Combined Biological and Chemical Mechanisms for Degradation of Insensitive Munitions

The insensitive munitions (IM) 2,4-dinitroanisole (DNAN) and nitroguanidine (NQ) have been used in newly developed explosives as a replacement for the more sensitive munition, 2,4,6-trinitrotoluene (TNT). These new formulations are less sensitive to external shocks, for example heat or strikes, and as such they are safer to store, transport and use in battle conditions. While these compounds have not yet been considered as groundwater contaminants, they may be released to the environment during detonation tests or in wastewater streams from manufacturing facilities. Explosives are toxic and mutagenic in humans and wildlife, thus it is important to understand their degradation pathway and processes that facilitate their attenuation. The presented work is an investigation of chemical and biological degradation of DNAN and NQ by ferrous iron or hydroquinones using several metabolically diverse microorganisms: Geobacter metallireducens strain GS-15, Shewanella oneidensis strain MR-1, Rhodobacter sphaeroides, and Clostridium geopurificans strain MJ1. Fe(III), as a common element found in subsurface environment, can be employed for in-situremediation when Fe(III)-reducing microorganisms are active and generating Fe(II). Humic material present in the aquifer can also be used to promote degradation of insensitive munitions, hence the quinone/hydroquinone couple was used in this study to represent humics. Fe(III) reducers, such as Geobacter and Shewanella, are ubiquitous in the subsurface and competitive for available substrates, which makes them suitable for explosives bioremediation. Rhodobacter and Clostridium species can be interchangeably employed for clean-up of contaminated sites due to their metabolic capabilities that enable them to thrive in different environmental conditions. IM degradation with ferrous iron and hydroquinone was tested in anoxic bottles buffered at pH range from 6 to 9. In these experiments, ferrous iron and hydroquinone served as reduced electron shuttles. Additionally, several Fe(II)-ligand complexes were tested at pH 6 and 7. Microbially mediated IM degradation was evaluated using oxidized electron shuttles, ferric iron and quinone, which were reduced by active microorganisms at pH 7. IM were degraded via electron transfer directly from microorganisms and indirectly by sequential reduction and oxidation of shuttling compounds. Experimental results showed that DNAN can be readily reduced by chemical and biological processes at pH 7 within 24 hours, and at pH 8 and 9 on the order of minutes. At pH 6, organic ligands had to be used to increase Fe(II) reactivity and to stimulate degradation. Same experiments were performed for the mixture of DNAN and the highly energetic explosive hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) as both compounds are used together in one of the newly developed explosive formulations. The mixture of DNAN and RDX was effectively degraded by the same chemical and biological electron transfer reactions and DNAN was a more preferential electron acceptor compared to RDX. NQ was less susceptible to reductive degradation and most easily degraded at alkaline pH by organically complexed Fe(II) within several days; therefore, NQ may persist in the anoxic subsurface environments. While the new munitions have not yet been detected outside of military test ranges, this work helps to evaluate environmental risks associated with potential contamination. Once IM are introduced to the environment, they may migrate into groundwater and travel over a long distance. IM and explosives contamination of aquifers used as drinking water source poses a hazard to human health. Also, these compounds may have adverse effect on wildlife and microbial communities. The results of this study help understand fate of IM in the environment and identify the biological-chemical processes that may contribute to natural attenuation of IM