Effective green treatment of sewage sludge from Fenton reactions: Utilizing MoS₂ for sustainable resource recovery

Significance

Our MoS₂-modified iron sludge, with exposed Mo⁴⁺ and low-coordination sulfur, achieves over 95% phenol degradation in eco-friendly Fenton systems. It maintains robust activity for at least 14 d across diverse conditions, ensuring compliance with discharge standards by efficiently eliminating COD during preparation. This highlights responsible utilization of sewage sludge resources. Optimizing the preparation method through a simpler boiling process and obtaining Fenton catalyst particles via high-temperature calcination granulation holds promise for large-scale iron sludge resource application in the future.

Abstract

Effectively managing sewage sludge from Fenton reactions in an eco-friendly way is vital for Fenton technology’s viability in pollution treatment. This study focuses on sewage sludge across various treatment stages, including generation, concentration, dehydration, and landfill, and employs chemical composite MoS₂ to facilitate green resource utilization of all types of sludge. MoS₂, with exposed Mo⁴⁺ and low-coordination sulfur, enhances iron cycling and creates an acidic microenvironment on the sludge surface. The MoS₂-modified iron sludge exhibits outstanding (>95%) phenol and pollutant degradation in hydrogen peroxide and peroxymonosulfate-based Fenton systems, unlike unmodified sludge. This modified sludge maintains excellent Fenton activity in various water conditions and with multiple anions, allowing extended phenol degradation for over 14 d. Notably, the generated chemical oxygen demand (COD) in sludge modification process can be efficiently eliminated through the Fenton reaction, ensuring effluent COD compliance and enabling eco-friendly sewage sludge resource utilization.

Water is the source of life, but the sewage and industrial wastewater produced by human production and life pose a huge challenge to the sustainable development of water resources (1). The Fenton reagent, comprising divalent iron ions (Fe²⁺) and H₂O₂, possesses potent oxidation capabilities, capable of targeting various organic substances present in water without discrimination. It proves particularly effective in treating organic wastewater resistant to biological or traditional chemical oxidation processes (2, 3). However, it is crucial to underscore that while it disintegrates organic pollutant molecules, a considerable amount of solid waste, known as iron sludge, is produced. Improper handling of this waste could potentially lead to significant secondary environmental pollution. Therefore, how to deal with Fenton iron sludge will determine the economy and feasibility of the Fenton oxidation process in future projects. Currently, various methods are employed for the treatment of Fenton iron sludge. These methods encompass recovering iron from the sludge for coagulant preparation (4, 5), reutilizing the iron mud as a source of iron (6–9), creating Fe-based catalysts or adsorbents (10–13), using it as a sludge conditioner (14), and employing it as an electron acceptor for anaerobic digestion (15, 16), among others. While these techniques are effective in utilizing iron sludge, they come with drawbacks such as high energy consumption and elevated costs, which hinder their widespread adoption in large-scale production. Consequently, there exists a pressing need to explore an economical, uncomplicated, and efficient approach for the reclamation of iron sludge.

MoS₂ serves as a widely employed co-catalyst in the Fenton reaction. In this process, exposed Mo⁴⁺ can engage in electron exchange with adsorbed Fe³⁺ on the surface, resulting in the generation of Mo⁶⁺ and Fe²⁺ (17, 18). This pivotal interaction enables the activation of H₂O₂ under acidic conditions. Subsequently, with the oxidation of H₂O₂, Mo⁶⁺ was transformed into pristine Mo⁴⁺. A series of valence state transitions between Mo and Fe species enhances the utilization of H₂O₂ and provides the possibility for the continuous degradation during the reaction (19). Hence, in theory, the valence state transformation between Fe³⁺ and Mo⁴⁺ can be extended to the initial preparation phase of the Fenton catalyst. This could enable the conversion of Fe³⁺ within the iron source to Fe²⁺ during the preparation process, thereby introducing a method for the recycling and effective utilization of iron sludge.

In this paper, we have successfully developed a green process for the environmentally friendly recycling of iron sludge. We prepared iron sludge-molybdenum sulfide binary composites (Sludge-MoS₂) through a simple one-step hydrothermal method, realizing pollution-free treatment of iron sludge from generation to final disposal. During the reaction, the catalyst utilizes the exposed Mo⁴⁺ active sites on its surface to significantly improve the cycle efficiency of Fe³⁺/Fe²⁺ in the Fenton reaction, achieving efficient degradation of phenols, antibiotics, organic dyes, etc. To validate the practicality of our approach, we investigated the effects of various water qualities and anions, which further supporting its validity. Concurrently, in order to expand the applicability of this method in real-world iron sludge treatment scenarios, we systematically combined iron slime at various industrial treatment stages with MoS₂ to investigate its degradation capabilities, thus fully realizing the resource utilization of Fenton iron sludge.

Results

Characterization of Materials.

