Effects, uptake, and fate of 2,4,6-trinitrotoluene aged in soil in plants and worms

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.

Analysis of the biodegradation and phytotoxicity mechanism of TNT, RDX, HMX in alfalfa (Medicago sativa)

The aim of this study was to reveal the mechanism underlying the toxicity of TNT (trinitrotoluene), RDX (cyclotrimethylene trinitroamine), and HMX (cyclotetramethylene tetranitramine) explosives pollution in plants. Here, the effects of exposure to these three explosives were examined on chlorophyll fluorescence, antioxidant enzyme activity, and the metabolite spectrum in alfalfa (Medicago sativa) plants. The degradation rates for TNT, RDX, and HMX by alfalfa were 26.8%, 20.4%, and 18.4%, respectively, under hydroponic conditions. TNT caused damage to the microstructure of the plant roots and inhibited photosynthesis, whereas RDX and HMX induced only minor changes. Exposure to any of the three explosives caused disturbances in the oxidase system. Non-targeted metabolomics identified a total of 6185 metabolites. TNT exposure induced the appearance of 609 differentially expressed metabolites (189 upregulated, 420 downregulated), RDX exposure induced 197 differentially expressed metabolites (155 upregulated and 42 downregulated), and HMX induced 234 differentially expressed metabolites (132 upregulated and 102 downregulated). Of these differentially expressed metabolites, lipids and lipid-like molecules were the main metabolites induced by explosives poisoning. TNT mainly caused significant changes in the alanine, aspartate, and glutamate metabolism metabolic pathways, RDX mainly caused disorders in the arginine biosynthesis metabolic pathway, and HMX disrupted the oxidative phosphorylation metabolic pathway. Taken together, the results show that exposure to TNT, RDX, and HMX leads to imbalances in plant photosynthetic characteristics and antioxidant enzyme systems, changes the basic metabolism of plants, and has significant ecotoxicity effects.

Multiple stressors in multiple species: Effects of different RDX soil concentrations and differential water-resourcing on RDX fate, plant health, and plant survival

Response to simultaneous stressors is an important facet of plant ecology and land management. In a greenhouse trial, we studied how eight plant species responded to single and combined effects of three soil concentrations of the phytotoxic munitions constituent RDX and two levels of water-resourcing. In an outdoor trial, we studied the effects of high RDX soil concentration and two levels of water-resourcing in three plant species. Multiple endpoints related to RDX fate, plant health, and plant survival were evaluated in both trials. Starting RDX concentration was the most frequent factor influencing all endpoints. Water-resourcing also had significant impacts, but in fewer cases. For most endpoints, significant interaction effects between RDX concentration and water-resourcing were observed for some species and treatments. Main and interaction effects were typically variable (significant in one treatment, but not in another; associated with increasing endpoint values for one treatment and/or with decreasing endpoint values in another). This complexity has implications for understanding how RDX and water-availability combine to impact plants, as well as for applications like phytoremediation. As an additional product of these greenhouse and outdoor trials, three plants native or naturalized within the southeastern United States were identified as promising species for further study as in situ phytoremediation resources. Plumbago auriculata exhibited relatively strong and markedly consistent among-treatment mean proportional reductions in soil RDX concentrations (112% and 2.5% of the means of corresponding values observed within other species). Likewise, across all treatments, Salvia coccinea exhibited distinctively low variance in mean leaf chlorophyll content index levels (6.5% of the means of corresponding values observed within other species). Both species also exhibited mean wilting and chlorosis levels that were 66% and 35%, and 67% and 84%, of corresponding values observed in all other plants, respectively. Ruellia caroliniensis exhibited at least 43% higher mean survival across all treatments than any other test species in outdoor trials, despite exhibiting similar RDX uptake and bioconcentration levels.

