Reductive Transformation of 3-Nitro-1,2,4-triazol-5-one (NTO) by Leonardite Humic Acid and Anthraquinone-2,6-disulfonate (AQDS)

Abstract

3-Nitro-1,2,4-triazol-5-one (NTO) is a major and the most water-soluble constituent in the insensitive munition formulations IMX-101 and IMX-104. While NTO is known to undergo redox reactions in soils, its reaction with soil humic acid has not been evaluated. We studied NTO reduction by anthraquinone-2,6-disulfonate (AQDS) and Leonardite humic acid (LHA) reduced with dithionite. Both LHA and AQDS reduced NTO to 3-amino-1,2,4-triazol-5-one (ATO), stoichiometrically at alkaline pH and partially (50−60%) at pH ≤ 6.5. Due to NTO and hydroquinone speciation, the pseudo-first-order rate constants (k Obs ) varied by 3 orders of magnitude from pH 1.5 to 12.5 but remained constant from pH 4 to 10. This distinct pH dependency of k Obs suggests that NTO reactivity decreases upon deprotonation and offsets the increasing AQDS reactivity with pH. The reduction of NTO by LHA deviated continuously from firstorder behavior for >600 h. The extent of reduction increased with pH and LHA electron content, likely due to greater reactivity of and/or accessibility to hydroquinone groups. Only a fraction of the electrons stored in LHA was utilized for NTO reduction. Electron balance analysis and LHA redox potential profile suggest that the physical conformation of LHA kinetically limited NTO access to hydroquinone groups. This study demonstrates the importance of carbonaceous materials in controlling the environmental fate of NTO.

Quinone Moieties Link the Microbial Respiration of Natural Organic Matter to the Chemical Reduction of Diverse Nitroaromatic Compounds

“…We tested the ability of the NTO-respiring enrichment culture to reduce the NOM surrogate AQDS (Figure S1) to AQDS red . In this study, AQDS red , a compound with p K a values of 7.6 and 10.2, , represents the sum of the two reduced AQDS species at pH 7.2, 71.5% AH 2 QDS, and 28.5% AHQDS – . The culture completely reduced the AQDS to AQDS red while using acetate as the electron donor.…”

“…Different from 1.8 mg L –1 of humic acids, 1 μM AQDS, which can shuttle 2 μe – equivalents, supported the complete reduction of DNAN. The superior ability of the lowest concentrations of AQDS in shuttling electrons compared to the lowest concentrations of the humic acids can be explained by the quinone moieties of NOM not being as readily accessible as the quinone groups of AQDS due to NOM’s physical conformation. , Only a fraction of the quinone and hydroquinone moieties of humic acids are accessible to solutes due to the humic acids’ complex macromolecular structure, which is constantly rearranged as a function of environmental conditions. , …”

“…As an alternative to biotic pathways, the abiotic reduction of nitroaromatic compounds can be carried out by bulk reducing agents, such as reactive minerals, , hydroquinones, − or reduced natural organic matter (NOM). , Due to its reversible electron transfer and storage capacity, NOM is a crucial participant in several biogeochemical processes, comprising nitroaromatic transformation. ,,, NOM’s redox reactivity has been demonstrated to occur due to the presence of quinone moieties as a major source of reversible reducing groups in NOM. For instance, electron paramagnetic resonance (EPR) detection of semiquinone radicals in NOM indicates that semiquinones were derived from one-electron redox transformations of quinone or hydroquinone moieties .…”

“…For instance, electron paramagnetic resonance (EPR) detection of semiquinone radicals in NOM indicates that semiquinones were derived from one-electron redox transformations of quinone or hydroquinone moieties . Humic acids, which constitute around 60% of NOM, and model quinone compounds have similar reactivity to transport electrons for the reduction of aromatic pollutants, including nitroaromatics. ,, During the nitroaromatic reduction, the reduced quinone moieties of NOM (hydroquinone moieties) are oxidized back to quinones.…”

“…Interestingly, NOM was also reported as an electron acceptor for anaerobic microbial respiration. − The NOM-respiring microorganisms grow by reducing the quinone moieties in NOM to hydroquinones while using simple substrates, such as acetate, as electron donors . In this regard, nitroaromatic compounds may play a significant role in recycling the quinone moieties of NOM available for microbial respiration by reoxidizing hydroquinones to quinones. ,, This reoxidation enables the recycling of quinones during the nitroaromatic reduction. Previously, important considerations of this cycle were overlooked.…”