| Abstract: |
A surface reaction model for the kinetics of quartz dissolution is derived from an analysis of low-temperature dissolution rate data published by seven experimenters. The model correlates seventynine quartz dissolution rate measurements with calculated surface species distributions to predict reaction kinetics. Using the triple layer electrostatic surface model, we compute the distributions of predominant acid-base surface species for the solution composition associated with each reported rate measurement. Reactivities of the modeled species are then obtained by fitting the calculated site distributions and the experimental rate data to a simple rate equation. This results in an expression valid for 25°C, pH 2–13 and 0-0.5 molal sodium chloride given by
rate = 10−13.0(θ–SiOH* + 10−10.8(θ≡SiOsum)1 + 10−9.2(θ≡SiOsum)2
, where rate has units of mol m−2s−1, θ≡4;SiOH is the fraction of ≡SiOH surface complexes and ∗ indicates that the reaction order dependence on this species is ill-determined. The θ≡SiOsum term gives the sum of ≡SiO-Na+ and ≡SiO− fractions. This correlation removes much of the apparent scatter that is observed between experimenters and demonstrates that most measurements follow a consistent trend when a sodium complex, ≡SiO-Na+, is included in the model. The model predicts that reaction rates are increased by a factor of 12 at pH 8 with the addition of 0.2 molal sodium to the reacting solutions.
While this approach is an indirect measure of the reactivity of bonds at mineral surfaces, when compared to previous models, it offers a better understanding of rate-controlling processes because it specifically includes the calculated distributions of surface reactants. This analysis, in conjunction with evidence from the literature for cation-specific influences on silicon dioxide reactivity, supports a dissolution model incorporating cation interactions. |
