Chromium Ferrite Supported into Activated Carbon from Olive Mill Solid Wastes for Photo-Fenton Degradation of Pollutants from Wastewater using LED Irradiation

1. Introduction

The accumulation of hazardous and non-degradable organic compounds in wastewater, as a result of 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 wastewater is considered the most prevalent form of organic wastewater and has garnered significant researchers’ attention. 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 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 which has 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 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 CO_2, H₂O and inorganic ions [19,20]. Some of the examples of AOP include, photocatalysis, Fenton, photo-Fenton, sono-photo-Fenton, ozonation [21,22,23]. 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 of 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 have 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 mostly used semiconductors (TiO_2, CdS, ZnO, g-C₃N₄ and SnO₂ ). Additionally, MFe₂O₄ have lower band gap (2-3 eV) which permit 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 relevance in photocatalytic applications. Thus, the aforementioned issues can be mitigated by loading metal ferrites into suitable support materials.

Activated carbon (AC) is commonly employed in 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 supports in playing this role [43]. The sustained high cost of commercial AC limits its application in large industrial scale [44]. Olive mill agricultural solid wastes (OMSW) could 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 [45]. In photocatalysis application, AC is extensively used as a support material for the deposition of metals and metal oxides. Recent research has shown that the combination of AC with suitable semiconductors, improved the efficacy in catalytic oxidation and acts as a highly stable photocatalyst for decomposing numerous contaminants [46,47]. Additionally, the loading of semiconductors on AC resulted in band gap reduction, increased solar energy collection, improved pollutant adsorption and decreased electron-hole recombination [48,49]. 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 FeCr/AC was prepared by hydrothermal method. The primary objective of this research is to reduce the leaching rate associated with the use of iron oxide in aqueous solutions while simultaneously decreasing the band gap, which enhances photocatalytic activity [50,51]. Additionally, chromium ferrite exhibits moderate hydrogen peroxide scavenger properties, further contributing to its potential in various applications [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 key challenges in photocatalytic applications. The degradation performance was further tested using methyl orange (MO) and the tetracycline 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) 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.

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 uniform particle size. The resulting particles were impregnated with ZnCl₂ solution (impregnation ratio of ZnCl₂ to OMSW = 2:1) for 24 hours at ambient temperature. Following impregnation, the sample was oven-dried at 110 ºC for 24 h. The dried mixture was transferred to a sealed crucible and subjected to thermal treatment in a programmable muffle furnace at 500 ºC for 2 h with a heating rate of 5 ºC 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 ºC 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 of metal nitrate solutions, Fe(NO₃)₃·9H₂O and Cr(NO₃)₃·9H₂O, were each dissolved in 10 mL 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 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 surface. Then, an ammonia solution (25% w/w) was added into 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 hours 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 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. 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. 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), 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).

2.5. Process for 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 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 minutes). 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).

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

(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 30º, 35.5º, 43º, 53.5º,57º, 62.5º, and 74º, 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.

3.2. Brunaeur-Emmet-Teller (BET)

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 type IV isotherm associated with H1 hysteresis loop, typical for mesoporous materials [56].

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. The AC exhibited a high S_BET 1148 m² g^-1 and a V_p of 0.6 cm³ g^-1. After 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 AC pores.

3.3. Fourier Transform Infrared (FT-IR)

Figure 3 presents the FTIR spectra of AC and FeCr/AC. For both spectra there existed 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 decreased following the incorporation of metals in the FeCr/AC spectrum. The stretching vibration band of C=C in benzene ring is confirmed by the peak observed at 1570 cm^-1 in the spectrum of AC [57]. While, in FeCr/AC, a band was detected at 1620 cm^-1, which can be assigned to the asymmetric stretching vibration of C=O [58] or the bending vibration of H₂O [59]. The absorption band at 946 cm^-1 in FeCr/AC confirmed the presence of aromatic group [60]. Additionally, the band observed at 2970 cm^-1 corresponded to the symmetric stretching vibration of C-H [61]. A sharp and intense absorption band appeared at 570 cm^-1 in the spectrum of FeCr/AC, which was absent in AC spectrum. This newly formed band can be attributed to the stretching vibration of M-O at the tetrahedral sites [62].

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 revealed an irregular, heterogeneous, rough surface full of cavities and voids. In Figure 4b, a rigid surface with some stacks was also observed, indicating that chromium ferrite nanoparticles are dispersed throughout the AC surface.

