Low-Pressure Nanofiltration Hollow Fiber Membranes for Effective

Feb 22, 2018 - In this work, novel loose nanofiltration (NF) hollow fiber membranes with ultrahigh water permeability and well-defined nanopore and su...
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Article Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Low-Pressure Nanofiltration Hollow Fiber Membranes for Effective Fractionation of Dyes and Inorganic Salts in Textile Wastewater Gang Han,† Tai-Shung Chung,*,† Martin Weber,‡ and Christian Maletzko§ †

Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore, 117585, Singapore Advanced Materials and Systems Research, BASF SE, RAP/OUB-B001, Ludwigshafen, 67056, Germany § Performance Materials, BASF SE, G-PMF/SU-F206, Ludwigshafen, 67056, Germany ‡

S Supporting Information *

ABSTRACT: In this work, novel loose nanofiltration (NF) hollow fiber membranes with ultrahigh water permeability and well-defined nanopore and surface charge characteristics were developed for effective fractionation of dyes and inorganic salts in textile wastewater treatment. The as-spun NF hollow fiber possesses a high pure water permeability (PWP) of 80 L·m−2·h−1·bar−1 with a small pore size of 1.0 nm in diameter and a MWCO of 1000 Da. The surface modification by means of hyperbranched polyethylenimine (PEI) further lowers the pore diameter to 0.85 nm and MWCO to 680 Da. The membrane surface also becomes more hydrophilic and positively charged after the PEI modification. Because of the synergistic effects from size exclusion and charge repulsion, the newly developed NF hollow fibers show high permeation fluxes of 7.0−71.2 L·m−2·h−1 and great rejections of 95.5−99.9% to various dyes at a low operating pressure of 1 bar. At the same time, they have ultralow rejections of less than 10% to inorganic salts (i.e., Na2SO4), suggesting that more than 90% of the salts would permeate through the fibers. In addition, the two hollow fibers exhibit outstanding performance stability, low fouling tendency, and great fouling reversibility. Their fluxes can be brought back to be more than 80% of the original values by a simple physical backwash. The newly developed loose NF hollow fiber membranes may have great potential for effective fractionation and treatment of textile wastewater. and electrostatic repulsion.9−11 NF has been increasingly adopted for color removal and textile wastewater treatment because of its energy efficiency, manufacturing scalability, and compact design.12−16 However, the current commercially available NF membranes are generally made from thin-film composite (TFC) approaches via the laborious and complicated interfacial polymerization.14,17−19 Although the TFC NF membranes have good rejections to dyes, they possess a relatively low water permeability and high rejections against inorganic salts. The former requires a high operating pressure to obtain a sufficient flux and thus increases the energy consumption, while the latter fails to fractionate and recover valuable dyes and inorganic salts such as Na2SO4. Furthermore, the accumulation of inorganic salts in the feed would not only lead to a low permeation flux because of the enhanced osmotic pressure but also undermine the dye rejection and induce more severe fouling.20,21 As a result, the development of advanced NF membranes that can effectively fractionate textile dyes and

1. INTRODUCTION Water is intensively consumed throughout the manufacture and applications of dyes. As a result, a large amount of wastewater is generated annually from the textile industry.1,2 Textile wastewater is chemically infused effluent that generally consists of dyes, inorganic salts, raw materials, suspended solids, oil and grease, and other auxiliary chemicals.3,4 Without adequate treatment, discharge of such textile wastewater would cause severe pollution to aquatic ecosystems which not only influences the photosynthetic activity of aquatic life but also induces severe public health problems.5,6 The rapidly growing water scarcity and the stringent environmental legislation on wastewater disposal have compelled the textile industry to effectively manage and reuse the highly polluted textile wastewater.7 Conventional methods such as adsorption, coagulation, oxidation, and biological degradation are widely utilized, but they are insufficient to treat the textile wastewater efficiently and sustainably.8 These treatments aim to take out or destroy compounds foreign to the water but have problems in the recovery and reuse of valuable dyes and salts in the wastewater. Nanofiltration (NF), a pressure-driven membrane separation process with a pore size of ∼0.5−2.0 nm, has shown great rejections toward water-soluble dyes by means of size exclusion © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

December 19, 2017 January 24, 2018 February 22, 2018 February 22, 2018 DOI: 10.1021/acs.est.7b06518 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology

