Calcite dissolution kinetics in the system CaCO 3–H 2O–CO 2 at high undersaturation

Abstract

Dissolution rates of limestone covered by a water film open to a CO 2-containing atmosphere are controlled by the chemical composition of the CaCO 3–H 2O–CO 2 solution at the water-mineral interface. This composition is determined by the Ca 2+-concentration at this boundary, conversion of CO 2 into H + and HCO 3 - in the solution, and by diffusional mass transport of the dissolved species from and towards the water–limestone interface. A system of coupled diffusion–reaction equations for Ca 2+, HCO 3 - , and CO 2 is derived. The Ca 2+ flux rates at the surface of the mineral are defined by the PWP-empirical rate law. These flux rates by the rules of stoichiometry must be equal to the flux rates of CO 2 across the air–water interface. In the solution, CO 2 is converted into H + and HCO 3 - . At low water-film thickness this reaction becomes rate limiting. The time dependent diffusion–reaction equations are solved for free drift dissolution by a finite-difference scheme, to obtain the dissolution rate of calcite as a function of the average calcium concentration in the water film. Dissolution rates are obtained for high undersaturation. The results reveal two regimes of linear dissolution kinetics, which can be described by a rate law F = α i( m i c eq − c), where c is the calcium concentration in the water film, c eq the equilibrium concentration with respect to calcite. For index i = 0, a fast rate law, which here is reported for the first time, is found with α 0 = 3 × 10 −6 m s −1 and m 0 = 0.3. For c > m 0 c eq, a slow rate law is valid with α 1 = 3 × 10 −7 m s −1 and m 1 = 1, which confirms earlier work. The numbers given above are valid for film thickness of several tenths of a millimetre and at 20 °C. These rates are proven experimentally, using a flat inclined limestone plate covered by a laminar flowing water film injected at an input point with known flow rate Q and calcium concentration. From the concentration measured after flow distance x the dissolution rates are determined. These experiments have been performed at a carbon-dioxide pressure of 0.00035 atm and also of 0.01 atm. The results are in good agreement to the theoretical predictions.