Nanofiltration membranes with hydrophobic microfiltration substrates for robust structure stability and high water permeation flux

Journal of Membrane Science 593 (2020) 117444

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Nanofiltration membranes with hydrophobic microfiltration substrates for robust structure stability and high water permeation flux

T

Xi Zhanga,b, Chang Liua,b, Jing Yanga,b,∗∗, Cheng-Ye Zhua,b, Lin Zhangc, Zhi-Kang Xua,b,∗ a

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Hangzhou, 310027, China Key Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, 310027, China c Key Laboratory of Biomass Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, 310027, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Nanofiltration membrane Microfiltration substrate Surface wettability Water permeation flux Structure stability

Traditional nanofiltration membranes (NFMs) suffer from ultrafiltration substrates with low porosity, small pore size and relatively poor solvent stability. Herein, NFMs have been fabricated on a series of hydrophobic polymer microfiltration substrates to address these issues. Polyphenol-based coatings of tannic acid/diethylenetriamine (TA/DETA) were co-deposited on the hydrophobic substrates to improve their surface wettability and to make them appropriate for interfacial polymerization. The spreading behaviors of aqueous solutions, which are of significant importance to the formation of defect-free polyamide layers, were directly visualized by laser confocal microscopy. The influences of TA/DETA coatings on the interfacial polymerization were further demonstrated by both dynamic molecular simulation and nanofiltration performance evaluation. The as-prepared NFMs exhibit higher water permeation flux compared with traditional ones because of the large pore size and high porosity of the microfiltration substrates, as well as the relatively low cross-linking degree of polyamide layers. Internal stress during the nanofiltration process was calculated by the theory of thin plates and the results claim good pressure resistance for these NFMs. Therefore, the as-prepared NFMs can be steadily used under high operation pressures even up to 0.9 MPa, which are in accordance with the theoretical calculation. Furthermore, these NFMs also present good solvent resistance since the chemical stability of the no-polar hydrophobic substrates.

1. Introduction Nanofiltration membranes (NFMs) have drawn growing attention over the past years due to their high water permeation flux, low operation pressure and high rejection to multivalent ions [1–3]. They have been widely used in waste-water treatment, biological engineering, medicine/food industry, and seawater desalination [4–7]. Currently, these NFMs usually consist of a polyamide selective layer and a porous support substrate [8–11]. Interfacial polymerization is one successful method to fabricate NFMs with such structures [10]. Most of the commercialized NFMs use polysulfone (PSF), polyethersulfone (PES) or polyacrylonitrile (PAN) ultrafiltration membranes as porous substrates due to their proper surface hydrophilicity for carrying out the interfacial polymerization [12–14]. However, these substrates exhibit low chemical resistance to ketones, esters and alcohols, limiting the structure stability of the NFMs [15–17]. Moreover, It has been reported that NFMs can obtain improved water permeability by increasing the pore ∗

size and porosity of the porous substrates because of the reduced transmembrane resistance and shortened water permeation pathway [18–20]. Some commercialized non-polar polymer membranes, such as polypropylene, polyethylene and poly(vinylidene fluoride) microfiltration membranes (PPMM, PEMM and PVDFMM), thereby emerge as satisfactory candidates for support substrates of NFMs due to their advantages of high solvent resistance, good mechanical strength, low cost for raw materials, large pore size and high porosity [21,22]. However, it is difficult to directly fabricate a polyamide selective layer on these microfiltration substrates by interfacial polymerization because aqueous solutions cannot be well spread on them due to their poor surface wettability [23]. Improving the surface hydrophilicity of these hydrophobic substrates is required for carrying out the interfacial polymerization. There are many available methods to hydrophilize the surfaces of hydrophobic membranes, such as UV, plasma, or ozone-induced grafting polymerization and chemical treatment [24–31]. For example, PPMMs were hydrophilized by UV-induced grafting or

Corresponding author. MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Hangzhou, 310027, China. Corresponding author. MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Hangzhou, 310027, China. E-mail addresses: [email protected] (J. Yang), [email protected] (Z.-K. Xu).

