Investigating the role of dissolved inorganic and organic carbon in fluoride removal by membrane capacitive deionization

Natural fluoride-containing waters are characterized by high contents of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC). For the first time, the impact of DIC and DOC on fluoride removal by membrane capacitive deionization (MCDI) was unravelled. A series of MCDI experiments were conducted by mixing NaF solutions with three DIC species (H2CO3, NaHCO3, and Na2CO3) and a DOC representative (humic acid). All three DIC species decrease fluoride removal, among which CO32− causes the greatest reduction. This is because the divalent CO32− is preferably adsorbed by MCDI over monovalent DIC ions. When the initial concentrations of F− and DIC ions are equal, F− is less adsorbed than DIC because the stronger hydration energy of F− makes its interaction with the electrode more difficult. DIC species also act as a buffering agent, reducing pH fluctuations during the adsorption and desorption cycle. On the other hand, DOC at a moderate concentration (10.9 mg/L) has an insignificant impact on fluoride removal. DIC significantly decreases both fluoride and DOC removal in ternary fluoride-rich water. This work highlights the importance of water characteristics in the selective removal of ions and demonstrates the potential applicability of MCDI to treat natural waters.