In this work, MCM-41, magnetite (MAG), and a composite of magnetite and MCM-41 (MCM-MAG) were synthesized by a simple route for the production of active systems in the decomposition of organic waste. The materials were characterized by N2 adsorption/desorption, X-ray diffraction analysis (XRD), temperature programmed reduction (TPR), Fourier transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM). Our results indicated that the decolorization kinetics of the dyes were in the order of MCM-MAG > MCM-41 > magnetite. Mineralization of the dyes was monitored by total organic carbon (TOC) decrease. The dye solutions could be completely decolorized and effectively mineralized, with an average overall TOC removal 43% for a Fenton-like reaction time of 180 min. The degradation activity of the MCM-MAG was stable during four consecutive experiments, confirming their stability and reusability of the composite. The great advantage of this composite is that it may be easily magnetically recovered and reused.
Currently the efficient use of water is important for industry, as it is crucial for sustainable development and public health. In many cases the generation of contaminated effluents with various types of organic waste, which are rich in microbial or bacterial activity, are involved. This is not appropriate for reuse in agriculture and human consumption [
With regard to organic pollutants, we can highlight the textile dyes, which possess a high capacity to modify the environment due to their strong color and visual pollution and also cause changes in biological cycles mainly affecting photosynthesis processes. Besides these facts, studies have shown that some classes of dyes and their byproducts may be carcinogenic and/or mutagenic [
In this context, the development of new processes for wastewater treatment in order to immobilize or degrade these compounds in textile industry effluents is very important. An extensively studied alternative is the use of advanced oxidation processes (AOP). These processes are based on the formation of hydroxyl radicals, which are capable of oxidizing contaminants to smaller and less polluting molecules or even mineralize them, turning them into CO2, H2O, and inorganic ions from atoms [
The development of active heterogeneous systems to promote Fenton chemistry is of considerable interest, since it could offer some advantages over the classical homogeneous Fenton; because there is no sludge formation, the operation is carried out in near neutral pH and there is the possibility of recycling the iron promoter [
The main feature of this technology is the use of iron as a solid catalyst supported on material with high surface area or in the form of poorly soluble magnetic oxides, facilitating the removal of the catalyst for later reuse [
The purpose of this study was to report the preparation of a composite based on magnetic iron oxide nanoparticles dispersed inside mesoporous silica MCM-41 and its application to the degradation of methylene blue (MB) by heterogeneous Fenton process.
First, magnetite was prepared by a coprecipitation method, already described in the literature [
The MCM-composite was obtained using 5% of magnetite (m/m). For this, 16 g of Cetyltrimethylammonium surfactant (CTAB) was dissolved in 300 mL of sodium hidroxide solution (0.33 mol L−1). Next, it was slowly added to 37 mL of tetraethyl orthosilicate (TEOS), which was left under stirring for 24 hours at room temperature. Afterwards, the magnetite was added to this suspension and left for 4 more hours. Finally, the composite was washed with distilled water, oven-dried for 6 hours at 60°C, and calcined slowly at 600°C for 4 h under a nitrogen flow (100 mL min−1). Likewise, the synthesis of MCM-41 was performed in the absence of magnetite.
The materials were characterized by N2 adsorption/desorption measurements at 77 K in a Quantchrome Autsorb 1 equipment. The surface areas were calculated by the Brunauer-Emmett-Teller (BET) equation and pore size distribution was determined by the Barrett-Joyner-Halenda (BJH) equation [
The adsorption kinetics tests were performed in batch at a temperature of 25 ± 1°C. Aqueous solutions of MB (10 mL) at a concentration of 50 mg
The degradation of MB was performed using 9.9 mL of 50 mg L−1 of dye suspension, 0.1 mL of 30% H2O2, and 10 mg of catalysts. The suspension was maintained under constant stirring (100 rpm) for 0, 15, 30, 60, 120, and 180 minutes. The temperature was set to 25 ± 1°C during the tests.
