Hexahydro-1,3,5-trinitro-1,3,5-triazine mineralization by zerovalent iron and mixed anaerobic cultures

Soil microcosms were used to evaluate the potential benefits of an integrated microbiol-Fe⁰ system to treat groundwater contamination by RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine). Microcosms amended with both Fe⁰ filings and municipal anaerobic sludge mineralized RDX faster and to a greater extent than separate treatments, with up to 51% ¹⁴CO₂ recovery after 77 d. The nitroso byproducts 1,3-dinitro-5-nitroso-1,3,5-triazacyclohexane (MNX), 1,3-dinitroso-5-nitro-1,3,5-triazacyclohexane (DNX), and 1,3,5-trinitroso-1,3,5-triazacyclohexane (TNX) were detected in all microcosms, although these compounds never accumulated above 5% of the added RDX on a molar basis. A soluble intermediate that was tentatively identified as methylene-dinitramine [(O₂NNH)₂CH₂] was relatively persistent, although it accumulated to lower extent in combined-treatment reactors than in sets with Fe⁰ or sludge alone. Some of the radiolabel was bound to soil and Fe⁰ and could not be extracted with CH3CN. This fraction, which was recovered by combustion with a biological oxidizer, was also found at lower concentrations in combined-treatment reactors. This work suggests that permeable reactive Fe⁰ barriers might be an effective approach to intercept and degrade RDX plumes and that treatment efficiency might be enhanced by biogeochemical interactions through bioaugmentation.