MoS₂-assisted iron-driven peroxydisulfate activation for green and sustainable water purification

Abstract

Peroxydisulfate (PDS)-based advanced oxidation processes are promising for reclaimed water treatment; however, they are often hindered by high energy demands, costly and complex catalyst synthesis, and the requirement for external energy inputs. Commercially available materials, commercial catalytic iron powder (CCI) and MoS₂ were selected to activate PDS (CCI/MoS₂/PDS system) for the degradation of high-concentration dye wastewater (Acid Orange 7, AO7) within 5 minutes. CCI generates Fe²⁺ and Fe³⁺, while MoS₂ facilitates the rate-limiting Fe³⁺/Fe²⁺ cycle. The reaction rate of the CCI/MoS₂/PDS system is approximately 2 times higher than that without a co-catalyst. Therefore, the system can efficiently generate a variety of reactive species (e.g., radicals) for the degradation of organic wastewater. The primary active species responsible for the degradation of AO7 in the CCI/MoS₂/PDS system was the sulfate radical (SO₄˙⁻). Following both a small-scale continuous-flow experiment and a pilot-scale continuous-flow reactor equipped with a catalyst-filled column, the system maintained high efficiency and catalytic activity, enabling long-term, stable, and effective removal of AO7. Remarkably, the CCI/MoS₂/PDS system has an operating cost of only US$ 0.16 per ton of wastewater, highlighting its great potential for large-scale industrial applications. This work provides valuable guidance for the development of green, low-cost and efficient PDS-activation systems for industrial wastewater treatment.

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Green foundation

1. This work proposes a new chemical process simply driven by MoS₂ and commercial catalytic iron powder (CCI), investigates the performance enhancement and underlying mechanism of PDS activation and develops a green, highly active and sustainable CCI/MoS₂/PDS system for water purification with long-term operation. It reveals the broad application prospects of environmentally friendly water treatment, which advances the field of green chemistry.

2. This work has successfully treated printing and dyeing wastewater into clear water through a simple and effective system without secondary pollution and carried out large-scale practical applications, contributing its due strength to the field of green chemistry.

3. This work is expected to become greener and more environmentally friendly by regulating the reaction parameters and integrating with deep wastewater treatment technology to achieve high efficiency with low energy consumption. We will continue striving hard to achieve this goal.

Introduction

In recent decades, rapid industrialization and population growth have led to the discharge of large amounts of pollutants, such as dye-containing wastewater, into the environment,^1,2 posing significant challenges to conventional treatment systems.³ Most synthetic dyes are associated with carcinogenicity and toxicity, raising serious environmental and health concerns.^4–6 There is an increasing demand for the treatment of highly contaminated wastewater to mitigate the environmental impact of these pollutants.^7–9 Conventional treatment methods, such as biological and physical processes, are often ineffective at removing organic pollutants from water.^10,11 Due to the high concentrations and toxicity of organic pollutants,^12,13 industrial wastewater generally exhibits low biodegradability, which can be improved through advanced oxidation processes (AOPs) that generate reactive species (e.g., radicals) to effectively degrade organic compounds.^14–17 Nevertheless, many AOPs are not widely applied in practice due to high operational costs associated with catalysts and external energy inputs.¹⁸

