Sludge-Derived Hercynite–Carbon as a Low-Cost Catalyst for Efficient Degradation of Refractory Pollutants in Wastewater

Abstract

Developing a robust Fenton-like catalyst through a feasible method is a significant challenge and is crucial for sustainability in wastewater treatment. Herein, we report a novel dual-phase H₂O₂ activation for ^•OH generation via both heterogeneous surface-mediated reactions and homogeneous radical propagation pathways. Mechanistic investigations revealed that the surface Fe²⁺/Fe³⁺ redox cycle was the primary driver of catalysis at pH 5. Notably, the catalyst produced fewer secondary pollutants than Fenton reactions and was effective in treating pollutants with high concentrations. The oxidative performance of the PAS-ISe was comparable to that of commercial FeSO₄·7H₂O in terms of chemical oxygen demand (COD) removal efficiency and reaction kinetics. Besides, the utility of the catalyst was 2-75-fold greater than that of state-of-the-art Fenton or photo-Fenton-like catalysts. A detailed techno-economic analysis confirmed the feasibility of this strategy and significant cost advantages over existing heterogeneous catalyst synthesis methods. This study concurrently proposes a low-cost approach to valorizing hazardous sludge and effectively treating industrial wastewater, which may support circular economic principles.

1. Introduction

The Fenton reaction is a widely used method for treating complex organic pollutants in industrial wastewater. This process involves the organics’ oxidation by ^•OH radicals generated through H₂O₂ activation by Fe²⁺ under acidic conditions. The ferrous ions rapidly react with H₂O₂, converting into Fe³⁺ and producing ^•OH (Equation (1)). These radicals are powerful oxidative species with a standard reduction potential (E⁰) of +2.8 V [1]. The Fe³⁺ can undergo secondary reactions with H₂O₂, regenerating Fe²⁺ (Equation (2)). However, the secondary reaction proceeds at a much slower rate compared to the primary reaction [2]. As a result, the production rate of ^•OH decreases over time, leading to the generation of a significant amount of iron sludge as a byproduct. This iron sludge is considered solid waste and is typically disposed of at a minimal cost. Recently, several studies have focused on reusing iron sludge by creating heterogeneous catalysts [3]. The controlled thermochemical treatment of Fenton sludge eliminates volatile organic compounds and reduces iron species, which can perform as a catalyst. This strategy not only reduces waste volume but also creates a high-value product, advancing circular economy principles by repurposing waste into a resource.

Fe²⁺ + H₂O₂ → Fe³⁺ + ^•OH + OH⁻

(1)

Fe³⁺ + H₂O₂ → Fe²⁺ + ^•OOH + H⁺

(2)

Typically, the iron species in iron sludge are Fe(OH)₃ or α-FeOOH, which contains Fe³⁺ and is less effective in catalysis [4]. When iron sludge is heated in an oxygen-rich environment, it produces Fe₂O₃, containing Fe³⁺ species similar to the iron sludge. A pyrolysis process can reduce these iron oxidation states from Fe³⁺ to Fe²⁺ (e.g., Fe₃O₄ or FeO) strategy for fabricating a sludge-derived hercynite catalyst through low-temperature complexation of iron sludge extract onto activated sludge-derived porous carbon. The optimized catalyst, named PAS-ISe, facilitated spontaneous electron transfer, enabling or even to Fe⁰, depending on the pyrolysis conditions [5]. This process reintroduces catalytic characteristics. However, the phase transition of iron in sludge during pyrolysis is complex due to the presence of other minerals and organic matters, resulting in a composite of Fe, Al, Si, and various other minerals. As a result, the material can facilitate heterogeneous catalysis in the presence of H₂O₂. Nonetheless, the reaction kinetics of these catalysts are considerably slower due to poor electron transfer efficiency from the catalyst to the reactant and are less effective than the homogeneous Fenton reaction process [6]. To enhance the performance of Fenton-like catalysts, various efficient catalysts have been recently developed, including single-atom catalysts, mixed metallic catalysts, and organic-supported metal catalysts [7]. Some recently reported high-performance Fenton-like catalysts include FeN₄ catalyst, synthesized via the atom-sacrificing method using cetyltrimethylammonium bromide [8]; FeP catalyst, synthesized from sodium hypophosphite and α-ferric oxide [9]; Fe-based magnetic catalyst, produced from iron precursors with a NaOH dosage exceeding 5 wt% [10]; ferrihydrite-loaded CNTs catalysts [11]; and Fe-SAC, prepared by pyrolysis-ball milling and HF treatment [12]. These catalysts exhibit activity comparable to or exceeding the Fenton reaction by several orders of magnitude. Some catalysts are also efficient at neutral pH, which may reduce pH-adjusting chemical consumption during wastewater treatment [8,11,12]. Nevertheless, most of these catalysts are synthesized using large quantities of high-cost materials, which can be 10 to 100 times more costly than FeSO₄·7H₂O. Additionally, previous studies have overlooked the actual yield achieved after the synthesis process. Typically, the final yield of these catalysts is low, especially when they are produced from waste materials and involve multiple preparation steps, such as carbonization, etching, and rinsing [13]. Considering the yield and preparation process cost, the industrial application of these catalysts is highly challenging despite their good catalytic performance. Thus, developing highly efficient Fenton-like catalysts through a low-cost synthesis method is in high demand.

