Bioremediation of TNT

2, 4, 6- Trinitrotoluene, also known as TNT, is a yellow colored chemical compound known for its explosive behavior. TNT has a molecular formula of C7H5N3O6. TNT is very poisonous and upon contact with the skin can cause irritation and discoloration. Long-term exposure to TNT can cause abnormal liver function, anemia, spleen enlargements, infertility and disorders of the immune system. TNT does not occur naturally in the environment. It enters through solid waste and waste water from the manufacture and destruction of bombs, grenade, and other explosives. TNT is known to bind to organic matter and is one of the most difficult contaminants to eliminate.

To reduce the levels of TNT at a toxic waste site, a plan to use a treatment train involving composting and phytoremediation is a good choice. The first phase in this treatment train consists of composting. This allows the individual to remediate large fragments of TNT by adding acetone and a carbon source for microorganisms inside the composting mixture. "Composting mixes natural organic amendments, such as manure, wood chips, alfalfa and vegetable processing wastes with 30% contaminated soil and adds water to 50% of moisture holding capacity" (Tech Trends).

Phase two consists of phytoremediation via transgenic tobacco plants. Tobacco thrives best in areas with a frost-free growing season of 120 to 170 days. Good-quality tobacco requires fertile, well-drained, moist soil and warm temperatures; it is mostly grown in sunny climates. Environmental factors influence the plant's characteristics. Soil, for example, can affect leaf size, texture, and color. Sandy soils tend to produce a relatively large leaf that is light in color and body, fine in texture, and burns with a weak aroma. Heavier soils, which contain silt and clay, tend to produce a small, dark leaf with a heavy body and a strong aroma when burned" (MSN Encarta). This process is accomplished through the creation of two bacterial enzymes, pentaerythritol tetranitrate (PETN) reductase and a classical type I nitroreductase (NR), both enzymes deriving from strains of Enterobacter cloacae which have been shown to catalyze a two electron reduction of TNT to produce hydroxylamino- and amino-dinitrotoluene derivatives" (ACS Publication). To overcome the high phytotoxic effects of TNT, we expressed bacterial nitroreductase in tobacco plants. Nitroreductase catalyzes the reduction of TNT to hydroxyaminodinitrotoluene (HADNT), which is subsequently reduced to aminodinitrotoluene derivatives (ADNTs). Therefore, "transgenic plants expressing nitroreductase show a striking increase in ability to tolerate, take up, and detoxify TNT" (Nature).

Neil C. Bruce of the University of Cambridge and colleagues created the transgenic tobacco and conducted a series of toxicity experiments. Plants grown on a medium containing high concentrations of TNT removed the entire compound within 72 hours (SCIAM). The plant helps to transform the TNT into a nonlethal substance. It has been proven by Bruce that genetically modified tobacco plants may prove to be a cheaper way to clean up polluted firing ranges, munitions dumps, and even post war zones (Science Central).

When putting together a cost analysis report, two key factors influence the total cost. The area of contamination is the primary driver of cost, since it will affect the scale of effort. Also, the density of sampling will affect sampling cost. Based on a cost analysis chart of sites with similar characteristics, there is an estimated cost of cleaning up a 2.7 million sq. ft site to be $3,691,490 or $1 per square foot (pre marked-up costs). The estimated time for the operation and management of the cleanup project would be about 20 years (Federal Remediation Technologies).

There are many advantages to using composting and phytoremediation to reduce TNT levels, such as the fact that it is mostly an in situ process, which is cheaper and less invasive. Also, little waste is generated, it uses readily available equipment, and the land can be reclaimed after completion of the remediation. However, there are a few disadvantages to using composting and phytoremediation. The growth of the tobacco plant is dependent on abiotic and biotic factors that may be out of our control. Also, the tobacco plant was created via genetic modifications, which may compete with nearby indigenous plant species. The biggest disadvantage is the time frame. It is not a short term process, but rather a long term endeavor. By using the transgenic tobacco plants and composting, our expected result is that TNT will be transformed into a nonlethal form that will not affect human health or the surrounding ecological environment.

Works Cited

"Explosive discovery on genetically engineered tobacco plant." Huliq News. 13 Aug. 2007. 10 Mar. 2009. http://www.huliq.com/30353/explosive-discovery-on-genetically-engineered-tobacco-plant .

"Explosive Discovery On Genetically Engineered Tobacco Plant." Science Daily. 15 Aug. 2007. 10 Mar. 2009. http://www.sciencedaily.com/releases/2007/08/070813103354.htm .

"Tobacco." MSN Encarta. 10 Mar. 2009. http://encarta.msn.com/encnet/features/dictionary/DictionaryResults.aspx?lextype=3&search=tobacco .

Craig, Harry . "The Composting Alternative to Incineration of Explosives Contaminated Soils." Tech Trends. 10 Mar. 2009. http://www.clu-in.org/products/newsltrs/ttrend/ttcmpost.htm .

Hager, Emily . "Good Tobacco." ScienCentral. 14 Jun. 2005. 10 Mar. 2009. http://www.sciencentral.com/articles/view.php3?article_id=218392571 .

Travis, Emma R. "Impact of Transgenic Tobacco on Trinitrotoluene (TNT) Contaminated Soil Community." Environmental Science and Technology. 12 Jul. 2007. 10 Mar. 2009. http://pubs.acs.org/doi/abs/10.1021/es070507a .

Wong, Kate . "Transgenic Tobacco Detoxifies TNT." Scientific American. 3 Dec. 2001. 10 Mar. 2009. http://www.scientificamerican.com/article.cfm?id=transgenic-tobacco-detoxi .