Outer-Selective Pressure-Retarded Osmosis Hollow Fiber Membranes

*Fax: 65-67791936. ... developed membranes can stand over 20 bar with a peak power density of 7.63 W/m2, ... Advanced Anti-Fouling Membranes for Osmot...
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Outer-Selective Pressure-Retarded Osmosis Hollow Fiber Membranes from Vacuum-Assisted Interfacial Polymerization for Osmotic Power Generation Shi-Peng Sun†,‡ and Tai-Shung Chung*,†,‡,§ †

Department of Chemical and Biomolecular Engineering, National University of Singapore (NUS), Singapore 119260, Singapore NUS Environmental Research Institute, National University of Singapore (NUS), 5A Engineering Drive 1, 02-01, Singapore 117411, Singapore § Water Desalination and Reuse (WDR) Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia ‡

S Supporting Information *

ABSTRACT: In this paper, we report the technical breakthroughs to synthesize outer-selective thin-film composite (TFC) hollow fiber membranes, which is in an urgent need for osmotic power generation with the pressure-retarded osmosis (PRO) process. In the first step, a defect-free thin-film composite membrane module is achieved by vacuum-assisted interfacial polymerization. The PRO performance is further enhanced by optimizing the support in terms of pore size and mechanical strength and the TFC layer with polydopamine coating and molecular engineering of the interfacial polymerization solution. The newly developed membranes can stand over 20 bar with a peak power density of 7.63 W/m2, which is equivalent to 13.72 W/m2 of its inner-selective hollow fiber counterpart with the same module size, packing density, and fiber dimensions. The study may provide insightful guidelines for optimizing the interfacial polymerization procedures and scaling up of the outer-selective TFC hollow fiber membrane modules for PRO power generation.

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environments.8−10 According to Statkraft, who built the first osmotic power generator in 2009, a membrane power density of more than 3 or 5 W/m2 is necessary for this technology to be economically viable if hollow fibers or flat-sheet membranes are used, respectively.7,11 To date, both phase inversion and thinfilm composite (TFC) membranes have been explored to fabricate PRO membranes.12,13 The phase inversion membrane is convenient and low-cost to make but suffers from a low water flux and a low power density because of the low porosity of the polymeric support layer. The TFC membrane, which is composed of a highly porous polymeric support and an ultrathin polyamide selective layer (99%, Sigma), n-dodecane (>99%, Sigma), dopamine hydrochloride (99%, Alfa-Aesar), camphorsulfonic acid (98%, Sigma), n-hexane (>99.9%, Fisher Chemical), and methanol (99.99%, Fisher Chemical) were employed to perform interfacial polymerization. Sodium chloride (NaCl, ≥99.5%, Merck) was used to prepare feed solutions and draw solutions. Fabrication and Characteristics of the Hollow Fiber Membrane Support. Matrimid polyimide hollow fiber supports for interfacial polymerization were fabricated by a non-solvent-induced phase-inversion process with a dry-jet wet spinning line. The detailed spinning conditions are listed in Table S1 of the Supporting Information. Process for Synthesizing the Outer-Selective TFC PRO Hollow Fiber Membranes. The schematic for preparing the outer-selective TFC hollow fiber membrane is shown in Scheme 1. (1) A total of 50 pieces of hollow fibers with a

