1 Introduction

Nowadays, environmental problems due to water pollution have recently drawn much attention from researchers. One of the main pollution sources comes from wastewater containing dyes discharged from textiles, foodstuffs, and leather industries [1]. The presence of colored organic compounds in dye-bearing effluents generally reduces sunlight transmission, affecting photosynthesis and harming aquatic ecosystems [2,3,4,5]. Besides, dyes are complex structures with high molecular weight, which are soluble in water, degradation-resistant, potentially carcinogenic, and mutagenic. Thus, the development of easy, cheap, and green methods for water pollution treatment has been a priority area in the field of environmental sciences [6, 7].

Methylene blue (MB) is an aromatic heterocyclic basic dye. MB is a well-known cationic and primary thiazine dye, having λmax of 664 nm absorbance. It is highly water-soluble and very stable at room temperature. It is a persistent pollution that has many potential applications in the textile, pharmaceutical, paper, dyeing, printing, paint, medicine, and food industries [8]. It is the most common dye in the textile industry. Advanced oxidation processes (AOPs) were developed to treat toxic organic pollutants, such as MB, through strong redox processes with specific radicals generated in this process without generating any additional harmful substances. AOPs approaches employed for the photodegradation of MB are ozonation, ultraviolet/H2O2 oxidation, electrochemical oxidation/degradation, catalytic oxidation, Fenton reaction, photocatalytic degradation, etc. It is desirable to have more than one mechanism that eliminates the presence of methylene blue. Besides, efficiency of the heterogeneous photocatalyst process depends on variables such as irradiation time, light source, dye concentration, pH, oxidant compounds and radical scavengers [9, 10].

Photocatalysis has emerged as a promising technology way to solve pollution problems. Using photocatalytic processes to remove dyes from such effluents might result in decolorization and complete degradation [11]. Semiconductors, due to large band gap and properties, have been used as photocatalytic material with UV/Visible light and, potentially, the practical application of natural solar light [12]. For example, in 2021 Al-Jemeli et al. reported degradation of anti-inflammatory drugs by solar photocatalysis process [13]. Recently, several authors reported photocatalysis as a possible remediation method for the elimination of organic pollution in water and studied different semiconductor hybrid materials for this application, for example in 2023 Bassim et al. reported a green synthesis of CuO/TiO2 nanoparticles were obtained using a natural extract for degrading methylene blue with photocatalysis under ultraviolet light irradiation [14]. In 2023, ** for the sample of MFe100 was obtained, and results are presented in Fig. 3. Al, Si, O, Na and Fe presence is revealed, with Fe atoms well distributed on the mordenite support. This homogeneous distribution of Fe atoms is very convenient since they are used as the catalytic active center for the photodegradation of contaminants.

Fig. 3
figure 3

SEM–EDS elemental map** of MFe100 sample composition where the first image is the secondary electron image and analogous elemental map** of the element in the other images

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4.3 UV–Vis optical diffuse reflectance

Optical properties of materials are related to the electronic structure. The calculated results of diffuse reflectance and band gap energy of studied samples are presented in Fig. 4. As shown in Fig. 4a, M sample has high diffuse reflectance for wavelengths > 400 nm, corresponding to visible light. However, mordenite samples with iron oxides present low diffuse reflectance for wavelengths > 400 nm, corresponding to important visible light absorption, from green to violet. Low optical diffuse reflectance for visible light of Fe3O4–Fe2O3 supported on mordenite samples can be used for photodegradation reaction of contaminants excited by visible light. Direct optical band gap energy (Eg) was estimated for M, MFe, MFe100, MFe200 and MFe300 samples using the equation proposed by Kubelka and Munk in 1931:

$$\frac{K}{S}=\frac{(1-R)}{2R}=F(R)$$

where S, R, K and F are the scattering, reflectance, absorption coefficients, and Kubelka–Munk function, respectively. Eg and the absorption coefficient are related through the Tauc relation [48]:

$${(\alpha h\nu )}^{n}=A(h\nu -{E}_{g})$$

where α is the linear absorption coefficient, \(\nu\) is light frequency, h is the Planck constant, and A is the proportionality constant. The power of the parenthesis, n, is taken equal to 2 for direct band gap materials. When incident radiation scatters in a perfectly diffuse manner, the absorption coefficient K becomes equal to 2α. In this case, considering constant the scattering coefficient S, concerning wavelength, the Kubelka–Munk function is proportional to the absorption coefficient α. Applying the last equation, we obtain the relation [49]:

