WS₂-Assisted Electrochemical Activation of Peroxymonosulfate for Eliminating Organic Pollutant in Water

Abstract

Advanced oxidation process based on heterogeneous activation of peroxymonosulfate (PMS) has received significant attention in wastewater remediation. Herein, a facile and effective electrochemical method was introduced in a tungsten sulfide (WS₂)-activated PMS process for the removal of a typical azo dye Acid Orange 7 (AO7) in aqueous solution. It was found that the electrochemical activation could remarkably promote the removal of organic pollutants by coupling with WS₂/PMS system. The elimination of AO7 in the electro-assisted WS₂-activated PMS (E/WS₂/PMS) system achieved 95.8% of AO7 removal in 30 min, with the optimal conditions of 1.0 g/L WS₂, 1.0 mM PMS, current density of 1.0 mA/cm² and initial pH of 6.5. Based on quenching experiments and EPR techniques, mechanistic studies confirmed that hydroxyl radical (^•OH) and singlet oxygen (¹O₂) are the primary reactive oxygen species for the oxidation of pollutants. In addition, the influences of pH, WS₂ dosage, PMS concentration, current density, common anions and humic acid on the AO7 removal are also investigated in detail. Furthermore, the system exhibited resistance to aqueous matrices, verifying the accepted applicability in real water (i.e., Yangtze River water and Shahu Lake water). In summary, this study demonstrates a green system for the effective removal of contaminants in water, holding significant implications for practical application.

1. Introduction

In recent years, peroxymonosulfate (PMS)-based advanced oxidation techniques have received significant attention owing to their exceptional oxidative potential [1]. These techniques have been applied in the removal of refractory organic contaminants in water. However, due to the limited oxidizing ability of PMS itself, the enhancement of its oxidizing ability through various activation methods like chemical agents and catalytic materials has become a hot research field [1,2]. Among them, the heterogeneous catalytic materials have been assumed to effectively activate PMS in generating highly reactive oxidative species for the oxidation of pollutants.

Currently, the inorganic transition metal disulfide compounds like tungsten sulfide (WS₂) have received much attention due to their graphene-like layered structure, large surface area, more active sites and low charge transfer resistance [2,3]. Moreover, the two-dimensional structure of WS₂ has good electrical conductivity and reducing ability, as well as electrocatalytic properties and excellent dispersibility [4], while this nanostructured surface always possesses abundant defects that can serve as active sites for small molecule adsorption [5], thus showing efficient catalytic performance.

These properties render WS₂ a promising candidate as activators for PMS, particularly in the removal of organic pollutants from water. For instance, Chen found that WS₂ could activate PMS to generate singlet oxygen (¹O₂) and superoxide radicals (O₂^•−) for the removal of diclofenac [6]. In Gou’s study, the simultaneous introduction of FeS₂ and WS₂ could also efficiently activate PMS in generating sulfate radical (SO₄^•−) and hydroxyl radical (^•OH) for the almost complete removal of carbamazepine in water within 40 min [7]. It has been demonstrated that WS₂ has a good reducing property and can promote the conversion of Fe(III) to Fe(II), which results in excellent co-catalytic effects [8]. Thus, WS₂ is a promising activator of PMS for accelerating the catalytic oxidation process. However, the oxidation efficiency of the sole WS₂ activation of persulfate process is limited. To deal with this issue, a variety of coupling technologies have been developed to further improve the oxidative performance of the system on organic pollutant removal.