Figure 1. FESEM images of the inner surface, outer surface, and cross-section of Torlon&sPPSU (a−d) and PEI-Torlon&sPPSU (e−h) hollow fiber membranes.

microscope (FESEM, JEOL, JSM-6700F). The membrane surface charge properties as a function of pH were characterized by streaming zeta potential measurements performed with a SurPASS electrokinetic analyzer (Anton Paar GmbH, Austria). Water contact angles were measured through a contact angle geniometer (Rame Hart, USA) with deionized water as the probe liquid at 23 ± 0.5 °C. The surface chemistry of hollow fibers was examined by using an X-ray photoelectron spectroscopy (XPS) performed with a Kratos AXIS UltraDLD spectrometer (Kratos Analytical Ltd., Manchester, UK). The molecular weight cutoff (MWCO), effective mean pore size, and pore size distribution of the hollow fibers were determined via solute filtration experiments using polyethylene glycols (PEG) with various molecular weights.30,31 The detailed specification of the experimental setup and steps are described in the Supporting Information. 2.3. Nanofiltration (NF) Experiments. The NF experiments were conducted in a lab-scale cross-flow filtration system at 23 ± 1 °C.23 In terms of permeation flux and solute rejection, the filtration performance of the hollow fibers was characterized by NF experiments using (1) deionized water to measure the pure water permeability (PWP); (2) singlecomponent solutions containing an inorganic salt (i.e., NaCl, Na2SO4, MgCl2, or MgSO4) or a dye at various solute concentrations; (3) 200 ppm dye solutions at various pH values. The main characteristics of the textile dyes used in this study are tabulated in Table S2. After that, the dye/salt fractionation performance was assessed by using multicomponent feed solutions consisting of 200 ppm INCA or AB-8 dye in the presence of different amounts of Na2SO4. The membrane long-term performance and fouling behaviors were evaluated under a total recirculation mode and concentrate operation mode using an AB-8/Na2SO4 solution consisting of 200 ppm AB-8 and 2000 ppm Na2SO4 as the feed. In order to ensure the repeatability of the experiments, at least three fibers were tested for each testing condition and the averaged results were reported. It is worthy to note that the error bars were not presented in the figures in order to make them easy to read.

salts is highly desirable for direct treatment and reuse of textile wastewater. As a result, the main objective of this work is to design and fabricate novel loose NF hollow fiber membranes for effective fractionation of dyes and salts in textile wastewater. We aim to not only remove dyes but also reuse salts from the wastewater. The hollow fiber configuration is chosen because it can provide a higher surface area to volume ratio, greater packing density, self-supporting characteristics, and spacer-free module fabrication when compared to a flat-sheet one. The separation performance (i.e., permeation flux and rejection) of the newly developed NF hollow fibers would be systematically characterized using single-component solutions made from various salts and dyes. Subsequently, filtration of multicomponent dye/ salt mixtures would be conducted followed by long-term performance evaluation and fouling tests for fractionation of dyes and Na2SO4. This study may provide valuable insights for the fabrication of effective low-pressure NF hollow fiber membranes to treat and reuse high salinity textile wastewater.

2. MATERIALS AND METHODS 2.1. Fabrication of Loose NF Hollow Fiber Membranes. A nonsolvent induced phase separation (NIPS) process was used to fabricate the Torlon&sPPSU hollow fiber membrane via a dry-jet wet spinning technique.22−28 The hydrophilic sulfonated polyphenylenesulfone (sPPSU) was added into the Torlon dope solution to tailor the membrane formation process and thus optimize the membrane pore size and permeability in addition to introduce negative charge properties on the membrane surface.22 Hyperbranched polyethylenimine (PEI, Figure S1) was then applied as the modification agent to further optimize the membrane mechanical stability, pore size, and surface chemistry and charge properties.23,29 Table S1 summarizes the specific spinning parameters, and the detailed experimental procedures for spinning and PEI modification were disclosed in the Supporting Information. The PEI modified Torlon&sPPSU hollow fiber was termed as PEI-Torlon&sPPSU. 2.2. Membrane Characterizations. The surface and cross-section morphology of the hollow fiber membranes were observed through a field emission scanning electron B

DOI: 10.1021/acs.est.7b06518 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Table 1. Summary of Mean Pore Diameter (μp), Geometric Standard Deviation (σp), Pure Water Permeability (PWP), Molecular Weight Cut-Off (MWCO), Water Contact Angle, and Isoelectric Point of the Hollow Fiber Membranes fiber code

PWP (LMH/bar) at 1 bar

water contact angle (deg.)

