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

Cathodic electrochemical degradation of legacy (HMX, RDX, TNT) and insensitive (NTO, NQ, DNAN) munitions constituents

This research evaluated the cathodic electrochemical treatment of wastewater contaminated with energetic compounds, including "legacy" explosives octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), hexahydro-1,3,5-trinitro-s-triazine (RDX), and trinitrotoluene (TNT), as well as compounds insensitive munitions constituents, including 3-nitro-1,2,4-triazol-5-one (NTO), nitroguanidine (NQ), 2,4-dinitroanisole (DNAN). Rate constants and transformation products observed using electrochemical degradation performed under constant voltage (4 V) and constant current (0.5 A), as well as degradation via alkaline hydrolysis, were compared. Electrochemical degradation rate constants for all the energetics were greater than rate constants measured during alkaline hydrolysis. Degradation rate constants for individual energetics were generally similar to those observed in a mixture of all six compounds, with the exception of TNT (0.41 vs. 1.08 h-1). Many of the transformation products detected (e.g., HCHO, dinitrophenol (DNP), NH4+) evidenced further electrochemical degradation, but remained as residuals during alkaline hydrolysis. Utilizing 13C/15N labeled parent compounds, varying degrees of mineralization to 13CO2 and 15N2O were confirmed for RDX, NTO, and DNAN. The calculated electrical energy per order of removal (EEO) was generally lower under constant voltage compared to constant current conditions, and ranged from 2 Wh/L for TNT to 10 Wh/L for NTO. These results provide proof-of-concept data for cathodic electrochemical treatment of mixed energetics wastewater.

Electron exchange capacity of dissolved natural organic matter: further method development and interpretation using square wave voltammetry in dimethyl sulfoxide

Most measurements of the electron exchange capacity (EEC) of natural organic matter (NOM) have been done in water using mediated chronoamperometry (MCA), which gives precise results that are believed to be representative of the samples' current redox condition, but the broader significance of these EECs is less clear. In a recent study, we described a novel but complementary electrochemical approach to quantify EECs of 10 pyrogenic dissolved organic matter (pyDOM) and 6 standard/reference natural organic matter (NOM) materials without mediation using square-wave voltammetry (SWV) in dimethyl sulfoxide (DMSO). Comparison of the results obtained by MCA and SWV showed that SWV in DMSO gave larger EECs than MCA, by several-fold for NOM and 1-2 orders of magnitude for pyDOM. In this study, we describe an improved protocol for calibration of the SWV/DMSO method, which largely eliminates the difference in EECs from SWV and MCA for the standard/reference NOM samples. The results show that values obtained via the SWV method depend on the specific redox standards used for calibration (i.e., calibrant model compounds), with slopes that span 1.5 orders of magnitude due to variations in current response factors. For pyDOM, the higher values of EEC obtained by SWV were further verified and rationalized. Like the calibrant model compounds, it is proposed that the relatively large EECs for some pyDOM samples from high-temperature chars reflect a combination of hydrodynamic influences in our electrochemical cell, primarily related to electrode surface area to volume ratio and pyDOM size. A detailed explanation of the calibration method, choice of working electrode, DOM sorption effects, and cosolvent effects are discussed. The results obtained with this method suggest that the capacity of NOM for donating, accepting, and storing elections is an operationally defined property, the significance of which will depend on application, e.g., to carbon, metal, or nutrient cycling, pollutant attenuation, etc.

Predicting Abiotic Reduction Rate Constants of Munition Compounds in Soils

We report an empirical poly-parameter linear free energy relationship (LFER) for estimating the mass-normalized rate constants for the abiotic reduction of munition compounds (MC) in soil. A total of 131 kinetic experiments were performed, using three classes of MC (nitroaromatic [TNT, DNAN], nitramine [RDX], and azole [NTO]) and 11 soils having highly varied organic carbon and iron contents and reduced with dithionite to different electron contents. The LFER has the same form as that for MC reduction by FeIII (oxyhydr)oxide-FeaqII redox couples and predicts MC reduction rate constants to within an order of magnitude, using only the aqueous-phase one electron reduction potential (EH1) of the MC and the pe and pH of the soil. As previously shown for azoles, which exhibited markedly higher reactivity toward iron than toward carbon reductants relative to all neutral MC, NTO reduction rate depended on soil composition and hence a correction to model prediction was necessary at soil iron-to-carbon mass ratios ≲1. This is the first successful attempt to predict the reduction kinetics of structurally diverse nitro compounds in compositionally complex soils based on their thermodynamic properties. The LFER would be useful in the management/restoration (e.g., natural or enhanced attenuation) of soils impacted by MC or other nitro pollutants.

