Electro-Fenton removal of TNT: Evidences of the electro-chemical reduction contribution

[Display omitted] According to TNT degradation pathways the majority of the by-products found by electro-Fenton treatment are obtained by cathodic reduction. ► These results are evidenced through high resolution LC–MS/MS. ► These are in contrast with the previous studies, where a dominant hydroxyl oxidation of nitro moieties was found and no cathode reduction process was reported. ► The proposed reaction scheme shows a complex alternation of oxidation and reduction mechanisms. ► This reaction scheme drastically differs from that obtained under Fenton's conditions. This study aims in analyzing, at laboratory scale, the mechanisms and performances of 2,4,6-Trinitrotoluene (TNT) removal from contaminated water using electro-Fenton process. This treatment belongs to the group of advanced oxidation processes (AOPs), regarded as the most effective approach to decontaminate wastewater from persistent and toxic organic pollutants. In this study, the TNT degradation was investigated through high resolution LC–ESI–MS/MS to tentatively identify the aromatic by-products. The proposed degradation scheme shows a complex alternation of oxidation and reduction mechanisms, evidencing the importance of the electrochemical reduction in the whole process. Furthermore, oxidative opening of the highly reduced/oxidized TNT aromatic ring occurred rapidly during the treatment, leading to the transient accumulation of short chain carboxylic acids such as malonic, glyoxylic, oxalic and formic acids. Mineralization occurred but was delayed in front of TNT and carboxylic acids, showing the persistence of some recalcitrant by-products.