Biodegradation of hexahydro-1,3,5-trinitro-1,3,5-triazine by novel fungi isolated from unexploded ordnance contaminated marine sediment

Marine-derived fungi as biocatalysts

Marine microorganisms account for over 90% of ocean biomass and their diversity is believed to be the result of their ability to adapt to extreme conditions of the marine environment. Biotransformations are used to produce a wide range of high-added value materials, and marine-derived fungi have proven to be a source of new enzymes, even for activities not previously discovered. This review focuses on biotransformations by fungi from marine environments, including bioremediation, from the standpoint of the chemical structure of the substrate, and covers up to September 2022.

An Overview of Treatment Approaches for Octahydro-1, 3, 5, 7-tetranitro-1, 3, 5, 7-tetrazocine (HMX) Explosive in Soil, Groundwater, and Wastewater

Octahydro-1, 3, 5, 7-tetranitro-1, 3, 5, 7-tetrazocine (HMX) is extensively exploited in the manufacturing of explosives; therefore, a significant level of HMX contamination can be encountered near explosive production plants. For instance, up to 12 ppm HMX concentrations have been observed in the wastewater effluent of a munitions manufacturing facility, while up to 45,000 mg/kg of HMX has been found in a soil sample taken from a location close to a high-explosive production site. Owing to their immense demand for a variety of applications, the large-scale production of explosives has culminated in severe environmental issues. Soil and water contaminated with HMX can pose a detrimental impact on flora and fauna and hence, remediation of HMX is paramount. There is a rising demand to establish a sustainable technology for HMX abatement. Physiochemical and bioremediation approaches have been employed to treat HMX in the soil, groundwater, and wastewater. It has been revealed that treatment methods such as photo-peroxidation and photo-Fenton oxidation can eliminate approximately 98% of HMX from wastewater. Fenton's reagents were found to be very effective at mineralizing HMX. In the photocatalytic degradation of HMX, approximately 59% TOC removal was achieved by using a TiO₂ photocatalyst, and a dextrose co-substrate was used in a bioremediation approach to accomplish 98.5% HMX degradation under anaerobic conditions. However, each technology has some pros and cons which need to be taken into consideration when choosing an HMX remediation approach. In this review, various physiochemical and bioremediation approaches are considered and the mechanism of HMX degradation is discussed. Further, the advantages and disadvantages of the technologies are also discussed along with the challenges of HMX treatment technologies, thus giving an overview of the HMX remediation strategies.

Reduction of a Heme Cofactor Initiates N-Nitroglycine Degradation by NnlA

Linear nitramines are potentially carcinogenic environmental contaminants. The NnlA enzyme from Variovorax sp. strain JS1663 degrades the nitramine N-nitroglycine (NNG)-a natural product produced by some bacteria-to glyoxylate and nitrite (NO₂^-). Ammonium (NH₄⁺) was predicted as the third product of this reaction. A source of nonheme Fe^II was shown to be required for initiation of NnlA activity. However, the role of this Fe^II for NnlA activity was unclear. This study reveals that NnlA contains a b-type heme cofactor. Reduction of this heme-either by a nonheme iron source or dithionite-is required to initiate NnlA activity. Therefore, Fe^II is not an essential substrate for holoenzyme activity. Our data show that reduced NnlA (Fe^II-NnlA) catalyzes at least 100 turnovers and does not require O₂. Finally, NH₄⁺ was verified as the third product, accounting for the complete nitrogen mass balance. Size exclusion chromatography showed that NnlA is a dimer in solution. Additionally, Fe^II-NnlA is oxidized by O₂ and NO₂^- and stably binds carbon monoxide (CO) and nitric oxide (NO). These are characteristics shared with heme-binding PAS domains. Furthermore, a structural homology model of NnlA was generated using the PAS domain from Pseudomonas aeruginosa Aer2 as a template. The structural homology model suggested His⁷³ is the axial ligand of the NnlA heme. Site-directed mutagenesis of His⁷³ to alanine decreased the heme occupancy of NnlA and eliminated NNG activity, validating the homology model. We conclude that NnlA forms a homodimeric heme-binding PAS domain protein that requires reduction for initiation of the activity. IMPORTANCE Linear nitramines are potential carcinogens. These compounds result from environmental degradation of high-energy cyclic nitramines and as by-products of carbon capture technologies. Mechanistic understanding of the biodegradation of these compounds is critical to inform strategies for their remediation. Biodegradation of NNG by NnlA from Variovorax sp. strain JS 1663 requires nonheme iron, but its role is unclear. This study shows that nonheme iron is unnecessary. Instead, our study reveals that NnlA contains a heme cofactor, the reduction of which is critical for activating NNG degradation activity. These studies constrain the proposals for NnlA reaction mechanisms, thereby informing mechanistic studies of degradation of anthropogenic nitramine contaminants. In addition, these results will inform future work to design biocatalysts to degrade these nitramine contaminants.

