Kevlar aramid nanofiber nanofiltration membranes with high permselectivity in water desalination

Journal of Membrane Science 592 (2019) 117396

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Fabrication of composite polyamide/Kevlar aramid nanofiber nanofiltration membranes with high permselectivity in water desalination

T

Yi Lia, Eric Wongb, Zhaohuan Maia,c, Bart Van der Bruggena,d,∗ a

Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B-3001, Leuven, Belgium Department of Management and Technology, UC Leuven-Limburg, Herestraat 49, 3000, Leuven, Belgium c Institute of Energy Conversion, Jiangxi Academy of Sciences, 7777 Changdong Rd, Nanchang, 330096, China d Faculty of Engineering and the Built Environment, Tshwane University of Technology, Private Bag X680, Pretoria, 0001, South Africa b

A R T I C LE I N FO

A B S T R A C T

Keywords: Thin film composite (TFC) membrane Aramid nanofiber (ANF) Kevlar Desalination Molecular separation

Conventional piperazine (PIP)-based nanofiltration (NF) membranes feature a high water flux and a high retention for divalent salt ions. However, it remains a challenge to obtain permselective NF membranes with high water permeance and a good selectivity for monovalent ions. In this work, a new m-phenylenediamine (MPD)based thin-film composite (TFC) NF membrane with excellent desalination performance was developed by interfacial polymerization on a solvent resistant Kevlar nanofibrous hydrogel substrate. The desalination performance of the ANF TFC membrane shifted from reverse osmosis (RO) into NF with a facile solvent treatment. The decreased membrane surface roughness, reduced surface zeta potential and increased surface hydrophilicity after solvent treatment yielding a high water permeability (14.4 L m−2 h−1 bar−1) for ANF TFC membrane, which is one order of magnitude higher than that of the pristine membrane and the hand-cast poly (m-phenylene isophthalamide) (PMIA) TFC membrane. The ANF TFC membrane showed an outstanding water-salt separation performance, with excellent rejections for multivalent salts (Na2SO4, 100%; MgSO4, 99.4%; MgCl2, 92.7%) and a high rejection for monovalent salt (NaCl, 80.3%), which is competitive with reference commercial membranes (NF90, NF270) tested in cross-flow filtration with 1000 mg L−1 salt solution at 6 bar, 25 °C. The newly developed TFC membrane was demonstrated to have great potential applications in water desalination, separation of organic compounds and dye wastewater treatment.

1. Introduction Nanofiltration (NF) as a pressure-driven membrane process has properties in between ultrafiltration (UF) and reverse osmosis (RO) [1–3]. NF can be considered as low-pressure reverse osmosis (RO), because of its advantages of lower energy consumption resulting from lower operating pressures, higher fluxes and high rejection of divalent ions compared to RO [1,2]. These properties have allowed NF to be used in many niche applications especially in water and wastewater treatment and desalination as well as some other interesting areas including pharmaceutical, biotechnology and food industry, etc [1]. However, to achieve a high selectivity for monovalent ions is still remains one of the challenges for NF membranes [3,4]. Recent advances in nanofiltration technology improved the efficiency of NF membranes for removal of monovalent ions and organic matters from sea water, brackish water, surface water and underground water [2,3,5–14]. As a pretreatment process for RO desalination, a higher monovalent ions

∗

retention from NF process is highly desirable to reduce the burden of the RO stage, increase membrane lifetime, achieve higher RO design flux and recovery in water desalination plants [8,9,15]. Van der Bruggen revealed that the drawback of insufficient monovalent removal efficiency also challenges NF membranes in some other specific applications in separations of trace elements [16], such as nitrate [17,18], boron [19], and organic micropollutants [20], in applications that require a high purity of water. NF membranes with superior flux and excellent retention of di-/mono-valent ions are attractive to the contemporary market [2,16]. Piperazine (PIP)-based NF membranes prepared from interfacial polymerization (IP) between the PIP in aqueous solution and trimesoyl chloride (TMC) in the organic phase are typical NF membranes [14–23]. The PIP-based NF membrane often have a high flux and a superior multivalent rejection (> 95%), whereas the monovalent ion rejection varies between 0 and 60% [1,16,21–30]. The ionic rejections of current commercially available NF membranes are also varied

Corresponding author. Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B-3001, Leuven, Belgium. E-mail address: [email protected] (B. Van der Bruggen).

https://doi.org/10.1016/j.memsci.2019.117396 Received 10 June 2019; Received in revised form 8 August 2019; Accepted 20 August 2019 Available online 21 August 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.

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designed MPD-based NF membrane was examined by desalination experiment with a single salt solution and the polyethylene glycols (PEGs) rejection experiment. One self-made poly (m-phenylene isophthalamide) (PMIA) TFC membrane and two commercial membranes (NF90, NF270) were also studied for comparison in this work to demonstrate the competitive applications of our fabricated TFC membrane in water desalination and small molecular separation. This work may pave the way for designing the next-generation NF membranes with high water-solute separation performance in water desalination and molecular separation.

