Fluoride removal using membrane capacitive deionization: The role of pH-dependent dissolved inorganic carbon

Defluorination technology is crucial for ensuring safe and accessible water. The application of capacitive deionization (CDI) technology faces challenges due to competitive adsorption of fluoride ions within complex natural fluoride-rich brackish water matrices, which often contain high levels of dissolved inorganic carbon (DIC) species (mainly HCO₃^– and CO₃^2–). These DIC species are pH-dependent, playing a significant role in the selective removal of fluoride by the CDI process. Thus, there is a knowledge gap in understanding the effects of membranes in membrane capacitive deionization (MCDI) on fluoride removal. In this study, we examined the key operating parameters in CDI and MCDI, including applied constant voltages and different types of anion-exchange membranes (AEMs), on the desalination performance in F^- and dissolved inorganic carbon water matrices. The application of AEMs significantly improved the salt adsorption capacity (SAC) for both F^- and DIC species, and reduced energy consumption. However, it simultaneously resulted in a notable decrease in F^- selectivity as membranes control mass transfer. Higher applied voltages enhance the SAC performance for F^- and DIC species, but also induce more severe Faradaic reactions, leading to increased energy consumption and lower energy efficiency. Additionally, ion species and pH changes during CDI and MCDI processes are interrelated, indicating that stability tests of CDI electrodes in batch mode are not reliable when using the same testing solution repeatedly. The diverse valence states of ions in the solution impact pH variations under different voltages in the CDI/MCDI process. These findings provide valuable insights into the development of water purification and desalination technology, particularly for the application and further advancement of selective fluoride removal by the CDI process.