Adsorption and Removal Kinetics of 2,4-Dinitroanisole and Nitrotriazolone in Contrasting Freshwater Sediments: Batch Study.

Environmental release of 2,4-dinitroanisole (DNAN) and 3-nitro-1,2,4-triazol-5-one (NTO) is of great concern due to high migration potential in the environment. In the present study we evaluated the adsorption and microbially-mediated removal kinetics of dissolved DNAN and NTO in contrasting freshwater sediments with different total organic carbon (TOC) content. River sand (low TOC), pond silt (high TOC), clay-rich lake sediment (low TOC), wetland silt (high TOC), carbonate sand (low TOC), and iron-rich clay (low TOC) were evaluated. Separate abiotic and biotic bench-top sediment slurry incubations were carried out at 23, 15, and 4 °C for DNAN and NTO. Experiments were conducted over 3 weeks. Time series aqueous samples and sediment samples collected at the end of the experiment were analyzed for DNAN and NTO concentrations. The DNAN compound equilibrated with sediment within the first 2 h after addition whereas NTO showed no adsorption. 2,4-Dinitroanisole adsorbed more onto fine-grained organic-rich sediments (Kd = 2-40 L kg-1 sed-1 ) than coarse-grained organic-poor sediments (Kd = 0.2-0.6 L kg-1 sed-1 ), and the TOC content and cation exchange capacity of sediment were reliable predictors for abiotic DNAN adsorption. Adsorption rate constants and equilibrium partitioning constants for DNAN were inversely proportional to temperature in all sediment types. The biotic removal half-life of DNAN was faster (t1/2 = 0.1-58 h) than that of NTO (t1/2 = 5-347 h) in all sediment slurries. Biotic removal rates (t1/2 = 0.1-58 h) were higher than abiotic rates (t1/2 = 0.3-107 h) for DNAN at 23 °C. Smaller grain size coupled with higher TOC content enhanced biotic NTO and DNAN removal in freshwater environments. Environ Toxicol Chem 2023;42:46-59. © 2022 SETAC.

Degradation of RDX (Hexahydro-1,3,5-trinitro-1,3,5-triazine) in contrasting coastal marine habitats: Subtidal non-vegetated (sand), subtidal vegetated (silt/eel grass), and intertidal marsh.

Hundreds of explosive-contaminated marine sites exist globally, many of which contain the common munitions constituent hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). Quantitative information about RDX transformation in coastal ecosystems is essential for management of many of these sites. Isotopically labelled RDX containing 15N in all 3 nitro groups was used to track the fate of RDX in three coastal ecosystem types. Flow-through mesocosms representing subtidal vegetated (silt/eel grass), subtidal non-vegetated (sand) and intertidal marsh ecosystems were continuously loaded with isotopically labelled RDX for 16-17 days. Sediment, pore-water and overlying surface water were analyzed to determine the distribution of RDX, nitroso-triazine transformation products (NXs) and nitrogen containing complete mineralization products, including ammonium, nitrate+nitrite, nitrous oxide and nitrogen gas. The marsh, silt, and sand ecotypes transformed 94%, 90% and 76% of supplied RDX, respectively. Total dissolved NXs accounted for 2%-4% of the transformed 15N-RDX. The majority of RDX transformation in the water column was by mineralization to inorganic N (dissolved and evaded; 64%-78% of transformed 15N-RDX). RDX was mineralized primarily to N2O (62-74% of transformed 15N-RDX) and secondarily to N2 (1-2% of transformed 15N-RDX) which exchanged with the atmosphere. Transformation of RDX was favored in carbon-rich lower redox potential sediments of the silt and marsh mesocosms where anaerobic processes of iron and sulfate reduction were most prevalent. RDX was most persistent in the carbon-poor sand mesocosm. Partitioning of 15N derived from RDX onto sediment and suspended particulates was negligible in the overall mass balance of RDX transformation (2%-3% of transformed 15N-RDX). The fraction of 15N derived from RDX that was sorbed or assimilated in sediment was largest in the marsh mesocosm (most organic carbon), and smallest in the sand mesocosm (largest grain size and least organic carbon). Sediment redox conditions and available organic carbon stores affect the fate of RDX in different coastal marine habitats.