Fig. 1A illustrates the synthetic procedure, where Sludge 1 was added to the molybdenum precursor reagents. The resulting flower-like Sludge 1-MoS₂-m (m represents the atomic ratio of Mo and Fe) was obtained through a one-step hydrothermal process at 200 °C. The microscopic morphology of the above compounds can be seen by a scanning electron microscope (SEM) (Fig. 1 B and C) and a transmission electron microscope (TEM) (Fig. 1 D and E). The illustration reveals that in the Sludge 1-MoS₂-m samples with different ratios, MoS₂ presents a flower ball shape, which can provide a large number of edge unsaturated S active sites. Notably, Sludge 1 particles became considerably smaller after hydrothermal treatment compared to untreated Sludge 1 (SI Appendix, Figs. S1 and S2). The structure of smaller particles allows Sludge 1 to disperse evenly and attach to the edge sheets of flower-like MoS₂, improving the utilization of iron in iron sludge. It can be seen from SI Appendix, Fig. S3, that after hydrothermal treatment, Sludge 1 turns into the well-known Fe₂O₃ brick red. The high-resolution transmission electron microscope (HRTEM) image shows that there are classical MoS₂ and Fe₂O₃ lattice stripes in Sludge 1-MoS₂-m (Fig. 1 F and G) (20). These observations strongly support the inference that the primary component of hydrothermally treated Sludge 1 is Fe₂O₃, which effectively combines with MoS₂, ultimately resulting in the formation of the final black product, Sludge 1-MoS₂-m (Fig. 1H). The Fourier transform infrared spectroscopy (FT-IR) spectrum also confirms that M-O (M represents Fe or Mo elements) vibrations exist in Sludge 1-MoS₂-0.07 and Sludge 1-MoS₂-0.14 (SI Appendix, Fig. S4). The X-ray diffraction (XRD) spectrum of the sample is shown in Fig. 1I. The characteristic peaks associated with Fe₂O₃ (JCPDS 33-0664) and MoS₂ (JCPDS 37-1492) are evident in the Sludge 1-MoS₂-m, reaffirming the successful amalgamation of Sludge 1 and MoS₂ through a straightforward hydrothermal process (21, 22). It is noteworthy that in Sludge 1-MoS₂-0.07 and Sludge 1-MoS₂-0.14 samples, the peak at 33.2° (104) shifts to a lower angle. This shift is attributed to the overlapping of the MoS₂ (101) crystal plane with the Fe₂O₃ (104) crystal plane and the presence of Mo and S atoms with larger atomic radius on the (104) crystal surface of Fe₂O₃. This further demonstrates that we have successfully synthesized the flower-like Sludge 1-MoS₂-m.

Fig. 1.

(A) Schematic illustration showing the preparation of Sludge 1. SEM images of (B) Sludge 1-MoS₂-0.07 and (C) Sludge 1-MoS₂-0.14. TEM images of (D) Sludge 1-MoS₂-0.07 and (E) Sludge 1-MoS₂-0.14. HRTEM images of (F) Sludge 1-MoS₂-0.07 and (G) Sludge 1-MoS₂-0.14. (H) Picture of the mass preparation of catalysts. (I) XRD spectrum of hydrothermally Sludge 1, MoS₂, Sludge 1-MoS₂-0.07 and Sludge 1-MoS₂-0.14.

Catalyst Oxidation Activity.

The oxidizing capability of the Sludge 1-MoS₂-m/H₂O₂ (PMS) system was evaluated using phenol as the model organic compound. The influence of various objective factors on the degradation efficiency has been systematically studied, such as the ratio of molybdenum and iron in Sludge 1-MoS₂-m, the catalyst dose, the oxidant (H₂O₂, PMS) dose, and the initial pH (detailed discussion is in SI Appendix, Figs. S5–S7). Notably, Sludge 1-MoS₂-0.07 exhibited the highest degradation efficiency for phenol, reaching 99.7% within 30 min, which was faster than other systems (SI Appendix, Fig. S8). This emphasizes the pivotal role played by the bonding between Sludge 1 and MoS₂ in enhancing pollutant degradation performance. In order to further explore the effect of compounding Sludge 1 with a single precursor in molybdenum sulfide, the degradation rate of phenol by Sludge 1-S (obtained by Sludge 1 and thioacetamide hydrothermally) and Sludge 1-Mo (obtained by Sludge 1 and sodium molybdate hydrothermally) within 30 min was investigated under the same conditions, and it was found that it was still at a negligible level, which confirms the importance of edge S active sites in MoS₂ for the activity of Sludge 1-MoS₂-m catalysts (SI Appendix, Fig. S9). Fig. 2A provides a clear and intuitive depiction of the superior pollutant degradation rate of Sludge 1-MoS₂-0.07. The first-order kinetic degradation rate constant for Sludge 1-MoS₂-0.07 is 0.187 min⁻¹, a value significantly surpassing other systems. Subsequently, we opted to examine the advanced oxidation performance of the catalyst using another commonly employed oxidant: PMS. Fig. 2B and SI Appendix, Fig. S10 clearly illustrate that the phenol degradation efficiency achieved by the Sludge 1-MoS₂-0.14 catalyst in the PMS system significantly surpasses that of the Sludge 1 system, the physical mixture of Sludge 1 and MoS₂ system, and the PMS-only system. Furthermore, catalysts generated from the hydrothermal reaction of either sulfur or molybdenum individually with Sludge 1 fail to enhance the activity for phenol degradation (SI Appendix, Fig. S11). This confirms the applicability of the Sludge 1-MoS₂-m catalyst to all types of Fenton/Fenton-like reactions, demonstrating its excellent universality. Especially, the total organic carbon (TOC) removal supports the more efficient mineralization of phenol in the Sludge 1-MoS₂-m system (Fig. 2 C and D). Whether in the H₂O₂ system or the PMS system, Sludge 1-MoS₂-m demonstrates a remarkable capacity for TOC removal, underscoring its significant environmental friendliness.

