Fabrication of positively charged composite nanofiltration membranes with “multilayer interlocking” structure based on ZIF-8 layer anchored constrained growth strategy for Mg2+/Li+ separation

Abstract

In recent years, with the rapid development of new energy vehicles, the demand for lithium resources has been increasing sharply. Positively charged nanofiltration (NF) membranes can accurately separate magnesium ions (Mg²⁺) and lithium ions (Li⁺), which have become an important technology for extracting lithium resources from salt lakes. The introduction of intermediate layer in thin-film composite (TFC) NF membranes could improve the permeability and selectivity for Mg²⁺/Li⁺ separation. However, there is a problem of poor bonding between the intermediate layer and the supporting membranes through the simple physical combination on the membrane surface. Inspired by the controlled growth of succulent plants, the positively charged PA/ZIF-8/PSF composite NF membranes with “multilayer interlocking” structure were prepared successfully based on ZIF-8 layer anchored constrained growth and further interfacial polymerization. The zinc ion ligands of ZIF-8 were pre-anchored on the PSFsolution to provide reaction sites, and the grown ZIF-8 nanoparticles were embedded in the PSFsupporting membrane surface layer. The embedded ZIF-8 nanoparticles can enhance the interface bonding between the PSFsupporting membranes and the ZIF-8 layer. Additionally, the confined growth method can solve the problems of ZIF-8 particle aggregation and poor dispersion. Compared with membranes without ZIF-8 layer, the prepared PA/M − 6 membrane had a high flux of 47.2 L m⁻²h⁻¹, which was 1.82 times that of PA/M-0. Meanwhile, the Mg²⁺/Li⁺ separation factor of the PA/ZIF-8/PSF composite NF membranes could reach 47.6. In particular, the prepared NF membranes exhibited good long-term stability for Mg²⁺/Li⁺ separation.