At present, refractory organic pollutants are frequently detected in wastewater worldwide. For instance, carbamazepine (CBZ), tetracycline (TC), and sulfamethoxazole (SMX) are commonly found in wastewater from both China and the United States [14,15]. The growth of industries and increased pharmaceutical consumption release large quantities of industrial organic pollutants and antibiotics into the aquatic environment. These pollutants have numerous toxic effects on ecosystems, and antibiotics in particular can promote the development of antibiotic-resistant bacteria. While some of these organics are easily biodegradable, others require advanced treatment processes due to their persistent nature, ability to bioaccumulate in organisms, and consequent threat to public health. Various photocatalysts are able to effectively decompose these pollutants using sunlight [16,17,18]. However, scaling up photocatalytic processes for industrial wastewater treatment remains a significant challenge. Therefore, producing an effective Fenton-like catalyst through a feasible method is a more practical solution.

Herein, we prepared a series of catalysts derived from iron sludge and activated sludge using feasible acid treatment and carbonization methods. The performance of catalysts was evaluated by assessing their ability to activate H₂O₂, remove wastewater COD, and degrade refectory organics, as well as their recyclability. In addition, we identified the catalytically active sites, proposed a catalytic mechanism, and compared the performance of our catalysts with high-performance catalysts reported in the literature. Finally, we conducted a detailed techno-economic analysis to highlight the applicability of these catalysts in industrial wastewater treatment plants.

2. Materials and Methods

2.1. Materials

Dry activated sludge, iron sludge, and agrochemical-pharmaceutical wastewater were obtained from United Environmental Water Treatment (Dafeng) Co., Ltd., Yancheng, China. Acetaminophen (APAP), sulfamethoxazole (SMX), tetracycline (TC), carbamazepine (CBZ), methylene blue (MB), tert-butanol (TBA), methanol (MeOH), p-benzoquinone (BQ), L-histidine (L-his), methyl phenyl sulfoxide (PMSO), phenyl methyl sulfone (PMSO₂), sulfuric acid, potassium iodide (KI), sodium hydroxide, ferrous sulfate heptahydrate (FeSO₄·7H₂O), and hydrogen peroxide (30%) were sourced from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. All chemicals were of analytical grade and used as received.

2.2. Catalyst Preparation Methods

Firstly, the as-received sludge was ground and sieved through a 100-mesh screen. A porous carbon substrate was synthesized from activated sludge via a literature-reported method [19]. Briefly, activated sludge was carbonized at 800 °C for 1 h under N₂. The resulting carbon was acid-washed with 3 M HCl for 48 h, rinsed with water, dried, and post-pyrolyzed at 800 °C for 30 min to yield the final product (labeled PAS). Slurry iron sludge extract (ISe) was prepared by treating 10 g of iron sludge powder with 50 mL of 3 M H₂SO₄ for 24 h, followed by filtration. ISe was coated onto PAS by mixing in water at 80 °C, followed by thermal treatment at varying temperatures. Among tested ISe loadings (5–50 wt%) and heating conditions (220–800 °C in air/N₂), the catalyst with 10 wt% ISe baked at 220 °C for 2 h in air showed optimal COD removal (Figure S1a,b) and was designated PAS-ISe. Besides, several iron-based reference catalysts were prepared for comparison, such as Raw IS: untreated iron sludge powder; IS-220: iron sludge baked at 220 °C (2 h, air); IS-900: iron sludge pyrolyzed at 900 °C (1 h, N₂), acid-washed (0.2 M H₂SO₄), and rinsed; ISe-dry: ISe dried at 100 °C; and ISe-220: ISe-dry baked at 220 °C (2 h, air).

2.3. Catalytic Performance Test

The catalytic performance of synthesized catalysts was assessed based on their ability to (1) decompose H₂O₂, (2) remove wastewater COD, (3) degrade refractory pollutants (e.g., acetaminophen), and recycling ability. Performance was benchmarked against commercial FeSO₄·7H₂O to evaluate practicality. The optimal catalyst was further tested with other refractory organics, identifying catalytic influencing factors and reaction mechanisms. In a typical experiment, 200 mg/L catalyst was reacted with 1.96 mM H₂O₂, with or without 96 mg/L COD wastewater or 200 ppm pollutant. All experiments were performed in triplicate, and the mean values with error bars are presented. H₂O₂ and pollutant concentrations were measured at baseline and selected intervals by collecting 1 mL samples, filtering (0.45 µm nylon), and analyzing each via: H₂O₂: potassium titanium (IV) oxalate colorimetry (SI Text S1); COD: Dichromate method; SI Text S2); organics: HPLC (APAP, PMSO, PMSO₂) or UV-Vis spectrophotometer (others; SI Text S3). Solution pH was adjusted (0.1–1 M NaOH/H₂SO₄) to probe pH-dependent activity. For the recycling experiments, the spent catalyst was regenerated by removing adsorbed organics and detaching iron particles through washing and treatment with H₂O₂. Then, ISe was recoated onto the PAS surface following the method described in Section 2.2 (details provided in SI Text S4).