Over the past few decades, the electrochemical activation system has received considerable development and shown excellent capacities of organic pollutant elimination in complex wastewater due to being environmentally friendly and highly efficient. During the electro-activation process, the electric field is assumed to provide electrons as co-catalysts to activate the peroxides in the generation of reactive species for degrading the contaminants. This process might also avoid the secondary pollution risks associated with the exogenous catalyst [9]. For instance, the previous literature has reported that electrochemistry can effectively promote the continuous cycling of redox pairs of transition metals on the catalyst surface, thus enhancing the activation of PMS [10]. Moreover, electrochemistry also facilitates electron transfer on the catalyst surface, directly transferring electrons to the PMS [11], which further facilitates the generation of reactive species, accelerating the degradation of pollutants. However, despite some heterogeneous catalysts being available for activating PMS in the electrochemical process, there is a paucity of research on metal disulfide (i.e., WS₂) with a layered structure for the removal of pollutants by activating PMS in the electrochemical oxidation process, which deserves to be fully investigated.

In this study, we have proposed the use of WS₂ catalyst combined with an electrochemical process for the activation of PMS in the removal of organic pollutants. As known, Acid Orange 7 (AO7) is a kind of commonly used azo dyes and widely used in numerous industrial applications [12,13], which is harmful to the ecosystem and human health. Therefore, the degradation experiments herein were investigated using AO7 as the target compound. Firstly, the obtained WS₂ was characterized using various characterization techniques. The effects of operating factors such as PMS dosage, WS₂ dosage, current density and solution pH on the AO7 removal, as well as the recycling usability of WS₂ catalyst, were evaluated. In addition, the effects of coexisting chlorine ion (Cl⁻), bicarbonate ion (HCO₃⁻), dihydrogen phosphate ion (H₂PO₄⁻) and humic acid (HA) on the AO7 removal were also investigated. Furthermore, the reactive oxidative species produced in the electrochemical activation process were identified, and the reaction mechanism was proposed by the scavenger experiments and electron paramagnetic resonance (EPR) techniques. In brief, this study will provide a theoretical basis and technical support for the application of WS₂ catalyst in the electrochemical activation of the PMS process in wastewater treatment.

2. Results and Discussion

2.1. Characterizations

The XRD of pristine WS₂ is shown in Figure 1. Four typical diffraction peaks at 14.2°, 28.8°, 43.8° and 59.8° belong to the lattice planes (002), (004), (006) and (008), respectively, which match well with the standard patterns of tungsten sulfide (JCPDS 84-2417). Figure 2a,b present the SEM image of WS₂, confirming its layered structure. In addition, EDS analysis (Figure 2c) demonstrates the uniform distribution of W and S on the surface of WS₂ nanosheets.

2.2. Decolorization of AO7 Under Different Systems

Four systems, including the WS₂/PMS system, electro-activated PMS system (E/PMS), electro-activated WS₂ system (E/WS₂) and electro-assisted, WS₂-activated PMS system (E/WS₂/PMS), were established under the following conditions: WS₂ dose of 1 g/L, PMS dosage of 1 mM and current density of 1 mA/cm² at room temperature. As depicted in Figure 3a, the electrochemical oxidation alone and E/PMS system could remove ~40% of AO7 in 30 min, respectively, indicating that the sole electrochemical process could not efficiently activate PMS and that the removal of AO7 was due to the anodic oxidation process. In addition, the adsorption efficiency of AO7 by using sole WS₂ or graphite electrode was only 35.9% and 5.1%, respectively, during the 30 min (inset of Figure 3a), which are significantly lower than that of WS₂/PMS system. In contrast, the AO7 removal for the WS₂/PMS system can reach 82.7% at 30 min, which implies that WS₂ is able to activate PMS to produce a lot of reactive species for eliminating AO7. Importantly, the degradation efficiency of AO7 in the E/WS₂/PMS system significantly outperforms other systems, achieving a remarkable removal of 95.8% within 30 min with the apparent reaction rate constant (k) of 0.2062 min⁻¹ (Figure 3b). This confirms that the presence of electrochemical process could significantly enhance the activation of PMS by WS₂ for producing abundant reactive oxidation species, thus accelerating the degradation of contaminants. Notably, the degradation efficiency of AO7 in the E/WS₂/PMS system also significantly outperforms that of other similar systems in

Table 1. Comparison of AO7 removal under different advanced oxidation processes.

