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

The efficient separation of Li + /Mg 2+ 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 + /Mg 2+ 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 Mg 2+ , and accordingly a high MgCl 2 rejection of 99.2%. The modified membranes exhibited a high Li + /Mg 2+ separation factor of up to 60.1, when applied for MgCl 2 and LiCl binary solution, which exceeded all known PA-based nanofiltration membranes. • A novel quaternary ammonium salt was developed for surface modification of polyamide nanofiltration membranes. • A high positive charge density up to +5.16 mC m −2 of the membrane surface was realized. • The modified membrane achieved a high Li + /Mg 2+ separation factor of 60.1.