Cosolvent-modulated polyamide nanostructures enable precise Mg2+/Li+ separation for lithium recovery from brines

Nanofiltration (NF) membranes prepared by traditional interface polymerization (IP) have garnered widespread attention for Mg 2+ /Li + separation. However, traditional NF membranes, like thin film composite polyamide (PA) membranes, show inadequate selectivity and permeance. The regulation of monomer diffusion plays a crucial role in adjusting the structure of nanofiltration membranes and improving their separation performance. In this study, we introduced the co-solvent N , N -dimethylformamide (DMF) into the aqueous phase to efficiently modulate the IP process of polyethyleneimine (PEI) and Trimesoyl chloride (TMC) without additional steps. The DMF-induced activation of the PA layer resulted in a more uniform pore structure across the layers, and the optimized membrane (CAIP (4)) had a pure water permeance of 13.10 ± 1.01 L/m 2 hbar, which was approximately three times that of conventional nanofiltration membranes. Molecular dynamics (MD) simulations indicated that the co-solvent DMF reduced the surface tension and accelerated the diffusion rate of PEI monomers, which diffused through the pores of the PA layer to leave more unreacted positively charged amine groups on the membrane surface. Consequently, the membrane achieved efficient separation of magnesium and lithium while maintaining good water permeance. The MgCl 2 removal was 95.09 % ± 1.65 %, and the Li + /Mg 2+ selectivity was 21.05. This study elucidates the mechanisms by which aqueous cosolvents influence IP reactions and offers novel strategies for modulating membrane structures and separation characteristics. These advancements are crucial for enhancing the efficiency of magnesium‑lithium separation processes.