Complete degradation of 3-amino-1,2,4-triazol-5-one (ATO) by manganese oxide

Abstract

Insensitive high-explosive compounds like 3-nitro-1,2,4-triazol-5-one (NTO) can contaminate soil and water at military sites. This study compared the ability of synthesized and commercial MnO₂ materials to degrade the reduced daughter product of NTO, 3-amino-1,2,4-triazol-5-one (ATO), and characterized the solid-phase transformations that govern the products formed in continuous-flow reactors. Synthetic birnessite degraded ATO fastest (k = 15.0 h⁻¹) but had poor hydraulic characteristics. Whereas the commercial material Pro-OX™️ had a good degradation rate (k = 0.64 h⁻¹) and its granular morphology ensured good hydraulic performance. Results show ATO is oxidized by MnO₂-type minerals to benign end-products (urea, NH₄^+, CO_2(g), and N_2(g)). Spent Mn oxides were fully regenerated using KMnO₄ or NaOCl, restoring or even surpassing their original oxidative capacity. Further investigation of Pro-OX in packed-bed columns under a simulated wastewater treatment regime (1 mM ATO, empty bed contact time [EBCT]= 1 h) showed that the column reached breakthrough (C/C₀ ≥ 0.05) at 2400 pore volumes (PVs), with a capacity to degrade 190 mg ATO g⁻¹. Under accelerated groundwater flow regime (0.1 mM ATO, EBCT= 24 h), the reactor removed ATO effectively for the duration of operation (660 PVs). After reacting with ATO, the Mn oxide material initially dominated by Mn^(IV) (e.g., pyrolusite, ramsdellite, todorokite) was converted to Mn^(III) minerals (e.g., groutite, manganite) and soluble Mn^(II) (recovered in effluent) indicating reduction of MnO₂ concurrent with ATO oxidation. Collectively, these findings demonstrate that MnO₂ is a promising material for application in permeable reactive barriers or packed-bed filters to treat waters contaminated with ATO at military sites.