Homogeneous catalytic activation of peroxymonosulfate and heterogeneous reductive regeneration of Co2+ by MoS2: The pivotal role of pH

Science of the Total Environment 712 (2020) 136447

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Homogeneous catalytic activation of peroxymonosulfate and heterogeneous reductive regeneration of Co2+ by MoS2: The pivotal role of pH Cong Pan, Libin Fu, Yaobin Ding ⁎, Xueqin Peng, Qihang Mao College of Resources and Environmental Science, South-Central University for Nationalities, Wuhan 430074, PR China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• MoS2 enhanced catalytic performance of Co(II)/PMS system. • The enhancement effect of MoS2 was dependent on reaction pH and MoS2 dosage. • Co redox cycle in Co(II)/PMS system was enhanced by MoS2. • MoS2 showed excellent chemical, structure and catalytic stability. • Co ions can be recycled via pH regulation to 10 in the MoS2-Co(II)/PMS system.

a r t i c l e

i n f o

Article history: Received 5 November 2019 Received in revised form 20 December 2019 Accepted 30 December 2019 Available online xxxx Editor: Yifeng Zhang Keywords: Advanced oxidation processes Peroxymonosulfate Co2+ MoS2 pH dependent

a b s t r a c t The application of MoS2 to enhance Co(II)/peroxymonosulfate (Co(II)/PMS) system for organic pollutants degradation was developed, and the mechanism for pH dependent catalytic activity in the MoS2 co-catalyzed Co(II)/ PMS processes was systematically investigated. It was found that MoS2 presented enhancement effect for Co (II)/PMS system in the tested pH range from 4.0 to 7.0, especially at pH 5.5 and 6.0. The pseudo first order reaction rates for Rhodamine B (RhB) degradation in MoS2-Co2+/PMS system at pH 5.5 and 6.0 were 3.2 and 1.8 times that in Co2+/PMS system (Co2+ 2 μmol L−1, PMS 0.2 mmol L−1, MoS2 0.5 g L−1). The redox recycle of Co3+/Co2+ was promoted by Mo(IV) and S(-II) on MoS2 surface and regenerated Co2+ induced homogeneous activation of PMS for the robust production of free radical with the major of hydroxyl radicals. Increasing MoS2 dosage, Co2+ and PMS concentration can linearly raise RhB degradation rate in MoS2-Co(II)/PMS system. Moreover, MoS2 exhibited excellent catalytic and chemical stability in recyclability and reuse for catalytic decontamination in MoS2Co(II)/PMS system. This work gains new insight into the enhancement effect of MoS2 in the meal ions/PMS system, and provides a high performance wastewater treatment process of Co(II)/PMS at low concentrated Co2+. © 2020 Published by Elsevier B.V.

1. Introduction

⁎ Corresponding author. E-mail address: [email protected] (Y. Ding).

https://doi.org/10.1016/j.scitotenv.2019.136447 0048-9697/© 2020 Published by Elsevier B.V.

Sulfate radicals based advanced oxidation processes (SRs-AOPs) have attracted much attention due to its diverse source (persulfate, sulfite and sulfate) via different activation methods (UV, heat, metal and metal oxides, and electrochemical methods), high oxidation potential

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(E = 2.6–3.2 V), and wide applicability in the complex water matrixes (Yang et al., 2019; Farhat et al., 2015; Matzek and Carter, 2016; Zhou et al., 2018). Co(II)/peroxymonosulfate (Co(II)/PMS) system was regarded as the most efficient one for the production of SRs and able to decompose various refractory organics (Anipsitakis and Dionysiou, 2003; Anipsitakis et al., 2006; Chen et al., 2007; Ji et al., 2015). In the consideration of the toxicity of Co ions to induce interstitial lung disease (Lison, 1996) and cardiomyopathy (Leyssens et al., 2017), the concentration of Co should be strictly controlled as less as possible. Permissible limit of cobalt in drinking water is recommended as 50 μg L−1 by United States Environmental Protection Agency (USEPA) (USEPA, 2015). In the reported Co(II)/PMS systems, the concentration of Co2+ mainly ranged in several hundred μmol L−1 with the maximum dose of 1244 μmol L−1 for 2,4-DCP transformation in the neutral aqueous solution (Anipsitakis and Dionysiou, 2003; Anipsitakis et al., 2006). The dosage of so highly concentrated Co2+ is of a great environmental risk and not feasible for practical application of Co(II)/PMS system for water and wastewater treatment. To solve the problem, two ways was developed: one is the utilization of Co-based heterogeneous catalysts, and the others is the use of low-concentrated Co2+ via enhancement of oxidation ability of Co(II)/PMS system. As for the first way, tremendous studies were carried out, in which Co3O4, CoFe2O4, Co3O4 based composites and supported Co3O4 catalysts exhibited good catalytic activity for PMS (Chen et al., 2007; M. Chen et al., 2019; L. Chen et al., 2019; J. Deng et al., 2019, Y. Ding et al., 2019, Ding et al., 2012, J. Li et al., 2018, Liang et al., 2012, Song et al., 2019, Zhang et al., 2018). In some cases, the concentration of leached Co2+ can be controlled lower than 10 μmol L−1 through enhancing the interaction of Co catalysts and supports such as nitrogendoped graphene aerogel (Yuan et al., 2018), MgO (Zhang et al., 2010) and NaBiO3 (Y. Ding et al., 2019). However, as compared with homogeneous Co catalysis, such heterogeneous catalysts suffer from the low atomic utilization of Co2+ because of only a small proportion of exposed cobalt as reactive sites on the surface. Therefore, based on the consideration above, homogeneous Co catalysis is still regarded as a feasible oxidation process but at low dose of Co2+. The key point to realize the goal is to improve the catalytic activity of Co(II)/PMS system. As described by Eqs. (1)–(4), the rate-limiting step in the Co(II)/PMS system is the reductive cycle of Co3+ to Co2+ especially at a low concentration of Co2 + , which was evidenced by the observation of the formation and accumulation of Co3+ intermediates during the reaction process by using EDTA as a probe (Y. Chen et al., 2019; M. Chen et al., 2019; L. Chen et al., 2019). Therefore, it is significant to develop efficient methods to rapid Co3+/Co2+ cycle for the application of homogenous Co(II)/PMS with low concentrated Co2+ in decontamination. Co2þ þ H2 O→CoOHþ þ Hþ