Aerobic biodegradation of high explosive hexahydro-1,3,5- trinitro-1,3,5-triazine by Janibacter cremeus isolated from contaminated soil

OBJECTIVE

To evaluate the ability of Janibacter cremeus a soil bacterium isolated from explosive contaminated site in degradation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and to study enzyme responsible for degradation.

RESULTS

The isolate exhibited 88% degradation of RDX in 30 days of incubation. The biodegradation process followed the first order kinetics. The half- life of RDX was calculated to be 11.088 days. The RDX degradation process was complemented by concomitant release of nitrite ions with 0.78 mol of nitrite released per mole of RDX. The metabolites; Trinitroso- RDX, diamino-RDX, trimino-RDX, bis- (hydroxymethyl) nitramine and methylenedintramine derivative, viz, methylene- N- (hydroxy- methyl)- hydroxylamine- N-(hydroxymethyl) nitroamine corresponding to the molecular weights 174, 162, 132, 122 and 167 Da respectively were also detected. Nitroreductase enzyme was found to be responsible for RDX degradation.

CONCLUSION

J. cremeus could degrade RDX as sole source of nitrogen, via three different pathways wherein, Nitroreductase enzyme was found to play a major role. The efficient degradation of RDX makes J. cremeus suitable in treatment of contaminated water and soil at field scale levels.

Bioremediation of Explosive TNT by Trichoderma viride

Nitroaromatic and nitroamine compounds such as 2,4,6-trinitrotoluene (TNT) are teratogenic, cytotoxic, and may cause cellular mutations in humans, animals, plants, and microorganisms. Microbial-based bioremediation technologies have been shown to offer several advantages against the cellular toxicity of nitro-organic compounds. Thus, the current study was designed to evaluate the ability of Trichoderma viride to degrade nitrogenous explosives, such as TNT, by microbiological assay and Gas chromatography-mass spectrometry (GC-MS) analysis. In this study, T. viride fungus was shown to have the ability to decompose, and TNT explosives were used at doses of 50 and 100 ppm on the respective growth media as a nitrogenous source needed for normal growth. The GC/MS analysis confirmed the biodegradable efficiency of TNT, whereas the initial retention peak of the TNT compounds disappeared, and another two peaks appeared at the retention times of 9.31 and 13.14 min. Mass spectrum analysis identified 5-(hydroxymethyl)-2-furancarboxaldehyde with the molecular formula C6H6O3 and a molecular weight of 126 g·mol-1 as the major compound, and 4-propyl benzaldehyde with a formula of C10H12O and a molecular weight of 148 g mol-1 as the minor compound, both resulting from the biodegradation of TNT by T. viride. In conclusion, T. viride could be used in microbial-based bioremediation technologies as a biological agent to eradicate the toxicity of the TNT explosive. In addition, future molecular-based studies should be conducted to clearly identify the enzymes and the corresponding genes that give T. viride the ability to degrade and remediate TNT explosives. This could help in the eradication of soils contaminated with explosives or other toxic biohazards.

Aerobic biodegradation of HMX by Planomicrobium flavidum

In this report, aerobic biodegradation of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine or high melting explosive (HMX), a highly explosive chemical by Planomicrobium flavidum strain S5-TSA-19, an isolate from an explosive-contaminated soil, was investigated. The isolate S5-TSA-19 degraded 70% of HMX in 20 days during which time nitrite ion was produced with the subsequent formation of metabolites, viz. methylenedintramine and N-methyl-N,N'-dinitromethanediamine with molecular weights 136 Da and 149 Da, respectively. The degradation mechanism was found to follow first-order kinetics with a half-life of 11.55 days and formation of above intermediates indicate single nitrite elimination pathway. The proliferation of isolate S5-TSA-19 in the absence of nitramines indicates the cometabolic degradation of HMX. Isolate S5-TSA-19 can thus be used as futuristic microbe for degradation of HMX at explosive-contaminated site.

