Microbial degradation of explosives: biotransformation versus mineralization

The nitroaromatic explosive 2,4,6-trinitrotoluene (TNT) is a reactive molec ule that biotransforms readily under both aerobic and anaerobic conditions to give aminodinitrotoluenes. The resulting amines biotransform to give sev eral other products, including ate, azoxy, acetyl and phenolic derivatives, leaving the aromatic ring intact. Although some Meisenheimer complexes, in itiated by hydride ion attack on the ring, can be formed during TNT biodegr adation, little or no mineralization is encountered during bacterial treatm ent. Also, although the ligninolytic physiological phase and manganese pero xidase system of fungi can cause some TNT mineralization in liquid cultures , little to no mineralization is observed in soil. Therefore, despite more than two decades of intensive research to biodegrade TNT, no biomineralizat ion-based technologies have been successful to date. The non-aromatic cycli c nitramine explosives hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and oc tahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) lack the electronic st ability enjoyed by TNT or its transformed products. Predictably, a successf ul enzymatic change on one of the N-NO2 or C-H bonds of the cyclic nitramin e would lead to a ring cleavage because the inner C-N bonds in RDX become v ery weak (<2 kcal/mol). Recently this hypothesis was tested and proved feas ible, when RDX produced high amounts of carbon dioxide and nitrous oxide fo llowing its treatment with either municipal anaerobic sludge or the fungus Phanaerocheate chrysosporium. Research aimed at the discovery of new microo rganisms and enzymes capable of mineralizing energetic chemicals and/or enh ancing irreversible binding (immobilization) of their products to soil is p resently receiving considerable attention from the scientific community.