『Abstract
The dissolution of calcite in a full range of saturation conditions
is investigated to explore the kinetic effect of Gibbs free energy,
dislocation density, and the presence of atmospheric pCO2. Experiments are carried out in a mixed-flow
reactor at room temperature (25℃) in both closed and open (to
air) settings, and calcite samples are prepared by fragmentation
and milling to generate different defect densities. Experimental
observations show a highly nonlinear dependence of the dissolution
rates on the Gibbs free energy; however, the kinetics does not
seem to be affected by the samples' dislocation density, nor the
presence of atmospheric pCO2 at any saturation
condition. Fitting the conventional transition state model (TST)
to the observed rate - free energy relationship indicates that,
though the TST rate equation is sufficient to describe the dissolution
kinetics near and far from equilibrium, it clearly overestimates
the dissolution rate when the system sits in between. These results
suggest that: (i) the classic TST model may not be sufficient
to depict the relation between dissolution rate and Gibbs free
energy once solution saturation falls from its extreme. (ii) The
steps associated with increased crystal defects may be overwhelmed
by those regenerated at corners and edges of calcite particles
through layer-by-layer dissolution along the cleavage directions.
(iii) The presence of CO2 in ambient environments
bears little importance to calcite dissolution possibly due to
the slow response of aqueous HCO3- to pCO2
change at low CO2 partial pressure conditions.
Keywords: Mineral dissolution; Calcite; Transitional state theory;
Kinetics; Dislocation density』
1. Introduction
2. Theoretical consideration
2.1. Calcium carbonate dissolution kinetics
2.2. Etch pit formation
3. Material and methods
3.1. Mixed-flow reactor system
3.2. Sample and solution preparation
3.3. Imaging by fluid contact AFM
4. Experimental results and dissolution
4.1. Calcite dissolution rates as a function of ΔG
4.2. Role of dislocation density in calcite dissolution
4.3. Effect of atmospheric carbon dioxide on calcite dissolution
5. Conclusions
Acknowledgments
Appendix A. Supplementary data
References