In situ pilot test for bioremediation of energetic compound-contaminated soil at a former military demolition range site

Bioremediation was performed in situ at a former military range site to assess the performance of native bacteria in degrading hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and 2,4-dinitrotoluene (2,4-DNT). The fate of these pollutants in soil and soil pore water was investigated as influenced by waste glycerol amendment to the soil. Following waste glycerol application, there was an accumulation of organic carbon that promoted microbial activity, converting organic carbon into acetate and propionate, which are intermediate compounds in anaerobic processes. This augmentation of anaerobic activity strongly correlated to a noticeable reduction in RDX concentrations in the amended soil. Changes in concentrations of RDX in pore water were similar to those observed in the soil suggesting that RDX leaching from the soil matrix, and treatment with waste glycerol, contributed to the enhanced removal of RDX from the water and soil. This was not the case with 2,4-DNT, which was neither found in pore water nor affected by the waste glycerol treatment. Results from saturated conditions and Synthetic Precipitation Leaching Procedure testing, to investigate the environmental fate of 2,4-DNT, indicated that 2,4-DNT found on site was relatively inert and was likely to remain in its current state on the site.

Uptake and fate of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in coastal marine biota determined using a stable isotopic tracer, (15)N - [RDX]

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is globally one of the most commonly used military explosives and environmental contaminant. (15)N labeled RDX was added into a mesocosm containing 9 different coastal marine species in a time series experiment to quantify the uptake of RDX and assess the RDX derived (15)N retention into biota tissue. The (15)N attributed to munitions compounds reached steady state concentrations ranging from 0.04 to 0.67 μg (15)N g dw(-1), the bulk (15)N tissue concentration for all species was 1-2 orders of magnitude higher suggesting a common mechanism or pathway of RDX biotransformation and retention of (15)N. A toxicokinetic model was created that described the (15)N uptake, elimination, and transformation rates. While modeled uptake rates were within previous published values, elimination rates were several orders of magnitude smaller than previous studies ranging from 0.05 to 0.7 days(-1). These small elimination rates were offset by high rates of retention of (15)N previously not measured. Bioconcentration factors and related aqueous:organism ratios of compounds and tracer calculated using different tracer and non-tracer methods yielded a broad range of values (0.35-101.6 mL g(-1)) that were largely method dependent. Despite the method-derived variability, all values were generally low and consistent with little bioaccumulation potential. The use of (15)N labeled RDX in this study indicates four possible explanations for the observed distribution of compounds and tracer; each with unique potential implications for possible toxicological impacts in the coastal marine environment.

Mineralization of RDX-derived nitrogen to N2 via denitrification in coastal marine sediments

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is a common constituent of military explosives. Despite RDX contamination at numerous U.S. military facilities and its mobility to aquatic systems, the fate of RDX in marine systems remains largely unknown. Here, we provide RDX mineralization pathways and rates in seawater and sediments, highlighting for the first time the importance of the denitrification pathway in determining the fate of RDX-derived N. (15)N nitro group labeled RDX ((15)N-[RDX], 50 atom %) was spiked into a mesocosm simulating shallow marine conditions of coastal Long Island Sound, and the (15)N enrichment of N2 (δ(15)N2) was monitored via gas bench isotope ratio mass spectrometry (GB-IRMS) for 21 days. The (15)N tracer data were used to model RDX mineralization within the context of the broader coastal marine N cycle using a multicompartment time-stepping model. Estimates of RDX mineralization rates based on the production and gas transfer of (15)N2O and (15)N2 ranged from 0.8 to 10.3 μmol d(-1). After 22 days, 11% of the added RDX had undergone mineralization, and 29% of the total removed RDX-N was identified as N2. These results demonstrate the important consideration of sediment microbial communities in management strategies addressing cleanup of contaminated coastal sites by military explosives.

Environmental fate of 2,4-dinitroanisole (DNAN) and its reduced products

Several defense departments intend to replace 2,4,6-trinitrotoluene (TNT) in munitions formulations by the less sensitive 2,4-dinitroanisole (DNAN). To help understand environmental behavior and ecological risk associated with DNAN we investigated its key initial abiotic and biotic reaction routes and determined relevant physicochemical parameters (pKa, logKow, aqueous solubility (Sw), partition coefficient (Kd)) for the chemical and its products. Reduction of DNAN with either zero valent iron or bacteria regioselectively produced 2-amino-4-nitroanisole (2-ANAN) which, under strict anaerobic conditions, gave 2,4-diaminoanisole (DAAN). Hydrolysis under environmental conditions was insignificant whereas photolysis gave photodegradable intermediates 2-hydroxy-4-nitroanisole and 2,4-dinitrophenol. Physicochemical properties of DNAN and its amino products drastically depended on the type and position of substituent(s) on the aromatic ring. Sw followed the order (TNT<DNAN<2-ANAN<4-ANAN<DAAN) whereas logKow followed the order (DAAN<4-ANAN<2-ANAN<DNAN<TNT). In soil, successive replacement of -NO2 by -NH2 in DNAN enhanced irreversible sorption and reduced bioavailability under oxic conditions. Although DNAN is more soluble than TNT, its lower hydrophobicity and its tendency to form aminoderivatives that sorb irreversibly to soil contribute to make it less toxic than the traditional explosive TNT.

