RDX Biodegradation Column Study: Extent of RDX Mineralization and Influence of Temperature on Rate of RDX Biotransformation

A series of column studies, with aquifer material from the former Nebraska Ordnance Plant (NOP), were performed to evaluate the influence of temperature on the rate of RDX biodegradation, and to assess the ultimate fate of RDX in groundwater under biologically induced reductive conditions. In treatment columns, RDX-contaminated water was amended with acetate as a readily available carbon source, and in control columns no electron donor was used. The results of the temperature study demonstrated clear indications of adverse effects of lower temperature on biological activity of RDX degraders. As the temperature decreased from 15 to 10 and eventually to 5°C, the concentration of nitroso-substituted metabolites and untreated RDX increased in the effluent stream. The estimated first-order biodegradation rate coefficient (k) for RDX at 15 °C was 0.155 1/h (±0.019, n = 3). This rate coefficient decreased by about 37% to 0.098 1/h (±0.017, n = 3) at 10 °C, and by another 38% to 0.061 1/h (±0.016, n = 3) at 5°C. An activation energy of 63.54 kJ/mol RDX was estimated from these reaction rate coefficients at three different operating temperatures. Results of the radiolabel study demonstrated that the ultimate fate of RDX under in situ reductive conditions is highly dependent on redox conditions in the aquifer. In treatment columns (ΔE_h =–550 to –700 mV), 23–46% of initial radiocarbon was mineralized to ¹⁴CO₂ compared to <5% in control columns, where ΔE_h ranged between 50 to –50 mV. The dissolved fraction of the initial radiocarbon in treatment columns was estimated between 46 and 64%. No or very low levels of nitroso-substituted RDX transformation products were identified in dissolved fraction from the treatment columns. In control columns dissolved fraction accounted for about 86% of initial ¹⁴C and was composed of mainly untreated RDX.