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PH05 Fate and Effects of Energetic Compounds
(PH020) Transport and Degradation of the Explosive CL-20 in Subsurface Sediments.
Szecsody, J1, Girvin, D1, Devary, B1, Qasim, M2, Crocker, F2, Fredrickson, H2, 1 Pacific Northwest National Laboratories, Richland, WA, USA2 US Army ERDC, Vicksburg, MS, USA
ABSTRACT- The fate of CL-20 (hexanitrohexaazaisowurtzitane) was investigated to quantify the potential for transport in the subsurface environment. Experiments showed that CL-20 abiotic and biotic reactions occur which significantly influence the fate of CL-20 in the subsurface environment from a surface spill, storage, or low-order detonation. Sorption of CL-20 to subsurface sediments (fraction organic carbon < 2%) is relatively small (Kd = 0.02 to 4.2 cm3/g). Abiotic and biodegradation half-lives of CL-20 range from minutes to 100s of days. Thus, CL-20 in some sediments will move quickly through unsaturated sediments to groundwater, so has the potential to be a groundwater contamination problem similar to the fate of RDX. Biodegradation was 10 to 100 times slower than abiotic degradation in the same sediment. Although CL-20 and RDX both have nitroso- functional groups, biodegradation pathways appear to differ. An RDX-degrading aerobe does not degrade CL-20, and a CL-20-degrading aerobe does not degrade RDX. CL-20 aerobic biomineralization (up to 51% after 42 days) was observed in a subsurface sediment with trace nutrient addition (no C or N). Abiotic CL-20 experiments showed that adsorbed ferrous iron on iron oxides or 2:1 clays promoted the most rapid degradation rate (minutes), whereas most oxides and clays without adsorbed ferrous iron degraded CL-20 slowly. Therefore, iron-reducing conditions in sediments promote rapid CL-20 degradation. Differences in activation energies for oxic and reduced sediments imply different mechanisms. The hypothesized degradation pathway (from molecular modeling) shows cleavage of the CL-20 cage structure with removal of 4-6 nitro groups. Experimentally 2-6 moles of nitrite/nitrate have been observed per mole of CL-20 degraded. Further investigation of CL-20 degradation products and the extent of mineralization is needed. Our observations of a wide range of reactivity with subsurface sediments suggest that geochemical and microbial characterization of sediments will enable prediction of CL-20 subsurface behavior.
Key words: sorption, CL-20, degradation
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