Highly selective Mg2+/Li+ separation membranes prepared by surface grafting of a novel quaternary ammonium bromide

The efficient separation of Mg2+ and Li+ is the crucial step in the process of extracting lithium from salt lake brine. Nanofiltration (NF) membranes exhibit promising application potential in Mg2+/Li+ separation but the Mg2+/Li+ selectively of negatively charged NF membranes prepared by conventional interfacial polymerization process is far from desirable. In this study, a novel positively charged quaternary ammonium bromide, 3-bromopropyl trimethylammonium bromide (BTAB), is grafted on the polyethyleneimine (PEI)/trimesoyl chloride (TMC) NF membrane surface. This is achieved by a chemical reaction known as nucleophilic substitution, which involves the replacement of a bromine atom (-Br) with an amine group. Owing to the enhanced Donnan effects, the BTAB-modified NF membranes exhibit a MgCl2 rejection of up to 99.2 %, while maintaining a water flux of approximately 50 LMH under 5 bar. Meanwhile, the LiCl rejection is only similar to 30 %. The Mg2+/Li+ selectively of the BTAB-modified NF membranes reaches 95.9 when filtrating a simulated brine with a Mg2+/Li+ ratio of 20 (2000 ppm MgCl2 and LiCl mixture), which is a twofold improvement compared with the pristine NF membranes. The working stability of the BTAB-modified NF membranes is confirmed by a 50-h continuous operation process. A two-stage NF treatment is conducted using a simulated East Taijinar salt lake brine, achieving Li2CO3 powder with a purity level of 99.4 %. The nucleophilic substitution reaction between -Br and the amine groups offers a good reference for the surface functionalization of NF membranes.