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

The main objective of the study is to explore the removal characteristics of Cu²⁺ and Zn²⁺ ions in activated carbon-based capacitive deionization (CDI). In this work, CDI experiments were performed to remove divalent ions (e.g., Cu²⁺, Zn²⁺, and Ca²⁺) 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 Cu²⁺ ions over Zn²⁺ and Ca²⁺ ions was observed in the charging step. The removal capacities for Cu²⁺, Zn²⁺, and Ca²⁺ 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 Cu²⁺ and Zn²⁺ were much lower than that of Ca²⁺, suggesting the occurrence of irreversible Faradaic reactions on the cathode. X-ray photoelectron spectroscopy analysis demonstrated that Cu²⁺ ions were reduced to Cu(I) and Zn²⁺ ions were transformed to ZnO/Zn(OH)₂ 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.