Fig. 2.

First-order kinetic curves of phenol degradation with different catalysts in the (A) H₂O₂ system and (B) PMS system. TOC removal ratios of phenol in the (C) H₂O₂ system and (D) PMS system. (E) EPR spectrum of •OH in the H₂O₂ system and PMS system. (F) Zeta potentials of Sludge 1, MoS₂, Sludge 1-MoS₂-0.07. (G) Py-IR spectra of Sludge 1 and Sludge 1-MoS₂-0.07. (H) Variation curves of Fe²⁺ concentration of Sludge 1, Sludge 1-MoS₂-0.07, and Sludge 1-MoS₂-0.14 in H₂O₂ and PMS systems. (I) DFT calculations of Sludge 1-MoS₂.

Exploration of the Degradation Mechanism.

Taking the H₂O₂ system as an illustrative case, we delved into the elemental composition and chemical states of the sample surface through X-ray photoelectron spectroscopy (XPS). SI Appendix, Fig. S12A, illustrates that upon compounding with MoS₂, the relative ≡Fe²⁺ content on the surface of Sludge 1 increases, coupled with the emergence of a Fe-S bond evident at 706.84 eV (Sludge 1-MoS₂-0.07). During the process of catalyst preparation, thioacetamide facilitates the reduction of surface ≡Fe³⁺ on Sludge 1 to ≡Fe²⁺ under conditions of elevated temperature and pressure. Hence, not only was the ≡Fe-S bond observed at 706.84 eV in Sludge 1-MoS₂-0.07, but also the splitting peaks at 162.21 eV for S 2p_1/2 and 163.32 eV for S 2p_3/2 exhibited shifts toward lower binding energies (SI Appendix, Fig. S12 A and B).This suggests the potential existence of Fe-S-Mo bonds. In addition, the O 1s XPS peak location shifts toward a high binding energy (SI Appendix, Fig. S12C). We speculate that this may be due to the introduction of the more electronegative Mo element. This conjecture is verified in the Mo 3d XPS spectra (SI Appendix, Fig. S12D). It is demonstrated that Mo-O-Fe and Mo-S-Fe bonds form after the composite of MoS₂ with Sludge 1. Due to the lower electronegativity of Fe compared to Mo, this leads to a shift in the positions of the peaks belonging to ≡Mo⁴⁺ toward lower binding energies, specifically at 231.62 eV and 228.54 eV.

In order to identify the dominant free radicals of the catalyst in the process of degrading phenol in the H₂O₂ system and PMS system, the active oxygen species were preliminarily determined by adding different quenchers [methanol (MeOH), tert-butanol (TBA), p-benzoquinone (p-BQ) and β-carotene] (23). As illustrated in SI Appendix, Fig. S13, in the H₂O₂ system, hydroxyl radical (•OH) plays the dominant role among reactive oxygen species (ROS) in the degradation of phenol, followed by superoxide radicals (•O₂⁻). In contrast, within the PMS system, the prominent ROS contributing significantly is sulfate radical (SO₄^•−). Electron paramagnetic resonance (EPR) can further qualitatively reflect the contribution of ROS. As shown in Fig. 2E, compared to the Sludge 1 + H₂O₂ system, a stronger 1:2:2:1 quadruplet peak was observed in the Sludge 1-MoS₂-0.07 + H₂O₂ system. This indicates that the Sludge 1-MoS₂-0.07 catalyst is capable of efficiently activating H₂O₂, leading to increased generation of •OH for the degradation of organic pollutants. For the PMS system, compared to Sludge 1, Sludge 1-MoS₂-0.14 can effectively activate PMS to generate an increased amount of •OH and SO₄^•− for the degradation of organic pollutants. According to Eqs. 1 and 2, SO₄^•− is the main source of •OH in the PMS system, explaining the limited •OH inhibition observed in quencher experiments (24, 25). SI Appendix, Fig. S14 shows that the •O₂⁻ signal generated by Sludge 1-MoS₂-m in both the H₂O₂ and PMS systems is significantly stronger than that of Sludge 1, indicating that •O₂⁻ also plays a crucial role in the degradation of pollutants. Within the Sludge 1-MoS₂-0.07 + H₂O₂ system, the intensity of the singlet oxygen (¹O₂) peak is faint, suggesting that ¹O₂ does not predominantly function as the active oxygen species during the degradation process (SI Appendix, Fig. S15A). The intensity of the ¹O₂ peak in the Sludge 1-MoS₂-0.14 + PMS system is marginally lower than that in the Sludge 1 + PMS system (SI Appendix, Fig. S15B), possibly owing to interactions between ¹O₂ and other oxygen species during the degradation process. These interactions also explain the limited impact on ¹O₂ inhibition observed in the quenching experiment.

