peroxymonosulfate process

Separation and Purification Technology 235 (2020) 116170

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Improved sulfamethoxazole degradation by the addition of MoS2 into the Fe2+/peroxymonosulfate process Songlin Wanga,b,c, Wenxin Xua,b,c, Junfeng Wua,b,c, Qing Gonga,b,c, Pengchao Xiea,b,c,d,

T

⁎

a

School of Environmental Science and Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074, China Key Laboratory of Water & Wastewater Treatment (HUST), MOHURD, Wuhan 430074, China c Hubei Provincial Engineering Research Center for Water Quality Safety and Pollution Control, Wuhan 430074, China d Center for the Environmental Implications of Nanotechnology (CEINT), Duke University, Durham 27708-0287, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: Sulfate radical (SO4%−) Hydroxyl radical (%OH) Ferrous ion (Fe2+) Peroxymonosulfate (PMS) Molybdenum disulfide (MoS2) Sulfamethoxazole

The low efficiency of transforming ferric ion (Fe3+) to ferrous ion (Fe2+) always challenges the activation of peroxymonosulfate (PMS) by (Fe2+) processes in the degradation of micro-organic contaminants. In this study, molybdenum disulfide (MoS2) which accelerated PMS activation and the Fe2+/Fe3+ cycle through the reaction of Mo4+ and Fe3+ was employed to significantly improve the efficiency of Fe2+/PMS on sulfamethoxazole (SMX) degradation. Through the analysis of electron paramagnetic resonance (EPR) and quenching experiments, both sulfate and hydroxyl radicals have been proven to account for SMX degradation in the MoS2/Fe2+/PMS process. The optimal pH of 3 has been determined for SMX degradation in the process, and an equation to express the relationship between the rate of SMX degradation and the dosages of Fe2+, MoS2 and PMS was simulated. Chloride ion and humic acid had positive and negative effects on the process, respectively, whereas nitrate and bicarbonate had no influences on SMX degradation. Through the identified intermediates by LC/MS, a transformation pathway of SMX in the MoS2/Fe2+/PMS process was proposed. Additionally, the good efficiency of SMX degradation (> 80%) in real water matrices and good stability of MoS2 after reuse for 6 times further suggest that the MoS2/Fe2+/PMS process can be effectively used to degrade SMX.

1. Introduction In recent decades, antibiotics have caused serious water pollution and attracted significant attention [1,2]. Sulfonamide antibiotics are well-known aqueous micro-pollutants for the difficult degradation, causing drug resistance of human and environmental organisms [3], and prompting the continuous emerging of super bacteria [4–6]. However, conventional treatment processes, such as coagulation, sedimentation and filtration, perform low efficiency on the removal of antibiotics from aquatic solution [7]. Advanced oxidation processes (AOPs) through the formation of hydroxyl radical (%OH) and sulfate radical (SO4%−) have been successfully applied to degrade antibiotics [7]. Compared to %OH, SO4%− contains similar oxidative capacity and higher selection, which has gained increasing attention to the destruction of recalcitrant organics in water [8]. SO4%− can be generated by scission of peroxo-bond of either peroxymonosulfate (PMS) or peroxydisulfate (PDS) [9,10]. Although PMS and PDS are thermodynamically strong oxidants, their direct reaction with the majority of the pollutants are very slow, meaning that the

⁎

direct reaction would be always ignored compared to the indirect reactive radical oxidation through the activation [11,12]. The activation methods of PDS and PMS usually include homogenous and heterogeneous transition metals [13–15], metal-free heterogeneous catalysts [16], ultraviolet [17], ultrasound [18], and thermo activation methods [19]. Among these activation methods, the low-cost ferrous iron (Fe2+) that is environmental-friendly has been evidenced to contain high activity and attracted worldwide attention. Due to the asymmetrical molecular structure and lower bond dissociation energy, PMS is relatively easier to be activated by transition metals compared to PDS [12]. Therefore, the application of Fe2+/PMS process in the treatment of contaminates has attracted extensive attention. However, as a Fentonlike process, Fe2+/PMS process also contains some drawbacks when it is selected to treat micro-pollutants, with the slow transformation of ferric ion (Fe3+) to Fe2+ as a major drawback [7]. To accelerate the Fe3+/Fe2+ cycle in Fenton and other Fenton-like processes, the addition of co-catalysts have been developed as an efficient method [20–22]. Among the co-catalysts, metal sulfite which can be easily separated from the reaction solution has been proposed to

Corresponding author at: School of Environmental Science and Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074, China. E-mail address: [email protected] (P. Xie).

https://doi.org/10.1016/j.seppur.2019.116170 Received 6 August 2019; Received in revised form 21 September 2019; Accepted 4 October 2019 Available online 04 October 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

