Preparation of positively charged composite nanofiltration membranes by quaternization crosslinking for precise molecular and ionic separations

Journal of Colloid and Interface Science 531 (2018) 168–180

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Regular Article

Preparation of positively charged composite nanofiltration membranes by quaternization crosslinking for precise molecular and ionic separations Chuanjie Fang a,b, Jian Sun a, Bin Zhang a, Yuchen Sun b, Liping Zhu a,⇑, Hideto Matsuyama b a b

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, PR China Center for Membrane and Film Technology, Department of Chemical Science and Engineering, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan

g r a p h i c a l a b s t r a c t Preparation route of positively charged PVI/PSF composite NF membranes.

a r t i c l e

i n f o

Article history: Received 25 April 2018 Revised 9 July 2018 Accepted 10 July 2018

Keywords: Nanofiltration membrane Poly(N-vinyl imidazole) Positive charge Molecular and ionic separation

⇑ Corresponding author. E-mail address: [email protected] (L. Zhu). https://doi.org/10.1016/j.jcis.2018.07.034 0021-9797/Ó 2018 Elsevier Inc. All rights reserved.

a b s t r a c t Developing nanofiltration (NF) membranes with highly efficient and precise separation ability is of great significance for the molecular and ionic separations. In this work, positively charged composite NF membranes were engineered via soaking of polysulfone (PSF) ultrafiltration membranes in poly(N-vinyl imidazole) (PVI) solutions followed by a quaternization crosslinking step. The PVI was firmly attached to the PSF membrane by this method and acted as an active separation layer of the composite NF membrane. The obtained composite NF membrane featured a high rejection (83%) to vitamin B12 (molecular radius: 0.74 nm) but a low rejection (24.6%) to vitamin B2 (molecular radius: 0.47 nm), exhibiting a great potential in precisely molecular separation. Furthermore, the ionic separation ability of the composite NF membrane was confirmed with a rejection order of Na2SO4 < MgSO4 < NaCl < MgCl2 and the MgCl2 rejection reached up to 90.1%. Compared to conventional polyamide NF membranes, the developed PVI/PSF composite NF membranes were characterized with high separation precision to organic molecules, higher rejection over cationic ions than over anionic ones, better chlorine resistance and stability in long-term operation. In addition, the membrane fabrication process is convenient and easily scaled up in industry. This work offers a novel alternative of NF membranes for high precision in molecular and ionic separations. Ó 2018 Elsevier Inc. All rights reserved.

C. Fang et al. / Journal of Colloid and Interface Science 531 (2018) 168–180

169

Nomenclature A t V J R CF Cp PWL

lp

effective area of membrane (m2) filtration time (h) filtration volume (L) pure water flux (L m2 h1) rejection (%) solute concentration (mol/L, ppm) solute concentration (mol/L, ppm) pure water flux (L m2 h1) mean effective pore radius (nm)

1. Introduction Highly efficient and energy-saving molecular and ionic separations are widely required in water treatment, seawater desalination, chemical engineering, food and beverage purification, and so on [1–3]. Nanofiltration (NF) is a pressure-driven separation process based on a semipermeable membrane with a pore size of
1 nm [4]. NF can be used in both the molecular separation with molecular weight cut-off ranging from 150 to 2000 Da and the ionic separation with the removal of multivalent ions from liquids, due to the unique separation mechanism combining with size sieving and charge repulsion [5,6]. Since formally introduced by FILMTEC Corporation in the early 1980s [7], NF membranes have broadly been investigated and used in many fields such as water softening [8], color removal [9], chemical oxygen demand (COD) reduction [10], drug separation [11,12], and in wastewater treatment for the removal of heavy metal ions [13–15]. The majority of commercially available NF membranes are so far polyamide (PA) thin film composite (TFC) membranes prepared by interfacial polymerization (IP). These membranes are negatively charged at normal operating conditions due to the existence of carboxylic acid functional groups generated from the partial hydrolysis of the acyl chloride unit of trimesoyl chloride (TMC) during IP process [16–18]. Often, the negatively charged PA TFC membranes 3 have higher rejection over multivalent anions (e.g. SO2 4 , PO4 ) 2+ 2+ than over multivalent cations (e.g. Mg , Ca ) under the effect of the charge repulsion effect. Therefore, in some occasions of requiring a removal of multivalent cations (Mg2+, Ca2+, and other heavy metal ions), such as separation of amino acids below isoelectric points, purification of cationic dyes, or a recovery of cathode electrophoretic lacquers etc., positively charged NF membranes are more suitable to guarantee high separation effectivity [19]. Another attractive feature of NF membranes is the molecular sieving ability. Conventional PA TFC membranes (RO and NF membranes) often remove nearly all molecules larger than 1 nm in size and it is difficult to achieve high selectivity to different molecules with size in the range of 1–10 nm [20,21]. In particular, in the fractionation of neutral molecules 1–3 nm in size (e.g. sugars, polypeptides, insulin, synthetic drugs, dyes, etc.), NF membranes with controllable and tunable effective pore size are highly needful [22,23]. Currently, such ‘‘loose” and high precision NF membranes are hard to get in commerce, and a limited number of newly synthesized membranes reported only in academia can separate molecules 1–3 nm in size [24–27]. Additionally, poor chlorine resistance of PA TFC NF membranes also limits their applications in aqueous systems sterilized with chlorine. Hence, to overcome the drawbacks of conventional PA TFC membranes and to provide more choices for the NF process, the development of positively charged, loose and chlorine-resistant NF membranes is greatly significant. In the preparation of NF membranes, the positively charged characteristic can be realized by chloromethylation, IP of TMC with aliphatic amines or triethanolamine, quaternization etc [28–31].

