SPPO-based cation exchange membranes with a positively charged layer for cation fractionation

Desalination 472 (2019) 114145

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SPPO-based cation exchange membranes with a positively charged layer for cation fractionation ⁎

Noor Ul Afsara, Wengen Jia, Bin Wub, Muhammad A. Shehzada, Liang Gea,c, , Tongwen Xua,

T

⁎⁎

a CAS Key Laboratory of Soft Matter Chemistry, iCHEM (Collaborative Innovation Center of Chemistry for Energy Materials), Department of Chemistry, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026, PR China b School of Chemistry & Chemical Engineering, Key Laboratory of Environment-Friendly Polymeric Materials of Anhui Province, Anhui University, Hefei 230601, PR China c Applied Engineering Technology Research Center for Functional Membranes, Institute of Advanced Technology, University of Science and Technology of China, Hefei 230088, PR China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Monovalent cation perm-selective membranes Cation fractionation Perm-selectivity Ionic flux Electrodialysis

The synthesis of monovalent cation perm-selective membranes (MCPMs) to efficiently discriminate amongst cations from seawater is of great importance for several industrial applications. However, a technical approach is highly desired to construct MCPMs to obtain a high ionic flux and sustain perm-selectivity, simultaneously. In the present work, the thickness of the quaternized poly (2, 6-dimethyl-1, 4-phenylene oxide) (QPPO) layer on the surface of SPPO-PVA (SPVA) composite membrane was adjusted using a facile procedure to achieve high permselectivity without scarifying the ionic flux. The thickness of the selective layer was precisely controlled using various concentrations of the QPPO solution. By the introduction of the cationic layer on the SPVA membrane, the monovalent cation can be separated from the divalent cation by their difference in charge density. The influence of the selective barrier (thickness) endows MCPMs with high perm-selectivity up to 12.7 for 0.1 mol L−1 Li+/Mg2+ system, which is very satisfactory for polymeric membranes. The fabricated membranes have low electrical resistance and high limiting current density (ilim). Keeping in view the ED results, the prepared membranes with selective surface layer could be a viable candidate for Li+ selective separation from divalent cation Mg2+.

⁎

Correspondence to: L. Ge, CAS Key Laboratory of Soft Matter Chemistry, iCHEM (Collaborative Innovation Center of Chemistry for Energy Materials), Department of Chemistry, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026, PR China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (L. Ge), [email protected] (T. Xu). https://doi.org/10.1016/j.desal.2019.114145 Received 3 July 2019; Received in revised form 9 September 2019; Accepted 10 September 2019 0011-9164/ © 2019 Elsevier B.V. All rights reserved.

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1. Introduction

expand the number of layers (note: the CEM terminated with the anionic layer as the last layer decreased the perm-selectivity). The augmented perm-selectivity and better flux can be achieved without the infusion of an anionic supporting layer. To the best of our knowledge, rare information is available regarding the thickness of the surface layer with one cationic polymer to obtain high flux and perm-selectivity of CEMs. Thus, we focus our study on finding out the influence of modifying layers thickness on the ion flux and perm-selectivity. In the present work, a layer coating technique was adopted to prepare high perm-selective MCPMs. The SPVA membrane was prepared by mixing polyvinyl alcohol (PVA) with sulfonated poly (2, 6dimethyl-1, 4-phenylene oxide) (SPPO) in an optimized ratio and then cross-linked with glutaraldehyde (GA) according to our previous work [32]. The protic SPPO initiated the crosslinking reaction and produced a dense membrane. The SPVA dense membrane was then modified with the QPPO (having hydrophilic wings) in a proper concentration to control the thickness of the surface layer. The interaction of the surface layer comprising opposite charges concerning the base membrane produced a stable membrane. The observed high perm-selectivity and ionic flux can be accredited to the proper thickness of the surface layer. Membrane characterization by ATR-FTIR and SEM revealed that the adopted protocol was fruitful to fabricate cation perm-selective membranes with desired characteristics.