∗∗

https://doi.org/10.1016/j.memsci.2019.117444 Received 2 May 2019; Received in revised form 29 July 2019; Accepted 4 September 2019 Available online 05 September 2019 0376-7388/ © 2019 Published by Elsevier B.V.

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oxidation with chromic acid solution and then they were used as substrates for NFMs [23,28,29]. PPMMs and PVDFMMs could also be hydrophilized by plasma treatment for the fabrication of NFMs [30,31]. However, these methods usually have complex operation process and high energy cost. The substrate structure may even be destroyed and pore blockage will occur, resulting in the increased trans-membrane resistance and reduced water permeation flux [22,23,29,30]. Another disadvantage is the lack of university, which means one can only hydrophilize one kind of hydrophobic substrate with a specific condition, limiting the selection of substrates for NFMs. Therefore, it is still a challenge to fabricate NFMs on hydrophobic microfiltration substrates with desirable nanofiltration performances. Polyphenols have strong solid-liquid interface activities and can form coatings on various polymer materials [32–35]. Recently, we have demonstrated a novel type of polyphenol coating by simple co-deposition of tannic acid (TA) and diethylenetriamine (DETA), which merits the advantage of simplicity, versatility and university [32,34,36]. It should be noticed that such coating is particle-free and uniform, thus exhibits a negligible influence on the surface structures of microfiltration substrates.34 Herein, we used the TA/DETA coatings to hydrophilize PPMM, PEMM and PVDFMM substrates according to our previous work [34], making them proper substrates for directly carrying out the interfacial polymerization (schematically shown in Fig. 1). The interfacial polymerization was then conducted on these microfiltration substrates to prepare NFMs. Laser confocal microscopy (LSCM) was used to visualize the spreading behaviour of aqueous solution on the studied substrates. Water can well spread on these TA/DETA co-deposited substrates while it shows a de-wetting phenomenon on the nascent ones, corresponding to whether the interfacial polymerization can be carried out or not. The resulting NFMs show improved water permeation flux compared with traditional ones with ultrafiltration substrates while maintaining a high rejection to divalent ions (> 95% for Na2SO4). Internal stress on the polyamide selective layer was calculated to confirm the good pressure resistance of such NFMs. On the other hand, our NFMs show stable nanofiltration performances against ethanol and acetone treatment, demonstrating high solvent resistance which is profited from the chemical stability of the hydrophobic substrates.

Table 1 Average pore diameter, porosity and water contact angle for three kinds of hydrophobic microfiltration membranes used in this study. Substrate

Average pore diameter (nm)

Porosity (%)

Water contact angle (°)

PPMM PEMM PVDFMM

338.3 143.6 307.8

81.14 52.29 61.81

144.25 116.56 126.65

phenylenediamine (MPD, 99%), tannic acid (TA, Analytical Reagent) and fluorescein sodium (Analytical Reagent) were bought from Aladdin Chemistry Co. Ltd. (China). Acetone, ethanol, hexane, sodium chloride (NaCl), sodium sulfate (Na2SO4), magnesium chloride (MgCl2), magnesium sulfate (MgSO4), hydrogen chloride (HCl) and sodium hydroxide (NaOH) were all analytical reagents and obtained from Sinopharm Chemical Reagent Co. Ltd. (China). All chemicals were used as received without further purification. Bicine buffer (pH = 7.8) was prepared from bicine and NaOH as reported in our previous work [34]. Ultrapure water (18.2 MΩ) was produced by an ELGA Lab Water System (France). 2.2. Dynamic molecular simulation Materials Studio 2017 R2 was used to carry out dynamic molecular simulation. Fig. 2 shows the molecular structures of PIP, MPD, PP and TA/DETA coating. The Forcite module with a task of geometry optimization was used to optimize all the structures applied in the dynamic molecular simulation. Especially, PP molecule with 20 repeat units was used to simplify the simulation. Mixing energies of PIP-PP, PIP-TA/ DETA, MPD-PP and MPD-TA/DETA were calculated by the Blends module. 2.3. Preparation of NFMs The hydrophobic substrates were co-deposited by TA/DETA coating for hydrophilization. Briefly, TA was dissolved in bicine buffer to prepare a solution with a concentration of 2 g/L. Then DETA was added into the freshly prepared TA solution with a TA/DETA mass ratio of 1/ 10. Substrates were pre-wetted in ethanol for 10 min and then quickly immersed into the freshly prepared TA/DETA solution for co-deposition. The substrates were shaken for desired time at 30 °C and then washed by ultrapure water three times. Subsequently, the TA/DETA codeposited substrates were dried in a vacuum oven overnight to a constant weight. Interfacial polymerization was carried out on the TA/DETA co-deposited substrates to prepare NFMs. First, the aqueous solution of PIP or MPD and the hexane solution of TMC were prepared with a concentration of 2.0 g/L and 1.0 g/L, respectively. The substrates were held in a module (Fig. S1 in Supporting Information) and the upper side of substrates was immersed in aqueous solution for the interfacial polymerization. The aqueous solution was poured off after immersion for 5 min and the residual solution on the substrate surface was subsequently drained off in air. The aqueous solution saturated substrate was then immersed in the hexane solution of TMC for 2 min to form a polyamide selective layer via interfacial polymerization. After removing the excess hexane solution by air-drying, the as-prepared NFM was placed in an oven at 60 °C for 30 min to stabilize the membrane structure. The as-prepared NFM was then washed by ultrapure water

