HR: 0800h
AN: B21B-0867    [Abstracts]
TI: Integrated Approach for Understanding Impurity Adsorption on Calcite: Mechanisms for Micro-scale Surface Phenomena
AU: * Vinson, M D
EM: mvinson@rice.edu
AF: Rice University, Department of Earth Science-MS 126, Houston, TX 77251-1892 United States
AU: Arvidson, R S
EM: rsa4046@ruf.rice.edu
AF: Rice University, Department of Earth Science-MS 126, Houston, TX 77251-1892 United States
AU: Luttge, A
EM: aluttge@rice.edu
AF: Rice University, Department of Earth Science-MS 126, Houston, TX 77251-1892 United States
AB: A longstanding goal within the field of environmental geochemistry has been the development of a fundamental understanding of the kinetics that governs the interactions of solution-borne impurities with the calcite mineral surface. Recent dissolution experiments using Mg$^{2+}$, Mn$^{2+}$, and Sr$^{2+}$ have shown distinct differences in the interaction of these three impurity ions with the calcite crystal surface. Because the dissolution of carbonate minerals in soils and sediments influences the uptake and migration of groundwater contaminants, a rigorous understanding of the basic processes that occur at the mineral-fluid interface is necessary. We have used vertical scanning interferometry (VSI) coupled with scanning probe microscopy (SPM) to examine calcite crystal dissolution in the presence of Mg$^{2+}$, Mn$^{2+}$, and Sr$^{2+}$, all known dissolution inhibitors and possible groundwater contaminants. We have studied the kinetics of impurity-crystal interactions at a pH 8.8, and in the presence or absence of dissolved inorganic carbon. Our data show that, when individually introduced into undersaturated solutions, Mg$^{2+}$ and Mn$^{2+}$ are shown to activate the calcite crystal surface, resulting in enhanced etch pit nucleation rates and step density. Conversely, Sr$^{2+}$ is shown to cause passivation of the calcite surface. The effect is intensified when solutions are saturated with respect to atmospheric CO$_2$. Results indicate that aqueous CO$_3^{2-}$ (or HCO$_3^-$) may influence how aqueous metal ionic complexes interact with the crystal surface. Furthermore, the influence is differently exhibited, and passivation or activation ultimately depends on the properties of the diffusing metal ion or metal-hydroxide complex. These properties include for example, differences in hydration enthalpy, the effective ionic radius, and electron shell configuration.
DE: 9800 GENERAL OR MISCELLANEOUS
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting