Microbes enlisted to capture uranium

Researchers at Georgia Institute of Technology are experimenting with ways to use bacteria to help clean up uranium. Colonies of cleanup microbes living in soil could also prevent the spread of radioactive material in the event of a leak at a power plant.

Uranium is dangerous to human health, but it's also tough to contain. It dissolves in water and thus is often found in groundwater near where the element was mined or enriched in the manufacture of weapons. Because of solubility, extracting it from water supplies or soil has always proved difficult and expensive.

It turns out that a species from the bacteria genus Rhanella secretes phosphate after it eats certain organic compounds containing phosphate. The excreted phosphate will subsequently, in the right conditions, bind with uranium (VI), a soluble form of the element, in a process called biomineralization. The result is an insoluble phosphate-uranium compound.

Fixed in this new compound, the uranium effectively becomes immobile. In a real-life situation, bulldozers could then come scrape away the infected soil, and little would get into the groundwater.

"The uranium is not going to lose its radioactivity, but it won't move from the site it is at," said Martial Taillefert, an assistant professor of earth and atmospheric sciences at Georgia Tech. "It (the phosphate-uranium reaction) is a pretty fast reaction. It is difficult to keep uranium in solution in the lab."

Similar experiments in employing microbes to immobilize uranium are taking place at Stanford University in conjunction with Pacific Northwest National Laboratory. In these experiments, the goal is to convert uranium (VI) into uranium (IV), which is easier to extract.

Despite the different approaches to the problem, the basic theme is the same. Microbes, like other species, have a metabolic pathway that will convert one substance (food) into another through a chain of complex and often not completely understood chemical reactions.

By harnessing the power of microbes, scientists in other fields are turning manure into natural gas, creating organic pesticides and trying to lay down circuit patterns on chips.

One of the advantages of using bacteria in nuclear cleanup comes in their small size, according to Patricia Sobecky, an associate professor of biology at Georgia Tech. Millions can live in a relatively small patch of ground. Permeating the soil with bacteria can help ensure contact between phosphate and uranium.

Phosphate overkill is crucial; other minerals will compete to react with it. Therefore, the source of the phosphate needs to be close to the uranium and plentiful. Spraying phosphate isn't that effective because it will react with other materials first.

Uranium, meanwhile, will not extract phosphate out of the soil on its own.

The species of Rhanella the team is experimenting on is fairly common, Sobecky said. She and Taillefert are also examining two other microbes. Potentially, these species could live in the soil around mines and nuclear power plants, digesting organic substances, making phosphate and breeding successive generations.

Unlike other scientists experimenting with microbes in energy production and medicine, Sobecky and Taillefert are not genetically manipulating their subjects. "The more you can rely on their inherent properties, the better," she said. "When you start experimenting, you wonder if they will be competitive and robust in the environment."

Not changing the microbes is crucial. Right now, the species of Rhanella continues to thrive in the presence of uranium when releasing phosphate.

Significant research needs to be completed. Sobecky and Taillefert have so far only fed the microbe species an organophosphate substance similar to soil. They need to document how well the microbes will extract phosphate out of the real thing. The two have also only conducted how well the microbes convert uranium into the insoluble compound in the lab.