Extracellular electron shuttle mediated biodegradation of hexahydro-1,3,5-trinitro-1,3,5-triazine

Bioremediation is one suggested technology for hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), as well as alternate explosive contaminants. The potential use of extracellular electron shuttling compounds to mediate electron transfer between iron (hydr)oxides and Fe(III)-reducing microorganisms was investigated for decontaminating RDX. The ultimate goal of this study is to develop an optimal strategy for the biodegradation of nitramine explosive compounds.;This study demonstrated that Geobacter metallireducens, a model Fe(III)- and electron shuttle-reducing microorganism, utilize electron shuttles to stimulate cyclic nitramine reduction at rates significantly faster than those previously reported. This work was the first to demonstrate electron shuttle-mediated nitramine biodegradation with environmentally relevant pure cultures. Electron shuttle-mediated explosives biodegradation was also investigated among four additional species of Fe(III)- and quinone-reducing bacteria. All species showed the fastest RDX degradation when electron shuttling compounds were present although biotransformation kinetics were different among four Bacterial species. In addition, the critical issue for RDX remediation is how fast the ring-cleavage metabolites are produced, and whether they are amenable to further degradation or ultimate mineralization. The higher production of desirable intermediate and mineralization compounds occurred with electron shuttling compounds and suggested that in situ strategy can lead to more rapid and complete RDX mineralization by targeting Fe(III)- and electron shuttle-reducing microorganisms and specific biotic-abiotic reactions. RDX reduction directly stimulated by the electron shuttling compounds in Fe(III)-poor environments and indirectly stimulated by reactive Fe(II) in Fe(III)-rich environments. This suggest that electron shuttle-mediated RDX biodegradation is a reasonable strategy in both Fe(III)-rich and Fe(III)-poor environments.;Electron shuttling compounds also stimulated RDX mineralization in RDX-contaminated aquifer material. The microbial communities associated with RDX biodegradation were characterized using amplified 16S rDNA restriction analysis. The data demonstrated that a novel Fe(III)-reducing community develops as RDX is degraded. This study also led to isolation and characterization of a novel species from RDX contaminated aquifer material that is capable of reducing RDX. For in situ applications, different electron shuttling compounds were tested to identify an option that is both mechanistically feasible and cost efficient. G. metallireducens stimulated RDX reduction with raw humics extract from commercial mulch and military smoke dye, suggesting that these quinone-containing compounds could be an alternative source of electron shuttling compounds for in situ application.