Biodegradation of the emerging contaminant 3-nitro-1,2,4-triazol-5-one and its product 3-amino-1,2,4-triazol-5-one in perlite/soil columns.

3-Nitro-1,2,4-triazol-5-one (NTO) is an ingredient of new safer-to-handle military insensitive munitions formulations. NTO can be microbially reduced to 3-amino-1,2,4-triazol-5-one (ATO) under anaerobic conditions if an electron donor is available. Conversely, ATO can undergo aerobic biodegradation. Previously, our research group developed an anaerobic enrichment culture that reduces NTO to ATO. A second culture could aerobically mineralize ATO. This study aimed to combine anaerobic/aerobic conditions within a down-flow perlite/soil column for simultaneous NTO reduction and ATO oxidation. Acetate biostimulation was investigated to promote oxygen depletion and create anaerobic micro-niches for NTO reduction, whereas perlite increased soil porosity and oxygen convection, allowing ATO oxidation. Two columns packed with a perlite/soil mixture (70:30, wet wt.%) or 100% perlite were operated aerobically and inoculated with the NTO- and ATO-degrading cultures. Initially, the influent consisted of ∼280 µM ATO, and after 30 days, the feeding was switched to ∼260 µM NTO and ∼250 µM acetate. By progressively increasing acetate from 250 to 4000 µM, the NTO removal gradually improved in both columns. The perlite/soil column reached a 100% NTO removal after 4000 µM acetate was supplemented. Additionally, there was no ATO accumulation, and inorganic nitrogen was produced, indicating ATO mineralization. Although NH4+ was produced following ATO oxidation, most nitrogen was recovered as NO3- likely via nitrification reactions. Microbial community analysis revealed that phylotypes hosted in the enrichment cultures specialized in NTO reduction (e.g., Geobacter) and ATO oxidation (e.g., Hydrogenophaga, Ramlibacter, Terrimonas, and Pseudomonas) were established in the columns. Besides, the predominant genera (Azohydromonas, Zoogloea, and Azospirillum) are linked to nitrogen cycling by performing nitrogen fixation, NO3- reduction, and nitroaromatics degradation. This study applied a bulking agent (perlite) and acetate biostimulation to achieve simultaneous NTO reduction and ATO oxidation in a single column. Such a strategy can assist with real-world applications of NTO and ATO biodegradation mechanisms.