Mitigating the Hydraulic Compression of Nanofiltration Hollow Fiber

Nov 10, 2014 - The fabricated hollow fiber membrane has a narrow pore size distribution with a molecular weight cutoff. (MWCO) of 470 Da. The outer la...
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Mitigating the Hydraulic Compression of Nanofiltration Hollow Fiber Membranes through a Single-Step Direct Spinning Technique Yee Kang Ong and Tai-Shung Chung* Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585, Singapore S Supporting Information *

ABSTRACT: Most nanofiltration (NF) membranes have been made through complicated multistep or thin-film composite processes. They also suffer the compaction issue that reduces permeate flux in pressure-driven filtration processes. A single-step coextrusion hollow fiber fabrication technique via immiscibility induced phase separation (I2PS) process is presented in this study to fabricate NF hollow fiber membranes. A protective layer is concurrently formed on top of the selective layer during the phase inversion process. The fabricated hollow fiber membrane has a narrow pore size distribution with a molecular weight cutoff (MWCO) of 470 Da. The outer layer of the I2PS hollow fiber is found to serve as a buffering layer that mitigates hydraulic compression on the compaction of dense-selective layer and sublayer and helps to retain membrane performance during nanofiltration operations. The newly fabricated NF hollow fiber membrane exhibits an average pure water permeability of 3.2 L m−2 h−1 bar−1 and shows good rejections toward the testing dyes. This study may offer a simple, direct, and cost-effective approach to fabricate NF hollow fiber membranes.

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for the flux decline in order to maintain the constant-permeate operation. As a result, the repetitive pressure increase will lead to irreversible membrane compaction. Pendergast et al. proposed the use of the constant-pressure operation mode to tackle this issue because additional membrane elements can be added into the system to offset the flux decline owing to membrane compaction and fouling.11 Mixed matrix membrane materials have been employed to enhance membranes’ compact resistance and mechanical properties. The inorganic fillers in the polymer matrix were reported to act as reinforced elements to strengthen membrane mechanical properties.11,13−17 Hoek and his co-workers incorporated nanoparticles (i.e., zeolite) into the substrate of thin film composite (TFC) RO membranes and improved their mechanical properties,11 while Homaeigohar and Elbahri fabricated zirconia-reinforced poly(ether sulfone) (PES) nanofibrous membranes with enhanced compaction resistance for microfiltration.17 Since marcovoids are the mechanical weak points of the membranes,12,18 one can design macrovoid-free spongelike membranes with better mechanical properties by properly controlling the phase inversion mechanisms and dope formulations.19−23

INTRODUCTION Pressure-driven filtration processes such as micro/ultra/nanofiltration (MF/UF/NF) and reverse osmosis (RO) have been widely applied for environmental separations such as wastewater treatment, water reclamation, and desalination.1−5 It has been generally accepted that membrane compaction occurs for all pressure-driven filtration processes during the early stages of operation which reduces the permeate flux and increases the solute rejection of the membranes.1,6 In addition, the degree of membrane compaction is greatly dependent on the type of filtration processes. NF and RO membranes operated under high pressures are more vulnerable to serious compaction that reduces the overall membrane porosity and flux. Prior studies reported that compaction and deformation of the macroporous sublayer were the dominant causes for flux decline due to the reduction of membranes porosity and the increase in overall tortuosity.7−12 Persson et al.8 and Li et al.12 observed that membranes with macrovoids structure have higher tendency to suffer from serious compaction as compared with those with a spongelike structure. They concluded that highly porous materials were more susceptible to compaction as compared with the less porous ones. Several approaches have been proposed to mitigate membrane compaction during the pressure-driven filtration processes. From a process point of view, the constant-permeate filtration mode has higher tendency to result in membranes with “overcompaction”. This is due to the fact that this mode requires a constant increment in feed pressure to compensate © 2014 American Chemical Society