Fig. 2F illustrates the zeta potential (ζ-potential) of Sludge 1, MoS₂, and Sludge 1-MoS₂-0.07 under varying pH conditions. In the case of Sludge 1, an increase in pH leads to a shift in surface charge from positive to negative. This transition is associated with the presence of “Lewis acidic sites”, a characteristic commonly observed on the surfaces of iron-based catalysts. In contrast, MoS₂, owing to the presence of Mo empty orbitals, exhibits a high affinity for OH⁻ ions, resulting in a notable concentration of negative charges on its surface. It is worth noting that the formation of a composite between MoS₂ and Sludge 1 reverses the previously observed inverse relationship between the catalyst’s zeta potential and pH. This unique behavior can be attributed to the significant presence of unsaturated sulfur sites located at the edges of MoS₂ nanosheets (26). These sites exhibit a remarkable capability to adsorb H⁺ ions from the solution, thereby creating an acidic microenvironment on the catalyst’s surface. Consequently, this acidic environment contributes to a decrease in the absolute value of the catalyst’s zeta potential. Furthermore, in this acidic microenvironment, the Fe-O bonds at the Sludge-MoS₂-m interface become more prone to breaking, leading to an increased concentration of Fe²⁺ within the Stern layer and Slipping Plane (27). By introducing 1,10-phenanthroline, we complexed the Fe²⁺ present at the catalyst’s Stern layer and Slipping Plane into the solution. We observed a significant increase in Fe²⁺ concentration in the solution upon the addition of 1,10-phenanthroline, confirming our hypothesis (SI Appendix, Fig. S16). This not only contributes to a further reduction in the absolute value of the zeta potential but also promotes the generation of a higher quantity of ROS by the oxidants, consequently expediting the reaction. Simultaneously, the pyridine infrared (Py-IR) spectra in Fig. 2G illustrates that the peak area related to Lewis acidic sites of Sludge 1-MoS₂-0.07 at 1,450 cm⁻¹ and 1,600 cm⁻¹ is lower than that of Sludge 1. This elucidates the reason for the stabilization of the surface zeta potential of Sludge 1-MoS₂-0.07 as pH increases. Hence, this provides further evidence for the favorable performance of Sludge 1-MoS₂-m across a wide range of pH values (SI Appendix, Figs. S5D and S7D), effectively surmounting the challenges faced by conventional Fenton reactions, which often suffer deactivation in neutral and alkaline conditions.

Fe²⁺ was determined to be of vital importance in assisting the decomposition of H₂O₂. Therefore, 1, 10-phenanthroline was utilized to detect changes in the concentration of Fe²⁺ in the solution (28). As shown in Fig. 2H, the mere presence of Sludge 1 alone does not ensure an ample supply of Fe²⁺ for active participation in the Fenton reaction. Only Sludge 1 that forms chemical bonds with MoS₂ can generate a significant amount of Fe²⁺ within the system. This is attributed to the fact that, under hydrothermal conditions, thioacetamide transforms Fe³⁺ within Sludge 1 to Fe²⁺, and concurrently, the addition of molybdenum sulfide contributes to the formation of an acidic microenvironment on the surface, thereby promoting Fe²⁺ generation. Therefore, throughout the reaction, we tracked the concentrations of Fe²⁺ and Fe³⁺, noting a consistent increase in Fe²⁺ and a continuous decrease in Fe³⁺. This confirms the presence of Fe²⁺/Fe³⁺ cycle in the Sludge 1-MoS₂-0.07 system. Moreover, upon extending the reaction time, we observed continual dynamic fluctuations in the concentrations of Fe²⁺ and Fe³⁺, offering evidence for the ongoing cycling of Fe ions within the system (SI Appendix, Fig. S17). From SI Appendix, Fig. S18 and Table S1, it is evident that after the reaction of MoS₂ with H₂O₂, the content of Mo⁶⁺ decreases, while the Mo⁴⁺ content increases. This indicates that H₂O₂ can reduce Mo⁶⁺ to Mo⁴⁺, thereby enabling the reduction of Fe³⁺ generated in the system to Fe²⁺, facilitating the continuous degradation of pollutants within the system. Additionally, density functional theory (DFT) calculations in Fig. 2I demonstrate that H₂O₂ is more likely to react with ≡Mo⁶⁺ to produce ≡Mo⁴⁺ (ΔG_{Mo(6+)/Mo(4+)}: −6.39 eV < ΔG_{Fe(3+)/Fe(2+)}: −5.39 eV), thereby facilitating the cyclic regeneration of molybdenum ions within the system. The ≡Mo⁴⁺ species on the surface of MoS₂ exhibits a reductive capability, sustaining a continuous generation of Fe²⁺ within the system, thus engendering a profusion of free radicals that expedite pollutant degradation (Eqs. 1–8) (29). In addition, Sludge 1-MoS₂-m effectively mitigates iron ion hydrolysis, further promoting the cyclic conversion between Fe²⁺ and Fe³⁺. This endows Sludge 1-MoS₂-m with the capacity to maintain prolonged catalytic activity throughout the degradation process. This suggests that Sludge 1-MoS₂-m facilitates the cyclic regeneration of iron and molybdenum ions, enabling sustained and efficient pollutant degradation. This not only establishes a bridge between solid waste and wastewater treatment but also bestows a renewed value upon Fe sludge.

