More than 60 years of nuclear power and weapons production in the United States has left this country with a large burden of radioactive legacy waste containing long-lived transactinide elements, as well as highly radioactive fission products. To face the challenge of contaminated site remediation, a solid understanding of the fundamental chemical processes behind the transport and migration of actinides in the environment is necessary. Especially important is the elucidation of interfacial reactions at the boundary between aqueous radionuclide solutions and minerals. This knowledge will allow us to more accurately model and predict the ultimate fate of radionuclides in the geosphere.
The work presented in this thesis focuses on understanding the binding mechanisms and structure of plutonium on the surface of metal oxide minerals. Plutonium(VI) sorption on the surface of well-characterized synthetic manganese-substituted goethite minerals (Fe1-xMnxOOH) was studied using X-ray absorption spectroscopy. Because manganese is often found in the environment as a minor component associated with major mineral components, such as goethite, understanding the molecular-level interactions of plutonium with such substituted-mineral phases is important for risk assessment purposes at radioactively contaminated sites and long-term underground radioactive waste repositories.
The first chapter of this thesis is an introduction and motivation to the work presented in the following chapters. The second chapter describes fundamental aqueous solution chemistry and covers the calculation of species in solution including methods for handling statistical errors. The third chapter provides an overview of the fundamental interactions that occur at the interface between aqueous solution and the surface of a mineral. The fourth chapter presents the methods and characterization techniques used in synthesizing goethite (α-FeOOH) and manganese-substituted goethite (Fe1-xMnxOOH) minerals. The fifth chapter presents an overview of the plutonium chemistry needed to conduct and understand the experiments. The sixth chapter presents a fundamental overview of X-ray absorption spectroscopy methods used in this study, including X-ray absorption near-edge structure spectroscopy, extended X-ray absorption fine-structure spectroscopy, micro X-ray absorption spectroscopy, and synchrotron X-ray fluorescence spectroscopy. The seventh chapter presents the data and discussion of experiments with plutonium sorbed on the surface of manganese-substituted goethite minerals. Work relating to the modeling of stochastic noise in extended X-ray absorption fine-structure spectra and synchrotron X-ray beam-induced chemistry on plutonium are also presented. The eighth chapter gives an overview of the work and offers suggestions for future work on this topic. The appendices at the end of this thesis are a collection of methods, instructions, suggestions, and computer code that was helpful when conducting experiments.