Enhancement of H2O2 Decomposition by the Cocatalytic Effect of

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Remediation and Control Technologies

Enhancement of H2O2 Decomposition by the Cocatalytic Effect of WS2 on the Fenton Reaction for the Synchronous Reduction of Cr(VI) and Remediation of Phenol Chencheng Dong, Jiahui Ji, Bin Shen, Mingyang Xing, and Jinlong Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02403 • Publication Date (Web): 04 Sep 2018 Downloaded from http://pubs.acs.org on September 5, 2018

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Environmental Science & Technology

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Enhancement of H2O2 Decomposition by the Cocatalytic Effect of WS2 on the

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Fenton Reaction for the Synchronous Reduction of Cr(VI) and Remediation of

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Phenol

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Chencheng Dong, Jiahui Ji, Bin Shen, Mingyang Xing* and Jinlong Zhang*

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Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of

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Chemistry & Molecular Engineering, East China University of Science and

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Technology, 130 Meilong Road, Shanghai 200237, P.R. China.

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ABSTRACT

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The greatest problem in the Fe(II)/H2O2 Fenton reaction is the low production of •OH

11

owing to the inefficient Fe(III)/Fe(II) cycle and the low decomposition efficiency of

12

H2O2 ( 420 nm, 500 W).

94

Simultaneous Oxidation of Phenol and Reduction of Cr(VI). The Fenton

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oxidation of phenol and the synergistic reduction of Cr(VI) were tested under visible

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light irradiation by using a tungsten lamp (> 420 nm, 500 W). In a typical experiment,

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certain amounts of ferrous sulfate, hydrogen peroxide, and WS2 were suspended in 100

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mL 10 mg/L phenol solution with the addition of 1.0 mL K2Cr2O7 (40 mg/L) aqueous

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solution. In addition, the pH of the suspension was adjusted by using 0.1 M H2SO4

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solution and 0.1 M NaOH solution. During visible light irradiation, a certain amount of

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suspension was collected from the reaction cell at given time intervals and then

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centrifuged to remove the catalysts. The concentration of Cr(VI) was determined by the

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diphenylcarbazide (DPC) method.54 The absorbance of sample solutions was measured

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by a UV-vis spectrometer at 540 nm after full color development. The concentration of

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phenol was measured by a High Performance Liquid Chromatography (HPLC) system.

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Detection of Generated Hydroxyl Radicals. The number of •OH molecules

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generated during the WS2 cocatalytic AOP was detected by measurement of the

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photoluminescence (PL) signal of hydroxybenzoic acid resulting from the capture of

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•OH by benzoic acid. The details are as follows: 400 mg WS2, 4 mg FeSO4·7H2O and

ACS Paragon Plus Environment

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a certain amount of benzoic acid (0.2 mmol/L) were mixed into 100 mL H2O. Then, 4.0

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μL H2O2 (30% wt) was added into the solution by pipette. After vortexing for 10

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seconds, the mixed solution was placed under visible light irradiation for 30 min. The

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solution was filtered, and the filtrate was measured by PL emission spectroscopy to

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indirectly measure the amount of •OH (excitation wavelength: 330 nm).

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Detection of Fe(II) Ions. In this work, to detect the ferrous ions, we employed

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deionized water instead of phenol solution. Certain amounts of ferrous sulfate,

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hydrogen peroxide, and WS2 were added into the water. In addition, the pH of the

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suspension was adjusted by using 0.1 M H2SO4 solution and 0.1 M NaOH solution.

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During visible light irradiation, approximately 1.0 mL of the suspension was collected

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from the reaction cell at given time intervals. The 1.0 mL supernatant was mixed with

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1.0 mL 1 mg/mL 1,10-phenanthroline monohydrate. After that, the absorbance was

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investigated by UV-vis spectroscopy to evaluate the corresponding amounts of Fe2+

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ions.

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Characterization. The concentration of the pollutant was measured using a UV-vis

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spectrophotometer (Shimadzu, UV-2450). Raman measurements were performed at

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room temperature using a Via+ Reflex Raman spectrometer with an excitation

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wavelength of 514 nm. Transmission electron microscopy (TEM) was conducted on a

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JEOL JEM-2100EX electron microscope, operated at an accelerating voltage of 200 kV.

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The intensity of hydroxyl radicals was also measured using luminescence spectrometry

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(Cary Eclipse) at room temperature under an excitation wavelength of 330 nm. X-ray

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diffraction (XRD) measurements were performed with a Rigaku Ultima IV (Cu Ka

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radiation, λ = 1.5406 Å) in the range of 10-80°(2θ). The instrument employed for XPS

133

measurements was a Perkin-Elmer PHI 5000C ESCA system with Al Kα radiation

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operated at 250 W. The total organic carbon (TOC) concentration of the degradation

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agent was recorded using a SHIMADZU TOC-L CPN analyzer.

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3. RESULTS AND DISCUSSION



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WS2 Cocatalytic AOP Performance for the Remediation of Phenol. Phenol is

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commonly known as a general reagent in chemical analysis, especially for the

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manufacture of medical and industrial organic compounds.55, 56, 57 In addition, phenol

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is a common component of oil refinery wastes, which may enter the environment via

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discharges from oil refineries, coal conversion plants, and municipal waste treatment

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plants.58 Due to the stability of its aromatic ring and the hydrophilicity of its hydroxyl

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group, phenol has served as a common model pollutant for the development of AOPs.59-

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61

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performance. Thus, we firstly carried out systematic studies to reveal the substantial

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impact of WS2 on cocatalytic performance for H2O2 decomposition. To be more specific,

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we used the degradation of phenol as a model reaction and investigated a number of

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factors that may influence the performance of AOPs, such as the pH value and the

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dosages of WS2, Fe(II) and H2O2, as shown in Figure 1.

