Microwave heating assistant preparation of high permselectivity polypiperazine-amide nanofiltration membrane during the interfacial polymerization process with low monomer concentration

Journal of Membrane Science 596 (2020) 117718

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Microwave heating assistant preparation of high permselectivity polypiperazine-amide nanofiltration membrane during the interfacial polymerization process with low monomer concentration Ben-Qing Huang , Yong-Jian Tang *, Zuo-Xiang Zeng , Zhen-Liang Xu ** State Key Laboratory of Chemical Engineering, Membrane Science and Engineering R&D Lab, Chemical Engineering Research Center, East China University of Science and Technology, Shanghai, 200237, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Microwave Interfacial polymerization Nanofiltration Polyamide

By causing the water molecule vibration to generate heat, microwave has become a rapid and efficient heating method which is widely used in industrial production. Herein, microwave heating assistant interfacial poly­ merization (IP) process was innovatively applied to fabricate polypiperazine-amide nanofiltration (NF) mem­ brane for the first time. With microwave heating, a uniform and faultless polyamide (PA) layer could be generated with low monomer concentration. The pure water flux (PWF) of the NF membrane fabricated with microwave heating was 157.1 L m-2 h-1 with the Na2SO4 rejection of 97.7% under 0.6 MPa. Meanwhile, such NF membrane showed extraordinary selectivity to Cl- and SO24 for a series of mixing solution with different con­ centrations. This new insight into microwave heating assistant IP provides a feasible method for the highpermselectivity NF membranes fabrication.

1. Introduction With the rapid growth of population and industrialization, the crisis of water resources has become a critical issue which endangers the survival and development of mankind. Due to the low energy con­ sumption, easy-operation and slight pollution, membrane technology shows great potential in water treatment [1]. Nanofiltration (NF), with nanoscale pore structure and charged surface, can been applied for seawater purification, drinking water and wastewater treatment [2]. NF possesses irreplaceable advantage in separation of monovalent and divalent anions, thus has become a feasible way for treating industrial sewage from dyeing, coal and some other chemical industry [3]. Inter­ facial polymerization (IP) is the common method to fabricate commer­ cial thin-film composite (TFC) NF membranes. Diamine and acyl chloride polymerize on the interface of two mutually insoluble phases and an ultrathin polyamide (PA) layer is generated on the porous support. With the purpose of high permeability and retention, different stra­ tegies have been applied to improve the structure of NF membranes including developing new monomers [4], designing interlayer [5,6], improving the IP process [7], introducing porous nanomaterials like

polydopamine-piperazine nanoparticles [8], mental organic frameworks (MOFs) [9,10] and covalent organic frameworks (COFs) [11]. Ren et al. [3] fabricated cation-affinitive membranes via IP process by using monomers with oligo-ethylene-glycol units. The membrane showed high rejection to Na2SO4 and excellent selectivity to Na2SO4 and NaCl in mixed solution. Wang et al. [12] found that introducing a cellulose nanocrystal (CNC) interlayer between microporous substrate and PA layer could reduce the polymerization degree of PA layer. The nega­ tively charged CNC interlayer could store piperazine (PIP) in aqueous phase and slow down the diffusion rate of PIP. As a result, the PA layer with low crosslinking degree was generated under low PIP concentra­ tion. Zhu et al. [13] fabricated PA nanofilm with thickness less than 12 nm on a free interface of water-hexane. Via this free-standing IP process, the PIP concentration was very low and the PA surface was smooth. Then the PA nanofilm was vacuum-filtrated onto a dopamine modified polymer substrate to generate a TFC membrane. The free-standing PA film was ultrathin and defect-free, thus showed unexpected performance in both permeability and retention. Li et al. [14] synthesized COFs nanoparticles and added into the m-phenylenediamine aqueous phase to fabricate NF membrane by IP. The surface hydrophilicity improved a lot and the thickness of the skin layer decreased. The membrane showed

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y.-J. Tang), [email protected] (Z.-L. Xu). https://doi.org/10.1016/j.memsci.2019.117718 Received 28 September 2019; Received in revised form 26 November 2019; Accepted 3 December 2019 Available online 4 December 2019 0376-7388/© 2019 Elsevier B.V. All rights reserved.

