Hexanitrohexaazaisowurtzitane (CL-20 or HNIW) is a promising replacement for existing propellants and explosives. CL-20 can be released to the surface and subsurface terrestrial environment by manufacturing processes, munitions storage, and use with low-order detonation or unexploded ordnance. Wastes associated with energetic materials constitute a major fraction of the Department of Defense’s (DoD) hazardous waste inventory. In addition, CL-20 is one to four orders of magnitude more toxic to terrestrial and aquatic organisms than other energetics. Therefore, it is critical that DoD consider the environmental fate and reactivity of CL-20 in surface/subsurface terrestrial and aquatic environments before it replaces currently used energetic materials.

The objective of this project was to characterize the fate and transport of CL-20 in subsurface sediments by identifying and quantifying geochemical and microbial reactions, the effects of weathering, and the influence of transport on these reactions in the subsurface environment.

Geochemical and microbial reactions, coupled effects, and effects of sediment weathering and flow were investigated using both simple (i.e., uncoupled batch experiments) and complex (i.e., one-dimensional unsaturated/saturated transport, coupled reactions in natural sediments) systems. Researchers quantified geochemical individual and coupled reactions during unsaturated and saturated transport. CL-20 reactivity initially was predicted with structure-activity modeling. Experimental data then was used to develop mechanism-based correlations between geochemical and microbial characterization and CL-20 reactivity in order to predict natural fate and degradation in engineered remediation systems.

Sorption mass and degradation rates define the potential for deep migration of CL-20. Sorption is relatively small for CL-20 (Kd = 0.22 to 3.83 cm³/g). In subsurface (low organic carbon) sediments, CL-20 sorption is correlated with the mass of iron oxides in the sediment. In surface (high organic carbon) and subsurface sediments, CL-20 sorption can be estimated from geochemical characterization information, including fraction organic carbon, extractable iron (Fe), percent clay, and water content. Based on sorption, CL-20 can be expected to move nearly unretarded through water-saturated sediments in groundwater. While CL-20 subsurface transport is influenced only slightly by sorption, CL-20 degradation has a major influence on the distribution of mass. CL-20 will degrade rapidly (minutes, hours) in alkaline waters (pH greater than 9.5), in sediments with high adsorbed Fe(II) and with some smectite clays. Abiotic degradation occurs more rapidly by electron transfer (35kJ/mol) than nonelectron transfer reactions. Most of the sediments tested had very slow degradation (100 to 6400 hour half-life) such that CL-20 would be persistent in the subsurface for years. At low water content, the CL-20 degradation rate decreased significantly. Geochemical properties can be used to estimate CL-20 abiotic degradation. The CL-20 biodegradation and mineralization rate can be estimated from the microbial population in oxic and reducing environments. CL-20 mineralization was slightly more rapid in reducing environments. Sequential initial abiotic transformation followed by biomineralization was most rapid.

By characterizing the fate and transport of CL-20 in the subsurface, DoD can determine the appropriateness of CL-20 as a replacement for currently used propellants and explosives that are known to contribute to the hazardous waste inventory. Although CL-20 will move rapidly through most sediments in the terrestrial environment, subsurface remediation can be utilized for cleanup. Transformation of CL-20 to intermediates can be rapidly accomplished under reducing conditions, pH greater than 0, and biostimulation. Biostimulation by carbon and nutrient condition can mineralize CL-20. (Project Completed – 2005)