2.4. Amperometric Experiments

The electrocatalytic activity of the PAS-ISe material was evaluated via amperometric testing. A working electrode was prepared by depositing the catalyst onto a carbon cloth. The electrode comprised approximately 85% active material (PAS-ISe), 10% polyvinylidene fluoride (PVDF) binder, and 5% multi-walled carbon nanotubes (MWCNTs) conductive materials. The electrochemical cell consisted of this prepared working electrode, a platinum counter electrode, and an Hg/HgO reference electrode, immersed in a 0.1 M Na₂SO₃ electrolyte. The current-time response and open-circuit potential of the PAS-ISe electrode were measured and compared against a reference electrode made of bare carbon cloth. Detailed experimental procedures are provided in the Supporting Information (Text S5).

2.5. Characterization

The catalyst’s surface morphology was examined using scanning electron microscopy (SEM) (ZEISS MERLIN Compact, Ostalbkreis, Germany). The textural properties, including surface area and pore characteristics, were determined by Brunauer-Emmett-Teller (BET) analysis (Santa Cruz, CA, USA) and pore size distribution measurements (Micromeritics ASAP 2460, Norcross, GA, USA). The crystal phase structure was identified via X-ray diffraction (XRD) (Rigaku Ultimate IV, Tokyo, Japan). Elemental composition was analyzed using X-ray fluorescence (XRF) (Rigaku ZSX Primus III, Tokyo, Japan) and ultimate analysis (Elementar UNICUBE, Langenselbold, Germany). Additionally, surface chemical species were characterized by X-ray photoelectron spectroscopy (XPS) (Thermo Fisher ESCALAB 250Xi, Waltham, MA, USA).

3. Results and Discussion

3.1. Catalysts Characterization

Raw powder-activated sludge biochar exhibited a nonporous and irregular surface structure (Figure 1a). Acid pickling significantly improved its pore volume and surface area from 0.16 cm³/g and 96.4 m²/g to 0.42 cm³/g and 358.4 m²/g, respectively, and the resulting material was named PAS. The pore volume of the PAS consists of micropores and mesopores, as confirmed by the steep rising curve at <0.02 P/P₀ and a type-IV hysteresis loop in the 0.4–1 P/P₀ region (Figure 1b). DFT calculations revealed a microporous surface area of 113.5 m²/g (total surface area: 251.4 m²/g). This enhanced porosity facilitated uniform ISe dispersion. After ISe coating, the surface area and pore volume decreased to 228.1 m²/g and 0.31 cm³/g, respectively. The inset in Figure 1b confirms pore blockage, as PAS-ISe showed reduced micropore/mesopore intensity than PAS. Despite pore reduction, PAS-ISe still maintains a sufficiently large surface area and likely has well-dispersed surface active sites. Various metal species associated with the ISe dispersion on the PAS surface are displayed in Figure 1c.

The phase structure of PAS was primarily halite (PDF# 75-0306) and calcite (PDF# 05-0586) (Figure 1d). These components may activate the biochar during carbonization and contribute to pore formation. The phase structure in iron sludge was goethite (PDF# 81-0462), the typical iron species in sludge. Heating iron sludge in the air ambiance facilitates the dehydroxylation of FeOOH (Equation (3)), resulting in the formation of hematite (Figure S2). The pyrolysis of iron sludge reduces these iron species by reacting with reducing agents, such as CO and H₂. Resultantly, IS-900 yielded magnetite, which consists of both Fe³⁺ and Fe²⁺ [20]. In contrast, the ISe-dry sample did not exhibit any iron phase structure and instead yielded germanium phosphate. When it was heated to 220 °C, iron sulfate and rhomboclase phase structures were formed. This indicates that the extraction process allows complete dissolution of FeOOH, and the dissolved ions can form crystalline structures upon heating. Consequently, the PAS-ISe sample exhibits a different crystal structure of hercynite (PDF# 82-0579) and hematite (PDF# 85-0599). The dissolved ion species on the surface of PAS may complex with aluminum and other components, leading to the formation of hercynite and hematite structures. Hematite may exhibit poor catalytic activity. However, hercynite contains a divalent iron species, and hercynite-anchored carbon surfaces can function as heterogeneous catalysts (Equation (4)) [21]. Additionally, the substitution of Al³⁺ ions in hercynite may alter the electronic properties and enhance the catalytic performance.

