Oxalate enhanced mechanism of Fe 3 O 4 @γ-Fe 2 O 3 was developed to provide novel insight into catalytic process regulation of iron oxide catalysts in heterogeneous UV-Fenton system. And the iron oxide composite of Fe 3 O 4 @γ-Fe 2 O 3 was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), fourier transform infrared (FTIR) spectroscopy and nitrogen adsorption-desorption isotherms. The results showed that large amount of iron could be leached from catalyst in the presence of oxalate, which promoted the homogeneous UV-Fenton reactions in solution. Orange II degradation could be significantly enhanced with the increase of the ratio of homogeneous UV-Fenton process to heterogeneous UV-Fenton process. The optimum concentration of oxalate determined by experiment was 0.5 mM in oxalate enhanced heterogeneous UV-Fenton system. On this condition, the pseudo-first-order rate constant value of Orange II degradation was 0.314 min −1 , which was 2.3 times as high as that in heterogeneous UV-Fenton system. The removal rates of color and TOC were 100% and 86.6% after 20 min and 120 min treatment, respectively. In addition, the iron ions in solution could be almost completely adsorbed back to the catalyst surface in later degradation stages of Orange II. During the recycle experiments, the results showed that the increase of pH in solution and the sorption of intermediates on the catalyst surface would hinder oxalate enhanced process and lead to a decrease of degradation rate of Orange II in oxalate enhanced heterogeneous UV-Fenton system.
An enhanced method of hydroxyl radical generation was studied to lead a better understanding of the catalytic mechanism of α-FeOOH in UV-Fenton system. The catalyst was characterized by X-ray diffraction, X-ray photoelectron spectroscopy, nitrogen adsorption–desorption isotherms and scanning electron microscope. The Orange II degradation and hydroxyl radical concentration under different conditions were studied in detail. The results showed that the hydroxyl radical was mainly oxidizing species for Orange II degradation. And the hydroxyl radical generation included both homogeneous and heterogeneous UV-Fenton processes. In heterogeneous UV-Fenton system, the additional oxalate could increase the ratio of homogeneous UV-Fenton process for hydroxyl radical generation. In oxalate enhanced system, the peak value of hydroxyl radical concentration would increase to 11.314 μmol/L which was nearly twice as it in heterogeneous UV-Fenton system (5.978 μmol/L). The k values of Orange II discoloration and mineralization increased to 0.386 min −1 and 0.044 min −1 , which were 1.169 and 0.692 times as that in heterogeneous UV-Fenton system (0.178 min −1 and 0.026 min −1 ). After treatment, the total iron ions concentration decreased to 2.357 μmol/L in oxalate enhanced system. The results indicated that additional oxalate was a promising enhanced method for hydroxyl radical generation in heterogeneous UV-Fenton system with α-FeOOH as catalyst.
The redox cycle of iron is a well-known rate-determining step for hydroxyl radical generation in photo-Fenton system. In this study, oxalate was employed as regulator to enhance the degradation of Orange II in Fe 3 O 4 magnetic nanoparticles (NPs)-catalyzed heterogeneous UV-Fenton system. Results showed that the oxalate could interact with the surface ≡Fe III species of catalyst, which weakened the bond of ≡Fe III –O and promoted the leaching of iron ions. Then the redox cycle of iron and generation of HO· would be accelerated via the homogeneous UV-Fenton reaction. The degradation rate constant of Orange II reached 0.220 min −1 when additional oxalate concentration was 0.4 mM, which was 2.5 times as high as that without oxalate in heterogeneous UV-Fenton system. In this case, the removal efficiencies of color and TOC were 99.3% and 92.0% after 30 and 120 min treatment, respectively. In addition, based on the results of XRD and XPS characterization, it could be deduced that the crystal structure and elemental configuration of Fe 3 O 4 magnetic nanoparticles could be maintained after reaction. Besides, the results of FTIR and magnetization characterization indicated that the C 2 O 4 2− on surface of catalyst could be degraded and the catalyst could be easily separated from aqueous by applying an external magnetic field. The Fe 3 O 4 magnetic nanoparticles showed high catalytic stability and reusability under the regulation of oxalate due to the fact that the leached iron ions could be re-adsorbed on the catalyst after treatment.
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