Understanding the Role of Water Content on Single Cation Diffusion in Negatively Charged Membranes

Abstract

Membrane-based technologies, which utilize synthetic polymer membranes offer advantages such as compact size, high configurability, and lower energy demands compared to thermal or liquid separation processes. Specifically, charged membranes capable of discriminating between similarly charged ions hold significant promise for target metal ion (resource) recovery, where selective membranes could preferentially transport one ion species over others. However, the incomplete fundamental understanding of how membrane properties influence ion transport in charged polymers hampers the design of advanced membranes with desired ion selectivity, particularly for separating similarly charged ions. This dissertation investigates the role of membrane water content on cation diffusion in negatively charged membranes. The results demonstrate that cation transport is strongly influenced by the quantity of freezable water, which is water that interacts weakly with ions or hydrophilic groups of the polymers. At low freezable water content, Coulombic interactions between mobile ions and fixed charges significantly hinder cation diffusion by increasing the activation energy of diffusion in these membranes. A straightforward Coulombic interaction model accurately predicts this increase in activation energy for all studied ions without requiring adjustable parameters. The agreement between model predictions and experimental data suggests that diffusion is more restricted as the ion valence increases, thereby providing negatively charged membranes having low freezable water content with a diffusion selectivity advantage for monovalent over multivalent ions. The insights of this study enhance the fundamental understanding of ion transport in charged polymers and pave the way for the development of next-generation selective membranes.