Biodegradation of RDX and HMX Mixtures: Batch Screening Experiments and Sequencing Batch Reactors

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Abstract
Biodegradation is a potential treatment method for the high explosives RDX and HMX. In batch biodegradation tests, a facultative microbial consortium consumed the dissolved oxygen (DO) or reduced the oxidation-reduction potential (ORP) of the culture medium and biodegraded mixtures of RDX and HMX when supplied with various carbon and nitrogen sources. In batch tests with a mixture of carbon sources (glucose, glycerol, and succinic acid), RDX was reduced from 1.1 mg/L to below detectable levels within 4 days in the presence of 0.6 mg HMX/L. With ethanol as the carbon source, the culture degraded 90% of the initial 2.5 mg RDX/L over 8 days in the presence of 0.5 mg HMX/L. In batch mineralization tests, the culture converted 8 and 30% of the initial 14C-RDX to 14CO2 when supplied with ethanol and the mixed carbon sources, respectively. In both cases, a significant fraction of the initial 14C-RDX was converted to water-soluble 14C compounds, indicating triazine ring cleavage. However, mineralization was limited by a lack of a suitable electron acceptor or metabolic limitations of the culture. Sequencing batch reactors (SBR) were used to test the stability of the culture and its ability to continuously degrade the high explosives over 64 days. Over each 4-day SBR cycle, with average initial concentrations of about 1 mg RDX/L and 0.5 mg HMX/L, the culture removed an average of 95% of the RDX when supplied with mixed carbon, and an average of 68% of the RDX when supplied with ethanol. This research shows the diversity of conditions under which RDX can be biodegraded and the limitations of certain types of microbial cultures.