Polyelectrolyte-based nanofiltration membranes with exceptional performance in Mg2+/Li+ separation in a wide range of solution conditions

To separate Li + ions from Mg2+ ions rich salt-lake brine using nanofiltration (NF), high-performance NF membranes capable of achieving a high Mg2+/Li+ separation factor in a wide range of solution conditions are critical. In this study, we investigated the impacts of three key solution parameters, including pH, salinity, and Mg2+/Li+ mass ratio, on the performance of Mg2+/Li+ separation by poly(styrene sulfonate) and poly(allylamine hydrochloride) (PSS/PAH) hollow fiber NF membranes fabricated using in-module layer-by-layer deposition with and without crosslinking. We found that lower pH generally favors better Mg2+/Li+ separation, whereas the impacts of salinity and Mg2+/Li + mass ratio on Mg2+/Li + separation depend on whether the membranes are crosslinked. The dependence of Mg2+/Li+ separation performance on the solution conditions and membrane properties is intricate but can nonetheless be qualitative explained using the existing framework of multi-ion transport described by the solution-diffusion-electromigration model (SDEM). Regardless of solution conditions and whether crosslinking was applied, the PSS/PAH NF membranes were capable of achieving exceptional Mg2+/Li+ separation unmatched by commercial NF membranes.