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a reverse-phased XDB-C18 column (5 μm particle size, 4.6 × 150 mm, Waters, USA) was applied to measure SMX concentration. The mobile phase comprised by methanol and formic acid-contained water (0.1%, v/v) at a ratio of 35:65 (v/v) was set at a flow rate of 1 mL min−1. The column temperature, injection volume and detection wavelength were set at 30 °C, 20 μL, and 270 nm, respectively. After the separation of suspended solid by a membrane (0.22 μm), the dissolved iron ions in the solution were determined using 1,10-phenanthroline spectrophotometry method [26]. PMS concentration was determined by a rapid iodometric spectrophotometry method [27], and solution pH was measured using a pH meter (PHS-3C, Leici, China). The dissolved Mo ions were detected by an inductively coupled plasma optical emission spectrometer (ICP-OES, Leeman, USA). X-ray powder diffraction (XRD) patterns were analyzed using an X-ray diffractometer (B.V.X’Pert PRO, PANalytical, Netherlands) with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was obtained from a Shimadzu-Kratos AXIS-ULTRA DLD-600W system, Japan. The surface structure and the size distribution of MoS2 were analyzed by a scanning electron microscope combined with energy dispersive X-ray (SEM-EDX, Tecnai G2 20, FEI, USA) and a Malvern Zetasizer granulometer (ZEN 1600, Malvern Instrument, UK), respectively. The intermediates derived from SMX degradation were analyzed by liquid chromatography/mass spectrometry (LC/MS, 1100 LC/MSD Trap, Agilent, USA). By using 5,5-dimethhyl-pyrroline-oxide (DMPO) as the spin trapping reagent, an electron paramagnetic resonance (EPR) spectroscopy (EMX-EPR, Bruker) was applied for determining the formed reactive radicals in the process. The chemical solutions of Fe2+ (1.4 mM), PMS (8.0 mM), MoS2 (1.67 g/L) and DMPO (8.3 mM) were fast mixed for 10 s. Then samples were transferred into capillary tubes (100 μL) which were subsequently fixed in the cavity of the EPR spectroscopy for analysis.

improve the efficiency of Fenton and other Fenton-like processes on the degradation of organic pollutants and inactivation of Escherichia coli [12,20,23]. As a common seen metal sulfide, molybdenum sulfide (MoS2) could effectively reduce Fe3+ to Fe2+ by the exposed Mo4+, as a result to improving the efficiency of Fenton and Fenton-like processes [12,20,23]. Recently, MoS2 has been used to improve the efficiency of Fe2+/PMS process on the degradation of 2,4,6-trichlorophenol [24]. Therefore, the combination of MoS2 and Fe2+/PMS process would be also efficient in the treatment of typical antibiotics. The objective of this study was to investigate the efficiency of the combined process of MoS2 and Fe2+/PMS (MoS2/Fe2+/PMS) on the degradation of typical antibiotics in water solution. Sulfamethoxazole (SMX), one of the most frequently detected antibiotic compounds in aquatic environment, has been selected as a model sulfonamide antibiotic due to its universality, persistence, and toxicity [25]. The mechanisms including the role of MoS2 and the involvement of reactive radicals were disclosed, and the influences of some inorganic ions and humic acid (HA) were also studied. Through studying SMX degradation using the re-used MoS2, the stability of MoS2 in the process was evaluated. By identification of the intermediates of SMX degradation, a transformation pathway of SMX in the MoS2/Fe2+/PMS process was proposed. 2. Materials and methods 2.1. Reagents All the chemicals are of analytic grade at least and commercially purchased. MoS2 (D50 = 21.40 μm) and MoS2 (D50 = 1.018 μm) whose size distribution and morphologies are shown in Figs. S1 and S2 were purchased from Alfa Aesar, Tewksbury, USA and Aladdin Chemistry Co., Ltd, Shanghai, China, respectively. PMS (2KHSO5·KHSO4·K2SO4, ≥47%), Nitric acid (HNO3, ≥65%) and 5,5-dimethyl-1-pyrroline (DMPO, > 99.0%) were supplied by Aladdin Chemistry Co., Ltd, Shanghai, China. Ferrous sulfate (FeSO4·7H2O, ≥99%), SMX (≥98%), sodium hydroxide (NaOH, ≥96%), sodium bicarbonate (NaHCO3, ≥99.5%), sodium chloride (NaCl, ≥99.8%), potassium nitrate (KNO3, ≥99%), sodium perchlorate (NaClO4, ≥99.0%), and tert-Butanol (TBA) were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Methanol (MeOH, ≥99.9%) with high-performance liquid chromatography (HPLC) grade was obtained from Tedia Co., Inc., Fairfield, USA. All solutions were prepared by dissolving chemicals in ultrapure water (18.2 MΩ) that produced by a purifier system (Thermo Fisher Scientific, Micropure UV, USA).