greatly [2,5]. NF90 (Dow Filmtec) as a fully aromatic polyamide (PA) membrane was suggested to be the best choice as the pre-treatment of seawater desalination by considering the ideal NaCl rejection [5,8,31]. However, the low permeability remains one of the drawbacks compared with other commercial NF membranes such as NF200, NF270 (Dow Filmtec), K-SR2 (Koch), ESNA1-LF2 (Hydranautics) and NF99HF (Alfa Laval) [31]. Generally, m-phenylenediamine (MPD)-based RO membranes prepared from MPD and TMC are non porous and can remove nearly all ions in addition to uncharged solutes of molecular weight greater than around 100 Da because of their highly crosslinked dense PA layer [32]. Transferring a RO membrane into a NF membrane in a facile way, in order to obtain a high water flux but maintain the high monovalent salt rejection is a promising way, albeit challenging because of the highly crosslinked PA network developed from MPD and TMC. Several studies have attempted to elevate the membrane performance by tailoring the PA thin film layer including addition of chemicals in the aqueous/organic phase [1,25,28], incorporation of nanomaterials into the selective layer [33–35] and post treatment membranes after IP reaction [36–40]. Among these strategies, posttreatment of membranes via contacting the pre-fabricated membrane with a solvent is a facile, simple and effective technique for promoting the membrane permeance [21,36,38,40]. A variety of organic solvents have been explored to activate PA membranes including mild alcohols (ethanol, isopropanol, benzyl alcohol), dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) [36–39]. DMF is considered to be the strongest activation agents for the permeability enhancement due to its strong solvency power but sacrificing the salt rejection [38]. It was speculated that upon exposure to the harsh solvent, the lower molecular weight polyamide fragments of the membrane may dissolve after IP reaction, providing more transportation passages [36–38]. However, it is a challenge to employ the solvent activation on PA layer with a conventional polymeric substrate, such as the commercial polysulfone (PSf) and polyethersulfone (PES), because of their inferior solvent resistance. Therefore, the harsh organic solvent resistant substrate in is the key to realize the solvent activation on the TFC membrane. In recent years, Kevlar aramid nanofibers (ANFs) are emerged as new building blocks for the membrane fabrication. The fibers in nanometer range consisting of poly (p-phenylene terephthalamide) (PPTA) macromolecular chains have great resistance in organic solvent and have strong mechanical strength [41–44]. In previous work, ANFs were used in the preparation of an organic solvent nanofiltration (OSN) membrane in a facile way by phase inversion and thermal treatment [41,43]. The hydrogel membrane underwent an irreversible transition from a hydrogel into a mechanical strong NF membrane [41]. Recently, ANFs constructed hydrogel membrane was found to be an excellent substrate for the TFC membrane fabrication [42]. The nanofilm membrane prepared via interfacial polymerization on the nanofibrous hydrogel substrate was found to have ultrafast transport performances in various organic solvents. The strong solvent resistance of ANFs enable the solvent activation applicable for the ANF TFC membrane and their potential application in water desalination is promising. Constructing a composite membrane through interfacial polymerization based on a solvent resistant substrate, followed by solvent activation may provide a new pathway for the preparation of high performance NF membranes. Moreover, the effects of solvent activation on the membrane in water desalination were not elucidated; more specifically, the transport behavior of water molecules and ions with different charge and valence through the modified membrane are not investigated. Herein, this work aims to develop a new MPD-based NF membrane by interfacial polymerization on a solvent resistant Kevlar nanofibrous hydrogel substrate, followed by solvent treatment using N-dimethylformamide (DMF). The effect of solvent treatment on the surface morphology, thin film thickness, surface chemistry, surface hydrophilicity, surface charge and the desalination performance of the membrane were investigated. The separation mechanism for the newly

2. Experiments 2.1. Chemicals and materials The non-woven support polypropylene/polyethylene (PP/PE) fabric Novatex 2471 was obtained from Freudenberg (Germany). Kevlar and Nomex® fiber were purchased from Zhangjiagang Free Trade Zone Fengduo International Trade Co., Ltd., China. Potassium hydroxide (KOH, 85%) in the form of pellets was obtained from Acros Organics NV. M-phenylenediamine (MPD, 99%) and trimesoyl chloride (TMC, 98%) were obtained from Sigma-Aldrich. Dimethyl sulfoxide (DMSO, 99.5%) and n-hexane (95%) were purchased from VWR International BVBA. Four inorganic salts (Na2SO4, MgSO4, MgCl2, NaCl), lithium chloride (LiCl, 99%), methanol (99.9%), acetone (99.9%), N-methyl-2pyrrolidone (NMP), N-dimethylacetamide (DMAc, 99%) and N-dimethylformamide (DMF, 99.8%) were obtained from Sigma-Aldrich BVBA. The dyes ((rose Bengal (RB, 1017 g mol−1, -), eosin Y (EY, 648 g mol−1, 0), Sudan black B (SBB, 457 g mol−1, 0), methyl orange (MO, 327 g mol−1, -), methyl blue (320 g mol−1, +) and disperse orange 3 (DO3, 242 g mol−1, 0)) with different molecular weight and charges were obtained from Sigma-Aldrich. Polyethylene glycols (PEGs) with molecular weight of 200, 300, 400, 600 and 1000 Da were obtained from Sigma-Aldrich BVBA. The commercial nanofiltration membranes NF90 and NF270 were kindly provided by Dow Water and Process Solutions. 2.2. Preparation of hydrogel substrate and TFC membrane The casting solution was prepared as reported in previous work [41]. Typically, 4.0 g of Kevlar fibers and 6.0 g of KOH mixed in 200 ml of DMSO solvent, stirred at 200 rpm at room temperature for over two weeks to obtain an ANF concentration of 2.0%. An ANF hydrogel membrane was formed via non-solvent phase inversion. The non-woven fabric was placed on a clean glass plate. The dope solutions were casted onto the PP/PE support using a casting knife set to a thickness of 250 μm at a temperature of 25 °C. The gel membranes precipitated from the water after phase inversion and immersed in 1.0 mM HCl solution for 1 min, then transferred into deionized (DI) water where they remained overnight. The poly (m-phenylene isophthalamide) (PMIA) substrate membrane was also prepared by phase inversion method. The dope solution was obtained by dissolving Nomex® fiber (15 wt%) and LiCl (5 wt%) in DMAc at 80 °C overnight. The obtained dope solution was centrifugated to remove the air bubbles and casted on the PP/PE non-woven support using a casting knife with a thickness of 200 μm. The membrane was followed by phase inversion in water and kept in DI water overnight before use. The TFC membrane was developed by IP techniques based on the hydrogel membrane and conventional polymeric substrate. The support hydrogel membrane was rinsed with DI water and subsequently immersed in an aqueous solution of 2.0 wt% MPD. The excess solution was gently absorbed using a tissue paper and dried in air for 2 min. The prepared hydrogel membrane was immersed in hexane containing 0.1 w/v% TMC for 20 s to form a selective PA layer. The resulting membrane was drained and rinsed with pure hexane. The nascent membrane 2

