Mineralization of RDX-Derived Nitrogen to N₂ via Denitrification in Coastal Marine Sediments

Abstract

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.

Review and synthesis of evidence regarding environmental risks posed by munitions constituents (MC) in aquatic systems

“…In the sister experiment to the marine mesocosm investigation of the fate of 15 N TNT, Smith et al (2015a) and Ariyarathna (unpublished work) investigated the fate of RDX in coastal marine environments using 15 N labeled RDX spiked into a complex marine mesocosm. Both Smith et al…”

“…Both experiments contained the same type and size (7-8 cm deep) sediments collected from subtidal waters of LIS. Smith et al (2015a) added the 15 N-labeled RDX as a single addition, while Ariyarathna's unpublished work added an initial 15 N-labeled RDX addition followed by metering in 15 N-labeled RDX using a peristaltic pump to target steady-state RDX. The 15 N was detected in all pools measured and was found in the following pools in decreasing order: aqueous RDX > N2 > sediments > biota tissue > aqueous RDX derivatives > NH4 + .…”

“…Unlike TNT, 15 N2 was measured much higher than 15 NH4 + in the 15 N RDX spiked marine mesocosms with 15 N2 accounting for as much as 33% and 15 NH4 + accounting for only 1% of total added RDX. 15 N2 accounted for 21% of the spiked 15 N labeled RDX in similar marine mesocosms without biota (Smith et al 2015a). A significant amount of measured 15 N2 explains how studies using 14 C labeled RDX have indicated that RDX undergoes a transformation process that result in unidentified products that contain a higher solubility in water and partition more readily to the water column (Lotufo et al 2001;Pennington et al 2011).…”

“…A common reaction in anaerobic sediment layers is denitrification, which uses organic carbon to reduce nitrogen by the following pathway: NO3 - NO2 - N2O  N2 (Hawari et al 2000a;Hattori 1983). The production of N2O through mineralization pathways in municipal sludge (Hawari et al 2000a) and with fungus (Shermata and Hawari 2000), followed by denitrification to form N2, is a potential pathway of RDX removal from sediments (Smith et al, 2015a). This pathway along with denitrification of both NO3 -and NO2 -, which both can be generated during bacterial denitrification within bacterial cells, can account for the loss of RDX from sediments as well as the production of compounds that partition more readily to the water column and are transported away from the system.…”

“…This pathway along with denitrification of both NO3 -and NO2 -, which both can be generated during bacterial denitrification within bacterial cells, can account for the loss of RDX from sediments as well as the production of compounds that partition more readily to the water column and are transported away from the system. Smith et al (2015a) has amended proposed mineralization pathways of RDX (Hawari et al 2000b;Bhushan et al 2002), which include denitrification that better illustrate a more complete mineralization of RDX in sediments.…”