The sulfate radical (SO₄˙⁻)-based advanced oxidation process (SR-AOP) has been recognized as one of the robust techniques for the removal of recalcitrant pollutants in water.¹⁹ Various oxidants have been widely studied for wastewater treatment, such as peroxydisulfate (PDS) and peroxymonosulfate (PMS), which generate highly reactive radical intermediates through the homolytic and heterolytic dissociation of peroxide bonds.^20,21 SO₄˙⁻ can be generated by the activation of PDS and PMS. PMS-based systems provide a strategy for the removal of pollutants. However, the high dosage requirements for PMS and the deactivation of catalysts limit their practical application.^22,23 The inefficient utilization and activation of PMS also significantly impede their practical application.¹¹ In contrast, the degradation of organic pollutants via the activation of PDS is more efficient than that of PMS.²⁴ This is attributed to the longer peroxide bond length of PDS (1.497 Å) compared to PMS (1.46 Å) and the lower O–O bond energy of PDS (140 kJ mol⁻¹) versus PMS (140–213.3 kJ mol⁻¹).²⁵ Consequently, PDS-based systems are activated, leading to the homolytic cleavage of the O–O bond in the PDS molecule to generate two SO₄˙⁻ radicals per PDS.^26,27 It is relatively inexpensive ($0.74 per kg) and more stable compared to other peroxides, such as PMS (commercially known as Oxone, $2.2 per kg).²⁸ A series of approaches (e.g., UV irradiation, heating, and the use of alkaline conditions, metal ions, metal oxides, and carbonaceous materials) have been developed to activate PDS, primarily aimed at generating the powerful SO₄˙⁻. However, these methods are energy-intensive and have seldom been implemented in practical applications. Therefore, from a practical perspective, it is imperative to develop highly efficient heterogeneous PDS activation systems that can be readily prepared on a large scale.²⁹

In this study, we propose the CCI/MoS₂/PDS system. Compared to conventional PDS activation methods, transition metal (TM = Co, Cu, Ni, Mn, Fe)-based heterogeneous catalysts for PDS are widely researched, benefiting from their variety, low cost and no additional energy consumption.^16,30 However, many of these catalysts face limitations, including the use of expensive reagents, complex preparation procedures, and challenges in storage and preservation.³¹ To achieve or approach engineering objectives, we utilized low-cost commercial materials (commercial catalytic iron powder (CCI) and MoS₂). Low-valent metals have been widely employed in PDS activation,³² but they often suffer from particle aggregation, a tendency to dissolve, and instability during the reaction process. CCI generates Fe²⁺ and Fe³⁺, and the Fe³⁺/Fe²⁺ cycle participates in radical production. However, the CCI/PDS process faces intrinsic limitations, including sluggish Fe³⁺ to Fe²⁺ conversion. Due to the challenges in Fe²⁺ regeneration and its continuous depletion, the reduction of Fe³⁺ to Fe²⁺ constitutes the rate-determining step (RDS) of the overall reaction. Recently, metal sulfides have been employed as co-catalysts in PDS systems to enhance radical generation by accelerating the Fe³⁺/Fe²⁺ rate-limiting step, thereby improving water detoxification and disinfection.³³ MoS₂ is inexpensive and does not require complex synthesis procedures. As a cocatalyst, MoS₂ can not only enhance the Acid Orange (AO7) degradation efficiency via CCI to nearly 100% within a very short time but also exhibits outstanding reusability. The currently developed CCI/MoS₂/PDS system does not require complex synthesis steps and is cost-effective. Additionally, the consumption of the oxidant (PDS) and energy input can be significantly reduced due to more efficient reaction pathways. Moreover, the potential application of the PDS-based process for engineered wastewater treatment was demonstrated using both a small-scale continuous-flow system and a pilot-scale continuous-flow reactor equipped with a catalyst-filled column. This work will provide guidance for the development of low-cost and efficient PDS-activation systems.

Materials and methods

Materials and chemicals

Materials and chemicals are summarized in Text S1. The details of characterization, experimental procedures and analyses are provided in Texts S2–S5, respectively.

Computational calculations

The geometry optimizations of BPA were performed at the B3LYP def2-TZVP level using the ORCA software package (version 6.0.0).^34,35 Orbital composition analysis was performed using Multiwfn 3.8.^36–38 The calculation method for operating cost is presented in Text S6.

Results and discussion

Catalyst performance

For the sake of investigating the degradation performance of the CCI/MoS₂/PDS system, acid orange 7 (AO7) was selected as the model pollutant. Furthermore, X-ray photoelectron spectroscopy (XPS) (Fig. S1) was employed to characterize the valence states of the elements. AO7 was relatively stable in PDS alone, CCI alone, MoS₂ alone, CCI/MoS₂ systems, CCI/PDS systems and MoS₂/PDS systems, indicating that both the adsorption and oxidation of AO7 by PDS, CCI and MoS₂ could be ignored (Fig. 1a). However, when CCI, MoS₂ and PDS coexisted, the degradation of AO7 showed significant improvement, with 92.0% degradation efficiency in 5 min, indicating that the CCI/MoS₂/PDS system would effectively degrade AO7. Furthermore, the reaction rate of AO7 in the CCI/MoS₂/PDS system was exponentially higher than that in the other systems (Fig. 1b).

