Facile Preparation of Antifouling Hollow Fiber Membranes for

Research Article pubs.acs.org/journal/ascecg

Facile Preparation of Antifouling Hollow Fiber Membranes for Sustainable Osmotic Power Generation Sui Zhang,† Yu Zhang,‡ and Tai-Shung Chung*,†,‡ †

Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117576 NUS Graduate School for Integrative Science and Engineering, National University of Singapore, Singapore 117456

‡

S Supporting Information *

ABSTRACT: Organic fouling in the membrane support is one of the major causes for the flux decline and low efficiency in the pressure retarded osmosis (PRO) process for osmotic power generation, especially when the fouling is complicated by inorganic salt ions. A facile method to fabricate antifouling hollow fiber membranes was demonstrated in this study, which employed the readily available poly(vinyl alcohol) (PVA) as the modification agent. The poly(ether sulfone) (PES) support for the thin film composite (TFC) membranes was first coated by polydopamine (PDA) and then coated with PVA with the aid of glutaraldehyde (GA). PDA was found to detach from the support in the first 2 h and gradually stabilized at pH 2, verifying its applicability for PRO processes. In addition, the existence of a PVA layer was confirmed by X-ray photoelectron spectroscopy. It is important to note that by controlling the reaction conditions, the water flux and salt reverse flux in the PRO process were not sacrificed, proving that the modification can well maintain the porous structure of the support. The modified membranes showed significantly improved fouling resistance to not only alginate but also complex alginate−calcium solutions. The water flux remained ∼80% instead of ∼64% in the latter case. Moreover, much of the fouling was converted from irreversible to reversible, which helped enhance the efficiency of physical cleaning to ∼90%, and hence improved the sustainability of the PRO process. KEYWORDS: Membranes, Antifouling, Poly(vinyl alcohol), Pressure retarded osmosis, Osmotic power

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sheet and hollow fiber membranes for PRO.10,11 By proper design of the hydrophilicity and structure in the support layer,12−15 manipulation of the interfacial polymerization,16 and post-treatment,17 the maximal power density of thin film composite (TFC) membranes have reached 27 W m−2.18 In addition, wholly integral membranes made directly from phase inversion have demonstrated their potential for osmotic power generation.19 However, membranes suffer from severe fouling in the PRO process.18,20−23 A rapid flux decline was observed in the first few hours, and the power density dropped by more than 3 folds when realistic water sources were used.18,22,23 Since both surfaces of the membranes are exposed to the feed streams, fouling might occur simultaneously on the selective and

INTRODUCTION The production of clean and sustainable energy is imperative in today’s world. Pressure retarded osmosis has been studied extensively in recent years for osmotic power generation.1−3 It is a membrane process consisting of a semipermeable membrane and two solutions of different salinity. Due to the osmotic pressure difference across the membrane, water permeates naturally from the dilute solution (referred to as the feed solution) to the pressurized concentrated solution (referred to as the draw solution). The permeate water increases the volume of the pressurized draw solution and is then used to generate power. The global osmotic energy from the sea is estimated to be 1750−2000 TWh per year.1 The technical and economic feasibility of PRO for power generation was investigated in the 1970s.4,5 However, due to the lack of efficient membranes,6−8 the development of PRO ceased until 2009 when Statkraft set up the prototype.3,9 The last few years have witnessed the rapid development of both flat © XXXX American Chemical Society

