Electro-nanofiltration membranes with positively charged polyamide layer for cations separation

Journal of Membrane Science 594 (2020) 117453

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Electro-nanofiltration membranes with positively charged polyamide layer for cations separation

T

Fangmeng Shenga, Linxiao Houa, Xiuxia Wangb, Muhammad Irfana, Muhammad A. Shehzada, Bin Wua, Xuemei Renc, Liang Gea,d,∗∗, Tongwen Xua,∗ a

CAS Key Laboratory of Soft Matter Chemistry, iCHEM (Collaborative Innovation Center of Chemistry for Energy Materials), Department of Applied Chemistry, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, 230026, PR China b Center for Micro- and Nanoscale Research and Fabrication, University of Science and Technology of China, Hefei, 230026, PR China c CAS Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute of Plasma Physics, Chinese Academy of Sciences, P. O. Box 1126, Hefei, 230031, PR China d Applied Engineering Technology Research Center for Functional Membranes, Institute of Advanced Technology, University of Science and Technology of China, Hefei, 230088, PR China

ARTICLE INFO

ABSTRACT

Keywords: Electrodialysis Ion separation Electro-nanofiltration membrane Interfacial polymerization

High performance and low-cost monovalent cation perm-selective membranes (MCPMs) are extremely desirable since the widespread demand for monovalent/multivalent cations separation. Herein, we developed a simple fabrication method for the preparation of electro-nanofiltration membranes (ENFMs), which were used as MCPMs for cations separation. The ultra-thin polyamide layer of ENFMs could offer selective channels for the transport of monovalent cations. Meanwhile, the positive charges in the polyamide layer act as useful barriers to divalent cations via electrostatic repulsion. The results show that the ENF-Q3 membrane with the highest quaternization degree owns outstanding perm-selectivity for Li+/Mg2+ system +

P Li 2 + = 11.3 , which is 7 times greater than that of the commercial CSO membrane, and high Li+ flux Mg

(JLi+ = 5.79×10 8 mol·cm 2·s 1) at the current density of 10 mA cm−2. Moreover, the ENFMs exhibit high limiting current density and excellent long-term stability, indicating that the developed strategy is promising to scale up for industrial applications.

1. Introduction Electrodialysis (ED) is an important membrane separation technology and generally known as a “no phase transition” and “near zero liquid discharge” process [1]. Ion exchange membranes (IEMs) are the key component of ED which can selectively separate counter-ions and co-ions based on the Donnan effect and have been widely used for brine desalination, cleaner production, and water reuse [2,3]. However, the conventional IEMs cannot meet the requirement to separate some special ions such as lithium extraction from magnesium-rich salt lakes [4,5], edible salt extraction from seawater or brine [6], and acid recovery from waste acidic solution containing a large number of metal ions [7]. Especially, in recent years, the demand for lithium resources is increasing rapidly owing to the continuous development of new energy,

automotive and electronics industries [8]. However, during lithium production from salt lakes, the extremely low concentration of Li+ and high concentration of other cations such as Mg2+, produce many technical challenges and reduce the process efficiency. To overcome this problem, monovalent cation perm-selective membranes (MCPMs) with preferential transport of monovalent cations are urgently needed nowadays. Till to date, numerous methods have been used to fabricate highperformance MCPMs. For example, Lee et al. [9] prepared a covalently cross-linked graphene oxide (GO) membrane by using tannic acidfunctionalized GO and polyethyleneimine. The cross-linked GO membrane showed excellent ions separation performance and K+/Mg2+ selectivity can attain 18.89 at a 0.05 mol L−1 mixed solution. Different from the chemical crosslinking, our previous work proposed an

Corresponding author. Corresponding author. CAS Key Laboratory of Soft Matter Chemistry, iCHEM (Collaborative Innovation Center of Chemistry for Energy Materials), Department of Applied Chemistry, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, 230026, PR China. E-mail addresses: [email protected] (L. Ge), [email protected] (T. Xu). ∗

∗∗

https://doi.org/10.1016/j.memsci.2019.117453 Received 16 April 2019; Received in revised form 4 September 2019; Accepted 6 September 2019 Available online 10 September 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.