The reuse of the catalysts was also investigated. In a typical test, 100 mg of the MCM-MAG was placed in contact with 100 mL of MB solution (50 mg L−1) and 1 mL of H2O2 under shaker at 130 rpm for 180 min. The catalysts were then recovered by filtration, water-washed in order to remove the excess of adsorbed MB, and dried at 60°C for 4 h. The materials were subsequently used in another oxidation cycle keeping the same standard conditions and ratio of catalyst/dye.
The degradation was monitored by UV-visible spectroscopy (Shimadzu UV-1601 PC) at 645 nm, the maximum absorption wavelength of MB, and total carbon analysis was performed in the dye suspension on a Shimadzu 500A TOC analyzer, in synthetic air atmosphere.
In order to investigate the textural properties, N2 adsorption/desorption was evaluated (Figure
N2 adsorption and desorption isotherms measured at 77 K (a) and pore size distributions (b).
The specific surface area was calculated by the BET equation (Figure
The infrared spectra are presented in Figure
FTIR spectrum of the synthesized samples.
The synthesized samples were studied by temperature-programmed reduction (TPR) using H2 as a reducing gas (Figure
Temperature programmed reduction profiles.
The diffractogram patterns were presented in Figure
The XRD diffraction patterns of MCM-41, MCM-MAG, and magnetite samples.
The crystallite size plays an important role and has a high influence on the surface area of synthesized samples and the surface reactions. Thus, we evaluated the average size of the magnetite crystals produced. To this end, we used the information and the domain of the XRD crystallographic coherence. It was calculated employing Scherrer’s equation. The particle size was about 12 nm [
Typical TEM micrographs for magnetite and MCM-MAG are shown in Figure
TEM image and histogram analysis for magnetite, (a) and (b), and MCM-MAG, (c) and (d).
To verify the adsorption and the oxidation effect of the synthesized samples, MB was used as an organic dye model molecule. This compound was chosen due to its ease of monitoring by simple techniques such as UV-visible spectroscopy at its maximum absorption wavelength, 645 nm. The adsorption kinetics process was controlled by measuring the discoloration of the dye solution in a batch adsorption experiment, with the magnetite, MCM-MAG, and MCM-41 (Figure
Adsorption kinetics (a) and MB oxidation (b) using the synthesized samples.
The kinetic study of Fenton process can be performed by assuming that the reaction between hydroxyl radicals and the pollutant is the rate determining step. Thus, by assuming that
The
From these results we can conclude that the composite was more active in carbon removal from the dye solution after 180 min of reaction, removing 43% of the carbon solution, followed by MCM-41 with 18% and 3% for magnetite, which is consistent with the analysis obtained by UV-VIS, as expected, evidencing the mineralization of the organic compound.
The reuse test was performed in order to evaluate the catalytic activity of MCM-MAG during successive experiments and thus to observe the possibility of catalyst reuse. Figure
Multicycle tests for degradation of MB for 180 min with MCM-MAG.
In this work MCM-MAG was successfully synthesized, with magnetite particles well dispersed on the surface of MCM-41. The composite showed very good catalytic performance for MB organic dye oxidation in H2O2 presence, after 90 min of reaction, compared with magnetite and MCM-41, suggesting that the combination of magnetite oxide with mesoporous MCM-41 may offer synergistic reaction routes for the catalytic oxidation of target compounds. These studies revealed that the dye removal occurs through a Fenton process system by the composite, whereas for the MCM-41 the dye removal occurs mainly via adsorption. In addition to the high catalytic activity for the MCM-MAG composite in the heterogeneous Fenton reaction, it should also be reported that this process provides an easy recovery of the catalyst due to presence of magnetic properties in the composite. It was also observed that catalytic behavior could be reproduced in consecutive experiments without a considerable drop in the process efficiency.
The authors declare that there is no conflict of interests regarding the publication of this paper.