Oxalate regulate the redox cycle of iron in heterogeneous UV-Fenton system with Fe3O4 nanoparticles as catalyst: Critical role of homogeneous reaction.

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 Fe3O4 magnetic nanoparticles (NPs)-catalyzed heterogeneous UV-Fenton system. Results showed that the oxalate could interact with the surface ≡FeIII species of catalyst, which weakened the bond of ≡FeIII-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 Fe3O4 magnetic nanoparticles could be maintained after reaction. Besides, the results of FTIR and magnetization characterization indicated that the C2O42- on surface of catalyst could be degraded and the catalyst could be easily separated from aqueous by applying an external magnetic field. The Fe3O4 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.

[Experiment to Enhance Catalytic Activity of α-FeOOH in Heterogeneous UV-Fenton System by Addition of Oxalate].

α-FeOOH was prepared by a symmetrical precipitation method and characterized by XRD, FT-IR, SEM, BET, and EDS techniques. The oxalate enhanced experiment for the catalytic activity of α-FeOOH during the degradation of Orange Ⅱ by the heterogeneous UV-Fenton process was evaluated. The results showed that the rate of Orange Ⅱ degradation in this heterogeneous UV-Fenton system could increase significantly with oxalate as an enhancing reagent, with the peak values of enhanced efficiency reaching 116.9% when the concentration of additional oxalate was 0.4 mmol·L-1. Under this condition, more than 99% of Orange Ⅱ (0.2 mmol·L-1) was degraded in 15 min. However, under the same condition, the inhibition ratios of malonate, acetate, EDTA and citrate were 5.2%, 8.1%, 23.2%, and 25.7%, respectively. Compared with the basic system, the enhanced efficiency of the mineralization rate constant can reach 69.9% when the concentration of additional oxalate was 0.4 mmol·L-1. As a result, the time required for mineralization could be reduced greatly. In the heterogeneous UV-Fenton system, the additional oxalate could provide a new photo-reduction pathway for Fe3+ to Fe2+, enhancing the ratio of the homogeneous UV-Fenton process by increasing the Fe leaching of α-FeOOH and the concentration of hydroxyl radicals, leading to higher degradation efficiency of Orange Ⅱ. As a result, an enhancement effect of 101.5% could be obtained even after three cycles. The iron ions leached from catalysts could be re-adsorbed after treatment, avoiding the loss of active species from catalysts and additional pollution caused by iron ions. The results indicated that additional oxalate was a stable and reliable enhancing reagent on the catalytic activity of α-FeOOH in the heterogeneous UV-Fenton system.

Oxalate enhanced degradation of Orange II in heterogeneous UV-Fenton system catalyzed by Fe3O4@γ-Fe2O3 composite.

Oxalate enhanced mechanism of Fe3O4@γ-Fe2O3 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 Fe3O4@γ-Fe2O3 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.