ARTICLE pubs.acs.org/est
Synthesis and Characterization of Novel Forward Osmosis Membranes based on Layer-by-Layer Assembly Qi Saren,†,‡ Chang Quan Qiu,†,‡ and Chuyang Y. Tang*,†,‡ †
School of Civil and Environmental Engineering and ‡Singapore Membrane Technology CentreNanyang Technological University, Singapore 639798
bS Supporting Information ABSTRACT: Forward osmosis (FO) has received considerable interest for water- and energyrelated applications in recent years. FO does not require an applied pressure and is believed to have a low fouling tendency. However, a major challenge in FO is the lack of high performance FO membranes. In the current work, novel nanofiltration (NF)-like FO membranes with good magnesium chloride retention were synthesized using layer-by-layer (LbL) assembly. The membrane substrate was tailored (high porosity, finger-like pores, thin cross-section, and high hydrophilicity) to achieve a small structural parameter of 0.5 mm. Increasing the number of polyelectrolyte layers improved the selectivity of the LbL membranes while reducing their water permeability. The more selective membrane 6#LbL (with 6 polyelectrolyte layers) had much lower reverse solute transport compared to 3#LbL and 1#LbL. Meanwhile, the FO water flux was found to be strongly affected by both membrane water permeability and solute reverse transport. Severe solute reverse transport was observed for the active-layer-facing-drawsolution membrane orientation, likely due to the suppression of Donnan exclusion as a result of the high ionic strength of the draw solution. In contrast, the active-layer-facing-feedsolution orientation showed remarkable FO performance (15, 20, and 28 L/m2.h at 0.1, 0.5, and 1.0 M MgCl2, respectively, for membrane 3#LbL using distilled water as feed solution), superior to other NF-like FO membranes reported in the literature. To the best of the knowledge of the authors, this is the first work on the synthesis and characterization of LbL based FO membranes.
1. INTRODUCTION Forward osmosis (FO) is a membrane process driven by the osmotic pressure gradient across a semi-permeable membrane, where water is extracted through the membrane from a lowosmotic-pressure feed solution (FS) to a high-osmotic-pressure draw solution (DS).1 In recent years, FO has attracted significant interest for its potential applications including seawater desalination, water and wastewater treatment, food processing, and so forth.1 One potential advantage of FO is its low prime energy demand (i.e., electricity consumption) provided a suitable draw solution is naturally available or can be regenerated economically (e.g., via the use of low grade heat 1,2). FO membranes also tend to have low fouling propensity 3,4 despite the complicated mechanisms involved.5,6 Priority research areas in FO include the development high performance FO membranes in addition to the search for suitable draw solutions.1 Water flux in FO processes tends to be limited due to internal concentration polarization (ICP), which refers to (1) the accumulation of solutes in the porous membrane support when the active rejection layer faces the draw solution (i.e., concentrative ICP in the active-layer-facing-draw-solution (AL-DS) membrane orientation) or (2) the dilution of the draw solution inside the support layer when the active layer faces the feed solution (dilutive ICP in active-layer-facing-feed-solution r 2011 American Chemical Society
(AL-FS)).7 For both membrane orientations, the effective osmotic driving force across the rejection layer can be drastically lower than the bulk osmotic pressure difference between the DS and FS.79 ICP is generally more severe for support layers with large structural parameter S (i.e., substrate thickness tortuosity/ porosity).8 Currently, the only commercially available FO membranes are the cellulose triacetate (CTA) membranes from Hydration Technology Inc. (HTI). The HTI membranes have tailored support structures (thin membrane sections, relatively high porosity, and straight pores) to minimize ICP and thus to enhance their FO water flux.1,10 Nevertheless, the relatively low water permeability of these membranes means that further dramatic performance enhancement is possible by improving FO membrane separation properties. Thus, an ideal FO membrane calls for (1) a rejection layer with high water permeability and low solute permeability as well as (2) a support layer with high mass transfer coefficient 1 in addition to chemical and mechanical stability. Previous work has also suggested the importance of the support layer hydrophilicity on FO water Received: January 11, 2011 Accepted: April 29, 2011 Revised: April 29, 2011 Published: May 18, 2011 5201
