Fabrication of BaSO₄-based mineralized thin-film composite polysulfone/polyamide membranes for enhanced performance in a forward osmosis process

Abstract

Novel BaSO₄-based mineralized thin-film composites (TFC) as forward osmosis (FO) membranes were fabricated through depositing barium sulfate on the surface of the prepared polysulfone/polyamide (PSf/PA) membranes by adopting an approach named surface mineralization. BaSO₄ particles were deposited by an alternate soaking process (ASP) with aqueous solutions of barium chloride (BaCl₂) and sodium sulfate (Na₂SO₄), separately. Membranes with different mineralization degrees were prepared by changing the number of ASP cycles. The mineralized TFC PSf/PA FO membranes were characterized by a variety of methods in accordance with membranes structure and surface properties. It turned out that the mineral coating made of BaSO₄ particles was evenly distributed on the membrane surface and the existence of this coating made the membrane surface became more hydrophilic and negatively charged after mineralization. The FO performances of the mineralized TFC membranes were also tested and compared with the original PSf/PA membrane and the commercial CTA-W FO membrane with pure water as the feed solution and a 1 M NaCl solution as the draw solution. The mineralized TFC PSf/PA membranes displayed better water permeability and salt rejection than the original PSf/PA membrane and the commercial FO membrane. The results revealed that mineralized TFC PSf/PA membranes showed great potential for further development of FO applications.

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1. Introduction

As an osmotically driven process, forward osmosis (FO) has gained increasing popularity among many fields in recent years, such as seawater desalination,^1,2 wastewater treatment,^3,4 food processing,^5,6 controlled drug release⁷ and power generation.^8,9 In FO processes, water was driven through the semipermeable membrane from the feed solution side of low concentration to the draw solution side of high concentration due to osmotic pressure gradient. Compared with conventional pressure-driven processes like reverse osmosis (RO), ultrafiltration (UF) and nanofiltration (NF), FO has certain advantages such as higher water recovery,¹⁰ lower fouling tendency,¹¹ easier cleaning¹² and less energy input.¹³

Despite the advantages of FO processes, the lack of adequate FO membranes remains challenging for researchers.^14,15 A promising FO membrane should show high water flux, low reverse salt diffusion and good chemical resistance.^14,16 The current commercially available flat FO membrane made of cellulose triacetate (CTA) and the majority of thin film composite (TFC) membranes^14,16,17 prepared in laboratory are asymmetric membranes with a dense skin layer and a porous support layer. One of the major obstacles in using these asymmetric membranes for FO processes is the presence of internal concentration polarization (ICP). ICP is an intrinsic phenomenon occurs inside the thick and hydrophobic porous support layer.^18–20 Studies showed that ICP was the key factor that caused water flux decline in FO, which is capable of decreasing water flux by more than 80%.^21,22 Increasing researches have focused on preparing new FO membranes to reduce ICP and a variety of methods for the fabrication and modification of FO membranes are carried out. Chung's group prepared hollow fiber and flat sheet modules cellulose ester-based membranes separately for FO applications.^23,24 Wang et al. developed high-performance FO membranes using PES/sulfonated PSf-alloyed membranes as the substrates during interfacial polymerization.²⁵ Song et al. adopted electrospinning followed by interfacial polymerization (ES-IP) to fabricate a novel nanofiber TFC FO membrane.²⁶ Setiawan et al. developed a type of hollow fiber FO membrane with a positively charged NF-like selective layer by polyelectrolyte post-treatment of a polyamide (PAI) microporous substrate using polyethyleneimine (PEI).²⁷

The surface properties of FO membranes play an important role in permeable performance and surface modification is one of the common membrane modification methods. Arena et al. used polydopamine (PDA) as a novel bio-inspired hydrophilic polymer to modify the support layers of commercial TFC RO membranes for engineered osmosis applications.²⁸ Hye et al. fabricated polyvinyl alcohol (PVA)-coated cellulose acetate (CA)-based flat-sheet membranes and improved water flux performance for FO processes.²⁹ Inger et al. used different linking molecules to bind the polyamide (PA) active layer to the cellulose triacetate (CTA) support and fabricated membranes with performance characteristics less dependent upon salt concentration.³⁰

