Fe

Phenol degradation using the mixed material clay/Fe immobilized on glass slides

Lidiane Yumi Taketa, Franciély Ignachewski, Juan Carlo Villalba, Fauze Jacó Anaissi & Sérgio Toshio Fujiwara Environmental Science and Pollution Research ISSN 0944-1344 Volume 22 Number 2 Environ Sci Pollut Res (2015) 22:894-902 DOI 10.1007/s11356-014-3239-3

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Author's personal copy Environ Sci Pollut Res (2015) 22:894–902 DOI 10.1007/s11356-014-3239-3

ADVANCED OXIDATION TECHNOLOGIES: ADVANCES AND CHALLENGES IN IBEROAMERICAN COUNTRIES

Phenol degradation using the mixed material clay/Fe immobilized on glass slides Lidiane Yumi Taketa & Franciély Ignachewski & Juan Carlo Villalba & Fauze Jacó Anaissi & Sérgio Toshio Fujiwara

Received: 21 December 2013 / Accepted: 19 June 2014 / Published online: 29 July 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract The mixed material clay/Fe was prepared and immobilized on glass slides and calcined at 550 and 750 °C. The calcined material X-ray powder pattern (XRD) diffractograms indicate that there is no intercalation of iron compounds inside the lamella clay. The experimental design revealed that the most suitable phenol degradation conditions were obtained using the material calcined at 750 °C in a pH 7 and 140 mg/L of hydrogen peroxide solution. The material MMAFe750 showed excellent performance as a catalyst for Fenton-like reaction; in 125 min, 50 % of phenol was removed in the absence of leaching-supported iron. These results indicate that the reaction occurs by a heterogeneous process. Furthermore, the material showed no loss of catalytic activity after five degradation studies. It was noted that the adsorption of phenol in the synthesized materials does not occur and the mixed material is strongly adsorbed onto glass slides. Keywords Mixed material clay/Fe . Phenol degradation . Immobilized Fenton . Immobilization on glass slides . Factorial design . Advanced oxidation processes

Introduction The development of processes capable to remedy wastewater containing recalcitrant organic compounds has become the focus of a large amount of research. This is due to the Responsible editor: Philippe Garrigues L. Y. Taketa : F. Ignachewski : J. C. Villalba : F. J. Anaissi Departamento de Química, Universidade Estadual do Centro Oeste, Guarapuava, PR 85040-080, Brazil S. T. Fujiwara (*) Departamento de Química, Universidade Estadual de Ponta Grossa, Ponta Grossa, PR 84030-900, Brazil e-mail: [email protected]

increased concern about the environment, especially when it comes to water resources (Garrido-Ramirez et al. 2010; Muthuvel et al. 2012; Herney-Ramirez et al. 2010; Hassan and Hameed 2011; Anaissi et al. 2009; Ayodele et al. 2012; Catrinescu et al. 2012; Purceno et al. 2012). Phenol is widely used in different industries, contributing to the pollution of industrial wastewater. The world production of phenol is over 3 million tons per year, being used in the synthesis of resins, dyes, pharmaceuticals, paint, petrochemicals, pesticides, and many other important industrial products (Ayodele et al. 2012; Iurascu et al. 2009). Long-term exposure to phenol paralyzes the human central nervous system and damages kidneys and lungs. It is also classified as a teratogenic and carcinogenic agent (Iurascu et al. 2009). Traditional wastewater treatment methods such as adsorption, stripping, and biological treatment represent an arduous process due to the recalcitrant nature of phenol and cannot readily mineralize it (Ayodele et al. 2012; Iurascu et al. 2009). The development of a persistent organic pollutant degradation process has attracted the attention of many researchers, and advanced oxidation processes (AOPs) have a special focus in this area (Garrido-Ramirez et al. 2010; Ayodele et al. 2012; Catrinescu et al. 2012; Zeng et al. 2013; Luo et al. 2009). Advanced oxidation processes involve the generation of reactive radicals, notably hydroxyl radicals (HO•), that are highly oxidative and capable of decomposing a wide range and variety of organic compounds (Garrido-Ramirez et al. 2010; Muthuvel et al. 2012; Herney-Ramirez et al. 2010; Hassan and Hameed 2011; Purceno et al. 2012; Iurascu et al. 2009). Among all the AOPs, Fenton and photo-Fenton oxidations have been proposed as efficient systems for the treatment of priority pollutants, considering the overall process efficiency based on pollutant removal and operation costs (Ayodele et al. 2012; Catrinescu et al. 2012). The tradional homogeneus Fenton and photo-Fenton reactions use a high concentration (normally >10 mg L−1) of iron

