Heterogeneous Fenton-like degradation of methylene blue (MB) by magnetic nanoparticles Fe3O4@TiO2 in neutral condition

The traditional Fenton system (Fe2+–H2O2) only works in an acidic environment and produces a large quantity of sludge. In this study, magnetic nanoparticles Fe3O4@TiO2 were used as a high-performance Fenton-like catalyst for the degradation of MB. The surface morphology was characterized by Transmission Electron Microscopy (TEM). And the Fenton-like catalytic activity of Fe3O4@TiO2 was evaluated under different pH condition and various H2O2 in feed concentration, respectively. The results indicated that nanoparticles Fe3O4@TiO2 show high efficiency for methylene blue (MB) degradation under the reaction condition of H2O2 40mM, Fe3O4@TiO2 20mM, MB 300mg/L, pH 7 and 30 °C, the mineralization of persistent MB can achieve 72% COD removal ratio.


Introduction
Wastewater produced by chemical industry, textile industry and leather manufacturing, often contains considerable amounts of organic pollutants, which can cause negative impacts for ecosystems and humans for their toxicity, carcinogenic or mutagenic properties. Therefore, it is significant to degrade the organics in wastewater before discharge [1] . Fenton oxidation is one of the simplest and most efficient AOPs in environment remediation. However, one disadvantage of this method is that the reaction achieves its high activity at relatively low pH range, usually at approximate pH=3, which is not beneficial for the chemical equipment and operation. In addition, the wastewater after treatment is preferred to be neutralized before discharge, so a large amount of slurry will be produced, which causes the problem of solid waste for further treatment and greatly increasing the running cost.
In recent years, heterogeneous Fenton-like catalysts, such as iron-based clays and zeolites, and iron oxide minerals [2,3] , have received considerable interests due to their advantages of facile recovery and recycling as well as dramatically decreased slurry. And among these catalysts, Fe 3 O 4 nanoparticles not only have unique characteristic of magnetic feature but also showed higher reactivity in neutral condition. For example, when the concentrations of Fe 3 O 4 and H 2 O 2 were 5g/L and 1.2M, phenol and aniline could be removed completely after 6h at 308K. However, it is found that the total organic carbon abatement efficiency for phenol and aniline were only 42.79% and 40.38%, respectively [4] .
Recently, TiO 2 coupled with iron oxide has been reported to have a good magnetic response and displayed high photocatalytic efficiency [5][6][7] . In this work, we have successfully synthesized the Fe 3 O 4 @TiO 2 NPs, which were applied to the Fenton-like degradation of methylene blue (MB) as a model of recalcitrant contaminants. The effects of operational parameters as concentrations of catalyst and H 2 O 2 , and pH value on the degradation efficiency of TC were systematically evaluated. Extraordinarily high and stable catalytic activity has been achieved on Fe 3 O 4 @TiO 2 with a wide working pH range of 5-9.

Fe 3 O 4 @TiO 2 MNPs preparation
Fe 3 O 4 @TiO 2 MNPs were prepared by coprecipitation of Fe 2+ and Ti 4+ in aqueous solutions. In a typical procedure, 0.06mol/L FeCl 3 and TiCl 4 solutions were mixed uniformly, and then dilute alkaline solution was added drop wise to the solution. After 2h vigorous stirring under an Ar stream, the black particles were deposited and separated from solution by a powerful magnet, washed with de-ionized water and ethanol to neutral pH. Then Fe 3 O 4 @TiO 2 MNPs were dried under vacuum and stored in the desiccator for further experiments.

Characterization of Fe 3 O 4 @TiO 2 MNPs
TEM images were conducted with a CM200 transmission electron microscope.

Conclusion
Highly active Fe 3 O 4 @TiO 2 MNPs were successfully synthesized by co-precipitation method, and used as heterogeneous Fenton-like catalysts that can degrade MB from aqueous solution efficiently. Unlike the traditional Fenton process, the COD removal efficiency remained relatively high at a wide pH range from 5~9 for MB.