Li+ ratio

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Positive charged PEI-TMC composite nanofiltration membrane for separation of Li+ and Mg2+ from brine with high Mg2+/Li+ ratio ⁎

T ⁎

Ping Xu, Wei Wang, Xiaoming Qian , Haibo Wang, Changsheng Guo, Nan Li, Zhiwei Xu , Kunyue Teng, Zhen Wang State Key Laboratory of Separation Membranes and Membrane Processes, School of Textiles, Tianjin Polytechnic University, Tianjin 300387, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Positive charged composite nanofiltration membrane Salt lake brine Polyethyleneimine Interfacial polymerization Mg2+ and Li+ separation

To effectively separate Mg2+ and Li+ from salt lake brine with high Mg2+/Li+ mass ratio, a positive charged nanofiltration membrane with abundant -NH3+ and -NH2+ was designed and fabricated via interfacial polymerization between polyethyleneimine (PEI) and trimesoyl chloride (TMC) on the polyethersulfone ultrafiltration membrane. Zeta potential test revealed that at pH below 9.3, the resulting membrane exhibited a high positive charge due to a large amount of unreacted amine groups on the PEI branch. In the separation process, Donnan exclusion effect dominated the separation performance of resulting membranes for Mg2+ and Li+. The results showed that the Mg2+/Li+ mass ratio declined from initial 20 to 1.3 and the separation factor SMg,Li was as low as 0.05 with a pure water permeation of 5.02 L/(m2hbar) after filtration with resulting NF membrane. The difference between the rejection for Mg2+ and Li+ reached up to 76%. Compared with membrane in previous studies, the separation performance for Mg2+ and Li+ of positive charged NF membrane prepared in this work increased more than one time. This research demonstrated the potential of positive charged PEI-TMC composite NF membrane for the separation of Mg2+ and Li+ from salt lake brine with high Mg2+/Li+ mass ratio.

1. Introduction Lithium, which is the lightest metal with many special physical and chemical properties, such as the highest redox potential value and perfect specific curing capacity compared with other solid element [1–3]. The demand of lithium is growing fast in response to the rapid development of electric vehicle industry [4–6]. Traditionally, lithium is extracted from ore, but it has disadvantages of complicated steps and large energy consumption [7]. Now, lithium resources are mainly recovered from salt lake brine. However, the brine is heavily loaded with magnesium [8,9], which can increase the difficulty of lithium extraction from brine due to the similar ion hydration radius of magnesium and lithium [10]. Therefore, in order to effectively extract lithium resources from the salt lake, the problem of separating Mg2+ and Li+ in brine must be solved. Currently, many methods have been used to recover lithium from brine, such as precipitation, lithium ion-sieve, liquid-liquid extraction, membrane process and so on [11–18]. The conventional precipitation technology is not suitable for the separation of Mg2+ and Li+ in brine with high Mg2+/Li+ mass ratio. And other methods which were used to separate Mg2+ and Li+ from brine may lead to serious problems, such as large energy consumption, low

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economic efficiency and unfriendly to the environment. In recent years, nanofiltration (NF) membrane separation technology is drawing more attention owing to its perfect properties, such as high permeation flux and low cost [19]. NF is a pressure-driven membrane separation technology with a molecular weight cut-off (MWCO) ranging from 200 to 1000 Da and a pore size of around 0.5–2.0 nm [20–28]. NF membranes can be used to remove divalent and multivalent ions from monovalent ions selectively owing to its special separation mechanisms: Donnan exclusion theory and the steric hindrance effect [29–33]. Based on above characteristics of NF membrane, it can be used for the separation of Mg2+ and Li+ in brine to achieve the purpose of lithium extraction. At present, many researchers have concentrated on the separation of Mg2+ and Li+ in brine with NF membranes. Wen et al. [34] investigated and reported the application of NF for recovering LiCl from diluted brine concluding LiCl and large concentrations of Mg by Desal-5 DL membrane with negative charge which is supplied by GE Osmonics, but the performance was poor, and the separation factor SMg,Li of Desal-5 DL was about 0.286. Yang et al. [35] explored the separation of Mg2+ and Li+ from the East Taijinaier Salt Lake brine using the spiral-wound Desal DK membrane with negative charge supplied by GE Osmonics. The

Corresponding authors. E-mail addresses: [email protected] (X. Qian), [email protected] (Z. Xu).

https://doi.org/10.1016/j.desal.2018.10.019 Received 28 June 2018; Received in revised form 17 October 2018; Accepted 17 October 2018 0011-9164/ © 2018 Published by Elsevier B.V.

