Abiotic reduction of insensitive munition compounds by sulfate green rust

Environmental context: There is a growing need to understand how insensitive munitions compounds behave in natural environments, particularly in soils, where non-combusted residues accumulate. Here, we tested the ability of sulfate green rust, a naturally occurring mineral, to transform munitions compounds by reacting with the mineral surface. Our results show that both the munitions compounds and the mineral structures are transformed in an oxidation-reduction reaction that alters the compounds' environmental fates. Abstract: Abiotic transformation of contaminants by redox-active mineral surfaces plays an important role in the fate and behaviour of pollutants in soils and sediments. However, there is very little information on such transformations for the insensitive munitions compounds (IMCs), 3-nitro-1,2,4-triazol-5-one (NTO) and 2,4-dinitroanisole (DNAN), developed in recent years to replace the traditional munition compounds in explosive mixtures. We tested the ability of sulfate green rust to transform NTO and DNAN (0.5 mM) under anoxic conditions at pH 8.4 in laboratory experiments, by using green rust supplied at 10 g kg^-1 (w/w) solid concentration. Results indicate that NTO and DNAN underwent rapid abiotic reduction to their organic amine daughter products. NTO was completely transformed to 5-amino-1,2 4-triazol-3-one (ATO) within 20 min of reaction. This is the first report of NTO reduction by a naturally occurring mineral. Similarly, DNAN was rapidly transformed to 2-methoxy-5-nitroaniline (MENA) and 4-methoxy-5-nitroaniline (iMENA). The reduction occurred with an intriguing staggered regioselectivity. Over the first 10 min, the para-nitro group of DNAN was selectively reduced to generate iMENA. Thereafter, the ortho-nitro group was preferentially reduced, generating MENA. Both iMENA and MENA were subsequently transformed to the final reduction product 2,4-diaminoanisol (DAAN) within 1 day. Iron Kα X-ray absorption near-edge spectroscopy (XANES) studies of reacted solids indicated oxidative transformation of the green rust to lepidocrocite-like mineral forms. These results indicate that the IMCs can be rapidly transformed in soil, sediment or aquatic environments containing green rust.