Torlon&sPPSU PEI-Torlon&sPPSU

82.5 ± 10.1 16.3 ± 3.6

75.6 ± 3.0 69.5 ± 2.8

pH for isoelectric point mean pore diameter, μp (nm) 3.6 6.4

3. RESULTS AND DISCUSSION 3.1. Membrane Characterizations. Figure 1 shows the membrane morphology of the newly developed NF hollow fiber membranes. Both fibers have great concentricity with an inner diameter of 788 μm and an outer diameter of 1177 μm. Due to the fast phase inversion induced by the water-rich bore fluid and external water coagulant as well as their intensive intrusion from both sides during the phase inversion,22,32 the as-spun Torlon&sPPSU fiber has a smooth and tight inner surface but a highly porous cross-section morphology with two layers of finger-like macrovoids located at the inner and outer edges. Additionally, a relatively dense and nodule-like sublayer (about 150 nm) is observed underneath the inner surface. Such a sponge-like microstructure provides small nanopores for good rejection while the finger-like macrovoids across the fiber wall would minimize the transport resistance for water permeation. After the PEI modification, the inner surface and the spongelike sublayer under it become denser, while the outer surface of the PEI-Torlon&sPPSU hollow fiber shows no visible changes. These indicate that the PEI cross-linking modification not only happens on the inner surface but also takes place inside the sublayer due to the diffusion and penetration of PEI molecules.23 The XPS data depicted in Table S3 shows an increase in nitrogen content on the inner surface of the PEITorlon&sPPSU fiber, further confirming the cross-linking modification.33 As more amine groups are introduced onto the membrane surface by the PEI modification, the membrane hydrophilicity and charge property are altered. As displayed in Table 1, the water contact angle of the Torlon&sPPSU membrane is around 75.6°. After the PEI modification, the contact angle of the PEI-Torlon&sPPSU fiber decreases to 69.5°, suggesting an increment in surface hydrophilicity. Figure 2 presents the zeta potential versus pH curves of the two hollow fibers, and Table 1 summarizes their isoelectric points. The Torlon&sPPSU fiber possesses an isoelectric point of pH = 3.6. As a consequence, it has positive charge below pH 3.6 due to the protonation of the amide−imide and sulfonic

1.02 0.85

σp

MWCO (Da)

1.30 1.36

1001.5 680.5

acid groups but becomes negatively charged above pH 3.6 because of the deprotonation of the carboxyl and sulfonic acid groups. Since the PEI modification introduces more amine groups on the membrane surface, the zeta potential curve of the PEI-Torlon&sPPSU fiber shifts to the right significantly and the isoelectric point increases to pH = 6.4. This implies that the PEI-Torlon&sPPSU fiber becomes more positively charged particularly at higher pH values. 3.2. Pure Water Permeability (PWP), Molecular Weight Cutoff (MWCO), Pore Size, and Pore Size Distribution of the Hollow Fiber Membranes. Table 1 summarizes the PWP value at 1 bar, MWCO, and effective mean pore diameter μp of the hollow fiber membranes, while Figure 3 shows their cumulative pore size distribution curves.

Figure 3. Pore size distribution curves of Torlon&sPPSU and PEITorlon&sPPSU hollow fiber membranes.

The Torlon&sPPSU fiber has a narrow pore size distribution with a small mean pore size of 1.02 nm in diameter and a low MWCO of 1001.5 Da. However, it possesses a high PWP of 82.5 LMH/bar, surpassing all the reported NF membranes with a similar pore size.9,14,18 The PEI cross-linking modification further reduces the pore size. The mean pore diameter of the PEI-Torlon&sPPSU fiber becomes 0.85 nm with a MWCO of 680.5 Da. Because of the smaller pore size and the more rigid structure induced by the PEI cross-linking,23 the PWP value drops to 16.3 LMH/bar. The mechanical stability of the NF hollow fibers was evaluated by measuring their PWP and rejections to NaCl at various hydraulic pressures. As illustrated in Figure S2, both fibers can withstand a high pressure of about 10 bar. Compared to the PWP measured at 1 bar, the PWP of the Torlon&sPPSU fiber gradually decreases and drops to 46% of the original value when the pressure goes up to 10 bar, while its rejection to NaCl slightly increases and then decreases with an increase in operating pressure. The decline of PWP and the rise of NaCl rejection with increasing pressure are mainly due to membrane compaction, while the rejection decrease after 6 bar is possibly due to the effects of high hoop stresses on the inner dense-

