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 MnO 2 -mediated system in the presence of humic acid. To evaluate the performance of different MnO 2 /mediator systems, 16 mediators are examined for their effects on MnO 2 reactions with SMX. The key role of the bidentate Mn(II)- o -SQ • complex and MnO 2 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 MnO 2 surface, EDTA was used to quench Mn(II)- o -SQ • . The high stability of o -SQ •- on the MnO 2 surface is attributed to the higher potential of o -SQ •- (0.9643 V) than the MnO 2 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 ) = 6.002 E 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.