Relating carbon and nitrogen isotope effects to reaction mechanisms during aerobic or anaerobic degradation of RDX (hexahydro-1,3,5- trinitro-1,3,5-triazine) by pure bacterial cultures

Abstract

Kinetic isotopic fractionation of carbon and nitrogen during RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) biodegradation was investigated with pure bacterial cultures under aerobic and anaerobic conditions. Relatively large bulk enrichments in 15N were observed during biodegradation of RDX via anaerobic ring cleavage (ε15N=-12.7‰±0.8‰) and anaerobic nitro reduction (ε15N=-9.9‰±0.7‰), in comparison to smaller effects during biodegradation via aerobic denitration (ε15N=-2.4‰± 0.2‰). 13C enrichment was negligible during aerobic RDX biodegradation (ε13C=-0.8‰±0.5‰) but larger during anaerobic degradation (ε13C=-4.0‰±0.8‰), with modest variability among genera. Dual-isotope ε13C/ε15N analyses indicated that the three biodegradation pathways could be distinguished isotopically from each other and from abiotic degradation mechanisms. Compared to the initial RDX bulk δ15N value of+9‰, δ15N values of the NO2- released from RDX ranged from-7‰ to+2‰during aerobic biodegradation and from-42‰ to-24‰ during anaerobic biodegradation. Numerical reaction models indicated that N isotope effects of NO2- production were much larger than, but systematically related to, the bulk RDX N isotope effects with different bacteria. Apparent intrinsic ε15N-NO2- values were consistent with an initial denitration pathway in the aerobic experiments and more complex processes of NO2- formation associated with anaerobic ring cleavage. These results indicate the potential for isotopic analysis of residual RDX for the differentiation of degradation pathways and indicate that further efforts to examine the isotopic composition of potential RDX degradation products (e.g., NOx) in the environment are warranted.