Figure 2. Zeta potential vs pH curves of Torlon&sPPSU and PEITorlon&sPPSU hollow fiber membranes. C

DOI: 10.1021/acs.est.7b06518 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 4. PWP and rejection of (a, b) Torlon&sPPSU and (c, d) PEI-Torlon&sPPSU hollow fiber membranes using different salt solutions with various concentrations as the feeds. Testing pressure was 1 bar.

selective layer. Interestingly, the PEI-Torlon&sPPSU fiber shows a much lower decline and fluctuation in PWP. Its PWP drops slightly to 90% of the original value at 8 bar and then slightly increases when the pressure reaches 10 bar because of a thinner dense-selective layer. However, its NaCl rejection continuously increases with an increase in operating pressure and approaches 52% at 10 bar, suggesting the superior membrane stability to the Torlon&sPPSU fiber at high pressures. This is mainly because of its small pore size, rigid structure, and enhanced mechanical strength induced by the PEI cross-linking modification. As a result, the Torlon&sPPSU hollow fiber may be suitable for operations at relatively low pressures of less than 6 bar, while the PEI-Torlon&sPPSU fiber can be applied for higher operation pressures. 3.3. Salt Rejection and Permeation Flux. Figure 4 portrays the permeation flux and rejection of the NF hollow fibers against various inorganic salts at 1 bar using 50−6000 ppm NaCl, MgCl2, Na2SO4, and MgSO4 solutions as feeds. In general, both fibers show much higher permeation fluxes but lower salt rejections than the conventional NF membranes,14,18 and the salt rejections undergo a dramatic decline with an increase in feed concentration. Specifically, the Torlon&sPPSU fiber has low rejections of less than 25% for all salts even at a very low concentration of 50 ppm. The salt rejections rapidly drop and become negligible when the salt concentrations are beyond 2000 ppm. The low rejections to inorganic salts are very consistent with the relatively large pore size of the Torlon&sPPSU fiber. Due to the smaller pore size and highly positively charged surface, the PEI-Torlon&sPPSU fiber exhibits higher salt rejections than the Torlon&sPPSU fiber to all the salts except for Na2SO4 particularly at relatively low

salt concentrations. With a salt concentration of 50 ppm, for example, the rejections to salts decrease in the order of R (MgCl2, 92%) > R (NaCl, 70%) > R (MgSO4, 34%) > R (Na2SO4, 12%). Since the PEI-Torlon&sPPSU fiber is highly positively charged (see Figure 2), it has higher rejections to divalent cations (i.e., Mg2+) than monovalent cations (i.e., Na+) and a lower rejection to divalent anions (i.e., SO42−) than monovalent anions (i.e., Cl−) because of the Donnan exclusion effects.23 With an increase in salt concentration, electrostatic screening is significantly enhanced and the charge induced electrostatic repulsion is suppressed. As a result, the rejections dramatically drop to 2−15% when the salt concentrations exceed 1000 ppm. Due to the high water permeability and low salt rejections, the hollow fibers have high permeation fluxes even when the feed concentration goes up to 6000 ppm. As shown in Figure 4, the permeation flux of the Torlon&sPPSU fiber gently drops and then stays almost constant at a value of larger than 57 LMH for all the salts. Interestingly, the PEI-Torlon&sPPSU fiber shows an increase in flux with the increase of salt concentration and the fluxes become even higher than the benchmark value of 16.3 LMH when using deionized water as the feed (see Table 1). These improved fluxes are likely due to the adsorption of ions on the membrane surface and pore wall of the PEI-Torlon&sPPSU fiber which would swell the fiber and enhance the hydrophilicity.34,35 Clearly, because of the well-defined pore sizes and unique surface charge properties, more than 90% of the inorganic salts can permeate through the loose NF hollow fibers. The unique separation characteristics offer a great possibility to fractionate and reuse the salts by means of these two loose NF fibers. D

DOI: 10.1021/acs.est.7b06518 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Table 2. Rejection and Permeation Flux of Torlon&sPPSU and PEI-Torlon&sPPSU Hollow Fiber Membranes to Different Dyesa fiber code Torlon&sPPSU

PEI-Torlon&sPPSU

a

dye

molecular weight (g/mol)

permeation flux Jw (LMH)

rejection (%)

solution pH

INCA RB AB-8 INCA RB AB-8

466 1018 1299 466 1018 1299

60.5 65.0 71.2 10.7 7.0 15.8

95.5 99.0 99.9 98.5 99.9 99.9

6 4.5 5 6 4.5 5

The feed solutions were prepared by dissolving the dyes in deionized water with a concentration of 200 ppm; testing pressure was 1 bar.