Fig. 4
figure 4

a UV–Vis diffuse reflectance spectra and b Band gap estimate by Kubelka–Munk method of, pristine mordenite and Fe3O4–Fe2O3 supported on mordenite samples

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$${\left[F\left(R\right)h\nu \right]}^{2}=A(h\nu -{E}_{g})$$

The \({\left[F\left(R\right)h\nu \right]}^{2}\) vs. \(h\nu\) (photon energy) graph is plotted, and the energy band gap of the powder sample can be easily extracted.

The value of Eg for studied samples was obtained by plotting \({\left[F\left(R\right)h\nu \right]}^{2}\) as function of \(h\nu\) (Fig. 4b) and extrapolation of the linear portion of the curve. The obtained results are presented in Fig. 4b. Estimated band gap values for pristine mordenite and Fe3O4–Fe2O3/mordenite samples with different thermal treatments in air atmosphere are ~ 3.4 eV and ~ 2.25 eV, respectively. These results agree with reports of other authors using iron oxide nanoparticles with different organic modifiers [40] or using iron oxide nanoparticles [39, 50]. Moreover, the band gap energy value estimated for the Fe3O4–Fe2O3 supported on mordenite samples shows the possibility of promoting the generation of free charge carriers by using visible light, improving the photodegradation process.

4.4 X-ray photoelectron spectroscopy

Survey XPS was obtained to confirm the chemical composition of Fe3O4–Fe2O3 supported on mordenite samples. Figure 5a shows six signal peaks located at 1072 eV, 103 eV, 532 eV, 75 eV, 285 eV and 712 eV, corresponding to Na 1s, Si 2p, O 1s, Al 2p, C 1s, and Fe 2p3/2, respectively. Also, high-resolution spectra of the Fe 2p region were obtained for MFe, MFe100, MFe200 and MFe300 samples. They were obtained with the purpose of detecting changes in oxidation, according to the chemical equation:

Fig. 5
figure 5

a XPS full Survey spectra of Fe3O4–Fe2O3 supported on mordenite samples and deconvolution of high-resolution XPS spectra of Fe 2p b MFe, c MFe100, d MFe200 and e MFe300 samples

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$${Fe}_{3}{O}_{4}+ {O}_{2}\to {Fe}_{2}{O}_{3}\cdot {Fe}_{3}{O}_{4}$$

Figures 5b–e show high-resolution spectra and deconvolution into six peaks of MFe, MFe100, MF200 and MFe300 samples. Fe 2p high resolution spectrum is composed of two spectral bands located at 725.3 eV, and 711.9 eV, corresponding to 2p1/2, and 2p3/2 of Fe3+ species, respectively. Also, the other two peaks at a binding energy of 723.8 eV and 710.6 eV are attributed to 2p1/2 and 2p3/2 of the Fe2+ species, respectively. The remaining two weak peaks at 719 eV and 733 eV are satellite peaks. These results and assignments agree with reports of other authors for Fe3O4 and Fe2O3 samples, indicating the successful formation of iron oxide compounds in the mordenite matrix [51,52,53]. Table 2 presents oxidation states of deconvolution estimates of species present in studied samples. They were calculated through the integral of deconvoluted signals in individual XPS peaks. Fe3+ peak signal increases with temperature treatment, from 46 At% to 55 At%, which could be related to an oxidation process. At the same time, the Fe2+ peak signal decreases with temperature treatment, from 54 At% to 45 At%. Then, because of thermal treatment, Fe2+ in Fe3O4 partially becomes Fe3+ in Fe2O3 in studied samples, allowing to obtain Fe2O3 and Fe3O4 in different relative concentrations in the same sample through a simple oxidative thermal process.