rp rp Dx Ak kD kr PRCO MWCO

geometric standard deviation effective pore radiuses (nm) membrane effective thickness (lm) membrane porosity Debye length (nm) Debye ratio pore radius cut-off of 90% (nm) molecule weight cut-off of 90% (Da)

Thereinto, quaternization is an effective and controllable method to introduce positive charges into membranes. Moreover, the membrane performance can be easily tailored by tuning the quaternization degree. For example, Li et al. fabricated an NF membrane by a UV-initiated graft polymerization of poly(N,Ndimethylaminoethyl methacrylate) from polysulfone (PSF) ultrafiltration (UF) membranes and subsequent quaternization crosslinking. The resulted NF membrane was positively charged and demonstrated a high rejection of MgCl2 (93.2%) [32]. Cui et al. reported the fabrication of positively charged NF membranes by a quaternization and crosslinking of poly(methyl methacrylateco-dimethylaminoethyl methacrylate) with polyvinyl chloride (PVC) membranes as the substrates, and the developed membranes showed high salt rejection and flux [33]. Huang et al. developed a positively charged NF membrane with enhanced separation performances by using quaternized chitosan as the active layer [34]. These efforts have contributed greatly to the design and preparation of NF membranes. Nevertheless, the tunable and controllable positively charged NF membranes with high separation precision still remains challenging. Poly(N-vinyl imidazole) (PVI) is an important water-soluble synthetic polymer which has been widely applied in heavy metal removal, CO2 separation, and methanol fuel cells, due to its diverse characteristics including metal chelation, catalysis, antibacterial activity, biocompatibility, biodegradability and thermal stability [35–43]. The tertiary amines of imidazole rings in PVI molecules hold lone electron pairs and can be readily quaternized by benzyl chloride. In addition, the PVI coating enabled PA NF membranes better chlorine resistance [44]. In our previous work, a PVI gel was filled into a porous PSF membrane by UV-initiated graft polymerization of N-vinyl imidazole [45]. The obtained PVI gel-filled membrane exhibited NF separation characteristic and was used for desalination and organic molecule removal from water [46]. These works showed that PVI is a promising material in the membrane technology. However, the UV-initiated graft process is complicated and it is difficult to control the uniformity of the separation layer. As we know, the uniformity of the separation layer plays a crucial role in the selectivity of membranes. Therefore, in this work, we attempted to engineer positively charged PVI-based ‘‘loose” NF membranes with controllable and tunable separation ability by simple soaking and chemical treatment process. It is expected that such method for membrane production here can bring more uniform separation layer to NF membranes and can be carried out on a large scale. In this work, positively charged PVI/PSF composite NF membranes were fabricated via soaking of PSF ultrafiltration (UF) membranes in PVI aqueous solutions followed by a quaternization crosslinking step with p-xylylene dichloride (XDC). The route for membrane preparation is schemed in Fig. 1. The effects of the synthesis conditions of the separation layer including PVI concentration, XDC concentration, crosslinking time and crosslinking temperature on membrane structures and performances were

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Fig. 1. Preparation route of positively charged PVI/PSF composite NF membranes.

investigated in detail. The structure-performance relationship for the composite NF membrane was discussed scientifically. The objective of this work is to develop a novel positively charged NF membrane qualified for high precision in molecular and ionic separations through a simple and scalable process. 2. Experimental

Table 1 Preparation of PVI-soaked membranes. Membranes

PVI concentration (wt%)

Soaking time (h)

Soaking temperature (°C)

S1 S2 S3 S4

0.25 0.5 0.75 1

8 8 8 8

25 25 25 25

2.1. Materials PSF UF membranes with a molecular weight cut-off (MWCO) of 90 kDa were kindly supplied by Vontron Membrane Technology Co. Ltd. (Guiyang, China) as the support membrane. N-vinyl imidazole (VI), azobisisobutyronitrile (AIBN), N,N-dimethylformamide (DMF), p-xylylene dichloride (XDC), vitamin B2 (VB2), vitamin B12 (VB12), glucose and sucrose were purchased from Aladdin Reagent Co. Ltd. (Shanghai, China). Magnesium chloride (MgCl2), magnesium sulfate (MgSO4), sodium chloride (NaCl), sodium sulfate (Na2SO4), sodium hypochlorite solution (NaClO, 5.2 wt% free chlorine), raffinose, and heptane were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All received reagents were of analytical grade and were used without further purification. Deionized water was used in all experiments.

were labeled S1–S4 based on different PVI concentrations, as listed in Table 1. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jcis.2018.07.034. Second, the as-soaked PVI membrane was then immersed into an XDC solution (0–1 wt%) with heptane as a solvent for quaternization crosslinking. The crosslinking temperature was set at 20–80 °C and the crosslinking time varied from 1 to 5 h. After quaternization crosslinking, the membrane was taken out and washed to remove unreacted XDC by heptane, and then remove noncrosslinked PVI polymer with ethanol and water. The prepared PVI/PSF composite NF membrane was stored in deionized water for further tests. The prepared PVI/PSF composite NF membranes were labeled M1-M14 in terms of preparation conditions, as listed in Table 2.

