Fluoride in the context of the environment

Abstract

An estimated 70 million people around the world suffer from fluorosis due to chronic exposure to high levels of fluoride in drinking water. The sources of this element in water are mostly geogenic, although important contributions also come from industrial activities and coal burning. Fluorine is the 13th more abundant element in the earth?s crust, as it is contained in several rock forming minerals. Among these, micas, apatites and fluorite are the most common minerals responsible for the release of elevated concentrations of fluoride in natural waters. Fluoride is also commonly associated with volcanic activity, which on a global scale may release important amounts of gaseous fluoride compounds to the atmosphere and produce large deposits of F-rich lavas and volcanic ashes. The mobility of fluorine in aqueous reservoirs depends on the interplay of a number of geochemical processes that determine its removal or release into the solution. The main processes that affect the dynamics of fluoride in natural environments are the dissolution and precipitation of F-bearing minerals and the adsorption/desorption from metal (hydr)oxides and clay minerals. Most of the world?s high-fluoride districts coincide with areas affected by volcanic activity, regions underlain by crystalline igneous and metamorphic rocks, and large sedimentary basins in arid and semiarid conditions. Critical zones include the Pacific volcanic belt, cratonic areas in central Africa, Asia and North and South America, the East African Rift valley, the large sedimentary basins in southern South America, China and the arid region on the border between USA and Mexico.