Biotic and abiotic transformations of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) mediated by Geobacter metallireducens (GS-15) and iron oxides

RDX is a toxic munitions compound that often contaminates soil and groundwater at military installations. One promising technology for RDX remediation is the use of permeable reactive barriers (PRB) with Fe0 as the reactive medium. This work was undertaken to identify contributions to RDX transformation that Geobacter metallireducens (GS-15), a dissimilatory iron reducing bacteria, may make in a PRB environment.; Results from this work demonstrate that GS-15 may transform RDX by two mechanisms; (1) directly via biotransformation, and (2) indirectly via the production of biologically-reduced iron solids. The biotransformation of RDX occurred in the presence and absence of exogenous electron donor and acceptor. Reduction of the nitro-groups of RDX was not observed. The transformation resulted in the accumulation of unidentified metabolite(s) and mineralization to CO₂. The respiration of GS-15 using ferrihydrite as an electron acceptor produced a reduced iron solid phase that is a mixed mineral phase of siderite and magnetite or maghemite. The bioreduced iron solids removed RDX in a pH-dependent reaction. This transformation resulted in the transient production of nitroso-metabolites of RDX and the accumulation of unidentified metabolite(s). RDX was also transformed by FeII adsorbed on magnetite. The transformation of RDX was pH dependent and resulted in the transient production of the nitroso-metabolites and unidentified metabolite(s).; The Fe0 introduced into the subsurface by installation of a PRB can create conditions that are favorable for enrichment of bacteria that may take advantage of cathodic hydrogen and/or iron oxides. In column studies, Fe0 was shown to decrease bacterial diversity in the downgradient soil but have little effect on total biomass. Using DGGE, iron-reducing bacteria of the family Geobacteracea were shown to be enriched in the downgradient soil. This was also confirmed by an increase of Fe II production in the downgradient soil as the column aged.