ð1Þ

CoOHþ þ HSO5 − →CoOþ þ SO4 ˙− þ H2 O

ð2Þ

CoOþ þ 2Hþ →Co3þ þ H2 O

ð3Þ

Co3þ þ HSO5 − →Co2þ þ SO5 ˙− þ Hþ

ð4Þ

Molybdenum sulfide (MoS2) was previously mainly used in photo/ electro-catalysis for H2 generation (Z. Li et al., 2018), energy storage (Theerthagiri et al., 2017) and sensors (Barua et al., 2017), and recently reported as co-catalyst in Fe(II)/H2O2 and Fe(II)/PMS processes (Sheng et al., 2019; Xing et al., 2018; Zhang et al., 2020). Although rapid reduction of Fe3+ to Fe2+ was realized in these processes by MoS2, to the best of our knowledge, its application in enhancing Co(II)/PMS system has rarely been investigated so far. Moreover, pH is an important parameter in the practical application of Co(II)/PMS system for wastewater treatment. Although the influence of pH on the catalytic activity was studied in the MoS2 co-catalyzed Fe(II)/PMS processes (Zhang et al., 2020), and it was reported that decreased acetaminophen degradation was observed with increasing solution pH in the range of 3.0–11.0, the mechanism for pH dependent catalytic activity in the MoS2 co-catalyzed Fe(II)/ PMS processes was not investigated systematically. Therefore, in the study, the role of MoS2 in the Co(II)/PMS system was checked, and the influence of reaction pH was assessed on the catalytic activity of MoS2-Co(II)/PMS system by using RhB as model organic pollutant. The reaction mechanism for enhancement effect of MoS2 in the MoS2-Co (II)/PMS system was proposed based on the results of radicals identification, Co distribution as a function of pH, Co3+ detection and X-ray photoelectron spectra (XPS)/Raman characterization of the used MoS2. This work can gain new insight into the enhancement effect of MoS2 in the Co(II)/PMS system, also provide a high-performance wastewater treatment process of Co(II)/PMS at low concentrated Co2+. 2. Experimental 2.1. Materials and characterization Bisphenol A (BPA), 4-chlorophenol (4-CP), rhodamine B (RhB), methylene blue (MB), Orange II, cobalt nitrate (Co(NO3)2), methanol (MeOH), tert-butanol (TBA), molybdenum(V) chloride (MoCl5), sodium molybdate dehydrate (Na2MoO4·2H2O), ethylenediaminetetraacetic acid (EDTA), 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO), tetramethylpiperidine (TEMP), molybdenum dioxide (MoO2), NaCl, NaHCO3, humic acid (HA), NaOH and HNO3 were provided by Sinopharm Chemical Reagent Co., Ltd. Oxone (PMS, 4.7% active oxygen) was purchased from Shanghai D&R Finechem Co., Ltd. Ultra-pure water was used in the present work. Tapping water was obtained from a water tap in our laboratory, and lake water was collected from Nan Lake, to the south of our campus. Prior to use, the collected water was filtered with 0.22 μm filter to remove the suspension. Molybdenum sulfide (MoS2, 99.5% metals basis, size was lower than 2 μm) was obtained from Shanghai Aladdin Bio-Chem Technology Co., Ltd. The fresh and used MoS2 was characterized by scanning electron microscope (SEM, FEI Quanta 200, the Netherlands), transmission electron microscopy (TEM, FEI Tecnai TF20), X-ray powder diffraction (XRD) diffractometer (PANalytical B.V. X'Pert PRO), Raman spectra (DXR Raman microscope, Thermo Scientific), X-ray photoelectron spectra (XPS) on a spectrometer (VG Multilab 2000, Thermo Electron Corporation). 2.2. Experiment setup

There are few literatures on developing methods to enhance the performance of Co(II)/PMS system, and the reported efficient methods include heat-assistance (Liu and Wang, 2018), small organic acids complexing (Y. Chen et al., 2019; M. Chen et al., 2019; L. Chen et al., 2019) and photosensitization of AO7 (Chen et al., 2007). Heat assistance is only applicable in sites where heat sources are easily accessed; otherwise the treatment cost will be too high. Small organic acids complexing and photosensitization of AO7 would induce the potential secondary pollution, consume oxidants and need continuing addition due to their decomposition (The advantages and disadvantages of these methods were summarized in Table S1). Therefore, efficient methods by using recycled materials have been needed to improve the catalytic activity of Co(II)/PMS system.

The degradation of organic pollutants such as BPA, 4-CP, RhB, MB and Orange II was used to assess the oxidation ability of Co/PMS system enhanced by MoS2. Typically, 25 mg MoS2 particles were added into 50 mL aqueous solution containing 2 μmol L−1 Co2+. The mixture solution was treated by ultrasonic for 10 min and stirred for another 10 min to facilitate the contact of Co2+ and MoS2 particles. After that, a specific amount of RhB stock solution was added to obtain the concentration of RhB at 30 μmol L−1. After magnetic stirring for 30 min, the adsorptiondesorption equilibrium of RhB on MoS2 particles was reached. Then, 0.2 mmol L−1 PMS was added to initiate the catalytic degradation of organic pollutants. The pH value of the reaction solution was quickly adjusted and controlled during the reaction with 0.1 mol L−1 NaOH and

C. Pan et al. / Science of the Total Environment 712 (2020) 136447

HNO3 solution. During the reaction, 1.0 mL solution was spiked and analyzed by using a UV–visible spectrophotometer or a high performance liquid chromatography (HPLC) after removal of MoS2 particles via centrifugation at 14,000 rpm for 3 min. All measurements were repeated three times at lest and the results were reproducible within the experiments errors. Electron spin resonance (ESR) was used to identify the formation of reactive species. Specifically, 0.5 mL of the reaction solution was sampled from the reaction solution containing 0.1 mol L−1 DMPO or TEMP in the systems of Co2+/PMS and MoS2-Co2+/PMS. The ESR spectra were immediately measured on ESR spectrometer (Bruker ESR EMX nano) with microwave bridge.

presented catalytic performance for PMS activation (Y. Chen et al., 2019; M. Chen et al., 2019; L. Chen et al., 2019; Zhou et al., 2019). However, we found that lower than 15% RhB degradation was achieved in 10 min by MoS2/PMS system in the pH range of 4 to 8 (Fig. S2). The low RhB degradation MoS2/PMS system was possibly due to the lower PMS concentration (0.2 mmol L−1) than that (0.5 and 0.6 mmol L−1) used in above two literatures (Y. Chen et al., 2019; M. Chen et al., 2019; L. Chen et al., 2019; Zhou et al., 2019). Co2+/PMS system is a highly efficient process for the decomposition of various organic pollutants (Anipsitakis and Dionysiou, 2003; Anipsitakis et al., 2006). However, in consideration of toxicity of Co2+, an ultra-low Co concentration (2 μmol L−1) was used in the present work. As one can see, about 55% and 87% removal of RhB was obtained in 5 and 10 min by Co2+/PMS system. After adding MoS2 as co-catalyst (0.5 g L−1), RhB was completely degraded in 5 min in MoS2-Co2+/PMS system, indicating that the use of MoS2 can prompt the oxidation capability of Co2+/ PMS system. Moreover, RhB degradation in MoS2-Co2+/PMS system at pH 5.5 was rapider than that of sum of MoS2 adsorption and degradation of Co2+/PMS system. The results confirmed the synergistic effect of Co2+ and MoS2 for PMS activation and RhB degradation. To better reveal the enhancement effect of MoS2 as co-catalyst in the Co2+/PMS system, the apparent rate constant (kobs) for RhB degradation was obtained by fitting RhB degradation profile with pseudo first order reaction kinetic (ln(c/c0) = −k obst). As seen in Fig. S3, the pseudo first order reaction rate kobs for RhB degradation was calculated as 0.68 min−1 in MoS2-Co2+/PMS system, 3.2 times that by Co2+/PMS system. The enhancement effect of MoS2 as co-catalyst in the Co2+/PMS system was further confirmed by the higher TOC removal in MoS2-Co2 + /PMS system than that in Co2+/PMS system. As displayed in Fig. S4A, TOC removal after degradation for 120 min in MoS2-Co2+/PMS system was 72%, much higher than that (23.6%) in Co2+/PMS system, and also higher than that the sum of MoS2 adsorption and degradation of Co2 + /PMS system (55% in 120 min). The results above further confirm MoS2 can enhance the catalytic activity of Co2+ for PMS activation and RhB mineralization. The use of MoS2 also improved the utilization efficiency of PMS in Co2+/PMS system. Specific oxidant efficiency (SOE) based on TOC removal was defined in the study as follows (Jaafarzadeh et al., 2017).