2.2. Membrane preparation 2.3. Membrane characterizations PVI was synthesized by free-radical polymerization and then characterized in detail, as described in the Supplementary information. The preparation procedure of the PVI/PSF composite NF membranes is shown in Fig. 1. Firstly, a cleaned PSF UF membrane was soaked in a PVI aqueous solution with designed concentration (0–1.0 wt%) under a continuous shaking at 25 °C for 8 h and then taken out. After removing excessive PVI solution on the membrane surface with a filter paper and then drying in vacuum at 40 °C, the membrane with PVI active materials was prepared and named as the PVI-soaked membrane. The prepared PVI-soaked membranes

Membrane surface chemistry was analyzed by attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR, Nicolet, NEXUS 670, USA) and X-ray photoelectron spectroscopy (XPS, PHI-5300 ESCA System) using Al Ka as radiation source with a take-off angle of 60°. The membrane surface and cross-section morphology were observed by a field-emission scanning electronic microscope (FE-SEM, JSF-7500F, JEOL Co. Ltd. Tokyo, Japan). The surface potential of the membrane was measured by a SurPass Electrokinetic Analyser (Anton Paar, GmbH, Austria) based on the

C. Fang et al. / Journal of Colloid and Interface Science 531 (2018) 168–180 Table 2 Preparation of PVI/PSF composite NF membranes. Membranes

PVI concentration (wt%)

XDC concentration (wt%)

Crosslinking time (h)

Crosslinking temperature (°C)

M1 M2 M3 M4

0.25 0.5 0.75 1

0.5 0.5 0.5 0.5

4 4 4 4

60 60 60 60

M5 M6 M7

0.75 0.75 0.75

0.25 0.75 1

4 4 4

60 60 60

M8 M9 M10 M11

0.75 0.75 0.75 0.75

0.25 0.25 0.25 0.25

1 2 3 5

60 60 60 60

M12 M13 M14

0.75 0.75 0.75

0.25 0.25 0.25

4 4 4

25 40 80

2.4. Membrane performance tests A flat-sheet dead-end NF test cell (Millipore, USA) with an effective area of 15 cm2 was used to measure the permeation and rejection performances of the PVI/PSF composite NF membranes under a pressure of 0.4 MPa. A magnetic stirrer was added to eliminate concentration polarization on the membrane surface. Before measurement, the membrane was pressurized for 30 min using ultrapure water at 0.4 MPa to obtain a stable permeate flux. Pure water flux, J (L m2 h1), was calculated using the following equation:

J¼

V At

VB2/VB12 mixtures with different concentrations. Salt concentrations of the feed and the permeate were measured by an electric conductivity meter (DDS-11A, Shanghai Leichi Instrument, China). VB12 and VB2 concentrations were tested by a UV–Vis spectrophotometer (UV-1601, Shimadzu, Japan) at 362 and 445 nm, respectively. The concentration of other neutral molecules was measured by a total organic carbon (TOC) analyzer (TOC-VCHP, Shimadzu, Japan). Rejection, R (%), was defined as:

R¼

streaming potential method. All zeta potential tests were carried out with 1 mM KCl background solution at 25 °C and the solution pH values were adjusted by adding 0.1 M HCl or 0.1 M NaOH.

ð1Þ

where V is the volume of pure water penetrated through the membrane (L), A is the effective filtration area of the membrane (m2) and t is the filtration time (h). Rejection performance of the NF membrane was characterized with different solutes and conditions such as inorganic salts (MgSO4, Na2SO4, NaCl, and MgCl2), neutral molecules (VB12, raffinose, glucose, and sucrose) with a concentration of 300 ppm, and

171

  CP  100% 1 CF

ð2Þ

where CP and CF are the solute concentrations in the permeate and the feed, respectively. Each value is the average value of three consecutive measurements. Chlorine resistance of PVI/PSF composite NF membranes was evaluated via testing the permeation and rejection performance of the membrane after immersion in a 2000 ppm NaClO aqueous solution for different time periods. The pHs of NaClO solutions were 4.0 and 7.0, respectively, adjusted by adding 1.0 M HCl solution. After soaking for the predetermined time in the NaClO solution, the membrane was taken out and thoroughly rinsed to remove residual NaClO with deionized water. Subsequently, pure water flux and MgCl2 rejection were measured to evaluate the chlorine resistance of the PVI/PSF composite NF membrane. The long-term stability of the PVI/PSF composite NF membrane was evaluated by testing pure water flux and MgCl2 rejection over a period of 0–15 day of shaking in hot water at 60 °C. 3. Results and discussion 3.1. Membrane surface chemistry The surface chemistry of the membranes was characterized by ATR-FTIR and XPS to analyze the construction of the PVI/PSF composite NF membrane by a facile soaking and subsequent quaternization crosslinking. The ATR-FTIR spectra of the membranes are shown in Fig. 2(left). As shown in Fig. 2(left), compared to the spectrum of the PSF pristine membrane, new peaks appeared in those of the PVI-soaked and the PVI/PSF composite NF membranes. The characteristic absorption peaks at 3106, 1673 and 914 cm1 were attributed to the CAH stretching vibration, C@N stretching and ring stretching vibrations, and in-plane bending of the imidazole ring, respectively. PSF characteristic peaks were also observed at 1587 and 1150 cm1 (S@O and C@C stretching vibration) in the spectra of the PVI-soaked and the PVI/PSF composite NF

Fig. 2. ATR-FTIR (left) and wide-scan XPS (right) spectra of (PSF) the PSF pristine membrane, (S3) one PVI-soaked membrane with 0.75 wt% PVI, and (M5) one PVI/PSF composite NF membrane with 0.75 wt% PVI, 0.25 wt% XDC and crosslinking 4 h at 60 °C.

