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Phosphate Transport in Calcium-Saturated Systems: I. Theory

Dep. of Soil Science, Univ. of Manitoba, Winnipeg, MB, Canada R3T 2N2

*Corresponding author.

ABSTRACT

A theory of diffusive transport of orthophosphate in a Ca²⁺-saturated cation-exchange resin medium with free CaCO₃ was formulated based on a model in which the cation of the applied phosphate salt replaced the exchangeable Ca²⁺ on the exchange complex, and this Ca²⁺ reacted with phosphate to precipitate CaHPO₄. The precipitation reaction produced H⁺ ions, lowering the pH and affecting the relative distribution of H₂PO^–₄, HPO^2–₄, HCO^–₃ and CO^2–₃ in solution. The local pH caused either the precipitation or dissolution of CaCO₃, thus affecting the concentration of Ca²⁺. Four transport equations, one each for the displacing cation, displaced Ca²⁺, phosphate, and H⁺, were necessary to describe the phosphate transport. The analyses indicated that the phosphate transport was retarded due to the precipitation of phosphate as CaHPO₄ in Ca-saturated cation-exchange media. The magnitude of CaHPO₄ precipitation was dependent on phosphate concentration, CEC, pH, and solid CaCO₃. In the presence of solid CaCO₃, the H⁺ produced reacted with CaCO₃ to supply Ca²⁺ to the solution, which in turn reacted with phosphate and produced H⁺. The buffering effect of the media, due to OH^–, HCO^–₃, CO^2–₃, the cation-exchange reaction, and solid CaCO₃, was found to be important in controlling the media pH, which was primarily responsible for the ionic composition of the system. The proposed theory assumed that the phosphate ion and the displaced or dissolved Ca²⁺, both of which were mobile, interacted with each other. Consequently, the interaction of phosphate was not restricted to the solid surface. In addition, the amount of locally immobilized phosphate was not restricted to an arbitrary value such as maximum adsorption capacity.

Contribution from the Dep. of Soil Science, Univ. of Manitoba.

Received for publication March 15, 1990.

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