3. Results and discussion 3.1. Degradation efficiency of SMX in the MoS2/Fe2+/PMS process As shown in Fig. 1, the variation of SMX concentration was less than 4% within 6 min in PMS, MoS2, FeSO4 or Fe2+/MoS2 process, suggesting the negligible removal of SMX by these processes. While SMX was fast degraded by around 35% in the initial 0.5 min in the Fe2+/ PMS process, then the SMX concentration kept nearly stable in the time range of 0.5–6 min, which might be due to the slow transformation of

2.2. Experimental procedures A 100 mL double-jacketed beaker under magnetic stirring was used to carry out the experiments, and the speed of magnetic stirring was fixed at 400 r/min. The reaction temperature was maintained at (25 ± 1 °C) by a thermostatic system (THD-1015, Tianheng Instrument, Ningbo, China). Unless otherwise noted, MoS2 (D50 = 1.018 μm) was selected in this study. MoS2, FeSO4 and SMZ at preset dosage were introduced into the beaker followed by adjusting the reaction pH with 0.1 M NaOH and 0.1 M HNO3 solutions. After the addition of PMS, the reaction was initiated. Then aliquots (1 mL) were withdrawn from the reactor at predetermined time intervals and immediately quenched by excess methanol (0.5 ML). Prior to analysis, each sample was filtered by a membrane with pore size of 0.22 μm. All the experiments were conducted in duplicate at least, and the errors shown in figures stand for the standard deviations. Fig. 1. Degradation of SMX in different systems. Conditions: [MoS2]0 = 0.3 g/ L, [Fe2+]0 = 70 μM, [PMS]0 = 75 μM, [SMX]0 = 25 μM, temperature 25 °C, initial pH 3.0, stirring speed 400 rpm. The error bars represent the standard deviations from duplicate tests.

2.3. Analytical methods A high performance liquid chromatography (HPLC, Shimadzu LC16, Japan) coupled with a UV detector (SPD-16, Shimadzu, Japan) and 2

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Fe3+ to Fe2+ [22]. Surprisingly, the concentration of SMX was reduced by approximate 88.5% within 6 min in the MoS2/Fe2+/PMS process, which is much higher than the sum of SMX removal in MoS2/PMS and Fe2+/PMS processes, suggesting that MoS2 could improve the efficiency of traditional Fe2+/PMS process through serving as an excellent cocatalyst. Similar results were also achieved in some previous studies focusing on the improvement of Fenton or Fenton-like processes by MoS2 [7,20,23,24]. It should be noted that SMX was also gradually degraded within 6 min in the MoS2/PMS process and the removal of SMX reached 22.8% in the end, meaning that PMS would be also activated by MoS2 to produce reactive radicals accounting for the degradation of SMX, which was also reported by a previous publication [24]. TOC removal of 33.9% was observed in the MoS2/Fe2+/PMS process in 6 min (Fig. S3), suggesting the MoS2/Fe2+/PMS process also possessed certain mineralization capacity for organic pollutants and some organic intermediates.

It is known that the reactions in Fe2+/PMS process mainly contains three equations expressed as Eqs. (1)–(3). The catalyzed decomposition of PMS by Fe2+ directly generates SO4%− and %OH following Eqs. (1) and (2), respectively, along with fast conversion of Fe2+ to Fe3+. The formed Fe3+ can be slowly transformed to Fe2+ following Eq. (3), which determines the overall efficiency of Fe2+/PMS process. Therefore, to facilitate the Fe3+/Fe2+ cycle is believed to be a good method to improve the efficiency of PMS/Fe2+ process, which can promote PMS decomposition to generate SO4%− and %OH [22]. Previous studies have suggested that the MoS2 surface could capture protons in the hydrogen evolution reaction due to the formed sulfur vacancies on the surface of MoS2 through the dispersion of commercial MoS2 in water [20,23,30]. Therefore, the transformation of Fe3+ to Fe2+ is expected to be facilitated by the Mo4+ contained on the MoS2 surface [20,23,30]. Based on the discovery in traditional Fenton process with the addition of MoS2, the transformation of Mo4+ and Mo6+ is proposed as Eqs. (4) and (5) in the MoS2/Fe2+/PMS process. To further confirm the reaction between Mo4+ and Fe3+, the concentrations of Fe2+ and total iron ions in the MoS2/Fe2+/PMS process were monitored during the reaction, where total iron ions corresponding to the sum of Fe2+ and Fe3+. As shown in Fig. 3(a), the concentration of Fe2+ fast decreased to around zero in the initial 30 s in the Fe2+/PMS process, indicating the rapid response to the reaction between Fe2+ and PMS. The results are in accordance with the variations of SMX (Fig. 1) and PMS (Fig. 3(b)) in the Fe2+/PMS, suggesting the absence of Fe2+ after 30 s inhibited the decomposition of PMS, as a result to inhibiting the degradation of SMX. As for the MoS2/Fe2+/PMS process, the fast decrease of Fe2+ in the initial 30 s also evidenced the fast reaction between Fe2+ and PMS (Fig. 3(a)). However, the gradual increase of Fe2+ in the reaction solution during the reaction time range of 1–6 min evidenced that the presence of MoS2 accelerated the transformation of Fe3+ to Fe2+. The variation of PMS in the MoS2/Fe2+/PMS process was also monitored (Fig. 3(b)), showing that the concentration of PMS kept gradual decrease to around zero in the initial 4 min, which is different to that in the Fe2+/PMS process. The results also suggest that the addition of MoS2 into the Fe2+/PMS process can enhance the decomposition of PMS, as a result to improving the formation of reactive radicals that caused the efficient degradation of SMX (Figs. 1 and 2(c)). Additionally, Mo 3d XPS spectrum of pristine and 6-times used MoS2 were studied (Figs. S5), finding that the spectrum for both pristine and 6-times used MoS2 are similar without the peak centered at 236 eV which corresponds to Mo6+, which is different from the result achieved in Fenton process [23]. This result might be due to the rapid reduction of Mo6+ to Mo4+ through Eq. (5) in the MoS2/Fe2+/PMS process.

