Co-Catalytic Effect of Molybdenum Disulfide on Fenton Reaction for the Degradation of Acid Orange 7 Dye: Reaction Mechanism,Performance Optimization,and Toxicity Evaluation

Abstract

Fenton reaction is extensively used in wastewater treatment to produce ^•OH, which can be used to remove organic pollutants excellently. However, the greatest issue of the process is the low efficiency of Fe²⁺/Fe³⁺ conversion cycle and hydrogen peroxide (H₂O₂) decomposition rate (<30%). In this study, molybdenum disulfide (MoS₂) was used as a co-catalyst in Fenton system to accelerate the oxidation of acid orange 7 (AO7) dye. The enhanced removal efficiency was attributed to that the unsaturated S atoms on the surface of MoS₂ could capture the protons in solutions and turn into H₂S, exposing the active sites with reducibility and accelerating the conversion of Fe²⁺/Fe³⁺. A high degradation rate of 98% was obtained in 30 min under the optimal conditions of AO7 concentration 100 mg/L, MoS₂ 6 g/L, FeSO₄·7H₂O 50 mg/L, H₂O₂ concentration 1.5 mmol/L, and pH 3.5. Furthermore, the degradation pathway and mechanism of AO7 were thoroughly discussed. The toxicity of the intermediates during the degradation progress slightly increased initially, which then decreased to nontoxicity. MoS₂ can keep stable, which can maintain high AO7 removal efficiency even after being reused for seven times. Moreover, the obtained results provide a theoretical basis and guidance for the application of inorganic co-catalyst in practical wastewater remediation applications.

Introduction

Dye wastewater is a kind of complex industrial wastewater containing various organic compounds (Qi et al., 2009; Katheresan et al., 2018). The untreated organic wastewater significantly affects water quality, public health, and welfare (Brillas and Martínez-Huitle, 2015). Because of the complex composition, high chemical oxygen demand, and toxicity value, it is difficult to remove and degrade (Gazi et al., 2010). As a result, it is necessary to put forward a useful removal method for wastewater before it is discharged into the environment. At present, wastewater treatment methods can be clarified into membrane separation, adsorption (Asfaram et al., 2015), magnetic separation, biological method, and chemical method like chlorination (Du et al., 2016), ozonation (Soltermann et al., 2017), and flocculation (Fan et al., 2017; Zhuang et al., 2017). Recently, advanced oxidation processes (AOPs) have received considerable attentions in the treatment of dye wastewater (Pera-Titus et al., 2004). Fenton reactions, based on the generation of hydroxyl radicals (^•OH), through the reaction of hydrogen peroxide (H₂O₂), are frequently used to oxidize complex organic constituents (Mu et al., 2017). Nevertheless, this method is restricted by the low production of ^•OH and low efficiency of H₂O₂ decomposition (Qin et al., 2015). To solve the current problem, researchers began to search for a co-catalyst to decompose H₂O₂ in the Fenton reaction. Co-catalysts such as salen (Gazi et al., 2010), cysteine (Qian et al., 2014), and protocatechuic acid (Qin et al., 2015) were selected to enhance the generation of ^•OH. However, these organic co-catalysts usually cause secondary pollution during the treatment progress. Although many inorganic catalysts were used in photocatalysis (Lang et al., 2015; Xiang et al., 2016), inorganic catalysts were seldom used in wastewater treatment (Sheng et al., 2019). Therefore, it is urgent to develop higher chemical stability inorganic co-catalysts (Xing et al., 2018; Sheng et al., 2020). According to Xing et al. (2018), metal sulfides (including WS₂, molybdenum disulfide [MoS₂], CoS₂, Cr₂S₃, ZnS, and PbS) might be used to be the favorable co-catalysts for substantially increasing the H₂O₂ degradation efficiency and markedly reducing the Fe²⁺ and H₂O₂ doses needed within AOPs. In addition, Dong et al. (2018) investigated how WS₂ affected Fenton reaction during Cr(VI) synchronous reduction and phenol remediation. Moreover, as suggested by Wang et al. (2019), the MoS₂ quantum dots exhibited favorable performance to co-catalyze Fenton reactions because those surface Mo⁴⁺ active sites facilitated the reaction between Fe(III) and Fe(II) while increasing the H₂O₂ degradation rate. But few studies are conducted for the systemic investigation on the dye degradation under the co-catalysis of MoS₂.

