An experimental study of dolomite dissolution rates at 80 °C as a function of chemical affinity and solution composition Maud Gautelier, Jacques Schott, Eric H. Oelkers ⁎ Géochimie et Biogéochimie Experimentale, CNRS/UMR 5563-OMP-Université Paul-Sabatier, 14 rue Edouard Belin, 31400 Toulouse France Received 9 November 2006; received in revised form 8 May 2007; accepted 16 May 2007 Editor: D. Rickard Abstract Steady-state dissolution rates of Haute Vallée de l'Aude dolomite were measured as a function of chemical affinity and aqueous solution composition in mixed-flow reactors at 80 °C. Experiments were performed in HCl–NaHCO 3 bearing aqueous solutions with in-situ pH ranging from 6.36 to 7.17. Rates are found to be a strong function of aqueous carbonate concentrations at far-from- equilibrium conditions. Measured rates can be described using the dissolution mechanism of Pokrovsky and Schott [Pokrovsky, O.S., Schott, J., 2001. Kinetics and mechanism of dolomite dissolution in neutral to alkaline solutions revisited. Am. J. Sci., 301, 597–626] where rates are controlled by the detachment of the N MgOH 2 + species at the dolomite surface. Within this mechanism, dolomite dissolution rates can be described using r ¼ k þ Mg K ⁎ CO3 K ⁎ Ca K ⁎ CO3 K ⁎ Ca þ K ⁎ Ca a CO 2 3 þ a CO 2 3 a Ca 2þ ( ) n 1  exp nA=RT ð Þ ð Þ where r refers to the overall BET surface area normalized dolomite dissolution rate at both near and far-from-equilibrium conditions, k Mg + designates a rate constant equal to 4.0 × 10 - 12 mol cm - 2 s - 1 , K Ca ⁎ and K CO 3 ⁎ denote equilibrium constants equal to 3.5×10 - 5 and 4.5×10 - 5 , respectively, a i refers to the activity of the subscripted aqueous species, A corresponds to the chemical affinity of the dissolving dolomite, R represents the gas constant, T symbolizes absolute temperature and n designates a stoichiometric coefficient equal to 1.9. © 2007 Elsevier B.V. All rights reserved. Keywords: Dolomite dissolution rates; CO 2 sequestration; Kinetics; Weathering 1. Introduction The motivation for this work is the improved under- standing of the role of dolomite-fluid reactions in the global carbon cycle and CO 2 sequestration strategies (c.f. Oelkers and Schott, 2005). The effect of dolomite dissolution on the global carbon cycle is complex. Dolo- mite dissolution, whether on the Earth's surface or in the subsurface, liberates carbonate, calcium, and magnesium to the fluid phase (c.f. Szramek et al., 2007; Williams et al., 2007). The degree to which carbonate liberated by this reaction is either compensated by subsequent car- bonate precipitation reactions or liberated to the atmo- sphere as CO 2 is unclear. Whereas the ultimate fate of Ca liberated from carbonate dissolution reactions is the for- mation of calcite (c.f. Gislason et al., 2006a), Mg liberated Chemical Geology 242 (2007) 509 – 517 www.elsevier.com/locate/chemgeo ⁎ Corresponding author. Tel.: +33 5 61 33 25 75; fax: +33 5 61 33 25 60. E-mail addresses: schott@lmtg.obs-mip.fr (J. Schott), oelkers@lmtg.obs-mip.fr (E.H. Oelkers). 0009-2541/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2007.05.008