Chromium Ferrite Supported on Activated Carbon from Olive Mill Solid Waste for the Photo-Fenton Degradation of Pollutants from Wastewater Using LED Irradiation

Abstract

In this study, chromium ferrite (FeCr; CrFe₂O₄) nanoparticles supported on activated carbon (AC), obtained from agricultural olive mill solid waste, were synthesized via a simple hydrothermal process. The structural, morphological, optical, and chemical properties of the FeCr/AC composite were characterized using XRD, SEM, EDX, DRS, BET, and FTIR techniques. The FeCr/AC composite was applied as a heterogeneous photo-Fenton catalyst for the degradation of methylene blue (MB) dye in an aqueous solution under 25 W visible-light LED irradiation. Critical operational factors, such as FeCr/AC dosage, pH, MB concentration, and H₂O₂ levels, were optimized. Under optimal conditions, 97.56% of MB was removed within 120 min of visible-light exposure, following pseudo-first-order kinetics. The composite also exhibited high efficiency in degrading methyl orange dye (95%) and tetracycline antibiotic (88%) within 180 min, with corresponding first-order rate constants of 0.0225 min⁻¹ and 0.0115 min⁻¹, respectively. This study highlights the potential of FeCr/AC for treating water contaminated with dyes and pharmaceuticals, in line with the Sustainable Development Goals (SDGs) for water purification.

1. Introduction

The accumulation of hazardous and non-degradable organic compounds in wastewater, as a result of the growing population and increased industrialization, has been recognized as a serious concern in recent years. By 2030, approximately 47% of the global population will face the challenge of clean freshwater scarcity [1]. Global water resources are being contaminated by a variety of contaminants, such as heavy metals, colorants, detergents, drugs, pesticides, and phenols, discharged by industries and municipalities [2]. Among these, dye effluents have garnered significant researchers’ attention due to their complex structure, high stability, and resistance to conventional treatment methods. Organic dyes have been extensively employed in various industrial activities, including printing, plastics, painting, textiles, and food [3]. According to the statistical findings, around 30,000 tons of industrial dyes are released into the ecosystem annually [4,5]. However, many types of dyes are lethal, carcinogenic, and resistant to degradation, resulting in severe environmental pollution and posing irreparable damage to human health and the ecosystem [6,7]. Recently, the pursuit of efficient techniques to reduce organic pollutants in wastewater has prompted the investigation of numerous innovative technologies. Over the past decades, conventional methods such as coagulation, electrocoagulation, adsorption, filtration, bioremediation, electrolysis, and chemical oxidation have been the primary techniques for treating large-scale dye-contaminated wastewater [8,9,10,11,12,13]. The adsorption process is a fast, facile, and economic method for color removal from aquatic bodies [14,15,16]. However, such a method has a major limitation of secondary waste being produced after the contaminants are adsorbed. Additionally, there are environmental concerns related to the reuse of adsorbents for successive batches, the leaching or desorption of pollutants, and the disposal of spent adsorbents. Advanced oxidation processes (AOPs) indeed stand out as a potent method for addressing these challenges [17]. AOPs are used to break down many toxic and biologically persistent organic pollutants in water to acceptable levels, without generating harmful by-products [18]. This method relies on the production of •OH having an oxidation potential of 2.8 V, for the mineralization of pollutants into CO₂, H₂O, and inorganic ions [19,20]. Some examples of AOPs include, photocatalysis, Fenton, photo-Fenton, sono-photo-Fenton, and ozonation [21,22,23]. The heterogeneous photo-Fenton process is considered an economical and environmentally friendly technique for breaking down resistant contaminants due to its fast reaction rate, broad pH range application, and excellent recyclability [24,25,26,27,28].

Currently, spinel ferrite nanoparticles with the general molecular formula MFe₂O₄ (M = Ni, Mg, Zn, Co, Cu, Au) are widely valued due to their unique characteristics [29,30,31,32,33,34]. These include low price, good chemical and thermal stability, strong electrical resistivity, and magnetic anisotropy [35]. In particular, MFe₂O₄ has been widely employed in diverse applications, such as drug delivery, magnetic resonance image (MRI), cancer diagnosis, electronic devices, adsorption, and photocatalysis [36,37]. As one of its advantages, MFe₂O₄ semiconductors exhibit light-response properties, demonstrating efficient photocatalytic activity comparable to the most commonly used semiconductors (TiO_2, CdS, ZnO, g-C₃N₄, and SnO₂). Additionally, MFe₂O₄ has a lower band gap (2–3 eV), which permits them to absorb light in the visible region [38]. Ferrite nanoparticles still encounter several challenges as they tend to accumulate as a result of Van der Waals forces and magnetic dipolar interactions [39,40]. Also, the leaching of metals in water and the fast electron–hole recombination restrict their application in photocatalytic applications. Thus, the aforementioned issues can be mitigated by loading metal ferrites into suitable support materials.

