Trimeric quaternary ammonium monomer anchored polyamide nanofiltration membrane with high permeate flux and enhanced positive charge density for superior Mg2+/Li+ separation

Lithium is a strategic resource critical for national defense and renewable energy applications. However, in most salt lake brines, lithium coexists with high concentrations of Mg 2+ , posing a major challenge for lithium extraction. The similar hydration radius of Mg 2+ (0.428 nm) and Li + (0.382 nm) lead to poor separation selectivity, significantly hindering efficient lithium recovery. Therefore, achieving efficient and selective separation of Mg 2+ and Li + is of paramount importance for sustainable lithium extraction from salt lake brines. To address this challenge, we designed a novel positively charged nanofiltration membrane (NFM) by interfacial polymerization using a tri-quaternary ammonium monomer, N,N,N′,N″,N″-pentamethyldiethylenetriamine (QPMDETA), anchored onto a polyethyleneimine (PEI)-based selective layer. This approach significantly enhances both the positive charge density and surface hydrophilicity, enabling superior Mg 2+ /Li + separation performance. Through rational modification, the QPMDETA-NFM exhibited significantly enhanced positive charge density and hydrophilicity, demonstrating an isoelectric point of 9.39 and an outstanding MgCl 2 permeability of 128.41 L·m −2 ·h −1 ·MPa −1 . The membrane achieved excellent separation performance, with a 96.04 % rejection rate for Mg 2+ and a stable Mg 2+ /Li + separation factor of approximately 11, even under challenging conditions of a 50:1 Mg 2+ /Li + ratio in a 2000 ppm mixed salt solution. Molecular dynamics simulations and density functional theory calculations further elucidated the water molecule transport mechanisms in QPMDETA-NFM. This study presents a novel strategy for developing high performance positively charged NFM for efficient lithium extraction from aqueous systems.