Exploring the electrosorption selectivity of nitrate over chloride in capacitive deionization (CDI) and membrane capacitive deionization (MCDI)

The electrosorption selectivity of nitrate (NO3−) over chloride (Cl−) was investigated in capacitive deionization (CDI) and membrane capacitive deionization (MCDI). In CDI, the selectivity depended on several key operational factors, including the charging time, initial NO3− and Cl− concentration ratio (CNO3−/CCl−) and applied voltage. The NO3− selectivity increased with prolonged charging time and in proportion to the initial CNO3−/CCl−, suggesting that the differences in ion-carbon affinity results in the preferential electrosorption of NO3−. Increasing the applied voltage decreased the NO3− selectivity, revealing that the electrical force kinetically controlled the competitive electrosorption of anions. Therefore, our results indicate that the electrosorption selectivity for NO3− ions in CDI was determined by the contributions of ion-carbon affinity and electrical force during the charging period. In comparison, the NO3− selectivity was found to be significantly reduced in MCDI due to the presence of an ion-exchange membrane controlling ion kinetics on the basis of charge rather than affinity. The electrosorption selectivity of NO3− over Cl− in CDI was 2.44, which was approximately 1.9-fold higher than that in MCDI (1.28). These results provide a practical understanding of the NO3− selectivity in the studied electrosorption processes.