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that must subsequently be removed and can result in sludges. Additionally, it requires acidic conditions, typically below pH 3.0, which makes the wastewater treatment processes difficult and more expensive (Muthuvel et al. 2012; Hassan and Hameed 2011; Purceno et al. 2012; Iurascu et al. 2009; Luo et al. 2009). The development of active heterogeneous systems to promote the Fenton and photo-Fenton reaction is of considerable interest since it can offer several advantages such as no sludge formation, operation at almost neutral pH, and the possibility to recycle the iron promoter (Purceno et al. 2012; Iurascu et al. 2009; Luo et al. 2009; Muthuvel et al. 2012). Heterogeneous solid catalysis can mediate Fenton-like reactions over a wide range of pH values; this is because the Fe(III) species in such catalysis is immobilized within the structure and in the pore/interlayer space of the catalyst. As a result, the catalyst can maintain its ability to generate hydroxyl radicals from H 2 O 2 , and iron hydroxide precipitation is prevented (Garrido-Ramirez et al. 2010). Several materials have been used as support to Fe(III) immobilization and applied to the degradation of the recalcitrant organic compounds through Fenton-like reactions (Hadjltaief et al. 2014; GarridoRamirez et al. 2010). Some examples are iron species supported on polymers (González-Bahamón et al. 2011a), hydrogel (Wang et al. 2014), nanotubes (Jang and Park 2014), alginate gel beads (Rosales et al. 2012), molecular sieve (Ignachewski et al. 2010), and clay (Iurascu et al. 2009; Luo et al. 2009; Garrido-Ramirez et al. 2010; Muthuvel et al. 2012; Herney-Ramirez et al. 2010). The use of clays as catalysts for Fenton-like reactions is a promising alternative for decontamination of soils, groundwater, and industrial effluents. Clay is a natural, inexpensive, and abundant mineral. Furthermore, the clay minerals have a high surface area; their structure is constituted of aluminosilicate sheet stacking. Lamellae are formed from silicon and aluminum tetrahedric and octahedric monolayers, respectively, linked by oxygen atoms (Coelho et al. 2007; Teixeira-Neto and Teixeira-Neto 2009; Garrido-Ramirez et al. 2010; Muthuvel et al. 2012; Hassan and Hameed 2011; Anaissi et al. 2009; Iurascu et al. 2009; Luo et al. 2009). Some papers describe the use of clays modified with iron in the degradation of phenol in very reduced times in the absence of leaching of supported iron (Catrinescu et al. 2012; Iurascu et al. 2009; Luo et al. 2009); however, these materials are used in the form of a suspension, requiring a final process of filtration of the treated effluent, and reduce the degradation efficiency due to the radiation being prevented from penetrating into the solution. Despite these problems, the possibility of immobilization of clay in inert supports is not mentioned. The immobilization of mixed material clay/Fe allows an increase in the efficiency of the degradation process as well as provides the material with increased functionality.

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In this paper, the immobilization of mixed material clay/Fe on glass slides and its use in phenol degradation by photoFenton process are reported.

Experimental Preparation of the catalysts The mixed material was prepared according to Villalba et al. (2010). First, a suspension of bentonite clay (Al2O3·Si4O8.H2O, Aldrich) 2 % (w/v) (Anaissi et al. 2009) was prepared. The suspension was stirred for 24 h. After that, 5.41 g of FeCl 3 ·6H 2 O (SigmaAldrich) was added to give a 0.1-mol L−1 Fe3+ concentration in the suspension. The resulting mixture was stirred for 4 h at room temperature and maintained for 48 h at 70 °C. Finally, the mixed material was successively washed with deionized water until the pH stabilized from 1 to 5 and designated as MMAFe. The suspension MMAFe was immobilized on glass slides of 25.4×76.2×1 mm. The glass slides were sanded and washed, and then, the MMAFe was added in the form of drops. The slides with the deposited material were dried for 30 min at 70 °C. After drying, the mass of material deposited on the slide was measured, yielding an average of 0.040± 0.012 g. The materials were calcined at two different temperatures, 550 and 750 °C, for 30 min. Thus, the resulting materials were designated MMAFe550 and MMAFe750, respectively.