2. Experimental 2.1. Materials PPMM, PEMM and PVDFMM were obtained from Membrana GmbH (Germany), Hebei Jinli Plastic Materials Factory (China) and Haining Chuangwei Filter Equipment Factory (China), respectively. Typical properties, including average pore diameter, porosity, as well as surface wettability, are listed in Table 1 for all these hydrophobic microfiltration substrates. The samples were cut into round pieces with a diameter of 4 cm. Then, the substrates were washed by acetone to remove adsorbed impurities and dried in a vacuum oven overnight to a constant weight before use. Polyethersulfone microfiltration membrane (PESMM) was bought from Haiyan Xindongfang plastic Co. Ltd. (China) and used as received. Trimesoyl chloride (TMC, > 99%) was purchased from Qingdao Sanlibennuo Co. Ltd. (China). Piperazidine (PIP, 99%), N,N-bis(2-hydroxyethyl) glycine (bicine, > 99.5%), N-(2-aminoethyl)1,2-ethylenediamine (diethylenetriamine, DETA, Chemically Pure), m-

Fig. 1. Schematic illustration of the preparation process of NFMs with hydrophobic microfiltration substrates. 2

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Fig. 2. Molecular structures of PIP, MPD, PP and TA/DETA coating.

7.07 cm2. All the samples were pre-compacted under 0.7 MPa for 30 min before the evaluation. Both the water and organic solvents permeation flux (F, L/m2⋅h) were calculated by equation (2):

for three times and stored in ultrapure water for further characterizations and measurements. 2.4. Characterization

F=

The pore size and porosity of microfiltration substrates were measured using a mercury porosimetry (AutoPore IV 9510, USA). The surface wettability of all samples was characterized by measuring the water contact angle with a drop Meter A-200 contact angle system (MAIST Vision Insection & Measurement Co. Ltd. China). A droplet of 2 μL water was used as the probe in these measurements. The water spreading behavior on the substrate surface was detected by a laser confocal microscopy (LSCM, LSM780, ZEISS, Germany). The substrate was immersed in an aqueous solution of fluorescein sodium with a concentration of 5 μg/mL for 5 min. It was taken out and the excess aqueous solution on the surface was drained off. Then the substrate was characterized using the LSCM and the excitation wavelength is 442 nm. The surface and cross-sectional morphologies of membranes were observed by a field emission scanning electron microscopy (FE-SEM, Hitachi, S4800, Japan) and a transmission electron microscopy (TEM, Hitachi, H-7650, Japan). A UV–vis spectroscope (Shimadzu, Japan) was applied to record the UV–vis spectra of MPD solution. The substrate was first immersed in an MPD solution with a concentration of 2 g/L for 5 min, and the excess solution was drained off from the out surface of substrate. Then the substrate was immersed in a sample of 5 mL hexane for 5 min. The adsorption of MPD in hexane was simultaneously determined. The surface chemistries were analyzed by a Fourier transform infrared spectroscope (FT-IR/ATR, Nicolet 6700, USA) equipped with an attenuated total reflectance accessory (ZnSe crystal, 45o). The surface charge properties were measured by an electro kinetic analyzer (SurPASS Anton Paar, GmbH, Austria) using a streaming potential method with KCl (1 mmol/L) as electrolyte solution. The pH of electrolyte solution was adjusted by HCl and NaOH solutions to investigate the influence of pH on the surface zeta potential. XPS spectra were collected with an X-ray photoelectron spectroscope (XPS, PerkinElmer, USA) using Al Kα excitation radiation (1486.6 eV) at a detected depth less than 10 nm. The cross-linking degree (D) of the polyamide layer is calculated by equation (1):