Participation of Strong H-Bonding to Acidic Groups Contributes to the Intense Sorption of the Anionic Munition, Nitrotriazolone (NTO) to the Carbon, Filtrasorb 400

5-Nitro-1,2-dihydro-3H-1,2,4-triazin-3-one ("nitrotriazolone," NTO) is an insensitive munition compound used in modern weaponry. It poses a potential threat to soil and water quality at relevant sites due to its physical properties that cause high mobility in the environment. NTO is polar and predominantly monoanionic (NTO-) at environmental pH (pKa1 = 3.64 and pKa2 = 11.06; determined by two independent methods in this study). Nevertheless, NTO- sorbs strongly to the carbon, Filtrasorb 400 (Freundlich coefficient KF = 1.26 × 104 Ln μmol1-n kg-1 at pH 9.5). We present evidence that sorption is contributed by the interaction of NTO- with functional groups of similar acidity on the carbon to form exceptionally strong, negative charge-assisted hydrogen bonds, (-)CAHB, written (-N-···H+···-O-[Formula: see text]), where -N- is the deprotonated N of NTO, and -O-[Formula: see text] represents a deprotonated surface group. Behaviors consistent with (-)CAHB include (1) a "hump" in the pH-sorption profile centered around pH 4, where maximal complexation is expected to occur; (2) an apparent ∼2.4-unit upward shift in pKa1 due to the enhanced H-bond strength; (3) the consumption of a proton from water to form the complex; and (4) sorption suppression by competing (-)CAHB-capable solutes. Anion exchange played a minor role. The findings help advance our understanding of weak acids sorption by carbonaceous materials.

Comparative Evaluation of Mediated Electrochemical Reduction and Chemical Redox Titration for Quantifying the Electron Accepting Capacities of Soils and Redox-Active Soil Constituents

The electron accepting capacity (EAC) of soil plays a pivotal role in the biogeochemical cycling of nutrients and transformation of redox-labile contaminants. Prior EAC studies of soils and soil constituents utilized different methods, reductants, and mediators, making cross-study comparison difficult. This study was conducted to quantify and compare the EACs of two soil constituents (hematite and Leonardite humic acid) and 12 soils of diverse composition, using chemical redox titration (CRT) with dithionite as the reductant and mediated electrochemical reduction (MER) with diquat as the mediator. The EACs of hematite and humic acid measured by CRT (EACCRT) and MER (EACMER) are similar and close to the theoretical/reported values. For soils, EACCRT and EACMER increased with iron and organic carbon (TOC) contents, suggesting iron and carbon were the main contributors to soil EAC. EACCRT > EACMER for all soils, and their difference (ΔEAC = EACCRT - EACMER) increased with TOC, presumably due to the longer contact time in CRT and thus more complete reduction of carbonaceous redox moieties. We propose an equation that relates EACCRT to EACMER (ΔEAC = 1796fTOC + 32) and another that predicts EACCRT from dithionite-reducible Fe and TOC (EACCRT = 2705 μmol e-/g C × fTOC + 17907 μmol e-/g Fe × fFedithionite-reducible). Our results suggest that at least 10-15% of soil organic carbon contributed to EACCRT.

Carbon and Nitrogen Isotope Fractionations During Biotic and Abiotic Transformations of 2,4-Dinitroanisole (DNAN)

2,4-Dinitroanisole (DNAN) is a main constituent in various new insensitive munition formulations. Although DNAN is susceptible to biotic and abiotic transformations, in many environmental instances, transformation mechanisms are difficult to resolve, distinguish, or apportion on the basis solely of analysis of concentrations. We used compound-specific isotope analysis (CSIA) to investigate the characteristic isotope fractionations of the biotic (by three microbial consortia and three pure cultures) and abiotic (by 9,10-anthrahydroquinone-2-sulfonic acid [AHQS]) transformations of DNAN. The correlations of isotope enrichment factors (ΛN/C) for biotic transformations had a range of values from 4.93 ± 0.53 to 12.19 ± 1.23, which is entirely distinct from ΛN/C values reported previously for alkaline hydrolysis, enzymatic hydrolysis, reduction by Fe2+-bearing minerals and iron-oxide-bound Fe2+, and UV-driven phototransformations. The ΛN/C value associated with the abiotic reduction by AHQS was 38.76 ± 2.23, within the range of previously reported values for DNAN reduction by Fe2+-bearing minerals and iron-oxide-bound Fe2+, albeit the mean ΛN/C was lower. These results enhance the database of isotope effects accompanying DNAN transformations under environmentally relevant conditions, allowing better evaluation of the extents of biotic and abiotic transformations of DNAN that occur in soils, groundwaters, surface waters, and the marine environment.