2.3. Chemical analysis The concentrations of RhB, MB and orange II were analyzed by Evolution 201 UV–visible spectrometer (Thermo Scientific). The concentration of BPA and 4-CP was measured with a HPLC system (ultimate 3000) with a G1315D 12600 DAD detector with Agilent C18\\P column (5 μm, 4.6 × 150 mm) as the separation column. The used mobile phase was a mixture of methanol and water (55:45, v/v). The flow rate was 1 mL min−1. The injection volume was 50 μL. The detection wavelength used for analysis of BPA and 4-CP was 220 nm and 230 nm. Total organic carbon (TOC) was analyzed by using a TOC analyzer (Vario TOC select, Elementar, Germany). Co ions were analyzed by Agilent 7700 ICP-MS. The electrochemical current response was performed on a CHI660 electrochemical workstation (Chenghua, Shanghai, China) at the potential (0.2 V) in 0.5 mol L−1 Na2SO4 electrolyte. A three-electrode cell was used, comprised of the MoS2 coated conductive glassy carbon electrode as a working electrode, a Pt electrode as auxiliary electrode, and a standard calomel electrode as reference electrode. The working electrodes were prepared via the drop-coating of 200 μL Nafion solution containing MoS2 (5 g L−1) on glassy carbon electrode and then natural drying under dark conditions. 3. Results and discussion 3.1. Enhancement of PMS activation by Co2+ with MoS2 Fig. 1A shows the comparison of degradation of RhB in the different systems including two systems of Co2+-PMS and MoS2-Co2+/PMS at pH 5.5. The single use of PMS could not effectively decompose RhB at the tested pH values of 4, 5.5 and 7 although it has oxidation ability (Fig. S1). The reason is possibly due to the low concentration of added PMS. RhB adsorption was rapid by MoS2, and about 41% RhB can be removed in 10 min by adsorption effect. It was reported that MoS2

A

TOC 0 −TOC t ½oxidant 0 −½oxidant t

ð5Þ

where TOC0 and TOCt are the Total Organic Carbon (TOC, mmol L−1) at t = 0 and t = t min, respectively. Accordingly, [oxidation]0 and [oxidation]t are the concentration of oxidant PMS (mmol L−1) at t = 0 and t = t min, respectively. Fig. S4B shows the concentration profile of PMS

Degradation

4

2+

MoS2-Co /PMS

B

PMS

2+

Co /PMS

0.6

3 R

MoS2

0.6

kobs / min

-1

0.8

c/c0

SOE ¼

0.8 Adsorption

1.0

0.4

3

0.4

2

Co2+/PMS

0.2

1

MoS2-Co2+/PMS

0.0

MoS2/PMS

MoS2adsorption 2+ +Co /PMS

0.2 0.0 -10

-5

0

Time/min

5

10

0 4

5

6 pH

7

8

Fig. 1. (A) Adsorption-degradation of RhB in the different systems. (B) Degradation kinetic rates of RhB in the systems of Co2+/PMS and MoS2-Co2+/PMS at different pH values. Reaction condition: Co2+ 2 μmol L−1; PMS 0.2 mmol L−1; MoS2 0.5 g L−1; pH 5.5. MoS2 adsorption+Co2+/PMS in Fig. 1A means RhB degradation by the sum of MoS2 adsorption and Co2+/PMS system. R in Fig. 1B was the ratio of kobs values in MoS2-Co2+/PMS system and Co2+/PMS system.

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in the systems of MoS2-Co2+/PMS and Co2+/PMS. PMS concentration was decreased by 78.5% from 0.2 mmol L−1 to 0.043 mmol L−1 in 120 min in MoS2-Co2+/PMS system, a little higher than that (67.5% decline of PMS concentration) in Co2+/PMS system. Based on the Eq. (5), SOE was calculated. As seen in Fig. S4C, the SOE value in MoS2-Co2+/PMS system was higher than that in Co2+/PMS system. The results indicate that the utilization efficiency of PMS for RhB mineralization in MoS2-Co2+/PMS system was higher than in Co2+/PMS system. 3.2. The pivotal effect of pH Fig. 1B depicts the effect of reaction pH on RhB degradation in Co2+/ PMS and MoS2-Co2+/PMS systems. Co2+/PMS system presented good catalytic activity for RhB degradation in the pH range from 4 to 7. The result was consistent with the previous report on Co2+/PMS system for degradation of phenolic and dyes (Anipsitakis et al., 2006; Huang and Huang, 2009; Huang et al., 2009). The best performance was observed at pH 5.5 in Co2+/PMS system for RhB degradation. According to Eq. (2), CoOH+ rather than Co2+ was regained as the real catalyst for the decomposition of PMS to free radicals. To facilitate the formation of CoOH+, weakly acidic condition was required (Huang et al., 2009). Under strongly acidic conditions, Co exists mainly in the form of Co2+, while Co precipitates in the state of Co(OH)2 when pH is too high. The addition of MoS2 as co-catalyst only presented significant enhancement effect in the narrow pH range from 5.5 to 6.0, rather than at all the pH values in the wide range from 4 to 7. The kobs values for RhB degradation in MoS2-Co2+/PMS system at pH 5.5 and 6.0 were 3.2 and 1.8 times that in Co2+/PMS system. We infer the kobs-pH effect was strongly related to the interaction of Co and MoS2 surface, which affected the activation of PMS as discussed later. 3.3. The effect of MoS2 load, Co2+ and PMS concentration The effect of MoS2 load on RhB degradation in MoS2-Co2+/PMS system was also investigated at pH 5.5, and the results were shown in Fig. S5A. As MoS2 load climbed from 0 g L−1 to 0.5 and 1.0 g L−1, kobs value for RhB degradation in MoS2-Co2+/PMS system was linearly raised from 0.21 min−1 to 0.68 and 0.98 min−1 by 3.2 and 4.7 times, respectively. The result fully indicates that the enhancement effect of MoS2 for RhB degradation in MoS2-Co2+/PMS system was load-dependent. Further, the dependence of RhB degradation on concentration of Co2+ and PMS in MoS2-Co2+/PMS system was also checked. Similarly, a linear dependence of k value for RhB degradation on concentration of PMS and Co2+ was observed as shown in Figs. S5C and S5E. Based on these results, the RhB degradation kinetics can be described by the pseudo first order reaction kinetic equation of r = k[MoS2]0.52[Co2+]0.98[PMS]0.92. 3.4. Important role of Co2+-MoS2 interaction As depicted in Fig. 2A, changing the addition order of the reaction agents obviously affected the degradation of RhB in MoS2-Co2+/PMS system. In the set of experiment, pre-adsorption of RhB on MoS2 before Co2+ was conducted instead of pre-adsorption of Co2+ on MoS2 before RhB. The pre-adsorption of RhB on MoS2 before Co2+ did not enhance the degradation of RhB as observed significantly in the heterogeneous catalysis, instead inhibited RhB degradation as compared with the result as seen in Fig. 1. The kobs value for RhB degradation declined from 0.68 min−1 to 0.28 min−1 after exchange the addition order of RhB and Co2+. Pre-added RhB can be adsorbed on MoS2, decreasing the contact of Co2+ with the reactive site on surface of MoS2. Consequently, the generated Co3+ from the reaction of Co2+ and PMS was not highly efficiently reduced to Co2+. Unavoidably, the recycle of Co3+/Co2+ and radicals formation was depressed. The results indicate the great important role of Co2+-MoS2 interaction in enhancing Co catalytic cycle for promoted PMS enhancement and RhB degradation.

