Degradation of Sulfamethoxazole by Manganese(IV) Oxide in the Presence of Humic Acid: Role of Stabilized Semiquinone Radicals
Zhong Chen, Hongbin Cao, Qingguo Huang, Yongbing Xie, He Zhao
Abstract
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.