High-Resolution Studies of Cadmium Uptake on Carbonate Surfaces: Mechanisms, Rates, and Structures

The research described in this dissertation provided systematic answers to questions about how and to what extent the toxic heavy metal cadmium is removed from aqueous solutions by carbonate mineral surfaces. The first part of the dissertation was focused on elucidation of fundamental mechanisms and the complex interrelationships among factors controlling Cd uptake. Thermodynamic and kinetic controls on the sequestration process were revealed. In particular, morphologies and the crystallographic structure of Cd-rich precipitates on the (104) cleavage surfaces of calcite and dolomite, rates of Cd uptake on dolomite, and the stability of the precipitates on dolomite during desorption in water were investigated. This was achieved through application of a suite of in situ and ex situ analytical techniques, namely, Atomic Force Microscopy (AFM), specular and non-specular X-ray Reflectivity (XR), and X-ray Fluorescence (XRF). These techniques are sensitive to the near-surface region and are ideal probes for examining processes on mineral surfaces at the atomic-scale. Monolayer-scale topography of the carbonate substrate and of the Cd-containing overgrowth film during the sorption and desorption processes was monitored by AFM. Synchrotron X-ray techniques XR and XRF enabled measurements of Cd atomic densities on the compositionally heterogeneous Cd-reacted surfaces. By considering these observations together, a holistic understanding of these mineral surface phenomena was obtained. It was demonstrated that Cd uptake occurs through the formation of coherently strained epitaxial Cd-rich overgrowths that with time can eventually cover the substrate, and that solution supersaturation and substrate morphology are the dominant controls on the manner and rate of uptake. It was shown from non-specular XR that starting at total Cd occupancies greater than ~6 monolayers, lateral compressive strain in the film is relieved, indicating that film structure evolves as the film thickens. Finally, the reversibility of Cd incorporation as a function of film thickness was evaluated by determining dissolution rates of the film in water; apparently slower rates for thinner films were indicated.