Evaluating biodegradation as a primary and secondary treatment for removing RDX (Hexahydro-1,3,5-trinitro-1,3,5-triazine) from a perched aquifer

Ground water beneath the U.S. Department of Energy (USDOE) Pantex Plant is contaminated with the high explosive RDX (hexahydro-1,3,5- trinitro-1,3,5- triazine). The authors evaluated biodegradation as a remedial option by measuring RDX mineralization in Pantex aquifer microcosms spiked with ¹⁴C-labeled RDX (75 g soil, 15 ml of 5 mg RDX/L). Under anaerobic conditions and constant temperature (16°C), cumulative ¹⁴CO ₂ production ranged between 52% and 70% after 49 days, with nutrient-amended (C, N, P) microcosms yielding the greatest mineralization (70%). The authors also evaluated biodegradation as a secondary treatment for removing RDX degradates following oxidation by permanganate (KMnO ₄) or reduction by dithionitereduced aquifer solids (i.e., redox barriers). Under this coupled abiotic/biotic scenario, we found that although unconsumed permanganate initially inhibited biodegradation, >48% of the initial ¹⁴C-RDX was recovered as ¹⁴CO ₂ within 77 days. Following exposure to dithionite-reduced solids, RDX transformation products were also readily mineralized (>47% in 98 days). When we seeded Pantex aquifer material into Ottawa Sand that had no prior exposure to RDX, mineralization increased 100%, indicating that the Pantex aquifer may have an adapted microbial community that could be exploited for remediation purposes. These results indicate that biodegradation effectively transformed and mineralized RDX in Pantex aquifer microcosms. Additionally, biodegradation may be an excellent secondary treatment for RDX degradates produced from in situ treatment with permanganate or redox barriers.