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Article

Pilot-Scale Fenton-like System for Wastewater Treatment Using Iron Mud Carbon Catalyst

1
School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China
2
School of Mechanical Engineering, Tianjin University of Commerce, Tianjin 300134, China
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2025, 15(15), 8210; https://doi.org/10.3390/app15158210
Submission received: 18 June 2025 / Revised: 18 July 2025 / Accepted: 21 July 2025 / Published: 23 July 2025
(This article belongs to the Section Green Sustainable Science and Technology)

Abstract

Fenton oxidation can contribute to meeting effluent standards for COD in actual wastewater treatment plant effluents. However, Fenton oxidation is prone to produce iron sludge waste. The application of heterogeneous Fenton-like systems based on Fenton iron mud carbon in wastewater treatment plants is essential for Fenton iron mud reduction and recycling. In this study, a Fenton iron mud carbon catalyst/Ferrate salts/H2O2 (FSC/Fe(VI)/H2O2) system was developed to remove chemical oxygen demand (COD) from secondary effluents at the pilot scale. The results showed that the FSC/Fe(VI)/H2O2 system exhibited excellent COD removal performance with a removal rate of 57% under slightly neutral conditions in laboratory experiments. In addition, the effluent COD was stabilized below 40 mg·L−1 for 65 days at the pilot scale. Fe(IV) and 1O2 were confirmed to be the main active species in the degradation process through electron paramagnetic resonance (EPR) and quenching experiments. C=O, O-C=O, N sites and Fe0 were responsible for the generation of Fe(IV) and 1O2 in the FSC/Fe(VI)/H2O2 system. Furthermore, the cost per ton of water treated by the pilot-scale FSC/Fe(VI)/H2O2 system was calculated to be only 0.6209 USD/t, further confirming the application potential of the FSC/Fe(VI)/H2O2 system. This study promotes the engineering application of heterogeneous Fenton-like systems for water treatment.

1. Introduction

With the rapid development of industrialization and urbanization, the content of refractory organic pollutants in wastewater has gradually increased, posing a serious threat to water ecosystems and human health [1]. In addition, some key regions have more stringent effluent standards for COD in wastewater. For example, the effluent from the Wuxi wastewater treatment plant in China should meet the “Discharge standard of main water pollutants for municipal wastewater treatment plant & key industries of Taihu area (DB32/1072-2018)” [2], with a COD effluent standard of 40 mg·L−1. However, the COD concentration of the effluent after A2O treatment was 60–80 mg·L−1, which did not meet the local effluent standards in Wuxi. Therefore, it is necessary to introduce deep treatment to reduce the COD value of the effluent. The Fenton reaction has been widely applied in the field of treating refractory organic wastewater with simple operation, rapid reaction, and thorough mineralization [3]. Specifically, Fenton oxidation can generate hydroxyl radicals (OH) by activating H2O2 with Fe(II). However, the traditional Fenton process can produce large amounts of iron sludge and react under acidic conditions (pH = 2–4), which limits its comprehensive application in industrial wastewater treatment [4].
To overcome the limitations of the traditional Fenton process, researchers have proposed a heterogeneous Fenton-like system based on hydrogen peroxide. This system shows promising application prospects in the treatment of refractory organic wastewater with a wide pH applicability range, reusable catalysts, and no iron sludge generation [5]. At present, the heterogeneous Fenton-like catalysts include metal and non-metal catalysts [6,7], metal carbon-based catalysts [8], transition metal-modified composite catalysts [9], nanomaterials, and magnetic catalysts [10], etc. Fenton iron mud, as a by-product of the Fenton process, is rich in iron and carbon elements and can be used as a raw material for carbon-based catalysts [11]. Specifically, Fenton iron mud carbon catalyst (FSC) preparation via pyrolysis can achieve “reducing waste with waste”, which presents practical application potential. However, the efficiency of the direct use of FSC for the activation of H2O2 for the degradation of organic pollutants is relatively low [12]. Previous studies have demonstrated that the catalyst prepared from Fenton iron sludge can activate the single oxidant (H2O2). It achieves a COD removal rate of 65% at an initial concentration of 200 mg/L [13]. However, the experiments were conducted under laboratory water conditions, and the catalytic effect of the catalyst under actual wastewater conditions remains unknown.
Research showed that the combination of H2O2 with other oxidants could synergistically degrade organic pollutants and enhance oxidation efficiency [14]. For instance, the combination of H2O2 and sulfite can rapidly degrade polycyclic aromatic hydrocarbons with a reaction rate constant (k) of 10.20 × 10−3 M−1·s−1. However, the single oxidant has a relatively low response to polycyclic aromatic hydrocarbons, with the k value being only 0.26 × 10−3 M−1s−1 [14]. In addition, the combination of H2O2 and calcium peroxide (CaO2) can generate singlet oxygen (1O2) within a wide pH range (3–12). At the same time, calcium peroxide is reduced to Ca(OH)2, and tetracycline (TC) is degraded efficiently. The degradation rate of TC increased from 4% to 91% [15], indicating that the combination of oxidants can produce a synergistic effect and effectively enhance the degradation efficiency of pollutants. Therefore, introducing a second oxidant into the FSC/H2O2 system could be beneficial for enhancing the oxidation performance of the system. However, the activation of dual oxidants by Fenton iron peat catalysts still presents a research gap. Moreover, the research on heterogeneous Fenton-like systems based on H2O2 is mainly in the laboratory stage, lacking engineering application exploration, and it is difficult to evaluate its practical application value. Therefore, it is crucial to explore the removal performance of COD in actual wastewater by FSC activation for the dual oxidants process at the pilot scale, which promotes the industrial application of heterogeneous Fenton-like technology.
Ferrate (Fe(VI)) has attracted attention in treating heavy metals, recalcitrant organics, and antibiotic resistance gene spread due to its multifunctional properties (oxidation, flocculation, disinfection) [16]. In this study, a demonstration line was built to prepare the sludge-based catalyst via pyrolysis. FSC was obtained using the demonstration line. Ferrate (Fe(VI)) was introduced into the FSC/H2O2 system, and the FSC/Fe(VI)/H2O2 system was constructed to remove the COD in the secondary effluent of the sewage treatment plant at the pilot scale. The effects of the ratio and total concentration of the Fe(VI)/H2O2 oxidant and solution pH on COD degradation were investigated. The mechanism of the FSC/Fe(VI)/H2O2 system was revealed via catalyst characterization, quenching experiment, and an electron paramagnetic resonance (EPR) test. In addition, the actual application effect of the system was evaluated after 65-day COD monitoring. Finally, the catalyst preparation cost and pollutant treatment benefits are evaluated.

