Quaternary ammonium-regulated fabrication of hollow fiber nanofiltration membrane for enhanced Mg2+/Li+ separation

Abstract

Efficient Mg²⁺/Li⁺ separation is vital for lithium recovery from salt-lake, necessitating the advancement of nanofiltration membrane technologies. Current nanofiltration membranes have been enhanced through advanced materials and surface modification to enhance the targeted separation of Mg²⁺ and Li⁺. However, significant challenges still exist, including maintaining an optimal balance between high selectivity and permeability and ensuring cost-effectiveness for large-scale industrial adoption. Herein, we developed a novel nanofiltration membrane by integrating 3-bromopropyl trimethylammonium bromide (BPTAB) into the aqueous phase as an additive during interfacial polymerization. The modified membrane’s physicochemical properties and performances were evaluated against a control membrane prepared using tris(2-aminoethyl)amine and trimesoyl chloride on a polyethersulfone substrate. With BPTAB added, the positive charge densities on both top and bottom surfaces of the membrane selective layer were enhanced. Besides, the introduction of the BPTAB led to a reduced membrane pore size and a more uniform pore size distribution. Both the increased positive charge density and refined pore structure collectively contributed to the improved MgCl₂ rejection and enhanced Mg²⁺/Li⁺ separation. The BPTAB-modified membrane exhibited a high pure water permeability of 23.5 L m⁻² h⁻¹ bar⁻¹ and achieved a Mg²⁺/Li⁺ selectivity (S_Li/Mg) of 24.8 in a 2000 ppm mixed solution with a Mg²⁺/Li⁺ ratio of 20, significantly surpassing the control membrane (S_Li/Mg = 5.2). The results emphasize the considerable ability of BPTAB-modified membrane for efficient Mg²⁺/Li⁺ separation in lithium extraction applications.