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

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 (kobs) 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)] could account 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.