Abiotic reduction is an important process that governs the fate of munitions compounds (MCs) in reducing environments and represents a potential approach to remediate MC-contaminated soil and groundwater. Organic carbon (OC) in soils contains redox-active functional groups that behave like rechargeable electron reservoirs which, when reduced, can react with MCs. Quinones are believed to be the predominant functional groups conferring OC its capacity to accept and donate electrons. To understand the kinetics and mechanism of MC transformation by carbonaceous reductants, we investigated the reactivity of dithionite-reduced anthraquinone-2,6-disulfonate (AQDS, a model quinone and surrogate of dissolved organic carbon) and Leonardite humic acid (LHA) toward 3-nitro-1,2,4-triazol-5-one (NTO) and 2,4-dinitroanisole (DNAN), insensitive munitions constituents in the IMX-101 and IMX-104 formulations. NTO was reduced to 3-amino-1,2,4-triazol-5-one (ATO) with higher yields at increasing pH. DNAN was reduced to 2,4-diaminoansiole (DAAN) through 2-amino-4-nitroanisole as the predominant intermediate. DAAN formation was transient, particularly in the presence of LHA, presumably due to further transformation. LHA exhibited measurable reactivity toward NTO and DNAN only when reduced above a threshold percent of its electron storage capacity (ESC), beyond which the reduction rates and extents of both MCs increased with LHA electron content. The reaction rates generally increased with pH, likely due to the speciation of NTO and hydroquinone functional groups; this was used successfully as the basis to model the kinetics of reduction. The pseudo-first-order reduction rate constant for NTO varied by 2 orders of magnitude from pH 2 to 9.5 and increased 4 orders of magnitude for DNAN between pH 2 and 6. This study illustrates the reactivity and potentially important role of soil organic carbon in influencing the natural and enhanced attenuation of NTO and DNAN in munitions constituent-impacted sites.

Abiotic reduction of the munitions compounds DNAN and NTO by AQDS and Leonardite humic acid

ACS Fall 2020 Virtual Meeting

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