Experimental fluorine liberation from Precambrian granites and Carboniferous-Permian sedimentary rocks associated with crystalline and sedimentary aquifers, Paraná Basin, southeastern Brazil

High natural fluoride (F–) concentrations (up to 8.75 mg dm–3) were detected in the groundwater near Salto city (São Paulo State, Brazil), on the eastern border of the Paraná Sedimentary Basin, with an average of 1.19 mg dm–3 in a fissured aquifer and 2.01 mg dm–3 in the sedimentary aquifer systems. The aim of the study was to evaluate granitic and sedimentary rocks (using whole rock powders at 150 m). In Experiment II, F– was progressively released from all rocks. Granites released more F– (up to 8.69 mg dm–3), which was controlled by the supersaturation of fluorite, due to Ca2+ from plagioclase dissolution, as well as biotite dissolution (2–5% by vol. of rocks). A good F/K value (r = 0.87) correlation implies that the F– was released from biotite and K+ from biotite+K-feldspar. In sedimentary rocks, the F– concentrations (0.12 to 0.91 mg dm–3) are related to the low fluorine rock concentrations, and Ca2+ removal was controlled by calcite precipitation and/or cationic exchange of Ca2+ for Na+ in the clay minerals. With regard to biotite alteration, the influence of higher temperatures proved to be noteworthy in the process of releasing F–, which also was more efficient under high salinity conditions (800 μS cm–1, NaCl 0.1%) when compared with alkaline conditions (pH 8.0, NaOH 0.1%).