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(HP4750) with an effective testing area of 14.6 cm2. The separation equipment was operated with a constant stirring of 600 rpm to minimize polarization concentration effects. The pure water permeability was tested continuously for 10 h at 4 bar. The desalination performances were measured in saline water after the membrane was compacted for 30 min at 6 bar and tested at 4 bar, 600 rpm at 25 °C in a dead-end filtration cell. The molecular separation in aqueous solution of the prepared membrane was evaluated using a single charged solute with concentration of 100 mg L−1 at pressure of 4 bar after compacted at 6 bar for 30 min. The negatively charged methyl orange and positively charged methylene blue were used as solutes. The long-term stability of the prepared ANF TFC membrane were tested using inorganic salts (Na2SO4, MgSO4, MgCl2 and NaCl) in water with concentration of 1000 mg L−1 (1.5 L feeding solution). The membrane was tested continuously in a cross-flow filtration device at a pressure of 6 bar at 25 °C for 10 h. The effective membrane area was 22.9 cm2. The solvent filtration performances were measured in dye/organic solution with dye concentration of 20 mg L−1. Membranes were tested in a deadend filtration cell at pressure of 6 bar, 600 rpm, 25 °C. The water or solvent permeability (P , L m−2 h−1 bar−1) was calculated from the following equation:

was dried in air for 1 min and immediately immersed into DI water or in DMF for different time (0.25 h, 0.5 h, 1 h, 3 h) and stored in methanol. The membranes were rinsed with DI water before testing. The control PMIA TFC membrane was also prepared by the same method as mentioned above. The resulting PMIA TFC membrane after IP reaction was immersed into DI water before testing. 2.3. Characterization The Kevlar nanofibers prepared in DMSO/KOH diluted aqueous solutions were observed by transmission-electron microscopy (TEM, JEOL ARM200F equipped with a cold-field emission source and operated at 200 kV). One drop of the solutions was placed on the surface of a copper grid coated with carbon and dried in the oven at 60 °C before testing. The diameter of the Kevlar fibers and ANFs were measured using ImageJ software. The Kevlar fibers, membrane surfaces and crosssectional morphologies were observed using a XL30 FEG field-emission scanning electron microscope (FE-SEM, Netherlands). The membrane samples were mechanically fractured in liquid nitrogen and dried before cross-section observation. Each sample was sputter-coated with a 1.5–2 nm Au layer before testing. TEM was performed at 100 kV accelerating voltage to study the thickness and morphologies of crosssection of ANF TFC membrane. Small pieces of the membranes without non-woven support were embedded in Epon resin and cut by an ultramicrotome. The water contact angles measured with a 3.0- μL DI water drop using the sessile drop method on a video contact angle system (OCA20, Dataphysics, German). The average value of six measurements at random positions for each sample was reported. The membrane surface chemistry was examined by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR, NEXUS670). The surface morphologies and roughness of prepared membranes were observed by atomic force microscopy (AFM) (Bruker, USA) with taping mode measurements in air. All the samples were dried with supercritical CO2 before characterization.

P=

Cp ⎞ R = ⎜⎛1 − ⎟ × 100% Cf ⎠ ⎝

dd p

(lnd p − lnμp)2 ⎤ 1 exp ⎡ − ⎥ ⎢ 2(lnσp)2 d plnσp 2π ⎦ ⎣

(3)

where Cp and Cf are the salt or dye concentrations in the permeate and feed solutions, respectively. The salt concentrations were determined by a conductivity meter (Thermo Scientific Orion Star A212). The dye concentrations were measured using a PerkinElmer lambda 12 UV–vis spectrophotometer.

