Fenton Process for the Degradation of Methylene Blue using Different Nanostructured Catalysts

Corresponding Author: María del C CottoMaldonado Nanomaterials Research Group, School of Natural Sciences and Technology, University of Turabo, 00778PR, USA Email: mcotto48@suagm.edu Abstract: In many places of the world, water is a scarcity resource. The possibility of reusing water increases the relevance of developing different water treatment methods. The goal of this research was to determine if the photo-Fenton processes are efficient for the degradation of organic compounds, as a possible alternative for wastewater treatment. In order to carry out this study, different catalysts were characterized by different techniques (XRD, FE-SEM, SBET and TGA) and catalytic tests. Iron oxide nanowires (Fe2O3NWs) and a commercial catalyst (FeCl2) were tested in the methylene blue photo-Fenton degradation reaction. The catalysts used were able to degrade the methylene blue and could eventually be used to remove contaminants from the water.


Introduction
One of the most discussed issues around the world is the water chemical pollution by organic, inorganic, bionutrients and microorganisms (Garriga I Cabo, 2007;UNEP et al., 2002). In many regions of the world, water is a scarce resource and in these places the reuse of the water is a major issue (Marin et al., 2007). The production and use of synthetic chemicals has experienced a significant increase over the past century and mankind is responsible for releasing many of the pollutants into the environment in their normal activities like industrial processes, wastewater discharges, overuse of pesticides, fertilizers, etc. (Ameta et al., 2013).
Aromatic compounds are an important source of environmental pollution that reaches the atmosphere and groundwater, since they are widely used as intermediates in the production of pesticides, synthetic polymers and dyes (Huang et al., 2010). The presence of these substances in the environment is worrisome, since they have carcinogenic, teratogenic and toxic properties (especially the azo dyes), they diminish the penetration of light through the water column and although it is not so relevant, they also affect aesthetically to nature (Houas et al., 2001;Karadag et al., 2006;Dafnopatidou et al., 2007).
Effluents from the textile industry have high concentrations of organic and inorganic dyes which are heavily colored, have a high Chemical Oxygen Demand (COD), fluctuate significantly in pH and are toxic to organisms (Abdelmalek et al., 2006). Common techniques used to remove dyes include chemical, physical and biological processes (Dafnopatidou et al., 2007). However, these conventional processes for the treatment of wastewater including the degradation of residual dyes are ineffective because these compounds have a high molecular weight and high biochemical stability due to the presence of aromatic rings in their structure (Panizza et al., 2006;Ma et al., 2007).
To maintain aesthetics and to reduce the environmental impact of industrial effluents, discoloration of the wastewater is necessary (Hussein Sharif Zein and Boccaccini, 2008). Currently, the most commonly used treatment methods for the removal of water pollutants are reverse osmosis, ion exchange technology, precipitation of materials, adsorption of pollutants by using activated carbon (charcoal) and biological degradation (Gupta et al., 2004;Mezohegyi et al., 2007). Other processes as Fenton, photochemistry, radiolysis or sonolysis generate highly reactive hydroxyl radicals for bleaching, finally arising to the mineralization of recalcitrant compounds (Ozen et al., 2005). In the boom of the eco-conservation and the eco-friendly techniques to degrade the pollutants in water and wastewater, the Advance Oxidation Processes or AOPs are seen as alternative techniques (Gupta et al., 2004) to the traditional processes. The AOPs use chemical procedures based on the use of catalysts or photochemical compounds which generate highly reactive transient species as the hydroxyl radical which possesses high efficiency for the oxidation of organic compounds (Marin et al., 2007;Mohabansi et al., 2011;Salehi et al., 2012;Samet et al., 2012). AOPs have many advantages such as complete mineralization of the pollutant, are non-selective processes, are used in low concentration of contaminants and can be combined with other methods (Garriga I Cabo, 2007).
The Fenton reaction, as part of the AOPs, generates hydroxyl radicals. This process is clearly non selective (Ai et al., 2007a) and represents a viable technique to degrade hazardous organic compounds (Barbusiński, 2005). Horstman Fenton and Jackson (1899) demonstrated the importance of iron and hydrogen peroxide during the oxidation of some substances. A characteristic of the Fenton process is that the reaction requires acid conditions to work more efficiently (pH ranging from 2 to 3) (Ai et al., 2007a). According to Ai et al. (2007b), the Fenton reagent can be defined as the combination of hydrogen peroxide and iron (II) (Fe 2+ /H 2 O 2 ). Fenton-like reagent does not include iron (II) species and normally this term is used for the combination of Fe 3+ /H 2 O 2 although both reagents (Fenton and Fenton-like) are present during the reaction because both iron species are in equilibrium during the reaction. An alternative to the typical Fenton reactions based on the use of soluble Fe 2+ /Fe 3+ species is the use of Fe 0 , as a supported or immobilized catalyst and hydrogen peroxide as oxidizer (Ai et al., 2007b). Examples of possible reagents for environmental remediation include Fe 0 , Fe 3 O 4 (magnetite) and Fe 2 O 3 (maghemite) (Ai et al., 2007b;Reza et al., 2016).
Methylene Blue (MB) (Fig. 1) is a heteropolyaromatic dye commonly used for printing cotton, as textile tannin and for coloring leather (Ma et al., 2007;Salmin and Shamali, 2013). MB is also used in chemistry as a base-acid indicator and in the medical field.
In this research, we have studied the efficiency of the photo-Fenton degradation process of MB in aqueous solution. The results obtained point to the fact that these catalytic processes could be implemented in the efficient degradation of recalcitrant pollutants present in wastewater.

