Enhanced polyamide nanofiltration membranes via a dual-reinforcement strategy for efficient Li+/Mg2+ separation

Nanofiltration (NF) membranes are emerging as promising candidates for lithium extraction. However, their performance is challenged by the comparable physicochemical characteristics of Li + and Mg 2+ . In this work, a dual-reinforcement strategy is proposed to simultaneously enhance both selectivity and permeability by combining the densification of the polyamide (PA) network and the construction of Li + -selective transport channels. During the interfacial polymerization of piperazine (PIP) and trimesoyl chloride (TMC), the cross-linking density of the PA layer is increased by adding sodium dodecyl sulfate (SDS), while Li + -selective transport nanochannels are constructed by incorporating 15-crown-5 ether (CE). This synergistic modification enhances Mg 2+ rejection via intensified size exclusion and facilitates Li +  permeation through ion-specific recognition. The resulting PIP/SDS/CE-PA membrane exhibits a high cross-linking degree of 91.26 %, a reduced pore size of 0.70 nm, and a narrow pore size distribution (σ = 1.22). Molecular dynamics simulations confirm the preferential complexation of CE with Li + , and that Li + exhibits faster and more favourable translocation in the PIP/SDS/CE-PA membranes, thereby facilitating its selective transport. Experimentally, the membrane achieves a high MgCl 2 rejection of 97.1 % and a low LiCl rejection of 56.7 %, yielding an exceptional Li + /Mg 2+ selectivity of 30 in a MgCl 2 /LiCl binary system, while maintaining a water permeance of 5 L m −2  h −1 bar −1 . These findings demonstrate the strong potential of the PIP/SDS/CE-PA membrane for the direct treatment of high Mg 2+ /Li + ratio salt-lake brines and offer a versatile strategy for designing next-generation NF membranes for selective ion separations.