Sorption and Metabolism of Explosives in Sediment of Coastal Marine Ecosystems

The lack of knowledge on fate and transport of explosive compounds, 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in marine ecosystems limits the ability to predict toxicological impacts and natural attenuation of these suspected carcinogens in contaminated coastal sites. This study focuses on improving our understanding of the sorption and transformation of TNT and RDX in coastal ecosystems by using stable nitrogen isotopes. Abiotic and biotic bench-top experiments using sediment slurries evaluated sorption kinetics and anaerobic biotransformation. Marine silt showed higher compound-uptake rates (\u3e ~100) than freshwater silt for both compounds though equilibrium partition constants (Kp’s) were on the same order of magnitude. Kp’s of TNT and RDX varied linearly with total organic carbon (TOC) in sediment and were inversely correlated to temperature. TNT was transformed from the slurry water at a faster rate than RDX and accumulation in sediment was higher in the TNT microcosms than for RDX. TNT was mineralized to NOX (NO2- and NO3-) and NH4+ via denitration, and deamination, possibly facilitated by iron and sulfate reducing bacteria. RDX was mineralized anaerobically to NOX, NH4+ and N2 gas via denitration, ring breakdown and denitrification. Studies were extended to mesocosm scales representing subtidal non-vegetated, subtidal vegetated and intertidal marsh to evaluate the fate and transport of RDX in multi-component, coastal settings at steady state conditions. Time series of dissolved RDX, derivatives and mineralization products (NH4+, NOX, N2 and N2O) in surface water, porewater, and solids were analyzed. Transformation of RDX was enhanced by microbial assemblages and lower redox potentials. Nitroso-derivatives were further converted to N2O (primarily) and N2 (secondarily). Subtidal vegetated and intertidal marsh (TOC-rich, fine grained sediments and sulfate reducers) showed higher mineralization of RDX. Subtidal non-vegetated mesocosm (TOC-poor, sandy sediment and iron reducers) yielded the highest persistence of RDX in the system. Sediment sorption decreased from intertidal marsh \u3e subtidal vegetated \u3esubtidal non-vegetated and was correlated to the available TOC (positively) and grain size (negatively) of the sediment though partitioning of RDX and derivatives onto sediment was a negligible sink for RDX. The greatest predictor of RDX fate was prevailing sediment redox conditions in the ecosystem

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. ¹⁵N nitro group labeled RDX (¹⁵N-[RDX], 50 atom %) was spiked into a mesocosm simulating shallow marine conditions of coastal Long Island Sound, and the ¹⁵N enrichment of N₂ (δ¹⁵N₂) was monitored via gas bench isotope ratio mass spectrometry (GB-IRMS) for 21 days. The ¹⁵N 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 ¹⁵N₂O and ¹⁵N₂ 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

Tracing the Cycling and Fate of the Explosive 2,4,6-Trinitrotoluene in Coastal Marine Systems with a Stable Isotopic Tracer, ¹⁵N‑[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 (¹⁵N-[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 δ¹⁵N analysis of sediments, SPM, and tissues revealed large quantities of ¹⁵N 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 ¹⁵N 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