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.