Congratulations to Sandy Taylor who successfully defended her dissertation on August 10, 2015.
Advisor: Udo Becker
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 Pu and 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.
Mechanisms involved in the formation of non-fcc PuO2-x structures sorbed on goethite (as observed in experimental studies by Powell et al., (2011)) are elucidated by molecular simulations on the Pu oxide-goethite interface. This study proposes that the observance of a non-fcc Pu oxide phase is due to the distortion of the fcc PuO2 lattice through combined structural changes caused by nanoparticulate properties and from sorption onto goethite.
The remainder of the dissertation 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 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. The influence of a mineral’s electronic properties on the redox rate is specifically probed. 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. With this insight, Marcus theory 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 energetic barriers may also need to be overcome during ET through a semiconducting surface via the proximity effect. Through these studies a better understanding of heterogeneous catalysis of redox reactions is obtained.