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Article

Application of Iron Oxides in the Photocatalytic Degradation of Real Effluent from Aluminum Anodizing Industries

by
Lara K. Ribeiro
1,
Matheus G. Guardiano
1,
Lucia H. Mascaro
1,
Monica Calatayud
2 and
Amanda F. Gouveia
2,*
1
Interdisciplinary Laboratory of Electrochemistry and Ceramics, Federal University of São Carlos, São Carlos 13565-905, SP, Brazil
2
Sorbonne Université, CNRS, MONARIS, CNRS-UMR 8233, 4 Place Jussieu, F-75005 Paris, France
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(15), 8594; https://doi.org/10.3390/app15158594
Submission received: 30 June 2025 / Revised: 25 July 2025 / Accepted: 30 July 2025 / Published: 2 August 2025
(This article belongs to the Special Issue Application of Nanomaterials in the Field of Photocatalysis)

Abstract

This study reports the synthesis and evaluation of iron molybdate (Fe2(MoO4)3) and iron tungstate (FeWO4) as photocatalysts for the degradation of a real industrial effluent from aluminum anodizing processes under visible light irradiation. The oxides were synthesized via a co-precipitation method in an aqueous medium, followed by microwave-assisted hydrothermal treatment. Structural and morphological characterizations were performed using X-ray diffraction, field-emission scanning electron microscopy, Raman spectroscopy, ultraviolet–visible (UV–vis), and photoluminescence (PL) spectroscopies. The effluent was characterized by means of ionic chromatography, total organic carbon (TOC) analysis, physicochemical parameters (pH and conductivity), and UV–vis spectroscopy. Both materials exhibited well-crystallized structures with distinct morphologies: Fe2(MoO4)3 presented well-defined exposed (001) and (110) surfaces, while FeWO4 showed a highly porous, fluffy texture with irregularly shaped particles. In addition to morphology, both materials exhibited narrow bandgaps—2.11 eV for Fe2(MoO4)3 and 2.03 eV for FeWO4. PL analysis revealed deep defects in Fe2(MoO4)3 and shallow defects in FeWO4, which can influence the generation and lifetime of reactive oxygen species. These combined structural, electronic, and morphological features significantly affected their photocatalytic performance. TOC measurements revealed degradation efficiencies of 32.2% for Fe2(MoO4)3 and 45.3% for FeWO4 after 120 min of irradiation. The results highlight the critical role of morphology, optical properties, and defect structures in governing photocatalytic activity and reinforce the potential of these simple iron-based oxides for real wastewater treatment applications.

Graphical Abstract

1. Introduction

Environmental issues have been receiving increasing attention in recent years, particularly those related to water pollution, which remains a major global concern, as clean water is essential for sustaining life. Among the most common pollutants found in aquatic environments are dyes from textile industries [1], pharmaceutical residues excreted by the human body [2], and microplastics originating from plastics [3], among others. To address these challenges, semiconductor materials have been increasingly explored for their potential as photocatalysts capable of degrading pollutants under light irradiation.
However, water treatment becomes significantly more challenging when addressing real industrial effluents. These wastewaters are typically complex and variable in composition, often containing mixtures of dyes, chemical additives, organic compounds, and heavy metals, depending on the industrial source. As a result, many photocatalytic studies have focused on model pollutants such as methylene blue (MB), tetracycline, and rhodamine B (RhB). In contrast, studies employing real wastewater as the target pollutant remain relatively scarce, despite their crucial importance for practical and scalable applications.
Recent reviews have highlighted the catalytic potential of iron-based materials in environmental remediation processes. For instance, nanoscale zero-valent iron (nZVI) systems have been successfully applied in advanced oxidation processes (AOPs) for the degradation of organic contaminants, demonstrating both reactivity and environmental compatibility in real wastewater conditions [4]. Similarly, magnetite-based catalysts (Fe3O4) have attracted attention for their excellent catalytic activity, magnetic recyclability, and low toxicity, reinforcing the viability of iron-based oxides in sustainable catalytic systems [5]. While these systems often rely on composite formation or magnetic recovery, this study focuses on the potential of simpler iron-based semiconductors—iron molybdate (Fe2(MoO4)3) and iron tungstate (FeWO4)—as visible light-driven photocatalysts. These materials combine structural stability, natural abundance, and environmental safety, making them promising candidates for real-world wastewater treatment without the need for doping or heterojunction engineering. Table 1 summarizes previously reported degradation efficiencies of Fe2(MoO4)3 and FeWO4 photocatalysts against standard reference pollutants, including dyes and tetracycline. These contaminants were selected because dyes are commonly used as model molecules to evaluate photocatalytic performance, while tetracycline is a widely prescribed antibiotic considered an emerging pollutant and frequently studied in the context of iron-based photocatalysis.
The efficiency of the process generally decreases when using tetracycline as the contaminant due to its low interaction with light compared to dyes and the structural complexity of tetracycline—a polycyclic antibiotic containing four fused rings, multiple functional groups (e.g., hydroxyl, amide, ketone), and chiral centers [17]. However, these materials have typically been evaluated under ideal and controlled laboratory conditions. In real-world systems, parameters such as temperature, pH, pressure, ionic composition, light penetration, and humidity are highly variable. This variability makes the performance of these catalysts more difficult to predict and reproduce, especially considering that dyes are frequently reported in industrial effluents, while antibiotics are commonly found in wastewater treatment plant effluents [18,19].
For example, when comparing the activity of Fe2(MoO4)3 and FeWO4 for tetracycline degradation [7,12], it becomes evident that the choice of catalyst plays a crucial role in process efficiency, as the target contaminant was the same. Using Fe2(MoO4)3, a removal efficiency of 20% was achieved after 120 min under Xe lamp irradiation, while FeWO4 reached 68% removal after 160 min using visible light (Table 1). These results highlight the importance of directly comparing the photocatalytic efficiencies of the materials to better understand their intrinsic activity—which was the main objective of this study.
These results were obtained under ideal laboratory conditions, which do not fully replicate the complexity of real-world systems, such as actual industrial effluent. In practical environments, the presence of multiple pollutants, fluctuating pH and temperature, and competing ions can significantly affect photocatalytic efficiency. Therefore, while these studies demonstrate the high potential of iron-based oxides under controlled conditions, it remains essential to evaluate their performance in real scenarios to determine their true applicability and robustness.
In this study, Fe2(MoO4)3 and FeWO4 samples were synthesized via a simple co-precipitation method in an aqueous medium, followed by microwave-assisted hydrothermal treatment. Their photocatalytic activity was evaluated through the degradation of a real industrial effluent obtained from an aluminum anodizing facility under visible-light irradiation. Comprehensive characterization was performed using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), Raman spectroscopy, and ultraviolet–visible (UV–vis) spectroscopy. Additionally, photoluminescence (PL) spectroscopy was employed to assess defect states, and total organic carbon (TOC) analysis was conducted to evaluate the degree of mineralization of the treated effluent.

