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Calcium Phosphate Precipitation: Effects of Common and Foreign Ions on Hydroxyapatite Crystal Growth¹

ABSTRACT

Calcium phosphate precipitation from slightly supersaturated solutions was induced by addition of hydroxyapatite (HAP) seed crystals. The rate of seed crystal growth was measured during the first 60 min by disappearance of P from solution. Effects of solution Ca²⁺, F^-, initial P, Mg²⁺, HCO₃^-, and HAP seed concentrations on crystal growth were measured by evaluating the empirical reaction order (n) and the rate constant (K) for P-removal by fitting the data to the equation for interface-controlled crystal growth. Phosphate disappearance had second-order dependence on the P concentration when initial F was > 0.05 mM. An increase of either 1 mM Ca or 0.1 mM F caused an increased-P removal rate of about 10 times. Both Mg and HCO₃ interfered with crystal growth. The P-removal rate was aproximately seven times more rapid at the 1-mM HCO₃ level than at the 6-mM level. Apatite crystal growth was strongly reduced in the presence of Mg. An increase of 1 mM Mg decreased the P-removal rate by greater than two orders of magnitude. Reactions exhibited second-order dependence on the P concentration when interfering ion concentrations were minimal and F was present. Values of (n) increased regularly when increasing concentrations of either Mg or HCO₃ were present. Surprisingly, relatively high initial P concentrations caused temporary crystal growth interference although this could also have been due to Mg. Apatite crystal growth was strongly influenced by seed concentrations. An increase of 100 mg/liter HAP resulted in a rate constant increase for P-removal of more than three orders of magnitude.

¹ Contribution from the Dep. of Soil Science, Inst. of Agric., Univ. of Minnesota, St. Paul, MN 55108.

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

Received for publication May 5, 1980. Accepted for publication December 26, 1980.