Low-temperature transformation of florfenicol mediated by δ-MnO2: the role of Mn(III) and reactive oxygen species.

Florfenicol (FF), a typical emerging contaminant, has potential environmental and health risks, arousing widespread concern. However, the role of δ-manganese dioxide (δ-MnO2), a natural mineral, in the transformation of FF in mid-to-high latitude regions under low-temperature conditions remains unclear. In this study, reaction systems of δ-MnO2 and FF were constructed to reveal the reaction kinetics, role of active substances, and FF transformation pathways under low-temperature conditions (5.0 °C). The results showed that the equilibrium oxidation amount and reaction rate of FF at 5.0 °C were 7.0 ± 0.2 µg mg-1 and 0.02 ± 0.005 min-1. After the reaction, the concentration of adsorbed Mn(II) reached 2.6 times that of free Mn(II), which was measured at 3.7 ± 0.3 µmoL L-1. The adsorbed Mn(II) occupied the surface-active sites of δ-MnO2, thereby terminating the transformation of FF. Mn(III) induced the formation of ⋅OH, O2˙-, and H2O2, which reacted with FF. The promoting order of these substances was Mn(III) > ⋅OH > O2˙- > H2O2. Under low-temperature conditions, the transformation pathways of FF mediated by δ-MnO2 involved hydroxyl group oxidation, defluorination, dechlorination, and desulfonylation. Overall, the toxicity of most transformation products showed a decreasing trend. This study provides a theoretical basis for the natural transformation of antibiotics mediated by natural minerals in aquatic environments with low temperatures.