In this study, MoS₂ was introduced as a co-catalyst to increase the Fe³⁺/Fe²⁺ conversion in photo-Fenton progress for the removal of acid orange 7 (AO7). In addition, the reaction mechanism, performance optimization, and toxicity evaluation were systematically studied. Exposed Mo⁴⁺ active sites with reductive properties could speed up the Fe³⁺/Fe²⁺ recycle and improve H₂O₂ decomposition. The removal efficiencies of AO7 were investigated under different experimental conditions including pH, MoS₂ dosage, H₂O₂ concentration, FeSO₄·7H₂O concentration and initial AO7 concentration. Then, the degradation pathway of AO7 was proposed by the gas chromatography–mass spectrometry (GC-MS) analysis. In the meanwhile, the toxicity evaluation was also discussed. MoS₂ co-catalytic Fenton progress has a positive application prospect in environmental remediation. Moreover, the obtained results provide a theoretical basis and guidance for the practical use of inorganic co-catalyst in environmental applications.

Materials and Methods

Chemicals and materials

MoS₂ and AO7 (C₁₆H₁₁N₂NaO₄S) were purchased from Aladdin Industrial Corp. (Shanghai, China). H₂O₂ (30 wt%) and FeSO₄·7H₂O were obtained from Sinopharm (Shanghai, China). All the chemicals were of analytical grade and used without further purification. In addition, all the solutions were prepared using deionized water.

Fenton experiments

A certain amount of MoS₂ (diameter <2 μm) and FeSO₄·7H₂O were added into 100 mL AO7 aqueous solution in a 150 mL quartz tube. About 0.1 mol/L HCl solution was used to adjust pH as needed. A certain amount of H₂O₂ solution was added into the above solution. The photo-assisted Fenton oxidation of AO7 was carried out under visible-light irradiation by a tungsten lamp (500 W, ∼550 nm). At the same time, all experiments were performed at constant temperature (25°C).

Analytical methods

The pH of the solution was monitored by a portable pH meter (Mettler-Toledo, Shanghai, China). The absorbance of AO7 during the degradation process was tested using a ultraviolet-visible (UV-Vis) spectrophotometer (UV-1700; Shimadzu, Japan). About 1.0 mL supernatant was mixed with 1.0 mL 1,10-phenanthroline monohydrate (1 mg/mL). Then, the absorbance was studied by UV-vis spectroscopy with the aim to make the evaluation of the concentration of Fe²⁺ ions. To measure the concentration of Fe³⁺ ions, potassium thiocyanate was used to replace 1,10-phenanthroline monohydrate. The intermediates formed during the AO7 degradation process were identified based on GC-MS (GC7890/MS5975; Agilent Technologies). Samples at different time intervals were extracted in dichloromethane (DCM) (ratio: dye solution:DCM of 1:10). Subsequently, 1.0 μL sample was injected at 40°C for 2 min. The column temperature was increased to 100°C at 10°C/min rate, and the temperature was then increased to 200°C at 5°C/min rate and maintained for 5 min. Afterward, the temperature was increased to 280°C at 20°C/min rate for 10 min. The microstructure of the MoS₂ was tested by scanning electron microscope (SEM; Merlin Compact, Zeiss, Germany).

Toxicity test

Vibrio fischeri was selected as a model bacteria to evaluate the toxicity by using bioluminescence inhibition assay. Before the toxicity test, an ampoule of the V. fischeri lyophilized powder was reactivated in 1 mL of 2 wt% NaCl solution for 10 min at 4°C and subsequently inoculated rapidly. AO7 and the degradation intermediates were arranged in the standard opaque plates. Each sample contained 20 μL of toxicants and 180 μL of the prepared bacteria solution. Regarding the control experiment, each sample was mixed with 20 μL 2% NaCl solution and 180 μL prepared bacteria solution. After exposing to the contaminant for 15 min, the luminescence intensity was detected with a microplate reader (GloMax-Multi; Promega). The percentage of inhibition of V. fischeri compared with the control group was used to evaluate the toxicity effect:

where L denotes the inhibition ratio; L_Sample and L_Blank refer to the average luminescence intensity of V. fischeri to the test samples and control group, respectively.

