S26: Microbial reactions with iron and extracellular electron shuttles that degrade RDX and insensitive munitions (IM)

The explosive hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and the insensitive munitions (IM) compound 2,4-dinitroanisole (DNAN) were investigated to determine the mixed biological-chemical reactions with iron and extracellular electron shuttling molecules that degrade them in both defined systems and contaminated environmental media.  We utilized Fe(III)-reducing pure cultures and contaminated aquifer material from the Picatinny Arsenal (NJ) for our experiments.  RDX biodegrades directly as an electron sink (i.e. an electron acceptor that is not linked to energy conservation as ATP), and this reaction is catalyzed by several microbial genera including Geobacter, Shewanella, and Anaeromyxobacter.  However, the fastest and most complete RDX degradation pathway is via extracellular reduced intermediates such as anthrhydroquinone-2,6-disulfonate (AH₂QDS), or ferrous iron, or both of these in distinct series.  HCHO is the dominant reaction product with the extracellular electron shuttles, and this pH dependent reaction was very fast – with rates on the order of hours between pH 8 and 9.  Strictly biological reactions (with pure cultures and aquifer material) were slower, and the reaction product distribution was different amongst several amendments including ferric iron and the oxidized form of the electron shuttle, anthraquinone-2,6-disulfonate (AQDS).  Fe(III) reducers dominated the sediment incubations that degraded RDX, demonstrating that Fe(III) reduction is a relevant in situ metabolism for RDX transformation.   An emerging contaminant of concern, DNAN, was also quickly degraded by hydroquinones via direct reduction.  Reaction rates were on the order of days, and these are the first data demonstrating DNAN reduction by electron shuttling molecules.  Biodegradation with mixed cultures was much slower.