Experimental Section
Reagents All chemicals used in this research were of analytical grade and were used as received without any further purification. Methylene Blue (MB) p.a. was provided by Sigma Aldrich. Isopropyl alcohol and ethyl alcohol (95% purity) were provided by Acros Organics. The iron substrate (99.999%) was provided by Good fellow (USA). Ferrous Chloride (99% purity) was provided by Fisher Scientific. Milli-Q water (18.2 MΩ.cm resistivity at 25°C) was used for all experiments.
Characterization Techniques X-Ray powder Diffraction patterns (XRD) were collected using an X´Pert PRO X-ray diffractometer (PANalytical, The Netherlands) in the Bragg-Brentano geometry. The thermogravimetric analyses were done using a TGA Q-500 instrument (TA Instruments) under an inert atmosphere of nitrogen. The specific surface areas of the catalysts used in the present research were determined by the BET method using a Micromeritics ASAP 2020. The variation in magnetization and coercivity of the Fe 2 O 3 NWs was determined by using a Lake Shore-7400 Vibrating Sample Magnetometer (VSM) at room temperature. Field emission scanning electron microscopy (FE-SEM) images were obtained using a JEOL JM-6400 microscope. High Resolution Transmission Electron Microscopy (HRTEM) images were recorded on a JEOL 3000 with an acceleration voltage of 300 kV. The Total Organic Carbon (TOC) concentration was determined by using both a Tekmar Dohomann, Phoenix 8000 UV-Persulfate TOC Analyzer and a Leco CHNS-932 Analyzer. The catalytic degradation of MB was studied by using UV-vis absorption (Varian Cary 3 spectrophotometer) and fluorescence (Varian Cary Eclipse spectrometer).

Synthesis of Iron Oxide Nanowires
Crystalline iron oxide nanowires were synthesized by a simple catalyst-free growth procedure. The iron substrates (Goodfellow, 99.999%) were thermally treated in a tubular oven. In a standard synthesis procedure, the iron substrates were incorporated into the furnace after being degreased by rinsing with acetone and isopropanol. The thermal treatment consists of a ramp-up rate of 25°C/min to the final temperature of 500°C, followed by one hour plateau, in flowing air (300 mL/min); and finally a cooling ramp rate of 25°C/min in vacuum (Fig. 2) (Bonilla et al., 2011).