Activated carbon (AC) is commonly employed for water purification either as an adsorbent or a carrier [41,42]. Its high surface area and porosity allow AC to effectively, quickly, and completely capture targeted contaminants, offering a notable advantage over other support systems playing this role [43]. The sustained high cost of commercial AC limits its application at a large industrial scale [44]. To address this limitation, lignocellulosic biomass has emerged as a sustainable, renewable, and cost-effective alternative for the development of carbon-based materials. Olive mill, agricultural solid waste (OMSW), a by-product of olive oil production, is a lignocellulosic biomass rich in cellulose, hemicellulose, and lignin [45]. It can be valorized into low-cost AC as one of the eco-friendly methods used to reduce their negative environmental impacts. Around 20 million tons of OMSW are produced every year [46]. In photocatalysis application, AC is extensively used as a support material for the deposition of metals and metal oxides. The recent research has shown that the combination of AC with suitable semiconductors improves the efficacy in catalytic oxidation and acts as a highly stable photocatalyst for decomposing numerous contaminants [47,48]. Additionally, the loading of semiconductors on AC resulted in a band gap reduction, increased solar energy collection, improved pollutant adsorption, and decreased electron–hole recombination [49,50]. Hamieh et al. [51] in their study developed a hybrid mixture of FeCr-SBA-15/AC to investigate the combined effect of adsorption and photo-Fenton processes on the degradation of methyl orange (MO) dye. The AC in the mixture improved MO adsorption and reduced the electron–hole recombination, thereby improving the overall photocatalytic activity. Therefore, it is feasible to enhance the photo-Fenton activity of metal ferrites by coupling them with AC.

In the present work, a composite mixture of FeCr/AC was prepared by the hydrothermal method. The objective of this research was to investigate the photocatalytic performance of the hybrid system while assessing the stability of the CrFe₂O₄ phase through leaching measurements under reaction conditions, with the aim of exploring potential improvements in the band gap and operational stability compared to conventional systems. Additionally, the presence of FeCr facilitates charge separation and enhances hydroxyl radical generation, which contributes to an improved photocatalytic performance [52]. On the contrary, the high surface area of AC plays a crucial role in improving catalytic activity. The production of AC with desirable physicochemical properties from agricultural biomass waste aligns with the ongoing efforts in sustainable development and the promotion of green energy within the scientific community. AC from olive mill solid waste was prepared by chemical impregnation with zinc chloride. The synthesized materials were characterized by different techniques (XRD, BET, SEM-EDX, FT-IR, UV-Vis) to study their physicochemical properties. The FeCr/AC composite was used to study the photo-Fenton degradation of different types of model pollutants. The photo-Fenton performance of MB was evaluated by modifying several factors, such as contact time, catalyst loading, dye concentration, and pH, on the degradation of MB. Additionally, the innovative design of our composite promotes higher reusability and stability, addressing the key challenges in photocatalytic applications. The degradation performance was further tested using methyl orange (MO) and the tetracycline (TCH) antibiotic to validate the composite’s versatility across different pollutant classes.

2. Materials and Methods

2.1. Chemicals

All reagents utilized in this study were of analytical grade and employed without any additional purification. Zinc chloride (ZnCl₂), iron nitrate nonahydrate (Fe(NO₃)₃·9H₂O), chromium nitrate nonahydrate (Cr(NO₃)₃·9H₂O), ammonia (NH₃, 25% w/w), hydrochloric acid (37% HCl), sodium hydroxide (NaOH), hydrogen peroxide (H₂O₂, 35% w/w, density 1.13 g/mL), methylene blue (C₁₆H₁₈N₃SCl, MW: 373.88 g/mole), methyl orange (C₁₄H₁₄N₃NaO₃S, MW: 327.33 g/mole), and tetracycline hydrochloride (C₂₂H₂₄N₂O₈·HCl, MW: 480.90) were all procured from Sigma-Aldrich (Beirut, Lebanon).

2.2. Preparation of Activated Carbon Support

The collected olive-processing residues were initially washed using deionized water and then crushed and sieved to obtain a uniform particle size. The resulting particles were impregnated with a ZnCl₂ solution (impregnation ratio of ZnCl₂ to OMSW = 2:1) for 24 h at an ambient temperature. Following impregnation, the sample was oven-dried at 110 °C for 24 h. The dried mixture was transferred to a sealed crucible and subjected to thermal treatment in a programmable muffle furnace at 500 °C for 2 h with a heating rate of 5 °C min⁻¹. The obtained AC was then cooled and washed with 0.1 M HCl and distilled water to eliminate any residual chlorides and mineral matter. Finally, the AC was dried again at 110 °C and stored for further use.

