Mechanisms for enhanced transport selectivity of like-charged ions in hydrophobic-polymer-modified ion-exchange membranes

Effectively removing contaminants or extracting resources from water necessitates selective separation of ions with toxicity or economic value from common ions. Existing ion exchange membranes (IEM) effectively separate ions with opposite charges, but are not optimized for separating ions of like charges. An understanding of ion separation mechanisms is imperative for developing IEMs with improved selectivity between like-charged ions. In this work, we modify a commercial anion exchange membrane (AEM) with a hydrophobic polymer, polypyrrole (PPy), and show that PPy-modified AEM exhibits enhanced transport selectivity of nitrate over both chloride and sulfate. Combining ion sorption experiments and molecular dynamics simulations, we show that the enhanced selectivity between nitrate and chloride is attributed to the decoupling of the inverse relation between the ion partition into and diffusion within the polymer phase, which improves the overall ion transport selectivity between like-charged ions.