Uranium contamination in soils is one of the most complex and intractable problems in environmental geochemistry. Remediation of U contamination requires understanding of solid-phase sources and sinks for U. Investigation of soil samples is difficult because many U hosts are fine-grained, sparsely distributed, and intergrown with other phases.

Samples of U-contaminated soils from U.S. Department of Energy sites at Oak Ridge, TN, and Hanford, WA, were investigated using electron microscopy. Uranium hosts identified by electron microprobe analysis (EMPA) were prepared using a focused ion beam (FIB) instrument for more detailed characterization in the transmission electron microscope (TEM). Three hosts were identified at Oak Ridge: (1) iron oxyhydroxides; (2) mixed Mn-Fe oxides; and (3) Al-U-phosphates, likely of the meta-autunite group. Six hosts were identified at Hanford: (1) metatorbernite, a meta-autunite-group mineral; (2) coatings on sediment clasts; (3) amorphous zirconium (oxyhydr)oxide; (4) void linings in basalt; (5) palagonite; and (6) Fe- and Mn-oxides. These findings are testimony to the complexity of U geochemistry at Oak Ridge and Hanford, illustrate the importance of microscopic features in controlling the fate of contaminants, and illuminate the role meta-autunite-group minerals play in U uptake in oxidizing environments.

The meta-autunites [M(UO₂)₂(PO₄) ₂·nH₂O, M=cation, n variable] are sheet-structured and hydrous, containing H₂O and cations in their interlayer regions. Understanding their structures under various physical and chemical conditions is essential to phase identification and prediction of the fate of contaminant uranium. While numerous studies have investigated their structures and dehydration behavior, none have completely described the structures of the dehydrated variants. It has been generally assumed that dehydration entails collapse of the spacing between the sheets, and that the sheets remain otherwise intact.

A major reconstruction of the sheets was discovered using Rietveld refinement and synchrotron x-ray diffraction from heated, powdered metatorbernite. Although the autunite-type sheets remain intact through the first dehydration event (102°C), the second (138°C) triggers a transformation to uranophane-type sheets. This transformation enables the structure to overcome steric constraints on basal spacing while maintaining Cu within the interlayer. These results provide fundamental insights into the crystal chemistry and structural behaviors of the meta-autunite group.