Column transport studies of 3-nitro-1,2,4-triazol-5-one (NTO) in soils

A laboratory rill study of IMX-104 transport in overland flow

The deposition of explosive contaminants in particulate form onto the soil surface during low-order detonations can lead to ground and surface water contamination. The vertical fate and transport of insensitive munitions formulation IMX-104 through soil has been thoroughly studied, however the lateral transport of explosive particles on the surface is less known. The objective of this research was to understand the impact of overland flow on the transport of IMX-104 constituent compounds 3-nitro-1,2,4-triazol-5-one (NTO), 2,4-dinitroanisole (DNAN), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The effect of overland flow was examined in a rill flume using several flow rates (165-, 265-, and 300-mL min^-1) and IMX-104 particle sizes (4.75-9.51 mm, 2.83-4.75 mm, 2-2.83 mm, and <2 mm). We found that the smaller particles were transported more in solution and with the sediment compared to the larger particles, which had a higher percent mass remaining on the surface. As flow rate increased, there was an increase in the percent mass found in solution and sediment and a decrease in the percent mass remaining on the surface. NTO fate was dominated by transport in solution, while DNAN, RDX and HMX were predominantly transported with the sediment. This research provides evidence of the role of overland flow in the fate of energetic compounds.

Outdoor dissolution and photodegradation of insensitive munitions formulations IMX-101 and IMX-104: Photolytic transformation pathway and mechanism study

New munition compounds have been developed to replace traditional explosives to prevent unintended detonations. However, insensitive munitions (IM) can leave large proportion of unexploded charge in the field, where it is subjected to photodegradation and dissolution in precipitation. The photolytic reactions occurring on the surfaces of IMX-101 and IMX-104 formulations and the subsequent fate of photolytic products in the environment were thoroughly investigated. The constituents of IMX-101 and IMX-104 formulations dissolve sequentially under rainfall in the order of aqueous solubility: 3-nitro-1,2,4-triazol-5-one (NTO) > nitroguanidine (NQ) > 2,4-dinitroanisole (DNAN) > 1,3,5-hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). A linear relationship between DNAN dissolution and rainwater volume was observed (r²: 0.86-0.99). It was estimated that it would take 16-228 years to completely dissolve these formulation particles under natural environmental conditions in Oracle, AZ. We used LC/MS/MS and GC/MS to examine the dissolution samples from IMX-101 and 104 particles exposed to rainfall and sunlight and found six DNAN photo-transformation products including 2-methoxy-5-nitrophenol, 4-methoxy-3-nitrophenol, 4-methoxy-3-nitroaniline, 2-methoxy-5-nitroaniline, 2,4-dinitrophenol, and methoxy-dinitrophenol, which are in good agreement with computational modeling results of bond strengths. The main DNAN photodegradation pathways are therefore proposed. Predicted eco-toxicity values suggested that the parent compound DNAN, methoxy-nitrophenols, methoxy-nitroanilines and the other two products (2,4-dinitrophenol and methoxy-dinitrophenol) would be harmful to fish and daphnid. Our study provides improved insight about the rain dissolution and photochemical behavior of IM formulations under natural conditions, which helps to form target-oriented strategies to mitigate explosive contamination in military training sites.

A review of treatment methods for insensitive high explosive contaminated wastewater

Insensitive high explosive materials (IHE) such as 3-nitro-1,2,4-triazol-5-one (NTO) and 2,4-dinitroanisole (DNAN) are increasingly being used in formulations of insensitive munitions alongside 1,3,5-trinitroperhydro-1,3,5-triazine (RDX). Load, assembly and packing (LAP) facilities that process munitions produce wastewater contaminated with IHE which must be treated before discharge. Some facilities can produce as much as 90,000 L of contaminated wastewater per day. In this review, methods of wastewater treatment are assessed in terms of their strengths, weaknesses, opportunities and threats for their use in production of IHE munitions including their limitations and how they could be applied to industrial scale LAP facilities. Adsorption is identified as a suitable treatment method, however the high solubility of NTO, up to 16.6 g.L⁻¹ which is 180 times higher that of TNT, has the potential to exceed the adsorptive capacity of carbon adsorption systems. The key properties of the adsorptive materials along the selection of adsorption models are highlighted and recommendations on how the limitations of carbon adsorption systems for IHE wastewater can be overcome are offered, including the modification of carbons to increase adsorptive capacity or reduce costs.

