Abiotic reductive transformation of 3-nitro-1,2,4-triazol-5-one by zero-valent iron

The military is switching over to insensitive munitions compounds (IMCs) to avoid unintentional detonations during handling and use of explosives. 3-nitro-1,2,4-triazol-5-one (NTO) is an important component of IMCs. NTO may contaminate the subsurface due to its high aqueous solubility. Thus, there is a need to develop remediation technologies for the treatment of NTO-containing (waste) water. This study demonstrated that zero-valent iron (ZVI) reductively transformed NTO to its daughter product, 3-amino-1,2,4-triazol-5-one. The pseudo first-rate constant (k ₁) of NTO reduction by micron-sized ZVI at pH 3 was 192.6 h⁻¹. Kinetic degradation experiments performed at different pH values showed that ZVI did not effectively reduce NTO at pH 6 (k ₁ = 0.6 h⁻¹) or higher. The rapid NTO reduction in acidic conditions may be due to dissolution of iron precipitates on the ZVI surface. Additional experiments were conducted to assess the effectiveness of various depassivating pretreatments with deionized water, acetic acid, hydrochloric acid, or bicarbonate. Treatment with 1 M HCl for 15 min was the most effective depassivation method for a ZVI material containing a thick passivating layer (ca. 880 nm), achieving 84.0% NTO removal after 10 min of reaction. On the other hand, a milder treatment involving washing with a diluted bicarbonate solution (60 mM) was sufficient for a ZVI material that was less passivated (estimated thickness of the passivating layer ≈ 300 nm). This study demonstrates that ZVI treatment is a promising approach for the remediation of NTO-contaminated sites or wastewater and provides critical information to optimize this process.