The effect of redox potential on the removal characteristic of divalent cations during activated carbon-based capacitive deionization.

The main objective of the study is to explore the removal characteristics of Cu and Zn ions in activated carbon-based capacitive deionization (CDI). In this work, CDI experiments were performed to remove divalent ions (e.g., Cu, Zn, and Ca) from single- and multicomponent aqueous solutions. As evidenced, divalent heavy metals could be successfully removed by charging the CDI cell at 1.2 V. Notably, the preferential removal of Cu ions over Zn and Ca ions was observed in the charging step. The removal capacities for Cu, Zn, and Ca ions in a competitive environment were 29.6, 19.6, and 13.8 μmol/g, respectively. In contrast, the regeneration efficiencies for the removal of Cu and Zn were much lower than that of Ca, suggesting the occurrence of irreversible Faradaic reactions on the cathode. X-ray photoelectron spectroscopy analysis demonstrated that Cu ions were reduced to Cu(I) and Zn ions were transformed to ZnO/Zn(OH) on the cathode. Therefore, there were two major mechanisms for the removal of divalent heavy metal ions: capacitive electrosorption and cathodic electrodeposition. Specifically, the reduction potential played a crucial role in determining the removal characteristics. When regarding divalent cations with similar hydrated sizes, the divalent cation with a higher reduction potential tended to be separated by cathodic electrodeposition rather than double-layer charging, indicating the high removal selectivity of activated carbon-based CDI. This paper constitutes a significant contribution to promoting the application of CDI for contaminant sequestration.