Result and Discussion

MoS₂ co-catalytic Fenton degradation of AO7

Effect of MoS₂ dosage

To investigate the effect of MoS₂ dosage, a series of experiments were conducted at different concentrations of 2, 4, 6, and 8 g/L with the initial AO7 of 100 mg/L, Fe(SO₄)·7H₂O of 50 mg/L, H₂O₂ of 1.5 mmol/L, pH = 3.5. Figure 1a shows the effect of MoS₂ dosage on the removal of AO7. The catalytic efficiency without MoS₂ is much lower than that of the addition of MoS₂. Obviously, the AO7 removal efficiency increased from 68.19% to 98.05% with the MoS₂ concentration increasing from 2 to 6 g/L in 30 min. However, no significant increment of AO7 removal efficiency was observed at MoS₂ concentration of 8 mg/L, which is mainly owing to that too much MoS₂ would weaken the light absorption (Dong et al., 2018). As given in Fig. 1b, under the condition [AO7 of 100 mg/L, Fe(SO₄)·7H₂O of 50 mg/L, H₂O₂ of 1.5 mmol/L, pH = 3.5, MoS₂ of 6 g/L], the Fe²⁺/Fe³⁺ ratio increased to 3.32 in the presence of MoS₂, indicating that MoS₂ could enhance the conversion efficiency from Fe³⁺ to Fe²⁺.

FIG. 1.

(a) The removal efficiency of AO7 under different concentrations of MoS₂ in the Fenton reaction. (b) Fe²⁺/Fe³⁺ ratio during the Fenton reaction. AO7, acid orange 7; MoS₂, molybdenum disulfide.

Effect of FeSO₄·7H₂O dosage

Iron ion is essential to the homolysis of H₂O₂. To investigate the effect of FeSO₄·7H₂O on AO7 removal, different FeSO₄·7H₂O concentrations of 10, 30, 50, and 70 mg/L were investigated and the results are given in Fig. 2a. For the same initial AO7 concentration (100 mg/L), a low FeSO₄ concentration (10–50 mg/L) has a great influence on the reaction. It was observed from the curve that the AO7 removals increased with the increase of FeSO₄ concentration. However, the AO7 removal efficiency decreased when the FeSO₄ concentration was >50 mg/L. This was because ^•OH was scavenged by the Fe²⁺ and H₂O₂, which was harmful to the enhanced degradation of AO7 (Tang and Huang, 1996). Therefore, FeSO₄ concentration of 50 mg/L was used in the following experiments.

FIG. 2.

The removal efficiency of AO7 by adding MoS₂ cocatalyst at different conditions: (a) different concentrations of FeSO₄; (b) different concentrations of H₂O₂; (c) different initial pH; (d) different initial AO7 concentration [initial AO7 of 100 mg/L, MoS₂ of 6 g/L, Fe(SO₄) · 7H₂O of 50 mg/L, H₂O₂ of 1.5 mmol/L, pH = 3.5]. H₂O₂, hydrogen peroxide.

