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Biodegradation of RDX and MNX with Rhodococcus sp. strain DN22 : New insights into the degradation pathway

National Research Council Canada; NRC Biotechnology Research Institute
Published in:
Environmental Science & Technology
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Previously we demonstrated that Rhodococcus sp. strain DN22 can degrade RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) aerobically via initial denitration. The present study describes the role of oxygen and water in the key denitration step leading to RDX decomposition using 18O2 and H 218O labeling experiments. We also investigated degradation of MNX (hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine) with DN22 under similar conditions. DN22 degraded RDX and MNX giving NO2-, NO3-, NDAB (4-nitro-diazabutanal), NH 3, N2O, and HCHO with NO2-/NO 3- molar ratio reaching 17 and ca. 2, respectively. In the presence of 18O2, DN22 degraded RDX and produced NO 2- with m/z at 46 Da that subsequently oxidized to NO 3- containing one 18O atom, but in the presence of H218O we detected NO3- without 18O. A control containing NO2-, DN22, and 18O2 gave NO3- with one 18O, confirming biotic oxidation of NO2- to NO3-. Treatment of MNX with DN22 and 18O 2 produced NO3- with two mass ions, one (66 Da) incorporating two 18O atoms and another (64 Da) incorporating only one 18O atom and we attributed their formation to bio-oxidation of the initially formed NO and NO2-, respectively. In the presence of H218O we detected NO2- with two different masses, one representing NO2- (46 Da) and another representing NO2- (48 Da) with the inclusion of one 18O atom suggesting auto-oxidation of NO to NO 2-. Results indicated that denitration of either RDX or MNX and denitrosation of MNX by DN22 did not involve direct participation of either oxygen or water, but both played major roles in subsequent secondary chemical and biochemical reactions of NO and NO2-. © 2010 American Chemical Society.
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