For further analysis of elemental composition, the relevant regions of AC and FeCr/AC are detected by EDS mapping. As shown in Figure 4c, AC comprises mainly two components (carbon and oxygen) with relative abundances 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 [63]. 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 (2).

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

(2)

Here, 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 (3):

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

(3)

In this relation, $h$ 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 literature, spinel ferrites have a direct band gap [64]. Therefore, in our case n is equal to ½. The Tauc plot of FeCr/AC is shown in Figure 5b. As revealed, the band gap energy for FeCr/AC is 1.9 eV. These findings reveal the narrow band gap of these nanoparticles, emphasizing their potential as an efficient visible-light photocatalyst.

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 minutes, resulting in a 25% removal of MB attributed to the adsorption process. Interestingly, the results revealed a complete decolorization of MB (97.56%) after 120 min when FeCr/AC was irradiated with 25W LED lamp in the presence of H₂O₂. This result is significantly higher 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 the oxidation reactions in photocatalytic activities [65]. The aforementioned findings showed that the combination of FeCr/AC, H₂O₂, and visible light are necessary for 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 a varying FeCr/AC quantity of 125, 250, and 400 ppm, at fixed MB concentration of 20 ppm, 0.25 mL H₂O₂ (35%), and a near-neutral pH of 6.47 (Figure 7). The results revealed 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, improved 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 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 is reduced from 92% to 43.6% as the MB concentration rose from 10 ppm to 100 ppm. This reduction could be ascribed to the increase in the number of MB molecules to be degraded which hinders a significant portion of light from reaching the FeCr/AC surface (Figure 8). This reduction in light exposure decreases the production of hydroxyl radicals responsible for degrading MB [66], Furthermore, the intermediates produced during MB decomposition compete with MB molecules for the available catalyst surface [67].

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 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. The reduction in MB degradation at acidic pH was expected due to the electrostatic repulsion between MB cationic dye and surface of FeCr/AC. In contrast, in alkaline conditions, despite the negatively charged environment, the degradation of MB was reduced to 50%. This reduction is attributed to production of weak oxidant ferryl ions (FeO²⁺) which are formed at pH > 5 according to Equation (4) [68]. Additionally, in a basic medium, hydrogen peroxide oxidant is unstable and its degradation produces water and oxygen (Equation 5). In this study, the optimum pH was determined to be 6.47.

$${Fe}^{2+}+H_2{O}_2\to {FeO}^{2+}+H_{2}O$$

(4)

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

(5)

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 concentrations of H₂O₂ ranging from 0.25 mL to 1.0 mL at 250 ppm FeCr/AC, MB concentration of 20 ppm, and neutral pH. As shown in Figure 10, almost complete degradation of MB (97%) was attained when the H₂O₂ concentration was 0.25 mL. However, the degradation decreased gradually from 97% to 86.6% as the concentration 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 6) [69]. Therefore, in our study, the addition of only 0.25 mL of H₂O₂ was sufficient to optimize the degradation of MB.

$$H_2{O}_2+·OH\to H_{2}O+·OOH$$

(6)

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, pH of 6.47, and 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 of ln (C₀/C_t) versus time (min). The rate constant (k_app) was determined from the slope of the linear graph (Equation 7). Table 2 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 2, 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. At elevated concentrations, the increased amount of intermediate products restricts the availability of active hydroxyl radicals, causing a decrease in the degradation rate constant. 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.

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

(7)

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

(8)

where C_o: 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 the 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 in the solution as detected by atomic absorption spectroscopy was very low (0.1 ppm), which is below the acceptable level. The level of Cr leaching was below the detection limit.

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 9). The e^- produced in the CB can interact with the absorbed O₂ molecules, leading to the formation of superoxide anions (O₂ ^{- •}) (Equation 10). These anions can directly degrade MB molecules or react with H₂O₂ to produce hydroxyl radicals (OH•) (Equation 11). On the contrary, the h⁺ in the VB may react with H₂O molecules to form OH• (Equation 12). These radicals are potent oxidizers responsible for MB degradation (Equation 13) [70].

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

(9)

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

(10)

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

(11)

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

(12)

$$MBdye+{OH}^{.}or{O}_2^{-.}\to {CO}_2+H_{2}O+mineralizationproducts$$

(13)

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 revealed 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 were observed.

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 3. The obtained k₁ values for TCH and MO were 0.0225 min^-1 and 0.0115 min^-1, respectively.

4. Conclusions

FeCr/AC composite was prepared by hydrothermal synthesis process and used to study the degradation of MB dye and different pollutants (MO and TCH) using 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 confirmed 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 a 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 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 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.