Synergistic Mechanism of Photocatalysis and Photo-Fenton by Manganese Ferrite and Graphene Nanocomposite Supported on Wood Ash with Real Sunlight Irradiation

Abstract

The present research aimed to evaluate the photocatalytic activity of reduced graphene oxide and manganese ferrite nanocomposite supported on eucalyptus wood ash waste (WA) from industrial boilers, for the decolorization of methylene blue (MB) solutions, using sunlight as an irradiation source. For this, the photocatalyst named MnFe₂O₄-G@WA was synthesized by a solvothermal method and characterized by analyzes of scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, Brunauer–Emmett–Teller and zeta potential. Firstly, the photocatalyst was evaluated for photocatalytic decolorization of MB under different reaction conditions. Then, the influence of pH, photocatalyst dose and H₂O₂ was evaluated. MnFe₂O₄-G@WA showed 94% of efficiency for photocatalytic decolorization of MB under operating conditions of solar irradiation, 0.25 g/L of catalyst, 300 mg/L of H₂O₂. The proposed degradation reaction mechanism suggested that the photodegradation of MB was through a synergistic mechanism of photocatalysis and photo-Fenton reactions, with the combined action of the three materials used. The data adjusted to the first order kinetics from the Langmuir–Hinshelwood model. In addition, MnFe₂O₄-G@WA showed high stability, maintaining its efficiency above 90% after 5 cycles. The results indicated that the nanophotocatalyst is a potential technology for the decolorization of MB solutions.

1. Introduction

Synthetic organic dyes are frequently used in numerous industrial activities, such as textiles, food and plastics [1]. Various techniques for removing these types of contaminants from water have been studied. Advanced oxidant processes (AOP) are effective wastewater treatment methods, especially for degradation of non-biodegradable compounds such as dyes. These approaches, when under appropriate conditions, can oxidize pollutants from the production of a large number of reactive species, mainly hydroxyl radicals (•OH) [2,3,4].

The heterogeneous photocatalysis, an emerging AOP, is a degradation method that employs semiconductors as catalysts. The chemical or photochemical interaction (UV/Visible) with the solid surfaces of these materials induce redox reactions and catalytic formation of hydroxyl radicals (•OH) from water oxidation as well superoxide radicals from the reduction of oxygen (O₂^•−) [5]. The process starts when the energy irradiated is higher than that of its band gap, then electrons from valence band are displaced to the conduction band, forming electron/photo-hole pairs that are responsible for inducing these redox reactions [6].

Many studies have been developed exploring the photocatalytic potential of inorganic nanoparticles. Among them, titanium dioxide (TiO₂) has become one of the most used materials in the area of photodegradation due to its high effectiveness [7,8]. However, despite its wide application, it is known that TiO₂ has some limitations, such as the wide band gap (3.2 eV) that limits the absorption of photons only of UV light (λ < 380 nm), which corresponds to approximately 4% of sunlight, making its application difficult in systems using real sunlight as a source of irradiation [7,9,10].

The heterogeneous photocatalysis can be considered a greener technology and more economically attractive when sunlight is used as irradiation source. The solar light is a naturally and inexhaustible source of energy that can be used on heterogeneous photocatalysis processes, with an appropriate photocatalyst that have good response to visible light [11,12].

In contrast, the spinel-type oxides stand out in the middle of photocatalysis due to a narrow band gap, specifically manganese ferrite (MnFe₂O₄) has a gap of around 1.9 eV, thus having a good response to visible light, in addition to being highly thermally and chemically stable [13]. This material also has characteristics such as great adsorption capacity, biocompatibility, good magnetic properties and functional surface, however when used on a large scale, there is a difficulty regarding its recovery due to its small size, magnetic interaction and tendency to cluster [14,15].

Another widely used AOP is the photo-Fenton system, which uses the incidence of UV/Visible radiation to induce the photoreduction of ferrous compounds from the catalyst, and also the photolysis of hydrogen peroxide (H₂O₂), with •OH as a product that can oxidize pollutants [16,17]. It is known that the association of these two mechanisms increases the degradation capacity of catalysts, since the photogenerated electrons induce a rapid conversion of Fe (II) and Fe (III) in the photocatalyst, resulting in extra •OH radicals. Thus, the use of catalysts containing iron may be preferred for the synergistic application of both mechanisms. However, it is also known that pure ferrous compounds have an accelerated recombination of electron-hole pairs, which can compromise the degradation efficiency [18,19].

The graphene-family encompass materials with structures similar to graphite, including graphene; graphene oxide (GO); reduced graphene oxide (G) [20,21], have been shown to be allies to solve issues of spinel-type oxides, since they have properties that prevents excessive agglomeration of the magnetic nanoparticles from occurring [22]. In addition, they also improve the photocatalytic capacity of semiconductors due to the excellent electron mobility, which reduces the rate of recombination of electron-hole pairs [13]. They also exhibit outstanding properties such as good conductivity, good adsorption capacity, thermal and chemical stability, are material with a large surface area and mechanical strength and also perform the function of electron dissipators [20,23,24,25,26].

Furthermore, the use of carbonaceous materials associated with semiconductors has already been reported, Gautam et al. [27] used MnFe₂O₄ dispersed in graphitic carbon sand composite and bentonite to mineralize the antibiotics ampicillin and oxytetracycline under sunlight irradiation. Huang et al. [13] used MnFe₂O₄ associated with G, for degradation of methylene blue dye (MB) under visible irradiation from a Xenon lamp.

