Calcite, dolomite and magnesite dissolution kinetics in aqueous solutions at acid to circumneutral pH, 25 to 150 °C and 1 to 55 atm pCO 2: New constraints on CO 2 sequestration in sedimentary basins

• Published in: Chemical geology Vol. 265; no. 1; pp. 20 - 32
• Main Authors: Pokrovsky, Oleg S., Golubev, Sergey V., Schott, Jacques, Castillo, Alain
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 2009
• Subjects: Calcite; Carbon dioxide sequestration; Dissolution; Dolomite; Kinetics; Magnesite; Magnesite; Carbon dioxide sequestration; Dolomite; Kinetics; Calcite; Dissolution
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2009.01.013

Dissolution rates of calcite, dolomite and magnesite were measured at 25, 60, 100 and 150 °C at far from equilibrium conditions in 0.1 M NaCl solutions of pH from 3 to 6 as a function of bicarbonate ion concentration (10 − 5  M ≤ [NaHCO 3] ≤ 0.1 M) and CO 2 partial pressure (1 ≤ pCO 2 ≤ 55 atm). In solutions without initially added NaHCO 3, calcite dissolution rates ( R) increase with pCO 2 (and equilibrium H + activity); however, recalculation of rates normalized to a constant pH = 4.0 yields only a weak dependence of R on pCO 2 at 25, 60, 100, and 150 °C. The apparent activation energy for calcite dissolution at 25–100 °C is equal to 48.2 ± 4.6 kJ mol − 1 . Dolomite dissolution rates increase with increasing pCO 2 at 1 ≤ pCO 2 ≤ 10 atm and stay constant when pCO 2 is further increased to 50 atm in HCO 3-free solutions at 3.1 ≤ pH ≤ 4.2. The apparent activation energy for dissolution is equal to 34, 21 and 16 kJ mol − 1 at pH = 4 (1–50 atm pCO 2), pH = 4.8 (30 atm pCO 2) and pH = 5.5 (50 atm pCO 2), respectively. Magnesite dissolution rates increase by a factor of 3 at 1 to 5–10 atm pCO 2 but remain constant from 5 to 55 atm pCO 2 in NaHCO 3-free solutions at pH ≤ 4. Apparent activation energy for magnesite dissolution at pCO 2 from 2 to 50 atm decreases from 44 ± 2 kJ mol − 1 at 3.1 ≤ pH ≤ 4.0 to 34 kJ mol − 1 at pH = 5.4. The rates of magnesite and dolomite dissolution at 25–100 °C and far from equilibrium conditions can be rationalized using a surface complexation approach with a unique set of surface adsorption and kinetic constants. The dissolution rates increase with increase of pCO 2 in carbonate-free acid solutions can be explained by the increase of >CO 3H° species concentration with pH decrease. In circumneutral solutions in the presence of added NaHCO 3, the dissolution rates decrease with pCO 2 increase is due to the inhibition by dissolved (HCO 3 −/CO 3 2−), which favors the formation of >MgCO 3 − and >MgHCO 3° at the expense of the rate-controlling >MgOH 2 + species. Finally, very weak effect of temperature on dissolution rates of all carbonate minerals between 100 and 150 °C in acidic solutions can be explained by the increase of the enthalpy of >CO 3 − protonation reaction. At the conditions of CO 2 storage, the effect of dissolved CO 2 on carbonate mineral reactivity is expected to be of second order importance compared to that of pH and dissolved carbonate/bicarbonate ions. Both high temperature and high partial CO 2 pressure decrease carbonate mineral reactivity in aqueous solution which is likely to help carbon dioxide sequestration in deep carbonate sedimentary basins.