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Journal of Membrane Science 596 (2020) 117718

Fig. 1. The schematic diagram of IP process with microwave and oven heating.

better ethanol permeability and solvent resistance compared with membrane prepared without COFs. In addition, there were some works focused on the supporting polymers [15,16] and solvent system [17,18] of IP process. Ang et al. [19] altered the physicochemical properties of polysulfone (PSf) supports by adding polyethylene glycol with molecu­ lar weight ranged from 200 to 35,000 g/mol. The influence of PSf supports on the formation of PA layer was studied systematically and the conclusion was that supports with low surface porosity, small surface pore size and opportune hydrophilicity was beneficial to the high water flux of PA layer. Kong et al. [20] added acetone as co-solvent in the hexane solution of TMC to control the reaction and adjust the membrane structure. Membranes with higher flux were successfully fabricated though this co-solvent assisted IP process. In the IP process, monomers contact at the interface and generate an original PA layer immediately. Then the membrane need to be posttreated and diamine in aqueous phase permeates across the PA layer and comes into contact with acyl chloride for further polymerization. Generally, the membrane is heated in oven for a few minutes with a temperature range between 60 and 80 � C. At higher temperature, the monomers diffusion speeds up and further polymerization takes place. By the oven heating, the heat transmitted to the membrane by convec­ tion and radiation, which is slow with poor efficiency. Up to now, most of the reports of TFC NF membranes focus on the reaction of IP process or the structure of porous support, few insights have been put in the heat-treatment. Developing new heating method is of great significance for the fabrication and industrialization of TFC NF membranes. Microwave refers to the electromagnetic wave with a frequency of 300 MHz–3000 GHz and a wavelength of 0.1 mm–1 m. It is widely used in communication, energy industry [21], separation process [22], food industry [23], materials processing [24], etc. Microwave can transfer energy to materials by selectively causing the molecular vibration. Polar molecules like water, with high dielectric constant and high dielectric loss factor, have strong absorption ability to microwave. Microwave heating has several advantages compared with traditional heating mode such as high selectivity, energy conservation and precise process control [25]. It has been widely used in industrial production such as microwave-associated catalysis [26], fuel production [27], rock and concrete processing [28], polymer synthesis [24] and nanomaterials synthesis [29]. Microwave has also been applied in zeolite membranes fabrication since late 20th century [30,31]. Microwave assistant process can shorten the synthesis time and decrease the thickness of the zeolite membranes, consequently enhancing the permeance [32,33]. Micro­ wave assistant MOF membrane fabrication has also attracted much attention in recent years [34,35]. Hillman et al. [36] fabricated zeolitic-imidazolate framework (ZIF) membranes by a short-time mi­ crowave assistant in-situ synthesis of ZIF on α-Al2O3 substrates. This rapid one-pot synthesis process provided application potential of MOF membrane in gas separation.

