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 (kobs, 7.0) = 6.002EHOMO(o-Qred) + 33.744(ELUMO(o-Q) – EHOMO(o-Qred)) – 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.