Wet air oxidation of indole, benzopyrazole, and benzotriazole: Effects of operating conditions and reaction mechanisms

Three simple structured nitrogen heterocyclic compounds (NHCs), namely indole (ID), benzopyrazole (BP) and benzotriazole (BT), were degraded via wet air oxidation (WAO) in an autoclave-type reactor. The elimination of target pollutants became significant only above 220 °C. The reactivity order of the three substrates was ID > BP > BT. The chemical oxygen demand (COD) removal levels were lower than 70%. Many partial oxidation intermediates were produced, as detected by electrospray ionization-mass spectrometry (ESI-MS). The presence of different anions, such as CO32−, SO42−, and PO43−, had a detrimental effect on the removal performances. On the other hand, Cl− and NO3− had a beneficial effect on the WAO of the target pollutants. By combining quantum chemistry calculations and ESI-MS analysis, detailed degradation pathways of ID, BP, and BT could be proposed. It was concluded that ring breakage occurred on both the benzene and heterocyclic ring in the initial stage of WAO for all three substrates.