Facile design of the nanofiltration membrane with Cu2+-incorporated aminated polyethylene substrate for highly selective magnesium/lithium separation

Efficient separation of magnesium and lithium ions (Mg2+/Li+) from high Mg2+/Li+ ratio salt-lake brines using nanofiltration (NF) technology is crucial for stabilizing lithium resources in light of the increasing demand for lithium-based products. Although positively charged NF membranes show promise in selectively separating Mg2+ and Li+ ions, the issues of inefficiency and undesirable trade-offs in perm-selectivity remain pressing. In this study, we presented a breakthrough in creating a highly permeable NF membrane tailored for selective Mg2+/Li+ separation using the cost-effective polyethylene (PE) as the substrate for the preparation of the NF supporting layer. Employing a facile and low-cost Cu2+-incorporated amination process, the hydrophilization of PE substrate was accomplished. This innovative Cu2+-incorporated amination technique not only facilitated rapid hydrophilization but also established a positively charged PE surface while simultaneously regulating the formation of a thinner and loose polyethyleneimine (PEI)-based polyamide (PA) skin layer (∼26 nm) through the coordination bonding between the Cu2+ and amine groups. Consequently, the resultant NF membrane achieved simultaneous enhancements in permeability (∼16 L m−2 h−1 bar−1) and selectivity for Li+/Mg2+ (33) during the nanofiltration of the simulated brine, effectively reducing high Mg2+/Li+ ratios in the feed close to 1 in the permeate, and making it suitable for the subsequent precipitation to produce Li2CO3. Furthermore, this membrane demonstrated remarkable long-term stability, with stable NF operation continuously for over 7 days, owing to its robust Cu2+-induced aminated PE supporting layer and PEI-based PA skin layer. Our work underscores the simplicity and cost-effectiveness of the Cu2+-incorporated amination process in achieving superior PE-based NF membranes, specifically targeting lithium extraction from salt-lake brines.