Uranium biogeochemistry

Uranium (U) is the heaviest naturally occurring element and is present in the subsurface either through geological processes or as a contaminant from the mining or nuclear processing industry. Interest in U biogeochemistry stems from two fields: subsurface remediation and paleo-redox reconstruction. 

Uranium redox transitions results in a change in its physical state. When oxidized, hexavalent U (U(VI)) is mainly soluble as a complex with carbonate or organic ligands. When reduced, tetravalent U (U(IV)) largely precipitates as a uranium oxide mineral (e.g., UO2) or as a non-crystalline U form (poorly characterized).  

The reduction can be catalyzed by microorganisms or by reduced minerals (e.g., Fe(II)-bearing minerals such as magnetite (Fe3O4)). In addition to U(VI) and U(IV), pentavalent U(V) is increasingly detected as a persistent form of uranium, particularly in the presence of iron oxides or organic ligands able to stabilize it. 

 

Microbial uranium reduction 

Microbial uranium reduction is known to induce U isotope fractionation, typically with the accumulation of the heavy isotope 238U in the reduced form. This is due to a process known as the Nuclear Field Shift effect. The current work focuses on deciphering the controls on uranium isotope fractionation during microbial reduction (by Shewanella spp.), to establish a mechanistic basis for the fractionation, its extent and direction. 

This project uses a combination of geochemistry, isotope geochemistry (multi-collector ICP-MS), microbiology, as well as synchrotron-based and luminescence spectroscopy tools.

People: Katharina Reinhold (Ph.D. student) 

Funding: SNSF

 

Uranium reduction by magnetite 

U(VI) is also reduced by magnetite, resulting in the production of the uranium oxide (UO2). However, the mechanism of U(VI) reduction by magnetite includes intermediate steps that result in the formation of transient uranium oxide nanowires.  

We use STEM-EELS and computational geochemistry to reveal the underlying mechanism and provide an understanding of the formation of the nanowires and their collapse. 

People: Jacopo Carbone (Ph.D. student); Katharina Reinhold (Ph.D. student); Barbora Bartova (Scientist). 

Collaboration: Thomas LaGrange (EPFL), Kevin Rosso (PNNL), Zezhen Pan (Fudan U.) 

Funding: SNSF