RDX Biodegradation Under Metal-Reducing Conditions

Laboratory data from microcosms showing the rate RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) tagged with radioactive carbon (14C) degraded to the final end product, radioactive carbon dioxide (14CO2), under different chemical conditions by adding different electron acceptors. Two different electron acceptors were added – manganese (Mn(IV)), the red triangles in the graph: and iron (Fe(III)), the blue squares. The circles were different control microcosms where electron acceptors were not added. The microcosms were constructed with contaminated sediments from the Naval Submarine Base (NSB) Bangor, Washington, site. The graph demonstrates that RDX can degrade under metal-reducing conditions (click image for a larger version)

USGS scientists have demonstrated that microbial degradation of hexahydro-1,3,5-trinitro-1,3,5-triazine – more commonly known as the explosive RDX ­– can be significant in soil and ground-water environments under commonly occurring, metal-reducing conditions. RDX is one of the most common military explosives in use today and is a soil and ground-water contaminant in many areas where explosives have been and are used. A lack of information on the biodegradation of RDX has been an obstacle to the broad application of biodegradation-based strategies to remediate RDX at contamination sites.

When oxygen is not available, many microorganisms resort to using other chemicals in the environment known as electron acceptors, such as sulfate or nitrate, to help them degrade contaminants. Anoxic (without oxygen), microbial degradation of RDX has been demonstrated previously under nitrate-reducing, sulfate-reducing, and methanogenic conditions, but not under metal-reducing conditions. USGS scientists observed the microbiological degradation of RDX under manganese-reducing conditions in laboratory experiments that used contaminated sediments from the Naval Submarine Base (NSB) Bangor, Washington. The data and understanding gained from the laboratory experiments led the scientists to conclude that RDX may potentially be degrading under naturally occurring metal-reducing conditions in the subsurface at NSB Bangor. The results of the experiments indicate that microbial degradation of RDX may contribute to natural attenuation of RDX in metal-reducing conditions at other contamination sites. Developers of engineered bioremediation systems can use this information to develop bioremediation approaches that take advantage of metal-reducing conditions to clean up RDX contamination.

References

• Bradley, P.M., and Dinicola, R.S., 2005, RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) biodegradation in aquifer sediments under manganese-reducing conditions: Bioremediation Journal, v. 9, no. 1, p. 1-8, doi: 10.1080/10889860590929574.

More Information

• RDX Biodegradation Assessment, Naval Submarine Base Bangor, Washington
• Navy Investigations and Technical Oversight and Scientific Investigations at U.S. Navy Facilities in Washington and Alaska
• Microbial Degradation of Chloroethenes in Ground Water Systems
• SUBASE Bangor Hydrologic Investigations

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