2. Materials and Methods

2.1. Synthesis

The Fe2(MoO4)3 and FeWO4 semiconductors were obtained by using the co-precipitation method in an aqueous medium, followed by irradiation in a microwave hydrothermal (MH) process. The synthesis was carried out following the steps below: separate solutions were made of Na2MoO4•2H2O/Na2WO4•2H2O and of the soluble salts of Fe (FeCl2•4H2O) with molar ratios of 1:1 (1 × 10−3 mol L−1). The two solutions were heated to a temperature of 70 °C, and then the Fe metal solution was added to the Na2MoO4•2H2O/Na2WO4•2H2O solution under constant agitation. After the addition, the suspension remained at this temperature for 20 min. The system was then transferred to a Teflon reactor, sealed, and placed in the MH system (operating at 2.45 GHz and 800 W). The reaction mixtures were heated to 160 °C for 32 min and naturally cooled to room temperature. The precipitates were washed with deionized water. Next, the samples were centrifuged, and this process was repeated until reaching pH neutrality (≅7). Finally, the materials were dried at 60 °C overnight. The resulting powders were then ground using a mortar and a pestle to homogenize the particle size and subsequently stored in Eppendorf tubes.

2.2. Characterization

The samples were characterized by XRD using a D/Max-2500PC diffractometer (Rigaku, Japan) with Cu Kα radiation (λ = 0.154184 nm), angle of diffraction 2θ ranging from 10° to 110°, and a scanning step of 0.02°/min. The morphology of the samples was observed with FE-SEM, which operated at 5 kV (Supra 35-VP, Carl Zeiss, Germany). Raman measurements were performed by means of an iHR550 spectrometer (Horiba Jobin-Yvon, Japan) coupled to a silicon CCD detector and an argon-ion laser (Melles Griot, USA) operated at 514.5 nm with maximum power of 200 mW. UV–vis spectroscopy using a Varian Cary spectrometer model 5G (USA) in the diffuse reflectance mode, with a wavelength range of 200 to 800 nm and a scan speed of 600 nm min−1, was employed to investigate the optical properties of the samples. The optical band gap energy (Egap) was calculated using the Kubelka and Munk-Aussig method [20] from UV–vis diffuse reflectance measurements.