solution containing trimesoyl chloride (TMC). The ridge-andvalley TFC layer is thus formed by MPD diffusion from the pores of the support into the oil phase, where MPD reacts with TMC and forms the cross-linked polyamide structure. Thus far, the TFC membranes for PRO purposes are all in the configurations of flat-sheet and inner-selective hollow fiber membranes. Recently, outer-selective TFC hollow fiber membranes have been attracting both industrial and academic attention because of the following reasons. First, in comparison to flat-sheet membranes, the hollow fiber design provides a much higher surface area per module, self-mechanical support, and ease of module fabrication. Second, in comparison to the inner-selective hollow fiber, the outer-selective hollow fiber is less inclined to fiber blockage and has less pressure drop during the real PRO application, because of the less mass-transfer resistance of the high-pressure draw solution in the shell side than the fiber lumen side.20,21 As a result, the outer-selective PRO hollow fiber membranes offer much smaller outer and inner diameters than inner-selective hollow fibers. In addition, the former will have much more surface area per module than the latter in commercial uses. However, it has been widely recognized by membrane researchers that forming a perfect polyamide layer on the outer surface of hollow fibers, especially for large-scale production, is much more challenging than on flat-sheet membranes and inner-selective hollow fibers.22,23 The major technical hurdle is how to remove the excess MPD and water solution before applying the TMC solution. It is convenient to employ a rubber roller for flat-sheet membranes and air purge/intermediate solvent for inner-selective hollow fibers.22 However, neither of these two methods is applicable for the outer-selective hollow fiber configuration. It was proposed to make reverse osmosis (RO) TFC hollow fiber membranes with an outer-selective configuration by passing a hollow fiber membrane continuously through several monomer solutions. However, this method suffers from several disadvantages: (1) the outer surface inevitably contacts the driving roller in the interfacial polymerization process, rendering an imperfect surface; (2) the sophisticate fabrication process requires excessive control parameters; and (3) the continuous flowing of thousands of meters of fibers in largescale production is time-consuming. Therefore, conducting interfacial polymerization with a bundle of fibers is a preferable way.24 However, the main problem for bundle coating is that the TFC layer tends to form in the space between the fibers rather than on each fiber surface if an excess of MPD and water is not sufficiently removed before applying the TMC solution. Therefore, the objectives of this paper are (1) to develop an innovative method for preparing a bundle of defect-free outerselective PRO hollow fiber membranes from vacuum-assisted interfacial polymerization, (2) to enhance the water/salt selectivity by polydopamine (PDA) coating, and (3) to improve the robustness of the support for high-pressure durance. To the best of our knowledge, no outer-selective TFC PRO hollow fiber membranes have been reported before.

Scheme 1. Simplified Process for Preparing the OuterSelective TFC Hollow Fiber Membrane Bundles for PRO Power Generation

length of 40 cm were bundled together with Teflon tape. The bundle was inserted into a short plastic tube with an inner diameter of 1/2 in. and a length of 2 cm. Epoxy is filled into the tube and the other end of the fibers. After curing, a blade was used to open the lumen of the fibers, while the other end was maintained closed. (2) The bundle was immersed in a polydopamine (PDA) aqueous solution for 1 h. The PDAcoating solution was prepared by dissolving 0.2 g of dopamine− HCl in a 1 L, 0.01 mol/L Tris−HCl buffer solution at pH 8.5. The top half of the tube was lifted over the PDA solution, so that only the outer surface of the fibers was facing the solution. (3) The bundle was then immersed in a 0.5 wt % PEG 400 aqueous solution for 30 min. (4) After that, the bundle was immersed into an aqueous solution with the composition of 2.0:0.5:0.1 (wt %) MPD/TEA/SDS. (5) After 5 min, the bundle was taken out of the MPD solution. Vacuum was then applied from the lumen side of the hollow fibers to maintain a transmembrane pressure of 800 mbar for 10 min. (6) The bundle was then immersed into a solution of 0.15% (w/v) TMC/n-dodecane. (7) After the reaction, the fibers were rinsed with n-hexane and methanol to remove excess TMC and MPD, respectively. Then, both ends of the fibers were cut, opened, immersed in a 20:80 (vol %) glycerol/water solution overnight,

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MATERIALS AND METHODS Materials. Matrimid 5218 polyimide was purchased from Vantico. N-Methyl-2-pyrrolidinone (NMP, ≥99.5%, Merck) was used as the solvent, and diethylene glycol (DG, 99%, AlfaAesar), tetrahydrofuran (THF, 99.99%, Fisher Chemical), and polyvinylpyrrolidone 360K (PVP 360K, Sigma) were used as additives for the spinning solutions. Polyethylene glycol (PEG) 2K, PEG 10K, PEG 20K, and PEG 35K were purchased from 13168