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2. Experiment

separation factor SMg,Li was found around 0.31. Somrani et al. [18] studied the separation of Mg2+ and Li+ from diluted brine with Mg2+/ Li+ mass ratio of 50 by NF90 and XLE membrane, and the result showed that NF90 is more efficient for the extraction of Li from brine. Bi et al. [36] carried out an experiment for the extraction of Li from high Mg2+/Li+ ratio salt lake brine, and the rejection for Mg2+ reached to 92% under a pressure of 8 bar. Sun et al. [17] adopted the DL-2540 membrane to investigate the separation of Mg2+ and Li+ from brine with a high Mg2+/Li+ ratio. According to the result, the rejection for Mg2+ and Li+ was nearly 65% and −20%, respectively for the simulated brine. Series of research results showed that NF membranes have the potential for the extraction of lithium from brine with high Mg2+/ Li+ mass ratio. Since positively charged membranes are prone to adsorb microorganisms and natural organics, causing membrane fouling. As a result, commercial NF membranes typically employ negative charged membranes. However, the negative charged NF membrane is not the best choice for the separation of Mg2+ and Li+ from salt lakes with high ratio of Mg2+/Li+. Up to now, there are few reports on the extraction of lithium from brine with high Mg2+/Li+ mass ratio using positive charged NF membranes. According to Li's research [37], the NF membranes with positive charge are more effective for the separation of Mg2+ and Li+ from salt lake brine on account of Donnan exclusion theory. They fabricated a positive charged NF hollow fiber membrane via interfacial polymerization of 1,4-Bis (3-aminopropyl) piperazine and trimesoyl chloride for the extraction of lithium from simulated brine with Mg2+/Li+ mass ratio of 20. After the filtration of feed solution, the mass ratio of Mg2+/Li+ decreased to 7.7. Recently, in order to improve the rejection for Mg2+, Li et al. [38] synthesized the positive charged NF membrane by interfacial polymerization via trimesoyl chloride and branched poly (ethylene imine) on the cross-linked polyetherimide support layer, then the membrane was modified with ethylenediaminetetraacetic acid. The result showed that the separation factor SMg,Li was about 0.109. It was shown that the positive charged NF membranes have a promising potential for the separation of Mg2+ and Li+ from brine. The difference in aqueous monomer during interfacial polymerization directly determines the overall performance of the membrane. Polyethylenimine (PEI) with long branched polymeric chains contains a large amount of amine groups [39]. Polymerization between PEI as a aqueous monomer and trimesoyl chloride (TMC) will lead to a polyamide thin-film with a large number of unreacted primary and secondary amine positive groups [40]. So, the polyamide active layer has a strong positive charge, which is beneficial to improve the rejection of the resulting membrane for divalent and high-valence cations, which is ideal for separating Mg2+ and Li+. The purpose of present work is to fabricate a positive charged NF membrane containing much -NH3+ and -NH2+ for the separation of Mg2+ and Li+ from salt lake brine with high Mg2+/Li+ mass ratio, and the NF membrane was prepared via interfacial polymerization between PEI and TMC on the support of polyethersulfone (PES) ultrafiltration membrane. In order to get the optimal structure and property of the NF membrane for the perm-selective of Mg2+ and Li+ from simulated brine consisting of MgCl2 and LiCl with a Mg2+/Li+ ratio of 20, we investigated the effects of the preparation parameters on the structure of NF membrane, such as the concentration and immersion time of PEI and TMC, respectively. At the same time, the structure-activity relationship between the membrane structure and the separation of Mg2+ and Li+ was analyzed. The control method of NF membrane microstructure and the mechanism of the separation of magnesium and lithium in positively charged composite NF membrane were investigated.

2.1. Chemicals and materials PES ultrafiltration membrane was prepared by the method named phase inversion which was stated in previous reports [41–45]. The permeability of pure water and the MWCO of the substrate membrane were 449 L/(m2 h) and 10 kDa, respectively. PEI with molecular weights of 70,000 (CAS#9002-98-6, Aladdin Reagent Co., Ltd., China), TMC (CAS#4422-95-1, Aladdin Reagent Co., China), sodium dodecyl sulfate (SDS) (CAS#151-21-3, Tianjin Yuanli Chemical Reagent Co., Ltd., China), Sodium carbonate (NaCO3) (CAS#497-19-8, Tianjin Yuanli Chemical Reagent Co., Ltd., China), nhexane (CAS#110-54-3, Tianjin Yuanli Chemical Reagent Co., Ltd., China) were used for the fabrication of polyamide thin-film selective layer by interfacial polymerization without further purification. Several analytical grade inorganic solutes including LiCl, MgCl2 (hexahydrate), NaCl, Na2SO4 and MgSO4 (heptahydrate) were provided by Tianjin Kermel Chemical Reagent Co., Ltd., which were utilized for the preparation of various feed solutions. 2.2. Preparation of composite NF membrane The fabrication process of composite NF membrane was presented in Fig. 1. Firstly, the PES substrate membrane was prepared by the method named phase inversion [41]. Then PEI aqueous solution (containing 0.1 wt% SDS as a surfactant, 0.1 wt% Na2CO3 as an acid acceptance agent) was poured into the cap device to contact with the surface of the PES substrate membrane for a desired time. After removing excess aqueous solution with a rubber roller, the membrane was covered by the TMC organic solution in n-hexane in the room temperature for a certain time to allow the interfacial polymerization to occur. The membrane was washed with the de-ionized to remove excess unreacted TMC organic solution after the membrane was placed in an oven at 70 °C for 10 min. Finally, the resulting composite NF membrane was stored in the de-ionized water before test. 2.3. Membrane characterization 2.3.1. Chemical structure To analyze the structure and function groups of the NF membranes surface, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was applied. X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical composition of the selective layer in NF membrane. 2.3.2. Morphology To investigate the surface and cross-sectional morphologies of the composite NF membranes, a field emission scanning electron microscopy (FE-SEM) (Hitachi S-4800, Japan) was used [46]. Atomic force microscope (AFM) (Agilent AFM 5500, USA) was applied to examine the surface roughness of composite NF membrane before and after interfacial polymerization. The AFM measurement was conducted with the mode of tapping, and the type of tip was Tap 300al-G. 2.3.3. Surface properties The surface zeta potential of the membrane was characterized with a Sur-PASS electrokinetic analyzer (Anton Paar GmbH, Austria) [47]. Hydrophilicity measurements of the air-dried composite NF membranes were carried out by dynamic contact angle goniometer (JC2000C Contact Angle Meter, Powereach Co.). At least five contact angles at different locations of each membrane sample were tested to evaluate the reliable value. 2.3.4. MWCO and pore size MWCO of the composite NF membrane were examined by the 58

