Degradation of Sulfamethoxazole by Manganese(IV) Oxide in the Presence of Humic Acid: Role of Stabilized Semiquinone Radicals

In this work, we demonstrate for the first time the abatement of sulfamethoxazole (SMX) induced by stabilized ortho-semiquinone radicals (o-SQ^•-) in the MnO₂-mediated system in the presence of humic acid. To evaluate the performance of different MnO₂/mediator systems, 16 mediators are examined for their effects on MnO₂ reactions with SMX. The key role of the bidentate Mn(II)-o-SQ^• complex and MnO₂ surface in stabilizing SQ^•- is revealed. To illustrate the formation of the Mn(II)-o-SQ^• complex, electron spin resonance, cyclic voltammetry, and mass spectra were used. To demonstrate the presence of o-SQ^• on the MnO₂ surface, EDTA was used to quench Mn(II)-o-SQ^•. The high stability of o-SQ^•- on the MnO₂ surface is attributed to the higher potential of o-SQ^•- (0.9643 V) than the MnO₂ surface (0.8598 V) at pH 7.0. The SMX removal rate constant by different stabilized o-SQ^• at pH 7.0 ranges from 0.0098 to 0.2252 min^-1. The favorable model is the rate constant ln (k_{obs, 7.0}) = 6.002E_HOMO(o-Q_red) + 33.744(E_LUMO(o-Q) - E_HOMO(o-Q_red)) - 32.800, whose parameters represent the generation and reactivity of o-SQ^•, respectively. Moreover, aniline and cystine are competitive substrates for SMX in coupling o-SQ^•-. Due to the abundance of humic constituents in aquatic environments, this finding sheds light on the low-oxidant-demand, low-carbon, and highly selective removal of sulfonamide antibiotics.