Molasses recruits bacteria for cleanup.

From the 1940s to the 1960s, workers at U.S. Army bases that manufactured 2,4,6-trinitrotoluene, or TNT, for weapons routinely flushed out the plants to reduce the risk of accidental explosions. Although that practice ensured worker safety, the wastewater left a legacy of TNT-contaminated soil.

To correct this problem, scientists are now using a sweet, gooey incentive to recruit another army-of bacteria. Mixing molasses into vats of soil encourages the bacteria naturally present to multiply. As the microbes munch on the molasses, they simultaneously break down the TNT into harmless molecules.

In a pilot study, researchers at Argonne (Ill.) National Laboratory treated 12,000 pounds of TNT-laced soil from the nearby Joliet Army Ammunition Plant. They mixed 300-gallon batches of soil and water in bioreactors and added a few kilograms of molasses each week. In a matter of weeks, the TNT concentration plummeted from a high of 7,000 milligrams per kilogram of soil to just 20 mg per kg.

A threshold level of bacterial activity must be reached for significant TNT degradation to begin, says Mark L. Hampton of the Army Environmental Center in Aberdeen, Md., which sponsored the study. Unrefined, agricultural-grade molasses is a great bacterial food source, says John Manning of Argonne's Environmental Research Division. "It has lots of sugars, proteins, and amino acids. A little goes a long way."

Although scientists aren't sure which bacteria are doing the work, this remediation process is attractive because it doesn't introduce any foreign microbes to the soil, says Hampton. In addition, bacterial activity seems to return to normal levels once the molasses is completely consumed.

This bioreactor process joins incineration and composting as Army-approved methods for cleaning up contamination. In many cases, cost determines the choice of method, Hampton says. The bioreactor process compares well to composting, which costs about half as much as incineration. The researchers are exploring the even cheaper possibility of adding molasses directly to the contaminated ground.

The method can be used to clean up other kinds of contamination too, including chlorinated solvents, wood-preserving chemicals, and petroleum (SN: 4/14/90, p. 236). "Explosives are tougher compounds to degrade than others," Hampton says, so most likely the technique will work for other contaminants.