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annealing treating method to adjust the crystalline degree of PVA-based cation exchange membranes (CEMs) [10]. The improvement in membrane density resulted from the increased crystalline degree obviously enhanced the ion perm-selectivity. However, sharply increased membrane area resistance and decreased ion flux usually accompany the improved ion perm-selectivity. Alternatively, surface modification is a more promising strategy to avoid the above-mentioned deficiency. White and co-workers [11] coated polyelectrolyte multilayers on a Nafion membrane to improve its ion perm-selectivity. The modified membranes exhibited excellent Li+/Co2+ perm-selectivity and maintained almost the same Li+ flux compared with the pristine Nafion membrane. Coincidentally, Le et al. [12] proposed a diazonium-induced anchoring process to graft a polyaniline-like thin layer on the commercial CEM surface. Compared with the bare membrane, the

2. Experimental 2.1. Materials Polyacrylonitrile (PAN) porous membrane supported by non-woven (Guo Chu Technology Co., China) acted as a substrate for ENFMs fabrication. Commercial anion exchange membrane AMX (Neosepta, Tokuyama Co., Japan) was used as an auxiliary membrane while the standard monovalent cation perm-selective membrane CSO (Selemion, Japan) acted as a reference. DETA and TMC were obtained from Shanghai Macklin Biochemical Co. Ltd. (Shanghai, China). Iodomethane was from Saan Chemical Technology Co. Ltd. (Shanghai, China). The other reagents were supplied by China National Pharmaceutical Group Industry Co. Ltd. (Beijing, China). All these reagents were of analytical grade and used as received. Deionized (DI) water was prepared by a reverse osmosis filtration system and used in all experiments.

+

perm-selectivity P H 2 + of the modified membrane increased from 17.9 Ni

to 166.7 while the area resistance increased slightly (about 8%). Li et al. [13] used a one-pot method to prepare MCPMs by a fast co-deposition of polydopamine/polyethylenimine composites with MIL-53(Al) on the commercial CEM surface. The perm-selectivity achieved an obvious elevation and the Na+ flux maintained at a higher level compared with the pristine membrane. Undoubtedly, surface modification is an effective method to improve the perm-selectivity of the membranes and meanwhile incapable of deteriorating membrane area resistance and monovalent ion flux. Furthermore, it is generally adopted and deemed as a hopeful solution for commercial MCPMs production. Recently, our group proposed a methodology similar to surface modification, i.e., constructing electro-nanofiltration membranes (ENFMs) for cations separation [14]. During this study, the porous substrate of ENFMs is beneficial to decrease the area resistance of the membrane and improve the ion flux. The ultra-thin polyamide layer of ENFMs effectively rejected the ions with larger hydrated ionic radius. Subsequently, the optimization of the pore size of the polyamide layer was systematically investigated to further elevate the ion separation performance [15]. Encouraged by our previous results, the novel ENFMs with positively charged polyamide layer were designed. In this case, the dense polyamide layer can guarantee the high perm-selectivity between monovalent and divalent cations based on the pore-sieving effect. Additionally, the difference in electrostatic repulsion between mono-/divalent cations and the positively charged polyamide layer may further improve the rejection of divalent cations. Based on the adopted strategy, water-soluble diethylenetriamine (DETA) was selected to react with 1,3,5-benzenetricarbonyl trichloride (TMC) to produce polyamide layer on the surface of hydrolyzed polyacrylonitrile (HPAN) porous substrate. The polyamide layer including secondary and tertiary amines was further quaternized by iodomethane. Afterwards, the quaternization degree was adjusted while the chemical composition, microstructures, physicochemical properties and ion separation performance of the produced membranes were systematically and successfully investigated.

