The kinetics of calcite dissolution and precipitation in geologically relevant situations of karst areas: 1. Open system

Abstract

A general theory of dissolution and precipitation rates at CaCO3 surfaces from calcareous solutions in contact with an atmosphere containing CO2 is presented. The three rate-determining processes, kinetics at the CaCO3 surface, diffusion into the bulk and conversion of CO2 into HCO−3 are treated simultaneously. In all cases the dissolution or precipitation rates are given by R = α([Ca2+]eq − [Ca2+]), where α is a function of CO2 pressure, thickness of the water film covering the CaCO3 surface, and temperature. α depends also on the hydrodynamic conditions of flow. Under turbulent flow the rates increase by one order of magnitude, since in comparison to laminar flow diffusion is significantly enhanced by eddies.We have carried out experiments to measure the time dependence of [Ca2+] in stagnant and turbulently-stirred water films covering CaCO3 surfaces for various temperatures, CO2 pressures and film thicknesses. From the exponential behaviour of [Ca2+](t) the value of α can be determined. Good agreement with the theoretical predictions is obtained. Furthermore, precipitation rates from supersaturated solutions were measured and found to be in good agreement with the theory.Our results are summarized in a table and figures, which provide the geologist data from which in all situations, being geologically relevant in karst areas as far as open system is concerned, dissolution and precipitation rates can be derived easily.