2.3. Photocatalytic Assays

The photocatalytic activity was evaluated through the degradation of a real effluent under visible light. The effluent was sourced from an aluminum anodizing company based in São Carlos, Brazil, specializing in industrial applications.
In a typical process, 50 mg of the synthesized materials were dispersed in 100 mL of the residue and subjected to ultrasonication for 10 min in an ultrasonic bath (42 kHz, model 1510). The mixture was then transferred to a 100 mL glass bottle and stirred in the dark for 30 min to ensure homogeneous dispersion of the catalyst and allow adsorption processes to occur. Subsequently, the suspensions were irradiated with six visible-light lamps (PHILIPS TL-D, São Paulo, Brazil, 15 W) positioned 10 cm from the reactor under vigorous stirring of ~600 rpm throughout the photocatalytic process, while the temperature was maintained at 20 °C using a thermostatic bath. The estimated irradiance at the sample surface was approximately 12–24 mW cm−2, and the photon flux was approximately 4.35 × 10−4 mol s−1, assuming emission centered in the visible spectrum (around 550 nm). These values were used for comparison and reproducibility purposes. At predetermined intervals (0, 5, 10, 15, 20, 30, 60, 90, and 120 min), 1 mL aliquots were withdrawn from the photocatalytic system and transferred to plastic tubes. The suspensions were then centrifuged at 6000 rpm for 5 min to completely remove the catalyst particles. The resulting solutions were analyzed using UV–vis absorption spectroscopy with a V-660 spectrophotometer (JASCO, Japan) to monitor changes in the absorption band of the residue, which exhibited a maximum at λ = 616 nm in all photocatalytic tests. The percentage of degradation was estimated using the relationship shown in Equation (1):
% D e g r a d a t i o n = C o     C f C o × 100
where C o and C f are the initial and final concentrations, respectively. A photolysis control experiment, conducted without a photocatalyst, was also carried out under identical conditions. In the following, the total organic carbon (TOC) concentration was analyzed to investigate the extent of mineralization using a TOC analyzer (TOC-L CSH/CSN, Shimadzu, Kyoto, Japan). The effluent composition was monitored using a 930 Compact IC Flex system (Metrohm, Switzerland) equipped with an 863 Compact Autosampler (Switzerland), a CO2 suppression system, and an IC Conductivity Detector. Anion separation and quantification were performed on a Metrosep A Supp 5 column (150 mm × 4.0 mm) with a Metrosep A Supp 5 Guard/4.0 column, and cation separation and quantification were performed on a Metrosep C4 column (150 mm × 4.0 mm) with a Metrosep C4 Guard/4.0 column, both at 25 °C. Anion analysis was performed at a flow rate of 0.7 mL min−1 with an isocratic elution of 3.2 mmol L−1 of Na2CO3 with 1.0 mmol L−1 of a NaHCO3 solution prepared in ultrapure water. For cations, an isocratic elution of 100% 1.7 mmol L−1 HNO3 with 0.7 mmol L−1 dipicolinic acid at a flow rate of 1.0 mL min−1 was employed.

