Abiotic transformation of high explosives by freshly precipitated iron minerals in aqueous Fe^II solutions

Zerovalent iron barriers have become a viable treatment for field-scale cleanup of various ground water contaminants. While contact with the iron surface is important for contaminant destruction, the interstitial pore water within and near the iron barrier will be laden with aqueous, adsorbed and precipitated Fe^II phases. These freshly precipitated iron minerals could play an important role in transforming high explosives (HE). Our objective was to determine the transformation of RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), and TNT (2,4,6-trinitrotoluene) by freshly precipitated iron Fe^II/Fe^III minerals. This was accomplished by quantifying the effects of initial Fe^II concentration, pH, and the presence of aquifer solids (Fe^III phases) on HE transformation rates. Results showed that at pH 8.2, freshly precipitated iron minerals transformed RDX, HMX, and TNT with reaction rates increasing with increasing Fe^II concentrations. RDX and HMX transformations in these solutions also increased with increasing pH (5.8-8.55). By contrast, TNT transformation was not influenced by pH (6.85-8.55) except at pH values <6.35. Transformations observed via LC/MS included a variety of nitroso products (RDX, HMX) and amino degradation products (TNT). XRD analysis identified green rust and magnetite as the dominant iron solid phases that precipitated from the aqueous Fe^II during HE treatment under anaerobic conditions. Geochemical modeling also predicted Fe^II activity would likely be controlled by green rust and magnetite. These results illustrate the important role freshly precipitated Fe^II/Fe^III minerals in aqueous Fe^II solutions play in the transformation of high explosives.