Synergistic enhancement of size-sieving and electrostatic repulsion in nanofiltration membranes via a Tp-mediated pre-crosslinking strategy for Li+/Mg2+ separation

The efficient extraction of lithium from salt-lake brine still faces challenges due to the insufficient performance of nanofiltration (NF) membranes. In this study, a nanofiltration membrane was fabricated by first pre-crosslinking polyethyleneimine (PEI) with 1,3,5-triformylphloroglucinol (Tp), followed by interfacial polymerization (IP) with trimesoyl chloride (TMC). The pre-crosslinking step incurred no amine loss. However, the higher molecular weight and increased steric hindrance of the product reduced its subsequent crosslinking with TMC. An optimal balance enables the simultaneous enhancement of both the overall crosslinking density, which is contributed by both pre-crosslinking and TMC crosslinking, and the positive charge density of the membrane. The optimized membrane (Tp 0.12) achieved a pure water permeability of 8.57 L·m −2 ·h −1 ·bar −1 and an ideal LiCl/MgCl₂ selectivity of 32.9, representing 179 % and 700 % of the values for the pristine membrane, respectively. Moreover, the optimal membrane remained effective even when the Mg 2+ /Li + ratio in the feed was as high as 140 or the total salt concentration reached up to 10,000 ppm. This work provides a simple and efficient new pathway for preparing high-performance selective nanofiltration membranes for lithium extraction.