Reductive biotransformation as a pretreatment to enhance in situ chemical oxidation of nitroaromatic and nitroheterocyclic explosives

In Situ Chemical Oxidation (ISCO) is increasingly being used to remediate a variety of organic contaminants in soil and groundwater. Explosives, such as 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), 2,4-dintroanisole (DNAN), and 3-nitro-1,2,4-triazol-5-one (NTO), are not easily oxidized due to the presence of nitro groups in their structures. However, these nitro groups can be reduced to amino groups by soil microorganisms, making them more prone to oxidation. In this study, we propose bioreduction as a pretreatment to ISCO. The oxidation of DNAN, DNT, NTO, and their respective reduced transformation products, 2-methoxy-5-nitroaniline (MENA), 4-amino-2-nitrotoluene (NAT), and 3-amino-1,2,4-triazol-5-one (ATO), by potassium permanganate showed that lower permanganate concentrations and shorter time-scales are needed to oxidize the compounds once they are reduced. Significant fractions of the daughter products were mineralized to CO₂ (30% and 22% for MENA and NAT, respectively, using 2.5 mM permanganate, and 48% for ATO using 1.0 mM permanganate). The bioreductive pretreatment was demonstrated by incubating 1.0 mM NTO anaerobically using soil as inoculum and pyruvate as electron donor. Once NTO was completely reduced to ATO, 1.0 mM permanganate was added, immediately oxidizing 86% of the compound. In controls without pretreatment, no oxidation was observed. NTO and ATO oxidation was tested in soils that differed in organic matter content. The results indicate that natural organic matter decreases the efficiency of NTO oxidation. However, ATO oxidation was not affected by the presence of soil. Our results suggest that the proposed reductive pretreatment followed by ISCO presents a safe and cost-effective alternative for explosives remediation.