TNT oxidation by Fenton reaction: Reagent ratio effect on kinetics and early stage degradation pathways

► TNT degradation by Fenton reaction at laboratory scale was investigated. ► The absolute rate constant between TNT and hydroxyl radicals was determined. ► Early stage TNT degradation by-products were identified by LC–ESI-MS/MS. ► Four intermediates never described in the literature were identified. The removal of TNT from contaminated water can be achieved through advanced oxidation treatment. In the present study, its degradation by Fenton reaction was investigated at laboratory scale. Degradation efficiencies were compared by varying Fenton's reagents concentration and ratios. The parallel monitoring of Fenton's reagents concentrations allowed evidencing hydrogen peroxide or ferrous ion contents as limiting factors for TNT removal. The absolute rate constant of the reaction between TNT and hydroxyl radicals was also determined. TNT by-products corresponding to early stage degradation were identified by LC–ESI-MS and LC–ESI-MS/MS after solid phase pre-concentration. For the first time, formation of 2,4,6-trinitro-cyclohexa-2,4-dienol (uncommon), 2,4,6-trinitrobenzylalcohol, 2,4,6-trinitro-benzaldehyde, and one of the dinitro-hydroxy-benzaldehyde isomers were evidenced. Degradation pathways involving hydroxyl radical attacks resulting in methyl group oxidation, decarboxylation, aromatic ring breakage and hydrolysis, were finally proposed.