Effect of H₂O₂ dosage

As the hydroxyl radicals were formed in the reaction solution, the effect of H₂O₂ concentration was explored. To investigate the effect of H₂O₂ decomposition for the removal of AO7, the pH of the prepared solution was adjusted as 3.5, the initial concentration of AO7 was 100 mg/L, the concentration of MoS₂ was 6 g/L, the concentration of FeSO₄·7H₂O was 50 mg/L, and the concentration of H₂O₂ was 0.5, 1.0, 1.5, and 2.0 mmol/L, respectively. As given in Fig. 2b, the removal of AO7 was almost proportional to the H₂O₂ concentration. The degradation rate of AO7 reached the highest value when the concentration of H₂O₂ was 1.5 mmol/L. The AO7 removal efficiency increased from 54.64% to 98.05% with the H₂O₂ concentration increasing from 0.5 to 1.5 mmol/L in 30 min. However, the AO7 removal efficiency showed no significant increment at 2.0 mmol/L of H₂O₂ concentration. This was mainly because of the fact that ^•OH was scavenged by the Fe²⁺ in the solution, as given in Equation (3). In the photo-Fenton process, the optimum H₂O₂ concentration was 1.5 mmol/L. The Equation (4) also showed that the Fe³⁺ might react with H₂O₂, resulting in a decreased AO7 removal. Excessive H₂O₂ ions would act as ^•OH scavengers, leading to equipment corrosion and high economic costs (Sharma and Kumar, 2012).

Effect of pH

pH is a crucial factor determining reaction effect in this MoS₂-photo-Fenton system. The effect of pH in operating time of 30 min was measured at 50 mg/L Fe(SO₄)·7H₂O and 6 g/L MoS₂, 1.5 mmol/L H₂O₂, and initial pH range of 2.5–6.5. The results are given in Fig. 2c. Clearly, low pH could promote the degradation of AO7. It could also be found that the removal efficiencies of AO7 increased at initial pH 2.5–4.5. The present results were in consistence with previous studies (Rivas et al., 2001; Szpyrkowicz et al., 2001). In addition, the Zeta potentials of the MoS₂ aqueous solution under different pH values (pH = 2.5, 3.5, 4.5, 6.5) could be measured and are given in Supplementary Fig. S1. Consequently, the isoelectric point of MoS₂ is ∼2.98. Therefore, it can be concluded that the MoS₂ surface is negatively charged under a pH value of 3.5. Under the action of exposed Mo⁴⁺, the Fe³⁺ adsorbed may be reduced in a continuous manner for the formation of Fe²⁺. Thereafter, the resultant Fe²⁺ ions were further distributed in the water solution to decompose H₂O₂.

It also indicated that the higher the pH, the worse the dispersion of MoS₂. That is why low pH can lead to better AO7 removal. In addition, the excessively high or low pH values reduce FeOH²⁺ content. As a matter of fact, at pH >4.0, the solubilized iron deposits in the form of ferric hydroxide, thereby resulting in catalyst poisoning.

The variation of pH during the reaction at different initial pHs was measured and presented in Supplementary Fig. S2. During the reaction, the unsaturated S atoms could capture the protons to form H₂S, causing the low and steady pH value of the MoS₂ aqueous solution. This explanation was also according to the pH variation. The pH decreased slightly at first, then increased and kept stable. The proper pH values could promote the concentration of reactive species of Fe(OH)²⁺ (Clarizia et al., 2017). When the initial pH was >4.5, lower AO7 removal efficiency was observed. Indeed, at higher pH values of >4.5, dissolved iron would precipitate to form ferric hydroxide, leading to catalyst poisoning. When the solution was alkaline, Fe²⁺ in the solution was extremely easy to convert into precipitation.

Effect of initial AO7 concentration

As given in Fig. 2d, after 30 min, the AO7 removals reached 100, 97.54, 95.15, and 86.25% for the initial concentrations of 25, 50, 100, and 200 mg/L, respectively. The results indicated that the AO7 decreased with the increasing initial AO7 concentration. It could be explained that under the same condition, the number of reactive species produced in the photo-Fenton progress was maintained constant. The AO7 solution at high concentration could not be degraded effectively because there were no sufficient reactive oxidative species (Yao et al., 2019). As a result, combining all the factors mentioned previously, 100 mg/L of the AO7 solution was the appropriate initial concentration.

