Nitroaromatic explosives have contaminated millions of acres of soil and water across the globe since World War II with known mutagenic, carcinogenic, and ecotoxicological effects. Recently, the U.S. Army initiated a shift away from traditional explosive compounds, such as trinitrotoluene (TNT) and hexahydrotrinitrotriazine (RDX), towards new, insensitive high explosive formulations. The new formulations approved for use include “IMX-101” and “IMX-104,” which contain 2,4-dinitroanisole (DNAN), 3-nitro-1,2,4-triazol-5-one (NTO), and nitroguanidine (NQ). These mixtures are less prone to accidental detonation making storage, transport, and implementation of these formulations safer for soldiers. Furthermore, initial research indicates that these compounds are less toxic than the older analogues. Despite the apparent benefits, the new explosives have higher solubility (approximately 3-300 times) than the compounds they are replacing, and NTO and NQ are fairly recalcitrant to aerobic biodegradation. The refractory nature and high solubility of the compounds raises concerns about leaching and water contamination considering the previous scale of environmental contamination from production and use of legacy explosives, while feasible strategies for cleaning up the new chemicals from soil and water have not been developed. Therefore, there is a critical need for understanding of the mechanisms of biodegradation these compounds will undergo in the environment and in engineered systems. In addition, a number of questions remain about the photochemistry of the compounds and how they may transform in sunlit surface water.
Accordingly, this thesis examines biological transformations of DNAN and NTO in vegetative, fungal, and bacterial organisms, as well as photolysis of NTO and NQ in aqueous solution and DNAN in plant leaves. I identified 34 novel biotransformation products of DNAN using stable-isotope labeled DNAN and high resolution mass spectrometry. Most identified biotransformation products were the result of a nitro-group reduction as the first metabolic step. Arabidopsis plants, a Rhizobium bacterium, and a Penicillium fungus all further metabolized DNAN to produce large, conjugated compounds, and no mineralization was observed in the systems studied. All three organisms reduced both para- and ortho-nitro groups of DNAN, with a dramatic preference for ortho reduction. I found that photodegradation of DNAN and its plant metabolites within Arabidopsis leaves could impact the phytoremediation of DNAN and other contaminants. Soil slurries acclimated to nitroaromatic wastewater degraded DNAN with and without carbon and nitrogen amendments and NTO with added carbon. Organisms capable of degrading DNAN and NTO were isolated, and NTO was transformed to urea, amino-triazolone, and hydroxyl-triazolone. Photolysis of NTO sensitized singlet oxygen formation and yielded hydroxyl-triazolone, nitrite, nitrate, and ammonium. The rate of photolysis of NTO increased over the neutral pH range, and natural organic matter quenched the photolysis of NTO. An unknown volatile product accumulated in the headspace of sealed reactors after NTO photolysis. Singlet oxygen degraded NTO and formed nitrite in stoichiometric yield. Photolysis of NQ produced nitrite and nitrate, but at high pH, the reaction occurred much faster than at neutral pH, and the mass balance of inorganic nitrogen was much lower. Further work should be done to investigate the mechanisms of and products from NTO and NQ photolysis.