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and finally dried in air. The dried fibers were mounted into a stainless-steel module for performance tests. The module was designated as “M1IP1”. Characterizations. The morphology of hollow fiber membranes was observed by a field emission scanning electron microscope (FESEM, JEOL JSM-6700F). Before observation, the freeze-dried hollow fibers were immersed in liquid nitrogen, fractured, and then coated with platinum using a JEOL JFC1300 platinum coater. The contact angles of water on hollow fiber membranes were conducted using a Sigma 701 tensiometer from KSV Instruments, Ltd. Each hollow fiber with an effective length of 5 mm was immersed in ultrapure water, and the advancing contact angle was calculated with the aid of the computer software. Five readings were measured, and an average was obtained from the results. The mechanical properties of the hollow fiber supports were tested by an Instron tensiometer (model 5542, Instron Corporation). The fiber sample was clamped at both ends and pulled in tension at a constant elongation rate of 10 mm min−1 with an initial gauge length of 50 mm. Tensile strength, Young’s modulus, and extension at break were obtained from the stress−strain curves. A total of 10 samples were measured, and an average was generated from the results. Determinations of Pure Water Permeability, Molecular Weight Cutoff (MWCO), Pore Size, and Pore Size Distribution of Hollow Fiber Membrane Supports. Pore structural parameters, including pure water permeability, MWCO, pore size, and pore size distribution of hollow fiber membrane supports were determined through NF experiments in a lab-scale circulating filtration unit, which is described previously.25 Because the outer surface of hollow fibers was the selective layer, the feed solution was pumped into the shell side, while the permeate solution exited from the lumen side of hollow fibers. The pure water permeation experiment was conducted at a constant flow rate of 0.2 L min−1 and pressure of 1 bar to measure the pure water permeability, PWP (L m−2 bar−1 h−1), which was calculated using the equation PWP =

Q ΔPA m

where cp and cf are the solute concentrations in the permeate and feed solution, respectively. Reverse Osmosis Tests of the TFC PRO Hollow Fiber Membranes. Each TFC PRO hollow fiber membrane module was subjected to the pure water permeation experiment at a constant flow rate (0.2 L/min) and pressure (1 bar) to measure the pure water permeability, A (or PWP, L m−2 bar−1 h−1), which was calculated according to eq 1. The salt rejection was then tested with 200 ppm NaCl at 0.2 L/min and 1 bar. The salt rejection was calculated using eq 2. Accordingly, the salt permeability B can be calculated on the basis of eq 328,29 1 − RT 1 = B RT (ΔP − Δπ )A

where ΔP is the transmembrane hydraulic pressure applied and Δπ is the osmotic pressure difference between the feed and permeate. Performance in the Forward Osmosis (FO) Process. FO tests were conducted with a bench-scale FO setup, which is shown in Figure S3 of the Supporting Information. The draw solution (1 M NaCl) and the feed solution (DI water) were recirculated counter-currently in the active layer facing draw solution mode. The volumetric flow rates at the shell and lumen sides were 0.2 and 0.05 L min−1, respectively. The lumen side pressure was kept below 2 psi. The water permeation flux, (Jw, L m−2 h−1, abbreviated as LMH) was calculated from the below equation Jw =

Δv A m Δt

(4)

where Δv is the volume change of the feed solution over a predetermined time t (h) in the duration of tests and Am (m2) is the effective membrane area. The salt reverse flux, Js, in g m−2 h−1 (abbreviated as gMH), was determined from the below equation Js =

(1)

where Q is the water permeation volumetric flow rate (L/h), Am is the effective filtration area (m2), and ΔP is the transmembrane pressure drop (bar). The membranes were then characterized by solute separation experiments with 200 ppm neutral organic solutes, i.e., PEG 2K, PEG 10K, PEG 20K, and PEG 35K, at pH 5.75 to estimate pore size, pore size distribution, and MWCO according to the solute transport method described previously.25−27 For each experiment, the feed solution was circulated at 1 bar for 1 h before the concentrations of both feed and permeate were measured. The samples were collected 3 times for consecutive time intervals of 0.5 h. The variation of rejection was less than 2%. Between runs of different solutes, the membrane was flushed thoroughly with deionized (DI) water. Concentrations of neutral solute solutions were measured by a total organic carbon analyzer (TOC, ASI-5000A, Shimazu, Japan). The solute rejection RT (%) was calculated using the equation ⎛ cp ⎞ RT (%) = ⎜1 − ⎟ cf ⎠ ⎝

(3)

