Physics
Scientific paper
Jul 1988
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988gecoa..52.1883i&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 52, Issue 7, pp.1883-1893
Physics
3
Scientific paper
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 Ca 2+ and PO 4 3- 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 m 2 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 = k f s 2 3 [ Ca 2+ ][ PO 4 3- ], where R = rate of HAP precipitation ( mol HAP L -1 s -1 ), k f = rate constant ( L 2 mol -1 m -2 s -1 ), 2 and 3 = divalent and trivalent ion activity coefficients, s = surface area ( m 2 L -1 ), and BRACKETS = concentrations of Ca 2+ and PO 4 3- (mol L -1 ), was derived based on the reaction orders with respect to S, [Ca 2+ ] and [PO 4 3- ]. The equation was also verified using the integral method, and the average value for the precipitation rate constant was 173 ± 11 L 2 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 Ca 2+ and HPO 4 2- into lattice vacancies, with subsequent dehydration and incorporation into the crystal lattice.
Inskeep William P.
Silvertooth Jeffrey C.
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