Iron(III) hydroxides are abundant in near-surface natural environments and play an important role in
geochemical processes and the fate of contaminants. The issue of the structure of the common nanophase
material ferrihydrite (ferric hydroxide) is controversial and has been debated in the literature for many
years without definitive resolution. At least two types of ferrihydrite, the 2-line and 6-line forms, are
conventionally recognized. It has been suggested that these forms possess different structures built up by
different mixtures of distinct nanophase components. However, traditional crystallographic methods provide
depictions of structure that are most sensitive either to short-range order (X-ray absorption) or long-range periodicity (X-ray diffraction or electron diffraction). We used high-energy X-ray total scattering
for pair distribution function analysis to observe both the short- and intermediate-range ordering (exceeding
∼15 Å) of synthetic ferrihydrite with three distinct average domain sizes of 2, 3, and 6 nm. We show
that there are no significant differences in the underlying structures of these materials and that the
differences in the diffraction patterns can be entirely interpreted by variations in the average size of the
coherent scattering domains. The average crystallite sizes inferred from the PDF analysis are in good
agreement with direct observation by high-resolution transmission electron microscopy.
Decontamination of metal surfaces contaminated with low levels of radionuclides is a major concern at Department of Energy facilities. The development of an environmentally friendly and cost-effective decontamination process requires an understanding of their association with the corroding surfaces. We investigated the association of uranium with the amorphous and crystalline forms of iron oxides commonly formed on corroding steel surfaces. Uranium was incorporated with the oxide by addition during the formation of ferrihydrite, goethite, green rust II, lepidocrocite, maghemite, and magnetite. X-ray diffraction confirmed the mineralogical form of the oxide. EXAFS analysis at the U L III edge showed that uranium was present in hexavalent form as a uranyl oxyhydroxide species with goethite, maghemite, and magnetite and as a bidentate innersphere complex with ferrihydrite and lepidocrocite. Iron was present in the ferric form with ferrihydrite, goethite, lepidocrocite, and maghemite; whereas with magnetite and green rust II, both ferrous and ferric forms were present with characteristic ferrous:total iron ratios of 0.65 and 0.73, respectively. In the presence of the uranyl ion, green rust II was converted to magnetite with concomitant reduction of uranium to its tetravalent form. The rate and extent of uranium dissolution in dilute HCl depended on its association with the oxide: uranium present as oxyhydroxide species underwent rapid dissolution followed by a slow dissolution of iron; whereas uranium present as an inner-sphere complex with iron resulted in concomitant dissolution of the uranium and iron.
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