CHEMICAL EQUILIBRIA OF FLUORINE IN SOILS: A THEORETICAL DEVELOPMENT

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Abstract

We selected thermodynamic data for 45 fluorine minerals and solution species that may play important roles in the fluorine chemistry of soils. Equilibrium reactions and stability constants for these minerals and species were calculated and used to develop stability diagrams in soils.

Aluminum fluoride is likely to precipitate in strongly acid soils of pH below 4.5. In these soils AlF3(c) in equilibrium with Al(OH)3 (gibbsite) can maintain F− below 10−4M in solution. Fluorite appears to be stable in slightly acid and near-neutral soils. When fluorite is in equilibrium with 10−2.5M Ca2+, it can support 10−3.96M F− in soil solution. Fluorophlogopite mineral is probably stable in alkaline soils. However, it is a complex mineral and its solubility is also affected by the activities of H4SiO40, Al3+, K+, Mg2+, and pH. Fluorapatite is unstable in acid soil, but it is the most stable F mineral in alkaline and calcareous soils. Fluorapatite supports extremely low F− activity (<10−6.5M) in soil solution when it is in equilibrium with hydroxyapatite.

Other metal fluorides, e.g., NaF, KF, CuF2, ZnF2, PbF2, CdF2, MgF2, and FeF3 are too soluble to persist in soils. Also, fluorosilicate, fluoroaluminate, and fluoroaluminosilicate minerals, such as Na2SiF6 (malladrite), (NH4)2SiF6 (bararite), K2SiF6 (hieratite), Na3AlF6 (cryolite), and NaCa2Mg4Al3Si6O22F2 (fluorpargasite), are thermodynamically unstable in soils.

Solution complexes of aluminum fluorides, e.g., AlF2+, AlF2+, AlF30, and AlF4−, and iron fluorides, e.g., FeF2+, FeF2_, and FeF30, are the major solution species in strongly acid soils below pH 6. Above this pH F− is the predominant species in soil solution. Silicon and lead fluoride complexes, along with KF0, NaF0, CuF+, ZnF20, and HF2− species, are not important in most cultivated soils.

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