Received: October 4, 2015 Revised: January 13, 2016

A

DOI: 10.1021/acssuschemeng.5b01228 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering support layers. Owing to reverse flushing by the water permeation flux, it was found that the fouling on the external surface of the selective layer is not quite significant.18,23 On the contrary, solutes, particles, and other compounds in the feed solution are carried into the porous support by water permeation. Since this porous side of the support layer is sheltered from hydrodynamic shear forces along the flow, foulants tend to deposit on the inner wall, block water passage, and hinder salt transport, leading to a reduced water flux. Two strategies have been developed to enhance the antifouling properties of PRO membranes. One is the structural design by coating a second dense layer on the surface of the membrane support, which then functions as a barrier to prevent foulant entrance, or the so-called “double-skin membranes”. The concept of double-skin membranes was first proposed by Wang et al. and Zhang et al.24,25 Less and more reversible fouling against nanoparticles was achieved when the membrane was operated in the PRO mode. More studies were reported later that proved the applicability of the concept under no or low hydraulic pressure.26,27 The other strategy is the chemical modification of the support layer to enhance its hydrophilicity as well as other properties. A hyperbranched glycerol was synthesized and grafted onto the support.28 Good resistance to protein fouling was reported. Due to the structural constraints, it is difficult to eliminate fouling using this type of membrane, but the degree and reversibility of fouling may be greatly improved. Poly(vinyl alcohol) (PVA) is a commonly used commercial polymer. It features ready availability, relatively low price, and hydrophilicity with its abundant hydroxyl groups in the polymer chains. PVA has been widely used as the antifouling agent for conventional pressure-driven membranes.29,30 However, since most polymers for the membrane support in PRO are inert to PVA, prior treatment by polydopamine (PDA) is necessary to active the fiber wall. Despite of the wide use of PDA in membrane modification, the study on its stability in acidic conditions is insufficient. Since acidic adjustment of the feed solution in the PRO process may be applied to mitigate the inorganic scaling,22 it would be useful to investigate this issue first. This study explores the facile preparation of antifouling hollow fiber membranes based on PVA. TFC hollow fiber membranes are first prepared using poly(ether sulfone) (PES) as the supporting material. Subsequently, the fibers are precoated with PDA to activate the wall surface, and the fiber stability under pH 2 is investigated. Then PVA molecules are grafted onto the support layer via co-cross-linking with PDA. The modified membranes not only exhibit much improved fouling resistance to alginate and the alginate−calcium complex but also convert a large portion of the fouling from irreversible into reversible, making the physical cleaning process highly efficient.

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50% in water, Sigma-Aldrich), hydrochloride acid (37%, Merck Millipore), and poly(vinyl alcohol) (PVA, Sigma-Aldrich) were used to modify the membrane support. Sodium chloride (NaCl, 99.5%, Merck) was used for the membrane transport characterizations and PRO performance tests. Sodium alginate (Sigma-Aldrich) and calcium chloride anhydrous (>97%, Sigma-Aldrich) were dissolved in aqueous solutions for fouling tests. Fabrication of PES TFC Hollow Fiber Membranes and Modifications of the Support. The PES TFC hollow fiber membranes were prepared in the similar way as described elsewhere.15 Briefly, the polymer dope solution containing 20.4 wt % PES and additives in NMP was used to fabricate the PES hollow fiber support through spinning. The fibers were bundled into modules with four fibers in each. Lastly, interfacial polymerization was conducted by flowing a MPD aqueous solution in the lumen side for 3 min, flushing with air for 5 min, and then flowing a TMC solution in hexane for 3 min. The subsequent modification of the support layer involved two steps. First, a 1 mg mL−1 dopamine solution in 10 mM Tris-HCl buffer (pH 8.5) was pumped through the shell side of the membrane modules for 5 min, kept within the modules for 3 h, and disposed. A polydopamine (PDA) layer was hence coated on the support. The modules were washed with DI water to remove the residual solution. Second, a PVA aqueous solution containing 0.5 wt % PVA, 0.05 wt % GA, and 0.01 M HCl was flowed through the shell side for a predetermined period. The modules were again washed by DI water to remove the unreacted solution. As a comparison, modules were also exposed to a 0.01 M HCl (pH 2) solution to study the stability of the PDA coating layer under the acidic condition. Table 1 summarizes the modification conditions of the six types of membranes in this study.