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Table 3 Element contents on the membrane surfaces determined by XPS. Membranes

C 1s (%)

O 1s (%)

N 1s (%)

S 2p (%)

Cl 2p (%)

PSF S3 M5

78.6 74.5 73.8

19.1 13.5 14.7

0.6 10.7 9.9

1.7 1.3 1.0

– – 0.6

membranes. These results indicated that the thickness of obtained PVI layers on membrane surfaces was relatively low and less than the probing depth of ATR-FTIR. However, the difference was not observed between the spectrum of the PVI-soaked membrane and that of the PVI/PSF composite NF membrane. This was possible because XDC with a low concentration was used here for the quaternization crosslinking. In the wide-scan XPS spectra shown in Fig. 2(right), it is observed that a strong peak of the N 1s core-level signal appeared at 401 eV in the spectra of both the PVI-soaked membrane (S3) and the PVI/PSF composite NF membrane (M5). The nitrogen element was reasonably attributed to the imidazole rings in PVI, further indicating the successful integration of PVI on the PSF membrane. On the other hand, a small peak was observed at 198 eV in the XPS spectrum of the PVI/PSF composite NF membrane (M5), which resulted from the Cl 2p signal. The appearance of the Cl 2p peak demonstrated that the introduced PVI on the support membrane was quaternized or quaternization crosslinked by XDC. The mole percentages of carbon, oxygen, nitrogen, sulfur, and chlorine of the PSF pristine, the PVI-soaked, and the PVI/PSF composite NF membranes are shown in Table 3. It is clear from Table 3 that, after introducing PVI onto the PSF membrane surface, the oxygen content decreased from 19.1% to 14.7%, while the nitrogen content increased from 0.6% to 9.9%. The sulfur content decreased from 1.7% to 1.0% due to the coverage of PVI. The Cl content in the PVI/PSF composite NF membrane was at a low value of 0.6%, which was in line with the expected one because of a low XDC concentration used here. A small amount of nitrogen (0.6%) was detected on the PSF membrane, which may result from the residual polyvinylpyrrolidone in the PSF membrane, or nitrogen adsorbed from the air. The results above revealed the successful introduction of PVI onto the PSF support membrane by a facile soaking step. However, it is significant to know the amount of PVI on the support membrane, affecting the subsequent quaternization crosslinking to tailor the separation performance of the NF membrane. Hence, the effect of PVI concentration on the amount of PVI on the support membrane was analyzed by XPS and the results are shown in

Fig. 3. Amount of N element of PVI-soaked membranes as a function of PVI concentration.

Fig. 3. From Fig. 3, it is evident that with the increase of PVI concentration, the N content on the PVI-soaked membrane increased, indicating a higher amount of PVI on the support membrane. When PVI concentration was 0.75 wt%, the PVI-soaked membrane showed a high amount of PVI of 10.7%, and a saturation trend also started appearing. To obtain more information about the quaternization crosslinking mechanism of the PVI/PSF composite NF membrane, the chemical states of elements were further analyzed by high-resolution XPS core-level N 1 s and Cl 2p spectra. It is shown in Fig. 4(left) that the N 1s peak of the PVI/PSF composite NF membrane was resolved into three component peaks with different areas at 400.2 eV, 401.8 eV, and 402.7 eV, corresponding to the signals of ANA, @NA, and @N+A of the imidazole ring, respectively, to characterize the membrane quaternization [47]. The Cl 2p signal was divided into two statuses of CACl and Cl to analyze the membrane crosslinking [48], as shown in Fig. 4(right). The covalently bonded (CACl) Cl 2p spectrum consisted of two component peaks of CACl 2p3/2 (198.4 eV) and CACl 2p1/2 (200.8 eV) due to the spin–orbit coupling split. The Cl 2p spectrum was separated into two component peaks of Cl 2p3/2 (196.8 eV) and Cl 2p1/2 (197.9 eV). The quaternization degree was defined as the molar ratio of @N+A/ (@N+A + @NA), and the crosslinking degree was calculated using the molar ratio of Cl/(CACl + Cl). The resulted quaternization and crosslinking degrees are shown in Fig. 5. As shown in Fig. 5, with the increase of XDC concentration, the quaternization degree increased, while the crosslinking degree increased firstly, and then decreased slightly. As we know, the crosslinking occurs only when double benzyl chloride groups in XDC molecule react with tertiary amines in imidazole rings of PVI [33]. Therefore, the continuous increase of XDC concentration was beneficial for the enhancement in the quaternization degree, but disadvantageous to get a higher crosslinking degree. Furthermore, it is found in Fig. 5 that when XDC was 0.25 wt%, a high crosslinking degree of 78.8% was showed, although the quaternization degree was only 24.1%. The high crosslinking degree indicated that the imidazole rings of PVI were easily crosslinked by the quaternization reaction with XDC. More importantly, a high crosslinking degree was advantageous to the stable fixation of the PVI separation layer on the substrate. 3.2. Membrane morphology and charge The surface and cross-sectional morphologies of the PVI-soaked and the PVI/PSF composite NF membranes were observed by SEM and the images are shown in Figs. 6 and 7, respectively. In Fig. 6(PSF-S), it is observed that the PSF support membrane was porous, and there was no visible separation layer near the outer surface, as shown in Fig. 6(PSF-C). After soaking in the PVI solution, the membrane surface was gradually dense. When PVI concentration was 0.75 wt%, the membrane surface was almost covered by PVI without the obvious presence of pores, as shown in Fig. 6(S3S). On the other hand, in Fig. 6(S3-C), the PVI layer with a thickness of 43.8 nm is observed. In the comparison with the PVI-soaked membrane (Fig. 6(S3)), after quaternization crosslinking by XDC of 0.25 wt%, the PVI layer was denser without the existence of pores, as shown in Fig. 7(M5-S). Furthermore, in Fig. 7(M5-C), it is observed that the PVI layer thickness increased to 59 nm. With the increase of XDC concentration from 0.25 to 1 wt%, the surface was similar and dense, while the thickness of the PVI layer increased to 72 nm shown in Fig. 7(M7-C)). These results indicated that the PVI layer was successfully fabricated on the PSF support membrane. Fig. 8 shows the surface charges of the membranes. It is observed in Fig. 8 that the PSF membrane was negatively charged over a wide pH range, and the isoelectric point was at pH = 4.5. This was likely due to the preferential adsorption of anions such

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173

Fig. 4. Core-level XPS spectra of (left) N 1s and (right) Cl 2p of the PVI/PSF composite NF membranes.

