Sorption and oxic degradation of the explosive CL-20 during transport in subsurface sediments.
The abiotic sorption and oxic degradation processes that control the fate of the explosive CL-20, Hexanitrohexaazaisowurtzitane, in the subsurface environment were investigated to determine the potential for vadose and groundwater contamination. Sorption of aqueous CL-20 is relatively small (K(d) = 0.02-3.83 cm3 g(-1) for 7 sediments and 12 minerals), which results in only slight retardation relative to water movement. Thus, CL-20 could move quickly through unsaturated and saturated sediments of comparable composition to groundwater, similar to the subsurface behavior of RDX. CL-20 sorption was mainly to mineral surfaces of the sediments, and the resulting isotherm was nonlinear. CL-20 abiotically degrades in oxic environments at slow rates (i.e., 10s to 100s of hours) with a wide variety of minerals, but at fast rates (i.e., minutes) in the presence of 2:1 phyllosilicate clays (hectorite, montmorillonite, nontronite), micas (biotite, illite), and specific oxides (MnO2 and the ferrous-ferric iron oxide magnetite). High concentrations of surface ferrous iron in a dithionite reduced sediment degraded CL-20 the fastest (half-life < 0.05 h), but 2:1 clays containing no structural or adsorbed ferrous iron (hectorite) could also quickly degrade CL-20 (half-life < 0.2 h). CL-20 degradation rates were slower in natural sediments (half-life 3-800 h) compared to minerals. Sediments with slow degradation rates and small sorption would exhibit the highest potential for deep subsurface migration. Products of CL-20 oxic degradation included three high molecular weight compounds and anions (nitrite and formate). The 2-3.5 moles of nitrite produced suggest CL-20 nitro-groups are degraded, and the amount of formate produced (0.2-1.2 moles) suggests the CL-20 cage structure is broken in some sediments. Identification of further degradation products and CL-20 mineralization rates is needed to fully assess the impact of these CL-20 transformation rates on the risk of CL-20 (and degradation product) subsurface movement.
Szecsody
J E
JE
Pacific Northwest National Laboratory, Geosciences Department, Battelle, P.O. Box 999, MS K3-61 Richland, WA 99352, USA. jim.szecsody@pnl.gov
Girvin
D C
DC
Devary
B J
BJ
Campbell
J A
JA
D000327:Adsorption; D001372:Aza Compounds / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid; D004785:Environmental Pollutants / Q000032:analysis*; D005296:Ferrous Compounds / Q000737:chemistry; D019015:Geologic Sediments / Q000032:analysis*; D006207:Half-Life; D006571:Heterocyclic Compounds / Q000737:chemistry*; D007700:Kinetics; D008903:Minerals / Q000737:chemistry; D010100:Oxygen / Q000737:chemistry*; D014872:Water Movements
C482139:2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaaza-isowurtzitane; D001372:Aza Compounds; D004785:Environmental Pollutants; D005296:Ferrous Compounds; D006571:Heterocyclic Compounds; D008903:Minerals; D010100:Oxygen