Experimental Determination of Calcite Dissolution Rates and Equilibrium Concentrations in Deionized Water Approaching Calcite Equilibrium

The calcite dissolution rates at 50–250 ℃ and 20 MPa in deionized water with flow rate varying from 0.2 to 5 mL/min were experimentally measured in a continuous flow column pressure vessel reactor. Equilibrium concentration (c_eq) of calcite dissolution in deionized water at 20 MPa was determined using dissolution data according to the iterative method presented by Jeschke and Dreybrodt. The equilibrium concentrations at 50, 100, 150, 200 and 250 ℃ are 1.84×10⁻⁴, 2.23×10⁻⁴, 2.25×10⁻⁴, 2.31×10⁻⁴ and 2.24×10⁻⁴ mol/L, respectively. The c_eq increases first and then decreases with temperature varying from 50 to 250 ℃ at 20 MPa, and the same variation trend occurs at 10 MPa with lower values. The maximum value (or extremum) of c_eq would increase with temperature at constant pressures. The dissolution reaction of calcite in this experiment is approaching the calcite equilibrium, and the reaction order doesn't keep a constant at different temperatures, which could imply that a change of the reaction mechanism was occurring. The Arrhenius equation shouldn't be used to calculate apparent activation energy using rate constant data at different temperatures when the reaction order or reaction mechanism changed.