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

Abstract

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.