In this research, the thin-film composite polysulfone/polyamide (PSf/PA) FO membranes were prepared by interfacial polymerization. Then a novel approach named surface mineralization was adopted to modify the TFC PSf/PA FO membrane. BaSO₄ particles were deposited on the surface of the TFC PSf/PA FO membrane by an alternate soaking process (ASP) with aqueous solutions of barium chloride (BaCl₂) and sodium sulfate (Na₂SO₄), separately. BaSO₄ was chosen as modifier because Ba²⁺ has two positive charges and it is easy to be captured by negative ion groups on the membrane surface. In addition, BaSO₄ has a very small solubility product constant and it is easy to form the deposition. Membranes with different mineralization degrees were prepared by changing the numbers of ASP cycles. Membrane morphology and surface properties were analyzed to demonstrate the effects on the membranes by deposition of BaSO₄-based mineral surface coating layer. The FO performances of the mineralized membranes were evaluated and compared with the original PSf/PA membrane and the commercial CTA-W FO membrane.

2. Material and methods

2.1. Materials

Polysulfone beads (M_n: 75 000 Da, Solvay Advanced Polymers), 1-methyl-2-pyrrolidinone (NMP, ≥99.0%, Sinopharm Chemical Reagent Co., Ltd) and polyvinylpyrrolidone (PVP K-30, Sinopharm Chemical Reagent Co., Ltd) were used to prepare the substrates. 1,3-Phenylenediamine (MPD, ≥99.5%, Sinopharm Chemical Reagent Co., Ltd), trimesoyl chloride (TMC, ≥98.0%, Aladdin) and n-hexane (≥97%, Sinopharm Chemical Reagent Co., Ltd) were used for interfacial polymerization. Barium chloride (BaCl₂) and sodium sulfate (Na₂SO₄) of analytical grade (Sinopharm Chemical Reagent Co., Ltd) were used to prepare the aqueous solutions for surface mineralization. Sodium chloride (NaCl, ≥99.5%, Sinopharm Chemical Reagent Co., Ltd) was used as draw solute for FO tests. Disodium carbonate (Na₂CO₃, ≥99%, Sinopharm Chemical Reagent Co., Ltd) was used as an effluent for ion chromatography (ICS-900, Dionex, CA, USA). The fabricated membranes were evaluated against the commercial membranes (Hydration Technology Innovations, HTI): a cartridge membrane made of cellulose triacetate with an embedded polyester screen mesh (CTA-W).

2.2. Preparation of mineralized TFC PSf/PA FO membranes

2.2.1 Preparation of PSf substrate. The PSf support membranes were prepared by phase inversion method via immersion precipitation technique. The PSf dope solutions were prepared by dissolving 14 wt% PSf and 0.5 wt% PVP in 85.5 wt% NMP and stirred by a mechanical stirrer (JJ, Yitong Electron Co., Ltd, China) at 70 °C for 8 hours. Then the homogenous polymer solutions were placed in a desiccator at room temperature (25 °C) for 12–15 h to remove air bubbles. The dope solution was then spread on a dry glass plate using a casting knife at a gate height of 140 μm. The glass plate was immediately immersed into deionized water at room temperature for phase inversion process to take place. After the phase separation finished, the substrate was peeled off and kept in deionized water at room-temperature, which was changed every 4 h for 24 h to wash out the solvents. Then the substrate was stored in deionized water till next use.