$$\equiv {\mathrm{Mo}}^{4+}+2{{\mathrm{HSO}}_5}^{-}\to \equiv {\mathrm{Mo}}^{6+}+2{{\mathrm{SO}}_4}^{\bullet -}+2{\mathrm{OH}}^{-},$$ [1]

$${{\mathrm{SO}}_4}^{\bullet -}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}^{+}+{{{\mathrm{SO}}_4}^2}^{-}+\bullet \mathrm{OH},$$ [2]

$$\equiv {\mathrm{Fe}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to \equiv {\mathrm{Fe}}^{3+}+\bullet \mathrm{OH}+{\mathrm{OH}}^{-},$$ [3]

$$\equiv {\mathrm{Mo}}^{4+}+\equiv {\mathrm{Fe}}^{3+}\to \equiv {\mathrm{Mo}}^{6+}+\equiv {\mathrm{Fe}}^{2+},$$ [4]

$$\equiv {\mathrm{Mo}}^{6+}+{\mathrm{H}}_2{\mathrm{O}}_2\to \equiv {\mathrm{Mo}}^{4+}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2,$$ [5]

$$\bullet \mathrm{OH}+{\mathrm{H}}_2{\mathrm{O}}_2\to {{\mathrm{HO}}_2}^{\bullet }+{\mathrm{H}}_2\mathrm{O},$$ [6]

$${{\mathrm{HO}}_2}^{\bullet }\to \bullet {{\mathrm{O}}_2}^{-}+{\mathrm{H}}^{+},$$ [7]

$$\bullet \mathrm{OH}+\bullet {{\mathrm{O}}_2}^{-}+\mathrm{organic}\to {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}.$$ [8]

Environmental Implications.

In order to verify the universal applicability of Sludge 1-MoS₂-m, different types of organic compounds such as phenols (4-CP, BPA, and phenol), dyes (RhB, MB, and MO), and antibiotics (SD, TH, and Enro) were used as target pollutants to evaluate its activity in the heterogeneous Fenton system. As depicted in Fig. 3A, regardless of whether in the PMS system or the H₂O₂ system, within a 30-min timeframe, the degradation efficiency of the majority of pollutants can exceed 90%. This validates the broad applicability of Sludge 1-MoS₂-m across various pollutants. The effect of natural water resources was also evaluated. It indicated that the catalysts demonstrated remarkable degradation efficiencies when dealing with phenol in various solvents (Fig. 3B). Considering the complexity of real wastewater, we investigated the performance of the catalyst by introducing different anions (Cl⁻, NO₃⁻, SO₄²⁻, CO₃²⁻, H₂PO₄⁻, and HCO₃⁻) and humus acid (HA). In the H₂O₂ system (Fig. 3C), the addition of CO₃²⁻, H₂PO₄⁻, and HCO₃⁻ has a noticeable inhibitory effect on the degradation rate of phenol, reducing it to 15.8%, 4.2%, and 4.3%, respectively. On the one hand, the addition of CO₃²⁻, H₂PO₄⁻, and HCO₃⁻ can seriously affect the initial pH of phenol (30). On the other hand, the addition of these anions forms a competitive relationship with pollutants, which seriously consumes •OH (31). For the PMS system, the existence of CO₃²⁻ can enhance the decomposition of PMS and improve the degradation efficiency (32). However, the pH of the reaction is elevated due to the presence of H₂PO₄⁻ and HCO₃⁻, its inhibitory effect is still the focus of subsequent research to be overcome.

Fig. 3.

(A) Phenol degradation of Sludge 1-MoS₂-0.07 and Sludge 1-MoS₂-0.14 for different pollutants, (B) different water sources, and (C) different anions and humus interference. (D) Longevity experiments with Fe powder, Sludge 1-MoS₂-0.07 and Sludge 1-MoS₂-0.14. (E) Photographs of Sludge 1, Sludge 2, Sludge 3, Sludge 4, and the corresponding Sludge x-MoS₂-m. (F) Flow diagram of the sludge from generation to green recovery. (G) Degradation behavior of phenol with different catalysts prepared from actual sludge. (H) Photos of different moments in the process of waste liquid treatment. ([Pollutants] = 20 mg/L. [Sludge x-MoS₂-m] = 30 mg, [H₂O₂] = 500 μL, pH = 4; [PMS] = 30 mg, pH = 5.5. [Cl⁻, NO₃⁻, SO₄²⁻, CO₃²⁻, H₂PO₄⁻, HCO₃⁻] = 10 mmol/L, [HA] = 10 mg/L. [Fe powder] = 30 mg).