Effects of TNT contaminated soil on vegetation at an explosive range by probing UPLC-qTOF MS profiling method

Three tree species (Wild olive, Stinkwood and Cape Holy) and a shrub (Dovyalis caffra) were each potted in 20 L pots in order to evaluate the effect of 1,3,5-trinitrotoluene (TNT)-contaminated soil on vegetation. TNT contamination was established by dissolving flake TNT in acetone at 300 and 600 mg per kilogram soil concentrations. One pot for every species was left uncontaminated as control elements. A set of 16 samples, four contaminated, four uncontaminated aerial parts and their corresponding soils, were gathered. These were processed and subjected to a solid phase extraction method to isolate analytes of interest. A laboratory analytical method was applied using ultra-performance liquid chromatography coupled to quadrupole time-of-flight mass spectrometry (UPLC-qTOF MS). For the UPLC-qTOF MS a gradient for the mobile phase was found which allowed the profiling and separation of metabolites in the aerial parts of the vegetation. This method allowed identification and quantification of major changes caused by TNT contaminated soil on vegetation. The Synapt High Definition Mass Spectrometer SYNAPT HDMS G1 was operated using the electrospray ionisation (ESI) technique in both positive and negative mode. A clear comparison of profiles was achieved and this has been demonstrated by the distinct newly-formed metabolites in the TNT contaminated vegetation understudy. The results have also shown that the chlorophyll region in the contaminated profile was also affected by the uptake of TNT degradation products. This has been observed in the contaminated profiles of Wild olive, Stinkwood and Cape Holly extracts indicating enhanced nutrient availability.

Bioconcentration factors and plant-water partition coefficients of munitions compounds in barley

Plants growing in the soils at military ranges and surrounding locations are exposed, and potentially able to uptake, munitions compounds (MCs). The extent to which a compound is transferred from the environment into organisms such as plants, referred to as bioconcentration, is conventionally measured through uptake experiments with field/synthetic soils. Multiple components/phases that vary among different soil types and affect the bioavailability of the MC, however, hinder the ability to separate the effects of soil characteristics from the MC chemical properties on the resulting plant bioconcentration. To circumvent the problem, this work presents a protocol to measure steady state bioconcentration factors (BCFs) for MCs in barley (Hordeum vulgare L.) using inert laboratory sand rather than field/synthetic soils. Three MCs: 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4-DNT), and 2,4-dinitroanisole (2,4-DNAN), and two munition-like compounds (MLCs): 4-nitroanisole (4-NAN) and 2-methoxy-5-nitropyridine (2-M-5-NPYNE) were evaluated. Approximately constant plant biomass and exposure concentrations were achieved within a one-month period that produced steady state log BCF values: 0.62 ± 0.02, 0.70 ± 0.03, 1.30 ± 0.06, 0.52 ± 0.03, and 0.40 ± 0.05 L kg_{plant dwt}^-1 for TNT, 2,4-DNT, 2,4-DNAN, 4-NAN, and 2-M-5-NPYNE, respectively. Furthermore, results suggest that the upper-bounds of the BCFs can be estimated within an order of magnitude by measuring the partitioning of the compounds between barley biomass and water. This highlights the importance of partition equilibrium as a mechanism for the uptake of MCs and MLCs by barley from interstitial water. The results from this work provide chemically meaningful data for prediction models able to estimate the bioconcentration of these contaminants in plants.

Impacts of explosive compounds on vegetation: A need for community scale investigations

Explosive compounds are distributed heterogeneously across the globe as a result of over a century of human industrial and military activity. RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) and TNT (2-methyl-1,3,5-trinitrobenzene) are the most common and most abundant explosives in the environment. Vegetation exhibits numerous physiological and morphological stress responses in the presence of RDX and TNT. Varied stress responses act as physiological filters that facilitate the proliferation of tolerant species and the extirpation of intolerant species. Contaminants alter community composition as they differentially impact plants at each life stage (i.e. germination, juvenile, adult), subsequently modifying larger scale ecosystem processes. This review summarizes the current explosives-vegetation literature, focusing on RDX and TNT as these are well documented in the literature, linking our current understanding to ecological theory. A conceptual framework is provided that will aid future efforts in predicting plant community response to residual explosive compounds.