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Environmental Science & Technology flux enhancement.11 The existing membrane fabrication works have mainly focused on thin film composite (TFC) polyamidebased FO membranes formed by interfacial polymerization 1216 and asymmetric FO membranes formed by phase inversion.17,18 Other FO fabrication methods are yet to be explored.19 One potential method to synthesize high performance FO membranes is the layer-by-layer (LbL) assembly method. LbL membranes have been successfully used for nanofiltration applications,2022 although their large scale production is still limited due to their relatively high production cost.20 These membranes tend to have high water permeability and good retention against divalent ions,2224 which makes them suitable candidates for FO membranes with nanofiltration (NF)-type rejection properties. In addition, LbL polyelectrolyte layers generally have high solvent resistance (except in some ternary solvent mixtures) and high thermal stability (e.g., no deterioration under 200 °C annealing temperature).22 Chemical crosslinking methods are also available to enhance their long-term stability.22 Up to now, no prior studies have been published on the use of LbL membranes for FO applications. The objectives of the current work were to synthesize novel FO membranes using LbL assembly method and to understand their behavior in FO conditions. To the best knowledge of the authors, this is the first study reporting the synthesis and characterization of LbL FO membranes.
2. EXPERIMENTAL DETAILS 2.1. Materials and Chemicals. Unless stated otherwise, all solutions and reagents were prepared with analytical grade chemicals and deionized (DI) water (Millipore Integral 10 water purification system, Singapore). Polyacrylonitrile (PAN, weightaveraged molecular weight Mw ≈ 150 000, Sigma-Aldrich), N,N-dimethylformamide (DMF, g 99.8%, Sigma-Aldrich), and lithium chloride (Sinopharm) were used as the polymer, solvent, and pore former, respectively, for the fabrication of the membrane substrate. Sodium hydroxide (Sigma-Aldrich) was used as the alkali solution for the post-treatment of PAN membrane substrate to enhance its surface charge density and
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hydrophilicity.21 Poly(allylamine hydrochloride) (PAH, Mw ≈ 56 000) and Poly(sodium 4-styrene-sulfonate) (PSS, Mw ≈ 70 000, 30 wt.% in H 2 O), both obtained from Sigma-Aldrich, were used as the respective polycation and polyanion for LbL assembly. Sodium chloride (Merck) was used to adjust the ionic strength of the polyelectrolyte solutions during LbL assembly. Membrane rejection properties were determined using magnesium chloride and sodium chloride (Merck). Magnesium chloride was used as draw solute during FO performance tests. 2.2. PAN Substrate Preparation and Post Treatment. PAN (18 wt.%) and LiCl (2 wt.%) were dissolved in DMF in a sealed container at 60 °C. To ensure complete dissolution, the solution was stirred for at least 24 h followed by filtration through a 0.15 μm filter. The polymer solution was cooled down to room temperature (23 °C) before use. A casting knife (Elcometer Pte Ltd., Asia) was used to spread the polymer solution onto a clean glass plate at a gate height of 175 μm. The plate was then immersed into a coagulant bath containing tap water at room temperature. The nascent substrate was washed with DI water for 1 min to remove excess solvent and additives. The resulting PAN substrate was soaked in 1.5 M NaOH at 45 °C for 1.5 h (adapted from ref 21). The purpose of the NaOH treatment was to impart negative charges to the substrate via partial hydrolysis, which is important for the subsequent LbL assembly.21 In the current study, we also relied on the partial hydrolysis to enhance the substrate hydrophilicity (see Section 3.1). Finally, the substrate was rinsed with DI water until the pH of the rinsewater became neutral, at which point it was transferred to PAH and PSS solutions for LbL treatment. 