The ash originating from the burning of eucalyptus from industrial boilers can act as a promising support material for the deposition of catalyst nanoparticles, since it has low cost, chemical stability, porous structure, and enhances the degradation capacity of photocatalyst materials, in addition to facilitate their recovery at the end of the process [11]. Research in this regard has also been reported, Kanakaraju et al. [10] synthesized a photocatalyst combining titanium dioxide doped with copper and fly ash to remove dye by adsorption and photocatalysis. Lum et al. [28] developed a bibliographic review highlighting the use of ash-based nanocomposites in terms of preparation methods and conditions, morphologies, physicochemical properties and their photocatalytic performances for dyes degradation.

Considering the aforementioned, it is known that the use of renewable energy sources, such as sunlight as a source of irradiation, can make photocatalytic methods more economically and environmentally attractive [12,29]. Thus, the present research aimed to evaluate the photocatalytic efficiency of manganese ferrite reduced graphene oxide nanocomposite supported on eucalyptus wood ash (MnFe₂O₄-G@WA) to remove MB under real sunlight irradiation. The experimental conditions were varied (pH, catalyst and oxidant concentration), in order to evaluate the influence of each parameter on the decolorization efficiency of the catalyst.

2. Materials and Methods

2.1. Chemicals and Reagents

Graphite powder (Biotec, 100%), hydrochloric acid (HCl, Alphagec, 37%), sulfuric acid (H₂SO₄, Química Moderna, 98%), potassium persulphate (K₂S₂O₈, Biotec, 99%), phosphorus pentoxide (P₂O₅, Êxodo Científica, 99.5%), potassium permanganate (KMnO₄, Biotec, 99%), hydrogen peroxide solution (H₂O₂, Synth, 30%,), ethylene glycol (C₂H₆O₂, Nuclear, 99%), ferric chloride hexahydrate (FeCl₃·6H₂O, Biotec, 97%), sodium acetate (C₂H₃NaO₂·3H₂O, Biotec, 99%), manganese dichloride (MnCl₂·4H₂O, Biotec, 98%), ethanol (CH₃CH₂OH, Biotec, 95%), MB (C₁₆H₁₈ClN₃S, All Chemistry do Brasil Ltd.a) were all purchased with analytical grade and were used without additional decontamination. WA was was kindly provided from Eucalyptus burning in a biodiesel industry with a particle size of 435–600 μm. Distilled water was used in all processes of aqueous solutions, suspensions and washing processes.

2.2. Synthesis of Nanophotocatalysts

GO was synthesized using the modified Hummers method [22,30]. The process, in short, encompasses the stages of pre-oxidation and oxidation of graphite. Initially, 5 g of powdered graphite, 2.5 g of K₂S₂O₈ and 2.5 g of P₂O₅ were added to a 250 mL flask with 18 mL of H₂SO₄ (98%) was kept under constant stirring at 80 °C for 5 h. Then, the pre-oxidized graphite was collected by filtration and dried at 60°C for 12 h. The dried material was then subjected to the oxidation process, and 1 g of pre-oxidized graphite was dissolved in 23 mL of sulfuric acid (98%) under stirring. Still under constant stirring, 3 g of KMnO₄ was slowly added and kept at 35 °C for 2 h. After this period of time, 140 mL of deionized water and 2.5 mL of H₂O₂ (30%) was added to the solution. The material was then washed with 10% HCl and allowed to decant at room temperature for 24 h. Finally, the mixture was washed with deionized water, centrifuged (3700 rpm) three times for 15 min and the supernatant was removed and dried in an oven at 60 °C for 12 h, thus, obtaining the GO.

The nanocomposite synthesis method was based on the methodology proposed by Yamaguchi et al. [22,31]. In summary, 3.7 mmol of iron chloride and 1.85 mmol of manganese chloride (MnCl₂·4H₂O) were added to a suspension of ethylene glycol and GO, which was kept in ultrasonication for 30 min. After, 3 g of sodium acetate, 15 g of washed and sieved WA and an additional 20 mL of ethylene glycol were added to the solution, which was kept under magnetic stirring for 30 min. This mixture was taken to a stainless-steel autoclave with an internal Teflon capsule and kept at 200 ° C for 10 h. Finally, the resulting material was washed with 100 mL of ethanol and 2 L of deionized water and dried in an oven at 60 °C for 12 h. The resulting material obtained was the hybrid nanocomposite of reduced graphene oxide and manganese ferrite supported on WA (MnFe₂O₄-G@WA). In this research, reduced graphene oxide will be treated only as G to facilitate understanding. All these steps are represented in Figure 1. Adopting the same methodology, a composite was synthesized only with manganese ferrite and G without WA, named MnFe₂O₄-G.

2.3. Nanophotocatalysts Characterization

Several techniques were used to characterize the WA, MnFe₂O₄-G and MnFe₂O₄-G@WA. The morphology of the materials was analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The Shimadzu SS-550 Scanning Electron Microscope and the JEOL transmission electron microscope model JEM-1230 were used. The X-ray diffraction (XRD) analysis aimed to determine the purity of prepared catalysts and their crystal structure. It was executed in equipment model Shimadzu XRD 6000, with Cu Ka radiation source (wavelength k = 1.54 Å), over a range from 10 to 80° of 2θ value.

