Degradation of fluoroquinolones and macrolides by solar light-driven heterogeneous photocatalysis - Proposed drug transformation pathways

Antimicrobial micropollutants are commonly detected pollutants in aquatic environments. Wastewater treatment plants inefficiently remove pharmaceuticals from wastewater, hence, they remain in treated wastewater. However, heterogeneous photocatalysis has drawn significant interest due to its high removal efficacy of antimicrobial micropollutants from water. In the presented study, the photocatalytic degradation of five selected antimicrobials (ciprofloxacin, enrofloxacin, levofloxacin, clarithromycin, and tylosin) in aqueous suspensions of five semiconductors (TiO2-anatase, TiO2-P25, ZnO, ZrO2, and WO3) was analyzed. Fluctuations in antimicrobial concentration were monitored during the processes using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). Screening analysis was employed to identify the pharmaceutical transformation products in the post-reaction mixtures. The total organic carbon in the samples before and after the process was determined to examine the samples' mineralization degree. TiO2-anatase (clarithromycin) and TiO2-P25 (ciprofloxacin, levofloxacin, enrofloxacin, tylosin) gave the highest removal efficiency of pharmaceuticals (over 90 %). The utilization of the most effective catalysts resulted in 50.5-73.2 % degree of mineralization of the samples after 120 min. A total of 33 transformation products of selected pharmaceuticals, formed due to aromatic ring cleavage and oxidation, were identified. The obtained results demonstrated that heterogeneous photocatalysis was a practical approach for the degradation of antimicrobials in an aquatic environment.