Received: Revised: Accepted: Published: 13933

July 5, 2014 October 5, 2014 November 10, 2014 November 10, 2014 dx.doi.org/10.1021/es503258s | Environ. Sci. Technol. 2014, 48, 13933−13940

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Figure 1. Polarized light microscope (PLM) images of polymer blends.

were rinsed in a water bath for 2 days to ensure complete removal of the residual solvent. All of the fibers were then immersed in a glycerol/water (50/50 wt %) solution for another 2 days. The removal of the outer layer was carefully carried out in a similar bath containing glycerol/water (50/50 wt %) to preserve pore structures throughout the removal process. The membrane was air-dried at ambient conditions prior to module fabrication. Characterizations. The morphology of the hollow fiber membrane was observed using a field emission scanning electron microscope (FESEM JEOL JSM-6700LV). Mean surface roughness (Ra) of the membranes was analyzed by using a Bruker Dimension Icon atomic force microscopy (AFM). A tapping mode was operated on the samples in air at room temperature with the image scanning size of 20 μm × 20 μm. The pore size distribution, mean pore size, and molecular weight cutoff (MWCO) of the membrane were characterized by solute rejection experiments using neutral solutes (shown in Table S2, Supporting Information) based on the method described in the literature.30−32 Filtration Experiments. Filtration experiments were carried out in a self-fabricated setup. The effective membrane area of each hollow fiber membrane module was around 45 cm2. A shell-feed cross-flow mode was applied in this study whereby the feed solution was circulated at the shell side of the hollow fiber membrane with the superficial velocity of >0.24 m/s at the trans-membrane pressure (TMP) of 1−7 bar. The permeate solution was collected from the lumen side of the membrane. The hollow fiber modules were conditioned with DI water for at least 2 h prior to performance evaluation and sample collection. The water flux permeating through the membrane (J) can be determined from the total volume of the permeate (Q) collected at a specific period (t) over the effective membrane area (A) applied in the filtration process:

The concept of frugal engineering in product development has recently received a great deal of attention. This concept attempts to simplify the complex engineering processes into its basic components in the most economical manner. Scientists and engineers are striving to deliver more product values to the end users at a lesser cost. To meet the demand, the hollow fiber membrane fabrication process has been extended to the duallayer spinning technique.23−26 Not only can these approaches enhance membrane performance but also reduce the time to produce hollow fiber membranes.27 Ong and Chung proposed a cost-effective single-step cospinning method; namely, the immiscibility induced phase separation (I2PS) process. A protective layer was simultaneously created on top of the selective layer of the membrane during the formation process. They have fabricated I2PS pervaporation membranes for dehydration of ethanol with enhanced separation performance.28,29 This study attempts to broaden the application of the I2PS single-step direct spinning method to fabricate hollow fiber membranes for liquid filtration applications and to alleviate the issue of membrane compaction. In this regard, relatively hydrophilic polymers were selected as both inner and outer layer materials. The pore size distribution analyses showed that the fabricated hollow fiber membranes were within the nanofiltration (NF) range. The function of the outer protective layer in liquid filtration experiments was investigated. The newly developed NF membranes were subsequently explored for dye removal in textile wastewater using simulated feed solutions.

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EXPERIMENTAL SECTION Hollow Fiber Spinning. The outer protective layer of the I2PS hollow fiber membrane was polyacrylonitrile (PAN) and P84 copolyimide consisting of 3,3′4,4′-benzophenone tetracarboxylicdianhydride and 80% methylphenylenediamine + 20% methylenediamine was employed as the inner layer material. The dual-layer hollow fibers were fabricated via dry-jet wetspinning by means of I2PS as reported in previous studies.28,29 The detailed spinning procedures are described in the Supporting Information and the spinning parameters are listed in Table S1of the Supporting Information. The as-spun fibers

J=

Q At

(1)

Chemical Analyses. The concentration of electrolytes, neutral solutes, and dyes of the feed and permeate were characterized by using an electrical conductivity meter (Metrohm, Singapore), total organic carbon analyzer (TOC 13934