The images were recorded in a digital file. The functional groups were detected by Fourier-transform infrared spectroscopy (FTIR), Shimadzu, model IR PRETIGE 21. KBr pellet method was used in the scanning range of 4000–400 cm⁻¹. The surface area of the composites was determined by BET (Brunauer–Emmett–Teller), under low pressure range and degassed temperature at 70 °C for 3 h.

The zeta potential analysis was carried out with the objective of determining the nanocomposite surface charge, the isoelectronic point (IEP), and the effect of pH on the behavior of surface charges. The materials were suspended in deionized water and 0.1 M solutions of NaOH and HCl were used to reach different pH (3 to 11). The zeta potential was analyzed with Delsa NanoTM C Beckman Coulter equipment.

2.4. Photocatalytic Activity

The photocatalytic activity of the MnFe₂O₄-G@WA nanocomposite was evaluated based on the decolorization of an artificially contaminated MB solution with a known concentration of 10 mg L⁻¹, following methodological processes previously cited in the literature [13,31]. First, 50 mg of the nanocatalyst was dispersed in a beaker containing 200 mL of this solution, which was kept under constant agitation. The total reaction time was 120 min. In the first 40 min, the samples were kept without the presence of light irradiation, to obtain the adsorption equilibrium. After that time, 300 mg/L of H₂O₂ solution was added, and then the solutions were exposed to real sunlight irradiation. At regular intervals of 20 min, throughout the process, 4 mL aliquots were collected, centrifuged for 10 min at 3200 rpm, and then their respective absorbances were measured by UV-VIS spectrophotometry, with the equipment adjusted at 664 nm. The intensity of sunlight was measured with the aid of an Instrutemp digital luxmeter, model ITLD260 (90–140 W/m²). The experiments were conducted in Maringá-PR (Brazil), between 10:00 a.m and 3 p.m., on predominantly sunny days during the months of August to December. All experiments were carried out in triplicate in order to improve the level of accuracy. The decolorization efficiency was calculated using Equation (1):

$$\mathrm{Decolorization}\mathrm{efficiency}(\%)=\frac{({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{t}})}{{\mathrm{C}}_0}\times 100$$

(1)

where C₀ is the final concentration of MB and C_t is the concentration of MB at time t.

In order to find the optimum conditions and also evaluate the influence that each parameter has on the decolorization efficiency, the catalyst dose (1 g/L; 0.5 g/L; 0.25 g/L; 0.125 g/L; 0.05 g/L), dose of H₂O₂ (0 mL; 1 mL; 5 mL; 10 mL) resulting in different concentrations of peroxide (0 mg/L; 300 mg/L; 1500 mg/L; 3000 mg/L) and the pH of the medium (2; 7; 12) were varied. The same procedures elucidated previously were adopted in all tests.

With the established optimal conditions, the tests were also performed without the presence of light, aiming to evaluate the influence of sunlight irradiation on the MB photocatalytic decolorization efficiency. Furthermore, the efficiency results using MnFe₂O₄-G@WA were compared with the results obtained using different photocatalysts: MnFe₂O₄-G, WA, MnFe₂O₄@WA and no photocatalyst.

Finally, the recyclability of the synthesized material was evaluated under optimal conditions obtained in the tests previously described, where it was subjected to 5 consecutive cycles of MB photocatalysis, being recovered by filtration, washed with distilled water and dried at 60 °C after each cycle. The recovery of the nanocomposite was carried out with a filtration system (Millipore Sterifil, Burlington, MA, USA) using a 0.45 µm acetate membrane (Sartorius, Göttingen, Germany).

3. Results and Discussion

3.1. Nanophotocatalysts Characterization

3.1.1. SEM and TEM Analysis

The morphology analysis of WA, MnFe₂O₄-G and MnFe₂O₄-G@WA samples were analyzed by SEM and TEM, as shown in Figure 2. In regard to the WA micrograph (Figure 2A,D), it can be seen that its surface has irregularities, showing to be rough and compact, characteristics also observed by Lum et al. [32], who analyzed the morphology of ash based photocatalytic nanocomposites. The same authors pointed out that this type of porous structure, with the presence of cavities on its surface, indicates that it has good surface area availability for semiconductor deposition. In addition, it is possible to verify that the particles have grooves, cracks and small heterogeneous agglomerations arranged on larger surfaces, characteristics also observed by Kim et al. [33] and Mushtaq et al. [11].

In SEM and TEM micrographs of MnFe₂O₄-G (Figure 2B,E), the MnFe₂O₄ nanoparticles have spherical shape and showed to be adhered to the graphene nanosheets homogeneously, which are characterized by having a rough and translucent surface [13]. Additionally, it was observed larger MnFe₂O₄ microspheres, as a result of the grouping of MnFe₂O₄ nanoparticles, measuring from 150 to 350 nm and from 10 to 20 nm, respectively, in accordance with other results in the literature [22,34,35] and with our previous results [22,31].

