Co-deposition modification of cation exchange membranes with

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Co-deposition modification of cation exchange membranes with dopamine and crown ether to achieve high K+ electrodialysis selectivity Shanshan Yang, Yuanwei Liu, Junbin Liao, Huawen Liu, Yuliang Jiang, Bart Van der Bruggen, Jiangnan Shen, and Congjie Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21031 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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ACS Applied Materials & Interfaces

Co-deposition modification of cation exchange membranes with dopamine and crown ether to achieve high K+ electrodialysis selectivity Shanshan Yanga, Yuanwei Liua,b, Junbin Liaoa, Huawen Liua, Yuliang Jianga, Bart Van der Bruggenc, Jiangnan Shena*, Congjie Gaoa a

Center for Membrane Separation and Water Science & Technology, Ocean College, Zhejiang University of Technology, Hangzhou 310014, China

b

Department of Chemical Engineering and Safety, Binzhou University, Binzhou 256600, China

c

Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B–3001, Leuven, Belgium

*

E-mail: [email protected]

Keywords:

K+-selective

cation

exchange

membrane,

electrodialysis,

dopamine,

4’-aminobenzo-15-crown-5, co-deposition. Abstract Surface modification has been proven as an effective approach for ion exchange membrane to achieve separation of counter-ions with different valences by altering interfacial construction of membrane to improve ion transfer performance. In this work, we have fabricated a series of novel cation exchange membranes (CEMs) by modifying sulfonated polysulfone (SPSF) membrane via co-deposition of mussel-inspired dopamine (DA) and 4’-aminobenzo-15-crown-5 (ACE), followed by glutaraldehyde cross-linking, aiming at achieving selective separation of specific cations. The as-prepared membranes before and after modification were systematically characterized in terms of their structural, physicochemical, electrochemical and electrodialytic properties. In electrodialysis (ED) process, the modified membranes exhibit distinct perm-selectivity to K+ ions in binary (K+/Li+, K+/Na+, K+/Mg2+) and ternary (K+/Li+/Mg2+) systems. In particular, at a constant current density of 5.0 mA·cm–2, modified membrane 

K + 2+ system and M-co-0.50 shows significantly prominent perm-selectivity ( PMg 2 = 5.99) in K /Mg

1

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

M-co-0.75 exhibits remarkable performance in K+/Li+ system ( PLiK = 2.87), superior to commercial monovalent-selective CEM (CIMS,



K PMg 5.36, 2 =



PLiK = 1.16). Besides, in

K+/Li+/Mg2+ ternary system, K+ flux reaches 30.8 nmol·cm–2·s–1 for M-co-0.50, while 25.8 nmol·cm–2·s–1 for CIMS. It possibly arises from the effects of pore-size sieving as well as the synergistic action of electric field driving and host-guest molecular recognition of ACE and K+ ions. This study can provide new insights into the separation of specific alkali metal ions, especially on reducing influence of coexisting cations K+ and Na+ on Li+ ions recovery from salt lake and seawater. 1. Introduction Electrodialysis (ED), as one of the most environmentally-friendly and promising electrochemical separation process, has elicited significant attention in various practical applications.1-3 Ion exchange membranes (IEMs), as the core components of ED process, possess high permeability to the counter-ions, but usually present poor selectivity to the counter-ions of the same valance.1,4 Nevertheless, in various practical applications, such as the removal of F– or NO3– from groundwater,5,6 separating Ca2+, Mg2+, SO42– from K+, Na+, Cl– in seawater,7 extracting Li+ from salt lake brines with high concentration of Mg2+,8,9 IEMs with high perm-selectivity to specific ions are urgently desirable. The perm-selectivity of IEMs is mainly dependent on three factors: counter-ions mobility in the solution, the affinity between counter-ions and membranes, and ion migration speed in the membrane matrix.1,10 Following this, numerous efforts to improve perm-selectivity of monovalent cations have been focused on increasing the cross-linking degree of membrane matrix and surface modification of membranes. In general, covalent cross-linking makes IEM matrix denser and further suppresses the migration of cations with larger hydrated radii, whereas it tends to increase surface area resistance of membranes.11,12 Thus far, surface modification is the most facile and efficient approach to improve monovalent cation perm-selectivity.4,10 The substances reported for membrane surface modification mainly contain polyelectrolyte, such as protonated

polyaniline

(PANI),13,14

polyethyleneimine 2


(PEI),15

polypyrrole

(PPy),16,17

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polyquaternium-7,18 and quaternized chitosan.19,20 As a common feature, these positively charged or rigid polymers can generally render relatively compact modified surface and thus enhance electrostatic repulsion to multivalent cations. Layer-by-layer self-assembly is an effective surface modification strategy via alternating electrostatic adsorption or deposition polycations and polyanions on membrane surface.21-24 However, the long-term stability of the functionalized layer is yet a serious problem for practical application. For the approaches mentioned above, the essence of improving perm-selectivity to monovalent cations is mainly restriction of multivalent cations transporting through the membrane. Particularly, separation of ions with the same valence, especially specific monovalent cations (e. g., Li+, Na+ and K+) from mixed aqueous environments is still a challenge for many applications like ED. Ion channels are integral membrane proteins that can control the flow of specific ions in and out of biological cell by changing the membrane potential or binding to a ligand.25,26 Mimicking the biological host-guest interactions between membrane proteins and ions, crown ether can selectively bind specific metal ions through ion-dipole interaction between metal ions and the oxygen atoms of ether ring,27 and transport specific ions imitating one-dimensional ion channel.28 Therefore, crown ethers and their derivatives have been the optimal candidates of constructing ion-transporting units in many fields.29,30 Considerable research interest has been devoted to crown ethers, their derivatives and polymers for separation of alkali metal ions by means of solvent extraction and adsorption,27,29,31 whereas the major drawbacks are that the processes require equivalent amount of counter-ions, stripping agents and desorbents in order to obtain specific cations.32,33 Herein, combination of crown ether and its derivatives with ion exchange resins or membranes may be a promising approach to overcome the disadvantages and efficiently separate specific ions. 18-crown-6, beneficial to form stable K+~18-crown-6 complexes and thus lower permeation of K+ ions, has been achieved to modify the commercial CEMs via impregnation method34 and change the presence modality of K+ ions in desalting-side solution during ED process.35 In addition, Bhattacharyya et al revealed that loading of debenzo-18-crown-6 in Li+ form of Nafion-117 membranes enhanced the selectivity of Cs+ over Li+ about 6 times than the 3