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Fig. 1. Schematic representation of composite NF membrane fabrication.

Mg2+ and Li+ in the feed, respectively. The concentrations for Mg2+ and Li+ were examined by inductively coupled plasma optical emission spectroscopy (ICP-OES) (Varian 715ES) (Agilent). The inductively coupled plasma is an ionized gas (Ar, which is less than 1% ionized in the plasma) produced in a quartz torch using a 1.5 kW radio frequency power supply. Samples are atomized into the atomization system by atomization, and enter the center of plasma in the form of an aerosol. Then it is fully evaporated, atomized, ionized and excited in a high temperature and inert atmosphere to emit a characteristic spectrum of the contained element. The concentration of the corresponding element in the sample can be evaluated according to the characteristic spectrum intensity (quantitative analysis). The concentration of sample solution was diluted to less than 50 ppm. The concentrations of standard solution were 0, 10, 20, 30, 40 and 50 ppm, respectively. To investigate the salt rejection performance of NF membrane, kinds of single salt solution were applied, including LiCl, NaCl, Na2SO4, MgCl2 and MgSO4. The concentration of each salt solution was 2000 ppm, and the tests were conducted under a pressure of 8 bar. All tests were carried out at least three times to get the average values. A conductivity meter (DDS-11A, Shengbang Science and Technology Co., Tianjin, China) was applied to measure the concentration of single salt solution including LiCl, NaCl, Na2SO4, MgCl2 and MgSO4.

permeation test of 50 mg/L PEG aqueous solution with various molecular weights of 200, 400, 600, 800, 1000 and 2000 Da under a pressure of 3 bar, using the lab-scale cross-flow filtration flat-sheet membrane module [48]. The flow rate of membrane surface was around 0.1 m/s. A total organic carbon analyzer (TOC-5050A, Shimadzu, Japan) was utilized to measure the concentration of PEG solutions. The MWCO of the composite NF membrane was defined as the molecular weight of solute which had a rejection of 90%. The effective pose radius rp (nm) of composite NF membrane was evaluated by the MWCO sterichindrance pore model that reported in previous study [49,50].

rp = 0.0397MWCO 0.43

(1)

2.4. Membrane performance tests The lab-scale cross-flow filtration flat-sheet membrane module was carried out to evaluate the pure water permeation PWP (L/m2hbar) [51] and rejection R [37] of the membrane samples with an effective area of 12.56 cm2. The tests were carried out at a trans-membrane pressure of 8 bar and a temperature of 25 °C. The flow rate of membrane surface was around 0.1 m/s. Each membrane was stabilized for 0.5 h with de-ionized water before testing. In order to test the selective separation property of Mg2+ and Li+, simulated brine solution, a mixture of LiCl and MgCl2, was utilized, in which the mass ratio of the Mg2+/ Li+ is about 20. The concentration of the simulated mixed solution was 2000 ppm. All tests were carried out at least three times to get the average values. In order to investigate the selective separation performance of the composite NF membranes for the mixed solution consisting of MgCl2 and LiCl, the separation factor SMg,Li was evaluated by the following equation:

SMg, Li =

3. Result and discussion 3.1. Optimization of fabrication conditions for composite NF membrane Several experiments were carried out to study the influence of different interfacial polymerization parameters on the separation performance. All tests were conducted under the following conditions: 2000 ppm MgCl2/LiCl mixed solution with a Mg2+/Li+ mass ratio of 20 under a pressure of 8 bar.

CMg, p/ CLi, p CMg, f / CLi, f

(2) 2+

+

and Li in where CMg,p and CLi,p represent the concentration of Mg the permeate, respectively. CMg,f and CLi,f represent the concentration of

3.1.1. Effect of PEI concentration and immersion time Fig. 2(a)–(b) showed the relationship between PEI concentration 59

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Fig. 2. Effect of PEI concentration and immersion time on the separation performance of composite NF membrane: (a)–(c) rejection of Mg2+ and Li+; (b)–(d) separation factor SMg,Li and PWP (conditions: 0.1% (w/v) TMC aqueous solution; curing at 70 °C for 10 min).