System	Pollutants	Reaction Conditions	Degradation Efficiency (%)	Ref.
Electro/PDS/MnO2	Acid Orange 7 (0.14 mmol/L)	[MnO2] = 0.6 g/L, [PDS] = 4.2 mmol/L, j = 12 mA/cm2, reaction time 30 min	98.1	[14]
Electro/Fe2+/S2O82−	Acid Orange 7 (0.1 mmol/L)	[Na2SO4] = 0.1 mol/L, [S2O82−] = 12 mmol/L, j = 16.8 mA/cm2, reaction time 30 min	85.1	[15]
Electro/Fe3O4/PDS	Acid Orange 7 (25 mg/L)	[PDS] = 10 mmol/L, [Fe3O4] = 0.8 g/L, j = 8.4 mA/cm2, [Na2SO4] = 50 mmol/L, reaction time 30 min	79.6	[16]
Electro/GAC/PMS	Acid Orange 7 (100 mg/L)	[PMS] = 10 mmol/L, [GAC] = 0.5 g/L, j = 16 mA/cm2, [Na2SO4] = 50 mmol/L, reaction time 30 min	75.8	[17]
Electro/Fe-B/PMS	Acid Orange 7 (50 mg/L)	[PMS] = 10 mmol/L, [Fe-B] = 0.5 g/L, j = 2 mA/cm2, [Na2SO4] = 50 mmol/L, reaction time 30 min	73.6	[18]
Electro/NM/PS	Acid Orange 7 (50 µmol/L)	[NM] = 0.5 g/L, [PDS] = 10 mmol/L, j = 100 mA/cm2, [Na2SO4] = 50 mmol/L, reaction time 30 min	~58	[19]
Electro/Cu2+/PDS	Acid Orange 7 (0.1 mmol/L)	[PDS] = 4 mmol/L, j = 0.5 mA/cm2, reaction time 30 min	~54	[20]
Electro/WS2/PMS	Acid Orange 7 (10 mg/L)	[PMS] = 1 mmol/L, [WS2] = 1 g/L, j = 1 mA/cm2, [Na2SO4] = 50 mmol/L, reaction time 30 min	95.8	This study

, demonstrating its superiority in terms of higher efficiency and lower energy consumption. In terms of electrical energy consumption, the E/WS₂/PMS composite activation is determined as 0.4364 kWh/m³, which is 62.5% less energy consumption compared to the E/PMS system (1.1346 kWh/m³), confirming the pivotal role of tungsten sulfide catalyst in the electrochemical activation of PMS process.

2.3. Influence of Operational Factors

2.3.1. Effect of WS₂ Dosage

The effect of WS₂ catalyst dosage in the range of 0.5~1.5 g/L on the removal of AO7 by the E/WS₂/PMS system was investigated in Figure 4a. As can be seen, the presence of WS₂ below 0.75 g/L⁻¹ in this system could not achieve a significant removal of AO7 within 30 min. The removal rate of AO7 increased with the increase of WS₂ dosage, which may be attributed to more surface adsorption and activation sites of catalyst provided for the activation of PMS towards reactive species generation. Specifically, 95.8% removal of AO7 could be achieved at 30 min with a concentration of 1.0 g/L WS₂. However, increasing the WS₂ dosage from 1.0 to 1.5 g/L did not enhance the removal efficiency, likely due to the side reactions between excessive WS₂ and radicals [21].

2.3.2. Effect of PMS Concentration

Figure 4b depicts the impact of PMS concentration on AO7 removal in the E/WS₂/PMS system. As an important source of reactive species, the concentration of PMS in a range of 0.25~1.0 mM was positively correlated with the removal rate of AO7. However, when the PMS concentration is excessive (>1.0 mM), increasing the PMS dosage will have an inhibitory effect on the degradation of contaminants. This might be owing to the competitive reactions between excessive PMS and target contaminant with the reactive species. For instance, the superfluous SO₄^•− could react with PMS to form the less active SO₅^•− (Equation (1)), and the inter reaction of both SO₄^•− and ^•OH would also happen (Equation (2)). These processes undoubtedly compromised the oxidation capacity of the whole system.