2.2. Preparation of ENFMs The preparation schematic of the ENFMs is shown in Fig. 1. The PAN porous substrate was hydrolyzed in 2 mol L−1 NaOH solution at 60 °C for 2 h to form –COOH groups, which improves the hydrophilicity and facilitates the spreading of aqueous amine solution on the HPAN substrate surface [16]. Then, the HPAN substrate was acidified by immersing in 1 mol L−1 HCl solution at room temperature for 12 h. Subsequently, it was rinsed thoroughly with DI water to wipe off the residual acid. After that, the HPAN substrate saturated by amine (by immersing in a 1 wt% DETA aqueous solution for 15 min) was fixed on a clean glass plate. A squeegee roller was employed to get rid of the excess aqueous amine solution on the HPAN substrate surface. Finally, the HPAN substrate was immersed in a 0.1 wt% TMC hexane solution for 3 min to perform interfacial polymerization (IP) reaction. The obtained membrane was washed repeatedly with hexane and DI water, and designated as ENF membrane. The ENF membrane was further modified in a 10 wt% iodomethane/methanol solution under alkaline condition at 25 °C. Then the reaction times were set for 12 h, 24 h, and 48 h to obtain the final membranes with different quaternization degrees. The prepared membranes were denoted as ENF-Q1, ENF-Q2, and ENF-Q3, respectively. 2.3. Membrane characterization 2.3.1. Structural characterization The chemical structure of the prepared ENFMs was analyzed by attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR, Thermo Nicolet FTIR spectrometer, USA) with the resolution of 2 cm−1. The elemental composition and quaternization degree of the polyamide layers were determined by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB250, USA). The surface and crosssectional morphologies of HPAN substrate and ENFMs were

Fig. 1. Schematic representative of the preparation procedure of ENFMs.

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effective exposed area of 7.07 cm2 was placed between the two middle compartments. The AMX membrane closing to the electrode worked as an auxiliary membrane to alleviate the influence of electrode reactions. Prior to the test, the membrane was equilibrated in 0.5 mol L−1 NaCl solution for 24 h to achieve a balance. Then, 0.5 mol L−1 NaCl solution was fed to the two middle compartments, while 0.3 mol L−1 Na2SO4 was supplied as an electrode solution. The applied current was 0.04 A and the potential difference between both sides of the investigated membrane was recorded with a multimeter (VC890C+, Shenzhen Victor Hi-Tech Co., Ltd., China). All the compartments were well circulated by peristaltic pumps (YZ15, Baoding Lead Fluid Co., Ltd., China) during the testing process. The membrane area resistance (Rm) can be calculated from the change in the potential difference with and without the investigated membrane [18]. The Rm was expressed as follows:

investigated by field emission scanning electron microscope (SEMmask-Hitachi 8220). Ion Beam Sampling (IBS-Leica-EMTIC3X) was used to obtain a high quality in situ cross-section microtopography of the membranes. The roughness of the membrane surface was measured with a scanning probe microscope (SPM, DI Innova, USA). 2.3.2. Water uptake, area swelling, contact angle and cation exchange capacity The water uptake and area swelling were measured by the change in weight and area between the dried and the wet membranes, respectively. A square-shaped membrane sample (1.50 × 1.50 cm2) was kept in an oven at 60 °C for 24 h. Then, the weight (Wdry) and area (Adry) of the dried membrane sample were measured accurately. Afterwards, the membrane sample was immersed in DI water at room temperature for 24 h. After removing the redundant water on the membrane sample surface, the weight (Wwet) and area (Awet) of the membrane sample were re-measured, respectively. The water uptake and area swelling were calculated by Eq. (1) and Eq. (2), respectively:

Water uptake (%) =

Area swelling (%) =

Wwet

Wdry

Wdry Awet

Adry

Adry

× 100

× 100

Rm =

M Wdry

U0 I

× Am

(4)

where Um (V) and U0 (V) are the potential difference with and without the investigated membrane, respectively; I (A) is the current and Am is the effective area of the membrane. The current-voltage (I–V) curves were tested using the same apparatus. A mixed solution of 0.1 mol L−1 LiCl and 0.1 mol L−1 MgCl2 was fed into the two middle compartments, while 0.3 mol L−1 Na2SO4 solution was selected as an electrode solution. All the solutions were well circulated by peristaltic pumps during the tests. Before the test, the prepared membrane sample was equilibrated in the mixed solution for 24 h. During the testing, the current density was kept a steady increment from 0 to 70 mA cm−2. At the same time, the corresponding potential difference across the investigated membrane was recorded by a multimeter.

