UC Davis

Iron oxides may reside in an abundance of environments where their chemical and physical properties may assist in combating repercussions of human industrialization. Uranium species, such as from leaked radiological waste repositories or from ore mine tailings, may easily mobilize and spread throughout their immediate aqueous environments. Readily present iron oxide nanoparticles may favorably adsorb these radiotoxic contaminants and limit the extent by which the actinide may spread by. They may further inhibit their mobility by trapping the atoms within their structures – a mechanism which we utilize and offer as a potential filtration and processing method. This dissertation aims to address the lack of thermodynamic data for uranium-incorporated iron oxide polymorphs. Results from this work may assist in understanding the fate of uranium in geological disposal sites over extended timescales and the transport of uranium in the environment. These insights will help guide the improvement of safe and scalable radioactive waste practices and water remediation applications, which ultimately improve water safety for humans and surrounding ecosystems. Chapter 2 discusses the role that uranium incorporation plays on the structure and thermodynamic stability of hematite. The synthesis method for these samples addresses challenges present in existing literature, namely high uranium concentrations and largely reduced reaction times. Formation enthalpies relative to constituent U and Fe oxides become increasingly positive with increasing uranium content. Based on structural measurements and calculations, the described coprecipitation and accelerated hydrothermal treatment process produce UxFe2-2xO3 analogous to conventionally aged samples studied in previous literature. Chapter 3 expands this work to explore U-Fe interactions in conditions where goethite, rather than hematite, is the energetically favorable polymorph. This chapter provides an alternative synthesis method for uranium incorporation into goethite, using strictly Fe(III) solutions as opposed to conventional mixtures of Fe(II) and Fe(III). Thermodynamic and structural studies were then employed to discern the stability of uranium incorporation, as well as the mechanism by which this occurs under previously unexplored aerobic synthesis conditions. Chapter 4 provides supplemental work by exploring kinetics and thermodynamics of aging synthetic ferrihydrite colloids. These metastable, poorly ordered nanoparticles gradually crystallize and transform into hematite. Measured calorimetric data were analyzed with existing data on ferrihydrites and nanosized goethite. Results highlight the time frame in which the formation enthalpy of 2-line ferrihydrite sharply drops, possibly indicative of the amorphous colloids beginning to crystallize and stabilize. Continued work may validate this hypothesis and elucidate the local atomistic behavior during this structural transition. Chapter 5 concludes with currenting findings, as well as future directions that this work may be taken to. Appendix A provides supporting information for uranium-incorporated hematite experiments. Appendix B provides supporting information for uranium-incorporated goethite experiments.