HSO₅⁻ + SO₄^•− → SO₅^•− + H⁺ + SO₄²⁻

(1)

^•OH + SO₄^•− → HSO₅⁻

(2)

2.3.3. Effect of Current Density

The effects of current densities on the AO7 removal were exhibited in Figure 4c. The AO7 removal was promoted by increasing the current density in the range of 0.25~1.0 mA/cm². However, the removal efficiency decreased withfurther increasing the current density above 1.0 mA/cm². Obviously, the degradation efficiency increases with the increase in current density in a certain range, probably because more reactive species could be produced near the cathode [22]. In this regard, PMS can directly gain electrons to generate SO₄^•−, or the dissolved oxygen can receive electrons to generate O₂^•− and then transfer electrons to PMS in producing SO₄^•− [23]. Increasing the current density can accelerate the electron transfer rate, thus promoting the degradation reaction. Nevertheless, the decrease in pollutant removal at a higher current density is presumed to be due to the effect of oxygen evolution reaction, which inhibits the electro-activated PMS process and the degradation of pollutants [24].

2.3.4. Effect of Initial pH

The effects of pH on the degradation of AO7 were investigated in Figure 4d. The increased pH seems to promote the removal of pollutant to some degree, possibly due to the effect of alkaline activation process. The removal of AO7 after 30 min of reaction was 90.5%~95.8% and 71.3% when the initial pH was 2.0~6.5 (natural pH) and 8.5, respectively. As the solution pH is above 8.5, the PMS is more likely to undergo invalid decomposition into low oxidation capacity of reactive species [25], thus leading to the slight decrease in pollutant removal. Briefly, the E/WS₂/PMS system has demonstrated a good oxidation performance of organic pollutants over a wide pH range.

2.4. Effect of Coexisting Substances and Application in Real Water

Given the ubiquity of inorganic anions (i.e., Cl⁻, NO₃⁻, HCO₃⁻ and H₂PO₄⁻) and humic acid (HA) in natural water, their interference with AO7 removal in the E/WS₂/PMS system was investigated (Figure 5a). Obviously, the presence of these anions has a weak influence on the removal of AO7, indicating a good adaptability of the system in complex water matrices. In contrast, as shown in Figure 5b, the removal of AO7 was slightly inhibited by ~23% in 30 min in the presence of 5~20 mg/L HA but still degraded 72.7% of AO7 in the E/WS₂/PMS system. It is assumed that the humic substances with complex structures of benzene rings, carboxyl groups and carbonyl groups will compete with target organic pollutants for reactive oxidation species, thus reducing the elimination efficiency of target contaminants. In view of this, certain inhibitions of AO7 by ~20% were observed in real water including Yangtze River water and Shahu Lake water (Figure 5c).

2.5. Mechanistic Investigation

Advanced oxidation techniques based on PMS activation could produce a variety of reactive oxidation species such as SO₄^•−, ^•OH, O₂^•−, et al. [26,27]. To gain insight into the mechanisms of the E/WS₂/PMS system, EPR tests and quenching experiments were conducted to identify the involved reactive species. As shown in Figure 6, a distinct four-line characteristic DMPO-^•OH adduct with an intensity multiplication of 1:2:2:1 and a weak typical three-line characteristic TEMP-¹O₂ adduct with an intensity ratio of 1:1:1 could be observed at 1.0 min of reaction. This confirms the generation of ^•OH and ¹O₂ in the system towards the oxidation of pollutants. Meanwhile, the typical DMPO-^•O₂⁻ adduct was not detected in the reaction, thus ruling out the involvement of ^•O₂⁻ in the degradation of contaminants.

