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 (MnO) is a commonly used oxidant in drinking water treatment. This study investigated the impact of humic acid (HA) on LF degradation by aqueous MnO from both kinetic and mechanistic aspects. In the absence of HA, the second-order rate constant (k) of LF degradation by MnO was determined to be 3.9 M s at pH 7.5, which increased with decreasing pH. In the presence of HA, the pseudo-first-order rate constant (k) of LF degradation at pH 7.5 was significantly increased by 3.8- and 2.8-fold at [HA]:[KMnO] (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 MnO with HA, to induce the successive formation of superoxide radicals (O) 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 MnO 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 MnO in drinking water treatment.