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PEG nanocapsules as water transport channels
Journal of Membrane Science 586 (2019) 115–121
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Highly permeable thin-film nanocomposite membranes embedded with PDA/PEG nanocapsules as water transport channels
T
Leilei Zhang, Mengxiao Zhang, Jingyu Lu, Anqi Tang, Liping Zhu∗ MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanocapsules Thin-film nanocomposite membranes Defect-free High water permeance
The incorporation of nanofillers in interfacial polymerization is a promising approach to elevate the permeability of thin-film nanocomposite (TFN) membranes. However, commonly-used inorganic nanomaterials (e.g., silica, zeolites, and carbon nanotubes) often bring defects, and thus low selectivity to the membranes due to their poor interfacial compatibility with polymeric matrix. In this work, organic polydopamine/poly(ethylene glycol) (PDA/PEG) nanocapsules were designed as fillers of the TFN membranes prepared by interfacial polymerization of piperazine with trimesoyl chloride. No aggregation phenomenon was observed for the nanocapsules in the membranes, which is attributed to their good dispersity brought by the grafted PEG brushes on the shells. Defectfree separation layer was obtained for the TFN membranes due to good miscibility of polymeric nanocapsules with polyamide matrix. The inner cavities of the embedded nanocapsules played a role of shortcuts for water permeation through the thin-films. The resultant PDA/PEG-TFN membranes showed a high water permeance of 11.7 L·m−2·h−1·bar−1, approximately 2.2 times that of the control TFC membrane, while maintained high solute rejections (RNa2SO4 = 95% and RVitamin B12 = 98%). Moreover, the PDA/PEG-TFN membranes exhibited high selectivity towards di-/mono-valent salts. This work presents a novel strategy of constructing water transport channels in selective layer to develop high performance nanofiltration membranes.
1. Introduction Nanofiltration (NF) membranes have been widely applied in water treatment (e.g., fractionation of multivalent ions [1–3], separation of pharmaceuticals [4,5], dyes [6,7] and natural organics [8]) with fairly low energy consumption. Interfacial polymerization (IP) is the most useful technique to fabricate a thin-film composite (TFC) NF membrane consisting of a porous substrate and a dense polyamide (PA) selective layer [9–11]. The PA selective layer is usually generated by the polymerization of an acyl chloride monomer and an amine monomer at the interface of two liquid phases. The TFC structure endows NF membranes with a relatively high permeation flux and multivalent salt (e.g. Na2SO4) rejection simultaneously [12]. To further improve the permeation and separation performances of TFC membranes, researchers have incorporated a variety of nanofillers into PA selective layer and developed so-called thin-film nanocomposite (TFN) membranes [13]. Such nanofillers include zeolite [14–16], silica [17,18], carbon nanotubes [19–21] and graphene oxide [22,23] etc. The key challenges for constructing thin defect-free selective film, which is an essential prerequisite to achieve both high permeability and
∗
high selectivity for TFN membranes, are the dispersity of nanofillers in the selective layer and the compatibility between nanofillers with PA matrix [24,25]. However, the existing nanomaterials to date hardly meet these requirements satisfactorily [17,26]. Therefore, developing novel nanomaterials which can overcome poor dispersity and compatibility is significant for synthesizing high performance TFN membranes. Polymeric nanocapsules with particular core-shell structure have been widely used in drug delivery [27–29] and microwave absorption [30,31]. However, to the best of our knowledge, so far rare work has been reported on the potential application of nanocapsules as fillers in TFN membranes. The polymeric characteristic offers nanocapsules good compatibility with PA matrix [32], which is advantageous to eliminate the interfacial defects. Moreover, the inner cavities of nanocapsules are potential to provide additional shortcuts for the permeation of water molecules through the selective layer. Hence, the incorporation of nanocapsules may be a good idea to achieve high permeability and high rejection simultaneously for TFN membranes. The motivation of this work is to explore the feasibility of polydopamine/poly(ethylene glycol) (PDA/PEG) nanocapsules as nanofillers in TFN membranes. The preparation procedure of the TFN
Corresponding author. E-mail address: [email protected] (L. Zhu).
https://doi.org/10.1016/j.memsci.2019.05.065 Received 23 March 2019; Received in revised form 10 May 2019; Accepted 24 May 2019 Available online 25 May 2019 0376-7388/ © 2019 Published by Elsevier B.V.
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annealed in an oven at 60 °C for 10 min and stored in deionized water for characterization. The weight percentages of PDA/PEG@ZIF-8 nanoparticles in the aqueous phase were 0.02%, 0.04%, 0.06%, respectively and the corresponding TFN membranes were referred as PDA/ PEG-TFN1, PDA/PEG-TFN2, PDA/PEG-TFN3. Further increasing the dosage of the nanoparticles in aqueous phase contributes little to the improvement of water permeance due to the limited nanoparticle loading in the membranes (Fig. S1). So in this work, the concentrations of PDA/PEG@ZIF-8 nanoparticles in aqueous phase were controlled less than 0.06%. As a control sample, a pristine TFC membrane was fabricated by the IP of PIP and TMC without the addition of PDA/PEG@ ZIF-8 nanoparticles following an identical procedure. 2.3. Membrane performance tests The filtration performance of the investigated membranes was tested using a dead-end stirred cell (XFUF047, Millipore Co., USA) at room temperature (25 °C) under a stirring speed of 600 rpm. In a typical procedure, a membrane was pre-pressured in the membrane cell with pure water under 0.4 MPa for 5 min until the flux reached a steady value that was recorded as the pure water flux (F, L·m−2·h−1). Then the feed was replaced with aqueous solutions of inorganic salts (Na2SO4 or NaCl) or pharmaceuticals (VB12 or OTC) and the solute rejection ratio was tested under 0.4 MPa of transmembrane pressure. The feed concentration of inorganic salts was 0.01 mol/L and the concentrations of VB12 or OTC were 10 ppm. The water flux F was calculated by equation (1):
Fig. 1. Schematic diagram for the preparation of TFN membranes with PDA/ PEG nanocapsules as fillers.
membranes is illustrated in Fig. 1. Core-shell nanoparticles with zeolitic imidazolate framework-8 (ZIF-8) as the core and PDA/PEG as the shell (PDA/PEG@ZIF-8) were beforehand prepared as the precursor and then added into aqueous phase during the IP of piperazine (PIP) and trimesoyl chloride (TMC). The PDA/PEG@ZIF-8 nanoparticles can be embedded into the PA selective layer and transformed into PDA/PEG nanocapsules due to the etching and removal of ZIF-8 cores by the acid generated in IP. The aim of grafting PEG brushes onto PDA@ZIF-8 nanoparticles is to improve the dispersity of particles in aqueous phase. By combining with the good compatibility of nanocapsules with polymeric matrix, defect-free PA selective layer is expected to be created for high solute rejection. We hypothesize that the inner cavities of nanocapsules can act as additional water transport channels, which is advantageous to the enhancement of water permeance. The developed TFN membranes can find broad applications in ionic and molecular separation.
