Structure-reactivity relationship in the degradation of three representative fluoroquinolone antibiotics in water by electrogenerated active chlorine

Abstract

In this work electrogenerated active chlorine was used to treat fluoroquinolones, a class of antibiotics that is highly consumed and frequently found in waste and natural waters. Taking into account the structural variety and diverse molecular forms of these pharmaceuticals, their structure-reactivity relationship toward active chlorine was studied. Three representative fluoroquinolones: ciprofloxacin, norfloxacin and levofloxacin were considered. Despite structural differences, the primary by-products evidenced that the major reactive part of fluoroquinolones was the piperazyl ring. However, ciprofloxacin and norfloxacin were degraded faster than levofloxacin, due to the fact that secondary amines on ciprofloxacin and norfloxacin were more reactive toward active chlorine than tertiary amines on levofloxacin. The reactivity of cationic and neutral forms of the fluoroquinolones was also evaluated using both experimental and theoretical procedures. Experimental results evidenced that the neutral forms have higher degradation rates than the cationic structures. Theoretical analysis of net charge on atoms (obtained by the natural bonding orbitals method) indicated that the reactive moieties on the neutral form of antibiotics have a high nucleophilicity. Therefore, this form is more susceptible to electrophilic attacks by chlorine species than the cationic one. Additionally, the environmental impact was evaluated considering the evolution of antimicrobial activity and its correlation with the antibiotics transformations. Finally, simulated hospital wastewater containing norfloxacin was treated. The electrogenerated active chlorine removed 100% of the antimicrobial activity in a short period (40min), indicating that this system is an interesting option for treating hospital wastewaters and decreasing the negative environmental impact of the input of fluoroquinolones into natural media.