Physiological and transcriptional responses of Baccharis halimifolia to the explosive "composition B" (RDX/TNT) in amended soil

Unexploded explosives that include royal demolition explosive (RDX) and trinitrotoluene (TNT) cause environmental concerns for surrounding ecosystems. Baccharis halimifolia is a plant species in the sunflower family that grows naturally near munitions sites on contaminated soils, indicating that it might have tolerance to explosives. B. halimifolia plants were grown on 100, 300, and 750 mg kg(-1) of soil amended with composition B (Comp B) explosive, a mixture of royal demolition explosive and trinitrotoluene. These concentrations are environmentally relevant to such munitions sites. The purpose of the experiment was to mimic contaminated sites to assess the plant's physiological response and uptake of explosives and to identify upregulated genes in response to explosives in order to better understand how this species copes with explosives. Stomatal conductance was not significantly reduced in any treatments. However, net photosynthesis, absorbed photons, and chlorophyll were significantly reduced in all treatments relative to the control plants. The dark-adapted parameter of photosynthesis was reduced only in the 750 mg kg(-1) Comp B treatment. Thus, we observed partial physiological tolerance to Comp B in B. halimifolia plants. We identified and cloned 11 B. halimifolia gene candidates that were orthologous to explosive-responsive genes previously identified in Arabidopsis and poplar. Nine of those genes showed more than 90% similarity to Conyza canadensis (horseweed), which is the closest relative with significant available genomics resources. The expression patterns of these genes were studied using quantitative real-time PCR. Three genes were transcriptionally upregulated in Comp B treatments, and the Cytb6f gene was found to be highly active in all the tested concentrations of Comp B. These three newly identified candidate genes of this explosives-tolerant plant species can be potentially exploited for uses in phytoremediation by overexpressing these genes in transgenic plants and, similarly, by using promoters or variants of promoters from these genes fused to reporter genes in transgenic plants for making phytosensors to report the localized presence of explosives in contaminated soils.

Sustainable remediation--the application of bioremediated soil for use in the degradation of TNT chips

Environmental contamination by TNT (2,4,6 trinitrotoluene), historically used in civilian industries and the military as an explosive is of great concern due to its toxicity. Scientific studies have however shown that TNT is susceptible to microbial transformation. The aim of this study was to assess the potential of a previously bioremediated hydrocarbon contaminated soil (PBR) to increase TNT degradation rates. This was investigated by adding TNT chips to PBR and uncontaminated soils (PNC) in laboratory based studies (up to 16 weeks). Residual TNT chip analysis showed greater TNT degradation in PBR soils (70%) and significantly higher metabolic rates (4.5 fold increase in cumulative CO(2) levels) than in PNC soils (30%). Molecular analysis (PCR-DGGE-cluster analysis) showed substantial shifts in soil microbial communities associated with TNT contamination between day 0 and week 4 especially in PBR soils. Bacterial communities appeared to be more sensitive to TNT contamination than fungal communities in both soils. Quantitative PCR analysis showed ~3 fold increase in the abundance of nitroreductase genes (pnrA) in PBR soils with a gradual reduction in community evenness (Pareto-Lorenz curves) in contrast to PNC soils. These results suggest that microbial response to TNT contamination was dependent on the history of soil use. The results also confirm that the microbial potential of waste soils such as PBR soil (usually disposed of via landfill) can be successfully used for accelerated TNT chip degradation. This promotes sustainable re-use of waste soils extending the life span of landfill sites.

Plant tissue analysis for explosive compounds in phytoremediation and phytoforensics

Plant tissue analysis methods were evaluated for six explosive compounds to assess uptake and phytoforensic methods development to quantify explosives in plant to obtain the plant data for the evaluation of explosive contamination in soil and groundwater. Four different solvent mixtures containing acetonitrile or methanol were tested at variable extraction ratios to compare the extraction efficiency for six explosive compounds: 2,4,6-trinitrotoluene (TNT), pentaerythritoltetranitrate (PETN), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), 2-amino-4,6-dinitrotoluene (2ADNT), and 2,4-Dinitroanisole (DNAN), in Laurel Willow (Salix pentandra) stem and range grass Big Bluestem (Andropogon gerardii) using LC-MS/MS. Plant tissues were spiked with 500 ng/g of explosives and extracted using ultrasonically-assisted solvent extraction. With the ratio of fresh plant mass to solvent volume of 1:20 for willow and 1:40 for big bluestem grass, results indicated that all explosives in willow except HMX were extracted at higher than 73.3% by using 20 mL of methanol, 50:50 (v/v) methanol:water, or acetonitrile, whereas HMX was extracted with the highest recovery of 61.3% by 20 mL of acetonitrile. In big bluestem grass, the most effective solvents were 20 mL of either methanol or 50:50 (v/v) methanol:water for PETN extraction with a recovery of higher than 101.2% and 20 mL of 50:50 (v/v) methanol:water for HMX, RDX, TNT, 2ADNT, and DNAN extraction with a recovery of 83.8%, 104.4%, 97.5%, 80.7%, and 108.2%, respectively. However, unlike methanol and acetonitrile, 50:50 (v/v) methanol:water provided no problem of leading or split peak in chromatogram; therefore, it was preferred in the test and performed a method validation. Results indicated that 50:50 (v/v) methanol:water provided good repeatability and recovery and method detection limits at 0.5-20 ng/g fresh weight or 8.8-61.3 ng/g dry weight. Overall, results suggested that solvent extraction efficiency of explosives in plant was influenced by plant species and solvent used, and method presented here was believed to provide the preliminary data with respect to the analysis of simultaneous explosives in plants with LC-MS/MS.