The separation performance of positive charged NF membrane is dominated by Donnan exclusion theory when the polyamide active layer was well formed. According to Donnan exclusion theory, in the process of separate Mg2+ and Li+, the concentration of cations in feed solution is higher than in membrane phase, and the anions is higher in membrane phase. High valence cations (Mg2+) own a higher positive charge than low valence (Li+) [37], leading to a stronger exclusion between high valence cations and positive charged NF membrane which resulted a high rejection for Mg2+ than Li+. The influence of immersion time in PEI solution on properties of the obtained NF membrane was illustrated in Fig. 2(c)–(d). The concentration of PEI aqueous and immersion time in TMC were 0.5 wt% and 180 s, respectively. As the immersion time of PEI solution increased from 0.5 min to 5 min, the rejection for Mg2+ and Li+ increased by 17.3% and 427.8%, respectively. The opposite trend was observed for the separation factor SMg,Li and PWP. The SMg,Li decreased by 73.7%. When the resulting NF membrane attained the optimal separation performance, the performance changed little and remained stable as the immersion time in PEI solution further increased. This is because when the immersion time in PEI aqueous solution is too short, the polymerization reaction between PEI and TMC is not sufficient, which resulted in a loose and thin layer on the PES substrate membrane. As a result, the permeation of the membrane is high, but the separation performance is poor. The polyamide function layer became denser as the increase of the immersion time in PEI solution, which led to better separation performance. But when the polyamide thin layer was well formed, the continuous increase of the immersion time in PEI solution had little influence on the separation performance of NF membrane due to the self-limited of the polymerization, because the polyamide thin layer was dense and thick enough to prevent the diffusion of one phase to another. Thus, the optimal immersion time in PEI aqueous solution chosen for formation of composite NF membrane was

and separation performance of NF membrane. The immersion time in PEI and TMC aqueous were 3 min and 120 s, respectively. When the concentration of PEI increased from 0.25 wt% to 0.5 wt%, the rejection for Mg2+ and Li+ increased by 3% and 5%, respectively. Meanwhile, the SMg,Li and PWP deceased by 37% and 4.4%, respectively. While a further increase of PEI concentration led to a decrease in rejection of both Mg2+ and Li+ and an increase in both SMg,Li and PWP. With lower PEI concentration, the interfacial polymerization process is expected to be slow, resulting in a loose polyamide selective layer with lower selective separation performance, higher SMg,Li and PWP. At this time, the separation performance of polyamide positive charged NF membrane was dominated by steric hindrance effect. The polyamide active layer was too loose to effectively intercept ions, leading to a poor separation performance for Mg2+ and Li+. The rate of interfacial polymerization increased with the increase of PEI concentration and created a denser polyamide selective layer. However, the continuous increase of PEI concentration would reduce the selective separation performance and enhance water flux. This is caused by the nature that under a high PEI concentration, the supply of TMC is in deficit and a large of unreacted amine groups from PEI was therefore left in the polyamide active layer of membrane, which would reduce the degree of cross-link. So the optimal concentration of PEI used to form composite NF membrane was chosen to be 0.5 wt%. That is, when the concentration ratio of PEI/TMC is 5, the separation performance of membrane is best. According to the Donnan-Steric partitioning Pore Model [52], the NF membrane is considered as a charged porous film, and there are two main factors that affect the rejection of the NF membrane which coincide with Donnan exclusion theory and steric hindrance effect [53,54]: the hydration radius of ions and relative size of the NF membrane pore size, and the charging performance of the NF membrane. Due to the similar ion hydration radius of Mg2+ and Li+, the steric hindrance effect plays a minor role in the separation process. 60

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Fig. 3. Effect of TMC concentration and immersion time on the separation performance of composite NF membrane: (a)–(c) rejection of Mg2+ and Li+; (b)–(d) separation factor SMg,Li and PWP (conditions: immersed in 0.5 wt% PEI aqueous; curing at 70 °C for 10 min).

the SMg,Li and PWP reduced by 69.6% and 4.2%, respectively. With the continuous increase of the immersion time in TMC organic phase solution, the selective separation performance of Mg2+/Li+ and permeability of the resulting NF membrane changed a little and maintained relatively stable. The phenomenon can be explained by the fact that the degree of cross-link of PEI and TMC increased with the increase of immersion time in TMC organic solution, which caused the polyamide thin layer became compact and thicker [56]. The enhanced selective separation property for Mg2+/Li+ of NF membrane and the decreased in PWP was resulted from the existence of trade-off between rejection and PWP. When the polyamide function thin film is thick enough to hinder the diffusion of the PEI monomer from aqueous phase into the organic phase, further increase in the immersion time of TMC solution would have little influence on the separation performance and permeability of NF membrane. In sum, the optimal immersion time in TMC organic solution chosen for formation of composite NF membrane was 180 s.

5 min.