F=
V A×t
(1)
where V (L) is the volume of the collected permeate in a designed filtration time t (h), and A (m2) is the effective membrane area for permeation. The permeance (L·m−2·h−1·bar−1) here is defined as the flux per unit transmembrane pressure. The rejection ratio (R) of the membranes towards inorganic salts or pharmaceuticals was calculated by equation (2):
2. Experimental
Cp ⎞ R = ⎜⎛1 − ⎟ × 100% Cf ⎠ ⎝
2.1. Materials and chemicals
(2)
where cp and cf are the concentrations of the permeate and the feed. The concentration of inorganic salts was determined by the solution conductivity tested with an electrical conductivity meter (DDS-11A, INESA Scientific Instrument Co. Ltd). The concentrations of VB12 and OTC were measured with an UV–vis spectroscopy (UV-1601, Shimadzu, Japan) at 362 nm and 280 nm of wavelength, respectively.
Dopamine hydrochloride (DA-HCl) was purchased from SigmaAldrich Co. Ltd. Polyethylene glycol amine (PEG-NH2, Mw = 5 kDa) was obtained from Jenkem Technology Co, Ltd. Trimesoyl chloride (TMC, 98%), piperazine (PIP, 99%) and oxytetracycline (OTC, 97%) were purchased from Aladdin Reagent Co. Ltd. Sodium sulfate (Na2SO4), sodium chloride (NaCl), vitamin B12 (VB12) and n-hexane were provided by Sinopharm Group Chemical Reagent Co. Ltd. A polyethersulfone (PES) ultrafiltration membrane with molecular weight cutoff of 20 kDa was used as the substrate of the TFN membranes and supplied by Beijing Separate Equipment Co. Ltd. All the reagents were used as received without further purification.
2.4. Characterizations The morphology of PDA/PEG@ZIF-8 nanoparticles was observed with transmission electron microscopy (TEM, JEM1200, Japan). Surface and cross-sectional morphologies of the control TFC and the PDA/PEG-TFN membranes were observed with field emission-scanning electron microscopy (FE-SEM, HitachiS-4800, Japan) after being sputtered with a 10–20 nm gold layer. The roughness of the membrane surface was obtained from atomic force microscopy (AFM, SPI3800 N, Seiko Instrumental) with a scanning size of 5 × 5 μm under tapping mode. Nanoscope Analysis software (Bruker, MA) was used for the acquisition of root-mean-square roughness (Rq). The surface chemistry of the membranes was analyzed by infrared spectrophotometer equipped with an attenuated total reflection accessory (ATR-FTIR, Nicolet 6700, USA) and X-ray photoelectron spectroscopy (XPS, PHI 5000C ESCA system, USA). The surface zeta potentials of the membranes were determined via streaming potential measurements on an electrokinetic analyzer (Anton Paar, GmbH, Austria).
2.2. Preparation of PDA/PEG-TFN membranes PDA/PEG@ZIF-8 nanoparticles were beforehand synthesized and characterized as shown in supplementary materials, then used as nanofillers in TFN membranes fabricated by a conventional IP procedure. In a typical process, a PES substrate was immersed in an aqueous solution of 0.2% w/v PIP containing a predesigned dosage of PDA/PEG@ ZIF-8 nanoparticles for 5 min. Then the PES substrate was taken out and dried in the air for 5 min to remove surface water followed by soaking in a TMC solution of 0.2% w/v in hexane for 30 seconds. This process resulted in the formation of a thin PA selective layer containing PDA/ PEG nanocapsules due to the etching and removal of ZIF-8 cores by the acid generated in IP. Finally, the resultant composite membrane was 116
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Fig. 2. TEM image and structure illustration of PDA/PEG@ZIF-8 nanoparticles.
3. Results and discussion 3.1. Characterization of PDA/PEG@ZIF-8 nanoparticles The beforehand synthesized ZIF-8 nanoparticles with an average diameter of ∼40 nm (Fig. S2b) were used as templates of PDA/PEG nanocapsules. PDA was coated onto the ZIF-8 nanoparticles by conventional oxidation and self-polymerization of DA in alkaline aqueous solution. To improve the dispersity of nanoparticles, PEG brushes were then grafted onto the PDA-coated ZIF-8 particles by the Michael addition and/or Schiff-base reaction between amines in PEG-NH2 with PDA. The coating of PDA and the subsequent grafting of PEG were confirmed by the FITR spectra (Fig. S4a). From the TEM image as shown in Fig. 2, it can be clearly observed that the PDA/PEG@ZIF-8 nanoparticles demonstrate a spherical morphology that is different from the rhombic dodecahedral shape of ZIF-8 nanoparticles (Fig. S4c). The PDA/PEG@ ZIF-8 nanoparticles have a mean diameter of ∼70 nm, which is higher than that of pristine ZIF-8 nanoparticles (∼40 nm). The increase in diameter is reasonably attributed to the coating of PDA and the grafting of PEG. Moreover, the PEG-grafted nanoparticles exhibited better dispersity in water than ZIF-8 and PDA@ZIF-8 (Fig. S4b).
