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Abstract 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 N 2) 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 (15NH 4 +, found in sediment and overlying water) and nitrate + nitrite (15NO X , found in overlying water only) together represented 10% of the total added 15N-[RDX]. The production of 15N labeled N 2 (15N 2), 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 N 2 , most of which diffused through porous sediments into the water column and escaped to the atmosphere. Graphical abstract Unlabelled Image Highlights • RDX transforms to inorganic nitrogen via series of organic derivatives. • Coupled oxic and anoxic degradation help to transform RDX in marine environments. • Major breakdown product of RDX is N 2 that majority escapes to the atmosphere. • The low redox conditions in porewaters were likely important for RDX mineralization. • RDX breakdown products can adsorb onto the sandy sediment in marine settings. [ABSTRACT FROM AUTHOR]

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 N 2 O (62–74% of transformed 15N-RDX) and secondarily to N 2 (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. Unlabelled Image • Majority of RDX transforms to inorganic nitrogen via a series of organic derivatives. • RDX is mineralized primarily to N 2 O which then exchanges with the atmosphere. • Sorption of RDX breakdown products account for a negligible portion of RDX transformation. • Silt/eel grass and marsh ecotypes transformed more RDX than non-vegetated sand. • Redox status and organic matter of sediment affects the fate of RDX in coastal habitats. [ABSTRACT FROM AUTHOR]