Abstract:

Excessive use of ciprofloxacin (CIP) has proven to be a significant threat to the ecological environment. In this work, a novel Fe-free photo-electro-Fenton system was designed for the degradation of CIP in water. The NiO/g-C₃N₄ composites were synthesized by a simple solvothermal method. The crystalline phases and chemical compositions of the different catalysts were determined via X-ray diffraction (XRD) analysis. Fourier transform infrared (FT-IR) spectroscopy further confirmed the molecular structures of the different composites. The results proved the successful synthesis of NiO/g-C₃N₄ composites. The morphology of the material was obtained using scanning electron microscopy (SEM), which showed that the structure of the optimal NiO/g-C₃N₄-60% was two-dimensional and flower-like. The transmission electron microscopy (TEM) analysis further proved that the NiO/g-C₃N₄-60% possessed a layered structure. Owing to the layered structure, the NiO/g-C₃N₄-60% boasts of a large specific surface area and abundant active sites, which were beneficial for the transmission of electrons and oxidation of CIP. Furthermore, it was evident from the X-ray photoelectron spectroscopy (XPS) analysis that the Ni²⁺ and Ni³⁺ coexisted, and there was low coordination oxygen with defects in the NiO/g-C₃N₄-60% composite. The electron paramagnetic resonance (EPR) spectrum also proved the existence of oxygen vacancies, which not only facilitated the activation of H₂O₂, but also promoted the formation of stable mixed valence states of metal ions. UV-vis diffuse reflection spectrum (UV-Vis DRS), photoluminescence (PL), and electrochemical tests showed that NiO/g-C₃N₄-60% exhibited the strongest light absorption capacity, lowest charge transfer resistance, and fastest charge separation efficiency, which was beneficial for the generation of active species and the rapid degradation of CIP. Therefore, the flower-like NiO/g-C₃N₄-60% composites exhibited photoelectric synergy in the photo-electro-Fenton process. They not only effectively decomposed the H₂O₂ produced in the electro-Fenton process into ·OH by the conversion of Ni³⁺/Ni²⁺, but also generated photogenerated electrons and holes to promote the production of ·OH, ·O₂^–, and h⁺ under light irradiation to improve the degradation efficiency of CIP. When the optimal NiO/g-C₃N₄-60% served as a catalyst in the photo-electro-Fenton system, the degradation efficiency of CIP reached approximately 100% in 90 min and the mineralization efficiency reached 82.0% in 120 min. In addition, compared with the traditional Fenton system (the optimal pH value of which is 2.8–3.5), the novel photo-electro-Fenton system possessed a wider range of pH, with a final CIP degradation efficiency of 78.8% at a pH value of 6. The NiO/g-C₃N₄-60% also demonstrated excellent structural stability in the photo-electro-Fenton system. After five consecutive cycles, the degradation efficiency was maintained at 96.3%. Based on the results of high-performance liquid chromatography-mass spectrometry (HPLC-MS), two possible pathways for CIP degradation were proposed. This study provides a theoretical basis for the rapid degradation of antibiotics in wastewater.

Key words: Two-dimensional layered, NiO/g-C₃N₄, Photo-electro-Fenton system, Ciprofloxacin, Flower-like composite