Mismatch and mix: Making use of electrokinetic aspects of spacers for intensified electrodialysis

Electrodialysis (ED) processes are limited in terms of practical current densities due to the emergence of concentration boundary layers. A great deal of research has focused on methods to promote mixing of these boundary layers, particularly through developing new spacers. Spacers are crucial to the performance of electrodialysis stacks, forming the flow channels between membranes and leading to hydrodynamic mixing that reduces mass transport limitations. Conventional analysis of spacers in ED has focused on characterizing the hydrodynamic mixing resulting from spacer geometry and assessing how this impacts the overall stack resistance. In this work, we explore the electrokinetic aspect of spacer performance: the electric-field bending induced by polymeric spacers and resulting electro-osmotic mixing. We carried out an experimental study to characterize the ED performance of a lab-scale stack using spacers with different surface charge densities, using polyelectrolyte adsorption on the spacers. Our results show that substantial increases in the current density or reductions in power consumption can be achieved when making use of this enhanced mixing due to electro-osmosis. We also explain this enhancement through the use of a simplified theoretical model to highlight the potential of utilizing electrokinetic mixing from spacers in electrodialysis.