Doctor of Philosophy (PhD)
Clay surrogates containing single redox-active transition metal centers are known to generate significant amounts of environmentally persistent free radicals (EPFRs), under controlled pressure conditions, in the gas phase, and at room temperature. To understand the role of transition metal oxide minerals in EPFR formation in real soil samples using model soils, it is important to consider multi-metal systems as a way of mimicking the complexity of natural systems. It is the fundamental basis upon which more complex soil mineral models can be developed and presents a step forward towards understanding the dynamics of EPFR generation and behavior under natural environmental conditions. To this end, EPFR formation on dual metal-loaded clay surrogates and their behavior in ambient and controlled conditions were investigated. Naturally sourced and synthetic montmorillonite clays were modified by cation exchange and exposed to phenol and catechol, in the gaseous phase and aqueous phases respectively. The EPFR-type and concentrations were characterized using electron paramagnetic spectroscopy (EPR) and the metal redox activity was probed using X-ray absorption spectroscopy (XAS). EPFR aging analyses under ambient conditions revealed three decay patterns: a rapid decay which then slows down with varying lifetimes as short as minutes and as long as months. The varying EPFR lifetimes of the synthetic clays as compared to the naturally sourced may be attributed to structural and cation content differences. EPFR aging further confirmed previous assertions about the role of oxygen as the main reactant as shown by the general EPFR g-factor trends which are indicative of predominantly oxygen-centered radicals. X-ray absorption near-edge (XANES) analysis of the EPFRs showed that both metals are reduced in the formation process, suggesting an electron transfer from the test organic precursors to the metal center of the EPFR complex; iron being largely reduced relative to copper. The toxicity potential of the EPFRs as relates to negative health implications was estimated based on reactive oxygen species generation. In an aqueous medium, dissolved O2 initiates the production of significant amounts of hydroxyl (•OH) radicals, even in the absence of H2O2.
Ampiah, Emmanuel, "Formation and Stability of Environmentally Persistent Free Radicals (EPFRs) in Dual-Metal Loaded Clay Surrogates" (2022). LSU Doctoral Dissertations. 5910.
Cook, Robert L.
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