Dissolution Kinetics of Calcite in 0.1 M NaCl Solution at Room Temperature: An Atomic Force Microscopic (AFM) Study

Abstract

Atomic force microscopy (AFM) was used to study the rates of migration of the (10¯1 4) plane of a single-crystal of calcite dissolving in 0.1 M NaCl aqueous solutions at room temperature. The solution pH and PCO 2 controlled in the ranges 4.4 < pH < 12.2 and 0 < PCO 2 10.8, only the velocity of the obtuse steps increased as pH increased, whereas that of acute steps gradually decreased. The dissolution rate of the mineral can be calculated from the measured step velocities and average slope, which is proportional to the concentration of exposed monomolecular steps on the surface. The average slope of the dissolving mineral, measured at pH 5.6 and 9.7, was 0.026 (±0.015). Using this slope, we calculate bulk dissolution rates for 5.3 < pH < 12.2 of 4.9(±3.0) × 10-11 to 1.8(±1.0) × 10-10 mol cm-2 s-1. The obtained dissolution rate can be expressed by the following empirical equation: Rdss = 10-4.66(±0.13)[H+] + 10-3.87(±0.06)[HCO3 -] + 10-7.99(plusmn; 0.08)[OH-] We propose that calcite dissolution in these solutions is controlled by elementary reactions that are similar to those that control the dissolution of other amphoteric solids, such as oxides. The mechanisms include the proton-enhanced hydration and detachment of calcium-carbonate ion pairs. The detachments are enhanced by the presence of adsorbed nucleophiles, such as hydroxyl and bicarbonate ions, and by protons adsorbed to key oxygens. A molecular model is proposed that illustrates these processes.