In the present era, the membrane-based technologies such as electrodialysis (ED) [1,2], reverse osmosis (RO) [3], ultrafiltration (UF) [4], and Donnan dialysis process (DD) [5] have been widely accepted by many industries to address the imperious issues in the separation processes. Amongst these processes, even though ED is not very practical for organics and colloidal systems; however, it is competent when coupled with ion exchange membranes (IEMs) to segregate ions without phase change. Moreover, it is more feasible in terms of high recovery efficiency and low energy consumption for various cations/anions separation systems [6,7]. Generally, the membrane component of the ED process is classified into two categories, i.e., heterogeneous and homogeneous IEMs. Heterogeneous IEMs are prepared via the dispersion of ion-exchanger particles in the polymer matrix [8–10], while the homogeneous IEMs only comprise of polymer backbones with ion exchange groups [11,12]. However, these traditional IEMs can only separate oppositely charged ions but do not provide selectivity amongst the same charged ions. Thus scheming these membranes, certain features such as high flux, high perm-selectivity, and low electrical resistance must be considered for anticipated applications like brackish water desalination [13], acid recovery [14], the extraction of lithium from brines [15], and the production of edible salt from seawater [16]. During the selective cation separation, the existence of divalent cations such as Mg2+ causes scaling on the membranes' surface in the ED process. The scaling effect may reduce the flux of the monovalent cation, which significantly affects the process efficiency [17]. Therefore, it is indispensable to induce new capability in the membrane's architecture for anticipated applications. Previously, commercial monovalent cation perm-selective membranes have been combined with ED to contend the membranes' fouling/scaling, and to advance the monovalent cation flux and permselectivity, respectively [18–20]. However, these membranes are either expensive or less productive in terms of cation perm-selectivity. To boost either the potency of the traditional CEMs or to develop efficient MCPMs, various approaches have been adopted such as cross-linking [21], blending [22], construction of acid-base pairs [23], layer by layer (LBL) polyelectrolyte deposition [18,24] and surface layer modifications [1]. Currently, the surface modification of IEMs with polyelectrolyte multilayers using the LBL procedure has become an attractive technique to develop MCPMs. The bipolar junction between the base membrane and a surface-active layer executed a high perm-selectivity. For example, Jiang et al. modified the surface of commercial cation exchange membranes (CEMs) with polyethyleneimine (PEI) and applied for pilot-scale ED (brackish water desalination), and reported high monovalent perm-selectivity [25]. Rijnaarts et al. synthesized MCPMs with PAH/PSS polyelectrolytes multilayers. The prepared membranes unveiled high perm-selectivity for monovalent cations due to the PAH with excess positive charges [26]. For MCPMs preparation, various cationic polymers such as polyaniline [27], quaternized chitosan [28], and PEI [29] have been considered. The withholding capacity (either electrodeposition or immersion) of these polymers on the surface of CEMs depends on the interaction between the surface layer and the base membrane. However, the surface charged layer on the CEM, designed by aforesaid two approaches is unstable during long term ED application [30]. Recently, we practiced the LBL coating technique and executed an excellent perm-selectivity. The surface of the prepared CEM was covered with alternate polymers having different charges [31]. The infusion of an anionic layer in-between the two cation layers constructed the LBL assembly, and the outermost layer needs to be cationic. This practice will create more positive layers on the membrane surface, and a high perm-selectivity may be obtained [26]. The positivity layer (which provides high resistance) can be customized by changing the concentration of the cationic polymer in the coating solution, which is mandatory to trim the trade-off between the ion flux and perm-selectivity. In this approach, the anionic layer is no more obliged to

