Capacitive deionization technology based on the principle of electrical double layer is one of desalination technologies which remove ions in water. Capacitive deionization(CDI) is an electrochemically controlled process for removing ionic salts from ...
Capacitive deionization technology based on the principle of electrical double layer is one of desalination technologies which remove ions in water. Capacitive deionization(CDI) is an electrochemically controlled process for removing ionic salts from aqueous solutions by alternatively adsorbing and desorbing excess ions in the electrical double layer region at an electrode-solution interface when a pair of electrodes are electrically charged by some external power supply. When the electrodes are charged, all counter ions in aqueous solutions are adsorbed on electrode surface nonselectively. Because removing all ions including non-targeting ions consume much energy, it is needed to remove specific target ions selectively to minimize consuming energy in the desalination process. In this study, we fabricated selective composite carbon electrodes using hardness-selective ion-exchangeable materials and performed desalination experiments by means of the CDI cells with selective composite carbon electrodes to selectively remove hardness materials. CMP28 resin was introduced as hardness selective material which has excellent hardness ion selectivity. Adsorption equilibrium experiment was performed to evaluate permselectivity of hardness ions, i.e., Ca2+ and Mg2+, for the CMP28 resin. The CMP28 resin showed much higher selectivity of Ca2+ and Mg2+ than Na+ and had more higher selectivity of Ca2+ than Mg2+. The adsorption equilibrium of the CMP28 resin represents that the selectivity increases with the following order: Na+ ≪ Mg2+ < Ca2+ due to the number of charge, hydrated radius, ionic concentration and etc. Thus, we fabricated selective composite carbon electrodes by coating the CMP28 resin powder on the original carbon electrodes and made up a selective capacitive deionization(SCDI) cell using the selective composite carbon electrode only in cathode. To investigate the improvement of hardness-selective removal in the SCDI cell, we performed the desalination experiments in two solutions (SC solution : Na+ + Ca2+, SM solution : Na+ + Mg2+). In the SC and SM solutions, selective removal of hardness materials was increased by using the selective composite carbon electrode in the SCDI cell up to 10% and 7% higher than the original CDI cell, respectively. To investigate the change of hardness-selective removal performance with various applied current densities to the SCDI cell, we carried out desalination experiments by applying various current densities (5, 7.5, 10, 12.5, 15 A/m2) in the SCM solution containing three ions (Na+ + Ca2+ + Mg2+). When the lowest current density (5 A/m2) is applied to the SCDI cell, most of ions were removed and the selective removal of hardness materials were increased because of the longest residence time for reaching the equilibrium state between the selective layer on the carbon electrode and the solution. It is concluded that we successfully developed the SCDI technology using the proprietary composite selective carbon electrode to increase selective removal of hardness materials.