Dissolution kinetics of calcite, dolomite and magnesite at 25 °C and 0 to 50 atm pCO 2

Abstract

Dissolution rates of calcite, dolomite and magnesite were measured at 25 °C and pH from 3 to 4 as a function of salinity (0.001 M≤[NaCl]≤1 M) and partial pressure of CO 2 (10 −3.5≤ pCO 2≤55 atm). Experiments on calcite and dolomite crystal planes dissolution were performed in a batch reactor under controlled hydrodynamic conditions using the rotating disk technique. Dissolution experiments using mixed-flow reactors were also conducted on calcite and dolomite powders of 100–200 μm. Magnesite dissolution rates were measured using a batch titanium high-pressure reactor on 100–200 μm powders. The pH was measured in-situ using a solid-contact electrode in a cell without liquid junction. At pH∼4.0 and constant hydrodynamic conditions pH-independent calcite dissolution rate increases by a factor of 3 from 1 to ∼20 atm pCO 2 and stays constant at 25 to 50 atm. These rates do not depend on NaCl concentration from 0.01 to 1.0 M and pH of 4 to 8. Calcite dissolution rates depend strongly on stirring between 200 and 2000 rpm at 2, 10, and 50 atm pCO 2 suggesting mass transport control at these conditions. Both for polycrystalline samples and cleavage planes, dolomite dissolution rate increases with increasing pCO 2 at 1≤ pCO 2≤10 atm and stays constant when pCO 2 is further increased to 50 atm. These rates depend on stirring velocity and increases by a factor of 2–3 from 200 to 2500 rpm reflecting moderate transport contribution to dissolution at these conditions. Within the experimental uncertainty, dolomite dissolution rates are independent of ionic strength between 0.1 and 1 M NaCl and 5 to 50 atm pCO 2. This is also confirmed by powder dissolution experiments performed in mixed-flow reactors. Magnesite dissolution rate increases by a factor of 3 at 0 to 5 atm pCO 2 but remains constant from 5 to 55 atm pCO 2. The results obtained in this study demonstrate that carbonate mineral dissolution rates are not proportional to H 2CO 3*(aq) and depend only weakly on pCO 2. For dolomite (cleavage planes) and magnesite, the surface complexation model (SCM) of Pokrovsky et al. [Pokrovsky, O.S., Schott, J., Thomas, F., 1999a. Processes at the magnesium-bearing carbonates/solution interface. I. A surface speciation model of magnesite. Geochim. Cosmochim. Acta, 63, 863–880; Pokrovsky, O.S., Schott, J., thomas, F., 1999b. Dolomite surface speciation and reactivity in aquatic systems. Geochim. Cosmochim. Acta, 63, 3133–3143.] predicts dissolution rates up to 50 atm pCO 2 with a good accuracy. Because the dissolution of calcite at 3<pH<4 is controlled by transport processes, experimental and theoretical difficulties do not permit this mineral dissolution rate to be accurately modeled within the framework of SCM.