Polyamide nanofiltration membranes with rigid–flexible microstructures for high-efficiency Mg2+/Li+ separation

• New polyamide layer with rigid-flexible microstructure was constructed. • Both internal pores and positive charge density were remolded at molecular level. • “Useless pores” were widened and “defect pores” were repaired simultaneously. • Modified membrane exhibited much higher Mg 2+ /Li + selectivity factor of 15.38. Lithium extraction from salt-lake brine with a high Mg 2+ /Li + ratio is crucial for mitigating lithium supply shortage. Positively charged polyamide nanofiltration membranes, which are often prepared by performing interfacial polymerization (IP) using polyethyleneimine and trimesoyl chloride, are considered to be promising candidates for Mg 2+ /Li + separation owing to their high energy efficiency and low environmental impact. However, the performance of these membranes is severely hindered by “useless pores” and “defects” present in the flexible polyamide layer. Herein, we provide a facile strategy for modifying flexible semi-aromatic polyamide chains by intercalating a rigid electropositive monomer, histamine, to remold the internal pores and positive charge density at a molecular level. Two routes, namely, “swelling-embedding-reacting-shrinking (SERS)” and “in-situ interposition (SIP)”,),” were used to construct a new polyamide layer with rigid–flexible microstructures. The fabricated SERS-0.50 membrane showed a 5-fold higher water flux and 2-fold increase in the Mg 2+ /Li + separation factor compared with the IP membrane. Moreover, the SIP-0.15 membrane with an enhanced positive charge density exhibited a considerably higher Mg 2+ /Li + separation factor (15.38, 500% increase compared with that of the IP membrane) for brine with a Mg 2+ /Li + ratio of 20. This study provides a reference for developing nanofiltration membranes used for high-efficiency Mg 2+ /Li + separation.