(539e) Elucidating the Role of Water Content on Counter-Ion Selectivity in Ion Exchange Membranes Contacted By Mixed Salt Systems

Ion exchange membranes (IEMs) could enable resource recovery and industrial separation applications such as lithium extraction, water softening, and nitrate recovery in electrodialysis-type modules. IEMs facilitate the transport of ions opposite to their fixed-charged groups (counter-ions) across the membrane, while effectively rejecting co-ions that bear the same charge as the fixed-charge groups. However, these applications require membranes that can efficiently discriminate between different counter-ions, particularly those that are similarly charged. Such IEMs are presently unavailable. Membrane water content and charge density govern ion partitioning and diffusion in IEMs, but the incomplete understanding of the relationship between these parameters and IEM counter-ion/counter-ion selectivity hinders the rational design of IEMs with high counter-ion/counter-ion selectivities. Additionally, studies probing counter-ion/counter-ion selectivity in IEMs with mixed salt solutions are scarce in the literature. In this study, we systematically investigated the effect of membrane water content on ion sorption in binary salt mixtures to better understand how this parameter affects mixed ion partitioning in IEMs.

Most studies that investigated mixed ion transport in IEMs have used commercially available membranes, which limits the development of structure/property relationships as the structures of these membranes are not controlled and are often unknown. In this study, a set of well-defined cross-linked cation-exchange membranes (CEMs) with varying water content and high charge density were synthesized via free radical polymerization. Cross-linked IEMs allow systematic and independent variation of these two parameters, creating opportunities for systematic studies probing the effect of membrane structure on IEM counter-ion/counter-ion selectivity. Equilibrium counter-ion partitioning within the CEMs was measured with monovalent/monovalent as well as monovalent/divalent salt mixtures with total concentrations and equivalence fractions varying over a broad range. The states of water molecules in the IEMs were quantified via Differential Scanning Calorimetry. The changes in counterion selectivity were correlated to the changes in the membrane water content and states of water. Results of this study demonstrate that lowering the water content of IEMs significantly increases monovalent/divalent counterion selectivity whereas monovalent/monovalent selectivity is only slightly affected. These results suggest that differences in ion hydration and polymer-ion interactions are likely the origin of this behavior.