Fig. 4. Surface and cross-sectional SEM images of TFC and PDA/PEG-TFN membranes.
and PEG [35]. The appearance of the new peak indicates the successful loading of PDA/PEG into the membrane surfaces. The XPS wide-scan spectra of the membranes are shown in Fig. 3b. Nearly no zinc element (key element in ZIF-8) was detected in the spectra of the TFN membranes, indicating that ZIF-8 was removed from the membranes. Also, no crystalline species was found in the TFN membranes from XRD tests (Fig. S5), further confirming the complete removal of ZIF-8. These results allow us to conclude that PDA/PEG shells without ZIF-8 cores were successfully introduced into the PA selective layer of the PDA/ PEG-TFN membranes. The surface and cross-sectional morphologies of the TFC and PDA/ PEG-TFN membranes were observed by SEM and the typical SEM images are shown in Fig. 4. The control TFC membrane shows a typical nodular surface morphology [36,37], which results in a high roughness. With the addition of PDA/PEG@ZIF-8 nanoparticles, the obtained PDA/
3.2. Characterization of PDA/PEG-TFN membranes PDA/PEG@ZIF-8 nanoparticles were used as the precursor of PDA/ PEG nanocapsules and added into the aqueous phase in the preparation of NF membranes by IP of piperazine (PIP) and trimesoyl chloride (TMC). The surface chemistry of the pristine TFC and the PDA/PEGTFN membranes was characterized by ATR-FTIR and XPS, respectively. Fig. 3a shows the ATR-FTIR spectra of the membranes. A peak at 1620 cm−1 is observed in the spectrum of the TFC membrane, which is attributed to the stretching vibration of C=O groups in PA structure [33,34]. In comparison, the PDA/PEG-TFN membranes show a new broaden peak at about 1660 cm−1, which is ascribed to the C=N stretching vibration resulted from the Schiff-base reaction between PDA
Fig. 3. (a) ATR-FTIR and (b) XPS spectra of TFC and PDA/PEG-TFN membranes. 117
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Fig. 5. 3D AFM images of the TFC and PDA/PEG-TFN membranes.
It's well known that the separation performance of nanofiltration membranes is dramatically influenced by surface charged characteristic of membranes due to the Donnan exclusion effect [41]. In this work, the zeta potentials of the control TFC and the PDA/PEG-TFN membranes were measured at different pH and the results are shown in Fig. 6. It can be seen that the zeta potentials decrease with the increase of pH. At pH ∼6.5, the condition that NF process often works, all of the investigated membranes are negatively charged. This is due to the existence of carboxylic acids hydrolyzed from the unreacted acyl chloride groups of TMC. Compared with the TFC membranes, the PDA/PEG-TFN membranes show lower zeta potentials in a range from −50 to −70 mV, indicating the PDA/PEG-TFN membranes are strong negatively charged. It is thought that the incorporation of nanocapsules reduced the cross-linking degree of polyamide and more acryl chloride groups of TMC were hydrolyzed into carboxylic acid groups [34,40]. According to the Donnan exclusion mechanism, the strong negatively charged character may be helpful to improve the rejection of anionic molecules or salts in NF operation.
PEG-TFN membranes show smoother surfaces. The contour of nanocapsules can be clearly observed. The diameter of nanocapsules observed by SEM is notably larger than the that of PDA/PEG@ZIF-8 nanoparticles observed by TEM (∼70 nm). The nanocapsules are easier to be swollen than the nanoparticles in water and thus had bigger size due to the removal of rigid ZIF-8 cores. The nanocapsules dispersed uniformly on membrane surface due to the surface-tethered PEG hydration layer that prevents the aggregation of PDA/PEG nanocapsules [38]. The thickness of the selective layers generated by the interfacial polymerization can be estimated from the cross-section SEM images. It can be seen that the formed selective layers range from 120 to 135 nm for both the control TFC and the PDA/PEG-TFN membranes, indicating that the introduction of nanocapsules didn't bring a distinct variation on the thickness of selective layer. Fig. 5 shows the AFM images of the control TFC and the PDA/PEGTFN membranes. The TFC membrane has a root mean square roughness (Rq) of 21.5 nm. However, it is clear that the incorporation of PDA/PEG nanocapsules brought a remarkable influence on membrane surface roughness. When a small dosage of PDA/PEG nanocapsules was used, the Rq of the obtained PDA/PEG-TFN1 membrane decreased to 12.0 nm, which is in well agreement with the SEM observation. Similar phenomenon was reported by Ji et al. [32], in which the surface roughness of polyamide nanocomposite membranes decreased with increasing the dosage of zwitterionic dopamine nanoparticles. It is believed that the addition of PDA/PEG nanocapsules may decrease the polymerization reaction rate and more time was needed to reach reaction end point. As a result, PDA/PEG nanocapsules and polyamide macromolecules had enough time to arrange more regularly, and thus smoother surfaces were obtained [32,39]. With the increase of the dosage of PDA/PEG nanocapsules, surface roughness parameter Rq of the membrane PDA/PEG-TFN3 increased up to 23.8 nm. The increasement in surface roughness can be attributed to the semi-embedded nanocapsules in the PA selective layer. In addition, the uneven diffusion rate of PIP at the interface of polyamide film and PDA/PEG nanocapsules also contributes to the increased roughness [40]. The rough membrane surface provides large contacting area for the penetration of water molecules through the membrane, which is advantageous to the improvement of water permeation flux.
Fig. 6. Zeta potential of the TFC and PDA/PEG-TFN membranes at different pH. 118
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Fig. 7. (a) Water flux of TFC and TFN membranes under different pressures and (b) schematic illustration of water molecules through PA selective layer.