Although the microwave has been widely used in membrane prep­ aration, it has not been used for polymer membrane fabrication. Herein, we use the microwave as heating source to prepare the TFC NF mem­ brane by IP process for the first time. Compared with the oven heating, the microwave heating is a more rapid and homogeneous heating method which heats the membrane by causing vibrations of water molecules. Under microwave heating, the temperature of aqueous phase rises up rapidly and the monomer diffusion rate accelerates. As a result, the PA layer can be generated under very low monomer concentration and hold a smooth top surface. The membrane heated by microwave showed preeminent water permeability and rejection. What’ more, the membrane exhibited unexpected selectivity in separating monovalent and divalent anions. This innovative insight proposes a feasible strategy for the commercial process of TFC membranes. 2. Experimental 2.1. Materials The polyether sulfone ultrafiltration (PES UF) membrane (MWCO ¼ 50,000) was produced by Development Center for Water Treatment Technology (Hangzhou, China). Trimesoyl chloride (TMC, �98%) was purchased from Qingdao Benzo Chemical Company (China). Carbamide (AR), glucose (AR), sucrose (AR), raffinose (AR), piperazine (PIP, GR), nhexane (AR), hydrochloric acid (HCl, AR), potassium hydroxide (KOH, AR), inorganic salts including Na2SO4(AR), MgSO4(AR), MgCl2(AR), NaCl(AR) and KCl(GR) were purchased from Sinopharm Chemical Re­ agent Co. Ltd (China). 2.2. Preparation of TFC NF membranes by oven and microwave heating The TFC NF membranes were prepared by IP with oven heating and microwave heating as shown in Fig. 1. The detailed preparation process was graphically represented in our previous work [37]. Firstly, 20 ml aqueous solution with different concentrations of PIP (from 25 to 100mg/100 ml) was poured onto the surface of PES UF substrate and hold for 3 min. After that, the excess fluid was removed by compressed air with pressure of 0.2 MPa and 20 ml n-hexane of 50mg/100 ml TMC was poured onto the membrane surface immediately and reacted for 15 s. At last, the prepared membrane was heated in 80 � C oven for 3min or microwave-heated (microwave frequency of 2450 MHz and output power of 700 W) for 1 min. Fig. S1 showed the oven and microwave oven used in this work. The membrane heated with oven and microwave was denoted as PA/O-X and PA/M-X, respectively, where X represented the PIP concentration (mg/100 ml) in aqueous solution.

2

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Journal of Membrane Science 596 (2020) 117718

Fig. 2. (a) XPS wide scan spectra of PES UF substrate, PA/O-50 and PA/M-50. (b and c) XPS narrow scan spectra of C 1s of PA/O-50 and PA/M-50. Table 1 Elemental composition of PES UF, PA/O-50 and PA/M-50. Sample PES UF PA/O-50 PA/M50

Atom percent (%) C 1s

N 1s

O 1s

S 2p

77.9 � 1.0 73.9 � 0.9 74.8 � 2.1

/

17.1 � 0.7 13.3 � 0.5 12.9 � 1.2

5.0 � 0.3 0.3 � 0.1 /

12.5 � 0.7 12.4 � 0.9

The ratio of N/ O / 0.95 � 0.01 0.96 � 0.02

Table 2 Peak percentage of C 1s score level. Sample PA/O-50 PA/M-50

Peak percentage (%) C–C

C–N

C¼O

58.5 � 2.3 61.2 � 2.0

30.5 � 1.6 27.6 � 1.1

11.0 � 0.8 11.2 � 0.9

2.3. Characterization The surface composition of the membranes was detected by an X-ray photoelectron spectrometer (XPS, ThermoFisher, ESCALAB 250Xi, USA) with a hemispherical energy analyzer and Al Kα radiation as the X-ray source. The morphology was characterized by field emission scanning electron microscopy (FESEM, FEI, Nova NanoSEM 450, USA). Before FESEM testing, the membranes were sprayed with Pt to make the sam­ ples electrically conductive. The roughness of the membranes was analyzed with atomic force microscopy (AFM, Veeco, NanoScope IIIa Multimode AFM, USA) with a testing area of 5 μm � 5 μm. The hy­ drophilicity of the membrane surface was measured by contact angle meter (JC2000A, Shanghai Zhong Cheng Digital Equipment Co., Ltd., China). The volume of water droplets was 4 μL and the contact angle was measured every 2 s within 80 s. The zeta potential of the membranes was tested with a streaming potential analyzer (SurPASS, Anton Paar, Austria) with 0.001 mol/L KCl solution. The pH range of the measure­ ment was from 4 to 10 which adjusted by 0.05 mol/L HCl and KOH.

Fig. 3. The top surface (left) and cross-section (right) SEM images of PES UF (a and b), PA/O-50 (c and d) and PA/M 50 (e and f) with 50,000 � magnification.