AOPs are always considered complicated systems, and many factors influence their

150 151

Figure 1. Different influencing factors of the WS2-cocatalytic Fenton process under

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visible light irradiation (λ> 420 nm) for the degradation of phenol (10 mg/L). (a) pH

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value influence (100 mL solution including 0.04 g/L Fe(SO4)·7H2O, 4.0 g/L WS2, and

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0.1 mmol/L H2O2). (b) Various concentrations of Fe(SO4)·7H2O (100 mL solution

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including 4.0 g/L WS2 and 0.1 mmol/L H2O2, pH = 3.8). (c) Various concentrations of


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WS2 (100 mL solution including 0.02 g/L Fe(SO4)·7H2O and 0.1 mmol/L H2O2, pH =

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3.8). (d) Various concentrations of H2O2 (100 mL solution including 0.02 g/L

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Fe(SO4)·7H2O and 4.0 g/L WS2, pH = 3.8).

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Some reports have demonstrated that photoassisted AOPs are a very efficient way to

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further improve the mineralization of organic pollutants.62-64 Hence, in our case, we

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carried out all the Fenton reaction experiments under visible light irradiation (λ > 420

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nm). First, we explored the impact of pH value on the photo-Fenton reaction for the

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degradation of phenol. As seen from Figure 1a, when the pH value is fixed at 3.8, WS2

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exhibits the optimal cocatalytic activity for the photo-Fenton reaction. L. Clarizia et

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al.46 have reported that too-low or too-high pH values decrease the concentration of

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reactive species of FeOH2+. Indeed, at pH values higher than 4.0, dissolved iron

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precipitates as ferric hydroxide, which leads to catalyst poisoning. Afterward, we tested

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the activity of WS2 cocatalytic AOPs with various amounts of Fe(II) ions. The results

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demonstrate that when ferrous sulfate is fixed at 0.04 g/L, it shows the highest

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efficiency (Figure 1b). Then, we investigated the effects of the amounts of added WS2

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and H2O2 on the degradation of phenol and found that the optimal amounts of WS2 and

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H2O2 were fixed at 4.0 g/L and 0.4 mmol/L, respectively (Figure 1c, d). Excessive Fe(II)

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ions or H2O2 would act as •OH scavengers, which are harmful to the enhancement effect

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of the Fenton reaction.65, 66 Meanwhile, too much WS2 powders would weaken the light

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absorption.



176 177

Figure 2. (a) Activity comparison of various Fenton reactions for the degradation of

178

phenol (10 mg/L). (100 mL solution including 0.04 g/L Fe(SO4)·7H2O, 4.0 g/L WS2,

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and 0.4 mmol/L H2O2, pH= 3.8; Vis: under visible light illumination (λ> 420 nm), Dark:

180

under dark conditions). (b) EPR spectra for the detection of •OH in the presence of 50

181

µL 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO, 0.14 M) at room temperature. The

182

EPR signals are marked as follows: green rhombus, hydroxyl free radicals.67 (c) PL


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spectra of hydroxybenzoic acid generated under different conditions. The

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decomposition efficiency of H2O2 can be obtained by the division of PL intensities.

185

Ultimately, in our case, the optimal visible-light-driven (λ > 420 nm) Fenton reaction

186

conditions were as follows: 100 mL of phenol solution at pH 3.8, containing 4.0 mg

187

FeSO4·7H2O, 400 mg WS2, and 4.0 µL H2O2. As shown in Figure 2a, under the optimal

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conditions, the WS2 cocatalytic AOPs using 0.04 g/L Fe(SO4)·7H2O, 4.0 g/L WS2 and

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0.4 mmol/L H2O2 display a high phenol degradation rate of 81% under visible light

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irradiation for 1 min only, which is much better than the degradation rate of the Fenton

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reaction in the absence of WS2 (43%). The low efficiency of pure WS2 in the dark and

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under light irradiation could negate the adsorption capacity and photocatalytic activity

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of WS2. In addition, the poor activity of WS2+H2O2 indicates that WS2 cannot directly

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decompose H2O2. It is worth mentioning that the activity of WS2 cocatalytic AOPs

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under visible light irradiation is better than that in the dark because light is beneficial

196

to the decomposition of H2O2 and the conversion of Fe3+ to Fe2+.68 After introducing

197

visible light, the efficiency of conventional Fenton reaction has a little increase, owing

198

to the low recycling efficiency of iron ions in the absence of WS2. Although a

199

significant enhancement can already be obtained in the dark, performing the reaction

200

under visible light irradiation may further improve the oxidation power of AOPs. On

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the other side, the commercial WS2’s BET surface area (6 m2·g-1) is too small to adsorb

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phenol molecules. Nevertheless, we have done a comparison of WS2 co-catalytic

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Fenton system with the adsorbent systems for the removal of phenol, as shown in Table

204

S1. As a result, the WS2 co-catalytic Fenton system exhibits a high phenol removal rate

205

of 240 mg·g-1·h-1, which is much better than the reported adsorbent systems (0.44～

206

44.82 mg·g-1·h-1). 5,5-Dimethyl-1-pyrrolidine-N-oxide (DMPO) was used as the

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electron paramagnetic resonance (EPR) probe to detect the •OH in the Fenton reaction.