With the incorporation of PDA/PEG nanocapsuels into PA layer, the RNaCl declines obviously. Especially, in comparison with 47% of NaCl rejection for the control TFC membrane, that for the PDA/PEG-TFN3 membrane with 0.06 wt% of nanocapsule dosage is only 21.5%. Nevertheless, the Na2SO4 rejections for the TFN membranes are comparative with that of the control TFC membrane. As the typical sample, the PDA/PEG-TFN3 membrane shows 95% of Na2SO4 rejection. According to Donnan exclusion theory, there exists a strong electrostatic repulsive force between SO42− ions with negatively charged NF membranes [43]. Therefore, the majority of Na2SO4 can be rejected by both the TFC and the TFN membranes due to the high negative zeta potential of membranes. In the case of NaCl separation, the electrostatic interaction is weak and the rejection is greatly influenced by the pore size of selective layer [44]. The addition of PDA/PEG@ZIF-8 nanoparticles in water phase led to the looser PA layer. As a result, the RNaCl of the TFN membranes is lower than that of the control TFC one. Here, a parameter SNa2SO4/NaCl is defined as the ratio of RNa2SO4 to RNaCl to quantify the salt selectivity of the membranes [32]. It is found that the SNa2SO4/NaCl of the PDA/PEG-TFN3 membrane reaches 4.4, while that of the control TFC membrane is only 2.1. This phenomenon indicates that the membranes with the incorporation of PDA/PEG nanocapsules had higher selectivity towards di-/mono-valent salts than the control TFC membranes. The TFN membranes can find their potential application in separation of salt mixtures. VB12 and OTC were used as the typical neutral molecules to evaluate the separation performance of the membranes towards organics and the results are shown in Fig. 8b. It can be seen that the VB12 (Mw = 1355 Da) rejection was kept more than 98% for all of the
3.3. NF performance of the PDA/PEG-TFN membranes The pure water fluxes of the control TFC and the TFN membranes containing PDA/PEG nanocapsules at various pressures are presented in Fig. 7a. The water fluxes of the membranes enhanced linearly with the applied pressure ranging from 2 to 5 bar, indicating that NF operation can be performed stably in such a pressure range. It can also be seen that the water fluxes enhanced with the increase of PDA/PEG nanocapsule dosage. By linear fitting of the water flux data vs the applied pressure, the water permeance for the control TFC membrane can be calculated and the value is 5.4 L·m−2·h−1·bar−1. The water permeability for the PDA/PEG-TFN membranes is greater than that of the TFC one. In particular, the water permeance of the PDA/PEG-TFN3 membrane with a dosage of 0.06 wt% PDA/PEG nanocapsules reaches up to 11.7 L·m−2·h−1·bar−1, nearly 2.2 times that of the control TFC membranes. The increase of water permeability can be attributed to two aspects. Firstly, just as discussed above, the PA selective layer became looser due to the addition of nanocapsules, which is advantageous to water permeation through membranes. On the other hand, as presented in Fig. 7b, the inner cavities of PDA/PEG nanocapsules provide additional shortcuts and decrease the hydraulic resistance for water to transport through the PA layer of the TFN membranes [42]. The solute rejection performances of the control TFC and the TFN membranes were evaluated by NF operation. Na2SO4 and NaCl were used as the representatives of inorganic salts and their rejections were examined with 0.01 mol/L salt aqueous solutions at 25 °C under an operation pressure of 0.4 MPa. As shown in Fig. 8a, for both the TFC and the TFN membranes, the RNaCl values are much lower than RNa2SO4.
Fig. 8. Separation performance for (a) inorganic salts (Na2SO4 or NaCl) and (b) pharmaceuticals (VB12 or OTC) of TFC and PDA/PEG-TFN membranes. 119
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application in water softening, pharmaceutical separation and dyes desalination etc. Acknowledgements This work was supported by the financial supports from the National Natural Science Foundation of China (Grant No. 51573159, 51773175and 51828301) and the Fundamental Research Funds for the Central Universities, China. (2019QNA4062). The authors also thank Ms. Li Xu in State Key Laboratory of Chemical Engineering, China, Zhejiang University for the tests support. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.memsci.2019.05.065.
Fig. 9. Comparisons in water permeance and divalent salt rejection between the PDA/PEG-TFN3 membrane with commercial NF membranes and TFN membranes reported in literature.
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investigated membranes. The rejection of OTC is lower than that of VB12 due to the lower molecular weight for OTC (Mw = 496 Da). The OTC rejection decreases from 97% for the control TFC membrane to 90% for the PDA/PEG-TFN3 membrane. This shows that the molecular weight cut-off of the PDA/PEG-TFN3 membrane is about 500 Da, exhibit a typical NF character. Just as discussed above, the incorporation of PDA/PEG nanocapsules brought a looser PA selective layer to the TFN membranes. As a result, the rejection towards organic neutral molecules decreases to some extent. Seen from another aspect, the high Na2SO4 and VB12 rejections show the PA selective layers of the TFN membranes were kept defect-free. This can be attributed to the uniform dispersity of PDA/PEG nanocapsules in PA matrix as well as their good compatibility. The developed PDA/PEG-TFN membranes can be used for molecular and ionic separation with high water permeability in water softening, pharmaceutical separation, dye desalination etc. Both high permeability and high selectivity are desirable for separation membranes to achieve energy-efficient and time-saving separation. However, a trade-off between permeability and selectivity often exists for dense membranes [45]. Therefore, a compromise has to be considered in these two aspects. The water permeance and divalent salt rejection of the developed PDA/PEG-TFN3 membrane was compared with those of commercial NF membranes and TFN membranes reported in literature (listed in Table S1 of the supplementary information). From Fig. 9, it can be seen that the performance parameters for the PDA/PEG-TFN membranes reach the upper bound for most of existing membranes. The typical PDA/PEG-TFN3 membrane exhibits 95% of Na2SO4 rejection and 11.7 L·m−2·h−1·bar−1 of water permeance. These results indicate that the incorporation of PDA/PEG nanocapsules is useful to achieve a better balance between permeability and selectivity for NF membranes. 4. Conclusions In this study, novel PDA/PEG-TFN membranes containing additional water transport channels were constructed by incorporating PDA/PEG nanocapsules into the PA selective layer. The PDA/PEG nanocapsules were in-situ transformed from PDA/PEG@ZIF-8 nanoparticles by the etching out of ZIF-8 cores with the acid generated in IP. The resulting PDA/PEG-TFN membranes show significantly improved water permeability compared with the control TFC membrane, while still maintain high solute rejection. The typical PDA/PEG-TFN membrane exhibits 95% of Na2SO4 rejection and 11.7 L·m−2·h−1·bar−1 of water permeance. Moreover, the PDA/PEG-TFN membranes exhibit higher selectivity towards di-/mono-valent salts than the control membranes. This work develops a novel strategy to construct water transport channels in selective layer of TFN membranes for further 120
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Highly permeable thin-film nanocomposite membranes embedded with PDA/PEG nanocapsules as water transport channels
T
Leilei Zhang, Mengxiao Zhang, Jingyu Lu, Anqi Tang, Liping Zhu∗ MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanocapsules Thin-film nanocomposite membranes Defect-free High water permeance
The incorporation of nanofillers in interfacial polymerization is a promising approach to elevate the permeability of thin-film nanocomposite (TFN) membranes. However, commonly-used inorganic nanomaterials (e.g., silica, zeolites, and carbon nanotubes) often bring defects, and thus low selectivity to the membranes due to their poor interfacial compatibility with polymeric matrix. In this work, organic polydopamine/poly(ethylene glycol) (PDA/PEG) nanocapsules were designed as fillers of the TFN membranes prepared by interfacial polymerization of piperazine with trimesoyl chloride. No aggregation phenomenon was observed for the nanocapsules in the membranes, which is attributed to their good dispersity brought by the grafted PEG brushes on the shells. Defectfree separation layer was obtained for the TFN membranes due to good miscibility of polymeric nanocapsules with polyamide matrix. The inner cavities of the embedded nanocapsules played a role of shortcuts for water permeation through the thin-films. The resultant PDA/PEG-TFN membranes showed a high water permeance of 11.7 L·m−2·h−1·bar−1, approximately 2.2 times that of the control TFC membrane, while maintained high solute rejections (RNa2SO4 = 95% and RVitamin B12 = 98%). Moreover, the PDA/PEG-TFN membranes exhibited high selectivity towards di-/mono-valent salts. This work presents a novel strategy of constructing water transport channels in selective layer to develop high performance nanofiltration membranes.