3.2. Determination of the reactive radicals in the MoS2/Fe2+/PMS process For the catalyst-mediated decomposition of PMS, three reactive radicals including SO4%−, %OH, and SO5%− have been suggested to be produced [22]. Both SO4%− and %OH contain high oxidation ability and have been widely used for the treatment of aquatic organic contaminants [28,29]. Due to the fast reaction rate constants of MeOH % reacting with SO4%− (k = 3.2 × 106 M−1 s−1) and OH 8 −1 −1 (k = 9.7 × 10 M s ), MeOH is usually considered as a scavenger for both SO4%−and %OH [27]. While TBA is always selected as a scavenger for %OH (k = 6 × 108 M−1 s−1), but not for SO4%− (k = 4 × 105 M−1 s−1) [27]. Therefore, MeOH and TBA were selected to quench the formed reactive radicals to explore the roles of SO4%− and %OH on SMX degradation through comparing the different removal of SMX separately in the presence of MeOH and TBA. The variations of SMX degradation with the addition of MeOH and TBA into the MoS2/Fe2+/PMS process were illustrated in Fig. 2(a) and (b), respectively. Along with the concentrations of MeOH and TBA varying from 0 mM to 50 mM, the degradation of SMX gradually decreased by 82.1% and 45.4%, respectively, which suggests that both % OH and SO4%− contribute to the SMX degradation in the MoS2/Fe2+/ PMS process. The result was in agreement with a previous report applying hydroxylamine/Fe2+/PMS to degrade SMX [7]. In Comparison with Fe2+, PMS, MoS2 and Fe2+/MoS2, the degradation of SMX in the MoS2/Fe2+/PMS process is still a little higher with the addition of 50 mM MeOH, which could be attributed to the fast initial degradation, the SO5%− oxidation or the remained few %OH or SO4%− [7]. To further test the formation of %OH and SO4%− in the process, EPR using DMPO as the spin trapping reagent was also applied in this study. As shown in Fig. 2(c), the special hyperfine coupling constants of α(N) = 1.49 mT and α(H) = 1.49 mT corresponding to DMPO-OH and the special hyperfine coupling constants corresponding to DMPO-SO4 (α(N) = 1.38 mT, α(H) = 1.02 mT, α(H) = 0.14 mT and α(H) = 0.08 mT) could be found in both Fe2+/PMS and MoS2/Fe2+/ PMS processes, while the intensities of the signals in MoS2/Fe2+/PMS process significantly outweigh that in traditional Fe2+/PMS process. The result suggests that the addition of MoS2 could accelerate the formation of both %OH and SO4%−, which is in accordance with the remarkable improvement of SMX degradation with the addition of MoS2 into the Fe2+/PMS process (Fig. 1). Fe2+ + HSO5− → Fe3+ + SO4%− + OH− Fe

2+

+ HSO5− →

Fe

3+

+ HSO5− → 3+

Fe

+ SO42− + %OH

(2)

Fe

2+

+ SO5%− +

(3)

6+

2+

→ Mo

Mo

4+

+ Fe

Mo

6+

+ HSO5− →

+ Fe

4+

Mo

+ SO5

H

Fig. 4(a) shows the effect of initial solution pH on SMX degradation in the pH range of 1–9, where the direct dependence of SMX degradation on the initial solution pH was observed. It can be seen that the degradation rate of SMX increased with the initial pH increasing from 1 to 3, and then gradually decreased with further elevating the initial solution pH. When the pH was 1, some free Fe2+ would be transformed to (Fe (H2O))2+, resulting in the decrease of free Fe2+ in the solution and SMX degradation [31]. Furthermore, the point of zero charge (PZC) of MoS2 was about pH 2, revealing that the surface of MoS2 is positively charged in solution at pH < 2. Therefore, electrostatic repulsive force would exist between MoS2 and Fe3+ at pH 1, which would inhibit the reaction between MoS2 and Fe3+, as a result to lowering SMX degradation rate. When the solution pH was elevated from 3 to 9, the formation of iron precipitation can lower the catalytic activity of iron ions, which gradually stopped the activation of PMS and inhibited SMX degradation in the MoS2/Fe2+/PMS process [31]. For example, the no longer existence of rapid first stage kinetics at pH 7 and 9 indicates a deficiency

