Adsorption of Fluoride by Soils and Minerals

The adsorption of F by soils and common hydroxyl-containing minerals found in soils was studied over a solution concentration range of 2 to 16 mg./l. It was found that gibbsite, kaolinite, halloysite, precipitated Al(OH)/sub 3/, that all soils studied adsorb significant amounts of F with release of OH/sup -/, and that the adsorption is concentration-dependent and described by the Langmuir adsorption equation. At an equilibrium F concentration of 16 mg./l. the adsorption in mg./kg. was as follows: alkaline soils, 59 to 120; acid Aiken soil, 1060; gibbsite and kaolinites, 190 to 295, dehydrated halloysite, 1400; hydrated (expanded) halloysite, 1777; and precipitated Al(OH)/sub 3/, 32,600. Goethite, benonite (montmorillonite) and vermiculite adsorbed only traces of F. The similar adsorption by dehydrated and expanded halloysite, together with the very high F adsorption capacity of precipitated Al(OH)/sub 3/, suggest that F adsorption at low concentrations occurs primarily by exchange with OH of Al(OH)/sub 3/ rather than with the crystal lattice OH of clay minerals. If a soil having high permeability and high F adsorption capacity were available, it might be practical to remove excess F from water for municipal use by adsorption on soil in a system such as that used for ground-water recharge.more » 10 references, 1 table.« less