Degradation of sulfathiazole by electro-Fenton using a nitrogen-doped cathode and a BDD anode: Insight into the H₂O₂ generation and radical oxidation

In this work, nitrogen-doped cathodes for high H₂O₂ production and sulfathiazole (STZ) degradation in electro-Fenton (EF) systems were prepared by the carbonization of three carbon/nitrogen-enriched precursors. Among the cathodes elaborated from different precursors, the one using 1h-1,2,4-triazole-3,5-diamine as the precursor showed the best oxygen reduction reaction (ORR) ability with the normalized H₂O₂ accumulation of 9.49 ± 0.03 mg L⁻¹ h⁻¹ cm⁻² compared to the other two N-containing cathodes. The enhanced H₂O₂ accumulation was attributed to the high electroactive surface area and pyrrolic N (60.45%) content. Regarding reactive oxygen species in the absence of Fe²⁺, aside from the H₂O₂, [rad]O₂ ⁻and ¹O₂ were identified using spectroscopic techniques and chemical probes. As a result, a degradation and mineralization efficiency of 98.25 ± 0.14% and 70.57 ± 0.27% of STZ were attained in the 180-min treatment, mainly coming from the homogeneous [rad]OH from classical Fenton, anodic [rad]OH on BDD anode and direct/indirect oxidation of [rad]O₂ ⁻and ¹O₂. In addition, the plausible degradation pathway of STZ was proposed based on the density functional theory (DFT) combined with experimental data derived by ultra-performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS). The frontier orbital theory and Fukui function theoretically suggested the vulnerable sites of STZ for different active species including [rad]OH, [rad]O₂ ⁻ and ¹O₂. This study provides a new strategy for improving the ORR process and analyzing the generation and conversion of reactive oxygen species in the EF process.