Figure 5. Permeation flux and rejection of (a) Torlon&sPPSU and (b) PEI-Torlon&sPPSU hollow fiber membranes as a function of dye concentration. The feed solutions were as-prepared INCA and AB-8 dye solutions without pH adjustment. The testing pressure was 1 bar.

Figure 6. Effect of Na2SO4 on permeation flux and dye and salt rejection of (a, b) Torlon&sPPSU and (c, d) PEI-Torlon&sPPSU hollow fiber membranes. Dye concentration was 200 ppm, and the testing pressure was 1 bar.

3.4. Dye Rejection and Permeation Flux Using SingleComponent Dye Solutions. The NF hollow fiber membranes were examined for dye removal using single-component dye solutions as feeds which were prepared by directly

dissolving the solid dyes in deionized water. Table 2 summarizes the molecular weights and pH values of dye solutions, permeation fluxes, and rejections of the fibers to 200 ppm INCA, RB, and AB-8 feeds. The Torlon&sPPSU fiber E

DOI: 10.1021/acs.est.7b06518 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 7. Long-term stability of Torlon&sPPSU and PEI-Torlon&sPPSU hollow fiber membranes in the fractionation of a dye/Na2SO4 mixture as a function of (a, b) testing duration and (c, d) feed recovery, respectively. The feed contained 200 ppm AB-8 and 2000 ppm Na2SO4, and the testing pressure was 1 bar.

ppm. Similarly, the permeation flux of the PEI-Torlon&sPPSU fiber decreases to 8.0 and 5.5 LMH, respectively. The flux decline is mainly caused by the enhanced pore blocking and adsorption on the membrane surface at higher dye concentrations. However, both fibers show high rejections of 99.9% to AB-8 at all concentrations because of its large molecular weight. The rejections to INCA slightly decrease with an increase in the dye concentration but are still larger than 90% even when the dye concentration goes up to 1000 ppm. The lowered rejection to INCA at higher concentrations is due to the enhanced electrostatic screening and concentration polarization which promote the permeation of dye molecules.38−40 The high permeation fluxes and good dye rejections at various concentrations demonstrate the great potential of the newly developed loose NF hollow fibers for dye removal. 3.5. Filtration Performance Using Dye/Na2SO4 Mixtures as Feeds. Figure 6 shows the filtration performance of Torlon&sPPSU and PEI-Torlon&sPPSU hollow fibers using 200 ppm INCA and AB-8 dye solutions containing various Na2SO4 concentrations as feeds at a pressure of 1 bar. The two fibers exhibit high permeation fluxes and good dye rejections with the presence of Na2SO4. The Torlon&sPPSU fiber obtains a high flux of larger than 40 LMH for all dyes even at high Na2SO4 concentrations, while the PEI-Torlon&sPPSU fiber has fluxes larger than 11 and 7 LMH for AB-8 and INCA dyes, respectively. Even when the Na2SO4 concentration is 6000 ppm, the two fibers show ultrahigh rejections of greater than 99% to AB-8 because of their small pore sizes and the relatively large molecular weight of AB-8 molecules. The rejections to INCA slightly drop with the addition of Na2SO4 possibly due to two factors. The salt enhanced electrostatic shielding may reduce the electrostatic repulsion of the dye from the