(1) (2)

The contact angle was measured on a contact angle meter (SL200B, Solon Tech Co., Ltd., China) at room temperature. A dry membrane sample was placed flatly on a clean glass slide and a total of 20 μL DI water was dropped onto the sample surface by a microsyringe. Meanwhile, the static contact angle was captured by the video of the contact angle meter. The cation exchange capacity (CEC) was determined as follows. A pre-weighted membrane sample (Wdry) was equilibrated in 1 mol L−1 HCl solution for 24 h. Then, it was taken out and rinsed thoroughly with DI water to wipe off the excess HCl. Afterwards, the sample was transferred into 50 mL of 0.01 mol L−1 NaOH solution for 24 h. The released H+ ions were measured by titrating the initial and final concentration of NaOH solution, respectively. The CEC was obtained from the following Eq. (3):

CEC =

Um

2.3.4. Evaluation of cation flux and perm-selectivity The evaluation of cation flux and perm-selectivity were carried out using an ED stack with an exposed area of 20 cm2 (Fig. 3). The diluted compartment contained 200 mL of 0.1 mol L−1 LiCl/0.1 mol L−1 MgCl2 (0.1 mol L−1 NaCl/0.1 mol L−1 MgCl2) mixed solution and the concentrated compartment was fed with 400 mL of 0.01 mol L−1 KCl solution. Meanwhile, 200 mL of 0.3 mol L−1 Na2SO4 solution was poured into the electrode compartments. The ED experiments were performed at a current density of 10 mA cm−2 using a DC electrical potential. All the solutions were well circulated by peristaltic pumps at the flow rate of 5.2 L h−1 to avoid the concentration polarization. The AMX membrane closing to the electrode compartment worked as an auxiliary membrane. Each experiment was performed for 1 h. The concentrations of Li+, Na+, and Mg2+ in the concentrated compartment were detected using ICP-OES (Optima 7300 DV, USA). The cation flux was calculated using the following equation.

(3)

where M (mmol) is the difference of the initial and final concentration of NaOH moles, respectively [17]. 2.3.3. Area resistance and current-voltage curves A four-compartment ED device (Fig. 2) was used to evaluate the area resistance of the membrane. The investigated membrane with an

J=

(Ct

C0 ) × V Am × t −2 −1

(5) −1

where J (mol·cm ·s ) represents the ion flux, Ct (mol·L ) and C0 (mol·L−1) are the ion concentrations of the concentrated compartment at time t and 0, respectively; V (dm3) is the volume of the concentrated compartment. The perm-selectivity between monovalent and divalent cations can be obtained by the following equation as reported previously [19]: +

P MN2 + =

J N + c M2 + J M2 + c N +

(6)

+ where P N2 + denotes the perm-selectivity between cations N+ and M2+, M + c N (mol· L 1) and c M2 + (mol·L−1) are the concentrations of N+ and M2+

in the diluted compartment during the experiment, respectively. Especially, ENF and ENF-Q3 membranes were tested for 5 cycles to evaluate membrane stability. After each cycle operation, the solution of concentrated compartment was gathered for detecting and the whole device was washed with DI water for more than 30 min.

Fig. 2. Schematic illustration for the area resistance evaluation. 3

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Fig. 3. The schematic diagram of the ED stack for ion separation evaluation.