2. Experimental 2.1. Materials PVA (1750 ± 50) was supplied by Shanghai Yuanli Chemicals Co. (Shanghai, P. R China). Brominated poly (2, 6-dimethyl-1, 4-phenylene oxide) (BPPO) with 60% degree of benzyl bromination and sulfonated poly (2, 6-dimethyl-1, 4-phenylene oxide) (SPPO) in Na+ form (IEC = 2.15 mmol g−1) were obtained from Tianwei Membrane Corporation Ltd., (Shandong, P.R China). Other chemicals, N-methyl-2pyrrolidone (NMP, AR grade), dimethyl sulfoxide (DMSO, AR grade), methanol (AR grade), sodium chloride (NaCl), magnesium hexachloride (MgCl2·6H2O), sodium sulfate (Na2SO4), and potassium chloride (KCl) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, P. R China). An anion exchange membrane (Neosepta AMX, Tokuyama Co., Japan) and a monovalent cation perm-selective membrane (CSO, Selemion, Japan) were used during the ED procedure. 2.2. Preparation of SPVA cation exchange membrane SPVA membrane was prepared by mixing 2.5 g of PVA and 5 g of SPPO (in H+-form) in 92.5 g of DMSO at 100 °C and stirred for 2 h. The reaction mixture was cooled to room temperature, and GA (0.4 mL of 25 wt% solution) was added with stirring for 20 min to ensure the complete mixing of GA. Afterward, sonication was performed to remove gas bubbles for a certain time. The solution was cast over a glass plate and heated at 60 °C for 12 h. The membrane was peeled off and designated as SPVA. The preparation process was entirely repeatable, and a stable membrane can be obtained. 2.3. Surface modification of SPVA cation exchange membrane QPPO with a pair of hydrophilic wings was selected to improve the surface attachment with the SPVA membrane. In this regard, QPPO was prepared using the same procedure as reported in our previous work [31]. The surface of the SPVA membrane was modified with various concentrations of the QPPO/methanol solutions, i.e., 0.2, 0.6, 1 and 1.8 wt% to control the thickness of the surface layer. The SPVA membrane was cut into small square pieces with a dimension of 6 cm × 6 cm. An individual piece of the membrane was attached to a glass plate with an adhesive tape to make sure one side surface modification. The coating solution (1 mL) was uniformly spread on the 2

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Scheme 1. Surface modification of the SPVA membrane with the QPPO solution

membrane surface as portrayed in Scheme 1, followed by heating at 40 °C for 8 h. The membrane was removed from the glass plate and hydrated with water for further use. Four membranes were prepared and designated as SPQ-2, SPQ-6, SPQ-10, SPQ-18, respectively.

was calculated using the Eq. (3) (Nernst equation) as follows:

Em =

The thickness of the surface layer and the chemical composition of the SPQ membranes were explored by scanning electron microscope (SEM) (XT30 ESEM-TMP, Holland) and FTIR spectrometer (ATR-FTIR, Vector 22, Bruker) in the spectral range of 4000–650 cm−1. Thermal stability and mechanical stability of membranes were studied by the thermo-gravimetric analyzer (Shimadzu TGA-50H) in N2 environment at 10 °C min−1 and dynamic mechanical analyzer (DMA) Model Q800 TA Instruments, USA, at 25 °C. The cation exchange capacity (CEC) was tested using a titration method. The membrane samples (in H+ form) were immersed in a 0.5 mol L−1 NaCl solution for 24 h to replace the H+ ions with the Na+ ions. The amount of H+ released in NaCl solution was then titrated with a 0.05 mol L−1 NaOH using phenolphthalein as an indicator. The CEC (mmol g−1) was calculated using the following Eq. (1):

(VNaOH × MNaOH ) Wm

2.5. Electrodialysis The prepared membranes were inspected for the cation perm-selectivity via ED [33]. The membrane sample was held between the concentrated and diluted sections, as given in Fig. 1. A constant current of 15 mA was generated by potentiostat-galvanostat (WYL1703, Hangzhou Siling Electrical Instrument Ltd.) to initiate the ion migrations across the membrane. A 100 mL volume of each solution, i.e., 0.1 mol L−1 of Li+/Mg2+ and 0.01 mol L−1 KCl was used in the diluted and concentrated compartment, respectively. The electrolyte chamber was filled with 100 mL of 0.3 mol L−1 Na2SO4 solution. A flow rate of 20 mL min−1 was set to circulate 100 mL of each solution in the corresponding compartments. The sample was collected from the concentrated section after 1 h for the ICP analysis (ICP-AES, Optima 7300 DV, USA). The following expression was used to calculate the ionic flux (JM+) [33].

(1)

where VNaOH is the volume of NaOH used for titration, MNaOH is the molarity of NaOH solution, and Wm is the weight of dried membrane, respectively. The water uptake (WU) was measured using a weight difference method. The membranes were soaked in water for 24 h and weighed as W2 (wet membrane). The membranes were then dried at 60 °C for 12 h and re-weighed as W1 (dried membrane). The WU of the membranes was calculated using the following Eq. (2):

WU =

(W2 − W1 ) × 100 W1

(3)

where Em (volts) is the membrane potential; R is the universal gas constant, T is the absolute temperature (K), F (Faraday constant), z (ion charge = 1 for Na+) and a1 and a2 are the activities of the electrolytes in the solutions, respectively. The current-voltage curve was obtained by a four-compartment apparatus using 0.5 mol L−1 NaCl and 0.3 mol L−1 Na2SO4 solutions, respectively [31]. The current was increased progressively using a potentiostat/galvanostat (WYL1703, Hangzhou Siling Electrical Instrument Ltd.), and the voltage drop across the membrane (an effective area of 7.07 cm2) was observed with a multimeter attached with a couple of Ag-AgCl electrodes.

