Nanofiltration membrane with modified nano-gradient structure and positive charge for Li separation from high Mg/Li ratio brine

Nanofiltration membranes with high selectivity and long-term stability for magnesium and lithium ions are crucial to efficiently enriching lithium resources in salt lake brines. Herein, a nano-gradient-structured nanofiltration membrane was fabricated, in which PEI with different molecular weights and TMC were utilized in a gradient interfacial polymerization process to construct the target active layer. Via this facile strategy, the grafting density of PEI and the amino group introduced were properly adjusted, and the pore size distribution and surface charge of the active layer were optimized. The fabricated active layer simultaneously possesses sub-nanometer pore size with a modified gradient structure and high interfacial positive charges, which can accurately screen lithium and magnesium. The successful grafting of secondary PEI and the high positive charge and continuity of the active layer have been demonstrated. After 150 hour operation (Mg/Li ratio of 50), PEI-70000/PA/PES nanofiltration membrane achieved a lithium‑magnesium separation factor (SLi/Mg) as high as 150 while maintaining the stable permeation flux of approximately 20 L·m−2·h−1·MPa−1, and the terminal Mg/Li ratio on the permeable side falls to 0.32, which exhibits significant potential for extracting Li from salt lake brines with high Mg/Li ratio with promising long-term stability.