Both RDX and HMX are powerful, highly energetic chemicals whose widespread use has resulted in soil and groundwater contamination. Because of their moderate solubility in water and weak binding affinity with soil, both chemicals migrate through subsurface soil to cause groundwater contamination. Efforts over the past two decades to biodegrade these two chemicals have failed because the microbial processes and enzymes involved in degradation are poorly understood.
The primary objective of this project was to determine the enzymatic and microbial processes involved in the initial attack on RDX and HMX that lead to rapid auto decomposition. The secondary objective was to conduct similar experiments to determine how these biochemical processes function in model and natural soil systems.
Experiments were conducted to identify RDX and HMX degradation products, particularly early intermediates, and to determine the kinetics and mechanisms of their formation. Similar experiments were designed to determine the types of enzymes and microorganisms that initiate the degradation of RDX and HMX in liquid culture media. Subsequently, experiments were conducted to determine how these biochemical processes function in natural and representative model soil systems.
Several anaerobic bacteria (Clostridium and Klebsiella) that can degrade RDX and HMX via the intermediary formation of methylenedinitramine (MEDINA) were isolated from domestic sludge. Likewise, researchers obtained three aerobic Rhodococci soil isolates (strain A-Canada, strain 11Y-UK, and strain DN22-Australia) and found that all degraded RDX with subsequent ring cleavage to 4-nitro-2,4-diazabutanal (4-NDAB). Under both anaerobic and aerobic conditions, similar product distributions (NO2-, HCHO, NH3, N2O, CO2) were detected, suggesting that once a successful initial attack, e.g., denitration, takes place the resulting intermediates undergo spontaneous decomposition in water. In support of this hypothesis, researchers found that the denitration of RDX by photolysis, hydrolysis, or electrolysis led to product distributions similar to those obtained by enzymatic and microbial degradation. Finally, in soil, researchers found that both RDX and HMX can be degraded and mineralized under anaerobic (indigenous bacteria) and aerobic (P. chrysosporium) conditions. However, both RDX and HMX favor sequential reduction of the N-NO2 groups to form the corresponding N-NO groups to eventually produce trinitroso-RDX (TNX) and tetranitroso-HMX (4NO-HMX), respectively, before ring cleavage.
Knowledge of RDX and HMX degradation products and insight gained regarding microbial and enzymatic processes of degradation can be used to better understand metabolic routes and to optimize mineralization. This information will assist efforts to enhance bioremediation of both cyclic nitramines and to scale up the developed processes for future field applications. Initially this project involved a multidisciplinary team from Canada (NRC and DND) and the United States (US AFRL), but during the course of the project, collaboration was expanded to include other organizations from the United States, United Kingdom, and Canada.