Counter-ion Conductivity and Selectivity Trade-Off for Commercial Ion-Exchange Membranes at High Salinities

The growing interest in using ion-exchange membranes (IEMs) for high-salinity applications such as brine concentration and produced water treatment necessitates a better understanding of their properties under relevant conditions. In this study, we examine the ion transport properties of 40 different commercial membranes contacted by either 1 or 5 m NaCl solutions. To sample a broad materials space, we selected 28 membranes marketed toward desalination applications and 12 membranes marketed toward energy applications. We quantified the equilibrium ion concentrations, salt permeability coefficients, and ionic conductivities of these membranes. Using these results, we derived the effective counter-ion and co-ion diffusion coefficients in the membranes and calculated the counter-ion conductivity and counter-ion/co-ion selectivity. There is a clear trade-off between the counter-ion conductivity and selectivity, with desalination membranes generally exhibiting combinations of high selectivity/low conductivity and energy IEMs generally exhibiting combinations of high conductivity/low selectivity. We decoupled the total selectivity into the partition and diffusion selectivity and correlated these parameters with membrane structural properties to establish structure/property relationships. The results of this study highlight shortcomings in the performance of these membranes and identify gaps in our fundamental understanding of ion transport in IEMs at high salinities.