Fig. 1 (a) Removal efficiency and (b) the corresponding reaction rate of AO7 in different systems, (c) effect of MoS₂ concentrations, (d) effect of CCI concentrations, (e) influence of PDS concentrations, (f) effect of initial pH, and (g) comparison of the reaction rates with reported materials.

Subsequently, the influence of key reaction parameters on AO7 degradation was systematically evaluated. The reaction rate reached 0.32 min⁻¹ at a MoS₂ amount of about 0.02 g L⁻¹, while the reaction rate increased (0.39 min⁻¹) when the MoS₂ amount was elevated to 0.08 g L⁻¹. However, when excessive MoS₂ (0.12 g L⁻¹) was added, the degradation of AO7 declined (Fig. 1c and Fig. S2). The excess catalyst adsorbed the ˙OH and ˙SO₄⁻ during the process, bursting some active substances and hindering the reaction.³⁹ It follows that the optimal dosage of MoS₂ is 0.08 g L⁻¹. As the amount of CCI increases, the degradation performance of AO7 gradually improves (Fig. 1d and Fig. S3), and the addition amount of CCI was finally selected as 0.08 g L⁻¹. Fig. 1e and Fig. S4 show the effect of PDS dosage (from 1.5 to 3 mM) on AO7 degradation, where 2 mM PDS was found to be the optimal dosage in the experiment of different PDS. Excessive PDS addition inhibited the generation of free radicals.⁴⁰ Subsequently, AO7 degradation experiments were conducted across different initial pH values and concentrations, with the results presented in Fig. 1f, Fig. S5 and S6. The CCI/MoS₂/PDS system maintained efficient AO7 degradation over a wide pH range (3.0–9.0) and diverse concentrations (1–100 mg L⁻¹), highlighting its outstanding environmental adaptability. Notably, the CCI/MoS₂/PDS system exhibits superior catalytic activity compared to previously reported systems (Fig. 1g and Table S1).

Investigation of the organic wastewater treatment system

The actual natural water contains various inorganic anions (Cl⁻, NO₃⁻, and SO₄²⁻) and natural organic matter such as humic acid (HA).⁴¹ Previous studies suggest that the presence of these components may affect the performance of radical-dominated AOPs.⁴² However, their effects on AO7 removal by the CCI/MoS₂/PDS system were found to be negligible (Fig. 2a).⁴³ This underscores the remarkable anti-interference capability of the CCI/MoS₂/PDS system. Rhodamine B (RhB), sulfamethoxazole (SMX), methylene blue (MB), and methyl orange (MO) were utilized to assess the capability of the CCI/MoS₂/PDS system in degrading various persistent organic pollutants (Fig. 2b). Notably, the CCI/MoS₂/PDS system exhibited excellent degradation performance for various pollutants, underscoring its broad-spectrum applicability. These results further confirm the potential of the CCI/MoS₂/PDS system for organic pollutant remediation. The reusability of the CCI/MoS₂/PDS system was also confirmed through cyclic degradation experiments. As demonstrated in Fig. 2c, the degradation rate of AO7 remained almost 90% after five successive cycles, demonstrating its excellent stability and reusability. The catalysts before and after the reaction were characterized using X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FT-IR) spectroscopy. The results demonstrated that the chemical composition of the catalysts remained unchanged after the reaction, confirming their excellent structural stability and suggesting high reusability potential (Fig. S7).

Fig. 2 (a) The effects of various anions and HA on the AO7 degradation efficiencies, (b) removal of multiple pollutants, (c) the recycles of CCI/MoS₂/PDS, (d) schematic diagram and the corresponding photograph of the small-scale continuous-flow system, (e) photograph of the reactor and images of the CCI/MoS₂ catalytic membrane, and (f) AO7 removal in the small-scale continuous-flow system.