2.3. LbL Assembly. Polyelectrolyte solutions were prepared by dissolving 1 g/L PAH or PSS in 0.5 M NaCl solution. Both PAH and PSS are commonly used for LbL assembly.20,22 The pH of both PAH and PSS solutions were adjusted to 5.5 ( 0.2 using 0.1 M NaOH or HCl. The procedure for forming the LbL rejection layer is outlined in Figure 1 (also see refs 2124). The negatively charged PAN substrate was first soaked in the PAH polycation solution for 30 min, followed by rinsing the substrate with DI water to remove the excess PAH on its surface.22 The substrate was then soaked in the PSS polyanion solution for
Figure 1. Schematic of LbL assembly. PAH was used as the polycation, and PSS was used as the polyanion. 5202
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Environmental Science & Technology 30 min, followed by another DI water rinsing. The alternative PAH and PSS treatment can be repeated to fabricate membranes with multiple PAH/PSS polyelectrolyte layers. In the current work, membranes with 1, 3, and 6 PAH/PSS layers were fabricated (denoted as 1#LbL, 3#LbL, and 6#LbL, respectively). 2.4. Membrane Characterization. 2.4.1. Membrane Structure and Surface Properties. The structure and morphology of the FO membranes were characterized using a scanning electron microscope (Zeiss Evo 50 SEM). Cross-sections were prepared by fracturing membrane samples in liquid nitrogen. All of the samples were sputtered with a uniform gold coating (Emitech SC7620 Sputter Coater) before SEM examination. The membrane thickness was also measured by a digital microscope (VHX-500F, Keyence, Canada) at six different locations for each membrane. The surface roughness was measured using an atomic force microscope (AFM) following similar procedures by Tang et al.5 Membrane zeta potential was determined by an eletrokinetic analyzer (EKA, SurPASS, Anton Paar GmbH, Austria) at pH 5.5 in 10 mM NaCl. Contact angle measurements were performed with the sessile drop method according to ref 5 using an OCA Contact Angle System (DataPhysics Instruments GmbH, Germany). The reported contact angle is the average of at least 14 measurements for each sample. 2.4.2. Pure Water Permeability and Porosity Measurements of PAN Substrate. Detailed procedures for pure water permeability and porosity measurement have been reported elsewhere.12 Briefly, the pure water permeability coefficient (Ap) of PAN substrate was determined at an applied pressure of 100 kPa using DI water as feed. The substrate porosity was determined using gravimetric measurements following ref 12. 2.4.3. Membrane Intrinsic Separation Properties. The water permeability coefficient (A) and salt rejection (R) of the LbL FO membranes were evaluated in RO testing mode using a crossflow filtration setup according to Tang and co-workers.5,6 Briefly, pure water permeability was determined based on weighting permeate water samples. For rejection measurements, the feedwater contained either 7.5 mM NaCl or 5 mM MgCl2. Solute rejection was determined based on conductivity measurements of the feed and permeate water (Ultrameter II, Myron L Company, Carlsbad, CA). The salt permeability coefficients B for both NaCl and MgCl2 (BNaCl and BMgCl2) at a given pressure were calculated based on the average rejection value (3 replicates) using the following equation:5,6 1 B R ¼ 1þ ð1Þ AðΔP ΔπÞ where ΔP is the trans-membrane pressure, and Δπ is the osmotic pressure difference across the membrane, and A was determined from pure water tests. 2.4.4. FO Performance Evaluation. FO performance was evaluated using a cross-flow FO setup (Supporting Information S1). A membrane coupon with an effective membrane area of 42 cm2 was used in each test. The feed solution and draw solution were pumped by two independent variable-speed peristaltic pumps, and spacers were used in both FS and DS channels to minimize external concentration polarization.10 FO water flux was determined by measuring the weight changes of the feed tank at predetermined time intervals using a digital mass balance connected to a computer data logging system. Salt flux through the FO membrane was determined by monitoring the conductivity of the feedwater. Both AL-DS and AL-FS membrane