2.2.2 Preparation of polysulfone/polyamide composite membrane. The active layer of TFC FO membrane was fabricated by interfacial polymerization on the surface of the prepared PSf substrate. At first, the substrate was soaked in 50 mL 1% (w/v) aqueous solution of MPD for 120 s. The residual MPD solution on the surface of the substrate was removed by using compressed nitrogen. Subsequently, 50 mL 0.05% (w/v) TMC in n-hexane solution was poured onto the membrane surface and held on the substrate for 120 s to form a polyamide film. The composite membrane was then rinsed thoroughly with deionized water and stored in deionized water at room temperature before surface mineralization.

2.2.3 Membrane surface mineralization. Before mineralization, 0.05 M BaCl₂ and Na₂SO₄ aqueous solutions were prepared, separately. Surface mineralization was then carried out through an alternate soaking process (ASP) in a clean room as described in.³¹ A complete cycle of ASP contains four steps: first, the membrane sample was soaked in BaCl₂ aqueous solution for 60 s at 25.0 °C and then cleaned with deionized water for 60 s, next the sample was soaked in Na₂SO₄ aqueous solution for 60 s at 25.0 °C, and lastly cleaned again with deionized water for 60 s. The mineralization degree was changed by varying the cycle number. The original TFC membrane and mineralized membranes with ASP cycles of 1, 2, 4 and 6 were named as M0, M1, M2, M4 and M6, separately. The mineralization degree was calculated through the weight increment of every TFC membrane sample after mineralization as described in ref. 32. Each sample was washed with deionized water to remove residual salt ions. Then the samples were dried at 40 °C under vacuum till the weight was constant. The calculation equation of mineralization degree (MD) was as follows (1):where w₀ is the dry weights of original membrane sample and w₁ is the dry weight of mineralized membrane sample. A is the surface area of the membrane sample.

2.3. Membrane characterizations

To observe the membrane morphologies, the dried membranes were cut in liquid nitrogen, and then coated with a gold layer for observation by a sputter coater. The cross-sectional images of the membranes were measured using a field emission scanning electron microscopy (FE-SEM) (Sigma, Zeiss, Germany) with the operation voltage of 15 kV. In addition, the element compositions of the surface of the membrane before and after mineralization were determined by energy dispersion X-ray (EDX) analysis employing the SEM with a 20 keV energy beam and an ESCALAB 250Xi X-ray photoelectron spectroscopy (XPS, Thermo Fisher, USA) with a source gun type of Al Kα.

Atomic force microscopy (AFM) was used to determine the surface roughness of the membranes. The experiments were performed in non-contact mode on a SPM-9500J3 AFM analysis (SHIMADZU, Kyoto, Japan) and 5 μm × 5 μm images were obtained with a scan rate of 0.5 Hz. The surface roughness was reported in terms of root mean square roughness (RMS).

The surface hydrophilicity of membrane substrates was measured by a dynamic contact angle instrument (DSA100, KRŰSS GmbH, Hamburg, Germany) using Milli-Q deionized water as the probe liquid at room temperature (about 25 °C). To minimize the experimental error, the contact angle was randomly measured at more than 10 different locations for each sample and the average value was reported.

Membrane surface zeta-potential was measured using streaming potential in the pH range from 3 to 9 by a surpass electrokinetic analyzer (Anton Paar GmbH, Austria). 500 mL of 1.0 mM KCl solution was circulated through an adjustable gap cell.

2.4. Determination of FO performance

The FO performance of the fabricated TFC membranes such as water flux and salt permeability was determined in a laboratory-scale FO setup (Fig. 1). This unit was built containing two channels with 22.37 cm² effective area, which is the same in both sides. Two diaphragm pumps (PLD-2203, China) were applied to pump the solution with the same velocity of 250 mL min⁻¹ in a closed loop. The draw and feed solution were 1 M sodium chloride (NaCl) and Milli-Q water (18 MΩ cm), respectively. Both the volume of draw solution and feed solution was 1 L and the solutions were kept in two 2 L tanks separately.

Fig. 1 Laboratory-scale forward osmosis testing.

