Positively charged nanofiltration membranes for efficient Mg2+/Li+ separation from high Mg2+/Li+ ratio brine

Nanofiltration has gained increasing attention in lithium extraction from salt lake brine with high Mg2+/Li+ ratio. However, conventional nanofiltration membranes with negatively charged surfaces suffer from low Mg2+/Li+ selectivity. Herein, positive nanofiltration membranes with high charge density were fabricated via a two-step charge enhancement strategy. High concentration of polyethylenimine was used as the aqueous monomer to ensure the abundant amino groups on the membrane surface. To further enhance the electro-positivity, 2, 3-epoxypropyl trimethyl ammonium chloride was grafted through ring-opening reactions. The as-obtained membranes demonstrated positive zeta potentials over a large pH range (3-10), leading to significantly strengthened Donnan exclusion for Mg2+. The membrane rejection to MgCl2 was up to 99.3% while the rejection to LiCl was only -30%. The Mg2+/Li + separation factor was 167 when filtration simulated brine with a Mg2+/Li+ ratio of 20 (2000 ppm MgCl2 and LiCl mixture), which is the highest value achieved among polyamide-based nanofiltration membranes. In addition, the membranes exhibited good stability in 40 h' continuous testing. The modification strategy proposed in the present work is highly compatible with current industrial membrane preparation processes and easy to scale up with cost effectiveness.