Sewage sludge, the residue produced by wastewater treatment processes, is regarded as a significant risk to the environment and human health and has recently become a growing concern ^{[1, 2, 3]}. Most sludge is disposed of in landfills, by incineration or agricultural reuse, all of which have numerous associated hazards, such as the presence of pathogens or metals ^{[4, 5, 6, 7]}. Recently, there has been renewed interest in anaerobic digestion, one of the most widely used sludge treatment methods, because of the ability of this technique to transform organic matter into methane-rich biogas (60-70 vol% CH₄). Compared with traditional anaerobic digestion of low-solids sewage sludge, high-solids anaerobic digestion, which is characterized by feedstocks having a high content of total solids (TS, typically greater than 15 wt%) ^[8], has several advantages. These include higher loadings, smaller reactor volumes, and lower energy consumption ^[9].

Biogas (a mixture of CH₄ and CO₂) is generated through the biodegradation of organic matter during anaerobic digestion. However, tyrosine and tryptophan and their analogues, along with humic substances, are difficult to biodegrade ^[10]. In addition, although the bioavailability of heavy metals is increased to some extent after high-solids anaerobic digestion, the concentration of these metals is also increased ^[11], which can make traditional sludge disposal methods, such as incineration and agricultural reuse, difficult. The product of the anaerobic digestion of sewage sludge is known as digested sludge (DS), and this material retains refractory organic compounds, dead bacterial cells, and inorganic components in the form of various oxides and salts, in addition to Fe-based compounds.

The Fenton reaction is based on the transfer of electrons between H₂O₂ and metal ions, such as ferrous ions (Fe²⁺), that act as catalysts ^[12]. Although they exhibit high degradation efficiency, traditional Fenton systems have several defects, such as pH limitations and iron precipitation. Despite these disadvantages, Fenton and photo-Fenton systems have proven to be efficient at hazardous waste remediation and water disinfection. In recent years, a variety of heterogeneous Fenton catalysts, including Fe₂O₃, FeVO₄, and BiFeO₃, have been applied to the degradation of organic contaminants ^{[13, 14, 15]}, and many studies have been performed using various supports (zeolite, graphene oxide sheets, clays, carbons and resin) to enhance the efficiency of the heterogeneous Fenton process ^{[16, 17, 18]}. Unfortunately, many of these catalysts present leaching concerns and tend to produce high concentrations of Fe ions ^{[19, 20]} that are well above the European Union directives that allow only 0.0002% Fe in treated water transferred directly into the environment ^[21]. In light of this, it would be beneficial to develop a cost-effective, heterogeneous catalyst exhibiting high activity and long-term stability so as to obtain improved catalytic efficiency and a wider range of applications.

In a previous study, our group used sewage sludge as the raw ingredient for the facile synthesis of an effective and stable heterogeneous catalyst to promote the photo-Fenton reaction ^[20]. However, it was determined that a portion of the organic matter in the sludge (which could, alternatively, have been converted into biogas by advance anaerobic digestion) was either evaporated, combusted or carbonized during the calcination process. Therefore, to make more efficient use of the organic matter in the sewage sludge, we instead employed the less-useful DS to synthesize the catalysts. The refractory organic compounds, biomacromolecules and bacteria in DS can serve as scaffold templates for the preparation of a mesoporous material from sewage sludge. This may represent a more efficient and environmentally friendly way to use sewage sludge compared with energy generation via anaerobic digestion. The aim of the present study was to optimize the catalyst synthesis by varying the type of Fe-based salt, the concentration of iron compounds added and the calcination temperature.

The dewatered sewage sludge that was used for anaerobic digestion trials in this study was obtained from the Anting Wastewater Treatment Plant in Shanghai, China (with a design capacity of 150000 m³/d) ^[22]. The collected sludge was stored at 4 ℃ in preparation for daily feeding. An reactor with a working liquid volume of 6.0 L and equipped with helical stirring blades was operated at 60 r/min, using repeated cycles composed of 2-min stirring and an 8-min break. The volumes of biogas generated in the reactor were measured with wet gas meters (XMF-1, China) on a daily basis. On the first day of the experimental trials, 6.0 L of seed sludge was added to the reactor, which was operated semi-continuously with a once daily draw-off and feeding at 35 ± 1 ℃. During these trials, the reactor sludge retention time was on the order of 20 d. The DS parameters were found to stabilize 60 d after starting the reactor operation, and so DS samples obtained between the 60 and 70 d marks were used to synthesize catalysts. The parameters of the dewatered sludge and the DS used in this study are summarized in Table 1.