The permeate water flux (J_w) was calculated through the increment of the weight of the draw solution because of the water permeated over the membrane during the FO testing. The change of the weight was obtained by the weighting balance which the draw solution was placed on. Corresponding, the feed solution was put on a platform at the same height to eliminate any gravitational effects. The calculation equation of water flux was as follows.

where Δw is the change of the weight of the draw solution, ρ is the water density, A_m is the effective membrane area and Δt is the time of testing.

RSF (reverse salt flux) was defined as the mass of NaCl diffusing from the draw solution to the feed solution per unit time per unit membrane during FO testing. The mass of NaCl was obtained through the concentration of chloride ions in each solution which was determined by ion chromatography. 10 mL samples of both solutions were collected for determination before and after the testing. The calculation equation of RSF was as follows:

where w is the mass of NaCl, A_m is the effective membrane area and Δt is the time of testing.

The fabricated TFC membranes were all tested in two membrane orientations: AL-DS (active layer facing draw solution) and AL-FS (active layer facing feed solution) and compared against commercial CTA-W FO membrane under the same condition. Data were recoded after running of the system for 5 min to stabilize water flux and the testing time was 1 h long. All the tests were conducted at 25 °C and repeated three times to minimize experimental error.

3. Result and discussion

3.1. Surface mineralization of TFC PSf/PA FO membrane

In this work, BaSO₄ particles were deposited on the surface of the prepared flat-sheet thin-film composite (TFC) polysulfone/polyamide (PSf/PA) forward osmosis membrane through an alternate soaking process (ASP), which is a common way to deposit calcium salts in polymeric materials.³³ During the ASP, BaSO₄ particles were generated and deposited on membrane surface by soaking membrane samples in solutions of BaCl₂ and Na₂SO₄ which provided Ba²⁺ and SO₄²⁻, separately. Fig. 2 illustrates the basic principle of the deposition of BaSO₄-based mineral coating on the surface of the TFC PSF/PA membrane.

Fig. 2 Basic principle of the deposition of BaSO₄-based mineral coating on the surface of the TFC PSf/PA membrane.

During alternate soaking processes, Ba²⁺ ions will be captured by the pendent carboxyl (COO–) groups on polyamide active layer. When the membrane sample was soaked in Na₂SO₄ solution in the next step, the local environment is supersaturated for the reaction between Ba²⁺ and SO₄²⁻, resulting in a fast and homogeneous BaSO₄ deposition on membrane surface. The procedure of cleaning with deionized water after every soaking step was in order to remove the residual salt ions. Fig. 3 clearly shows that the mineralization degree (MD) almost increases linearly with the increase of ASP cycle. The SEM, AFM, EDX and XPS analyses further confirmed the mineralization. As shown in Fig. 4, the surface SEM images clearly show the apparent change of membrane surface morphology after mineralization. The membrane surface became denser due to the average deposition of BaSO₄ particles. AFM images were presented in Fig. 5, which illustrate that the membrane surface becomes smoother after mineralization. The surface root mean square roughness (RMS) decreases from about 64.903 nm of the base PSf/PA membrane to about 43.429 nm of the mineralized membrane with 4 ASP cycles. EDX cures shown in Fig. 6 and XPS wide scans presented in Fig. 7 also clearly demonstrate that barium element is existed on the membrane surface after mineralization. Table 1 lists the atomic percentage (C, O, S, Ba) of the TFC PSF/PA FO membranes from XPS and it is found that the content of S and Ba increase with the number of ASP cycles. In addition, as it can be seen from the cross-sectional SEM images of Fig. 8, the thickness of the skin layer of the original PSf/PA membrane (M0) is nearly the same as the mineralized membrane with 4 ASP cycles (membrane M4).

Fig. 3 Change of the mineralization degree with the number of ASP cycle.

Fig. 4 Surface SEM images of the TFC PSf/PA FO membrane: (a) original PSF/PA; (b) mineralization with 4 ASP cycles.

Fig. 5 Surface AFM images of the TFC PSf/PA FO membrane: (a) base PSf/PA (M0 = 64.903 nm); (b) mineralization with 4 ASP cycles (M4 = 43.429 nm).