208

Compared with the very weak EPR signal for WS2+H2O2, the FeSO4+H2O2 and

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FeSO4+WS2+H2O2 systems displayed the characteristic quartet signals of •OH (Figure

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2b).67 Furthermore, the FeSO4+WS2+H2O2 system showed the strongest EPR signals,

211

supporting the observation of the efficient cocatalytic activity of WS2 for the



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decomposition of H2O2. In addition, the production of •OH in AOPs was measured by

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observing the increase in the PL signal of hydroxybenzoic acid resulting from the

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capture of •OH by benzoic acid.69 Compared with the data for FeSO4+H2O2, the

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FeSO4+WS2+H2O2 solution displays a significantly enhanced PL signal (Figure 2c), an

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observation that further confirms the cocatalytic effect of WS2 on the decomposition of

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H2O2 into •OH. The tert-butyl alcohol (TBA) are always employed as the quenching

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agents of •OH radicals.70 Seen from Figure S1, after adding TBA (100 mg/L) into the

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phenol (10 mg/L) solution, the phenol removal rate of cocatalytic Fenton system has an

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obvious decrease from 81% to 22%. The result no doubt confirms that •OH radicals

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play a key role in cocatalytic Fenton reaction for the degradation of organic pollutants.

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In addition, we also employed AgNO3, 1,4-benzoquinone (PBA) and CH3OH as

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scavengers for electrons, superoxide radicals and holes, respectively. Seen from Figure

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S2, the phenol removal rate was obviously impeded by the adding of AgNO3 or CH3OH.

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It is because that the AgNO3 and CH3OH would react with the Fe2+ (or W4+) and •OH

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radicals respectively, which is harmful to the Fenton reaction for the oxidation of phenol.

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On the other hand, the adding of AgNO3 would greatly hinder the reduction of Cr6+,

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indicating the Fe2+ plays the key role for the Cr6+ reduction in the cocatalytic Fenton

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system. The AgNO3 would react with Fe2+ and W4+ to greatly decrease the

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concentration of Fe2+ in the Fenton process. The decomposition efficiency of H2O2

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represents an important index to measure in order to evaluate the performance potential

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of AOPs. The H2O2 (0.4 mmol/L) was heated at 90 °C for 120 min in order to

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completely decompose the H2O2 into •OH, as shown in Figure S3. After careful

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calculation by the division of PL intensities in Figure 2c, the conversion efficiency of

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H2O2 into •OH was as high as 60.1% by the promotion of WS2, which is much higher

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than the yield of 22.9% for the conventional Fenton reaction in the absence of WS2.


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Figure 3. Schematic diagram of the photo-Fenton degradation mechanism of phenol in

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the presence or absence of WS2. Insets are the mass spectra of intermediates observed

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from high-performance liquid chromatography (HPLC) that appeared in the

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degradation of phenol after 30 min.

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In addition, we investigated the degradation mechanism of phenol in the WS2

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cocatalytic Fenton system, as shown in Figures 3 and S4. In the original phenol solution,

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there was an obvious mass spectra peak at m/z=78, which was assigned to the benzene

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ring (Figure S4). After the Fenton reaction, the benzene ring is disintegrated into a

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variety of intermediate products. Among them, various organic acids and aldehydes are

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the main intermediate products. Interestingly, compared with the traditional Fenton

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process, there are significantly fewer intermediates from the WS2 cocatalytic Fenton

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reaction in the degradation of phenol (Figure 3), indicating that the cocatalytic system

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has a faster reaction rate for the remediation of organic pollutants. Another interesting

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finding is that after 30 min of reaction, the content of small-molecule fatty acids in the

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WS2 cocatalytic system is much lower than that of the traditional Fenton reaction (insets

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of Figure 3), suggesting the better mineralization of the cocatalytic system for the

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degradation of phenol.

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Apparent Kinetic Modeling. In the classic Fenton process, zero-,71 first-,72 and

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second-order73 reaction kinetics have been used to study the degradation of phenolic

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pollutants. In our case, the reaction rate equation can be described as the following Eq.

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4 or expressed in terms of logarithms (Eq. 5) owing to the presence of the cocatalyst



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WS2. In Eqs. 4 and 5, a, b, and c represent the reaction order, and K represents the total

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reaction rate constant. Here, we specifically analyzed three parameters: the

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concentrations of ferrous sulfate, hydrogen peroxide and tungsten disulfide. For this

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analysis, we assumed that the initial concentration of tungsten disulfide in the Fenton

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system is [WS2]0, the concentration of ferrous ion is [Fe2+]0, the concentration of

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hydrogen peroxide is [H2O2]0, and the rate at t = 0 can be expressed as in Eq. 6 or in the

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logarithmic form of Eq. 7.