2. Materials and Methods

2.1. Chemicals and Materials

This study utilized the secondary treatment effluent from a wastewater treatment plant in Wuxi, and the research was carried out in the sewage treatment plant. The process flow adopted the A2O process for organic matter removal combined with advanced treatment. The H2O2 used was purchased from Tianjin Zhengcheng Chemical Co., Ltd. (Tianjin, China), trichloromethane was purchased from Tianjin Jiangtian Chemical Technology Co., Ltd. (Tianjin, China), and potassium ferrate, furfuryl alcohol, borax, absolute ethanol, and tert-butanol were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China).

2.2. Preparation of Iron Sludge Based-Catalyst

Fenton iron mud used in this experiment was taken from the Fenton process of a sewage treatment plant in Wuxi, Jiangsu. After the Fenton mud was removed, it was placed in the refrigerator at 4 °C. Before the experiments, the Fenton clay was dried in a 105 °C-constant temperature drying box for 24 h, then pulverized by a grinder, and passed through a 200-mesh screen to obtain the Fenton clay sample, which was then sealed and stored for reserve. The main elements of the original Fenton clay were measured by the X-ray fluorescence spectrum analyzer (XRF, ZSX PRIMUS II, Rigaku, Japan), with the highest C content being 49.7%. Fe is one of the main components of Fenton clay (35.4%). Direct discarding will cause the waste of Fe resources. Moreover, the high Fe content facilitates the generation of Fe sites in the catalyst and promotes H2O2 activation. All the catalysts used in this study were prepared by the constructed demonstration line for pyrolysis of solid waste-based catalysts (3 t/h) and were named FSC. The reaction conditions were as follows: heating rate of 10 °C/min, pyrolysis temperature of 700 °C, pyrolysis time of 2 h, and the atmosphere was nitrogen.

2.3. Characterization of Catalyst

Fresh catalysts and used catalysts were collected for morphology and functional group characterization. The used catalyst was collected from the pilot plant after it had been in operation for 65 days. The morphology and microstructure characteristics of the iron mud-based catalyst were observed by scanning electron microscopy (SEM, Horiba Lab RAM HR Evolution spectrometer, HORIBA Jobin Yvon, Kyoto, Japan). The pore structure information of the Fenton iron mud samples was analyzed using a surface area and pore volume analyzer (Quantachrome Instrument, Boynton Beach, FL, USA). The specific surface area, average pore diameter, and total pore volume of the samples could be obtained by nitrogen adsorption–desorption isotherms at 77 K. To analyze the functional groups on the FSC surface, the FSC samples were characterized by Fourier transform infrared spectroscopy (FTIR, Nicolet iS50, Thermo Fisher Scientific, Waltham, MA, USA). The changes in functional groups before and after FSC use could be determined by the frequency of characteristic absorption bands. To analyze the crystal structure of the samples, the FSC samples were scanned by a Japanese Rigaku X-ray diffractometer equipped with a CuK α radiation source (XRD, Rigaku SmartLab 9 kW, Rigaku, Osaka, Japan). The reactive oxygen species were identified by electron paramagnetic resonance (EPR, JES-FA200, JEOL, Tokyo, Japan). The semi-quantitative analysis of elements such as N1s, O1s, C1s, and Fe2p in the Fenton iron mud was carried out by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250, Thermo Fisher Scientific, Waltham, MA, USA).

2.4. Pilot-Scale Device and Performance Testing

To save cost, the system with better COD removal performance was explored using sequential batch experiments. In order to explore the potential application of FSC/Fe(VI)/H2O2 in engineering, this study developed a pilot-scale device with a wastewater treatment capacity of 600 L/d. The sewage indicators of the sewage treatment plant are shown in Table 1. The COD of the influent water is 70 mg·L−1. Figure 1a,b are the schematic diagram and physical picture of the pilot-scale device. This device is equipped with three catalyst layers to ensure the full utilization of the catalyst and the full occurrence of the oxidation reaction. Glass beads are laid below the catalysts, which can provide a stable supporting structure for the catalysts and prevent them from aggregating or sinking due to gravity or air flow impact, ensuring that the catalysts are evenly distributed in the reaction area and maintaining a good reaction state. Four peristaltic pumps are set up in the device, including an acid-base pump, a hydrogen peroxide pump, a ferrate potassium pump, and a reflux pump. They operate in the mode of bottom-in and top-out, with some reflux. This ensures that the wastewater fully contacts with the catalysts. To monitor the pH of the reaction system in real time, an online pH control system is configured in the pilot-scale device. In addition, COD online monitoring equipment and an online controller are configured to monitor the COD concentration, as well as control the switches and flow rates of the pumps in real time. Specifically, the device operated for a total of 65 days. The flow rates of the acid-base pump, hydrogen peroxide pump, ferrate potassium pump, and reflux pump were 10 mL/min, 0.38 mL/min, 20.59 mL/min, and 1044 mL/min, respectively. Hydrogen peroxide is 1.5 mmol·L−1 and ferrate potassium is 8 mmol·L−1. The acid-base pump regulates the pH at 6 ± 0.5. The dosing pump uses a four-way device to uniformly add the reagents to the reactor. The reflux device is equipped with two pumps, and the reflux ratio of each pump is 1:2, with a total reflux ratio of 1:4.