PEGs with different molecular weights (200–1000 Da) were used as markers to determine the pore size and pore size distribution of ANF TFC membrane as reported elsewhere [42]. The filtration experiment was tested using a feed solution of 500 ppm of PEG molecular in DI water. PEG concentration in the feed and permeate were determined using TOC-VCPH analyzer (Shimadzu, Japan). The PEG rejection R (%) was calculated by the same equation used for the salt rejection. The molecular weight cut-off (MWCO) has been defined as the molecular weight of PEG which has a 90% rejection rate. The geometric mean diameter ( μs ) is defined as the Stokes diameter of a solute when the solute rejection is 50%. The geometric standard deviation (σg ) is the calculated from the ratio of Stokes diameters when the solute rejections at 84.13% and at 50%. By ignoring the hydrodynamic and electrostatic interactions between neutral solutes and membrane pores, the mean effective pore diameter ( μp ) and the geometric standard deviation (σp ) of the membrane can be considered to be the same as of μs and σg . The relationship between pore size distribution and solute Stokes diameters was mathematically fitted by an exponential probability density function as following:

=

(2)

where V is the volume of collected permeate (L), A is the effective area of the testing membrane (m2), t is the interval time (h) and ΔP is the applied transmembrane pressure (bar). The salt or dye rejections (R , %) were calculated from the following equation:

2.4. Pore size and size distribution

dR (d p)

V A × t × ΔP

3. Results and discussion 3.1. Characterization of ANF and TFC membrane Commercial Kevlar in the form of fibers with an uniform diameter of 12 μm was utilized to prepare the aramid nanofibers (Fig. 1a) [41]. The resultant aramid nanofibers in diluted DMSO/KOH dope solution were characterized using transmission electron microscopy (TEM) (Fig. 1b). The Kevlar fibers were successfully exfoliated into nanofibers with an average diameter of 13.7 nm at large scale. The dope solution was facilely casted onto a solvent-resistant and mechanically strong PP/PE non-woven fabric. The final porous hydrogel substrate was obtained by non-solvent induced phase inversion. As shown in Fig. 1c, the hydrogel surface showed a relatively smooth surface because of the nanofibrous structure. After interfacial polymerization, a defect-free thin film of PA layer was formed on the hydrogel/non-woven fabric support (Fig. 1d). The structure of the PP/PE fabric fibers can be easily observed from the top view; this demonstrates the ultra-thin characteristic of the nanofilm and the good attachment between the PA layer and the hydrogel substrate. This can also be demonstrated by the cross-sectional SEM image of the ANF TFC membrane in Fig. 1e. A thin “leaf-like” PA nanofilm was found adhered on the hydrogel surface without any defects. X-ray photoelectron spectroscopy (XPS) was used to demonstrate the successful preparation of the PA layer by examining the membrane surface chemistry (Fig. S1). The chemically stable nanofibers make the DMF activation applicable for ANF TFC membrane. Their ease of preparation endows the membrane with a scalable fabrication potential compared

(1)

where d p is the pore size of the membrane. 2.5. Membrane separation performance The separation performance of the prepared ANF TFC membranes was evaluated in a stainless-steel dead-end membrane module 3

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Fig. 1. (a) SEM image of Kevlar fiber. (b) TEM image of the exfoliated Kevlar aramid nanofibers (ANFs). (c) SEM image of hydrogel surface. (d) SEM observation of thin film composite nanofiltration with the PP/PE non-woven support. (e) The cross-sectional SEM image of polyamide layer on the hydrogel surface.

with recently reported “free-standing” membranes [29]. The effect of the duration of DMF activation on the morphology of the membrane surface was characterized using SEM (Fig. 2a). The prepared membrane without DMF treatment showed a characteristic “leaf-like” structure with ridges and valleys, which is a typical surface morphology resulting from the interfacial polymerization between MPD and TMC [33,35]. By immersing the fabricated membranes in DMF for different hours, the membrane surface exhibited a decreasing surface roughness, the polyamide top layers were effectively trimmed by the solvent due to the similar solubility parameters between DMF (24.8 MPa1/2) and polyamide (23.0 MPa1/2) [36,38,42]. It was found that even with a short solvent treatment (0.5 h), there was a significant change in surface morphology. However, longer treatment had no great influences on the morphology because of the highly cross-linked PA network. It indicated that the removed components were probably have a relatively loose structure. Furthermore, the cross-section of the membranes was characterized as shown in Fig. 2b. All membranes consist of a continuous PA layer and a nanofibrous substrate. To confirm the variations of the membrane surface morphologies, the surface roughness parameters of the ANF TFC membranes have been measured using atomic force microscopy (AFM). As shown in Fig. 3a, the nanofibrous substrate had a smooth surface with an average roughness (Ra)

of 10.1 nm. After interfacial polymerization, the ANF TFC membrane gave a Ra of 44.8 nm because of the produced ridge-and-valley protrusions (Fig. 3b). However, the TFC membranes after DMF posttreatment exhibited a decreasing surface roughness (Fig. 3c–f). The membrane with a 0.5 h of treatment reached the lowest Ra of 25.3 nm (Fig. 3d). The surface roughness parameters are summarized in Table 1. Rq and Rz represent the root mean square height and maximum height, respectively. The decreased membrane surface roughness is accordance to the observations of SEM. It may caused by the dissolution of the rough PA leaf-like layer, giving rise to a smooth surface [37,38]. The removal of the overlapped PA leaf-like layer is beneficial for reducing mass transportation resistance and enhancing the permeability [37]. The PA thickness of ANF TFC membranes before and after DMF treatment were characterized by transmission-electron microscopy (TEM) (Fig. 4). The pristine membrane showed a nanofibrous crosssection and a top skin PA layer in a thickness of ~79 nm, which is much lower than that of the MPD-based TFC membrane prepared on the conventional substrate (Fig. 4a). By contrast, the membrane after DMF treatment showed a decreased thickness of ~69 nm and a loose structure with large amounts of pores distributed on the PA layer (marked in red circle in Fig. 4b). Those discontinuities of nano-structure were may dissolved in DMF and created a porous surface selectively layer [36,39].

