Surface complexation modeling of fluoride sorption onto calcite

Calcite and various calcium salts have been used to remove fluoride from drinking water and waste water in many countries, and calcareous soils retard the mobility of fluoride in soil. Adsorption of fluoride onto calcite could be important at low fluoride concentration. In this study, a series of batch experiments with calcite equilibrated solutions was conducted to investigate the mechanisms of fluoride uptake by calcite. The range of initial fluoride concentration was from 2.5 to 10 mg/l. Effects of pH, ionic strength, and solid concentration were evaluated. Fluoride uptake was found to be strongly pH dependent whereas ionic strength also affected fluoride removal. A constant capacitance model was used to model fluoride sorption onto calcite, where fluoride adsorption was described by the formation of two surface complexes, >CaF0 and >CO3FCa0. The formation of >CaF0 complex was evidenced from thermodynamic approach, whereas inclusion of >CO3FCa0 was necessary to explain fluoride sorption to calcite surface. Fluoride sorption was modeled with and without precipitation. The results showed that, at lower fluoride concentration, adsorption was the governing mechanism for fluoride removal by calcite whereas a combined mechanism of adsorption and fluorite precipitation was necessary to describe fluoride interaction with calcite at low pH and higher fluoride concentration. Thus, adsorption of fluoride to calcite can influence the mobility of fluoride in soil under suitable conditions where calcite is abundant. Also, surface adsorption should also be considered in addition to precipitation in defluoridation methods, which use calcite or limestone as one of the defluoridating agents.