(242a) Electrically Driven Membrane-Based Ion Separations

Ion separations are vital for purifying the materials needed to create devices ranging from Li batteries to high-flux magnets. In principle, membrane-based processes can provide continuous, efficient ion separations that have a low environmental impact. Coating of ion-exchange membranes with polyelectrolyte multilayers yields remarkable K⁺/Mg²⁺, Li⁺/Mg²⁺, K⁺/La³⁺, and Li⁺/Co²⁺ electrodialysis selectivities approaching 1000. Although the polyelectrolyte films on ion-exchange membranes give rise to low limiting currents and water splitting, monovalent ion recoveries approaching 100% along with salt purities around 99.9% are possible when using an exponentially decreasing applied current. In principle, selective partitioning of trivalent cations, such as lanthanides, into unmodified cation-exchange membranes can lead to selective electrodialysis of the trivalent species over monovalent and divalent cations. Although partitioning selectivities for trivalent ions can be >100, the low mobility of the trivalent ion in the membrane along with significant concentration polarization limits selectivity to <10.

Separation of monovalent ions is much more challenging than separation of ions with different valences. Remarkably, simply flowing dilute solutions through negatively charged, 30-nm pores gives Li⁺/K⁺ transport selectivities up to 70. Both the Li⁺/K⁺ selectivity and Li⁺ passage initially increase with flow rate, breaking the permeability/selectivity trade-off. Modelling demonstrates that flow through the membranes creates spontaneous electric fields that selectively retard transport of more-mobile cations. Extension of the method to concentrated solutions requires an applied electrical potential, but high currents at high salt concentrations lead to increased energy costs.