Turing-structured polyamide membranes with surface charge engineering for high-efficiency Mg2+/Li+ separation

To address the critical challenge of lithium extraction from high Mg 2+ /Li + ratio brines, this study develops a high-performance separation membrane through synergistic regulation of binary and ternary acyl chloride monomers combined with a bis-quaternary ammonium salt surface modification strategy. Optimizing the ratio of terephthaloyl chloride (TPC) to trimesoyl chloride (TMC) facilitates the formation of a periodic stripe Turing structure on the piperazine-based polyamide membrane's surface, markedly increasing the effective permeation area and water flux. The study reveals that adjusting the TPC ratio finely tunes the polyamide network's crosslinking density, thereby affecting the size exclusion effect. Additionally, TPC incorporation, by diminishing surface carboxyl density, enhances the membrane's surface positive charge, significantly improving its Li + permeability and Mg 2+ rejection capabilities. The optimized TPC-20 membrane demonstrates exceptional ion selectivity, achieving a MgCl 2 rejection of 91.3 %, a LiCl rejection of 11.3 %, and a water flux of 61 LMH. Further enhancement via amino-terminated bis-quaternary ammonium salt (ATBAS) grafting achieves 98.7 % Mg 2+ rejection, a record Li + negative rejection of -37.3 %, and a Mg 2+ /Li + selectivity of 105.1. Extended testing for 120 h verifies the membrane's reliable performance, highlighting its potential for industrial application. This research provides an efficient and energy-saving membrane separation solution for lithium extraction from brines.