V A�t

(1)

2.4. Nanofiltration performance measurement

PWF ¼

The permeability and rejection of the TFC membranes were measured by a home-made stainless steel cross-flow device. The mem­ branes prepared with the same condition were tested by three identical parallel modules to ensure data reliability. The equipment run for 30 min under 0.6 MPa at 25 � C before each measurement for stability consideration. The pure water flux (PWF) was calculated with the formula below:

where V represented the volume of the permeation (L), A was the effective permeation area (m2) of the membrane and t was the time (h) of measurement. The rejection was calculated with the following equation: Rð%Þ ¼ 1

Cp Cf

(2)

where Cp and Cf was the concentration of permeation and feed solution, 3

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Journal of Membrane Science 596 (2020) 117718

Fig. 4. The AFM 2D (left) and 3D (right) images of PES UF (a and b), PA/O-50 (c and d) and PA/M

respectively. The conductivity of the inorganic salt solutions was detected by a conductivity meter (Shanghai Neici Instrument Company, DDS-11A, China) and matched with standard curve to get concentration. The concentration of organics was measured by a total organic carbon analyzer (TOC, Shimadzu, Model TOCVPN, Japan). The concentration of NaCl and Na2SO4 in the mixed solution was detected by ion chroma­ tography (ThermoFisher, ICS-5000, USA).

50 (e and f) with a 5 μm � 5 μm testing area.

3. Results and discussion 3.1. Surface composition The XPS analysis of PES UF, PA/O-50 and PA/M 50 was conducted to analyze the chemical composition of the membrane surface and the results were displayed in Fig. 2, Tables 1 and 2. Different from the PES UF substrate, the peak of nitrogen appeared in the wide scan spectrum of TFC membranes as shown in Fig. 2(a), demonstrated that PA layer was introduced successfully. A spot of sulfur (0.38%) could be detected in 4

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Journal of Membrane Science 596 (2020) 117718

chemical composition of them had no significant difference because these two membranes were prepared under the same aqueous phase and organic phase. 3.2. Morphologies SEM and AFM images in Figs. 3 and 4 showed the morphologies of the PES UF, PA/O-50 and PA/M-50. From the SEM image, the PES UF membrane hold a smooth surface and sponge-like cross section struc­ ture. There were some irregular folds in the top surface of PA/O-50 while the top surface of PA/M 50 was very smooth. The AFM result also indicated this, the roughness of PA/M 50 was very close to the PES UF membrane. The smooth surface may cause by the low concen­ tration of PIP and TMC in the IP process. There was no consecutive and intact PA layer could be detected like the classic polypiperazine-amide NF membranes [39] form the cross section image of PA/O-50 due to the relatively low monomer concentration. Under oven heating, the formation of PA layer relay on the free diffusion of monomers. With such low concentration of PIP and TMC (50mg/100 mL), it was hard to form the consecutive PA layer. The microwave heating had expected advan­ tages for IP process, the smooth PA layer could be formed under the low monomer concentration compared with the PA TFC membranes which were heated in oven [40]. The microwave delivered energy to the membrane by causing the vibrations of water molecules, so the tem­ perature rise was rapid and uniform. Even under the low concentration of PIP, the diffusion rate was suspected to speed up and the reaction rate expedited within a short time. Therefore, the PA layer with microwave heat-treated showed smooth and complete morphology. When the concentration of PIP increased, the top surface of the PA layer became rough with irregular protuberance structure (Figs. S2 and S3) like the structure of polypiperazine-amide membranes in literatures [5,41]. But

Fig. 5. (a) The dynamic water contact angle in 80 s and the photos of PES UF substrate (b), PA/O-50 (c) and PA/M 50 (d).

