Comparison of Dissolution Rate-Determining Mechanisms between Limestone and Dolomite

The dissolution rate-determining processes of carbonate rocks include: (1) heterogeneous reactions on rock surface; (2) mass transport of ions into solution from rock surface via diffusion; and (3) conversion reaction of CO₂ into H⁺ and HCO₃^-. Generally, it is the slowest of these three processes that limits the dissolution rate of carbonate rock. However, it was found from experiments and theoretical analysis that under similar conditions, not only are the initial dissolution rates of dolomite lower by a factor of 3 to 60 than those of limestone, but also there are different dissolution rate-determining mechanisms between limestone and dolomite. For example, under conditions of CO₂ partial pressures (p_CO2) >100 Pa, limestone dissolution rates increase remarkably, by a factor of about 10 after the addition of carbonic anhydrase (CA) into the solution, which catalyzes the conversion reaction of CO ₂. For dolomite, the increase of dissolution rate after the addition of CA appears at p_CO2 < 10 000 Pa. The enhancement factor of CA on dolomite dissolution rates is much lower (a factor of only about 3). In addition, though dissolution of both limestone and dolomite is also determined by hydrodynamics (rotation speed or flow speed), especially under p_CO2 < 1 000 Pa, the dissolution of limestone is more sensitive to hydrodynamic change than the dissolution of dolomite. These findings are of significance in understanding the differences in karstification and relevant problems of resources and environments of dolomite and limestone areas.