On the chemistry and mineralogy of boron in pure and in mixed systems: A review

Boron (B) is the third member of the third periodic group. It is a nonmetal with an atomic number of five and a constant valence of 3+. Boric acid [B(OH)3]and H2Bo3 − are the most common geologic forms of B, with boric acid the predominant form of B in agricultural soils. Boron occurs mainly in silicate minerals. The 10 μg/g B concentration of the earth's crust is distributed among common rock types. Less than five percent of total soil B is available for crop uptake. This helps to explain the occurrence of B deficiency in 41 of the 50 states. The availability of soil B depends on soil texture, pH, and soil moisture content. A number of soil components have shown an affinity for B. These include Al‐ and Fe‐ hydroxy compounds, ferromagnesian minerals found in sand and silt separates, Al‐ and Fe‐ oxides, and organic matter. Studies have shown B adsorption to increase with increasing pH. Boric acid predominates at pH values less than 7.0 and borate ions predominate in solutions of higher pH. Boron adsorption data are usually described by: the Langmuir equation at low concentrations, a quadratic curve described by a BET relationship; and phenomenological equations which describe B adsorption as a function of both the equilibrium B concentration and the pH of the suspension solution.