3. Results and Discussion

3.1. Structural and Morphological Analysis

The structural analysis of the samples was carried out using XRD and Raman spectroscopy, as shown in Figure 1. The XRD patterns were indexed to the crystalline phases of Fe2(MoO4)3 and FeWO4. Both samples correspond to monoclinic structures, with no secondary phases detected, according to PDF cards 100606 (Fe2(MoO4)3) and 26843 (FeWO4). The diffraction peaks for both materials were sharp and well-defined, indicating a high degree of crystallinity in the synthesized samples.
Fe2(MoO4)3 crystallizes in the monoclinic P21/a space group and is composed of [FeO6] octahedra (green clusters in Figure 1A) and [MoO4] tetrahedra (black clusters in Figure 1A). In contrast, FeWO4 crystallizes in the P2/c space group and contains [FeO6] and [WO6] (pink-colored clusters in Figure 1A) clusters.
The diffraction peaks corresponding to the (011) and (110) planes are commonly used to assess preferential crystal growth, as the enhanced intensity of a specific plane often indicates oriented crystallization and higher crystallinity. In the case of FeWO4, the XRD pattern shows that the intensities of the (011) and (110) reflections are comparable, suggesting the absence of preferential growth along a specific crystallographic direction [21]. These structural differences may influence the photocatalytic behavior of the materials, as the arrangement and type of polyhedral units affect charge separation and mobility.
As shown in Figure 1B, the Raman spectrum of Fe2(MoO4)3 crystals revealed four distinct spectral regions. These correspond to the symmetric stretching modes of the distorted Mo = O bonds in the [MoO4] clusters (1100–900 cm−1), the antisymmetric stretching modes of the [MoO4] clusters (900–700 cm−1), the bending modes of the [MoO4] clusters (500–300 cm−1), and the lattice modes (below 300 cm−1) [14,22].
In the Raman spectrum of FeWO4 crystals (Figure 1C), five characteristic spectral regions were identified: the symmetric Ag modes of terminal WO2 groups (1100–900 cm−1), the antisymmetric stretching modes of [WO6] clusters (900–750 cm−1), the antisymmetric bridging vibrations related to tungstate chains (750–650 cm−1), the bending modes of [WO6] clusters (500–200 cm−1), and the lattice modes (below 200 cm−1) [23,24]. Notably, the vibrational modes associated with [WO6] units exhibit reduced intensity, which is consistent with the lower degree of crystallinity observed in the FeWO4 sample, as also indicated by the XRD results.
The optical properties of the synthesized materials were investigated using UV–vis diffuse reflectance spectroscopy. The Egap values were estimated using the Kubelka–Munk function, followed by extrapolation of the linear portion of the Tauc plot to the energy axis, as shown in Figure 2A,B. The calculated Egap values were approximately 2.14 eV for Fe2(MoO4)3 and 2.00 eV for FeWO4, in good agreement with those reported in the literature [25,26,27,28]. These results confirm that both materials are active under visible light irradiation, with FeWO4 exhibiting a slightly narrower band gap, which may contribute to its higher photocatalytic performance. A lower band gap favors more efficient absorption of visible light and may enhance charge carrier generation.
To complement the UV–vis results, PL spectra were recorded to investigate the presence of defects in the Fe2(MoO4)3 and FeWO4 samples. Measurements were performed at room temperature over a wavelength range of 350–600 nm, as shown in Figure 2C,D. Both samples exhibited broad emission bands in the visible region. For Fe2(MoO4)3, the main emission band is centered between 450 and 500 nm, while for FeWO4, it appears between 420 and 470 nm.
According to the literature, these broad emissions originate from the recombination of charge carriers trapped at defect sites [29,30]. The electronic characteristics and charge migration behavior were further analyzed by deconvoluting the emission bands using Gaussian fitting, revealing two peaks for Fe2(MoO4)3 and three for FeWO4. Emissions in the red and yellow regions are typically associated with deeper defects, such as oxygen vacancies, whereas emissions in the blue region are linked to shallow defects caused by structural distortions in metal–oxygen bonds. Table 2 summarizes the main PL peaks, their relative intensities, and the corresponding defect types, offering insights into the recombination dynamics and defect structure of the material.
The Fe2(MoO4)3 crystals are composed of interconnected [MoO4]–[FeO6]–[MoO4] clusters, as illustrated in Figure 1A, suggesting that [MoO4] clusters play a key role in the emission behavior. The PL results for Fe2(MoO4)3 are consistent with those reported by Zhu et al. [31], who observed a maximum emission at 435 nm. Deconvolution of the PL spectrum revealed two main components: one at 435 nm (68%), corresponding to a blue emission, and another at 489 nm (32%), associated with blue–green emission. The lower-energy 489 nm component suggests radiative recombination via deep-level defect states, such as oxygen vacancies or structural distortions, which act as trap sites for charge carriers. These results indicate that deep defects significantly contribute to the overall emission behavior of the material.
As a result, the recombination process in Fe2(MoO4)3 is slower, as electrons and holes are first captured by these deep-level states before recombining. A schematic illustration is provided in the inset of Figure 2C, depicting the excitation of electrons (e) to the conduction band (CB), followed by recombination through deep defect states before returning to the valence band (VB) via holes (h+).
In contrast, FeWO4 exhibits predominantly shallow-level defects, with deconvoluted PL components centered at 473 nm (30%), 441 nm (56%), and 421 nm (14%). According to the XRD pattern shown in Figure 1A, the FeWO4 sample displays lower crystallinity, which correlates with the higher PL intensity observed at 441 nm. Typically, lower PL intensity is associated with reduced recombination of photoinduced charge carriers (e/h+ pairs), indicating more effective charge separation due to the compact interaction between distinct structural units.
FeWO4 consists of interconnected [WO6]–[FeO6]–[WO6] clusters, as illustrated in Figure 1A, suggesting that the [WO6] units play a key role in the PL behavior. The two high-energy (blue) emission components are attributed to the radiative recombination of self-trapped electrons localized on [WO6] clusters, while the lower-energy (blue–green) emission likely arises from shallow defect states or distorted [WO6] units. Almeida et al. [21] demonstrated that variations in synthesis methods directly affect the atomic arrangement within these clusters, altering the density and nature of the defect states. These structural modifications, in turn, impact charge separation efficiency and recombination dynamics, ultimately influencing the photocatalytic performance.
Therefore, synthesis-dependent differences in the crystalline and local structures modulate the electronic landscape of FeWO4. A more disordered arrangement may promote shallow-level recombination paths (as indicated by the PL peak at 441 nm), while simultaneously reducing the density of deep traps and enhancing the charge carrier mobility. The schematic in the inset of Figure 2D illustrates this mechanism, showing electrons excited to the CB and recombining through defect states located near the CB rather than deeply trapped ones—leading to relatively faster but less detrimental recombination.
Together, the PL and UV–vis results demonstrate that iron oxides are promising materials for photocatalytic applications. Moreover, their optical properties can be effectively tailored through controlled synthesis routes, enabling direct and efficient optimization of their photocatalytic performance.
To investigate the morphology and surface texture of the iron oxides, FE-SEM analysis was performed, and the results are shown in Figure 3. The Fe2(MoO4)3 crystals exhibited an elongated morphology with well-defined exposed surfaces. Based on the Wulff construction, this morphology is likely dominated by the (001) and (110) surfaces. The main morphological differences observed between the samples highlight the greater number of exposed faces and surfaces in Fe2(MoO4)3, which may enhance the adsorption capacity and promote the formation of vacancies—features supported by the PL results. In a study by Dutta et al. [14], Fe2(MoO4)3 synthesized via a conventional 24 h hydrothermal route exhibited an aggregated morphology and a band gap of 2.66 eV. In contrast, the present synthesis produced a more well-defined crystalline structure with a narrower band gap of 2.06 eV. This suggests that enhanced crystal growth along specific (001) and (110) surfaces may contribute to improved photocatalytic performance by facilitating charge transport and increasing the density of active surface sites in Fe2(MoO4)3.
On the other hand, the FeWO4 crystals exhibit a noticeably different morphology. The FE-SEM images reveal a highly porous and fluffy texture with irregularly shaped particles. These smaller particles suggest a larger specific surface area compared to Fe2(MoO4)3. The synthesis of FeWO4 with tailored morphologies, such as hierarchical plates, nanorods, and complex microstructures, has recently attracted significant attention due to its impact on catalytic performance. In our study, the combination of a co-precipitation route with microwave-assisted hydrothermal treatment resulted in an aggregated morphology composed of ultrafine crystallites. This approach not only promotes particle size control, but also enables crystal formation within a significantly shorter synthesis time, representing a promising strategy for scalable production. He et al. [23] reported that FeWO4 microcrystals synthesized by means of a similar method exhibited significant photocatalytic activity for RhB degradation under visible light. Likewise, Ojha et al. [32] synthesized FeWO4 nanorods via a hydrothermal route using a precursor complex, achieving a narrow band gap of 1.98 eV. Their findings demonstrated that reducing crystal size can enhance photocatalytic performance compared to bulk or larger particles. These results support the promising photocatalytic potential of the FeWO4 synthesized in this study.