Effect of operation method with/without visible-light

According to the experiments mentioned previously, it could be concluded that the optimal MoS₂-photo-Fenton reaction conditions were as follows: AO7 concentration of 100 mg/L, MoS₂ of 6 g/L, Fe(SO₄) · 7H₂O of 50 mg/L, H₂O₂ of 1.5 mmol/L, and pH of 3.5. To compare different operation methods for AO7 degradation, a series of experiments were conducted to investigate the MoS₂ co-catalysis effect in the Fenton reaction. The MoS₂ adsorption in dark/under visible light indicated the dye adsorption by MoS₂ alone. As given in Fig. 3, the AO7 removal by MoS₂ adsorption was very low. Therefore, for pure MoS₂, its efficiency is low under light irradiation and in dark, which can offset its photocatalytic and adsorption performances (Xing et al., 2018). The poor activity of MoS₂ + H₂O₂ indicated that MoS₂ could not directly decompose H₂O₂. In addition, the activity of MoS₂ under visible light irradiation was much better in comparison with that in dark environment. It could be explained by the fact that light could improve the decomposition of H₂O₂ as well as the conversion of Fe³⁺ to Fe²⁺ (Sang et al., 2013). Through introducing visible light, the efficiency of traditional Fenton reaction experienced little increase. The removal of AO7 was >70% for 10 min under visible light irradiation by the addition of MoS₂. Even through an obvious enhancement could be acquired in the lack of visible light irradiation, carrying out the reaction under visible light irradiation could make further improvement of the dye degradation. The above results indicated that the MoS₂ could enhance the oxidation of Fenton under visible light.

FIG. 3.

Degradation of AO7 under visible light and in dark. [AO7 of 100 mg/L, MoS₂ of 6 g/L, Fe(SO₄)·7H₂O of 50 mg/L, H₂O₂ of 1.5 mmol/L, pH = 3.5].

Mechanism investigation

Degradation pathways of AO7

To further understand the degradation pathways of AO7 in this MoS₂ co-catalytic Fenton reaction, GC-MS analysis was used to identify the intermediates and detect the degradation pathway. Table 1 provides the main intermediates and their structures. According to the intermediates, we could propose a possible degradation pathway of AO7, as given in Fig. 4. First of all, the azo bond (−N = N−) of AO7 undergoes cleavage under the attack of hydroxyl radicals, which leads to the decolorization of the solution. The GC-MS results demonstrated that AO7 was decomposed to 4-aminobenzenesulfonic acid (A1) and 1-amino-2-naphthol (A3). Then, A1 was degraded into p-dihydroxybenzene (A2) with the releasing NO₃⁻/ and ions (Garcia-Segura et al., 2013). Meanwhile, A3 was hydroxylated and converted to naphthalene-1, 3-diol (A4) and naphthalene-1, 2, 4-triol (A5) (Guinea et al., 2009). Subsequently, the smaller aromatics A2 and A5 were oxidized by hydroxyl radicals. The benzene rings were opened and many linear carboxylic acids were produced. Consequently, the AO7 was efficiently removed by the co-catalytic Fenton reaction.

FIG. 4.

The degradation pathway of AO7 during the MoS₂ co-catalytic Fenton reaction.

Table 1.

The Intermediates During the Molybdenum Disulfide Co-Catalytic Fenton Reaction

No.	m/z	Molecular formula	Name
A1	173	C₆H₇NO₃S	4-Aminobenzenesulfonic acid
A2	110	C₆H₆O₂	p-Dihydroxybenzene
A3	159	C₁₀H₉NO	1-Amino-2-naphtol
A4	160	C₁₀H₈O₂	Naphthalene-1, 3-diol
A5	176	C₁₀H₈O₃	Naphthalene-1, 2, 4-triol

In this study, MoS₂ was introduced as a co-catalyst to this photo-assisted Fenton system. The presence of MoS₂ effectively enhanced the efficiency of Fenton progress. Admittedly, conventional Fenton progress involved Fe²⁺/H₂O₂ processes through the following equations:

Equation (4) reveals that a large quantity of H₂O₂ is required owing to the presence of Fe³⁺.