PA/O-50 as seen in Table 1. With the oven heating mode, the PA layer fabricated with such low concentration of PIP and TMC was partly defective and the PES substrate exposed to the testing environment. The morphology and inferior NF performance (Fig. S6) of PA/O-50 also mediately illustrated the fragmentary structure of PA layer. The C 1s narrow scan data of PA/O-50 and PA/M 50 was displayed in Fig. 2(b and c). The peak at 286.0 eV belonged to the C–N species of unreacted diamine. The content of C–N of PA/O-50 was higher than PA/M-50. The – O species of amido bond which peak at 287.9 eV represented the C– could illustrate the crosslinking degree of the PA layer [38]. There was –O no significant difference between PA/O-50 and PA/M 50 in C– percentage. Besides, the ratio of N/O elements of the two PA NF mem­ branes which reveal the degree of cross-linking was very close [3]. The

Fig. 6. (a) The rejection and flux of PA/M 50 to different inorganic salts. (b) Separation efficiency of PA/M curves of PA/M-50. (d) Nanofiltration performance of PA/M 50 under different operation pressure. 5

50 for NaCl and Na2SO4. (c) Pore size distribution

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Journal of Membrane Science 596 (2020) 117718

Table 3 The NF performance of TFC NF membranes in literatures and this work. Membrane

Monomer concentration (w/v %)

PWF (L m-2 h-1 bar-1)

Rejection (%) Na2SO4

MgSO4

NaCl

MgCl2

PA/M-50

PIP 0.05 TMC 0.05 PIP 0.2 þ PVA 0.14 TMC 0.2 PIP 1 TMC 0.15 þ TAC 0.04 PIP 1% þ CNCs0.01 TMC 0.2 PIP 2 TMC 0.7 BAPBS 0.75 TMC 0.1

26.2

97.7

81.2

17.0

11.4

This work

26.0

99.6

99.2

49.6

91.2

[7]

13.2

97.6

92.7

34.0

/

[40]

16.2

98.8

97.5

/

/

[42]

22.9

99.0

91.3

22.0

42.3

[47]

12.1

92.5

80.0

45.0

38.0

[48]

TS-II NFM-4 CNC-TFC-M2 PA-SGO BAPBS/TMC

it was interesting that with the TMC concentration decreasing or increasing, the surface of the membranes showed no significant changes (Fig. S4). This was because that the IP process took place on the organic side [7], the amount of PIP molecule which migrated to the reaction site was unaltered with the same PIP concentration, the TMC concentration made less influence on the reaction rate and polymer structure.

Reference

consistent with the negatively charged PA membranes in literatures [3]. The zeta potential of PA/M 50 was measured with a pH value range of 4–10 (Fig. S7) to further investigate the surface charge of membranes. The zeta potential was 39.6 mV at pH ¼ 5.7, which demonstrated electronegativity of the membrane [41]. The zeta potential decreased with the pH value increment due to the deprotonation of functional groups [3]. As a contrast, the zeta potential of PA/O-50 was also tested at the same condition. The two membranes exhibited similar tendency of zeta potential due to the similar surface composition. The separation performance of PA/M 50 to monovalent and divalent anions was tested with mixed solution of Na2SO4 and NaCl (1:1) with total concentration ranged from 1000 to 9000 ppm. The rejection to Na2SO4 decreased slightly with the increment of feed concentration. It was interesting to note that the negative rejection of NaCl appeared with the concentration increased. At higher concentration of salts, the phe­ nomenon of concentration polarization was more obvious. The con­ centration of sodium near and inside the negatively charged membrane was higher than the feed solution. Based on the electroneutrality con­ dition, the chloride ion concentration was equal to sodium [46]. Therefore, the rejection calculated with the chloride ion concentration in permeation and feed was a negative value. Overall, the PA/M 50 showed excellent performance in separating Cl- and SO24 . The pore size and MWCO of PA/M 50 was simulated by MATLAB (the detailed computing method was enumerated in Supplementary Material). Fig. 6(c) exhibited the simulated pore size distribution curves of PA/M-50. The calculated pore diameter and MWCO were 1.03 nm and 383, respectively. This result illustrated the application potential of the TFC membrane in the removal of small organic pollutants from water. To further investigate the utilization potentiality, the PWF and rejection to 2000 ppm Na2SO4 of the PA/M 50 under different oper­ ating pressure was tested and plotted in Fig. 6 (d). The rejection remained high with the operation pressure ranged from 0.1 to 0.7 MPa. The PWF increased linearly with the operating pressure increasing. The NF performance of TFC NF membranes in some published works was enumerated in Table 3 for comparison. The PA/M 50 in this work showed ideal permeability and good selectivity for separating Cl- and SO24 .