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Figure 2. Morphology of the hollow fiber membrane.

hollow fiber were tested using an Instron testing equipment38 and shown in Table S3 of the Supporting Information. The outer layer of the hollow fiber is full of macrovoids, whereas the inner layer consists of a spongelike layer immediately underneath the outer surface of the inner layer and a finger-like macrovoid supporting layer. The macrovoid structures at both inner and outer layers may reduce mass transport resistance during the filtration process. Since macrovoids are mainly formed via nonsolvent intrusion18,22,39 during the phase inversion process, the macrovoids at both inner and outer protective layers were postulated to be induced by the intrusion of strong nonsolvent (i.e., water) due to the rapid phase inversion process. Pure Water Permeability (PWP), Mean Pore Size, Pore Size Distribution, and Molecular Weight Cutoff (MWCO). The hollow fiber membrane was tested by various neutral solutes to characterize its pore properties. Table 1 summarizes

ASI-5000A, Shimazu, Japan), and UV/vis spectrophotometer (Pharo 300, Merck), respectively. The rejection (R) of each component was calculated as ⎛ Cp ⎞ R = ⎜1 − ⎟ × 100% Cf ⎠ ⎝

(2)

where Cf and Cp are the solute concentrations in the feed and the permeate solutions, respectively. Organic dyes consisting of Remazol Brilliant Blue R and Orange II sodium salt were employed as model solutes to evaluate the membrane performance and the wavelength (λ) of 484 and 590 nm were selected to determine the concentration of Orange II sodium salt and Remazol Brilliant Blue R, respectively.

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RESULTS AND DISCUSSION Compatibility of Outer-Layer and Inner-Layer Polymers. Since the incompatibility between inner- and outer-layer polymeric materials is the essential requirement to form the I2PS hollow fiber membranes, the compatibility of the selected polymers was first examined by dissolving both polymers at specific blend ratios of 95/5, 50/50, and 5/95 wt % in N,Ndimethylformamide (DMF) at a total polymer content of 5 wt %. Dope solutions made from individual inner- and outer-layer polymers were also prepared as the controls for comparison. All solutions were cast into dense membranes and their compatibility was probed using a polarized light microscope (PLM).33 As shown in Figure 1, no phase separation is observed for the controls, while phase separation is clearly visible for all the blend membranes. Therefore, the selected polymer pairs may fulfill the criteria of immiscibility for the I2PS process. The resultant dual-layer hollow fibers will have a dense selective layer on the outer surface of the inner layer. Morphology. Figure 2 depicts the morphology of the asspun hollow fiber. It is delamination-free, and both outer surfaces of the inner and outer layers are relatively dense. These are typical characteristics of I2PS hollow fibers.28,29 The hollow fiber possesses an outer diameter (OD) of ∼700 μm with a wall thickness (Δh) of ∼150 μm which translates to an OD/2Δh ratio of 2.3. Hence, it may satisfy the requirement for highpressure applications.34−37 The mechanical properties of the

Table 1. Mean Effective Pore Radius (rp), Geometric Standard Deviation (σp), Molecular Weight Cut-off (MWCO), and Pure Water Permeability (PWP) of the Hollow Fiber Membrane membrane ID

rp (nm)

σp

MWCO (Da)

pure water permeability (L m−2 h−1 bar−1)

NF-1

0.45

1.22

470

3.2 ± 0.5

a

Trans-membrane pressure = 1 bar.

its pure water permeability (PWP), mean pore size, and molecular weight cutoff (MWCO), while Figure S2 (Supporting Information) shows its pore size distribution. The relatively sharp pore size distribution is one of distinctive characteristics of I2PS dual-layer hollow fiber membranes, and it is essential to effectively separate targeted solutes. Since the outer layer prevents rapid nonsolvent intrusion (i.e., water) during the phase inversion process, it results in a uniform selective layer in the outer surface of the inner layer. As a result, the I2PS duallayer hollow fiber membranes have a narrow pore size distribution. The newly developed hollow fiber membrane has an average pure water permeability of 3.2 L m−2 h−1 bar−1 with a MWCO of 470 Da; it is a nanofiltration (NF) membrane 13935