2FeOOH ⟶ Fe₂O₃ + H₂O↑

(3)

Fe²⁺_(surf.) + H₂O₂ ⟶ Fe³⁺ + ^•OH + ⁻OH

(4)

XRF analysis was used to reveal the elemental contents in the catalysts. As shown in Figure 2a, Fe and Cl are the predominant elements in most of the catalysts. The mass ratio of Fe is significantly higher in the IS sample (62.6%). In the activated sludge sample, Cl is the dominant element (47.3%), followed by Fe (21.3%). After coating ISe onto PAS, the contents of Fe and Cl decrease due to acid washing and the coating of a lower amount of iron sludge extract on the PAS surface. Additionally, PAS-ISe exhibits higher peaks for other elements, such as Al, Si, and Ca. This change may be attributed to complexation of ISe onto the PAS surface. This composition aligns with the results from XRD and the XPS survey (Figure 2b). The complete list of elements and their corresponding oxides from XRF analysis is shown in Table S1. The high-resolution Fe 2p peaks of PAS-ISe were deconvoluted into five distinct peaks. The peaks at 710.2 eV and 723.3 eV are assigned to Fe²⁺ [22], whereas those at 711.5 eV and 725.3 eV correspond to Fe³⁺ [23], indicating the presence of mixed-valent iron species in the catalyst. Furthermore, PAS-ISe has a low nitrogen content of 2.3%. The presence of Fe²⁺ and nitrogen species may be conducive for catalytic activities [24].

3.2. Catalytic Performances

3.2.1. Catalytic Performance Comparison

The catalytic performance of the synthesized catalyst and commercial FeSO₄·7H₂O was initially assessed by their ability to decompose H₂O₂. The pH for these comparative studies was maintained at 3, as the optimal pH range for the Fenton reaction is between 2.8 and 3.2. As shown in Figure 3a and the corresponding UV-vis spectrum absorbance at 400 nm in Figure 3b, PAS-ISe exhibited a superior H₂O₂ removal efficiency of 97.6%, while the Fenton reaction process removed only 62.5%. The FeSO₄·7H₂O process relies entirely on homogeneous catalysis for the decomposition of H₂O₂. In contrast, the performance of PAS-ISe may involve a combination of adsorption, heterogeneous catalysis, and homogeneous catalysis. The H₂O₂ decomposition performance of raw IS and IS-220 was notably low. This can be attributed to the poor catalytic performance of Fe³⁺. Despite having Fe²⁺/Fe³⁺, IS900 demonstrated poor H₂O₂ removal efficiency, indicating the slow catalytic activity of Fe₃O₄ due to a limited number of surface active sites and the sluggish regeneration of Fe²⁺ [6]. Surprisingly, the iron sludge-extracted samples, ISe-dry and ISe-220, demonstrated excellent H₂O₂ decomposition performances of 53.1% and 68.6%, respectively, making them comparable to the Fenton reaction. To further investigate the effectiveness of catalysts, we examined the performance of PAS-ISe and FeSO₄·7H₂O in removing high concentrations of H₂O₂ (e.g., 0.98–7.84 mM) using a reduced dose of catalyst (50 mg/L) over a duration of 1 to 3 h. Figure 3c shows that, after 1 h of treatment, the PAS-ISe removed 79.6–71.2% of 0.98–7.84 mM of H₂O₂, while the performance of FeSO₄·7H₂O was 47.2–28.1%, respectively. Although the performance of both catalysts improved after 3 h of treatment, PAS-ISe exhibited significantly higher effectiveness than FeSO₄·7H₂O. Notably, the removal of H₂O₂ during catalysis may not only represent the degradation of it caused by the Fe²⁺/Fe³⁺ redox reactions but also the secondary consumption of H₂O₂ by already produced ^•OH (Equation (5)). To quantify only Fe²⁺/Fe³⁺ redox-induced H₂O₂ decomposition, we suspended ^•OH-based H₂O₂ consumption by applying different doses of TBA and MeOH. These scavengers can convert ^•OH (free or surface-bond) to inert products without affecting the Fe²⁺/Fe³⁺ and H₂O₂ reactions (Equations (6) and (7)). At a specific dose, TBA demonstrated better ^•OH scavenging than MeOH. When using TBA (1.96–19.6 mM), the performance of H₂O₂ removal decreased from 97.6% and 62.5% to 56.7% and 23.4% for PAS-ISe and FeSO₄·7H₂O, respectively (Figure S3). For a similar catalyst dose, PAS-ISe showed 2.7 times greater H₂O₂ decomposition compared to the Fenton reaction.

H₂O₂ + ^•OH → HO₂^• + H₂O

(5)

^•OH + (CH₃)₃COH ⟶ H₂O + (CH₃)₂•CCH₂OH ⟶ acetone    k_OH = 6 × 10⁸ M⁻¹ s⁻¹

(6)

^•OH + CH₃OH ⟶ ^•CH₂OH + H₂O ⟶ Formic acid/CO₂    k_OH = 9.7 × 10⁸ M⁻¹ s⁻¹

(7)