To further confirm the inference, the distribution and the reductive regeneration of Co ions were analyzed. Fig. S6 shows the concentration change of Co ions in the reaction solution in the MoS2-Co2+/PMS system as a function of reaction pH. The distribution of Co ions between the solution and MoS2 surface can equilibrium rapidly in 5 min. Accordingly, the distribution of Co ions in 5 min in the MoS2-Co2+/PMS system as a function of reaction pH was calculated as seen in Fig. 2B. With the increase of reaction pH, less Co ions were dissolved in the reaction solution. At pH 5.5, about 38.3% of added Co (2 μmol L−1) was detected in the reaction solution, while the others adsorbed on MoS2 surface. In comparison, 97.2% of Co existed in the reaction solution at pH 4, while only 6.7% of Co was dissolved in the reaction solution at pH 7. Solubility product constant (Ksp) of Co(OH)2 is 2 × 10−15, 29 orders of magnitude larger than that of Co(OH)3 (Ksp = 2 × 10−44). Therefore, the highest concentration of Co2+ in the aqueous solution is calculated as 2 × 105, 2 × 102 and 0.2 mol L−1 at 4, 5.5 and 7, respectively. In comparison, the highest concentration of Co3+ in the aqueous solution is calculated as 2 × 10−14, 2 × 10–18.5 and 2 × 10−23 mol L−1 at 4, 5.5 and 7, respectively. It means that Co3+ only precipitated on MoS2 surface if there were Co3+ generated during the reaction process in MoS2-Co2+/PMS system, and Co2+ most dissolved in the aqueous solution. So it can be inferred that 97.2% of Co existed as Co2+ in the reaction solution at pH 4, while only 6.7% of Co was dissolved as Co2+ in the reaction solution at pH 7. At pH 5.5, about 38.3% of added Co ions (2 μmol L−1) were in the state of Co2+ in the reaction solution. Fig. 2C gives the change of Co3+ amount in the reaction solution as a function of MoS2 dose with EDTA as a probe of Co3+ at pH 5.5. The complex of Co3+-EDTA showed a broad absorption peak at 535 nm (Y. Chen et al., 2019; M. Chen et al., 2019; L. Chen et al., 2019). The addition of MoS2 decreased the peak intensity of Co3+-EDTA complex. Raising dosage of MoS2 induced the lower absorption of Co3+-EDTA complex at 535 nm, suggesting that in the presence of MoS2, the concentration of Co3+ was reduced due to the reduction of Co3+ by MoS2. Meanwhile, the effect of pH on Co2+ reduction was also checked at pH 4.0, 5.5 and 7.0. As seen in Fig. S7, the absorption of Co3+-EDTA complex at 5.5 was lower than that at pH 4.0 and 7.0 in Co2+/PMS system, suggesting that the concentration of Co3+ at 5.5 was less than that at pH 4.0 and 7.0 in Co2+/PMS system. In other word, the reduction of Co3+ to Co2+ was easier at 5.5 than that at pH 4.0 and 7.0. The result maybe explain why Co2+ presented much higher catalytic performance for PMS activation and RhB degradation at pH 5.5 than at pH 4 and 7 in Co2+/PMS system. After the addition of MoS2, the absorption of Co3+-EDTA complex at 535 nm declined by 36% and 33% at pH 4 and 7. In comparison, the absorption of Co3+-EDTA complex at 535 nm was decreased by 57% at pH 5.5. The results indicate that the reduction of Co3+ to Co2+ in MoS2-Co2+/PMS system was also pH dependent, and it is more facile at 5.5 than that at pH 4.0 and 7.0. The interaction of Co2+, PMS and MoS2 was further studied by electrochemical test with MoS2 as a working electrode. As one can see in Fig. 2D, a visible positive current signal was observed corresponding to the addition of PMS (in 30 s), suggesting that MoS2 was oxidized by PMS. After that (in 60 s), Co2+ was subsequently spiked. However, the current was not increased. In comparison, no current was generated after Co2+ was singly added, suggesting that there are no chemical reaction between Co2+ and MoS2. The subsequent spiking of PMS in 60 s induced the occurrence of an obvious current signal, and current intensity was higher that with the signal addition of PMS, indicating that the formed Co3+ generated from the reaction between Co2+ and PMS was reduced by MoS2. These results indicate that the addition of MoS2 can promote the reductive regeneration of Co2+. 3.5. Radical identification and reaction mechanism Radical and non-radical mechanism was reported on PMS activation by various catalysts (Ding et al., 2013; Duan et al., 2015; Zhang et al., 2013). In the proposed radical mechanism, SO4•− or •OH were

C. Pan et al. / Science of the Total Environment 712 (2020) 136447

1.0

A

100

5

In solution

B

On surface of MoS2

0.8

Co / %

c/c0

80

0.6 -1

changed addition order k=0.28 min

0.4

60 40

0.2 20

2+ -1 MoS2-Co /PMS k=0.68 min

0.0 0

2

4

6

0

8

4

Time / min 0.06 Without MoS2

C

-1

30 g L MoS2

0.02

0.00 450

500

550

600

650

Wavelength/ nm

Current / nA

decreased by 57%

Abs

0.04

-1

10 g L MoS2 -1 20 g L MoS2

500 400 300 200 100 0 -100

5

pH

6

7

Co2+

D PMS

317

500 400 300 200 100 0 -100

Co2+

0

20

PMS

429 40

60

80

100

120

Time / s

Fig. 2. (A) Effect of the addition order of Co2+ and RhB on RhB degradation in the MoS2-Co2+/PMS system. (B) Distribution of Co in the MoS2-Co2+/PMS system in 5 min. (C) The absorption spectrum of Co3+-EDTA in the MoS2-Co2+/PMS system with the addition of MoS2 at various load. (D) Current response of MoS2 electrode with the addition of PMS, Co2+ and Co2+-PMS.

