High Selectivity Among Cations or Anions in Transport through Modified Ion-Exchange Membranes

Although ion-exchange membranes exhibit high selectivities between cations and anions, modest selectivities among cations or among anions limit the applications of these materials in ion separations. This presentation will explore modification of ion-exchange membranes with polyelectrolyte multilayers to dramatically enhance selectivity among cations or anions. Remarkably, adsorption of polyelectrolyte multilayers on Nafion and Fujifilm cation-exchange membranes yields Li⁺/Co²⁺, K⁺/La³⁺, and K⁺/Mg²⁺ selectivities that may exceed1000, whereas the corresponding selectivities of bare membranes are around 2. Such selectivities might prove important in battery recycling or water softening. New anion-exchange membranes coated with polyelectrolyte films also show high Cl^-/SO₄^2- selectivities. Moreover, in Donnan dialysis through modified membranes K⁺/Li⁺ selectivities are >50, demonstrating remarkable discrimination among monovalent ions, which may result from selective partitioning into the polyelectrolyte multilayer. Recent studies show that under a limited set of conditions, even bare ion-exchange membranes show essentially infinite selectivities between K⁺ and Mg²⁺, with Mg²⁺ ions moving from a low- to a high-concentration phase.