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

• Published in: Desalination Vol. 497; p. 114764
• Main Authors: Tsai, Shao-Wei, Hackl, Lukas, Kumar, Arkadeep, Hou, Chia-Hung
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.01.2021
• Subjects: Competitive electrosorption; Ion selectivity; Ion-exchange membrane; Membrane capacitive deionization; Nitrate; Ion-exchange membrane; Nitrate; Competitive electrosorption; Membrane capacitive deionization; Ion selectivity
• ISSN: 0011-9164; 1873-4464
• DOI: 10.1016/j.desal.2020.114764

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. [Display omitted] •Preferential electrosorption of NO3− over Cl− is discovered in both CDI and MCDI.•The ion-carbon affinity effect determines NO3− selectivity in CDI.•The ionic charge promotion effect governs NO3− selectivity in MCDI.•A high initial concentration ratio of NO3− promotes preferential electrosorption.•Increasing the applied voltage reduces NO3− selectivity.