3.3. Hydrophilicity Water contact angle (WCA) is a common method to represent the hydrophilicity of materials. It is generally assumed that low WCA sym­ bolizes better hydrophilicity [42]. For membrane materials, better hy­ drophilicity can enhance the permeability and antifouling property of the membrane [43]. The WCA of PES UF membrane, PA/O-50 and PA/M 50 was recorded every 2 s in 80 s and was plotted in Fig. 5(a). The WCA decreased gently because of the spreading of water drops on the membrane surface [44]. The WCA of PA/O-50 and PA/M 50 was much lower compared with the substrate. The PA layer fabricated by IP was more hydrophilic than PES. The PA/O-50 had more hydrophilic surface than PA/M-50. From the XPS result, the content of oxygen groups on the surface of the PA/O-50 was a little higher than that of PA/M-50. It can be speculated that more carboxyl groups may be generated with oven heating. The permeation rate of PIP was slow by oven heating, and gave rise to more hydrolysis of TMC. Besides, the PA/O-50 hold a rougher surface, leading larger contact area with water, which contributes to lower WCA as well [45]. 3.4. Nanofiltration performance The TFC membranes were prepared by microwave heating and oven heating with different PIP concentration and the NF performance was measured (Fig. S5). The membranes prepared by microwave heating showed much better performance in both permeability and rejection compared with the membranes prepared by oven heating. The optimal performance (PWF of 157.1 L m-2 h-1, Na2SO4 rejection of 97.7%) was obtained when PIP and TMC concentrations were 50mg/100 ml. This contrasting result showed the great advantages of the microwave heat­ ing in low concentration IP process. The influence of microwave heating times was also explored and the performance of PA/O-50 with different microwave heating time was shown in Fig. S6. With the microwave heating time increased, the flux of the membranes decreased while the rejection of Na2SO4 increased due to the increment of cross-linking de­ gree. When the microwave heating time was over 1min, the flux declined slightly and the rejection remained a similar level. So the mi­ crowave heating time of 1 min was chosen as the optimal heating time. The permeability and rejection of PA/M 50 were demonstrated in Fig. 6 (a) which was tested with solutions of 2000 ppm Na2SO4, NaCl, MgSO4 and MgCl2. The rejection followed the order: Na2SO4(97.7%)> MgSO4(81.2%)>NaCl(17.0%)>MgCl2(11.4%). This result was

4. Conclusions High-performance polypiperazine-amide NF membrane was suc­ cessfully fabricated with low monomer concentration via microwave heating. Different from the oven heating, the microwave heating is rapid and uniform, shows great advantages in fabricating TFC NF membrane. The PA layer fabricated with the low monomer concentration was defect-free and smooth. The PWF and rejection of the membrane both improved enormously compared with the membrane prepared by oven heating. In addition, the membrane exhibited excellent performance in separating monovalent and divalent anions. Overall, the microwave 6

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Journal of Membrane Science 596 (2020) 117718

heating is a simple and efficient heating method to fabricate highperformance TFC membranes and is of great application potential for the industrialized production.