as hydroxide on the PSF membrane surface [49]. With the introduction of PVI, the isoelectric point of the membrane increased. When PVI concentration was 0.75 wt%, the isoelectric point of

the PVI-soaked membrane was at pH = 6.8, as shown in Fig. 8 (S3), which was close to the isoelectric point of the PVI polymer (around pH = 7.0) [50]. After reaction with XDC, the isoelectric

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Fig. 5. The effect of XDC concentration on the quaternization and the crosslinking degrees of the PVI/PSF composite NF membranes.

point of the membrane further increased. When XDC of 0.25 wt% was used, the PVI/PSF composite NF membrane (M5 in Fig. 8) showed a strongly positively-charged property with an isoelectric point at pH = 10. Enhancing further XDC concentration did not clearly increase the isoelectric point of the NF membrane, as shown in Fig. 8(M7), indicating that XDC of 0.25 wt% was enough for followed quaternization crosslinking of the PVI-soaked membrane to obtain a strongly positive charge. The charge change in the membrane surfaces further demonstrated that PVI was successfully incorporated onto the PSF support membrane, and tertiary amine groups in the imidazole rings of PVI were quaternized with XDC. The strongly positive charge of the composite NF membrane was advantageous to the cationic removal based on the electrostatic repulsion mechanism. 3.3. Permeation and rejection performances of PVI/PSF composite NF membranes The permeation and rejection performances of the PVI/PSF composite NF membranes are shown in Fig. 9. In Fig. 9(a), it is observed that with the increase of PVI concentration, the pure water flux decreased, while the MgCl2 rejection increased. This was because elevating PVI concentration in the soaking step resulted in higher

loading amount of PVI on the membrane, as shown in Fig. 3. Hence, the membrane surface was gradually dense, and the thickness of the PVI layer increased, as shown in Fig. 6. In turn, the obtained PVI separation layer became thicker and denser after crosslinking. As a result, the membrane permeability decreased, while the rejection increased. When the PVI concentration was 0.75 wt%, the MgCl2 rejection went up to 90.1%, and the pure water flux remained at a moderate level. Therefore, 0.75 wt% was used as the appreciate PVI concentration to prepare the PVI-soaked membrane for subsequent quaternization crosslinking. The effect of XDC concentration on the permeation and rejection performances of the membranes is shown in Fig. 9(b). It is observed that, with the increase of XDC concentration, the pure water flux decreased gradually, while the MgCl2 rejection increased firstly and then decreased slightly. This phenomenon can be explained by the variation in the quaternization and the crosslinking degrees of the tertiary amine of PVI [33]. When XDC concentration increased from 0 to 0.25 wt%, both the quaternization degree and the crosslinking degree of PVI increased simultaneously, as shown in Fig. 5. The membrane surface became more compact and the PVI separation layer was thicker, as shown in Fig. 7. Moreover, the charge density of the membrane surface also increased (Fig. 8). Hence, the pure water flux decreased, while the MgCl2 rejection increased. With the further increase of XDC concentration from 0.25 to 0.75 wt%, the MgCl2 rejection increased slowly, while the pure water flux decreased dramatically. This was due to a further increase in the crosslinking degree, as shown in Fig. 5. When XDC concentration was 1 wt%, the rejection decreased slightly because of a slight decrease in the crosslinking degree, while the pure water flux still decreased clearly. This decline in pure water flux may be due to the clear increase in the thickness of the PVI layer, as shown in Fig. 7(M7-C), resulted in higher permeation resistance in the membrane. Hence, it was known that 0.25 wt% of XDC was suitable for quaternization crosslinking to achieve both high rejection and permeability. Fig. 9(c) shows the effect of crosslinking time on the membrane performances. From Fig. 9(c), it is observed that the pure water flux decreased, while the MgCl2 rejection increased with the prolongation of reaction time. Within a short time, the quaternization crosslinking was incomplete with a low crosslinking degree (Fig. S3). The obtained PVI separation layer was relatively loose

Fig. 6. SEM images of the PSF support membrane (PSF) and the PVI-soaked membranes prepared in different PVI concentrations of 0.25 wt% (S1) and 0.75 wt% (S3), and S and C are surface and cross-section, respectively.

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Fig. 7. SEM images of the PVI/PSF composite NF membranes prepared in different XDC concentrations of 0.25 wt% (M5) and 1 wt% (M7), and S and C are surface and crosssection, respectively.

tion, 4 h of crosslinking time and 60 °C of crosslinking temperature for the quaternization crosslinking step. In the conditions, the prepared PVI/PSF composite NF membrane showed high MgCl2 rejection of 90.1% and moderate pure water flux of 22 L m2 h1 (0.4 MPa). This membrane was used to further evaluate the molecular and ionic separation performances of PVI/PSF composite NF membranes. If not specified otherwise, the PVI/PSF composite NF membrane in the following sections was prepared at the optimal conditions. 3.4. Pore structure of the PVI/PSF composite NF membrane

Fig. 8. The surface charge of the membranes.

with a low isoelectric point at pH = 8.4, as shown in Figs. S4 and S5, respectively, which resulted in high permeability but low MgCl2 rejection. The PVI separation layer was gradually compact with the prolongation of crosslinking time. When the crosslinking time was 4 h, the MgCl2 rejection was 90.1%, and the pure water flux was 22 L m2 h1. Fig. 9(d) shows the effect of crosslinking temperature on the membrane performances. With increasing crosslinking temperature, the pure water flux decreased, while the MgCl2 rejection increased. Generally, the high temperature is advantageous to the progress of quaternization reaction. Thus, a more compact PVI separation layer with a high isoelectric point was created with increasing crosslinking temperature, as shown in Figs. S4 and S5. From the results above, the preferable preparation conditions for the PVI/PSF composite NF membrane were 0.75 wt% of PVI concentration for the soaking step, and 0.25 wt% of XDC concentra-

