Nanofiltration membrane with a zwitterion-g-C3N4 composite interlayer for Mg2+/Li+ separation

Nanofiltration (NF) membrane with high selectivity and permeance is important for recovering lithium from salt lake brine. In this work, sandwich-like NF membranes were prepared using polyamide (PA) as the interfacial polymerization (IP) layer, dopamine (DA) and zwitterion-g-C 3 N 4 as the composite interlayer and polyethersulfone (PES) ultrafiltration (UF) membrane as the substrate. Three NF membranes were prepared with different zwitterion-g-C 3 N 4 : M3 membrane modified with C 5 H 12 ClNO 2 -g-C 3 N 4 , M4 membrane modified with C 12 H 24 N 2 O 4 S-g-C 3 N 4 and M5 membrane modified with C 19 H 39 NO 2 -g-C 3 N 4 , and their physical and chemical properties, separation capacity, and antifouling behaviour were analyzed. The zwitterion-g-C 3 N 4 composite interlayer improves the hydrophilicity and pore structure of NF membranes. The water contact angle (CA) of M5 membrane is 25.5°and its pore size is 0.49 nm. M5 membrane shows a high retention rate of 96.66 % for Mg 2+ in mixed salt solution (Mg 2+ /Li +  = 84) and performs well in salt lake brine with a rejection of 97.90 % to Mg 2+ . The flux recovery ratio (FRR) can be kept above 93 %. This study may give ideas for the preparation of composite NF membrane for water softening and lithium utilization.