Laboratory studies for in situ treatment of an RDX-contaminated aquifer

Ground water beneath the U.S. Department of Energy (USDOE) Pantex Plant is contaminated with the high explosive RDX. The USDOE Innovative Treatment and Remediation Demonstration (ITRD) program identified three treatment technologies for further testing: (i) oxidation by KMnO₄, (ii) anaerobic biodegradation, and (iii) abiotic reduction by reduced (dithionite-treated) aquifer material. We evaluated KMnO₄ and anaerobic biodegradation by determining degradation kinetics and carbon mass balances with ¹⁴C-RDX. Aqueous RDX solutions (2.8 mg/L), with and without aquifer material, were treated with KMnO₄ at 1000, 2000, 4000, and 20000 mg/L. Initial results with the highest KMnO₄ concentration (20000 mg/L) indicated that RDX concentrations were below detection limits (50 μg/L) within 7 d, and cumulative mineralization proceeded for 14 d until >85% of the labeled carbon was trapped as ¹⁴CO₂. Soil microcosms (75 g soil, 15 mL aqueous solution with 5 mg RDX/L) incubated at 16□°C inside an anaerobic chamber also showed high cumulative ¹⁴CO ₂ production (52-70%) with C, N and P amended microcosms yielding the greatest mineralization. We also assessed subsequent biodegradation of products resulting from the treatment of RDX with dithionite-reduced aquifer sediment. Biotic mineralization rates were initially much greater for the reduced products over parent RDX but cumulative ¹⁴CO₂ production (∼40%) became similar after 40 d. These results indicated that all three technologies have the capacity to transform RDX and directly or indirectly enhance mineralization.