Article pubs.acs.org/est
High Performance Thin-Film Composite Forward Osmosis Hollow Fiber Membranes with Macrovoid-Free and Highly Porous Structure for Sustainable Water Production Panu Sukitpaneenit and Tai-Shung Chung* Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent 4 Engineering Drive 4, Singapore 117576, Singapore S Supporting Information *
ABSTRACT: The development of high-performance and well-constructed thin-film composite (TFC) hollow fiber membranes for forward osmosis (FO) applications is presented in this study. The newly developed membranes consist of a functional selective polyamide layer formed by highly reproducible interfacial polymerization on a polyethersulfone (PES) hollow fiber support. Using dual-layer coextrusion technology to design and effectively control the phase inversion during membrane formation, the support was designed to possess desirable macrovoid-free and fully spongelike morphology. Such morphology not only provides excellent membrane strength, but it has been proven to minimize internal concentration polarization in a FO process, thus leading to the water flux enhancement. The fabricated membranes exhibited relatively high water fluxes of 32−34 LMH and up to 57−65 LMH against a pure water feed using 2 M NaCl as the draw solution tested under the FO and pressure retarded osmosis (PRO) modes, respectively, while consistently maintaining relatively low salt leakages below 13 gMH for all cases. With model seawater solution as the feed, the membranes could display a high water flux up to 15−18 LMH, which is comparable to the best value reported for seawater desalination applications.
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have attracted considerable attention in many fields of science and engineering including liquid food processing,9 pharmaceutical and protein enrichment,10 and power generation,4,11,12 in addition to its great potential for seawater desalination, fertigation, water treatment, and reclamation processes.5,13−16 However, one of the major challenges to the current FO technology is the lack of effective membranes, which is the heart of the system. The ideal FO membranes need to provide high water permeability, low solute permeability, sufficient high mechanical strength, good chemical stability, and low internal concentration polarization (ICP).3,4 To meet the above characteristics, most FO membranes are designed to have an ultrathin active layer supported by a relatively thin and fully porous substrate. One promising approach is the fabrication of thin-film composite (TFC) hollow fiber membranes made by interfacial polymerization because they possess higher surface areas and a better flow pattern for the FO process.17−19 While several studies have reported the preparation of flat sheet TFC membranes in the recent FO development,20−22 few studies
INTRODUCTION To address or alleviate the water crisis, fresh water production through desalination and wastewater reclamation processes has received worldwide attention.1 Currently, reverse osmosis (RO) is a dominant technology in seawater desalination and water treatment markets. However, it is energy intensive in nature. Furthermore, there have been questions concerning the negative environmental impact of current RO desalination technologies.1 With these concerns, there is an urgent need for a novel sustainable technology that consumes less energy and chemicals, and has minimal impact on the environment for the future water-energy nexus. Forward osmosis (FO) is an emerging, low-energy, and green membrane process in comparison to conventional desalination and wastewater treatment processes. FO utilizes the osmotic pressure difference between two solutions separated by a semipermeable membrane to induce spontaneous water transport from the feed solution of low osmotic pressure to the draw solution of high osmotic pressure while most solutes are rejected by the FO membrane.2−4 In addition to this low hydraulic operating pressure, FO may offer advantages of higher rejections to a wide range of contaminants and lower membrane-fouling propensities compared to the conventional RO process.5−8 These superior FO properties © 2012 American Chemical Society
Received: Revised: Accepted: Published: 7358
April 19, 2012 May 28, 2012 June 4, 2012 June 4, 2012 dx.doi.org/10.1021/es301559z | Environ. Sci. Technol. 2012, 46, 7358−7365
Environmental Science & Technology
Article
Figure 1. SEM micrographs of different bulk and surface morphologies of PES hollow fiber membrane supports (spinning code: PESwater).
