Comparative study of dissolution rate-determining mechanisms of limestone and dolomite

The dissolution rate-determining processes of carbonate rocks include: (1) heterogeneous reactions on rock surfaces; (2) mass transport of ions into solution from rock surfaces via diffusion; and (3) the 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, from experiment and theoretical analysis under similar conditions not only were the initial dissolution rates of dolomite lower by a factor of 3-60 than those of limestone, but also there are different dissolution rate-determining mechanisms between limestone and dolomite. For example, for limestone under the condition of CO2 partial pressures PCO2 > 100 Pa dissolution rates increased significantly by a factor of about ten after addition of carbonic anhydrase (CA) into solution, which catalysed the conversation reaction of CO2, whereas CA had little influence on dolomite dissolution. For dolomite, the increase of dissolution rate after addition of CA into solution appeared at PCO2 < 10,000 Pa. Moreover, the enhancement factor of CA on dolomite dissolution rate was much lower (by a factor of about 3). In addition, when dissolution of both limestone and dolomite was determined by hydrodynamics (rotation speed or flow speed), especially under PCO2 < 1,000 Pa, the dissolution of limestone was more sensitive to hydrodynamic change than that of dolomite. These findings are of significance in understanding the differences in karstification and relevant problems of resource and environment in dolomite and limestone areas.