Date of Award

5-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Committee Member

Dr. Christopher L. Kitchens, Committee Chair

Committee Member

Dr. Stephen J. Klaine

Committee Member

Dr. Julia Brumaghim

Committee Member

Dr. Elizabeth Carraway

Committee Member

Dr. Aaron Roberts

Abstract

Interest in inclusion of titanium dioxide nanoparticles in a multitude of industrial and personal products has driven production over the past two decades. Concurrent with increases in nanoparticle production, an increase in nanoparticle movement from use to environment can be expected. Particular concern is focused on TiO2 nanoparticles moving to freshwater compartments. Inherent photocatalytic nanoparticle properties generate reactive oxygen species upon exposure to water, oxygen, and ultraviolet light. While this particular feature is utilized for surface-cleaning and pollution mitigating applications, it poses a significant risk to organisms exposed to these nanoparticles. This risk can be difficult to quantify, exhibited by the variation in toxicity reports from various labs. These variations are a result of differing conditions. Environmental factors such as presence of natural organic matter (NOM), intensity of ultraviolet (UV) light, and the wavelengths of UV light exposure will affect toxicity as well as physical characteristics of the nanoparticle, including size and crystallinity. These variations impart uncertainty to toxicity measurements creating a knowledge gap regarding conditional effects acting on TiO2 to modulate toxicity. The goal of the present research was to develop a comprehensive understanding of the effects that environmentally relevant conditions have on TiO2 radical generation and correlate these conditionally affected rate changes to toxicity measurements. To accomplish these goals, a systematic approach of a full factorial exposure design to quantify the interacting effects of simulated environmental conditions on irradiated TiO2 nanoparticles at eight TiO2 concentrations, five NOM concentrations (measured as dissolved organic carbon), and four UV-A intensities was utilized. Radicals generated by irradiated TiO2 were characterized as hydroxyl radicals using Electron Paramagnetic Resonance spectroscopy. The exposure conditions were characterized and compared to existing literature and natural conditions. The changes in hydroxyl radical generation rates were monitored using fluorescence spectroscopy. Linear regression techniques were used to determine how the conditional effects regulated hydroxyl generation rate. A number of trends were well correlated with conditions. Rate of hydroxyl generation was positively correlated with concentration of TiO2 nanoparticles as a result of increased total available surfaces for photon impingement. Increases in light intensity were likewise positively correlated to increases in hydroxyl generation rate, a result of a greater number of photons interacting with the nanoparticle surface. The reciprocal interaction of these conditions demonstrates classic phototoxic behavior wherein a low concentration of TiO2 and a high intensity of UV-A generates an equivalent response compared to high concentrations of TiO2 and low intensities of UV-A. This reciprocal effect is complicated by the addition of NOM. Increasing total amounts of NOM to suspensions results in decreased hydroxyl generation rate. Decreased generation rates are related to the large number of oxidizable functionalities that exist within the NOM conglomeration of molecules. Readily available functionalities can competitively quench radicals, resulting in the attenuated hydroxyl generation rate measurements. Additional rate reduction as a result of coating effects and aggregation/agglomeration to reduce available surface area may also occur.

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