2.4. Membranes characterization

CEC =

RT a (2ti − 1) ln 1 zF a2

(2)

JM+ =

where W1 is the weight of the dried membranes and W2 represents the weight of the wet membranes, respectively. The transport number (tNa+) of the prepared membranes was measured using a two-compartment cell. The membrane sample was held between two compartments containing NaCl solution of different concentrations, i.e. 0.01 mol L−1 and 0.05 mol L−1, respectively. The potential difference generated across the membrane was observed with a multimeter attached with two Ag-AgCl electrodes. The (tNa+) of Na+

(Cf − Ci ). V Am . t

(4)

In Eq. (4), Cf and Ci are the final and initial molar concentration of ions after time t (60 min). V is the volume (100 mL) circulated in the concentrated compartment. Am is the effective area of a membrane (7.07 cm2). The perm-selectivity of the membrane was calculated by using the following Eq. (5) as reported [34]. 3

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Fig. 1. The representation of the ED setup for the ion fractionation.

+

P MN2 + =

J N + C M2 + J M2 + C N +

one can notice the pristine SPPO signals, and a small band at 1104 cm−1, which is verifying the existence of the acetal linkage (-C-OC-) produced after cross-linking [37]. These acetal linkages were formed between the chains of PVA via GA. The acetal linkages made the membrane structure more compact due to a reduction in free volume and were beneficial for the cation removal with a bigger size (size exclusion) [38]. After surface coating with the QPPO polymer, a few new bands appeared at 1647 cm−1, 1035 cm−1, and 837 cm−1, which explicated the -N+R3 group with water content, and a phenyl group, respectively. The surface of the SPQ membrane has revealed the same absorption pattern, which is identical to the pristine QPPO spectrum, hence proves the successful insertion of the QPPO layer on the SPVA membrane surface [39–41]. The FTIR results showed that the crosslinked SPVA composite membrane was produced and the surface QPPO layer was superimposed successfully.

(5)

where JN+ and JM2+ represent the ion flux of N+ and M2+ in the concentrated compartment, respectively.

3. Results and discussion 3.1. ATR-FTIR The chemical structure of the prepared membranes was investigated using the ATR-FTIR as in Fig. 2. The fundamental bands observed in the pristine SPPO and QPPO spectra were compared with the SPVA and SPQ-10 spectra to prove the cross-linking amongst PVA chains via GA and the existence of QPPO layer. In Fig. 2, the absorption band appeared at 2990–2850 cm−1 was associated with the alkyl groups (-CH3) of the QPPO and SPPO. The characteristics bands at 1602 cm−1 and 1187 cm−1 were ascribed to the presence of aromatic ether (-C-O-C-) of the PPO backbone in both SPPO and QPPO, respectively [35]. The important band at 1067 cm−1 verified the sulfonic group (symmetric stretching) present in the SPPO skeleton [36]. In the SPVA membrane,

3.2. Morphology of the membranes The cross-sectional analysis of the SPQ-2 (a), SPQ-6 (b), SPQ-10 (c), and SPQ-18 (d) membranes was studied using SEM, as given in Fig. 3. It was noticed that the SPVA membrane showed uniform morphology with no cavities/cracks. The dense structure was attributed to the acetal linkages, which brought the PVA chains closer and confined the SPPO polymer (as discussed in the ATR-FTIR section). The cross-sectional analysis of the SPQ-2 and SPQ-6 membranes revealed dense layers of the QPPO with the thicknesses of about 43 nm and 0.65 μm, respectively. These layers are firmly attached to the base membrane. Further increasing the contents of the QPPO from 1 wt% to 1.8 wt%, the thickness was increased from 1.18 μm to 3.62 μm, respectively. The adherent structure of the QPPO layer with the SPVA membrane could be linked to the electrostatic interaction between the -N+R3 and the -SO3− groups and the π-π interactions between the aromatic rings of the SPPO and QPPO polymers [42,43]. The SEM results acknowledge that increasing QPPO concentration may increase the thickness of the surface layer. The optimization of the thicknesses (surface layer) is very crucial in the present work to improve the permselectivity without lowering the ion flux. Equating the SEM images with the ED results, we may deduce that 1 wt% solution of the QPPO is the optimal choice to construct a densely layered membrane (SPQ-10) to overcome the trade-off between the flux and perm-selectivity.

