Individual and competitive removal of heavy metals using capacitive deionization

This study presents the viability and preference of capacitive deionization (CDI) for removing different heavy metal ions in various conditions. The removal performance and mechanisms of three ions, cadmium (Cd²⁺), lead (Pb²⁺) and chromium (Cr³⁺) were investigated individually and as a mixture under different applied voltages and ion concentrations. It was found that CDI could effectively remove these metals, and the performance was positively correlated with the applied voltage. When 1.2V was applied into solution containing 0.5mM individual ions, the Cd²⁺, Pb²⁺, and Cr³⁺ removal was 32%, 43%, and 52%, respectively, and the electrosorption played a bigger role in Cd²⁺ removal than for the other two ions. Interestingly, while the removal of Pb²⁺ and Cr³⁺ remained at a similar level of 46% in the mixture of three ions, the Cd²⁺ removal significantly decreased to 14%. Similar patterns were observed when 0.05mM was used to simulate natural contaminated water condition, but the removal efficiencies were much higher, with the removal of Pb²⁺, Cr³⁺, and Cd²⁺ increased to 81%, 78%, and 42%, respectively. The low valence charge and lack of physical sorption of Cd²⁺ were believed to be the reason for the removal behavior, and advanced microscopic analysis showed clear deposits of metal ions on the cathode surface after operation.