Reactions between boron and clays

Abstract

Sorption of boron by Kent sand kaolinite, WilIalooka illite, and Marchagee montmorillonite in 0.01M CaCl2 is consistent with a mechanism described by the Langmuir equation, except that deviations occur at solution concentrations greater than about 10 µg B/ml. The quantity of boron adsorbed increases markedly with pH, and is rapid, reversible, and unaffected by the concentration of calcium chloride present. Complete desorption of surface boron, without significant solution of lattice boron, is achieved with 0.01M mannitol solution. Comparing the three clays at pH 7.5 on a surface area basis, the illite is most reactive (1.18 µg B/m2), followed by the kaolinite (0.28 µg B/m2) and the montmorillonite (0.02 µg B/m2). For illite, which is the most reactive of the three minerals, the magnitude of the above values emphasizes the small proportion (less than 1.4%) of the total surface area occupied by boron. Consideration of the reaction between boron and clays suggests that either boric acid molecules or borate ions could be the adsorbed species. Increasing sorption with pH could be explained either by the increasing proportion of borate ions to boric acid with pH or by a greater number of sites on the clay surface at higher pH values.