Accumulation of Insensitive Munition Compounds in the Earthworm Eisenia andrei from Amended Soil: Methodological Considerations for Determination of Bioaccumulation Factors

The present study investigates the bioaccumulation of the insensitive munition compounds 2,4-dinitroanisole (DNAN) and 3-nitro-1,2,4-triazol-5-one (NTO), developed for future weapons systems to replace current munitions containing sensitive explosives. The earthworm Eisenia andrei was exposed to sublethal concentrations of DNAN or NTO amended in Sassafras sandy loam. Chemical analysis indicated that 2- and 4-amino-nitroanisole (2-ANAN and 4-ANAN, respectively) were formed in DNAN-amended soils. The SumDNAN (sum of DNAN, 2-ANAN, and 4-ANAN concentrations) in soil decreased by 40% during the 14-d exposure period. The SumDNAN in the earthworm body residue increased until day 3 and decreased thereafter. Between days 3 and 14, there was a 73% decrease in tissue uptake that was greater than the 23% decrease in the soil concentration, suggesting that the bioavailable fraction may have decreased over time. By day 14, the DNAN concentration accounted for only 45% of the SumDNAN soil concentration, indicating substantial DNAN transformation in the presence of earthworms. The highest bioaccumulation factor (BAF; the tissue-to-soil concentration ratio) was 6.2 ± 1.0 kg/kg (dry wt) on day 3 and decreased to 3.8 ± 0.8 kg/kg by day 14. Kinetic studies indicated a BAF of 2.3 kg/kg, based on the earthworm DNAN uptake rate of 2.0 ± 0.24 kg/kg/d, compared with the SumDNAN elimination rate of 0.87 d^-1 (half-life = 0.79 d). The compound DNAN has a similar potential to bioaccumulate from soil compared with trinitrotoluene. The NTO concentration in amended soil decreased by 57% from the initial concentration (837 mg NTO/kg dry soil) during 14 d, likely due to the formation of unknown transformation products. The bioaccumulation of NTO was negligible (BAF ≤ 0.018 kg/kg dry wt). Environ Toxicol Chem 2021;40:1713-1725. © 2021 SETAC. This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Computational Investigation on Interactions between Some Munition Compounds and Humic Substances

Humic acid substances (HAs) in natural soil and sediment environments affect the retention and degradation of insensitive munition compounds and legacy high explosives (MCs): 2,4-dinitroanisole (DNAN), DNi^-NH₄⁺, N-methyl-p-nitroaniline (nMNA), 1-nitroguanidine (NQ), 3-nitro-1,2,4-triazol-5-one (NTO; neutral and anionic forms), 2,4,6-trinitrotoluene (TNT), and 1,3,5-trinitro-1,3,5-triazinane (RDX). A humic acid model compound has been considered using molecular dynamics, thermodynamic integration, and density functional theory to characterize the munition binding ability, ionization potential, and electron affinity compared to that in the water solution. Humic acids bind most compounds and act as both a sink and source for electrons. Ionization potentials suggest that HAs are more susceptible to oxidation than the MCs studied. The electron affinity of HAs is very conformation-dependent and spans the same range as the munition compounds. When HAs and MCs are complexed, the HAs tend to radicalize first, thus buffering MCs against reductive as well as oxidative attacks.

Sorption, transport, and transformation of natural and synthetic progestins in soil-water systems

Natural and synthetic progestins are emerging endocrine disruptors that can be transported from livestock farms and agricultural fields to receiving waters via surface runoff. The transformation of progestins during transport is expected to affect the efficiencies of runoff management systems. Therefore, this study explored the sorption, transport, and transformation of progesterone, norethisterone acetate, medroxyprogesterone acetate, cyproterone acetate, dydrogesterone, and norethisterone in agricultural soil-water systems. The sorption coefficients and retardation factors (R) were positively correlated with the progestin hydrophobicities, indicating that hydrophobic interactions dominated the sorption and transport processes. During transport, dydrogesterone and progesterone were transformed into 9-10 products. The breakthrough curves of the parents and products exhibited periodical patterns over extended times. Specifically, the R values of the parents and products were positively correlated with chromatographic retention times (hydrophobicities) when the products were generated before transport. In contrast, a negative correlation (R² = 0.75-0.88) was observed when products were successively generated during transport, indicating that the transformation kinetics changed the retardation of these solutes in the columns. These observations also demonstrated that the transport potential estimates based on traditional metrics of steroid hydrophobicity are not always accurate and that runoff management measures are less effective for metastable progestins.