In the organic pollutants catalytic degradation experiments, the catalyst, H₂O₂, and pollutant doses were set at a ratio of 2:1:1 for COD removal experiments and 1:1:1 for other organic pollutant degradation. This was because the recommended ratio of Fe²⁺ and H₂O₂ for the Fenton reaction is 1:1 to 1:5 by mole, and the ratio of H₂O₂ (30%) to COD is 1:1 to 1:3 by weight. The COD removal performance of PAS-ISe and FeSO₄·7H₂O was comparable, with efficiencies of 27.4% and 27.1%, respectively (Figure 3d). Although PAS-ISe demonstrated greater effectiveness in decomposing H₂O₂, its performance in removing wastewater COD was comparable to that of the Fenton reaction. This may be due to the wastewater hindering some of PAS-ISe’s catalytic activities or the combined effects of adsorption and catalysis contributing to the high removal of H₂O₂, indicating PAS-ISe’s higher surface area and well-dispersed active sites are highly beneficial for H₂O₂ activation. The ISe-dry, ISe-220, and IS-900 showed 6.8%, 12.4%, and 5.3% COD removal efficiency, whereas Raw IS and IS-220 performances were quite low. Since wastewater pollutants vary significantly across industries, the catalysts’ ability to degrade refractory organic pollutants was assessed to compare their performance with catalysts in the literature. APAP was selected for detailed studies due to its higher solubility and good stability compared to TC, SMX, CBZ, and MB. Figure 3e shows that APAP removal efficiency correlated closely with COD decomposition. The best APAP removal of 36.4% was achieved with PAS-ISe, while FeSO₄·7H₂O removal performance was 34.8%.

3.2.2. Effect of pH

Solution pH is a crucial factor in the Fenton reaction, and adjusting the pH significantly increases the cost of the wastewater treatment process. Figure 3f shows that the PAS-ISe catalyst also performs better at pH 5 for APAP degradation. Under these conditions, the adsorption performance slightly increased from 4.6% to 6.3%, while the degradation performance remains comparable. PAS-ISe also effectively removed various antibiotics and dyes at pH 5, achieving 24.7% removal of CBZ, 14.4% of TC, 13.2% of SMX, and 39.3% of MB (Figure S4). It is noteworthy that MB, CBZ, and TC showed better catalysis at pH 3 while APAP and SMX showed better catalysis at pH 5. It could be due to their water solubility, ionization state, and electron density. In the advanced oxidation process, more soluble chemicals tend to degrade effectively due to their greater interactions with reactive oxygen species (ROS). APAP is more soluble in alkaline conditions because of the deprotonation of its phenolic hydroxyl group, leading to enhanced degradation at elevated pH. APAP degradation favorability at near-neutral pH in the Fenton system was also reported in previous studies [25,26]. SMX also showed better degradability at high pH due to its ionic state. The Zwitterionic (SO₂NH⁻, NH₃⁺) SMX shows more ^•OH-induced degradation susceptibility than neutral SMX (SO₂NH₂, NH₃⁺). In this study, we select these pollutants and different solution pH to demonstrate the catalyst’s effectiveness in treating chemicals with various ionization states. For example, APAP and CBZ are neutral at pH 3 and 5, TC is zwitterionic at pH 3 and partially deprotonated at pH 5, SMX is neutral at pH 3 and zwitterionic at pH 5, and MB is protonated at pH 3 and neutral at pH 5. Besides, these pollutants, particularly CBZ, TC, and SMX, are often detected in Chinese and American wastewater and treated water [14,15]. The performance of PAS-ISe is compared with high-performance Fenton and photo-Fenton-like catalysts from the literature. Table S2 indicates that the utility of our catalyst, calculated as the mole fraction of removed pollutant per mole of H₂O₂ and weight of catalyst at pH 5, is approximately 2-75-fold higher than that of the catalysts reported in the literature. Additionally, this performance is close to that of single-atom catalysts’ PMS activation performance for some pollutants [12], indicating excellent oxidative effectiveness of PAS-ISe.

3.2.3. Catalytic Reaction Path

The APAP degradation kinetics for PAS-ISe can be fitted using both pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models (Figure 4a). Both models fit well with the degradation data, achieving R² values of 0.998 and 0.991, respectively. This fit indicates that the catalytic process might combine homogeneous and heterogeneous catalysis, and the APAP adsorption onto the surface also has some role. To quantify the role of homogeneous catalysis in the degradation process, the PAS-ISe catalysts were treated with water at pH 5 for 1 h to facilitate the leaching of metals into the solution. The supernatant containing leached metal ions was filtered and then mixed with APAP (200 mg/L) and H₂O₂ (1.96 mM). After treatment for 1 h, the APAP removal efficiency was 16.2%, suggesting that homogeneous catalysis contributed nearly half of the total catalytic performance (Figure 4a inset). Homogeneous catalysis is typically performed by soluble metal ions leached from the catalyst, and high metal leaching during catalysis is unacceptable for treated wastewater discharge. To measure the leached iron content, an ICP-AES analysis of the supernatant was performed for both wastewater treatment and APAP degradation. We focused specifically on iron leaching because it is the primary metal content in PAS-ISe (Figure 2a). As listed in Table S3, 0.189 mg/L Fe leached during the APAP treatment. However, when the pH was adjusted after the Fenton reaction and 2 mg/L of polyacrylamide (PAM) flocculant was added to remove treated intermediates, the leached iron was reduced to 0.041 mg/L. Under these conditions, FeSO₄·7H₂O showed 0.044 mg/L iron leaching. On the other hand, 0.056 mg/L of iron was detected in the treated wastewater supernatant. The pH adjustment and trace particle removal are essential post-treatment steps to prevent particle carryover into discharge water or tertiary adsorption columns. According to the Chinese standard for wastewater discharge (GB 8978-1996) [27], iron concentrations must be <1 mg/L for industrial effluent and <0.3 mg/L for domestic WWTPs. Considering the iron leaching propensity of PAS-ISe, if the COD of the influent is 5.3 times higher than the wastewater used in this study, our catalyst at proportionally higher doses maintains COD removal efficiency while keeping iron leaching <0.3 mg/L, indicating the effectiveness of the material in treating highly concentrated wastewater. Although PAS-ISe and FeSO₄·7H₂O showed comparable pollutant degradation performance. The significant advantage of PAS-ISe, however, lies in its heterogeneous nature. It not only maintains faster reaction kinetics but also offers considerable practical benefits, most notably remarkably low iron leaching. This characteristic is crucial for applications, as it minimizes catalyst loss and significantly reduces iron sludge production compared to the homogeneous FeSO₄·7H₂O system.