recognized as the main free radicals (Ding et al., 2013; Zhang et al., 2013), while singlet oxygen was identified involved in non-radical mechanism (Duan et al., 2015, Nie et al., 2017, Luo et al., 2019). In the group of experiments, quenching and competition experiments were used firstly to identify the involved the main reactive species in the MoS2-Co2+/PMS system. As seen in Fig. 3A, the use of 0.2 mol L−1 tert-butanol (TBA) as a quench agent of •OH shows little effect on RhB degradation in the MoS2-Co2+/PMS system. When TBA concertation was increased to 1 mol L−1, RhB degradation rate was decreased by 63% to 0.25 min−1. In comparison, the addition of methanol (MeOH) having the ability to react with SO4•− and •OH at a comparable rate remarkably presented stronger inhibition effect. In the presence of 0.2 and 1 mol L−1 MeOH, degradation rate of RhB declined to 0.11 and 0.009 min−1. The degradation of RhB was nearly completely depressed by the addition of 1 mol L−1 MeOH. The results hint that both SO4•− and •OH made the main contribution to RhB removal, and other reactive species had little contribution to RhB removal. Moreover, the comparison of degradation rate of phenol and nitrobenzene in the MoS2-Co2+/ PMS system was given in Fig. 3B. Compared with •OH having the comparable reaction rate with phenol and nitrobenzene (k•OH–phenol = 2.8 × 109 M−1 s−1, k•OH–nitrobenzene = (3.0–3.9 × 109 M−1 s−1), the reaction rates of SO4•− with phenol and NB are much lower (kSO•− = 4 −phenol 6 × 107 M−1 s−1, kSO•− b 106 M−1 s−1) (Liang and Su, 2009). There4 −NB fore, slower degradation of NB was observed as compared with that of phenol in the Co2+/PMS and MoS2-Co2+/PMS systems. The results above suggest that both SO4•− and •OH made the main contribution to RhB removal. Further, ESR was used to confirm the generation of free radicals by using DMPO as a probe. Unexpectedly, the strong ESR signal of DMPO-

OH adducts (aN = 14.9 and aH = 14.9) was detected in MoS2-Co2+/ PMS system, with no observation of visible DMPO-SO4 adducts. Moreover, the formation of singlet oxygen was also checked by using TEMP as a probe. As seen in Fig. S8, no signals assigned to TEMP-1O2 adducts were detected. The results confirm the generation of SO4•− as predominant reactive species. No observation of ESR signal assigned to DMPOSO4 adducts was attributed to the rapid conversion of SO4•− to •OH under the reaction conditions (Fig. S9), although SO4•− has been confirmed as the major oxidant for degradation of RhB in MoS2-Co2+/ PMS system. The similar result was also reported in the several literatures about PMS activation (Qi et al., 2017; Zou et al., 2013). The contribution of SO4•− and •OH to RhB degradation was estimated by using RhB degradation rate in the presence of TBA and MeOH. In the presence of 1 mol L−1 TBA, degradation of RhB can be considered as the contribution of SO4•−, because the concentration of TBA used was high as 3.3 × 104 times that of RhB. Therefore, the contribution rate of SO4•− to RhB degradation can be estimated as 0.25/0.68 = 37%. Accordingly, the contribution rate of •OH to RhB degradation was calculated as 63%. As seen in Fig. 3C, the ESR signal intensity of DMPO-OH adducts in different systems declined in the order of MoS2-Co2+/PMS system N Co2+/PMS system N MoS2/PMS system, which is consistent with the order of RhB degradation in these systems in Fig. 1A. The results further support that RhB degradation was a free radical-dominant reaction process in the MoS2-Co2+/PMS system. The effect of reaction pH on the radical generation was also investigated by ESR. Fig. 3D clearly shows the ESR peak intensity at pH 5.5 in the MoS2-Co2+/PMS system was much higher than that at 4.0 and 7.0, being consistent with the pH dependent degradation rate (kobs) of RhB in the MoS2-Co2+/PMS system as displayed in Fig. 1B, and further

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C. Pan et al. / Science of the Total Environment 712 (2020) 136447

A

1.0

B

1.0

-1

0.8

-1

c/c0

0.2 mol L MeOH

0.6

c/c0

1 mol L MeOH

0.8

0.6

-1

1 mol L TBA

0.4

0.4

-1

0.2 mol L TBA

without MoS2

NB

adding MoS2

Control

0.2

0.2

without MoS2

phenol

adding MoS2

0.0

0.0 0

2

4

6

8

0

10

5

10

Time / min

C

15

20

25

D

2+

2+

MoS2-PMS

pH 5.5 Intensity / a.u.

Intensity / a.u.

MoS2-Co -PMS

Co -PMS

pH 4.0

pH 7.0

PMS

3360

3390

3420

3450

3480

30

Time / min

3510

3540

3400

3450

3500

3550

Magnetic field (G)

Magnetic field (G)

Fig. 3. Radicals identification: (A) Effect of MeOH and TBA addition on degradation of RhB in the MoS2-Co2+/PMS system. (B) Degradation of 0.1 mmol L−1 phenol and nitrobenzene in the MoS2-Co2+/PMS and Co2+/PMS systems. (C) ESR spectrum of the reaction solution with DMPO as a probe in the MoS2-Co2+/PMS, Co2+/PMS and MoS2/PMS systems at pH 5.5. (D) ESR spectrum of the reaction solution with DMPO as a probe in the MoS2-Co2+/PMS at different pH values.

Further, used MoS2 was recycled after RhB was completely degraded in 5 min, and investigated by Raman and XPS. As depicted in Fig. 4, the fresh MoS2 exhibited two characteristic Raman peaks at 372 and 398 cm−1, corresponding to E12g and A1g modes of the Mo\\S bonds, respectively (Li et al., 2012). After reaction in the system of MoS2-Co2+/ PMS, these two characteristic Raman peaks were still observed,

398 333 372 Intensity / a.u.

confirming the pivotal role of reaction pH in tuning catalytic oxidation activity of the MoS2-Co2+/PMS system. To reveal the reaction mechanism of MoS2 enhanced Co2+/PMS system, several control experiments were conducted. Firstly, whether Mo ions can enhance the performance of Co2+/PMS system was checked by using Mo5+ and MoO2− 4 due to unavailability of commercial product Mo4+ ions. The molar concentration of Mo5+ and MoO2− used was 4 equal to that of MoS2 in the MoS2-Co2+/PMS system (3.1 mmol L−1). As seen in Fig. S10, little RhB degradation was observed in the systems 5+ of Mo5+/PMS and MoO2− and MoO2− have 4 /PMS, suggesting Mo 4 low catalytic reactivity for PMS activation and free radicals production. Although RhB was readily degraded in the systems of Mo5+-Co2+/ 2+ PMS and MoO2− /PMS, the degradation efficiency and reaction 4 -Co rate for RhB degradation in the two systems were lower than that in Co2+/PMS and MoS2-Co2+/PMS systems. The results indicate that neither Mo5+ nor MoO24 promotes the reactivity of Co2+ with PMS, and meanwhile hint that dissolved Mo ions have low reactivity for PMS activation and make little contribution to the enhanced RhB degradation in MoS2-Co2+/PMS system. Secondly, the comparison of MoS2 and MoO2 used for enhancing Co2+/PMS system was conducted. As displayed in Fig. S11, RhB degradation rate in MoO2-Co2+/PMS system was 0.29 min−1, a little higher than that (0.21 min−1) in Co2+/PMS system, suggesting that MoO2 can enhance the catalytic activity of Co2+. However, it was much lower than that (0.68 min−1) in MoS2-Co2+/ PMS system, indicating the important role of sulfur species in MoS2. These results demonstrate that both Mo and S species in MoS2 made contribution to enhanced performance of Co(II)/PMS system for RhB degradation.