(ctvt ) − (c0v0) 1 Δt Am

(5)

where c0 and v0 are the salt concentration and volume of the feed at the beginning of the FO tests and ct and vt are the salt concentration and volume of the feed at the end of FO tests, respectively. The experiments were repeated 3 times, and average values were reported. Performance in the PRO Process. PRO performance for osmotic power generation was tested with a lab-scale PRO setup, which is similar to the FO setup, but a high-pressure pump (Hydra-cell, Minneapolis, MN) was used to recirculate the draw solution (1 M NaCl) at 0.5 L/min under gradually increased pressure. A variable-speed peristaltic pump (ColePalmer, Vernon Hills, IL) was employed to recirculate the feed solution (DI water) at 0.06 L/min to maintain the feed side pressure less than 2 psi. The power density is calculated by eq 6 W = Jw ΔP

(6)

where ΔP is the hydraulic pressure difference across the membrane and Jw is the water permeation flux, which can be either experimentally measured in PRO tests by eq 7 or theoretically calculated by

(2) 13169

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Figure 1. General morphology of the hollow fiber support for preparing the outer-selective PRO membranes: (a) cross-section, (b) outer edge, (c) inner surface, and (d) outer surface.

Table 1. Dimensions, Contact Angles, Mean Pore Size, Standard Deviation, MWCO, and Pure Water Permeability (PWP) of Matrimid Hollow Fiber Membrane Supports

a

ID

outer diameter/inner diameter (μm)

contact angle (deg)a

dp (nm)

σp

MWCO (kDa)

PWP (L m−2 bar−1 h−1)

M1 M2

404/234 417/232

68.76 ± 5.14 78.43 ± 2.85

3.1 4.8

1.42 1.48

13.8 34.9

93.8 ± 2.5 152.6 ± 5.0

The contact angle of Matrimid hollow fibers without PVP is 91.13° ± 5.69°. ⎤ ⎡ ⎛ J ⎞ B Jw = A⎢πD,b exp⎜ − w ⎟(1/(1 + [exp(Jw K m) − 1])) − ΔP ⎥ ⎢⎣ ⎝ k⎠ Jw ⎦⎥ (7)

methods developed for flat-sheet and inner-selective hollow fibers are not capable of totally removing the excess water solution. For example, if the fibers are drained in the air after immersing in the MPD solution, the interfacial reaction takes place in the space between the fibers. Thus, the TFC layer becomes very thick (∼1 μm) and becomes a defective layer, as shown in Figure 2a. The surface is also very rough with many large ridges. Such a membrane shows nearly no salt rejection and no potential for PRO applications. However, with an optimized vacuum pressure of 800 mbar through the lumen side of the fibers, the excess water is effectively drawn into the fiber lumen, while the MPD stays in the pores of the hollow fibers. After interfacial polymerization, the fiber forms an ultrathin layer (∼100 nm) of cross-linked polyamide, as shown in Figure 2b. The membrane possesses a water permeability of 1.42 L m−2 h−1 bar−1, a salt permeability of 0.40 L m−2 h−1, and a salt rejection of 74.53% determined under the RO mode and shown in Figure 3a. FO experiments were conducted using a cross-flow system described in the Supporting Information. The performance is shown in Figure 3b. The membrane has a water flux of 21.78 L m−2 h−1 and a Js/Jw ratio (i.e., the ratio of reverse draw solute flux to water flux) of 0.59 g/L using 1 M NaCl as the draw solution facing the active layer (i.e., PRO mode). The TFC membrane was further evaluated by high-pressure PRO experiments. As shown in Figure 4, the membrane flux slightly decreases as the pressure increases from 0 to 4 bar. A sharper flux drop is observed from 4 to 12 bar. A peak power density of

where πD,b is the osmotic pressure of the bulk draw solution and k is the mass-transfer coefficient. The experiments were repeated 3 times, and average values were reported.

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RESULTS AND DISCUSSION Effects of Vacuum-Assisted Interfacial Polymerization on the Formation of Defect-Free TFC Hollow Fiber Membranes. The general morphology is shown in Figure 1, and the detailed morphology is shown in Figure S1 of the Supporting Information. The hollow fiber is asymmetric with fully macrovoids sandwiched by two spongy-like layers; the outer layer is dense, while the inner layer is porous. The membranes possess ultrafiltration (UF) characteristics listed in Table 1, and the pore size distributions are shown in Figure S2 of the Supporting Information. The first-generation support was designated as “M1” and prepared from a dope solution of 15:15:15:1:54 Matrimid/DG/THF/PVP/NMP. Unlike the normal FO membranes with a large outer diameter and thin wall, the hollow fibers in this work are specially designed to have a small cross-sectional dimension of 404/234 μm, providing a collapse pressure of 14.5 bar. Using vacuum to remove water on the outer surface is the crucial step in the whole process. As mentioned above, the 13170

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Figure 2. Top-row images are the cross-sectional morphology of the outer edge, while the bottom-row images are the outer surface of the outerselective hollow fiber membranes: (a) control membrane without vacuum-assisted IP, (b) M1IP1 with vacuum-assisted IP but without PDA coating, and (c) M1IP2 with both vacuum-assisted IP and PDA coating.