Supporting Information (SI). The spinning conditions for the PES hollow fiber membrane supports are listed in Table S1 of the SI. The detailed procedures for the polymer dope preparation, spinning process, membrane post-treatment, and module fabrication are described in the SI. Interfacial Polymerization of Thin-Film-Composite (TFC) Forward Osmosis Hollow Fiber Membranes. The formation of a polyamide thin layer on the inner surface of PES hollow fiber supports was achieved by an interfacial polymerization (IP) between MPD monomers in the aqueous phase and TMC monomers in the organic phase. The detailed specification of the experiment setup and preparation steps is disclosed in the SI. Characterizations of PES Hollow Fiber Membrane Supports and TFC-FO Hollow Fiber Membranes. The fabricated hollow fiber membrane supports were first tested to measure pure water permeability (PWP) (L m2− h−1 bar−1 or LMHbar−1) using a lab-scale circulating filtration unit, as described in SI or eslewhere.10,25 The membranes were then characterized by solute rejection measurements. Feed solutions containing 200 ppm of different molecular weights of PEG or PEO were utilized as the neutral solutes to estimate pore size, pore size distribution, and molecular weight cutoff (MWCO) according to the solute transport method described in the SI or elsewhere.17,26 The water permeability A, salt rejection Rs, and salt permeability B of TFC-FO hollow fiber membranes were determined by testing the membranes under the RO mode following the method described elsewhere.19,25 Water Reclamation through Forward Osmosis Tests of TFC-FO Hollow Fiber Membranes and Evaluation of Structural Parameters of Supporting Layers. Forward osmosis (FO) experiments were conducted on a lab-scale circulating filtration unit.25 The detailed experimental setup, operating conditions, and the determination of structural
have been devoted to the fabrication and characterization of TFC hollow fiber membranes but without revealing the details of interfacial polymerization.18,19 Moreover, most investigated TFC hollow fiber membranes consist of a top thin polyamide (PA) rejection layer and large finger-like macrovoids in a porous membrane support. These macrovoids may be undesirable because they are mechanically weak points, which may cause membrane failure under continuous vibration and backwashing operations.23 On the other hand, hollow fiber membranes with a favorable macrovoid-free structure could be the more preferential design in the industrial applications. In this work, we have demonstrated novel TFC-FO hollow fiber membranes with a high flux and a favorable macrovoidfree substructure for water production through FO processes. The feasibility of fabricating well-defined FO hollow fiber membrane supports has been demonstrated by employing the advanced coextrusion technologies to effectively control the phase inversion and create appropriate membrane supports for interfacial polymerization. A simple and highly reproducible interfacial polymerization method, using the m-phenylenediamine (MPD) and trimesoyl chloride (TMC) monomers, was developed. The simple and cost-effective fabrication technique using a dual-layer spinneret demonstrated through this study could provide another platform for future FO membrane development.
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MATERIALS AND METHODS Fabrication of Macrovoid-Free PES Hollow Fiber Membrane Supports. The hollow fiber membranes were prepared by a dry-jet wet spinning process employing the advanced coextrusion technology through a dual-layer spinneret. A detailed description of the hollow fiber spinning process is documented elsewhere.24 The dual-layer spinneret and its dimension used in this study are described in Figure S1 of the 7359
dx.doi.org/10.1021/es301559z | Environ. Sci. Technol. 2012, 46, 7358−7365
Environmental Science & Technology
Article
exchange) in the air-gap region prior to entering the water coagulant, attempting to form a macrovoid-free cross-section and a porous structure at the membrane’s outer surface. In contrast, an instantaneous demixing (rapid phase inversion and fast solvent exchange) was developed by introducing a nonsolvent at the bore-fluid channel to create a relatively dense inner surface. Moreover, based on our observation during experiments, the fabricated hollow fiber membranes spun from the dual-layer coextrusion technique tend to have a uniform dimension and no irregular shape, which reflects the uniform solvent exchange rate during the phase inversion process. Another important parameter that plays an important role in eliminating larger finger-like macrovoids and meanwhile creating highly porous and open-cell structure is the addition of PEG and water nonsolvent additives with optimum concentrations into dope solutions. PEG, a highly hydrophilic polymer and a weak nonsolvent of a PES/NMP system, is known as an effective additive to enhance pore formation, improve pore interconnectivity, and prevent macrovoid formation. Water, a strong nonsolvent of the system, is added (with a relative small amount) to enhance dope viscosity and to bring the dope formulation close to the binodal composition which could favor the macrovoid suppression.27 The inner surface roughness of hollow fibers was examined by AFM and is displayed in Figure S4. All as-spun fibers have a similar surface roughness of 2−3 nm regardless of bore-fluid spinning conditions. It can be seen that the membranes spun with pure water exhibited slightly higher surface roughness compared to others. Similar phenomenon has been reported by Widjojo et al.28 and Sukitpaneenit and Chung29 where the instantaneous demixing induced by strong nonsolvent, e.g. water, tends to result in membranes with a rougher surface (Ra = 2.85 nm). In other words, the delayed demixing could rather result in a smoother surface and a smaller surface roughness as shown in the case of membranes spun with water/NMP (Ra = 2.25 nm) and water/NMP/PEG (Ra = 2.69 nm). Nevertheless, one may observe a slight increase in surface roughness of the membranes PESwater/NMP/PEG, compared to the membrane PESwater/NMP. This may be due to the fact that (1) adding PEG, known as a pore forming additive, in the bore-fluid may enhance the pore evolution at the contacting inner surface during membrane formation, and (2) the highly hydrophilic PEG polymer in the bore-fluid may complicate the precipitation rate and preferentially create a relatively higher surface roughness. In other words, introducing PEG into a bore-fluid solution could enhance the delayed demixing and slow the precipitation rate to some certain extent, leading to formation of a membrane with relatively higher surface porosity/
parameters are described in the SI. A schematic diagram of FO setup is shown in Figure S2 of the SI.