3.1.2. Effect of TMC concentration and immersion time The effect of TMC concentration on the selective separation performance of NF membrane was illustrated in Fig. 3(a)–(b). The immersion time in PEI and TMC aqueous were 5 min and 180 s, respectively. The separation performance of Mg2+ and Li+ enhanced with the increase of TMC concentration in n-hexane from 0.01% (w/v) to 0.1% (w/v), which manifested as the decrease of the Mg2+/Li+ mass ratio and the separation factor SMg,Li decreased by 81.5%. The rejection for Mg2+ and Li+ increased by 27.7% and 113%, respectively. And the corresponding PWP dropped by 49.8%. However, the further addition of TMC concentration not only had negative effect on the rejection for Mg2+ and Li+, but also reduced the separation property and increased the water flux. This is mainly caused by the reason that there was no enough TMC to contact with PEI when the TMC concentration was low, which led to a loose film with a lower selective separation performance. The active polyamide polymerization layer was gradually well formed with the increase of the TMC concentration [55]. However, once the optimum performance achieved, continued increase in the TMC concentration would actually degrade the selective separation performance of resulting NF membrane. Moreover, unreacted acyl chloride groups from TMC would hydrolyze to carboxyl groups, reducing the degree of polymerization between PEI and TMC but enhancing the hydrophilicity of polyamide thin layer owing to the presence of carboxyl groups. Consequently, the optimal concentration of TMC was 0.1% (w/v). Fig. 3(c)–(d) showed the influence of immersion time in TMC on selective separation performance of the resulting NF membrane. The immersion time in PEI aqueous and the concentration of TMC aqueous were 3 min and 0.1% (w/v), respectively. When prolong the immersion time in TMC organic phase solution from 30 s to 180 s, the rejection for Mg2+ and Li+ increased by 52% and 261%, respectively. Accordingly,

3.1.3. Effect of curing treatment The effect of curing temperature on the separation performance of resulting NF membrane was shown in Fig. 4(a)–(b). As the curing temperature rose from 50 °C to 70 °C, the difference between the rejection of Mg2+ and Li+ increased up to 76.3%, respectively. The corresponding PWP and the separation factor SMg,Li decreased by 16% and 72.2%,respectively, which indicated the enhance of the separation performance of membrane. However, when the temperature gradually rose to 100 °C, the rejection decreased and water flux increased. This phenomenon implied that the increase of the curing temperature accelerated the rate of polymerization between PEI and TMC, resulting in a dense polyamide thin layer with better selective separation performance but lower water flux. But a further rise of curing temperature would lead to shrinkage of the polyamide thin layer and increase of pore size. As a result, the larger pore size of the resulting NF 61

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Fig. 4. Effect of curing temperature and time on the separation performance of composite NF membrane: (a)–(c) rejection of Mg2+ and Li+; (b)–(d) separation factor SMg,Li and PWP (conditions: immersed in 0.5 wt% PEI aqueous for 5 min and in 0.1% (w/v) TMC solution for 180 s).

to a large number of protonated primary and secondary amine positive groups (-NH3+ and -NH2+). When the positive charged polyamide layer is well formed, the separation performance of composite NF membrane is dominated by Donnan exclusion theory. However, the separation performance of composite NF membrane is dominated by steric hindrance effect if the polyamide layer was loose. Because the composite membrane with loose polyamide layer possessed a surface with larger and loose hole, leading to poor separation properties for Mg2+ and Li+. Based on above results, the optimal interfacial polymerization conditions were immersion time of 5 min in 0.5 wt% PEI aqueous and 180 s in 0.1% (w/v) TMC solution, then curing at 70 °C for 10 min. NF membranes prepared under the optimal interfacial polymerization conditions possessed better separation performance for Mg2+ and Li+ compared with membranes prepared under other conditions. Next, we analyzed physical and chemical characteristics of the PEI-TMC composite membrane prepared under optimal condition.

membrane facilitated the permeation of water and metal ion through one side of the membrane to another. However, the corresponding separation factor SMg,Li and PWP increased and the rejection for both Mg2+ and Li+ decreased. Therefore, the optimal curing temperature was chosen as 70 °C. The relationship between curing time and the separation performance of resulting NF membrane was shown in Fig. 4(c)–(d). It can be seen clearly that the separation performance of resulting membrane is extremely poor when the curing time was short. With the increase of the curing time from 0.5 min to 10 min, the polyamide function film with better performance was gradually formed, which manifested as a decrease of the separation factor SMg,Li about 91.7% and an increase of the rejection for Mg2+ and Li+ about 111.6% and from 55.7%, respectively. But the PWP declined by 76.7% accordingly. Subsequently, the separation and permeability of NF membrane changed little when the curing time continued to increase. Above results can be explained by the reason that there is no enough time to form a dense polyamide thin layer when the curing time is too short. Thus with the increase of the curing time, the degree of polymerization between PEI and TMC was accelerated and the rate of crosslink was enhanced, resulting a dense polyamide active thin layer on the PES substrate membrane. As a result, the selective separation performance of Mg2+ and Li+ was enhanced, but the water flux reduced owing to the existence of trade-off between rejection and flux. When the polyamide thin layer is well formed, further increase of curing time had little influence on the performance of membrane. The rejection reduced a little and the water flux increased a little, which indicated that when the curing time is too long, it would cause a little shrinkage of the polyamide active film. Thence, the optimal curing time for the preparation of positive charged composite NF membrane was 10 min. After polymerization between PEI and TMC on PES substrate, a dense polyamide thin-film formed with a strong positive charge owing

