Structure and positive charge regulation in nanofiltration membrane by novel nanomaterial g-C3N5 for efficient Li+/Mg2+ separation

The positively charged nanofiltration (NF) membrane prepared via interfacial polymerization (IP) of polyethyleneimine (PEI) and trimesoyl chloride (TMC) has drawn considerable attention for lithium-magnesium separation, particularly crucial for lithium recovery from salt lakes with high Mg2+/Li+ mass ratios. However, NF membranes prepared solely from amine monomers have not achieved efficient Li+ and Mg2+ separation. In this study, PEI-g-C3N5/TMC composite NF membranes were successfully fabricated by mixing amino-rich graphitic carbon nitride (g-C3N5) with PEI, followed by the IP reaction of the mixed solution with TMC. Molecular dynamics (MD) simulation results showed that g-C3N5 had an attractive effect on PEI, which led to a decrease in the diffusion rate of PEI, thereby resulting in a thinner separation layer. Density functional theory (DFT) revealed that g-C3N5 competed with PEI, leading to a reduction in the crosslinking within the polyamide (PA) layer and subsequently causing an increase in the surface pore size of the membrane. Besides, the hydrophilicity and positive charge of the modified membrane were both improved. MD simulations, transition state theory, and DLVO principles indicated that the modification of the PA layer by g-C3N5 enhanced the repulsion of Mg2+ and the transport of Li+ during the NF process. This breakthrough effectively overcame the trade-off effect between membrane separation and permeability, especially at a g-C3N5 content of 0.06 wt%, where the lithium-magnesium selectivity coefficient (SLi, Mg = 18.18) and pure water flux (Flux = 58.59 L m−2 h−1) reached the optimum values. Meanwhile, the PEI-g-C3N5/TMC membrane demonstrated stable separation performance during prolonged filtration and in solutions with varying Mg2+/Li+ mass ratios. This study not only provides an effective strategy for the design of NF membranes but also expands the application prospects of nanomaterials in the field of membrane separation.