The separation performance of NF membranes mainly depends on pore structure and surface charge [51]. The pore radius and pore radius distribution of the PVI/PSF composite NF membrane were studied in detail in this section by the pore-flow model and the solute transport model (the detailed equations and steps are shown in the Supplementary information). The effective pore radii (rp) obtained by using different probing molecules are shown in Table S1. The chemical structures of probing molecules are shown in Fig. S6. The effective pore radius of the prepared PVI/PSF composite NF membrane was determined to be 0.82 nm and close to that of the reported PVI gel-filled NF membranes [46]. Meanwhile, the pore radius distribution of the PVI/PSF composite NF membrane was obtained. The mean effective pore radius (lp) and the geometric standard deviation (rp) of the membrane are listed in Table 4. Based on the lp and rp, the cumulative pore radius distribution and probability density function curves were determined according to Eqs. (S6) and (S8) and the results are shown in Fig. 10. The values of lp and rp determine the position and sharpness of the pore radius distribution curve, respectively. Compared to the previously reported PVI gel-filled NF membranes with the rp range of 2.3–2.86 [45,46], the as-prepared NF membrane had a

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Fig. 9. Effects of fabrication conditions (a) PVI concentration, (b) XDC concentration, (c) time and (d) temperature of the quaternization crosslinking on the pure water flux and MgCl2 rejection of PVI/PSF composite NF membranes.

Table 4 Pore structure of the PVI/PSF composite NF membrane and the PVI gel-filled NF membranes [45,46].

* **

Membranes

PWL*/L m2 h1

lp/nm

rp

rp/nm

Dx/Ak

PRCO**/nm

PVI/PSF composite NF membrane PVI gel-filled NF membranes

22 6–11

0.56 0.51–0.84

1.25 2.3–2.86

0.82 0.895–0.91

5.8 10.1–33

0.75 0.89–1.07

PWL is pure water flux at 0.4 MPa. PRCO is pore radius cut-off of 90%.

smaller rp = 1.25, corresponding to a narrower and less diffusive pore size distribution. Furthermore, no big pores with a pore radius larger than 1 nm is observed in Fig. 10, corresponding to a smaller pore radius cut-off (PRCO) at 0.75 nm (Fig. S7). The PRCO was close to the rp, indicating the accuracy of two models in estimating the pore radius. According to the Hagen-Poiseuille equation (Eq.

Fig. 10. Cumulative pore size distribution (red curve) and probability density function curves (black curve) of the PVI/PSF composite NF membrane. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

(S10)), the ratio of effective thickness to porosity (Dx/Ak) of the PVI/PSF composite NF membrane was obtained and is listed in Table 4. A lower Dx/Ak value implies better permeability [52]. In this case, the PVI/PSF composite NF membrane showed higher pure water flux than the reported PVI gel-filled NF membranes despite having similar pore radii. It was clear therefore that the PVI separation layer fabricated by quaternization crosslinking was more uniform in the pore structure than the PVI gel layer filled by UVinduced in situ grafting polymerization. The UV-initiated grafting polymerization and crosslinking of VI on/in the substrate occurred in a UV-irradiation depth of several microns, and the chain transfer and termination usually happened during free-radical polymerization [45]. In addition, the VI monomer was easy to penetrate into the pores of the substrate, resulting in a uniformly defect-free separation layer being fabricated hard [45]. As a result, PVI gel-filled NF membranes showed thicker PVI layers with wider pore radius distribution, compared to the PVI/PSF composite NF membrane engineered in this work. The molecular weight cut-off (MWCO) of the PVI/PSF composite NF membrane was calculated to be 1170 Da from Eq. (S9). From the results above, it was concluded that the developed PVI/PSF composite NF membrane was a positively charged NF membrane with ‘‘loose” pore structure. 3.5. Molecular sieving ability of the PVI/PSF composite NF membrane Fig. 11 shows the molecular sieving performance of the PVI/PSF composite NF membrane. As shown in Fig. 11(a), the similar

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Fig. 11. The molecular sieving performance of the PVI/PSF composite NF membrane (a) 40 ppm VB2 aqueous solution, (b) 40 ppm VB12 aqueous solution and (c) 20 ppm VB2/ 20 ppm VB12 mixed solution.

UV–Vis spectra of the VB2 solutions before and after filtrations indicated that VB2 was not rejected by the PVI/PSF composite NF membrane with a low rejection of 16.3%. The low rejection of VB2 was due to a smaller radius of 0.47 nm than the effective pore radius of the NF membrane. The digital photos before and after filtrations are shown in the inset of Fig. 11(a), where the colors of the two solutions were almost at same. Fig. 11(b) shows the UV–Vis spectra of VB12 solutions before and after filtrations. From Fig. 11(b), it is clear that VB12 was rejected by the NF membrane with a high rejection of 92.6% because of the larger radius of 0.74 nm and the network-like molecular structure. After filtration, the color also changed from light red to transparent, as shown in the inset of Fig. 11(b). The NF membrane was further used to separate a VB2/VB12 mixture and the UV–Vis spectra of the feed and the permeate are shown in Fig. 11(c). From Fig. 11(c), it is obvious that VB2/VB12 were separated with a high VB12 rejection of 83% and a low VB2 rejection of 24.6%, similar to those of the single VB2 and VB12 filtrations. The color also varied from pale orange (VB2/VB12 color mixture) in the feed to pale yellow (VB2 color) in the permeate. Hence, it was concluded that the developed PVI/PSF composite NF membrane possessed the molecular-level sieving ability and a high separation factor can be obtained for a mixture of molecules with a larger difference in the molecular size. 3.6. Salt rejection of the PVI/PSF composite NF membrane Salt rejection performance of the PVI/PSF composite NF membrane was studied under different experimental conditions. The rejection of salts mainly depends on the electrostatic repulsion rather than the size sieving in NF membranes, due to the hydrodynamic radii of ions are far smaller than the pore radius of the membrane. The rejections for different salts are shown in Fig. 12(a) and