membranes without crown ethers.33 Tas et al demonstrated that poly(arylene ether ketone)s (PAEKs) and sulfonated PAEKs (SPAEKs) containing dibenzo-18-crown-6 moieties in main chains, showed almost four times lower K+ diffusion rate than that in native SPAEK IEM.36 These examples demonstrate that crown ethers and their derivatives achieve enhanced selectivity to certain cations by sacrificing the mobility of specific ions in solution or in IEM matrix. Meanwhile, the strong complexation between crown ethers and cations greatly increases the membrane resistance. The surface modification of CEMs by crown ether might provide specific ion channels for the given ions and promote ion transportation under electric field force. Chaudhury et al fabricated an ion-gated Nafion composite membrane, with Cs+ driven loading of a thin layer of dibenzo-21-crown-7 on a single side of CEM. The high selectivity of Cs+ over Na+ can be achieved in simulated nuclear waste solution under electric field.37,38 Nevertheless, this approach of loading crown ether is unstable and the study lacks systematic investigations in different cationic systems to confirm the membranes specificity to Cs+ ions. As an attractive representative for surface modification and functionalization of abundant materials,39,40 dopamine (DA) can self-polymerize to form polydopamine (PDA), which can adhere easily onto almost all substrates. During polymerization process, the catechol groups can react with thiol- or amino-terminated reagents through Michael addition or Schiff base reaction.41 Along this, numerous investigations on improving membrane properties (such as perm-selectivity42,43, antifouling43,44) of IEMs have been reported. Benzo-15-crown-5 is able to selectively form 2:1 “sandwich” host-guest complex with K+ ions.45,46 Based on the characteristics of biomimetic materials DA and benzo-15-crown-5, we have fabricated a novel CEM with specific selectivity to K+ ions by co-deposition of DA and 4’-aminobenzo-15-crown-5 (ACE) on the surface of sulfonated polysulfone (SPSF) CEMs, followed by cross-linking of glutaraldehyde (GA). The modified membranes have been characterized with respect to their physicochemical and electrochemical behavior. Furthermore, we have systematically investigated the effect of ACE/DA molar ratio on the mobility of different cations (Li+, Na+, K+ and Mg2+), and the galvanostatic perm-selectivity to cations in binary (K+/Li+, K+/Na+ and K+/Mg2+) and ternary (K+/Li+/Mg2+) systems for the modified CEMs.

4


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2. Experimental section 2.1 Materials Sulfonated polysulfone (SPSF, the molar percent of bis(4-fluorophenyl) sulfone with respect to the total molar of difluoro monomers (bis(4-fluorophenyl) sulfone and sulfonated bis(4-fluorophenyl) sulfone which used in the synthesis procedure was 60%, Mw = 65,000, PDI = 2.25, Yanjin Technology Co. Ltd., China), benzo-15-crown-5 (98%, Macklin Biochemical Co. Ltd, China), dopamine hydrochloride (98%, Aladdin Reagent Co. Ltd., China), glutaraldehyde (GA, 50% in water, Energy Chemical Co. Ltd, China), Tris(hydroxymethyl) aminomethane (Tris, 99.5%, Energy Chemical Co. Ltd, China), N,N-dimethylformamide (DMF, 99.5%), chloroform (99%), concentrated nitric acid (HNO3, 68%), acetic acid (HAc, 98%), lithium chloride (LiCl, 99.9%), sodium chloride (NaCl, 99.5%), potassium chloride (KCl, 99.5%), magnesium chloride (MgCl2, 99%), sodium sulfate (Na2SO4, 99.0%) etc. were received from Aladdin Reagent Co. Ltd. and used without further purification. Deionized (DI) water was used throughout the experiments. Commercial monovalent-selective CEM NEOSEPTA CIMS and conventional AEM NEOSEPTA AMX (ASTOM Co., Japan) were both received from ASTOM Co., of which the properties were listed in Table S1. 2.2 Synthesis of 4’-aminobenzo-15-crown-5 4’-Aminobenzo-15-crown-5 (ACE) was synthesized from benzo-15-crown-5 by nitrification and reduction reaction in sequence.47,48 5.0 g of benzo-15-crown-5 was dissolved in the mixture of chloroform (70 mL) and HAc (60 mL), and then 17 mL of HNO3 was dropwise added into the mixture. The mixture was reacted under stirring at 25 oC for 24 h, followed by neutralization with 3.0 M Na2CO3 solution. The yellow product of 4’-nitrobenzo-15-crown-5 was obtained after extraction of inorganic phase by chloroform and further evaporation of solvent, followed by recrystallization from ethanol, with a yield of 75%. Furthermore, ACE was obtained by reduction of 4’-nitrobenzo-15-crown-5 with hydrazine. In detail, 2.5 g of 4’-nitrobenzo-15-crown-5, 1.0 g of 10% Pd/C catalyst and 20 mL of hydrazine 5



were mixed in 375 mL of ethanol solvent and stirred for 6 h at 80 oC. Then the mixture was filtered and the filtrate was evaporated to collect the crude product. The product was received after further purification in the yield of 68%. The chemical structures of purified products were confirmed by 1H NMR spectra (Figure S1). 2.3 Membrane Formation and Modification. The fabrication process of surface modified SPSF CEMs was co-deposition of ACE/DA and further cross-linking by glutaraldehyde (GA). The SPSF-based CEM was firstly prepared by solvent-casting method. In detail, 25 wt% SPSF casting solution was formed by stirring 5.0 g of SPSF in 15 g of DMF solvent at 60 oC for 4 h. The casting solution was cast on a smooth glass plate and then heated at 60 oC for 24 h. The as-prepared CEMs were washed with DI water and stored in it for use. Under oxidizing conditions, the catechol groups can form covalent bond with thiol- or amino-terminated reagents through Michael addition or Schiff base reaction during DA polymerization,41 and it has been verified by various literature reports.49-51 In this work, DA and ACE were co-deposited on SPSF CEM surface, and the possible polymerization mechanism was depicted in Figure 1. Dopamine hydrhloride (2.0 g·L–1) and ACE were dissolved in Tris-HCl buffer solution (pH 8.5, 10 mM) with a certain molar ratio of ACE/DA as co-deposition solution. The as-prepared SPSF CEM was fixed in a customized device horizontally and the co-deposition solution was poured onto the upper surface. Subsequently, the device was shaken in a constant speed at 25 oC for 24 h. Up to the desired time, the residual co-deposition solution was removed and the modified CEM was rinsed by DI water several times. The same co-deposited modification process was also performed on the other surface. The modified CEMs were immersed in 2.5 wt% GA of ethanol solution at 50 oC for 3 h, and then washed with ethanol solution three times and stored in DI water for further evaluation. For convenience, the nomenclatures employed to designate the pristine and modified CEMs are described herein. The pristine CEM is denoted as M-0 and CEM modified by DA is as M-DA. CEMs modified by co-deposition of ACE/DA and further cross-linking with GA are denoted as 6


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M-co-x (x represents the molar ratio of ACE to DA, x = 1:4, 2:4, 3:4, 4:4, it is denoted as 0.25, 0.50, 0.75, 1.0 for simplicity)), and CEMs modified by co-deposition of ACE/DA without cross-linking are denoted as M-co-x-UC.