Results and Discussion
Material Characterization-Iron Oxide Nanowires (Fe 2 O 3 NWs) Fe 2 O 3 NWs were synthesized according to the procedure described above. The SEM images of the synthesized samples are shown in Fig. 3. Iron oxide nanowires are characterized by having a high aspect ratio, with lengths above 5 microns and diameters below 100 nm. However, in preliminary studies, it was found that Fe 2 O 3 NWs become thicker (with diameters that can be three times greater), when the synthesis temperature increases above 600°C, indicating the effect of temperature on the morphology of these nanostructures. The BET surface analysis revealed an unexpectedly high specific surface area (S area ) of 180 m 2 g −1 (Table 1) which has been related to the high aspect ratio of this material. TG analysis revealed only a weight loss of 3.62% at 520°C (Table 1), indicating that iron nanowires do not possess porosity capable of absorbing an appreciable amount of water or other solvents. This result clearly points to the fact that the specific area of this material is due to the high aspect ratio and not to a possible porosity produced during the synthesis of the material.
The morphology and size of the as-synthesized Fe 2 O 3 NWs was also investigated by TEM. Figure 4 shows a TEM image of an isolated Fe 2 O 3 nanowire (a) and the corresponding Selected Area Electron Diffraction Pattern (SAED) (b). As can be seen there, the surface of the nanowire does not have a smooth appearance, showing small particles that could also contribute to the surface area value measured for this material. The SAED (Fig. 3b) is consistent with a single crystal structure. The halo-rings could be due to the presence of small crystals on the nanowire surface.
Magnetic properties of the as-synthesized Fe 2 O 3 NWs were also characterized. Figure 5 shows the hysteresis loop of the iron nanowires at room temperature. The coercivity value obtained for this material was 20.485 G, with saturation Magnetization (Ms) of 22.376 emu g −1 , indicating a high ferromagnetic behavior (Wu et al., 2010). Fe 2 O 3 NWs was also characterized by XRD. Figure 6 shows the diffractions patterns corresponding to samples synthesized in oxidative atmosphere (in flowing air) at 500°C (4a) and 700°C (4b). The results indicated the presence of highly crystalline magnetite (Fe 3 O 4 ). However, as can be seen in Fig. 6, some small differences are observed as a function of the synthesis temperature, showing possible changes in the structure (phase) and crystallinity (Bonilla et al., 2011).
Iron oxide as hematite phase (α-Fe 2 O 3 ) was not observed, due to the lack of peaks corresponding to reflections (210) and (211), that are always present in this crystallographic phase (Daou et al., 2006).

Ferrous Chloride (FeCl 2 )
Ferrous chloride (FeCl 2 , Fisher Scientific, 99%) was used in this research to compare its efficiency with respect to the synthesized catalyst. The thermogravimetric analysis of this compound established a weight loss of approximately 16.14% (Table 1). The TG curve showed a single step of weight loss and this behavior was attributed to the removal of water molecules adsorbed on the surface of the compound (Niederberger et al., 2002). The specific surface area (Sarea), measured by the BET method, was 55 m 2 g −1 (Table 1).

