Catalytic oxidation of 4-chlorophenol with magnetic Fe3O4 nanoparticles: mechanisms and particle transformation

Magnetite (Fe3O4) is usually inert and when combined with metal catalysts or enzymes it forms a composite that exhibits both magnetism and catalytic activity. However, it has been reported that Fe3O4 nanoparticles have intrinsic peroxidase-like activity. In this study, super paramagnetic Fe3O4 nanoparticles with a diameter of about 30 nmwere synthesized using self-designed experimental devices under mild conditions. Moreover, 4-chlorophenol (4-CP), which is a priority pollutant that widely exists in the environment but is recalcitrant towards chemical and biological degradation, was used as a model compound to test the catalytic activity of the synthesized Fe3O4 nanoparticles and analyse the mechanisms for 4-CP removal. Besides, surface analysis techniques, such as SEM, XRD and Raman spectroscopy, were used to investigate the transformation of the nanoparticles and further verify the interaction between the nanoparticles and 4-CP. The results revealed that the synthesized Fe3O4 nanoparticles show high catalytic activity even after being used several times, and acidic conditions are favourable for the dechlorination of 4-CP. However, 4-CP could also be degraded under neutral and alkali conditions. In the process 4-CP was transformed to formic acid, acetic acid and other byproducts. Adsorption tests indicated that the adsorption process does not play an important role in 4-CP removal, but it occurs between 4-CP and Fe3O4. The surface morphology of the Fe3O4 nanoparticles changed a lot and the reactive sites on the surface increased, which resulted in the higher activity of the particles after being used. The crystal structure of the nanoparticles did not change, suggesting the role of Fe3O4 nanoparticles as catalysts. Moreover, Raman spectra reflected that the adsorption and catalytic oxidation were surface reaction processes. It is proposed that the hydroxyl radical produced during the reaction is the main cause for the degradation of 4-CP. The reaction of H2O2 with ferrous to produce hydroxyl radical is the initial step, and is very important for the overall process.