2.3. Preparation of Composite Mixture (FeCr/AC)

Chromium ferrite supported on activated carbon (AC), derived from OMSW, was prepared using the hydrothermal method. First, a known mass for each metal nitrate solution, Fe(NO₃)₃·9H₂O and Cr(NO₃)₃·9H₂O, was dissolved in 10 mL of distilled water in two separate beakers to achieve final percentages of 8% iron and 4% chromium in the final prepared catalyst. The two solutions were then mixed together, after which one gram of AC was added to them. Following this, the suspension was sonicated for 1 h to obtain a uniform suspension and to ensure the proper dispersion of iron and chromium precursors on the AC’s surface. Then, an ammonia solution (25% w/w) was added to the suspension until the pH reached 10. The resulting solid was collected through filtration, thoroughly rinsed with deionized water, and then left to dry overnight at 110 °C. Subsequently, the dried powders were subjected to calcination at 500 °C for 2 h using a heating rate of 5 °C per minute.

2.4. Characterization Techniques

Various analytical tools were employed to characterize the synthesized materials. The crystal structure and phase composition of both AC and FeCr/AC composites were examined via X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer (Karlsruhe, Germany) equipped with a Cu Kα radiation source (λ = 1.5418 Å), operated at 40 kV and 40 mA, and scanned over a 2θ range of 10–60°. The textural properties, including surface area and porosity, were investigated by nitrogen adsorption–desorption measurements at 77 K using a Micromeritics Gemini VII 2390p analyzer (Norcross, GA, USA). Before analysis, the samples were degassed under vacuum at 120 °C overnight. The specific surface area (S_BET) was derived using the BET method over a relative pressure range of 0.01–0.1. Pore distribution was determined by the Barrett–Joyner–Halenda (BJH) method applied to the desorption curve, while micropore volume was obtained using the t-plot technique.

To assess the surface morphology and elemental content, scanning electron microscopy (SEM) analysis was conducted using a Tescan MIRA3 microscope fitted with an EDX detector (Hillsboro, OR, USA). A small amount of catalyst powder was affixed to a carbon-coated aluminum stub and coated with approximately 5 nm of gold by sputtering. Functional groups present on the AC and FeCr/AC surfaces were identified using FTIR spectroscopy (Thermo Nicolet Summit-LITE, Darmstadt, Germany), scanning in the 400–4000 cm⁻¹ range at a resolution of 4 cm⁻¹. The optical properties and band gap of FeCr/AC were assessed through diffuse reflectance spectroscopy (DRS) with a Perkin Elmer UV-Vis-NIR spectrophotometer (Lambda 1050+ model, Thermo Fisher Scientific, Karlsruhe, Germany). To study the leaching of metals, an atomic absorption spectroscopy machine (AAS) was employed using a flame-based AAS instrument (Thermo Fisher Scientific ICE 3000 Series spectrometer, Karlsruhe, Germany). Calibration was performed using standard solutions for each metal. The detection limits were determined to be <12 ppm for Fe and <8 ppm for Cr. Each measurement was performed in triplicate, and the results are reported as the average of the three runs to ensure accuracy and reproducibility.

2.5. Process for the Photo-Fenton Degradation Experiment

The photocatalytic degradation experiment was conducted in a 250 mL beaker under visible light using a 25 W white LED lamp. Different pollutants, including cationic dye (methylene blue, MB), anionic dye (methyl orange, MO), and pharmaceutical (tetracycline hydrochloride), were taken as model pollutants. For the degradation of the MB pollutant, varying doses of FeCr/AC were suspended in 100 mL of MB solution with an initial concentration of 20 ppm. The suspension was stirred in the dark for 30 min before irradiation to achieve an equilibrium state between the pollutant and catalyst. Next, the lamp was turned on and 0.25 mL H₂O₂ was introduced into the beaker. Samples were then withdrawn at different time intervals (t = 0 to t = 180 min). These samples were filtered using a 0.45 µm syringe filter. The concentration of residual pollutants in the filtered solution was recorded using a UV-visible spectrophotometer by measuring the absorbance at maximum wavelengths of 664, 464, and 365 nm for MB, MO, and TCH, respectively. The degradation efficiency was computed using Equation (1).

$$Degradation efficiency \left(\%\right)={\displaystyle \frac{{(C}_{0-}{C}_t)}{C_0}}\times 100$$

(1)

where C₀: initial pollutant concentration (mg L⁻¹) and C_t: pollutant concentration at time t (mg L⁻¹).