Fig. 2. The effect of solvent treatment on morphology of membrane surface (a) and cross-section (b). (DMF treatment time from left to right: 0 h, 0.25 h, 0.5 h, 1 h, 3 h). 4

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Fig. 3. AFM observations of the ANF hydrogel substrate (a) and ANF TFC membranes with DMF treatment of 0 h (b), 0.25 h (c), 0.5 h (d), 1 h (e) and 3 h (f), respectively.

to the hydrolysis of polyamide oligomers when the membrane was immersed in the DMF, which giving rise to the exposure of hydrophilic groups (-COOH and –NH2) on the membrane surface. The discontinuity in the PA matrix may also be partially dissolved as we observed the changes in the membrane surface morphology (Fig. 2a) and cross-section of PA layer (Fig. 4). To verify our hypothesis, Fourier transforminfrared spectroscopy (FT-IR) spectra were used to examine the membrane surface chemistry. The N–H stretching vibration at 3320 cm−1 is belongs to the amine group (Fig. 5c) [41,42]. The characteristic peaks for fully aromatic polyamide are 1644 cm−1 (C]O stretching vibration, amide I band), 1610 cm−1 (H-bonded C]O stretching) and 1540 cm−1 (N–H in-plane bending and C–N bending vibration stretching vibration of the group –CO–NH-, amide II band) (Fig. 5d) [36,38,40,42]. The absorption peaks at 1722 cm−1 (Figs. 5c) and 1250 cm−1 (Fig. 5d) are attributed to carboxylates [40]. All the DMF post-treated TFC membranes showed FT-IR spectra identical to that of the pristine membrane, indicating that those membranes have the same surface chemistry [38,39]. However, the intensified absorption peaks for the carboxyl and amine groups demonstrated that the membrane surface has an increasing content of hydrophilic motifs along with the DMF treatment time. This result is consistent with the hypothesis reported by other researchers that the severe swelling effect induced by DMF partially etches the PA layer, creating a hydrophilic and loose PA surface layer [36,38,39].

Table 1 Surface roughness parameters, contact angle and zeta potential for ANF substrate, ANF TFC membranes. Membrane

ANF ANF ANF ANF ANF ANF

substrate TFC- 0 TFC- 0.25 TFC- 0.5 TFC- 1 TFC- 3

Surface roughness parameters Ra (nm)

Rq (nm)

Rz (nm)

10.1 44.8 31.4 25.3 30.5 30.9

12.8 57.7 41.1 32.0 39.2 39.3

104 435 377 259 328 316

Contact angle (°)

Zeta potential (mV)

43.6 59.7 15.1 20.1 23.9 23.0

−21.1 −24.8 −27.2 −29.5 −43.6 −40.9

± ± ± ± ± ±

4.2 6.9 0.5 0.9 1.5 5.6

± ± ± ± ± ±

0.8 1.2 1.1 1.4 1.6 1.8

Fig. 5a displays the as-prepared ANF hydrogel TFC membrane. The hydrophilicity of the membrane surface was measured by water contact angle analysis (Fig. 5b), the membrane prepared on the nanofibrous hydrogel substrate showed a water contact angle of 59.7 ± 6.9°. The water contact angle dropped to 15.1 ± 0.5° instantly when the membrane treated with DMF for 0.25 h and increased slightly to 23.0 ± 5.6° by further prolonging the post-treatment time. It manifested that the hydrophilicity of the ANF TFC membrane was enhanced tremendously by the facile DMF post-treatment. It is probably ascribed

Fig. 4. TEM observations of cross-section of ANF TFC membranes without (a) and with (b) DMF post-treatment. 5

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Fig. 5. (a) The photograph of the fabricated ANF TFC membrane with large area (15 cm in length and 12 cm in width); (b) Contact angle of ANF TFC membranes with different DMF post-treatment time; (c, d) FT-IR spectrums of ANF TFC membranes.

Fig. 6. (a) The effect of DMF treatment time on zeta potential of ANF TFC membrane. (b) Zeta potential variations for ANF TFC membrane before and after solvent treatment.

Fig. 7. Effect of DMF post-treatment on the transportation behavior of ANF TFC membrane from reverse osmosis membrane to nanofiltration membrane.