Importantly, the potential for the practical application of the CCI/MoS₂/PDS system was preliminarily evaluated using a small-scale continuous-flow system. The schematic diagram and photographs of the device are shown in Fig. 2d. A CCI/MoS₂ catalytic membrane was also successfully fabricated for the system (Fig. 2e).⁴⁴ To further evaluate the feasibility of practical application, experiments were conducted to treat AO7-containing wastewater. Notably, the system maintained an AO7 removal efficiency of nearly 100% over a 6-hour period (Fig. 2f), indicating excellent catalytic activity and long-term durability.

Pilot-scale application evaluation

Despite their superior performance in conventional water treatment, membrane reactors have seen limited widespread adoption due to prohibitive operation and maintenance costs, as well as limited scalability for high-flow wastewater treatment. Therefore, to facilitate the practical application of the CCI/MoS₂/PDS system in water purification,⁴⁵ a pilot-scale continuous-flow reactor consisting of a catalyst-filled column was constructed (Fig. 3a).^43,46 Continuous removal of AO7 was achieved by activating PDS using CCI/MoS₂−sodium alginate gel spheres as the column packing material (Fig. 3b and c). Specifically, the packed column had an inner diameter of 45 cm and a height of approximately 50 cm and employed an upflow configuration for wastewater introduction. This arrangement allowed the sodium alginate pellets to remain suspended in the wastewater, thereby enhancing interactions among the catalyst, oxidant, and contaminants. As shown in Fig. 3d, the degradation rate of AO7 was maintained at nearly 100% throughout the continuous reaction over a 72-hour period, demonstrating the strong potential of the CCI/MoS₂/PDS system for practical applications. Additionally, the system demonstrated effective purification performance at various flow rates, consistently maintaining an AO7 removal efficiency close to 100% (Fig. 3e). The design of the sodium alginate pellets facilitated the recovery and reuse of the catalyst after degradation, reducing costs and minimizing the risk of secondary contamination (Fig. S8). Additionally, the detected leakage of metal ions in the two systems is lower (Fig. S9), further proving the robust structural stability of CCI/MoS₂/PDS and sustained removal performance during long-term operation. In addition, the operating cost of treating one ton of wastewater is only US$ 0.16, indicating its great potential for industrial applications.³¹

Fig. 3 (a and b) Schematic diagram and the corresponding photograph of the pilot-scale continuous-flow reactor, (c) photograph of the packed column, (d) AO7 removal efficiency within 72 h, and (e) the removal efficiency of wastewater in a pilot-scale continuous flow reactor at different flow rates.

AO7 degradation pathways and toxicity evaluation

To further explore the possible degradation pathway, DFT calculations were employed. The highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) and the ESP map can be used to explore the ability of AO7 to be oxidized (Fig. 4a and b).^47,48 The detailed charge distribution and the Fukui index of AO7 are illustrated in Table S2, along with the optimized structure (Fig. S10). To identify the oxidative degradation pathway of AO7, the intermediates produced during the degradation of AO7 by the CCI/MoS₂/PDS system were identified via LC-MS analysis. The proposed oxidative degradation pathways for AO7 are illustrated in Fig. 4c and Table S3. The degradation of AO7 initially proceeds through the cleavage of the azo bond (–N=N–), leading to decolorization. Subsequently, three distinct degradation pathways can be identified. In the first pathway, 1-amino-2-naphthol (P2) is formed and undergoes deamination to produce naphthalene-1,2-diol (P3), which subsequently decomposes into catechol. In the second pathway, P5 is formed first and then converted to 4-methylaniline and sulfur trioxide (P6), which is further oxidized to quinone (P7). In pathway III, the C–C bond in 5-hydroxy-2-vinylbenzaldehyde (P8), between the methylene group and the benzene ring, is cleaved due to a strong electrophilic attack, forming P9; this intermediate subsequently transforms into 4-hydroxyphthalaldehyde (P10). These intermediates were degraded into ring-opening products (P11–P13), which were subsequently oxidized to generate carbon dioxide and water. Fig. S11 shows that 35% of total organic carbon (TOC) and 44% of chemical oxygen demand (COD) were removed, which supports the proposed degradation pathway.⁴⁹

Fig. 4 (a) The HOMO and LUMO of AO7, (b) ESP map of AO7, (c) possible AO7 degradation pathways, (d) development toxicity, (e) bioconcentration factor, and (f) growth of zebrafish cultured.

