The dissolution kinetics and apparent solubility of natural apatite in closed reactors at temperatures from 5 to 50 °C and pH from 1 to 6

The apparent solubility and dissolution rates of natural apatite were measured in closed-system reactors as a function of temperature from 5 to 50 °C and pH from 1 to 6. The temporal release rates of Ca, P, and F during the experiments are approximately consistent with stoichiometric dissolution in all experiments. One advantage of closed-system experiments is that they allow determination of reactive fluid evolution and dissolution rates at far-from to near-to equilibrium conditions. Surface area normalized apatite dissolution rates, r, obtained in all experiments are consistent withr=AAaH+nexp(EA/RT)(1−exp(−A/σRT))where AA stands for a rate constant equal to 4 × 10− 3 mol/cm2/s, aH+ denotes the activity of the aqueous H+, n designates a reaction order equal to 0.6, EA symbolizes an activation energy equal to 11.0 kcal/mol, A refers to the chemical affinity of the dissolving apatite, σ stands for Temkin's average stoichiometric number equal to 5; R designates the gas constant, and T represents absolute temperature. Logarithms of apparent equilibrium constants obtained from experiments performed at 3 ≤ pH ≤ 5.6 for the apatite dissolution reactionCa5(PO4)3F + 3H+ = 5Ca2+ + 3HPO42− + F−are found to be − 29.5 ± 0.6, − 29.4 ± 0.9 and − 29.9 ± 1.3 at 5, 25, and 50 °C, respectively.