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 orthosemiquinone radicals (o-SQ(center dot-)) 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(center dot) complex and MnO2 surface in stabilizing SQ(center dot) is revealed. To illustrate the formation of the Mn(II)-o-SQ(center dot) complex, electron spin resonance, cyclic voltammetry, and mass spectra were used. To demonstrate the presence of o-SQ(center dot) on the MnO2 surface, EDTA was used to quench Mn(II)-o-SQ(center dot). The high stability of o-SQ(center dot-) on the MnO2 surface is attributed to the higher potential of o-SQ(center dot-) (0.9643 V) than the MnO2 surface (0.8598 V) at pH 7.0. The SMX removal rate constant by different stabilized o-SQ(center dot) 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(center dot), respectively. Moreover, aniline and cystine are competitive substrates for SMX in coupling o-SQ(center dot-). 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.