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 -semiquinone radicals (-SQ) in the MnO-mediated system in the presence of humic acid. To evaluate the performance of different MnO/mediator systems, 16 mediators are examined for their effects on MnO reactions with SMX. The key role of the bidentate Mn(II)--SQ complex and MnO surface in stabilizing SQ is revealed. To illustrate the formation of the Mn(II)--SQ complex, electron spin resonance, cyclic voltammetry, and mass spectra were used. To demonstrate the presence of -SQ on the MnO surface, EDTA was used to quench Mn(II)--SQ. The high stability of -SQ on the MnO surface is attributed to the higher potential of -SQ (0.9643 V) than the MnO surface (0.8598 V) at pH 7.0. The SMX removal rate constant by different stabilized -SQ at pH 7.0 ranges from 0.0098 to 0.2252 min. The favorable model is the rate constant ln () = 6.002(-Q) + 33.744((-Q) - (-Q)) - 32.800, whose parameters represent the generation and reactivity of -SQ, respectively. Moreover, aniline and cystine are competitive substrates for SMX in coupling -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.