Table 2 Atomic percent oxidation states of iron oxide species in Fe3O4–Fe2O3 supported on mordenite samples as a function of temperature in oxidative thermal treatment
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4.5 Surface area analysis

Figure 6 shows results of N2 adsorption–desorption isotherms of (a) MFe, (b) MFe100, (c) MFe200, (d) MFe300 samples. They exhibit low adsorption at low relative pressure and hysteresis characteristic of interparticle mesopores of the aggregates present in zeolites [54,55,56]. Table 3 shows textural parameters of the synthesized catalysts. Adsorbed amount of nitrogen, represented by SBET, slowly decreases with thermal treatment temperature, which could be due to sintering and/or dealumination. Volume and diameter of pore show little change within the uncertainty of measurement. A similar effect was observed in previous work with hydrotreated Cu-Ag/mordenite catalysts for NO reduction [57]. The textural properties are related to organic compounds adsorption application and its possible elimination for the remediation process, due to the surface area of the catalyst interacting with organic dye pollution via electrostatic forces. Moreover, the pore of the catalyst is related to the active catalytic centers where dye will be degraded by the photocatalysis and fenton process [58, 59].

Fig. 6
figure 6

Pore size distribution Adsorption–desorption profile of a MFe, b MFe100, c MFe200, d MFe300 samples

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Table 3 Textural parameters of Fe3O4–Fe2O3 supported on mordenite samples
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4.6 Photocatalytic MB degradation

Photocatalytic methylene blue (MB) degradation by the catalysts synthesized was evaluated with visible light excitation. Figure 7 presents the results of UV–visible absorption spectra after photodegradation of MB with visible light for different times of (a) M, (b) MFe, (c) MFe100, (d) MFe200 and (e) MFe300; and (f) MFe100 together with H2O2. Studied catalysts with iron oxides present different decrements in absorbance as function of time, indicating MB degradation with visible light exposition. Absorbance curves show an important contribution from the adsorption effects of MB in the dark (black line, − 30 min) before visible light irradiation exposure on Fe3O4–Fe2O3 supported on mordenite which is the typical behavior in synthetic zeolites with a high Si/Al ratio and could be due to organic residues adsorbed. Figure 8a exhibits results of relative photocatalytic efficiency, represented by C/Co as a function of time of exposure, and Fig. 8b shows kinetics of photocatalytic degradation of MB with visible light of studied catalysts, pristine mordenite and Fe3O4–Fe2O3 supported on mordenite samples. MFe100 shows the best photocatalytic activity with almost 70% MB degradation after 120 min, without adding a co-catalyst (Fig. 8a). The Fenton effect was proved by adding 2.5 ml of H2O2 to MFe100 sample during the MB photocatalytic degradation [60, 61]. Figures 8a and b show how adding H2O2 improves photocatalytic efficiency and kinetics of photocatalytic degradation of MB with visible light, reaching ~ 90% MB degradation after 120 min. On the other hand, M and MFe300 samples do not present considerable photocatalytic activity. Therefore, increasing the temperature of the thermal treatment slightly favored more oxidation in the samples, according to XPS results, increasing Fe2O3 concentration in comparison with Fe3O4, also modification of the surface area, crystallinity and morphology, which could be related with properties for photocatalytic applications.

Fig. 7
figure 7

UV–visible absorption spectra of photodegradation of MB with visible light excitation for 120 min with previous agitation in dark condition (− 30 min to 0 min) of catalysts of a M, b MFe, c MFe100, d MFe200, e MFe300 and f MFe100 together with H2O2

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Fig. 8
figure 8

a The relative photocatalytic efficiency (C/Co) as function of reaction time (t) and b kinetics of photocatalytic degradation of MB with visible light of mordenite pristine and Fe3O4–Fe2O3 supported on mordenite as a function of reaction time (t) in logarithmic scale [−ln(C/Co)], and matched straight lines to first-order reaction kinetics

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Curves of relative concentration as a function of time based on the first order kinetic reaction equation [62, 63] was matched to study the kinetics of MB photodegradation, according to the equation:

$$-ln\frac{C}{{C}_{0}}=kt$$

where C0 and C are respective initial and real-time MB concentrations, and k is the first-order degradation rate constant with visible light. Matched lines are shown in Fig. 8b, and obtained kinetic constants are described in Table 4. Rate constant increases from 0.0043 to 0.0069 min−1 from MFe samples to MFe100 corresponding to ~ 60% increase. Also, rate constant increases about four times from 0.0043 to 0.016 min−1 after MFe100 and a small portion of hydrogen peroxide addition on the Fe3O4–Fe2O3 supported on mordenite catalysts. Considering the rate constant reaction of MB degradation of this catalyst, an extrapolation indicates it could eliminate 99%, approximately, after 180 min of visible irradiation.