D=

4 − 2RO / N 1 + RO / N

Q A×t

(2)

where Q, A and t represent for the filtrated solvents volume, the filtration area and the filtration time, respectively. Four kinds of salts (Na2SO4, MgSO4, MgCl2 and NaCl) were used to analyze the rejection performance with a concentration of 2 g/L and a fixed cross-flow rate of 30 L/h. And the salt rejection (R) was calculated by equation (3):

R = (1 −

Cp Cf

) × 100%

(3)

where Cp is the concentration of permeation solution and Cf is the concentration of feed solution. The concentration was proportional to the conductivity, which was detected by an electrical conductivity meter (FE30, Mettler Toledo, China). The nanofiltration performance was measured under different operating pressures and cross-flow rates. The structure stability of NFMs against organic solvents is examined in two aspects. First, NFMs were immersed in ethanol or acetone for 24 h. After washing by ultrapure water for three times to remove the excess solvent, the nanofiltration performance of NFMs was measured. The organic solvent permeation flux of NFMs was characterized for 12 h to further analyze their structure stability against organic solvents. The pH stability of NFMs was examined by evaluating the nanofiltration performance after immersing them in aqueous solutions with different pH values for 24 h. 3. Results and discussion 3.1. Hydrophilization of substrates and fabrication of NFMs The hydrophilic coating of TA/DETA can be easily formed by the cross-linking of TA and DETA via Michael addition reaction [34,37]. This coating is able to adhere onto various materials via hydrogen bonding, electrostatic adsorption and hydrophobic interaction [33,38]. Therefore, we can simply adjust the surface wettability of the hydrophobic substrates by the co-deposition of TA/DETA. Fig. 3(a–c) presents dynamic water contact angles for PPMM, PEMM and PVDFMM substrates with different co-deposition times, respectively. PPMM shows a water contact angle around 90° when the co-deposition time is 10 min. There is no decrease tendency along with the measurement time, which means the water droplet cannot spread well on this substrate. Furthermore, the water contact angle decreases with increasing the codeposition time, indicating an enhanced hydrophilicity of the substrate surface (Fig. 3(a)). Co-deposition of TA/DETA on PEMM endows the substrate with nearly the same trend as PPMM. Fig. 3(b) shows that the water contact angle decreases to lower than 90° and the water droplet

(1)

where RO/N is the molar ratios of oxygen to nitrogen determined by XPS spectra. 2.5. Nanofiltration performance measurement A laboratory scale cross-flow flat membrane module was used to evaluate the nanofiltration performance of the prepared NFMs. Each sample was measured under 0.6 MPa at 30 °C with a filtration area of 3

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Fig. 3. Dynamic water contact angle of (a) PPMM, (b) PEMM and (c) PVDFMM substrates with different co-deposition times; (d) Water contact angles for different hydrophobic microfiltration substrates before and after co-deposition of TA/DETA for 60 min; (e) 3-Dimensional LSCM images of different microfiltration substrates after wetted by the aqueous solution of fluorescein sodium; (f) Schematic diagrams of the microfiltration substrates after wetted by aqueous solution.

