Continuous cycling of an asymmetric capacitive deionization system: An evaluation of the electrode performance and stability

•Asymmetric CDI system tested with different operating parameters and salt solutions.•Two different mechanisms for ion adsorption appeared to occur.•CDI system performance was shown to be anion dependent.•Sulfate interactions were shown to decrease ion desorption from the electrodes.•Anode oxidation led to electrosorption capacity loses and solution acidification.

Capacitive deionization (CDI) shows great promise as an energy efficient water treatment technology. However, much of the research has focused on brackish water desalination and/or NaCl removal while ignoring other ions that may pose challenges for CDI and should be addressed. In this work, we studied the effects of modifying operational parameters in a CDI system operating in a continuous adsorption/desorption cycle using CaSO4, as well as other electrolytes. Low-cost, high surface area carbons coated with two different metal oxides (SiO2 on the cathode and Al2O3 on the anode) were employed as electrodes in the CDI reactor. Two ion adsorption mechanisms, electrosorption in the electrochemical double layer (EDL) and specific adsorption on the oxide surfaces and carbon support, appeared to play a role in the ion removal process. During regeneration of the electrodes, poor CaSO4 desorption likely associated with specific adsorption on the metal oxide coatings, along with diffusional issues, led to salt accumulation on the electrodes. Electrode regeneration values above 95% were obtained when chloride-containing electrolytes were employed, indicating that SO42− interactions were the main cause of the poor desorption. The mode of regeneration also strongly influenced ion desorption kinetics and charge efficiencies during subsequent removal cycles. Long-term experiments revealed that oxidation of the anode affected the electrosorption of ions and induced acidification of the solution.