Chloramphenicol removal by zero valent iron activated peroxymonosulfate system: Kinetics and mechanism of radical generation

Zero valent iron (Fe-0) was used as a catalyst to activate peroxymonosulfate (PMS) for removal of chloramphenicol (CAP). About 95.2% of 10 mg/L CAP was removed by 0.2 mM peroxymonosulfate and 0.5 g/L Fe-0 at neutral pH, with little Fe3+ leaching. The degradation reactions well followed a pseudo-first-order kinetics pattern (R-2 > 0.95). The production of hydroxyl radicals (OH center dot) and sulfate radicals (SO4-center dot) was directly identified in in-situ Electron paramagnetic resonance (EPR) tests with 0.1 M 5,5-dimethyl-1-pyrrolidine N-oxide (DMPO) and indirectly identified through quenching tests. The second order rate constant between CAP and SO4-center dot was ascertained to be 6.18 x 10(10) M-1 s(-1) in competition reaction. According to SEM and XRD analysis, the average diameter of catalyst was elevated to be about 88 nm after use. Possible mechanisms on the radical generation were proposed based on the results of EPR and XPS analyses. It emerged that the cycle of Fe2+-Fe3+ on the Fe-0 was answerable for the OH% and SO4-center dot generation, and iron sulfate layer was assembled on the Fe-0 following exposure to peroxymonosulfate compared to the FeOOH layer present on the fresh Fe-0. While the presence of common anions including Cl-, CO32-, Fe2+, Cu2+ and Mn2+ will inhibit the degradation of CAP in Fe-0/PMS system. Besides, Fe-0/PMS pre-oxidation before chlorine or chloramine engendered in a sharp increment in the concentration of disinfection by-products (DBPs) precursors. The results manifested that Fe-0/PMS was effective in CAP removal at neutral pH, whereas it should be more cautiously investigated when combined with chlorine disinfection.