276

817 989

recyceld MoS2

fresh MoS2 200

300

400

500

600

700

800

900 1000

-1

Raman shift / cm

Fig. 4. Raman spectra of the fresh and recycled MoS2 in the MoS2-Co2+/PMS system.

C. Pan et al. / Science of the Total Environment 712 (2020) 136447

indicating the structure of MoS2 was stable during the reaction process. Moreover, four new Raman peaks at 276, 333, 817 and 989 cm−1 appeared. The Raman signals at 276 and 333 cm−1 was assigned to the B2g and B1g vibrational modes of δ(Mo = O) and δ(OMo3), respectively, while the peaks at 817 and 989 cm−1 were ascribed to Mo–O–Mo bonds and vibrational modes of Mo_O, respectively (Sheng et al., 2019). The similar result was also observed in oxygen-implanted MoS2 and the used MoS2 in the Fenton-like reaction (Sheng et al., 2019; Xing et al., 2018), and indicates the oxidation of MoS2 happened during the reaction process in the MoS2-Co2+/PMS system, which should couple with the reductive recycle of Co3+ to Co2+ and the activation of PMS. Fig. 5 displays XPS of fresh and used MoS2 after 5 cycles in the MoS2Co2+/PMS system. As displayed in Fig. 5A, for fresh MoS2, there were two typical peaks in its spectra corresponding to Mo 3d5/2 and Mo 3d3/2 with the binding energy of 229.3 and 232.4 eV, demonstrating that Mo on surface of fresh MoS2 existed in Mo(IV). After reaction for 5 runs, in XPS of the used MoS2, the peaks corresponding to Mo 3d5/2 and Mo 3d3/2 shifted to higher binding energy, suggesting Mo on surface of fresh MoS2 was oxidized. Moreover, an visible peak of Mo(VI) at 236.1 eV was observed (Sheng et al., 2019), further confirms the oxidation of MoS2 during the reaction process. The same oxidation was also observed in S element of MoS2. As compared with the high resolution S2p of fresh MoS2, the peaks of S2p1/2 and S2p3/2 of the used MoS2 shifted to higher binding energy. More obvious evidence for the oxidation of MoS2 during the reaction process was the detection of SO2− 4 at 169 eV in the S 2p spectra in Fig. 5B. Based on the discussion above, the reaction mechanism for heterogeneous reductive regeneration of Co2+ by MoS2 and homogeneous catalytic activation of peroxymonosulfate for production of radicals in the system of MoS2-Co2+/PMS was proposed as seen in Scheme 1. Firstly, as confirmed by ESR in Fig. 6, the obtained commercial product MoS2 powder presented an ESR signal at g = 2.0, confirming the formation of sulfur vacancies. And the ESR signal for the sulfur vacancies was changed little for the recycled MoS2 in the system of MoS2-Co2+/PMS without RhB and with RhB. The results indicate that the sulfur vacancy can be easily regenerated during the reaction process. The presence of sulfur vacancies induced the expose of Mo4+ reactive sites. In the aqueous solution, Co2+ reacted with PMS to produce SO4•− and •OH (Eqs. (1) and (2)), accompanied with the generation of Co3+ (Eq. (3)). In the absence of MoS2, the reductive recycle of Co2+ via Eq. (4) was very slow. In comparison, in the presence of MoS2, the generated Co3+ adsorbed or precipitated on the surface of MoS2 at pH 5.5. And then, the exposed Mo4+ can reduce Co3+ to Co2+, enhancing completion of the cycle of Co3+/Co2+ (Eq. (6)).

Co3þ þ Mo4þ →Co2þ þ Mo5þ

240

MoS2

Mo3d5/2

used MoS2

229.8

Mo3d3/2 232.9 232.4

235 230 Binding Energy/eV

aqueous interface solid

Co2+

Co3+

S

Co3+ S

S 2s

225

Co2+

CYCLE S

S S vacancy

HSO5-

SO4- / OH

Mo

e

Co2+

S

S

S

e

Mo

Mo

Mo

Mo

e S

S

S

S

S

S

Scheme 1. Heterogeneous reductive regeneration of Co2+ by MoS2 and homogeneous catalytic activation of peroxymonosulfate for production of free radicals.

S2− →SO4 2− þ n e−

ð7Þ

Co3þ þ e− →Co2þ

ð8Þ

Mo5þ þ e− →Mo4þ

ð9Þ

Sulfur species in MoS2 also directly involved in reaction of Co2+ and PMS as supported by the more significant enhancement effect of MoS2 to Co(II)/PMS system for RhB degradation than MoO2 as displayed in Fig. S11 and the occurrence of SO2− 4 in the S 2p spectra in Fig. 5B. Sulfur species in MoS2 is in the state of −2, and can donate electrons to Co3+ to complete the cycle of Co3+ to Co2+ (Eqs. (7) and (8)). The other way MoS2 involved in reaction of Co2+/PMS was to regenerate Mo5+ to Mo4+ (Eq. (9)), which can further promote the reductive cycle of Co3 + to Co2+. Due to the great difference in solubility product constant (Ksp) of Co (OH)2 (2 × 10−15) and Co(OH)3 (2 × 10−44), the regenerated Co2+ was much more easily dissolved in the aqueous solution, further induced the homogeneous activation of PMS and the high-performance production of free radicals. The reaction for the regeneration of Co2+ was pH dependent. The highest catalytic activity of MoS2-Co2+/PMS system at pH 5.5 can be explained in the following two aspects: (1) The catalytic activity of Co2+ for PMS activation was pH dependent. As seen in Fig. 1B, at the same concentration of Co ion (2 μmol L−1), RhB degradation rate kobs in Co2+/PMS system was 0.21 min−1 at pH 5.5, which was 5.2 and 11.8 times that at pH 4 and pH 7, respectively (black line in Fig. 1B). Further, the different concentration of Co ions in the solution of MoS2-Co2+/PMS system at different pH was taken into consideration. As seen in Fig. S6, the concentration of Co ions in the solution in 5 min in MoS2-Co2+/

B

229.3

Mo(VI)

HSO5-

Intensity/a.u.