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orientations were tested. Unless stated otherwise, the following reference testing conditions were adopted: DS: MgCl2 at various concentrations FS: DI water or 10 mM NaCl Cross flow velocity for both FS and DS: 18.75 cm/s (corresponding to typical cross-flow velocity in spiral wound RO modules) Temperature: 23 ( 1 °C
3. RESULTS AND DISCUSSION 3.1. Membrane Substrate Characterization. The PAN substrate was treated with NaOH to enhance its surface charge and hydrophilicity. The untreated substrate was only slightly negatively charged (zeta potential of 6.4 ( 0.6 mV at pH 5.5). The NaOH treatment dramatically increased the surface charge (zeta potential of 32 ( 0.8 mV). Similar observation has been reported in the literature,25 which is attributed to the hydrolysis of the surface CN groups to form negatively charged carboxyl groups. Such charge enhancement is essential to form more stable ionic bonding between the polycation and the substrate during the subsequent LbL assembly.21 In addition, the NaOH treatment also significantly enhanced the substrate hydrophilicity. The contact angle was reduced from 52.1 ( 0.9° before treatment to 27.1 ( 0.9° after treatment, which is consistent with the formation of hydrophilic carboxyl functional groups on the substrate surface.21 The NaOH-treated PAN was significantly more hydrophilic compared to commercially available HTI membranes (∼70° 5,16) as well as substrates used for TFC FO membranes (∼7080° for polyethersulfone 12 and ∼5060° for polysulfone/polyvinyl pyrrolidone blend 16). Such hydrophilic nature of the treated PAN is desirable to allow water to penetrate the fine pores in the substrate, which tends to enhance FO water flux.11 SEM micrographs of the NaOH-treated PAN substrate are presented in Figure 2. The cross-sectional images (Figures 2(a) and 2(b)) show that the PAN substrate had a thin spongy-like skin layer (∼1 μm in thickness) on top of characteristic straight finger-like pores. This resulted in a highly porous substrate (porosity of >80% based on gravimetric measurements). Substrates with high porosity and low tortuosity are generally favored in FO applications to minimize the structural parameter S,8,12,15,16 a length scale of ICP that is analogous to the boundary layer thickness of external concentration polarization in RO. The overall thickness of the substrate was only ∼60 μm, which is comparable to the thickness of HTI membranes (4090 μm for woven fabric supported membranes 10,16) but is significantly thinner than that of conventional TFC RO membranes (∼150 μm16,26). These features (high porosity, straight pores, and thin cross-section) have been tailored in order to reduce the structural parameter of the resulting FO membranes and thus to minimize their ICP.8,12 The top surface of the substrate was relatively smooth (AFM root-mean-square roughness ∼10 nm, also see Figure 2(c)). In contrast, the bottom surface of the substrate had large pores (pore diameter on the order of 10 μm, Figure 2(d)). This confirms that the pores observed in Figure 2(a) penetrated all the way through the bottom of the substrate, which helps to minimize ICP within the substrate. As a result of its thin skin layer, high porosity, low tortuosity, thin cross section, and excellent hydrophilicity, the NaOH-treated substrate had a low S value of 0.5 ( 0.2 mm (Supporting Information S2). This value is slightly smaller than commercial HTI membranes (0.71.4 mm), 5203
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Figure 2. SEM micrographs of membrane structure (a) cross-section, (b) magnified view of cross-section, (c) top surface, and (d) bottom surface of the PAN support layer.
Table 1. Intrinsic Separation Properties of LbL FO Membrane with Different PAH/PSS Layers (Measured at 100 kPa) RMgCl2
NaOH-treated
B/A
A ( 1011 m/s Pa)
(%)
B ( 107m/s)
KPa
52.3 ( 1.5
0
N.A.
