Modelling Ion Transfer Through Ion Exchange Membranes In Electrodialysis For Multi-Ionic Solutions.

Poster and abstract:
Water reuse is one of the solutions to prevent depletion of freshwater resources. However, continuous use of water in closed cycle systems can result in accumulation of specific ions in the water cycle, which limits the possibilities for re-use applications. Especially in agriculture, accumulation of sodium ion (Na+) in the irrigation water negatively affects the soil permeability and limits the crop growth. Developing ion selective desalination technologies that remove specific ions could in many cases increase the potential for water reuse. Electrodialysis (ED) is a desalination technology that has been mainly used for brackish water desalination and reuse. ED has a potential to selectively remove or recover specific ions. It is an electrically driven membrane process in which ion transport takes place as a result of electro-migration and diffusion across the membranes.
Understanding these mechanisms behind ion transport mechanisms in ED is important to enhance the ion selectivity of the process. In this research we aim to explain these mechanisms based on theoretical and experimental studies considering multi-ionic solutions. For that purpose, a two-dimensional theoretical model was developed in order to describe the transport of ions and water through membranes in an ED cell. The computational domain of this model is a repeating unit of an ED cell, which includes one diluate channel, one cation exchange membrane (CEM), one concentrate channel and one anion exchange membrane (AEM). The two-dimensional process model describes transport of ions across the membranes with the extended Nernst-Planck equation. Furthermore, chemical (acid-base) equilibria were included. Different material characteristics of the membranes (AEM and CEM) used in the ED cell, such as thickness, membrane charge and porosity were also considered in the model. Furthermore, ion specific properties, including the chemical affinity of the membrane materials for specific ions, and the diffusion coefficients were analysed and included in the model. In order to validate the model desalination experiments were performed using a laboratory scale batch-mode ED setup. The effect of various operational parameters on selectivity was studied, such as the current density and the water flowrate. During the experiments the pH, electrical conductivity, temperature and water level of the three solutions (diluate, concentrate and electrolyte) were recorded continuously.