Intensity/a.u.

A

ð6Þ

7

S2p 3/2

MoS2 used MoS2

S2p1/2

2-

SO4

174 172 170 168 166 164 162 160 158 Binding Energy/eV

Fig. 5. (A) Mo3d and (B) S2p XPS of the fresh and used MoS2 in the MoS2-Co2+/PMS system.

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MoS2 2+

MoS2-Co /PMS 2+

Intensity / a.u.

MoS2-Co /PMS-RhB

3350

3400

3450

3500

3550

Magnetic field (G) Fig. 6. ESR of fresh MoS2 and recycled MoS2 in the system of MoS2-Co2+/PMS and MoS2Co2+/PMS-RhB.

PMS system was 1.7, 0.7 and 0.12 μmol L−1 at pH 4, 5.5 and 7, respectively. Therefore, 1.7, 0.7 and 0.12 μmol L−1 Co2+ was used to degrade RhB at pH 4, 5.5 and 7 in the presence of 0.2 mmol L−1 PMS. The result was seen in Fig. S12. The degradation rate of RhB at pH 4, 5.5 and 7 was 0.047, 0.16 and 0.0042 min−1, respectively. The degradation rate of RhB at 5.5 was 3.4 times that at pH 4. The results indicate that Co2+ had the best catalytic performance for PMS activation at pH 5.5 even when the concentration of Co ion in the solution of MoS2-Co2+/PMS system at pH 5.5 was lower than that at pH 4. (2) The addition of MoS2 further amplified the pH-effect of Co ion catalysis. As displayed by red line in Fig. 1B, RhB degradation rate kobs in MoS2-Co2+/PMS system was 0.68 min−1 at pH 5.5, which was 12.1 and 27.2 times that at pH 4 and pH 7, respectively. As seen in Fig. 2B, when the reaction pH was 4, 97.2% of Co ions existed as Co2+ in the reaction solution due to the low catalytic activity of Co2+ at the pH value of 4. Meanwhile, most of Co ions existed in the reaction solution and only a small proportion of Co3+ contacted with exposed Mo4+ and S2− on MoS2 surface. Therefore, the reduction of Co3+ on surface of MoS2 surface was depressed. On the other hand, at higher pH of 7, only 6.7% of Co ion was dissolved as Co2+ in the reaction solution, while most of Co precipitated in the reaction solution or on MoS2 surface, suggesting that the regeneration of Co2+ was also inhibited and not facile at the pH value. In comparison, at pH 5.5, in the presence of MoS2, the formed Co3+ from the reaction of Co2+ with PMS preferred to precipitate on the surface of MoS2. Afterwards, the precipitated Co3+ can be reduced to Co2+, which tended to dissolve in the reaction solution. In the way, the reductive cycle of Co3 + /Co2+ and a homogeneous activation of PMS were achieved. 3.6. Catalytic stability of MoS2 The stability and recyclability of MoS2 in the MoS2-Co2+/PMS system was investigated by monitoring of leaching solution, reusability experiments, XRD, Raman and SEM/TEM. Firstly, the leaching of Mo was monitored during the reaction process in the MoS2-Co2+/PMS system. As seen in Fig. S13, the concentration of Mo ions was increased with the reaction time, and reached 1.47 and 2.6 mg L−1 in 5 and 30 min, accounting for 0.49% and 0.87% of the total amount of Mo in added MoS2, indicating that the MoS2 nanosheet had excellent chemical stability under reaction conditions. For reuse test, two sets of experiments were conducted. One is the addition of fresh Co2+ and PMS for RhB degradation by the recycled MoS2 in each run. After RhB degradation was completed, MoS2 was recycled via filtration and washing by water several times. The recycled MoS2 was used again to degrade RhB at pH 5.5 by adding fresh Co2+ and PMS. The result for reusability test in the way

was shown in Fig. 8A. Degradation of RhB still kept nearly 100% in 5 min after 10 runs, suggesting MoS2 exhibited extreme stability for catalytic degradation of RhB in the MoS2-Co2+/PMS system. The other is degradation of RhB by just adding fresh PMS in the presence of the recycled MoS2 containing Co catalysts. In the set of experiment, after the degradation of RhB was completed, pH of the reaction solution was firstly adjusted to 10 to ensure the adsorption/precipitation of all Co catalysts on MoS2. After further stirring for 10 min, the reaction solution was filtrated to recycle Co and MoS 2 . Only 2.2 μg L −1 Co was detected in the filtrate, suggesting 98% of Co was kept on MoS2 surface. The recycled MoS2 containing Co was dispersed again in the aqueous solution containing RhB, and the pH of suspended solution was adjusted to 5.5. And then, RhB degradation was initiated by adding PMS. pH of the reaction solution was controlled at 5.5 by adding HNO 3 or NaOH. As seen in Fig. 8B, 90.4% removal of RhB can be still obtained in 5 min after 5 runs, suggesting the recycling of Co and MoS2 by adjusting the reaction solution to basic condition is feasible and further confirming the excellent stability of MoS2 for catalytic reaction. Fig. S14 shows that XRD of the recycled MoS2 had no visible change after reuse for 10 times as compared with that of fresh one, indicating the crystal structure of MoS2 is well stable during the reaction process. Raman spectra of the recycled MoS2 shows that characteristic Raman peaks at 372 and 398 cm−1 was not obviously decreased from the first recycle to the tenth recycle. SEM/TEM images in Fig. S15 shows that nanosheets structure of MoS2 was clearly seen, and the crystal lattice of MoS2 was not changed. All results confirm the excellent stability of MoS2 for catalytic reaction in the reusability experiments. 3.7. Application in the treatment of other pollutants The oxidation ability of the system of MoS2-Co2+/PMS was further checked by degradation of other organic refractory pollutants including two phenolic pollutants (BPA and 4-CP) and two dyes (MB and Orange 11). Fig. 7 shows that removal of MB and orange II was rapid, and the removal of MB and orange II in 10 min was 83.2% and 89.1% in the MoS2Co2+/PMS system, and 62.4% and 74.7% in Co2+/PMS system. In comparison, the degradation of BPA and 4-CP was much slower. The degradation of BPA and 4-CP in 30 min was 71.6% and 76.9% for MoS2-Co2+/ PMS system, and 51.1% and 59.7% for Co2+/PMS system. The difference on the degradation of these pollutants is possibly related to their functional moieties. As previously reported, SO4•− and •OH have high reactivity with organic pollutants with electron donating groups in structure via direct single electron transfer (Cvetnić et al., 2019). As seen in Fig. S16, RhB and MB have strong electron donor moieties such as two diethylamino groups in RhB and two dimethylamino groups in MB. Orange II has a nitrogen‑nitrogen double bond, which also has high reactivity with SO 4 •− and •OH (Cai et al., 2015). In comparison, in two phenolic pollutants of BPA and 4-CP, alkane groups in BPA are weaker in electron donor capacity, while 4CP has a high electron withdraw moiety of \\Cl. So BPA and 4-CP was observed to have lower degradation rates in the systems of Co2 + /PMS and MoS2-Co2+/PMS with SO4•− and •OH as the main reactive species. Also seen in Fig. 7, the reaction rate constant for BPA, 4-CP, MB and Orange 11 in the MoS2 -Co2+/PMS system was about 1.6–1.9 times that in the Co2+ /PMS system. The results demonstrate that the use of MoS2 can promote the catalytic oxidation capability of Co2+/PMS system for degradation of various pollutants with diverse structures. 3.8. Effect of real water matrix To better evaluate the effect of real water matrix on performance of MoS2-Co2+/PMS system, the influence of Cl−, HCO–3 and natural organic

