The pollution of soil and water with explosives and related compounds caused by military activities has been known for a long time, but progress in understanding the environmental fate of such substances has only been made in the last few years. Microbial processes could be used for the remediation of explosives-contaminated soils and waste waters because it has been shown that a variety of different microorganisms are able to metabolize these chemical compounds. In some cases even a complete mineralization has been found, whereas in others only biotransformation reactions took place, producing more or less toxic and/or recalcitrant metabolites. Studies with pure cultures of bacteria and fungi have given detailed insights into the biodegradation pathways of at least some nitroorganic compounds. Additionally, some of the key enzymes have been isolated and purified or studied in crude extracts. This review summarizes information on the biodegradation and biotransformation pathways of several important explosives. This may be useful in developing microbiological methods for a safe and economic clean-up of soil and water contaminated with such compounds. It also shows the necessity of further investigations concerning the microbial metabolism of these substances.

An anaerobic/aerobic composting experiment with 15N-2,4,6-trinitrotoluene (TNT) spiked soil was performed to investigate the fate of the explosive under the applied conditions. For a qualitative description of TNT-residues formed during the composting process, bulk soil and different soil fractions were subjected to solid-state 15N NMR spectroscopy. Major resonance signals could be detected in the chemical shift regions of five-ring heterocyclic nitrogen and in the area of aniline derivatives and primary amines. Distinct nitro peaks were found in the bulk samples and in the humic fractions obtained with a mild extraction procedure. This signal disappeared in the material extracted with a more drastic procedure. Quantitative investigations of the 15N distribution in the composted material revealed that 33% of the stable nitrogen isotope was incorporated into the humic- and fulvic acid, and 23% was present in the humin. Furthermore 38.8% of the 15N present in the composted material could be allotted to condensed TNT residues, whereas 1.9% are assigned to nitro functions and 15.2% to amino functions. In the investigation presented here a bioremediation method was simulated with 15N-TNT spiked soil. The nonradioactive label allowed a qualitative and quantitative characterization of residues of the explosive. Our results give strong evidence for a stable incorporation of the nitroaromatics into the humic material of soils. However, further investigations will be necessary to prove a long-time stability of bound TNT residues and to assess toxicological effects of the treated soil.

Solid-state 15 N NMR was applied to a humic acid, extracted from 15 N-2,4,6-trinitrotoluene (TNT) enriched soil treated in an anaerobic/aerobic composting system to characterize the nitrogen functionality of the transformation products bound to the soil organic material. Signals assignable to aniline derivatives and condensation products were identified, indicating that the anaerobic/aerobic treatment caused a reduction of nitro groups followed by condensation reactions with the soil organic material. Relevant parameters for routine application of the cross polarization magic angle spinning technique were determined and optimized. The proton spin-lattice relaxation times of all peaks in the 15 N NMR spectrum of the humic acid did not exceed 30 ms. Due to the fast relaxation, the application of 15 N NMR spectroscopy to soils with lower enrichment of 15 N-TNT is feasible. The influence of spinning sidebands on the intensity distribution was shown to be minimal at spinning speeds between 5.5 and 6.5 kHz. Contact times between 0.7 and 1 ms resulted in spectra with representative intensity distribution of all visible 15 N-TNT transformation products. However an underestimation of unreacted TNT must be considered. The results imply that CPMAS 15 N NMR is a valuable tool for the examination of bound residues of TNT in soils. (Figure 1).