2.5. Quenching Experiments

Through quenching experiments, the types and contribution levels of reactive oxygen species in the Fenton-like oxidation system can be determined. In this experiment, methyl phenyl sulfoxide (PMSO), tert-butanol (TBA), trichloromethane (chloroform), and furfuryl alcohol were used to identify Fe(VI), OH, O2•−, and 1O2. PMSO can capture high-valence metals (k = 101 M−1∙s−1) [17]. TBA has high reactivity towards OH and can act as a OH catcher (k = (3.8~7.6) × 108 M−1∙s−1) [18]. Trichloromethane can act as an O2•− catcher (k = 3.0 × 1010 M−1∙s−1) [19]. Furthermore, furfural can capture OH and 1O2 (kOH = 5.0 × 109 M−1∙s−1, k1O2 = 1.5 × 108 M−1∙s−1) [20].

2.6. Analysis Methods

In this study, the water samples were filtered through a 0.45 μm membrane filter before COD determination to remove suspended solids and possible FSC fragments. After filtration, the samples were heated in a digestion apparatus at 100 °C for 5 min to ensure the complete decomposition of residual H2O2. COD was determined by the standard potassium dichromate method. After digestion at 150 °C for 120 min, the absorbance was measured at a wavelength of 600 nm. The COD removal rate (RCOD) could be calculated by Equation (1). According to the method of calculating the cost of a wastewater treatment plant in Wuxi, China, the cost of the FSC/Fe(VI)/H2O2 system can be determined by Equations (2)–(4).
R C O D = C O D i n f l u e n t C O D e f f l u e n t C O D i n f l u e n t
CostFSC = CFSC × PriceFSC
Costoxidant = Coxidant × Priceoxidant
Total Cost = CostFSC + Costoxidant + Costreagent
where CODinfluent represents the COD of influent water, CODeffluent represents the COD of effluent water, CostFSC represents the cost of FSC, CFSC represents the amount of FSC per ton of water, PriceFSC represents the price of FSC, Costoxidant represents the cost of oxidant, Coxidant represents the amount of oxidant, Priceoxidant represents the price of oxidant, Total Cost represents the cost of this technology, and Costreagent represents the cost of reagent.

