A photo of yellow cake uranium, a solid form of uranium. Image credits: NRC.
At a flooded uranium mine in Germany, bacteria pulled off an extremely useful chemical trick.
Given glycerol and 130 days without oxygen, the native microbial community removed as much as 96% of the dissolved uranium from samples of contaminated mine water. While the bacteria didn’t make the uranium disappear, it changed it into a solid form that stays put and doesn’t get dissolved and transported by water.
A Mine That Needs Treatment
Schlema-Alberoda is one of the largest and most well-studied uranium ore deposits in the world. Around 80,000 tons of uranium were extracted from there between 1949 and 1990. When mining ended, the underground workings were flooded. But flooding did not make the pollution problem go away.
Water moving through the mine still picks up uranium and other contaminants. Before that water can be discharged, operators aerate it, strip out carbon dioxide, add alkaline chemicals and separate the resulting precipitates. The process produces contaminated sludge that must also be managed.
The treatment works, but it requires continuous operation.
Uranium concentrations in the untreated mine water remain around one milligram per liter, above the discharge limits applied in Saxony. Researchers are therefore looking for complementary methods that might immobilize more uranium inside the flooded mine and reduce the burden on the treatment plant.
Microbial Chain Reaction
The researchers collected untreated water from the inlet of the Schlema-Alberoda treatment plant. They placed it in two-liter vessels, removed the oxygen and added carefully calculated quantities of glycerol.
The bottles were kept in darkness at about 28 degrees Celsius for 130 days. They already contained microorganisms native to the mine water; the researchers did not introduce a specially engineered bacterial strain.
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Over the course of the experiment, dissolved uranium fell from one milligram per liter to 0.04 milligrams per liter — a decrease of 96%.
Some uranium also disappeared from solution in the control experiments, sticking to particles, biomass, container surfaces or naturally occurring minerals. But the decline was much larger when living microbes and glycerol were present together.
Bacteria play a key role in breaking down compounds in nature. Some specialize in breaking down harmful substances. Apparently, that can also include radioactive materials.
Fermenting bacteria broke the glycerol into smaller compounds such as acetate and lactate, while also producing hydrogen gas. Those products could then feed sulfate- and metal-reducing microorganisms.
Two sulfate-reducing groups, Desulfobulbus and Desulfovibrio, became particularly prominent. Some members of Desulfovibrio are known to transfer electrons directly to uranium. Both groups can also produce hydrogen sulfide or reduced iron, chemicals that may transform uranium without direct contact between the metal and a bacterial enzyme.
So, it wasn’t really one species doing the magic. Rather, the study showed that the bacterial community acted together.
Either way, the uranium was not being used like food. It was acting more like an electrical outlet: a place where electrons released by microbial metabolism could ultimately end up.
Changing Uranium
That transfer changed the uranium’s chemistry, not its radioactivity. Chemists keep track of those changes with Roman numerals, which describe how many electrons an atom has lost.
In oxygenated, carbonate-rich mine water, uranium occurs mainly as U(VI). Carbonate molecules form complexes around it, helping keep it dissolved and allowing it to move with flowing water.
When U(VI) gains two electrons, it can become U(IV). Under oxygen-poor conditions, U(IV) is much less soluble. So, it can precipitate into solid uranium dioxide, better known as the mineral uraninite.
Turning mobile U(VI) into less mobile U(IV) has long been one of the principal goals of uranium bioremediation.
Microscope observations confirmed that the experiment produced uranium-rich clusters on bacterial surfaces. Some contained uraninite crystals only a few nanometers across.
Could This Work Practically?
The practical question is whether the same reaction could happen realistically inside the mine. Here, there are still some unanswered questions.
The researchers worked with sealed bottles in which they could control the temperature, oxygen level and glycerol concentration. Distributing glycerol throughout a flooded network of tunnels would be much harder. Engineers would also have to maintain sufficiently reducing conditions without disrupting water management elsewhere in the mine.
Stimulating these microbes would affect more than uranium. In the experiment, iron, sulfate and arsenic concentrations also changed substantially. Any field trial would need to track how the treatment altered the mobility and toxicity of those and other elements.
The solids themselves would need close attention. Uraninite nanoparticles can oxidize, while very small particles may travel through groundwater rather than remaining fixed in one location. Researchers would therefore need to establish whether the uranium-bearing material stayed attached to microbial films, sediment and rock — or whether some of it could move as colloidal particles.
The study nevertheless offers a useful new direction.
Instead of relying only on an above-ground plant to remove uranium after contaminated water emerges, operators might one day stimulate underground microbial communities to immobilize part of it before it reaches the treatment system.
That would not eliminate the uranium. Nor would it make the material non-radioactive. But it could help keep more of it out of the water—and, ideally, in forms that are less likely to spread.
The study was published in Nature Communications.