Figure 3. (a) Water permeability, A, and salt permeability, B, of outerselective TFC hollow fiber membranes tested by RO experiments with a constant pressure of 1 bar and a NaCl concentration of 200 ppm. (b) Water flux, Jw, and salt reverse flux, Js, of outer-selective TFC hollow fiber membranes tested by FO experiments in the active layer facing draw solution mode without hydraulic pressure. The draw solution is 1 M NaCl. The feed solution is DI water. Figure 4. PRO performance of outer-selective TFC hollow fiber membranes: (a) water flux as a function of pressure and (b) power density as a function of pressure. The draw solution is 1 M NaCI. The feed solution is Dl water.

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3.47 W/m is obtained at 12 bar, after which the membrane is collapsed. In fact, the collapse pressure of the UF support is 14.5 bar, as listed in Table 2. Therefore, the performance deterioration is mainly due to the weak strength of the thin TFC layer. 13171

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polymer concentration of the new generation support, which is designated as “M2”, was increased from 15 to 17 wt %, although the porosity of the support may be sacrificed. (2) In comparison to NMP, THF is more volatile in the air gap region and has a more rapid exchange rate with water, resulting in a smaller pore size. Therefore, the THF concentration was reduced from 15 to 10 wt % to compensate for the flux reduction and to enlarge the substrate pore size. (3) PVP serves as a hydrophilizing agent, which not only promotes uniform reaction in the interfacial polymerization process but also enhances water transport in the PRO operation. However, the presence of PVP may decrease the mechanical strength of the support. Therefore, in the modified dope, the PVP concentration was reduced from 1 to 0.5 wt %. Although the contact angle of the support is increased from 68.76° to 78.43°, it is still more hydrophilic than a plain Matrimid hollow fiber without PVP addition (91.13°). As shown in Table 2, the modified support, i.e., “M2”, possesses a much enhanced mechanical strength in terms of Young’s modulus, tensile stress, and elongation at break than the first-generation support “M1”. The enhanced mechanical strength of the support increases the collapse pressure from 14.5 to 22.5 bar. Furthermore, as shown in Table 1, the modified support also has an enlarged pore size, which may lead to better penetration of the TFC layer into the support layer, thus increase the robustness of the TFC layer. As observed in Figure 3a, although the water permeability of M2IP2 is reduced by 26.8% to 3.59 L m−2 h−1 bar−1, the salt permeability is decreased more sharply by 45.5% to 1.82 L m−2 h−1 with a salt rejection of 62.39%. As a result, the flux is slightly reduced to 33.17 L m−2 h−1, while the Js/Jw ratio is much reduced to 0.72 g/L (Figure 3b). From the comparison of PRO performance in Figure 4, although the flux of M2IP2 at 0 bar is lower than that of M1IP2, the reduced salt permeability lowers the ICP tendency, thus preventing a sharp flux drop. The enhanced mechanical strength of both the support and the TFC layers extends the operation limits of the membrane to over 20 bar, where the peak power density is substantially increased to 7.63 W/m2. The power density is equivalent to 13.72 W/m2 of its inner-selective hollow fiber counterpart (i.e., membrane area calculated on the basis of the inner diameter) with the same module size, packing density, and fiber dimensions.12 Alternatively, considering two 8 in. modules that consist of outer- and inner-selective hollow fibers with the same dimension and power density, 7.63 W/m2, the power output of the outer-selective 8 in. module could reach 1695 W ideally, while the inner-selective counterpart can only harvest 1.8 times less power output, as estimated in Table S2 of the Supporting Information.12 Furthermore, as indicated by the higher predicted performance (dash lines in Figure 4) than the experimental performance, there is still space for PRO membranes to improve the water permeability and the tolerance of membrane compaction and ICP effect. In this work, PDA coating and vacuum-induced interfacial polymerization are employed to fabricate novel outer-selective PRO membranes, which are in an urgent need for osmotic power generation. To prepare a bundle of hollow fibers with defect-free surface, an optimum vacuum pressure of 800 mbar is necessary to be applied for the removal of excess water before the interfacial polymerization reaction. The structures of both the TFC and the support layers are essential to influence the PRO performance of the membranes. The interpenetration of these two layers is also important for high-pressure operations. This can be achieved by optimizing the support in terms of