In order to explore the enduring pollutant degradation capabilities of Sludge 1-MoS₂-m, we conducted a 14-d experiment focused on the degradation of phenol (Fig. 3D). As a control, we employed Fe powder as the catalyst due to its pronounced Fenton reactivity (33). Sludge 1-MoS₂-m exhibits efficient and stable pollutant degradation in both the H₂O₂ and PMS systems. This validates our earlier hypothesis that the inclusion of Sludge 1-MoS₂-m effectively inhibits the hydrolysis of iron ions, preventing the formation of iron sludge (Fe(OH)₃). Simultaneously, it promotes the continual regeneration of Fe²⁺ within the system, thus ensuring the persistent degradation of aromatic organic pollutants. However, it is worth noting that iron powder nearly loses all its Fenton reactivity after the second cycle in the PMS system. Additionally, after the first cycle in the PMS system, setting the pH to 7 resulted in the rapid formation of a substantial amount of iron sludge. This led to catalyst deactivation and secondary contamination (SI Appendix, Fig. S19). Remarkably, through the collection of this iron sludge, Sludge-MoS₂-0.07 was successfully re-synthesized using a one-step hydrothermal method with iron sludge, sodium molybdate, and thioacetamide. It regained its ability to effectively degrade phenol (>95%), affirming the one-step hydrothermal method’s potential for resource recovery and reuse of iron sludge (SI Appendix, Fig. S20). Furthermore, as evident from SI Appendix, Table S2, within the Fe + H₂O₂ system, although long-lasting pollutant degradation is achieved, a substantial leaching of iron ions is observed. Concurrently, compared with the Sludge 1-MoS₂-m system, the supernatant in the Fe powder system appears more turbid (SI Appendix, Fig. S21), suggesting the potential hydrolysis of iron ions and subsequent formation of iron sludge during the reaction. As Fe powder reacts with oxidants (PMS and H₂O₂), soluble iron ions interact with OH⁻ ions in the aqueous solution, leading to the formation of amorphous precipitates (SI Appendix, Fig. S22). This occurrence hampers the breakdown of oxidants, diminishing the reaction activity and rendering it unsuitable for extended wastewater treatment. In contrast, the crystal structure of Sludge 1-MoS₂-m remained largely unaltered throughout the reaction (SI Appendix, Fig. S23). Additionally, there were no discernible peak shifts evident in the XPS spectra (SI Appendix, Fig. S24). These observations provide confirmation of the remarkable structural stability possessed by the Sludge 1-MoS₂-m catalyst.