Furthermore, MeOH, TBA and L-His were chosen as the quenching agents for the reactive species SO₄^•−/^•OH, ^•OH and ¹O₂, respectively [28,29]. As indicated in Figure 7, the addition of TBA and MeOH inhibited the removal of AO7 to some degree, and the inhibition of TBA was more severe, indicating that hydroxyl radical rather than sulfate radical was the dominant reactive species in the degradation of AO7. In addition, the added 10 mM L-Histidine led to a similar inhibitory effect, with the removal efficiency of AO7 decreasing from 95.7% to ~40%, further verifying that ¹O₂ contributes to the oxidative degradation of AO7.

Based on the aforementioned results and analyses, the plausible mechanism of AO7 degradation by the E/WS₂/PMS system is illustrated in Figure 8. At first, the partial removal of pollutants in the sole electrolysis system could be attributed to the direct electron transfer between the pollutant and the surface of the graphite electrode [30], while the PMS might accept the electrons from the cathode to produce reactive radicals (Equation (3)), which could also attack the pollutants. On the one hand, the catalyst WS₂ alone can activate PMS to generate various reactive species for the oxidation of pollutants (Equations (4)–(6)). On the other hand, the electrochemical process will facilitate the electron transfer reaction in the WS₂/PMS system. Specifically, the redox circulation of W⁴⁺/W⁶⁺ on the surface of WS₂ will be accelerated by obtaining electrons from the cathode, thus continuously activating the PMS into the production of reactive oxidation species for the removal of pollutants (Equations (4) and (7)). Additionally, these electrons from the cathode might create more surface defects on the catalyst and accelerate the activation of the PMS process.

HSO₅⁻ + e⁻ → SO₄²⁻ + ^•OH

(3)

W⁴⁺ + 2HSO₅⁻ → W⁶⁺ + 2SO₄²⁻ + 2^•OH

(4)

W⁶⁺ + 2HSO₅⁻ → W⁴⁺ + 2SO₅^•− + 2H⁺

(5)

2SO₅^•− + H₂O → 2SO₄²⁻ + 2H⁺ + 1.5 ¹O₂

(6)

W⁶⁺ + 2e⁻ → W⁴⁺

(7)

2.6. Stability of WS₂

Stability and recyclability are critical for the use of catalysts in practical application. Therefore, this study conducted cyclic experiments on the WS₂. As shown in Figure 9a, the WS₂ maintains acceptable efficiency for AO7 removal (>70%), though a slight decrease was observed after three cycles, indicating its relative stability in the oxidation process. The decreased removal efficiency might be attributed to the adsorption of AO7 and its degradation byproducts on the catalyst surface, thus covering its active sites on the surface of catalysts. In order to validate this hypothesis, the adsorption of AO7 by using the reused catalyst has been explored. As shown in Figure 9b, the used catalyst after the reaction had lost its adsorption effect on the pollutants further confirmed that the active sites were covered by organic intermediates.

To analyze the change of surface physicochemical properties of the catalyst, the used catalyst was then characterized using XRD and XPS. The XRD (Figure 10a) pattern shows that the catalyst’s crystal phase did not change before and after the reaction. XPS (Figure 10b–d) analysis indicated that two peaks observed around 32.8 eV and 34.9 eV belong to W⁴⁺ 4f_5/2 and W⁴⁺ 4f_7/2, respectively [31]. The peaks near 35.6 eV and 38.3 eV belong to W⁶⁺ 4f_5/2 and W⁶⁺ 4f_7/2, respectively. Compared to the fresh WS₂, the W⁶⁺/W⁴⁺ ratio in the used WS₂ slightly increased from 17.3% to 20.3%, indicating the redox role of the W element in the activation of the PMS process. In contrast, for the S2p peak, the peaks near 162.4 and 163.5 eV are due to the presence of both S²⁻ [32], and the peak appearing at 168.9 eV is observed owing to the presence of SO₄²⁻ in the fresh WS₂, which might be probably ascribed to a little oxidation of S²⁻. In brief, the structural and surface properties of the catalyst are relatively stable before and after the reaction, which is conducive to the reuse of catalyst.