3. Results

3.1. X-Ray Diffraction

Figure 1 displays the wide-angle XRD patterns of the AC support and the FeCr/AC composite. For AC, a broad peak appears within the 2θ range of 20–30°, which corresponds to the (002) reflection plane typically associated with hexagonal graphitic carbon structures [53]. FeCr/AC showed sharp and intense peaks at 2θ angles of 30°, 35.5°, 43°, 53.5°, 57°, 62.5°, and 74°, which can be indexed to planes (220), (311), (400), (422), (511), (440), and (533), respectively [54]. These peaks are in good agreement with the cubic spinel structure of metal ferrites with space group Fd-3m according to JCPDS card no.10-0325 [55]. No other impurities were detected in the XRD pattern, which indicates the high purity of the synthesized material. Therefore, CrFe₂O₄ nanoparticles are effectively incorporated into AC.

The crystallite size of FeCr/AC is determined from the Scherrer equation given below:

$$D= K\lambda /\beta cos\theta$$

(2)

where D represents the crystallite size, K is the shape factor (taken as 0.9 for cubic structure), λ is the X-ray wavelength (Cu Kα, 0.15418 nm), β is the full width at half maximum (FWHM) of the most intense diffraction peak (in radians), and$\theta$ is the Bragg angle.

The FeCr/AC crystallite size was determined to be 12 nm at the 311 diffraction plane. This value is consistent with the previously reported crystallite sizes of CoFe₂O₄ [56].

3.2. Textural Properties

The surface area and pore structure of the pure AC support and FeCr/AC were assessed by nitrogen adsorption–desorption isotherms at 77 K. The isotherms are displayed in Figure 2. The adsorption isotherm of AC exhibited a type I isotherm, typical for microporous materials. This is accompanied by a hysteresis loop of type H4 in the relative pressure range of 0.4–0.8 P/P₀. The prominent hysteresis loop, which is a property of mesoporous materials, is caused by capillary condensation in slit-shaped pores. Conversely, the sorption isotherm of modified FeCr/AC revealed a type IV isotherm associated with the H1 hysteresis loop, typical for mesoporous materials [57].

The textural parameters, including BET surface area (S_BET), total pore volume (V_p), pore diameter (D_p), and micropore volume (V_µp), of the prepared AC and FeCr/AC are listed in

Table 1. Textural characteristics of AC and FeCr/AC measured by N₂ sorption at 77K.

Sample	SBET (m2 g−1)	Vp (cm3 g−1)	Dpore (nm)	Vµp (cm3 g−1)
Pure AC	1148	0.6	3.2	0.055
FeCr/AC	222	0.32	9.7	0.025

. The AC exhibited a high S_BET 1148 m² g⁻¹ and a V_p of 0.6 cm³ g⁻¹. After the deposition of Cr and Fe, there was a significant reduction in the S_BET and V_p to 222 m² g⁻¹ and 0.3 cm³ g⁻¹, respectively. The reduction in surface area after metal deposition suggests that the nanoparticles are well incorporated within the AC pores.

3.3. Fourier Transform Infrared (FT-IR)

Figure 3 presents the FTIR spectra of AC and FeCr/AC. For both spectra, there exists a wide and large band at 3365 cm⁻¹, which refers to the stretching vibration band of OH in carbonyl, hydroxyl, or adsorbed H₂O molecules. The intensity of this band decreases following the incorporation of metals in the FeCr/AC spectrum. The stretching vibration band of C=C in the benzene ring is confirmed by the peak observed at 1570 cm⁻¹ in the spectrum of AC [58], while, in FeCr/AC, a band is detected at 1620 cm⁻¹, which can be assigned to the asymmetric stretching vibration of C=O [59] or the bending vibration of H₂O [60]. The absorption band at 946 cm⁻¹ in FeCr/AC confirms the presence of an aromatic group [61]. Additionally, the band observed at 2970 cm⁻¹ corresponds to the symmetric stretching vibration of C-H [62]. A sharp and intense absorption band appears at 570 cm⁻¹ in the spectrum of FeCr/AC, which was absent in the AC spectrum. This newly formed band can be attributed to the stretching vibration of M-O at the tetrahedral sites [63].