Removal rates of dissolved munitions compounds in seawater

The historical exposure of coastal marine systems to munitions compounds is of significant concern due to the global distribution of impacted sites and known toxicological effects of nitroaromatics. In order to identify specific coastal regions where persistence of these chemicals should be of concern, it is necessary to experimentally observe their behavior under a variety of realistic oceanographic conditions. Here, we conduct a mesocosm scale pulse addition experiment to document the behavior of two commonly used explosives, 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in simulated marine systems containing water and sediments collected from Long Island Sound, CT. The addition of sediments and sediment grain-size had a major influence on the loss rates of all compounds detected. RDX and reduced TNT products were removed from seawater only in the presence of sediment, and TNT degraded significantly faster in the presence of sediment. Both compounds were removed from the system faster with decreasing grain-size. Based on these findings and a thorough review of the literature, we hypothesize that in addition to bacterial abundance and nutrient availability, TNT removal rates in coastal marine waters may be controlled by sorption and rapid surface-mediated bacterial transformation, while RDX removal rates are controlled by diffusion into sedimentary anoxic regions and subsequent anaerobic bacterial breakdown. A comparison of published removal rates of RDX and TNT highlights the extreme variability in measured degradation rates and identifies physicochemical variables that covary with the breakdown of these munitions compounds.

Application of (13)C-stable isotope probing to identify RDX-degrading microorganisms in groundwater

We employed stable isotope probing (SIP) with (13)C-labeled hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) to identify active microorganisms responsible for RDX biodegradation in groundwater microcosms. Sixteen different 16S rRNA gene sequences were derived from microcosms receiving (13)C-labeled RDX, suggesting the presence of microorganisms able to incorporate carbon from RDX or its breakdown products. The clones, residing in Bacteroidia, Clostridia, α-, β- and δ-Proteobacteria, and Spirochaetes, were different from previously described RDX degraders. A parallel set of microcosms was amended with cheese whey and RDX to evaluate the influence of this co-substrate on the RDX-degrading microbial community. Cheese whey stimulated RDX biotransformation, altered the types of RDX-degrading bacteria, and decreased microbial community diversity. Results of this study suggest that RDX-degrading microorganisms in groundwater are more phylogenetically diverse than what has been inferred from studies with RDX-degrading isolates.

Biodegradation of RDX nitroso products MNX and TNX by cytochrome P450 XplA

Anaerobic transformation of the explosive RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) by microorganisms involves sequential reduction of N-NO(2) to the corresponding N-NO groups resulting in the initial formation of MNX (hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine). MNX is further reduced to the dinitroso (DNX) and trinitroso (TNX) derivatives. In this paper, we describe the degradation of MNX and TNX by the unusual cytochrome P450 XplA that mediates metabolism of RDX in Rhodococcus rhodochrous strain 11Y. XplA is known to degrade RDX under aerobic and anaerobic conditions, and, in the present study, was found able to degrade MNX to give similar products distribution including NO(2)(-), NO(3)(-), N(2)O, and HCHO but with varying stoichiometric ratio, that is, 2.06, 0.33, 0.33, 1.18, and 1.52, 0.15, 1.04, 2.06, respectively. In addition, the ring cleavage product 4-nitro-2,4,-diazabutanal (NDAB) and a trace amount of another intermediate with a [M-H](-) at 102 Da, identified as ONNHCH(2)NHCHO (NO-NDAB), were detected mostly under aerobic conditions. Interestingly, degradation of TNX was observed only under anaerobic conditions in the presence of RDX and/or MNX. When we incubated RDX and its nitroso derivatives with XplA, we found that successive replacement of N-NO(2) by N-NO slowed the removal rate of the chemicals with degradation rates in the order RDX > MNX > DNX, suggesting that denitration was mainly responsible for initiating cyclic nitroamines degradation by XplA. This study revealed that XplA preferentially cleaved the N-NO(2) over the N-NO linkages, but could nevertheless degrade all three nitroso derivatives, demonstrating the potential for complete RDX removal in explosives-contaminated sites.