3. Results and Discussion

3.1. Characterizations of Catalyst

To investigate the specific surface area, pore volume, and diameter of FSC, FSC was analyzed by the N2 adsorption–desorption isotherm. As demonstrated in Figure 2a, the N2 adsorption–desorption isotherm of both fresh and used catalysts presents a typical H3-type hysteresis loop (IUPAC classification), and the curve exhibits obvious adsorption–desorption separation in the region with high relative pressure (P/P0 > 0.4), indicating that there is a large amount of mesoporous structure formed by particle accumulation in the material. The aperture distribution map of fresh catalysts shows that the aperture distribution presents a dual-mode feature. The proportion of micropores (<2 nm) is less than 5%, and the mesoporous pore size of 10–20 nm contributes 68.3% of the total pore volume, which is highly consistent with the characteristics of mesoporous pore proportion of high-efficient catalysts [21]. In addition, the pore volume attenuation trend in the large pore region (>50 nm) conforms to the power law distribution. Therefore, FSC has a hierarchical pore structure, which is conducive to the mass transfer and diffusion of pollutant molecule [22]. Compared with fresh catalysts, micropores of used catalysts almost disappear. Mesoporous region decreases by 30%, while the macroporous region exhibits relatively insignificant changes. In addition, it was found by BET calculation that the SSA, average pore size, and total pore volume of FSC were 32.21 m2/g, 9.94 nm, and 0.0949 cm3/g, respectively. The used catalyst showed decreased SSA (21.46 m2/g) and total pore volume (0.784 cm3/g). Its average pore diameter slightly increased to 10.04 nm. The changes resulted from pollutant adsorption and pore blockage. Figure 2b shows the microscopic morphology of FSC. The results show that FSC has an irregular particle shape, an obvious porous structure on the surface, and an uneven, rough, and layered texture, which is conducive to promoting the mass transfer process and improving the reaction efficiency between oxidants and pollutants.
In order to explore the changes in FSC surface functional groups before and after the reaction, FTIR spectroscopy was used to analyze FSC. As shown in Figure 2c. The unused FSC showed a strong O-H stretching vibration peak at 3403 cm−1, indicating that the surface of FSC is rich in hydroxyl functional groups, which can promote H2O2 activation by providing H+ [23]. The characteristic peak of O=C-O at 1520 cm−1 corresponds to the ester group or carboxyl group, and the peak of C=O at 1427 cm−1 indicates the presence of carbonyl group. It has been confirmed that the C=O structure promotes the formation of 1O2. The strong C-O/Si-O stretching vibration peak at 1023 cm−1 involves both C-O bond in the organic ligand and Si-O bond as a carrier, which jointly affect the electronic structure and active site distribution of the catalyst. In addition, the characteristic peaks at 875 cm−1 (S-O) and 523 cm−1 (Fe-O) correspond to the vibration of sulfur-containing functional groups and ferro-oxygen bonds, respectively. Comparing the catalyst spectra before and after use, it was found that the O-H peak of the used catalyst almost disappeared, which may be the O-H group involved in the adsorption of organic matter. In addition, the peaks of O=C-O and C=O of FSC were significantly weakened after the reaction, suggesting that O=C-O and C=O may be involved in the activation of oxidants. FTIR spectra showed significantly weakened peaks at 3403 cm−1, 1520 cm−1, 1427 cm−1, and 523 cm−1 in used catalysts. The attenuated peaks correspond to consumption of carboxyl groups, carbonyl groups, and iron–oxygen bonds, confirming surface functional group depletion during catalytic reactions. The significant consumption of surface functional groups might also be one of the reasons for the decline in COD removal rate in the later stage of the pilot operation, as mentioned in Section 3.3.
Figure 2d shows the crystal structure changes in FSC before and after use. In the unused FSC, significant diffraction peaks appear at 2θ = 22.6°, 33.1°, and 36.4°, corresponding to SiO2, CaO, FeO, and Fe3O4 phases, respectively, indicating that FSC is composed of polycrystalline phases and provides multiple active sites for catalytic reactions. Compared with the XRD pattern of the used FSC, the corresponding peak intensity of FeO was significantly weakened, which was attributed to the reduction in high-value iron species to Fe0 during the oxidation–reduction cycle of Fe species in the catalytic reaction, which promoted the activation of oxidants and strengthened the formation of active species. In addition, the peaks and strengths of CaO, SiO2, and other carrier phases remain relatively stable, highlighting their structural stability and providing physical support for the catalytic system.
The fitting results of XPS characteristic peaks of C1s, N1s, O1s, and Fe2p of FSC were shown in Figure 3. Figure 3a shows that N1s can be divided into pyridine nitrogen (398.5 eV), pyrrole nitrogen (399.8 eV), and graphite nitrogen (400.8 eV). Table 2 shows that the relative atomic percentages of pyridinium, pyrrole, and graphite nitrogen in unused FSC are 1.05%, 2.07%, and 1.39%, respectively. The relative atomic percentage of pyridinium nitrogen, pyrrole nitrogen, and graphite nitrogen in FSC was reduced to 0.78%, 1.56%, and 1.17%, respectively, indicating that nitrogen species were involved in oxidant activation. The results show that pyridinium, pyrrole, and graphite nitrogen may promote 1O2 formation with the activation of H2O2.
Figure 3b shows the XPS sub-peak results of catalyst O1s. The peak of 530.1 eV corresponds to the Fe-O bond, 530.66 eV is surface-adsorbed oxygen (Oads), 531.3 eV and 532 eV belong to C=O and O-C=O, respectively, and 533 eV corresponds to the C-O bond. The results showed that the relative atomic percentage of C=O and O-C=O decreased from 27.19% and 27.41% before the reaction to 10.79% and 12.18%, respectively, indicating a role of the carbonyl functional groups in H2O2 activation [24].
Figure 3c is the Fe2p spectrum, showing a typical spin–orbit split bimodal structure, with 711.0 eV and 724.7 eV corresponding to Fe 2p3/2 and Fe 2p1/2, respectively. The results show that 710.6 eV and 724.3 eV are Fe (II) signals, 712.4 eV and 726.4 eV are Fe(III) signals, and 717.8 eV and 732.7 eV are satellite peaks. The results showed that the peak strength of Fe(II) and Fe(III) increased from 1.03% and 1.79% to 1.37% and 2.74%, respectively, and the Fe0 signal disappeared completely. The oxidation process of Fe0 → Fe(II) → Fe(III) in the reaction shows that the iron species may even form high-priced iron (e.g., Fe(IV)).
Figure 3d is a spectrum of C1s, which can be decomposed into four characteristic peaks. 284.7 eV belongs to sp2 carbon (C=C/C-C), 285.5 eV corresponds to the C-N/C-O bond, 286.5 eV belongs to the ketone/aldehyde group C=O, 288.8 eV belongs to the carboxylic acid group O-C=O. The results showed that the peaks of C=O and O-C=O of the used catalyst decreased significantly, from 6.79% and 6.05% to 5.67% and 3.62%, indicating that C=O and O-C=O participated in the reaction as the active sites of 1O2 and were consumed in the process of oxidizer activation. The peaks of C-C/C=C and C-N/C-O increased from 26.01% and 6.74% to 30.31% and 9.12%, indicating that C=O and O-C=O were partially transformed into C-C/C=C and C-O structures.

3.2. Catalytic Performance Test Using Sequential Batch Experiments

3.2.1. The Removal Performance of COD in Different Systems

This study investigates the COD removal performance of FSC/Fe(VI), FSC/H2O2 system, Fe(VI)/H2O2 system, and FSC/Fe(VI)/H2O2 system by a sequential batch experiment. Figure 4a shows the adsorption performance of FSC for COD. After 2 h, adsorption reached equilibrium, and only 1.4 mg COD was removed due to adsorption onto the catalyst surface. The adsorption phenomenon is related to the porous structure on the surface of the catalyst as described in Section 3.1. However, the removal rate of COD is only 4.3% by the adsorption of FSC, which can be ignored. Figure 4b shows the removal performance of COD in different oxidation systems under the same total oxidant concentration. In the FSC/Fe(VI) system, the COD degradation rate is the lowest, at only 11.69%, indicating that the activation performance of FSC for Fe(VI) is poor. The FSC/H2O2 system has a higher COD removal efficiency than the FSC/Fe(VI) system, reaching 43.03%. This is because FSC can activate H2O2 to generate hydroxyl radicals (OH) with strong oxidizing properties, thereby non-selectively attacking organic substances through hydrogen extraction and electron transfer pathways, significantly enhancing COD removal efficiency [25]. Moreover, the removal rate of the Fe(VI)/H2O2 dual-oxidant synergistic system is only 28.57%, indicating that the oxidation effect of the single oxidant is poor. Surprisingly, the FSC/Fe(VI)/H2O2 system achieved a 57% COD removal rate, and the removal rate exceeded 43.03% for the FSC/H2O2 system. It also surpassed the 30% removal rate of the single-oxidant Fenton system in real wastewater [26]. The excellent COD removal effect might be attributed to the possible synergistic effect between Fe(VI) and hydrogen peroxide in the presence of FSC.