From the SEM and TEM images of the nanocomposite MnFe₂O₄-G@WA (Figure 2C,F), it was possible to identify each nanomaterial used. As indicated in the images, graphene is characterized by having a translucent appearance [31,36]. Additionally, clusters of MnFe₂O₄ nanoparticles well distributed are also observed, this fact can be attributed to the graphene nanosheets, which prevents the excessive aggregation of MnFe₂O₄ nanoparticles, thus, providing a larger surface area [37]. As remarked also by Luciano et al. [31], it was possible to identify MnFe₂O₄ nanoclusters adhered to the graphene nanosheets, which are adhered to the surface of WA, the supporting material.

3.1.2. X-ray Diffraction (XRD)

The results of the structural characterization of the materials by the X-ray diffraction (XRD) analysis are presented in Figure 3. The sharp peaks (111), (220), (311), (400), (422), (511) and (440) at 2θ = 18.4°, 30.2°, 35.5°, 43.3°, 53.5°, 57.1° and 62,7°, respectively, are relative to crystal planes of spinel structure of MnFe₂O₄ and reveal the good crystallinity of the MnFe₂O₄ sample. The same peaks were observed in previous studies for XRD analysis of MnFe₂O₄-G and bare MnFe₂O₄ [22,31]. Furthermore, the peak (311) at 2θ = 35.5° was also observed in XRD pattern of MnFe₂O₄-G@WA indicating the presence of MnFe₂O₄ nanoparticles in WA (JCPDS 10-031) [38,39,40,41,42].

The Debye–Scherrer equation was used to estimate the average crystallite size of MnFe₂O₄ [22]. The nanoparticles of MnFe₂O₄ sizes were found to be 5.9 nm and 3.8 nm of MnFe₂O₄-G and MnFe₂O₄-G@WA samples, respectively. Similar results were reported before [22]. The decrease in the size of the nanoparticles in the nanocomposite with WA can be attributed due to the growth blockage of nanoparticles on the micropores of WA.

It is important to mention that, regarding the MnFe₂O₄-G sample, it is possible to notice that there is no diffraction peak of GO or G. This can be associated with the destruction of the regular layer stacking of GO by the crystal growth of MnFe₂O₄ between the interlayers during the reduction reaction, in accordance with previous studies [22,31,43].

The XRD pattern of wood ash exhibited an intense peak at 2θ = 23.5° as well, this result is characteristic of the amorphous silica and aluminosilicate, which is a component commonly found in ash from burning coal and/or biomass [28,44,45]. Additionally, it is possible to observe an intense peak at 2θ = 29.7°, which indicates the presence of CaCO₃ (JCPDS 47-1743), which is a compound normally found in the composition of biomass ash [46,47].

3.1.3. FTIR

The FTIR analysis results are shown in Figure 4. It was possible to identify peaks in the spectra of GO, WA, MnFe₂O₄-G and MnFe₂O₄-G@WA samples, in the range of 4000 to 400 cm⁻¹. The peak located at 3400 cm⁻¹ range corresponds to the stretching vibration of the O-H group, which can be attributed to the moisture adsorbed by the nanomaterials [36]. This group can also be observed on the GO spectra at a wavelength of 1405 cm⁻¹ [48].

The peaks at 1625 cm⁻¹ and 1740 cm⁻¹ found in GO and MnFe₂O₄-G spectra are located in a band that indicate carbon-oxygen bonds, specifically the stretching vibration of the C=O group [10,27]. The carbon-oxygen bonds, also appear in the GO spectra at 1210 cm⁻¹, which can be related to the epoxy group indicating C–O–C bonds.

Furthermore, the peak at 1050 cm⁻¹, found in GO spectra, is assigned to the stretching vibrations of C–O [48]. In the spectra of MnFe₂O₄-G@WA and WA, this peak is wider, reaching the characteristic wavelength of silica (1040 cm⁻¹), indicating the presence of the Si–O–Si bond, characteristic compound of biomass ash [28,45]. In addition, it was observed at MnFe₂O₄-G@WA spectra, the tendency of peaks at shorter wavelengths, that can be related to the intense interaction and chemical bond formation between Si present in WA and MnFe₂O₄ nanoparticles [11].

Specifically in the WA spectra, there is a peak at 1398 cm⁻¹, this band is attributed to polymorphic calcium carbonate (CaCO₃), but due to burning at high temperatures its presence is smaller and its peak is less prominent than the peak attributed to silica [49,50]. The presence of these compounds, both silica and carbonates, are minerals naturally present in plant materials or resulting from the combustion process [51,52]. It is also known that for temperatures at 600 °C, CaCO₃ is frequently found in wood ash while at higher temperatures such as at 1300 °C, CaO is more commonly found in wood ash samples [53]. Furthermore, Scheepers et al. [54] performed a review study on the use of wood ash and found that calcite was present in more than 40% of industrial samples.

The peaks identified at shorter wavelengths correspond to metal-oxygen bonds, showing the formation of these metal-oxygen bounds at tetrahedral and octahedral sites in the spinel structure [55]. The peaks at 592 and 575 cm⁻¹ are considered characteristics of the Fe–O bond present in iron, so its greatest intensity was observed in MnFe₂O₄ nanoparticles [56,57]_. The peak identified at 484 and 450 cm⁻¹ in MnFe₂O₄-G@WA and MnFe₂O₄ samples, represents the Mn-O stretching vibrations [27].

Additionally, comparing MnFe₂O₄-G and GO spectra, it was noted that some peaks of oxygen-containing groups disappear, indicating the efficient reduction of GO to reduced graphene oxide during the adopted solvothermal process, as previously observed by Yamaguchi et al. [22].