Fig. 3E depicts actual images of the four selected iron mud at different stages of our routine iron sludge treatment process. Sludge 1 is the sludge produced by simulating the traditional homogeneous Fenton reaction at the laboratory scale. Sludge 2 is the actual sludge from the Fenton process effluent of the Suzhou Industrial Park, without the dewatering step. Sludge 3 is the dewatered and compacted sludge from the Suzhou Industrial Park. Sludge 4 is the actual sludge transferred to landfill by Xinjiang industrial wastewater treatment plant. Based on the optimal reaction ratio of Sludge 1-MoS₂-m, Sludge 2-MoS₂-m, Sludge 3-MoS₂-m, and Sludge 4-MoS₂-m were prepared, respectively. Beneath each sludge variety, the accompanying photograph on the left is Sludge x-MoS₂-0.07 (x represents the type of sludge obtained from the Suzhou Industrial Park and other actual application sites) employed for H₂O₂ activation, while the image on the right is Sludge x-MoS₂-0.14 utilized for PMS activation. Fig. 3F further summarized the applicable stages of our developed iron sludge recovery and utilization technique in practical iron sludge treatment processes: a. generation (Sludge 1); b. concentration (Sludge 2); c. dehydration (Sludge 3); and d. landfill (Sludge 4). Fig. 3G investigates the activity of Sludge x-MoS₂-m in pollutant degradation. It is observed that Sludge x demonstrates a constrained activation capacity for both H₂O₂ and PMS. In contrast, Sludge x-MoS₂-m, synthesized through hydrothermal methods, demonstrates exceptional phenol degradation performance, with removal efficiencies ranging from 81.5 to 99.6%. This validates that regardless of the sludge type, it can undergo a straightforward hydrothermal process for chemical integration with MoS₂. The subsequently prepared catalyst can be reintroduced into the Fenton oxidation tank for reaction with the wastewater. Moreover, we also performed COD detection and degradation tests on potential waste liquids (obtained by mixing the supernatant after hydrothermal reaction with the washing solution of the catalyst) generated during the catalyst preparation process. For catalysts exhibiting outstanding degradation activity, such as Sludge 4-MoS₂-m, the degradation of COD in the waste liquid can be achieved through the direct addition of both the catalyst and oxidant. As shown in Fig. 3H, the left side is the wastewater generated during the preparation process of Sludge 4-MoS₂-0.07, and the right side is the wastewater generated during the preparation process of Sludge 4-MoS₂-0.14. The initial COD values of the waste liquids generated during the preparation process were 590 mg/L and 656 mg/L, respectively. After being subjected to a 6-h reaction with Sludge 4-MoS₂-0.07 and Sludge 4-MoS₂-0.14, the COD levels decreased to 173 mg/L and 193 mg/L (SI Appendix, Fig. S25), effectively meeting the discharge standards (GB 31573-2015). However, when dealing with waste liquid from a catalyst with relatively lower catalytic activity, such as Sludge 2-MoS₂-m, due to the presence of some thiolated sulfates in the supernatant, we can effectively target the stepwise removal of inorganic ions and organic compounds that impact COD in the wastewater by employing coagulants and catalysts (SI Appendix, Fig. S26A). The initial waste liquid for preparing Sludge 2-MoS₂-0.07 had a COD of 1024 mg/L. Following treatment with the coagulant, the COD decreased to 697 mg/L. As shown in SI Appendix, Fig. S26B, the sulfur content in the supernatant significantly decreases after the addition of a coagulant. This is the primary reason for the reduction in COD. It is worth mentioning that Fenton technology itself will introduce a large amount of sulfur (mainly from ferrous sulfate), and the resulting sulfate wastewater can be converted into sulfide precipitation by sulfate-reducing bacteria in an anaerobic environment for resource reuse. Ultimately, by introducing Sludge 2-MoS₂-0.07 to eliminate organic compounds from the waste liquid, the COD was reduced to 487 mg/L, thereby meeting the inlet water quality standards for wastewater treatment plants (GB/T 31962-2015). Furthermore, a one-step hydrothermal method was utilized to synthesize Cu-MoS₂-0.07 (obtained by copper sludge, thioacetamide and sodium molybdate hydrothermally) and Ni-MoS₂-0.07 (obtained by nickel oxide (NiO), thioacetamide and sodium molybdate hydrothermally) catalysts, with the aim of exploring the impact of metals such as Cu and Ni present in actual iron sludge on the Fenton degradation activity. SI Appendix, Fig. S27 demonstrates that, within 30 min, Ni-MoS₂-0.07 did not significantly degrade phenol. This observation may be attributed to the inability of Ni²⁺ to activate H₂O₂ (ΔG > 0; detailed calculations are provided in the Materials and Methods). Additionally, from the figure, it can be observed that Cu-MoS₂-0.07 under optimal conditions only degrades 35% of phenol. This is attributed to the weaker oxidative capability of •O₂⁻ generated by Cu²⁺ activation of H₂O₂. Moreover, the standard electrode potential of Cu (II)/Cu (I) (E⁰_(Cu2+/Cu+) = 0.166 V) is lower than that of Fe (III)/Fe (II) (E⁰_(Fe3+/Fe2+) = 0.770 V) (34). Therefore, within the same timeframe, the degradation rate of pollutants by the Cu-MoS₂-0.07 system is lower than that of the Sludge x-MoS₂-0.07 system, confirming the pivotal role of Fe ions in Fenton degradation.

Considering the issue of production costs, we conducted an assessment of the operating costs for recycling and resource utilization of one ton of iron sludge as an example. The current market cost of processing one ton of iron sludge ranges from $273 to $684. It is worth noting that the cost of preparing catalysts from one ton of iron sludge using the one-step hydrothermal method we employ is $111.33, which is significantly lower than the market processing cost (specific calculation process in the Materials and Methods). Subsequently, we optimized the catalyst preparation method by employing a facile water-bath method for the combination of iron sludge and molybdenum sulfide. The successful synthesis of the Sludge 1-MoS₂-m catalyst has been achieved, exhibiting efficacy in phenol degradation (>90%) (detailed discussion is in the Materials and Methods) (SI Appendix, Fig. S28).

Given the challenges in recovering powdered catalysts in industrial applications, we tackled this issue by granulating the powder catalyst with bentonite to enhance recoverability. As depicted in SI Appendix, Fig. S29A, the powder catalyst exhibits robust phenol degradation activity within the temperature range of 200 to 400 °C. Beyond 500 °C, a decline in catalyst activity is observed, speculated to be associated with a reduction in sulfur content (35). Therefore, particles were prepared using calcination conditions of 400 °C, which could effectively degrade phenol within 150 min (SI Appendix, Fig. S29B). Importantly, the prepared particles show great stability without any particle breakage post-reaction (SI Appendix, Fig. S30).

