Effects of the hydration ratio on the electrosorption selectivity of ions during capacitive deionization

The electrosorption performances of different ions in a capacitive deionization process were studied. The hydration ratio, i.e., the ratio of hydrated radius to ion radius, significantly affects the electrosorption capacity and selectivity. In a mono-ionic solution, ions with low hydration ratios exhibit high electrosorption capacities. The electrosorption capacity demonstrates a negative linear relationship with the hydration ratio for both anions and cations. In a multi-ionic solution, the monovalent ions with low hydration ratios exhibited high electrosorption selectivity. Divalent ions adsorb more easily onto electrode surface than monovalent ions do, resulting in higher electrosorption selectivity. In the late period of the electrosorption, the adsorbed ions were partly substituted by ions with lower hydration ratios or a higher valence. The hydration ratio and valence affect the ions' electrostatic attraction to the electrodes, determining the electrosorption capacity and selectivity of ions in capacitive deionization. (C) 2016 Elsevier B.V. All rights reserved.