Crystal Dissolution Kinetics: Linking Surface Processes at the Solid-Solution Interface over Multiple Length-Scales
Vinson, Michael David
Doctor of Philosophy
This thesis presents a multiple-scale experimental study of mineral dissolution kinetics, utilizing direct measurement of crystal surface morphology in great detail to determine rates and mechanisms acting at the reactive solid-solution interface. The overall approach uses primarily vertical scanning interferometry (VSI) to analyze threedimensional crystal surface morphological change and quantify rates of dissolution. Integration of VSI with atomic force microscopy (AFM), theoretical kinetic models, and thermodynamic calculations has permitted the recognition of rate-controlling processes and mechanisms, thus strengthening our ability to link dissolution kinetics over a broad range of length and time scales. The motivation for this thesis arises from an incomplete understanding of how molecular-scale surface processes, acting at the solid-solution interface, control large-scale natural dissolution phenomena. Mineral dissolution is a fundamental geologic process that exerts control over a number of significant geochemical events which can affect both man and the environment over a broad range of spatial and temporal scales. At the Earth surface, rates of dissolution are largely surface-controlled, and thus influenced by the threedimensional nature of the crystal surface. The goals of this thesis involve improving overall understanding of dissolution rate-scaling issues by investigation of crystal surface dissolution kinetics, identification ofrate-controlling mechanisms. This thesis reports investigations into the dissolution kinetics of calcite, rhyolite and uraninite over a range of far-from-equilibrium laboratory conditions. The results presented within this work demonstrate that: 1. Monomolecular "rough" step retreat controls the overall rate of calcite crystal dissolution, which in turn can be inhibited by impurity adsorption at a level dependent on the physical and chemical properties of the adsorbing impurity and the presence of carbonate in solution. 2. Heterogeneous retreat of volcanic glass phases control overall rhyolite dissolution rate and the steady-sate release of ions into solution, which in turn may be influenced by the formation of a surface reprecipitation phase. 3. Dissolved carbon species influence the steady-state dissolution uraninite, although this dissolution fails to produce resolvable surface-normal retreat. In total, this original work constructs a clearer understanding of the kinetics at the reactive solid-solution interface and reveals how dissolution phenomena can scale across time and space.