Fig. 6 EDX curves of the TFC PSf/PA FO membrane: (a) original PSf/PA; (b) mineralization with 4 ASP cycles.

Fig. 7 XPS wide scans of the original PSf/PA membrane (M0) and the mineralized membrane with 4 ASP (M4).

Table 1 Atomic Percentage (C, O, S, Ba) of the TFC PSF/PA FO membranes from XPS

Membranes	Atomic percentage (%)
	C	O	S	Ba
M0	85.87	13.72	0.41	0
M1	85.48	11.45	2.29	0.78
M2	82.4	12.62	3.81	1.17
M4	80.99	12.5	4.74	1.77
M6	78.33	13.56	5.83	2.27

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Fig. 8 Cross-sectional SEM images of the TFC PSf/PA FO membrane: (a) original PSf/PA; (b) mineralization with 4 ASP cycles.

3.2. Surface properties of the mineralized membrane

Fig. 9 shows the contact angle value of the fabricated TFC PSf/PA membranes and it is found that the value of the surface water contact angle of the mineralized membrane drops with the increase of ASP cycles. The water contact angle declines sharply from 60° of the original PSf/PA membrane to 35.5° of the mineralized TFC membrane with 6 ASP cycles, indicating a highly hydrophilic surface. The decrease of water contact angle indicates the increase of surface hydrophilicity of the membrane due to the smoother membrane surface after mineralization. The improved surface hydrophilicity of the mineralized membrane is significant both in enhancing water and reducing reverse salt flux since the improvement in surface hydrophilicity promotes the membrane adsorbing water preferably.

Fig. 9 Surface contact angle values of the TFC PSF/PA FO membranes.

Fig. 10 showed the surface zeta potential of the synthesized TFC PSF/PA FO membranes which demonstrated the change of membrane surface charge. It is clear that the value of surface zeta potential is declining from positive to negative, demonstrating that surfaces of the original PSf/PA and mineralized TFC FO membranes are amphoteric. In addition, the surface of the mineralized membrane is more negatively charged than the original PSf/PA membrane and the surface negative charge of the mineralized membrane increases with the number of ASP cycle which is due to the adsorption of anionic ions on the surfaces of the BaSO₄ particles deposited on membrane surface.³⁴ The BaSO₄-mineralized TFC PSf/PA FO membrane may improve FO performance since the more negatively charged surface makes the membrane electrostatically attracts water molecules and repel salt anions.

Fig. 10 Surface zeta potential of the TFC PSf/PA FO membranes.

