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Instruments / Technical Area	Publication Details	Published	Actions
I20-Scanning-X-ray spectroscopy (XAS/XES)	Persistence of the isotopic signature of pentavalent uranium in magnetite Zezhen Pan , Yvonne Roebbert , Aaron Beck , Barbora Bartova , Tonya Vitova , Stefan Weyer , Rizlan Bernier-Latmani Environmental Science & Technology DOI: 10.1021/acs.est.1c06865 Diamond Proposal Number(s): [17472] Open Access Show Abstract ▼ Abstract: Uranium isotopic signatures can be harnessed to monitor the reductive remediation of subsurface contamination or to reconstruct paleo-redox environments. However, the mechanistic underpinnings of the isotope fractionation associated with U reduction remain poorly understood. Here, we present a coprecipitation study, in which hexavalent U (U(VI)) was reduced during the synthesis of magnetite and pentavalent U (U(V)) was the dominant species. The measured δ238U values for unreduced U(VI) (∼−1.0‰), incorporated U (96 ± 2% U(V), ∼−0.1‰), and extracted surface U (mostly U(IV), ∼0.3‰) suggested the preferential accumulation of the heavy isotope in reduced species. Upon exposure of the U-magnetite coprecipitate to air, U(V) was partially reoxidized to U(VI) with no significant change in the δ238U value. In contrast, anoxic amendment of a heavy isotope-doped U(VI) solution resulted in an increase in the δ238U of the incorporated U species over time, suggesting an exchange between incorporated and surface/aqueous U. Overall, the results support the presence of persistent U(V) with a light isotope signature and suggest that the mineral dynamics of iron oxides may allow overprinting of the isotopic signature of incorporated U species. This work furthers the understanding of the isotope fractionation of U associated with iron oxides in both modern and paleo-environments.	Jan 2022
B18-Core EXAFS I20-Scanning-X-ray spectroscopy (XAS/XES)	Nanoscale mechanism of UO2 formation through uranium reduction by magnetite Zezhen Pan , Barbora Bartova , Thomas Lagrange , Sergei M. Butorin , Neil C. Hyatt , Martin C. Stennett , Kristina O. Kvashnina , Rizlan Bernier-Latmani Nature Communications 11 DOI: 10.1038/s41467-020-17795-0 Diamond Proposal Number(s): [20581, 15955] Open Access Show Abstract ▼ Abstract: Uranium (U) is a ubiquitous element 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 likely of critical importance to explain the geochemical behavior of U. Here, we tackle the mechanism of reduction of U(VI) by the mixed-valence iron oxide, magnetite. Through high-end spectroscopic and microscopic tools, we demonstrate that the reduction 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 as nanocrystals (~1–2 nm) with random orientations inside nanowires. Through nanoparticle re-orientation and coalescence, the nanowires collapse into ordered UO2 nanoclusters. This work provides evidence for a transient U nanowire structure that may have implications for uranium isotope fractionation as well as for the molecular-scale understanding of nuclear waste temporal evolution and the reductive remediation of uranium contamination.	Aug 2020

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