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Calcium Phosphate Precipitation in Aqueous Calcitic Limestone Suspensions¹

ABSTRACT

Calcium phosphate precipitation was studied by measuring the decline in phosphate concentration resulting from the reaction between dilute orthophosphate and aqueous calcitic limestone suspensions. The rate of phosphate disappearance from solution increased with an elevation of pH between 7.0 and 8.4, and increased strongly in the presence of hydroxyapatite seed crystals. The presence of from 0.1 to 0.2 mM sodium fluoride increased the phosphate disappearance rate strongly, but only when seed crystals were present. Participation of fluoride and hydroxyapatite in the reactions suggests that hydroxyapatite or fluorapatite may have precipitated in some of the experiments, although the phosphate solubility after a prolonged reaction period was intermediate between that of octacalcium phosphate and hydroxyapatite.

Key Words: hydroxyapatite • fluorapatite • phosphate solubility • octacalcium phosphate • seed crystals

¹ Contribution of Institute for Man and Environment, State Univ. of New York, Plattsburgh, NY 12901.

² Assistant Professor of Environmental Science, Institute for Man and Environment, State Univ. of New York, Plattsburgh, NY 12901.

Received for publication September 13, 1979.