Professor, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, China

Speech Title: Impact of humic acid on the degradation of levofloxacin by aqueous permanganate: Kinetics and mechanism

Abstract: Levofloxacin (LF) is a frequently detected fluoroquinolone in surface water, and permanganate (MnO₄⁻) is a commonly used oxidant in drinking water treatment. This study investigated the impact of humic acid (HA) on LF degradation by aqueous MnO₄⁻ from both kinetic and mechanistic aspects. In the absence of HA, the second-order rate constant (k) of LF degradation by MnO₄⁻ was determined to be 3.9 M^‒1 s^‒1 at pH 7.5, which increased with decreasing pH. In the presence of HA, the pseudo-first-order rate constant (k_obs) of LF degradation at pH 7.5 was significantly increased by 3.8^- and 2.8^- fold at [HA]_o:[KMnO₄]_o (mass ratio) = 0.5 and 1, respectively. Secondary oxidant scavenging and electron paramagnetic resonance tests indicated that HA could form a complex with Mn(III), a strongly oxidative intermediate produced in the reaction of MnO₄⁻ with HA, to induce the successive formation of superoxide radicals (O₂^•−) and hydroxyl radicals (^•OH). The resulting ^•OH primarily contributed to the accelerated LF degradation, and the complex [HA-Mn(III)] accounted for the rest of acceleration. The degradation of LF and its byproducts during MnO4− oxidation was mainly through hydroxylation, dehydrogenation and carboxylation, and the presence of HA led to a stronger destruction of LF. This study helps better understand the degradation of organic micropollutants by MnO₄⁻ in drinking water treatment.