Figure 1 Schematic illustration of the possible co-deposited reaction mechanism of 4’-aminobenzo-15-crown-5 (ACE) and dopamine (DA) on the surface of SPSF CEM. 2.4 Surface characterization Attenuated total reflectance Fourier transform infrared spectroscopy (FT-IR) spectra of dried samples were recorded on infrared spectrophotometer (Nicolet 6700, USA) in the range of 4000–400 cm–1. X-ray photoelectron spectroscopy (XPS) analyses were performed on a spectrometer (Thermo Fischer ESCALAB 250Xi, USA) with Al Kα excitation radiation (1486.6 eV). The scanning electron microscope (SEM, SU8010 Hitachi, Japan) was used to observe the surface morphology and elements distribution in cross-section of the CEMs. The samples were dried in a freeze dryer for more than 24 h before characterization. Atomic force microscope (AFM, Bruker, Dimension Icon) was used to estimate the thickness of modified layer for the modified membranes. Partially modified membrane samples were attached onto a magnetic sample disk using double sided tape. The thickness of modified layer was measured from the original surface to the modified surface of the membrane and the height difference between the unmodified and modified interfaces was used to estimate the thickness of the modified layer.52,53 7



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The change of hydrophilicity for the modified membranes was evaluated by contact angle analyzer (OCA50AF, Germany) according to the sessile-drop water method. Each membrane sample was measured for three times. 2.5 Physicochemical characterization 2.5.1 Ion exchange capacity Ion exchange capacity (IEC) of CEMs was determined by potentiometric titration (T50, Mettler Toledo, Switzerland). A piece of CEM was soaked in 25 mL of 1.0 M HCl solution for 48 h to exchange K+ in the membrane matrix with H+, then washed with DI water to remove the residual H+ on the membrane surface. Afterwards, the membrane was dried under vacuum at 60 oC for 24 h and weighed (Wdry, g) accurately. Subsequently, the membrane was immersed in 0.5 M NaCl solution for 48 h to release H+ in the membrane matrix. The concentration of H+ in the resulting solution was titrated with a known concentration of NaOH solution (cNaOH, mol·L–1), and the equivalent volume of NaOH was denoted as VNaOH (L). The IEC (mmol·g–1) of CEMs were obtained by Equation (1): IEC 

cNaOH  VNaOH Wdry

(1)

2.5.2 Water uptake Water uptake (WU) refers to the mass change rate after the membrane is immersed in water to reach equilibrium (Wwet, g) at a certain temperature, relative to the dry membrane (Wdry, g). Membrane samples were dried at 60 oC under vacuum for 24 h, and then soaked in water at 25 oC

for 24 h. The residual water on membrane surface was wiped out and the wet membrane

sample was rapidly weighed. WU was calculated by Equation (2): WU 

Wwet  Wdry Wdry

100%

2.5.3 Electrolyte uptake 8


(2)

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Electrolyte uptake (EU) was evaluated according to the measurement of weight difference before and after membrane samples immersed in a given electrolyte solution reaching equilibrium, similar to the measurement of WU. EU was calculated according to Equation (3). EU 

Ewet  Edry Edry

100% (3)

where Edry and Ewet are the weights (g) of the dry and wet membrane samples, respectively. 2.6 Electrochemical characterization 2.6.1 Surface area resistance The surface area resistance (R) of pristine and modified CEMs was measured by impedance measurement utilizing an Autolab electrochemical workstation with a four-electrode setup in 0.5 M chloride salt solution (KCl, NaCl, LiCl and MgCl2, respectively).54 As shown in Figure 2, the measurement was performed in a four-compartment cell. In the middle of the cell was the CEM to be investigated and on both sides of the cell were the auxiliary AEMs to eliminate the interference of electrode reaction on the measurement.21 The electrode solution was 0.5 M Na2SO4 solution. The cathode and anode made of titanium coating ruthenium were utilized as working electrode (WE) and counter electrode (CE), respectively. A couple of saturated Ag/AgCl electrodes, as the reference electrode (RE) and sensitive electrode (SE), were placed on both sides of the tested membrane as closely as possible. The electrochemical impedance spectroscopy (EIS) was measured with an AC signal of 10 mV amplitude and the frequency ranging from 103 kHz to 100 mHz. The measurement temperature maintained at 25 oC. Ri (Ω·cm2) of CEM was calculated as follows: Ri   Z m  Z

n

 S

m

(4)

where Z m and Z n represent the impedance value with and without the tested CEM, S m is the effective membrane area in this setup ( S m =7.07 cm2), and i represents electrolyte species. 9



2.6.2 Current-voltage test The current-voltage curves were measured by the four-electrode setup in Figure 2, similar to Ri measurement. The DC current was supplied with a scan rate of 250 μA·s–1. During the measurement, the electrode solution was 0.05 M Na2SO4 solution, and the work solution was equimolar binary or ternary mixture of chloride salts (0.05 M, K+/Na+, K+/Li+, K+/Mg2+, K+/Li+/Mg2+, respectively).

Figure 2 Schematic diagram of the experimental setup for the measurement of electrochemical impedance spectroscopy (EIS) and polarization current-voltage curves. 2.7 Electrodialysis experiments A homemade four-compartment setup was utilized to measure the membrane perm-selectivity to different cations under galvanostatic condition at room temperature. As shown in Figure 3, the device consisted of the dilute (II), the concentrate (III), and two electrode compartments (I and IV). The electrode compartments were circulated with 0.05 M Na2SO4 solution. To avoid the influence of concentration gradients between both sides of the tested CEM, the dilute and concentrate compartments were both filled with 80 mL of 0.05 M equimolar binary or ternary mixtures of chloride salts (K+/Li+, K+/Na+, K+/Mg2+, K+/Li+/Mg2+), respectively. Concentration of the cations in compartments (II) and (III) were determined by Cation Chromatography 10


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(ICS-1100, Thermo Fisher) at 60 min. The flux of a certain cation was determined by the concentration change of the ion (mol·L–1) in the concentrate compartment, according to Equation (5):

Ji 

V  dci    Am  dt 

(5)

where Ji is the flux of cation (i) through the CEM (mol·m–2·s–1), V is the volume (L) of electrolyte in the concentrate chamber (volume change was negligible within a certain time) and Am is the effective area (19.63 cm2) of the tested CEM. m

The perm-selectivity of CEM between Am+ and Bn+ (denoted as PBAn ) was generally defined by the following Equation (6).1,21 m