Table 1 Parameters of dewatered sludge and digested sludge. Parameter Dewatered sludge Digested sludge pH 7.31 ± 0.15 8.07 ± 0.13 TS (wt%) 16.50 ± 0.03 14.01 ± 0.04 VS/TS (wt%) 58.83 ± 0.04 46.34 ± 0.04 TA (mg/L) 5850.3 ± 103.4 15848.4 ± 135.8 TAN (mg/L) 759.9 ± 35.2 4120.7 ± 112.1 TS: total solids; VS: volatile solids; TA: total alkalinity; TAN: total ammonia nitrogen.

Table 1 Parameters of dewatered sludge and digested sludge.

All the organic and inorganic reagents used in this work were analytical grade unless otherwise stated. All solutions were prepared with water from a water purification system (Hitech Instrument Co., Shanghai, China).

The DS-derived, Fe-loaded mesoporous materials were prepared by adding 10 g DS to 20 ml of various Fe-loading solutions followed by stirring for 3 h at room temperature. The Fe-loaded DS was subsequently separated by centrifugation and dried overnight in air at 105 ℃. As a final step, the material was calcined in air for 3 h.

Four Fe-based compounds, (NH₄)₂Fe(SO₄)₂ (FAS), FeSO₄ (FS), FeCl₃ (FC) and Fe(NO₃)₃ (FN), were used. A DS sample was heated to 350 ℃ in a muffle furnace in air for 3 h to obtain the catalyst designated herein as DS-350, while the other samples are named based on the salt, the Fe(Ⅲ)/Fe(Ⅱ) concentration and the calcination temperature employed during the synthesis process. As an example, FS-1-350 indicates that the DS-derived, Fe-loaded mesoporous material was prepared by adding 10 g DS to 20 mL of a 1 mol/L FeSO₄ solution followed by stirring for 3 h at room temperature, recovery via centrifugation, drying in air at 105 ℃ overnight, and finally calcination in air at 350 ℃ for 3 h. Based on these same rules, the other samples are named FAS-1-350, FC-1-350, FN-1-350, and so on.

The TS, volatile solids (VS), total alkalinity (TA), and total ammonia nitrogen (TAN) contents of the dewatered sludge and DS were determined according to standard methods ^[23]. The elemental C, H, and N concentrations were measured using an elemental analyzer (Vario EL Ⅲ, Hanau, Germany). The concentrations of heavy metals (iron included) in the catalysts were assessed by inductively coupled plasma atomic emission spectrometry (ICP, Agilent 720ES, USA) following microwave digestion in Teflon vessels using a mixture of HNO₃, HCl and HF. Fourier transform infrared (FTIR) spectra were obtained employing the KBr disk method with a VERTEX 70 FT-IR (Bruker Co., Bremen, Germany) to determine the functional groups present in the catalysts. The crystal structures of the samples were elucidated using X-ray diffraction (XRD, X’ Pert PRO, Philips Co., Amsterdam, Netherlands). The morphology of each specimen was observed via scanning electron microscopy (SEM, FEI Co., Oregon, USA) and the surface areas were calculated with the Brunauer-Emmett-Teller method.

Rhodamine B (RhB), a widely used azo dye, was chosen as a model pollutant to evaluate the photo-catalytic activity of the synthesized catalysts. A 150-mL portion of a RhB (55.5 mg/L) solution was placed in a quartz glass cylinder (8.5 cm high, 5.5 cm in diameter) with constant magnetic stirring and irradiated using a 30-W low-pressure mercury lamp affixed next to the cylinder. The reaction temperature was maintained at 25 ℃ during irradiation by employing an air conditioning unit.

