Quantum-Mechanical Evaluation of the Thermodynamics and Kinetics of Environmentally-Relevant Actinide Reactions

Abstract

The mobility of the early actinide (An) elements uranium, neptunium, and plutonium is dependent on a variety of different chemical processes. Many of these processes, including complex formation, adsorption to mineral surfaces, (co-)precipitation of mineral phases, and redox, are controlled by interactions at the atomic and molecular scale. Understanding the chemistry and behavior of the U, Np, and Pu at this fundamental level is critical to making informed decisions about the long-term geologic storage of spent nuclear fuel and remediation of contaminated sites. This dissertation applies quantum-mechanical modeling to study the thermodynamics, kinetics, and mechanisms of some of the aforementioned processes.

Structural incorporation of actinides into growing mineral phases is a potential pathway for immobilization. Chapter 2 of this dissertation explores the incorporation of U and Pu into magnetite (Fe3O4), a corrosion product of steel. Actinide incorporation, from solid and aqueous sources with different oxidation states (Pu3+ and An4+/5+/6+), is explored using a multi-step computational approach. We find U and Pu assume the An5+ oxidation state when incorporated into the lattice via a coupled substitution mechanism. The atomic and electronic structures align with available data for synthetic U-incorporated magnetite and we present the first descriptions of Pu-incorporated phases. Comparable reaction energetics of Pu and U incorporation, preferentially from An4+ and An5+ sources, suggest magnetite may be an important sink for both elements in near-field environments of repositories.

Dissolved actinides, present in solution as actinyl molecules (AnV,VIO2x+(aq), x = 1 or 2), may also react with Fe2+ and other redox-active species in a homogenous manner without the involvement of a mineral phase. Chapter 3 presents a new computational approach that combines collision theory and quantum-mechanical calculation to determine the thermodynamics and kinetics of discrete reaction sub-processes: (1) the collision of dissolved species to form an outer-sphere complex, (2) the transition from outer- to inner-sphere complex, and (3) electron transfer. For the reactions of plutonyl hydrolysis complexes with Fe2+, Fe3+, and hydroxyl radical, energetically-favorable outer-sphere complexes are found to form rapidly. Subsequent conversion to the inner-sphere complex configuration is achieved after overcoming activation energy barriers which are mainly related to dehydration and reorganization of coordinating solvent molecules (H2O and OH-) as An-reactant distances are decreased. Inner-sphere complex formation is coincident with reduction of Pu6+ by Fe2+. The rate-limiting step for the tested reactions is predicted to be electron transfer given the favorable kinetics of the preceding sub-processes.

This computational approach is extended in Chapter 4 to study the reactions of uranyl, neptunyl, and plutonyl tricarbonate complexes with Fe2+ and H2S. These common, stable complexes are known to inhibit actinide reduction in natural environments. The calculated kinetic parameters are in line with this understanding and in some cases, significant activation energy barriers are observed for the formation of inner-sphere complexes which necessitate disruption of the carbonate coordination environment. Spontaneous one-electron reduction of An6+ and An5+ is not observed and proton transfer, tested manually, from water and/or bicarbonate is necessary to induce reduction of An6+ to An5+. These results suggest that the tricarbonate complex effectively shields actinyls from reduction and will maintain their solubility in environments with alkaline pH and sufficient aqueous carbonate. The method used in Chapters 3 and 4 can now be applied to other reactant pairs and also, with some modification, reactions catalyzed by mineral surfaces.