Vibrational Spectroscopic Investigations of Sulfate Behavior at Environmental Interfaces.

摘要

Chemical interactions that occur at the interface between two bulk isotropic media are vital to understand as these interactions dictate many fundamentally important phenomena influencing life on Earth as we know it. This dissertation presents investigations focused on elucidating behavior for the simple inorganic anion sulfate at a variety of environmentally relevant interfacial regimes, primarily mineral/solution interfaces. Sulfate is a ubiquitous anion in the environment that plays a key role in various physical and chemical phenomena. With perspective toward atmospheric chemistry, sulfate is a large component of aqueous acidic aerosols and has been shown to influence the reactivity and growth of these aerosols. Sulfate coatings have also been linked to the retardation of ice nucleation by mineral dust aerosols. Sulfate is also highly relevant to geochemical systems where it can inhibit, or encourage, aqueous contaminant uptake onto soil adsorption sites. This may lead to added complications when attempting to predict the spatial and temporal transport of aqueous contaminants within the environment.;Work presented in this dissertation utilizes two different vibrational spectroscopic methods to probe sulfate behavior at the interfaces of interest. Chapter 2 presents studies harnessing vibrational sum frequency generation spectroscopy, an inherently interface specific spectroscopic technique, to investigate sulfate at the vapor/solution and the buried fluorite/solution interfaces. Results for the vapor/solution interface indicate that protonated sulfate, bisulfate (HSO4-), features perturbed hydration compared to bulk solvated bisulfate. The presence of sodium (Na +) and magnesium (Mg2+) cations are found to disrupt bisulfate hydration to a greater extent within vapor/solution interfaces. The highly disruptive nature of Mg2+ is attributed to it's strongly solvated nature. The adsorption of sulfate to a mineral, fluorite, surface is observed in Chapter 2, as well. This adsorption is found to proceed with a bidentate inner-sphere structure preferred. The surface free energy of adsorption is calculated from the vibrational sum frequency results with a simple Langmuir adsorption model and is found to be -31 +/- 3 kJ/mole.;Chapter 3 presents a procedure to generate and characterize thin films of three iron oxide polymorphs: hematite, maghemite, and magnetite. It was necessary to develop this procedure as accessing buried interfaces with optical spectroscopic techniques requires the transmission of the probe light through one of the interfacial media, generally the solid mineral phase. Due to the highly absorptive nature of colored oxide minerals, such as iron oxides, transmission of the probe light to the interface of interest is not possible when using a bulk sample. This obstacle is overcome through the use of thin films allowing for the study of sulfate at iron oxide/solution interfaces.;Chapter 4 presents results using total internal reflection Raman spectroscopy to investigate sulfate behavior at silica/solution and hematite/solution interfaces. Total internal reflection Raman is an interface selective spectroscopy which generally probes only the first 100 nm of an interface. These results indicate that sulfate forms ternary adsorption structures as a function of cation identity at silica/solution interfaces. For buried hematite/solution interfaces sulfate is suggested to adsorb primarily in a bidentate inner-sphere manner at hematite/solution interfaces, in direct contrast to literature results.