3.4. Scanning Electron Microscope (SEM)

The surface morphology and elemental distribution of the synthesized samples were analyzed through SEM imaging combined with EDX spectroscopy, as presented in Figure 4. The SEM micrograph of AC displays a highly porous and rough surface morphology, characterized by an irregular and heterogeneous structure with numerous cavities and voids. These porous characteristics contribute to a high surface area, which enhances the adsorption capacity and provides suitable support for metal dispersion in photocatalytic applications. In contrast, Figure 4b shows a more compact and dense surface with reduced porosity. This is likely due to pore filling and surface coverage by the dispersed Fe and Cr metals. Additionally, the presence of large, flake-like structures coated with a rough, crust-like layer indicates that successful deposition of chromium ferrite nanoparticles across the AC’s surface.

For a further analysis of elemental composition, the relevant regions of AC and FeCr/AC were detected by EDS mapping. As shown in Figure 4c, AC comprises mainly two components (carbon and oxygen) with relative abundances of 89.16 and 10.84 wt.%, respectively. However, after modification with chromium ferrite, new peaks for Fe and Cr are observed in Figure 4d with relative abundances of 7.79 and 3.9 wt.%, respectively. These findings confirm the successful incorporation of Fe and Cr into the AC material.

3.5. UV-Vis Spectroscopy

In order to evaluate the optical characteristics and estimate the band gap of the synthesized materials, DRS spectra were recorded, and the findings are illustrated in Figure 5a. Spinel ferrite nanoparticles exhibit light absorption within the visible range due to electrons’ transition from the O-2p level to the Fe-3d level [64]. The optical behavior evident in the DRS data for ferrites is attributed to electron excitation occurring from the valence band (VB) to the conduction band (CB). This excitation energy is directly linked to the ferrite band gap. In general, the Kubelka–Munk method is utilized with DRS data to compute the band gap (Equation (3)).

$$F\left(R\right)=\alpha ={\displaystyle \frac{({1-R)}^2}{2R}}$$

(3)

where F(R) denotes the Kubelka–Munk function, α refers to the absorption coefficient, and R represents the reflectance.

The Tauc plot equation is represented as follows in Equation (4):

$${\left(F\left(R\right)hv\right)}^{{\displaystyle \frac{1}{n}}}=A{\displaystyle (}hv-E_{BG}{\displaystyle )}$$

(4)

In this equation, $h$ is Planck’s constant, A is a proportionality factor, $v$ is the frequency of incident photons, and E_BG indicates the band gap energy. The exponent n varies based on the type of electronic transition. Depending on the type of transition, n can be 2 for indirect or ½ for direct transitions. As reported in the literature, spinel ferrites have a direct band gap [65]. Therefore, in our case, n is equal to ½. The Tauc plot of FeCr/AC is shown in Figure 5b. As is revealed, the band gap energy for FeCr/AC is 1.9 eV.

Table 2. Comparison of band gap energies of different metal ferrites.

Ferrite Catalysts	Band Gap (eV)	Ref.
NiFe2O4/SBA-15	2.09	[66]
CoFe2O4@SiO2@Dy2Ce2O7	3.25	[67]
NiFe2O4/AC (10NFAC)	2.01	[68]
MnFe2O4/rGO	2.13	[69]
CrFe2O4/SBA-15	2.7	[51]
NiFe2O4/MWCNTs	2.32	[70]
CrFe2O4/AC	1.9	This study

presents a comparison of the band gap energies of various metal ferrite photocatalysts reported in the literature. Among the catalysts examined, our prepared CrFe₂O₄/AC photocatalyst showed the lowest band gap (1.90 eV), indicating better potential for visible-light-driven photocatalytic applications.

3.6. Photo-Fenton Catalytic Activity of FeCr/AC

Several experimental conditions were tested to compare the decolorization efficiency of MB, and the results are presented in Figure 6. These conditions included visible light only, FeCr/AC with visible light, FeCr/AC with H₂O₂ and visible light, and FeCr/AC in the dark. The blank experiment, conducted under visible-light irradiation and in the absence of FeCr/AC catalyst and H₂O₂, revealed only 1.6% MB degradation after 120 min, indicating that the photolysis reaction was negligible. Prior to illumination, a dark experiment was performed for 180 min to study the adsorption of MB onto FeCr/AC. Equilibrium between adsorption and desorption was achieved within 30 min, resulting in a 25% removal of MB attributed to the adsorption process. Interestingly, the results reveal a complete decolorization of MB (97.56%) after 120 min when FeCr/AC is irradiated with a 25W LED lamp in the presence of H₂O₂. This result is significantly better than that obtained using FeCr/AC alone (35%) in the presence of visible light without adding H₂O₂. Thus, H₂O₂ plays a pivotal function in the degradation mechanism of MB and facilitates oxidation reactions in photocatalytic activities [71]. The aforementioned findings show that the combination of FeCr/AC, H₂O₂ and visible light is necessary for the effective photo-Fenton degradation of MB dye.