1. Introduction Nanofiltration (NF) membranes have been widely applied in water treatment (e.g., fractionation of multivalent ions [1–3], separation of pharmaceuticals [4,5], dyes [6,7] and natural organics [8]) with fairly low energy consumption. Interfacial polymerization (IP) is the most useful technique to fabricate a thin-film composite (TFC) NF membrane consisting of a porous substrate and a dense polyamide (PA) selective layer [9–11]. The PA selective layer is usually generated by the polymerization of an acyl chloride monomer and an amine monomer at the interface of two liquid phases. The TFC structure endows NF membranes with a relatively high permeation flux and multivalent salt (e.g. Na2SO4) rejection simultaneously [12]. To further improve the permeation and separation performances of TFC membranes, researchers have incorporated a variety of nanofillers into PA selective layer and developed so-called thin-film nanocomposite (TFN) membranes [13]. Such nanofillers include zeolite [14–16], silica [17,18], carbon nanotubes [19–21] and graphene oxide [22,23] etc. The key challenges for constructing thin defect-free selective film, which is an essential prerequisite to achieve both high permeability and
∗
high selectivity for TFN membranes, are the dispersity of nanofillers in the selective layer and the compatibility between nanofillers with PA matrix [24,25]. However, the existing nanomaterials to date hardly meet these requirements satisfactorily [17,26]. Therefore, developing novel nanomaterials which can overcome poor dispersity and compatibility is significant for synthesizing high performance TFN membranes. Polymeric nanocapsules with particular core-shell structure have been widely used in drug delivery [27–29] and microwave absorption [30,31]. However, to the best of our knowledge, so far rare work has been reported on the potential application of nanocapsules as fillers in TFN membranes. The polymeric characteristic offers nanocapsules good compatibility with PA matrix [32], which is advantageous to eliminate the interfacial defects. Moreover, the inner cavities of nanocapsules are potential to provide additional shortcuts for the permeation of water molecules through the selective layer. Hence, the incorporation of nanocapsules may be a good idea to achieve high permeability and high rejection simultaneously for TFN membranes. The motivation of this work is to explore the feasibility of polydopamine/poly(ethylene glycol) (PDA/PEG) nanocapsules as nanofillers in TFN membranes. The preparation procedure of the TFN
Corresponding author. E-mail address: [email protected] (L. Zhu).
https://doi.org/10.1016/j.memsci.2019.05.065 Received 23 March 2019; Received in revised form 10 May 2019; Accepted 24 May 2019 Available online 25 May 2019 0376-7388/ © 2019 Published by Elsevier B.V.
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annealed in an oven at 60 °C for 10 min and stored in deionized water for characterization. The weight percentages of PDA/PEG@ZIF-8 nanoparticles in the aqueous phase were 0.02%, 0.04%, 0.06%, respectively and the corresponding TFN membranes were referred as PDA/ PEG-TFN1, PDA/PEG-TFN2, PDA/PEG-TFN3. Further increasing the dosage of the nanoparticles in aqueous phase contributes little to the improvement of water permeance due to the limited nanoparticle loading in the membranes (Fig. S1). So in this work, the concentrations of PDA/PEG@ZIF-8 nanoparticles in aqueous phase were controlled less than 0.06%. As a control sample, a pristine TFC membrane was fabricated by the IP of PIP and TMC without the addition of PDA/PEG@ ZIF-8 nanoparticles following an identical procedure. 2.3. Membrane performance tests The filtration performance of the investigated membranes was tested using a dead-end stirred cell (XFUF047, Millipore Co., USA) at room temperature (25 °C) under a stirring speed of 600 rpm. In a typical procedure, a membrane was pre-pressured in the membrane cell with pure water under 0.4 MPa for 5 min until the flux reached a steady value that was recorded as the pure water flux (F, L·m−2·h−1). Then the feed was replaced with aqueous solutions of inorganic salts (Na2SO4 or NaCl) or pharmaceuticals (VB12 or OTC) and the solute rejection ratio was tested under 0.4 MPa of transmembrane pressure. The feed concentration of inorganic salts was 0.01 mol/L and the concentrations of VB12 or OTC were 10 ppm. The water flux F was calculated by equation (1):
Fig. 1. Schematic diagram for the preparation of TFN membranes with PDA/ PEG nanocapsules as fillers.
membranes is illustrated in Fig. 1. Core-shell nanoparticles with zeolitic imidazolate framework-8 (ZIF-8) as the core and PDA/PEG as the shell (PDA/PEG@ZIF-8) were beforehand prepared as the precursor and then added into aqueous phase during the IP of piperazine (PIP) and trimesoyl chloride (TMC). The PDA/PEG@ZIF-8 nanoparticles can be embedded into the PA selective layer and transformed into PDA/PEG nanocapsules due to the etching and removal of ZIF-8 cores by the acid generated in IP. The aim of grafting PEG brushes onto PDA@ZIF-8 nanoparticles is to improve the dispersity of particles in aqueous phase. By combining with the good compatibility of nanocapsules with polymeric matrix, defect-free PA selective layer is expected to be created for high solute rejection. We hypothesize that the inner cavities of nanocapsules can act as additional water transport channels, which is advantageous to the enhancement of water permeance. The developed TFN membranes can find broad applications in ionic and molecular separation.