Fig. 2. ATR-FTIR spectra of the prepared membranes. 4

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Fig. 3. Cross-sectional SEM images of the SPQ membranes.

200 °C), represented the decomposition of -SO3−, -N+R3, and ether functional groups of the PPO backbone [31,44]. From the TGA curve, the second degradation temperature decreased for increasing the thickness of the QPPO layer. The dropped thermal stability was due to the presence of -N+R3 groups in the coating layer (QPPO), and this observation has been reported in our previous work [31]. The third phase which was initiated above 413 °C, can be accredited to the decomposition of the whole polymeric backbone. In conclusion, the prepared membranes displayed satisfactory thermal stability. Tensile strength (TS) and elongation at break (Eb) are the important parameters for ion exchange membranes. For the ED applications, the membranes should have enough mechanical stability to sustain against the physical impact of water during the ED test. Mechanical stability of the prepared membranes was studied, and the results are given in Fig. 4b. The TS and Eb values were observed in the range of 14–44 MPa and 12–33%, respectively. We used the pristine SPPO membrane to compare our results. Both TS and Eb values are increasing from the pristine SPPO to SPQ-6, except for the SPQ-10 and SPQ-18 membranes. From Fig. 4b, we can obviously evaluate the low mechanical stability of the pristine SPPO. In comparison with pristine SPPO, our approach was more encouraging to produce mechanically stable membranes. For instance, both TS and Eb values were recorded high for the SPVA and can be associated with the PVA backbone, which provided a matrix for crosslinking with GA and reinforced the membrane structure. After coating with the QPPO, the membrane during testing must behave as one material to improve the TS and Eb simultaneously. Further coating to 1% and 1.8% of QPPO, the TS showed anomalous behavior, and the Eb depreciated rather than being improved. The thickness of the surface layer can be accounted for the lower Eb value of the SPQ-18. The high thickness makes the composite membrane tough and stiff, and the movement of chains was restricted, which decreased the Eb (less ductile). It is very usual for a membrane with high TS value, always possesses lower Eb value and vice versa [40,45].

3.3. Physio-chemical characteristics of membranes The physio-chemical properties such as cation exchange capacity (CEC), water uptake (WU), and transport number (tNa+) were measured and collected in Table 1. The PVA and SPPO are highly hydrophilic due to the presence of –OH and SO3−H groups. After blending the two polymers and then crosslinking with GA, the WU and CEC were observed about 77.15% and 1.40 mmol g−1, respectively. These values are lower as compared to our previous PVA based CEMs [31]. The decrease in hydrophilicity could be linked to the reduction of –OH groups (due to crosslinking) and the compactness. After coating with the cationic layer, the WU and CEC values were declined due to the low WU of QPPO, i.e., 42% and the low contents of sulfonic groups in the membranes. By increasing the concentration of the QPPO in the coating solution, the regular decrease in CEC and WU can be observed as in Table 1. The transport number of the SPVA membrane as in Table 1 indicated that the prepared membrane possessed a good CEM nature with (tNa+) of 0.92. After coating with the QPPO, the apparent transport number decreased due to the presence of a cationic layer, which restricted the transport of the counter ions. It proposes a slight bipolar characteristic of the membrane due to the introduction of the cationic layer. This double-layer structure was the key to high perm-selectivity, as observed in the Electrodialysis section. 3.4. Thermal stability and mechanical stability Thermal stability of the SPVA and SPQ membranes was analyzed at 10 °C min−1 in N2 environment. From Fig. 4a, the membranes showed three main degradation phases. The first weight loss (up to 160 °C) was entirely associated with the evaporation of water molecules, and this weight loss is prominent for all membranes due to the hydrophilic behavior of membranes [12]. The second phase in the TGA curves (max Table 1 The physio-chemical properties of the membranes.

