Effect of Pyrophosphate on Orthophosphate Reactions in Calcareous Soils

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Abstract
Sodium pyrophosphate (NaPP) is known as a crystal growth inhibitor. Its effect on the kinetics of phosphorus (P) retention by calcium carbonate (CaCO3) and by two calcareous soils was investigated using orthophosphate (OP) solutions ranging in P concentration between 0.1 and 1 × 10−3M. Reaction of the most dilute solution with CaCO3 involved initial rapid adsorption of P followed by a gradual change in solution composition toward equilibration with hydroxyapatite (HAP). For the more concentrated solutions, the P concentration vs. time curves exhibited an initial, rapid adsorption followed by an induction period that preceded a second drop which was proportional to the initial concentration, and finally a gradual decrease toward a solubility equilibrium value. Amounts of P removed during the initial drop were close to the calculated Langmuir adsorption maximum. The second drop in P concentration was presumed to reflect precipitation involving growth of calcium phosphate nuclei adsorbed on the CaCO3 surface. Behavior of a desert soil (39% CaCO3) was quite similar to that of CaCO3, while the precipitation reaction was considerably less in an alluvial soil (4.1% CaCO3).Addition of NaPP at a concentration of at least 2 × 10−5M P increased both the induction period and the final P concentration. Effectiveness of NaPP depended on its addition before the end of the induction period as well as on the pyrophosphate‐to‐orthophosphate ratio. Results obtained with triammonium pyrophosphate on the two soils were similar to those with NaPP.
Funding Information
  • International Development Research Center

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