PNNL

Abstract

Hexanitrohexaazaisowurtzitane (CL-20) is being considered by DoD as a replacement for existing propellant and explosives. This SERDP-funded study is focused on quantification of geochemical and microbial reactions of CL-20 in sediments and transport in the subsurface environment. CL-20 is unstable in aqueous alkaline conditions (pH > 9.0), but is stable with some sediments for a year or more. Low sorption and slow degradation rates observed with oxic sediments indicate a high potential for deep migration of CL-20 (similar to RDX). Sorption of CL-20 is relatively small (Kd = 0.02 to 4.2 cm3 g-1, which results in only slight to moderate retardation relative to water (i.e., Rf 1.1 to 12). In oxic systems, CL-20 abiotically degrades at slow rates (i.e., 10s to 100s of hours) in a wide variety of sediments, but at fast rates (i.e., minutes) in the presence of 2:1 smectite clays and ferrous iron oxides. CL-20 was degraded rapidly in sediments under iron reducing conditions, demonstrating that reduced iron technologies for groundwater remediation of RDX and TNT may work for CL-20. The degradation process in reduced sediments was investigated further using specific ferrous iron phases. CL-20 was rapidly degraded with adsorbed Fe(II) on amorphous iron oxides or clays, but not adsorbed Fe(II) on silica or feldspar. Because iron oxides delocalize electrons from adsorbed Fe(II), this may increase the rate of CL-20 degradation. CL-20 degradation was also at moderate rates in the presence of minerals containing only structural Fe(II). CL-20 oxic degradation produced 2 to 4 moles of nitrate with little carbon mass, whereas anaerobic degradation produced mainly low-molecular weight compounds that are aromatic. This degradation pathway in a reducing environment may follow C-C bond breaking, as predicted from molecular modeling. 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.