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The zeta potential of the membranes with different duration of DMF treatment were measured. As shown in Fig. 6a, the membrane surface has decreasing zeta-potential values. Especially, the value decreased from −25 mV to −44 mV when applying DMF treatment for 1 h, which indicates a more negatively charged membrane surface. By considering the better desalination performance, the membrane with 0.5 h of DMF treatment was selected; for this membrane, the zeta potential was measured in a wide pH range (4–10) (Fig. 6b). It was found that the post-treated membrane has an enhanced electronegativity. The increased surface negative charge was mainly ascribed to the high dissolution ability and etching effect of DMF for the PA layer as we mentioned above. The membrane surface roughness parameters, contact angle and zeta potential are summarized in Table 1. These results demonstrate that the DMF treatment not only changed the membrane surface morphology by eliminating the leaf-like PA layer and decreasing the surface roughness but also gives rise to a more negatively charged surface and increased surface hydrophilicity. The MPD-based membrane was successfully transferred from a RO membrane to a NF membrane with a facile solvent treatment. As shown in Fig. 7, the decreased overlap of PA layers and the enhanced surface negative charge contribute to the improvement of water permeability as well as the excellent salt rejection. In addition, the nanofibrous hydrogel substrate provides a lower tortuosity and a high porosity, giving rise to a minimum transport resistance and less concentration polarization. Therefore, the newly designed MPD-based nanofiltration membrane has great advantages compared with conventional commercial NF membranes developed from PIP/TMC with a polymeric substrate.

Table 2 Comparison of separation performances against various conditions for ANF TFC membrane, PMIA TFC membrane and commercial NF membranes. Feed

Pure waterc Na2SO4d MgSO4d MgCl2d NaCld MOe MBe

ANF TFC

PMIA TFC

NF 90

NF 270

Pa (R%)b

P (R%)

P (R%)

P (R%)

14.4 10.9 (100%) 10.5 (99.4%) 7.4 (92.7%) 8.7 (80.3%) 8.4 (98.6%) 7.2 (99.7%)

1.6 1.3 1.2 1.2 1.3 1.2 1.5

8.3 7.2 7.1 6.0 6.2 4.0 3.7

15.2 10.3 (96.1%) 11.3 (95.3%) 11.5 (62.2%) 13.8 (25.9%) 9.3 (98.6%) 11.2 (94.9%)

(99.6%) (99.2%) (98.9%) (98.3%) (100%) (100%)

(100%) (99.7%) (99.2%) (87.6%) (100%) (100%)

Permeability (L m−2 h−1 bar−1). Rejection. c The pure water permeability was tested in a dead-end filtration cell using DI water at pressure of 4 bar, 25 °C. d Membrane were tested in a cross-flow filtration with salt concentration of 1000 mg L−1 at 6 bar, 25 °C. e Membranes were tested in a dead-end filtration cell using dye/water solution with dye concentration of 100 mg L−1, at 4 bar, 600 rpm, 25 °C. a

b

permeability by 29.9% was observed, resulting in a water permeability of 14.4 L m−2 h−1 bar−1 (Table 2). This permeability was competitive with the commercial NF270 membrane (15.2 L m−2 h−1 bar−1) and outperformed the NF90 membrane (8.3 L m−2 h−1 bar−1). The surface morphology was further characterized with SEM to observe the changes after long term filtration (Fig. 8c). It was found that the membrane was still defect-free and had a nodulous structure, which is consistent with prior observations. The molecular weight cut-off (MWCO) of the ANF TFC membrane was further examined using neutral polyethylene glycols (PEGs) as marker with different molecular weights (Fig. 8d). The as-prepared ANF TFC membrane showed a MWCO of around 329 Da. Fig. 9 Shows the effect of solvent treatment time on the desalination performance of ANF membranes. The membranes were tested in a saline water with a single salt. It can be seen in Fig. 9 (a-d) that the membrane without DMF treatment had a lower water permeability of 1.1–1.4 L m−2 h−1 bar−1, but a higher rejection for ions, with a rejection of 99.3% for Na2SO4, 99.1% for MgSO4, 97.9% for MgCl2 and

3.2. Filtration performances of ANF TFC membrane The mechanical strength of the fabricated ANF hydrogel NF membrane was examined in a wide range of trans-membrane pressures, ranging from 2 to 15 bar. Fig. 8a shows the water flux as a function of pressure, and indicates that the membrane can withstand high pressures, but the linear relation levels off at pressures above 10 bar. Furthermore, a long-term filtration test was conducted in pure water for 10 h at a transmembrane pressure of 4 bar (Fig. 8b). A decrease of water

Fig. 8. (a) Pure water flux against pressure; (b) Water permeability against operation time. (Membranes were treated in DMF for 0.5 h and tested in a dead-end filtration at 4 bar); (c) Membrane surface morphologies after long-term filtration. (d) The rejection of PEGs with different molecular weights through ANF TFC membrane.

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Fig. 9. Separation performances of the prepared ANF TFC membrane after different durations of DMF treatment (0, 0.25, 0.5, 1, 3 h) using 1000 mg L−1 of Na2SO4 (a), MgSO4 (b), MgCl2 (c) and NaCl (d), respectively in water. (Membranes were pressurized at 6 bar for 30 min and tested at 4 bar, 600 rpm, 25 °C in a dead-end filtration cell).