3.5. Current-voltage curves −1

S. no

Membranes

WU (%)

IECs (mmol g

1 2 3 4 5

SPVA SPQ-2 SPQ-6 SPQ-10 SPQ-18

77.15 74.52 72.34 70.41 70.33

1.40 1.31 1.30 1.29 1.28

)

tNa

+

Generally, the IEMs exhibit a typical profile comprises three kinds of behavior depending on the applied current density [46]. The I-V curves of the SPVA and SPQ membranes are given in Fig. 5. The relevant parameters, such as membrane resistance (Rm) and limiting current density (ilim) were calculated from these curves. The obtained I-V curves displayed three prominent characteristics regions, which are in

0.92 0.89 0.85 0.84 0.79

5

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Fig. 4. TGA (a) and DMA (b) of the prepared membranes.

agreement with the concentration polarization (Cp) and electro-convection theories as reported [47]. By increasing the concentration of the coating solution, the Rm also increased due to an increase in the thickness of the surface layer. Amongst these membranes, the lowest Rm value obtained for the SPVA membrane (1.13 Ω·cm2). Afterward, the Rm increased up to 4.19 Ω·cm2 as the QPPO concentration increased, which was predictable, and the SPQ-18 exhibited the highest Rm value (4.19 Ω·cm2). The high resistance can be associated with the dense cationic layer, which is accountable for the high resistance [48]. At the start of I–V curves (low current densities), we observed a linear relationship between the applied current and voltage drop, which specifies the membrane behavior as quasi-ohmic. Further increasing the current, a deviation takes place from linear behavior, and a limiting current density is reached (plateau). The ilim indicated the depletion of counter-ions on the dilute side of the membrane, which increased the resistance of the system (plateau region). The membrane's surface acts as a mimic electric double layer (EDL) due to the presence of the oppositely charged pair. The double-layer structure causes water dissociation, and the ions stream to the membrane surface was enhanced. Thus, the slope of the I-V curve again increased to establish a linear relationship amongst the current density and the voltage drop (overlimiting region). The positive charges on the membrane surface resisted the transport of cations and significantly led to high area resistance. Although the highest area resistance obtained for the SPQ-18 membrane was 4.19 Ω·cm2, it is lower than a commercial monovalent cation perm-selective membrane (CSO). From Fig. 5, the ilim values were approximately from 12.7 to 60 mA cm−2 of the prepared SPQ membranes.

Fig. 6. Li+/Mg2+ system, ion flux with perm-selectivity.

membranes except for SPQ-18 membrane. The proper and optimal thickness of the QPPO layer greatly influenced the Li+/Mg2+ permselectivity, as a high thickness of the SPQ-18 membrane decreased the Li+ and Mg2+ flux, which declined the perm-selectivity to 8.4. From Fig. 6, the SPVA membrane showed the Li+ flux up to 8.42 × 10−9 mol cm−2 s−1, while the Mg2+ flux was recorded to 8.4 × 10−9 mol cm−2 s−1 and the perm-selectivity was calculated around 1. In contrast to the pristine SPPO membrane [31], the SPVA has shown an improved perm-selectivity for Li+/Mg2+ and could be ascribed to the crosslinked structure of the membrane and the lower content of the -SO3H groups. After surface modification with the QPPO, the Li+ flux was gradually increased while the Mg2+ flux decreased, which accounted for a high perm-selectivity for Li+/Mg2+. More interestingly, for the SPQ-10 membrane, the Li+ flux remarkably increased to 2.47 × 10−8 mol cm−2 s−1, while the Mg2+ flux was

3.6. Ion flux and perm-selectivity The performance of the SPQ membranes for cation fractionation was explored for a feed solution of 0.1 mol L−1 LiCl/MgCl2 at 2.12 mA cm−2 current density. From Fig. 6, the Li+ concentration increased in the concentrated compartment during ED via the modified

Fig. 5. I–V curves of the SPVA (a), SPQ-6, SPQ-10, and SPQ-18 (b) membranes. 6

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electrostatic force of repulsion. The optimal thickness advances the monovalent ionic flux and perm-selectivity, simultaneously. 3.7. Operational stability The SQP-10 membranes showed the best ED results; therefore, it was selected to evaluate its stability for long-term application. The selected membrane was tested for eight consecutive cycles, and the results are given in Fig. 7. It was fascinating that the SPQ-10 membrane showed almost stable behavior for eight consecutive cycles. Both the ion flux and perm-selectivity persisted with the average value, and a slight variation was acceptable due to the fluctuation in the ICP results. These findings supported that the surface layer was quite stable and strongly adherent to the base membrane, which gave reproducible results. During the stability test, the membrane performed well without any fouling/scaling, which showed that the surface layer also acted as anti-scaling due to the presence of cationic head groups. 4. Conclusion

Fig. 7. Operational stability of SPQ-10 membrane.

