Ultra-fast interlayer construction strategy for the preparation of NF membranes with high Li+/Mg2+ separation performance

Nanofiltration (NF) membranes have demonstrated significant potential in lithium extraction from salt-lake brines due to their superior selectivity in separating monovalent cations from divalent cations. In this paper, NF membranes with high permeate flux and high lithium-magnesium separation factor (S_Li,Mg) were prepared by using a polyamide (PA) layer as the separation layer, RA-PEI as the interlayer, and a polysulfone (PSf) membrane as the substrate membrane. Notably, the RA-PEI co-deposition modification method is an ultra-fast substrate modification method, requiring only 1s for modifying PSf substrates. At the same time, RA-PEI firmly adheres to the surface of the PSf substrate by the synergistic effects of intermolecular hydrogen bonding interactions and π-π interactions. This was further verified by comparing the adsorption energies of different kinds of organic interlayers and PSf substrates obtained from molecular dynamics (MD) simulations. Correlation characterization showed that the RA-PEI interlayer endowed the NF membranes with reduced average pore size as well as attenuated electronegativity, thus realizing the efficient separation of Mg²⁺ and Li⁺. The performance assessment showed that the membrane containing RA-PEI interlayer exhibited excellent separation performance and permeate flux in simulated salt-lake brine (2000 ppm, W_Mg/Li = 20), where S_Li,Mg (92.8) and permeate flux (16.7 L·m⁻²·h⁻¹·bar⁻¹) were improved by 7.8 and 2.2 times, respectively, in comparison to the pristine NF membrane. Therefore, this work provides a useful contribution to the practical use of NF membranes for lithium extraction from salt lakes.