Dual-functional regulation of nanofiltration membranes using covalent organic framework nanosheets for efficient Li+/Mg2+ separation

With growing demand for lithium resources, developing high-performance Li + /Mg 2+ separation methods for salt-lake brines has become crucial, especially the highly efficient energy-saving membrane technology. For the traditional nanofiltration (NF) membranes for mono−/multivalent ion separation, achieving simultaneously high permeance and selectivity for lithium extraction from brine remained a critical challenge. Herein, we proposed a kind of cationic covalent organic frameworks (COF), TpEBr nanosheets with positive charge and structural regulation capability as dual-functional regulator for polyamide (PA) NF membranes. The introduction of TpEBr as additive reduced the diffusion rate of polyethyleneimine (PEI), forming a thin and loose membrane. Additionally, quaternary ammonium groups on TpEBr increased the membrane's surface Zeta potential, significantly strengthening the Donnan exclusion effect. The optimized membrane exhibited a significant Li + /Mg 2+ selectivity of 46.4 with a 25% increase in permeance. Such dual-functional regulation strategy for constructing high-performance NF membranes proposed in this work is promising for future highly efficient lithium extraction from brines.