Soils contaminated with explosives: Environmental fate and evaluation of state-of-the-art remediation processes (IUPAC Technical Report)

An explosion occurs when a large amount of energy is suddenly released. This energy may come from an over-pressurized steam boiler, from the products of a chemical reaction involving explosive materials, or from a nuclear reaction that is uncontrolled. In order for an explosion to occur, there must be a local accumulation of energy at the site of the explosion, which is suddenly released. This release of energy can be dissipated as blast waves, propulsion of debris, or by the emission of thermal and ionizing radiation. Modern explosives or energetic materials are nitrogen-containing organic compounds with the potential for self-oxidation to small gaseous molecules (N2, H2O, and CO2). Explosives are classified as primary or secondary based on their susceptibility of initiation. Primary explosives are highly susceptible to initiation and are often used to ignite secondary explosives, such as TNT (2,4,6-trinitrotoluene), RDX (1,3,5-trinitroperhydro-1,3,5-triazine), HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane), and tetryl (N-methyl-N-2,4,6-tetranitro-aniline).

Toxic effects of oral 2-amino-4,6-dinitrotoluene in the Western fence lizard (Sceloporus occidentalis)

The compound 2-amino-4,6-dinitrotoluene (2A-DNT) was evaluated under laboratory conditions in the Western fence lizard (Sceloporus occidentalis) to assess the potential for reptile toxicity. Oral LD(50) values were 1406 and 1867 mg/kg for male and female lizards, respectively. Based on responses from a 14-day subacute study, a 60-day subchronic experiment followed where lizards were orally dosed at 0, 5, 15, 20, 25, 30 mg/kg-d. At day 60, number of days and survivors, food consumption, and change in body weight were inversely related to dose. Signs of toxicity were characterized by anorexia and generalized cachexia. Significant adverse histopathology was observed in hepatic tissue at ≥ 15 mg/kg-d, consistent with hepatocellular transdifferentiation. Based on survival, loss of body weight, diminished food intake, changes in liver, kidney, and testes, and increased blood urea nitrogen, these data suggest a LOAEL of 15 mg/kg-d and a NOAEL of 5 mg/kg-d in S. occidentalis.

Uptake and transformation of soil [14C]-trinitrotoluene by cool-season grasses

This study investigated the fate and uptake of [(14)C]-TNT from soil into orchardgrass (Dactylis glomerata), perennial ryegrass (Lolium perenne), and tall fescue (Festuca arundinacea) over a one year period in a greenhouse-controlled environment. Pots (n = 4 for each grass, containing 10 mg cold TNT/kg soil + 1.2 mg [(14)C]-TNT/kg soil and controls with no TNT) were exposed to light and temperature conditions typical of June at 45 degrees N for 369 days. Three plant harvests were made (63, 181, and 369 days), and soil and plant materials were monitored for [(14)C]-TNT and metabolite concentrations. The 11.2 mg/kg TNT dose was not phytotoxic to the plant species tested. Continual uptake of TNT into grass blades was observed over the one-year period, with a total accumulation of 1.3%, 0.9%, and 0.8% of the initial soil [(14)C]-TNT dose for orchard grass, perennial ryegrass, and tall fescue, respectively. All [(14)C]-TNT residue in plant material was incorporated as bound residue. At final harvest, radioactivity was concentrated most highly in the root > crown > blade for all species. Soil TNT was gradually reduced to aminodinitro-toluenes and then further to an unidentified metabolite(s). Overall, orchardgrass appeared to be the most efficient species at taking up TNT.