3.3. Determination of FO performance

The FO performances including water flux and reverse salt flux of the synthesized TFC PSf/PA FO membranes are shown in Fig. 11 and Table 2 listed the detailed data of the fabricated membranes and the commercial CTA-W FO membrane. It is found that the BaSO₄-mineralized TFC PSf/PA FO membranes displayed higher water flux and lower reverse salt flux than the original PSf/PA membranes in both AL-DS and AL-FS orientations. Comparing to AL-FS orientation, AL-DS exhibited higher water flux but also higher reverse salt flux due to the more severe ICP in the AL-FS configuration than the AL-DS situation.³⁵ The range of water fluxes changed from 4.2–12.7 L m² h⁻¹ in AL-FS to 6.1–25.1 L m² h⁻¹ in AL-DS. In addition, it is found that the FO water flux of the original PSf/PA and BaSO₄-mineralized TFC FO membranes increased with the number of ASP cycle. The water flux was enhanced from 6.1 L m² h⁻¹ to 25.1 L m² h⁻¹ in AL-DS with the number of ASP cycle changed from 0 to 6. Although the BaSO₄ coating layer prevents water from permeating through the membrane and result in decline in water flux, the enhanced surface hydrophilicity promotes the membrane adsorbing water preferably and results in improvement in water flux. The fact of the enhancement of water flux of the mineralized membranes is mainly because the increase of water permeability from the improved surface hydrophilicity compensates for the reduction of water permeability from the additional coating layer. BaSO₄-mineralized TFC PSf/PA membranes also showed low reverse salt flux. The improvement in surface hydrophilicity decided it is water not salt that is preferential adsorbed by the membrane. Besides, the separation of the ionic compounds for charged membrane is also affected by the electrostatic interaction between the ionic permeating species and the membrane, which tends to exclude co-ions and favors sorption of counter-ions.³⁶ The surface of the TFC PSf/PA membrane acquires more negative charge (as shown in Fig. 10) after mineralization, thus enhancing the repulsion force between Cl⁻ ion and membrane surface. Hence, it becomes more difficult for Cl⁻ ions to permeate across the mineralized membrane. As a result, the reverse salt flux will decline. It is worthwhile to note that the increase of the number of ASP cycle led to the decrement of reverse salt flux in all FO membranes except M6. It might be caused by the agglomeration of BaSO₄ and under this condition the impeding effect of the coating layer plays a more important role in FO performance. Reverse salt flux is proportional to the salt rejection coefficient R (ref. 37) and the lower reverse salt flux represents the higher salt rejection.³⁸ Hence, the excessive BaSO₄ deposition may not be advisable for FO applications since the ideal FO membranes should exhibit the high water flux and salt rejection. In addition, it should be noted that the synthesized BaSO₄-mineralized TFC PSf/PA FO membranes exhibited superior performance to the commercial CTA-W FO membrane as listed in Table 2. In general, all of the above results and discussion prove that the BaSO₄-mineralized TFC PSf/PA FO membranes displayed better permeable performance.

Fig. 11 Water flux and reverse salt flux of synthesized FO membranes: (a) water flux and (b) reverse salt flux.

Table 2 Performances of original and mineralized TFC membranes in the FO process. DS was 1 M NaCl; FS was purified water at 25 °C

Membranes	Water flux Jw (LMH)		Reverse salt flux Js-NaCl (g NaCl m^−2 h^−1)
	AL-FS	AL-DS	AL-FS	AL-DS
M0	4.2	6.1	9.58	13.17
M1	6.6	13.1	7.67	11.69
M2	8.6	16.8	6.26	10.03
M4	10.7	22.6	3.99	7.85
M6	12.7	25.1	4.58	8.544
CTA-W	8.3	11.3	14.41	18.82

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4. Conclusions

Novel BaSO₄-based mineralized thin-film composite (TFC) FO membranes were synthesized through depositing barium sulfate on the surface of the prepared polysulfone/polyamide (PSf/PA) membranes by adopting the approach named surface mineralization. The deposition of BaSO₄-base mineral coating layer was confirmed by SEM, EDX and XPS analyses and the coating did not increase the thickness of the skin layer of the TFC membrane but made the membrane surface denser and smoother. The surface hydrophilicity and negative charge of the mineralized TFC membranes are both enhanced with the increase of mineralization degree. The synthesized BaSO₄-based mineralized TFC membranes exhibited superior FO performance than original TFC and commercial FO membranes. The surface mineralization simultaneously enhanced water flux and salt rejection of membranes in both AL-FS and AL-DS modes. However, excessive deposition of BaSO₄ might decrease the permeability and selectivity of the membranes. In conclusion, BaSO₄-based mineralized TFC membranes have great potential for further development of FO application due to their improved structural and separation properties.

Acknowledgements

This work was supported by the National Science and Technology Support Program (2012BAC02B03) and the Fundamental Research Funds for the Central Universities, China (Awards No. 2015203020213).

