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RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), a nitramine explosive, is often found as a subsurface contaminant at military installations. Though biological transformations of RDX are often reported, abiotic studies in a defined medium are uncommon. The work reported here was initiated to investigate the transformation of RDX by ferrous iron (Fe(II)) associated with a mineral surface. RDX is transformed by Fe(II) in aqueous suspensions of magnetite (Fe3O4). Negligible transformation of RDX occurred when it was exposed to Fe(II) or magnetite alone. The sequential nitroso reduction products (MNX, DNX, and TNX) were observed as intermediates. NH4+, N2O, and HCHO were stable products of the transformation. Experiments with radiolabeled RDX indicate that 90% of the carbon end products remained in solution and that negligible mineralization occurred. Rates of RDX transformation measured for a range of initial Fe(II) concentrations and solution pH values indicate that greater amounts of adsorbed Fe(II) result in faster transformation rates. As pH increases, more Fe(II) adsorbs and k(obs) increases. The degradation of RDX by Fe(II)-magnetite suspensions indicates a possible remedial option that could be employed in natural and engineered environments where iron oxides are abundant and ferrous iron is present.

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Upflow anaerobic filters fed acetate and propionate, and completely mixed, suspended growth reactors fed acetate, propionate, lactate, and glucose were used to investigate the effect of electron donor and reactor type on the interaction between sulfate‐reducing bacteria (SRB) and methanogens. Organic loading rates of 0.25–0.50 g chemical oxygen demand (COD)/L · d were used in suspended growth systems and 1.0–5.0 g COD/L · d in filters. COD/sulfur ratios ranged from 20/1 to 2/1 for completely mixed reactors, and 20/1 to 8/1 for anaerobic filters. Results indicated that organisms involved in the conversion of lactate and glucose into simpler products were not affected by sulfide toxicity. Levels of 60–75 mg sulfur/L of hydrogen sulfide and 150–200 mg/L of dissolved sulfide (DS) caused stress in all suspended growth reactors; 100–150 mg sulfur/L of hydrogen sulfide and 200–400 mg DS/L could be tolerated in lactate and glucose systems, although with diminished COD and sulfate removal. For similar loading conditions, lactate and glucose systems had higher DS levels than acetate and propionate systems. A cyclic pattern of variation of DS and hydrogen sulfide with volatile‐acids COD (VACOD) was observed in long‐term experiments with suspended growth reactors. Anaerobic filters were able to tolerate higher DS and hydrogen sulfide levels than suspended growth reactors. A propionate‐fed filter could withstand more than 150 mg hydrogen sulfide/L of hydrogen sulfide and 1000 mg DS/L, and an acetate‐fed filter could tolerate more than 125 mg sulfur/L of hydrogen sulfide and 400 mg DS/L.

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