PBAn 

t Am  / t B n + c Am  / c B n 

=

J Am   c B n 

(6)

J B n +  c Am 

where t Am + and t Bn+ are the transport numbers of Am + and B n  in the membrane phase, c Am and

cBn + are the concentrations of Am + and B n  (mol·L–1) in the dilute compartment during ED, respectively. J Am + and J Bn represent the fluxes of

Am + and

B n  through the membrane

(mol·m–2·s–1). Additionally, investigations on individual chloride salt removal (0.05M, KCl, NaCl, LiCl and MgCl2, respectively) for CEMs were also undertaken by the setup shown in Figure 3. Current efficiency (η, %) and special energy consumption (ESEC, kWh/mol Am+, Am+ is denoted as the given cation) in the process of ED were calculated according to the reference, shown in Equation (7) and (8).9

 (%) 

(nt  n0 ) zF t

N  I (t )dt

100

0

11


(7)


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t

ESEC (kWh / molAm  ) 

I  U (t )dt 0

nR

(8)

where nt and n0 are the number of moles of Am+ ion at time t and initial concentrations in the concentrate compartment; z is the valance of Am+; F is Faraday constant (96485C/mol); I is the constant current (0.1 A) and t is 60 min; N is the number of repeating units (N=1); U is the detected voltage (V) during ED process; nR is the molar number of Am+ increased in the concentrate compartment. The data investigated in this paper are the average values of at least three measurements.

Figure 3 Schematic of the ED apparatus for measuring the cation perm-selectivity of the pristine and modified CEMs. 2.8 Quantum-chemical calculations A recent literature reported catechol-amine reaction mechanism and the conclusion was that both primary and secondary amines react with catechol through Michael addition.40 Referring to this conclusion, we chose a simply probable product form for the reaction of DA and ACE to simulate the interaction between the possible molecule on modified CEM surface and metal ions K+, Na+ and Li+ in aqueous solution. As well-known host molecules, crown ethers can selectively recognize metal ions to form stable 1:1 or 2:1 (host : cation) host-guest complexes. Hence, the interaction energies of the host molecule (see Figure S2) and metal ions K+, Na+ and 12



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Li+ in types of 1:1 and 2:1, respectively, were calculated as follows. The Gaussian 09 program was employed to carry out all the density functional theory (DFT) calculations. The molecular structures of complexes were all optimized to their equilibrium position at B3LYP/6-31+G(d,p) level.55,56 Solvation effects were taken into consideration using the polarizable continuum method (PCM) proposed by Tomasi et al.57 3. Results and discussion 3.1 Structural and morphological characterization The surface chemical structures for the pristine and modified CEMs were analyzed by FT-IR spectroscopy. The spectra of DA (a), ACE (b), M-0 (c) and modified membranes (d-f) are shown in Figure 4(A). Figure 4(B) presents the spectra of co-deposited product of ACE/PDA (g) and PDA aggregates (h). The strong characteristic peaks at 1026 cm–1 and 1095 cm–1 are ascribed to the symmetric and asymmetric stretching vibrations of sulfonated group of the pristine membrane material.58 A strong peak around 1623 cm–1 in spectra (g) and (h) is assigned to the overlap absorption band of C=C stretching vibrations of aromatic ring, attributed to the aromatic rings of PDA and ACE.59 Furthermore, the obviously developed peak at 1513 cm–1 in spectra (g) and (h) is due to the N–H scissoring vibration of PDA. For ACE/DA co-deposited membranes and co-deposited product of ACE/PDA (spectra (e), (f) and (g)), the peaks at approximately 1128 cm–1 and 941 cm–1 correspond to the asymmetric stretching vibrations of C–O–C and the rocking vibration of –CH2– for the ether ring of ACE.60 In addition, the N–H scissoring vibration of ACE around 1610 cm–1 (spectra (e) and (f)) becomes much weaker during ACE/DA co-deposition on the membrane surface, possibly attributable to the –NH2 of ACE reacted with the catechol groups of PDA through Michael addition or Schiff base reaction.

13



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Figure 4 FT-IR spectra of (A) (a) dopamine (DA), (b) 4’-aminobenzo-15-crown-5 (ACE), (c) pristine CEM (M-0), (d) CEM modified by DA (M-DA), (e) CEM modified by ACE/DA with a molar ratio 0.50 and without further cross-linking (M-co-0.50-UC), (f) CEM modified by ACE/DA with a molar ratio 0.50 (M-co-0.50); (B) (g) co-deposited product of ACE/PDA and (h) PDA aggregates. Table 1 Chemical composition of the pristine (M-0) and modified (M-DA, M-co-0.50-UC and M-co-0.50) membrane surface from XPS spectra. Materials/Membranes

C (%)

O (%)

N (%)

S (%)

C/O

C/N

O/N

DA (Theoretical)

72.73

18.18

9.09

-

4.00

8.00

2.00

ACE (Theoretical)

70.00

25.00

5.00

-

2.80

14.00

5.00

M-0

76.39

20.08

-

3.53

3.80

-

-

M-DA

71.95

21.06

4.90

2.09

3.42

14.68

4.30

M-co-0.50-UC

72.45

22.30

4.34

0.91

3.25

16.69

5.14

M-co-0.50

73.29

21.95

3.97

0.79

3.34

18.46

5.53

XPS analysis was accomplished to further certify the chemical composition of the pristine and modified CEMs, and the results are displayed in Table 1. In comparison with M-0, N element was detected in the modified M-DA, further confirming that PDA was successfully deposited on surface. C/O ratio decreases from 3.42 to 3.25 and O/N ratio increases from 4.30 to 5.14 for M-co-0.50-UC, compared with M-DA, due to the higher ratio of O element (25.00%) than that of 14