F=
V A×t
(1)
where V (L) is the volume of the collected permeate in a designed filtration time t (h), and A (m2) is the effective membrane area for permeation. The permeance (L·m−2·h−1·bar−1) here is defined as the flux per unit transmembrane pressure. The rejection ratio (R) of the membranes towards inorganic salts or pharmaceuticals was calculated by equation (2):
2. Experimental
Cp ⎞ R = ⎜⎛1 − ⎟ × 100% Cf ⎠ ⎝
2.1. Materials and chemicals
(2)
where cp and cf are the concentrations of the permeate and the feed. The concentration of inorganic salts was determined by the solution conductivity tested with an electrical conductivity meter (DDS-11A, INESA Scientific Instrument Co. Ltd). The concentrations of VB12 and OTC were measured with an UV–vis spectroscopy (UV-1601, Shimadzu, Japan) at 362 nm and 280 nm of wavelength, respectively.
Dopamine hydrochloride (DA-HCl) was purchased from SigmaAldrich Co. Ltd. Polyethylene glycol amine (PEG-NH2, Mw = 5 kDa) was obtained from Jenkem Technology Co, Ltd. Trimesoyl chloride (TMC, 98%), piperazine (PIP, 99%) and oxytetracycline (OTC, 97%) were purchased from Aladdin Reagent Co. Ltd. Sodium sulfate (Na2SO4), sodium chloride (NaCl), vitamin B12 (VB12) and n-hexane were provided by Sinopharm Group Chemical Reagent Co. Ltd. A polyethersulfone (PES) ultrafiltration membrane with molecular weight cutoff of 20 kDa was used as the substrate of the TFN membranes and supplied by Beijing Separate Equipment Co. Ltd. All the reagents were used as received without further purification.
2.4. Characterizations The morphology of PDA/PEG@ZIF-8 nanoparticles was observed with transmission electron microscopy (TEM, JEM1200, Japan). Surface and cross-sectional morphologies of the control TFC and the PDA/PEG-TFN membranes were observed with field emission-scanning electron microscopy (FE-SEM, HitachiS-4800, Japan) after being sputtered with a 10–20 nm gold layer. The roughness of the membrane surface was obtained from atomic force microscopy (AFM, SPI3800 N, Seiko Instrumental) with a scanning size of 5 × 5 μm under tapping mode. Nanoscope Analysis software (Bruker, MA) was used for the acquisition of root-mean-square roughness (Rq). The surface chemistry of the membranes was analyzed by infrared spectrophotometer equipped with an attenuated total reflection accessory (ATR-FTIR, Nicolet 6700, USA) and X-ray photoelectron spectroscopy (XPS, PHI 5000C ESCA system, USA). The surface zeta potentials of the membranes were determined via streaming potential measurements on an electrokinetic analyzer (Anton Paar, GmbH, Austria).
2.2. Preparation of PDA/PEG-TFN membranes PDA/PEG@ZIF-8 nanoparticles were beforehand synthesized and characterized as shown in supplementary materials, then used as nanofillers in TFN membranes fabricated by a conventional IP procedure. In a typical process, a PES substrate was immersed in an aqueous solution of 0.2% w/v PIP containing a predesigned dosage of PDA/PEG@ ZIF-8 nanoparticles for 5 min. Then the PES substrate was taken out and dried in the air for 5 min to remove surface water followed by soaking in a TMC solution of 0.2% w/v in hexane for 30 seconds. This process resulted in the formation of a thin PA selective layer containing PDA/ PEG nanocapsules due to the etching and removal of ZIF-8 cores by the acid generated in IP. Finally, the resultant composite membrane was 116
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Fig. 2. TEM image and structure illustration of PDA/PEG@ZIF-8 nanoparticles.
3. Results and discussion 3.1. Characterization of PDA/PEG@ZIF-8 nanoparticles The beforehand synthesized ZIF-8 nanoparticles with an average diameter of ∼40 nm (Fig. S2b) were used as templates of PDA/PEG nanocapsules. PDA was coated onto the ZIF-8 nanoparticles by conventional oxidation and self-polymerization of DA in alkaline aqueous solution. To improve the dispersity of nanoparticles, PEG brushes were then grafted onto the PDA-coated ZIF-8 particles by the Michael addition and/or Schiff-base reaction between amines in PEG-NH2 with PDA. The coating of PDA and the subsequent grafting of PEG were confirmed by the FITR spectra (Fig. S4a). From the TEM image as shown in Fig. 2, it can be clearly observed that the PDA/PEG@ZIF-8 nanoparticles demonstrate a spherical morphology that is different from the rhombic dodecahedral shape of ZIF-8 nanoparticles (Fig. S4c). The PDA/PEG@ ZIF-8 nanoparticles have a mean diameter of ∼70 nm, which is higher than that of pristine ZIF-8 nanoparticles (∼40 nm). The increase in diameter is reasonably attributed to the coating of PDA and the grafting of PEG. Moreover, the PEG-grafted nanoparticles exhibited better dispersity in water than ZIF-8 and PDA@ZIF-8 (Fig. S4b).