References

1. S. Zhao, L. Zou, C. Y. Tang and D. Mulcahy, Recent developments in forward osmosis: opportunities and challenges, J. Membr. Sci., 2012, 396, 1–21.
2. R. V. Linares, Z. Li, S. Sarp, S. S. Bucs, G. Amy and J. S. Vrouwenvelder, Forward osmosis niches in seawater desalination and wastewater reuse, Water Res., 2014, 66, 122–139.
3. J.-J. Qin, W. C. L. Lay and K. A. Kekre, Recent developments and future challenges of forward osmosis for desalination: a review, Desalin. Water Treat., 2012, 39, 123–136.
4. X. Zhang, Z. Ning, D. K. Wang and J. C. D. Costa, Processing municipal wastewaters by forward osmosis using CTA membrane, J. Membr. Sci., 2014, 468, 269–275.
5. E. M. Garcia-Castello, J. R. McCutcheon and M. Elimelech, Performance evaluation of sucrose concentration using forward osmosis, J. Membr. Sci., 2009, 338, 61–66.
6. B. Jiao, A. Cassano and E. Drioli, Recent advances on membrane processes for the concentration of fruit juices: a review, J. Food Eng., 2004, 63, 303–324.
7. Y. K. Lin and H. O. Ho, Investigations on the drug releasing mechanism from an asymmetric membrane-coated capsule with an in situ formed delivery orifice, J. Controlled Release, 2003, 89, 57–69.
8. A. Altaee, G. Zaragoza and A. Sharif, Pressure retarded osmosis for power generation and seawater desalination: performance analysis, Desalination, 2014, 344, 108–115.
9. E. Nagy, A general, resistance-in-series, salt- and water flux models for forward osmosis and pressure-retarded osmosis for energy generation, J. Membr. Sci., 2014, 460, 71–81.
10. C. R. Martinetti, A. E. Childress and T. Y. Cath, High recovery of concentrated RO brines using forward osmosis and membrane distillation, J. Membr. Sci., 2009, 331, 31–39.
11. A. Achilli, T. Y. Cath, E. A. Marchand and A. E. Childress, The forward osmosis membrane bioreactor: a low fouling alternative to MBR processes, Desalination, 2009, 239, 10–21.
12. E. R. Cornelissen, D. Harmsen, K. F. de Korte, C. J. Ruiken, J.-J. Qin, H. Oo and L. P. Wessels, Membrane fouling and process performance of forward osmosis membranes on activated sludge, J. Membr. Sci., 2008, 319, 158–168.
13. R. L. McGinnis and M. Elimelech, Energy requirements of ammonia–carbon dioxide forward osmosis desalination, Desalination, 2007, 207, 370–382.
14. N. Y. Yip, A. Tiraferri, W. A. Phillip, J. D. Schiffman and M. Elimelech, High performance thin-film composite forward osmosis membrane, Environ. Sci. Technol., 2010, 44, 3812–3818.
15. D. Stillman, L. Krupp and Y. H. La, Mesh-reinforced thin film composite membranes for forward osmosis applications: the structure–performance relationship, J. Membr. Sci., 2014, 468, 308–316.
16. J. Wei, C. Qiu, C. Y. Tang, R. Wang and A. G. Fane, Synthesis and characterization of flat-sheet thin film composite forward osmosis membranes, J. Membr. Sci., 2011, 372, 292–302.
17. Y. H. Cho, J. Han, S. Han, M. D. Guiver and H. B. Park, Polyamide thin-film composite membranes based on carboxylated polysulfone microporous support membranes for forward osmosis, J. Membr. Sci., 2013, 445, 220–227.
18. G. T. Gray, J. R. McCutcheon and M. Elimelech, Internal concentration polarization in forward osmosis: role of membrane orientation, Desalination, 2006, 197, 1–8.
19. Y. Gao, Y. N. Wang, W. Li and C. Y. Tang, Characterization of internal and external concentration polarizations during forward osmosis processes, Desalination, 2014, 338, 65–73.
20. J. R. McCutcheon and M. Elimelech, Influence of concentrative and dilutive internal concentration polarization on flux behavior in forward osmosis, J. Membr. Sci., 2006, 284, 237–247.
