A MASS-TRANSFER MODEL FOR DISSOLUTION AND PRECIPITATION OF CALCITE FROM SOLUTIONS IN TURBULENT MOTION

A film theory model for dissolution and precipitation of calcite from a plane surface in a H2O-CO2-CaCO3 solution under turbulent flow conditions is presented. It takes into account: (1) molecular diffusion across a diffusion boundary layer of thickness epsilon adjacent to the liquid-solid interface; (2) slow conversion of CO2 into HCO3- and H+; and (3) heterogeneous chemical reactions at the surface of solid CaCO3. The rates depend heavily on the thickness of the boundary layer. The parameter epsilon is determined by the hydrodynamic conditions of the problem. Its value can be found either experimentally by dissolution experiments entirely determined by diffusional mass transport, such as dissolution of CaSO4, or by hydrodynamic correlations. We have calculated dissolution and precipitation rates of calcite for various values of Ca2+ and CO2 concentrations in the solution, and various values of epsilon. The results are compared to data of calcite dissolution by rotational disc experiments and show satisfactory agreement. Finally, we discuss the consequences of our results with respect to dissolution of limestone in karst terranes.