In this study, Toxicity Estimation Software (T.E.S.T.) was used to evaluate the developmental toxicity and bioaccumulation factor of AO7 and its degradation intermediates based on quantitative structure–activity relationship (QSAR) predictions,^44,48 as shown in Fig. 4d and e. Toxicity assessment results indicated the formation of less toxic or non-toxic products following AO7 degradation.⁵⁰ Additionally, experimental toxicity evaluations of the degradation products were conducted using zebrafish embryo assays (Fig. 4f).⁴⁵ Zebrafish embryo hatching experiments, using the original AO7, CCI/MoS₂/PDS-treated water, and deionized water as exposure media, revealed lethality in embryos exposed to the original AO7, indicating its substantial toxicity to aquatic organisms. In contrast, embryos exposed to the CCI/MoS₂/PDS-treated water developed comparably to those in the control group, confirming the system's safety and detoxification efficacy. These results demonstrate that the CCI/MoS₂/PDS system could effectively degrade organic pollutants without generating toxic byproducts, highlighting its dual advantages of high efficiency and detoxification for practical applications in water purification.

Reactive oxygen species (ROS) and their contribution to AO7 degradation

To further clarify the predominant oxidative species generated in the CCI/MoS₂/PDS system, five representative scavengers were selected, including tert-butyl alcohol (TBA), methanol (MeOH), trichloromethane (CHCl₃), sodium azide (NaN₃) and dimethyl sulfoxide (DMSO).⁵¹ TBA is widely recognized as a selective scavenger for ˙OH,⁵² with a reported reaction rate constant ranging from 3.8 to 7.6 × 10⁸ M⁻¹ s⁻¹.⁵³ As depicted in Fig. 5a and b, no inhibitory effect on AO7 degradation was observed in the presence of 100 and 150 mM TBA compared to the control experiment, suggesting a negligible contribution from ˙OH. In contrast, in the presence of MeOH (a known scavenger for SO₄˙⁻ and ˙OH), 100 and 150 mM MeOH significantly inhibited AO7 removal, providing evidence for the involvement of SO₄˙⁻. However, the presence of CHCl₃ had only a minor effect, suggesting that O₂˙⁻ plays a limited role in the system.⁵⁴ The addition of NaN₃, a known singlet oxygen (¹O₂) quenching agent, markedly reduced AO7 removal efficiency. It should be noted, however, that NaN₃ can also act as an electron donor and consume PDS, which may exaggerate the perceived contribution of ¹O₂ in the non-radical pathways.^44,55 Additionally, DMSO is widely recognized as an effective scavenger for high-valent metal species (HVMS).^56,57 The presence of 10 mM DMSO exerted an inhibitory effect on the degradation of AO7, with the removal efficiency decreasing to 67.3% at 5 min. The inhibitory effect of DMSO on AO7 oxidation in the CCI/MoS₂/PDS system can likely be attributed to the competition between DMSO and AO7 for HVMS. To further confirm the presence of HVMS, phenylmethyl sulfoxide (PMSO) was selected as the probe compound in this work, because high-valent metal oxides such as Fe(IV) have been shown to selectively oxidize PMSO to phenylmethyl sulfone (PMSO₂).⁵⁸ As shown in Fig. 5c, PMSO₂ production was observed in both CCI/MoS₂/PDS and CCI/PDS systems, confirming the generation of HVMS, although with modest activity.

Fig. 5 (a and b) AO7 removal efficiency in the presence of different quenchers. (c) Yield of PMSO₂. (d–f) EPR spectra of ˙OH, SO₄˙⁻, O₂˙⁻, and ¹O₂. (g) Degree of contribution of the oxidation pathway. (h) The ratio of Fe²⁺/Fe³⁺. (i) The consumption of PDS.