3.2.2. Optimization of the FSC/Fe(VI)/H2O2 System for COD Removal

Figure 5 shows the effects of different pH values, total oxidant concentration, and the ratio of H2O2 to Fe(VI) on COD removal in the FSC/Fe(VI)/H2O2 system under pilot-scale conditions. The results indicate that after 2.5 h of reaction, there are significant differences in COD removal rates under different pH values (4, 6, and 8) (Figure 5a). Specifically, when pH = 4, the COD removal rate in the FSC/Fe(VI)/H2O2 system is only 38%. This is because in an acidic environment, Fe(VI) undergoes acid-catalyzed decomposition, generating Fe(III) and O2, resulting in a decrease in oxidant concentration and accelerating the decomposition of H2O2, which inhibits the generation of free radicals. When the system is in a slightly neutral environment with pH = 6, the COD removal rate reached its maximum, with the highest value being 57%. When pH increases to 8, the COD removal rate in the FSC/Fe(VI)/H2O2 system is only 13%. The reason is that in an alkaline environment, the protonation degree of organic molecules decreases, weakening their electron transfer efficiency with Fe(VI). In addition, Fe(IV) may be generated in the FSC/Fe(VI)/H2O2 system, which is beneficial for the degradation of organic pollutants. As shown in Figure S1, the available Fe(IV) content decreases at pH 4 and 8 compared to pH 6 [27], which further proves that the reaction system at pH 6 promotes COD degradation.
The effect of the H2O2 and Fe(VI) concentration ratio on COD removal rate is shown in Figure 5b. As the concentration ratio of H2O2 and Fe(VI) increases from 1:1 to 8:1, the COD removal rate increases from 15.59% to 57%. At a 16:1 H2O2/Fe(VI) molar ratio, COD removal decreased sharply from 57% to 11.86%. The reduction correlates with H2O2 acting as a reductant. Excess H2O2 intensifies Fe(VI) decomposition through enhanced Fe(IV)/Fe(V) formation. Reaction conditions are optimized to enhance the oxidative activity of Fe(VI) and promote the generation of its reactive intermediates [28]. Figure 5c shows the effect of total oxidant concentration on COD removal rate. When the concentration of total oxidant is 0.563 and 0.844 mmol·L−1, the COD removal rate is similar at 25.00% and 28.57%, respectively. The removal rate reaches the highest (57.01%) when the total oxidant concentration is 0.9 mmol·L−1. When the concentration of total oxidant is 1.35 M, the removal rate drops to 8.20%, which is due to the rapid reaction of a too high concentration of H2O2 with Fe(VI) to produce O2 and Fe(II), resulting in a decrease in the concentration of active species [28]. Therefore, when pH = 6, H2O2/Fe(VI) = 8:1, and with a total concentration of 0.9 mmol·L−1, COD can be effectively removed, and excessive consumption of oxidants can be avoided, providing the best reaction conditions for the pilot scale.

3.3. The Removal Performance of COD in the FSC/Fe(VI)/H2O2 System at the Pilot Scale

Figure 6 shows the COD removal performance of the FSC/Fe(VI)/H2O2 system on secondary effluent at the pilot scale. As shown in Figure 6a, when pH increases from 6.33 to 8.15, the COD removal rate presents an overall decreasing trend. When pH = 6.5 ± 0.3, the COD removal rate is 60.7%, and the effluent COD is lower than 35 mg·L−1, reaching the discharge standard of main water pollutants for municipal wastewater treatment plant & key industries of Taihu area (DB32/1072-2018) (hereinafter referred to as Standard). With the increase in pH to about 8.1, the COD removal rate drops to 4.2%, and the effluent COD reaches 66 mg·L−1, which does not meet the effluent standard. Under high pH conditions, the reason for the decrease in COD removal rate might be that the accelerated decomposition of H2O2 limits the regeneration of hexavalent iron (Fe(VI)) [29]. Additionally, the precipitation of Fe(OH)3 may lead to the passivation of the catalyst surface [30] (Equation (5)).
Fe3+ + 3OH → Fe(OH)3
Figure 6b shows the dynamic trends of influent COD, effluent COD, and COD removal rate at the pilot scale within 65 days. At the initial stage of the pilot test, the influent COD experienced a brief fluctuation due to the variation in the actual secondary effluent. With the extension of the running time, the influent COD gradually tends to be stable (70.3 mg·L−1). However, the stable effluent COD concentration (<40 mg·L−1) demonstrated that the increased organic load did not affect the process efficiency, indicating that the system has sufficient active sites (Figure 2 and Figure 3) and oxidizing species to cope with load fluctuations. In addition, in the primary stage of oxidation, the removal rate of COD is the highest (64.59%), which is due to the presence of a large number of active sites when the catalyst is just added. When the catalyst was operated for 500–1000 h, the activity was still better, and COD removal rate was reduced by 9.59%. The average effluent COD value is 33 mg·L−1. After reaction for 1000 h, the COD removal rate decreased and the effluent COD increased slightly, because the catalyst sites gradually reached saturation and the degradation efficiency of organic matter slowed down. In particular, the COD degradation rate was 38.4% after 65 days of operation. However, effluent COD is still less than 40 mg·L−1. Therefore, the effluent COD treated by the FSC/Fe(VI)/H2O2 system can meet the Standard and has great engineering application potential.

