Sequential monomer immersion guided by density contrast tailors pore structure and surface chemistry in nanofiltration membranes for lithium recovery

Precise ion sieving by polyamide (PA) nanofiltration (NF) membranes relies on narrowly distributed subnanometer pores and tailored surface chemistry, yet simultaneously controlling both within a single interfacial polymerization (IP) process remains challenging. Here, we developed a density contrast-assisted sequential immersion strategy by preloading a porous substrate with a polyethyleneimine (PEI)/sucrose phase before piperazine (PIP) immersion. The preloaded phase regulated PIP storage and interfacial supply, enabling rapid PIP-dominated film formation and diffusion-limited growth, while limited PEI near the initial interface may participate in reaction before interfacial sealing. This reaction-diffusion process produced a thinner selective layer with an MWCO of approximately 190 Da, a narrowed pore-size distribution, and modified chemical and electrostatic characteristics. The resulting sequential immersion membrane (SIM-PA) achieved a Li + /Mg 2+ selectivity of 28.7 at a Mg 2+ /Li + mass ratio of 100:1 and maintained stable separation under moderately elevated salinity. Transport analysis suggested that the enhanced Mg 2+ /Li + discrimination mainly originated from strengthened steric and dielectric exclusion during ion partitioning and diffusion. Integrated with multistage NF and evaporation-precipitation, SIM-PA enabled lithium recovery from both simulated salt-lake brine and spent lithium-ion battery discharge wastewater. This work establishes density contrast-assisted sequential monomer loading as a practical route to NF membranes with narrow pore-size distributions for lithium recovery from complex saline streams.