Identifying degradation products responsible for increased toxicity of UV-degraded insensitive munitions

Abstract

Degradation of insensitive munitions (IMs) by ultraviolet (UV) light has become a concern following observations that some UV-degradation products have increased toxicity relative to parent compounds in aquatic organisms. This investigation focused on the Army's IM formulation, IMX-101, composed of three IM constituents: 2,4-dinitroanisole (DNAN), 3-nitro-1,2,4-triazol-5-one (NTO), and nitroguanidine (NQ). The IM constituents and IMX-101 were irradiated in a UV photo-reactor and then administered to Daphnia pulex in acute (48 h) exposures comparing toxicities relative to the parent materials. UV-degradation of DNAN had little effect on mortality whereas mortality for UV-degraded NTO and NQ increased by factors of 40.3 and 1240, making UV-degraded NQ the principle driver of toxicity when IMX-101 is UV-degraded. Toxicity investigations for specific products formed during UV-degradation of NQ, confirmed greater toxicity than the parent NQ for degradation products. Summation of the individual toxic units for the complete set of individually measured UV-degradation products identified for NQ only accounted for 25% of the overall toxicity measured in the exposures to the UV-degraded NQ product mixture. Given the underestimation of toxicity using the sum toxic units for the individually measured UV-degradation products of NQ, we conclude that: (1) other unidentified NQ degradation products contributed principally to toxicity and/or (2) synergistic toxicological interactions occurred among the NQ degradation product mixture that exacerbated toxicity.

Comparative Toxicological Evaluation of UV‐Degraded versus Parent‐Insensitive Munition Compound 1‐Methyl‐3‐Nitroguanidine in Fathead Minnow

“…Even though UV treatment clearly caused increased toxicity of MeNQ in fathead minnows (Table3), the magnitude of increased toxicity is lower than has been reported for UVdegraded NQ in previous experiments(Gust et al 2017;Kennedy et al 2017;Moores et al 2020b) Kennedy et al (2017). reported a 73-fold lower LC50 for UV-treated NQ compared with NQ parent compound in C. dubia Moores et al (2020b). demonstrated the greatest increase in toxicity yet observed for UV-degraded NQ, where 90% UV degradation of NQ elicited a 1240-fold increase in toxicity in Daphnia pulex.…”

“…Despite similar concentrations of MeNQ parent compound in the exposure water (716 and 709 mg/L for parent compound and UV-treated solutions, respectively), survival was significantly decreased in the UVtreated MeNQ exposure (14% mean survival) relative to the exposure to non-UV-treated MeNQ parent compound (100% survival). Even though UV treatment clearly caused increased toxicity of MeNQ in fathead minnows (Table3), the magnitude of increased toxicity is lower than has been reported for UVdegraded NQ in previous experiments(Gust et al 2017;Kennedy et al 2017;Moores et al 2020b) Kennedy et al (2017). reported a 73-fold lower LC50 for UV-treated NQ compared with NQ parent compound in C. dubia Moores et al (2020b).…”

“…Kennedy et al () reported a 73‐fold lower LC50 for UV‐treated NQ compared with NQ parent compound in C. dubia . Moores et al () demonstrated the greatest increase in toxicity yet observed for UV‐degraded NQ, where 90% UV degradation of NQ elicited a 1240‐fold increase in toxicity in Daphnia pulex . Comparatively, the effect of UV‐degraded NQ was perhaps less severe in fish compared with cladocerans, where 69.2% UV degradation of NQ caused a 17‐fold increase in toxicity in larval zebrafish (Gust et al ), whereas 7% UV degradation of NQ elicited an 85‐fold increase in toxicity for D. pulex (Kennedy et al ).…”

“…This contrasts with the 42 and 99% reductions reported by Kennedy et al (2017) for 1619 and 108 mg/L solutions, respectively, and the 69.2% reduction reported by Gust et al (2017) for a 3000 mg/L solution. The present study used the same UV-irradiation method as used previously (Gust et al 2017;Kennedy et al 2017;Moores et al 2020b), and all were conducted in the same laboratory. Therefore, the transformation system has remained constant among studies; however, starting concentrations have varied as well as the test species investigated to assess toxicity.…”

“…Computational and analytical investigative research on the UV-degradation products of MeNQ should be the focus of follow-up studies. Under UV radiation, aqueous NQ transforms relatively rapidly into a number of nitrogenous compounds, including primarily guanidine, nitrate, and nitrite, as well as cyanamide, cyanoguanidine, urea, ammonium, and trace amounts of cyanide under simulated natural conditions (Becher et al 2019;Moores et al 2020aMoores et al , 2020b. Given that MeNQ represents a methylsubstituted structural analog of NQ, where the methyl group replaces one hydrogen at the amino group in NQ, formation of similar compounds with the methyl group attached (e.g., methyl guanidine, methyl cyanamide, methyl cyanoguanidine) is expected.…”