Positively-charged nanofiltration membranes constructed via gas/liquid interfacial polymerization for Mg2+/Li+ separation

Nanofiltration (NF) shows great application potentials in selectively recovering lithium (Li) from natural brine to meet the rapidly growing demands in lithium-ion batteries. However, it remains challenges in how to fabricate positively-charged NF membranes via a convenient method since conventional polyamide-based NF membranes are negatively-charged, which exhibit undesirable Mg2+/Li+ selectivity according to the Donnan and electrostatic effects. Herein, we report a new kind of interfacial polymerization (IP) for preparing positively-charged NF membranes at the gas/liquid (GL) interface by replacing the amine solution with amine gas. Compared with the traditional liquid/liquid interfacial polymerization (LLIP), the amine monomers are always more abundant than the acyl chloride monomers during the GLIP process. Therefore, enough amino groups are desirably remained, resulting in positively-charged polyamide-based layers. In addition, the surface charges as well as the average pore sizes of the polyamide-based layers can be facilely adjusted by the reaction time due to the slow reaction kinetics in the GLIP process. The as-prepared NF membranes show a relatively high rejection for MgCl2 of 98.5% and low rejection for LiCl of 22.6%, demonstrating their excellent Mg2+/Li+ selectivity. Moreover, the NF membranes can also maintain the high Mg2+/Li+ separation factors as high as 28 during a long-term operation.