Soybean oil is especially versatile. It fries chicken. It makes a tasty salad dressing. Believe it or not, it can remove uranium from groundwater, too.
ORNL investigators have developed a method for reducing both nitrate and uranium in groundwater by using the energy found in soybean oil. Their approach is simpler, cheaper and less disruptive than traditional alternatives such as excavation and pump-and-treat technologies.
“It’s one of those things that is really elegant; there’s a lot of beauty in it,” explained ORNL geochemist Scott Brooks.
Brooks, hydrogeologist David Watson and microbiologist Chris Schadt developed their approach while working with a disposal site at the nearby Y-12 nuclear weapons plant. The site, known as the S-3 ponds, was used from the early 1950s to the early ’80s to hold liquid waste from uranium operations. Although the ponds have since been cleaned up and capped—the site is now a parking lot—a plume of contaminated groundwater extends several miles from the ponds.
That plume was the focus of a field research center sponsored by DOE’s Biological and Environmental Research program, with researchers looking for ways to understand and manipulate the interaction of chemistry, microbes and contaminants in groundwater. The center hosted investigators from around the country and sent samples of contaminated soil and groundwater to facilities around the world.
The soybean oil approach—Brooks and his colleagues refer to it as emulsified vegetable oil, or EVO—grew from a focused exploration among ORNL researchers. While the technique itself appears low-tech, the process to develop it was not. Along the way they took advantage of cutting-edge analysis tools, especially the Advanced Photon Source at Chicago’s Argonne National Laboratory.
“It went from very basic fundamental laboratory studies to applying it in the field and taking a look at the coevolution of the chemistry and the biology,” Brooks said.
“We would write our concept on a blackboard and say, ‘We think if we do this, that should happen, and if that happens, this should happen.’ Then we worked to solidify all that into a robust mathematical model, so we could say, ‘If we add exactly this much, we ought to see exactly that amount of change.’”
The soybean oil is emulsified—converted to small droplets to penetrate more deeply into the groundwater. It doesn’t reduce either the nitrate or the uranium directly. Rather, it feeds useful microbes that are already in the soil. First, the team needed to tackle the nitrate, relying on bacteria known as nitrate reducers. The bacteria are anaerobic, meaning they don’t need oxygen. Rather, they rely on nitrogen in the acid, transferring energy to and from it in the same way other organisms do with oxygen.
“We normally think of living things as requiring oxygen to live,” Brooks said. “Your body uses oxygen as a place to dump electrons, and its cells harness energy from that. Bacteria have much broader capabilities. They still generate energy by moving electrons through their cell machinery, but in the absence of oxygen they have to put those electrons someplace else. And nitrate reducers put those electrons into nitrate and reduce it into nitrite or nitrous oxide.”
Not only do the nitrate reducers act on the nitrate; they also break down fats within the oil and make them available for the next stage in the cleanup: reducing uranium.
“Many metal-reducing bacteria can reduce uranium, either directly or indirectly,” Brooks said. “In some cases, they produce reduced iron that can chemically reduce the uranium.”
The microbes don’t change uranium into something else. Rather, they cause it to stick to particles in the ground or to simply precipitate out of the water, much as the salt in a glass of salt water eventually drops to the bottom of the glass.
So far, it looks like the oil does just that. Researchers found that a single injection of the oil reduced the nitrogen and uranium for a year and can keep uranium levels in the water low enough that it’s not a hazard to people who consume that water.
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