ABSTRACT
Fluoride (F−) is an important element because it has a wide variety of applications and uses. It is utilized in the production of phosphate fertilizers, insecticides, and hightemperature plastics. Fluoride occurs naturally in soils. Additionally, fluorine is regarded as the most common phytotoxic pollutant. The most phytotoxic fluorine gases include hydrogen fluoride (HF) and silicon tetrafluoride (SiF4) (National Academy of Science, 1974). The total F− content of natural soils ranges between 200 and 300 mg/kg (Worl et al., 1973). The most important naturally occurring F− minerals include fluorite (CaF2), fluorapatite [Ca5(PO4)3F], cryolite (Na3AlF6), and fluorophlogopite (KMg3AlSi3O10F2) (Elrashidi and Lindsay, 1986). In addition to these natural sources, F− is deposited in soil through a variety of anthropogenic activities; such as mining, aluminum smelting, fertilizer production, burning fossil fuels, and longterm application of phosphate
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fertilizers (Drury et al., 1980; Polomski et al., 1982; Adriano, 1986; Rutherford et al., 1994; Arnesen and Krogstad, 1998; Vedina and Kreidman, 1999). Once F− is released into soil water, it undergoes several geochemical processes including metal complexation, adsorption, and precipitation processes (Figure 13.1). These chemical processes in turn control the solubility, plant uptake, and mobility of F− in the soil subsurface ecosystem. Fluoride is beneficial in small doses to animals and humans; however, in excess it becomes toxic. Relative to the risk of F− contamination, our knowledge of F− mineral dissolution and precipitation processes in natural soils, particularly in semi-arid soils, is very limited.
