How does pH influence ferrate(VI) oxidation of fluoroquinolone antibiotics?

Abstract

• Kinetic model of fluoroquinolones oxidation by Fe(VI) was built at pH 3.0–9.0. • Reactive sites and main reactions were proposed based on theoretical calculation. • Fe VI , Fe V , and Fe IV were primary reactive species at both alkaline and acidic pH. • HO was yielded via Fenton or Fenton-like process at acidic pH in Fe(VI) oxidation. This study investigated the impact of pH on the efficiency of ferrate (Fe(VI), K 2 FeO 4 ) toward fluoroquinolone antibiotics, built kinetic models of Fe(VI) oxidation, and elucidated the involved active species during Fe(VI) oxidation at acidic and alkaline conditions. It was found that the second-order reaction rate constants ( k app , M −1 s −1 ) of Fe(VI) with three fluoroquinolone antibiotics (i.e., enroxacin, ofloxacin, and norfloxacin) were strongly dependent on pH, and k app reached its maximum at pH about 6.5. The reactions between HFeO 4 - /H 2 FeO 4 and neutral/protonated fluoroquinolone antibiotics (X and XH + ) were the major ones. Moreover, the reactive species were identified as high-valent iron species, i.e., Fe(VI), Fe(V), and Fe(IV), in alkaline solution, while hydroxyl radicals was generated through Fenton or Fenton-like processes at acidic condition with limited contribution.