When the co-catalysts MoS₂ was added in this system as given in Fig. 5a, the initial pH of the 6 g/L MoS₂ aqueous dispersal is ∼4.5, which is acidic, indicating either donating protons or/and getting hydroxyl groups happened in water. Because MoS₂ is incapable of donating protons to H₂O, it is highly likely that MoS₂ is being able to capture hydroxyl groups from water. The possible route is the leaving of S on Mo with the aid of proton to form H₂S. Moreover, when washed, the MoS₂, the sample became less and less acidic, which might be owing to the removal of H₂S during washing (Fig. 5a). To investigate the generation of sulfur vacancies of MoS₂, the electron paramagnetic resonance (EPR) spectra of the MoS₂ was tested. As given in Fig. 5b, MoS₂ after water washing presents a significant EPR signal at g = 2.0, which can be explicated as the sulfur vacancies can be generated. In general, it is believed that the unsaturated S atoms can capture protons during H₂ evolution (Voiry et al., 2016).

FIG. 5.

(a) pH value of MoS₂ solution after washing with water for different times. (b) Electron paramagnetic resonance spectra of MoS₂ after being washed by water (g = 2.0). (c) The total organic carbon at different reaction time. (d) The proposed mechanism of MoS₂ co-catalytic Fenton reaction.

To sum up, the reaction was added with the conversion of Fe³⁺ to Fe²⁺, as given in the following Equation (5) (Xing et al., 2018).

First, the protons was captured by unsaturated S atoms on the surface of MoS₂, forming H₂S with the oxidization of Mo⁴⁺ to Mo⁶⁺. Furthermore, the Mo⁶⁺ was reduced to Mo⁴⁺ with the decomposition of H₂O₂ in Equation (6), ensuring the catalytic of MoS₂. The reason can be speculated in two aspects. One is that the MoS₂ can guarantee the recycling of Fe^3+/Fe²⁺. In addition, it can effectively promote the H₂O₂ to produce hydroxyl radicals.

The total organic carbon (TOC) of AO7 was also measured in the removal process. As given in Fig. 5c, the TOC of orange acid after 30 min was 18.7%. Figure 5d proposed the mechanism of MoS₂ as the co-catalyst in Fenton reaction. MoS₂ shows excellent co-catalytic activity for Fenton reactions owing to the Fe³⁺/Fe²⁺ cycle reactions and high H₂O₂ decomposition efficiency.

Toxicity evaluation and UV-vis absorption spectra during the degradation process

In conventional wastewater treatment, the toxicity of the intermediates was rarely tested during the degradation. To test the toxicity of accumulation of AO7 and its intermediates, the V. fisheri bioluminescence bacterial assay was used to determine the luminescence intensity and the results are given in Fig. 6a. The inhibition ratio of the initial toxicity of the AO7 solution (100 mg/L) was tested as 31.32%. During the first 5 min, the inhibition ratio rose slightly from 31.32% to 37.34%, indicating that there might be some new intermediates that were more toxic than AO7 in the solution. Then, the inhibition ratio rapidly reduced to 2.47% in 15 min of treatment. Subsequently, the inhibition ratio of the test solution was decreased to negative values. This was owing to the fact that luminescence of V. fisheri was not inhibited. These toxicity test results indicated that AO7 was effectively degraded by MoS₂ co-catalytic Fenton reaction. Furthermore, the UV-Vis absorption spectra during the Fenton degradation are given in Fig. 6b. The characteristic peaks of AO7 intensity decreased dramatically in 20 min and maintained unchanged. This can be attributed to the production of ^•OH, which comes from the decomposition of H₂O₂ by the efficient Fe³⁺/Fe²⁺ conversion cycle.

FIG. 6.

The inhibition to Vibrio fischeri (a) and ultraviolet visible absorption spectra of AO7 (b) during different reaction time [AO7 of 100 mg/L, MoS₂ of 6 g/L, Fe(SO₄)·7H₂O of 50 mg/L, H₂O₂ of 1.5 mmol/L, pH = 3.5].