Table 2. Mechanical Properties of Matrimid Hollow Fiber Membrane Supports ID

Young’s modulus (MPa)

tensile stress (MPa)

elongation at break (%)

collapse pressure (bar)

M1 M2

180.42 ± 11.46 245.11 ± 10.83

3.68 ± 0.65 5.55 ± 0.77

35.59 ± 2.90 46.98 ± 5.73

14.5 22.5

Enhancing the PRO Performance through PDA Coating. To enhance the PRO performance, various strategies were explored. First, the structure of the TFC layer was improved by coating a cushion layer of PDA prior to the interfacial polymerization step. PDA as a bio-inspired material has recently been proven to benefit PRO membranes in terms of mechanical strength and surface hydrophilicity because of its strong adhesive nature and covalent bonding between the polyimide support and TFC layer through free amine and hydroxyl groups.15,18,30,31 However, the PDA layer is not stable in the 2.0:0.5:0.1 MPD/TEA/SDS solution with a pH of 11.7. The MPD solution changes to dark once the fibers are transported from the PDA solution, indicating the PDA dissolution. In our modified TFC membrane, which is designated as “M1IP2”, camphorsulfonic acid (CSA) with an optimized concentration of 1.03 wt % was added to the MPD solution to lower the pH to 10.2. The PDA cushion layer is stable under this pH, and no color change was observed in the MPD solution. Besides the PDA coating, the introduction of TEA/CSA pair additives may promote the formation of organic salts between the amine group of TEA and the sulfonic group of CSA. The resultant water-soluble salts may increase the porosity of the TFC layer after being washed out after interfacial polymerization.32 Because of the above reasons, much more small nodules are formed in comparison to the membranes without PDA coating, as observed by the FESEM images shown in panels b and c of Figure 2. The water permeability is significantly increased to 4.91 L m−2 h−1 bar−1 because of the improved hydrophilicity and increased water channels. The salt permeability is also increased to 3.30 L m−2 h−1 with a decreased salt rejection of 55.32%. This is due to the fact that the pore size of the support, i.e., 3.1 nm, as listed in Table 1, tends to further reduce after the PDA coating, as evidenced from previous reports.15,33 Therefore, during the interfacial polymerization step, the TFC layer would prefer forming on top of the PDA-coated surface rather than penetrating into the pores of the support. In other words, the PDA coating serves as a cushion layer to support the thin film formation, which is discussed in details in our recent reports.15,33 As a result, the water flux of M1IP2 is increased substantially to 36.29 L m−2 h−1, while the Js/Jw ratio also increased to 0.93 g/L. Benefited by the enhanced water permeability compared to M1IP1, M1IP2 shows a higher water flux over 0−12 bar, leading to a higher peak power density, 4.91 W/m2, as shown in Figure 4. However, a steeper flux drop over the increased pressure is observed for M1IP2 than M1IP1, which arises from the severe ICP, as evidenced by the high salt permeability, as discussed above. Enhancing the PRO Performance through Tailoring the Hollow Fiber Support Structure. From the above observations, the PRO performance was further enhanced by keeping the interfacial polymerization condition of M1IP2 but tailoring the structure of the support layer through adjusting the dope compositions, as shown in Table S1 of the Supporting Information. (1) To enhance the mechanical strength, the 13172

