A Chemical Model of Phosphate Adsorption by Soils: I. Reference Oxide Minerals: II. Noncalcareous Soils

Abstract

The Constant Capacitance model is shown to provide a quantitative description of o-phosphate adsorption by a variety of aluminum and iron hydrous oxides, including the effect of varying pH values. This model, based on a ligand exchange mechanism for o-phosphate adsorption, comprises six adjustable parameters: two surface protonation-dissociation constants, three o-phosphate surface complexation constants, and a capacitance density parameter. The five surface equilibrium constants can be obtained from potentio-metric titration data and o-phosphate adsorption data. These constants are independent of pH but, in principle, can depend on the composition of the background electrolyte solution. The capacitance density parameter cannot be obtained directly from experiment. A working value can be chosen on the basis of previous applications of the Constant Capacitance model and other model parameters are not sensitive to this choice. Additional Index Words: anion adsorption, ligand exchange, surface chemistry, o-phosphate. Goldberg, S., and G. Sposito. 1984. A chemical model of phosphate adsorption by soils: I. Reference oxide minerals. Soil Sci. Soc. Am. J. 48:772-778. T HE REACTIONS between o-phosphates and soils often have been described mathematically with adsorption isotherm equations (Olsen and Watanabe,