(1)

3+

+

3.3. Effect of initial pH

(4) %−

+H

+

(5) 3

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Fig. 2. Effects of MeOH (a) and TBA (b) on SMX degradation in the MoS2/Fe2+/PMS process; and EPR spectra (c). Conditions: [MoS2]0 = 0.3 g/L (a and b) or 1.67 g/ L (c), [Fe2+]0 = 70 μM (a and b) or 1.4 mM (c), [PMS]0 = 75 μM (a and b) or 8 mM (c), [SMX]0 = 25 μM, [DMPO]0 = 8.3 mM, temperature 25 °C, initial pH 3.0, stirring speed 400 rpm. The error bars represent the standard deviations from duplicate tests.

activator of PMS for the generation of reactive radicals. However, as a reductant, excess Fe2+ can scavenge the formed reactive radicals at the same time. Thus, the optimum Fe2+ dosage in the MoS2/Fe2+/PMS process was identified with the Fe2+/MoS2 ratio ranging from 1:50 to 1:10 at a constant MoS2 dosage of 0.3 g/L (1874 μM) (Fig. 4(b)). As shown in Fig. 4(b), SMX was rapidly degraded in the first 30 s followed by a much slower decay when the Fe2+/MoS2 ≤ 1:20.

of catalyst. Additionally, PMS would be self-dissociated through nonradical pathways at a raised pH level, which might impair the oxidizing capacity of PMS towards the selected contaminants [31]. 3.4. Effect of dosages of Fe2+, MoS2 or PMS In the MoS2/Fe2+/PMS process, Fe2+ was used as the dominant

Fig. 3. (a) Variation of Fe2+ and total iron ions in reaction solution; (b) variation of PMS in reaction solution. Conditions: [MoS2]0 = 0.3 g/L, [Fe2+]0 = 70 μM, [PMS]0 = 75 μM, [SMX]0 = 25 μM, temperature 25 °C, initial pH 3.0, stirring speed 400 rpm. The error bars represent the standard deviations from duplicate tests. 4

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Fig. 4. The impacts of initial pH (a), initial Fe2+ concentration (b), MoS2 dosage (c), and PMS dosage (d) on SMX degradation in the MoS2/Fe2+/PMS process. Conditions: [MoS2]0 = 0.3 g/L except (c), [Fe2+]0 = 70 μM except (b), [PMS]0 = 75 μM except (d), [SMX]0 = 25 μM, initial pH 3.0 except (a), temperature 25 °C, stirring speed 400 rpm. The error bars represent the standard deviations from duplicate.

Similar to typical heterogeneous catalysts, the co-catalytic effect of MoS2 is supposed to depend on particle size distribution and dose [4,32]. As shown in Fig. S4, at the same dose of 0.3 g/L, MoS2 with larger size (D50 = 21.40 μm) showed much lower activity on the degradation of SMZ than that with smaller size (D50 = 1.018 μm) when applying MoS2/Fe2+/PMS process to degrade SMX. As discussed in some previous studies, the number of exposed edge S atoms and the specific surface area on MoS2 particles depend on the particle size distribution, which is considered to attribute to the size dependence of the activity [20,23]. There is an obvious positive correlation of activity and MoS2 dose as the degradation rate of SMX was significantly elevated from 57.4% to 88.5% with increasing MoS2 dose from 0.1 g/L to 0.3 g/L (Fig. 4(c)). However, when the MoS2 continues to increase, the activity-increasing trend slows down, which might be explained that the fast decomposition of PMS caused the limited concentration of PMS in the MoS2/Fe2+/ PMS process [23,24]. PMS concentration is a critical parameter as the source of %OH and SO4%− in the MoS2/Fe2+/PMS process. The influence of PMS dose on SMX degradation was evaluated by varying [PMS]/[SMX] ratio from 2:1 to 8:1 with an initial SMX concentration of 25 μM. As illustrated in Fig. 4(d), SMX decay rate increased with the elevation of PMS concentration when [PMS]/[SMX] ratio was below 3:1. However, no further acceleration of SMX degradation was achieved with further increasing PMS concentration. The higher PMS concentration might generate more %OH and SO4%− from the activation of PMS. On the

