Fed Glycerol, Mine Bacteria Stripped 95% of the Uranium From Water — and Locked It in a Form Chemists Called Impossible
Pentavalent uranium was supposed to be fleeting. German researchers found bacteria producing it in bulk, bound into a compound so stable it gained ground when exposed to oxygen.
Uranium is normally locked inside minerals in soil, where it stays put. Mining and other disturbances can convert it into forms that dissolve in water, and once it is mobile the toxicity becomes a problem that spreads.
Researchers at the Helmholtz-Zentrum Dresden-Rossendorf, working with the German mining remediation company Wismut GmbH and scientists at the University of Granada, have now shown for the first time that bacteria can take uranium dissolved in water and convert it into a stable chemical compound, provided they are given glycerol to eat. In the process, the uranium settles into a chemical state that chemists had written off as merely temporary. The findings were published in Nature Communications.
"There are bacteria that can metabolically utilize the heavy metal, uranium, which is toxic for humans," said Dr. Evelyn Krawczyk-Bärsch, a scientist in the center's Terrestrial Microbiology group and a co-author of the study. Her group had already established that such bacteria will use dissolved uranium in their metabolism when glycerol — a basic component of plant and animal fats, which also forms naturally when fungi break down wood — is available.
The open question was how much uranium the microbes could actually remove, and what they turned it into. To find out, the team collected mine water from a flooded uranium mine in the Ore Mountains operated by Wismut, added a controlled amount of glycerol in the laboratory, and kept the samples in an oxygen-free environment.
"We wanted to create natural conditions for the bacterial community already existing in the mine water because at a depth of approximately 2,000 meters there is usually little or no oxygen in the mine," said Dr. Antonio M. Newman-Portela, the study's lead author and a former doctoral candidate at both the Dresden center and the microbiology department at the University of Granada.
The microbes began consuming the glycerol, and the dissolved uranium started disappearing. "After 130 days, only around five percent of the uranium dissolved in the water remained in the samples," Newman-Portela said. "We suspected that the bacteria had incorporated the uranium in their cell walls." That suspicion was confirmed: the uranium had accumulated inside the bacterial cell walls.
Identifying the compound required a synchrotron. The team ran experiments at the Rossendorf Beamline, which the Dresden center operates at the European Synchrotron Radiation Facility in Grenoble, France, with complementary work at Granada. What they found in the bacterial membranes contradicted standard expectations.
Chemists describe an atom's bonding capacity as its valency — roughly, how many "hands" it has available. "Uranium usually occurs with a valency of 4 or 6. Pentavalent uranium does exist, but it is rare or only transient. Until now, it had been seen in an unstable oxidation state," Newman-Portela said. "So, the findings of our study were extremely surprising because in the biomass analyzed from our experimental runs, an unusually high proportion of the uranium identified was also pentavalent uranium."
The pentavalent uranium had combined with iron and oxygen to form FeU(V)O4, a compound so new it does not yet have a name. It was first demonstrated in a 2020 study of soil in parts of Croatia contaminated by uranium ammunition, where it had stayed stable for more than 25 years despite exposure to atmospheric oxygen. "But until now, we didn't know how this compound is formed in nature or that bacteria play a role in its formation," Krawczyk-Bärsch said.
Then the researchers exposed dried bacterial biomass to oxygen, expecting the fragile compound to break down. The amount of FeU(V)O4 increased instead. That durability is what makes the result more than a curiosity: a bacterial process that pulls uranium out of water and parks it in a form that survives contact with air is the kind of thing that could underpin future efforts to clean up contaminated sites.
Originally reported by ScienceDaily.