Enhancement of the advanced Fenton process by weak magnetic field for the degradation of 4-nitrophenol

A weak magnetic field (WMF) was employed to enhance the degradation of 4-nitrophenol (4-NP) by the advanced Fenton process (Fe⁰/H₂O₂) in this study. Although the oxidation rates of 4-NP by Fe⁰/H₂O₂ and WMF-Fe⁰/H₂O₂ dropped sharply upon increasing the initial pH (pH_ini), the introduction of WMF could remarkably improve the 4-NP degradation by Fe⁰/H₂O₂ at pH_ini ranging from 3.0 to 6.0. The quenching and electron paramagnetic resonance experiments verified that the hydroxyl radical was the primary oxidant responsible for the 4-NP degradation at pH_ini 4.0 and the cumulative concentration of HO in the WMF-Fe⁰/H₂O₂ system was about 3-fold that in Fe⁰/H₂O₂ system. The superimposed WMF increased the generation of HO in the Fe⁰/H₂O₂ process by accelerating the Fe⁰ corrosion and Fe^II generation, which was the limiting step of the Fe⁰/H₂O₂ process. The application of WMF largely enhanced the mineralization of 4-NP but it did not change the 4-NP degradation pathways, which were proposed based on the degradation products detected with LC-MS/MS. The optimum intensity of the magnetic field for 4-NP oxidation by WMF-Fe⁰/H₂O₂ was determined to be 20 mT. Response surface methodology (RSM) was applied to analyze the experimental variables and it was found that lower pH and higher Fe⁰ and H₂O₂ dosages were beneficial for 4-NP degradation by WMF-Fe⁰/H₂O₂. Among the three factors (pH_ini, Fe⁰ dosage, and H₂O₂ dosage) investigated, pH_ini was the most important factor affecting the performance of the WMF-Fe⁰/H₂O₂ process. The WMF-Fe⁰/H₂O₂ technology provides a new alternative for scientists working in the field of water treatment.