AT MUNITIONS FACTORIES, ammunition plants and military bases, the landscape often hides serious environmental hazards. Trinitotoluene (TNT) and other explosives, such as RDX and HMX, have accumulated over the decades in soils and some groundwaters. These pollutants now threaten to contaminate public water supplies or even trigger small explosions at abandoned and operational sites.

Phytoremediation could become a useful substitute for incineration, which is the conventional and rather costly method of disposing of TNT wastes.

The solution to this problem may be helping nature restore itself. To this end, researchers at Georgia Tech and their colleagues at Rice University and Louisiana State University are developing clean-up techniques relying on aquatic vegetation to defuse and detoxify TNT.

This approach, called phytoremediation, could become a useful substitute for incineration, the conventional method of disposing of TNT wastes. Incineration requires costly removal procedures.

In laboratory studies, researchers are growing different species of aquatic plants and assessing their ability to break down and assimilate these polluting compounds.

"The plants that seem to do the best job are submerged aquatic plants, which flourish in wetlands and are commonly viewed as nuisance plants in national waterways," says overall project director Dr. Michael Saunders, a professor of civil and environmental engineering at Georgia Tech. "We think that phytoremediation could provide an effective and economical alternative that is preferable at some sites."

Plants' Potential Well-Known

Louisiana State University faculty researchers are describing how TNT moves from contaminated soils to water and are using this knowledge to build models of the process. Dr. K.T. Valsaraj, an associate professor, is leading a team that drew samples of TNT from soils and is studying the fundamental physical and chemical processes involved when the compound dissolves.

Researchers at Georgia Tech and Rice University are conducting basic studies of the way plants transform TNT and the "pathways" the compound follows during breakdown and assimilation. Dr. Jackie Shanks, a professor of chemical engineering, has shown that TNT transformation results from vegetative processes and has described the key variables required to break down the contaminant. Environmental engineer Dr. Joe Hughes has tagged TNT with carbon-14 and followed its migration through living plants.

At Georgia Tech, Saunders and Dr. Spyros Pavlostathis, both professors of civil and environmental engineering, have led an effort to assess the TNT-reactivity of a variety of submerged aquatic plants. They have focused on optimal methods of cultivating plants, assessing phytoremediation kinetics and the role of microorganisms and sediments in breaking down TNT. They also are determining if plants need a period of acclimation or continuous exposure to TNT to attain effective reactivity.

The Georgia Tech researchers have assessed the phytoremediation potential of approximately 20 aquatic plants, both as monocultures and parts of polycultures. They judged the most promising to be opportuntistic, structurally simple species, such as Myriophyllum spp., Elodea spp. and selected algal species.

Transformation Mechanisms

PHOTO COURTESY MICHAEL SAUNDERS Aquatic vegetation in this experimental lagoon is continually fed with soluble TNT.

Georgia Tech and Rice researchers have found evidence that the aquatic plants assimilate TNT and use it for structural growth.

Three key processes appear to take place. TNT nitro groups are reduced to form a series of amino-nitro toluenes. This process takes place within several hours and makes the contaminant nonexplosive. The TNT derivatives may be conjugated, a process in which the plant attaches compounds of higher molecular weight, such as glucose, to the contaminant. Conjugation allows detoxification, cellular transport and cellular storage of TNT.

This process acts in concert with plant growth processes and takes weeks to complete in parallel with plant growth processes. The final key process in phytoremediation is assimilation of TNT constituents into plant tissues.

The Georgia Tech group found that aquatic plants do not use the TNT amino-nitro toluenes as nutritive sources of nitrogen. Instead, they incorporate these contaminant byproducts into storage, where they are used to produce cell tissue. The assimilation process also takes several weeks to finish. These remediation processes conform to the plant's life cycle, but they take place faster than when TNT is broken down by microbial systems.

Another important finding has been determining where TNT degradation occurs. As Hughes' carbon-14 study showed, transformation does not appear to take place in the soil or water, but instead happens inside the plant and at the plant surface. Some of the resulting amino-nitro toluenes leak into the surrounding water, where they are taken back up into the plant and assimilated.

Sunlight's Impact Studied

Reactivity with vegetation is not the only factor affecting the breakdown of TNT in solution. Dr. Chris Tiller, a Georgia Tech assistant professor of civil and environmental engineering, tested the effect of sunlight on TNT transformation.

When exposed to photolysis, TNT-contaminated water turns red or pink if it lacks reactive aquatic plants. The resulting "redwater" contains condensation and polymerization products so difficult to break down that they are considered a remediation dead end.

Tiller found that the presence of reactive vegetation in the water triggers such a rapid transformation of TNT that sunlight has little dampening effect.

In Tiller's words, "Photo doesn't hurt phyto."

Field Studies In Progress

Researchers at the Georgia Tech Research Institute have built a pilot-scale phytoremediation system on campus and are using it to test the conclusions of their laboratory studies.

In this experimental lagoon, aquatic vegetation is fed with soluble TNT on a continuous basis. The field study began in the summer of 1995 and is still underway. Georgia Tech researchers have supported another field demonstration at a military ammunition plant, with funding from the U.S. Army.

Through these field tests, the researchers are learning some of the problems that must be resolved before a phyotoremediation technology is ready for practical use. Much development work remains to be done, but the prospects look good that plants can effectively defuse this explosive national environmental problem.

Further information is available from Dr. F. Michael Saunders, School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0512. (Telephone: 404/894-7693) (E-mail: michael.saunders@ce.gatech.edu)

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Last updated: 27 Nov. 1996