New insights into the role of electron shuttles in enhancing permanganate oxidation for the removal of aniline compounds: Free radicals and intermediate manganese

Permanganate/electron shuttle systems for the enhanced transformation of anilines were evaluated. The presence of electron shuttles (2,2’-azino-bis(3-ethylbenzothiazoline)-6-sulfonate (ABTS), 1-hydroxybenzotriazole (HBT) and catechol (CAT) accelerated the oxidative transformation of aniline by Mn(VII). In the Mn(VII)/ABTS process, ABTS^•+ was partially consumed through fragmentation or polymerization with aniline, while the second-order rate constants (kapp₂, M⁻¹ s⁻¹) of the interaction of ABTS^•+ with aniline rose from 86.57 to 140.04 M⁻¹ s⁻¹ with increase in pH from 5 to 8. In the Mn(VII)/HBT system, aniline removal was markedly enhanced under acidic conditions due to the formation of polymerized products via radical cations (HBT^•). Furthermore, the oxidative transformation of aniline (82.5 %) by Mn(VII)/CAT was significantly higher than that by Mn(VII) (65.2 %) because of Michael addition reaction of aniline and ortho-benzoquinone (o-BQ). Moreover, the formation of Mn(III) was confirmed by the overall oxidation state (3.13) and the X-ray Photoelectron Spectroscopy (XPS) spectra of the Mn products in the CAT/Mn(VII) process, revealing a synergistic mechanism between o-BQ and Mn(III) for enhanced aniline degradation.