Additionally, the removal efficiency increased from 22.8% to 86.5% with the Fe2+/MoS2 ratio increasing from 0 to 1:20, and the rate constants of rapid degradation also increased as the Fe2+/MoS2 ratio increased until 1:20, where the retarded slow degradation stage showed no significant change of rate constant. However, as the ratio Fe2+/ MoS2 > 1:20, the SMX concentration curves leveled off at the second degradation stage, suggesting the reaction was totally inhibited at second-stage reaction. In the Fe2+-mediated PMS process, %OH and SO4%− generated through the decomposition of PMS via Eqs. (1) and (2) in the first few seconds, resulting in the rapid SMX degradation in the initial time stage [7]. Fe3+ generated in Eqs. (1) and (2) would serve as an electron acceptor as a result to decomposing extra PMS to generate peroxysulfate radicals containing weaker oxidizing capacity than %OH and SO4%− [11]. Fe2+ regenerated through Eqs. (3) and (4) can further react with PMS. In comparison with the Fe2+/PMS process, the combination of Fe3+ with PMS degraded organics at a much lower rate, which agrees with the slower second-stage reactions as Fe2+/MoS2 ≤ 1:20. However, excessive Fe2+ in the Fe2+/PMS process can also consume %OH and SO4%− through Eqs. (6) and (7). Therefore, unlimited increase of Fe2+ is not recommended and the molar ratio Fe2+/MoS2 = 1:20 is considered as the optimal in the MoS2/Fe2+/PMS process for the degradation of SMX. Fe2+ + %OH + H+ → Fe3+ + H2O Fe

2+

+ SO4

%−

→ Fe

3+

+ SO4

2−

(6) (7) 5

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other side, HSO5− can also react with the generated SO4%− through Eq. (8), which is believed to account for the no further improvement on SMX degradation with increasing [PMS]/[SMX] ratio from 2:1 to 8:1 [11]. HSO5−+SO4%− → SO5%−+SO42−+H+

influence the electrostatic interactions (outer-sphere interactions) in a heterogeneous system by compressing the thickness of the electric double layers, the result also suggests that the electrostatic interactions between MoS2, Fe2+ and PMS was not the dominant mechanism for the positive effect by increasing Cl− concentration in the MoS2/Fe2+/PMS process on SMX degradation [28]. As MeOH can efficiently scavenge all the possible reactive radicals including SO4%−, %OH, Cl% and Cl2%− in the MoS2/Fe2+/PMS process, the same efficient inhibition of SMX degradation in the MoS2/Fe2+/PMS process no matter the addition of Cl− or not shown in Fig. S10 suggests that the formed Cl% or Cl2%− participated in the transformation of SMX in the MoS2/Fe2+/PMS process [37]. As shown in Fig. 5(b) and (c), little to no influence of the presence of 0–2 mM NO3− and the presence of 0–10 mM inorganic carbon on SMX removal efficiency in the MoS2/Fe2+/PMS process was observed. The negligible impact of NO3− on SMX degradation in the MoS2/Fe2+/PMS process also illustrates that choosing nitric acid to adjust solution pH in this study is reasonable. Humic substances, the most widely occurring organic compounds in natural waters, can strongly affect the treatment efficiency of pollutants by AOPs [39]. SMX degradation experiments in the MoS2/Fe2+/PMS process in the presence of humic substances were carried out to investigate the impact of humic substances by selecting humic acid (HA) as a reference humic substance. As shown in Fig. 5(d), the degradation efficiency of SMX was significantly reduced by 34.7% with increasing the initial HA concentration from 0 to 2.0 mg/L. SO4%− and %OH which react with HA at quite high second-order reaction rate constants have been proven to be primary oxidants for SMX degradation in the activated PMS process [40,41], suggesting that the inhibitory effect of HA on SMZ oxidation was related to the quenching of SO4%− and %OH, which is also reported by many previous publications [40–42]. To examine the potential of practical application of the MoS2/Fe2+/ PMS process in the treatment of SMX, a real water sample that has gone through coagulation-sedimentation treatment was collected from a drinking water treatment plant. Some typical characteristics of the real water sample including TOC, HCO3−/CO32−, Cl− and pH are shown in Table S4. As shown in Fig. S11, approximately 81% of SMX were removed within 6 min in the MoS2/Fe2+/PMS process with the dosages of Fe2+, PMS and MoS2 being 70 μmol/L, 75 μmol/L, and 0.3 g/L, respectively. This result further conforms the good efficiency of MoS2/ Fe2+/PMS process in the treatment of SMX in aquatic environment.

(8) 2+

To further insight into the influence of dosages of Fe , MoS2 and PMS on SMX degradation in the MoS2/Fe2+/PMS process, an apparent kinetic model was proposed following the calculation procedures shown in Text S1. Then an equation expressing the relationships between SMX degradation and the initial concentrations of Fe2+, PMS and MoS2 can be achieved through simulating the experimental data shown in Fig. 4(b)-4(d) and is shown as Eq. (9).

