Doctor of Philosophy (PhD)
Environmentally persistent free radicals (EPFRs) have recently attracted attention due to their properties that detrimental to the health of living organisms. Earlier studies devoted to the formation of combustion generated particulate matter (PM) showed that EPFRs formed as a result of the association of aromatic chlorinated hydrocarbons with transition metal centers. Based on our previous studies, the amounts of EPFRs formed at Superfund soil sites contaminated with pentachlorophenol (PCP) are ~30 times higher compared to those formed at the neighboring uncontaminated soil sites, showing that EPFR formation is not confined to combustion generated particle. In order to design viable remediation procedures and technologies to minimize EPFR formation, it is important to understand the active constituents of these organic radicals and the mechanism(s) of their formation. This is a complicated task due to the fact that soil is a complex environmental matrix that can be described as a tri-component system consisting of the clay/mineral, organic, and the biological components. Our previous studies on EPFR-contaminated Superfund soil sites utilized a “top-down” approach to reduce the soil matrix into its constituting components in order to determine which soil component was associated with the majority of the formed EPFRs. However, no real mechanistic information was obtained from these studies. To overcome this limitation, the work presented in this dissertation was carried out using a “bottom-up” approach, which utilized surrogate systems to individually investigate the component(s) of the soil which are involved in the formation and stabilization of EPFRs in the Superfund soil. The first part of this work, as contained in chapter 2, utilized Fe(III)-loaded montmorillonite as a surrogate system the clay/mineral component of the soil and phenol as the organic pollutant to mimic the polluted, EPFR-containing Superfund soil. This approach allowed, for the first time, for a molecular level understanding of the mechanism of EPFR formation occurring in the clay/mineral component system of the soil and was accomplished by a combination of FT-IR, EPR, ICP, XRD, XPS, and XANES analyses It revealed that, once sorbed, the phenol transfers an electron to Fe(III), forming Fe(II) and an organic EPFR. In chapter 3, the work was extended to a bi-component clay/humin surrogate soil system, utilizing Cu(II)-loaded as well as poly-p-phenylene (PPP)-grafted Cu(II)-loaded montmorillonite clay surrogate soil systems. Both individually and in combination, the effects of the make-up of the surrogate soil system on the EPFR formation were compared in terms of EPFR concentrations, lifetimes, and their potential to generate reactive oxygen species (ROS). These studies indicated that the electron-rich aromatic backbone in the model surrogate organic component (PPP) utilize π-π stacking to prolong lifetimes of EPFRs formed in the bi-component system of PPP-Cu(II)-loaded montmorillonite clay system when compared to the single component Cu(II)-loaded clay system. Also, following the higher EPFR concentrations in the PPP-Cu(II)-loaded montmorillonite clay system, ROS generation potential is also higher in the bi-component systems, suggesting a synergistic affect. The effects of soil pH and other environmental conditions, such as humidity and solar irradiation, on EPFR formation were probed in chapter 4 of this dissertation. This work shows that EPFR formation is very dependent on the soil pH conditions for both Cu(II) and Fe(III)-loaded montmorillonite clays, with alkaline pHs being the most conducive to EPFR formation. The persistence of EPFR was shown to be greatly reduced by high humidity, while UV irradiations were seen to accelerate EPFRs formation. Finally, insight into the naturally (non-pollution) occurring EPFRs was obtained by comparing three different natural organic matter (NOM) samples sourced from the Humic Substance Society (IHSS), and revealed that NOM contains EPFRs with very similar characteristics to those detected in Superfund samples, but at much lower levels. This research reveals that EPFRs are ubiquitous in the environment
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Nwosu, Ugwumsinachi Godwin, "An Investigation of Environmentally Persistent Free Radical (EPFR) Formation in Contaminated Soils Using Surrogate-Based Model Systems" (2016). LSU Doctoral Dissertations. 4229.
Cook, Robert L.