Abstract
The earth’s surface is made up of about 70% water; of this amount, 98% is saline. The global abundant saline water is viewed as a potential source of freshwater production, but the challenge is removing the high concentration of Total Dissolved Salts (TDS), a process called desalination. Capacitive deionization (CDI) is considered a cost-effective and efficient desalination technology and a reliable alternative to the already established method of reverse osmosis. However, research in CDI technology is faced with two major setbacks, electrode instability, and a relatively low (salt) ion adsorption capacity. Hence, present scientific research on CDI is focused on the development of electrode materials with a good high ion adsorption capacity and thermal stability. In this study, three-electrode materials were prepared and applied for desalination studies by the hybrid capacitive deionization (HCDI) method. The electrode materials prepared were carbon aerogel Polypyrrole (CA/PPy), Polyaniline-Polypyrrole@Manganese dioxide - Zirconium dioxide (MnO2-ZrO2@PANI-PPy), and graphitic carbon nitride@Nickel oxide - Zirconium dioxide (NiO-ZrO2@g-C3N4) composites.
In the first experimental study, a CA/PPy composite material was used to modify activated carbon electrodes and applied for the desalination of NaCl solution by the HCDI method.
In the second experimental study, a MnO2-ZrO2@PANI-PPy electrode material was prepared and used to modify activated carbon electrodes. It was then applied for desalination studies by the HCDI method as in the first experimental study. In the third experimental study, a NiO-ZrO2@g-C3N4 electrode material was prepared and modified on an activated carbon electrode and applied for desalination studies by the CDI method as in the first two experimental studies.
This study showed that the MnO2-ZrO2@PANI-PPy and NiO-ZrO2@g-C3N4 composites had the best ion adsorption capacity. The optimal ion adsorption capacities recorded are 34.16 mg/g (MnO2-ZrO2@PANI-PPy), 21.51 mg/g (NiO-ZrO2@g-C3N4), and 15.7 mg/g (CA-PPy). The salt removal efficiency values of the electrode materials show that they are most efficient at lower concentrations of salt solutions. The optimal salt removal efficiency values recorded for each electrode material during the desalination studies using 300 mg/L solutions were 27.0%, 42.7%, and 48.5% for the CA-PPy, NiO-ZrO2@g-C3N4, and MnO2-ZrO2@PANI-PPy respectively. These materials also displayed excellent electrochemical stability after several cycles of ion adsorption/desorption studies. The BET and electrochemical tests show that the electrode materials have a decent porous network, large surface area, and high specific capacitance values which are ideal for ion adsorption in HCDI cell operation.