Abiotic reduction is an important process that governs the fate of munitions compounds (MCs) in reducing environments. It also represents a potential approach to remediate MC-impacted soil and groundwater. Humic substances (HS) constitute a major fraction of organic carbon (OC) in soils and can mediate important chemical reactions including redox reactions. Quinones are believed to be the predominant functional groups conferring HS the capacity to store and reversibly accept and donate electrons. To understand the kinetics and mechanism of MC transformation by humic substances, we investigated the reactivity of anthraquinone-2,6-disulfonate (AQDS, a model quinone and surrogate of dissolved organic carbon) and four humic acids (Leonardite (LHA), Elliott Soil (ESHA), Pahokee Peat (PPHA) and Suwannee River (SRHA)) toward 3-nitro-1,2,4-triazol-5-one (NTO) and 2,4-dinitroanisole (DNAN), two insensitive munitions compounds in explosive formulations. AQDS and humic acids were reduced to different extents with dithionite. Due to AQDS and/or NTO speciation, the pseudo-first-order rate constants (k_Obs) for NTO reaction varied by 3 orders of magnitude from pH 1.5 to 12.5 and by 4 orders of magnitude for DNAN from pH 2 to 6. HAs and AQDS reduced NTO stoichiometrically to 3-amino-1,2,4-triazol-5-one (ATO) at pH ≥ 8 and partially (~60%) at pH < 6.5. DNAN was partially reduced to 2,4-diaminoanisole (DAAN), presumably due to the accumulation of nitroso and hydroxylamino intermediates. When reduced to the same extent, the terrestrial HAs exhibited comparable reduction potential (E_H) and similar reactivity toward NTO. After wet oxidation by O₂, the only aquatic HA SRHA transformed NTO, whereas the three terrestrial HAs did not. The reduction rates and extents of both MCs increased with HA electron content and pH, which could affect the reactivity and/or accessibility of hydroquinone moieties. This study illustrates a potentially important role of soil organic carbon in influencing MCs' natural and enhanced attenuation at munitions-impacted sites.

Dr. Jimmy Murillo Gelvez

University of Delaware

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