3.4. Identification of Active Species

In order to identify the active species that play a major role in the FSC/Fe(VI)/H2O2 system, quenching experiments were carried out. COD cannot be determined after the quenching agent is added to the secondary effluent. Therefore, the quenching experiment of methylene blue was carried out in this study. As shown in Figure 7a, when furfuryl alcohol was added, the methylene blue degradation rate was reduced from 91% to 49%. However, the degradation rate did not change significantly after the addition of tert-butanol and trichloromethane. Therefore, the system contains almost no OH and O2, so 1O2 is the main active species in the FSC/Fe(VI)/H2O2 system. However, when 1O2 in the system is quenched, the degradation rate of methylene blue is still about 50%, so there may be other active species in the system. Furthermore, it was shown that Fe(VI) has oxidative performance at pH = 8 [28]. Similarly, the high-valent iron intermediates (Fe(IV/V)) generated by Fe(VI) activation have been proven to be the dominant active species for the degradation of organic compounds [31]. Therefore, Fe(VI) can be used directly as an oxidant for COD removal.
In order to verify the presence of high-priced metals in the system, PMSO is used as a quenching agent. As shown in Figure 7b, the degradation rate of PMSO reached 47% within 2.5 h, and the yield of PMSO2 increased by 45%. In the quenching experiment, the decrease in PMSO and the increase in PMSO2 are relatively obvious, indicating that there are some high-priced metal active species Fe(VI), Fe(V), and (Fe(IV)) in the FSC/Fe(VI)/H2O2 system. Figure 7c depicts the concentration profiles of Fe(VI) and Fe(II) over time. During the reaction, Fe(VI) could be converted to other Fe species, with a degradation rate of 80% achieved after 2.5 h. The temporal evolution of Fe(II) concentration was also monitored. The results indicated a rapid initial increase in Fe(II) concentration, reaching 22.61 μmol·L−1, followed by a stabilization phase with an average concentration of 19.76 μmol·L−1. The final effluent contained 21.16 μM of Fe2+ and 19.96 μM of Fe(VI), which was the result of both the leaching of iron from the catalyst and the transformation of the oxidant. It was reported that the generation of Fe(II) can be attributed to the reduction in Fe(IV) by H2O2 [28]. Thus, Fe(IV) was proven to exist in the FSC/Fe(VI)/H2O2 system via Fe(VI) transformation. Although Fe(II) may react with H2O2 to produce trace amounts of OH, their contribution to the overall reaction is negligible (Figure 7d). Thus, Fe(II) basically does not play a catalytic role. In summary, Fe(IV) and 1O2 were the main active species in the FSC/Fe(VI)/H2O2 system. The EPR map can further verify the active species in the system. As shown in Figure 7d, 1O2 and Fe(IV) had obvious characteristic peaks, while O2 has basically no response. In addition, a slight signal peak of OH was also presented.

3.5. The Catalytic Mechanism in the FSC/Fe(VI)/H2O2 System

Based on the above studies, Figure 8 describes the possible mechanism of COD removal in FSC/Fe(VI)/H2O2 system: (1) The generation of 1O2. C=O and O-C=O can activate Fe(VI) and H2O2 to co-generate 1O2 (Equation (5)). In addition, graphite nitrogen, pyridine nitrogen, and pyrrole nitrogen can activate H2O2 to produce 1O2 [32,33]. (2) High-value metals: The surface of Fe0 can adsorb and activate H2O2, reduce Fe(VI), generate highly active Fe(IV) species through REDOX reaction, and then oxidize organic pollutants, accompanied by the formation of Fe(III) or Fe(II) [34]. (3) In the solution, Fe(VI) can react with H2O2 to generate reactive Fe(IV) species [28], and the Fe(IV) from this process, together with that from process (2), oxidizes the organic pollutants. After 1O2 and Fe(IV) are produced, they will mineralize with COD to produce CO2 and H2O, respectively.
FeO42− + H2O2 → Fe3+ + 1O2 +2OH

3.6. Analysis of Economic and Environmental Benefits of Heterogeneous Fenton Pilot

According to the experimental results, the amount of FSC is about 144 kg (the total catalyst is 360 kg, according to the total catalyst 2/5), the operation hours are 1247 h, the inlet flow rate is 2.18 L/min, and the total amount of treated water is 2.18 × 60 × 1247 = 163.1 t. According to the current catalyst failure calculation, about 0.88 kg FSC per ton of water is added. According to the experience of biochar preparation in the park, the average preparation cost per ton of biochar is about 68.95 dollars, and the cost of biochar per ton of water is 0.0607 dollars. According to the optimal oxidation conditions optimized in this experiment, the concentration of H2O2 is 1.2 mmol·L−1 and the concentration of potassium ferrate is 0.15 mmol·L−1. The treatment of each ton of wastewater requires 169.83 mL H2O2 and 37.13 g potassium ferrate, with the market prices of H2O2 904 USD/t and potassium ferrate 9000 USD/t as references. The cost of the oxidizers required to treat each ton of water is 0.9270 USD/t, and sulfuric acid and potassium hydroxide are used to adjust the pH. The cost of the acid-base buffer solution per ton of water is 0.7324 USD/t, so the cost of treating each ton of wastewater in the advanced oxidation stage is 0.8409 USD/t. The project also carried out cost calculations for Fenton technology and ozone technology, as shown in Table 3. After accounting, the cost of Fenton technology and ozone technology is 1.0505 and 1.4052 USD/t of sewage, respectively, and the cost of FSC/Fe(VI)/H2O2 is lower than the other two technologies. The data is sourced from a water plant in Wuxi, China (Table 4). This result not only provides strong data support for the technology selection of enterprises, but also provides an important theoretical basis for the technical optimization and cost control of the sewage treatment industry.

