Optimizing Mg2+/Li+ selectivity of nanofiltration membrane: synergistically modification on steric and Donnan exclusion

The transition to renewable energy and the pursuit of carbon neutrality have heightened the demand for lithium extraction from salt lake brines to ensure a sufficient lithium supply. Nanofiltration (NF) membranes offer an efficient method for solute sieving; however, achieving highly selective Mg 2+ /Li + separation remains challenging due to their similar hydration radii. This study introduces a synergistic modification strategy to adjust the surface physicochemical properties of NF membranes through incorporation of a sodium alginate interlayer and post-treatment with piperazine. The resulting modifications improved Mg 2+ selectivity by optimizing steric and Donnan exclusion mechanisms, achieved through a refined pore size distribution and reduced surface negative potential. Consequently, the modified membrane exhibited an outstanding Mg/Li selectivity factor (>100), with Mg 2+ rejection efficiency exceeding 98%. The membrane also demonstrated stable performance under mixed feed conditions with high total dissolved solids (TDS, 2000 ppm) and low Li + concentration (100 MgCl 2 /LiCl mass ratio). This study provides a facile and effective approach to enhance multiple exclusion mechanisms for Mg 2+ separation, offering valuable insights into the design and fabrication of advanced NF membranes for efficient Mg/Li separation.