TY - JOUR

T1 - Persistence of the Isotopic Signature of Pentavalent Uranium in Magnetite

AU - Pan, Zezhen

AU - Roebbert, Yvonne

AU - Beck, Aaron

AU - Bartova, Barbora

AU - Vitova, Tonya

AU - Weyer, Stefan

AU - Bernier-Latmani, Rizlan

N1 - Funding Information: The work at EPFL was supported by the Swiss National Science Foundation Grant 200021E-164209 and the European Research Council Consolidator Grant 725675 (UNEARTH). We thank Luca Loreggian and Ashley Brown at EPFL for helpful instructions and discussions. We thank the Institute for Beam Physics and Technology for the operation of the storage ring of the KIT synchrotron radiation facility, the Karlsruhe Research Accelerator (KARA). We acknowledge Dr. Jo?rg Rothe, Dr. Kathy Dardenne, Jurij Galanzew, and Bianca Schacherl for help with the organization and running of synchrotron experiments. We acknowledge the Diamond Light Source for time on Beamline I20-Scanning (L3-edge EXAFS measurements) under proposal SP17472 and thank the beamline scientist Shu Hayama for beamtime assistance. We appreciate the comments of Associate Editor David Waite and three anonymous reviewers that helped us improve the presentation and interpretation of our study.

PY - 2022/2/1

Y1 - 2022/2/1

N2 - 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.

AB - 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.

KW - isotope fractionation

KW - pentavalent uranium

KW - redox tracer

KW - uranium remediation

UR - http://www.scopus.com/inward/record.url?scp=85123523626&partnerID=8YFLogxK

U2 - 10.1021/acs.est.1c06865

DO - 10.1021/acs.est.1c06865

M3 - Article

C2 - 35061941

AN - SCOPUS:85123523626

VL - 56

SP - 1753

EP - 1762

JO - Environmental Science and Technology

JF - Environmental Science and Technology

SN - 0013-936X

IS - 3

ER -