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Calcite Dissolution in Deionized Water from 50°C to 250°C at 10 MPa: Rate Equation and Reaction Order

Carbonate minerals and water (or geofluids) reactions are important for modeling of geochemical processes and have received considerable attention over the past decades. The calcite dissolution rates from 50°C to 250°C at 10 MPa in deionized water with a flow rate varying from 0.2 to 5 mL/min were experimentally measured in a continuous flow column pressure vessel reactor. The dissolution began near the equilibrium with c/ceq > 0.3 and finally reached the equilibrium at 100°C‐250°C, so the corresponding solubility was also determined as 1.87, 2.02, 2.02 and 1.88times10−4 · mol/L at 100°C, 150°C, 200°C and 250°C respectively, which was first increasing and then switching to decreasing with temperature and the maximum value might occur between 150°C and 200°C. The experimental dissolution rate not only increased with temperature, but also had a rapid increase between 150°C and 200°C at a constant flow rate of 4 mL/min. The measured dissolution rates can be described using rate equations of R = k(1‐c/ceq)R or R = kc−n. In these equations the reaction order n changed with temperature, which indicates that n was a variable rather than a constant, and the activation energy was 13.4 kj/mol calculated with R = k(1‐c/ceq)R or 18.0 kJ/mol with R = kc−R, which is a little lower than the surface controlled values. The varied reaction order and lower activation energy indicates that calcite dissolution in this study is a complex interplay of diffusion controlled and surface controlled processes.