Improved degradation of metronidazole in a heterogeneous photo-Fenton oxidation system with PAC/Fe3O4 magnetic catalyst: biodegradability, catalyst specifications, process optimization, and degradation pathway

A UV-assisted heterogeneous photo-Fenton process based on powder activated carbon (PAC)/Fe3O4 catalyst was investigated to degrade metronidazole (MTZ). Successful uniform synthesis of PAC/Fe3O4 has confirmed by X-ray diffraction (XRD), field emission scanning electron microscope ( FE-SEM), energy dispersive X-Ray spectroscopy mapping (EDX-mapping), vibrating sample magnetometer (VSM), and Fourier transform infrared spectroscopy (FTIR) techniques. The interaction of five independent variables in the MTZ degradation in the UV + PAC/Fe3O4+H2O2 photo-Fenton system was optimized by a three-level full-face-centered central composite design (CCD). Proposed model proved to have adequacy and validity in prediction of MTZ degradation (P value < 0.0001 and R-2 = 0.9512). Although the desirable pH for MTZ degradation was 3, Fenton-like reaction was ongoing over a wide range of pH. However, at alkaline pH, the process efficiency was more affected by MTZ concentration than acidic pH. MTZ degradation by UV+PAC/Fe3O4+H2O2 heterogeneous Fenton system followed the pseudo-first-order model (R-2 > 0.95). High stability, reusability, and separation capability of PAC/Fe3O4 were confirmed by EDX and VSM analyses and iron leach investigation. This process could represent the MTZ degradation, DOC, and COD removal efficiencies of 96.12%, 93.67, and 94.87%, respectively. Also, increasing of the AOS value from -0.422 to 0.437 and increasing of the COS value from -0.422 to 3.773 indicate that the MTZ has been highly mineralized and the solution biodegradability has been improved. MTZ degradation intermediates, mechanism, and pathways were proposed. The abundancy of intermediates with m/z < 70 confirmed the strong degradation of MTZ in the photo-Fenton system.