Stable ionic liquid-based polymer inclusion membranes for lithium and magnesium separation

The separation of lithium and magnesium is crucial for the exploitation of lithium resources with a high concentration of magnesium ions (Mg2+). In this study, polymer inclusion membranes (PIMs) based on the stable ionic liquid (IL) extractant system were developed for separating lithium ions (Li+) from Mg2+ in aqueous solutions. The mixture of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][NTf2]) and tributylphosphate (TBP) was used as the carrier to transfer Li+, and the cellulose triacetate (CTA) polymer was chosen as the matrix due to its high compatibility with [C4mim][NTf2] via hydrogen bond. The TBP was kept in the membrane matrix without obvious leakage in the membrane extraction process, and the lithium selectivity and extraction percentage of the membranes decrease by less than 10% after five continuous extraction operation cycles. In the transport experiment using the feed containing 7.2 mmol/L Li+ and 2.06 mmol/L Mg2+, the PIM with 40 wt% TBP-[C4mim][NTf2] exhibits the separation factor of 2.24 for Li+/Mg2+ and an initial flux of 0.89 × 10−6 mol m−2 s−1 for Li+, much higher than the results of the pure CTA membranes, indicating that TBP-[C4mim][NTf2] system is efficient for Li+ extraction in PIM membrane. Besides, the PIMs we prepared were applied to extract Li+ from simulative salt-lake brines with a high Li+/Mg2+ selectivity of 23.87. The IL-based PIMs provide a possible membrane technology to extract Li+ from salt-lake brines with high stability, low energy consumption and environment-friendly.