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N element (5.00%) in ACE molecule. O/N ratio further increases to 5.53 for M-co-0.50, after cross-linking by GA, owing to the improvement of O element ratio of the aldehyde group in GA. It is in accordance with the results from Lv et al.50 3.2 Physicochemical properties IEC, as a significant parameter, throws light on the ionic charged nature of the CEMs.61 Herein, IEC values of pristine and modified CEMs have been measured (see Table 2). Obviously, the membrane M-DA exhibits slightly higher IEC values (2.08 mmol·g–1), compared with the pristine M-0 (1.99 mmol·g–1), while the values for M-co-x (x=0.25, 0.50, 0.75) gradually decrease with increasing of ACE/DA molar ratio and fall down to 1.72 mmol·g–1 for M-co-1.0. The variation of IEC values of modified membranes can be possibly considered form the following aspects: (1) PDA is an ampholytic material containing amine groups and phenolic hydroxyl groups and it is positively charged in pH 3 solution62 and even in pH 0 solution (1 M HCl). (2) Sulfonic acid groups in membrane matrix can easily form an acid-base interaction with amine groups.63 (3) During the deposition of DA or ACE/DA on the membrane surface, the deposition solution is prone to permeate in the membrane matrix based on the high swellability of SPSF supporting membrane. We have certified it through the gradient distribution of N from EDX for N element mapping in the whole cross-section of single-sided modified M-DA and M-co-1.0 (see Figure S3). Herein, higher IEC of M-DA is dominantly ascribed to amounts of positively charged PDA in the membrane during the IEC measurement (membranes were soaked in 1.0 M HCl and H+ ions from –SO3H and PDA were released in NaCl solution). For the co-deposited membranes, all the factors stated above may affect the IEC values. Nevertheless, the increased amount of co-deposited product on membrane surface (see Figure S4 and Table S2) and in membrane matrix (see Figure S5, the membranes gradually darken and even change to completely opaque with increasing of ACE/DA molar ratio) probably promote the acid-base interaction between sulfonic acid groups from membrane matrix and amine groups from PDA, leading to decreased IECs values. Water uptake (WU) reflects the variation of the hydrophilic character, presence of pores/cavities in the membrane matrix and dimensional properties61 of CEMs as a result of proposed surface 15



Page 16 of 33

modification. Compared with M-0 (70.48%), WU of M-DA is evidently down to 51.98%. Meanwhile, WUs of co-deposited membranes are gradually reduced with increasing of ACE/DA molar ratio (from 43.85% to 31.06%). Significant drop in WUs of modified membranes is largely attributable to the effect of the modified medium on membrane matrix. As stated above, modified components distributing in the membrane (certified by Figures S3 and S4) occupy a portion of the interstitial space and cavities of the membrane matrix, leading to less water adsorption and lower WU. Besides, the presence of modified layer reduces the hydrophilicity of these membranes, reflected by measurement of water contact angle (see Figure S6). Additionally, EUs for the pristine and modified membranes in various electrolytes (0.1 M, KCl, NaCl, LiCl and MgCl2) are exhibited in Table S3. Particularly, the increase of EU comes from adsorbed cations and anions in solution for the equilibrium of salt sorption and water adsorption in the membrane matrix.64 Table 2 IEC, WU and the ratios of surface area resistances (RM/RKCl, M presents LiCl, NaCl and MgCl2) for pristine (M-0) and modified CEMs (M-DA, M-co-x, x = 0.25, 0.50, 0.75, 1.0). IEC

WU

mmol·g–1

%

RLiCl/RKCl

RNaCl/RKCl

RMgCl2/RKCl

M-0

1.99±0.02

70.48±0.10

2.21

1.41

3.70

M-DA

2.08±0.03

51.98±0.12

2.41

1.74

3.32

M-co-0.25

2.04±0.02

43.85±0.09

3.23

2.02

4.60

M-co-0.50

1.96±0.02

41.18±0.04

4.39

2.36

5.63

M-co-0.75

1.89±0.03

38.49±0.06

4.05

3.03

6.24

M-co-1.0

1.72±0.04

31.06±0.03

2.98

2.32

10.3

Membranes a

a SPSF

The ratio of surface area resistance

supporting membrane in acid-form (-SO3H)

3.3 Electrochemical properties The surface area resistances (Ri) in four electrolyte solutions for the CEMs are presented in Figure 5. Ri values of the modified CEMs increase as a function of ACE/DA molar ratio for a given electrolyte solution, which are far higher than that of M-0. It is closely related to the modified ingredient in the membrane. With the increasing of ACE/DA molar ratio, the thicker 16


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modified layer on membrane surface (see Figure S4) and increased amount of modified ingredient in membrane matrix (see Figure S5) reduces water passage (lower WU) and gives a great increment of ion transfer resistance. Furthermore, for the pristine and modified CEMs, Ri values in the four electrolyte solutions are different, whereas, they are in the same order for a given CEM: MgCl2 > LiCl > NaCl > KCl. In addition, cations with larger hydrated ion radii exhibit lower migration rate and self-diffusion coefficient,4 leading to higher surface area resistance. Particularly, RMgCl2 and RKCl are 13.3Ω·cm2 and 2.36 Ω·cm2, respectively, for modified M-co-0.50, in accordance with the size of hydrated ion radii for Mg2+ (4.28 Å) and K+ (3.31 Å) (see Table S4).

Figure 5 The surface area resistance (Ri) for pristine (M-0) and modified (M-DA, M-co-x, x = 0.25, 0.50, 0.75, 1.0) membranes in 0.5 M LiCl, NaCl, KCl and MgCl2 electrolytes. In order to further investigate the difference of ion transfer performance in CEMs matrix among different electrolytes, the ratios of surface area resistances between KCl and any other electrolyte (LiCl, NaCl and MgCl2) for the CEMs are listed in Table 2. As the ratio becomes higher, a greater difference of ion transfer property is observed for two electrolytes in the membrane system. The ratios of RLiCl/RKCl and RNaCl/RKCl gradually rise with the increasing of ACE/DA molar ratio for the modified CEMs. And the ratios reach the highest values of 4.05 (RLiCl/RKCl) 17



and 3.03 (RNaCl/RKCl) for M-co-0.75, compared with the M-0 (2.41 for RLiCl/RKCl and 1.41 for RNaCl/RKCl). It can reveal that it is more beneficial to K+ ions transfer than Li+ and Na+ ions under weak electric force with the higher modification ratio of ACE. However, the ratios drop to 2.98 (RLiCl/RKCl) and 2.32 (RNaCl/RKCl) for modified M-co-1.0, probably ascribed to the case that excessive ACE/DA coated on CEM surface leads to lowest WUs (Table 2) and forms densest membrane matrix (from the absolutely opaque macroscopical presence, Figure S5) thus suppresses the transport of all cations. The ratio of RMgCl2/RKCl remarkably increases as a function of ACE/DA molar ratio for the modified M-co-x, and it even attains 10.3 for M-co-1.0. It is attributable to the dense membrane matrix suppressing K+ ions transport, which is less important than Mg2+ ions transport.44,65 In comparison with RM/RKCl, the ratios order are following this: RMgCl2/RKCl > RLiCl/RKCl > RNaCl/RKCl for a given modified CEMs. The difference in ratios of RM/RKCl for modified CEMs is significantly pronounced, and it further predicts that modified CEMs may present outperforming perm-selectivity to K+ in K+/Mg2+ system than K+/Li+ and K+/Na+ systems.

