Adsorption of novel insensitive munitions compounds at clay mineral and metal oxide surfaces

Abstract

Environmental context Insensitive munitions compounds are increasingly used in the manufacture of military energetic materials because of their lower unintentional explosion risk during transport and handling. The current study was designed to better resolve the environmental chemistry of two of these insensitive munitions compounds. In particular, we investigated the solid–solution partitioning that occurs when aqueous solutions containing dissolved unexploded ordinances come into contact with soil mineral media. Abstract Insensitive munitions compounds (IMCs) are increasingly used for military energetic materials, yet their environmental fate is poorly understood. Prior work has shown that the nitroaromatic 2,4-dinitroanisole (DNAN) and the heterocyclic nitrogen compound 3-nitro-1,2,4-triazole-5-one (NTO), both newly introduced IMCs, can undergo microbially mediated reduction under anoxic conditions to form 2-methoxy-5-nitroaniline (MENA) and 3-amino-1,2,4,triazole-5-one (ATO) respectively. In the present work, DNAN, MENA, NTO and ATO were subjected to batch adsorption–desorption experiments with specimen soil mineral adsorbents that included montmorillonite, birnessite and goethite. DNAN and MENA exhibited high affinity, linear adsorption to montmorillonite, with enhanced surface excess at a given aqueous equilibrium concentration for K+-saturated relative to Na+-saturated forms, but negligible adsorption to the metal oxides. Powder X-ray diffraction data and surface occupancy calculations indicate interlayer intrusion by DNAN and MENA and adsorption at siloxane sites. Conversely, NTO and ATO exhibited low sorptive affinity and apparent anion exclusion upon reaction with the negatively charged layer silicate clays. However, both of the N-heterocycles showed positive adsorption affinities for goethite (Kd values of 11.1 and 3.1, and HI values of 1.8 and 0.50 respectively), consistent with anion adsorption to the positively charged goethite surface. Both ATO and MENA were subjected to apparent oxidative, abiotic chemical transformation during reaction with birnessite. The results indicate that the IMCs studied will exhibit adsorptive retardation – and their biodegradation products may undergo further abiotic transformation – upon reaction at soil mineral surfaces.