In each degradation trial, 0.05 g of the synthesized catalyst was transferred into the reactor. Prior to the reaction, the suspension of the catalyst in the RhB solution was stirred under dark conditions for 30 min to fully disperse the catalyst, after which the pH of the reaction solution was adjusted to 4.00, and a 1-mL portion of 3% H₂O₂ was added to initiate the degradation reaction. At specific time intervals during the reaction, aliquots of the reaction solution were obtained and centrifuged at 4 ℃ immediately to remove any catalyst particles. The RhB concentration was subsequently measured using UV-vis spectroscopy (PhotoLab 6100, WTW Co., Germany) and the total organic carbon (TOC) concentration was determined using a TOC analyzer (TOC-L CPH CN 200, Shimadzu Co., Japan). The Fe concentration in each solution after irradiation was measured via ICP spectrometry to assess any leaching of Fe from the catalyst. Each trial was conducted in triplicate. To evaluate the stability and recyclability of the catalyst, the FAS-1-350 was recycled and reused six times, being employed to decompose the RhB under the same conditions each time.

The functional groups and the chemical bonds present on the catalyst surfaces were determined from their FTIR spectra (Fig. 1(a)).The peaks at 3451 and 1638 cm⁻¹ are assigned to O-H stretching and bending vibrations, respectively ^[24]. Compared with the sample treated solely by drying at 105 ℃ ^[20], some peaks are seen to have disappeared. These include the O/N-H stretching vibration at 3298 cm⁻¹, the C-H stretching vibrations from CH₃ and CH₂ groups at 2932 and 2852 cm⁻¹, the N-H deformation and C-N stretching vibrations in -CO-NH at 1541 cm⁻¹ and the symmetrical CH₂ deformations at 1442 cm⁻¹, all of which are primarily attributed to the loss of adsorbed H₂O and organic molecules and biomacromolecules that are evaporated, combusted or carbonized during the calcination process. The broad peak in the 1300 to 800 cm⁻¹ region that is observed in all four spectra is actually composed of a set of peaks. The broad, high intensity band at 1080 cm⁻¹ is ascribed to the asymmetric Si-O-Si stretching vibrations, the shoulder at 930 cm⁻¹ corresponds to an Si-O-Fe linkage, and the Si-O-Si symmetrical stretching vibration is assigned to the peak at 800 cm^{−1 [25]}. Additional peaks at 475 and 671 cm⁻¹, characteristic of the symmetric stretching and bending vibrations of the Si-O-Fe moiety ^[26], further confirm the formation of chemical bonds between the SiO₂ in the DS and the loaded Fe compound in the synthesized catalysts. Compared with catalysts made with dewatered sewage sludge ^[20], new peaks are evident at 3150 and 1400 cm⁻¹, attributed to NH₄⁺ stretching and antisymmetric bending vibrations, respectively. Ammonium is a product of the anaerobic digestion of DS and may reach a concentration as high as 4000 mg/L in the digestion mixture ^[22], thus this process may account for the appearance of the NH₄⁺ peaks. After six recycling trials involving the degradation of RhB, the FTIR spectra of the FAS-350-1 do not show significant changes, demonstrating the stability of the catalyst.

Fig. 1. FTIR spectra (a) and XRD patterns (b) of the synthesized catalysts.

XRD was used to characterize the structure of the synthesized catalysts (Fig. 1(b)). Quartz (SiO₂), with two main characteristic peaks at 2θ= 20.9° and 26.6°, was the most recognizable crystallographic structure in the pattern of each material ^{[27, 28]}. The FC-1-350 generated three main diffraction peaks at 2θ = 33.1°, 35.6°, and 50.1°, which are assigned to the (104), (110), and (024) reflections of α-Fe₂O₃ (hematite) (JCPDS 84-0306). No obvious characteristic diffraction peak of α-Fe₂O₃ (hematite) was observed in the FAS-1-350, FS-1-350, or FN-1-350 patterns. These XRD results suggest that the FC-1-350 consisted mainly of α-Fe₂O₃ and SiO₂ crystallites, whereas the FAS-1-350, FS-1-350, and FN-1-350 were composed primarily of SiO₂ crystallites and amorphous iron. However, the Fe concentrations in these catalysts as determined by ICP spectrometry (Table 2) were similar, ranging from (18.05 ± 0.98)% to (20.76 ± 1.16)% ((18.05 ± 0.98)% for FN-1-350, (18.78 ± 1.62)% for FC-1-350, (20.76 ± 1.16)% for FS-1-350, and (19.15 ± 2.29)% for FAS-1-350). It is therefore evident that both crystalline and amorphous iron can play significant roles in the synthesized catalysts during photo-Fenton degradation ^{[29, 30]}.

