Competitive Electrosorption of Fluoride and Chloride in Capacitive Deionization: The Role of Membranes

Fluoride removal from solutions containing mixed monovalent ions, particularly F– and Cl–, is challenging due to fluoride’s high electronegativity, large hydration radius, and high hydration energy. This study investigates the performance of capacitive deionization (CDI) and membrane capacitive deionization (MCDI), focusing on initial concentrations, applied constant voltages, and different types of membranes. Membrane integration enhanced adsorption capacity and removal rate and minimized Faradaic reactions. However, these membranes were unable to enhance fluoride selectivity and achieve selective fluoride removal. Increasing initial concentrations was not a viable approach to enhance adsorption capacity since higher concentrations resulted in decreased removal rates. Although raising applied constant voltages improved both adsorption capacities and fluoride selectivity, detrimental Faradaic reactions occurred during the adsorption and desorption processes, decreasing energy efficiency. pH changes in CDI cells increased during the adsorption process and decreased during the desorption process at high applied voltages, resulting in a decrease in anion adsorption capacity and current efficiency. The pH change trend in MCDI was opposite, causing the increase of anion adsorption capacity. These findings offer valuable insights for improving membrane technologies in wastewater treatment and selective fluoride removal.