Sorption and stability of the polycyclic nitramine explosive CL-20 in soil

The polycyclic nitramine CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10, 12-hexaazaisowurtzitane) is being considered for use as a munition, but its environmental fate and impact are unknown. The present study consisted of two main elements. First, sorption-desorption data were measured with soils and minerals to evaluate the respective contributions of organic matter and minerals to CL-20 immobilization. Second, since CL-20 hydrolyzes at a pH of >7, the effect of sorption on CL-20 degradation was examined in alkaline soils. Sorption-desorption isotherms measured using five slightly acidic soils (5.1 < pH < 6.9) containing various amounts of total organic carbon (TOC) revealed a nonlinear sorption that increased with TOC [K-d (0.33% TOC) 2.4 L kg(-1); Kd (20% TOC) = 311 L kg(-1)]. Sorption to minerals (Fe2O3, silica, kaolinite, montmorillonite, illite) was very low (0 < K-d < 0.6 L kg(-1)), suggesting that mineral phases do not contribute significantly to CL-20 sorption. Degradation of CL-20 in sterile soils having different pH values increased as follows: sandy agricultural topsoil from Varennes, QC, Canada (VT) (pH = 5.6; K-d = 15 L kg(-1); 8% loss) < clay soil from St. Sulpice, QC, Canada (CSS) (pH = 8.1; K-d = I L kg(-1); 82% loss) < sandy soil provided by Agriculture Canada (SAC) (pH = 8.1, K-d = approximately 0 L kg(-1); 100% loss). The faster degradation in SAC soil compared with CSS soil was attributed to the absence of sorption in the former. In summary, CL-20 is highly immobilized by soils rich in organic matter. Although sorption retards abiotic degradation, CL-20 still decomposes in soils where pH is >7.5, suggesting that it will not persist in even slightly alkaline soils.