Nanoscale mechanism of UO2 formation through uranium reduction by magnetite

Significance:

Uranium (U) biogeochemical behavior is constrained by redox transformations. In anoxic environments, soluble hexavalent U is reduced and immobilized as tetravalent U. During abiotic U reduction, the formation of tetravalent U oxide (UO2) has been demonstrated and the persistence of an intermediate (pentavalent) valence state invoked. However, despite decades of study, there is little insight into the molecular mechanistic details of UO2 formation. Here, we show the formation of transient nanowires composed of randomly oriented UO2 nanoparticles followed by rearrangement into ordered UO2 nanoclusters. We also evidence the persistence of pentavalent U on the magnetite surface. These findings have implications for uranium isotopic fractionation, nuclear waste, and uranium remediation.

Abstract:

Uranium (U) is a ubiquitous element, present in the Earth’s crust at ~2 ppm. In anoxic environments, soluble hexavalent uranium (U(VI)) is reduced and immobilized. The underlying reduction mechanism is unknown but is likely of critical importance to explain variability in isotopic fractionation depending on the reducing agent and the chemical conditions. Here, we tackle the mechanism of reduction of U(VI) by the mixed-valence iron oxide, magnetite (Fe3O4). Through a combination of high-end spectroscopic and microscopic tools, we demonstrate that the reduction of U(VI) proceeds first through surface-associated U(VI) to form pentavalent U, U(V). U(V) persists on the surface of magnetite and is further reduced to tetravalent UO2 in the form of nanocrystals (~1-2 nm) arranged at random orientations in nanowires that extend hundreds of nanometers from the magnetite surface. Through re-orientation of the nanoparticles and their coalescence into larger nanoparticles, the nanowires collapse after several weeks to generate ordered UO2 nanoclusters. Thus, this work provides evidence for a transient U nanowire structure that may have implications for uranium isotope fractionation as well as for molecular-scale understanding of nuclear waste temporal evolution and the reductive remediation of uranium contamination.