Quaternary ammonium engineered polyamide membrane with high positive charge density for efficient Li+/Mg2+separation

The efficient separation of Li+/Mg2+ is the key to lithium extraction from salt lake brine. Herein, a multi-amino quaternary ammonium salt (3,5-dimethylhydrazide-benzyl trimethyl ammonium bromide, DHTAB) was designed and synthesized to modify pristine polyamide (PA) nanofiltration membranes. DHTAB was grafted on the surface of the PA nanofiltration membrane to construct a positively selective layer for Li+/Mg2+ separation. The introduction of DHTAB increased the membrane surface charge density to +5.16 mC m- 2, which was nearly three times that of the pristine PA membrane. The increased charge density of the modified membrane led to a stronger Donnan exclusion for Mg2+, and accordingly a high MgCl2 rejection of 99.2%. The modified membranes exhibited a high Li+/Mg2+ separation factor of up to 60.1, when applied for MgCl2 and LiCl binary solution, which exceeded all known PA-based nanofiltration membranes.