Figure 6 Current density-voltage curves for the pristine (M-0) and modified (M-co-x, x = 0.25, 0.50, 0.75, 1.0) CEMs in the mixture of chloride salt electrolytes (0.05 M, K+/Li+, K+/Na+, 18


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K+/Mg2+, K+/Li+/Mg2+, respectively). The evaluation of the current-voltage curve is a mean to describe the ion transport properties for IEMs, and can provide the limiting current density (Ilim) for ED process. Typical current-voltage curves exhibit three characteristic regions, including quasi-ohmic, plateau and over-limiting regions.21,22,43 The quasi-ohmic region appears at low current density, where the membrane voltage almost linearly increases with the applied current. The ohmic resistance (Rohm) of the membrane system can be obtained from the slope between voltage and current in this region. With the current increasing, the ions are rapidly migrated through the membrane, leading to ion depletion in the boundary layer. Consequently, the resistance of membrane system dramatically increases and then a plateau appears in the current-voltage curve. Therefore, the over-limiting region is formed as the current further increases, where the counter-ions in the dilute side are carried to the membrane interface and voltage slowly increases due to the water splitting and electroconvection. The intersection of the tangents between quasi-ohmic region and plateau region could obtain the Ilim value.66 The current density-voltage curves are displayed in Figure 6 for the pristine (M-0) and modified membrane (M-co-0.50) in different electrolytes with equimolar mixtures of chloride salts (0.05M, K+/Li+, K+/Na+, K+/Mg2+, K+/Li+/Mg2+, respectively). According to the trend of these curves, they can be divided into three characteristic regions as stated above. The Ilim in different electrolytes and Rohm of the membrane system are exhibited in Table 3. Apparently, Ilim values of the modified M-co-0.50 are lower than those of the pristine M-0 in the given electrolytes. This effect is in agreement with the results reported in the literatures.67,68 It is attributed to the modified ingredient on the membrane surface and in the membrane matrix, dramatically increasing the resistance of membrane system and further impeding somewhat cationic migration. Furthermore, crown ethers introduced on the membrane surface could complex easily with K+ ions, leading to the accumulation of positive charge on the dilute side of membrane. Therefore, the decrease of Ilim for the modified M-co-0.50 is also possibly resulted from the increase of the electrostatic repulsion between the modified layer and incompatible cations. Table 3 The Ilim and Rohm of the pristine (M-0) and modified (M-co-0.50) membranes in 19



Page 20 of 33

equimolar binary and ternary mixtures of chloride salt (0.05 M). Electrolyte

M-0

M-co-0.50

Ilim (mA·cm–2)

Rohm (Ω·cm2)

Ilim (mA·cm–2)

Rohm (Ω·cm2)

K+/Li+

9.63±0.03

75.00±0.12

8.72±0.02

123.03±0.14

K+/Na+

10.62±0.02

71.05±0.09

9.38±0.03

98.98±0.13

K+/Mg2+

14.44±0.05

52.82±0.13

12.41±0.03

96.25±0.11

K+/Li+/Mg2+

17.95±0.08

50.44±0.07

16.70±0.05

68.34±0.10

solution

3.4 Evaluation of K+ ion perm-selectivity Based on the measurement of current-voltage curves, ED experiments were investigated at a constant current density of 5.0 mA·cm–2, to separate different cations in binary mixture of K+/Li+, K+/Na+ and K+/Mg2+ systems for the pristine M-0, modified (M-DA, M-co-x, x = 0.25, 0.50, 0.75, 1.0) CEMs and commercial CIMS. Figure 7 displays that, the pristine M-0 presents poor 





K PNaK  =0.63 and PLiK =1.18) in binary mixture. For the perm-selectivity to K+ ions ( PMg 2 =1.23,

modified M-DA, the perm-selectivity to K+ ions is slightly improved in these three systems, possibly the result of the fact that abundant free catechol and amino groups in the DA polymerization process could tightly combine with metal ions by chelation.69 However, co-deposition modified membranes exhibit distinct perm-selectivity to K+ ions, especially in K+/Mg2+ system. It confirms that the ether rings of functionalized benzo-15-crown-5 are probably more compatible with K+ ions in salt solution, which provides new ion channels for the migration of K+ ions during ED. 

K For co-deposition modified membranes, PMg 2 value is dramatically improved with the increase

of ACE/DA molar ratio, and the value for M-co-0.50 reaches 5.99, superior to the commercial 

K CIMS ( PMg = 5.36). Additionally, considering the relatively low RKCl (Figure 5) for the 2

co-deposition modified M-co-x, we further confirm that ACE/DA modified CEMs have 20


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

K outperforming recognition and selectivity to K+ ions. Whereas, PMg 2 value for M-co-1.0 drops

down to 3.54. It is closely associated with its low IEC (Table 2) and high R (Figure 5), limiting the transfer rate for all the cations in the membrane phase.



Figure 7 Perm-selectivity PCKm (Cm+ expresses Li+, Na+ and Mg2+) for the pristine (M-0) and modified (M-DA, M-co-x, x=0.25, 0.50, 0.75, 1.0) CEMs in binary mixture of equimolar chloride salt (0.05 M, K+/Na+, K+/Li+, K+/Mg2+) at current density of 5.0 mA·cm-2. It is quite difficult to achieve the separation of specific ions by ED for a mixed system of monovalent cations, and there have been few reports until now. Thereupon, in this study, we investigated specific monovalent cation separation performance for the co-deposition modified 

CEMs in binary systems of K+/Li+ and K+/Na+ (see Figure 7). PLiK value is 1.18 for the pristine M-0. As the result of the coating of ACE/DA on the membrane surface, the permeability increases in comparison with the pristine M-0. And the optimal separation performance reaches to 2.87 for modified M-co-0.75. However, it is much more difficult to separate K+ and Na+ than K+ and Li+, as consequence of the size similarity between K+ and Na+ in aqueous solution. The hydrated ion radii of K+, Na+ and Li+ are respectively 3.31, 3.58 and 3.82 Å (see Table S3). 21



Page 22 of 33



Measurement results show that PNaK  is 0.63 for M-0 and rises to 1.45 for M-co-0.75. Obviously, the separation efficiency of K+/Li+ is close to twice as that of K+/Na+ for a certain CEM. Whereas, commercial monovalent-selective CEM (CIMS) exhibits almost no selectivity for K+/Li+ and K+/Na+ systems. In a word, the measurement results of separation K+/Mg2+, K+/Li+ and K+/Na+ by ED is consistent with the speculation of cation separation effect from the ratios of RMgCl2/RKCl, RLiCl/RKCl and RNaCl/RKCl, shown in Table 2. Furthermore, a ternary system of K+/Li+/Mg2+ mixed solution was measured by ED and the test condition was the same as that of binary system. Figure 8a shows the fluxes of Li+, K+ and Mg2+ for the pristine M-0, modified M-co-0.50 and commercial CIMS after ED was operated for 60 min. Obviously, M-co-0.50 exhibits the highest K+ flux among these three CEMs, and relatively low fluxes for Li+ and Mg2+ ions. The transport number (t) is defined as the ratio of the flux for a certain cation and the sum fluxes of all cations in the membrane phase. The transport numbers of Li+, K+ and Mg2+ for the membranes are presented in Figure 8b. It is significant that tLi+ is in the order: CIMS > M-0 ≈ M-co-0.50, and the order of tK+ is as follows:

M-co-0.50 > CIMS > M-0.

