Impact of acyl chloride monomers on the structure and performance of polyamide nanofiltration membranes for lithium-magnesium separation

Lithium extraction from salt lakes is crucial for lithium resource recovery, yet selective lithium-magnesium separation remains challenging in brines with high Mg 2+ /Li + ratios. Here, polyethyleneimine (PEI) was used in combination with trimesoyl chloride (TMC), terephthaloyl chloride (TPC), isophthaloyl chloride (IPC), and phthaloyl chloride (PTC) to successfully fabricate four nanofiltration (NF) membranes via interfacial polymerization (IP). Based on the structural characteristics of TMC, TPC, IPC, and PTC, two crosslinking models for PEI with the four acyl chloride monomers were proposed: inter-molecular crosslinking and intra-molecular crosslinking. The features of these models were consistent with the characterization results of the four NF membranes. Furthermore, characterization analysis showed that PEI/TPC membranes exhibited higher positive charge, narrower pore size distribution, and smaller pore size compared to other membranes. Under the combined effects of pore size sieving and charge repulsion, the lithium-magnesium selectivity of the PEI/TPC membrane was the highest among the four membranes, and after secondary NF, its Mg 2+ /Li + permeation ratio could be reduced to below 1. Although the PEI/TPC membrane enhanced the steric hindrance effect on both Mg 2+ and Li + , MD results further revealed that Li + , compared to Mg 2+ , dehydrated more easily and passed through the membrane pores, thereby achieving efficient lithium-magnesium separation. This work provides valuable insights for designing NF membranes via IP and introduces a promising candidate for efficient lithium extraction from salt lakes.