3.1.4. Zeta Potential Analysis

The polarity of the surface charge as a function of the pH of the nanomaterials and their IEP determination can be obtained through the analysis of zeta potential [58]. The IEP of MnFe₂O₄-G@WA, WA and MnFe₂O₄-G is approximately at pH 4. Thus, the results obtained of IEP indicate that at a pH higher than 4, the catalysts become negatively charged (Figure 5). As the photocatalysis tests were carried out at neutral pH, the MB adsorption by the nanocatalysts was favorable, as MB has positive charges [59].

In regard to GO, it was observed that in all tested pH the material presented a negative charge, which indicate that the incorporation of this material in the catalyst is advantageous [34], not only because it avoids the excessive agglomeration of the MnFe₂O₄ nanoparticles, as previously mentioned, but also because it intensifies the adsorption capacity of MB due to its negative charge. However, probably due to the small amount of graphene used, the charge of the other materials used (MnFe₂O₄ and WA) prevailed.

3.1.5. BET Analysis

The characteristics of surface area of the nanocomposites were investigated using N₂ adsorption–desorption volumetric analysis (Figure 6). WA presented type II adsorption isotherm according to International Union of Pure and Applied Chemistry (IUPAC) classification, which is characterized by multilayer adsorption [60]. The uptake continuously increases even when the pressure ratio is close to unity. This type of behavior is found for microporous adsorbents [61]. Furthermore, the nitrogen adsorption–desorption isotherm for the WA (Figure 6A) shows the hysteresis loop type H3 pores according to IUPAC classification, which is attributed to wedge-shaped pores with narrow necks and/or both open-ends [60].

The adsorption isotherm of MnFe₂O₄–G@WA sample (Figure 6B) presented the type I classification according to IUPAC. This type of isotherm is found mostly in narrow microporous adsorbents, normally with pore size less than 1 nm, with a small external surface, with the limiting uptake of adsorbate governed by the accessible micropore volume rather than by the internal surface area [60,61].

Moreover, the calculated BET surface area of WA was found to be 123 m²/g, comparing with the BET surface area of MnFe₂O₄-G@WA decreased with the addition of MnFe₂O₄-G to 111 m²/g. This behavior is an indicative that the magnetic material may have obstructed the pores of WA. Additionally, this is also an indicative that physisorption is the main governing adsorption mechanism process [62].

3.2. Photocatalytic Efficiency

3.2.1. Efficiency of Different Photocatalysts

Different photocatalysts were tested and the results are presented in Figure 7. In addition, the behavior of MB under the same conditions (light and H₂O₂) without any photocatalyst is also demonstrated. Firstly, without the presence of photocatalyst, it can be observed that the decolorization efficiency was much lower compared with the photocatalysts, not even reaching 30% of MB removal efficiency. In the absence of a catalyst material, the production of electron-hole pairs is compromised, consequently H₂O₂ has fewer active sites available to react and form radicals [63].

The efficiencies of the MnFe₂O₄@WA and WA showed a significant decolorization efficiency of MB, and reached 85% of MB removal. Indicating that both materials have favorable properties as photocatalysis. Manganese ferrite, specifically, has a narrow band gap, which is a determining factor as it favors its response to visible light due to the greater ease of generation of electron-hole pairs [13,64].

WA, in addition to being an excellent adsorbent (Figure S1), which is an important property for the efficiency of a photocatalyst, also contains the presence of CaCO₃ in its composition, which is a compound that has been widely studied in the area of photodegradation [65,66]. This is because during the photocatalytic process, calcite can generate, in an aqueous medium, chemical species such as CO₃²⁻, HCO₃⁻, which can react with the •OH produced and generate carbonate radicals (•CO₃⁻), which is also a potent oxidant [67,68].

Regarding the MnFe₂O₄-G system, the decolorization efficiency was only 44%, a result below the expected, since the G has the function of acting as an electron scavenger and prevents the rapid recombination of the electron-hole pairs of MnFe₂O₄, which theoretically enhances the efficiency of the semiconductor [35]. However, without the support of WA and at a concentration of 0.25 g/L of photocatalyst, an increase in the turbidity of the sample solution was observed, a fact that hinders the penetration of photons and compromises the synergistic reaction of photocatalysis and photo-catalyst. Fenton, a behavior also observed by Wei et al. [42]. Furthermore, due to the larger surface area of WA, the nanoparticles are better distributed, and their photocatalytic potential is optimized as there is a greater availability of active sites to react with the pollutant molecules [28].

Finally, it can be observed that the association of the three materials (MnFe₂O₄, graphene, WA) optimized the MB decolorization process, reaching a removal percentage of 94%. Therefore, the synergy between the photocatalytic potential of the three materials has been shown to play an important role in the efficiency of the system.

