The Electronic and Atomic Structure of Actinide Contaminants at the Mineral-Fluid Interface

Abstract

Geochemical reactions at the mineral-water interface are a critical factor in the mobility of actinide contaminants such as those intrinsic to nuclear waste storage facilities. Actinides can undergo a number of sorption, phase, and oxidation state changes that control their interactions with the surrounding environment. This dissertation investigates the redox behavior, sorption, thermodynamics, and kinetics of actinides at a near-atomistic scale in order to further the understanding of actinide chemistry in the environment. Novel and combined multi-method approaches utilizing experimentation and atomistic modeling were developed to achieve a detailed understanding of both naturally-occurring and synthetic actinides, specifically uranium (U) and plutonium (Pu), and the mechanisms that may lead to their immobilization at the mineral-fluid interface. In reducing conditions, soluble uranyl(VI) (UO22+) can be reduced to insoluble U(IV)O2 solid, resulting in the decrease of its mobility within the environment. Chapter 2 is an electrochemical investigation of U(VI) redox interactions in a relatively uncharacterized synthetic uranyl peroxide material called uranium-60 nanoclusters (U60) and their natural analog, the mineral studtite. Results indicate a two-step, one-electron irreversible reduction of U(VI) to U(IV) resulting in the fragmentation of the U60 cluster and the studtite crystalline structure. Utilizing a combined approach with spectroscopic and computational methods, electrochemical redox responses were assigned to specific or concurrent reactions, possibly indicating the existence of an uncommon U(V) superoxo intermediate phase within U60 clusters. Actinide contaminants such as uranyl peroxides can also be immobilized via redox or adsorption reactions catalyzed by mineral surfaces. Therefore Chapter 3 uses electrochemical atomic force microscopy (EC-AFM) in conjunction with spectroscopic methods to image redox reactions and sorbates at the mineral-fluid interface directly. Results indicate the growth of U60 nanostructures adsorbed to mineral surfaces in either face-centered cubic (FCC) crystalline or composite clusters, the fragmentation of U60 upon reduction, and the observation of a fibrous nanoparticle that could be completely uncharacterized in the literature. Actinides exceptionally mobile in aqueous conditions, such as pentavalent plutonyl (PuO2+), can be immobilized without a change in oxidation state via adsorption or incorporation into mineral hosts. Thus, Chapter 4 is a theoretical investigation of incorporation of plutonyl [Pu(V)] ions into carbonates and sulfates; minerals ubiquitous in subsurface environments. This chapter describes new ab initio modeling methodology using quantum mechanics to calculate equilibrium energetics, hydration, and thermodynamics to assess structural and electronic changes in the host mineral. Results suggest that barite group and aragonite mineral structures with orthorhombic symmetry, high coordination number, and large ionic radii have greater interstitial cell space and are therefore more favorable for PuO2+ incorporation than other host minerals. The contribution to existing incorporation methodology is the consideration of mineral surfaces. Findings from this study have substantial implications for the long-term sequestration of mobile actinide contaminants.