Table 2 Element concentrations (wt%) of the synthesized catalysts. Element DS-350 FN-1-350 FC-1-350 FS-1-350 FAS-1-350 C 0.78 ± 0.10 0.73 ± 0.08 1.06 ± 0.06 0.97 ± 0.08 0.59 ± 0.03 N 0.29 ± 0.05 0.15 ±0.01 0.32 ±0.03 0.28 ±0.02 0.27 ± 0.01 H 1.62 ± 0.06 1.16 ± 005 0.91 ±0.01 1.48 ± 0.04 1.72 ± 0.07 Fe 10.76 ± 0.57 18.05 ± 0.98 18.78 ±1.62 20.76 ±1.16 19.15 ± 2.29 Al 0.44 ± 0.03 0.40 ± 0.03 0.40 ± 0.04 0.38 ± 0.01 0.41 ± 0.02 Ca 0.17 ± 0.02 0.09 ± 0.01 0.19 ± 0.03 0.19 ± 0.01 0.20 ± 0.02 Cr 0.05 ± 0.004 0.05 ± 0.007 0.05 ± 0.01 0.03 ± 0.006 0.06 ± 0.01 Cu 0.02 ± 0.003 0.02 ± 0.001 0.04 ± 0.007 0.03 ± 0.002 0.06 ± 0.002 Mg 0.11 ± 0.01 0.02 ± 0.003 0.03 ± 0.008 0.04 ± 0.008 0.01 ± 0.001 Mn 0.03 ± 0.001 0.11 ± 0.009 0.09 ± 0.003 0.09 ± 0.006 0.12 ± 0.007 Na 0.47 ± 0.03 0.44 ± 0.04 0.35 ± 0.01 0.31 ± 0.02 0.41 ± 0.03 Ni 0.02 ± 0.005 0.02 ± 0.004 0.03 ± 0.007 0.02 ± 0.003 0.04 ± 0.002 Pb 0.004 ± 0.001 0.005 ± 0.001 0.008 ±0.001 0.006 ± 0.001 0.008 ± 0.002 Zn 0.07 ± 0.01 0.07 ± 0.02 0.18 ± 0.03 0.12 ± 0.01 0.26 ± 0.02

Table 2 Element concentrations (wt%) of the synthesized catalysts.

The surface morphologies of the synthesized catalysts are shown in Fig. 2. The coarse, porous surfaces seen here are due to the evaporation or decomposition of H₂O molecules and organic molecules during the calcination process. The N₂ adsorption-desorption results for the catalysts are presented in Fig. 3. According to the IUPAC classification system, all of the isotherms were type IV and are thus typical of large-pore mesoporous solids. The sample surface areas, pore volumes, and pore sizes are summarized in Table 3. Compared with the FC-1-350 and FN-1-350, the FAS-1-350 and FS-1-350 displayed porous structures that increased their surface areas and pore volumes without generating changes in the pore size. All of these data (Table 3 and Fig. 2) demonstrate that the Fe²⁺-loaded catalysts provided a larger contact area and thus had greater potential to act as stable, efficient heterogeneous catalysts for the photo-Fenton reaction compared with the Fe³⁺-loaded catalysts. It is also evident that the FAS-1-350 may exhibit the best catalytic activity.

Fig. 2. SEM images of (a) FAS-1-350, (b) FC-1-350, (c) FN-1-350 and (d) FS-1-350.

Fig. 3. N2 adsorption-desorption isotherms of the synthesized catalysts.

Table 3 Textural properties of the synthesized catalysts. Catalyst Specific surface area (m2/g) Pore volume (cm3/g) Pore diameter (nm) FN-1-350 4.21 0.010 22.39 FC-1-350 6.96 0.045 25.85 FS-1-350 11.86 0.077 26.09 FAS-1-350 15.17 0.076 20.04

Table 3 Textural properties of the synthesized catalysts.

To obtain information regarding the transformation of the structural and molecular features of RhB during the degradation process, ultraviolet (UV)-visible spectra of a typical degradation process were acquired during UV irradiation of a reaction solution composed of RhB with both H₂O₂ and FAS-1-350 (Fig. 4(a)). The RhB absorption band in the aqueous solution is seen at 547 nm, and its intensity reflects the RhB concentration in the solution. Rapid RhB degradation is clearly seen by the eventual disappearance of this peak within 20 min. Approximately 69% TOC reduction was determined for the same solution at the 30 min mark, suggesting that the FAS-1-350 catalyst resulted in a significant degree of RhB mineralization. A UV-visible spectrum of a blank solution (with FAS-1-350 and stirring but without UV irradiation, tracked for 150 min) indicated that very little dye is removed by physical adsorption.

