Batch dissolution of granite and biotite in water: Implication for fluorine geochemistry in groundwater

We performed batch dissolution experiments on different sized granite and biotite in pure water, in order to better understand the source and geochemical behavior of fluoride in groundwater. Very high concentrations of fluoride (up to 6-10 mg/l) were observed from granite leaching. Correlations between leached ions suggest that fluoride possibly originates from dissolution of biotite. After ∼500 hrs, fluoride concentration gradually decreased because of the supersaturation with respect to fluorite as a result of the build-up of sufficient Ca ions by dissolution of Ca-bearing plagioclase. Biotite dissolution experiments also showed that the fluoride concentrations are controlled by the attainment of fluorite saturation, which depends on the removal of Ca ion by adsorption and/or cation exchange on the surface of clay minerals. In summary, this study indicates that occurrence and behavior of fluorine in groundwater are mainly controlled by fluorite precipitation as a function of Ca concentration which depends on several geochemical processes such as dissolution of Ca-bearing minerals, calcite precipitation, and adsorption and/or cation exchange.