3.2.4. Catalyst Stability

The phase structure stability of the catalyst was evaluated using XRD analysis. Figure 4b shows that the hercynite and hematite phases in spent and regenerated PAS-ISe were unchanged compared to the fresh sample, confirming catalyst stability. However, the sharp peak for halite at 31.77° (2θ) is absent in the spent and regenerated catalysts, likely due to its dissolution caused by its high solubility. The amount of sludge produced is another crucial factor determining the cost of the catalytic process. Since APAP is a single pollutant, the sludge generated during its degradation may not accurately reflect the actual industrial wastewater treatment scenario. Therefore, we calculated the sludge production after the wastewater treatment. Following the COD removal method outlined in SI Text 2, we calculated the sludge amount upon filtration and drying. The Fenton process generated 64.8% sludge, compared to only 21.6% for the PAS-ISe catalytic system. The reduction in sludge generation in the PAS-ISe system is attributed to the regeneration of spent catalyst. After regenerating PAS-ISe, APAP removal performance decreased from 37.5% to 33.4% (Figure 4c) and COD removal performance reduced from 27.4% to 18.9% (Figure S5). It is a typical scenario, as the robustness of the reaction often reduces after recycling catalysts. We did not extend the reaction time due to the short catalytic duration typically used in most industrial catalytic processes. When the dosage of ISe re-coating was increased from 10% to 20%, the degradation performance of APAP improved to 39.2% after recycling. This enhancement is likely due to the more homogeneous catalytic process facilitated by the higher ISe loading. A recent study concluded that combining homogeneous and heterogeneous catalytic processes is an efficient approach for treating industrial wastewater, especially since conventional heterogeneous catalysts often fail to achieve the desired catalytic performance and robustness [6].

3.2.5. Electron Transfer Potential

Amperometric tests were conducted to investigate the electron transfer potential between PAS-ISe and H₂O₂/APAP system. The current versus time graph in Figure 4d (upper section) demonstrates that introducing H₂O₂ into the electrolyte after 60 s significantly changes the current amplitude of the PAS-ISe electrode, indicating a spontaneous electron transfer between the electrode and H₂O₂ [28]. After injecting 0.1 mM APAP at 120 s, a slight change in current amplitude was observed (Figure 4d inset; 107–140 s). However, when the APAP concentration was increased to 4 mM, a significant change in current amplitude occurred for the PAS-ISe electrode, while the carbon cloth exhibited no response. This phenomenon suggests that the injection of APAP enhances the electron transfer from the electrode to the system. It could be attributed to spontaneous electron transfer from the electrode to H₂O₂, resulting in the production of ROS. The catalytic process is likely dominated by a radical mechanism, as the non-radical process shows an opposite change in current amplitude after adding organics to the catalyst/oxidant system due to electron transfer from the pollutants to the catalyst/oxidant complex [29]. The open circuit potential curve in Figure 4d (lower section) shows that the potential of PAS-ISe significantly increased after adding H₂O₂. The subsequent addition of APAP after 120 s slightly affects the curve, indicating the formation of ROS in the PAS-ISe and H₂O₂ system [9]. In contrast, adding H₂O₂ and APAP had little impact on the carbon cloth electrode. The exceptional electron transfer resulting from low resistance may be a critical factor for PAS-ISe’s high catalytic performance, whereas conventional heterogeneous catalysts exhibit lower catalytic activity due to high electron transfer resistance and subsequent poor Fe²⁺/Fe³⁺ redox cycling [30].