4. Conclusions

In this study, the FSC/Fe(VI)/H2O2 system was used in the pilot test, which realized the efficient degradation of COD in the actual wastewater and had higher economic benefits. Specifically, the FSC/Fe(VI)/H2O2 system was applied in the pilot scale, and the degradation rate of COD could be up to 64.59% in a neutral environment (pH = 6.5 ± 0.5). Meanwhile, the COD in the effluent reached the Standard within 65 days. In addition, the abundant Fe0 and C=O groups on the surface of FSC promote the activation of Fe(VI) and H2O2. Fe(IV) and 1O2 were the main active species in the degradation process. The cost per ton of wastewater treated in the FSC/Fe(VI)/H2O2 system is 0.6209 USD/t. The FSC/Fe(VI)/H2O2 system has stability and operability on a pilot scale and has a wide application prospect in the resource utilization of Fenton iron mud and wastewater treatment. In the future, the activity of the FSC can be recovered by thermal regeneration, further reducing costs.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app15158210/s1, Figure S1: The Fe(IV)-oxo species at various pH conditions.

Author Contributions

L.W.: Conceptualization, methodology, analysis, writing, review. L.L.: Formal analysis, writing—original draft, validation. J.X.: Methodology, writing—original draft. Y.W.: Software, investigation. B.Y.: Writing—review and editing, validation. G.C.: Data curation. N.L.: Conceptualization, writing—review and editing, supervision, funding acquisition. L.H.: Formal analysis, writing—review and editing, conceptualization. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Young Scientific and Technological Talents (Level Two) in Tianjin (QN20230214), the Tianjin Natural Science Foundation (24JCYBJC01290), the Climbing Program of Tianjin University (2023XPD-0006), and the National Key R&D Program (2024YFC3908903).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Pilot plant schematic diagram and (b) actual plant diagram.
Figure 1. (a) Pilot plant schematic diagram and (b) actual plant diagram.
Applsci 15 08210 g001
Figure 2. (a) Nitrogen adsorption–desorption isotherm (illustrated as aperture distribution map of FSC), (b) SEM images of fresh FSC, (c) FTIR spectra, and (d) XRD pattern of fresh and used FSC.
Figure 2. (a) Nitrogen adsorption–desorption isotherm (illustrated as aperture distribution map of FSC), (b) SEM images of fresh FSC, (c) FTIR spectra, and (d) XRD pattern of fresh and used FSC.
Applsci 15 08210 g002
Figure 3. XPS spectra of N1s (a), O1s (b), Fe2p (c), and C1s (d) of the fresh and used FSC.
Figure 3. XPS spectra of N1s (a), O1s (b), Fe2p (c), and C1s (d) of the fresh and used FSC.
Applsci 15 08210 g003
Figure 4. (a) The adsorption effect of FSC and (b) removal efficiency of COD in different reaction systems. The reaction conditions: A: FSC/Fe(VI) system, [Fe(VI)] = 0.9 mmol·L−1, CFSC = 0.5 g·L−1, pH = 6, Time = 2.5 h; B: FSC/H2O2 system, [H2O2] = 0.9 mmol·L−1, CFSC = 0.5 g·L−1, pH = 6, Time = 2.5 h; C: Fe(VI)/H2O2 system, [H2O2] = 0.8 mmol·L−1, [Fe(VI)] = 0.1 mmol·L−1, pH = 6, Time = 2.5 h; D: FSC/Fe(VI)/H2O2 system, [H2O2] = 0.8 mmol·L−1, [Fe(VI)] = 0.1 mmol·L−1, CFSC = 0.5 g·L−1, pH = 6, Time = 2.5 h.
Figure 4. (a) The adsorption effect of FSC and (b) removal efficiency of COD in different reaction systems. The reaction conditions: A: FSC/Fe(VI) system, [Fe(VI)] = 0.9 mmol·L−1, CFSC = 0.5 g·L−1, pH = 6, Time = 2.5 h; B: FSC/H2O2 system, [H2O2] = 0.9 mmol·L−1, CFSC = 0.5 g·L−1, pH = 6, Time = 2.5 h; C: Fe(VI)/H2O2 system, [H2O2] = 0.8 mmol·L−1, [Fe(VI)] = 0.1 mmol·L−1, pH = 6, Time = 2.5 h; D: FSC/Fe(VI)/H2O2 system, [H2O2] = 0.8 mmol·L−1, [Fe(VI)] = 0.1 mmol·L−1, CFSC = 0.5 g·L−1, pH = 6, Time = 2.5 h.
Applsci 15 08210 g004
Figure 5. COD removal rate in FSC/Fe(VI)/H2O2 system at (a) different pH, (b) concentration ratio of H2O2 to Fe(VI), and (c) total concentration of oxidizer. The reaction conditions: (a) [H2O2] = 0.8 mmol·L−1, [Fe(VI)] = 0.1 mmol·L−1, CFSC = 0.5 g·L−1, Time = 2.5 h; (b) pH = 6, CFSC = 0.5 g·L−1, Total[oxidizer] = 0.9 mmol·L−1, Time = 2.5 h; (c) [H2O2]/[Fe(VI)] = 8:1, pH = 6, CFSC = 0.5 g·L−1, Time = 2.5 h.
Figure 5. COD removal rate in FSC/Fe(VI)/H2O2 system at (a) different pH, (b) concentration ratio of H2O2 to Fe(VI), and (c) total concentration of oxidizer. The reaction conditions: (a) [H2O2] = 0.8 mmol·L−1, [Fe(VI)] = 0.1 mmol·L−1, CFSC = 0.5 g·L−1, Time = 2.5 h; (b) pH = 6, CFSC = 0.5 g·L−1, Total[oxidizer] = 0.9 mmol·L−1, Time = 2.5 h; (c) [H2O2]/[Fe(VI)] = 8:1, pH = 6, CFSC = 0.5 g·L−1, Time = 2.5 h.
Applsci 15 08210 g005
Figure 6. (a) Changes in effluent COD and COD removal rate with pH at the pilot scale; (b) removal performance of COD at the pilot scale.
Figure 6. (a) Changes in effluent COD and COD removal rate with pH at the pilot scale; (b) removal performance of COD at the pilot scale.
Applsci 15 08210 g006
Figure 7. (a) Removal rate of methylene blue after adding quencher in the FSC/Fe(VI)/H2O2 system; (b) PMSO consumption rate and PMSO2 production rate in the FSC/Fe(VI)/H2O2 system; (c) the changes in the concentrations of Fe(VI) and Fe(II); (d) EPR testing.
Figure 7. (a) Removal rate of methylene blue after adding quencher in the FSC/Fe(VI)/H2O2 system; (b) PMSO consumption rate and PMSO2 production rate in the FSC/Fe(VI)/H2O2 system; (c) the changes in the concentrations of Fe(VI) and Fe(II); (d) EPR testing.
Applsci 15 08210 g007
Figure 8. Mechanism scheme of organics degradation in the FSC/Fe(VI)/H2O2 system.
Figure 8. Mechanism scheme of organics degradation in the FSC/Fe(VI)/H2O2 system.
Applsci 15 08210 g008
Table 1. Sewage indicators of the sewage treatment plant.
Table 1. Sewage indicators of the sewage treatment plant.
IndexCOD/mg·L−1SS/mg·L−1TN/mg·L−1NH3-N/mg·L−1TPpH
Inflow7022040856~7.5
Outflow405101.00.26~9
Table 2. The relative atomic percentage of different active sites in used and fresh FSCs.
Table 2. The relative atomic percentage of different active sites in used and fresh FSCs.
CatalystSitesContent (at.%)
Fresh FSCUsed FSC
C1sC-C/C=C26.0130.31
C-N/C-O6.749.12
C=O6.795.67
COOH6.053.62
Fe2pFe(III)1.792.74
Fe(II)1.031.37
N1sPyridine N1.050.78
Pyrrole N2.071.56
Graphite N1.391.17
O1sFe-O9.378.87
Oads6.2715.89
C=O27.1910.79
O-C=O27.4112.18
C-O20.148.22
Table 3. Economic comparison of mainstream treatment technologies.
Table 3. Economic comparison of mainstream treatment technologies.
TechnologyTechnical ContrastEconomic Contrast
Pharmacy NamePharmacy SpecificationsInvestment and QuantityUnit-Price
(USD/t)
Prime Cost (USD/t of Sewage)
This technologyStrong oxidation
No iron mud
pH wide range
H2O2Technical grade169.83 mL/t9040.1528
FSCSelf-made1.48 kg/t68.950.0607
Potassium ferrateTechnical grade37.13 g/t90000.3342
Sulfuric acidTechnical grade98 mL/t4000.06624
Caustic potashTechnical grade2.8 g/t25000.007
Power consumption-kW·h0.75 USD/(kW·h)0.2200
Total
(USD/t of wastewater)
---0.8409
Fenton technologyMore iron mud
Using a variety of chemical agents
Acid conditions
H2O2Technical grade199.91 g/t9040.235
FerrisulfasTechnical grade425.73 g/t5300.111
Sulfuric acidTechnical grade410.72 g/t4000.244
Calcium hydroxideTechnical grade190.07 g/t2000.0349
Nonionic polyacrylamideTechnical grade4.28 g/t16000.01261
Sludge disposal---0.303
Power consumption-kW·h0.75 USD/(kW·h)0.1100
Total
(USD/t of wastewater)
---1.0505
Ozone technologyHard to remove completely
Difficult to degrade organic matter
Low utilization rate of ozone
Liquid oxygenTechnical grade103 g/t7200.7416
Electric charge-0.8848
kW·h
0.75 USD/(kW·h)0.6636
Total
(USD/t of wastewater)
---1.4052
Table 4. Annual chemical consumption of sewage treatment plants for sewage treatment.
Table 4. Annual chemical consumption of sewage treatment plants for sewage treatment.
Water
Volume
(10,000 Tons)
Ferrous Iron
(PPM)
Sulfuric Acid
(PPM)
Hydrogen Peroxide
(PPM)
Lime
(PPM)
Sludge
(PPM)
874425.73410.72199.91190.073.90
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MDPI and ACS Style