3. Results and discussion 3.1. ATR-FTIR and XPS spectroscopies As shown in Fig. 4, the ATR-FTIR spectrum of the HPAN substrate showed a characteristic peak at 1730 cm−1 representing the stretching vibration of C]O in carboxyl groups [20]. Meanwhile, the peak at 2250 cm−1 represented the –CN stretching vibration. After the IP reaction, two new characteristic absorption peaks were observed at 1545 cm−1 and 1649 cm−1 corresponding to the stretching vibration of N–H (amide II) and C]O (amide I), respectively. In addition, the peaks intensity of –COOH and –CN groups decreased due to the formation of the PA layer on the HPAN substrate surface. After the quaternization, the characteristic peaks of N–H (amide II) and C]O (amide I) were still existed which indicated that the PA layer was reserved. XPS spectra were conducted to determine the surface chemical composition and element contents of the prepared membranes. The three major peaks were observed at 532.0 eV, 399.8 eV, and 285.2 eV corresponding to O1s, N1s, and C1s, respectively (Fig. 5). Meanwhile, the high-resolution spectra of N1s are shown in Fig. 6. There was only one emission peak at 399.5 eV corresponding to C–N species for the ENF membrane, while all the quaternized ENF membranes showed a new peak at 402.2 corresponding to C–N+ species, which indicated the

Fig. 5. XPS spectra of ENF, ENF-Q1, ENF-Q2, and ENF-Q3 membranes.

formation of quaternary ammonium groups in the PA layer. The quaternization degree can be estimated by the molar ratio of the C–N+ species to the total nitrogen species, defined as C–N+/(C–N + C–N+) [21]. As shown in Table 1, the quaternization degree gradually increased with reaction time and the ENF-Q3 membrane obtained the highest quaternization degree of 18.9%. 3.2. Microstructures characterization of the prepared ENFMs As shown in Fig. S1, the HPAN substrate has numerous mesopores with about 200 nm in diameter uniformly distributed on the smooth surface and a typical finger-like macrovoids structure. After the IP reaction, an ultra-thin (about 250 nm) PA layer formed on the HPAN substrate surface (Fig. 7e, Fig. S1c). The morphology of the PA layer surface showed a continuous striped structure (Fig. 7a), which is similar to the Turing structure [22]. As seen in Fig. 7b–d, no obvious pores could be observed and the PA active layer was still reserved after the quaternization reaction. However, the thickness of the active layer decreased with the increasing quaternization reaction time (Fig. 7f–h, Fig. S1d). The possible reason is the partial hydrolysis of the amide bond connected with a quaternary ammonium group in alkaline condition [23]. Note that in our work, considering the quaternized amides permitted hydrolyzes on strong alkaline conditions, we purposely used

Fig. 4. ATR-FTIR spectra of HPAN substrate, ENF, ENF-Q1, ENF-Q2, and ENFQ3 membranes. 4

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Fig. 6. The N1s high-resolution spectra of ENF, ENF-Q1, ENF-Q2, and ENF-Q3 membranes.

3.3. Water uptake, area swelling, contact angle and cation exchange capacity

Table 1 The element contents for ENF, ENF-Q1, ENF-Q2, and ENF-Q3 membranes. Membrane ENF ENF-Q1 ENF-Q2 ENF-Q3

Atomic Conc. (mol%) C 1s

N 1s

O 1s

68.73 67.64 61.51 69.13

12.55 13.33 8.72 10.76

18.72 19.03 29.77 20.11

Quaternization degree (%) C–N+/(C–N + C–N+)