Table 4 Kinetic constants of MB photodegradation of Fe3O4–Fe2O3 supported on mordenite samples
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Stability of the catalyst was investigated by monitoring the catalytic activity during successive cycles of degradation and results are shown in Fig. 9a, where C0 and C are the initial and real-time MB concentrations, respectively. MFe100 exhibits a very stable photocatalytic performance after five cycles of test without significant loss of activity. Additionally, Fig. 9b shows one of the synthesized catalysts with iron oxides before and after bringing closer a magnet, indicating clearly that thanks to the magnetic properties of the synthesized catalysts it is possible to recover the catalyst through a magnetic field, allowing its reusability in applications of wastewater photodegradation.

Fig. 9
figure 9

a Photocatalytic stability test to five cycles of MFe100 and b magnetic retrieval with a permanent magnet of MFe100 sample

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4.7 Mechanism of MB degradation

A possible main mechanism for degradation of MB in Fe3O4–Fe2O3/mordenite + H2O2 + Visible light system proposed is: (1) MB is adsorbed in the mordenite support pores through electrostatic interaction, (2) HO• radicals produced by H2O2 activated with photo oxide of Fe2+ and HOO• radicals produced by H2O2 activated with photoreduced of Fe3+ on the surface of Fe3O4–Fe2O3 supported on mordenite samples; and simultaneous process (3) photocatalysis with visible light photons to photogenerated electron–hole pairs that reduces and oxidizes, respectively, O2 and H2O present, generated reactive species OO• and HO•, (4) adsorbed MB in Fe3O4–Fe2O3 supported on mordenite sample is attacked by HO•, OO•, and HOO•, produced by Fenton reaction and photocatalyst. Active centers of MB around the catalyst are provided by the mordenite support with a large surface area. Incorporation of Fe3O4–Fe2O3 particles on mordenite seems to greatly enhance the transformation from Fe3+ to Fe2+ or the recycling of iron species [64, 65]. This possible photocatalytic mechanism of Fe3O4–Fe2O3 supported on mordenite for MB degradation is depicted in Fig. 10.

Fig. 10
figure 10

Catalytic oxidation mechanism of MB in Fe3O4–Fe2O3 supported on mordenite + H2O2 + Visible light system

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5 Conclusions

A set of Fe3O4–Fe2O3 supported on mordenite catalysts with different thermal treatments was successfully synthesized by using a simple chemical method and its physicochemical properties were confirmed by DRX, SEM, EDS, XPS, SBET, UV–Vis DR. Crystalline structure and morphology of studied samples are related to thermal treatment. Due to the presence of iron oxides, catalyst samples synthesized with iron oxides have more visible light absorption than pristine mordenite. Samples have a significant temperature dependence with crystallinity, particle agglomeration, surface area, and other textural properties. Sample MFe100 presented the best photodegradation capabilities with visible light excitation. This behavior could be attributed to methylene blue degradation by three processes: adsorption of organic residues in the mordenite matrix support due to electrostatic interactions, photocatalysis heterogeneous reaction with visible light and Fenton reaction catalyst with a small portion of H2O2 by presence of Fe3O4–Fe2O3. Due to the magnetic properties of the Fe3O4–Fe2O3 supported on mordenite samples the catalyst could be retrieved and reused in the photodegradation process. Obtained catalyst MFe100 was able to degrade MB ~ 90% after 120 min. The catalysts synthesized with the presence of Fe3O4–Fe2O3 using visible light for MB degradation were prepared following simple and economical thermal treatments without changing pH, temperature, dose or other conditions. Additionally, catalysts can be retrieved and reused at least five times by using a magnetic field. These catalysts could be proposed for the remediation of wastewater using visible light or solar excitation related to textile, food and pharmaceutical industries.