Membrane-Enabled Direct Extraction of Critical Metals

Water treatment devices designed to extract critical elements from wastewater and natural brines rely on polymer membranes that exhibit selective permeability towards dissolved ions. However, the underlying design principles that govern selectivity between ions of similar size and charge (e.g. Li⁺/Na⁺) remain unclear because of natural tradeoffs in the solubility and diffusivity of dissolved species within a membrane. Given that these properties arise from the solvation environments made available to permeating ions, we proposed a membrane platform wherein specific coordination modes are enforced in at sub-nanometer length scales with rigid polymer backbones and pendant groups with unique geometries. To this end, a library of systematically tuned microporous polymers have been synthesized and evaluated for ion uptake, ionic conductivity, and salt permeability. Our results lend insights to an energy landscape model of ion transport across membranes, where the pendant is shown to influence the energetics of partitioning at the water-membrane interface, as well as hopping between adjacent solvation cages within the membrane.