In conclusion, the surface-modified membranes developed in this work showed an excellent monovalent cation perm-selectivity. The surface layer was effectively superimposed on the SPVA membrane, which affected the resistance of the membrane in the ohmic region and thus had a substantial impact on achieving high monovalent cation perm-selectivity. The perm-selectivity was enhanced from 1 to 12.7 for 0.1 mol L−1 Li+/Mg2+ system by changing the concentration of the coating solution, which was due to the presence of cationic charges on the membrane surface (electrostatic repulsion for divalent ions) and the cross-linked nature of the base membrane. The high thickness of the surface layer provided an extensive and forbidding tract to exclude the hydrated Mg2+ cation. The surface-modified membranes unveiled that Li+ flux can be obtained to a maximum value of 2.47 × 10−8 mol cm−2 s−1 when using a concentration of 1% QPPO/ methanol. The representative membrane (SPQ-10) exhibited stable flux and perm-selectivity for eight consecutive cycles, which testified the stability of the membrane for long-term application.

significantly decreased to 1.93 × 10−9 mol cm−2 s−1 and the permselectivity was improved to 12.7. The observed perm-selectivity is higher than our previous LBL assembled membranes for Li+/Mg2+ with perm-selectivity of 5.16 [31], a nanofiltration (NF) membrane for Li+/ Mg2+ with perm-selectivity around 3.5 [49], a surface modified commercial membrane with perm-selectivity of 7.8 [26], and a commercial CSO membrane, as given in Fig. 6. For the layered structure of the membranes, two explanations can be given for high flux and perm-selectivity. The QPPO layer contains plenty of cationic head groups, which provided greater electrostatic repulsion to the divalent cations as compared to Li+ ions. As the QPPO concentration raised, the extent of repulsive force further increased in the surface layer, which hindered the active transport of Mg2+ions. The QPPO layer obstructed the active transport of Mg2+ ions and conceded the Li+ ions to carry more current. This explanation is very rational for IEMs coated with skin layer for cation fractionation, as reported previously [1,18]. The second explanation can be associated with the thickness of the surface layer. As the thickness of the layer increased, the resistance also raised, which reflected that the ions require a high driving force (an extended track) to penetrate across the membrane. For example, taking the SPQ-10 membrane, the higher thickness greatly affected the active transport of Mg2+ ions, which implied that Mg2+ ions had to travel an extended penetration zone to reach to the concentrated compartment. This extended penetration zone can be designated as a non-ohmic region, as described by Rijnaarts et al. [1]. However, this effect was beneficial for the transport of Li+ ions up to some extent. When the QPPO concentration was increased up to 1.8 wt%, the Li+ fluxes reduced to 1.45 × 10−8 mol cm−2 s−1 and the Mg2+ flux also declined, i.e., 1.72× 10−9 mol cm−2 s−1 and the perm-selectivity was negatively affected, i.e., 8.4. The decrease in the perm-selectivity was due to the high thickness (more extended track) of the surface layer. This non-ohmic region negatively affected ion fluxes and perm-selectivity, respectively. The bipolar characteristics provoked in the membrane (apparent transport number decreased to 0.79) greatly affected the cation fluxes. Moreover, this surface layer with positive charges and the underneath membrane with negative charges acted as a mimic EDL, which resulted in high transport resistance for cations. In conclusion, the ionic flux and perm-selectivity were increased with the increased thickness of the surface layer except for the SPQ-18 membrane. The electrical resistance was increased due to the change in the divergence of ionic groups (transition from the cationic layer to anionic layer) and the localization of ionic charges. The appearance of this resistance caused cation fractionation. Thus, it is indispensable to maintain the thickness of the QPPO layer on the SPVA membrane. The proper thickness confines the transport of divalent cations due to the

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