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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Ben-Qing Huang: Data curation, Investigation, Writing - original draft, Visualization. Yong-Jian Tang: Conceptualization, Methodology, Validation, Supervision, Writing - review & editing, Supervision, Writing - review & editing. Zuo-Xiang Zeng: Supervision, Writing review & editing. Zhen-Liang Xu: Supervision, Writing - review & editing. Acknowledgement Thanks for the support received from the National Natural Science Foundation of China (21808060) and the Fundamental Research Funds for the Central Universities (WA1814009). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2019.117718. References [1] Y. Ji, W. Qian, Y. Yu, Q. An, L. Liu, Y. Zhou, C. Gao, Recent developments in nanofiltration membranes based on nanomaterials, Chin. J. Chem. Eng. 25 (2017) 1639–1652. [2] C.Z. Torma, E. Csefalvay, Nanofiltration: a final step in industrial process water treatment, Period. Polytech-Chem. 62 (2018) 68–75. [3] D. Ren, X.T. Bi, T.Y. Liu, X. Wang, Oligo-ethylene-glycol based thin-film composite nanofiltration membranes for effective separation of mono-/di-valent anions, J. Mater. Chem. A. 7 (2019) 1849–1860. [4] Y.J. Tang, Z.L. Xu, S.M. Xue, Y.M. Wei, H. Yang, Improving the chlorine-tolerant ability of polypiperazine-amide nanofiltration membrane by adding NH2-PEG-NH2 in the aqueous phase, J. Membr. Sci. 538 (2017) 9–17. [5] G. Gong, P. Wang, Z. Zhou, Y. Hu, New insights into the role of an interlayer for the fabrication of highly selective and permeable thin-film composite nanofiltration membrane, ACS Appl. Mater. Interfaces 11 (2019) 7349–7356. [6] Z.Y. Zhou, Y.X. Hu, C. Boo, Z.Y. Liu, J.Q. Li, L.Y. Deng, X.C. An, High-performance thin-film composite membrane with an ultrathin spray-coated carbon nanotube interlayer, Environ. Sci. Technol. Lett. 5 (2018) 243–248. [7] Z. Tan, S. Chen, X. Peng, L. Zhang, C. Gao, Polyamide membranes with nanoscale Turing structures for water purification, Science 360 (2018) 518–521. [8] M.B.M.Y. Ang, Y.L. Ji, S.H. Huang, K.R. Lee, J.Y. Lai, A facile and versatile strategy for fabricating thin-film nanocomposite membranes with polydopamine-piperazine nanoparticles generated in situ, J. Membr. Sci. 579 (2019) 79–89. [9] C. Van Goethem, R. Verbeke, M. Pfanmoller, T. Koschine, M. Dickmann, T. TimpelLindner, W. Egger, S. Bals, I.F.J. Vankelecom, The role of MOFs in thin-film nanocomposite (TFN) membranes, J. Membr. Sci. 563 (2018) 938–948. [10] H.Y. Yang, N.X. Wang, L. Wang, H.X. Liu, Q.F. An, S.L. Ji, Vacuum-assisted assembly of ZIF-8@GO composite membranes on ceramic tube with enhanced organic solvent nanofiltration performance, J. Membr. Sci. 545 (2018) 158–166. [11] D.B. Shinde, G. Sheng, X. Li, M. Ostwal, A.H. Emwas, K.W. Huang, Z. Lai, Crystalline 2D covalent organic framework membranes for high-flux organic solvent nanofiltration, J. Am. Chem. Soc. 140 (2018) 14342–14349. [12] J.J. Wang, H.C. Yang, M.B. Wu, X. Zhang, Z.K. Xu, Nanofiltration membranes with cellulose nanocrystals as an interlayer for unprecedented performance, J. Mater. Chem. A. 5 (2017) 16289–16295. [13] J.Y. Zhu, J.W. Hou, R.J. Zhang, S.S. Yuan, J. Li, M.M. Tian, P.H. Wang, Y.T. Zhang, A. Volodin, B. Van der Bruggen, Rapid water transport through controllable, ultrathin polyamide nanofilms for high-performance nanofiltration, J. Mater. Chem. A. 6 (2018) 15701–15709. [14] C. Li, S. Li, L. Tian, J. Zhang, B. Su, M.Z. Hu, Covalent organic frameworks (COFs)incorporated thin film nanocomposite (TFN) membranes for high-flux organic solvent nanofiltration (OSN), J. Membr. Sci. 572 (2019) 520–531. [15] M. He, T. Li, M. Hu, C. Chen, B. Liu, J. Crittenden, L.Y. Chu, H.Y. Ng, Performance improvement for thin-film composite nanofiltration membranes prepared on PSf/ PSf-g-PEG blended substrates, Separ. Purif. Technol. 230 (2020) 115855.

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