Ciprofloxacin degradation in photo-Fenton and photo-catalytic processes: Degradation mechanisms and iron chelation.

Ciprofloxacin (CIP) is a broad spectrum synthetic antibiotic drug of fluoroquinolones class. CIP can act as a bidentate ligand forming iron complexes during its degradation in the photo-Fenton process (PFP). This work investigates on PFP for the degradation of CIP to understand the formation mechanism and stability of iron complexes under ultraviolet (UV)-light illumination. A comparison was made with the UV-photocatalysis (UV/TiO2) process where CIP doesn't form a complex. In PFP, the optimal dose of Fe2+ and H2O2 were found to be 1.25 and 10 mmol/L with pH of 3.5. An optimal TiO2 dose of 1.25 g/L was determined in the UV/TiO2 process. Maximum CIP removal and mineralization efficiency of 93.1% and 47.3% were obtained in PFP against 69.7% and 27.6% in the UV/TiO2 process. The mass spectra could identify seventeen intermediate products including iron-CIP complexes in PFP, and only seven intermediate products were found in the UV/TiO2 process with a majority of common products in both the processes. The proposed mechanism supported by the mass spectra bridged the routes of CIP cleavage in the PFP and UV/TiO2 process, and the decomposition pathway of Fe3+-CIP chelate complexes in PFP was also elucidated. Both in PFP and UV/TiO2 processes, the target site of HO• radical attack was the secondary-N atom present in the piperazine ring of the CIP molecule. The death of Escherichia coli bacteria was 55.7% and 66.8% in comparison to the control media after 45 min of treatment in PFP and UV/TiO2 process, respectively.