Sulfite activation for ciprofloxacin rapid degradation using an iron-based metal organic framework derivative in heterogeneous processes: Performance and mechanisms investigation

Owing to its low eco-toxicity and low cost, sulfite is considered a promising precursor for oxysulfur radicals. In this study, we report the rapid degradation of ciprofloxacin in water using a derivative of an iron-based metal-organic framework catalyst (S-MIL-101(Fe)) for the heterogeneous activation of sulfite. S-MIL-101(Fe) possessed a similar structure to the original MIL-101(Fe), and more active sites were exposed. Unlike previous systems used to activate sulfite with Fe-based catalysts, this system exhibited excellent performance under alkaline conditions. Ciprofloxacin (10 mg/L) removal efficiency of 94.7 % was observed at pH 8.7, implying a different activation mechanism. It is suggested that the iron coordinatively unsaturated metal sites (Fe CUSs) on the surface of S-MIL-101(Fe) can effectively complex with SO32 to form Fe(III) CUS-SO3+, followed by the generation of SO3- through single-electron transfer. Quenching and electron paramagnetic resonance experiments demonstrated that SO3-, O-2, SO5 and -OH were involved in the degradation of ciprofloxacin, in which SO3 center dot- played a significant role. Moreover, HSO5-(peroxymonosulfate ion), an important product produced during the sulfite activation process, also participated in the formation of free radicals. This study complements the mechanism of heterogeneous activation of sulfites by Fe-based materials and reinforces the important role played by SO3 in some cases. The influence of chloride, bicarbonate, nitrate, and humic acid on ciprofloxacin elimination was minimal. In addition, this system could operate efficiently in real water environments.