Apr. 13, 2000 Like the Wicked Witch of the West, another danger can be melted away, only this time it's with a bucket of fingernail polish remover. Soil found on old gunnery ranges and military wastewater lagoons is often contaminated with the explosive TNT. In some instances, the common composting method to clean the soil leaves small particles of the potentially carcinogenic substance. Researchers have found that by adding acetone -- the main component of nail polish remover -- to this composting method, they can safely dissolve away the chunks and biologically degrade the TNT.
Modifying a standard composting recipe, researchers at the Department of Energy's Idaho National Engineering and Environmental Laboratory successfully dissolved and degraded chunks of TNT from INEEL soil. They also found the microbial populations at work degrading the explosive lived quite happily under the modified conditions. The report by INEEL microbiologists Corey Radtke, Richard M. Lehman and Francisco F. Roberto will be appearing in this week's quarterly issue of Bioremediation Journal.
By the end of the process, the TNT is completely degraded and the acetone evaporates. "The beauty of this method is that when the experiment is done," said INEEL microbiologist Radtke, "the treated soil can go into INEEL's landfill and we never have to worry about it again."
Part of the land on which the INEEL sits was a World War II era Naval gunnery range. As a result of military activities, particles of TNT ranging in size from about 56 grams to less than a hundredth of a gram contaminate about a thousand cubic yards of soil. Although the TNT left to its own merits is unlikely to pose an explosion hazard, the Environmental Protection Agency denotes TNT as a possible human carcinogen under the agency's Integrated Risk Information System. As a part of its overall environmental mission and to safeguard the health of its workers, the INEEL is trying to clean up the TNT legacy.
Soil contaminated with TNT is not unique to the INEEL, however. According to the Environmental Protection Agency, soil at more than 30 munitions test areas across the U.S. are contaminated with TNT. For example, a World War II Naval range on the Hawaiian island of Kaho'olawe is the site of the largest funded Unexploded Ordnance Remediation Project. Last November, Radtke and colleagues traveled to a conference in Maui to make the remediation managers aware of their promising technique.
Although TNT is considered a carcinogenic compound, microbes normally found in the soil will eat it with great relish. That is, if the relish consists of old sawdust, hay, mashed sugar beets and a sampling of manures in the form of compost. Digging up soil and composting off the TNT is a tried and true method. It also provides a safe alternative to incinerating contaminated soil to burn off the explosive substance.
Unfortunately, chunk TNT isn't broken down as easily by the microbes and therefore remains in the soil at the end of the composting period. Workers need to treat the chunks of TNT to make the explosive more accessible to the microbes.
In their research, the microbiologists took advantage of earlier studies that showed acetone could be used as a solvent to extract the TNT from the dirt. Unfortunately -- or fortunately -- they couldn't find a way to safely recover the acetone containing dissolved TNT from the soil. They decided to try composting the contaminated soil without first removing the acetone.
The chunk size variation interfered with the researchers' ability to draw conclusions about how well the method worked. To reduce the variation, they sifted the soil with a mesh that removed TNT chunks over 2.4 millimeters (a little bigger than a ballpoint pen point). The researchers used the sifted soil in two-liter compost reactors. In these, they tested how much acetone was needed, if the presence of acetone would slow down the composting degradation, and what effect the acetone had on the microbial communities responsible for the TNT degradation.
After determining the optimal concentration of acetone to use, the team sampled the composting reactions to see how fast the microbes were breaking down the TNT. Without the acetone, the compost reactions still had an average of 2000 parts per million after 20 days. The addition of acetone allowed the microbes to degrade the TNT to less than 500 parts per million within one day. The concentration of TNT was down to safe levels within a week.
Treating the soil with acetone alone did not degrade the TNT as fast as with the addition of composting, but how the compost speeds up the contaminant degradation is not known. The researchers point out that the acetone may be dissolving the TNT, thus making it available to the microbial communities in the compost. Alternatively, since microorganisms can use acetone as a food source, the solvent may enable the microbes to metabolize the TNT chunks in a way that isn't yet clear. The researchers note that the compost doesn't heat up as fast when acetone is added, but that the temperature -- usually an indication of microbes working in high gear -- does not seem to be important for TNT degradation. They suggest that perhaps the acetone creates such a feast for the microorganisms that they don't have to work as hard to degrade the explosive substance.
To understand the effect acetone had on the microbial communities, Lehman tested whether the acetone substantially changed the communities. He used a method that categorizes bacterial communities based on what they eat -- similar to comparing a fast-food-eating community (the U.S.) to a rich-cream-sauce-and-pasta-eating community (the French).
Using a panel of 95 different bacterial foodstuffs, the INEEL team found that compost reactors with and without acetone had different microbial communities at work. However, the acetone-containing compost degraded the TNT without the researchers having to tailor the compost to TNT-containing soil.
"We're pretty sure there are at least three distinct communities doing the same thing," said Radtke. "The fact that there are so many gives us flexibility in the field. It makes the composting problem an engineering one and not one where we have to worry about the science behind it." For example, instead of having to grow certain bacterial strains in the laboratory to eat TNT, users of this method will only need to assure their soil conditions and recipes are consistent. The team has already scaled up its experiments to 30-gallon batches of soil at a time. Due to INEEL's strict safety and documentation requirements, however, they don't know how much money this method could save the national lab. The INEEL has about a thousand cubic yards of TNT-contaminated soil, about 40 garbage truckfuls.
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