The rate of hydroxyapatite (HAP) precipitation was studied using a reproducible seeded growth technique under pH stat conditions. Thirty different experiments were performed at initial Ca2+ and PO43- concentrations ranging from 0.37-0.86 and 0.29-1.0 mmol L-1, respectively, ionic strengths from 0.015-0.043 mol L-1, HAP seed concentrations from 7.1-28.4 m2 L-1, temperatures from 10-40°C, and pH from 7.4 to 8.4. Initial rates expressed as mole HAP L-1 s-1 were used to test several empirical rate equations and derive a rate equation based on experimentally determined reaction orders. The rate equation: R = kfsγ2γ3[Ca2+][PO43-], where R = rate of HAP precipitation (mol HAP L-1 s-1), kf = rate constant (L2 mol-1 m-2 s-1), γ2 and γ3 = divalent and trivalent ion activity coefficients, s = surface area (m2 L-1), and brackets = concentrations of Ca2+ and PO43- (mol L-1), was derived based on the reaction orders with respect to S, [Ca2+] and [PO43-]. The equation was also verified using the integral method, and the average value for the precipitation rate constant was 173 ± 11 L2 mol-1 m-2 s-1. The Arrhenius activation energy was 186 ± 15 kJ mol-1, indicative of a surface controlled precipitation mechanism. We speculate that the rate limiting steps include migration of surface Ca2+ and HPO42- into lattice vacancies, with subsequent dehydration and incorporation into the crystal lattice.
ASJC Scopus subject areas
- Geochemistry and Petrology