Electron Paramagnetic Resonance (EPR) studies were conducted to identify the active species and elucidate the reaction mechanism in the CCI/MoS₂/PDS system. Transient reactive species can be identified when spin traps rapidly bind to them, forming stable spin adducts that produce characteristic peaks in the EPR spectra.^52,59 In the CCI/MoS₂/PDS system, the peak intensity gradually increased with prolonged reaction time (Fig. 5d). At 5 min, a strong characteristic peak with an intensity ratio of 1 : 2 : 2 : 1 emerged, assigned to DMPO-˙OH,⁴⁸ while well-resolved DMPO-SO₄˙⁻ signals were also clearly observed.⁶⁰ As shown in Fig. 5e, six peaks with a 2 : 2 : 1 : 2 : 1 : 2 intensity ratio were identified as DMPO-˙O₂⁻ signals, further confirming the presence of ˙O₂⁻.⁴⁵ However, a 1 : 1 : 1 triplet signal diminished over time, indicating negligible ¹O₂ presence in the system (Fig. 5f).⁶¹ These observations confirmed the presence of SO₄˙⁻, ˙OH, and O₂˙⁻, all of which contributed to the reactivity of the CCI/MoS₂/PDS system.

Based on the results, the relative contributions of various reactive species were quantified to establish their functional significance in AO7 degradation (Fig. 5g and Fig. S12). The radical pathways contributed as high as 93.6% to the reaction, whereas the non-radical pathways contributed only 6.4%. Notably, the radical pathways exhibited differential contributions from SO₄˙⁻ (51.2%), ˙OH (34.3%), and O₂˙⁻ (8.1%). Non-radical pathways, including HVMS (4.9% contribution) and other possible pathways (1.5%), exhibited only negligible proportions. Moreover, the Fe²⁺ concentration critically determines the efficiency of Fenton and Fenton-like processes, highlighting the importance of enhancing Fe²⁺/Fe³⁺ cycling.²⁷ As shown in Fig. 5h, the CCI/MoS₂/PDS system significantly accelerated Fe²⁺/Fe³⁺ cycling, thereby promoting radical generation and enhancing pollutant removal. As the reaction time progresses, the content of Fe²⁺ gradually decreases, confirming its role in enhancing both catalytic activity and AO7 degradation. Additionally, a high concentration of Fe²⁺ can accelerate the activation of PDS to a certain extent. Therefore, we analyzed and compared the consumption of PDS in the two systems. As shown in Fig. 5i, the CCI/MoS₂/PDS system exhibited enhanced PDS consumption, indicating that MoS₂ incorporation promoted Fe²⁺ generation and improved PDS utilization efficiency. Collectively, the CCI/MoS₂/PDS synergy facilitates radical generation, establishing a radical-driven pathway for water purification.

Removal mechanism of AO7

In conclusion, the catalytic mechanisms of the CCI/MoS₂/PDS system were elucidated. As shown in Fig. 6, the results indicate that the primary active species responsible for the degradation of AO7 in the CCI/MoS₂/PDS system was SO₄˙⁻. When CCI and PDS were introduced into an aqueous solution, CCI was oxidized to Fe²⁺ in the presence of S₂O₈²⁻ (eqn (1)). In the solution, Fe²⁺ acted as an electron donor to PDS, facilitating its decomposition to generate SO₄˙⁻ radicals with strong oxidizing properties, along with Fe³⁺ (eqn (2)).⁶² In the CCI/MoS₂/PDS system, the generated SO₄˙⁻ radicals could react with water to form ˙OH radicals. Both SO₄˙⁻ and ˙OH radicals possess high oxidizing capacities and can effectively degrade a wide range of environmental pollutants (eqn (3)).^63,64 Therefore, CCI activates PDS to produce reactive radicals,⁶⁵ these radicals are generated both through redox reactions involving Fe²⁺ and Fe³⁺ (eqn (4)–(6)). MoS₂ also enhances PDS activation and promotes the Fe²⁺/Fe³⁺ cycle (eqn (7)–(9)). The CCI and MoS₂ participate directly or indirectly in activating PDS to generate SO₄˙⁻, ˙OH, and O₂˙⁻ radicals, facilitating AO7 degradation through a radical-mediated pathway. During this process, the Fe²⁺/Fe³⁺ and the Mo⁴⁺/Mo⁶⁺ cycles mutually promote each other, while a small amount of high-valent metal species is also generated to participate in the degradation of AO7. The potential role of S²⁻ in the degradation of the target pollutant was further investigated (eqn (10)–(12)). Ultimately, the efficient activation of PDS by core CCI and MoS₂ is attributed to enhanced mass transfer through these pathways and their intrinsic reactivity, which favors interactions with PDS over H₂O and O₂.