Tracing the cycling and fate of the munition, Hexahydro-1,3,5-trinitro-1,3,5-triazine in a simulated sandy coastal marine habitat with a stable isotopic tracer, 15N-[RDX].

Coastal marine habitats become contaminated with the munitions constituent, Hexahydro-1,3,5-trinitro-1,3,5-trazine (RDX), via military training, weapon testing and leakage of unexploded ordnance. This study used 15N labeled RDX in simulated aquarium-scale coastal marine habitat containing seawater, sediment, and biota to track removal pathways from surface water including sorption onto particulates, degradation to nitroso-triazines and mineralization to dissolved inorganic nitrogen (DIN). The two aquaria received continuous RDX inputs to maintain a steady state concentration (0.4 mg L-1) over 21 days. Time series RDX and nitroso-triazine concentrations in dissolved (surface and porewater) and sorbed phases (sediment and suspended particulates) were analyzed. Distributions of DIN species (ammonium, nitrate + nitrite and dissolved N2) in sediments and overlying water were also measured along with geochemical variables in the aquaria. Partitioning of RDX and RDX-derived breakdown products onto surface sediment represented 13% of the total added 15N as RDX (15N-[RDX]) equivalents after 21 days. Measured nitroso-triazines in the aquaria accounted for 6-13% of total added 15N-[RDX]. 15N-labeled DIN was found both in the oxic surface water and hypoxic porewaters, showing that RDX mineralization accounted for 34% of the 15N-[RDX] added to the aquaria over 21 days. Labeled ammonium (15NH4+, found in sediment and overlying water) and nitrate + nitrite (15NOX, found in overlying water only) together represented 10% of the total added 15N-[RDX]. The production of 15N labeled N2 (15N2), accounted for the largest individual sink during the transformation of the total added 15N-[RDX] (25%). Hypoxic sediment was the most favorable zone for production of N2, most of which diffused through porous sediments into the water column and escaped to the atmosphere.

Biodegradation and mineralization of isotopically labeled TNT and RDX in anaerobic marine sediments.

The lack of knowledge on the fate of explosive compounds 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), particularly in marine ecosystems, constrains the application of bioremediation techniques in explosive-contaminated coastal sites. The authors present a comparative study on anaerobic biodegradation and mineralization of 15 N-nitro group isotopically labeled TNT and RDX in organic carbon-rich, fine-grained marine sediment with native microbial assemblages. Separate sediment slurry experiments were carried out for TNT and RDX at 23°C for 16 d. Dissolved and sediment-sorbed fractions of parent and transformation products, isotopic compositions of sediment, and mineralization products of the dissolved inorganic N pool (15 NH4+ ,15 NO3- ,15 NO2- , and 15 N2 ) were measured. The rate of TNT removal from the aqueous phase was faster (0.75 h-1 ) than that of RDX (0.37 h-1 ), and 15 N accumulation in sediment was higher in the TNT (13%) than the RDX (2%) microcosms. Mono-amino-dinitrotoluenes were identified as intermediate biodegradation products of TNT. Two percent of the total spiked TNT-N is mineralized to dissolved inorganic N through 2 different pathways: denitration as well as deamination and formation of NH4+ , facilitated by iron and sulfate reducing bacteria in the sediments. The majority of the spiked TNT-N (85%) is in unidentified pools by day 16. Hexahydro-1,3,5-trinitro-1,3,5-triazine (10%) biodegrades to nitroso derivatives, whereas 13% of RDX-N in nitro groups is mineralized to dissolved inorganic N anaerobically by the end of the experiment. The primary identified mineralization end product of RDX (40%) is NH4+ , generated through either deamination or mono-denitration, followed by ring breakdown. A reasonable production of N2 gas (13%) was seen in the RDX system but not in the TNT system. Sixty-eight percent of the total spiked RDX-N is in an unidentified pool by day 16 and may include unquantified mineralization products dissolved in water. Environ Toxicol Chem 2017;36:1170-1180. © 2016 SETAC.

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.

