Codeposition Modification of Cation Exchange Membranes with Dopamine and Crown Ether To Achieve High K⁺ Electrodialysis Selectivity

Abstract

Surface modification has been proven to be an effective approach for ion exchange membranes to achieve separation of counterions with different valences by altering interfacial construction of membranes to improve ion transfer performance. In this work, we have fabricated a series of novel cation exchange membranes (CEMs) by modifying sulfonated polysulfone (SPSF) membranes via codeposition 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 the electrodialysis 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 M-co-0.50 shows significantly prominent perm selectivity in the K+/Mg2+ system and M-co-0.75 exhibits remarkable performance in the K+/Li+ system , superior to commercial monovalent-selective CEM (CIMS, , ). Besides, in the K+/Li+/Mg2+ ternary system, K+ flux reaches 30.8 nmol·cm–2·s–1 for M-co-0.50, while it reaches 25.8 nmol·cm–2·s–1 for CIMS. It possibly arises from the effects of pore-size sieving and 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+ ion recovery from salt lake and seawater.

Cation Exchange Membranes Coated with Polyethyleneimine and Crown Ether to Improve Monovalent Cation Electrodialytic Selectivity

“…FT-IR was used to analyze the chemical structures of the membrane surfaces for the pristine and modified membranes, and the spectra are exhibited in Figure 4 . The obvious characteristic peaks at 1026 and 1096 cm −1 in all of the spectra (a)–(e) are assigned to the symmetric and asymmetric stretching vibrations of the sulfonated group [ 28 , 38 ]. A broad band around 3460 cm −1 is ascribed to the stretching vibration of the O-H bond in the sulfonated group and residual moisture in the membrane matrix.…”

“…It can confirm the permeation of codeposition solution (12C4/DA and PEI/DA) to the membrane matrix easily. During the 12C4/DA codeposited reaction, 12C4 and DA gradually permeate into the membrane matrix, and the amine groups mainly from PDA interact with sulfonate groups in the membrane matrix, leading to decreased exchangeable sites [ 28 , 42 ]. The further variation of IECs for M-12C4-x-PEI (x = 0.50 and 0.75) is plausibly attributed to the acid–base interaction between sulfonate groups and DA small molecules in PEI/DA codeposition solution.…”

“…4′-Aminobenzo-12-crown-4 (12C4) was synthesized according to the preparation of 4′-aminobenzo-15-crown-5 in our previously reported work [ 28 ]. PEI and 12C4 were respectively codeposited with DA by means of rapid deposition using CuSO 4 /H 2 O 2 as a trigger [ 36 ].…”

“…The surface area resistance ( R ) of the as-prepared CEMs was measured by a custom-designed four-compartment cell and calculated according to electrochemical impedance spectroscopy (EIS) recorded by an Autolab electrochemical workstation (PGSTAT302 N, Metrohm, Herisau, Switzerland). As shown in Figure 2 , the measurement was performed according to our published work [ 28 ]. In the four-compartment cell, the CEM to be tested was clamped in the middle two compartments filled with 0.5 M chloride salt solution (LiCl and MgCl 2 , respectively), and two pieces of AEMs were placed respectively on both ends of the compartments full of 0.5 M Na 2 SO 4 .…”

“…In our previous work, the modification of a sulfonated polysulfone (SPSF) membrane through the codeposition of DA and 4′-aminobenzo-15-crown-5 (ACE) followed by GA cross-linking was beneficial to achieve pronounced K + ions electrodialytic selectivity. Apart from pore-size sieving effect, the electric field driving and host–guest molecular recognition of ACE and K + ions are essentially taken into account in evaluation of membrane permselectivity [ 28 ].…”