Positively charged zwitterion-carbon nitride functionalized nanofiltration membranes with excellent separation performance of Mg2+/Li+ and good antifouling properties

A nanofiltration (NF) membrane with positive charges capable of separating Mg 2+ and Li + from high Mg/Li ratio brine was prepared by interfacial polymerization of 1, 4-bis (3-aminopropyl) piperazine (BAPP)/trimesoyl chloride monomers, and then nano graphitic carbon nitride (g-C 3 N 4 ) functionalized with zwitterion (BHC-CN) was introduced into the active layer of the fabricated membrane. Salt permeation and selectivity were improved with the increase of BHC-CN content to 0.02% in BAPP solution. The morphology and surface chemical composition of BHC-CN were detected. The surface Zeta potential test indicates that the membrane fabricated with BHC-CN is positively charged at pH < 7.33. The presence of small aggregates on the surface of the membrane may increase the effective filtering area of the membrane and thus improve the permeability. It also exhibits long-time stability and excellent separation of Mg 2+ and Li + , and the Mg/Li ratio is decreased from 73 in the feed to 1.85 in the permeation. Moreover, the resulted membrane shows good fouling resistance. In conclusion, the membrane has potential advantages in water softening and recovering Li + from brine with high Mg/Li ratio.