The key physicochemical properties of HPAN substrate and ENFMs are presented in Table 3. After hydrolysis of PAN substrate, although the –CN group can be transformed into –COOH, the CEC value of the HPAN substrate is only 0.21 mmol g−1. This is due to the HPAN substrate supported by the non-woven fabrics without charged groups. Compared with the HPAN substrate, the PA layer has a higher density of –COOH groups because of the partial hydrolysis of acyl chloride groups. After the quaternization reaction, the value of CEC decreased with the increasing quaternization degree. This is mainly due to the decline in the thickness of PA active layer. Contact angle is another important parameter to evaluate the hydrophilicity of the membrane surface. The HPAN substrate has the minimum contact angle derived from the carboxyl groups and the pore structure. After the IP reaction, though the contact angle has a slight increment, the ENF membrane also shows good hydrophilicity because of carboxyl and amine group on the surface layer [24]. Meanwhile, the contact angle decreased with the increased quaternization of the membranes due to the decline in membrane surface roughness [25]. Moderate water uptake and low area swelling are the important parameters for the performance and application of the ENFMs [26,27]. Satisfyingly, the special structure of HPAN substrate, i.e., a typical finger-like macrovoids structure on the top layer with non-woven fabrics supporting endowed all the ENFMs moderate water uptake and low area swelling. Consequently, the prepared membranes showed low area resistance compared to commercial CSO (6.82 Ω cm2).

/ 5.9 12.0 18.9

a low concentration (0.14 M NaOH/methanol solution) to fulfill the quaternization reaction and producing the quaternized polyamide. AFM images of HPAN substrate and all the prepared ENFMs are shown in Fig. 8 and Fig. S2. The gaps between the peaks and the valleys on the HPAN substrate were uniform and smooth (Fig. S2), which illustrates the low surface roughness of the substrate. After the IP reaction, the surface of the PA layer showed more roughness as shown in Fig. 8. The ridges observed in the AFM images also agreed with the obvious striped-like structure in the SEM images. In addition, the surface roughness of the ENFMs decreased gradually with the increasing reaction time, indicating the quaternization reaction time greatly affected the PA active layer morphology. The detailed results of the root means square and the mean roughness are displayed in Table 2.

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Table 2 The root means square and the mean roughness for the surfaces of HPAN substrate, ENF, ENF-Q1, ENF-Q2, and ENF-Q3 membranes. Membrane

Rq (nm)

Ra (nm)

HPAN ENF ENF-Q1 ENF-Q2 ENF-Q3

9.58 75.3 49.0 36.6 13.1

7.81 61.4 39.7 28.2 10.5

3.4. Current-voltage curves The limiting current density (LCD) reveals the maximum current applied in the ED process, which appears on the inflection point between the ohmic and plateau regions in the I–V curves [28]. As shown in Fig. 9A, the LCD of commercial CSO membrane is 28 mA cm−2, while the ENF, ENF-Q1, ENF-Q2, and ENF-Q3 membranes were 63 mA cm−2, 56 mA cm−2, 50 mA cm−2, and 32 mA cm−2, respectively (Fig. 9B). Compared with the CSO membrane, the ENF membrane obtained higher LCD benefiting from the particular structure including porous HPAN substrate and ultra-thin PA layer. After the quaternization, the LCD declined with the increment in quaternization degree because the electrostatic repulsion between quaternary amines groups and cations increased the concentration of cations around membrane surfaces and resulting in the raising of the polarization concentration. In conclusion, the LCD of ENFMs can be tuned by changing the quaternization degree and is higher than that of the CSO membrane, which indicates a better operating flexibility in practical application. 3.5. Cationic flux and perm-selectivity of the membranes The separation performance of HPAN and ENF membranes was evaluated in Na+/Mg2+ and Li+/Mg2+ systems (Fig. S3). The ultrathin and dense PA layer reduced the divalent cations transfer by steric effect [15] because of the hydrated ionic diameter as dH-Na

Fig. 7. Surface (a–d) and cross-sectional (e–h) morphology of the prepared membranes: (a, e) ENF, (b, f) ENF-Q1, (c, g) ENF-Q2, (f, h) ENF-Q3.

Fig. 8. The three-dimensional AFM images of (a) ENF, (b) ENF-Q1, (c) ENF-Q2, and (d) ENF-Q3 membranes. 6

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Table 3 The values of thickness, CEC, area resistance, contact angle, water uptake and area swelling of HPAN substrate, ENF, ENF-Q1, ENF-Q2, and ENF-Q3 membranes. Membrane

Thickness (μm)

CEC (mmol·g−1)

Rm (Ω·cm2)

contact angle (°)

Water uptake (%)

Area swelling (%)

HPAN ENF ENF-Q1 ENF-Q2 ENF-Q3

137 139 136 139 137

0.21 0.44 0.41 0.38 0.30

3.62 5.41 4.07 4.47 4.75

42.9 48.9 53.2 58.3 65.4

113.5 103.6 110.0 118.1 121.3

2.8 2.0 2.5 2.3 2.3

Fig. 9. (A) I–V curves of the ENFMs and (B) the derivative of dE/di as a function of current density.

