Measurements of the dissolution rate of diopside (r) were carried out as a function of the Gibbs free energy of the dissolution reaction (Delta G(r)) in a continuously stirred flow-through reactor at 90 degrees C and pH(90 degrees C) = 5.05. The overall relation between r and Delta G(r) was determined over a free energy range of -130.9 < Delta G(r) < -47.0 kJ mol(-1). The data define a highly non-linear, sigmoidal relation between r and Delta G(r). At far-from-equilibrium conditions (Delta G(r) <= -76.2 kJ mol(-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 Delta G(r). A sharp decrease of the dissolution rate (similar to 1 order of magnitude) occurs in the transition Delta G(r) region defined by -76.2 < Delta G(r) <= -61.5 kJ mol(-1). Dissolution closer to equilibrium (Delta G(r) > -61.5 kJ mol(-1)) is characterised by a much weaker inverse dependence of the rates on Delta G(r). Modeling the experimental r-Delta G(r), 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-Delta G(r), 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-Delta G(r), 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. (C) 2010 Elsevier Ltd. All rights reserved.
Daval, Damien Hellmann, Roland Corvisier, Jérôme Tisserand, Delphine Martinez, Isabelle Guyot, Francois