Aquatic environmental fate of nitroguanidine

Abstract

The environmental fate of nitroguanidine (NQ) in surface waters is dominated by photolysis with surface half‐lives at 40°N ranging from 0.6 d in summer to 2.3 d in winter. The environmental quantum yield is 0.01. The NQ is initially photolyzed to nitrite and hydroxyguanidine; nitrite is photochemically converted to nitrate and hydroxyguanidine undergoes sensitized photolysis to unknown products. The photooxidation of nitrite is assisted by organic material in a process not involving H2O2 or singlet oxygen. Nitroguanidine biotransforms cometabolically; in the absence of extra organic nutrients the second‐order rate constant was (3.8 ± 0.9) × 10−10 ml cell−1 h−1. Half‐life estimates for aerobic, aquatic biotransformation range from 1 to 100 d. Cyanamide appears to be an end product of NQ use and no intermediate biotransformation products were observed. Nitroguanidine is expected to move readily through soils (soil sorption coefficient Kp < 0.1); however, anaerobic biotransformation occurs readily in soil, with an estimated half‐life of 4 d. Other fate parameters measured at 25°C are a water solubility of 2,600 ± 100 ppm, octanol/water partition coefficient of 0.148 ± 0.001 (dimensionless), Henry's constant of < 7 × 10−6 (dimension‐less), base hydrolysis constant of (3 ± 1) × 10−4 M−1 S−1, neutral hydrolysis constant ≤ 2 × 10−8 s−1, and biouptake constants of 110 g dry cells/g water for Anabena flos‐aquae and 150 g dry cells/g water for Selenastrum capricornutum.

Aquatic toxicity of photo‐degraded insensitive munition 101 (IMX‐101) constituents

“…For NQ, photodegradation products previously identified include nitrosoguanidine, hydroxyguanidine, and cyanoguanidine, which, after further irradiation, terminate in nitrite, nitrate, ammonia, urea, guanidine, cyanamide, nitrosamide, and melamine [18,19,37]. Cyanoguanidine potentially converts to cyanide under acidic conditions, such as those found after prolonged photodegradation experiments at elevated concentrations [38].…”

Biotransformation and photolysis of 2,4-dinitroanisole, 3-nitro-1,2,4-triazol-5-one, and nitroguanidine

“…Consistent with the large absorptivity in the UV region, NQ is amenable to photo degradation, especially by UV light. 23,56,80 We found that after four hours of photolysis, NQ absorbance decreased, but an increase in absorbance was observed below 225 nm ( Figure 34B).…”

“…There are conflicting reports about the organic products from NQ photolysis, including cyanamide, hydroxy-guanidine, or more recently, stoichiometric guanidine. 23,56,80 NTO apparently produces CO2 or other volatile organics upon extended irradiation, with another study finding hyrdoxy-triazolone and potential nitric oxide formation. 23,74 Given the conflicting reports of photolysis products and mechanisms, along with the surprising renitration of guanidine to form NQ, our purpose was to investigate the conditions under which NQ and NTO are photolyzed.…”

“…53 The toxicity of NQ is low across the domains of life, from bacteria and aquatic invertebrates to plants and mammals. [54][55][56][57][58] Interestingly, photolysis increases the toxicity of NQ to fathead minnows, algae, and Daphnia magna, as well as to Ceriodaphnia dubia by 1000 times for yet unknown reasons. 38,55 In non-contaminated soil, slurries augmented with carbon induced complete NQ degradation in about six days, while adding carbon and nitrogen only produced moderate degradation.…”

“…53 Surface water microcosms transformed NQ with the addition of nutrients, and degradation occurred slowly under anaerobic conditions amended with nutrients. 56 Soil columns did not fully transform NQ and land application was discouraged by Williams et al 60 In terms of plant uptake and transformation, tall fescue and soybean took up NQ under hydroponic and soil conditions, with no transformation products detected. 57 We found that switchgrass took up NQ into the blades, with no transformation and a full mass balance (Appendix B).…”