Electrode reactions and adsorption/desorption performance related to the applied potential in a capacitive deionization process

Desalination experiments were performed by constructing a capacitive deionization (CDI) unit cell with a carbon electrode prepared from activated carbon powder (ACP). Through CDI experiments, the mechanism of adsorption, desorption and electrode reactions were investigated by measuring conductivity, effluent pH, and the current passed through the cell under different electrode potentials. The salt-removal efficiency increased with increasing potential at the range of 0.8–1.5 V. Additionally, the pH of the solution varied significantly with a change in potential. At potentials less than 1.0 V, the pH increased due to the reduction of dissolved oxygen and the pH decreased at potentials over 1.2 V due to oxidation reactions at the anode. The change in current revealed that adsorbed ions were not completely desorbed and a fraction of ions were retained at the carbon electrode. These accumulated ions were re-adsorbed at the electrode surface when a potential was re-applied, which led to a decrease in the salt-removal efficiency of CDI.