Scientists have discovered a potential new approach to tackling uranium contamination, using bacteria to transform dissolved uranium into stable solid compounds. In a 130-day laboratory experiment involving water from a former uranium mine in Germany’s Ore Mountains, microbes stimulated with glycerol removed around 90–95% of dissolved uranium from the water.
The research is significant because uranium can become highly mobile in groundwater when it is present in its oxidised U(VI) form. This allows the metal to move through contaminated water and potentially reach surrounding ecosystems and water supplies.
Traditional uranium bioremediation research has largely focused on converting U(VI) into U(IV), a less soluble form that can precipitate as uraninite. However, the new research indicates that bacteria can also produce U(V), a chemical state that has generally been considered unstable in natural environments.
Researchers used water from the flooded Schlema-Alberoda uranium mine in Germany. Instead of introducing specially engineered bacteria, they studied the naturally occurring microbial community already present in the mine water.
The scientists supplied glycerol as an energy source for the microorganisms and maintained oxygen free conditions. As microbial activity increased, dissolved uranium concentrations declined and uranium bearing solid particles began forming.
Advanced microscopy and X-ray spectroscopy revealed that the bacteria were influencing uranium chemistry rather than simply absorbing the metal onto their surfaces.
Some of the uranium was converted into U(IV) and formed tiny uraninite particles. However, a significant amount was converted into U(V), with the U(V) fraction accounting for roughly 20–30% of the uranium in the analysed black precipitates.
The researchers also identified FeU(V)O₄, a rare iron-uranium compound containing pentavalent uranium. The compound formed nanoparticles alongside uraninite.
The scientists identified hundreds of nanoparticles, including FeU(V)O₄ and uraninite particles, with many measuring only around 2–3 nanometres across.
U(V) remained stable for weeks
One of the most surprising findings was the apparent stability of the U(V) compound.
Researchers initially expected exposure to oxygen to potentially push the reduced uranium back toward a more oxidised and mobile state. Instead, FeU(V)O₄ increased after dried bacterial biomass was exposed to oxygen.
U(V) remained detectable after four weeks of oxygen exposure, suggesting that the compound was not merely a short-lived chemical intermediate.
How bacteria help trap uranium
The bacteria appear to influence uranium through their metabolism. When they consume glycerol, they alter the chemical conditions around them, creating an environment that favours uranium reduction and the formation of uranium bearing minerals.
Researchers found enrichment of fermentative microorganisms and sulfate reducing bacteria under the experimental conditions. Their activity can influence the availability of elements such as iron and sulfur, which can participate in the formation of uranium minerals.
This is important because simply attaching uranium to bacterial surfaces would not necessarily provide a permanent solution. The metal could potentially become mobile again. Incorporating uranium into stable mineral structures could provide longer term immobilisation.
Potential applications for contaminated sites
The findings could eventually contribute to new methods for cleaning uranium-contaminated groundwater and mine sites. Glycerol is relatively inexpensive and is also produced as a byproduct of biodiesel manufacturing, making it an interesting potential electron donor for microbial remediation.
However, the researchers caution that the results come from controlled laboratory experiments. Applying the technique at real contaminated sites would require consideration of groundwater chemistry, geology, oxygen levels, microbial communities and the movement of glycerol through underground environments.
Further research will also be needed to determine how stable the uranium bearing compounds remain over much longer periods and whether the approach can work effectively under different environmental conditions.
The study nevertheless offers a new perspective on uranium contamination. Rather than simply converting dissolved uranium into U(IV), microbial activity may guide uranium through several oxidation states and ultimately help lock it into stable mineral phases such as FeU(V)O₄.
If the process can eventually be adapted for field use, naturally occurring microbes could become an important tool for preventing uranium from travelling through contaminated groundwater and reducing the long-term environmental risks associated with abandoned uranium mines.