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V  (dc / dt )  K [Fe 2+ ]a [H 2O2 ]b [WS2 ]c

(4)

-lg(dc / dt )  lg(K )  a  lg[Fe 2+ ]+b  lg[H 2O2 ]+c  lg[WS2 ]

(5)

V  (dc / dt )  K [Fe 2+ ]0a [H 2O2 ]0b [WS2 ]0c

(6)

-lg(dc / dt )  lg(K )  a  lg[Fe 2+ ]0 +b  lg[H 2O2 ]0 +c  lg[WS2 ]0

(7)

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We employed a series of ferrous solutions with different concentrations in the phenol

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solution, but the same concentrations of tungsten disulfide and hydrogen peroxide were

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used. Then, we recorded the curve of phenol concentration versus time. We can obtain

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the equations of these curves, which use t as the independent variable and c as the

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dependent variable, and the derivative of the equation that is equal to -lg(dc/dt) at

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different [Fe2+]0 values when t = 0. Meanwhile, we can obtain a group of related data,

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and -lg(dc/dt) is inversely related to lg[Fe2+]0. Since the amount of tungsten disulfide

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and hydrogen peroxide is fixed, lgK + b[H2O2]0 + c[WS2]0 is constant. Thus, -lg(dc/dt)

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is linear with respect to lg[Fe2+]0. We can obtain a straight line by using lg[Fe2+]0 as the

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abscissa and -lg(dc/dt) as the y-axis. The slope is a, and the intercept is lgK + b[H2O2]0

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+ c[WS2]0. Similarly, we can obtain the data for b, lgK + alg[Fe2+]0 + c[WS2]0, c and

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lgK + alg[Fe2+]0 + b[H2O2]0; thereby, we can obtain the value of K. Accordingly, the

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apparent kinetic equation of the WS2 cocatalytic Fenton reaction for the degradation of

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phenol can be obtained. The details of these calculations and results are shown in Figure

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4 and Tables S2~S4.


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Figure 4. Apparent kinetic calculations. (a) The fitting line between -lg(dc/dt) and

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lg[Fe2+]0 under visible light irradiation. (b) The fitting line between -lg(dc/dt) and

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lg[H2O2]0 under visible light irradiation. (c) The fitting line between -lg(dc/dt) and

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lg[WS2]0 under visible light irradiation. We chose the equation model ExpDec1: y =

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y0+A1e-x/t to fit our degradation data. As illustrated in Tables S2~S4, we acquired a

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series of fitting equations for the degradation rate under visible light. For t = 0, the

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derivatives of the equations are listed in Tables S2~S4.

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We employed a different initial concentration of FeSO4 to explore its kinetic model.

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By utilizing the above method, we obtained fitting equations for the degradation



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experiments (as listed in Table S2). The derivative of the equation -lg(dc/dt)0 is listed

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in Table S2 as well. Under visible conditions, -lg(dc1/dt) = 0.68999, -lg(dc2/dt) =

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0.73225, -lg(dc3/dt) = 0.31878, and -lg(dc4/dt) = 0.20616. As mentioned above, -

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lg(dc/dt) is inversely proportional to lg[Fe2+]0. Thus, we can obtain a fitting line (Figure

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4a) and Eq. 8. This indicates that the reaction order a is equal to -0.85691 and that lgK

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+ blg[H2O2]0 + clg[WS2]0 = -0.67339.

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-lg  dc/dt  = -0.67339-0.85691 lg[FeSO4 ]0

(8)

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Similarly, we chose a series of different initial concentrations of H2O2 to represent

300

its kinetic model. By employing the above calculation method, we obtained fitting

301

equations for the degradation experiments, as listed in Table S3. The derivative of the

302

equation -lg(dc/dt)0 is listed in Table S3 as well. As a result, under visible conditions,

303

we can obtain a fitting line (Figure 4b) and the equation of Eq. 9. Regarding the different

304

initial concentrations of the WS2 cocatalyst, the fitting equations supported by Origin

305

software are listed in Table S4. Unsurprisingly, we can obtain the fitting line shown in

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Figure 4c and Eq. 10.

307

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[ 0.523 -lg(dc/dt)=0.5896+( )e 0.514 π/2

-2(lg(H 2 O2 )0 +0.377)2

[ 0.0817 -lg(dc/dt)=0.1694+( )e 0.1231 π/2

0.5142

]

-2(lg(WS2 )0 -0.608)2 0.12312

(9)

]

(10)

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To further explore the total reaction rate constant K, we combined three equations

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(Eqs. 8~10) to obtain the apparent kinetic equation of Eq. 11: 3lgK + 2alg[FeSO4]0 +

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2blg[H2O2]0 + 2clg[WS2]0= 0.08561, where [FeSO4]0 = 0.02 g/L, [H2O2]0 = 0.1 mmol/L,

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[WS2]0 = 4 g/L, a =-0.85691, b = 0.81186, and c =0.52955. Thus, K equals 0.24359,

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and the total reaction order of a + b + c equals 0.4845. Careful kinetic measurements

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revealed that the reaction order of [WS2] is between zero- and first-order kinetics, which

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indicates that increased WS2 is beneficial for the degradation of phenol. In addition,

316

compared with the very slow rate-limiting step of Eq. 2 for the conventional Fenton


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reaction, the addition of the cocatalyst WS2 can greatly improve the total reaction rate.

