Efficient degradation of chloramphenicol by zero-valent iron microspheres and new insights in mechanisms

Chloramphenicol (CAP) has been extensively used in the broad-spectrum antibiotic therapy so that its excessive emission has brought about serious contamination in aquatic environments. It is of great concern to find an efficient removal method and understand the transformation pathways. In this study, we developed the zero-valence iron based catalytic technique by synthesizing regular zero-valence iron microspheres (ZVI MPs), which could completely remove the CAP pollutant (20 mg L-1) within 15 min. When the CAP concentration increased to 50 mg L-1, the removal efficiency achieved to 95.5% after 15 min of reaction. The chemical evolutions of iron species on the catalyst surface and changes in components of the solution demonstrated the activation of molecular oxygen and hydrogen ion by ZVI MPs. The generation of( center dot)OH and H-center dot radicals as well as their contributions to CAP degradation were confirmed by electron spin resonance and trapping experiments during the reaction. Further experiments including the intermediates identification revealed a possible degradation pathway. To the best of our knowledge, the mechanism of CAP degradation by ZVI was proposed for the first time involving both reductive and oxidative transformations with a prerequisite of efficient reductive dechlorination. The complete dechlorination of CAP molecules was confirmed by the detection of inorganic Cl- concentration (0.124 mmol L-1 after reaction), which generally lowered the toxicity of the products. These findings not only provided an efficient and economic zero-valent iron technique for eliminating the antibiotic pollutants in water, but also made important contributions to better understand the removal mechanism.