Related Experiment Videos
Role for Fe(III) minerals in nitrate-dependent microbial U(IV) oxidation
John M Senko1, Yasser Mohamed, Thomas A Dewers
1Department of Botany and Microbiology and Institute for Energy and the Environment, University of Oklahoma, Norman, Oklahoma 73019, USA.
Environmental Science & Technology
|May 12, 2005
Summary
Microbial reduction of uranium (U(VI)) to insoluble uranium (U(IV)) can immobilize it, but nitrate contamination can cause U(IV) re-mobilization. Iron (III) minerals, formed under nitrate-reducing conditions, significantly impact U(IV) stability in aquifers.
Area of Science:
- Environmental Science
- Geochemistry
- Microbiology
Background:
- Microbial reduction of soluble uranium (U(VI)) to insoluble uranium (U(IV)) is a key process for immobilizing uranium in groundwater.
- However, the presence of nitrate in uranium-contaminated sediments can lead to the oxidation and remobilization of U(IV) via nitrite or iron (III) oxyhydroxides.
- Understanding the factors controlling U(IV) oxidation under nitrate-reducing conditions is crucial for predicting uranium stability in aquifers.
Purpose of the Study:
- To determine the rate and extent of U(IV) oxidation by nitrite and iron (III) oxyhydroxides under nitrate-reducing conditions.
- To investigate the dominant mechanism of iron (III) oxyhydroxide production in aquifer sediments during microbial nitrate reduction.
- To assess the influence of iron (III) mineralogy and surface area on U(IV) oxidation rates and extent.
Main Methods:
- Quantified the oxidation rates of U(IV) by nitrite and iron (III) oxyhydroxides.
- Isolated and characterized a nitrate-dependent, Fe(II)-oxidizing bacterium from a contaminated aquifer.
- Compared U(IV) oxidation rates and extents mediated by amorphous Fe(III) (produced abiotically) and crystalline Fe(III) (produced microbially).
Main Results:
- Iron (III) oxidized U(IV) at a significantly greater rate (130 microM U(IV)/day) than nitrite (10 microM U(IV)/day).
- A isolated bacterium oxidized U(IV) more effectively under acetate-oxidizing conditions (leading to nitrite accumulation) than Fe(II)-oxidizing conditions.
- The mineralogy of Fe(III) produced (amorphous vs. goethite/lepidocrocite) significantly affected the rate and extent of U(IV) oxidation.
Conclusions:
- The mineralogy and surface area of Fe(III) minerals formed during nitrate reduction critically influence U(IV) oxidation and remobilization.
- Microbially produced Fe(III) minerals (goethite, lepidocrocite) are more effective at oxidizing U(IV) than abiotically produced amorphous Fe(III).
- These findings are vital for predicting the long-term stability and transport of uranium in contaminated aquifers.