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V=-lg  dc/dt  = 0.24359[FeSO4 ]-0.85691[H2O2 ]0.81186 [WS2 ]0.52955

(11)

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Mechanism Investigation. To investigate the mechanism of the WS2 cocatalytic

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effect in the Fe(II)/H2O2 Fenton system, we detected the soluble Fe2+ ions in solution

321

by adding 1,10-phenanthroline monohydrate (Figure 5a).74 Unsurprisingly, in the

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traditional Fenton reaction, there is a negligible peak ascribed to Fe2+ due to the rapid

323

oxidation of Fe2+ to form Fe3+ (Eq. 1). Interestingly, after adding the cocatalyst WS2,

324

an obvious peak at 511 nm assigned to Fe2+ can be observed owing to the acceleration

325

of the Fe(III)/Fe(II) cycle. In addition, the inset picture shows a color comparison

326

between the conventional Fenton and WS2 cocatalytic Fenton process. That is, the color

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of the solution in the presence of WS2 is much darker than that of the solution without

328

WS2, indicating the higher concentration of Fe2+ in the WS2 cocatalytic system. Hence,

329

we can conclude that WS2 can indeed promote the reduction of Fe(III) to Fe(II) in the

330

Fe(II)/H2O2 Fenton system, which plays the key role in H2O2 decomposition.

331



332 333

Figure 5. (a) Fe(II) concentration detection: UV-vis spectra of the conventional Fenton

334

process (Fe2++H2O2) and WS2 cocatalytic Fenton process with the addition of 1,10-

335

phenanthroline monohydrate (inset picture shows a photograph of this extracted

336

solution). (b) Low-temperature EPR spectra of WS2 after being washed by water (g =

337

2.0). (c) UV spectra of AgNO3, WS2 supernatant, and their mixture. (d) W4f XPS

338

spectra of WS2 before and after mixing with commercial Fe3O4 particles.

339

To investigate the reduction of Fe(III) by the cocatalytic effect of WS2 in the Fenton

340

process, we examined the surface chemical properties of WS2. We found an interesting

341

phenomenon in that WS2 is always acidic (pH = 2.5) in aqueous solution. Therefore,

342

we can infer that WS2 could release a large number of protons in solution. To determine

343

why, we performed EPR analysis at room temperature. Interestingly, WS2 after water

344

washing treatment shows an obvious EPR signal at g = 2.0 (Figure 5b), which can be

345

ascribed to the sulfur vacancies generated on the surface of WS2. It is generally accepted

346

that the unsaturated S atoms capture protons during H2 evolution.75-77 However, in our

347

case, the unsaturated S atoms could be removed from the WS2 surface by binding with


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protons to form H2S,76 leading to the low pH value of the WS2 aqueous solution. To

349

further demonstrate the release of H2S, we compared the extinction spectra of an

350

aqueous solution of pure AgNO3, the supernatant of a WS2 aqueous dispersion, and a

351

mixture of AgNO3+WS2, as shown in Figure 5c. A broad absorption curve at 267 nm in

352

the UV range corresponding to Ag2S was observed,78 indicating the loss of S atoms

353

(H2S) from WS2. The loss of S atoms exposes more W4+ active sites, which are easily

354

oxidized to W6+ by the adsorbed Fe3+. In addition, we calculated the released content

355

of H2S during the Fenton reaction, which is approximately 0.05 mg/L according to

356

inductively coupled plasma optical emission spectrometry (ICP-OES), indicating

357

negligible secondary pollution.

358

To further investigate the reduction capacity of WS2 in the Fenton reaction, we added

359

WS2 to the heterogeneous Fenton reaction of the Fe3O4/H2O2 system. The W3d XPS

360

spectra, shown in Figure 5d, can be used to detect the variation of states in WS2+Fe3O4.

361

The peaks at 31.9 and 34.1 eV were assigned to W4+, and a distinct characteristic peak

362

of W6+ was visible at 35.2 and 37.5 eV after WS2 was mixed with Fe3O4, indicating the

363

oxidation of W4+ to W6+ by forming a W6+-O-Fe bond. Meanwhile, there is an obvious

364

“blueshift” for the S2p characteristic XPS peak after the WS2 mixed with the Fenton

365

reagent (Figure S5) owing to the formation of sulfur vacancies on the surface of WS2.

366

Although the Raman peak positions of WS2 were almost unchanged after mixing with

367

Fe3O4 (Figure S6), the intensity of the peaks changed significantly: the characteristic

368

peak of the A1g mode decreased obviously after the Fenton reaction, which indicates

369

the generation of sulfur vacancies on the surface of WS2.79, 80 The above results indicate

370

that the release of H2S and the generation of S vacancies are responsible for the

371

production of W6+-O-Fe bonds between WS2 and Fe3O4, which results from the

372

reduction of Fe3+ to Fe2+ by the exposed W4+. Interestingly, bond formation can also be

373

evidenced by the fact that a simple mixture of WS2 and Fe3O4 powders in aqueous

374

solution can be completely separated by applying an external magnetic field (Figure

375

S7). To understand the source of oxygen in the W6+-O-Fe bonds, we mixed WS2 and

376

Fe3O4 powders in the anaerobic solvent of cyclohexane. As shown in Figure S8, the



377

resulting mixture can also be completely separated by magnet, indicating that the

378

oxygen in the W6+-O-Fe bonds originated from Fe3O4 rather than water molecules or

379

dissolved oxygen. To further eliminate the effect of dissolved oxygen on homogeneous

380

Fenton performance, we performed the WS2 cocatalytic Fe2+/H2O2 Fenton experiment

381

in the absence of oxygen by N2-bubbling treatment, as shown in Figure S9. In the

382

absence of dissolved oxygen, WS2 still shows a very high cocatalytic Fenton activity

383

for the degradation of phenol. To demonstrate the reduction of W6+ by H2O2 during the

384

Fenton reaction, we added H2O2 (4 mmol/L) to the mixed WS2 and Fe3O4 solution. The

385

WS2 powders could not be separated by magnet (Figure S10) mainly due to the

386

disruption of W6+-O-Fe bonds by the reduction of W6+ to W4+.