Fig. 4. (a) UV-visible spectra of a typical RhB degradation process under UV irradiation with the addition of both H2O2 and FAS-1-350; (b) Repeated RhB degradation trials under UV irradiation with addition of both H2O2 and FAS-1-350.

The stability of the FAS-350-1 under UV irradiation was also assessed as a means of examining the practical applicability of this catalyst (Fig. 4(b)). No obvious deactivation of the FAS-350-1 catalyst was observed over six repetitive trials, indicating its excellent long-term stability.

To optimize the DS-derived mesoporous material, RhB removal was assessed under different conditions, with the data shown in Fig. 5(a) and (b). The linear relationship of ln(C_t/C₀) versus t demonstrates that RhB degradation under different photo-Fenton conditions followed pseudo-first-order kinetics as in the equation: ln(C_t/C₀) = −kt, where C_t/C₀ is the normalized RhB concentration, t is the reaction time (min), and k is the reaction rate constant (min⁻¹).

Fig. 5. Catalytic activity and Fe leaching of the synthesized catalysts. (a) The reaction rate constant of RhB during typical degradation processes under various conditions (Fe type and temperature); (b) The reaction rate constant of RhB during typical degradation processes at different Fe concentrations; (c) Fe concentrations in solution after the photo-Fenton degradation of RhB (the red square indicates data for the FAS-1-350, which exhibited the best catalytic activity with sustainable Fe leaching); (d) Fe concentrations (wt%) of the synthesized catalysts.

RhB removal was also observed when employing the FAS-1-350 and FS-1-350 (Fig. 5(a)). The values of k for RhB during the degradation processes with various Fe-loaded mesoporous materials present were: FAS-1-T > FS-1-T > FN-1-T > FC-1-T, where T represents the synthesized catalysts calcined in air at a given temperature. As an example, the k values for RhB during the degradation processes with the FAS-1-350, FS-1-350, FN-1-350, and FC-1-350 were 0.308 ± 0.016, 0.282 ± 0.005, 0.094 ± 0.008, and 0.078 ± 0.009 min⁻¹, respectively. The k for RhB during the degradation processes with specific catalysts (FAS, for instance) that were calcined in air at different temperatures were FAS-1-350 (0.308 ± 0.016 min⁻¹) > FAS-1-250 (0.183 ± 0.009 min⁻¹) > FAS-1-450 (0.130 ± 0.022 min⁻¹) > FAS-1-550 (0.118 ± 0.021 min⁻¹) > FAS-1-650 (0.070 ± 0.008 min⁻¹) > FAS-1-750 (0.056 ± 0.006 min⁻¹). It is likely that a higher calcination temperature will remove a greater proportion of the organic/inorganic matter in the DS, resulting in damage to the scaffold template of the synthesized catalyst, whereas an overly low temperature will not be able to form a good scaffold. Therefore, the FM-1-350 (especially the FAS-1-350) exhibited the fastest reaction rates, followed by the FM-1-250, FM-1-450, FM-1-550, FM-1-650, and FM-1-750 (where FM indicates FAS/FS/FC or FN).

Figure 5(b) summarizes the RhB k values measured during typical degradation processes with FAS or FS materials at different concentrations (FAS/FS-C-350). When the FAS or FS amount was increased from 0.5 to 3 mol/L, the k increased from 0.149 ± 0.013 to 1.888 ± 0.284 min⁻¹, and from 0.134 ± 0.013 to 4.195 ± 0.714 min⁻¹, respectively. At the same time, the respective Fe concentration (wt%) of the synthesized catalysts increased from (17.85 ± 2.33) to (24.76 ± 2.99) wt% and from (19.43 ± 2.54) to (29.64 ± 3.38) wt%, respectively (Fig. 5(d)). The Fe concentrations in the solution after irradiation were 0.20 ± 0.13 and 0.94 ± 0.24 mg/L, respectively, with initial FAS concentrations of 0.5 and 1 mol/L, whereas they rapidly increased to 3.10 ± 0.40 and 10.58 ± 1.41 mg/L, respectively, when the initial FAS concentration reached 1.5 and 3 mol/L (Fig. 5(c)). Since the Fe concentrations in the solution after the degradation of RhB (over 20 min) increased rapidly when the initial FS concentration was 1.5 mol/L, it appears that a 1 mol/L concentration may be the best initial Fe-loading level with regard to avoiding significant leaching. Given its faster reaction rate (see Fig. 5(a) and (b)) and lesser degree of Fe leaching (Fig. 5(c)), the FAS-1-350 exhibits the best catalytic activity with a sustainable amount of Fe leaching, in good agreement with the earlier characterization data for the synthesized catalysts.

