Adsorption and Reduction of Actinides on Iron and Aluminum (oxyhydr)oxides.

Abstract

Mineral-water interface geochemistry plays a critical role in the understanding the integrity of underground geologic repositories where nuclear waste will be disposed of. This dissertation seeks to provide a fundamental understanding of how sorption and/or redox processes at mineral surfaces influence the mobility of actinides, specifically plutonium (Pu) and uranium (U). Unique and novel approaches combining experiments and atomistic modeling were utilized to make detailed studies on the structure, thermodynamics, kinetics, and reaction mechanisms between actinide/metal complexes and mineral surfaces.

In Chapter 1, a series of computational simulations are used to explain the how substrates can strain the lattice of fcc PuO2, in turn leading to the formation of non-fcc Pu nanocolloids on the surface of goethite. The remainder of the dissertation (Chapters 2-5) investigates synergistic effects between sorption and/or redox processes and mineral surfaces in controlling the mobility of U. First, the reduction U(VI)aq by Fe(II)aq is not observed in the absence of a solid substrate (at neutral pH, anoxic conditions) using batch experiments. Ab initio calculations coupled with Marcus Theory (MT) complement experimental observations, showing that electron transfer (ET) from Fe(II)aq to U(VI)aq is inhibited by high energetics associated with the dehydration and inner-sphere complexation of Fe and U. Heterogeneous catalysis of U(VI) reduction by Fe(II) in the presence of Fe and Al (oxyhydr)oxide minerals is also studied using batch experiments and ab initio models. These experiments specifically probe how a mineral’s electronic properties affect the redox rate. U(VI) reduction by Fe(II) is measured to be ten times faster in the presence of semiconducting Fe(oxyhydr)oxides compared to their insulating Al isostructures using batch experiments. Models demonstrate that the enhanced catalytic abilities on semiconducting mineral surfaces are potentially heavily influenced by the proximity effect, where a semiconducting surface transports electrons between adsorbed electron donors and acceptors. MT was applied to describe the kinetics of mineral-catalyzed redox reactions in ternary, coadsorbed systems for the first time. In particular, it is found that interfacial and surficial ET reactions in hematite may possibly be energetically limiting steps for ET through a semiconducting surface to occur via the proximity effect.