387

Figure S11 illustrates the proposed mechanism of the WS2 cocatalytic effect in the

388

Fe(II)/H2O2 Fenton reaction. The first step is the capture of protons by unsaturated S

389

atoms on the surface of WS2 to form H2S molecules. As H2S is formed, many W4+

390

active sites are exposed on the surface of WS2 and can be easily oxidized by Fe3+ ions

391

to generate W6+. In addition, the oxidation reaction is accompanied by the reduction of

392

Fe3+ to Fe2+, thus greatly improving the reaction rate of the originally rate-limiting step

393

Eq. 2 in the conventional Fe(II)/H2O2 Fenton system. Afterward, the further reduction

394

of W6+ back to W4+ with the help of H2O2 based on the Fenton reaction ensures the

395

cocatalytic cycling of WS2. WS2 greatly enhances the conversion efficiency from Fe3+

396

to Fe2+, which is very helpful to the decomposition of H2O2 and the suppression of iron

397

sludge in the Fenton reaction. As seen from the above cocatalytic mechanism, the

398

addition of WS2 can not only promote the Fe3+/Fe2+ cycle reaction but also cause the

399

Fenton reaction to exhibit oxidation (·OH) and reduction (Fe2+) activity at the same

400

time, which is different from conventional AOPs.81 Therefore, we can boldly predict

401

that the WS2 cocatalytic Fenton reaction could achieve the synchronous reduction of

402

heavy metal ions and remediation of organic pollutants.

403

Simultaneous Oxidation of Phenol and Reduction of Cr(VI). Toxic inorganic

404

pollutants, especially those such as hexavalent chromium (Cr(VI)) that result from the

405

manufacture of paints, ceramics and corrosion inhibitors, are usually associated with


Page 18 of 40

Page 19 of 40


406

toxicity and bioaccumulation, even at a low concentration.82 As a result, Cr(VI) has

407

been listed as one of the priority pollutants regulated by the United States

408

Environmental Protection Agency (USEPA) due to its potential threat to public health.83,

409

84

410

water treatment, even though Cr(VI) ions can be directly reduced by H2O2 to form

411

Cr(III), which also participates in a Fenton-like reaction.81, 85 However, the problem is

412

that Cr(III) is easily precipitated as insoluble chromium hydroxide [Cr(OH)3] in neutral

413

and alkaline conditions (pH > 5), which terminates the decomposition of H2O2,81 and

414

under acidic conditions, free Cr(III) in the form of [Cr(H2O)6]3+ is completely

415

unreactive toward H2O2.86 To maintain the oxidation activity of the Cr(VI)/H2O2

416

Fenton-like reaction, we must keep a high concentration of Cr(VI) in the system, which

417

means the reaction system is still very toxic. There is thereby an urgent need to develop

418

advanced AOPs for the synchronous reduction of Cr6+ and remediation of organic

419

pollutants, but this is still a significant challenge. Here, to highlight the potential of WS2

420

cocatalytic AOPs for industrial environmental applications, we attempted to apply the

421

WS2 cocatalytic Fenton system to the simultaneous oxidation of phenol and reduction

422

of Cr(VI), according to the excellent reduction performance of exposed W4+ in WS2.

The treatment of Cr(VI) has been an ongoing challenge in wastewater and drinking

423 424

Figure 6. Simultaneous (a) reduction of Cr(VI) and (b) degradation of phenol via

425

different Fenton reactions (100 mL solution including 0.04 g/L Fe(SO4)·7H2O, 4.0 g/L

426

WS2, 0.4 mmol/L H2O2, 10 mg/L phenol, and 40 mg/L Cr(VI), pH= 3.8, Vis: under



427

visible light illumination (λ> 420 nm)). (c) Cycle test of the visible-light-driven WS2

428

cocatalytic Fenton reaction for the simultaneous degradation of phenol and reduction

429

of Cr(VI). (d) Reduction of CrO42- ions in the dark (in the absence of phenol and H2O2,

430

100 mL solution including 40 mg/L Cr(VI) and 4.0 g/L WS2, 0.04 g/L Fe2(SO4)3 or 0.04

431

g/L Fe(SO4)·7H2O, pH= 3.8). (e) Reduction of CrO42- and Fe3+ with WS2 in the dark

432

(100 mL solution including 4.0 g/L WS2, 0.04 g/L Fe2(SO4)3 and 40 mg/L Cr(VI), pH=

433

3.8; KSCN and o-phenanthroline were employed as the color-developing agents for the

434

detection of Fe3+ and Fe2+ by UV-visible spectrophotometry, respectively). (f) Fe2+

435

concentration detection: statistics for the absorbance of the color-developing agent (o-

436

phenanthroline) after the complexing of Fe2+.