Catalytic Tests
Photo-Fenton degradation catalytic processes were carried out using a cylindrical reactor (semi-batch type), with continuous stirring, which was located in the center of a solar simulator as irradiation source. The experimental setup was composed by two annular white bulb lights with a total irradiation power of 60 watts. A vessel of 1 L was used during the irradiation of the sample. The sample was mechanically stirred to maintain a homogeneous mixture during the irradiation of the sample. Before the irradiation, the selected catalyst was dispersed in the solution and kept in the dark under stirring for 30 min (Hong et al., 2009) to ensure substrate-surface equilibrium .  All systems were covered to avoid any further irradiation on the sample. In this way, only the light from the solar simulator was able to reach the sample. Hydrogen peroxide was added to the sample batch to increase the oxygen source during the reaction, avoiding the decrease of the catalytic activity during the reaction due to lack of oxygen (Cotto-Maldonado, 2012). The volume of hydrogen peroxide added to the batch, the concentration of the dye and the catalyst concentration were 0.1 mL, 10 −5 M and 0.6 gL −1 , respectively (Velegraki and Mantvinos, 2008;Asiri et al., 2011;Cotto-Maldonado, 2012). A 10 mL-aliquot of the reaction mixture was extracted every 10 min and a dilution (1:5, v/v) was prepared to record the UV and fluorescence spectra and to determine the TOC concentration.
Fluorescence intensity and UV-vis absorption were measured at λ = 665 nm and λ = 670 nm, respectively. TOC was used as a complementary technique to spectroscopy. Figure 7 shows the results obtained with Fe 2 O 3 NWs as catalyst. As can be seen in Fig. 7a and 7b, the absorbance and fluorescence intensity are reduced by about 80% of the initial value, confirming the degradation of the compound. This result was also verified by total organic carbon measurements. Figure  7c shows the evolution of TOC at different times under different experimental conditions. Under the same reaction conditions as those used for monitoring degradation by UV-vis absorption spectroscopy and fluorescence, the greatest reduction in carbon level was observed (black dots). In the absence of catalyst (blue dots), the degradation is minimal, probably produced by oxidation of the MB in the presence of H 2 O 2 . When the reaction is carried out using only the oxygen present in water (red dots), the reaction proceeds very weakly until the oxygen present in the solution is exhausted, resulting in minimal MB degradation.
In the presence of FeCl 2 as catalyst and under similar experimental reaction conditions, the MB undergoes a degradation slightly higher than that observed with Fe 2 O 3 NWs (Fig. 8). Table 2 shows the percent of degradation (based on the decrease in TOC concentration). According with the data presented there, the FeCl 2 has the highest degradation rate during the photo-Fenton process, reaching a value of 92%. The high efficiency of the FeCl 2 as catalyst could be due to the high solubility of this catalyst with respect to Fe 2 O 3 NWs. The photocatalytic activity was quantitatively evaluated by calculating the pseudo first-order rate constants (k) according to the following equation (Wang et al., 2006;Asiri et al., 2011;Salehi et al., 2012): where, C 0 and C are the initial concentration of MB and its concentration at the time t, respectively. The semilogarithmic plots of the concentrations Vs time were adjusted using a linear regression to determine the rate constant ( Fig. 7d and 8d). Table 2 shows the k values for the degradation reaction of the MB during the photo-Fenton process. The kinetic rate represents a measure of the catalytic activity. Therefore, the larger this magnitude, the greater the catalytic activity of the system. As can be seen in Table 2, both catalysts (Fe 2 O 3 NWs and FeCl 2 ) have kinetic rates of the same order of magnitude, although higher in FeCl 2 . The higher efficiency shown by FeCl 2 may be due to the solubility of the catalyst in water. In fact, unlike Fe 2 O 3 NWs which is not soluble in water and can be recovered by centrifugation, once FeCl 2 is used, it cannot be recovered.

Conclusion
Fe 2 O 3 NWs was synthesized by heat treatment of an iron substrate in air. The material obtained was recovered and characterized by SEM, TEM, TGA and magnetometry. The activity of this catalyst has been compared with that of the commercial catalyst FeCl 2 , studying the efficiency of the photo-Fenton degradation process of a stable organic dye (methylene blue) in water. As a result, the activity of both catalysts was comparable, although a higher efficiency was obtained in the commercial catalyst (FeCl 2 ). This result has been justified as possibly due to the higher solubility of FeCl 2 in water. The activity of Fe 2 O 3 NWs, although this material is not soluble, could be due to its high specific area, compared to FeCl 2 .
The use of photo-Fenton degradation reactions opens a world of possibilities that in the future could be implemented at an industrial level for the abatement of organic pollutants in inland waters.