Adsorption of monomeric silica by volcanic ash soils

Abstract

The adsorption of monomeric silica at concentrations not exceeding 100 ppm SiO₂ was studied with volcanic ash soils containing allophane-imogolite with a SiO₂A1₂O₃ ratio of about 1, with or without humus and opaline silica; allophane with a SiO₂/Al₂O₃ ratio of about 2; and halloysite, respectively. The adsorption of silica increased with increasing pH from 6 to 10. The adsorption equilibrium was attained within a day at the higher pH, but not even after a week at the lower pH. All the adsorption isotherms conform to Freundlich's adsorption equation at and above certain Si0₂ concentrations, and suggest that the pH effect is primarily related to the total number of available adsorption sites. The zero points of silica adsorption (ZPSA) were found for all the examined soils at SiO₂ concentrations in the range from 2.5 to 26 ppm, which were lower than those at which the isotherms deviated from Freundlich's equation. The ZPSA values show dependence on the pH and soil, suggesting that the soluble silica equilibria in the soils are governed largely by the precipitation-dissolution equilibria of silicate and silica minerals in the clays. Approximate silica adsorption capacities of allophane-imogolite and halloysite at pH 8 to 9 may amount to 10 to 25 g and 2.5 g of SiO₂ per 100 g of clay, respectively. The capacities are reduced either by lowering the ambient pH or by the presence of humus. A fair stability of the adsorbed silica was indicated by repeating washings with water.