Stability of MoS₂ co-catalytic Fenton reaction

The recyclable property of MoS₂ used in the reaction was studied and the results are given in Fig. 7a. The recycled MoS₂ catalyst maintains nearly the same removal performance even after being reused seven times and the AO7 removal efficiency can keep up to 98%. Because the surface property determined the catalytic activity of the catalyst, X-ray photoelectron spectroscopy was used to characterize the surface information of MoS₂ co-catalyst before and after dye degradation. By comparing the Mo3d and S2s peaks in Fig. 7b, it was found that the surface of Mo and S on MoS₂ had no obvious change before and after the dye degradation. In other words, it is one of the reasons that the MoS₂ can be reused many times. In addition, the SEM images of MoS₂ before and after co-catalytic Fenton reaction (Fig. 7c) confirms the stability of MoS₂ during the reaction. The mass and microstructure of MoS₂ almost do not change after the cycle test, indicating that the MoS₂ can be used as a stable co-catalyst in the Fenton reaction for wastewater treatment. In addition, the content of Mo element in the supernatant of the treated solution was measured by inductively coupled plasma optical emission spectrometer. The actual content of Mo was 0.008 mg/L in the treated AO7 solution after filtering. During this process, a certain amount of dissolved Mo ions could be produced and caused a wastage of MoS₂. However, in this study, the mass loss of MoS₂ was very small and could be ignored.

FIG. 7.

(a) Cycle test of the MoS₂ cocatalytic Fenton reaction for the degradation of AO7. (b) XPS spectra of MoS₂ before and after reaction. (c) Scanning electron microscope images of MoS₂ before and after cycle test. XPS, X-ray photoelectron spectroscopy.

Conclusion

To conclude, MoS₂ was introduced as co-catalyst in the Fenton system for the enhanced removal of AO7. The co-effect speeded up the reduction cycle of Fe³⁺ to Fe²⁺ owing to the oxidation of Mo⁴⁺. The removal efficiency of AO7 in the MoS₂ co-catalytic Fenton reaction reached 98.05% in 30 min under the optimal conditions as follows: AO7 of 100 mg/L, MoS₂ of 6 g/L, Fe(SO₄)·7H₂O of 50 mg/L, H₂O₂ of 1.5 mmol/L, and pH = 3.5. The main intermediates of AO7 degradation were detected in Fenton's progress by performing the GC-MS analysis. Furthermore, the toxicity of intermediates during the degradation progress slightly increased at first and then decreased to nontoxicity. The cycle test shows that MoS₂ can be used as a stable co-catalyst in the Fenton reaction. Moreover, this study suggested that inorganic co-catalyst could be widely applied in Fenton progress for the treatment of wastewater polluted by organic dyes.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This study was supported by the Program National Natural Science Foundation of China (No. 41807469), the Scientific Research Fund of Yunnan Provincial Education Department (2018JS026), Yunnan Applied Basic Research Project (No. 2019FD038), and Yunnan Provincial Scientific Innovation Team of Soil Environment and Ecological Safety (2019HC008).

Supplementary Material

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Supplementary Material

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Co-Catalytic Effect of Molybdenum Disulfide on Fenton Reaction for the Degradation of Acid Orange 7 Dye: Reaction Mechanism,Performance Optimization,and Toxicity Evaluation

Abstract

Introduction

Materials and Methods

Chemicals and materials

Fenton experiments

Analytical methods

Toxicity test

Result and Discussion

MoS2 co-catalytic Fenton degradation of AO7

Effect of MoS2 dosage

Effect of FeSO4·7H2O dosage

Effect of H2O2 dosage

Effect of pH

Effect of initial AO7 concentration

Effect of operation method with/without visible-light

Mechanism investigation

Degradation pathways of AO7

Toxicity evaluation and UV-vis absorption spectra during the degradation process

Stability of MoS2 co-catalytic Fenton reaction

Conclusion

Footnotes

Author Disclosure Statement

Funding Information

Supplementary Material

References

Supplementary Material

MoS₂ co-catalytic Fenton degradation of AO7

Effect of MoS₂ dosage

Effect of FeSO₄·7H₂O dosage

Effect of H₂O₂ dosage

Stability of MoS₂ co-catalytic Fenton reaction