A novel polyelectrolyte-modified membrane for selective lithium extraction from water in an electrified process

The selective extraction of lithium ions (Li+) from salt lake brines holds substantial significance for environmental and energy applications. However, the effective extraction of Li+ is hindered by the low concentration of Li+ and the high Mg2+ to Li+ ratio in salt lakes. In this study, polyelectrolyte modified cation exchange membranes (CEMs) were prepared by coating poly(allylamine hydrochloride)/poly(sodium 4-styrenesulfonate) (PAH/PSS) bilayers using a layer-by-layer method, and their effects on the selective separation of Li+ and Mg2+ ions were investigated. By optimizing the bilayer number and current density, an average Li+/Mg2+ selectivity exceeding 10 was achieved. Increasing the bilayer number from 0.5 to 2.5 led to a rise in peak Li+/Mg2+ selectivity from 12.8 to 31.6, which was significantly higher than that of the unmodified CEM. An optimal Li+/Mg2+ selectivity of 97.2 was further achieved by increasing the current density to 2.0 mA/cm2. The mechanism of Li+/Mg2+ separation in PAH and PSS monolayers were also elucidated. PSS demonstrated a higher Li+ adsorption ability compared to Mg2+, while PAH enhanced Li+/Mg2+ selectivity through stronger electrostatic repulsion against Mg2+. This study highlights the potential of polyelectrolyte-modified membranes for Li+ extraction, inspiring a novel electrified process for the highly selective separation of valuable species using tailor-designed membranes with polyelectrolyte bilayers.