Characterization of catalyst The concentration of iron in the clay and the catalysts prepared was determined by atomic absorption spectroscopy (AAS) using a Varian Spectra AA-220. The measurements were performed at the Laboratory for Trace Analysis and Instrumentation, LabGAT/UNICENTRO. The Fourier transform infrared (FTIR) spectra were obtained on a spectrophotometer Shimadzu Prestige-21 with 4-cm−1 resolution, in the range 4,000–400 cm−1. Scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS) images were taken by Shimadzu model SSX-550; the samples were coated with a thin gold layer. X-ray powder patterns (XRD) were obtained using an X-ray diffractometer Rigaku Ultima 4, employing Cu Kα radiation (λ=1.541 Å) and settings of 40 kV and 20 mA. The scattered radiation was detected in the angular range 30°–100°, with 2° min−1 scanning speed, and a step of 0.02°. The measurements were performed at the Complex Multiuser Laboratories, CLABMU/UEPG.

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Factorial design The phenol degradation through Fenton-like process can be influenced by several factors; therefore, the optimization of variables such as pH, concentration of hydrogen peroxide, and catalyst is important for the process development. The physicochemical characteristics of the catalyst influence the availability of Fe(III) for the degradation process. The H2O2 concentration is directly related to the number of hydroxyl radicals generated and thus to the performance achieved; on the other hand, the higher H2O2 concentration scavenging of HO• radicals occurs, forming HO2• which is less reactive than HO• radicals. The pH of the medium is a crucial operating parameter, as it directly affects not only the photo-catalytic performance but also the extent of Fe leaching from the support. In homogeneous phase, at pH 2.8, approximately half of the Fe ions are present as Fe(III) and half as the complex ion Fe(OH)2+, which are the photo-active species. Lower values of pH result in decline of the Fe(OH)2+ concentration, while higher values result in precipitation of oxyhydroxides (Herney-Ramirez et al. 2010). In order to investigate the influence of pH and the concentration of hydrogen peroxide and catalyst on the degradation process, the 23 factorial design was performed. This study involves the three factors and two levels as shown in Table 1 (Barros-Neto et al. 2001). As the factorial design involves the catalyst as a qualitative variable, the planning, was replicated without midpoint. Thus, the t test is most suitable for analyzing the statistical significance. Catalytic activity measurements The reaction was developed and observed inside a discontinuous reactor refrigerated with a thermostat, containing 250 mL of phenol, 30 mg L−1 aqueous solutions, and 0.08 g catalyst. The substrate was irradiated using a 125-W mercury vapor lamp (without the original glass bulb), inserted into the solution through a glass bulb (UVA and Vis radiation). The pH was adjusted to the desired value by using 1.0 mol L−1 H2SO4 or 1.0 mol L−1 NaOH. The reaction was initiated by adding a predetermined amount of H2O2 solution. The material reuse study was developed by evaluating the performance of the proposed material as a catalyst in successive processes of phenol degradation. The catalyst used in the Table 1 Factors studied in the factorial design 23 Factors 1 2 3

pH [H2O2] (mg L−1) Catalyst

Level (−)

Level (+)

5 70 MMAFe550

7 140 MMAFe750

process of degradation was washed and dried at 70 °C and used again in another experiment under the same conditions of pH and concentration of hydrogen peroxide. Analytical methods The evaluation of the iron concentration in the liquid phase was developed by atomic absorption spectroscopy. The monitoring of the phenol concentration followed the colorimetric method using Folin Ciocalteu, according to NBR 10740/89 standards certified by the environmental assessment of ABNT, Brazilian Association standards (1989) (Sousa et al. 2007; Blainski et al. 2013). The monitoring of the consumption of hydrogen peroxide followed the method of ammonium vanadate proposed by Oliveira et al. (2001). All spectroscopic analyses were performed on UV–Vis, Varian 3000 spectrophotometer using quartz cuvettes with a 1-cm optical path.