methylene blue (MB, 319.86 g mol−1), and a negatively charged dye, methyl orange (MO, 327.33 g mol−1), were chosen as the solutes. As shown in Table 2, the membrane had a higher water flux in separation of MO (8.4 L m−2 h−1 bar−1) than in separation of MB (7.2 L m−2 h−1 bar−1). However, the rejections for MB were almost 100%, which was much higher than for MO (98.6%). This is mainly ascribed to the interactions between the negatively charged membrane surface and the positively charged solutes, which lead to membrane fouling and therefore, a lower water permeability and a higher rejection. The photographs in Fig. 10 (a, b) show the perfect permeate and the highly concentrated retentate of MB and MO. The chemical structure, the valence charge and the molecular weight of the solutes are illustrated in Fig. 10 (c, d). The membrane clearly has a superior selectivity for small molecules with a size below 0.48 nm. The molecular separation performance of the ANF TFC membrane was found to have a better water flux compared with the NF90 membrane, with similar rejection (see Table 2). The separation of organic molecules from aqueous solution is relevant in a number of industrial processes, in the chemical industry, food, textiles, metal finishing, pulp and paper, pharmaceutical and biotechnology applications, and in purification of wastewater containing trace organic compounds with concentration level down to ng/ L- μg/L [14,16,21]. In addition, the ANF TFC membrane also showed excellent molecular separation performances in organic solvent (Fig. S3 a). The membrane had a rejection of 98.9% for rose Bengal (RB, 1017 g mol−1), 99.4% for eosin Y (EY, 648 g mol−1), 99.2% for Sudan black B (SBB, 457 g mol−1), 99.2% for methyl orange (MO, 327 g mol−1) and 64.5% for Disperse orange 3 (DO3, g mol−1) in ethanol. Besides, the membrane also exhibited excellent solvent resistance in methanol, acetone, DMF and NMP (Fig. S3 b). The membrane with 0.5 h of DMF treatment showed a high permeance of 12.3 L m−2 h−1 bar−1 for methanol, 13.7 L m−2 h−1 bar−1 for acetone, 6.2 L m−2 h−1 bar−1 for DMF and 2.2 L m−2 h−1 bar−1 for NMP with nearly complete rejections of RB molecules. The outstanding organic solvent nanofiltration performances demonstrated that the ANF TFC had great potentials in organic compounds separation in organic media. The stability of the ANF TFC NF membranes was analyzed in a crossflow nanofiltration device using saline water with concentrations of 1000 mg L−1. As shown in Fig. 11, after 10 h filtration, a steady water flux of 47.2 L m−2 h−1 (Na2SO4), 62.9 L m−2 h−1 (MgSO4), 52.4 L m−2 h−1 (MgCl2), 44.5 L m−2 h−1 (NaCl) was observed. The

98.3% for NaCl. Thus, the prepared ANF TFC membrane without DMF treatment is a typical RO membrane. However, the water permeance was elevated by one order of magnitude by immersing the membrane in DMF for 0.25 h (with the largest effect obtained with 0.5 h treatment). The membrane had a water permeability of 15.0 L m−2 h−1 bar−1 for Na2SO4, 16.6 L m−2 h−1 bar−1 for MgSO4, 12.4 L m−2 h−1 bar−1 for MgCl2, 14.0 L m−2 h−1 bar−1 for NaCl. However, a further prolongation of the activation time had little effect on the water flux. The membranes showed a decreasing trend for salt rejections, which remained stable after 0.5 h of solvent treatment. A slight decrease of the rejection of Na2SO4 and MgSO4 was observed, but these were still over 98.0%. The rejection of MgCl2 and NaCl was greatly affected by solvent activation, and dropped significantly at 0.25 h of DMF treatment. This indicates that the pore structure of the membrane was altered relatively fast after contact of the membrane with the solvent [38]. It was assumed that the DMF opened up the tortuous polymeric channels in the PA selective layer by dissolving the non-selective oligomers and unreacted monomers, giving rise to a considerable increase of water flux. Those dredged pores are more sensitive to monovalent ions rather than divalent ions due to the smaller hydrated radius of monovalent ions. As shown in Table S1, Mg2+ and SO42− have a hydrated radius of 4.28 Å and 3.79 Å, respectively; these radii are larger than those of Na+ and Cl−, which are 3.58 Å, 3.32 Å, respectively. These differences also appear by comparing the Stokes radii of the salt ions. Based on the PEG rejection experiment, the pore size and size distribution of the ANF TFC membrane were determined from the probability density function curve (Fig. S2) [42]. The ANF TFC membrane had a mean effective pore size (μp) of 0.492 nm (pore radius: 2.46 Å) and a geometrical standard deviation (σp) of 1.639. The pore radius of the membrane is found smaller than the radius of the hydrated salts ions (Mg2+, 4.28 Å; SO42−, 3.79 Å; Na+, 3.58 Å; Cl−, 3.32 Å) (Table S1). This indicated that size-exclusion played a dominate role in water desalination of the MPD-based NF membrane. The high mono-/di-valent ions rejection as well as the excellent molecular separation of the ANF TFC membrane are mainly originated from the size sieving effect. However, the membrane showed a different rejection behavior for different salt ions, it implies that the electrostatic interactions may also played a role in the water desalination. The separation performance of the ANF membrane for small molecules in aqueous solution was investigated. A positively charged dye, 8