3.2. Photocatalytic Degradation

In this study, the photocatalytic activity of the synthesized materials was evaluated by monitoring the degradation of a real industrial effluent from aluminum anodizing processes under visible light irradiation. The effluent exhibited a dark blue color, with a maximum absorbance peak at 616 nm. Additional information regarding the effluent composition is summarized in Table 3.
In polluted environments such as industrial effluents or contaminated soils, a wide variety of chemical compounds can poison or deactivate catalysts, including heavy metals, sulfur, chlorine, and particulate matter. These contaminants tend to bind to the active sites of the catalysts, thereby blocking catalytic activity and significantly reducing overall efficiency. Moreover, the presence of color or high turbidity in the effluent can hinder light penetration, limiting the interaction between the catalyst and the irradiation source, and consequently reducing both process efficiency and the photogeneration of reactive species [33].
The synthesized Fe2(MoO4)3 and FeWO4 samples were applied for the photocatalytic degradation of a real effluent from aluminum anodizing industries (Figure 4). Both iron-based oxides demonstrated the ability to remove the effluent’s color under the tested conditions (Figure 4A). A control experiment, performed in the absence of a photocatalyst (photolysis), confirmed that light irradiation alone did not degrade the effluent, highlighting the essential role of the catalysts in the process. Specifically, Fe2(MoO4)3 achieved an average color removal efficiency of 35.6%, while FeWO4 reached 44.6%.
Several factors may account for the difference in photocatalytic performance between the two materials, particularly their morphology. Although Fe2(MoO4)3 exhibited a more defined morphology with exposed (001) and (110) surfaces, FeWO4 showed a more irregular morphology with smaller particle sizes, which is likely to enhance surface area and catalytic activity. These structural differences may directly influence the materials’ ability to generate reactive species and interact with pollutants during photocatalysis.
Maya et al. [34] investigated the synthesis of FeWO4 with different morphologies (nanoparticles, nanorods, and nanofibers) via the hydrothermal method and evaluated their influence on ciprofloxacin degradation using various Fenton-based processes. While all morphologies exhibited photocatalytic activity, the nanoparticles demonstrated superior performance, achieving complete antibiotic removal at neutral pH within 40 min using 100 mg L−1 of the catalyst and 2.0 mmol L−1 of H2O2.
Reusability tests demonstrated that both materials maintained their photocatalytic performance after two consecutive cycles (Figure 4B). However, a decrease in efficiency was observed for Fe2(MoO4)3 after the first cycle, possibly due to the blocking of active sites by contaminants, structural degradation, or morphological changes. In contrast, the FeWO4 sample exhibited a more stable performance across cycles, reinforcing its robustness and reusability. This behavior may be attributed to its porosity and the absence of well-defined crystalline facets, which likely contribute to a more accessible and resilient catalytic surface.
In a study evaluating FeWO4 nanorods as a photo–Fenton catalyst for methylene blue removal, the catalyst maintained its performance over multiple cycles [35], consistent with the results observed in the present work using heterogeneous photocatalysis. Similarly, Fe2(MoO4)3 was investigated as a catalyst for persulfate activation in the degradation of bisphenol A, where a slight decrease in activity was reported over successive cycles [36], also aligning with our findings. These comparisons suggest that, among the two iron-based catalysts, FeWO4 exhibits superior reusability, while Fe2(MoO4)3 shows a modest decline in performance, which could be attributed, for instance, to differences in the morphology achieved during synthesis.
To better understand the catalyst performance, TOC analysis was conducted. It was observed that over both cycles, Fe2(MoO4)3 removed 60 ± 7% of the organic content from the effluent, while FeWO4 achieved 67 ± 0.7% mineralization. This result suggests the degradation of additional organic compounds present in the effluent that do not absorb at the same wavelength (616 nm) as the monitored dye. While both catalysts demonstrated similar color removal efficiencies, the superior and consistent performance of FeWO4 across cycles was evident. In contrast, the 7% deviation observed for Fe2(MoO4)3 may indicate a decline in catalytic activity over time.
These findings underscore the importance of catalyst morphology in determining performance during real effluent treatment. Since photocatalytic activity is strongly influenced by factors such as particle size, geometry, and morphology, the enhanced performance of FeWO4 can be attributed to its porous, high-surface-area morphology, as confirmed by FE-SEM analysis and PL spectroscopy.
An additional advantage of the proposed process using simple iron-based oxides is the ability to treat the real effluent directly. In contrast, other studies highlight the need for pre-treatment—biological or physicochemical—before applying an advanced oxidation process [37]. Although such steps can enhance organic removal, pre-treatment was not required in this study, given the initial TOC levels.
The degradation of real effluents poses a significant challenge due to their complex and variable composition, which may include mixtures of dyes, chemical additives, and various organic and inorganic compounds, depending on the source. Given the heterogeneous nature of such effluents, proposing a specific photocatalytic degradation mechanism becomes difficult. For example, the presence of chloride species in the medium can reduce the removal efficiency, as Cl ions can act as scavengers of hydroxyl radicals [38]. These hydroxyl radicals are among the reactive oxygen species (ROS) typically generated during the heterogeneous photocatalysis process. After light-induced generation of e/h+ pairs, the holes (electron vacancies) in the VB can react with surface-bound water to produce hydroxyl radicals, whereas the electrons in the CB can reduce oxygen to generate superoxide radical anions [39].
The presence of various interfering species forming a complex matrix, along with the observed catalytic performance achieved without any effluent pre-treatment, highlights the capability of the materials to address real-world challenges. Therefore, we investigated the behavior of the catalysts in a real effluent to validate their practical applicability and identified certain limitations that could guide future optimization, such as morphological tuning, doping strategies, and the construction of heterostructures.
To contextualize the performance of the materials investigated in this study, a comparison with other photocatalysts reported in the literature was carried out. Table 4 presents a selection of materials used for real effluent degradation, including heterostructures such as Cu2O/ZnO/Ag3PO4, ZnO/Fe2O3/MnO2, and ZnO–iron oxide. These systems generally exhibit high degradation efficiencies due to synergistic effects between their components.
Despite being single-phase materials, the iron-based oxides evaluated in this study demonstrated promising photocatalytic activity under visible light irradiation within 120 min, highlighting their potential for practical wastewater treatment applications. Additionally, Fe2(MoO4)3 and FeWO4 are low-toxicity semiconductors composed of naturally abundant elements, which contribute to their economic viability.
These features suggest a favorable cost–benefit ratio for potential industrial implementation. However, further studies are required to evaluate the environmental impact of the photocatalytic process, including possible ecotoxicity and the risk of secondary pollution. Figure 5 presents a schematic representation of the degradation process of real effluents using iron-based oxides as catalytic agents. This model aims to provide a simplified illustration of the proposed mechanism for the studied system.

