S57: Mixed biological and abiotic reactions for RDX and DNAN biodegradation in groundwater and adsorbed to granular activated carbon

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and 2,4-dinitroanisole (DNAN) were investigated to determine the biological-chemical reactions with iron and extracellular electron shuttling molecules that degrade them in both defined systems and contaminated environments.  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.  An emerging contaminant, 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.  Fe(II) (ferrous iron) mediated reduction was fast, but pH dependent – with DNAN degraded at pH 7 and 9, but not 6 and 8.  We believe this is related to Fe(II) speciation at varying pH, and the ferric minerals that result from Fe(II) oxidation.  Ligands such as citrate and Tiron increased reaction rates.  Cysteine was also a strong DNAN reductant in these experiments.  We have coupled these reactions to a phototroph – Rhodobacter sphaeroides – so we can use sun energy to generate the reducing equivalents needed to drive the reactions.  Finally, we are investigating microbially-mediated electron transfer to all explosives when they are adsorbed to granular activated carbon, which is the de facto remediation strategy from a regulatory standpoint.