Chronic oral toxicity of 3-nitro-1,2,4-triazol-5-one (NTO) in rats

To evaluate the effects of chronic exposure to 3-nitro-1,2,4-triazol-5-one (nitrotriazolone, NTO), male and female rats were given ad libitum access to NTO in drinking water at concentrations of 0, 36, 110, 360, 1100, and 3600 mg/L for one year. NTO did not affect body weight, body weight gain, or food consumption in either sex. No treatment-related effects were observed in clinical chemistry and hematology parameters at the 6 month or one year sampling. At both the interim and final sampling, males and females from the 3600 mg/L group produced smaller volumes of urine that was darker, more concentrated, and contained more bilirubin than the controls. Total and motile sperm counts were not affected by NTO treatment. Absolute and relative organ weights did not differ between control and NTO treated groups for either sex. Spontaneous age-related neoplasms occurred in controls and NTO groups at rates consistent with published historic controls. NTO was generally non-toxic in females at the doses tested. Toxicity in males was limited to testicular toxicity as demonstrated in previous studies. Chronic exposure did not result in testicular toxicity at lower doses and the toxicity observed only in the high dose group in this study is less severe than that observed in shorter exposures of previous studies, suggesting differences may be associated with influences of study design on kinetics. A Benchmark Dose (BMD) of 1604 mg/L (76 mg/kg-day) and a Benchmark Dose Lower Bound (BMDL₁₀) of 921 mg/L (44 mg/kg-day) were determined for chronic effects of NTO in male rats.

Microbial Enrichment Culture Responsible for the Complete Oxidative Biodegradation of 3-Amino-1,2,4-triazol-5-one (ATO), the Reduced Daughter Product of the Insensitive Munitions Compound 3-Nitro-1,2,4-triazol-5-one (NTO)

3-Nitro-1,2,4-triazol-5-one (NTO) is one of the main ingredients of many insensitive munitions, which are being used as replacements for conventional explosives. As its use becomes widespread, more research is needed to assess its environmental fate. Previous studies have shown that NTO is biologically reduced to 3-amino-1,2,4-triazol-5-one (ATO). However, the final degradation products of ATO are still unknown. We have studied the aerobic degradation of ATO by enrichment cultures derived from the soil. After multiple transfers, ATO degradation was monitored in closed bottles through measurements of inorganic carbon and nitrogen species. The results indicate that the members of the enrichment culture utilize ATO as the sole source of carbon and nitrogen. As ATO was mineralized to CO₂, N₂, and NH₄⁺, microbial growth was observed in the culture. Co-substrates addition did not increase the ATO degradation rate. Quantitative polymerase chain reaction analysis revealed that the organisms that enriched using ATO as carbon and nitrogen source were Terrimonas spp., Ramlibacter-related spp., Mesorhizobium spp., Hydrogenophaga spp., Ralstonia spp., Pseudomonas spp., Ectothiorhodospiraceae, and Sphingopyxis. This is the first study to report the complete mineralization of ATO by soil microorganisms, expanding our understanding of natural attenuation and bioremediation of the explosive NTO.

The effect of soil type on the extraction of insensitive high explosive constituents using four conventional methods

Explosive contamination is commonly found at military and manufacturing sites (Hewitt et al., 2005; Clausen et al., 2004; Walsh et al., 2013). Under current environmental legislation the extent of the contamination must be characterized by soil sampling and subsequent separation of the explosive contaminants from the soil matrix by extraction to enable chemical analysis and quantification (Dean, 2009). It is essential that the extraction method can consistently recover explosive residue from a variety of soil types i.e. all materials that have not degraded or irreversibly bound to the matrix, so that any resultant risk is not underestimated. In this study, five different soil types with a range of organic content, particle size and pH, were spiked with a mixture of RDX, DNAN, NQ and NTO at 50 mg/kg and were extracted using one of four one-step extraction methods: stirring, shaking, sonication, and accelerated solvent extraction (ASE). Analysis of the extraction efficiencies of the four methods found that they were broadly successful for the extraction of all IHE constituents from all five soils (an average of 84% ± 14% recovery across 80 extractions). However, soils with high organic content (Total Organic Content (TOC) ≥ 2%) were found to significantly affect extraction efficiency and reproducibility. NTO and DNAN were the least consistent in extraction efficiency with poorest recovery of NTO as low as 37% ± 2%. Of the four tested methods shaking was found to be the most reproducible, though less efficient than stirring (64%-91%). ASE was found to have the most variable results for extraction of IHE constituents suggesting that ASE was the most affected by the different soil types. Therefore, it is recommended that the efficiency and reproducibility of the selected extraction method should be validated by extracting known concentrations of the IHE from the soil of interest and that any required correction factors are reported.

