Optimization of desorption potential for stable and efficient operation of membrane capacitive deionization systems

The optimal desorption potential (DP) for maximizing the salt adsorption capacity of a carbon electrode while maintaining the stable operation of a membrane capacitive deionization (MCDI) cell was studied. Cyclic voltammetry (CV) measurements were performed for the MCDI unit cell. Additionally, the adsorption and desorption characteristics of the MCDI cell were analyzed while changing the DP. The potential at which the charges of the carbon electrode become the minimum was defined as the potential of minimum charge (PMC). The PMC value was measured as -0.22 V from the cyclic voltammogram. In addition, as the cell potential increased, the adsorption capacity of the carbon electrode increased. However, the effect of the DP on the desorption rate was negligible. In DPs below the PMC value, all adsorbed ions were desorbed, but the effluent pH rapidly decreased due to the occurrence of a water splitting reaction at the interface between the carbon electrode and the ion exchange membrane. When the DP was set as greater than the PMC value, some ions remained on the carbon electrode, resulting in a decrease in the amount of adsorbed ions during the subsequent adsorption process. When the DP was equivalent to the PMC value, the adsorption capacity of the carbon electrode was maximally utilized, and the pH of the effluent was maintained in a stable state.