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(9) Park, M.; Lee, J. J.; Lee, S.; Kim, J. H. Determination of a constant membrane structure parameter in forward osmosis processes. J. Membr. Sci. 2011, 375 (1−2), 241−248. (10) Chung, T. S.; Li, X.; Ong, R. C.; Ge, Q.; Wang, H.; Han, G. Emerging forward osmosis (FO) technologies and challenges ahead for clean water and clean energy applications. Curr. Opin. Chem. Eng. 2012, 1 (3), 246−257. (11) Thorsen, T.; Holt, T. The potential for power production from salinity gradients by pressure retarded osmosis. J. Membr. Sci. 2009, 335 (1−2), 103−110. (12) Fu, F. J.; Zhang, S.; Sun, S. P.; Wang, K. Y.; Chung, T. S. POSScontaining delamination-free dual-layer hollow fiber membranes for forward osmosis and osmotic power generation. J. Membr. Sci. 2013, 443, 144−155. (13) She, Q. H.; Jin, X.; Tang, C. Y. Osmotic power production from salinity gradient resource by pressure retarded osmosis: Effects of operating conditions and reverse solute diffusion. J. Membr. Sci. 2012, 401, 262−273. (14) Zhang, S.; Fu, F. J.; Chung, T. S. Substrate modifications and alcohol treatment on thin film composite membranes for osmotic power. Chem. Eng. Sci. 2013, 87, 40−50. (15) Li, X.; Zhang, S.; Fu, F. J.; Chung, T. S. Deformation and reinforcement of thin-film composite (TFC) polyamide-imide (PAI) membranes for osmotic power generation. J. Membr. Sci. 2013, 434, 204−217. (16) Yip, N. Y.; Tiraferri, A.; Phillip, W. A.; Schiffrnan, J. D.; Hoover, L. A.; Kim, Y. C.; Elimelech, M. Thin-film composite pressure retarded osmosis membranes for sustainable power generation from salinity gradients. Environ. Sci. Technol. 2011, 45 (10), 4360−4369. (17) Chou, S. R.; Wang, R.; Shi, L.; She, Q. H.; Tang, C. Y.; Fane, A. G. Thin-film composite hollow fiber membranes for pressure retarded osmosis (PRO) process with high power density. J. Membr. Sci. 2012, 389, 25−33. (18) Arena, J. T.; McCloskey, B.; Freeman, B. D.; McCutcheon, J. R. Surface modification of thin film composite membrane support layers with polydopamine: Enabling use of reverse osmosis membranes in pressure retarded osmosis. J. Membr. Sci. 2011, 375 (1−2), 55−62. (19) Song, X. X.; Liu, Z. Y.; Sun, D. D. Energy recovery from concentrated seawater brine by thin-film nanofiber composite pressure retarded osmosis membranes with high power density. Energy Environ. Sci. 2013, 6 (4), 1199−1210. (20) Baker, R. W. Membrane Technology and Applications; John Wiley and Sons, Ltd.: West Sussex, U.K., 2004. (21) Sivertsen, E.; Holt, T.; Thelin, W.; Brekke, G. Pressure retarded osmosis efficiency for different hollow fibre membrane module flow configurations. Desalination 2013, 312, 107−123. (22) Verissimo, S.; Peinemann, K. V.; Bordado, J. Thin-film composite hollow fiber membranes: An optimized manufacturing method. J. Membr. Sci. 2005, 264 (1−2), 48−55. (23) Lau, W. J.; Ismail, A. F.; Misdan, N.; Kassim, M. A. A recent progress in thin film composite membrane: A review. Desalination 2012, 287, 190−199. (24) Kumano, A.; Ogura, H.; Hayashi, T. Composite hollow fiber membrane and process for its production. U.S. Patent 5,783,079, 1998. (25) Wang, K. Y.; Matsuura, T.; Chung, T. S.; Guo, W. F. The effects of flow angle and shear rate within the spinneret on the separation performance of poly(ethersulfone) (PES) ultrafiltration hollow fiber membranes. J. Membr. Sci. 2004, 240 (1−2), 67−79. (26) Sun, S. P.; Wang, K. Y.; Rajarathnam, D.; Hatton, T. A.; Chung, T. S. Polyamide-imide nanofiltration hollow fiber membranes with elongation-induced nano-pore evolution. AIChE J. 2010, 56 (6), 1481−1494. (27) Singh, S.; Khulbe, K. C.; Matsuura, T.; Ramamurthy, P. Membrane characterization by solute transport and atomic force microscopy. J. Membr. Sci. 1998, 142 (1), 111−127. (28) Lonsdale, H. K. Recent advances in reverse osmosis membranes. Desalination 1973, 13 (3), 317−332.

