Development of highly selective composite polymeric membranes for Li⁺/Mg²⁺ separation

To meet the exponentially rising demand for lithium, it becomes vital to develop environmentally friendly processes for its recovery from brines, salt lakes and/or seawater. In this work, novel composite lithium transport selective polymeric membranes were developed to separate lithium and magnesium ions. Hydrogen manganese oxide (HMO) (at weight percentage from 0 to 25%), polystyrene sulfonate sodium salt (PSS–Na) and lithium triflate (LiCF₃SO₃) were added into the sulfonated polyethersulfone (SPES) matrix to prepare composite membranes. The developed membranes showed high mechanical stability and a homogeneous distribution of HMO. The most promising membrane, containing 20% (w/w) of HMO, showed an almost 13 times higher Li⁺ ionic conductivity (8.28 mS/cm) compared to the control composite membrane (without HMO) and an average ideal selectivity of 11.75 for the Li⁺/Mg²⁺ pair. The composite-20% membrane had the lowest intermolecular distance between the polymer chains (according to X-ray diffraction (XRD) analysis), the most flexible structure (lowest T_g) and showed the homogeneous dispersion of HMO (SEM images), which explains its highest Li⁺/Mg²⁺ selectivity among the tested membranes. The lithium ion transport performance and separation efficiency were investigated through diffusion dialysis experiments, under different operating conditions. A binary separation factor of 9.10 for Li⁺/Mg²⁺ and Li⁺ molar flux of 0.026 mol/(m².h) was achieved without applying any external potential difference. When an external potential difference of 0.2 V was applied, the binary separation factor of Li⁺/Mg²⁺ pair was 5, while the Li⁺ molar flux increased almost 5 times. The obtained results provide the basis to design and develop composite lithium transport selective polymeric membranes, thus representing a promising step for future implementation of such membranes to recover lithium from saline streams.