Thesis (Ph. D.)--University of Missouri-Columbia, 2001. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file. Title from PDF of title page (University of Missouri--Columbia, viewed on January 21, 2010). Thesis advisor: Dr. Gary Solbrekken. Includes bibliographical references.

Thesis (Ph. D.)--University of Missouri-Columbia, 1998. / Typescript. Vita. Includes bibliographical references (leaves 153-156). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2001. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

Natural attenuation of trichloroethene (TCE) and technetium (99Tc) was studied for five consecutive seasons (from January 2002 to January 2003) in Little Bayou Creek. The stream receives ground water discharge from an aquifer contaminated by past waste disposal activities at the Paducah Gaseous Diffusion Plant (PGDP), a uranium enrichment facility near Paducah, Kentucky. Results from stream gaging, contaminant monitoring, tracer tests (with bromide, nitrate, rhodamine WT and propane) and simulation modeling indicate the TCE is naturally attenuated by volatilization and dilution, with volatilization rates related to the ambient temperature and surface discharge rate. The only apparent mechanism of 99Tc attenuation is dilution. Travel times of non-gaseous tracers were found to be similar and have highest values in October and lowest in June. It was also estimated from modeling that the transport of the solutes in the stream was mostly one-dimensional with insignificant secondary storage.

Thesis (Ph. D.)--University of Missouri-Columbia, 1996. / "m in ⁹⁹mTc on short title page is supercript." Typescript. Vita. Includes bibliographical references (leaves 131-136). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 1996. / "M in ⁹⁹mTc on short title page is supercript." Typescript. Vita. Includes bibliographical references (leaves 131-136). Also available on the Internet.

Thesis (Ph. D.)--Oregon State University, 2007. / Printout. Includes bibliographical references. Also available on the World Wide Web.

Made available in DSpace on 2014-10-09T12:43:19Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:57:44Z (GMT). No. of bitstreams: 1 06437.pdf: 4468034 bytes, checksum: 590a8fa00a608b9ee7764f05c4eca4d1 (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Subsurface contamination is common at nuclear sites and it is likely that radioactive wastes will be managed in the long-term via burial in a deep Geological Disposal Facility (GDF). The migration of radionuclides in the geosphere from such sites is a major societal concern. In particular, long-lived, redox-active radionuclides (in the case of this thesis: 99Tc and Np) can migrate over large distances due to their high solubility under oxic conditions. Bioremediation has been proposed as a mechanism to limit the migration of 99Tc and Np in the environment. Here, an electron donor is supplied to the subsurface and soluble Tc(VII) and Np(V) are reduced to poorly soluble Tc(IV) and Np(IV), respectively. Reduction occurs via direct microbial action (termed bioreduction) or through radionuclide reaction with the by-products of microbial metabolism (primarily Fe(II)). Given the ubiquity of microorganisms and Fe in the geosphere, similar reactions can be expected in the deep subsurface surrounding a GDF. Once reduced, the long-term stability of the Tc(IV) and Np(IV) phases will significantly impact migration rates. Oxidative dissolution of Tc(IV)- and Np(IV)-bearing solids has been demonstrated in the literature and can be pervasive, thus questioning the efficacy of bioreduction. However, these studies have been conducted over short time-scales and during a single period of oxidation. Given the long half-life of 99Tc and Np and the ephemeral nature of redox conditions in the subsurface, there is a need to better understand 99Tc and Np biogeochemistry during longer time-scales and across multiple redox cycles. In this thesis, microcosm experiments have been used to address this knowledge gap. Sediment and groundwater used in the microcosms were representative of the Sellafield Ltd. nuclear site. For Tc, three successive redox cycles (reduction followed by oxidation with O2) over 2 years, gradually reduced the extent of Tc remobilisation during oxidation, and molecular scale characterisation of solids revealed that sediment associated Tc was always present as Tc(IV). Further, over time sequential extractions and EXAFS revealed an increased significance of Tc-Fe bonding in the sediment at the expense of TcO2. Despite this, a small but significant fraction of Tc(IV) was also found to be stable in solution during the experiments and XAS and TEM analysis suggested this was Tc(IV) associated with magnetite colloids. In other experiments completed with higher concentrations of bioavailable Fe (added as ferrihydrite to sediments, and in pure culture experiments with Geobacter sulfurreducens), the significance of Tc-Fe bonding was again highlighted, and potential Tc(IV) incorporation into biogenic magnetite was also documented. In experiments with Np, virtually all of the Np(V) added to oxic groundwater was removed to the sediment commensurate with microbially mediated Fe(III) reduction. Further, in systems with elevated bioavailable Fe, Np removal from solution was more extensive. Taken together, the data for Tc and Np reveals critical links between redox-active radionuclides and Fe cycling in sediments over periods of years and across multiple redox cycles. Furthermore, these processes help to predict the long-term fate of radioactive contamination at the Sellafield Ltd. nuclear site and have implications for contaminated land worldwide.