Fig. 4. Surface and cross-sectional SEM images of TFC and PDA/PEG-TFN membranes.
and PEG [35]. The appearance of the new peak indicates the successful loading of PDA/PEG into the membrane surfaces. The XPS wide-scan spectra of the membranes are shown in Fig. 3b. Nearly no zinc element (key element in ZIF-8) was detected in the spectra of the TFN membranes, indicating that ZIF-8 was removed from the membranes. Also, no crystalline species was found in the TFN membranes from XRD tests (Fig. S5), further confirming the complete removal of ZIF-8. These results allow us to conclude that PDA/PEG shells without ZIF-8 cores were successfully introduced into the PA selective layer of the PDA/ PEG-TFN membranes. The surface and cross-sectional morphologies of the TFC and PDA/ PEG-TFN membranes were observed by SEM and the typical SEM images are shown in Fig. 4. The control TFC membrane shows a typical nodular surface morphology [36,37], which results in a high roughness. With the addition of PDA/PEG@ZIF-8 nanoparticles, the obtained PDA/
3.2. Characterization of PDA/PEG-TFN membranes PDA/PEG@ZIF-8 nanoparticles were used as the precursor of PDA/ PEG nanocapsules and added into the aqueous phase in the preparation of NF membranes by IP of piperazine (PIP) and trimesoyl chloride (TMC). The surface chemistry of the pristine TFC and the PDA/PEGTFN membranes was characterized by ATR-FTIR and XPS, respectively. Fig. 3a shows the ATR-FTIR spectra of the membranes. A peak at 1620 cm−1 is observed in the spectrum of the TFC membrane, which is attributed to the stretching vibration of C=O groups in PA structure [33,34]. In comparison, the PDA/PEG-TFN membranes show a new broaden peak at about 1660 cm−1, which is ascribed to the C=N stretching vibration resulted from the Schiff-base reaction between PDA
Fig. 3. (a) ATR-FTIR and (b) XPS spectra of TFC and PDA/PEG-TFN membranes. 117
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Fig. 5. 3D AFM images of the TFC and PDA/PEG-TFN membranes.
It's well known that the separation performance of nanofiltration membranes is dramatically influenced by surface charged characteristic of membranes due to the Donnan exclusion effect [41]. In this work, the zeta potentials of the control TFC and the PDA/PEG-TFN membranes were measured at different pH and the results are shown in Fig. 6. It can be seen that the zeta potentials decrease with the increase of pH. At pH ∼6.5, the condition that NF process often works, all of the investigated membranes are negatively charged. This is due to the existence of carboxylic acids hydrolyzed from the unreacted acyl chloride groups of TMC. Compared with the TFC membranes, the PDA/PEG-TFN membranes show lower zeta potentials in a range from −50 to −70 mV, indicating the PDA/PEG-TFN membranes are strong negatively charged. It is thought that the incorporation of nanocapsules reduced the cross-linking degree of polyamide and more acryl chloride groups of TMC were hydrolyzed into carboxylic acid groups [34,40]. According to the Donnan exclusion mechanism, the strong negatively charged character may be helpful to improve the rejection of anionic molecules or salts in NF operation.
PEG-TFN membranes show smoother surfaces. The contour of nanocapsules can be clearly observed. The diameter of nanocapsules observed by SEM is notably larger than the that of PDA/PEG@ZIF-8 nanoparticles observed by TEM (∼70 nm). The nanocapsules are easier to be swollen than the nanoparticles in water and thus had bigger size due to the removal of rigid ZIF-8 cores. The nanocapsules dispersed uniformly on membrane surface due to the surface-tethered PEG hydration layer that prevents the aggregation of PDA/PEG nanocapsules [38]. The thickness of the selective layers generated by the interfacial polymerization can be estimated from the cross-section SEM images. It can be seen that the formed selective layers range from 120 to 135 nm for both the control TFC and the PDA/PEG-TFN membranes, indicating that the introduction of nanocapsules didn't bring a distinct variation on the thickness of selective layer. Fig. 5 shows the AFM images of the control TFC and the PDA/PEGTFN membranes. The TFC membrane has a root mean square roughness (Rq) of 21.5 nm. However, it is clear that the incorporation of PDA/PEG nanocapsules brought a remarkable influence on membrane surface roughness. When a small dosage of PDA/PEG nanocapsules was used, the Rq of the obtained PDA/PEG-TFN1 membrane decreased to 12.0 nm, which is in well agreement with the SEM observation. Similar phenomenon was reported by Ji et al. [32], in which the surface roughness of polyamide nanocomposite membranes decreased with increasing the dosage of zwitterionic dopamine nanoparticles. It is believed that the addition of PDA/PEG nanocapsules may decrease the polymerization reaction rate and more time was needed to reach reaction end point. As a result, PDA/PEG nanocapsules and polyamide macromolecules had enough time to arrange more regularly, and thus smoother surfaces were obtained [32,39]. With the increase of the dosage of PDA/PEG nanocapsules, surface roughness parameter Rq of the membrane PDA/PEG-TFN3 increased up to 23.8 nm. The increasement in surface roughness can be attributed to the semi-embedded nanocapsules in the PA selective layer. In addition, the uneven diffusion rate of PIP at the interface of polyamide film and PDA/PEG nanocapsules also contributes to the increased roughness [40]. The rough membrane surface provides large contacting area for the penetration of water molecules through the membrane, which is advantageous to the improvement of water permeation flux.
Fig. 6. Zeta potential of the TFC and PDA/PEG-TFN membranes at different pH. 118
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Fig. 7. (a) Water flux of TFC and TFN membranes under different pressures and (b) schematic illustration of water molecules through PA selective layer.