Electron Transfer Energy and Hydrogen Atom Transfer Energy-Based Linear Free Energy Relationships for Predicting the Rate Constants of Munition Constituent Reduction by Hydroquinones

No single linear free energy relationship (LFER) exists that can predict reduction rate constants of all munition constituents (MCs). To address this knowledge gap, we measured the reduction rates of MCs and their surrogates including nitroaromatics [NACs; 2,4,6-trinitrotoluene (TNT), 2,4-dinitroanisole (DNAN), 2-amino-4,6-dinitrotoluene (2-A-DNT), 4-amino-2,6-dinitrotoluene (4-A-DNT), and 2,4-dinitrotoluene (DNT)], nitramines [hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and nitroguanidine (NQ)], and azoles [3-nitro-1,2,4-triazol-5-one (NTO) and 3,4-dinitropyrazole (DNP)] by three dithionite-reduced quinones (lawsone, AQDS, and AQS). All MCs/NACs were reduced by the hydroquinones except NQ. Hydroquinone and MC speciations were varied by controlling pH, permitting the application of a speciation model to determine second-order rate constants (k) from observed pseudo-first-order rate constants. The intrinsic reactivity of MCs (oxidants) decreased upon deprotonation, while the opposite was true for hydroquinones (reductants). The rate constants spanned ∼6 orders of magnitude in the order NTO ≈ TNT > DNP > DNT ≈ DNAN ≈ 2-A-DNT > DNP^- > 4-A-DNT > NTO^- > RDX. LFERs developed using density functional theory-calculated electron transfer and hydrogen atom transfer energies and reported one-electron reduction potentials successfully predicted k, suggesting that these structurally diverse MCs/NACs are all reduced by hydroquinones through the same mechanism and rate-limiting step. These results increase the applicability of LFER models for predicting the fate and half-lives of MCs and related nitro compounds in reducing environments.

Photolysis of 3-Nitro-1,2,4-triazol-5-one: Mechanisms and Products

Insensitive munitions formulations that include 3-nitro-1,2,4-triazol-5-one (NTO) are replacing traditional explosive compounds. While these new formulations have superior safety characteristics, the compounds have greater environmental mobility, raising concern over potential contamination and cleanup of training and manufacturing facilities. Here, we examine the mechanisms and products of NTO photolysis in simulated sunlight to further inform NTO degradation in sunlit surface waters. We demonstrate that NTO produces singlet oxygen and that dissolved oxygen increases the NTO photolysis rate in deionized water. The rate of NTO photolysis is independent of concentration and decreases slightly in the presence of Suwannee River Natural Organic Matter. The apparent quantum yield of NTO generally decreases as pH increases, ranging from 2.0 × 10-5 at pH 12 to 1.3 × 10-3 at pH 2. Bimolecular reaction rate constants for NTO with singlet oxygen and hydroxyl radical were measured to be (1.95 ± 0.15) × 106 and (3.28 ± 0.23) × 1010 M-1 s-1, respectively. Major photolysis reaction products were ammonium, nitrite, and nitrate, with nitrite produced in nearly stoichiometric yield upon the reaction of NTO with singlet oxygen. Environmental half-lives are predicted to span from 1.1 to 5.7 days. Taken together, these data enhance our understanding of NTO photolysis under environmentally relevant conditions.

Significant Contribution of Solid Organic Matter for Hydroxyl Radical Production during Oxygenation

Dark formation of hydroxyl radicals (•OH) from soil/sediment oxygenation has been increasingly reported, and solid Fe(II) is considered as the main electron donor for O₂ activation. However, the role of solid organic matter (SOM) in •OH production is not clear, although it represents an important electron pool in the subsurface. In this study, •OH production from oxygenation of reduced solid humic acid (HA_red) was investigated at pH 7.0. •OH production is linearly correlated with the electrons released from HA_red suspension. Solid HA_red transferred electrons rapidly to O₂ via the surface-reduced moieties (hydroquinone groups), which was fueled by the slow electron transfer from the reduced moieties inside solid HA. Cycling of dissolved HA between oxidized and reduced states could mediate the electron transfer from solid HA_red to O₂ for •OH production enhancement. Modeling results predicted that reduced SOM played an important or even dominant role in •OH production for the soils and sediments possessing high molar ratios of SOC/Fe(II) (e.g., >39). The significant contribution of SOM was further validated by the modeling results for oxygenation of 88 soils/sediments in the literature. Therefore, reduced SOM should be considered carefully to comprehensively understand •OH production in SOM-rich subsurface environments.