3.7. Impact of Catalyst Amount on MB Degradation

The influence of FeCr/AC dosage on MB degradation was investigated using varying FeCr/AC quantities of 125, 250, and 400 ppm at a fixed MB concentration of 20 ppm, 0.25 mL H₂O₂ (35%), and a near-neutral pH of 6.47 (Figure 7). The results reveal an improvement in MB degradation from 54.2% to 86.8% as the catalyst amount increased from 125 ppm to 250 ppm at 60 min under LED irradiation. This enhancement can be attributed to the increased availability of active sites on the FeCr/AC surface, which improves its light-harvesting capability. As more photons are absorbed, more charge carriers are generated, resulting in an increase in the number of hydroxyl radicals formed and, consequently, an improvement in the degradation of MB. However, there was almost no improvement in the degradation of MB as the catalyst amount exceeded 250 ppm, possibly due to the agglomeration of nanoparticles at high catalyst doses. Thus, in our study, the optimum dosage of FeCr is determined to be 250 ppm.

3.8. Impact of Initial MB Concentration

The effect of the starting dye concentration on MB degradation was investigated by varying MB levels between 10 and 100 ppm, with a fixed catalyst amount of 250 ppm, 0.25 mL H₂O₂, and a near-neutral pH of 6.47. The degradation of MB reduced from 92% to 43.6% as the MB concentration rose from 10 ppm to 100 ppm (Figure 8). At elevated dye concentrations, multiple factors contributed to the observed decrease in degradation efficiency. First, the increased optical density of the solution reduces light penetration, limiting the activation of the photocatalyst surface [72]. Second, a higher substrate concentration leads to competition for available hydroxyl radicals, since the radical generation rate remains constant while more dye molecules are present. Third, the accumulation of intermediate degradation products may also consume hydroxyl radicals [73], further decreasing the effective radical availability for parent dye molecules. These combined effects result in a lower degradation rate constant at higher methyl orange concentrations.

3.9. Impact of Initial pH

The influence of solution pH on the degradation of MB by FeCr/AC was examined using three distinct pH values (3, 6.47, and 10) with 250 ppm FeCr/AC, 20 ppm MB, and 0.25 mL H₂O₂. The results are represented in Figure 9. The degradation of MB was the highest at neutral pH and reached 87% in 60 min, indicating that neutral conditions are favorable for the photo-Fenton degradation of MB. However, a decline in MB degradation was observed under acidic and basic conditions, at 29% and 50%, respectively. Although homogeneous Fenton systems typically exhibit an optimal performance at pH ~3 due to the efficient generation of hydroxyl radicals via Fe²⁺/Fe³⁺ cycling [74], our system is based on a heterogeneous ferrite catalyst. In such systems, surface interactions play a key role in degradation efficiency. The reduction in MB degradation at an acidic pH was expected due to the electrostatic repulsion between MB cationic dye and the surface of FeCr/AC, which also becomes more positively charged under highly acidic conditions, thereby hindering effective adsorption and interaction. In contrast, in alkaline conditions, despite the negatively charged environment, the degradation of MB was reduced to 50%. This reduction is attributed to the production of weak oxidant ferryl ions (FeO²⁺), which are formed at pH > 5 according to Equation (5) [75]. Additionally, in a basic medium, a hydrogen peroxide oxidant is unstable and its degradation produces water and oxygen (Equation (6)). In this study, the optimum pH was determined to be 6.47.

$${Fe}^{2+}+H_2{O}_{2 }\to {FeO}^{2+}+H_{2}O$$

(5)

$${2H}_2{O}_2\to {2H}_{2}O+O_2$$

(6)

3.10. Impact of Oxidant Dosage on MB Degradation

In the photo-Fenton oxidation process, H₂O₂ plays a key role in generating hydroxyl radicals, which participate in the mineralization of MB dyes. The impact of H₂O₂ amount was evaluated using different dosages of H₂O₂ ranging from 0.25 mL to 1.0 mL at 250 ppm FeCr/AC, a MB concentration of 20 ppm, and a neutral pH. As shown in Figure 10, an almost complete degradation of MB (97%) was achieved when the H₂O₂ concentration was 0.25 mL. However, the degradation decreased gradually from 97% to 86.6% as the dosage of H₂O₂ increased to 1 mL. This reduction is due to the formation of hydroperoxyl radicals (HO₂^.) at excess concentrations, which act as scavengers of hydroxyl radicals (Equation (7)) [76]. Therefore, in our study, the addition of only 0.25 mL of H₂O₂ was sufficient to optimize the degradation of MB.

$$H_2{O}_{2 }+·OH \to H_{2}O +·OOH$$

(7)

3.11. Kinetic Study of MB

The degradation rate of MB can be evaluated using kinetic parameters. To ascertain the kinetics of the degradation of MB, the experiment was performed using 250 ppm FeCr/AC, a pH of 6.47, and a time interval up to 180 min under visible LED light. The degradation kinetics of MB at different concentrations (10, 20, and 100 ppm) were studied by plotting ln (C₀/C_t) versus time (min). The rate constant (k_app) was determined from the slope of the linear graph (Equation (7)).