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).

Biodegradation of RDX and MNX with Rhodococcus sp. strain DN22: new insights into the degradation pathway

Previously we demonstrated that Rhodococcus sp. strain DN22 can degrade RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) aerobically via initial denitration. The present study describes the role of oxygen and water in the key denitration step leading to RDX decomposition using (18)O(2) and H(2)(18)O labeling experiments. We also investigated degradation of MNX (hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine) with DN22 under similar conditions. DN22 degraded RDX and MNX giving NO(2)(-), NO(3)(-), NDAB (4-nitro-diazabutanal), NH(3), N(2)O, and HCHO with NO(2)(-)/NO(3)(-) molar ratio reaching 17 and ca. 2, respectively. In the presence of (18)O(2), DN22 degraded RDX and produced NO(2)(-) with m/z at 46 Da that subsequently oxidized to NO(3)(-) containing one (18)O atom, but in the presence of H(2)(18)O we detected NO(3)(-) without (18)O. A control containing NO(2)(-), DN22, and (18)O(2) gave NO(3)(-) with one (18)O, confirming biotic oxidation of NO(2)(-) to NO(3)(-). Treatment of MNX with DN22 and (18)O(2) produced NO(3)(-) with two mass ions, one (66 Da) incorporating two (18)O atoms and another (64 Da) incorporating only one (18)O atom and we attributed their formation to bio-oxidation of the initially formed NO and NO(2)(-), respectively. In the presence of H(2)(18)O we detected NO(2)(-) with two different masses, one representing NO(2)(-) (46 Da) and another representing NO(2)(-) (48 Da) with the inclusion of one (18)O atom suggesting auto-oxidation of NO to NO(2)(-). Results indicated that denitration of either RDX or MNX and denitrosation of MNX by DN22 did not involve direct participation of either oxygen or water, but both played major roles in subsequent secondary chemical and biochemical reactions of NO and NO(2)(-).

Black carbon-mediated destruction of nitroglycerin and RDX by hydrogen sulfide

The in situ remediation of sediments contaminated with explosives, including nitroglycerin and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), is desirable, particularly at bombing ranges where unexploded ordnance (UXO) renders excavation dangerous. Sulfides generated by biological sulfate reduction in sediments are potent nucleophiles and reductants that may contribute to the destruction of explosives. However, moderately hydrophobic explosives are likely to sorb to black carbons, which can constitute 10-30% of sediment organic carbon. In this study, we evaluated whether the black carbons accelerate these reactions or simply sequester explosives from aqueous phase reactions. Using environmentally-relevant sulfide and black carbon concentrations, our results indicated that black carbons accelerated the destruction of both compounds, yielding relatively harmless products on the time scale of hours. For both compounds, destruction increased with sulfide and graphite concentrations. Using sheet graphite as a model for graphene regions in black carbons, we evaluated whether graphene regions mediated the reduction of explosives by promoting electron transfer from sulfides. Our results demonstrated that the process was more complex. Using an electrochemical cell that enabled electron transfer from sulfides to explosives through graphite, but prevented nucleophilic substitution reactions, we found that nitroglycerin destruction, but not RDX destruction, could be explained by an electron transfer mechanism. Furthermore, surface area-normalized destruction rates for the same explosive varied for different black carbons. While black carbon-mediated destruction of explosives by sulfides is likely to be a significant contributor to their natural attenuation in sediments, a fundamental characterization of the reaction mechanisms is needed to better understand the process.

Identification of hexahydro-1,3,5-trinitro-1,3,5-triazine-degrading microorganisms via 15N-stable isotope probing

This study reported the application of 15N-stable isotope probing (SIP) to identify active hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)-utilizing microorganisms in groundwater microcosms. Fifteen 16S rRNA gene sequences were derived from the 15N-DNA fraction (contributed from active microorganisms capable of using RDX as a nitrogen source) of microcosms receiving cheese whey. The 16S rRNA gene sequences belonged to Actinobacteria (two clones), alpha-Proteobacteria (seven clones), and gamma-Proteobacteria (six clones). Except for five sequences with high similarity to two known RDX degraders (Enterobacter cloacae and Pseudomonas fluorescens I-C), our results suggested that phylogenetically diverse microorganisms were capable of using RDX as a nitrogen source. Six sequences of the xplA gene (a known RDX-degrading catabolic gene) were detected from the 15N-DNA fraction. The xplA gene sequences were 96-99% similar to the xplA gene of Rhodococcus sp. DN22(a known RDX utilizer), suggesting that other RDX utilizers might contain xplA-like genes. Twenty-five 16S rRNA gene sequences recovered from the unenriched, RDX-contaminated groundwater clustered differently from those obtained from the 15N-DNA fraction of the cheese-whey-amended microcosm. Our results suggested that active RDX utilizers can be stimulated by nutrient source additions even if they are present at low densities, and that use of 15N-SIP can identifythese functional members of the microbial community.

