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Local Reactions on Carbonate Surfaces: Structure, Reactivity and Solution D. R. Baer, J. E. Amonette, Y. Liang, A. S. Lea, L. Sorenson,(a,b) and P. Geissbuhler(b) Supported by Geosciences Program of the DOE Office of Basic Energy Sciences. (a) EMSL User. (b) University of Washington. Carbonate minerals are particularly important in the global carbon dioxide cycle and in subsurface contaminant migration processes. Carbonate rocks make up the majority of Karst landscapes that cover 20% of the earth’s dry land surface (White et al. 1995; Liang et al. 1996a). In arid environments, carbonates often exist as caliche layers, which influence the spread of surface contaminants into groundwater. Although many geochemical processes are controlled by the kinetics of reactions and movement of material to or from a surface, accurate quantitative descriptions of these processes (important for predicting contaminant spread, the success of environmental remediation efforts, or the effects of changes in the carbon dioxide cycle) are mostly lacking. This is primarily caused by an incomplete understanding of the molecular-scale processes controlling these reactions. Advances in both experimental and theoretical capability make it possible to begin to obtain information about the reaction rates at specific sites on some surfaces. The purpose of this program is to develop a fundamental, molecular-level understanding of the structure and chemistry of carbonate surfaces, with a focus on the interactions between adsorbates and calcite surfaces. The availability of large single-crystals allows fundamental measurements to be made on well-defined surfaces. By linking experimental studies of geochemical reactions on single-crystal surfaces with first-principles quantum-mechanical model calculations to describe the surface and interfacial structure and chemistry, a systematic study of the factors controlling the surface chemistry of carbonate minerals can be made. In particular, the effects of substitutional impurities and other point chemical defects on the structure and geochemical reactivity of carbonate mineral surfaces and interfaces can be isolated and quantified. This improved microscopic understanding will eventually provide insights into the behavior of these materials in natural systems. This program involves both 1) measurements of the reaction kinetics and structure of calcite surfaces in model geochemical solutions using scanning force microscopy (Liang et al. 1996a, b; McCoy and LaFemina 1997) and 2) development of ab initio, kinetic Monte Carlo (White et al. 1995; Liang et al. 1996a) and other models for the structure and chemistry of the calcite cleavage surface. The dissolution of calcium carbonate in aqueous solutions has been examined for solutions containing differing concentrations of divalent cations including Mn, Sr and Ca (Lea et al. submitted 1999). Rates of dissolution are measured by observing pit formation and growth with an atomic-force microscope (AFM). The influence of different concentrations of Mn in the solution on the rate of growth of the pits is shown in Figure 6.3 along with the nature of the surfaces, as observed by AFM. The observations show that the presence of Mn slows the rate of dissolution of calcite. At the higher Mn concentrations, a second Mn-rich phase is observed to precipitate and grow. This new phase appears while the calcite substrate is dissolving and grows in a direction along one diagonal of the dissolution pits that form. These rods have a number of interesting features that we are still working to understand. For specific solution conditions, they grow in three dimensions until a well-defined height and specific width are reached. Once the "critical" thickness and width dimensions are reached, they continue to grow only in length (more quickly in one direction than the other). Figure 6.3. Influence of Mn in solution on the rates of dissolution of calcite as observed by the growth of pits formed by dissolution. As the Mn concentration increases, the calcite dissolution decreases and, at higher Mn concentrations, a new phase is observed growing on the surface. The rates of growth also depend upon solution conditions as shown in Figure 6.4. The complex relationship between Mn solution concentration and growth rate likely stems from the impact of Mn on both precipitate formation and calcite matrix dissolution. The bulk solution concentrations of Mn2+ and CO32- are affected by both the incoming solution composition and the products added to the solution by calcite dissolution. As the Mn concentration increases, the overall calcite dissolution rate decreases, thus decreasing the amounts of CO32- and Ca+ added to the solution by dissolution. This slows growth until at higher Mn concentrations where the growth rate increases but the shape of the precipitate phase is more ragged, suggesting a greater rate of nucleation. Figure 6.4. (a) nature of the crystallographic orientation of Mn precipitates on the calcite surface and (b) variation of growth rate as a function of Mn in solution. The effects of Ca2+ ions in solution on both the dissolution and growth have been examined as part of a Chemical Engineering MS thesis by N. G. Colton (1999). She directly measured both pit growth during dissolution and the filling of pits when the Ca concentrations took the solution above saturation. The changing shape of these pits during fill-in and growth (Figure 6.5) provides site-specific information about surface reactions taking place during these processes. Of particular interest in this work was the observation that the sites most influenced by the amount of CO32- in solution were not those for which nucleation and growth occurred most rapidly. Figure 6.5. Schematic depicting proposed mechanism for pit fill-in that describes pit shapes observed in solutions just above observed saturation. The diagonal line indicates the <010> direction. The graduated shading depicts time-sequenced pit fill-in. The work with differing cations has been particularly interesting because the different metals influence (and apparently absorb at different rates) different surface sites. Since carbonates in natural environments interact extensively with metals and complex anions, the differences in the nature of the interactions will have significant effects on the incorporation and release of the metals and other contaminates in the environment. Furthermore, differences in incorporation or second-phase formation can significantly alter conditions appropriate for environmental remediation processes. References Colton, N. G., Solution Effects on Calcite Dissolution, A thesis submitted in partial fulfillment of the requirements for a degree of Master of Science in Chemical Engineering, Washington State University, August 1999. Lea, A. S., J. E. Amonette, D. R. Baer, and Y. Liang, "Microscopic Effects of Total Carbon Dioxide, Manganese, and Strontium on Calcite Dissolution," submitted to Geochim. Cosmochim. Acta, September 1999. Liang, Y., and D. R. Baer, "Anisotropic Dissolution at the CaCO3 (1014)-Water Interface," Surf. Sci. 373, 275 (1997). Liang, Y., D. R. Baer, J. M. McCoy, J. P. LaFemina, "Interplay Between Step Velocity and Morphology During the Dissolution of CaCO3 Surface," J. Vac. Sci. Technol. A 13(3), May/Jun (1996a). Liang, Y., D. R. Baer, J. M. McCoy, J. E. Amonette, and J. P. LaFemina, "Dissolution Kinetics at the Calcite - Water Interface," Geochim. Cosmochim. Acta 60, 4883 (1996b). McCoy, J. M., and J. P. LaFemina, "Kinetic Monte Carlo Investigation of Pit Formation at the CaCO3 (1014) Surface-Water Interface," Surf. Sci. 373, 288 (1997). White, W. B., D. C. Culver, J. S. Herman, T. C. Kane, and J. E. Mylroie, "Karst Lands," American Scientist 83, 450-459 (1995). William R. Wiley Environmental Molecular Sciences Laboratory Feedback: webmaster@emsl.pnl.gov Revised: June 12, 2001 Security & Privacy PNNL-13147