Date of Degree

6-2020

Document Type

Dissertation

Degree Name

Ph.D.

Program

Chemistry

Advisor

Lynn Francesconi

Advisor

Benjamin Burton-Pye

Committee Members

Michele Vittadello

Charles Drain

Subject Categories

Chemistry | Inorganic Chemistry | Materials Chemistry | Radiochemistry

Keywords

graphene oxide, titanium dioxide, photoreduction, Keggin

Abstract

Research into the sequestration of radionuclides from aqueous media is being conducted due to the contamination of aqueous waste effluents and groundwater with highly toxic long-lived radionuclides. These contaminants are introduced into water and streams from legacy nuclear sites, nuclear reactor operations, nuclear fuel reprocessing, nuclear weapons testing, and plutonium production. A major contributor (~6%) to the waste from thermal neutron fission of uranium-235 is the radionuclide technetium-99 (99Tc). It is considered a long-lived radioisotope with a half-life of 2.1x105 years. It is a weak beta emitter with a max energy of 0.29 MeV and is most prevalent in its oxidized form, which predominantly exists as pertechnetate (Tc(VII)O4-). This contaminant is of concern in aqueous streams because of its inert nature that allows it to spread at nearly the same rate of water flow. To mediate this feature, reducing 99Tc has been the common approach because it is more reactive and less mobile when in a reduced state. The foundation of this work involves utilizing various materials to investigate the reducing and binding potentials of different functionalized surfaces. The breadth of this work includes the nanomaterials of titanium dioxide, graphene oxide, a nanocomposite of those two materials, as well as a variety of polyoxometalates (POMs). Titanium dioxide is of great interest because it has already shown to be a good material for removing toxic materials from the environment. Furthermore, it has a strong reduction potential and is found to be chemically inert in a variety of environments. Graphene oxide (GO) is an eco-friendly platform with a large surface area that provides tunability due to its functionalization. It is speculated that the combination of these two materials will allow for robust reduction in aqueous solutions. By utilizing a sol-gel method, a titanium dioxide-graphene oxide composite (TGO) was made. Polyoxometalates (POMs) are inorganic metallic compounds. They typically have at least three transition metal oxyanions linked together by oxygen atoms that together make up a three-dimensional structure. Due to their multi-electron reducing capabilities, POMs are studied to not only facilitate reduction of 99Tc, but also for their ability to bind to metals and possibly sequester 99Tc.

The work contained herein involves several projects that all involve reducing 99Tc(VII) either in solution, onto the surface of a substrate, or incorporating it into a structure. The first two projects involve studying the incorporation of 99Tc into soluble metal oxides to gain molecular level understanding of the redox coordination of 99Tc. Both of these projects utilized POMs, more specifically the Keggin structure. The first Keggin POM project identified a novel 99Tc-substituted structure that could potentially be used to help expand standard reference data for 99Tc compounds in a variety of oxidation states. The second POM project examined strategies to elucidate reduction capabilities of various Keggin ions under an array of conditions, including photocatalysis and bulk electrolysis. The last project utilized nanostructures, mainly titanium dioxide (TiO2), graphene oxide (GO), and nanocomposites thereof, as platforms to investigate the reducing and binding potentials for 99Tc from solutions. As a complete body of work, these projects help to expand the known reduction chemistry of 99Tc and will help other researchers to further the study of this very interesting radioelement.

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