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Calcium Phosphate Precipitation: Identification of Kinetic Parameters in Aqueous Limestone Suspensions¹

ABSTRACT

Phosphate disappearance from aqueous calcitic limestone suspensions was described using an equation for interface-, controlled crystal growth following addition of hydroxyapatite seed crystals. Phosphate removal from solution exhibited second order dependence on the phosphate concentration. Hydroxyapatite seed crystals were more important than limestone in governing phosphate removal. Limestone functioned primarily to buffer the activities of (Ca²⁺) and (OH^-). At pH 8.5, an increase of 0.1-millimolar (mM) NaF caused a fivefold increase in the rate constant. Twenty successive reactions of limestone with phosphate caused a fourfold increase in the phosphate removal rate. A portion of freshly precipitated calcium phosphate probably served as seed for development of new crystals. Participation of both (F^-) and hydroxyapatite crystals suggests a mixture of fluorapatite and hydroxyapatite, or an apatite precursor formed in the experiments.

¹ Contribution from the Institute for Man & Environment, State University of New York, Plattsburgh, NY 12901.

² Assistant Professor of Environmental Science, Inst. for Man & Environ., State Univ. of New York, Plattsburgh, NY 12901.

Received for publication February 29, 1980. Accepted for publication December 22, 1980.