Table 1. Membrane codes that correspond to different modification conditions membrane code

PDA, 3h

PES PDA PES−PDA−PH2 PES−PDA−PH2-4h PES−PDA−PVA PES−PDA−PVA4h

√ √ √ √ √

0.01 M HCl

0.5 wt % PVA in 0.01 M HCl, 0.05 wt % GA

2h 4h 2h 4h

Surface Characterizations. The membranes were freeze-dried, fractured in liquid nitrogen, and sputtered with platinum by a Jeol JFC-1100E ion sputtering device. The membrane morphology was observed by a field emission scanning electron microscope (FESEM, JEOL JSM-6700). X-ray photoelectron spectroscopy (XPS, Kratos AXIS UltraDLD spectrometer, Kratos Analytical Ltd.) with a mono Al Kα X-ray source was employed to investigate the chemical compositions on the membrane surface. The extremely thin coating layers were also qualitatively characterized by Fourier transform infrared spectroscopy (FTIR) (Bio-Rad FTS-3500) over the range of 400−4000 cm−1. The coating polymers together with part of the supporting PES polymers were scratched from the outer surface, and then a pellet was prepared by grinding with KBr powder. The direct transmittance mode was applied. The total number of scans for each sample was 16. To measure the water contact angle of the membranes, flat sheet PES membranes were prepared from the same polymer dope, and similar modifications were conducted. Then the water contact angles of the membrane surfaces were measured by a contact angle geniometer (Rame Hart, Succasunna, NJ) at 22 ± 0.5 °C with DI water. PRO, Fouling, and Cleaning Tests. The PRO tests were conducted using a lab-scale cross-flow PRO setup. A variable-speed gear pump (Cole-Palmer, Vernon Hills, IL) was utilized to recirculate the feed solution (DI water, 1L) through the shell side of the hollow fibers at 0.2 L min−1, and a high-pressure hydra cell pump

EXPERIMENTAL SECTION

Materials. Radel A poly(ether sulfone) (PES, Solvay Advanced Polymer, L.L.C., GA), N-methyl-2-pyrrolidone (NMP, > 99.5%, Merck), polyethylene glycol 400 (PEG, Mw = 400 g/mol, SigmaAldrich), and deionized (DI) water were mixed in a round-necked bottle as the polymer, solvent, and additive, respectively, for the fabrication of hollow fiber supports. m-Phenylenediamine (MPD, > 99%, Sigma-Aldrich), trimesoyl chloride (TMC, 98%, Sigma-Aldrich), sodium dodecyl sulfate (SDS, > 97%, Fluka), and hexane (>99.9%, Fisher Chemicals) were employed to perform interfacial polymerization. Dopamine-HCl (98%, Sigma-Aldrich), glutaraldehyde (GA, B


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ACS Sustainable Chemistry & Engineering

Figure 1. Outer surface morphology of the hollow fibers: (A) original PES, (B) PES−PDA, (C) PES−PDA-pH2, (D) PES−PDA−PVA, and (E) PES−PDA−PVA4h. (Minneapolis, MN) was employed to recirculate the draw solution (1 M NaCl, 3.5 L) through the lumen side at 0.1 L min−1. No hydraulic pressure was applied to the feed solution. The fibers were stabilized at the hydraulic pressure of 15 bar in the draw solution for 15 min and was then tested at 10 bar. The water flux (Jw, LMH) was determined by monitoring the weight changes of the feed solution, and the salt reverse flux (Js, g m−2 h−1, abbreviated as gMH) was calculated based on conductivity measurements Jw =

ΔV SmΔt

(1)

Js =

C Ft V Ft − C F0V F0 S mΔt

(2)

where ΔV is the total permeate volume from the feed to the draw solution over the period Δt; C and V refer to the concentration and volume of the solutions, respectively. Subscript F refers to the feed solution, superscripts 0 and t refer to the time 0 and t, respectively, and Sm is the membrane area. The solution concentration can be calculated from the predetermined concentration−conductivity curves. The fouling tests were conducted by first applying DI water as the feed for 3 h, and then replacing it with the foulant solution. DI water was used as the control, while the foulant solution is either a 200 pmm sodium alginate solution or a 200 ppm sodium alginate solution with 1.5 mM CaCl2. After the tests, cleaning was conducted by flushing with DI water at 4.3 cm s−1 (or 200 mL min−1) at the feed side for 30 min, and then the recovered water flux was measured by applying DI water as the feed for PRO.