N.A.
PAN 1#LbL FO
10.1 ( 2.5
31
150 ( 7.1
149
3#LbL FO 6#LbL FO
2.84 ( 0.65 2.02 ( 0.60
64 80
9.60 ( 0.2 3.03 ( 0.2
33.8 15
and is an order of magnitude lower than that of a typical TFC RO membrane.16 3.2. Intrinsic Separation Properties of LbL Membranes. The water permeability coefficient A, the MgCl2 rejection RMgCl2, and the solute permeability coefficient BMgCl2 for the NaOH-treated PAN substrate and the LbL membranes are presented in Table 1. The NaOH-treated PAN substrate had no measurable rejection against MgCl2. Upon the deposition of the first PAH/PSS polyelectrolyte layer, the resulting membrane 1#LbL showed a MgCl2 rejection ∼30% at an applied pressure of 100 kPa. Correspondingly, the water permeability coefficient decreased to ∼10 1011 m/s.Pa (∼ 20% of that for the PAN substrate). The deposition of additional PAH/PSS layers resulted in further decrease of water permeability and enhancement of MgCl2 rejection. The membranes 3#LbL and 6#LbL (with 3 and 6 polyelectrolyte layers, respectively) had MgCl2
rejection of 64% and 80% at 100 kPa, respectively (Table 1). Significantly higher rejection was obtained at increased testing pressure (e.g., > 90% for 3#LbL at 700 KPa, see Supporting Information S2). However, the solute permeability coefficient was relatively constant at different testing pressures. Thus, the B value is a better parameter for comparing the retention of different membranes. The MgCl2 permeability coefficient was reduced from ∼150 107 m/s for 1#LbL to only ∼3 107 m/s for 6#LbL. The reduction of water and solute permeability are consistent with the formation of multiple polyelectrolyte layers, which increased the resistance against both water and solute transport. In addition, the B/A ratio decreased as the number of polyelectrolyte layers increased. The B/A ratio is an important selectivity parameter in FO applications, and it is directly related to the solute reverse transport (see Section 3.3 and refs 5,6, and 27). A lower B/A ratio (i.e., higher selectivity) is generally preferred for enhanced solute rejection, reduced fouling tendency, and more stable FO process operation.6,28,29 The current work suggests a strong trade-off between the membrane water permeability and selectivity.30 The separation properties of the LbL membranes can be easily tuned by selecting an appropriate number of polyelectrolyte layers for optimized FO performance (see further discussion in Section 3.3). Supporting Information S4 compares the MgCl2 and the NaCl rejection of the LbL membranes. Clearly, the membranes exhibited much better retention against MgCl2 compared to NaCl. Solutes are rejected by LbL membranes presumably due to the Donnan effect.21 Divalent magnesium ion has higher 5204
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superior FO water flux obtained for the LbL membranes (Section 3.3). 3.3. FO Water Flux and Salt Reverse Transport. 3.3.1. Effect of Number of Polyelectrolyte Layers. The FO water flux and solute flux of the LbL membranes were evaluated in the AL-FS orientation using MgCl2 as a draw solution (Figures 3(a) and 3(b)). Both the water flux and solute flux increased at higher DS concentration as a result of increased apparent osmotic driving force.5 Among the 3 LbL membranes, 6#LbL had the lowest solute flux, consistent with the fact that this membrane had the best selectivity (lowest B/A ratio, Table 1). However, 6#LbL also had the lowest water flux, which can be explained by its relatively low water permeability coefficient compared to those of 1#LbL and 3#LbL. Membranes 1#LbL and 3#LbL had nearly identical water flux over 0.52.0 M MgCl2 DS, while the value was higher for 3#LbL at 3.0 M MgCl2. This result is surprising considering the water permeability of 3#LbL was only 30% of that of 1#LbL (Table 1). This apparent discrepancy can be reconciled considering the significantly higher solute flux of 1#LbL. According to classical ICP models,5,8 one can show that the FO water flux Jv is governed by (see detailed derivation in Supporting Information S2): ( ) D Cdraw þ Js =Jv ðAL-FS orientationÞ Jv ¼ ln S Cf eed þ Jv =ðA 3 βRg TÞ þ Js =Jv ð2Þ ( ) Cdraw Jv =ðA 3 βRg TÞ þ Js =Jv D Jv ¼ ln ðAL-DS orientationÞ Cf eed þ Js =Jv S
ð3Þ
Figure 3. Effect of number of polyelectrolyte layers on (a) FO water flux and (b) FO solute flux. Testing conditions: 0.53 M MgCl2 in draw solution, DI water in feed solution; 23 ( 1 °C. Error bar was based the standard deviation of 3 replicate measurements.