3.2.2. Decolorization in Different Reactions Conditions

The photocatalytic efficiency for MB decolorization in different conditions was verified (Figure 8). It was observed that when MnFe₂O₄-G@WA was kept in contact with the MB solution in the absence of light irradiation, the catalyst presented an efficiency of 70% of MB removal, thus, demonstrating that it can be considered a good adsorbent for MB in aqueous solution [10], as well as WA alone, as demonstrated in the WA adsorption kinetics (Figure S1). However, with the addition of the sunlight irradiation factor (MnFe₂O₄-G@WA + Light), the percentage of decolorization reached 75% after 120 min, hence, indicating that the synthesized catalyst does not degrade MB only by the photogenerated holes [37]. When only adding H₂O₂ without sunlight irradiation (MnFe₂O₄-G@WA + H₂O₂) the maximum decolorization efficiency reached 66%. This behavior can be explained because in absence of light, the Fe (II, III) circulations of the photo-Fenton reaction is affected and then a reduction in extra hydroxyl radical generation occurs, fact that impair the degradation efficiency and indicate that the main mechanism of MB degradation by MnFe₂O₄-G@WA is through •OH radicals [64]. In addition, this behavior can also be attributed to the decomposition of the oxidant (H₂O₂) by the hydroxyl radicals generated. In other words, the H₂O₂ becomes a second contaminant that •OH tend to degrade, causing inefficient consumption of H₂O₂ by •OH radicals, as shown in Equation (2), thus compromising the effectiveness of MB removal [69].

$${\mathrm{H}}_2{\mathrm{O}}_2+•\mathrm{OH}\to {\mathrm{HO}}_2•+{\mathrm{H}}_2{\mathrm{O}}_{}$$

(2)

Although the MB dye removal by only adsorption reached an efficiency of 70%, the combination MnFe₂O₄-G@WA+H₂O₂ + light favored the faster and more efficient decolorization of MB reaching 94%, which indicates that the synergy between these three factors play an important role in the degradation of MB [70].

In addition, the immobilization of a photocatalyst on WA as support showed to be advantageous because it improves the initial adsorption, an important step for obtaining an efficient process. Due to the porous characteristic of the WA and its high surface area, the concentration of pollutant is increased on the MnFe₂O₄-G@WA porous, that enhance the photodegradation reaction [28].

3.2.3. Effect of Photocatalyst Concentration

The photocatalyst concentration resulted in different efficiencies of MB removal (Figure 9). The increase in the photocatalyst concentration from 0.05 g/L to 0.5 g/L resulted in an increase from 82% to 95% of MB removal efficiency. This increase can be attributed to a greater availability of active surfaces for adsorption of MB molecules. This fact can be confirmed by observing the portion of the graph that represents only adsorption (0–40 min), without sunlight irradiation. In addition, a greater number of active sites for the absorption of photons after 40 min (with sunlight irradiation), induce the potential formation of hydroxyl radicals, which allows better decolorization efficiency [32].

It is possible to observe that when using 1 g/L of photocatalyst the peak of decolorization efficiency is reached at 60 min and is followed by a decline. Such behavior can be attributed to the obstruction of the pores on the surface of the material and also to its saturation [71,72,73].

However, the use of 0.25 g/L or 0.5 g/L of photocatalyst dose did not result in a significant increase in the efficiency of final decolorization of MB, as it was 94% and 95%, respectively. This result revealed that it is not viable to use a greater mass of catalyst. Additionally, doubling the photocatalyst dose to 1 g/L, the MB removal efficiency dropped to 74%. This behavior can be justified primarily by the addition of catalyst above the saturation limit, which can lead to the excessive agglomeration of the catalyst particles and to the photon absorption block.

Further, the excess of Fe ions on the catalyst surface can lead to inefficient consumption of the active radicals that should react with the MB molecules, resulting in the reduction in the degradation efficiency [31,32]. Moreover, the excess of catalyst can also contribute to an increase in the turbidity of the solution, thus causing a weakening of the light penetration by scattering, which compromise the incidence of photons on the surface of the catalyst and hinder the induction of photodegradation process [24,74]. In addition, it can be observed that when using 1 g/L dose, a high decolorization was achieved by adsorption (0–40 min), which resulted in the adsorption saturation. The high concentration of MB molecules adhered to the surface of MnFe₂O₄-G@WA acted as a filter preventing the incidence of light irradiation, also compromising the effective photoactivity of the catalyst, resulting in less radical production •OH [27].

3.2.4. Effect of H₂O₂

The effect of H₂O₂ on the decolorization efficiency, is represented in Figure 10.

In general, an increase in H₂O₂ dose leads to an increase in the efficiency of MB removal due to the greater availability of oxidant for the generation of hydroxyl radicals, which act in the degradation of the dye [57]. However, it was observed that the use of 1500 mg/L and 3000 mg/L of H₂O₂ resulted in similar MB removal efficiency when compared with the use of only 300 mg/L of oxidant. This fact was also observed by Lai et al. [64] and Wang [69], who point out that the increase in the oxidant dosage, may result in residual H₂O₂ without active sites available to react. Thus, generating a kind of competition with MB for •OH radicals, due to the side reaction (Equation (1)) previously mentioned. Hence, the use of 5–10 times more reagent is not justifiable, considering that when using a smaller volume, the overall performance is practically the same [74].

3.2.5. Effect of pH

The efficiency of MB decolorization by the photocatalyst MnFe₂O₄-G@WA was not affected by the pH of the medium. In Figure 11, is observed that all pH tested obtained a similar result of efficiency, above 90%, after 120 min.