3.2. Physical and chemical characteristics for optimal composite membrane In this part, the PEI-TMC composite NF membrane was fabricated under the optimal conditions: immersion time of 5 min in 0.5 wt% PEI aqueous and 180 s in 0.1% (w/v) TMC solution, then curing at 70 °C for 10 min. 3.2.1. Surface functional group and element The ATR-FTIR spectra of membranes before and after interfacial polymerization are shown in Fig. 5(a). Compared with the PES substrate membrane, several new characteristic peaks were found after the reaction between PEI and TMC. The new peak of amide occurred at 1634 cm−1, which indicated that the polymerization between amine groups from PEI monomer and acyl chloride groups from TMC monomer successfully happened. Furthermore, the characteristic peak 62

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Fig. 5. Surface functional group and element analysis for PES substrate ultrafiltration membrane and composite NF membrane (a) ATR-FTIR spectra; (b) XPS spectra.

of amine were detected at 2914 cm−1 and 2865 cm−1 [51,57], which declared that excess amine groups from PEI monomer were remained in the surface of the selective thin layer. XPS was used to analysis the elemental composition of the PSE substrate membrane and the composite NF membrane. As shown in Fig. 5(b), the major emission peaks observed at 285.25 eV, 399.86 eV and 531.29 eV corresponding to C1s, N1 s and O1s, respectively. In addition, two small peaks observed at 168.21 eV and 229.45 eV were attributed to S2p and S2 s. It can be seen in Fig. 5(b) and Table 1, the peak intensity of S2p and S2 s of composite NF membrane were weaken than those observed for the PES substrate membrane. Furthermore, compared with the PES substrate membrane, the atomic percentage of S element on the composite NF membrane declined owing to the polyamide selective thin film layer of the composite membrane did not have any sulfur. The weak peak at 399.86 eV for PES substrate was attributed to the N1 s of the polyvinylpyrrolidone (PVP), which was added in the PES casting solution. After interfacial polymerization, the atomic concentration of N1 s increased from 2.03% to 12.07% and the atomic ratio of N/O increased from 0.10 to 0.78, indicating that a large quantity of N element were introduced to the surface of composite NF membrane. The N element of the polyamide selective layer was mainly related to the amine content in the PEI molecules, it indicated that the interfacial polymerization happened and the polyamide selective thin film layer was indeed formed.

interfacial polymerization, AFM measurement was applied with a scan size of 10 μm × 10 μm. All tests were carried out at least three times to get the average values of root-mean square roughness (Rms) and average roughness (Ra). As shown in Fig. 7 and Table 2, the PES substrate membrane possessed a smooth surface. While after the occurrence of interfacial polymerization, the Rms and Ra of composite NF membrane both increased, which confirmed the formation of the polyamide selective thin layer. Moreover, rougher surface may improve the water permeation of the membrane due to the expanded effective surface area of the membrane [62]. The phenomenon tested by AFM was consistent with the results of SEM image. 3.2.3. Surface properties The surface charge characteristics of the PES substrate membrane and composite NF membrane were evaluated by the streaming potential method. The 1 mmol/L KCl solution was utilized as the background, and 0.1 mol/L HCl and NaOH were utilized to adjust the pH. As shown in Fig. 8(a), the PES substrate membrane has isoelectric point below pH = 5, indicating the PES substrate membrane is negative charged. While the isoelectric point of the resulting composite NF membrane is about pH = 9.3 owing to a large number of unreacted primary and secondary amine positive groups (-NH3+ and -NH2+) from PEI. When the pH is below the isoelectric point, the composite NF membrane is positive charged as a result of the protonation of the unreacted amine groups from PEI monomer [63,64]. The composite NF membrane is negative charged in the case of pH above the isoelectric point, owing to the existence of the deprotonated carboxyl groups hydrolyzed from unreacted acyl chloride groups of TMC [65]. The composite NF membrane possessed high positive charges under the feed solution pH range involved in this research. Above phenomenon proved the positive charged polyamide interfacial polymerization layer was well formed, which facilitates the separation of divalent cations (Mg2+) and monovalent cations (Li+). The dynamic contact angles were applied to analysis the hydrophilicity of membrane surface. As shown in Fig. 8(b), the dynamic contact angle showed a reduce trend after interfacial polymerization, indicating the hydrophilicity of composite NF membrane was improved compared with the PES substrate membrane. The decrease of the dynamic contact angle is mainly due to hydrophilic amine or carboxyl groups presented in the polyamide thin film layer, which can further demonstrate the formation of the polyamide selective layer. The reason lies in that less unreacted acyl chloride groups of TMC hydrolyzed to carboxyl groups and some unreacted amine group from PEI is therefore left in the polyamide layer, which eventually enhanced the hydrophilicity of the resulting NF membrane.

