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Kinetic Model for Phosphate Transport and Transformation in Calcareous Soils: I. Kinetics of Transformation¹

ABSTRACT

Phosphorous in concentrations exceeding those generally observed through agricultural activities has been encountered at land application wastewater treatment sites. To predict the environmental impact of these systems, a model was developed to describe the reaction of orthophosphate with mineral soils. The model is based on instantaneous reversible absorption theory plus time-dependent solubility theory. It was assumed, in the model, that first-order kinetics would adequately describe the formation or dissolution of phosphate minerals. Statistical regression analyses were performed on batch sorption data for 14 mineral soils where two equilibrium compounds were considered. The two compounds considered were hydroxyapatite (Ca₁₀HPO₄·2H₂O) and dicalcium phosphate dihydrate (CaHPO₄·2H₂O). Comparisons of correlation coefficients between this model and previous models using a common data set indicate the current model with a mean correlation coefficient of 0.93 is more accurate in describing phosphate reactions in calcareous soils than earlier models with mean correlation coefficients ranging from 0.75 to 0.91.

¹ Contribution from U.S. Environmental Protection Agency, Office of Research and Development, Robert S. Kerr Environmental Research Laboratory, Ada, Okla. Presented at ASA Annual Meeting, 17 Nov. 1977, Los Angeles, Calif.

² Soil Scientist, EPA, Ada, Okla.; Member of Tec. Staff, Computer Science Corp., Huntsville, Ala.; Research Agronomist, EPA, Ada, Okla.; and Professor, Kansas State Univ., Manhattan, respectively.

Received for publication August 24, 1979. Accepted for publication July 1, 1981.