Efficient Removal of Metronidazole by the Photo-Fenton Process with a Magnetic Fe₃O₄@PBC Composite

Abstract

Fe3O4-coated pretreated biochars (Fe3O4@PBC) were prepared for the first time by a Fe(III)-ethanol solution impregnation-calcination method. When photo-Fenton catalysts were used, their effectiveness in removing metronidazole (MNZ) from aqueous media was evaluated. Fe3O4@PBC samples were characterized by X-ray diffraction, scanning electron microscope, vibrating sample magnetometer, X-ray photoelectron spectroscopy, and Brunauer–Emmett–Teller methods. The results showed that Fe3O4 coating was successfully formed on the surface of HNO3- pretreated biochar, and Fe3O4@PBC can be separated by applying an external magnetic field. The coating of Fe3O4 did not change the pore structure and maintained a high surface area of the biochar. Fe loading significantly affected the photo-Fenton degradation and adsorption ability of MNZ. The highest MNZ removal rate and the greatest catalytic ability were found in the PBC-6.6Fe sample containing 6.6% Fe by mass. Various operating parameters, such as solution pH, H2O2 concentration, and MNZ concentration, were tested during MNZ’s photo-Fenton catalytic degradation. The results indicate that the highest MNZ degradation efficiency can be derived from a moderate acidic solution, and the optimal pH is 3. Using PBC-6.6Fe, the increase of H2O2 concentration from 30 to 60 mmol·L−1 promotes the degradation of photo-Fenton, and both an excessive H2O2 and an increase in MNZ concentration suppressed the process. Under the conditions of 0.4 g·L−1 PBC-6.6Fe, 300 mg·L−1 MNZ, 60 mmol·L−1 H2O2, and initial pH of 3, 95.1% of MNZ was degraded. The PBC-6.6Fe had good stability, and its removal efficiency was still over 92% after five repeated uses. This study confirmed that •OH played a dominant role, while O2•− and h+ played a weaker role in the photo-Fenton system. The results indicated that Fe3O4@PBC served as a prospective visible-light-driven catalyst similar to Fenton for the treatment of wastewater containing MNZ.