membranes have been eliminated. A series of well-studied hydrophilic polymers, cross-linked poly(ethylene J

NOMENCLATURE AW = water permeance of a membrane (LMH/bar) B = salt permeance of a membrane (cm/s) CS,P = salt concentration in the permeate flow (g/cm3) CS,F = salt concentration in the feed (g/cm3) CmS,F = equilibrium salt concentration in the membrane surface in contact with the feed solution (g of salt/cm3 of swollen polymer) DS = salt diffusion coefficient (cm2/s) gMH = salt flux, grams of salt per m2 of membrane per hour H = height of the flow channel in the permeation cell (cm) JS = salt flux across the membrane (gMH) JW = water flux across the membrane (LMH) kD = mass-transfer coefficient in the draw solution adjacent to the membrane (cm/s) kD,ideal = mass-transfer coefficient in the draw solution adjacent to the membrane, estimated using film theory (cm/ s) kF = mass-transfer coefficient in the feed solution adjacent to the membrane (cm/s) KS = salt solubility in the polymer [(g of salt/cm3 of swollen polymer)/(g of salt/cm3 of solution)] l = membrane thickness (cm) L = length of the flow channel in the permeation cell (cm) LMH = water flux, liters per m2 of membrane per hour DOI: 10.1021/acs.iecr.5b03241 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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M∞ = salt mass in the extraction solution at equilibrium (g) m0 = weight of a polymer sample including sol and gel (g) mair = weight of the polymer sample in air (g) mdry = weight of the dry polymer sample (g) mgel = weight of the polymer gel (g) mliquid = weight of the polymer sample in a liquid (g) Mt = salt mass in the extraction solution at time t (g) MW,S = molecular weight of the salt (g/mol) mwet = weight of a swollen polymer sample at equilibrium (g) Δp = pressure difference across the membrane (bar) pF = pressure on the membrane feed side (bar) pp = pressure on the membrane permeate side (bar) PS = salt permeability of the membrane (cm2/s) PW = water permeability of the membrane (cm2/s) PDW = water diffusive permeability of the membrane (cm2/s) Re = Reynolds number Rg = gas constant (83.1 cm3 bar/mol K) RS = salt rejection (%) t = time (s) T = temperature (K) v∞ = flow velocity on the membrane surface (cm/s) vW = equilibrium volume fraction of water in the polymer V̅ W = molar volume of liquid water (18 cm3/mol) W = width of the flow channel in the permeation cell (cm) wgel = weight fraction of the gel in the polymer containing sol and gel wW = water sorption in the polymer (wt %) Greek Letters

δ = thickness of the boundary layer (cm) ν = kinetic viscosity (cm2/s) πD = osmotic pressure of the draw solution (bar) πF = osmotic pressure of the feed solution (bar) πP = osmotic pressure of the membrane permeate stream (bar) ρair = air density (g/cm3) ρliquid = density of a liquid (g/cm3) ρp = density of the dry polymer (g/cm3) ρW = water density (g/cm3) χ = Flory−Huggins interaction parameter

Subscripts

b = bulk property D = draw solution side of the membrane F = feed side of the membrane p = polymer P = permeate side of the membrane S = salt W = water

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