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/(m(2).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.

Automated summary

Novel composite lithium transport selective polymeric membranes were developed to separate lithium and magnesium ions, with the composite membrane containing 20% HMO showing the highest Li+ ionic conductivity and Li+/Mg2+ selectivity. Through diffusion dialysis experiments, the lithium ion transport performance and separation efficiency were investigated, achieving satisfactory results under different operating conditions.