Wang, L.; Liang, L.; Xu, J.; Wang, Y.; Yan, B.; Chen, G.; Li, N.; Hou, L. Pilot-Scale Fenton-like System for Wastewater Treatment Using Iron Mud Carbon Catalyst. Appl. Sci. 2025, 15, 8210. https://doi.org/10.3390/app15158210

AMA Style

Wang L, Liang L, Xu J, Wang Y, Yan B, Chen G, Li N, Hou L. Pilot-Scale Fenton-like System for Wastewater Treatment Using Iron Mud Carbon Catalyst. Applied Sciences. 2025; 15(15):8210. https://doi.org/10.3390/app15158210

Chicago/Turabian Style

Wang, Lia, Lan Liang, Jinglei Xu, Yanshan Wang, Beibei Yan, Guanyi Chen, Ning Li, and Li’an Hou. 2025. "Pilot-Scale Fenton-like System for Wastewater Treatment Using Iron Mud Carbon Catalyst" Applied Sciences 15, no. 15: 8210. https://doi.org/10.3390/app15158210

APA Style

Wang, L., Liang, L., Xu, J., Wang, Y., Yan, B., Chen, G., Li, N., & Hou, L. (2025). Pilot-Scale Fenton-like System for Wastewater Treatment Using Iron Mud Carbon Catalyst. Applied Sciences, 15(15), 8210. https://doi.org/10.3390/app15158210

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