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Dissolution kinetics of diopside as a function of solution saturation state: Macroscopic measurements and implications for modeling of geological storage of CO2 |
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| Authors: | Damien Daval Roland Hellmann Delphine Tisserand François Guyot |
| Institution: | a Institut de Physique du Globe de Paris, Centre de Recherches sur le Stockage Géologique du CO2, 4 Place Jussieu, 75005 Paris, France b Laboratoire de Géologie, UMR 8538 du CNRS, École Normale Supérieure, 24 Rue Lhomond, 75005 Paris, France c Géochimie de l’Environnement, Laboratoire de Géophysique Interne et Tectonophysique, CNRS UMR C5559, OSUG, Université Joseph Fourier, 38041, Grenoble Cedex 9, France d Institut de Minéralogie et de Physique des Milieux Condensés, CNRS, Université Paris 6 et 7, 140 rue de Lourmel 75252, Paris, France |
| Abstract: | Measurements of the dissolution rate of diopside (r) were carried out as a function of the Gibbs free energy of the dissolution reaction (ΔGr) in a continuously stirred flow-through reactor at 90 °C and pH90 °C = 5.05. The overall relation between r and ΔGr was determined over a free energy range of −130.9 < ΔGr < −47.0 kJ mo1−1. The data define a highly non-linear, sigmoidal relation between r and ΔGr. At far-from-equilibrium conditions (ΔGr ? −76.2 kJ mo1−1), a rate plateau is observed. In this free energy range, the rates of dissolution are constant, independent of Ca], Mg] and Si] concentrations, and independent of ΔGr. A sharp decrease of the dissolution rate (∼1 order of magnitude) occurs in the transition ΔGr region defined by −76.2 < ΔGr ? −61.5 kJ mo1−1. Dissolution closer to equilibrium (ΔGr > −61.5 kJ mo1−1) is characterised by a much weaker inverse dependence of the rates on ΔGr. Modeling the experimental r-ΔGr data with a simple classical transition state theory (TST) law as implemented in most available geochemical codes is found inappropriate. An evaluation of the consequences of the use of geochemical codes where the r-ΔGr relation is based on basic TST was carried out and applied to carbonation reactions of diopside, which, among other reactions with Ca- and Mg-bearing minerals, are considered as a promising process for the solid state sequestration of CO2 over long time spans. In order to take into account the actual experimental r-ΔGr relation in the geochemical code that we used, a new module has been developed. It reveals a dramatic overestimation of the carbonation rate when using a TST-based geochemical code. This points out that simulations of water-rock-CO2 interactions performed with classical geochemical codes should be evaluated with great caution. |
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