3.3. Catalytic Mechanisms

Various radical scavengers were employed to identify the role of specific ROS in the catalysis. Figure 5a shows that the degradation of APAP decreased by 83.5%, 54.6%, 76.2%, and 94.9% using TBA, BQ, L-his, and PMSO probes, respectively. The degradation inhibition using TBA suggests that the free ^•OH plays a significant role in the catalysis, while the inhibition using a PMSO probe indicates the presence of high-valent iron (≡Fe⁴⁺=O) besides ^•OH. Although L-his also showed high scavenging performance, which indicated ¹O₂ formation, this was due to pH alteration. We measured the pH of solutions before and after the scavenging experiments (Table S4). The pH changes before and after catalysis using no scavenger, TBA, and PMSO were minimal, accurately reflecting the impact of scavengers. In contrast, BQ caused the solution to become slightly acidic, while L-his resulted in an alkaline solution. To address this, we added acetate buffer for the BQ and L-his-induced catalysis. The inclusion of buffers resulted in a reduction of APAP degradation by 23.5% and 28.9% for BQ and L-his, respectively. This could be attributed to the iron chelation, slightly slowing the activation of H₂O₂ by stabilizing the pH and scavenging some ^•OH. A similar scavenging trend was observed for APAP degradation at pH 3 (Figure S6a); however, the TBA and PMSO scavenging effects were reduced compared to the inhibition observed at pH 5.

To verify the scavenging results in APAP degradation, we conducted a similar scavenging experiment with a cationic pollutant. Figure 5b,c, along with their corresponding UV-Vis spectra in Figure S6b,c, demonstrate that MB degradation at pH 5 and pH 3 is significantly inhibited using TBA and PMSO. However, unlike APAP, the use of BQ exhibited limited scavenging effects on MB degradation at both pH 3 and pH 5. This suggests that superoxide radicals (O₂^•−) are unlikely to be produced during catalysis. Since PMSO displayed optimal scavenging performance in both APAP and MB degradation, we identify the PMSO role in catalysis. The PMSO probe can be oxidized to produce PMSO₂ by iron-oxo species (Equation (8)) during catalysis [31]. Figure S6e shows that after a reduction of 2.84% (0.56 mM) during the first minute of the reaction, the concentration of PMSO remains unchanged in the H₂O₂/PAS-ISe system, and no production of PMSO₂ was detected, suggesting that high-valent iron species were not produced during the catalytic process. It appears that PMSO primarily acts as a scavenger for ^•OH radicals (Equation (9)) and as a chelator of iron species. The Fe²⁺ from PAS-ISe may be partially inhibited by thiol groups, which could impede the Fe²⁺/Fe³⁺ redox cycle. Furthermore, PMSO can also stabilize Fe³⁺, reducing the regeneration of Fe²⁺.

It is important to note that PMSO has a significant inhibitory effect in PAS-ISe catalysis. However, when using FeSO₄·7H₂O, the addition of 19.6 mM of PMSO did not remarkably scavenge the degradation of pollutants (Figure S6f). This phenomenon indicates that the PMSO addition significantly hindered the Fe²⁺-surface-induced catalysis of PAS-ISe, while the catalysis by dissolved Fe²⁺ from FeSO₄·7H₂O is less restricted. This result suggests distinct ^•OH production pathways in the PAS-ISe and FeSO₄·7H₂O systems, with heterogeneous catalysis playing a significant role in the PAS-ISe system. We also investigated the roles of KI and MeOH scavengers to identify surface-bonded hydroxyl radicals ^•OH [32]. However, the scavenging ability of MeOH was not as effective as that of TBA, and KI contributed to the degradation of APAP due to iodine-mediated oxidation in the presence of H₂O₂.

From the discussion above, it can be concluded that the ^•OH generated through both heterogeneous and homogeneous catalytic processes was crucial for the degradation of pollutants. The remarkable electron transfer capability of PAS-ISe activated H₂O₂ spontaneously, resulting in significant production of ^•OH. This production of ^•OH in the PAS-ISe/H₂O₂ system after a 5-min reaction was confirmed through EPR analysis using DMPO (Figure 5d). The homogeneous catalytic process likely dominated at pH 3, while at pH 5, both heterogeneous and homogeneous catalysis functioned effectively.

PMSO + ≡Fe⁴⁺=O → PMSO₂ + ≡Fe²⁺

(8)

PMSO + ^•OH → ^•PMSO–OH + H₂O    k_OH = 10⁹–10¹⁰ M⁻¹ s⁻¹

(9)

The degradation intermediates of APAP were identified via positive-ion mode mass spectrometry. The primary fragment ions at m/z 134.9, 132.8, 112.9, 129.9, 136.9, 167, 103, 146.9, 182, and 203 after 10 min of catalysis collectively suggest a multi-step oxidative degradation pathway (Figure S7). Based on the identified products, we proposed an APAP degradation path in the H₂O₂/PAS-ISe system (Figure 6). The para carbon adjacent to the phenolic –OH group in APAP has the highest π-electron density due to resonance from both the electron-donating –OH and –NHCOCH₃ groups, rendering it the primary site for electrophilic ^•OH radical attack, leading to the formation of N-acetyl-p-benzoquinone imine (NAPQI). This highly reactive intermediate was subjected to hydrolysis to yield p-benzoquinone or p-aminophenol [33], which further degraded via hydroxylation/oxidation to ring-opened products, such as 4-aminobutanoic acid (m/z 103) or malonic acid (m/z 104) [34]. Concurrently, APAP can undergo hydroxylation at the ortho carbon of the –OH group due to the increased electron density resulting from the resonance of –OH (ESP map in Figure 6), leading to the formation of 3-OH-APAP (m/z 167). This compound is highly unstable and can undergo hydrogen abstraction and bond homolysis to produce maleic acid and acetylaminyl radicals. Maleic acid can further participate in cyclization and oxidation reactions with TBA-generated radicals, forming sodium or water-dihydrofuranone, which corresponds to m/z 132.8 and 134.9, respectively. Na⁺ may be present from the system, as the removal of NaCl after catalysis was evidenced by XRD results (Figure 4b). Some higher m/z, such as 182 or 203, were also detected during the catalysis process. These may correspond to multiple oxidation states of APAP, radical (TBA) adducts, or Na⁺ ion-coupled intermediates.