Results and discussion The procedure adopted for MMAFe immobilization on glass slides was developed in order to obtain a more stable catalyst in aqueous solution and with great mechanical stability. Therefore, the experimental conditions used for the immobilization were obtained from several tests carried out in laboratory (results not shown). Certainly, the calcination process was the most relevant for the MMAFe immobilization on the glass slide. Two calcination temperatures were used: 550 and 750 °C. Thus, the materials obtained were designated MMAFe550 and MMAFe750, respectively. The heating resulted in the highest MMAFe fixing to the glass slide and conferring higher mechanical resistance to the material. The calcination temperatures were chosen on the basis of obtaining the clay’s best mechanical strength and preventing glass slides from melting (Pinheiro and Holanda 2010). Characterization of catalyst The concentration of iron in materials was verified by atomic absorption spectroscopy, obtaining values of 12.36±0.6 % (w/ w) for MMAFe550 and MMAFe750. The iron concentration in the bentonite clay is 2.03 % (w/w). In Fig. 1, the FTIR spectra of the prepared materials and bentonite clay can be observed; there is not much difference between the spectra obtained. The bands in common between the materials are related to the vibration characteristics of the clay. Bentonite clay presented bands at 3,430 and 1,644 cm−1 due to OH stretching of interlayer water. Typical bands of the silicate framework contributions were confirmed: at 1,045 cm−1 due to in-plane band

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Fig. 1 FTIR spectra of the bentonite clay (SI), MMAFe, MMAFe550, and MMAFe750

stretching of Si–O bonds and at 530 and 466 cm−1, corresponding to Si–O–Si and Al–O–Si vibrations, respectively. The band at 792 cm−1, due to Al–O vibrations, and 909 cm−1 is attributed to the deformation vibrations of hydroxyl groups, Al–O–H sitting on the alumina faces (Wu et al. 2011; Nogueira et al. 2011). The band at 3,629 cm−1 corresponds to hydroxyl groups on the bentonite clay surface; the main difference between the spectra of MMAFe and bentonite clay is that this band decreases in intensity, which should be promoted by the interaction between ferric ions and hydroxyl groups on the surface of the bentonite clay. In the calcined materials (MMAFe550 and MMAFe750), decreased bands at 3,629 and 1,644 cm−1 indicate that the heat treatment removed most of the hydroxyl groups that might have contributed to the adsorption of organic molecules. Stretches relating to iron hydroxide oxide, Fe–O–Fe at 916 cm−1 (Ayodele et al. 2012), were not found or were overlapped by the intense bands present in the clay (Hassan et al. 2011; Muthuvel et al. 2012; Nogueira et al. 2011).

Fig. 2 SEM images of materials: a Bentonite clay (SI) and b MMAFe (magnification ×1,000)

The SEM images (Fig. 2) show the morphologies of the bentonite clay and MMAFe. The bentonite clay (Fig. 2a) shows plates that demonstrate the lamellar structure, similar to that observed by Purceno et al. (2012). This morphology is altered by obtaining MMAFe (Fig. 2b) in which the formation of smaller particles on the surface of the plates occurs, indicating the formation of iron aggregates on the clay matrices. A similar phenomenon was observed by Muthuvel et al. (2012). After the calcination processes, the aspect presented by MMAFe morphology did not change significantly (figure not shown).The EDS was used to map the iron atoms in MMAFe750. Figure 3 shows that the iron atoms were homogeneously dispersed on the surface of the material. The bentonite clay (SI) XRD pattern (Fig. 4) shows the characteristic peaks of bentonite clay, indicating the presence of clay minerals of the smectite group. Peak d (001) relates to the interlayer distance between the planes, while the other peaks are due to the presence of crystalline phases of quartz, kaolinite, and illite in the clay analyzed (Muthuvel et al. 2012).

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Environ Sci Pollut Res (2015) 22:894–902 Table 2 Factors, levels, and percentage of degradation values for the 23 factorial design Test Factors

Percentage of degradation

pHa H2O2b Catalystc 1

−

−

−

65.27

63.43

64.35±1.30

2 3 4 5 6 7 8

+ − + − + − +

− + + − − + +

− − − + + + +

86.63 80.87 92.57 67.67 75.50 82.80 94.07

84.50 80.13 93.70 70.13 68.50 85.60 96.48

85.56±1.50 80.50±0.52 93.13±0.80 68.90±1.74 72.00±4.95 84.20±1.98 95.27±1.70

a

pH—5 (−), 7 (+)

b

[H2O2] (mg L−1 )—70 (−), 140 (+)

c

Catalyst—MMAFe550 (−), MMAFe750 (+)

Studies on phenol degradation Fig. 3 Mapping of the iron atoms in MMAFe750 by EDS