Nevertheless, tMg2+ for M-co-0.50 is comparative to that of CIMS, far lower than that of M-0. It profoundly proves that the introduction of functionalized benzo-15-crown-5 makes the modified CEMs perform specifity to K+ ions. In addition, the monovalent-selective CIMS presents much higher selectivity to monovalent cations K+ and Li+ than Mg2+.

Figure 8 The ion flux (a) and transport number (b) of K+, Li+ and Mg2+ for CEMs M-0, 22


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M-co-0.50 and commercial CIMS in equimolar K+/Li+/Mg2+ (0.05 M) system at current density of 5.0 mA·cm-2 after ED test 60 min. In a practical ED process, the concentration gradients cross IEMs are gradually enhanced, resulting in electro-diffusion of electrolytes through the membranes.54 It eventually has a significant influence on the productivity of electrolyte and current efficiency. Herein, current efficiency and energy consumption, as two key parameters, can efficiently evaluate the ED process. Figure S7 compares the current efficiency (η) and special energy consumption (ESEC) of M-0, M-co-0.50 and CIMS in individual and mixed electrolytes. It can be seen: in individual electrolyte, η almost gradually decreases with the size of hydrated cations increasing (K+ < Na+ < Li+ < Mg2+), resulting in higher ESEC (see Figure S7 (a) and (b)).17 It reveals that electro-transportation of KCl is much easier than other cations, especially MgCl2. M-co-0.50 presents the lowest ηMg2+ and relatively high ηK+, which is significantly related to its high perm-selectivity to K+ in K+/Mg2+ system. Additionally, M-0 and M-co-0.50 present superior current efficiency compared with CIMS. The performance in binary(K+/Li+, K+/Na+, K+/Mg2+) and ternary systems(K+/Li+/Mg2+) for the membranes are displayed in Figure S7 (c) and (d). By comparision, M-co-0.50 presents outperforming current efficiency and lower special energy consumption especially in the systems of K+/Li+, K+/Mg2+ and K+/Li+/Mg2+ in ED process, in consistent with the results of perm-selectivity to different cations (shown in Figures 7 and 8). It is highly beneficial for the practical application. 3.5 Analysis of specificity to K+ ions for the co-deposition modified CEM Co-deposition modified CEMs present distinctly specific selectivity to K+ ions in cationic binary and ternary systems as stated above. It is most likely the result of two effects: pore-size sieving effect as well as the synergistic effect of electric field driving force and host-guest molecular recognition of crown ether and K+ ions, exhibited in Figure 9.

23



Figure 9 Analysis of the possible factors for the specificity to K+ ions of the co-deposition modified CEMs It is well known that the presence modality of cations in aqueous environment is hydrated ions. Hydrated cations can be migrated from one side of the CEM to another side in ED process by a partial dehydration behavior under a certain potential difference.9 Certainly, cations with smaller hydrated radii possess lower hydration free energies (see Table S4) and may dehydrate more easily to pass through the channel of membrane matrix in solution.70 In this study, a thin layer of macromolecules and self-polymers by co-deposition on the surface of pristine CEM exhibits more homogeneous,43 relative to M-DA (see Figure S8 B and D), due to the destruction of the non-covalent interactions among PDA aggregates during the reaction of ACE and PDA on CEM surface. Furthermore, the distribution of modified components in the membrane matrix (Figures S3 and S4) results in lower IEC and sharply reduced WU, which in turn makes the membrane structure denser. Overall, for the co-deposited CEMs, pore-size sieving effect promotes the fact that cations with lower hydrated radii, especially K+ ions, permeate more easily through CEM matrix. The other effect is the synergistic effect of electric field driving force and specific ion channel driven by complexation.3737 Under the driving of electric field force, modified M-co-x 24


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present outperforming selectivity to K+ ions in binary and ternary mixtures, especially when x was 0.50 and 0.75 (shown in Figures 7 and 8). To further confirm the specific interaction between functionalized benzo-15-crown-5 of CEM surface and K+ ions, the density functional theory (DFT) was employed to calculate the interaction energy between the possible molecule structure of functionalized benzo-15-crown-5 and metal cations Li+, Na+ and K+ in aqueous solution. Herein, the interaction energies of two types of complexes were calculated as shown in Table 4. It is worth noting that the interaction energies in the type of 2:1 are lower than that in 1:1 type, indicating the stronger ion-dipole interaction between ether ring and cations in 2:1 type. Moreover, the interaction energy is the lowest when the host molecules interact with K+ ions in the form of 2:1. It can be considered that when various cations migrate to the modified membrane surface under electric field, the modified layer on membrane surface binds more easily with K+ ions. Table 4 The possible conformation of complexes between ether rings of functionalized benzo-15-crown-5 and cations, and the interaction energies calculated by the density functional theory (DFT). Host : cation

Interaction Energy (kJ·mol–1)

Possible conformation of complexes

Li+

Na+

K+

1:1

-105.02

-95.81

-72.76

2:1

-114.76

-131.17

-136.83

Based on the analysis above, the explanation of distinct selectivity to K+ ions for co-deposition modified membranes in this study is derived as follows. In ED process, cations in the bulk of the dilute side, migrate directionally to the cathode under the electric field force, and then gather on the interface of modified CEM towards the anode. Vast K+ ions with the smallest hydrated radii can be interacted with functionalized benzo-15-crown-5 immobilized on the CEM surface, at the same time, the complexed K+ ions can easily pass through the dense pore structure of CEM 25



