Degradation of 3-nitro-1,2,4-triazol-5-one (NTO), an Insensitive High Explosive, and its Reduced Daughter Product by Reactive Minerals

Abstract

Insensitive high explosive compounds like 3-nitro-1,2,4-triazol-5-one (NTO) have the potential to contaminate soil and water at Department of Defense sites. The objective of this study is to determine the ability of manganese oxide minerals to degrade the reduced daughter product of NTO, 3-amino-1,2,4-triazol-5-one (ATO). Batch experiments demonstrate that ATO can be oxidized by Mn oxide minerals, with urea, ammonium, CO2(g), and N2(g) as major end products. Synthetic birnessite degraded ATO most rapidly (k = 15.0 h-1), followed by the commercially available Mn oxide material Pro-OX™️ (k = 0.64 h-1). After exhaustion via reaction with excess ATO, Mn oxides can be fully regenerated to their original oxidative capacity using KMnO4 or NaOCl. In packed-bed continuous-flow columns fed 1 mM ATO (hydraulic retention time = 22 min), Pro-OX™️ reached breakthrough (effluent ATO concentration/influent ATO concentration, C/C0 ≥ 0.05) at 2400 pore volumes (PVs), as compared to only 55 PVs for Greensand Plus™️. Pro-OX™️ degraded 190 mg ATO per g Pro-OX™️, while Greensand Plus™️ only degraded 5 mg ATO per g Greensand Plus™️. After reacting with ATO, Mn oxide materials showed a loss of Mn(IV) minerals (e.g. pyrolusite, ramsdellite, todorokite) and the formation of Mn(III) minerals (e.g. groutite, manganite), indicating a general reduction of Mn oxides concurrent with ATO oxidation. In order to fully degrade NTO, a sequential reductive-oxidative process was evaluated consisting of a column packed with micron-sized zero-valent iron (m-ZVI) (HRT = 10 min) followed by a column packed with a mixture of Pro-OX™️ and quartz sand (HRT = 38 min). The m-ZVI column reduced >95% of the influent 1 mM NTO to ATO for 3300 PVs. The ATO produced in the ZVI column then flowed into the Pro-OX™️/sand column, where it was oxidized to benign end products. Breakthrough of ATO out of the Pro-OX™️/sand column occurred at 130 PVs. Dissolved iron from ZVI columns should be controlled in future studies to extend the lifespan of subsequent Mn oxide columns. These findings show that this ZVI-Mn oxide treatment train can be applied as permeable reactive barriers or packed-bed filters to treat groundwater and wastewater contaminated with NTO at military sites.