2.7. Degradation Pathway and Product Toxicity Prediction

LC–MS analysis was used to identify the formation of degradation intermediates. The chemical structures of five byproducts along with their m/z values are shown in

Table 2. Degradation products in the E/WS₂/PMS system determined by LC–MS.

Number	Compound	Molecular Formula	m/z	Molecule Structure
M1	(2-ethyl-4-hydroxy-3-nitrophenyl) acetic acid	C10H9NO7	255
M2	β-Naphtalenol	C10H8O	144
M3	2-nitrophenol	C6H5NO3 (H+)	140
M4	Salycilic acid	C7H6O3	138
M5	Pentanoic acid	C5H10O2	102

. The possible oxidation pathways of AO7 are proposed in Figure 11. Initially, the products M1 (m/z = 255) and M2 (m/z = 144) were probably produced due to the hydrogenation reaction of the -N=N- group, and the ring opening reaction by the subsequent oxidation [33]. Afterwards, the product M3 (m/z = 140) was generated through a decarboxylation reaction of product M1, while product M2 was transformed into M4 (m/z = 138) with the further ring opening and carboxylation of the benzene ring. Finally, the carboxylic acid structure in product M4 was unstable and oxidized through ring–rupturing reaction into aliphatic compound M5 (m/z = 102).

The developmental toxicity of the AO7 and degradation products was assessed through Toxicity Estimation Software (T.E.S.T.), v. 5.1.2 [34]. As depicted in Figure 12, it was worth noting that compared with the developmental toxicity index (0.96) of AO7, the other five degradation products exhibited relatively low values of developmental toxicity, indicating that the toxicity of solution could be efficiently reduced by the treatment of the E/WS₂/PMS process.

3. Materials and Methods

3.1. Chemicals

C₁₆H₁₁N₂NaO₄S (AO7), KHSO₅∙0.5KHSO₄∙0.5K₂SO₄ (PMS), methanol (MeOH), tert-butyl alcohol (TBA) and L-Histidine (L-His) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Sodium sulfate (Na₂SO₄), sulfuric acid (H₂SO₄), sodium hydroxide (NaOH), potassium chloride (KCl), potassium bicarbonate (KHCO₃), potassium dihydrogen phosphate (KH₂PO₄), potassium nitrate (KNO₃), tungsten disulfide (WS₂), 2,2,6,6-tethamethyl-4-piperidinol (TEMP), 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and humic acid (HA) were obtained from Aladdin Chemical Co., Ltd. (Shanghai, China). Unless otherwise specified, all the chemicals used in this work are of analytic grade and used without additional purification steps.

3.2. Experiments

All experiments were conducted in a 250 mL beaker containing 10 mg/L of AO7 solution (200 mL), which was stirred by using a mechanical stirrer (SN-MS-H280D, Shanghai Co., Ltd., Shanghai, China) at room temperature (25 °C), with 0.05 mol/L of Na₂SO₄ as the electrolyte. The initial pH (pH₀) of AO7 solution was measured with a pH meter (LC-PHB-1M, Shanghai Lichen Technology Instruments Co., Ltd., Shanghai, China) and adjusted by diluted H₂SO₄ and NaOH.

A piece of graphite anode and graphite cathode (4 cm × 10 cm) was placed in the aforementioned solution with an immersed area of 12 cm². The distance between the anode and cathode is 3 cm. During the experiments, a certain amount of PMS and WS₂ powder was added to the solution in sequence. Afterwards, the electricity is switched on by using a direct current power supply (IT6800A, EADEX Electronics Co., Ltd., Suzhou, China), and the reaction begins. At predetermined intervals, 3 mL of the AO7 solution was taken through a 0.22 μm membrane and immediately pipetted into a quartz cuvette and measured by a UV–Vis spectrophotometer (UV-1800, Suzhou Shimadzu Instruments Co., Ltd., Suzhou, China) at a wavelength of λ = 485 nm. Each experiment was carried out at least twice with error bars. The AO7 removal rate (η) could be calculated as Equation (8), while the removal of AO7 obeyed pseudo-first-order kinetic, which can be defined as Equation (9) [35].