shows high permeation fluxes and good rejections to all dyes. The orders of flux and rejections are well in agreement with the order of the dye molecular weights. A flux of 60.5 LMH and a rejection of 95.5% are obtained for the INCA dye which has the smallest molecular weight of 466 g/mol among the dyes used in this study. The flux increases to 65.0 and 71.2 LMH and the rejection goes up to 99.0% and 99.9% for RB and AB-8 dyes, respectively, which possess larger molecular weights. Although the Torlon&sPPSU fiber has a MWCO of 1000 Da, its high rejections to INCA and RB dyes with molecular weights of 466 and 1018 Da arise from the two factors. The negatively charged membrane surface promotes the rejections to these negatively charged dyes via electrostatic repulsion. In addition, the dye molecules may form larger clusters or aggregates via hydrophobic interactions between their aromatic rings. As a result, the fiber can effectively reject them via size exclusion.36,37 Since the PEI-Torlon&sPPSU fiber possesses a smaller pore size than the Torlon&sPPSU fiber, it exhibits higher rejections of 98.5% to INCA and 99.9% to RB and AB-8 but has relatively low permeation fluxes of 7.0−15.8 LMH. It is interesting to note that the RB dye shows the lowest flux although it has a medium molecular weight of 1018 Da among the dyes. This is possibly due to the high affinity between the RB molecules and the positively charged membrane surface at a low pH of 4.5. Figure 5 shows the permeation flux and dye rejection of hollow fibers as a function of dye concentration at 1 bar using INCA and AB-8 as model dyes. A rapid decline in permeation flux with an increase in dye concentration is observed for both hollow fibers, and the decrease is more significant for INCA than for AB-8. For the Torlon&sPPSU fiber, the permeation flux drops to 65 and 57 LMH for the AB-8 and INCA dyes, respectively, when the dye concentration increases to 1000 F

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high fouling reversibility.46 The high flux, consistent and effective removal of dyes, high permeation of salts, and good fouling reversibility demonstrate the promising advantages of the developed loose NF hollow fibers for effective treatment of textile wastewater with high salinity.

membrane surface,40−42 while the high salt concentration may swell the membrane and slightly increase the membrane pore size.43 However, it is important to note that both fibers still have >90% rejections to INCA even at high salt concentrations (i.e., 6000 ppm). Compared to pure Na2SO4 feed solutions (see Figure 4), Figure 6 also shows that the two hollow fibers possess similar salt rejections to Na2SO4 when being measured in the presence of the dyes. The Torlon&sPPSU and the PEI-Torlon&sPPSU hollow fibers exhibit low salt rejections of 20% and 15%, respectively, when a low Na2SO4 concentration of 50 ppm is employed. The rejections further drop to 5−6% when the salt concentration is increased beyond 1000 ppm. In other words, more than 90% of the Na2SO4 can permeate through the fibers, allowing effective fractionation of dyes and inorganic salts. This phenomenon also implies that the dye molecules have negligible effects on salt rejections of the hollow fibers. 3.6. Performance Stability in Long-Term NF Operations. Long-term tests were conducted to investigate the performance stability of the hollow fibers for the fractionation of dye/salt mixtures using a 200 ppm AB-8 solution containing 2000 ppm Na2SO4 as the model feed at 1 bar. The filtration was first operated under a total recirculation mode to examine the long-term membrane performance, during which permeate was recycled back to the feed to maintain its composition. Figure 7a presents the evolution of permeation flux as a function of testing duration. The Torlon&sPPSU fiber shows a slight decline of permeation flux over time, and the flux drops to 84% of the original value even after a continuous operation of 168 h (or 10 080 min), indicating the good performance stability and low fouling propensity. A rapid flux decrease is observed by the PEITorlon&sPPSU fiber during a relatively short duration of 20 h, and then, the flux becomes more stable. Since the feed composition and volume are kept constant, the flux reduction is mainly caused by dye adsorption, pore blocking, and cake layer formation.44,45 However, the final flux of the PEI-Torlon&sPPSU fiber is still around 60% of the initial value after 168 h. A concentrate operation mode was taken by continuously recycling the feed through the fibers until reaching the predetermined recovery rate. As depicted in Figure 7c, both fibers exhibit a rapid flux drop at a low recovery and then become relatively stable until a high recovery of 90%. Specifically, the flux of the PEI-Torlon&sPPSU fiber quickly drops to 80% of the initial value within a low recovery of 4% and then slowly decreases to 72% when the recovery reaches 90%. For the Torlon&sPPSU fiber, the flux decline is around 40% at a low recovery of 10% and then rapidly drops to 44% of the initial flux when the recovery reaches 90%. The first fast flux decline is mainly caused by the electrostatic interaction, enhanced adsorption, pore blocking, and cake layer formation while the following mild decrease is mainly due to the increased concentration and osmotic pressure of the feed solution. Figure 7b,d shows that the hollow fibers have negligible fluctuations in rejections to the dye and salt over the entire test of 168 h or up to 90% of feed recovery. The rejections to AB-8 under both modes remain higher than 99%, while the fibers provide almost complete passage of the Na2SO4 salt (i.e.,