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

Insensitive munitions compounds (IMCs) are emerging nitroaromatic contaminants developed by the military as safer-to-handle alternatives to conventional explosives. Biotransformation of nitroaromatics via microbial respiration has only been reported for a limited number of substrates. Important soil microorganisms can respire natural organic matter (NOM) by reducing its quinone moieties to hydroquinones. Thus, we investigated the NOM respiration combined with the abiotic reduction of nitroaromatics by the hydroquinones formed. First, we established nitroaromatic concentration ranges that were nontoxic to the quinone respiration. Then, an enrichment culture dominated by Geobacter anodireducens could indirectly reduce a broad array of nitroaromatics by first respiring NOM components or the NOM surrogate anthraquinone-2,6-disulfonate (AQDS). Without quinones, no nitroaromatic tested was reduced except for the IMC 3-nitro-1,2,4-triazol-5-one (NTO). Thus, the quinone respiration expanded the spectrum of nitroaromatics susceptible to transformation. The system functioned with very low quinone concentrations because NOM was recycled by the nitroaromatic reduction. A metatranscriptomic analysis demonstrated that the microorganisms obtained energy from quinone or NTO reduction since respiratory genes were upregulated when AQDS or NTO was the electron acceptor. The results indicated microbial NOM respiration sustained by the nitroaromatic-dependent cycling of quinones. This process can be applied as a nitroaromatic remediation strategy, provided that a quinone pool is available for microorganisms.

Modeling the Reduction Kinetics of Munition Compounds by Humic Acids

Dissolved organic matter (DOM) comprises a sizeable portion of the redox-active constituents in the environment and is an important reductant for the abiotic transformation of nitroaromatic compounds and munition constituents (NACs/MCs). Building a predictive kinetic model for these reactions would require the energies associated with both the reduction of the NACs/MCs and the oxidation of the DOM. The heterogeneous and unknown structure of DOM, however, has prohibited reliable determination of its oxidation energies. To overcome this limitation, humic acids (HAs) were used as model DOM, and their redox moieties were modeled as a collection of quinones of different redox potentials. The reduction and oxidation energies of the NACs/MCs and hydroquinones, respectively, via hydrogen atom transfer (HAT) reactions were then calculated quantum chemically. HAT energies have been used successfully in a linear free energy relationship (LFER) to predict second-order rate constants for NAC reduction by hydroquinones. Furthermore, a linear relationship between the HAT energies and the reduction potentials of quinones was established, which allows estimation of hydroquinone reactivity (i.e., rate constants) from HA redox titration data. A training set of three HAs and two NACs/MCs was used to generate a mean HA redox profile that successfully predicted reduction kinetics in multiple HA/MC systems.

Abiotic reduction of 3-nitro-1,2,4-triazol-5-one (NTO) and other munitions constituents by wood-derived biochar through its rechargeable electron storage capacity

The environmental fate of 3-nitro-1,2,4-triazol-5-one (NTO) and other insensitive munitions constituents (MCs) is of significant concern due to their high water solubility and mobility relative to legacy MCs. Plant-based biochars have been shown to possess a considerable electron storage capacity (ESC), which enables them to undergo reversible electron transfer reactions. We hypothesized biochar can act as a rechargeable electron donor to effect abiotic reduction of MCs repeatedly through its ESC. To test this hypothesis, MC reduction experiments were performed using wood-derived biochars that were oxidized with dissolved oxygen or reduced with dithionite. Removal of aqueous NTO, an anion at circumneutral pH, by oxidized biochar was minimal and occurred through reversible adsorption. In contrast, NTO removal by reduced biochar was much more pronounced and occurred predominantly through reduction, with concomitant formation of 3-amino-1,2,4-triazol-5-one (ATO). Mass balance and electron recovery with ferricyanide further showed that (1) the amount of NTO reduced to ATO was relatively constant (85-100 μmol per gram of biochar) at pH 6-10; (2) the fraction of biochar ESC reactive toward NTO was ca. 30% of that toward ferricyanide; (3) the NTO-reactive fraction of the ESC was regenerable over multiple redox cycles. We also evaluated biochar transformation of other MCs, including nitroguanidine (NQ), 2,4-dinitroanisole (DNAN), and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). While mass and electron balances could not be established due to sorption, DNAN and RDX reduction by reduced biochar was confirmed via detection of multiple reduction products. In contrast, NQ was not reduced under any of the conditions tested. This study is the first demonstration of organic contaminant degradation through biochar's rechargeable ESC. Our results indicate biochar is a regenerable electron storage medium and sorbent that can remove MCs from water through concurrent reduction and sorption, and is thus potentially useful for pollution control and remediation at military facilities.