Calcite, dolomite and magnesite dissolution kinetics in aqueous solutions at acid to circumneutral pH, 25 to 150 °C and 1 to 55 atm pCO2: New constraints on CO2 sequestration in sedimentary basins

Dissolution rates of calcite, dolomite and magnesite were measured at 25, 60, 100 and 150 degrees 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 <=[NaHCO3]<= 0.1 M) and CO2 partial pressure (1 <= pCO(2)<= 55 atm). In solutions without initially added NaHCO3, 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 degrees C. The apparent activation energy for calcite dissolution at 25-100 degrees 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 HCO3-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 NaHCO3-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 degrees 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 pCO2 in carbonate-free acid solutions can be explained by the increase Of >CO3H degrees species concentration with pH decrease. In circumneutral solutions in the presence of added NaHCO3, the dissolution rates decrease with pCO(2) increase is due to the inhibition by dissolved (HCO3-/CO32-), which favors the formation Of >MgCO3- and >MgHCO3 degrees at the expense of the rate-controlling >MgOH2+ species. Finally, very weak effect of temperature on dissolution rates of all carbonate minerals between 100 and 150 degrees C in acidic solutions can be explained by the increase of the enthalpy Of >CO3- protonation reaction. At the conditions Of CO2 storage, the effect of dissolved CO2 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 CO2 pressure decrease carbonate mineral reactivity in aqueous solution which is likely to help carbon dioxide sequestration in deep carbonate sedimentary basins. (C) 2009 Elsevier B.V. All rights reserved.