21. G. D. Mehta and S. Loeb, Performance of permasep B-9 and B-10 membranes in various osmotic regions and at high osmotic pressures, J. Membr. Sci., 1978, 4, 335–349.
22. G. D. Mehta and S. Loeb, Internal polarization in the porous substructure of a semipermeable membrane under pressure-retarded osmosis, J. Membr. Sci., 1978, 4, 261–265.
23. K. Y. Wang, R. C. Ong and T. S. Chung, Double-skinned forward osmosis membranes for reducing internal concentration polarization within the porous sublayer, Ind. Eng. Chem. Res., 2010, 49, 4824–4831.
24. J. Su, Q. Yang, J. F. Teo and T. S. Chung, Cellulose acetate nanofiltration hollow fiber membranes for forward osmosis processes, J. Membr. Sci., 2010, 355, 36–44.
25. K. Y. Wang, T. S. Chung and G. Amy, Developing thin-film-composite forward osmosis membranes on the PES/SPSf substrate through interfacial polymerization, AIChE J., 2012, 58, 770–781.
26. X. Song, Z. Liu and D. D. Sun, Nano gives the answer: breaking the bottleneck of internal concentration polarization with a nanofiber composite forward osmosis membrane for a high water production rate, Adv. Mater., 2011, 23, 3256–3260.
27. L. Setiawan, R. Wang, K. Li and A. G. Fane, Fabrication of novel poly(amide-imide) forward osmosis hollow fiber membranes with a positively charged nanofiltration-like selective layer, J. Membr. Sci., 2011, 369, 196–205.
28. J. T. Arena, B. McCloskey, B. D. Freeman and J. R. McCutcheon, 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, 55–62.
29. H. R. Ahna, T. M. Taka and Y. N. Kwonb, Preparation and applications of poly vinyl alcohol (PVA) modified cellulose acetate (CA) membranes for forward osmosis (FO) processes, Desalin. Water Treat., 2015, 53, 1–7.
30. I. L. Alsvik, K. R. Zodrow, M. Elimelech and M. B. Hägg, Polyamide formation on a cellulose triacetate support for osmotic membranes: effect of linking molecules on membrane performance, Desalination, 2013, 312, 2–9.
31. T. Tetsushi, K. Akio and A. Mitsuru, Hydroxyapatite formation on/in poly(vinyl alcohol) hydrogel matrices using a novel alternate soaking process, Chem. Lett., 1998, 27, 711–712.
32. S.-H. Zhi, L.-S. Wan and Z.-K. Xu, Poly(vinylidenefluoride)/poly(acrylic acid)/calcium carbonate composite membranes via mineralization, J. Membr. Sci., 2014, 454, 144–154.
33. X.-N. Chen, L.-S. Wan, Q.-Y. Wu, S.-H. Zhi and Z.-K. Xu, Mineralized polyacrylonitrile-based ultrafiltration membranes with improved water flux and rejection towards dye, J. Membr. Sci., 2013, 441, 112–119.
34. M. Zhang, B. Zhang, X. Li, Z. Yin and X. Guo, Synthesis and surface properties of submicron barium sulfate particles, Appl. Surf. Sci., 2011, 258, 24–29.
35. C. Y. Tang, Q. She, W. C. L. Lay, R. Wang and A. G. Fane, Coupled effects of internal concentration polarization and fouling on flux behavior of forward osmosis membranes during humic acid filtration, J. Membr. Sci., 2010, 354, 123–133.
36. M. Mulder, Transport in Membranes Basic Principles of Membrane Technology, Kluwer Academic Publishers, Dordrecht, 2nd edn, 1996.
37. T. Y. Cath, A. E. Childress and M. Elimelech, Forward osmosis: principles, applications, and recent developments, J. Membr. Sci., 2006, 281, 70–87.
38. H. Y. Ng, W. Tang and W. S. Wong, Performance of forward (direct) osmosis process: membrane structure and transport phenomenon, Environ. Sci. Technol., 2006, 40, 2408–2413.

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