valence than sodium, which explains its better retention by the polyelectrolyte layers.31 Since electrostatic repulsion can be strongly affected by the ionic environment, it is worthwhile to realize that the solute permeability coefficients reported in Table 1 are nominal values. Supporting Information S5 shows the effect of MgCl2 concentration on the rejection by membrane 3#LbL. Clearly, solute retention decreased at higher MgCl2 concentration, which is consistent with the increased electric double layer (EDL) compression at higher ionic strength. Nevertheless, the solute permeability coefficients listed in Table 1 can still provide a valid comparison on a relative scale, which is valuable to understand the FO performance of these membranes (Section 3.3). The LbL membranes in the current study are benchmarked against other NF-like FO membranes reported in the literature (Supporting Information S6). Among all of the membranes reported,1719,3234 the LbL membranes had the highest water permeability, and 3#LbL and 6#LbL also had relatively good selectivity against MgCl2. These separation properties, together with the tailored substrate properties (Section 3.1), were the likely reason for the
where D is the solute diffusion coefficient; S is the structural parameter of the support layer; Cdraw and Cfeed are the DS and FS concentrations, respectively; Js is the solute flux; β is the van’t Hoff coefficient;1 Rg is the universal gas constant; and T is the absolute temperature. In eqs 2 and 3, the term Js/Jv represents the effective solute concentration reverse diffused through an FO membrane.6,28 For given DS and FS types and concentrations, the above ICP equations clearly show that the FO water flux is affected by both membrane water permeability coefficient A and reverse solute flux Js. While the large A value of 1#LbL tends to enhance its FO water flux, its severe solute reverse diffusion tends to promote a more severe ICP in the membrane support layer. The latter effect severely limited the Jv attainable by 1#LbL. The solute reverse diffusion of 1#LbL became more severe at higher DS concentrations (Figure 3(b)), which may explain its inferior water flux compared to 3#LbL at 3.0 M DS. When evaluating FO membranes, one needs to consider both its water flux and solute flux. A higher Jv is preferred to reduce the capital cost of an FO plant. However, a low Js/Jv value is preferred to avoid excessive fouling and solute accumulation in FO processes and reduced operational cost for DS replenishment.6,28,29 While 1#LbL was inferior to 3#LbL (similar Jv but much severer Js), the comparison between 3#LbL and 6#LbL will strongly depend on the types of FO applications. For example, one may prefer 6#LbL over 3#LbL for its lower solute reverse diffusion for applications where severe fouling and solute accumulation may occur. The solute flux is governed by the membrane selectivity while the water flux is 5205
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Figure 4. Effect membrane orientations of 3#LbL FO membrane on (a) water flux, (b) solute flux, and (c) solute flux over water flux ratio. Testing conditions: 3#LbL FO membrane; DI water in feed solution; 23 ( 1 °C. Refer to Figure S5 in the Supporting Information for the FO performance data in the low DS concentration range (0.030.5 M MgCl2). Error bar was based the standard deviation of 3 replicate measurements.