Figure 2. Possible chemical structure of PDA, PVA, and GA.

percentage of S, which means that the thickness or density of the PDA coating is decreased. Some short PDA chains that are not well attached to the network or PES surface might have detached from the membrane. An extended exposure in 0.01 M HCl for another 2 h does not bring a significant reduction in nitrogen content, suggesting that the coating layer becomes relatively stable at pH 2 after a 2 h immersion. In the PRO and other membrane processes, the feed pH may be adjusted to a slightly acidic condition (typically between 4 to 6) to avoid extensive inorganic scaling.22,32 Since the PES−PDA-pH2 membrane is relatively stable at pH2, the membrane can be safely applied in the common PRO processes. Detailed analyses on the C 1s and O 1s spectra in Figure 3 provides more information on the chemical status of the PDA coating.33,34 The C 1s spectrum mainly proves the presence of CO bonds in the PDA-coated membrane, which is represented by the peak at the binding energy of 287.8 ev. The percentage of each peak area in Table S1 also reveals the increment and then decrement in the C−O and C−N/C−S bonds for PES−PDA and PES−PDA-pH2 membranes. In the O 1s spectra, the CO and C−OH peaks, which correspond to the quinone and catechol groups in PDA as shown in Figure 2, become more evident. Interestingly, after the treatment in 0.01 M HCl, the percentages of C−OH and C−O peaks are reduced, while the CO peak ratio is enhanced. There have been debates over the mechanism of PDA attachment to the inert surface. However, it is generally agreed that the catechols are active in the adhesion of PDA chains to the surface, and quinons improve the cohesion among the PDA chains.35 In this study, the reductions in C−OH and C−O peak areas are attributed to the release of some PDA molecules from the membrane. The intensified content in CO suggests that PDA chains rich in quinone adhere to each other more firmly and tend to stay within the coating network; other short chains might be released to the acidic solution.

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RESULTS AND DISCUSSION Stability of PDA Coating at pH 2. As shown in Figure S1, the outer surface of the PES fibers becomes dark after immersion in the dopamine solution for 3 h, indicating the successful coating of PDA layer on the support. The magnified outer surface image in Figure 1 also demonstrates a slightly denser structure after the coating. However, the dark color becomes gray when the fiber is subsequently immersed in 0.01 M HCl (pH 2) for 2 h. This implies that part of the PDA layer detaches from the surface under acidic conditions. No further significant change in color is observed when the immersion in 0.01 M HCl is continued for another 2 h. Figure 2 presents the possible chemical structure of PDA as proposed recently.31 Compared to the PES polymer, the PDA aggregates possess no sulfur but the nitrogen element. The XPS surveys in Figure 3 clearly show the appearance of nitrogen peaks after the PDA coating. Meanwhile, the sulfur peaks are present in both the original and modified membranes. These indicate that the coating layer thickness is less than the detection depth of XPS, or 10 nm. The FTIR spectra in Figure 4 confirm the presence of PDA polymers on the outer surface by showing a N−H stretching peak at around 1610 cm−1. Table 2 summarizes the ratios of N, C, and O over S. After the PDA coating, the N/S ratio increases from 0 to 0.86, and in the same time, the C/S and O/S ratios also increase. Immersion in the acidic environment for 2 h reduces the ratios of other elements to S, or in other words, increases the C


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Figure 3. XPS survey and C 1s and O 1s core-level spectra of the outer surface of four different fibers.

Figure 5. Water contact angles of different membranes.

the lower coating density and more quinone groups on the surface. Coating of the PVA Layer. The presence of the PVA layer on the membrane surface is proved by XPS. As shown in Table 2, upon contact with the PVA solution for 2 h, the O/S ratio for membrane PES−PDA−PVA has been largely increased compared to PES−PDA-pH2 due to the oxygen-rich PVA chains. The higher C/S ratio indicates that the total thickness of the two coated layers is larger than the PDA layer alone. However, the N/S ratio is slightly decreased, which suggests that the PDA layer in PES−PDA−PVA is thinner than that in PDA−PDA-pH2. The possible reaction between PVA and PDA with the aid of GA is proposed in Figure S2. Due to the fairly acidic environment, the free amine in the PDA molecules becomes positively charged and hence has a lower chance to participate in the reaction. As a result, the major cross-linking reaction occurs between GA and the hydroxyl groups in both PVA and PDA chains. The PVA molecules may be cross-linked with each other in the solution first, and along with the diffusion of small GA molecules, co-cross-linking between PVA and PDA chains gradually takes place. Two situations can be imagined during