3.4. Techno-Economic Analysis

The techno-economic performance was evaluated by analyzing the costs associated with chemicals and energy used in the preparation of PAS-ISe, and by comparing its performance to that of FeSO₄·7H₂O. Details regarding the calculation of energy consumption are provided in SI Text 6. The calculations utilize industrial-grade chemical prices from the current Chinese market. The preparation process for PAS involves three main steps: carbonization at 800 °C for 1 h, H₂SO₄ pickling for 48 h, and a final post-pyrolysis at 800 °C for 30 min. The material yields after these processes are as follows: 57.5% after carbonization, 19.4% after acid treatment, and 17.5% after post-pyrolysis. Taking into account the yield reductions at each stage, the total electrical energy consumed to produce 1 ton of PAS is 633.4 kWh, which costs 401.6 ¥. H₂SO₄ pickling reduces 33.7% of the material. Considering it and the dose of used H₂SO₄ (6 mL of 3M H₂SO₄ used for 1 g of carbon), the cost required to obtain 1 ton of treated carbon after pickling is 573 ¥. Hence, for 1 ton of PAS production, the total energy and chemical cost can be 974.6 ¥. To obtain 1 ton of PAS, about 5.71 tons of dry activated sludge is required, whose disposal cost can be 7428.5 ¥. Among these 5.71 tons of activated sludge, 61.9% is converted into char or volatile matters, while 38.1% still remains as highly concentrated slurry. Thus, 1 ton of PAS production can reduce the disposal cost of 4598.2 ¥. Activated sludge from the agrochemical and pharmaceutical wastewater treatment industry is considered hazardous sludge, and in China, its disposal cost can be 1300 ¥ per ton [19].

One ton of dry ISe preparation required 1.29 tons of H₂SO₄ (18M) as the yield of ISe is 64.4%, which costs 419.7¥. Producing ISe from iron sludge can reduce the IS disposal cost of 167.4 ¥/t. Dry IS is considered solid waste, whose disposal cost is 260 ¥/t. For 1 ton of PAS-ISe preparation, 90% of PAS and 10% of ISe are required. The energy and chemical cost of final PAS-ISe, including its backing energy consumed, can be 948.4 ¥/t. This cost is 2.3 times the FeSO₄·7H₂O price (420 ¥/t). However, 1 ton of PAS-ISe can reduce the AS disposal cost of 4138.4 ¥ and the IS disposal cost of 16.7 ¥. Besides, after each cycle, the total sludge production using PAS-ISe is reduced by 3 times than that of FeSO₄·7H₂O. PAS-ISe is highly recyclable. The recycling cost (energy and chemical) can be as low as 71.3 ¥/t because of using the backing cost of 29.3 ¥/t, and ISe (10%) cost 41.97 ¥. It is also worth mentioning that the catalytic reaction can be performed at pH 5, which significantly reduces H₂SO₄ and NaOH consumption. The amount of pH-adjusting chemicals is difficult to quantify as the consumption depends on the specific water’s pH buffers (HCO₃⁻, CO₃²⁻, ⁻OH), resisting chemicals (organic acid, phosphate, ammonia), organic matters, and suspended solids. Nevertheless, PAS-ISe is not a complete substitute for FeSO₄·7H₂O in industry, as neither dry iron sludge nor activated sludge can be fully recycled, and a continuous input of them is required. However, PAS-ISe may significantly reduce the cost of water treatment and recycle a high amount of waste, which supports the circular economic principle.

4. Conclusions

In summary, this work introduces a novel strategy for coating iron sludge extract onto activated sludge-driven porous substrates to produce a high-performance hercynite catalyst. The catalyst demonstrates excellent physicochemical properties and exhibits high electron transfer potential in an H₂O₂ environment. As a result, it rapidly activates H₂O₂ and subsequently generates ^•OH, which efficiently degrades refectory organic pollutants. The performance of this catalyst was comparable to commercial FeSO₄·7H₂O, and they showed similar effectiveness in both synthetic and actual wastewater treatment. Additionally, the leaching of Fe after catalysis was low, and the catalyst effectively treated high concentrations of pollutants with high recyclability. The preparation method for this catalyst has proven to be more feasible than the heterogeneous Fenton-like catalyst in the literature. This work not only presents an innovative approach to valorizing waste sludge but also offers a low-cost method for treating wastewater, which may promote the principles of a circular economy.