Based on the results above, we propose a possible mechanism associated with the functioning of Fe-based heterogeneous catalysts during the degradation of RhB. Initially, both the RhB and H₂O₂ are adsorbed on the surface of the iron catalyst. Subsequently, the Fe(Ⅱ)/Fe(Ⅲ) redox system on the catalyst surface is initiated in conjunction with the H₂O₂ to produce ^•OH radicals capable of oxidizing the organic dye into various intermediates and, eventually, non-toxic inorganic species. Moreover, the Fe(Ⅱ)/Fe(Ⅲ) redox cycle is enhanced by UV light irradiation, resulting in markedly accelerated degradation of the RhB ^{[31, 32]}. Eventually, the adsorbed RhB molecules are converted into CO₂ and H₂O by the ^•OH radicals on the surface of the catalyst, while the catalyst itself is restored to its original state ^[33]. The reactions in this heterogeneous system can be expressed as follows.

The Fenton reaction is sustained by the redox recycling of the Fe(Ⅱ)/Fe(Ⅲ)couple. Although Fe(Ⅲ) can also initiate a Fenton-like reaction ^[34], the reduction of Fe³⁺ to Fe²⁺ occurs much more slowly than the reverse process ^[35]. Thus, the addition of Fe(Ⅱ) to the DS-derived catalysts allows the formation of Fe(Ⅱ)/Fe(Ⅲ)compounds during the calcination process. Interestingly, much less Fe(Ⅱ) is formed in this process when Fe³⁺ is added to the DS-derived catalysts. This may represent the reason why the Fe²⁺-loaded catalysts are better able to act as stable and efficient heterogeneous catalysts for the photo-Fenton reaction compared with Fe³⁺-loaded catalysts.

The reaction rate constant for the Fenton reaction was reduced along with the mesoporous quality of the catalyst, an effect that was attributed to the evaporation, combustion, or carbonization of adsorbed H₂O, small organic molecules, and biomacromolecules in the DS during the calcination process. The original Fe content (10.07 mg/g dry material, Table 2) in the DS is evidently able to provide catalytic sites in addition to the added iron species. These properties make DS a useful raw ingredient for the synthesis of an efficient and stable catalyst ^{[1, 36, 37]}. Based on this study, more efficient use of sewage sludge could be achieved by employing the anaerobic digestion process to generate biogas from biodegradable organic matter and then applying the remaining DS (with less useful organic compounds and even more heavy metals and ferrous compounds) as an environmentally friendly material for the synthesis of Fe-based catalysts.

This work demonstrated the synthesis of an efficient photo-Fenton catalyst composed of a mesoporous material derived from DS via a facile method consisting of only three steps (stirring, centrifugation and heating). The surface area of the synthesized catalyst was relatively low compared with those of catalysts prepared by other reported methods, such as carbonization, chemical activation, and pyrolysis ^[38]. However, these other methods would be accompanied by an unavoidable cost increase and greater investment for equipment. Because of its ample supply and simple synthesis method, the DS-derived catalyst should lead to savings in operational costs and less investment for equipment, and thus may be more acceptable for practical applications and from an environmental standpoint.

A more environmentally friendly process was developed by using DS rather than sewage sludge in the synthesis of an Fe-loaded mesoporous material. Several DS-derived catalysts were synthesized via a facile method, and their efficiency and stability as heterogeneous catalysts for the photo-Fenton reaction were demonstrated. The results indicate that the Fe²⁺-loaded versions have more potential to act as stable and efficient heterogeneous catalysts for the photo-Fenton reaction than the Fe³⁺-loaded specimens. Of the synthesized catalysts tested in the present work, the faster reaction rate and lower Fe leaching of the FAS-1-350 indicate that it exhibits the best catalytic activity.