Rethinking nanofiltration membrane design for breaking the trade-off in Li/Mg separation: A comprehensive analysis based on the separation factor-lithium flux (S-JLi) framework

Abstract

Nanofiltration (NF) membranes are crucial for lithium (Li) recovery from salt-lake brine, but efficient Li/magnesium (Mg) separation remains challenging. This study employs the recently proposed S-J_Li (separation factor vs Li flux) framework to evaluate NF membrane performance for Li/Mg separation, addressing limitations in traditional S-A (separation factor vs water permeance) frameworks. Using the Donnan Steric Pore Model with Dielectric Exclusion (DSPM-DE), we systematically investigate how operating conditions, feedwater properties, and membrane characteristics affect Li/Mg separation. Our results reveal that positively charged membranes outperform negatively charged ones, despite experiencing performance drops in high-salinity environments. We identify a trade-off between Li/Mg selectivity and Li flux that cannot be overcome by adjusting single membrane parameters. Multi-parameter synergistic regulation, particularly minimizing effective membrane thickness while optimizing charge density and pore size, emerges as a promising strategy to enhance separation performance. Our numerical simulations align well with experimental data, providing theoretical insights for designing high-performance Li/Mg separation membranes and emphasizing the importance of considering both selectivity and Li recovery in membrane development and evaluation.