4. Conclusions

In this study, iron-based oxides Fe2(MoO4)3 and FeWO4 were successfully synthesized via co-precipitation followed by microwave-assisted hydrothermal treatment. Structural and morphological analyses confirmed the formation of well-crystallized phases with distinct morphologies, which directly influenced their photocatalytic behavior. Optical characterization revealed band gap values consistent with visible light absorption, while PL analysis indicated the presence of different types of lattice defects contributing to the charge carrier dynamics.
Photocatalytic tests using a real effluent from the aluminum anodizing industry demonstrated degradation efficiencies of 32.2% and 45.3% for Fe2(MoO4)3 and FeWO4, respectively, under visible light irradiation. FeWO4 exhibited superior performance, likely due to its porous morphology and higher surface area, which enhanced light absorption and charge separation.
Although the degradation of real effluents remains a significant challenge due to their complex composition, this study demonstrates that single-phase iron oxides can offer competitive photocatalytic performance without the need for complex heterostructures. These findings highlight the potential of iron-based oxides for sustainable and cost-effective wastewater treatment and reinforce the importance of tailoring morphological and electronic properties to improve photocatalytic efficiency.

Author Contributions

Conceptualization, L.K.R., M.G.G., L.H.M., M.C., and A.F.G.; methodology, L.K.R., M.G.G., and A.F.G.; formal analysis, L.K.R., M.G.G., and A.F.G.; investigation, L.K.R., M.G.G., and A.F.G.; resources, L.H.M. and M.C.; data curation, L.K.R., M.G.G., and A.F.G.; writing—original draft preparation, L.K.R., M.G.G., and A.F.G.; writing—review and editing, L.H.M. and M.C.; visualization, L.K.R., M.G.G., and A.F.G.; supervision, L.H.M. and M.C.; project administration, L.H.M. and M.C.; funding acquisition, L.H.M. and M.C. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financed, in part, by the São Paulo Research Foundation (FAPESP), Brazil (process Nos. 2023/12399-7, 2023/01415-1, 2013/07296-2, 2023/11548-9, and 2024/07586-5), the São Paulo Research Foundation’s National Council for Scientific and Technological Development (CNPq, grant Nos. 158689/2023-2, 200978/2024-1, and 311769/2022-5), the Financier of Studies and Projects (FINEP, grant No. 01.22.0179.00), and the MSCA Postdoctoral Fellowships–European Fellowships (TREX-MPs project No. 101154906).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets supporting the findings of this study, including raw and processed XRD, Raman, UV–vis, and PL data, are openly available on Zenodo at https://doi.org/10.5281/zenodo.15773426.

