High-efficiency membrane for Mg2+/Li+ separation prepared via grafting symmetrical bis-quaternary ammonium salt

Efficient separation of Mg²⁺/Li⁺ is the key to extracting lithium resources from salt lakes. In this work, a symmetrical bis-quaternary ammonium salt (quaternized 1, 4-dimethylpiperazine, QDMPIP) monomer was synthesized and grafted to the surface of a polyamide nanofiltration membrane. By modulating the grafting amount of QDMPIP monomers, the membrane properties—including positive charge density, pore size, and hydrophilicity—were tailored. The surface of optimally QDMPIP-grafted membrane has a strong positive property, which can reduce Mg²⁺/Li⁺ from 50 (feed) to 0.85 (permeate), and the corresponding S_Mg2+/Li + reaches 58.5, which is 7.4 times that of the unmodified membrane. Remarkably, the flux remained comparable to the pristine membrane, while achieving a MgCl₂ rejection rate exceeding 93 % at 6000 ppm. In addition, the grafting of QDMPIP also improves the crosslinking degree of the membrane surface, which makes it have stable performance at complex operating pressures. Owing to its exceptional separation efficiency, high stability, and straightforward fabrication process, the QDMPIP-modified membrane demonstrates great industrialization potential in the extraction of lithium resources from salt lakes with a high Mg²⁺/Li⁺ ratio.