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Selective partitioning and uphill transport enable effective Li/Mg ion separation by negatively charged membranes

Efficient separation of lithium (Li+) and magnesium (Mg2+) is critical for enhancing sustainable lithium extraction from natural brines, which is vital for battery production and renewable energy technologies. Here we present a method for highly selective Li+/Mg2+separation driven by concentration gradients across negatively charged membranes with high charge densities. In contrast to typical electric field-driven transport in negatively charged membranes, where divalent cations generally permeate faster than monovalent cations, Li+ions in our system permeate the membrane at substantially higher rates than Mg2+ions. This unexpected selectivity stems from the selective ion partitioning properties of the membrane and the uphill transport of Mg2+ions against their external concentration gradient. We demonstrate the efficacy of this separation approach through bench-scale dialysis experiments using a model Atacama brine solution, achieving efficient separation of monovalent and divalent cations. The high separation efficiency observed in this study suggests a promising approach for monovalent/divalent ion separations, offering higher selectivity compared to current technologies.