Capacitive deionization for water treatment: Screening of key performance parameters and comparison of performance for different ions

• Capacitive deionization is a promising low-energy desalination technology.
• Factorial design was applied to screen the key operational parameters.
• High ion removal and low water recovery and energy usage can be achieved.
• Low charge efficiencies were caused by concentration polarization.
• Effects were observed for ionic charge, but not for ionic mass and hydrated radius.

In capacitive deionization (CDI) ions are removed from a salt solution by temporarily storing them on carbon electrodes by an electro-sorption process at low voltage. In this study, the key performance parameters of a bench-scale CDI system (with ion-exchange membranes positioned in front of the electrodes) were studied in a structured and statistically founded manner by applying factorial experiment design. Results showed that it is possible to operate CDI at high ion removal efficiency, high water recovery and/or low energy usage, depending on the settings. This tunability results in a broad potential application field for CDI technology. In addition, many significant main and interaction effects were elucidated. Among the most remarkable, were the respective negative and positive effects of cell voltage and flow rate on charge efficiency, which point at the occurrence of concentration polarization phenomena. Because the charge efficiency immediately determines the energy usage, it should be considered a key parameter for economical and sustainable CDI operation. CDI experiments performed with different salt solutions showed higher charge capacities for divalent compared to monovalent ions. In contrast to some literature reports, no clear effects of ion hydrated radius and mass were observed.