Solute-controlled dissolution thresholding at near-equilibrium calcite-water interfaces

Direct measurements at the aqueous solution-calcite (104) interface were performed using in situ atomic force microscopy (AFM) at 50—70 °C to reveal the influence of solution saturation state and magnesium concentration on calcite dissolution kinetics and surface morphology. Under near-equilibrium conditions, time-sequential AFM images demonstrated a linear relationship between step speed and solution saturation state, Ωcalcite. Dissolved Mg2+ brought negligible inhibitory effects on calcite dissolution at [Mg2+] ≤ 10-4 molal. Upon the introduction of 10-3 molal Mg2+, however, Ωcalcite acted as a “switch” for magnesium inhibition. That is, no significant changes in step kinetics were observed at Ωcalcite ≤ 0.1, whereas a sudden inhibition from Mg2+ was activated at Ωcalcite ≥ 0.2. The presence of the Ω-switch in dissolution kinetics indicates the presence of critical undersaturation in accordance with thermodynamic principles. Etch pits formed in Mg-free near-equilibrium solutions exhibited a nearly-triangle shape, while in the presence 10-3 molal Mg2+ the etch pits displayed a unique distorted rhombic profile, different from those formed under far-from-equilibrium conditions. Such inconsistency in calcite step morphologies may be associated with the increased influence of the backward reaction and the anisotropy in net kink detachment rates. The calcite dissolution results presented in this work demonstrate a need to consider fundamental surface interactions that may lead to highly nonlinear near-equilibrium behavior and serve as a foundation for the development of robust geochemical models for the prediction of long-term calcite dissolution kinetics in natural water systems.