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because NF membranes usually have MWCO in the range of 200−1000 Da.40 Electrolyte Rejection. The water-softening capabilities and charge properties of the NF hollow fiber membrane were investigated by performing electrolytes rejections using various electrolytes. Figure 3 shows the results and the rejection follows

Figure 4. Pore-size distributions of NF hollow fiber membranes with/ without the protective layer.

5), the selective layer of this membrane is located at the outer surface of the inner layer. The performances of the NF hollow fiber membranes with or without the protective layer are summarized in Table 2 using the same testing conditions. Consistent with the pore size distributions, both membranes have similar rejections toward sucrose (molecular weight: 342 g/mol). However, the NF hollow fiber membrane without the protective layer suffers a drastic PWP reduction as a function of time compared to the membrane with the protective layer, especially during the first hour of the experiment (shown in Figure 6). This observation is contradicted to the initial postulation that the removal of the protective layer may increase mass transport and result in a higher PWP. Clearly, the NF hollow fiber membrane without the protective layer may experience serious compaction during the stabilization period which results in a drastic reduction of PWP. Figure 7 depicts the mean surface roughness (Ra) of the selective layer (i.e., the outside surface of the inner layer) before and after the NF experiment. All the surfaces become smooth after the nanofiltration experiment, as compared with the pristine hollow fiber membrane. As a consequence, the NF hollow fiber membrane without the protective layer is postulated to suffer serious compaction near the surface skin region during the stabilization process that results in PWP decline. The hydraulic pressure would concentrate at the outer surface of the inner layer due to its narrow pore size and therefore provides the highest transport resistance in the whole membrane. In addition, the loose surface nodules and the sublayer may be compressed which lead to lower porosity in the subsurface of the inner layer. On the other hand, the protective layer may act as a buffering layer to reduce the net hydraulic pressure from direct contacts with the dense selective layer and retains the membrane performance. As a result, it could serve as a protective/buffering layer to mitigate the dense selective layer from deformation and compaction at NF operations. Although the presence of protective/buffering layer may provide certain resistance for mass transport, this drawback can be compensated through the gain in alleviating the downside of membrane compaction during NF processes.

Figure 3. Rejections of various electrolyte solutions. (Feed: 1000 ppm electrolyte; TMP: 1 bar.)

a descending order of Na2SO4 > MgSO4 > NaCl > MgCl2. The membrane displays a higher rejection toward the divalent anion of SO42− over the monovalent anion of Cl− and a lower rejection toward the divalent cation of Mg2+ over the monovalent cation of Na+. This observation indicates that the NF hollow fiber membrane possesses a negatively charged surface. Thus, it has a similar performance trend toward positive- and negative-charged electrolytes as those negatively charged NF membranes in the literature.32,41−43 It is also further supported by the ζ-potential analyses on the membrane shown in Figure S3 (Supporting Information). Donnan exclusion and size exclusion are the key separation mechanisms for NF processes.3,44 On the basis of the hydrated radii of the ions shown in Table S4 (Supporting Information),45 the Donnan exclusion appears to be the dominant separation mechanism in this study because the electrolyte rejections are contradicted with their hydration radii. Functions of the Outer Protective Layer. In addition to produce a much perfect selective layer with a narrow pore size distribution, the outer protective layer has been found to minimize the swelling of the selective layer by preventing it from direct contact with the feed solution during pervaporation.28 However, this protective layer may result in additional mass transport resistance during NF operations. To understand its additional functions, we removed the protective layer and tested the resultant hollow fiber membrane in NF operations. Figure 4 shows the pore size distributions of both NF hollow fiber membranes. They display a similar mean pore size, and the membrane without the protective layer has a slightly broader pore size distribution. Apparently, the NF hollow fiber membrane was not damaged during the removal process. In addition, some pin holes are detected on the top surface of the protective layer using AFM (shown in Figure 5). This observation may be due to the rapid intrusion of the nonsolvent (i.e., water as the external coagulant) during the phase-inversion process. Since both of the outside and inside surfaces of the protective layer are defective (Figures 2C and 13936

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Figure 5. Pin holes at the outer surface of the protective layer.