C. Pan et al. / Science of the Total Environment 712 (2020) 136447

A

Co /PMS 2+ MoS2-Co /PMS

c/c0

0.8

B

orange II 2+

Co /PMS 2+ MoS2-Co /PMS

0.8

kobs =0. 091 min

-1

0.6

1.0

MB

2+

c/c0

1.0

9

0.6

kobs =0. 123 min

-1

0.4

0.4

0.2

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kobs =0. 173 min

-1

kobs =0. 200 min

-1

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0.0 0

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1.0

C

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BPA

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4-CP

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kobs =0. 024 min

-1

0.6

c/c0

c/c0

6

Time / min

Time / min

kobs =0. 025 min

-1

0.6

kobs =0. 041 min

-1

0.4

0.4 2+ kobs =0. 038 min Co /PMS 2+ MoS2-Co /PMS

-1

0.2

2+

0.2

Co /PMS 2+ MoS2-Co /PMS

0.0

0.0 0

5

10

15

20

25

0

30

5

10

15

20

25

30

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Time / min

Fig. 7. Degradation of (A) MB, (B) orange II, (C) BPA and (D) 4-CP in the systems of Co2+/PMS and MoS2-Co2+/PMS.

matter (NOM) with humic acid (HA) as a model was firstly evaluated. As seen in Fig. S17A and S17B, the presence of Cl− and HCO–3 greatly inhibited RhB degradation. RhB degradation in 5 min declined from 100% for the absence of any inorganic ions to 53% and 20% corresponding to the presence of 1 and 10 mmol L−1 Cl−. After the addition of 1 and 10 mmol L−1 HCO− 3 , RhB degradation was greatly decreased to 22% and 13% in 5 min. The inhibition effect of Cl− and HCO–3 was attributable to the conversion of highly oxidizing species of SO4•− to weaker oxidizing species Cl• and Cl2•− in the presence of Cl− (Eqs. (10) and (11)), and to HCO3• in the presence of HCO–3 (Eq. (12)). It was reported that Cl• and Cl2•− have redox potential of 2.4 V and 2.0 V vs NHE, a little lower than that of SO4•−

(2.5–3.1 V vs NHE). Carbonate radicals have much lower redox potential of 1.65 V (Y. Ding et al., 2019, 2020). −

SO4 •− þ Cl →SO4 2− þ Cl •

−

Cl þ Cl →Cl2

•−

•

k ¼ ð2:7–6:6Þ  108 M−1 s−1

k ¼ 8:5  109 M−1 s−1

SO4 •− þ HCO3 − →SO4 2− þ HCO3 •

k ¼ 9:1  106 M−1 s−1

80 60 40 20 0

2+

Without addition of Co

B

100

Degradation efficiency / %

Degradation efficiency / %

2+

100

ð11Þ

80 60 40 20 0

0

1

2

3

4

5 Cycles

6

7

8

9

10 11

ð12Þ

As seen in Fig. S17C, the presence of 10 and 50 mg L−1 HA exhibited inhibition effect on RhB degradation. RhB degradation in 5 min was

adding fresh Co in each cycle

A

ð10Þ

0

1

2

3

4

5

6

Cycles

Fig. 8. Degradation of RhB in 5 min co-catalyzed by (A) the recycled MoS2 and (B) the recycled MoS2 and Co catalyst in the MoS2-Co2+/PMS system.

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C. Pan et al. / Science of the Total Environment 712 (2020) 136447

decreased to 91% and 64% for the addition of 10 and 50 mg L−1 HA, while 100% removal of RhB was obtained in 5 min for no addition of HA. The inhibition effect of HA was due to the consumption of SO4•− by HA, which was also reported in other sulfate radicals based systems of PMS/CuFeO2 (Ding et al., 2016) and PMS/Co3O4-Bi2O3 (Hu et al., 2018). Finally, the RhB degradation in the practical water matrixes of tapping water and lake water by MoS2-Co2+/PMS system was also checked. As seen in Fig. S17D, although RhB degradation rate became slower in the practical water matrixes, RhB can be still readily decomposed with 68% and 79% removal of RhB in 10 min in tapping water and lake water. The slowed degradation of RhB in the practical water matrixes was possibly due to the presence of Cl−, HCO–3 and NOM (tapping water: Cl− 0.08 mol L−1, HCO–3 0.005 mol L−1, NOM 2.4 mg TOC L−1; lake water: Cl− 1.2 mol L−1, HCO–3 0.85 mol L−1, NOM 6.1 mg TOC L−1). However, the observation that RhB was still degraded suggests that MoS2-Co2+/PMS system has potential application in treatment of practical wastewater. 4. Conclusion The use of MoS2 for enhancing Co(II)/PMS system for organic pollutants degradation was investigated. The result indicates that reaction pH had significant effect on the enhancement effect of MoS2 for Co(II)/PMS system. The use of MoS2 only presented remarkable enhancement effect for Co(II)/PMS system at pH 5.5 and 6.0 due to the promoted redox cycle of Co3+/Co2+. Moreover, the enhancement rate for RhB degradation was linear with the dosage of MoS2 (0.1–1.0 g L−1) in MoS2-Co2+/ PMS system. Moreover, Co ion can be recycled with MoS2 by pH regulation to 10 via filtration, and exhibited excellent catalytic stability in catalytic degradation of organic pollutants in MoS2-Co(II)/PMS system. CRediT authorship contribution statement Cong Pan: Investigation, Writing - original draft, Writing - review & editing, Visualization, Formal analysis. Libin Fu: Methodology, Writing review & editing, Data curation. Yaobin Ding: Conceptualization, Writing - review & editing, Supervision, Project administration, Funding acquisition. Xueqin Peng: Investigation, Visualization, Writing - review & editing. Qihang Mao: Data curation, Writing - review & editing. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Funding is acknowledged from the National Natural Science Foundation of China (Grant No. 21876209), and the Natural Science Foundation of Hubei Province of China (Grant No. 2018CFB623). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2019.136447. References Anipsitakis, G.P., Dionysiou, D.D., 2003. Degradation of organic contaminants in water with sulfate radicals generated by the conjunction of peroxymonosulfate with cobalt. Environ. Sci. Technol. 37, 4790–4797. Anipsitakis, G.P., Dionysiou, D.D., Gonzalez, M.A., 2006. Cobalt-mediated activation of peroxymonosulfate and sulfate radical attack on phenolic compounds. Implications of chloride ions. Environ. Sci. Technol. 40, 1000–1007.

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