Quinone group enhances the degradation of levofloxacin by aqueous permanganate: Kinetics and mechanism

Abstract

Quinone group is an important fraction of humic acid. The pseudo-first-order rate constant (kobs) of levofloxacin (LF) degradation by permanganate (MnO4−) significantly increased from 0.010 (without benzoquinone, BQ) to 0.042–0.443 min−1 at [BQ]o:[MnO4−]o (molar ratio) = 0.03–0.25 at pH 7.5, and an acidic pH facilitated LF degradation. In the presence of BQ, MnO4− was first reduced to Mn(II). Then, Mn(II) reacted with BQ to produce Mn(III) and semiquinone radical, which was promoted under acidic conditions. With dissolved oxygen available, Mn(III) further oxidized semiquinone radical to produce singlet oxygen (1O2) and superoxide radical (O2−) as well as regenerate BQ. In addition, MnO4− could also react with Mn(II) to produce Mn(III), whose complexation with semiquinone radical in turn promoted this reaction. Due to the predominant scavenging of O2− by BQ, 1O2 and Mn(III) mainly contributed to the accelerated LF degradation, with a notable formation of hydroxyl, ketone and endoperoxide groups in the degradation byproducts. This study helps better understand the role of natural organic matter in the degradation of organic micropollutants by MnO4− in water treatment.