Native bacteria in water from a flooded German uranium mine removed up to 96% of the dissolved uranium when fed glycerol in oxygen-free conditions over 130 days — transforming a mobile pollutant into a solid mineral that stays put.

The water came from Schlema-Alberoda in Saxony, one of the largest and most-studied uranium ore deposits in the world, where some 80,000 tons of uranium were extracted between 1949 and 1990. When mining ended, the underground workings were flooded — but water moving through the mine still picks up uranium, keeping concentrations around one milligram per liter, above the discharge limits applied in Saxony.

Researchers collected untreated water from the inlet of the mine's treatment plant, placed it in two-liter vessels, removed the oxygen and added carefully calculated amounts of glycerol. The bottles sat in darkness at about 28 degrees Celsius for 130 days, relying on the microbial community already native to the mine water — no engineered strain was introduced.

Dissolved uranium fell from one milligram per liter to 0.04 milligrams per liter, a reduction of about 96%. Fermenting bacteria broke the glycerol into smaller compounds such as acetate and lactate while producing hydrogen gas, feeding sulfate- and metal-reducing microorganisms. Two sulfate-reducing groups, Desulfobulbus and Desulfovibrio, became especially prominent; some Desulfovibrio members are known to transfer electrons directly to uranium.

The uranium was not food — it behaved more like an electrical outlet, a place where electrons released by microbial metabolism could ultimately end up. That electron transfer changed the uranium's chemistry: mobile U(VI) gained electrons and became U(IV), which is far less soluble and precipitates as solid uranium dioxide, the mineral uraninite. Microscope observations confirmed uranium-rich clusters on bacterial surfaces, including uraninite crystals only a few nanometers across.

Practical hurdles remain: distributing glycerol through a flooded network of tunnels would be difficult, and the treatment would also change iron, sulfate and arsenic concentrations. Uraninite nanoparticles can oxidize, and tiny particles could travel through groundwater. Still, the study — published in Nature Communications — points to a complementary strategy: stimulating underground microbial communities to immobilize uranium before it ever reaches above-ground treatment systems.

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