Mobilization of non-crystalline uranium (U) and related U isotope fractionation in biotic environments

Environmental uranium (U) contaminations may be remediated by using microbes to reduce mobile hexavalent [U(VI)] to more immobile tetravalent U [U(IV)]1. However, the subsurface stability of U(IV), usually present as solid-phase non-crystalline U, can be affected by reactions with ligands, oxidants or bacteria. Uranium isotopes have the potential to monitor and assess these mobilization/immobilization processes, however, currently detailed knowledge of their fractionation in all involved individual processes is lacking.

Here, we present laboratory experiments, investigating the efficacy of Acidithiobacillus (At.) ferrooxidans to mobilize non-crystalline U(IV) and associated U isotope fractionation. As starting material an U(VI) isotope standard was reduced by Shewanella oneidensis MR-1 in a phosphate-containing medium (WLP) in order to produce non-crystalline U(IV). At. ferrooxidans mobilized between 74% and 91% U after one week, and interestingly, U mobilization was observed for both, living and inactive cells. This finding raises doubts on the long-term sustainability of in-situ bioremediation measures at U-contaminated sites.

In contrast to mobilization with ligands, which results in 238U enrichment in the mobilized phase2, mobilization by At. ferrooxidans did not cause U isotope fractionation, potentially because of a layer effect3. Thus, isotopic signatures found in nature are more likely associated with other processes such as U reduction, abiotic oxidation, or mobilization with ligands.

(1)         Basu, A. et al., Environ. Sci. Technol. 2015, 49, 5939–5947.

(2)         Roebbert, Y. et al., Environ. Sci. Technol. 2021, 55, 7959–7969.

(3)         Wang, X. et al., Geochim. Cosmochim. Acta 2015, 150, 160–170.