Persistence of the Isotopic Signature of Pentavalent Uranium in Magnetite.
Environ Sci Technol
; 56(3): 1753-1762, 2022 02 01.
Article
in En
| MEDLINE
| ID: mdl-35061941
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) (â¼-1.0), incorporated U (96 ± 2% U(V), â¼-0.1), and extracted surface U (mostly U(IV), â¼0.3) 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.
Key words
Full text:
1
Collection:
01-internacional
Database:
MEDLINE
Main subject:
Uranium
Language:
En
Journal:
Environ Sci Technol
Year:
2022
Document type:
Article
Affiliation country:
China