With the incorporation of PDA/PEG nanocapsuels into PA layer, the RNaCl declines obviously. Especially, in comparison with 47% of NaCl rejection for the control TFC membrane, that for the PDA/PEG-TFN3 membrane with 0.06 wt% of nanocapsule dosage is only 21.5%. Nevertheless, the Na2SO4 rejections for the TFN membranes are comparative with that of the control TFC membrane. As the typical sample, the PDA/PEG-TFN3 membrane shows 95% of Na2SO4 rejection. According to Donnan exclusion theory, there exists a strong electrostatic repulsive force between SO42− ions with negatively charged NF membranes [43]. Therefore, the majority of Na2SO4 can be rejected by both the TFC and the TFN membranes due to the high negative zeta potential of membranes. In the case of NaCl separation, the electrostatic interaction is weak and the rejection is greatly influenced by the pore size of selective layer [44]. The addition of PDA/PEG@ZIF-8 nanoparticles in water phase led to the looser PA layer. As a result, the RNaCl of the TFN membranes is lower than that of the control TFC one. Here, a parameter SNa2SO4/NaCl is defined as the ratio of RNa2SO4 to RNaCl to quantify the salt selectivity of the membranes [32]. It is found that the SNa2SO4/NaCl of the PDA/PEG-TFN3 membrane reaches 4.4, while that of the control TFC membrane is only 2.1. This phenomenon indicates that the membranes with the incorporation of PDA/PEG nanocapsules had higher selectivity towards di-/mono-valent salts than the control TFC membranes. The TFN membranes can find their potential application in separation of salt mixtures. VB12 and OTC were used as the typical neutral molecules to evaluate the separation performance of the membranes towards organics and the results are shown in Fig. 8b. It can be seen that the VB12 (Mw = 1355 Da) rejection was kept more than 98% for all of the
3.3. NF performance of the PDA/PEG-TFN membranes The pure water fluxes of the control TFC and the TFN membranes containing PDA/PEG nanocapsules at various pressures are presented in Fig. 7a. The water fluxes of the membranes enhanced linearly with the applied pressure ranging from 2 to 5 bar, indicating that NF operation can be performed stably in such a pressure range. It can also be seen that the water fluxes enhanced with the increase of PDA/PEG nanocapsule dosage. By linear fitting of the water flux data vs the applied pressure, the water permeance for the control TFC membrane can be calculated and the value is 5.4 L·m−2·h−1·bar−1. The water permeability for the PDA/PEG-TFN membranes is greater than that of the TFC one. In particular, the water permeance of the PDA/PEG-TFN3 membrane with a dosage of 0.06 wt% PDA/PEG nanocapsules reaches up to 11.7 L·m−2·h−1·bar−1, nearly 2.2 times that of the control TFC membranes. The increase of water permeability can be attributed to two aspects. Firstly, just as discussed above, the PA selective layer became looser due to the addition of nanocapsules, which is advantageous to water permeation through membranes. On the other hand, as presented in Fig. 7b, the inner cavities of PDA/PEG nanocapsules provide additional shortcuts and decrease the hydraulic resistance for water to transport through the PA layer of the TFN membranes [42]. The solute rejection performances of the control TFC and the TFN membranes were evaluated by NF operation. Na2SO4 and NaCl were used as the representatives of inorganic salts and their rejections were examined with 0.01 mol/L salt aqueous solutions at 25 °C under an operation pressure of 0.4 MPa. As shown in Fig. 8a, for both the TFC and the TFN membranes, the RNaCl values are much lower than RNa2SO4.
Fig. 8. Separation performance for (a) inorganic salts (Na2SO4 or NaCl) and (b) pharmaceuticals (VB12 or OTC) of TFC and PDA/PEG-TFN membranes. 119
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application in water softening, pharmaceutical separation and dyes desalination etc. Acknowledgements This work was supported by the financial supports from the National Natural Science Foundation of China (Grant No. 51573159, 51773175and 51828301) and the Fundamental Research Funds for the Central Universities, China. (2019QNA4062). The authors also thank Ms. Li Xu in State Key Laboratory of Chemical Engineering, China, Zhejiang University for the tests support. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.memsci.2019.05.065.
Fig. 9. Comparisons in water permeance and divalent salt rejection between the PDA/PEG-TFN3 membrane with commercial NF membranes and TFN membranes reported in literature.
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investigated membranes. The rejection of OTC is lower than that of VB12 due to the lower molecular weight for OTC (Mw = 496 Da). The OTC rejection decreases from 97% for the control TFC membrane to 90% for the PDA/PEG-TFN3 membrane. This shows that the molecular weight cut-off of the PDA/PEG-TFN3 membrane is about 500 Da, exhibit a typical NF character. Just as discussed above, the incorporation of PDA/PEG nanocapsules brought a looser PA selective layer to the TFN membranes. As a result, the rejection towards organic neutral molecules decreases to some extent. Seen from another aspect, the high Na2SO4 and VB12 rejections show the PA selective layers of the TFN membranes were kept defect-free. This can be attributed to the uniform dispersity of PDA/PEG nanocapsules in PA matrix as well as their good compatibility. The developed PDA/PEG-TFN membranes can be used for molecular and ionic separation with high water permeability in water softening, pharmaceutical separation, dye desalination etc. Both high permeability and high selectivity are desirable for separation membranes to achieve energy-efficient and time-saving separation. However, a trade-off between permeability and selectivity often exists for dense membranes [45]. Therefore, a compromise has to be considered in these two aspects. The water permeance and divalent salt rejection of the developed PDA/PEG-TFN3 membrane was compared with those of commercial NF membranes and TFN membranes reported in literature (listed in Table S1 of the supplementary information). From Fig. 9, it can be seen that the performance parameters for the PDA/PEG-TFN membranes reach the upper bound for most of existing membranes. The typical PDA/PEG-TFN3 membrane exhibits 95% of Na2SO4 rejection and 11.7 L·m−2·h−1·bar−1 of water permeance. These results indicate that the incorporation of PDA/PEG nanocapsules is useful to achieve a better balance between permeability and selectivity for NF membranes. 4. Conclusions In this study, novel PDA/PEG-TFN membranes containing additional water transport channels were constructed by incorporating PDA/PEG nanocapsules into the PA selective layer. The PDA/PEG nanocapsules were in-situ transformed from PDA/PEG@ZIF-8 nanoparticles by the etching out of ZIF-8 cores with the acid generated in IP. The resulting PDA/PEG-TFN membranes show significantly improved water permeability compared with the control TFC membrane, while still maintain high solute rejection. The typical PDA/PEG-TFN membrane exhibits 95% of Na2SO4 rejection and 11.7 L·m−2·h−1·bar−1 of water permeance. Moreover, the PDA/PEG-TFN membranes exhibit higher selectivity towards di-/mono-valent salts than the control membranes. This work develops a novel strategy to construct water transport channels in selective layer of TFN membranes for further 120
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