(470c) Facilitated Transport Mechanisms of Ion-Selective Membranes with Fixed Coordination Sites

Developing technology that supports a circular water economy and clean energy future requires advancements in ion-selective membranes, which are materials that separate a particular ionic species from other species. Ion-selective membranes are commonly utilized in water purification and energy conversion and storage technologies such as electrodialysis, fuel cells, and redox flow batteries. However, conventional polymeric membranes cannot attain the highly precise ion separations needed to meet emerging water and wastewater treatment objectives, such as resource recovery. To obtain selectivity between species of similar physicochemical characteristics, reversible coordination interactions between target ions and membrane binding sites, as seen with biological ion channels, may be required. We investigate transport mechanisms in polymeric membranes that demonstrate ion selectivity via strong but reversible bonding with immobile coordination sites. In our analysis, we utilize polyelectrolyte layer-by-layer deposition on anodic aluminum oxide substrates to produce polymeric membranes with amine and iminodiacetate binding sites. We report that among ions of nearly identical size and equal charge, ion flux in single-salt solutions does not correlate with binding strength to membrane fixed charges, which could be explained by ion pairing at binding sites. In mixed-salt solutions, stronger binding species preclude weaker binding species from complexing with the polymer, such that permselectivity between ions is dependent upon their difference in binding energy. Finally, we show that increasing membrane thickness reduces permselectivity when coordination reactions are rate-limiting for transport of weaker binding species. Our experimental findings, which are coupled with density functional theory calculations, enhance fundamental understanding of facilitated transport mechanisms that may inform design of ion-selective membranes for water and energy technologies.