Biological reduction of uranium—From the laboratory to the field

The chemical and biological processes underlying in situ bioremediation of uranium-contaminated groundwater have been studied in the laboratory and in the field. This article focuses on the long-term stability of uraninite (UO2) in the underground. A large tailings pond, ‘Dänkritz 1’ in Germany, was selected for this investigation. A single-pass flow-through experiment was run in a 100-liter column: bioremediation for 1 year followed by infiltration of tap water (2.5 years) saturated with oxygen, sufficient to oxidize the precipitated uraninite in two months. Instead, only 1 wt.% uraninite was released over 2.4 years at concentrations typically less than 20 μg/L. Uraninite was protected against oxidation by the mineral mackinawite (FeS0.9), a considerable amount of which had formed, together with uraninite. A confined field test was conducted adjacent to the tailings pond, which after bio-stimulation showed similarly encouraging results as in the laboratory. Taking Dänkritz 1 as an example we show that in situ bioremediation can be a viable option for long-term site remediation, if the process is designed based on sufficient laboratory and field data. The boundary conditions for the site in Germany are discussed.