Low-temperature regulated interfacial polymerization of nanofiltration membrane for efficient Li+/Mg2+separation

Selectively removing Mg2+ and enriching Li+ from salt lakes using positively charged polyamide nanofiltration (NF) membranes based on Donnan exclusion theory and size sieving effect is expected to address the issue of lithium resource scarcity. However, simultaneously achieving high selectivity and permeability remains a challenge for the currently reported membranes. In this study, we developed a novel positively charged NF membrane by the strategy of low-temperature-induced interfacial polymerization (IP). Low-temperature conditions during the IP reaction regulated the diffusion of amine monomers, resulting in changes in the morphology, chemical composition, and surface properties of the polyamide (PA) layer. Consequently, the obtained membrane features a positively charged PA layer with a relatively loose structure, owing to the low temperature slowing down both the amine monomer diffusion and the IP reaction, which finally reduces the crosslinking degree. The optimal NF membrane prepared in this study with a lower N/O ratio of 0.75 exhibited high water permeance (10.2 L m-2 h-1 bar-1) compared to the pristine membrane prepared at room temperature, high Li+/Mg2+ separation selectivity (29.9), excellent stability, and the purity of Li+ reached 60.0 %. Moreover, after two-stage filtration of the simulated brine using the prepared NF membrane, the Mg2+/Li+ mass ratio dropped significantly from the initial 40 in the initial feed solution to 0.22 in the final permeate solution, demonstrating its considerable potential for practical application in Li+ and Mg2+ separation. Since the high positive charge of the membrane also causes certain lithium rejection, future research should focus on optimizing the membrane charge for more efficient lithium extraction with high recovery.