Staged interfacial reaction for regulating polyamide selective layer formation toward efficient Mg2+/Li+ separation

In this work, we proposed a staged interfacial reaction strategy to regulate the formation of the polyamide selective layer by separating nascent-layer formation from subsequent densification, thereby addressing the mismatch between monomer diffusion and reaction kinetics in conventional interfacial polymerization (IP) and enabling high permeability and selectivity in nanofiltration (NF) membranes for Mg 2+ /Li + separation. The optimized membrane (SIP) was fabricated using 0.2 wt% BAPP and 0.1 wt% TMC for 120 s in Stage-I, followed by 0.5 wt% BAPP for 120 s in Stage-II. Compared with membranes prepared by conventional one-step IP, the SIP membrane exhibited a thinner selective layer (14.89 ± 1.16 nm), a higher amide-related O C–N content (50.96 at%), a more favorable hydration state, and a lower molecular weight cut-off (330 Da), indicating the formation of a thinner yet more compact and uniform polyamide network. Benefiting from these tailored structural and physicochemical features, the SIP membrane exhibited robust Mg 2+ /Li  +  separation over a broad range of feed Mg 2+ /Li + mass ratios, with high Mg 2+ rejection, low Li + rejection, and stable water flux. During long-term filtration tests, the Mg 2+ /Li + mass ratio in the permeate remained below 0.68, while the Mg 2+ rejection and separation factor were maintained at 95.57%–97.79% and 25.50–35.34, respectively. These results demonstrate that the staged interfacial reaction strategy is an effective platform for regulating polyamide selective layer formation and offers a promising route for lithium extraction from brines with high Mg 2+ /Li + mass ratios.