Factorial design The MMAFe550 and MMAFe750 XRD patterns were very similar and exhibited characteristic peaks of akaganeíta phase (β-FeOOH) in 27°, 33°, and 36° (Anaissi et al. 2009; Lübble et al. 2010). Observing the bentonite clay (SI), MMAFe550, and MMAFe750 XRD patterns, one can see that in the calcined materials (MMAFe550 and MMAFe750), the peak 001 is shifted to higher values of 2θ, indicating a decrease in the interlayer space in the calcined material. This fact provides evidence that the phases, related to iron, were structured on the clay surface and not within the clay lamellae. A similar phenomenon was observed by Nogueira et al. (2011). Fig. 4 XRD diffractograms of bentonite clay (SI), MMAFe550, and MMAFe750

The influence of variables (pH, catalyst, and concentration of hydrogen peroxide) in the phenol degradation was studied using 23 factorial design (study of three variables at two levels) resulting in eight different degradation conditions. The levels of the parameters studied were chosen from preliminary degradation studies carried out in laboratory (results not shown). Table 1 shows the parameters and the value of the levels measured in the factorial design. Table 2 shows the conditions used in each experiment as well as the percentage of phenol

Author's personal copy Environ Sci Pollut Res (2015) 22:894–902 Table 3 Effects calculated for the 23 factorial design

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Effects Average Main effects pH (A) H2O2 (B) Catalyst (C)

80.49±0.78 12.00±1.56 15.57±1.56 −0.79±1.56

Interaction of the second order AB −0.15±1.56 AC −4.92±1.56 BC 3.71±1.56 Interaction of the third order ABC 4.14±1.56

degradation obtained in each condition. Through the data presented in Table 2, it was found that the highest degradation percentages were obtained using higher levels of hydrogen peroxide. It was also observed that the experiments performed at pH 7 resulted in higher efficiency in the phenol degradation. This fact is very important because the possibility of working at neutral pH prevents the formation of sludge, allowing the implementation of a continuous system of waste treatment. Moreover, the elimination of the acidification step makes the process cheaper. The values of the effects were calculated using the equation (Box et al. 1978; Barros-Neto et al. 2001): Efi ¼ RiðþÞ−Rið−Þ where Ri(+) and Ri(−) are the averages of results when the ith factor is at its high (+) or low (−) level independent of the Fig. 5 Spectroscopic monitoring of the degradation of phenol. Phenol 250 mL and 30 mg L−1, pH 7, H2O2 140 mg L−1, and catalyst MMAFe750

signs of the other effects. The values of the effects calculated with Table 2 data are presented in Table 3. For the main effects, only pH and H2O2 are significant in the experimental range investigated. The effect of the catalyst reflects the similarity between MMAFe550 and MMAFe750 materials. As seen through the characterization study, the catalysts developed do not present significant differences. The effect of the interaction between pH and catalyst and the interaction between concentration of H2O2 and catalyst is also significant, so is the effect of the interaction between the three variables. Therefore, the main effects must be interpreted together. The increased concentration of H2O2 increased the rate of degradation, but this effect is more pronounced at pH 7 than at pH 5 when the reaction is catalyzed by both MMAFe550 and MMAFe750. The exchange of MMAFe550 to MMAFe750 at pH 7 and 70 mg L−1 of H2O2 decreases the degradation to 13.565 %. Similarly, for the rest of the experimental conditions, the exchange of MMAFe550 to MMAFe750 causes increase in the degradation percentage. The highest degradation percentage was obtained in the experiment when the catalyst MMAFe750, the highest level of H2O2 (140 mg L−1), and pH 7 were used; these levels were chosen as working condition to continue the phenol degradation studies. Another factor that contributed to the choice of the catalyst was the mechanical resistance shown by MMAFe750 in the preliminary studies on degradation. The mixed materials were observed to adhere with greater efficiency to the glass slide when undergoing the calcination at 750 °C.

