Aza-crown ether-coupled polyamide nanofiltration membrane for efficient Li+/Mg2+ separation

The scarcity of lithium can be effectively addressed by extracting it from salt-lake brines. In this work, aza-crown ether (diazo-18-crown-6, DA18C6) was coupled into a polyamide (PA) network interfacial polymerized by trimesoyl chloride and polyethyleneimine (PEI) for efficient Li+/Mg2+ separation. The embedment of DA18C6 molecule into PA network was confirmed by structural characterization and density functional theory simulation, which enhanced crosslinking degree and reduced average pore size of PA selective layer. In addition, the strong interaction between DA18C6 and Li+ was proven through molecular dynamics simulation. The resulting PEI@DA18C6-PA membrane displayed a high pure water permeance of 10.4 L m−2 h−1·bar−1 and an excellent Li+/Mg2+ separation factor (SLi,Mg) of 11.2, which was comparable with most reported Li+/Mg2+ separation membranes. Furthermore, the PEI@DA18C6-PA membrane was employed in a three-stage NF treatment process to filtrate simulated brines containing a high Mg2+/Li+ ratio of 57. The results demonstrated that Li+ can be effectively concentrated and extracted through the membrane to obtain a permeate with an ultralow Mg2+/Li+ ratio of only 0.03, suggesting that PEI@DA18C6-PA membrane has great application potential in lithium extraction from salt-lake brines.