affected by both the membrane water permeability and solute flux (thus membrane selectivity). Since there is generally a strong
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trade-off between water permeability and selectivity (Section 3.2), this calls for a compromise between water permeability and selectivity to achieve the optimized FO performance. 3.3.2. Effect of Membrane Orientation. The effect of membrane orientation on water flux and solute reverse diffusion was studied for 3#LbL (Figure 4(a),(b)). While most of the existing studies on tight RO-like FO membranes reported higher FO water flux in AL-DS orientation compared to AL-FS orientation due to the more severe dilutive ICP in AL-FS,1,57,12 a different trend was observed in the current study: (1) AL-FS had a higher Jv compared to AL-DS when DS concentration was greater than 2.0 M, although (2) the opposite was observed at lower DS concentrations (Figure 4(a)). Additionally, while recent studies on solute reverse diffusion of tight RO-like FO membranes concluded that the Js/Jv ratio is a constant independent of membrane orientation and DS concentration,5,6,27 we observed for the LbL membranes (1) that Js/Jv was significantly higher in AL-DS and (2) that the Js/Jv ratio increased with DS concentration in the AL-DS orientation while it remained nearly constant in the AL-FS orientation (Figure 4(c)). Similar observation was made for 6#LbL (Supporting Information S7). It is important to recognize that solute retention by the NFlike LbL membranes were achieved mainly via Donnan exclusion, such that lower rejections were obtained at higher ionic strength due to EDL compression (Supporting Information S5). In the AL-DS orientation, the membrane rejection layer was directly facing the high-concentration DS, which can cause a dramatic loss of membrane selectivity and thus severe solute reverse diffusion. This effect was more severe at higher DS concentrations, which explains the increased Js/Jv ratio in AL-DS under those conditions. In comparison, the ionic strength experienced by the membrane rejection layer in AL-FS was likely much lower,5,7 consistent with the low Js/Jv ratio in this orientation. The above explanation on solute flux behavior also provides insights into the water flux behavior of the LbL membranes. At high DS concentrations (>2.0 M), membrane 3#LbL experienced severe solute reverse diffusion in AL-DS, and the FO water was likely dominated by the Js/Jv term (eq 3), causing a lower Jv compared to the AL-FS orientation. In contrast, the reverse solute diffusion effect was less important at lower DS concentrations, such that Jv was higher in AL-DS (less severe concentrative ICP 1,57,12). For practical applications, the AL-FS orientation is preferred due to its better fouling resistance.3,5,6,10 3.3.3. Comparison with Other NF-like FO Membranes. The FO performance of the LbL membranes (particularly 3#LbL and 6#LbL) was superior compared to other NF-like FO membranes reported in the literature (Table 2). It is particularly worthwhile to highlight that relatively high FO water flux was achievable even at low DS concentrations (13, 15, 20, and 28.7 L/m2.h at 0.03, 0.1, 0.5, and 1.0 M of MgCl2, respectively, using DI water as FS) for 3#LbL in the AL-FS orientation while maintaining low solute flux (Figure 4 and Supporting Information S8). This was likely due to the superior separation properties for the LbL membranes (Section 3.2) as well as their excellent support layer properties (thin cross section, high porosity, finger-like pores, and excellent hydrophilicity, see Section 3.1). The inclusion of a 10 mM NaCl in the FS only slightly reduced the FO water flux as a result of increased ICP due to the feed solutes (Table 2). These results suggest that the LbL membranes developed in the current work may be applied for low-feed-concentration applications. 5206
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Table 2. Comparison of FO Performance of NF-like FO Membranes membranes
Jv (L/m2h)
Js (mol/m2h)
Js/Jv (mM)
FW
DS: MgCl2 (M) h
orientation
operation temp. (°C)
ref
1#LbL FO
27.7
0.66
23.8
DI
1
Al-FS
23 ( 1
present work
3#LbL FO
28.7
0.18
6.3
DI
1
Al-FS
23 ( 1
present work
3#LbL FO
25.3
N.A.