pore size and mechanical strength and TFC layer with PDA coating and molecular engineering of the interfacial polymerization solution. This study may provide insightful guidelines for optimizing the interfacial polymerization procedures and scaling up of the outer-selective TFC hollow fiber membrane modules for PRO power generation. The method is also applicable to preparing other semi-permeable membranes, such as low-pressure RO membranes for seawater desalination and nanofiltration membranes for use in removing pharmaceutical active compounds, heavy metal ions, and other emerging contaminants in an aquatic environment.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed spinning conditions, detailed hollow fiber morphologies, and pore size distributions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: 65-67791936. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank (1) GSK-EDB Trust Fund for the project with Grant R-706-000-019-592 and (2) Singapore National Research Foundation under its Environmental and Water Technologies Strategic Research Programme and administered by the Environment and Water Industry Programme Office (EWI) of the PUB under the project entitled “Membrane Development for Osmotic Power Generation, Part 1. Materials Development and Membrane Fabrication” (1102-IRIS-11-01) and NUS Grant R-279-000-381-279. Special thanks are due to Dr. Zhang Sui and Han Gang for their valuable suggestions. Shi-Peng Sun acknowledges IChemE for the award of “Singapore Young Chemical Engineer of the Year 2012”.

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REFERENCES

(1) Logan, B. E.; Elimelech, M. Membrane-based processes for sustainable power generation using water. Nature 2012, 488 (7411), 313−319. (2) In Modern Applications in Membrane Science and Technology; Escobar, I., Van der Bruggen, B., Eds.; American Chemical Society (ACS): Washington, D.C., 2011. (3) Agrawal, R.; Singh, N. R. Solar energy to biofuels. In Annual Review of Chemical and Biomolecular Engineering; Prausnitz, J. M., Doherty, M. F., Segalman, M. A., Eds.; Annual Reviews: Palo Alto, CA, 2010; Vol. 1, pp 343−364. (4) Ramon, G. Z.; Feinberg, B. J.; Hoek, E. M. V. Membrane-based production of salinity-gradient power. Energy Environ. Sci. 2011, 4 (11), 4423−4434. (5) Chung, T. S.; Zhang, S.; Wang, K. Y.; Su, J. C.; Ling, M. M. Forward osmosis processes: Yesterday, today and tomorrow. Desalination 2012, 287, 78−81. (6) Achilli, A.; Cath, T. Y.; Childress, A. E. Power generation with pressure retarded osmosis: An experimental and theoretical investigation. J. Membr. Sci. 2009, 343 (1−2), 42−52. (7) Gerstandt, K.; Peinemann, K. V.; Skilhagen, S. E.; Thorsen, T.; Holt, T. Membrane processes in energy supply for an osmotic power plant. Desalination 2008, 224 (1−3), 64−70. (8) Mi, B. X.; Elimelech, M. Organic fouling of forward osmosis membranes: Fouling reversibility and cleaning without chemical reagents. J. Membr. Sci. 2010, 348 (1−2), 337−345. 13173

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Environmental Science & Technology

Article

(29) Lee, K. L.; Baker, R. W.; Lonsdale, H. K. Membranes for power generation by pressure-retarded osmosis. J. Membr. Sci. 1981, 8 (2), 141−171. (30) Dreyer, D. R.; Miller, D. J.; Freeman, B. D.; Paul, D. R.; Bielawski, C. W. Elucidating the structure of poly(dopamine). Langmuir 2012, 28 (15), 6428−6435. (31) Wang, H. L.; Chung, T. S.; Tong, Y. W.; Jeyaseelan, K.; Armugam, A.; Duong, H. H. P.; Fu, F. J.; Seah, H.; Yang, J.; Hong, M. Mechanically robust and highly permeable AquaporinZ biomimetic membranes. J. Membr. Sci. 2013, 434, 130−136. (32) Kim, I. C.; Jeong, B. R.; Kim, S. J.; Lee, K. H. Preparation of high flux thin film composite polyamide membrane: The effect of alkyl phosphate additives during interfacial polymerization. Desalination 2013, 308, 111−114. (33) Han, G.; Zhang, S.; Li, X.; Widjojo, N.; Chung, T. S. Thin film composite forward osmosis membranes based on polydopamine modified polysulfone substrates with enhancements in both water flux and salt rejection. Chem. Eng. Sci. 2012, 80, 219−231.

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