Determination of Thermodynamic Properties of Select Uranyl Compounds

The continued environmental, technological, and industrial importance of actinide materials sustains the need for understanding their behavior from the nanoscale to macroscale. Understanding the physical and chemical properties of actinides is a necessary step towards predicting their long- and short-term behavior in different environments. Determination of thermodynamic properties of uranium minerals and associated compounds is fundamental for understanding the alteration pathways of uranium in geological systems, predicting transport of uranium in the environment, and ultimately developing a geologic repository for spent nuclear fuel in the US. Towards this end, I focus on experimentally determining the thermodynamic properties of uranyl compounds in this research.

The work described in this dissertation explores two major topics in actinide geochemistry. The first theme is focused on determination of standard-state thermodynamic properties of the zippeite-group minerals from the uranyl sulfate family that form on uranium mine wastes and that may be important in nuclear waste disposal. The result of the experimental thermochemistry of four members of the zippeite group with zippeite-type uranyl sulfate sheets and different interlayer cations is presented in this work. Pure zippeite, natrozippeite, cobaltzippeite, and zinczippeite mineral analogs were hydrothermally synthesized and thoroughly characterized using X-ray diffraction and analytical chemistry methods (ICP-OES, TGA). The standard-state enthalpy of formation of each sample was determined using high-temperature calorimetry. Solubility experiments were carried out for zippeite to measure it solubility product and hence calculate its standard state Gibbs free energy of formation. Calorimetric data revealed that there is a linear relationship between the formation enthalpies from oxides and the acidity of cation oxides, as well as the ionic radius of charge-balancing alkalis, indicating the importance of the nature and coordination environment of the interstitial complex in determining the thermodynamic properties of these minerals.

In the second component of this work, I studied the chemical thermodynamics of selected members of the family of nanoscale uranyl peroxide cage clusters. Uranyl nanoclusters, that are proposed to be energetic intermediates between dissolved aqueous uranyl species and uranyl minerals, have potential importance in an advanced nuclear fuel cycle and environmental transport of actinides following nuclear accidents. Their structures are composed of uranyl polyhedra bridged by bidentate peroxide, and other types of bridges in addition to peroxide ligands in many cases, that are charge-balanced by metal cations. Six members of this family were selected, synthesized under ambient conditions, and well-characterized using single crystal X-ray diffraction, ICP-OES, ESI-MS, and TGA. Our calorimetric data shows that the enthalpies of formation of the cluster compounds from oxides become more negative as the charge on the cluster increases. The results obtained from this work demonstrate that the energetics of uranyl peroxide cluster crystals are largely driven by the alkali cation oxide thermodynamics, rather than negatively charged uranyl cages.

The data obtained from this work proposes trends between enthalpies of formation of select uranyl phases and their chemical properties related to their crystal structure. The findings may be used to predict the energetics of other members of that family, or on a larger scale to better understand and improve the nuclear fuel cycle.