10–15 μm (Measured by 2D LSCM images with different focal planes, Fig. S2 in Supporting Information), which is significant for carrying out the interfacial polymerization on them (Fig. 3(f)). The TA/DETA coatings change the color of the substrates slightly (Fig. S3 in Supporting Information) but maintains their surface morphologies (Fig. S4 in Supporting Information). Moreover, the pore size and porosity of the substrates undergo negligible variations before and after the co-deposition of TA/DETA (Figs. S5 and S6 in Supporting Information). These features are beneficial to avoid the undesired pore blockage which is believed to be detrimental to NFMs with low transmembrane resistance [40]. The TA/DETA coatings are stable at acid or alkaline conditions, which is beneficial for the structure stability of NFMs (Fig. S7 in Supporting Information). In our case, the prepared NFMs are assigned as PP-NFM, PE-NFM and PVDF-NFM, respectively (Co-deposition time is 60 min in the following parts if there is no additional illustration). The monomer concentrations were fixed as 2 g/L and 1 g/L for PIP and TMC according to our optimization experiments (Figs. S8 and S9 in Supporting Information). Fig. 4 shows typical SEM and TEM images for the surface morphology, the cross-sectional structure of the polyamide selective layer for PP-NFM, PE-NFM and PVDF-NFM. The NFMs show a dense and poreless surface compared with original TA/DETA co-deposited substrates (Fig. S6 in Supporting Information) due to the formation of polyamide selective layers. It is worth noting that the dense and poreless polyamide layer cannot be formed on the nascent hydrophobic substrates. The thickness is around

can spread well on the substrate surface when the co-deposition time is above 10 min. However, the surface wettability of PVDFMM can be improved slightly compared with PPMM and PEMM. The TA/DETA coatings are adhered on the hydrophobic substrates due to the hydrophobic interactions between them [34]. But PVDF has a higher polarity compared with PP and PE, making PVDFMM relatively hard to be codeposited with TA/DETA [39]. Nevertheless, when the co-deposition time is 60 min, the PVDFMM substrate shows the water contact angle lower than 90° during all the measuring time (Fig. 3(c)). All these results indicate that the surface wettability can be promoted from the water contact angle of 144°, 117° and 127°–26°, < 20° and 85° by 60 min co-deposition of TA/DETA on the hydrophobic PPMM, PEMM and PVDFMM substrates (Fig. 3(d)), respectively. The surface wettability has great impact on the spreading behaviors of aqueous solution on the substrates, which are important for carrying out the interfacial polymerization. LSCM was used to visualize the spreading behavior of an aqueous solution of fluorescein sodium on the substrates. Fig. 3(e) presents 3D LSCM images from the nascent and TA/ DETA co-deposited substrates. It can be seen that there is no fluorescence from the nascent substrates, indicating the aqueous solution cannot well spread on the hydrophobic surfaces. On the other hand, the TA/DETA co-deposited PPMM, PEMM and PVDFMM substrates are able to emit strong green fluorescence from their surface, demonstrating homogeneous spreading of the aqueous solution on the hydrophilized surfaces. These hydrophilized substrates can hold an aqueous layer of 4

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Fig. 4. SEM images for (a–c) the surface and (d–f) the cross-sectional morphologies of PP-NFM, PE-NFM and PVDF-NFM, respectively; TEM images for the polyamide selective layer fabricated on co-deposited (g) PPMM, (h) PEMM and (i) PVDFMM substrates, respectively.

different surface wettability will influence the interfacial polymerization and then the nanofiltration performances. Fig. 5(a) shows the nanofiltration performances of PP-NFMs based on substrates with different co-deposition times, which is in accordance with the surface wettability. Polyamide selective layers cannot be fabricated when the co-deposition time is less than 10 min for PPMM substrates (We signed as N/A in the figures) due to the poor surface wettability. The Na2SO4

38 nm, 36 nm and 42 nm for these selective layers of PP-NFM, PE-NFM and PVDF-NFM (Fig. 4(g–i)), respectively. Moreover, the selective layers exhibit typical chemical structures of semi-aromatic polyamide films (Fig. S10 in Supporting Information). The PP-NFM, PE-NFM and PVDF-NFM were evaluated for their nanofiltration performances, including water permeation flux and salt rejection. One can envisage that the microfiltration substrates with

