Current efficiency and selectivity reduction caused by co-ion leakage in electromembrane processes

Abstract

In electromembrane processes such as electrodialysis (ED) and ion concentration polarization (ICP), the diffusion layers on both diluate and concentrate sides influence permselectivity of the ion-exchange membrane and current utilization. The diffusion layer in the diluate stream, due to lower salinity and higher resistivity, has been regarded as the primary source of energy loss. In contrast, very few studies have focused on the diffusion layer in the concentrate stream. In this paper, we evaluate the influence of hydrodynamic convective flow on the development of diffusion layers on both concentrate and diluate sides, specifically in the ICP desalination process. Interestingly, the higher convective flow in the concentrate side was shown to drastically improve the current utilization drop in high operating current, which has been a recurring challenge in electromembrane processes. We attribute this to the prevention of co-ion leakage into the membrane, confirmed by both experimentation and numerical modeling. This new insight has a clear design implication for optimizing electromembrane processes for higher energy efficiency.

Keywords: Co-ion leakage; Current efficiency; Electromembrane; Hydrodynamic convection; Ion concentration polarization.