Enhanced desalination efficiency in capacitive deionization with an ion-selective membrane

Abstract

In order to investigate ionic adsorption and desorption mechanisms on carbon electrode surfaces, capacitive deionization experiments were carried out with two types of capacitive deionization (CDI) cell configurations. A CDI cell equipped with carbon electrodes only and a membrane capacitive deionization (MCDI) cell having a cation-exchange membrane on the cathode surface were constructed, and desalination experiments were carried out at various operating conditions. The conductivity transients of effluents and currents passed through the cells were accurately measured by a potentiostat in order to analyze the transport mechanism at the carbon electrode surfaces. The salt removal efficiencies of the MCDI cell were enhanced by 32.8–55.9% compared to the CDI cell, depending on the operating conditions. Furthermore, the current efficiencies were improved for the MCDI cell: 83.9–91.3% versus 35.5–43.1% for the CDI cell. It was verified that ions were selectively transported between the electric double layer at the electrode surface and the bulk solution in the MCDI cell configuration when a potential was applied. On the contrary, in the case of the CDI cell, both anions and cations were transported during the sorption and desorption processes, which led to decreased salt removal and current efficiencies.