Fig. 5. Nanofiltration performances of (a) PP-NFM, (b) PE-NFM and (c) PVDF-NFM on substrates with different co-deposition times (The operation pressure is 0.6 MPa); (d) UV–vis spectra of MPD in hexane after diffusing from co-deposited PPMM substrates with different co-deposition times; (e) Mixing energies between different di-amine monomers and substrates calculated by dynamic molecular simulation. 5

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rejection reaches above 95% and maintains a stable value when the codeposition time is longer than 20 min, demonstrating the formation of a defect-free polyamide selective layer. Moreover, the water permeation flux increases along with the increased co-deposition time. For PE-NFM, the Na2SO4 rejection reaches maximum value of above 95% when the co-deposition time is 15 min while the water permeation flux increases with co-deposition time after that (Fig. 5(b)). However, Fig. 5(c) indicates that PVDF-NFM with a Na2SO4 rejection around 95% can only be obtained when the co-deposition time is 60 min, which can be assigned to the much poorer surface wettability of the co-deposited PVDFMM substrate compared with PPMM and PEMM. The pore size and porosity of substrates also influence the final nanofiltration performances of NFMs. As shown in Fig. 5(a)–(c), the water permeation flux has the order of PP-NFM > PVDF-NFM > PE-NFM when co-deposition time is 60 min. Given that the thicknesses of polyamide selective layers are almost the same for these three NFMs (Fig. 4(g)–(i)), the gradually increased water permeation flux from PE-NFM, PVDFNFM to PP-NFM can be rationalized to the increased pore size and porosity of the substrates (Figs. S4, S5 and S6), which cause the reduced trans-membrane resistance and shortened water pathway [19,20]. PP-NFM was used as a model to further analyze the influences of substrates on the polyamide selective layers. The thickness of the selective layers shows no obvious difference for PP-NFMs based on the TA/DETA coated substrates with different co-deposition times (Fig. S11 in Supporting Information). In the meanwhile, the cross-linking degree of the polyamide selective layers decreases from 0.44 to 0.13 with the increased co-deposition time (Fig. S12 and Table S1 in Supporting Information). Therefore, the increased water permeation flux can be rationalized to the decreased cross-linking degree, which is along with the increased co-deposition time of TA/DETA. We suggest the multiple and strong hydrogen bonding between the TA/DETA coatings and the diamine monomers hinder the diffusion of diamine monomers from the aqueous phase to the organic phase. And the increased co-deposition time leads to increasing the thickness of TA/DETA coatings on the substrates, which slows the diffusion of diamine monomers and then reduces the cross-linking degree of the polyamide selective layers. The diffusion of diamine monomers was detected by UV–vis spectra to prove our speculation. MPD was used as the typical diamine monomer because PIP shows very low UV absorption. Fig. 5(d) demonstrates the diffusion of MPD becomes slow with the increased codeposition time of TA/DETA. It should be noted that the same tendencies for water permeation flux, selective layer thickness and crosslinking degree were observed for PP-NFMs with either PIP or MPD based polyamide selective layers (Figs. S12–S14 and Table S1 in Supporting Information). Furthermore, Fig. 5(e) presents the mixing energies between different diamine monomers and substrates. They are 55.71 kJ/mol, 56.31 kJ/mol, −197.72 kJ/mol and −341.38 kJ/mol for PIP-PP, MPD-PP, PIP-TA/DETA and MPD-TA/DETA, respectively. The results mean that the nascent PPMM substrate has the tendency to “repulse” diamine monomers. Meanwhile, TA/DETA coating has mixing energies with diamine monomers below 0, hindering the diffusion of diamine monomers. Notably, the absolute value of mixing energy of MPD-TA/DETA is larger than PIP-TA/DETA, which means there are strong interactions between MPD and TA/DETA coating. These results match well with the decreasing tendency of cross-linking degrees for MPD and PIP based polyamide selective layers (Table S1 in Supporting Information).