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Fig. 6 Analytical monitoring of phenol degradation, reusing the catalyst MMAFe750. Phenol 250 mL and 30 mg L−1, pH 7, and H2O2 140 mg L−1

Degradation study The best conditions obtained in the factorial design (MMAFe750, 140 mg L−1 of H2O2, pH 7) were used in the degradation of 250 mL of a 30-mg L−1 phenol solution. In the first few minutes of the reaction, a large increase in the band of aromatic groups in the region of 200 nm was observed through spectroscopic monitoring (Fig. 5). This fact can be explained based on the phenol degradation mechanisms found in the literature. In the first stage of degradation, the formation of aromatic compounds such as hydroquinone, benzoquinone, and catechol is usually proposed (Devlin and Harris 1984; Kang et al. 2002; Alnaizy and Akgerman 2000). After 90 min Fig. 7 Proposed mechanism for the degradation of phenol through the MMAFe750 as a heterogeneous catalyst in the photo-Fenton process

of reaction, the absorbance in the aromatic region begins to decrease, indicating the elimination of phenol and aromatic compounds. Furthermore, the behavior described earlier occurs when the process is carried out in a heterogeneous system, that is, when ferric ions (responsible for cleavage of hydrogen peroxide) are located on the surface of the catalyst. In heterogeneous systems, substrates require a certain time (induction period) to adsorb on the catalyst surface and then suffer the attack of hydroxyl radicals. After the induction period, an abrupt increase in the reaction rate of phenol degradation occurs (Luo et al. 2009). This behavior of the degradation process is an indication that the reaction occurs in a heterogeneous system.

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The concentrations of iron present in the reaction medium were monitored. During the reaction time, it was seen that the leaching of iron to the reaction medium is very low, very close to the instrument error. These data indicate that the ferric ions immobilized on the surface of the catalyst may be mediating the phenol degradation. Studies have indicated that adsorption of phenol on the material does not occur during the degradation studies, discarding the influence of this phenomenon in the degradation studies performed. The reuse study to evaluate the catalytic activity of the material after successive phenol degradation tests enabled us to observe that the material obtained had excellent fixation on the glass slide. No detachment of the catalyst after any of the tests was observed. Figure 6 shows the phenol degradation using slides with MMAFe750 for five successive degradation tests. It can be seen that at the end of 300 min of reaction, all degradation tests reached approximately 100 % degradation. The efficiency of the material was not altered after each use. It was certainly associated with the negligible amount of ironleached MMAFe750 to the reaction medium. Results indicate that the material has a long lifetime, far exceeding the number of three to four cycles considered acceptable for materials using clay as support (González-Bahamón et al. 2011b; Barrault et al. 1998).

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Conclusions The immobilization of the mixed material MMAFe glass slides was performed successfully, and the material obtained was used in the phenol degradation study. Through the experimental design, it was found that the most suitable phenol degradation conditions were obtained using MMAFe750 material, at pH 7, and with 140 mg L−1 of hydrogen peroxide. The MMAFe750 material exhibited excellent performance as a catalyst for Fenton-like reaction; within just over 125 min of reaction, 50 % of the phenol had been eliminated. Furthermore, the material exhibited high efficiency for phenol degradation, since even after five degradation studies, the catalytic activity was not damaged and the material had not detached from the glass slide. This indicates that the catalyst can be used for extended periods of reaction; therefore, the material is promising to be used in continuous systems of waste degradation. Acknowledgments FI and JCV are indebted to CAPES for the fellowship, and LYT is indebted to CNPq for the fellowship. FJA and STF are indebted to PROCAD/CAPES, Fundação Araucária, and CNPq for financial support. The authors are also indebted to LabGAT/UNICENTRO and CLABMU/UEPG for the AAS, FTIR, SEM, and XRD analyses.

References Kinetic analysis As discussed earlier, at the beginning of the reaction, within the referred induction period, the reagents were adsorbed on the surface and fast transformation into aromatic products occurs. Therefore, the kinetic analysis took into consideration only the phenol concentration in solution after this period. The reaction order was checked through the graph method; as pointed out in the literature, the degradation follows a firstorder kinetics (Muruganandham and Swaminathan 2006) with 2.13×10−2-min−1 constant rate. Phenol degradation mechanism Based on the results discussed, a simple mechanism for MMAFe750 as a heterogeneous catalyst in the photo-Fenton process of phenol degradation was proposed. The mechanism is shown in Fig. 7. Firstly, the photo-reduction of Fe3+ on the surface the MMAFe750 to Fe2+ under irradiation of light occurs. After that, the Fe2+ formed takes the decomposition of H2O2 in the solution. The radicals formed attack the phenol molecule, giving rise to a product processing such as catechol and hydroquinone, which are degraded into organic acids. Finally, the organic acids are mineralized into CO2 and H2O.

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