Therefore, the NF hollow fiber membrane with the protective layer is utilized in subsequent experiments. Nanofiltration (NF) Experiments for Dye Removal. The pure water flux of the membrane as a function of transmembrane pressure is portrayed in Figure S4 (Supporting Information). The linear relationship between pure water flux and trans-membrane pressure indicates that the NF hollow fiber membrane is able to maintain its structural integrity throughout the selected trans-membrane pressures during NF experiments. The membrane was subsequently tested for dye removal in textile wastewater using simulated feed solutions. Figure 8 shows the separation performance of the NF hollow fiber membrane against Orange II sodium salt which has a molecular weight of 350.32 g/mol. The permeate flux increases with trans-membrane pressure in a linear trend, while the rejection suffers a slight reduction. Sucrose and Orange II have similar molecular weights (342 g/mol vs 350.32 g/mol), they are smaller than the MWCO of the membrane. However, the rejection of Orange II is much higher than that of sucrose (>99% vs 77 ± 6% (Table 2) at 1 bar). This difference is mainly attributed to the Donnan exclusion mechanism that governs the whole separation process.32 Interestingly, the dye rejection as a function of trans-membrane pressure is opposite the findings in conventional pressure-driven filtration processes.32,46,47 This phenomenon can be plausibly explained by the cushioning effect of the protective layer in mitigating hydraulic compression; thus, the structures of the denseselective layer and sublayer are retained under pressures. As a result, the effect of hydraulic compression that increases the membrane rejection as a function of trans-membrane pressure is reduced. In addition, the concentration polarization occurs at

Table 2. Performance of the NF Hollow Fiber Membrane with or without Protective Layer membrane ID NF-1 (protective layer) NF-1 (without protective layer) a

pure water permeability (L m−2 h−1 bar−1)

sucrose rejection (%)

3.2 ± 0.5 1.1 ± 0.3

77 ± 6 74 ± 2

Trans-membrane pressure = 1 bar.

Figure 6. Pure water permeability (PWP) of the NF hollow fiber membranes with/without the protective layer (trans-membrane pressure = 1 bar).

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Figure 7. Surface roughness of the selective layer (the outer surface of the inner layer) before and after the NF experiment.

Figure 8. Performance and permeate quality of the I2PS NF hollow fiber membrane as a function of trans-membrane pressure (TMP) in rejecting Orange II sodium salt (feed: 50 ppm dye).

Figure 9. Performance and permeate quality of the I2PS NF hollow fiber membrane as a function of trans-membrane pressure (TMP) in rejecting Remazol Brilliant Blue R (feed: 50 ppm dye).

Figure 8 in rejecting Orange II (2.51 L m−2 h−1 bar−1 vs 2.28 L m−2 h−1 bar−1). This difference is due to the fact that Orange II dye molecules are smaller, and they may preferentially foul the membrane through pore blocking or adsorption mechanisms.5,49 As most commercially available NF membranes are made from interfacial polymerization and thin-film composite (TFC) approaches,2,4,46 this study may offer a simple and direct approach to fabricate NF hollow fiber membranes through a single-step direct spinning process. It is worth mentioning that the preferred membrane materials are not limited to those

the membrane surface due to flux increase may also contribute to the reduction of solute rejection.48 On the other hand, the performance of the NF hollow fiber membrane against Remazol Brilliant Blue R under various trans-membrane pressures is depicted in Figure 9. Compared to Orange II sodium salt, this blue dye is larger and has a molecular weight of 626.54 g/mol. The NF hollow fiber membrane shows a good and stable separation performance in terms of flux and rejection. The average permeate permeability obtained from the slope of Figure 9 in rejecting Remazol Brilliant Blue R is higher than that obtained from the slope of 13938