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Fig. 10. The absorbance of feed (F), permeate (P) and retentate (R) solution of MB/water (a) and MO/ water (b); The molecular properties (size, charge, molecular weight) of methylene blue (c) and methyl orange (d). (Membranes were tested in a dead-end filtration cell using dye/water solution with dye concentration of 100 mg L−1, at 4 bar, 600 rpm, 25 °C). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

membrane compaction taking place in the first few hours leads to an increased salt rejection. In steady state, the membrane had a complete rejection for Na2SO4 (100%) (Figs. 11a) and 99.4% rejection for MgSO4 (Fig. 11b). Particularly, the membrane has a high rejection for MgCl2 and NaCl, with rejections of 92.7% and 82.3%, respectively (Fig. 11 (c, d)). Overall, the salt rejection followed the sequence Na2SO4 > MgSO4 > MgCl2 > NaCl, which is the same as for conventional PIP-based NF membranes with negatively charged surface [29,30]. One hand-cast control PMIA TFC membrane and two typical commercial membranes (NF90, NF270) were tested in the same conditions; the desalination performances were compared in Table 2. Poly (m-phenylene isophthalamide) (PMIA) as an aramid fibrous material was used to prepare the substrate membrane for the TFC membrane. The surface of the PMIA substrate showed a porous structure (Fig. S4 (a, b)). The PMIA TFC membrane was successfully prepared through interfacial polymerization between MPD and TMC (Fig. S4 (c, d)). However, the conventional polymeric substrate could not undergo the DMF

post-treatment due to the poor resistant to harsh organic solvent. Therefore, the hand-cast PMIA TFC membrane showed a typical RO desalination performance with high monovalent and divalent salt ions selectivity (> 98%) and low water permeability (1.6 L m−2 h−1 bar−1) (Table 2). The ANF TFC membrane has almost one order of magnitude higher of the water permeability than that of the PMIA TFC membrane. It was found that the PIP-based NF270 membrane has a higher water flux (13.8 L m−2 h−1 bar−1), but a lower monovalent salt rejection (RNaCl = 25.9%). In contrast, the MPD-based NF90 membrane has a superior salt rejection (RNaCl = 87.6%), but an inferior lower water permeance (6.2 L m−2 h−1 bar−1). The balance between monovalent rejection and water permeability was optimized for the ANF membrane, which combines a high water flux and a high NaCl rejection. Finally, the desalination performance of the ANF TFC membrane was compared with other recently reported high performance NF membranes and commercial NF membranes in terms of NaCl rejection and water permeability, as shown in Fig. 12. The commercial TFC NF

Fig. 11. Stability test of the prepared ANF TFC membrane with 0.5 h of DMF activation using Na2SO4 (a), MgSO4 (b), MgCl2 (c) and NaCl (d) as the solutes, in water with concentration of 1000 mg L−1 (Membrane tested at 6 bar in a cross-flow filtration at 25 °C). 9

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to the kind help from Chritine Wouters for the FT-IR measurement and Alexander Volodine for the AFM meausurement from KU Leuven. The authors are also thanks to Hongyun Ren, Shishi Yang, Huali Tian from the Institute of Urban Environment, Chinese Academy of Sciences for their help in sample preparation and TEM characterization. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.memsci.2019.117396. References Fig. 12. The NaCl rejection versus water permeability for ANF TFC NF membrane. PA NF membranes prepared from interfacial polymerization using different substrates: polyacrylonitrile (PAN) [29], polysulfone (PSf) [21,22,24,25], polyethersulfone (PES) [26,27,30,45,46] are compared. Typical nanofiltration data of commercial membranes reported in literature are included [29]. Data of NF270 membrane and NF90 membrane were tested in this work.

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membranes in the purple circle area have a low water permeability (< 2 L m−2 h−1 bar−1) and a moderate NaCl rejection (40–60%). The TFC membranes in the yellow circle area are all PIP-based NF membranes with a conventional polymeric substrate, such as PSf, PES, PAN. Many studies considered the introduction of nanomaterials to prepare an extremely thin film or a rough membrane surface to obtain a higher water permeance [29,30]. However, based on these data we could found that there is a “trade-off” relationship between permeability and selectivity for PIP-based NF membranes. To obtain a higher water permeance, sacrificing the rejection of monovalent ions is unavoidable. The MPD-based NF membrane fabricated in this work outperformed the PIP-based membrane in NaCl rejection and competitive to NF90 membrane, which has a similar salt rejection and a higher water permeance. 4. Conclusions A novel MPD-based TFC NF membrane was successfully developed by interfacial polymerization on a nanofibrous hydrogel substrate followed by solvent treatment. The DMF solvent treatment tailored the membrane surface morphology by removing the top “leaf-like” PA layers and decreased the membrane surface roughness. Besides, the membrane surface hydrophilicity improved significantly and the surface zeta potential reduced when exposure to the DMF. The desalination performance of the ANF TFC membrane transferred from RO into NF. The ANF TFC membrane had a PWP of 14.4 m−2 h−1 bar−1, which is one order of magnitude higher than that of the hand-cast PMIA TFC membrane. NF experiments demonstrated that the ANF TFC membrane had excellent rejections of divalent ions (RNa2SO4 = 100%, RMgSO4 = 99.4%, RMgCl2 = 92.7%) and a high rejection of monovalent ions (RNaCl = 80.3%). The size exclusion was found to be the main mechanism for the separation of salt ions by examining the PEGs rejection. Compared with commercial membranes (NF270, NF90), the synthesized membrane showed a competitive separation performance in water desalination and molecular separation. The ANF TFC membrane also out-performed the recently reported high performance PIPbased NF membranes in terms of NaCl rejection. The newly designed ANF TFC nanofiltration membrane has great potential in scale-up for industrial application due to the facile fabrication process and the reproducible and good desalination performance. Acknowledgments Yi Li acknowledges the support provided by the China Scholarship Council of the Ministry of Education. The authors are gratefully thanks 10

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