Conclusions

In summary, through the chemical combination of real iron sludge from different treatment stages with MoS₂, we have successfully realized the comprehensive utilization of iron sludge recovery technology throughout various phases of sludge treatment. The prepared Sludge-MoS₂-m efficiently degrades organic pollutants over an extended period and is suitable for various aquatic environments. Compared to other sludge treatment methods, our approach boasts lower energy consumption and costs, making it suitable for large-scale production and implementation in industrial facilities. Additionally, the catalyst prepared using this method for Sludge x-MoS₂-m can also treat the wastewater generated during its own production process, achieving the goal of “self-production and self-sale”. This truly embodies a green and pollution-free process, contributing to the realization of the “dual-carbon” strategic objectives.

Materials and Methods

Material Synthesis.

Preparation of Sludge 1.

Sludge 1 was obtained by simulating the Fenton reaction in the laboratory. Specifically, 20 mg of FeSO₄•7H₂O was added to 100 mL of aqueous solution at pH = 4, followed by 5 mL of H₂O₂ (30 wt%) to initiate the reaction, and a large amount of ginger precipitate was obtained after 30 min. After a water washing and drying step, a simulated Sludge 1 can be obtained.

Preparation of Sludge x-MoS₂-m.

The synthesis of Sludge x-MoS₂-m (x represents the type of sludge obtained from the Suzhou Industrial Park and other actual application sites; m represents the atomic ratio of Mo and Fe) was carried out by a simple hydrothermal method. Specifically, different proportions of thioacetamide (Y = 90, 180, 270, 360, and 450 mg) and sodium molybdate (Z = 45, 90, 135, 180, and 225 mg) were added to a hydrothermal kettle liner of polytetrafluoroethylene (PTFE) containing 20 mL of aqueous solution. Following a 30-min stirring period, we added 200 mg of Sludge x to the aforementioned homogenous solution and subsequently subjected it to a hydrothermal process at 200 °C. After a straightforward centrifugation and washing procedure, we successfully acquired the desired samples of Sludge x-MoS₂-m with varying ratios. It is worth noting that unless specifically stated otherwise, all instances of Sludge x used in this experiment underwent hydrothermal treatment.

Preparation of Sludge 1-MoS₂-m using the water-bath method.

The synthesis of Sludge 1-MoS₂-m was carried out by a simple water-bath method. Specifically, different proportions of the commercialized MoS₂ were added to a beaker containing 60 mL of an aqueous solution. After a 30-min stirring period, 200 mg of Sludge 1 was added to the aforementioned homogeneous MoS₂ solution, and subsequently, the mixture was subjected to a process at 80 °C for 2 h. After a straightforward centrifugation and washing procedure, we successfully acquired the desired samples of Sludge 1-MoS₂-m with varying ratios.

Degradation of Organic Pollutants.

The all degradation experiments were carried out in a 150-mL beaker under magnetic stirring. Taking the phenol degradation experiment as an example, the initial pH of the phenol solution (20 mg/L) was first adjusted to 4 with 1 M NaOH and H₂SO₄ (In the H₂O₂ system, the initial pH was 4; in the PMS system, the initial pH was not adjusted). Then, Sludge x-MoS₂-m was added and dispersed homogeneously by ultrasonication. The reaction was timed while the oxidant (H₂O₂, PMS) was added to trigger the reaction, sampled at specific times, terminated by the rapid addition of methanol, filtered through a 0.22-μm filter and the phenol removal efficiency was quantified by high-performance liquid chromatography at 272 nm.

In various water source degradation experiments, only the deionized water was substituted with equal volumes of tap water, Pearl River water, Huangpu River water, and Bailang River water, while keeping all other conditions unchanged. The degradation experiment process of other pollutants [4-chlorophenol (4-CP), bisphenol A (BPA), sulfadiazine (SD), tetracycline hydrochloride (TH), and enrofloxacin (Enro)] is basically the same as the above steps, and the removal efficiency of dye pollutants [rhodamine B (RhB), methylene blue (MB), and methyl orange (MO)] is quantitatively detected by measuring the absorbance by UV-Vis spectrophotometry.

Removal of Chemical Oxygen Demand.

Collect the supernatant from the concluded catalytic reaction and proceed to wash the residual solid catalyst. Wash three times with water, collect the washing liquid, and mix it with the above supernatant. After thorough mixing, add a coagulant for reaction. Subsequently, collect the supernatant and adjust the solution’s pH to 4. Take 2 mL of the solution and add it to a pre-prepared reagent tube for the initial COD measurement. COD detection was carried out by an ultraviolet spectrophotometer (DR3900) with a 15-hole benchtop multi-function digester (DRB200). Subsequently, add 30 mg of catalyst and 500 μL of H₂O₂ for the degradation reaction. Sample at specific time intervals, using the same detection method as described above.

Calculation of Operating Cost.

When recycling one ton of iron sludge, 225 kg Na₂MoO₄ and 450 kg C₂H₅NS were required. The formula of cost is unit price × m (Na₂MoO₄ or C₂H₅NS).

Statistical Analysis.

All data were gained directly from the source experiment and processed using Origin. All experiments were carried out in duplicate. Full details for DFT computations, control experiments, and materials characterizations were introduced in SI Appendix.

Supplementary Material

Appendix 01 (PDF)

Data, Materials, and Software Availability

All study data are included in the article and/or SI Appendix.