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RESULTS AND DISCUSSION Morphology, Microstructure, and Separation Characteristics of PES Hollow Fiber Supports. A typical morphology of PES hollow fiber membrane supports spun using water as a bore-fluid (PESwater) is illustrated in Figure 1. The hollow fibers have a macrovoid-free and fully sponge-like structure with a high degree of concentricity. The inner surface of as-spun membranes exhibits relatively small pores and smooth surface, compared to the outer surface which possesses a highly porous structure and relatively large pore sizes of 1 μm (estimated from the FESEM observation, Figure 1). Similar bulk and surface morphologies as discussed above are obtained for the hollow fibers spun with other conditions (70/30 wt % water/NMP and 40/30/30 wt % water/NMP/PEG as nonsolvent bore-fluids), as illustrated in Figure S3 of the SI. These morphological features are consistent with our strategy, employing dual-layer spinneret to facilitate membrane phase inversion during fiber spinning, as depicted in Figure 2. The
Figure 2. Strategies to control the phase inversion process with the aid of coextrusion technology employing a dual-layer spinneret.
pure NMP solvent was fed at the outer channel to induce a delayed demixing23 (slow phase inversion and mild solvent
Table 1. Summary of Mean Effective Pore Size (μp), PWP, MWCO, Porosity, Water Contact Angle, and Mechanical Properties of PES Hollow Fiber Membrane Supports membrane ID
PESwater
PESwater/NMP
PESwater/NMP/PEG
mean pore size, μp (nm) (at the inner surface) geometric standard variation, σp PWP (L m2− h−1 bar−1) (at 1 bar) MWCO (Da) porosity (%) water contact angle, θ (°) (at the inner surface) tensile strength (MPa) elongation at break (%) Young’s modulus (MPa)
15.01 1.30 835 106 967 80.0 ± 1.4 64.6 ± 2.0 5.88 ± 0.27 48.13 ± 5.78 273.80 ± 21.58
16.50 1.33 856 132 349 82.0 ± 1.0 56.5 ± 2.1 6.97 ± 0.23 49.82 ± 8.34 228.30 ± 49.56
17.05 1.34 1021 142 672 80.9 ± 1.6 54.8 ± 2.8 5.81 ± 0.09 50.90 ± 4.65 196.27 ± 27.31
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dx.doi.org/10.1021/es301559z | Environ. Sci. Technol. 2012, 46, 7358−7365
Environmental Science & Technology
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Figure 3. Typical morphology of TFC-FO hollow fiber membranes with PESwater/NMP supports layer after interfacial polymerization.
in nanorange, high porosity, and high hydrophilicity, macrovoid-free with enhanced mechanical properties. Characteristics and Performance of TFC-FO Hollow Fiber Membranes. The active polyamide layer of the TFCFO hollow fiber membranes after interfacial polymerization on the aforementioned membrane supports is depicted in Figure 3. The selective layer has a uniform ridge-and-valley morphology, which is a typical characteristic of polyamide membranes formed using an interfacial polymerization.32 The estimated thickness of the selective polyamide layer is 380 ± 25 nm regardless of different membrane supports. There is no significant change observed on the outer surface morphology after IP, indicating no IP solutions penetrating across the membrane bulk. The fabricated TFC-FO hollow fiber membranes with different membrane supports were evaluated for their FO performance in both FO and PRO operating modes using DI water feed and 2 M NaCl as a draw solution. The membrane performance was expressed in terms of water flux and salt reverse flux as summarized in Table 2. Interestingly, all membranes demonstrate remarkable high water fluxes and reasonably low salt leakages. Based on the experimental results,
roughness. Besides, the highly hydrophilic characteristic of PEG may further hinder the solvent/nonsolvent exchange rate during phase inversion at the inner surface due to high affinity with added PEG in the polymer dope, and thus enhance the surface roughness.27 Basic characteristics of PES hollow fiber membrane supports before interfacial polymerization, which include the mean pore size, pure water permeability (PWP), molecular-weight cutoff (MWCO), porosity, and water contact angle, are listed in Table 1. All spun PES hollow fiber membranes possess a small pore size in a range of 15−17 nm and a narrow pore size distribution profile (small geometric standard variation of 1.30−1.34), which is essential to produce a continuous and homogeneous polyamide layer through interfacial polymerization.28,30 A clear comparison of pore size and pore size distribution of the resultant membranes is depicted in Figure S5. As shown in Table 1, the PESwater, PESwater/NMP, and PESwater/NMP/PEG supports display high PWP values of 835, 856, and 1021 L m2− h−1 bar−1 at 1 bar, respectively. This tendency is apparently attributed to their corresponding pore size, pore size distribution, and high overall porosity (80−82%) for each respective membrane. Additionally, the membranes possess moderately low water contact angles of 55−65° (