Investigation into the environmental fate of the combined Insensitive High Explosive constituents 2,4-dinitroanisole (DNAN), 1-nitroguanidine (NQ) and nitrotriazolone (NTO) in soil

Contamination of military ranges by the use of explosives can lead to irreversible environmental damage, specifically to soil and groundwater. The fate and effects of traditional explosive residues are well understood, while less is known about the impact of Insensitive High Explosives (IHEs) that are currently being brought into military service. Current research has focussed on the investigation of individual constituents of IHE formulations, which may not be representative of real-world scenarios when explosive residues will be deposited together. Therefore, this study investigated the fate and transport of the combined IHE constituents 2,4-dinitroanisole (DNAN), 1-nitroguanidine (NQ) and 3-nitro-1,2,4-triazol-5-one (NTO) in two UK soil types. Static experiments ran for 9weeks to determine the fate of the combined explosive constituents in soil by monitoring the rate of degradation. Transport was examined by running soil column experiments for 5weeks, with a watering regime equivalent to the average yearly UK rainfall. Both static and soil column experiments confirmed that DNAN and NTO started to degrade within twenty-four hours in soil with high organic content, and were both completely degraded within sixty days. NQ was more stable, with 80% of the original material recovered after sixty days. The major degradation product of DNAN in the test soils was 2-amino-4-nitroanisole (2-ANAN), with trace amounts of 4-amino-2-nitroanisole. NTO was rapidly degraded in soil with high organic content, although no degradation products were identified. Results supported work from literature on the individual constituents DNAN, NQ and NTO suggesting that the three explosives in combination did not interact with each other when in soil. This study should provide a useful insight into the behaviour of three combined Insensitive High Explosive constituents for the predication of soil and water contamination during military training.

Dissolution and transport of insensitive munitions formulations IMX-101 and IMX-104 in saturated soil columns

Military training exercises can result in deposition of energetic residues on range soils, which ultimately can contaminate groundwater with munitions constituents. Column experiments followed by HYDRUS-1D modeling were conducted to evaluate dissolution and transport of energetic constituents from the new insensitive munitions (IM) formulations IMX-101, a mixture of 3-nitro-1,2,4-triazol-5-one (NTO), nitroguanidine (NQ), and 2, 4-dinitroanisole (DNAN), and IMX-104, a mixture of NTO, 1,3,5-hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and DNAN. NTO and DNAN are emerging contaminants associated with the development of insensitive munitions as replacements for traditional munitions. Flow interruption experiments were performed to investigate dissolution kinetics and sorption non-equilibrium between soil and solution phases. The results indicated that insensitive munitions compounds dissolved in order of their aqueous solubility, consistent with prior dissolution studies conducted in the absence of soil. Initial elution of the high concentration pulse of highly soluble NTO and NQ was followed by lower concentrations, while DNAN had generally lower and more constant concentrations in leachate. The sorption of NTO and NQ was low, while RDX, 1,3,5,7-octahydro-1,3,5,7-tetranitrotetrazocine (HMX, an impurity in technical grade RDX), and DNAN all exhibited appreciable sorption. DNAN transformation was observed, with formation of amino-reduction products 2-ANAN (2-amino-4-nitroanisole) and 4-ANAN (4-amino-2-nitroanisole). HYDRUS-1D model, incorporating one-dimensional advective-dispersive transport with particle dissolution and first-order solute transformation was used to simulate the measured breakthrough curves. Optimized dissolution parameters varied widely but were correlated between compounds in the same formulation. Determined adsorption coefficients generally agreed with values determined from batch and column studies conducted with pure NTO and DNAN, while mass-loss rate coefficients were in better agreement with ones from batch than column studies possibly due to suppression of microbial transformation during elution of high concentrations of explosives. Even in the low organic matter soils selected in this study DNAN experienced significant retardation and transformation, indicating potential for its natural attenuation.