Oxyanion Sorption on Soil Surfaces

Abstract

The non-metal elements in the groups IIIA to VIA form oxyanions in water solutions. Some of the inorganic oxyanions commonly found in the aqueous phase in the soil solution are: carbonate, nitrate, silicate, phosphate, selenate and molybdate. The negative charge on the oxyanion is dependent on the pH and the dissociation constant of any specific group. Phosphate, for example, has three OH groups and is present in the solution in all these forms at different proportions as H₃PO₄, H₂PO ⁻₄ , HPO ²⁻₄ and PO ³⁻₄ in the pH range of 0–14. The proportion of each anion form in solution at any specific pH could be calculated from the dissociation constants of each anion and the total amount of P present (Fig. 1). The soil pH can vary within short distances around active living organs like plant roots due to differential absorption of cations (Marschner et al. 1986) or due to respiration that affects the partial CO₂ pressure. Due to the sensitivity of the oxyanion charge to pH and the unpredictable variations in soil pH, abundant literature can be found which deals with the reactions of oxyanions in soils, clays and oxide surfaces.