Positively charged polyamide nanofiltration membrane via grafting a bis-pyridinium molecule for high-efficiency Mg2+/Li+ separation

Efficient separation Mg 2+ and Li + is crucial for lithium extraction from salt-lake brines because the two cations have very similar chemical and physical properties. In this work, we synthesized a novel bis-pyridinium amino monomer, 1,1'-(ethane-1,2-diyl)bis(3-aminopyridin-1-ium) (BAPE), and grafted it onto the PEI-TMC membrane surface. By regulating the BAPE grafting concentration, both the surface charge and effective pore size were systematically modulated. The optimized BAPE-grafted membranes exhibited a strongly positive zeta potential (19.9 mV at pH 7) and a water permeance of 24.5 LMH/bar, 4.4 times that of the pristine membrane, while maintaining a high MgCl 2 rejection of ∼98.6 % using a 1000 ppm MgCl 2 feed solution. Notably, a single filtration stage reduced the Mg 2+ /Li + ratio from 20 in the feed to 0.48 in the permeate, corresponding to a high selectivity (S Mg 2+ /Li + ) of 41.6 (2000 ppm, Mg 2+ /Li + ratio of 20). The modified membranes also demonstrated excellent long-term stability over 72 h. These results underscore the value of rational molecular design, introducing pyridinium functionality by surface grafting, as a promising route to high-performance nanofiltration membranes for brine with high Mg 2+ /Li + ratios.