matrix under the applied electric field. However, the dense membrane surface and relatively weak interaction between those cations with larger hydrated radii and functionalized benzo-15-crown-5 lead to lower permeability for Mg2+, Li+, and Na+ ions. 4. Conclusions A series of co-deposition modified CEMs by ACE/DA have been fabricated, with the main objective of endowing the membranes with specific perm-selectivity to K+ ions. Compared with the pristine M-0, the modified CEMs present distinguishing Ri and the dense membrane matrix makes it difficult for cations with larger hydrated radii to transport through the membrane. ACE/DA modified CEMs selectively recognize and complex with K+ ions, which results in a large number of K+ migrating easily to the membrane surface. In ED experiments, the modified CEMs present outperforming perm-selectivity to K+ in binary (K+/Li+, K+/Na+, K+/Mg2+) and ternary (K+/Li+/Mg2+) systems, which is superior to commercial CIMS, particularly on the separation of monovalent cations. It mainly attributes to the pore-size sieving effect from the compact modified layer and membrane matrix and the synergistic effect of electric field force and host-guest molecular recognition of crown ether complexation with K+ ions. This work provides a potential approach to effectively overcome the problem of separation of alkali metal ions in ED. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. More details on the characteristic properties of commercial membranes used in measurement, electrolyte uptake for the membranes in various electrolytes, properties of hydrated cations Li+, Na+, K+ and Mg2+, 1H NMR spectra of benzo-15-crown-5, 4’-nitrobenzo-15-crown-5 and 4’-aminobenzo-15-crown-5, the possible molecular unit of the reaction between dopamine and 4’-aminobenzo-15-crown-5, the EDX elemental maps in the cross-section for the single-sided modified membranes, the photographs of the pristine and modified membranes with different ACE/DA molar ratio, the thickness of modified layer for the modified membranes and total thickness for the pristine and modified membranes, SEM imges of the pristine and modified membranes, water contact angle 26


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for the investigated membranes and current efficiency and energy consumption for the investigated membranes in individual and mixed electrolytes. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes: The authors declare no competing financial interest ACKNOWLEDGMENTS We gratefully acknowledge the funding support of National Natural Science Foundation of China (No. 21676249 & No. 21878273) and Project for Assistance of Qinghai from Science Technology Department of Zhejiang Province (No. 2018C26004). References (1) Sata, T.; Sata, T.; Yang, W. K. Studies on Cation-exchange Membranes Having Permselectivity between Cations in Electrodialysis. J. Membr. Sci. 2002, 206, 31-60. (2) Xu, T. Ion Exchange Membranes: State of their Development and Perspective. J. Membr. Sci. 2005, 263, 1-29. (3) Nagarale, R. K.; Gohil, G. S.; Shahi, V. K. Recent Developments on Ion-exchange Membranes and Electro-membrane Processes. Adv. Colloid Interface Sci. 2006, 119, 97-130. (4) Luo, T.; Abdu, S.; Wessling, M. Selectivity of Ion Exchange Membranes: A Review. J. Membr. Sci. 2018, 555, 429-454. (5) Vaselbehagh, M.; Karkhanechi, H.; Takagi, R.; Matsuyama, H. Surface Modification of an Anion Exchange Membrane to Improve the Selectivity for Monovalent Anions in Electrodialysis-experimental Verification of Theoretical Predictions. J. Membr. Sci.2015, 490, 301-310. (6) Pan, J.; Ding, J.; Tan, R.; Chen, G.; Zhao, Y.; Gao, C.; der Bruggen, B. V.; Shen, J. Preparation of a Monovalent Selective Anion Exchange Membrane through Constructing a Covalently Crosslinked Interface by Electro-deposition of Polyethyleneimine. J. Membr. Sci. 2017, 539, 263-272. (7) Asraf Snir, M.; Gilron, J.; Oren, Y. Gypsum Scaling of Anion Exchange Membranes in 27



Electrodialysis. J. Membr. Sci. 2016, 520, 176-186. (8) Nie, X. Y.; Sun, S. Y.; Song, X.; Yu, J. G. Further Investigation into Lithium Recovery from Salt Lake Brines with Different Feed Characteristics by Electrodialysis. J. Membr. Sci. 2017, 530, 185-191. (9) Chen, Q. B.; Ji, Z. Y.; Liu, J.; Zhao, Y. Y.; Wang, S. Z.; Yuan, J. S. Development of Recovering Lithium from Brines by Selective-electrodialysis: Effect of Coexisting Cations on the Migration of Lithium. J. Membr. Sci. 2018, 548, 408-420. (10) Ge, L.; Wu, B.; Yu, D.; Mondal, A. N.; Hou, L.; Afsar, N. U.; Li, Q.; Xu, T.; Miao, J.; Xu, T. Monovalent Cation Perm-selective Membranes (MCPMs): New Developments and Perspectives. Chin. J. of Chem. Eng. 2017, 25, 1606-1615. (11) Sata, T. Modification of Properties of ion-exchange Membranes. IV. Change of Transport Properties of Cation-exchange Membranes by Various Polyelectrolytes. J. Polym. Sci.: Polym. Chem. Edition 1978, 16, 1063-1080. (12) Sata, T.; Nojima, S. Transport Properties of Anion Exchange Membranes Prepared by the Reaction of Crosslinked Membranes Having Chloromethyl Groups with 4-Vinylpyridine and Trimethylamine. J. Polym. Sci. Part B: Polym. Phys.1999, 37, 1773-1785. (13) Moon, D. K.; Maruyama, T.; Osakada, K.; Yamamoto, T. Chemical Oxidation of Polyaniline by Radical Generating Reagents, O2, H2O2-FeCl3 Catalyst, and Dibenzoyl Peroxide. Chem. Lett. 1991, 1633-1636. (14) Farrokhzad, H.; Darvishmanesh, S.; Genduso, G.; Van Gerven, T.; Van der Bruggen, B. Development of Bivalent Cation Selective Ion Exchange Membranes by Varying Molecular Weight of Polyaniline. Electrochim. Acta 2015, 158, 64-72. (15) Amara, M.; Kerdjoudj, H. Modification of Cation-exchange Membrane Properties by Electro-adsorption of Polyethyleneimine. Desalination 2003, 155, 79-87. (16) T., S.; Yamaguchi, T.; Matsusaki, M. Preparation and Properties of Composite Membranes Composed of Anion-Exchange Membranes and Polypyrrole. J. Phys. Chem. 1996, 100, 16633-16640. (17) Gohil, G. S.; Binsu, V. V.; Shahi, V. K. Preparation and Characterization of Mono-valent Ion Selective Polypyrrole Composite Ion-exchange Membranes. J. Membr. Sci. 2006, 280, 210-218. (18) Li, J.; Zhou, M. L.; Lin, J. Y.; Ye, W. Y.; Xu, Y. Q.; Shen, J. N.; Gao, C. J.; Bruggen, B. V. d. Mono-valent Cation Selective Membranes for Electrodialysis by Introducing Polyquaternium-7 in a Commercial Cation Exchange Membrane. J. Membr. Sci. 2015, 486, 89-96. (19) Hu, Y.; Wang, M.; Wang, D.; Gao, X.; Gao, C. Feasibility Study on Surface Modification of Cation Exchange Membranes by Quaternized Chitosan for Improving its Selectivity. J. Membr. Sci. 2008, 319, 5-9. (20) Wang, M.; Jia, Y. X.; Yao, T. T.; Wang, K. K. The Endowment of Monovalent Selectivity 28


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Co-deposition modification of cation exchange membranes with

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