Kinetics of hydroxyapatite precipitation at pH 7. 4 to 8. 4

Journal Article · Fri Jul 01 00:00:00 EDT 1988 · Geochimica et Cosmochimica Acta; (USA)

DOI:https://doi.org/10.1016/0016-7037(88)90012-9· OSTI ID:5181341

Inskeep, W P; Silvertooth, J C ^[1]

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1. Oklahoma State Univ., Stillwater (USA)

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{sup 2+} and PO{sup 3{minus}}{sub 4} concentrations ranging from 0.37-0.86 and 0.29-1.0 mmol L{sup {minus}1}, respectively, ionic strengths from 0.015-0.043 mol L{sup {minus}1}, HAP seed concentrations from 7.1-28.4 m{sup 2}L{sup {minus}1}, temperatures from 10-40{degree}C, and pH from 7.4 to 8.4. Initial rates expressed as mole HAP L{sup {minus}1}s{sup {minus}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{sub f}s{gamma}{sub 2}{gamma}{sub 3}(Ca{sup 2+})(Po{sup 3{minus}}{sub 4}), where R = rate of HAP precipitation (mol HAP L{sup {minus}1} s{sup {minus}1}), k{sub f} = rate constant (L{sup 2} mol{sup {minus}1} m{sup {minus}2} s{sup {minus}1}), {gamma}{sub 2} and {gamma}{sub 3} = divalent and trivalent ion activity coefficients, s = surface area (m{sup 2} L{sup {minus}1}), and brackets = concentrations of Ca{sup 2+} and PO{sup 3{minus}}{sub 4} (mol L{sup {minus}1}), was derived based on the reaction orders with respect to s,(Ca{sup 2+}) and (PO{sup 3{minus}}{sub 4}). The equation was also verified using the integral method, and the average value for the precipitation rate constant was 173 {plus minus} 11 L{sup 2} mol{sup {minus}1} m{sup {minus}2} s{sup {minus}1}. The Arrhenius activation energy was 186 {plus minus} 15 kJ mol{sup {minus}1}, indicative of a surface controlled precipitation mechanism. The authors speculate that the rate limiting steps include migration of surface Ca{sup 2+} and HPO{sup 2{minus}}{sub 4} into lattice vacancies, with subsequent dehydration and incorporation into the crystal lattice.

• OSTI ID: 5181341
• Journal Information: Geochimica et Cosmochimica Acta; (USA), Vol. 52:7; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
APATITES
ACTIVATION ENERGY
NUCLEATION
PRECIPITATION
CHEMICAL REACTION KINETICS
CONCENTRATION RATIO
CRYSTAL LATTICES
CRYSTALLOGRAPHY
GEOCHEMISTRY
MINERALIZATION
PH VALUE
SOLUTIONS
THERMODYNAMIC ACTIVITY
CHEMISTRY
CRYSTAL STRUCTURE
DISPERSIONS
ENERGY
KINETICS
MINERALS
MIXTURES
PHOSPHATE MINERALS
REACTION KINETICS
SEPARATION PROCESSES
580000* - Geosciences