Figure 4. FTIR spectroscopy of the original and modified membranes. Thin layers of polymers were scratched from the outer surface of the membranes, blended with KBr, and then characterized to provide qualitative implications.

Table 2. XPS Analysis of Atomic Ratio on Outer Surface of Different Fibers PES PDA PES−PDA−PH2 PES−PDA-PH2-4h PES−PDA−PVA

N/S

C/S

O/S

0 0.9 0.7 0.7 0.5

24.7 32.2 32.1 33.4 37.7

4.6 6.9 6.2 6.4 10.8

Figure 5 compares the water contact angles of the original and modified membranes. A more hydrophilic surface is resulted from the PDA coating. The slightly enhanced water contact angle after immersion in pH2 may be attributed to both D


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disappeared. An extended reaction time is hence not preferable for the PRO membranes. Antifouling Performances. Figure 7A presents the fouling behaviors of the original and modified membranes against 200 ppm sodium alginate in the PRO process. A control line is plotted when DI water is used as the feed. A slight flux decline is observed, which is mainly attributed to the dilution of draw solution and increased feed salinity over 15 h. When the fouling tests start for the original PES membrane, the normalized water flux decreases fast in the initial stage and keeps decreasing in the later at a slightly lower rate. The final water flux is around 77% of the initial value. The rapid flux reduction is due to the attachment of alginate to the porous and relatively hydrophobic PES support. The PDA-modified membrane slows the flux decrement and reserves 82% of the initial water flux at the end because the relative hydrophilicity of the PDA coating helps resist alginate fouling to some extent. A further improvement in antifouling performance is observed when PVA is coated onto the support. The final water flux is around 87% of the initial value. Physical flushing of the feed side by DI water at a moderate flow velocity of 4.3 cm s−1 is performed to wash the membrane. It is shown in Figure 7B that only a very small portion of the water flux is recovered for the PES membrane. In addition to the hydrophobicity of the support, due to the shielding effects of the support layer, it is difficult for the shear force along the flow to reach into the porous structure. Foulants inside the support cannot be removed easily, and irreversible fouling is experienced. A bit more water flux is recovered for the PES− PDA membrane. The most substantial difference is observed for the PVA-modified membrane. The water flux almost completely returns to the starting value after the simple physical cleaning process, indicating that the interaction between alginate and the PVA layer is weak and the fouling is almost reversible. Therefore, the PVA coating not only reduces the extent of fouling but also weakens the foulant−membrane interaction. Since calcium ions exist in real water sources and the organic fouling is deteriorated by the complexion with calcium,19,20 1.5 mM CaCl2 is added into the alignate solution to see the fouling behaviors of different membranes (Figure 8). Apparently more serious fouling is observed on the original PES membrane. Only 64% of the initial water flux is left after 12 h. In addition, the PDA modification does not bring much improvement. In

the co-cross-linking reaction: either the PVA molecules adhere to the PDA layer on the membrane surface, or the PDA chains are pulled off the wall and dragged into the bulk PVA network in the solution. From the XPS results, it is observed that both processes might take place. It is likely that in the first stage some PDA molecules detach from the surface due to the crosslinking with PVA in the solution, and then the PVA grafting onto the surface dominates. The coating of PVA to the PDA layer is also supported by the C 1s and O 1s spectra in Figure 3 and the FTIR spectra in Figure 4. More C−O and C−OH bonds, which are characteristics of PVA, are detected after the PVA coating (Table S1). In addition, a strong O−H stretching peak appears in the FTIR spectrum. The PRO performances against 1 M NaCl and DI water are compared in Figure 6. No significant difference is observed