Sorption kinetics of TNT and RDX in anaerobic freshwater and marine sediments: Batch studies.

Examination of the partitioning of explosives onto sediment in marine environments is critical to predict the toxicological impacts of worldwide explosive-contaminated sites adjacent to estuaries, wetlands, and the coastal ocean. Marine sediments have been identified as sites of enhanced munitions removal, yet most studies addressing these interactions focus on soils and freshwater sediments. The present study measured the kinetics of 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) sorption onto 2 marine sediments of varying grain sizes (silt vs sand) and organic carbon (OC) content. Abiotic sediment sorption tests were performed at 23 °C, 15 °C, and 4 °C by spiking TNT and RDX solutions directly into anaerobic sediment slurries. Marine sediments showed significantly higher compound uptake rates (0.30-0.80 h(-1) ) than freshwater silt (0.0046-0.0065 h(-1) ) for both compounds, probably because of lower compound solubilities and a higher pH in marine systems. Equilibrium partition constants are on the same order of magnitude for marine silt (1.1-2.0 L kg(-1) sediment) and freshwater silt (1.4-3.1 L kg(-1) sediment) but lower for marine sand (0.72-0.92 L kg(-1) sediment). Total organic carbon content in marine sediments varied linearly with equilibrium partition constants for TNT and was moderately linear for RDX. Uptake rates and equilibrium constants of explosives are inversely correlated to temperature regardless of sediment type because of kinetic barriers associated with low temperatures.

Tracing the Cycling and Fate of the Explosive 2,4,6-Trinitrotoluene in Coastal Marine Systems with a Stable Isotopic Tracer, (15)N-[TNT].

2,4,6-Trinitrotoluene (TNT) has been used as a military explosive for over a hundred years. Contamination concerns have arisen as a result of manufacturing and use on a large scale; however, despite decades of work addressing TNT contamination in the environment, its fate in marine ecosystems is not fully resolved. Here we examine the cycling and fate of TNT in the coastal marine systems by spiking a marine mesocosm containing seawater, sediments, and macrobiota with isotopically labeled TNT ((15)N-[TNT]), simultaneously monitoring removal, transformation, mineralization, sorption, and biological uptake over a period of 16 days. TNT degradation was rapid, and we observed accumulation of reduced transformation products dissolved in the water column and in pore waters, sorbed to sediments and suspended particulate matter (SPM), and in the tissues of macrobiota. Bulk δ(15)N analysis of sediments, SPM, and tissues revealed large quantities of (15)N beyond that accounted for in identifiable derivatives. TNT-derived N was also found in the dissolved inorganic N (DIN) pool. Using multivariate statistical analysis and a (15)N mass balance approach, we identify the major transformation pathways of TNT, including the deamination of reduced TNT derivatives, potentially promoted by sorption to SPM and oxic surface sediments.

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.

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.

Dose-dependent Na and Ca in fluoride-rich drinking water--another major cause of chronic renal failure in tropical arid regions.

Endemic occurrence of chronic kidney disease with unknown etiology is reported in certain parts of the north central dry zone of Sri Lanka and has become a new and emerging health issue. The disease exclusively occurs in settlements where groundwater is the main source of drinking water and is more common among low socio-economic groups, particularly among the farming community. Due to its remarkable geographic distribution and histopathological evidence, the disease is believed to be an environmentally induced problem. This paper describes a detailed hydrogeochemical study that has been carried out covering endemic and non-endemic regions. Higher fluoride levels are common in drinking water from both affected and non-affected regions, whereas Ca-bicarbonate type water is more common in the affected regions. In terms of the geochemical composition of drinking water, affected households were rather similar to control regions, but there is a large variation in the Na/Ca ratio within each of the two groups. Fluoride as shown in this study causes renal tubular damage. However it does not act alone and in certain instances it is even cytoprotective. The fine dividing line between cytotoxicity and cytoprotectivity of fluoride appears to be the effect of Ca(2+) and Na(+) of the ingested water on the F(-) metabolism. This study illustrates a third major cause (the other two being hypertension and diabetes) of chronic kidney diseases notably in tropical arid regions such as the dry zone of Sri Lanka.