The high efficiency of MB removal of the nanocomposite at pH 7 and 12 can be attributed to the negative surface charge of MnFe₂O₄-G@WA, as the IEP of the nanocomposite is 4 (Figure 5), and at pH greater than the IEP, the nanocomposite is negatively charged, favoring the adsorption of cationic materials, such as MB [59]. Furthermore, in alkaline media, H₂O₂ and NaOH can reacted and produce O₂ which can react with h⁺ and generate •O₂⁻, potentiating the photocatalytic effect [42].

However, at pH 2 the MnFe₂O₄-G@WA obtained an efficiency similar to that observed in an alkaline and neutral medium. This fact can be attributed to the negative surface charge of graphene in this pH range, as shown in the IEP analysis (Figure 5). According to Anjum et al. [75] and Luciano et al. [31], at very acid pH, occurs a dissolution of the metal ions, and, consequently, the surface charges of graphene predominate, favoring the adsorption of the cationic MB molecules. In addition, with the H₂O₂ a classic Fenton reaction mechanism occurs, that are more efficient around pH 3 [76].

Nevertheless, the decolorization efficiency was maintained at high level in all pH tested, indicating the photocatalyst synthetized can be applied in a wide pH range and without pH adjustment, what is more conducive to practical application [77,78,79].

3.2.6. Mechanism and Kinetic Analysis

According to the results obtained for the photocatalytic efficiency for MB removal in different conditions, different degradation reaction mechanisms were proposed (Figure 12). One of the pathways of MB photodegradation occurs from the formation and of •OH radicals. The mechanism for the generation of hydroxyl radicals happens by the reaction between H₂O₂ and Mn²⁺, the circulation of Mn (II/III) that reacts with H₂O₂ and also by the reaction between Fe²⁺ and the H₂O₂ as the classic Fenton reaction producing more •OH radicals. The H₂O₂ can further reduce Mn³⁺ resulting in superoxide radicals. The photoreduction can also occur from Fe³⁺ to Fe²⁺, which can either be oxidized by Mn³⁺ and generate Mn²⁺ and Fe³⁺, or react with H₂O₂ and by photolysis generate •OH radicals. In addition, this reversible reaction mechanism of MnFe₂O₄ also favors the adsorption of MB molecules, as there are more holes available in the valence band and electrons in the conduction band [20,64,69,80,81]. The oxidative states and electron circulation of both Fe and Mn have already been confirmed by XPS analysis in previous reports using manganese ferrite as photocatalysts [41,82,83]. These possible photocatalytic process pathways are described by the Equations (3)–(7). Thus, the decolorization of MB by MnFe₂O₄-G@WA can be attributed to the photodegradation (photocatalysis and photo-Fenton) increased by adsorption, due to the presence of the oxidizer H₂O₂ and sunlight irradiation [24].

$${\mathrm{Mn}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Mn}}^{3+}+{\mathrm{HO}}^{-}+•\mathrm{OH}$$

(3)

$${\mathrm{Mn}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Mn}}^{2+}+{\mathrm{H}}^{+}+{\mathrm{HO}}_2^{•}$$

(4)

$${\mathrm{Fe}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Fe}}^{3+}+{\mathrm{HO}}^{-}+•\mathrm{OH}$$

(5)

$${\mathrm{Fe}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2+\mathrm{hv}\to {\mathrm{Fe}}^{2+}+{\mathrm{H}}^{+}+{\mathrm{HO}}_2^{•}$$

(6)

$${\mathrm{Fe}}^{2+}+{\mathrm{Mn}}^{3+}\to {\mathrm{Fe}}^{3+}+{\mathrm{Mn}}^{2+}$$

(7)

Throughout this process, G acts as a kind of electron scavenger, as it has excellent electron transport and charge separation properties. Thus, the recombination of electrons pairs (e⁻) and holes (h⁺) becomes more difficult, which potentiates the photodegradation process of the nanocomposite [35,84]. Furthermore, these electrons attracted by G can still react with oxygen and result in superoxide radicals (Equations (8)–(10)), which are also highly oxidative species and can optimize the decolorization process [85,86].

$${\mathrm{MnFe}}_2{\mathrm{O}}_4+\mathrm{hv}\to {\mathrm{MnFe}}_2{\mathrm{O}}_4\left({\mathrm{e}}^{-}+{\mathrm{h}}^{+}\right)$$

(8)

$${\mathrm{MnFe}}_2{\mathrm{O}}_4\left({\mathrm{e}}^{-}+{\mathrm{h}}^{+}\right)+\mathrm{G}\to {\mathrm{MnFe}}_2{\mathrm{O}}_4({\mathrm{h}}^{+})+\mathrm{G}\left({\mathrm{e}}^{-}\right)$$

(9)

$$\mathrm{G}\left({\mathrm{e}}^{-}\right)+{\mathrm{O}}_2\to \mathrm{G}+•{\mathrm{O}}_2^{-2}$$

(10)

Furthermore, WA has the ability to act as a potential adsorbent to initiate the photodegradation process [32]. In addition, the presence of CaCO₃ was found in WA and MnFe₂O₄-G@WA characterization analysis. This compound can potentiate the decolorization process because in aqueous medium it can generate chemical species (HCO₃⁻ and CO₃²⁻) capable of reacting with •OH and producing •CO₃, which are considered oxidants with high potential for degradation of organic compounds (Equations (11)–(14)) [67,68].