Acknowledgments

The authors have reviewed and edited the output and take full responsibility for the content of this publication. L.K.R. thanks the CNPq. M.G.G. thanks the FAPESP. L.H.M. thanks the FINEP the CNPq, and the FAPESP. A.F.G. acknowledges the MSCA Postdoctoral Fellowships–European Fellowships. We would like to express our sincere gratitude to Márcio Daldin Teodoro for conducting the photoluminescence analysis.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this review article.

Abbreviations

The following abbreviations are used in this manuscript:
Fe2(MoO4)3Iron molybdate
FeWO4Iron tungstate
MBMethylene blue
RhBRhodamine B
XRDX-ray diffraction
FE-SEMField-emission scanning electron microscopy
UV-visUltraviolet–visible
PLPhotoluminescence
TOCTotal organic carbon
MHMicrowave hydrothermal
EgapBand gap energy
CBConduction band
VBValence band
eElectrons
h+Holes
e/h+ pairsCharge carriers
ROSReactive oxygen species

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Figure 1. (A) XRD patterns and Raman spectra of the as-synthesized catalysts (B) Fe2(MoO4)3 and (C) FeWO4. The crystal structure of Fe2(MoO4)3 corresponds to a monoclinic system with a P21/a space group, featuring a unit cell composed of alternating [FeO6] octahedra (green) and [MoO4] tetrahedra (black). The crystal structure of FeWO4 adopts a monoclinic system with a P2/c space group, with its unit cell containing interconnected [FeO6] (green) and [WO6] (pink) octahedra, forming a robust three-dimensional framework.
Figure 1. (A) XRD patterns and Raman spectra of the as-synthesized catalysts (B) Fe2(MoO4)3 and (C) FeWO4. The crystal structure of Fe2(MoO4)3 corresponds to a monoclinic system with a P21/a space group, featuring a unit cell composed of alternating [FeO6] octahedra (green) and [MoO4] tetrahedra (black). The crystal structure of FeWO4 adopts a monoclinic system with a P2/c space group, with its unit cell containing interconnected [FeO6] (green) and [WO6] (pink) octahedra, forming a robust three-dimensional framework.
Applsci 15 08594 g001
Figure 2. UV–vis Kubelka–Munk function for (A) Fe2(MoO4)3 and (B) FeWO4. PL spectra and the illustration of the VB and CB for (C) Fe2(MoO4)3 and (D) FeWO4.
Figure 2. UV–vis Kubelka–Munk function for (A) Fe2(MoO4)3 and (B) FeWO4. PL spectra and the illustration of the VB and CB for (C) Fe2(MoO4)3 and (D) FeWO4.
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Figure 3. FE-SEM images of (A) Fe2(MoO4)3 and (B) FeWO4 samples. A Wulff construction was proposed, in which the (001) and (110) surfaces are represented by yellow and gray, respectively, in the Fe2(MoO4)3 morphology.
Figure 3. FE-SEM images of (A) Fe2(MoO4)3 and (B) FeWO4 samples. A Wulff construction was proposed, in which the (001) and (110) surfaces are represented by yellow and gray, respectively, in the Fe2(MoO4)3 morphology.
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Figure 4. (A) Removal of color and (B) recycling of the catalyst. Conditions: 50 mg catalyst and 100 mL effluent, without pH adjustment.
Figure 4. (A) Removal of color and (B) recycling of the catalyst. Conditions: 50 mg catalyst and 100 mL effluent, without pH adjustment.
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Figure 5. Schematic representation of the photodegradation process of real effluents mediated by iron oxides.
Figure 5. Schematic representation of the photodegradation process of real effluents mediated by iron oxides.
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Table 1. Reported degradation rate (%) from the literature of FeWO4 and Fe2(MoO4)3 photocatalysts against standard reference pollutants and laboratory-prepared solutions. The corresponding exposure times are reported in minutes.
Table 1. Reported degradation rate (%) from the literature of FeWO4 and Fe2(MoO4)3 photocatalysts against standard reference pollutants and laboratory-prepared solutions. The corresponding exposure times are reported in minutes.
PhotocatalystEffluentDegradationExposure timeLampRef.
