Intellectual Merit: Aqueous complexation with organic ligands strongly influences the chemical speciation of U(VI) in nature, and under anoxic conditions is likely to also affect the distribution of U(IV) and the stability of U(IV) mineral phases, such as UO2. It is difficult to quantitatively evaluate the importance of organic complexation on UO2 stability, because there are few available thermodynamic values for potentially important U(IV)-organic complexation reactions. UO2 is the predominant ore mineral for uranium, the most common form of U in nuclear reactor fuel and spent nuclear fuel, a widely-used component in military ballistic technology, and a potential catalyst for removing sulfur contaminants from petroleum. Thus, understanding the geochemical stability of U(IV) oxides is a significant research concern.
In the proposed study, the PI will investigate the solubility of synthetic amorphous U(IV) oxide (UO2(am)) in the presence of citric acid as a U(IV)-complexing ligand, which was selected as a model tricarboxylic organic acid encountered in natural sediments as a result of in situ fungal and microbial activity. Solubility data will be obtained primarily from initially U(IV) undersaturated (UO2(am) dissolution) experimental systems, as a function of pH, ligand activity and ionic strength (NaCl). Solubility data for U(IV) in the presence of dissolved citrate will be used to regress optimal model-dependent values for stoichiometric and equilibrium stability constants for U(IV)-organic complexation reactions. Reversibility of complexation-related solubility relations will be tested by conducting U(IV) oxide precipitation experiments (from initially supersaturated conditions) in the presence of citrate.
Broader Impacts: The proposed study will develop estimates for the properties of U(IV)-citrate complexation reactions, based on U(IV) solubility in the presence of citrate as a chelating agent. Uranium is an important trace geochemical indicator in sedimentary, metamorphic and igneous rocks, and its distribution in hydrated systems is controlled by both complexation and redox processes. UO2 is an important strategic ore mineral and the primary constituent of nuclear reactor fuel and spent fuel waste. For these reasons it is necessary to obtain accurate thermodynamic values describing the major reactions governing U geochemistry. The experimental data obtained from this proposed investigation will improve understanding of the geochemical properties of U(IV) under reducing conditions in ground waters.
Western Michigan University (WMU) is a student-centered research university. WMU and the PI are committed to promoting a productive and meaningful graduate and undergraduate research environment, where student investigations provide a sound footing for future professional advancement. The PI has a strong and productive record of student research mentoring, which will be further promoted through the undergraduate and graduate support requested here. WMU encourages the enrollment and success of minorities and students from under-represented groups, and this project will actively engage the university in recruiting minority and female students to this project. The educational opportunities engendered through this project will help to advance productive and creative student research and training activities, enriching and advancing the development of young earth science professionals.