$${\mathrm{CaCO}}_3\to {\mathrm{Ca}}^{2+}{\mathrm{CO}}_3^{2-}$$

(11)

$${\mathrm{CO}}_3^{2-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{HCO}}_3^{-}+{\mathrm{OH}}^{-}$$

(12)

$$•\mathrm{OH}+{\mathrm{HCO}}_3^{-}\to •{\mathrm{CO}}_3^{-}+{\mathrm{H}}_2\mathrm{O}$$

(13)

$$•\mathrm{OH}+{\mathrm{CO}}_3^{2-}\to •{\mathrm{CO}}_3^{-}+{\mathrm{OH}}^{-}$$

(14)

The role of the initial adsorption is detached analyzing the reaction kinetics of the decolorization process. To demonstrate the kinetics of heterogeneous photocatalysis processes, usually the data are adjusted to the first order kinetics derived from the Langmuir–Hinshelwood (LH) model, as follows Equation (15) [87]:

$$\ln \left(\frac{{\mathrm{C}}_0}{\mathrm{C}}\right)=\mathrm{kt}$$

(15)

where C₀ (mg/L) is the initial concentration of MB in t = 0, C (mg/L) is the dye concentration of time t (minute) and k is the pseudo-first order rate constant. The constant can be obtained from a plot of ln(C₀/C) versus irradiation time, as shown in Figure 13.

The results suggest that data were well adjusted to this kinetic model, because of the high coefficient of determination (R²= 0.9815) and constant k of 0.0341 (min⁻¹). These data indicate the photocatalysis process followed a kinetics of pseudo-first order, which indicate that the oxidizing species and the molecules of MB are adsorbed to the surface of the nanocomposite before the oxidation process, as proposed by the LH model [88,89] and is also in accordance with the N₂ adsorption results (Figure 6) that indicated the presence of micropores.

Table 1. Degradation efficiency of different dyes of hybrid photocatalysts with manganese ferrite.

Photocatalysis	Pollutant	Light Source	Pollutant Concentration (ppm)	Catalyst Concentration (g/L)	Reaction Time (min)	Decolorization (%)	Reference
g-C3N4/TiO2-MnFe2O4 halloysite	Crystal violet	Xenon lamp	10	0.05	90	91	[83]
MnFe2O4@SnS2	MB	Xenon lamp	-	0.05	120	93	[41]
MnFe2O4-rGO	MB	Xenon lamp	-	0.60	150	62	[42]
MnFe2O4-G@WA	MB	Sunlight	10	0.25	120	94	This work

shows the degradation efficiency of different dyes of hybrid photocatalysts with manganese ferrite. Firstly, the three mentioned studies rely on the use of an artificial lamp to investigate the degradation of their respective dyes, which makes the system more expensive and complex, taking into account that they presented lower efficiency than that achieved in this study. Wei et al. [42] studied a similar photocatalyst used in this study, but without the support of WA, however, they reached an efficiency of only 62%. These results are another indication that the association of MnFe₂O₄ with G and WA is advantageous for the synergistic photo-Fenton photocatalysis process for MB dye decolorization.

3.2.7. Stability and Reusability

The stability and reusability of a photocatalyst are important factors when trying to evaluate the possibility of using it in practical applications [90]. Thus, MnFe₂O₄-G@WA was tested in five consecutive cycles under the optimal conditions previously found and are showed in Figure 14.

The maximum efficiency reached in the first cycle was 94%, and after five consecutive cycles, MnFe₂O₄-G@WA presented a MB removal efficiency of 91%. The negligible reduction in the efficiency demonstrated that MnFe₂O₄-G@WA is stable and that it can be recycled preserving its high catalytic activities even after five cycles, being effective in degrading MB [13,91].

4. Conclusions

The synthesis of the nanophotocatalyst MnFe₂O₄-G@WA by the solvothermal method was successful, as the images of the morphological analyzes showed the MnFe₂O₄ nanoparticles, with clusters of average size of 200 nm, adhered to the exfoliated graphene nanosheets and well distributed on the WA. The photocatalytic efficiency reached a maximum efficiency of 94% in MB decolorization after 120 min under optimal conditions of 0.25 g/L of catalyst, 300 mg/L concentration of H₂O₂ and solar irradiation. The pH did not show significant effect on MB removal efficiency, indicating the photocatalyst synthetized can be applied in a wide pH range. The photocatalyst MnFe₂O₄-G@WA was more efficient when compared with MnFe₂O₄-G, demonstrating that the use of WA act as an excellent support material and enhances the adsorption capacity of the photocatalyst. The decolorization of MB by MnFe₂O₄-G@WA can be attributed to the photodegradation (photocatalysis and photo-Fenton) increased by adsorption, due to the presence of the oxidizer H₂O₂ and sunlight irradiation, with the synergic combined action of the three materials used. MnFe₂O₄ with its narrow band gap makes the decolorization process faster due to its good response to visible light and electron circulation capacity, graphene acts as an electron scavenger preventing the rapid recombination of photoexcited of electron-hole pairs. which potentiates the formation of •OH, and the ash, in addition to being an excellent adsorbent, also has CaCO₃ in its composition, a compound that collaborates with photodegradation due to the formation of carbonate radicals (•CO₃⁻). Therefore, for future research, it is suggested the investigation of the efficiency of the novel material on a pilot and/or industrial scale, as well as its use in the treatment of real effluents.