Interfacial Engineering of Janus Nanofiltration Membranes Incorporating a PDA/PAA Interlayer for Enhanced Lithium-Magnesium Separation

Efficient Li + /Mg 2+ separation remains a critical challenge in lithium extraction from salt lake brines. In this study, a Janus-structured composite nanofiltration (NF) membrane was developed through interfacial regulation. A polyethersulfone (PES) substrate with a sponge-finger transitional morphology was first fabricated via non-solvent induced phase separation (NIPS). Then, a polydopamine (PDA) intermediate layer was introduced through dopamine self-polymerization to facilitate the deposition of a hydrophilic poly(acrylic acid) (PAA) interlayer, which exhibited negative surface charge under near-neutral brine conditions (pH ≈ 6.0). Subsequently, a positively charged polyamide (PA) selective layer was constructed in situ via interfacial polymerization (IP) using polyethyleneimine (PEI) and trimesoyl chloride (TMC), resulting in a Janus charge configuration. The influence of PAA molecular weight and PDA charge density on membrane structure and separation performance were systematically evaluated. FTIR, contact angle, and zeta potential analyses confirmed successful construction of the Janus-structured NF membrane with enhanced hydrophilicity and controllable surface charge. The optimized membrane prepared using 2000 Da PAA achieved 98.8% ± 0.33% Mg 2+ rejection and only 39.9% ± 0.77% Li + rejection, corresponding to a Li + /Mg 2+ separation factor of 48 ± 10. This study provides an effective strategy for designing high-selectivity NF membranes for lithium recovery. Graphical This work illustrates the fabrication of PES-PDA/PAA composite nanofiltration (NF) membrane via dopamine modification and PAA coating, followed by interfacial polymerization. The prepared membrane achieves efficient Mg 2+ rejection while permitting Li + permeation for Li + /Mg 2+ separation.