Analysis of an underlimiting and overlimiting current regime in a single electrodialysis channel

Abstract

Overlimiting ionic transfer is considered a mechanism to intensify transport processes in electrodialysis, potentially providing a smaller required footprint of electromembrane units. However, its applicability in desalination is still questionable. To address this aspect of electrodialysis, we characterized desalination in a single-channel electrodialysis unit operated under various current densities, yielding various diluate desalination degrees. Our analysis revealed that the conditions providing <100 % desalination were pertinent to an underlimiting regime with no indication of overlimiting processes. Specifically, no (electro-) convective structures intensifying ionic transfer occurred in the channel under these conditions. Applying electric current densities yielding theoretical desalination larger than 100 % significantly increased the necessary voltage, driving electroconvective vortices. The observation suggested that overlimiting phenomena occur upon setting conditions corresponding to complete desalination and, thus, do not positively contribute to desalination. Notably, the partial loss of membrane ion selectivity was associated with an operation in the overlimiting regime, worsening desalination efficiency. Our results imply that the overlimiting regime is, at least with the tested membranes, unsuitable for enhancing ionic transfer. Conversely, the small voltage required to remove ions in the underlimiting regime was attributed to the effect of natural convection, which readily occurs in the model system.