Effect of the chemical bond on the electrosorption and desorption of anions during capacitive deionization.

This paper is concerned with the effect of the chemical bond on the electrosorption and desorption of anions during capacitive deionization (CDI). An empirical equation developed firstly based on the experimental data in the electrosorption of Cl-. The empirical equation shows that the electrosorption capacity exhibits a logarithmic relationship with anode potential. Electrosorption of ReO4- and NO3- at different anode potentials were studied and the obtained data used to compare with empirical equation. The empirical equation provided results that were in good agreement with experimental data. According to parameters (A and b) of empirical equation, the chemical bond mainly affected adsorption mechanism and desorption performance. For Cl- with the weaker chemical bond, the main mechanism of electrosorption of Cl- is formation of electrical double layers and the desorption is easier, while for ReO4-, the main adsorption mechanism is chemical bonding and adsorbed anion basically fail to desorb at the equal conditions with Cl-. The revelation of all the CDI performance here would appear to be a useful tool for selection of more suitable electrodes materials to improve adsorption capacity and desorption efficiency of anions.

Effect of the electronegativity on the electrosorption selectivity of anions during capacitive deionization.

The effect of electronegativity on the electrosorption selectivity of anions during capacitive deionization was investigated via a combination of experimental and theoretical studies. A model was developed based on chemical thermodynamics and the classic Stern's model to reveal the role of the anode potential and to describe electrosorption selectivity behavior during capacitive deionization. The effects of the anode potential on the adsorption of Cl- and ReO4- were studied and the obtained data were used to validate the model. Using the validated model, the effects of the anode potential and electronegativity of various anions, including Cl-, ReO4-, SO42- and NO3- were assessed. The experimental results for the electrosorption of Cl- and ReO4- corresponded well with the developed model. The electrosorption capacity demonstrates a logarithmic relationship with the anode potential. The model showed that the electronegativity significantly affects the selectivity. In a mixed Cl-, ReO4-, SO42- and NO3- solution, ReO4- was preferentially adsorbed over the other three anions, and the following selectivity was exhibited: ReO4- > NO3- > Cl- > SO42-. The results showed that the effect of flow rates on the electrosorption selectivity can be considered negligible when the flow rates are higher than 112 mL min-1. The anions selectivity can be further enhanced by increasing the anode potential, and electrosorption selectivity is no appreciable decline after 6 experiments.