N.A.
10 mM NaCl
1
Al-FS
23 ( 1
present work
6#LbL FO
22
0.07
3.2
DI
1
Al-FS
23 ( 1
present work
HTI
15.8
0.02
1.27
DI
1
Al-FS
23 ( 1
present work
ST#2a
10.4
0.033 g
3.2 g
DI
1
Al-FS
23
19
ST#3a
11
0.046 g
4.2 g
DI
1
Al-FS
23
19
CA NF (#3)b PBIc
3.1 5.2
N.A. N.A.
N.A. N.A.
DI DI
1 2
Al-FS Al-FS
room temp. 22.5
17 32
PBI 2 h d
7.5
N.A.
N.A.
DI
1
Al-FS
23 ( 1
18
PBI 4 h d
5
N.A.
N.A.
DI
1
Al-FS
23 ( 1
18
PBI 9 h d
1.7
N.A.
N.A.
DI
1
Al-FS
23 ( 1
18
Dl - PBIe
12.5 g
0.004 g
0.32 g
DI
1
Al-FS
23 ( 0.5
33
0.053 g
3.79 g
DI
1
Al-FS
22 ( 0.5
34
1#LbL FO
13.2
1.62
122.7
DI
1
Al-DS
23 ( 1
present work
3#LbL FO 3#LbL FO
31.7 23.8
0.49 N.A.
15.5 N.A.
DI 10 mM NaCl
1 1
Al-DS Al-DS
23 ( 1 23 ( 1
present work present work
CA f
14 g
6#LbL FO
25.1
0.15
5.98
DI
1
Al-DS
23 ( 1
present work
HTI
36.3
0.13
3.58
DI
1
Al-DS
23 ( 1
present work
ST#2a
15.4
0.146 g
9.5 g
DI
1
Al-DS
23
19
ST#3a
14.6
0.269 g
18.4 g
DI
1
Al-DS
23
19
CA NF (#3)b
4.1
0.001
0.24
DI
1
Al-DS
room temp.
17
PBIc
9.02
N.A.
N.A.
DI
2
Al-DS
22.5
32
N.A. N.A.
N.A. N.A.
DI DI
1 1
Al-DS Al-DS
23 ( 1 23 ( 1
18 18
PBI 2 h d PBI 4 h d PBI 9 h d Dl - PBI e CA f
14.5 11
N.A.
N.A.
DI
1
Al-DS
23 ( 1
18
0.006 g
0.38 g
DI
1
Al-DS
23 ( 0.5
33
0.026 g
2.6 g
DI
1
Al-DS
22 ( 0.5
34
5 15.7 g 10 g
Poly(amideimide) FO hollow fiber membranes. b Cellulose acetate hollow fiber membranes. c Polybenzimidazole (PBI) hollow fiber membranes. d Cross-linked PBI hollow fiber membrane. e Dual layer polybenzimidazole-polyethersulfone (PBI-PES) hollow fiber membranes. f Cellulose acetate double-skinned forward osmosis membranes. g These values were estimated from published figures. h FO water and solute flux data for 1 M MgCl2 are used where available, otherwise data at the next higher MgCl2 concentration is included in this table. a
’ ASSOCIATED CONTENT
bS
Supporting Information. S1. Schematic of the cross-flow FO test setup; S2. Internal concentration polarization model; S3. Separation properties as a function of applied pressure; S4. B/A ratio for MgCl2 and NaCl for the LbL membranes; S5. Effect of feed concentration on MgCl2 rejection; S6. Comparison of separation properties of LbL membranes and other membranes reported in literature; S7. Effect membrane orientations of 6#LbL FO membrane; S8. Effect of draw concentration on water and solute flux for membrane 3#LbL; S9. References used in SI. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This project (Reference No. MEWR C651/06/173) is supported by the Environment and Water Industry Program
Office of Singapore (under the funding of National Research Foundation).
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