437

As seen from Figure 6a, when phenol and Cr(VI) ions coexist, the WS2 cocatalytic

438

system exhibits a significant enhancement to the reduction rate of Cr(VI), compared

439

with the conventional Fenton reaction. On the other hand, the removal rate of phenol

440

via the WS2 cocatalytic system also shows an increasing trend compared with that of

441

the Fenton reaction (Figure 6b). We have explored the individual effect of H2O2 towards

442

chromium reduction and phenol oxidation, as shown in Figure S12. The result shows

443

the pure H2O2 can only remove about 12% phenol, however, the Cr6+ ions can be

444

reduced about 85% with 30 minutes, owing to the reduction of H2O2.87 Nevertheless,

445

the WS2+H2O2+Fe2+ Fenton system can achieve 80% reduction rate of Cr6+ just within

446

5 minutes (Figure 6a), which is much higher than that of the individual H2O2 system

447

(30% within 5 minutes). In conclusion, the individual H2O2 cannot possess the ability

448

of simultaneous oxidation and reduction. Most importantly, the WS2 cocatalytic system

449

shows an excellent cycling stability for the remediation of phenol and reduction of

450

Cr(VI), as shown in Figure 6c. In addition, the TOC degradation rate of phenol can

451

reach up to 54% after the 5th cycle test. The TEM and XRD results of WS2 before and

452

after the Fenton reaction indicate the microstructure stability of WS2 during the cycle

453

test (Figure S13). After 5 cycle test, the microstructure of WS2 is almost unchanged

454

(Figure S13a), and the HRTEM images indicate that the WS2 after cycle test maintains

455

the lattice spacing of 0.279 nm (Figure S13b), which corresponds to the (100) plane of


Page 20 of 40

Page 21 of 40


456

WS2.88 The corresponding plane in XRD patterns is shown in Figure S13c. The

457

diffraction peaks located at 14.2o, 28.8o, 33o, 33.7o,39.4o, 44.1o, 49.7o, 58.4o and 60.6o

458

are correspond to (002), (004), (100), (101), (103), (006), (105), (110) and (112) planes,

459

respectively. The data agree well with the standard values of WS2 (JCPDS 08-0237).

460

After 5 cycles test, the planes of WS2 are almost unchanged, indicating the stability of

461

WS2 cocatalytic Fenton system. In the reaction system of coexistent phenol and Cr(VI),

462

the exposed W4+ in WS2 not only can act as the cocatalyst for the Fenton reaction but

463

also plays a key role in the reduction of Cr(VI). We have taken various concentration

464

of phenol and chromium for simultaneous oxidation and reduction. Seen from Figure

465

S14, when the concentration of Cr6+ was confined, the phenol removal rate could be

466

limited slowly with the increasing concentration of phenol. And when phenol was fixed

467

at 5 mg/L, the phenol removal rate can achieve above 99%. When we fixed the

468

concentration of phenol, it was found that the reduction rate of Cr6+ showed a downward

469

trend with the increase concentration of Cr6+ from 40 mg/L to 80 mg/L. In summary,

470

the WS2 co-catalytic Fenton system can indeed realize the synchronous reduction of Cr

471

(VI) and remediation of phenol. To understand the remediation mechanism of Cr(VI)

472

and phenol, the reduction of CrO42- ions in the absence of phenol and H2O2 with

473

different catalysts is shown in Figure 6d. As a result, Fe3+ cannot reduce Cr(VI), but

474

Fe2+ can efficiently reduce Cr(VI). Interestingly, after adding WS2, the Fe3++WS2

475

system shows a significant enhancement in the Cr(VI) reduction rate (98.3%), which is

476

much higher than the sum of the reduction rates of WS2 (53.5%) and Fe3+ (0.4%), owing

477

to the generation of Fe2+. The pure WS2 shows a relatively low activity for the reduction

478

of Cr(VI) because CrO42- is not readily adsorbed on the surface of WS2. In Fig. 6d, it

479

was observed that the WS2+Cr6+ exhibited ～40% efficiency. This is mainly ascribed

480

to the reduction ability of exposed W4+. Compared with CrO42- ions, WS2 can much

481

more easily adsorb Fe3+ ions in an acidic solution (pH ≈ 4.0) owing to the negatively

482

charged surface of WS2.89 We have tested the Zeta potentials of the WS2 aqueous

483

solution under different pH values (pH=2, 3, 5, 7, 9, 11), as shown in Figure S15. As a

484

result, the isoelectric point of WS2 is about 2.29. Hence, we can conclude that the WS2

485

surface is negatively charged at a pH value of 4.0. The adsorbed Fe3+ ions are



Page 22 of 40

486

continuously reduced by the exposed W4+ to form Fe2+. The regenerated Fe2+ ions are

487

redistributed into aqueous solution for H2O2 decomposition due to the excellent water

488

solubility of Fe2+.90 Therefore, the only possibility is that Fe2+ ions or H2O2 molecules

489

directly reduce the Cr(VI) in the WS2 cocatalytic Fenton system. Actually, Fe2+ or H2O2

490

can individually directly reduce Cr(VI), as shown in Figure S16. However, the mixture

491

of Fe2+ and H2O2 (Fenton system) cannot further improve the Cr(VI) reduction rate

492

owing to the occurrence of the reaction between Fe2+ and H2O2. As a result, the excess

493

Fe3+ produced not only inhibits H2O2 decomposition but also limits Cr(VI) reduction.

