Supercritical Water Oxidation vs Supercritical Water Gasification: Which Process Is Better for Explosive Wastewater Treatment?

2,4,6-Trinitrotoluene (TNT), as a representative component of explosive wastewater, is treated in supercritical water gasification (SCWG) and supercritical water oxidation (SCWO) using molecular dynamic simulations based on ReaxFF reactive force field as well as density functional theory (DFT). The detailed reaction processes, important intermediates and products distribution, and kinetic behaviors of SCWG and SCWO systems have been analyzed at the atomistic level. For the SCWG system, TNT is activated by water cluster or H radical and the N atom is mainly converted into NH₃ more than N₂ through two significant intermediates NOH and C–N fragment. In addition to water cluster and H radical, the TNT is activated by O₂ in the SCWO system. Besides, the N atom is transferred into N₂ more than other N-containing products after 750 ps simulation. Combined with the calculated cracking energy of the bonds in TNT, SCWG can accelerate its degradation and is easier for C–N bond breaking or changing through other reactions because of its low cracking energy (69.6 kcal/mol in thermal decomposition and 59.0 kcal/mol in SCWG). In addition, a large amount of H₂ molecules is produced in SCWG, which is a meaningful way of transforming waste to assets. On TNT degradation, SCWO with inadequate O₂ that can be treated as partial oxidation reaction (SCWPO) can combine the advantages of SCWG and SCWO (with enough O₂) to convert TNT into CO₂, H₂O, as well as H₂ and NH₃ with high economic value. Finally, a kinetic description is performed whose activation energies (17.6 and 18.4 kcal/mol) are theoretically consistent with experimental measurements.