V = lg (−dc / dt ) = 0.50450[Fe 2 +]0.90160 [PMS ]0.00193 [MoS2]−0.44639

(9)

−1

where V is the rate of SMX degradation (s ), c is the concentration of SMX (μM), t is the reaction time (s), [Fe2+] is the concentration of Fe2+ (μM), [PMS] is the concentration of PMS (μM), and [MoS2] is the concentration of MoS2 (g/L). 3.5. Effect of various inorganic salts, humic acid and real water matrices SMX may co-exist with various inorganic ions whose detected concentration is ranged from μM to mM in various waters [33]. It is known that the presence of some inorganic ions, such as Cl− and HCO3−, can seriously affect the treatment efficiency of organic contaminants in the hydroxyl and sulfate radicals-based advanced oxidation processes [34,35]. To evaluate the performance of these inorganic ions in the MoS2/Fe2+/PMS process, the effects of typical anions including Cl−, inorganic carbon and NO3− on SMX decay were evaluated in this study (Fig. 5). As a widely existing ion, chloride ions have exhibited dual role on the degradation of different organic contaminants in AOPs [36]. As illustrated in Fig. 5(a), a gradual acceleration of SMX degradation in the first-stage as Cl− concentration increased in the range of 0–10 mM was observed when the MoS2/Fe2+/PMS process was applied to treat SMX. It is known that a sequence of reactive chlorine species, such as Cl%, Cl2%− and HClO, would be produced in activated PMS processes in the presence of Cl−, which may result in the generation of a series of secondary oxidants accounting for SMX degradation [34,36]. Therefore, the enhancement of SMX degradation with the addition of Cl− into the process was probably because of the reaction between SMX and the formed reactive chlorine species. To verify the involvement of the reactive chlorine species in SMX degradation in the MoS2/Fe2+/PMS process in the presence of Cl−, the following experiments including the degradation of SMX with addition of Cl− into PMS solution, adding ClO4− in the MoS2/Fe2+/PMS process, and simultaneous addition of radical quencher (methanol) and Cl− in the MoS2/Fe2+/PMS process were carried out. Due to the direct generation of HClO from redox reaction between PMS and Cl− without any radical species [8,37,38], the negligible variation of SMX degradation in the Cl−/PMS system with the Cl− concentration varying from 0 mM to 10 mM suggests that the improvement of SMX degradation in the MoS2/Fe2+/PMS system in the presence of Cl− was not derived from the possible formed HClO (Fig. S8). The addition of Cl− into the solution can increase the ionic strength, which might affect the degradation of organic pollutants in activated PMS processes [28,37]. Therefore, the impacts of ionic strength on SMX degradation in the MoS2/Fe2+/PMS process was investigated. As shown in Fig. S9, the addition of 10 mM ClO4− had no influence on the degradation efficiency of SMX when applying the MoS2/Fe2+/PMS process to degrade SMX, which is different from the significant enhancement of SMX degradation in the presence of 10 mM Cl−. Such a result suggests that the acceleration of SMX degradation rate with addition of Cl− into the MoS2/Fe2+/PMS process was not accounted for the ionic strength. As increasing ionic strength tends to

3.6. Intermediates and degradation pathways The degradation intermediates of SMX by various oxidation processes including ozonation, chlorination, permanganate oxidation, and various AOPs have been identified [5,43–45]. As for different AOPs, the species of intermediates and degradation pathways of SMX are always different [46]. In this study, LC/MS was employed to detect the reaction intermediates, and the degradation pathways of SMX in the MoS2/ Fe2+/PMS process were proposed based on the detected intermediates (Fig. 6). As shown in Fig. S12, eight intermediate products including P-98, P116, P-117, P-155, P-226, P-283, P-287, and P-502 were identified by LC/MS, which was also reported in hydroxylamine/Fe2+/PMS process [7]. The MS spectra of P-98 (m/z 99 for MH+) with fragments of m/z 72 was identified as 3-amino-5-methylisoxazole which can be produced from the cleavage of S-N bond in SMX [6]. Similar to previous reports, another primary product P-155 (m/z 156 for MH+) containing the fragments of m/z 92 and m/z 108 corresponded to 4-sulfonylcyclohexa2,5-dien-1-imine, which can be also produced from the cleavage bond reaction of SMX [6,7]. According to Fig. S12, the detected fragment P283 (m/z 284 for MH+) with its ion fragments (m/z 99 and m/z 133) during the degradation of SMX in the process would be considered as nitro SMX through the oxidation of the susceptible –NH2 group on the aromatic ring of SMX [7,46,47]. Furthermore, the reaction of reactive 6

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Fig. 5. Effect of chloride ion (a), nitrate ion (b), inorganic carbon (c), and humic acid (d) on SMX degradation in the MoS2/Fe2+/PMS process. Conditions: [MoS2]0 = 0.3 g/L, [Fe2+]0 = 70 μM, [PMS]0 = 75 μM, [SMX]0 = 25 μM, temperature 25 °C, initial pH 3.0, stirring speed 400 rpm. The error bars represent the standard deviations from duplicate tests.

