Quantifying removal and mineralization of nitrotriazolone in contrasting freshwater sediment systems using ¹³C and ¹⁵N stable isotope tracers

The environmental fate of insensitive high explosive compound nitrotriazolone (NTO) has not been adequately studied in surface freshwater environments. The goal of this study was to evaluate and quantify removal and complete mineralization of NTO in distinct freshwater sediment systems. We conducted aquaria-scale experiments using four freshwater sediment types: low organic carbon (OC) river sand, low OC silt, high OC wetland sediment, high OC pond silt with and without submerged aquatic plants to investigate natural attenuation of NTO. Isotopically labeled NTO (¹³C and ¹⁵N) was added to the aquaria and time series aqueous and sediment samples were collected over three to four weeks. Aqueous NTO half-lives ranged from 6 to 63 days in the presence of sediments while no NTO loss from the aqueous phase was measured in the absence of sediment. Nitrotriazolone completely disappeared within one month in pond silt and wetland sediment aquaria that had higher sediment organic contents, facilitating NTO biotransformation in freshwater ecosystems. Approximately 7%-50% of the NTO loss was demonstrated to be complete mineralization, and NTO mineralization half-lives ranged from 17 to 533 days, with the shortest mineralization half-lives measured in the highest organic content wetland. There was good fidelity between the appearance of ¹³C tracer and ¹⁵N tracer in mineralization products, providing high confidence that both tracers were quantifying complete mineralization. For NTO, increased sediment organic content corresponded to both faster rates of loss of NTO from overlying water and a higher fraction of that loss representing mineralization.