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.

The transformation of several mono-and dinitroaromatic compounds (tested at 50-200 p~) by methanogenic bacteria, sulphate-reducing bacteria and clostridia was studied. Some of the nitroaromatics tested were transformed chemically by 1.5 mM quantities of culture media reducing agents, like cysteine or sulphide. This abiotic reduction occurred at the o-nitro-groups preferentially. Nitrophenols, p-nitroaniline and p-nitrobenzoic acid were completely transformed biologically into the corresponding amino derivatives. The nitroaromatics were transformed by all of the bacterial strains tested. While growing cells of sulphate-reducing bacteria and Clostridium spp. carried out nitroreduction, methanogen cells lysed in the presence of nitroaromatics. Nevertheless these culture suspensions converted nitroaromatics to the corresponding amino derivatives. This was also confirmed by crude cell extracts of methanogenic bacteria. The rate of nitroreduction by sulphate-reducing bacteria depended on the electron donors supplied and the cell density, with molecular hydrogen being the most effective donor of reducing equivalents. The toxicity of p-nitrophenol to some of the organisms tested depended on the concentration of the nitroaromatic compound and the type of organism.

No abstract

Experiments were conducted to evaluate the level of incorporation of 14 C-labeled 2,4,6-trinitrotoluene (TNT) and metabolites into the organic soil matrix of anaerobic and sequential anaerobic-aerobic treated soil/molasses mixtures. After 9 weeks of anaerobic-aerobic incubation with an optimized experimental setup, we determined nearly 84% of the initially applied radioactivity immobilized in different soil fractions, whereas only 57% of the radioactivity was measured as immobilized in the soil organic matrix at the end of the anaerobic treatment (after 5 weeks). After alkaline hydrolyses of the solventextracted soil/molasses mixtures, small amounts of radioactivity were found in the humic acid and fulvic acid fraction, whereas the major part of radiolabel was found to be strongly bound to the humin fraction. In agreement with these findings, the amount of extractable radioactivity (water, methanol, and ethyl acetate extractions) decreased from 40% after the anaerobic phase to nearly 9% after the aerobic treatment phase. The transformation of TNT at the end of the experiments was above 95% and 97% after anaerobic and sequential anaerobic-aerobic treatment, respectively. We propose a two-step treatment process (anaerobic-aerobic bioremediation process) with some special procedures during the anaerobic and the aerobic treatment phases as the most promising method for effective, economic, and ecologically acceptable disposal of TNT from contaminated soils by means of immobilization (for example, humification) of this xenobiotic.