Regulating pore and surface properties of nanofiltration membrane by reactive ionic liquid for fast Li+/Mg2+ separation

Nanofiltration membranes possess great potential in ion separation, meanwhile, the pore and surface properties determining membrane permeance and selectivity, should be designed and regulated. Herein, we utilized reactive ionic liquid (1-aminoethyl-3-methylimidazolium tetrafluoroborate, AMIM-BF 4 ) simultaneously as aqueous phase additive and surface modifier to regulate pore and surface properties for fast and efficient Li + /Mg 2+ separation. AMIM-BF 4 was added in aqueous phase to undergo a competitive interfacial polymerization with polyethyleneimine (PEI), effectively increasing the average pore size of the membrane. Then, amino group in AMIM-BF 4 could react with the unreacted acyl chloride groups to modify membrane surface, increasing its positive charge and hydrophilicity. Accordingly, the membrane possessed high water permeance of 33.3 L m −2  h −1  bar −1 with MgCl 2 rejection rate of 98.9%, and separation factor of 22.7 for the mixed solution with magnesium-to‑lithium ratio (MLR) of 50:1. Meanwhile, during the 168-h continuous cross-flow test, the rejection rate for MgCl 2 remained above 98.1% with stable water permeance. Moreover, through the three-stage nanofiltration experiment, the membrane could reduce the MLR of simulated salt lake brine with 2000 ppm from 50:1 to only 0.02:1, demonstrating its potential for efficient lithium enrichment from complex brine.