3.2.2. Morphologies and structures The top surface and the cross-section of the PES support membrane and composite NF membrane are shown in Fig. 6. As shown in Fig. 6(a), the PES substrate membranes possessed an asymmetric structure and a micro-porous finger-like sub-layer. It can be seen in the Fig. 6(d)–(f) that a clearly visible thin selective layer was formed on the PES substrate layer after interfacial polymerization. The PES substrate membrane contained a smooth surface with uniformly distributed pores. The composite NF membrane exhibited a denser and rougher surface morphology without perceptible pores, which demonstrate that the polyamide selective layer was successfully formed on the surface of the PES substrate membrane. According to several previous study [58,59], the aqueous phase monomers (PEI) with lager amount of amine groups can migrate into the organic phase, which can push the formation of the ridge and valley cross-linked network structure of the selective layer, leading to an uneven surface of the composite NF membrane [60,61]. To evaluate the surface roughness of membrane before and after Table 1 Elements content of membrane surface from XPS analysis. Membrane

C (%)

N (%)

O (%)

S (%)

N/O

PES support membrane Composite NF membrane

71.99 68.72

2.03 12.07

19.6 15.4

5.66 0.47

0.10 0.78

3.3. Stability of separation performance We can know about the stability performance of the composite NF membrane for the separation of Mg2+ and Li+ from Fig. 9. Fig. 9(a)-(b) 63

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Fig. 6. SEM images of PES substrate ((a): the surface; (c): the cross section) and composite NF membrane ((b): the surface; (d)-(e)-(f): the cross section).

Fig. 7. AFM images of PES substrate (a) and composite NF membrane (b).

investigate the separation performance for Mg2+ and Li+ over multiple cycles, a 3 h cycle filtration was conducted, and the membrane was cleaned with de-ionized water before each 3 h cycle filtration test. As shown in Fig. 9(c)–(d), the separation performance remained in each cycle test. The rejection of Mg2+ and Li+ can reach up to the initial level after being cleaned with de-ionized water, and the PWP changed a little over three cycle tests. The results indicated that the stability of the functional groups was less affected after long time filtration, and the separation performance of the composite NF membrane changed a little over multiple cycles. In conclusion, the composite NF membrane exhibited a stable performance for the separation of Mg2+ and Li+.

Table 2 Root-mean square roughness (Rms) and average roughness (Ra) of membranes. Membranes

Rms (nm)

Ra (nm)

PES substrate Composite NF membrane

11.4 ± 2.4 28.3 ± 0.9

8.73 ± 1.3 21.4 ± 0.7

showed the changes of PWP and rejection of the composite NF membrane during a 24 h filtration test. It can be seen obviously that the rejections of Mg2+ and Li+ remained relatively stable values of around 95% and 19%, respectively. The corresponding SMg,Li in Fig. 9(b) showed the same trend of rejection in Fig. 9(a). At the same time, the PWP only decreased a little within 24 h filtration test. In order to 64

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Fig. 8. Zeta potential and dynamic contact angle of PES substrate membrane and composite NF membrane (a) Zeta potential; (b) dynamic contact angle.

Fig. 9. Variation of separation performance of composite NF membrane for: (a)–(b) 24 h filtration and (c)–(d) multiple cycles (2000 ppm feed solution with a Mg2+/ Li+ mass ratio of 20 at 8 bar).

Fig. 10. (a) MWCO measurement tested with 50 ppm PEG at 3 bar; (b) separation performance tested with 2000 ppm different inorganic salt at 8 bar.

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Table 3 Comparison of NF membrane with other reported NF membranes and commercial NF membrane on the separation property for Mg2+ and Li+. Membranes

Composite NF hollow fiber membrane EDTA modified NF membrane NF90 DK DL-2540 PEI-TMC composite NF membrane

Rejection (%) 2+

SMg,Li

PWP (L/m2hbar)

Reference

0.384 0.108 0.476 0.31 0.35 0.05

– 0.6 – – – 5.02

[37] [38] [37] [35] [17] This work

+

MgCl2

LiCl

MgSO4

Na2SO4

NaCl

Mg

Li

70.4 84.6 – – – 94.8

21.8 68.1 – – – 30.6

36 91.7 – – – 84.1

– 83.1 – – – 81.4

23 69.1 – – – 36.9

46 91.9 60.5 – 60 95

−40.7 35 15 – −20 19

3.4. Salt rejection performance of optimal composite membranes

for the recovery of lithium resource from salt lake brine.

