Nanofiltration membrane based on a dual-reinforcement strategy of support and selective layers for efficient Mg2+/Li+ separation

With the consumption of lithium resources increasing year by year, the investigation of high-performance salt lake lithium extraction technology is extremely important. In order to effectively separate Mg2+ and Li+ from brine, a novel charge-positive nanofiltration (NF) membrane with high permeability and excellent Mg2+/Li+ separation factor was designed by interfacial polymerization (IP) in this work. A B15C5-MX-NF membrane was fabricated by blending hydrophilic two-dimensional (2D) MXene nanosheets into a PES membrane substrate to enhance the permeability, followed by the modification of polyamide (PA) layer with a crown ether molecule (Benzo-15-crown-5) to optimize the surface properties of membrane. The PA layer thickness was effectively reduced (58 nm) due to the enhancement in the interaction between the substrate membrane and PEI by the incorporation of MXene, while the addition of B15C5 provided additional Li+ permeation channels. The B15C5-MX-NF membrane exhibited excellent permeability (16.1 L·m−2·h−1·bar−1) and superior selectivity for Mg2+/Li+ (SLi, Mg of 15.8) under the dual optimization of membrane substrate and PA layer. In addition, it was demonstrated by molecular dynamics (MD) simulations that the B15C5 molecules in the B15C5-MX-NF membranes had a specific recognition ability of Li+ to allow its preferential permeation, thereby enhancing the Mg2+/Li+ selectivity of the membranes. This study provides a new idea for the preparation of NF membranes for efficient Mg2+/Li+ separation.