ENF-Q3 membranes also proved this speculation (Table S1 and Fig. S4). Ultimately, the prepared ENFMs exhibited better perm-selectivity after quaternization. The greater electropositivity hindered the Mg2+ flux significantly compared to Li+. The ENF-Q3 membrane with the highest quaternization degree exhibited outstanding performance. The perm+

selectivity P Li 2 + = 11.3 was 7 times higher than that of CSO memMg

brane P Li 2 + = 1.6 , while the Li+ flux (JLi+ = 5.79 × 10 8·mol·cm 2·s 1) +

Mg

almost kept the same level (CSO, JLi+ = 6.19 × 10 8·mol·cm 2·s 1). To evaluate the stability of ENFMs during the long-term ED application, ENF and ENF-Q3 membranes were selected as the representative and their fluxes as well as perm-selectivities were evaluated by five sequential cycles. As shown in Fig. 11, the perm-selectivities of ENF and ENF-Q3 membranes were changed slightly around 11.4 and 7.3, respectively. Meanwhile, the fluxes of Li+ and Mg2+ kept steadily. The excellent stability of ENF and ENF-Q3 membranes implied the better capability of prepared ENFMs in practical applications.

Fig. 10. The ion flux and perm-selectivity of ENFMs.

(7.16 Å) < dH-Li (7.64 Å) < dH-Mg (8.56 Å) [29,30]. Meanwhile, the small-sized monovalent cations contributed to higher transport rate through the negatively charged HPAN porous substrate and the ultrathin PA layer at the same potential difference. Consequently, a significant enhancement in perm-selectivity was observed. The perm-selectivity of ENF membrane was further improved by tuning the positive charge density on the PA layer surface. As shown in Fig. 10, both the Li+ and Mg2+ fluxes of ENF-Q1 were reduced because of the electrostatic repulsion induced by the quaternary ammonium. Interestingly, for ENF-Q2 and ENF-Q3 membranes, the Li+ flux increased. The results do not seem to be what was expected due to the enhanced repulsion force between the Li+ ions and the positively charged polyamide layer. The increase in Li+ flux is due to the declined thickness of PA selective layer of ENF-Q2 and ENF-Q3 that compensated the reduced flux. The pure water permeance of all the ENFMs and the rejection of polyethylene glycol (PEG) with different molecular weight for ENF and

4. Conclusion In conclusion, an effective and facile approach to fabricate high perm-selectivity ENFMs was reported. The ultrathin positively charged PA active layer formed on the negatively charged HPAN substrate through IP reaction and following quaternization reaction. The results showed the dense polyamide layer guaranteed the high perm-selectivity between monovalent and divalent cations, and the positively charged PA layer further improved the rejection of the divalent cations. The quaternization degree was adjusted and its effects on Li+/Mg2+ separation was systematically and successfully investigated. Although the partial hydrolysis of the amide bond is still inevitable, the final separation performance of the membranes is satisfactory. Compared with 7

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Fig. 11. The ion flux and perm-selectivity of (A) ENF and (B) ENF-Q3 membranes. +

Li commercial CSO ( JLi+ = 6.19 × 10 8·mol·cm 2·s 1, PMg 2 + = 1.6 ), the ENF-Q3 membrane with the highest quaternization degree attained high Li+ flux (5.79 × 10−8 mol cm−2 s−1) and Li+/Mg2+ perm-selectivity as high as 11.3. Moreover, the high LCD and outstanding longterm stability are the promising features of the prepared ENFMs to scale up in industrial applications.

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