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(8) Persson, K. M.; Gekas, V.; Trägårdh, G. Study of membrane compaction and its influence on ultrafiltration water permeability. J. Membr. Sci. 1995, 100, 155−162. (9) Peterson, R. A.; Greenberg, A. R.; Bond, L. J.; Krantz, W. B. Use of ultrasonic TDR for real-time noninvasive measurement of compressive strain during membrane compaction. Desalination 1998, 116, 115−122. (10) Bohonak, D. M.; Zydney, A. L. Compaction and permeability effects with virus filtration membranes. J. Membr. Sci. 2005, 254, 71− 79. (11) Pendergast, M. T. M.; Nygaard, J. M.; Ghosh, A. K.; Hoek, E. M. V. Using nanocomposite materials technology to understand and control reverse osmosis membrane compaction. Desalination 2010, 261, 255−263. (12) 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. (13) Aerts, P.; Greenberg, A. R.; Leysen, R.; Krantz, W. B.; Reinsch, V. E.; Jacobs, P. A. The influence of filler concentration on the compaction and filtration properties of Zirfon®-composite ultrafiltration membranes. Sep. Purif. Technol. 2001, 22−23, 663−669. (14) Ebert, K.; Fritsch, D.; Koll, J.; Tjahjawiguna, C. Influence of inorganic fillers on the compaction behaviour of porous polymer based membranes. J. Membr. Sci. 2004, 233, 71−78. (15) Pendergast, M. M.; Hoek, E. M. V. A review of water treatment membrane nanotechnologies. Energy Environ. Sci. 2011, 4, 1946−1971. (16) Edwie, F.; Chung, T. S. Development of hollow fiber membranes for water and salt recovery from highly concentrated brine via direct contact membrane distillation and crystallization. J. Membr. Sci. 2012, 421−422, 111−123. (17) Homaeigohar, S. S.; Elbahri, M. Novel compaction resistant and ductile nanocomposite nanofibrous microfiltration membranes. J. Colloid Interface Sci. 2012, 372, 6−15. (18) Strathmann, H.; Kock, K.; Amar, P.; Baker, R. W. The formation mechanism of asymmetric membranes. Desalination 1975, 16, 179− 203. (19) Smolders, C. A.; Reuvers, A. J.; Boom, R. M.; Wienk, I. M. Microstructures in phase-inversion membranes. Part 1. Formation of macrovoids. J. Membr. Sci. 1992, 73, 259−275. (20) Boom, R. M.; Wienk, I. M.; Van Den Boomgaard, T.; Smolders, C. A. Microstructures in phase inversion membranes. Part 2. The role of a polymeric additive. J. Membr. Sci. 1992, 73, 277−292. (21) Tsai, H. A.; Ruaan, R. C.; Wang, D. M.; Lai, J. Y. Effect of temperature and span series surfactant on the structure of polysulfone membranes. J. Appl. Polym. Sci. 2002, 86, 166−173. (22) Widjojo, N.; Chung, T. S. Thickness and air gap dependence of macrovoid evolution in phase-inversion asymmetric hollow fiber membranes. Ind. Eng. Chem. Res. 2006, 45, 7618−7626. (23) Sukitpaneenit, P.; Chung, T. S. High performance thin-film composite forward osmosis hollow fiber membranes with macrovoidfree and highly porous structure for sustainable water production. Environ. Sci. Technol. 2012, 46, 7358−7365. (24) Li, S. G.; Koops, G. H.; Mulder, M. H. V.; Van Den Boomgaard, T.; Smolders, C. A. Wet spinning of integrally skinned hollow fiber membranes by a modified dual-bath coagulation method using a triple orifice spinneret. J. Membr. Sci. 1994, 94, 329−340. (25) He, T.; Mulder, M. H. V.; Wessling, M. Preparation of porous hollow fiber membranes with a triple-orifice spinneret. J. Appl. Polym. Sci. 2003, 87, 2151−2157. (26) Yang, Q.; Wang, K. Y.; Chung, T. S. Dual-layer hollow fibers with enhanced flux as novel forward osmosis membranes for water production. Environ. Sci. Technol. 2009, 43, 2800−2805. (27) Kopeć, K. K.; Dutczak, S. M.; Wessling, M.; Stamatialis, D. F. Chemistry in a spinneret-On the interplay of crosslinking and phase inversion during spinning of novel hollow fiber membranes. J. Membr. Sci. 2011, 369, 308−318. (28) Ong, Y. K.; Chung, T. S. High performance dual-layer hollow fiber fabricated via novel immiscibility induced phase separation (I2PS)