the rejection order was MgCl2 > NaCl > MgSO4 > Na2SO4, exhibiting the typical rejection characteristic of positively charged NF membranes. The strong electrostatic repulsion between high valence cations and the PVI quaternary ammonium (@N+A) largely improved the rejection of MgCl2, while the strong electrostatic screening effect from attractive interactions between SO2 and 4 @N+A led to lower rejections of Na2SO4 and MgSO4, which was in line with the results reported in the literature [53]. To further investigate the effect of the electrical charge on salt rejection, the filtration experiments of MgCl2 solutions with different concentrations (ionic strength) were carried out. Fig. 12(b) shows that the MgCl2 rejection and water flux decreased with the increase of MgCl2 concentration. The flux decline was attributed to the increased viscosity and osmotic pressure of feed solution [51,52]. The electrostatic screening on the membrane surface results in the decrease of ion rejection. For any charged surface, an electrical double layer forms when it is surrounded by a salt solution and this layer thickness is defined as Debye length (kD), which is inversely proportional to ionic strength. The Debye ratio (kr), defined as the ratio of the Debye length (kD) to the effective pore radius (rp), is often used to qualitatively explain the effect of charge on salt rejection [54,55]. When kr is larger than 2, the electrostatic repulsion between the pore surface and ions is prominent, leading to a strong Donnan exclusion of co-ions. Hence, a high rejection still exhibited, even though the Debye ratio reduced from 3.44 at 0.005 mol/L to 2.44 at 0.01 mol/L. When the Debye ratio decreased from 2.44 to 1.41, corresponding to the increase in MgCl2 salt concentration from 0.01 to 0.03 mol/L, the ion exclusion sharply decreased, resulting in low MgCl2 rejection. This result demonstrated the significance of the electrostatic repulsion mechanism of the PVI/PSF composite NF membrane in the rejection of inorganic salts.

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Fig. 12. Salt rejection performances of the PVI/PSF composite NF membrane under different conditions including (a) different salts with a concentration of 0.01 mol/L, (b) MgCl2 concentration from 0.005 to 0.03 mol/L and (c) feed pressure from 0.1 to 0.5 MPa with an MgCl2 concentration of 0.01 mol/L.

Furthermore, the effect of operating pressure on ion rejection was explored here. Fig. 12(c) shows that the water flux linearly increased with the increase in operating pressure, indicating a high-pressure resistance of the PVI/PSF composite NF membrane. On the other hand, the ion rejection initially increased and then kept stable with increasing pressure. According to the extended Nernst-Planck equation, the electrolyte transport through a membrane is controlled by convection, diffusion, and migration [56]. Because of these factors, the ion rejection increased firstly and subsequently approached a limiting value with the increase of the flux. When the water flux was low at low operating pressure, diffusion was the dominant mechanism contributing to the MgCl2 permeation through the membrane. Hence, a relatively low MgCl2 rejection was observed at low operating pressure. When the flux increased, the contribution of both convection and migration to ion transport increased, while the contribution of diffusion decreased. In this case, the ion exclusion increased initially and subsequently kept unchanged because the sorption of MgCl2 into

the membrane became generally saturated under the effect of the Donnan exclusion. The comparison of the PVI/PSF composite NF membrane engineered in this work with NF membranes reported in the literature is shown in Table 5. The reported NF membranes were prepared by the quaternization crosslinking or the interfacial polymerization, and claimed to show a positively charged property. It is observed in Table 5 that the PVI/PSF composite NF membrane showed a good comprehensive performance for the cationic and molecular separations, compared with the reported positively charged NF membranes. Furthermore, various commercial NF membranes were also listed in Table 5. Negative charge property of the most commercial NF membranes resulted in higher Na2SO4 and lower MgCl2 rejections, compared to the PVI/PSF composite NF membrane developed in this work. In addition, low MWCO or rp resulted in them hard exhibiting the ‘‘loose NF” property for the precisely molecular separation. UTC-20 was an exception and had a cationic selective skin as claimed by the manufacturer, and thus presented

Table 5 Comparison of various NF membranes.

a b *

Membranes

Water flux (L m2 h1 bar1)

Salt rejection (%) CaCl2

MgCl2

NaCl

Na2SO4

MgSO4

PVI/PSF composite NF HACC/PAN composite NF PEI/PSF composite NF HPEI-TMC/PAN TFC NF TFC NF hollow fiber NTR-7450 NF-40 NF-270 UTC-20

5.5 1.6 4.6 9.5 4.9 9.2 0.2 11 9.7

– 87 – – – – – –

90 96 – 80 96.2 13 70 50 98

54 56 42 45 85.5 51 45 98 55

41 20 21 35 61 92 95 – 93

18 48 29.5 78 82 – – – –

Molecular weight cut-off. Pore radius. Molecular weight of VB12, raffinose and sucrose are 1355, 504 and 342 Da, respectively.

Solute used or MWCOa

R (%)

Ref.

