Biodegradation of explosives mixture in soil under different water-content conditions.

Soil redox potential plays a key role in the rates and pathways of explosives degradation, and is highly influenced by water content and microbial activity. Soil redox potential can vary significantly both temporally and spatially in micro-sites. In this study, when soil water content increased, the redox potential decreased, and there was significant enhancement in the biodegradation of a mixture of three explosives. Whereas TNT degradation occurred under both aerobic and anaerobic conditions, RDX and HMX degradation occurred only when water content conditions resulted in a prolonged period of negative redox potential. Moreover, under unsaturated conditions, which are more representative of real environmental conditions, the low redox potential, even when measured for temporary periods, was sufficient to facilitate anaerobic degradation. Our results clearly indicate a negative influence of TNT on the biodegradation of RDX and HMX, but this effect was less pronounced than that found in previous slurry batch experiments: this can be explained by a masking effect of the soil in the canisters. Fully or partially saturated soils can promote the existence of micro-niches that differ considerably in their explosives concentration, microbial community and redox conditions.

Sequential biodegradation of TNT, RDX and HMX in a mixture.

We describe TNT's inhibition of RDX and HMX anaerobic degradation in contaminated soil containing indigenous microbial populations. Biodegradation of RDX or HMX alone was markedly faster than their degradation in a mixture with TNT, implying biodegradation inhibition by the latter. The delay caused by the presence of TNT continued even after its disappearance and was linked to the presence of its intermediate, tetranitroazoxytoluene. PCR-DGGE analysis of cultures derived from the soil indicated a clear reduction in microbial biomass and diversity with increasing TNT concentration. At high-TNT concentrations (30 and 90 mg/L), only a single band, related to Clostridium nitrophenolicum, was observed after 3 days of incubation. We propose that the mechanism of TNT inhibition involves a cytotoxic effect on the RDX- and HMX-degrading microbial population. TNT inhibition in the top active soil can therefore initiate rapid transport of RDX and HMX to the less active subsurface and groundwater.