Degradation of RDX using granular iron and nickel-plated granular iron

Using granular iron (Fe) and nickel-plated iron (Ni/Fe), this paper examines the effectiveness of these two types of reactive materials for the treatment of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), a common groundwater and soil contaminant at military facilities. RDX degraded very rapidly in the presence of both Fe and Ni/Fe in column and batch experiments. Enhancement by Ni/Fe did not prove to be effective as the half-lives of RDX ranged from 3 to 24 seconds and 3 to 11 seconds in the Fe and Ni/Fe columns, respectively. Reaction vessel experiments and estimation of the mass transfer coefficient in the column indicated that reaction kinetics was mass transfer limited. Detailed analyses of reaction intermediates and products suggest that RDX degradation proceeds through direct electron transfer processes and following to the same pathways in the presence of Fe and Ni/Fe. The formation of carbon-containing products, including formaldehyde (up to 60%), CO2 (up to 45%) and formic acid (1%) and the nitrogen containing products of ammonium (up to 48%) and N2O (up to 13%), provides convincing evidence that RDX was completely decomposed to non-toxic end products. CO2, previously reported to form only in biological or Fe-microbial combined systems, was detected as one of the main C-bearing end product. Therefore, this study shows that Fe is an effective material for remediating groundwater and industrial effluents containing RDX; and the use of additional enhancement, either biological or with Ni catalyst, does not provide additional advantages.

Bioremediation of nitroexplosive wastewater by an yeast isolate Pichia sydowiorum MCM Y-3 in fixed film bioreactor

Nitroexplosives are essential for security and defense of the nation and hence their production continues. Their residues and transformed products, released in the environment are toxic to both terrestrial and aquatic life. This necessitates remediation of wastewaters containing such hazardous chemicals to reduce threat to human health and environment. Bioremediation technologies using microorganisms become the present day choice. High Melting Explosive (HMX) is one of the nitroexplosives produced by nitration of hexamine using ammonium nitrate and acetic anhydride and hence the wastewater bears high concentration of nitrate and acetate. The present investigation describes potential of a soil isolate of yeast Pichia sydowiorum MCM Y-3, for remediation of HMX wastewater in fixed film bioreactor (FFBR). The flask culture studies showed appreciable growth of the organism in HMX wastewater under shake culture condition within 5-6 days of incubation at ambient temperature (28 +/- 2 degrees C). The FFBR process operated in both batch and continuous mode, with Hydraulic Retention Time (HRT) of 1 week resulted in 50-55% removal in nitrate, 70-88% in acetate, 50-66% in COD, and 28-50% in HMX content. Continuous operation of the reactor showed better removal of nitrate as compared to that in the batch operation, while removal of acetate and COD was comparable in both the modes of operation of the reactor. Insertion of baffles in the reactor increased efficiency of the reactor. Thus, FFBR developed with baffles and operated in continuous mode will be beneficial for bioremediation of high nitrate and acetate containing wastewater using the culture of P. sydowiorum.

Abiotic degradation of hexahydro-l,3,5-trinitro-1,3,5-triazine in the presence of hydrogen sulfide and black carbon

We report that hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) was rapidly destroyed by sulfides in the presence of black carbon, forming nitrite and formaldehyde, rather than toxic nitrosated reduction products. Although traditionally viewed as inactive sorbents, black carbons have been noted to participate in the destruction of certain contaminants, such as azo dyes, via quinonoid groups. However, in our experiments sulfide modification of quinones did not seem to be involved. Although at least 1.2 mM sulfides were needed for the reaction to proceed, abiotic natural attenuation of RDX in marine sediments may occur, because these concentrations are found in certain marine sediments, together with black carbon. In the absence of natural black carbons, synthetic black carbons, such as activated carbon, may be added to sediments. As compared with other in situ techniques, such as bioremediation and zero-valent iron cutoff trenches, which often generate nitrosated byproducts, this in situ, abiotic technique may be an attractive alternative.