Fig. 6. Comparison among NFMs fabricated by interfacial polymerization with ultrafiltration membranes (UM) or microfiltration membranes (MM) as substrates. (The operation pressure is 0.6 MPa for our NFMs).

Na2SO4 rejection above 95%. Moreover, it has a salt rejection order of Na2SO4 > MgSO4 > MgCl2 > NaCl due to the negatively charged surface of the polyamide selective layer (Figs. S15 and S16 in Supporting Information). In addition to the decreased cross-linking degree of the polyamide selective layers mentioned above, the high water permeation flux can be ascribed to the large pores and high porosity of the microfiltration substrate than those ultrafiltration ones, which decrease the trans-membrane resistance and shorten the water pathway during nanofiltration process [19,20]. NFMs prepared with nanostructured materials as interlayers or sacrificial layers show a comparable or even higher water permeation flux compared to our PP-NFM [54–57]. However, these interlayers or sacrificial layers may impact the structure stability of NFMs and thus limit their practical applications. During the pressure-driven nanofiltration process, internal stress will occur in both the porous substrate and the selective layer of the NFMs. The internal stress in the selective layer is negligible for traditional NFMs due to the small pore size of the ultrafiltration substrates. However, this internal stress increases dramatically with enlarging the pore size of substrates, which may result in break of the selective layer. Therefore, it is necessary to take the effects of internal stress on the selective layer into account when microfiltration substrates are used for the preparation of NFMs. We calculated the internal stress of selective layers by the theory of thin plates [58]. As schematically shown in Fig. 7(a), we consider the selective layer as a round thin plate covering on a single round pore. It will have a micro deformation (M) under the applied pressure during the nanofiltration process, leading to internal stress in the selective layer. There are two extreme cases: simply supported structure and built-in supported structure. The maximum tensile stress for these two cases can be calculated by equations (4) and (5), respectively.

σs =

3(3 + μ) PR2 8t 2

(4)

σb =

3PR2 4t 2

(5)

where σs, σb represent the maximum internal stress of the simply supported and built-in supported structures, respectively, and P, t, R, μ are pressure applied on the membrane, thickness of the selective layer, average pore diameter of substrate and Poisson's ratio, respectively. In reality, the maximum tensile stress (σmax) of the selective layer is between σs and σb. In this work, the maximum applied pressure P is 0.9 MPa, μ is 0.3 and the average pore diameters of substrates are shown in Table 1. Thus, σmax of PP-NFM, PE-NFM and PVDF-NFM can be calculated and the values are shown in Fig. 7(a). These values are definitely much lower than the tensile strength of polyamide films (usually 70–100 MPa), indicating that the selective layers cannot be

3.2. Nanofiltration performance and structure stability of NFMs Fig. 6 shows a comparison of the nanofiltration performance among NFMs prepared under optimized conditions in this work and other ones reported in literatures [15–17,36,41–53]. Most of the traditional NFMs using ultrafiltration membranes as substrates and their water permeation flux are usually less than 15 L/m2 h bar. Our PP-NFM exhibits a water permeation flux up to 33 L/m2⋅h⋅bar and maintains a high 6

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Fig. 7. (a) Schematic diagrams for the calculation of the internal stress of the selective layer; nanofiltration performances of PP-NFM, PE-NFM and PVDF-NFM under different operation pressures (b–d) and different cross-flow rates (e–g), respectively. (Normalized water permeation flux (F/F0) and salt rejection (R/R0) were used, where F0 and R0 represent for the water permeation flux and salt rejection for NFMs measured with an operation pressure of 0.6 MPa and a cross-flow rate of 30 L/h.).

