A study of electrosorption selectivity of anions by activated carbon electrodes in capacitive deionization

• The electrosorption capacity: trivalent anion > divalent anion > monovalent anion.
• The electrosorption capacity was dependent on the ionic charge and hydrated radius.
• Electrosorption selectivity was studied in a competitive mixed solution.
• Cl− and NO3− show obvious competitive electrosorption in mixed solution.

The capacitive deionization (CDI) performance of activated carbon electrodes with various anions and competitive electrosorption between two anions in mixed solution were studied. It was found that the electrosorption capacity was strongly dependent on the ionic charge and hydrated radius. With the same concentration, smaller monovalent anions showed their size-affinity to access the pores. The order of normalized equivalent capacity was as follows: trivalent anion > divalent anion > monovalent anion. In mixed Cl− and NO3− solution, Cl− is preferably electrosorped over NO3− in early period of the electrosorption process. However, during the end of the electrosorption process, part of Cl− would be exchanged by NO3− due to the stronger competitiveness of NO3− than Cl−. There was no obvious selectivity between Cl− and SO42 −.