Fe + 2S₂O₈²⁻ → Fe²⁺ + 2SO₄˙⁻ + 2SO₄²⁻ (1)

Fe²⁺ + S₂O₈²⁻ → Fe³⁺ + SO₄˙⁻ + SO₄²⁻ (2)

SO₄˙⁻ + H₂O → SO₄²⁻ + ˙OH + H⁺ (3)

Fe²⁺ + S₂O₈²⁻ → Fe⁴⁺ + 2SO₄²⁻ (4)

Fe²⁺ + O₂ → Fe³⁺ + O₂˙⁻ (5)

2Fe³⁺ + Fe → 3Fe²⁺ (6)

2Fe³⁺ + Mo⁴⁺ → 2Fe²⁺ + Mo⁶⁺ (7)

Mo⁴⁺ + 2S₂O₈²⁻ → Mo⁶⁺ + 2SO₄˙⁻ + 2SO₄²⁻ (8)

Mo⁶⁺ + 2S₂O₈²⁻ + 2H₂O → Mo⁴⁺ + 2SO₅˙⁻ + 2SO₄²⁻ + 4H⁺ (9)

S²⁻ + 8Fe³⁺ + 4H₂O → SO₄²⁻ + 8Fe²⁺ + 8H⁺ (10)

2S²⁻ + Fe²⁺ → Fe + S₂²⁻ (11)

S²⁻ + 4Mo⁶⁺ + 4H₂O → SO₄²⁻ + 4Mo⁴⁺ + 8H⁺ (12)

Fig. 6 Catalytic mechanisms of CCI/MoS₂/PDS system activation and AO7 degradation.

Conclusion

In summary, we have developed a low-cost PDS activation system that eliminates the need for complex material synthesis. This CCI/MoS₂/PDS system exhibited excellent and consistent catalytic performance in the degradation of organic pollutants, including Acid Orange 7 (AO7). This system maintained a degradation efficiency of 90% within 5 minutes, even after five consecutive operational cycles. This remarkable performance can be attributed to the activation of dominant sulfate radical (SO₄˙⁻) pathways, facilitated by the Fe²⁺/Fe³⁺ cycle. Liquid chromatography–mass spectrometry (LC–MS) was employed to identify the degradation products of AO7 in the CCI/MoS₂/PDS system, revealing possible degradation pathways. To assess the practical applicability of this system, the potential toxicity of AO7 and its intermediates was preliminarily assessed using toxicity prediction software. The findings indicated that the toxicity levels of the degradation products were nearly non-toxic in the CCI/MoS₂/PDS system and validated through zebrafish toxicity assays. The feasibility of applying the PDS-based process to engineered wastewater treatment was demonstrated through both a small-scale continuous-flow setup and a pilot-scale reactor featuring a catalyst-packed column. This work offers valuable guidance for the development of low-cost and efficient PDS-activation systems.

Conflicts of interest

There are no conflicts to declare.

Data availability

The authors declare that the data supporting this article have been included as part of the supplementary information (SI). The supplementary information mainly contains other water treatment experiments and related characterizations. See DOI: https://doi.org/10.1039/d5gc04923g.

Any further information will be made available from the corresponding author upon request.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (52027815 and 52470020) and the Hefei Postdoctoral Research Foundation (2021).

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