Introduction
The easy dissolution of Fe under acidic conditions results in the decreased activity of traditional Fenton-like catalysts. It was reported that the encapsulated or framework stabilized Fe-based materials showed excellent stability. Zeolite is a crystalline silicate/aluminosilicate with a stable framework structure, high specific surface area, uniform microporous and mesoporous structure, good acid-base resistance property. These characteristics make it a promising carrier or main material for Fenton-like catalysts. Therefore, a Fe-doped nanosheet MFI (MFI-Fe) zeolite was synthesized in this study. The Fe sites was embedded in the framework of MFI-Fe zeolite to inhibit the dissolution of Fe under acidic conditions, which could improve the performance of MFI-Fe zeolite for activating H2O2 towards acidic red G (ARG) degradation in water.
Methods
With the assistance of diquaternary ammonium-type structural directing agents, the solid products were obtained through hydrothermal reactions (160 ℃, 5 d) using tetraethyl orthosilicate and ferric chloride as the main raw materials. Then, MFI-Fe zeolite was obtained by calcining the solid products in air at 550 ℃ for 3 h. X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), fourier transform infrared spectroscopy (FTIR), N2 adsorption-desorption and X-ray photoelectron spectroscopy (XPS) were applied to determine the physicochemical properties of MFI-Fe zeolite. The prepared MFI-Fe zeolite was applied to active H2O2 for ARG degradation. The effects of catalyst dosage, H2O2 dosage, solution pH value, reaction temperature, ARG concentration and coexisting ions on the degradation of ARG were studied. The concentration of ARG in the water was determined by an ultraviolet and visible (UV–Vis) spectrophotometer.
Results and discussion
According to the XRD and FTIR results, the prepared MFI-Fe zeolite exhibited the characteristic peaks of MFI zeolite, and there were not impurity peaks of silicon oxide or iron oxide, indicating that both silicon and iron entered into the framework of MFI zeolite. The XPS spectra of MFI-Fe zeolite exhibited peaks of Fe 2p3/2 and Fe 2p1/2 at 711.8 eV and 725 eV, respectively. They were very close to the binding energies of the 2p3/2 and 2p1/2 peaks of Fe2O3. Moreover, there was no Fe2+ peak (approximately 708 eV) in MFI-Fe zeolite. These results indicated that Fe in MFI-Fe zeolite existed in the form of +3 valence and was directly connected to O. The SEM images showed that MFI-Fe zeolite was composed of many nanosheets. Si and Fe were uniformly distributed in the crystals. According to the N2 adsorption and desorption results, the specific surface area and external surface area of MFI-Fe zeolite reached to 343.71 m2·g–1 and 176.78 m2·g–1, respectively. The average pore size of MFI-Fe zeolite reached 4.19 nm. These structural characteristics endowed MFI-Fe zeolite with excellent catalytic activity. The catalytic experiment results showed that ARG could only be rapidly removed when both H2O2 and MFI-Fe zeolite existed. The reaction mechanism results indicated that HO• was generated by the activating of H2O2 with MFI-Fe zeolite, leading to the degradation of ARG. In solution having pH values of 3–9, the removal rates of ARG gradually decreased with the increase of pH values. It can be explained that a lower pH value conduced to the activation of H2O2, thereby accelerating the degradation of ARG. Under the optimum conditions (initial pH=3, 25 ℃, MFI-Fe 0.40 g/L, H2O2 20 mmol/L), the removal rate of ARG (20 mg/L) reached to 98.40% within 180 min. The coexistence of NO3–, Cl–, and SO42– in the solution had not effect on the removal of ARG. The degradation of ARG was slightly inhibited by humic acid (HA). The removal rate of ARG was significantly slowed down with the coexistence of HCO3–. It can be explained that the pH value of the solution would increase and partial HO• would be consumed when HCO3– is added into the reaction system. After reuse for 5 times, the concentrations of leached Fe were less than 0.42 mg/L, and the removal rates of ARG exceeded 87.64%.
Conclusions
A nanosheet MFI-Fe zeolite was synthesized and applied to active H2O2 for ARG degradation. When the pH value of the solution was between 3 and 9, the removal rates of ARG gradually decreased with the increase of the solution pH values. The reaction rate reached to the maximum (0.023 min–1) at a pH value of 3. Under the optimal conditions (pH=3, 25 ℃, 0.40 g/L MFI-Fe and 20 mmol/L H2O2), the removal rate of ARG (20 mg/L) reached approximately 98.40% within 180 min. The coexistence of NO3–, Cl–, and SO42– ions in the solution had not effect on the removal of ARG, while the degradation of ARG was slightly inhibited by humic acid (HA). The removal rate of ARG decreased significantly with the coexistence of HCO3–. After reuse for 5 times, the concentrations of leached Fe were less than 0.42 mg/L, and the removal rates of ARG exceeded 87.64%.