covered in this study. Cost-effective and water/chemical resistant materials can be applied as long as they meet the principles of the I2PS process to fabricate liquid filtration membranes. In addition, the concept of introducing a buffering layer on top of the selective layer of the membrane may provide a possible solution in fabricating next generation filtration membranes. It can alleviate the compaction issue that reduces the productivity/permeate flux and is often regarded to be irreversible in pressure-driven filtration processes.11 Concentration polarization or fouling may occur at the interface of the membrane. Significant research is needed to minimize these phenomena through the morphological design and modifications of the protective layer.

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ASSOCIATED CONTENT

S Supporting Information *

Materials, hollow fiber spinning, characterizations, pore size distribution, mechanical properties, and ζ-potential. This material is available free of charge via the Internet at http:// pubs.acs.org/

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +65 6516 6645. Fax: +65 6779 1936. E-mail: chencts@ nus.edu.sg. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded under the project entitled ‘‘Membrane development for osmotic power generation, Part 1. Materials development and membrane fabrication’’ (1102IRIS-11-01) and NUS Grant No. of R-279-000-381-279. This research grant is supported by the Singapore National Research Foundation under its Environmental & Water Technologies Strategic Research Programme and administered by the Environment & Water Industry Programme Office (EWI) of the PUB. Special thanks are given to Prof. Juin-Yih Lai and Prof. Hui-An Tsai at R&D Center for Membrane Technology, Chung Yuan Christian University of Taiwan, for the provision of PAN polymer, Dr. Kaiyu Wang and Dr. Youchang Xiao for providing valuable suggestions to this research, Mr. Zhiwei Thong for ζ-potential analyses.

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REFERENCES

(1) Mulder, M. Basic principles of membrane technology; Kluwer Academic: Dordrecht, 1991. (2) Matsuura, T. Synthetic membranes and membrane separation processes; CRC Press: Boca Raton, 1993. (3) Verliefde, A. R. D.; Heijman, S. G. J.; Cornelissen, E. R.; Amy, G. L.; Van Der Bruggen, B.; Van Dijk, J. C. Rejection of trace organic pollutants with high pressure membranes (NF/RO). Environ. Prog. 2008, 27, 180−188. (4) Baker, R. W. Membrane Technology and Applications; John Wiley & Sons: New York, 2012. (5) Ong, Y. K.; Li, F. Y.; Sun, S. P.; Zhao, B. W.; Liang, C. Z.; Chung, T. S. Nanofiltration hollow fiber membranes for textile wastewater treatment: Lab-scale and pilot-scale studies. Chem. Eng. Sci. 2014, 114, 51−57. (6) Hussain, Y. A.; Al-Saleh, M. H. A viscoelastic-based model for TFC membranes flux reduction during compaction. Desalination 2014, 344, 362−370. (7) Jonsson, G. Methods for determining the selectivity of reverse osmosis membranes. Desalination 1977, 24, 19−37. 13939

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dx.doi.org/10.1021/es503258s | Environ. Sci. Technol. 2014, 48, 13933−13940