Figure 6. Water flux and salt reverse flux of the PES and modified membranes. Feed, DI water; draw solution, 1 M NaCl.

among the original PES, PES−PDA, and PES−PDA−PVA membranes. It proves that the coating layer is thin enough so as not to block the pores. However, when the reaction time with PVA is increased to 4 h, the water flux is reduced by a large extent. It is shown in Figure 1 that the surface is apparently covered by a thick PVA layer, and many surface pores have

Figure 7. Fouling (A) and cleaning (B) performance of the PES and modified membranes. Feed, 200 ppm alginate; draw solution, 1 M NaCl. Prior to each test, the fibers were stabilized with DI water as the feed for 3 h. The normalized water flux was then plotted against the operating time. The control line was obtained by employing DI water as the feed. Cleaning after the fouling tests was performed by flowing DI water at the feed side at 4.3 cm s−1 (or 200 mL min−1) for 30 min. The water flux was subsequently tested with DI water as the feed, and its normalized value was labeled as “cleaned”. E


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Figure 8. Fouling (A) and cleaning (B) performance of the PES and modified membranes Feed, 200 ppm alginate +1.5 mM CaCl2; draw solution, 1 M NaCl. Prior to each test, the fibers were stabilized with DI water as the feed for 3 h. The normalized water flux was then plotted against the operating time. The control line was obtained by employing DI water as the feed. Cleaning after the fouling tests was performed by flowing DI water at the feed side at 4.3 cm s−1 (or 200 mL min−1) for 30 min. The water flux was subsequently tested with DI water as the feed, and its normalized value was labeled as “cleaned”.

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fact, due to the free amine and other nitrogen-containing groups in the PDA chains, the PDA-modified surface is positively charged.36 Because of the electrostatic interactions between the positive charged PDA and the negative charged carboxylic acid groups in alginate, alginate might deposit more easily onto the membrane surface and attract more alginate molecules through the bridging effects of calcium ions. Such electrostatic interaction, after magnified by Ca2+, almost counteracts the benefits brought by the higher hydrophilicity via PDA modification and results in a similar degree of fouling as the PES membrane. Interestingly, the grafting of PVA greatly enhances the resistance toward complicated organic fouling. Approximately 78% of the initial water flux remains at the end of the test. It is not surprising to see that the fouling on the PES membranes is irreversible. Physical cleaning by DI water only exhibits a tiny effect on flux recovery. A similarly low flux recovery is observed on the PDA-modified membrane. However, up to 90% recovery is achieved for the PES− PDA−PVA membrane. Therefore, the PVA modification can effectively reduce the irreversible fouling and improve the cleaning efficiency.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01228. Tables and figures on the peak ratio analysis from XPS results, images of original and coated membranes, and the proposed reaction scheme. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +65-65166645. Fax: +65-67791936. E-mail: chencts@ nus.edu.sg. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the Singapore National Research Foundation under its Competitive Research Program for the project entitled, “Advanced FO Membranes and Membrane Systems for Wastewater Treatment, Water Reuse and Seawater Desalination” (Grant R-279-000-336-281) and was also 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 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-279-000381-279. The authors also thank Wan Chunfeng for his help.

CONCLUSIONS

Chemical modifications of the PES TFC hollow fiber support by PVA for low-fouling PRO processes have been demonstrated in this study. PVA was chosen as the hydrophilic agent for its relativley low cost and ready availability, which made the current work more prospective for real applications. Innerselective PES TFC hollow fiber membranes were first prepared, followed by surface functionization of the fiber support by PDA. The stability of the PDA coating at pH 2 was studied by XPS, which proved its applicability for the PRO processes. The coating of PVA onto the PDA layer with the aid of GA was also confirmed. The PDA/PVA/GA-modified membranes showed much improved resistance to alginate and alginate−calcium complex fouling. The flux was maintained at more than 90% instead of 77% in the first case and ∼80% instead of 64% in the latter. Moreover, much of the fouling was changed from irreversible to reversible states, and hence, a high physical cleaning efficiency of ∼98% or ∼90% for different foulants was achieved for the modified membranes.

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REFERENCES

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