Table 3. First-order kinetic parameters for MB degradation using FeCr/AC under different initial concentrations.

Concentrations (ppm)	k (min−1)	R2	t1/2 (min)
10	0.0297	0.9785	23.33
20	0.0258	0.9739	26.9
100	0.0077	0.9873	89.78

presents the values of the decomposition rate constant (k) and the half-life time (t_1/2) for the different initial concentrations of MB (Equation (8)). As shown in Table 3, the Langmuir–Hinshelwood pseudo-first-order kinetic model fits the experimental data well, with R² > 0.97. The calculated rate constants for different concentrations of MB were 0.0297 min⁻¹, 0.0258 min⁻¹, and 0.0077 min⁻¹ for 10 ppm, 20 ppm, and 100 ppm concentrations of MB, respectively. The reaction rate constant reduced from 0.0297 min⁻¹ to 0.0077 min⁻¹ with increasing MB concentration from 10 ppm to 100 ppm. As previously discussed, the observed decrease in the degradation rate at higher dye concentrations is attributed to the combined effects of reduced light penetration, competition for hydroxyl radicals, and radical scavenging by intermediate products (see Section 3.8). Conversely, the half-life of MB increased from 23.33 min⁻¹ to 89.78 min⁻¹ as the concentration of MB increased from 10 ppm to 100 ppm.

$$\ln \left({\displaystyle \frac{C_0}{C_{t }}}\right)=k_{app}t$$

(8)

$$t_{1/2}={\displaystyle \frac{ln2}{k_{app}}}$$

(9)

where C₀: MB initial concentration (mg L⁻¹), C_t: final concentration (mg L⁻¹), k_app: first-order rate constant (min⁻¹), t: reaction time (min), and t_1/2: half-life time (min).

3.12. FeCr/AC Recyclability and Leaching Test

The reusability of catalysts is a crucial requirement for industrial applications. To assess the reusability of FeCr/AC under visible light, the catalyst was tested over three consecutive cycles. Following each cycle, the catalyst was recovered from the MB solution, washed with deionized water, and then dried at 100 °C prior to its reuse in subsequent reactions. The photocatalytic efficiency of FeCr/AC remained high, starting at 98% in cycle 1, achieving 92.76% in cycle 2, and maintaining a robust 85.41% in cycle 3. These results validate the reusability and durability of the prepared photocatalyst nanocomposite. Moreover, the leaching of Fe into the solution as detected by atomic absorption spectroscopy was 0.1 ppm after the third cycle, while Cr leaching remained below the detection limit of the instrument.

3.13. Possible Mechanism for MB Degradation by FeCr/AC

When chromium ferrite nanoparticles supported on AC absorb visible light, electron-hole pairs are generated. The electrons (e⁻) in the valence band (VB) of CrFe₂O₄/AC are promoted to the conduction band (CB), leaving behind holes (h⁺) in the VB (Equation (10)). The photogenerated electrons in the conduction band can reduce dissolved O₂ molecules, forming superoxide anions (O₂⁻•), which can contribute to additional degradation pathways alongside classical Fenton reactions (Equation (11)), as similarly reported for semiconductor-based hybrid systems. These anions can directly degrade MB molecules or react with H₂O₂ to produce hydroxyl radicals (OH•) (Equation (12)). On the contrary, the h⁺ in the VB may react with H₂O molecules to form OH• (Equation (13)). These radicals are potent oxidizers responsible for MB degradation (Equation (14)).

$$Cr{Fe}_2{O}_4/AC+hv\to e_{CB}^{-}+h_{VB}^{+}$$

(10)

$$e_{CB}^{-}+O_2\to O_2^{-.}$$

(11)

$$H_2{O}_2+O_2^{-.}\to {OH}^{.}+{OH}^{-}+O_2$$

(12)

$$h_{VB}^{+}+H_{2}O\to {OH}^{.}+H^{+}$$

(13)

$$MB dye+{OH}^{.} or O_2^{-.}\to {CO}_2+H_{2}O+mineralization products$$

(14)

3.14. Degradation of MO and TCH

The photocatalytic performance of FeCr/AC under visible LED irradiation is investigated for the degradation of MO and TC using 250 ppm FeCr/AC, 20 ppm MO, 20 ppm TCH, and 0.5 mL H₂O₂, without modifying the solution pH (5.47 for MO and 4.4 for TCH). Figure 11 presents the degradation results for both pollutants. Dark experiments were also performed for 180 min to assess the adsorption of MO and TCH. The results reveal no adsorption of MO and around 17% adsorption of TCH. According to Figure 11, maximum removal rates of 88% for MO and 97% for TC can be observed.

