Kinetics and mechanisms of dolomite dissolution in neutral to alkaline solutions revisited

Abstract

Steady-state dissolution rates of dolomite were measured at 25°C in a mixed-flow reactor as a function of pH (from 5–12), ionic strength (0.002 < I < 0.1 M), total dissolved carbonate (10⁻⁵ < ∑CO₂ < 0.1 M), calcium (10⁻⁶ −0.003 M), magnesium (3 · 10⁻⁷ −0.005 M), and inorganic (sulfate) and organic (acetate, ascorbate, formiate, tartrate, oxalate, citrate, and EDTA) ligands concentration. Dissolution rates were found to be pH-independent at 6 ≤ pH ≤ 8 and to decrease with increasing pH at pH > 8 and ∑CO₂ > 10⁻³ M. In the alkaline pH region, carbonate and bicarbonate ions significantly inhibit dissolution rates at far from equilibrium conditions. Dissolved Ca was found to be a strong inhibitor of dolomite dissolution at pH above 7, whereas dissolved Mg has no effect on the dissolution rate. The surface complexation model developed by Pokrovsky, Schott, and Thomas (1999b) was used to correlate dolomite dissolution kinetics with its surface speciation. At the conditions of this study (5 < pH ≤ 12), dissolution is controlled by the hydration of Mg surface sites and formation of >MgOH₂⁺ species. This Ca and CO₃-free surface precursor complex allows us to account for the inhibiting effect of aqueous calcium and carbonate ions on dolomite dissolution. Based on these results and those of Pokrovsky, Schott, and Thomas (1999b), the following rate equation, consistent with transition state theory, was used to describe dolomite dissolution kinetics over the full range of solution composition: \[\mathrm{R}{=}{[}\mathrm{k}_{\mathrm{CO}3}{\cdot}{\{}{>}\mathrm{CO}_{3}\mathrm{H}{^\circ}{\}}^{2.0}{+}\mathrm{k}_{\mathrm{Mg}}{\cdot}{\{}{>}\mathrm{MgOH}_{2}^{{+}}{\}}^{1.9}{]}{\cdot}(1{-}\mathrm{exp}({-}1.9\mathrm{A}/\mathrm{RT}))\] or, alternatively, at pH above 6 and I = 0.1 M, \[\mathrm{R}{=}\mathrm{k}_{\mathrm{Mg}}{\ast}{\cdot}{\{}\frac{\mathrm{K}_{\mathrm{CO}3}{\ast}{\cdot}\mathrm{K}_{\mathrm{Ca}}{\ast}}{\mathrm{K}_{\mathrm{CO}3}{\ast}{\cdot}\mathrm{K}_{\mathrm{Ca}}{\ast}{+}\mathrm{K}_{\mathrm{Ca}}{\ast}{\cdot}\mathrm{a}_{\mathrm{CO}_{3}^{2{-}}}{+}\mathrm{a}_{\mathrm{CO}_{3}^{2{-}}}{\cdot}\mathrm{a}_{\mathrm{Ca}^{2{+}}}}{\}}^{1.9}{\cdot}{[}1{-}(\frac{\mathrm{Q}}{\mathrm{K}_{\mathrm{sp}}^{0}})^{1.9}{]}\] where {>i} stands for surface species concentration (mol/m²), A refers to the chemical affinity of the overall reaction, k_CO₃, k_Mg, k_Mg*, K_CO₃*, K_Ca* are constants, and (Q/K_sp⁰) stands for dolomite saturation index. This equation reflects the formation of two different precursor complexes that contain two protonated >CO₃H° and two hydrated >MgOH₂⁺ groups in acid and in neutral and alkaline solutions, respectively. Crystallization of dolomite was found to occur in highly supersaturated solutions as confirmed by outlet solutions analysis and SEM observation of reacted grains. Very low dolomite crystallization rates (that is, ∼10⁻¹⁶ mol/cm²/s) are consistent with those observed in natural conditions and predicted by the empirical model of Arvidson and Mackenzie (1997). Dolomite dissolution rate is promoted by the addition of inorganic and organic ligands with the following effectiveness: sulfate ≈ formiate ≈ tartrate < acetate < ascorbate ≤ oxalate < citrate ≪ EDTA. The effect of these ligands can be modeled within the framework of the surface coordination theory.