FeWO4MB dye72.1120 Light irradiation[6]
FeWO4Tetracycline68160Light irradiation[7]
FeWO4Tetracycline67.5105Xe arc lamp[8]
FeWO4
(nanosheet) *
RhB dye96.5120Xe lamp[9]
FeWO4
(hexagonal sheet) *
72.7
FeWO4
(nanorod) *
74.4
Fe2(MoO4)3–MO3Tetracycline75300LED lamp[10]
Fe2(MoO4)3Tylosin~9920LED lamp[11]
Fe2(MoO4)3Tetracycline20120Xe lamp[12]
Fe2(MoO4)3Doxycycline45.2120Xe lamp[13]
Fe2(MoO4)3MB dye9670Solar radiation[14]
Fe2(MoO4)3Crystal violet62120LED lamp[15]
Fe2(MoO4)3 *Methyl orange80.9330UVC light source[16]
Brilliant blue72.5830
* In addition, H2O2.
Table 2. Summary of PL emission wavelengths (nm) and their energies (eV) and the associated electronic transitions.
Table 2. Summary of PL emission wavelengths (nm) and their energies (eV) and the associated electronic transitions.
Fe2(MoO4)3
PL Emission(%)WavelengthEnergyLikely Origin
Dark blue68435~2.85Self-trapped excitons in [MoO4] clusters
Blue–green *32489~2.53Extrinsic defect states/deep-level emissions
FeWO4
PL Emission(%)WavelengthEnergyLikely Origin
Blue14421~2.95Intrinsic/shallow state ([WO6] in an ordered lattice)
Dark blue56441~2.81Self-trapped excitons in [WO6] clusters
Blue–green30473~2.62Shallow defect states or distorted [WO6] clusters
* Green–blue transition.
Table 3. TOC removal efficiency and wastewater initial composition characterization.
Table 3. TOC removal efficiency and wastewater initial composition characterization.
TOC removal%
Fe2(MoO4)360.6 ± 0.01
FeWO466.7 ± 0.07
ParametersResult
Total organic carbon (mg L1)14.2
pH6.0
Conductivity (mS cm1)126
Fluoride (mg L1)1.18
Chloride (mg L1)1.14
Nitrite (mg L1)<0.04
Bromide (mg L1)<0.01
Chlorate (mg L1)< 0.04
Chlorite (mg L1)0.71
Nitrate (mg L1)2.48
Phosphate (mg L1)<0.01
Sulfate (mg L1)30.69
Sulfite (mg L1)<0.04
Lithium (mg L1)<0.04
Sodium (mg L1)12.36
Ammonium (mg L1)0.36
Potassium (mg L1)2.10
Calcium (mg L1)5.84
Magnesium (mg L1)1.16
Table 4. Photocatalytic degradation efficiencies (%) of different materials applied to a real effluent. The corresponding exposure times are reported in minutes.
Table 4. Photocatalytic degradation efficiencies (%) of different materials applied to a real effluent. The corresponding exposure times are reported in minutes.
PhotocatalystEffluentDegradationExposure TimeLight SourceConditionsRef.
Fe2(MoO4)3Aluminum anodizing32.2120Visible50 mg catalyst, 100 mL effluentThis work
FeWO445.3
Cu2O/ZnO/Ag3PO4Sewage (textile company)78.2180Tungsten140 mg/L catalyst, 100 mL effluent, pH = 6.0[40]
ZnO/Fe2O3/MnO2Sewage (textile company)63120Tungsten125 mg/L catalyst, 100 mL effluent, pH = 6.0[41]
ZnO
ZnO–iron oxide
Textile effluent50
77
120UV-A0.2 g catalyst, 150 mL effluent, pH = 12.4[42]
ZnO–iron oxide85Natural solar radiation (sunny days)
ZnO–iron oxide98.60.2 g catalyst, 150 mL effluent, pH = 12.4, 1 mL of H2O2
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Ribeiro, L.K.; Guardiano, M.G.; Mascaro, L.H.; Calatayud, M.; Gouveia, A.F. Application of Iron Oxides in the Photocatalytic Degradation of Real Effluent from Aluminum Anodizing Industries. Appl. Sci. 2025, 15, 8594. https://doi.org/10.3390/app15158594

AMA Style

Ribeiro LK, Guardiano MG, Mascaro LH, Calatayud M, Gouveia AF. Application of Iron Oxides in the Photocatalytic Degradation of Real Effluent from Aluminum Anodizing Industries. Applied Sciences. 2025; 15(15):8594. https://doi.org/10.3390/app15158594

Chicago/Turabian Style

Ribeiro, Lara K., Matheus G. Guardiano, Lucia H. Mascaro, Monica Calatayud, and Amanda F. Gouveia. 2025. "Application of Iron Oxides in the Photocatalytic Degradation of Real Effluent from Aluminum Anodizing Industries" Applied Sciences 15, no. 15: 8594. https://doi.org/10.3390/app15158594

APA Style

Ribeiro, L. K., Guardiano, M. G., Mascaro, L. H., Calatayud, M., & Gouveia, A. F. (2025). Application of Iron Oxides in the Photocatalytic Degradation of Real Effluent from Aluminum Anodizing Industries. Applied Sciences, 15(15), 8594. https://doi.org/10.3390/app15158594

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