Expanding Actinyl Peroxide Chemistry through Dissolution of Uranium-Based Materials and Studies of Neptunyl Peroxide Compounds

A growing subset of actinide compounds comprised of actinide oxide clusters are of interest in the processing and environmental transport of actinides. The most numerous of these reported structures are known as actinyl peroxide clusters: the first uranyl and neptunyl peroxide clusters were discovered in 2005, and in the years since, more than 70 unique structures have been described. Uranium peroxide chemistry has long been studied, as the interaction between uranium and peroxide is responsible for the only known peroxide-containing minerals, studtite and metastudtite, and hydrogen peroxide found early use in processing of uranium. Since the emergence of actinyl peroxide clusters, studies have suggested potential roles of the clusters in actinide transport and applications to nuclear processes; however, more information is needed about the controls on cluster formation to accurately predict or control actinide speciation.

In this work, uranyl peroxide cluster chemistry is expanded by applying the synthesis conditions of uranyl peroxide clusters to the dissolution of uranium-based materials, uranium dioxide and uranium nitride. These studies demonstrate the significance of counter cation and hydrogen peroxide availability in directing uranyl peroxide speciation and limiting uranium loading in solutions.

Neptunyl peroxide chemistry is also explored. One neptunyl peroxide cluster was previously reported, but it was not extensively characterized and neptunyl peroxide cluster properties and chemistry remain unexplored. I investigate neptunyl peroxide chemistry with the synthesis of a novel neptunyl peroxide compound, the first to be exclusively composed of hexavalent neptunium. Additionally, new salts of the previously described neptunyl peroxide cluster are reported and characterized. These studies reveal interesting characteristics, such as ordering of Np(V) in specific sites on the cage in the solid state and lower solubility in water relative to uranyl peroxide clusters.