η = (C₀ − C_t)/C₀ × 100%

(8)

ln(C₀/C_t) = kt

(9)

where t is the reaction time (min), C₀ and C_t are the pollutant concentrations (mg/L) at 0 min and t min, respectively, and k is the apparent kinetic constant (min⁻¹).

Energy consumption per stage (EE/O) is adopted to evaluate the electrical energy consumption. EE/O is defined as the electrical energy required to remove pollutants by one order of magnitude in 1 m³ of wastewater [36], and its calculation is shown in Equation (10).

$$\mathrm{E}\mathrm{E}/\mathrm{O}={\displaystyle \frac{\mathrm{U}\times \mathrm{I}\times \mathrm{t}}{\mathrm{V}\times \log \left({\mathrm{C}}_0/{\mathrm{C}}_{\mathrm{t}}\right)}}$$

(10)

where U is the average voltage (V), I is the output current (A), t is the reaction time (h), V is the volume of reaction solution (L) and EE/O is the energy consumption per stage (kWh/m³).

The catalyst WS₂ was recovered by just washing several times and drying at 80 °C overnight to evaluate its reusability. The degradation products of AO7 were determined by Liquid Chromatography–Mass Spectrometry (LC–MS; UFLC–MS 2020, Shanghai Shimadzu Instruments Co., Ltd., Shanghai, China). Data were recorded with a high-resolution accurate-mass (HR/AM) Orbitrap mass analyzer and operated in a negative mode with a mass range of 50–900. A sample injection of 20 µL was separated on a Zorbax RX-C18 column (4.6 × 250 mm, 5 μm) at 30 °C with the flow rate of 0.8 mL/min. Isocratic elution was employed by a mixture of C₂H₃N and water at 75:25 v/v.

3.3. Characterization

The structural information of catalyst was analyzed by X-ray diffraction instrument (XRD; MiniFlex 600, Rigaku Holdings Corporation, Tokyo, Japan) and field emission scanning electron microscopy (FESEM; SIGMA 500, Carl Zeiss AG, Jena, Germany). The elemental composition and chemical valence of the catalyst were determined by X-ray photoelectron spectroscopy (XPS; Thermo Scientific ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA) spectrometer. Additionally, the electron paramagnetic resonance (EPR; Bruker EMX plus, Bruker Corporation, Karlsruhe, Germany) technique was employed to detect the generation of reactive oxygen species during the reaction, with DMPO and TEMP being trapping agents for ^•OH/SO₄^•− and ¹O₂, respectively.

4. Conclusions

In this study, the electrochemical-assisted, WS₂-activated PMS system is carried out for the efficient removal of AO7 in water. Under the optimal conditions of 1.0 g/L WS₂, 1.0 mM PMS and a current density of 1.0 mA/cm² at pH 6.5, 95.8% of AO7 removal could be achieved in 30 min during the E/WS₂/PMS system. In addition, the removal efficiency of AO7 in neutral and acidic conditions was higher than that in the alkaline condition. The AO7 removal tended to be increased with increasing the catalyst dosage, PMS concentration and the current density, to some degree, while a further increase in these factors will have an inhibitory effect on the AO7 removal. Importantly, WS₂ had good reusability during the reaction, with excellent anti-interference capabilities. The degradation of AO7 was slightly affected by the presence of various anions, humic acid and real water, demonstrating the potential for practical application. Mechanism investigations further revealed that ^•OH and ¹O₂ are the dominant reactive species on the oxidation of organic pollutants during the E/WS₂/PMS system.