494

Figure 6e shows the reduction of CrO42- and Fe3+ with WS2 in the dark. As expected,

495

with decreasing Fe3+ and Cr(VI), there is an increasing trend of Fe2+, confirming that

496

the generation of Fe2+ is responsible for the reduction of Cr(VI) in the WS2 cocatalytic

497

Fenton system.

498

In our case, we demonstrated that the addition of WS2 to the Fenton system can

499

greatly improve the reduction of Fe3+ to Fe2+. Hence, we can infer that in the system

500

designed for the synchronous reduction of Cr(VI) and remediation of phenol, Fe2+ is

501

responsible for the reduction of Cr(VI) and the decomposition of H2O2. We measured

502

the Fe2+ concentration in the synchronous system, as shown in Figure 6f. Interestingly,

503

the Fe2++Cr6++phenol and Fe2++H2O2+Cr6++phenol systems display a decreasing trend

504

of

505

Fe2++H2O2+Cr6++WS2+phenol system shows a significant increasing trend in Fe2+

506

concentration, further confirming that the concentration of Fe2+ is the reason for the

507

synchronous reduction of Cr(VI) and remediation of phenol. Additionally, the activity

508

of the reduction of Cr(VI) and remediation of phenol is directly proportional to the mass

509

of WS2 added, as shown in Figure S17. No obvious poisoning effect was observed,

510

which is consistent with the nature of the cocatalytic effect of WS2 in the Fenton

511

reaction. When the amount of WS2 was increased from 400 mg to 500 mg, the

512

degradation rate of phenol was increased from 57.8% to 80.9%. Meanwhile, the

513

reduction rate of Cr(VI) was improved from 74.0% to 90.9% within merely 1 min of

514

irradiation; both of these rates are much higher than that obtained for the conventional

Fe2+

concentration

with

increasing

reaction


time.

Conversely,

the

Page 23 of 40


515

Fenton reaction in the absence of WS2. Furthermore, we performed a careful

516

comparison between past reports and our research on the Fenton reaction for the

517

remediation of pollutants, as shown in Table S5. In all the reported literature, it often

518

took a long time for the complete degradation of phenol (> 5 min), but in our case, the

519

WS2 cocatalytic Fenton system required only 1 min. Importantly, the Fenton systems

520

reported in the literature require a very high level of H2O2 (136~6800 mg/L) and Fe2+

521

(10~500 mg/L), but the WS2 cocatalytic Fenton system involved here only needs 13.6

522

mg/L H2O2 and 1.47 mg/L Fe2+. Moreover, the WS2 cocatalytic Fenton system exhibits

523

remarkable stability for the synchronous reduction of Cr(VI) and remediation of phenol,

524

as shown in Figure 6c. Therefore, compared with the reported Fenton reaction, the WS2

525

cocatalytic Fenton system has an efficient and steady capacity for the remediation of

526

organic/inorganic pollutants.

527

4. ENVIRONMENTAL IMPLICATIONS.

528

Unlike traditional organic chelators such as the Fe(III)-salen complex, PCA, and

529

cysteine, WS2 is an inorganic compound that shows very stable physicochemical

530

properties and is widely found in many tungsten ores. The addition of WS2 could

531

effectively promote H2O2 decomposition in the Fenton reaction and enhance the

532

degradation of phenol because it can expose active W4+ to realize the effective

533

Fe(III)/Fe(II) cycle and prevent the precipitation of iron sludge. More importantly, WS2

534

cocatalytic AOPs can achieve the simultaneous mineralization of phenol molecules and

535

reduction of toxic Cr(VI), which can reduce the environmental risk caused by

536

multicomponent wastewater. In addition to the Fe(II)/H2O2 Fenton system, the WS2

537

cocatalytic effect in the remediation of phenol is also suitable for other Fenton-like

538

systems, such as Fe(III)/H2O2 and Ni(II)/H2O2 (Figure S18). Thus, we expect that this

539

work could lead to the development of an attractive and reliable platform for promoting

540

the conversion and mineralization of organic-inorganic contaminants in natural aquatic

541

environments via the WS2 cocatalytic Fenton reaction.

542

ASSOCIATED CONTENT



543

Supporting Information. The Supporting Information is available free of charge on the ACS

544

Publications website at DOI: XXXXX.

545

Detailed reaction kinetics data; Mass spectra of intermediates; H2O2 total

546

decomposition by heating treatment; EPR, TEM and XRD spectra of WS2; XPS spectra

547

of S2p; Digital image of magnetic separation; Simultaneous degradation of phenol and

548

reduction of Cr(VI) with different amounts of WS2; WS2 cocatalytic Fenton-like

549

reactions.

550

AUTHOR INFORMATION

551

Corresponding Author

552

*Email: [email protected]; [email protected]

553

Author Contributions

554

The manuscript was written with contributions from all authors. All authors have

555

approved the final version of the manuscript.

556

Notes

557

The authors declare no competing financial interest.

558

ACKNOWLEDGMENT

559

This work was supported by the State Key Research Development Program of China

560

(2016YFA0204200), the National Natural Science Foundation of China (21822603,

561

21773062, 21577036, 21377038, 5171101651), the Shanghai Education Development

562

Foundation and Shanghai Municipal Education Commission (16JC1401400,

563

17520711500), the Shanghai Pujiang Program (17PJD011), and the Fundamental

564

Research Funds for the Central Universities (22A201514021, 22221818014).

565

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