VB12* PEG 600 PEG 4000 PEG 6000 Raffinose* Sucrose* rp = 0.45 nmb 200–400 Da 180 Da

93 95 96 90 90 0.36 – – –

This work [34] [57] [58] [59] [52,60] [60,61] [62,63] [63,64]

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Fig. 13. Stability of the PVI/PSF composite NF membrane during (a) chlorine exposure and (b) long-term operation.

a high MgCl2 rejection. However, it cannot still achieve a high precision in the molecular separation because of its low MWCO. 3.7. Chlorine resistance and stability of the PVI/PSF composite NF membrane Chlorine resistance is one of the most significant factors affecting the application of NF membranes in water treatment due to the abundance of residual chlorine from chlorine oxidants used to inhibit the growth of microorganisms [44]. In addition, the periodic chemical wash of NF membranes often involves chlorine-based detergents [44]. The chlorine resistance of the PVI/PSF composite NF membrane is shown in Fig. 13(a). Regardless of pH, the pure water flux sharply decreased upon exposure to chlorine and then slowly increased with higher chlorination degree, while the MgCl2 rejection continually decreased. Although the permeation and rejection performances of the PVI/PSF composite NF membrane changed in the existence of chlorine, the degree of deterioration was less than that of conventional TFC PA membranes [65], indicating a better chlorine resistance. In addition, it is observed in Fig. 13 (a) that the chlorine resistance at pH = 7 was better than that at pH = 4. This lower chlorine resistance at pH = 4 was due to a higher active chlorine concentration in the acidic solution. The crosslinked PVI layer was stable in NaClO aqueous solutions because there is no active point in the PVI molecule vulnerable to the attack by the active chlorine [44]. The performance variations in the PVI/PSF composite NF membrane likely resulted from the PSF substrate [65]. The stability of the separation layer is of great significance for practical applications. The long-term operation experiment was carried out to evaluate the stability of the PVI/PSF composite NF membrane and the results are shown in Fig. 13(b). The permeation and the rejection performances of the NF membrane remained stable, demonstrating that the PVI separation layer was stable and tightly attached to the substrate. This high stability was mainly attributed to the high quaternization crosslinking degree of 78.8%, which resulted in the PVI separation layer being anchored on the substrate in the form of a crosslinked network, similar to interfacial polymerization. 4. Conclusions In this work, a positively charged PVI/PSF composite NF membrane with high selectivity in ionic and molecular separations was fabricated via soaking followed by a quaternization crosslinking step. The optimized PVI/PSF composite NF membrane showed a high MgCl2 rejection of 90.1% and pure water flux of 22 L m2 h1 under 0.4 MPa. The exclusion of different inorganic salts followed the order of Na2SO4 < MgSO4 < NaCl < MgCl2. The effective pore

radius of the composite NF membrane was
0.82 nm, corresponding to an MWCO of 1170 Da. The molecular sieving ability was further demonstrated with a high VB12 rejection of 83% and a low VB2 rejection of 24.6%. Furthermore, the PVI/PSF composite NF membrane exhibited high chlorine tolerance and durability. This work offers a new alternative of positively charged NF membranes with controllable and tunable separation ability used in molecular and ionic separations. Acknowledgments We are grateful for the financial support from the National Natural Science Foundation of China (Grant No. 51573159 and 51773175). References [1] S. Karan, Z. Jiang, A.G. Livingston, Sub-10 nm polyamide nanofilms with ultrafast solvent transport for molecular separation, Science 348 (6241) (2015) 1347–1351. [2] Q. Song, S. Cao, P. Zavala-Rivera, L.P. Lu, W. Li, Y. Ji, S.A. Al-Muhtaseb, A.K. Cheetham, E. Sivaniah, Photo-oxidative enhancement of polymeric molecular sieve membranes, Nat. Commun. 4 (2013) 1918. [3] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marinas, A.M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (7185) (2008) 301–310. [4] Y. Zheng, G. Yao, Q. Cheng, S. Yu, M. Liu, C. Gao, Positively charged thin-film composite hollow fiber nanofiltration membrane for the removal of cationic dyes through submerged filtration, Desalination 328 (2013) 42–50. [5] R.J. Petersen, Composite reverse osmosis and nanofiltration membranes, J. Membr. Sci. 83 (1) (1993) 81–150. [6] P. Marchetti, M.F. Jimenez Solomon, G. Szekely, A.G. Livingston, Molecular separation with organic solvent nanofiltration: a critical review, Chem. Rev. 114 (21) (2014) 10735–10806. [7] P. Eriksson, Nanofiltration extends the range of membrane filtration, Environ. Prog. Sustain. Energy 7 (1) (1988) 58–62. [8] J. Schaep, B. Van der Bruggen, S. Uytterhoeven, R. Croux, C. Vandecasteele, D. Wilms, E. Van Houtte, F. Vanlerberghe, Removal of hardness from groundwater by nanofiltration, Desalination 119 (1–3) (1998) 295–301. [9] J. Huang, K. Zhang, The high flux poly (m-phenylene isophthalamide) nanofiltration membrane for dye purification and desalination, Desalination 282 (2011) 19–26. [10] L.P. Raman, M. Cheryna, N. Rajagopalan, Consider nanofiltration for membrane separations, Chem. Eng. Progress; (United States) 90 (3) (1994). [11] T.A. Ternes, M. Meisenheimer, D. McDowell, F. Sacher, H.-J. Brauch, B. HaistGulde, G. Preuss, U. Wilme, N. Zulei-Seibert, Removal of pharmaceuticals during drinking water treatment, Environ. Sci. Technol. 36 (17) (2002) 3855– 3863. [12] A. Schäfer, L. Nghiem, T. Waite, Removal of the natural hormone estrone from aqueous solutions using nanofiltration and reverse osmosis, Environ. Sci. Technol. 37 (1) (2003) 182–188. [13] C. Bellona, J.E. Drewes, Viability of a low-pressure nanofilter in treating recycled water for water reuse applications: a pilot-scale study, Water Res. 41 (17) (2007) 3948–3958. [14] B. Mi, B.J. Mariñas, D.G. Cahill, RBS characterization of arsenic (III) partitioning from aqueous phase into the active layers of thin-film composite NF/RO membranes, Environ. Sci. Technol. 41 (9) (2007) 3290–3295. [15] A.W. Mohammad, R. Othaman, N. Hilal, Potential use of nanofiltration membranes in treatment of industrial wastewater from Ni-P electroless plating, Desalination 168 (2004) 241–252.

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