MWCO of the resulting composite NF membrane were defined by permeation test of PEG as mentioned previously. The rejection for different molecular weight of PEG molecules was illustrated in Fig. 10(a). As shown in Fig. 10(a), the MWCO of the composite NF membrane was about 340 Da, and the pore radius of composite NF membrane was around 0.48 nm in according to Eq. (1) in Section 2.3.4. The property of positive charged composite NF membrane for different salt (MgCl2, LiCl, MgSO4, Na2SO4, NaCl) with concentration of 2000 ppm were further investigated and shown in Fig. 10(b). The permeate flux of MgCl2, MgSO4, Na2SO4, NaCl and LiCl were 3.7, 3.9, 4.1, 4.2 and 4.4 (L/m2hbar), respectively. It is obviously that the order of rejection for different salt was as follows: MgCl2 (94.8%) > MgSO4 (84.1%) > Na2SO4 (81.4%) > NaCl (36.9%) > LiCl (30.6%), which revealed that the resulting composite NF membranes were all positively charged. According to the theory of Donnan exclusion, high valence cations (Mg2+) carry a higher positive charge than low valence (Na+, Li+), resulting a stronger exclusion between high valence cations and positive charged NF membrane, leading to a higher rejection for high valence cations. Similarly, the positive charged NF membrane has a higher rejection for low valence anions (Cl−) than high valence anions (SO42−), resulting a higher rejection of MgCl2 than MgSO4. As for positive charged membranes, the rejection of Na2SO4 should be lower than NaCl and LiCl according to Donnan exclusion theory. However, the results were contrary to the theory. This phenomenon was due to the fact that the SO42− possessed a larger hydrated radius than Cl− [40]. In addition, the higher retention of MgSO4 than NaSO4 is due to the double load of Mg2+, and the higher rejection of Na2SO4 than NaCl and LiCl was due to the larger size of the hydrated SO42− ions [37]. In a conclusion, the separation performance of positive charged NF membrane for inorganic salt is determined by theory of Donnan exclusion and steric hindrance. According to the equation of the separation factor SMg,Li, we can know that the smaller the SMg,Li, the better the separation performance of the membrane for Mg2+ and Li+. It is clearly shown in Table 3 that the separation factor SMg,Li of composite NF membrane fabricated in this work was only 0.05. While the SMg,Li of other membrane were all greater than 0.05. Compared with commercial NF membrane (NF90, DK, DL-2540) shown in Table 3, the composite NF membrane prepared in this study exhibited a higher rejection for Mg2+ and the separation factor SMg,Li decreased by more than 5 times. This is because these commercial NF membranes were negative charged, which were not suitable for the separation of Mg2+ and Li+. Compared with other positive charged NF membrane in Table 3, the separation factor SMg,Li of the positive charged NF membrane prepared in this study was less than half of them. The results indicated that the difference in aqueous monomer during interfacial polymerization directly determines the separation performance of the membrane. PEI carries a large number of primary and secondary amine positive groups. Polymerization between PEI and TMC will lead to a higher positive charged membrane compared with other aqueous monomer. This result indicated that the positive charged PEI-TMC NF membrane has a good application prospect

4. Conclusion A positive charged NF membrane containing abundant -NH3+ and -NH2+ was designed and successfully fabricated via interfacial polymerization between PEI and TMC on the PES substrate membrane, which was dedicated to separate Mg2+ and Li+ from salt lake brine with high Mg2+/Li+ mass ratio. The results in this study indicated that the properties of resulting composite NF membrane were mainly affected by PEI aqueous owing to a large number of primary and secondary amine positive groups of PEI. The separation performance was dominated by Donnan exclusion theory owing to the similar ion hydration radius of Mg2+ and Li+. The optimal interfacial polymerization conditions were 0.5 wt% PEI aqueous for 5 min and 180 s in 0.1% (w/v) TMC solution, then curing at 70 °C for 10 min. Under the optimal interfacial polymerization condition, the resulting NF membrane exhibited good performance on the separation of Mg2+ and Li+ for simulated brine consisting of MgCl2 and LiCl with Mg2+/Li+ mass ratio of 20 under a pressure of 8 bar. After the filtration of feed solution, the Mg2+/Li+ mass ratio declined to 1.3, the separation factor SMg,Li and the rejection for Mg2+ and Li+ were 0.05, 95% and 19%, respectively. Compared with other NF membranes in previous reports, the SMg,Li of positive charged NF membrane prepared in this study was less than half of them. And the water flux was 5.02 L/ (m2hbar), which dedicated the existence the trade-off between the rejection and the flux. Characterization contained ATR-FTIR, XPS, AFM and zeta potential, etc., which all demonstrated the fabrication of positive charged polyamide function layer on PES substrate. After the polymerization between PEI and TMC, the corresponding dynamic contact angle decreased on account of hydrophilic amine or carboxyl groups from polyamide layer. The isoelectric point of composite NF membrane reached up to pH = 9.3 owing to the presence of a large number of protonated primary and secondary amine positive groups (-NH3+ and -NH2+) on PEI. Furthermore, the salt rejection followed the order: MgCl2 (94.8%) > MgSO4 (84.1%) > Na2SO4 (81.4%) > NaCl (36.9%) > LiCl (30.6%). The rejection performance of resulting membranes for salt was affected not only by the Donnan exclusion theory, but also by the steric hindrance effect. All the results proved that the positive charged NF membrane fabricated via interfacial polymerization between PEI and TMC has a prospect application for the separation of Mg2+ and Li+ from salt lake brine with high Mg2+/Li+ mass ratio so as to improve the recycling rate of lithium resources. Acknowledgments The work was funded by the Qaidam Salt Chemical Joint Fund of National Natural Science Foundation of China - People's Government of Qinghai Province (U1607117), the National Natural Science Foundation of China (51708409), the Natural Science Foundation of Tianjin (16JCZDJC36400) and the Science and Technology Plans of Tianjin (15PTSYJC00230). 66

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