traditional NFMs, can be easily swollen in organic solvents. For the meanwhile, the polyamide selective layer can be hardly swollen in such conditions. These different swelling behaviors will cause the detaching of the selective layers from the porous substrates and deteriorates the nanofiltration performances of NFMs [16,17]. Typically, PPMM, PEMM and PVDFMM are all solvent-resistant substrates [29,31,59]. The solvent resistance of PP-NFM, PE-NFM and PVDF-NFM were assessed by immersing them in water, ethanol and acetone for 24 h and then measuring their nanofiltration performances. Fig. 8(a) demonstrates that PP-NFM, PE-NFM and PVDF-NFM show nearly the same water permeation flux and Na2SO4 rejection, indicating their good solvent resistance. For comparison, the water permeation flux increases while salt rejection decreases obviously after 24 h ethanol treatment for PESNFM (NFMs prepared on PESMM substrates), demonstrating the structure damage of such NFM. Moreover, PESMM was completely dissolved in acetone and the structure of PES-NFM is completely destroyed, making it can no longer be used in nanofiltration process. Moreover, PP-NFM, PE-NFM and PVDF-NFM also exhibit stable organic solvent permeation flux (ethanol and acetone) for 12 h operation (Fig. 8(b) and (c)), which further confirm their good solvent resistance and structure stability. Furthermore, these NFMs show stable water permeation flux and salt rejection after they are immersed in aqueous solution in the pH range of 3–11 for 24 h (Fig. S17 in Supporting

broken during the nanofiltration process even if the applied pressure is as high as 0.9 MPa. In practice, we measured the nanofiltration performances of PP-NFM, PE-NFM and PVDF-NFM under different applied pressures. Normalized water permeation flux and salt rejection are shown in Fig. 7(b)–(d). For all of our NFMs, the water permeation flux increases with the applied pressure while the Na2SO4 rejection maintains almost the same values throughout the whole measurements, demonstrating that the selective layer cannot be fractured with enhancing the operation pressure. Therefore, we can conclude that our NFMs with microfiltration substrates possess a sufficient structural robustness for the pressure-driven nanofiltration process. It should also be noted that the as-prepared NFMs have a good adhesion between polyamide layers and substrates for nanofiltration evaluation. As shown in Fig. 7(e)–(f), PP-NFM, PE-NFM and PVDF-NFM all maintain a stable nanofiltration performance with varying the cross-flow rate from 30 to 100 L/h (The cross-flow rate reported in literatures is usually 10–30 L/h for testing nanofiltration performances [36,55,57]), indicating that the polyamide layers cannot be peeled off from the substrate under such a wide range of the cross-flow rate during the tests and confirming a robust adhesion between the polyamide layers and substrates. The solvent resistance is equally important for NFMs in industrial applications. However, polysulfone and polyethersulfone ultrafiltration membranes, which are commonly used as the porous substrates in

7

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Fig. 8. (a) Nanofiltration performances of PP-NFM, PE-NFM, PVDF-NFM and PES-NFM after immersed by different solvents for 24 h; (Normalized water permeation flux (F/F0) and salt rejection (R/R0) were used, where F0 and R0 represent for the water permeation flux and salt rejection for NFMs immersed in water.) Normalized (b) ethanol and (c) acetone permeation flux for PP-NFM, PE-NFM and PVDF-NFM tested for 12 h (F0 represents for the permeation flux of NFMs measured when the operation time is 0 and the operation pressure is 0.6 MPa)

Information). It means that our NFMs own good stability in acid or alkaline solutions.

wettability can be improved by TA/DETA coatings for these substrates to carry out the interfacial polymerization. The microfiltration substrates have much larger pore size and higher porosity than the ultrafiltration ones, leading to reduced trans-membrane resistance and shortened water pathway. These properties greatly improve the water permeation flux of NFMs compared with traditional ones. The NFMs show high structure stability and can be used under a wide range of

4. Conclusions NFMs were prepared on three kinds of commercial hydrophobic microfiltration substrates by interfacial polymerization. The surface 8

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operation pressures with a stable nanofiltration performance. Thanks to the good chemical stability of the substrates, the as-prepared NFMs show a superior solvent resistance over traditional ones against organic solvents such as ethanol and acetone. Therefore, they are promising to be applied in practical applications because of their excellent nanofiltration performance and structure stability.

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[22]

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Acknowledgements [24]

This work is financially supported by the National Natural Science Foundation of China (Grant No. 21534009). We are grateful for the support of the Research Computing Center in College of Chemical and Biological Engineering at Zhejiang University for assistance with the dynamic molecular simulations carried out in this work.

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Appendix A. Supplementary data [27]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.memsci.2019.117444.

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