Phosphate Transport in Calcium‐Saturated Systems: I. Theory

A theory of diffusive transport of orthophosphate in a Ca 2+ ‐saturated cation‐exchange resin medium with free CaCO 3 was formulated based on a model in which the cation of the applied phosphate salt replaced the exchangeable Ca 2+ on the exchange complex, and this Ca 2+ reacted with phosphate to precipitate CaHPO 4 . The precipitation reaction produced H + ions, lowering the pH and affecting the relative distribution of H 2 PO − 4 , HPO 2− 4 , HCO − 3 and CO 2− 3 in solution. The local pH caused either the precipitation or dissolution of CaCO 3 , thus affecting the concentration of Ca 2+ . Four transport equations, one each for the displacing cation, displaced Ca 2+ , 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 CaHPO 4 in Ca‐saturated cation‐exchange media. The magnitude of CaHPO 4 precipitation was dependent on phosphate concentration, CEC, pH, and solid CaCO 3 . In the presence of solid CaCO 3 , the H + produced reacted with CaCO 3 to supply Ca 2+ 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 CaCO 3 , 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 Ca 2+ , 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.