Highly positively charged nanofiltration membrane through bis-quaternary ammonium modification for Li+/Mg2+ separation

Increasing the positive charge density (PCD) of nanofiltration (NF) membranes is crucial to acquiring the high selectivity of Li + /Mg 2+ separation and remains extensive exploitation. Herein, a novel modifier (1-methyl hydrazide-3,5-dibenzyl trimethyl ammonium bromide, MDTAB) containing two quaternary ammonium groups and an amino group was designed and synthesized, which was grafted onto the polyamide NF membrane surface. The surface PCD of the MDTAB modified membrane (+7.02 mC m −2 ) was almost four times that of the unmodified membrane, which is the highest among the existing positively charged NF membranes. The MDTAB modified membrane acquired MgCl 2 rejection as high as 99.3 %, and the pure water permeance was 15.92 L·m −2 ·h −1 ·bar −1 . When utilized for Li + /Mg 2+ separation, the Mg 2+ /Li + mass ratio was reduced from the original 100 to 1.73. This strategy may provide valuable guidance for the rational design of the positively charged membrane in salt lake lithium extraction applications.