The Langmuir–Hinshelwood first-order kinetic model is suitable to fit the degradation of MO and TCH. The first-order kinetic parameters k, R², and t_1/2 for MO and TCH are mentioned in

Table 4. First-order kinetic parameters for MO and TCH.

Pollutant	First-Order Kinetic
	k (min−1)	R2	t1/2 (min)
MO	0.0115	0.9988	60.27
TCH	0.0225	0.9731	30.8

. The obtained k₁ values for TCH and MO are 0.0225 min⁻¹ and 0.0115 min⁻¹, respectively.

3.15. Comparison with the Literature

The catalytic performance of the FeCr/AC composite is evaluated for the degradation of MB and compared with other ferrites-based photocatalysts reported in the literature. As revealed in

Table 5. Comparison of the performance of various ferrite photocatalysts for MB degradation under different experimental conditions.

Photocatalyst	Catalyst Concentration (g L−1)	[MB]0 (ppm)	pH	Light Source	MB Degradation (%)	Time (min)	Ref.
NiFe2O4/MWCNTs	1	10	-	Metal halide (400 W)	99	360	[77]
Ni0.3Co0.2Cu0.5Fe2O4	1	10	-	Visible (125 W)	75	120	[78]
Ni0.1Co0.9Fe2O4/g-C3N4/biochar	0.5	20	-	Xenon lamp (500 W)	96.7	120	[79]
Mg0.5Co0.5Fe2O4	2	25	10.5	UV lamp (40 W)	95.76	120	[80]
FeCo/SiO2	1	20	3	UV lamp (λ = 365 nm, 36 W)	100	60	[81]
CaFe2O4/g-C3N4	2	10		Tungsten halogen lamp (150 W)	94	120	[82]
MnFe2O4-G@WA	0.25	10	7	Sunlight (90–140 W/m2)	94	120	[83]
FeCr/AC	0.25	20	6.47	LED (25 W)	97.56	120	This study

, the FeCr/AC photocatalyst developed in this study exhibits notable advantages over previously reported ferrite-based catalysts for the degradation of MB. The FeCr/AC composite achieved a high removal efficiency of 97.56% using lower catalyst loading (0.25 g L⁻¹) under a near-neutral pH (6.47) within 120 min of irradiation. The use of a low-power LED light source further underscores the energy efficiency and cost-effectiveness of the process under ambient conditions. Moreover, the utilization of AC derived from OMSW as a catalyst support not only reduces material costs but also enhances the environmental sustainability and structural stability of the photocatalyst, which are essential for practical wastewater treatment applications.

4. Conclusions

A FeCr/AC composite was prepared by the hydrothermal synthesis process and used to study the degradation of MB dye and different pollutants (MO and TCH) using a heterogeneous photo-Fenton process under visible 25 W LED illumination. The physicochemical characteristics of FeCr/AC were investigated using techniques including XRD, BET, SEM, EDX, FT-IR, and DRS. The characterization results confirm the successful incorporation of chromium ferrite nanoparticles into AC support. Moreover, the band gap of FeCr/AC, determined from DRS spectra, was 1.9 eV, demonstrating its efficiency in visible-light photocatalytic applications. The prepared FeCr/AC catalyst exhibited high efficacy in the decolorization of MB, achieving 97% removal efficiency within 120 min in a neutral environment using only 0.25 mL H₂O₂ under visible LED illumination. The degradation kinetics of MB using FeCr/AC followed a pseudo-first-order model, yielding a rate constant of 0.0258 min⁻¹ at an initial concentration of 20 ppm. Furthermore, FeCr/AC demonstrated notable reusability and stability, with only a minor reduction in degradation the performance observed after three consecutive cycles. Additionally, FeCr/AC showed high removal rates for MO and TCH, 88% and 97%, respectively, within 180 min under the following specified reaction conditions: 250 ppm FeCr, 0.5 mL H₂O₂, and an unmodified pH. The corresponding rate constants of MO and TCH were 0.0115 and 0.0225 min⁻¹, respectively. Therefore, the prepared FeCr/AC catalyst shows promise for large-scale application in the effective treatment of contaminated water containing diverse pollutants.