Photocatalytic degradation of ciprofloxacin using Zn-doped Cu2O particles: Analysis of degradation pathways and intermediates

Abstract

Currently, the occurrence of ciprofloxacin in the water environment has raised serious concerns. For utilizing the sunlight for degradation of ciprofloxacin it is important to develop visible light semiconductor photocatalysts with high catalytic activity. In this paper, Zn-doped Cu2O particles were prepared by a solvothermal method. The effects of Zn-doped on the structures and performances of Cu2O were examined via XRD, XPS, SEM, TEM, BET, and UV–Vis techniques. The photocatalytic mechanism and the factors responsible for the enhanced photocatalytic performance of the Zn-doped Cu2O were investigated. The main intermediate products of the photocatalytic degradation were elucidated using HPLC-MS/MS analysis. Frontier electron densities were used to predict the reaction sites of ciprofloxacin. The results indicate that the prepared samples have high purities, and the Cu2O, R1, R2, and R3-Cu2O exhibit dodecahedron, polyhedral, octahedral, and hexahedral morphologies, respectively. Compared with pure Cu2O, the specific surface areas of the Zn-doped Cu2O increased, the absorption intensity improved significantly in the visible light range, and the band gap is greater than that of the undoped Cu2O. The fitting resistance values decreased significantly, and the transfer and separation efficiencies of the photogenerated electrons and holes were improved. Among the samples, R2-Cu2O has the best photocatalytic performance and reusability. 94.6% of ciprofloxacin was degraded in presence of R2-Cu2O. After 5 cycles, the degradation percentage still exceeded 91%. The degradation of ciprofloxacin begins with direct attack of the photogenerated holes and the addition of hydroxyl radicals. Two possible degradation pathways from the cleavage of the piperazine ring and the decarboxylation of the quinolonic ring were suggested by combining the experimental data with the theoretical calculations of the frontier electron densities. This research work provides an insight into the mechanism and pathways of ciprofloxacin degradation by Zn-doped Cu2O, and opens-up a new opportunity for the treatment of ciprofloxacin-containing wastewater using semiconductor photocatalytic oxidation technology.