Boosting lithium/magnesium separation performance of selective electrodialysis membranes regulated by enamine reaction

Abstract

Monovalent cation exchange membranes (MCEMs) have progressively played an important role in the field of ion separation. However, according to transition state theory (TST), synchronously tuning the enthalpy barrier (△H) and entropy barrier (△S) for cation transport to improve ion separation performance is challenging. Here, the enamine reaction between the -NH- and -CHO groups is applied to regulate the subsequent Schiff-base reaction between the -CHO and -NH₂ groups, which reduces the positive charges of the selective layer but increases the steric hindrance. The increased -T△S (△S term) for cation transport plays an important role in improving Li⁺/Mg²⁺ separation performance. The optimal positively-charged glutaraldehyde@piperazine/polyethyleneimine assembled membrane (M-Glu@PIP/PEI) has a perm-selectivity (Li⁺/Mg²⁺) of 31.83 with a Li⁺ flux of 1.87 mol·m^-2·h^-1, surpassing the Li⁺/Mg²⁺ separation performance of state-of-the-art monovalent ion selective membranes (MISMs). Most importantly, the selective electrodialysis (S-ED) process with M-Glu@PIP/PEI can be directly applied to treat simulated salt-lake brines (SLBs), and its superior Li⁺/Mg²⁺ separation performance and operational stability enables 74.44 % of the lithium resources with a Li⁺ purity of 34.02 % to be recovered. This study presents new insights into the design of high-performance MCEMs for energy-efficient resource recovery.