Naturally occurring bacteria, already seen as a promising tool for cleaning up oil spills and toxic chemicals, also could help provide a solution for radioactive uranium wastes, a new study said Wednesday.
Scientists at the U.S. Geological Survey have identified a strain of bacterium that not only "eats" iron, but also has an appetite for uranium, a common environmental contaminant at many nuclear fuel facilities, uranium mining operations and federal atomic weapons plants.The study found the iron-eating microbes, known to scientists as GS-15, gobble up dissolved uranium in water, obtaining energy and forming solid deposits of the uranium metal.
By fixing the uranium in insoluble clumps, the bacteria could help prevent the spread of uranium contamination in rivers, lakes or groundwater, the study suggested.
"The potential for uranium contamination of surface and groundwaters through uranium mining activities, irrigation of agricultural lands and disposal of nuclear wastes is an environmental concern," said the study published in the journal Nature.
"The results presented here suggest that, in many instances, it may be possible to immobilize uranium contamination by stimulating microbial (activity) in aquatic sediments or groundwater."
The study said the same method also might work with other radioactive metals, such as plutonium and technetium, both of which have been discharged into soil at federal nuclear weapons facilities.
The biological process described in the study differs from the growing use of bacteria to break down and detoxify oil spills or toxic wastes, a cleanup method known as "bioremediation."
The most publicized application of bioremediation techniques has come in Alaska where Exxon and federal scientists have fertilized naturally occurring bacteria in Prince William Sound to accelerate their decomposition of oil spilled from the supertanker Exxon Valdez in March 1989.
Unlike traditional bioremediation, in which toxic chemicals are broken apart and neutralized, GS-15 does not chemically change uranium or eliminate the health threat posed by its radioactivity. It only solidifies dissolved uranium, preventing it from spreading and making it easier to clean up.
However, Derek Lovley, lead author of the study, said that, in addition to its ability to immobilize uranium, GS-15 has shown it can break down aromatic hydrocarbons, a family of toxic organic chemicals.
While its ability to attack other types of organic wastes has not been tested, Lovley said the microbes' versatility could make it ideal for addressing extensive "mixed waste" contamination at government nuclear weapons plants.
Mixed wastes contain both toxic and radioactive elements, posing an especially difficult cleanup problem in soil and groundwater.
"In the case of `mixed wastes,' the activity of uranium-reducing bacteria might be able to couple the decomposition of toxic organic compounds with the immobilization of uranium," the study said.
Lovley said the ability of GS-15 to filter out dissolved uranium in water was demonstrated in both laboratory tests and in nature.
He said uranium deposits often were found in iron-rich ocean bottom sediments, suggesting the bacteria acted on both minerals.
Lovley noted scientists generally recognized the formation of uranium ores was associated with certain bacteria, but believed microbes played only an indirect role. For example, the bacteria were thought to produce chemical substances, such as hydrogen sulfide, that reacted with dissolved uranium to form ore deposits.
However, Lovley said that in testing sediments from the Potomac River, he found uranium deposition was much greater in samples where bacteria were active, as compared to sediments were bacteria were killed off.