radicals with –NH2 group could produce N-centered radical too, which can subsequently go through coupling reaction to produce TP502 [2,46]. In particular, m/z 288 as dihydroxylated SMX could be produced through the electrophilic addition on C14 and C13 of the isoxazole ring by SO4%− and %OH, which is also widely detected in other AOPs such as CuFe2O4/hydroxylamine/PMS, Fe0/bisulfite/O2, UV, UV/ PDS and UV/H2O2 [46]. The attack on S7-N11 by %OH might generate m/z 133 (not detected here) which can be further transformed to m/z 117 and m/z 118 via %OH substitution. Then the coupling of the Ncentered radical derived from –NH2 group of m/z 99 and m/z 133 could subsequently form m/z 227 [47]. The reactive site N8 of SMX was attacked by SO4%− and %OH, as a result to forming m/z 284 (NO2-SMX) which is considered as a classical oxidative product of amino group in SMX through H-abstraction that caused by reactive radicals attack. As shown in Fig. S3, the concentration of TOC was reduced by more than 30%, suggesting that partial organic intermediates could be mineralized by the formed SO4%− and %OH. Therefore, the degradation pathway of SMX in the MoS2/Fe2+/PMS process should be ended with the formation of CO2 (Fig. 6).

for SMX degradation during the 6-times cycle tests of MoS2. The SMX degradation efficiency kept around 90% for all the six cycles, suggesting good stability of MoS2 in the MoS2/Fe2+/PMS process. Additionally, we also monitored the variation of dissolved Mo ions in the solution by an ICP-OES, finding that the concentration of Mo ions in the solution kept undetectable (data not shown). The result suggests that the MoS2 can not release Mo ions into the solution during the reaction in the MoS2/Fe2+/PMS process, meaning that the acceleration of SMX degradation with the addition of MoS2 occurred on the surface of MoS2 and the MoS2 would contain good stability. To further identify the stability of MoS2, The charges of Mo and the crystalline structure of MoS2 after re-used for 6 times were also evaluated by XPS and XRD, respectively. As shown in Fig. S5, the similar XRD patterns between pristine and re-used MoS2 were observed, indicating that the commercial MoS2 contains good structural stability. Additionally, the similar Mo 3d XPS spectrum of pristine and 6-times used MoS2 also evidenced the good recovery of Mo4+ (Fig. S6). All these results indicate the good stability and reusability of MoS2 in the MoS2/Fe2+/PMS process, which further suggests that MoS2 co-catalytic Fe2+/PMS reaction would have great potential in the treatment of organic pollutants in aquatic environment [20].

3.7. Stability of MoS2 in the MoS2/ Fe2+/PMS process Cycling test of MoS2 for SMX degradation in the MoS2/Fe2+/PMS process was carried out to evaluate the stability of MoS2. The MoS2 was recycled following the sequence procedure of centrifugation and dried in a vacuum oven. As shown in Fig. 7, there are no obvious differences

4. Conclusion The MoS2/Fe2+/PMS process was efficiently adopted to degrade SMX under different experimental conditions. Based on the EPR 7

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Fig. 6. The proposed pathways of SMX degradation in the MoS2/Fe2+/PMS process.

spectrum and the results of scavenging experiments by either MeOH or TBA, both SO4%− and %OH have been determined to be the primary oxidants for SMX degradation in the process. The fast transformation of Fe3+ to Fe2+ through the reaction between Fe3+ and the exposed Mo4+ in MoS2 can accelerate the PMS decomposition as a result to improving SMX degradation. Reducing the particle size of commercial MoS2 benefits the efficiency of MoS2. The optimal pH of MoS2/Fe2+/PMS process for SMX degradation was found to be 3. The SMX degradation was enhanced as the MoS2 or PMS dosage increased from 0 g/L and 0 μM to 0.5 g/L and 200 μM, respectively. However, the optimum Fe2+ dose depended on the concentration of PMS and MoS2, and the SMX degradation was inhibited by the excess Fe2+. SMX degradation was inhibited by HA and promoted by Cl−, whereas no influences of nitrate and bicarbonate on SMX degradation were observed. Based on the eight detected intermediates by LC/MS, pathways of SMX degradation with primary reaction of the cleavage of S-N bond, hydroxylation of the aromatic ring and oxidation of amine group were proposed. The good efficiency of SMX degradation (> 80%) in real water matrices and the good stability of MoS2 after re-used for 6 times further suggest that there is a great potential to apply MoS2 as a co-catalyst to improve the efficiency of Fe2+/PMS process for the degradation of organic pollutants in aquatic environment.

Fig. 7. Cycle test of MoS2 for the degradation of SMX in the MoS2/Fe2+/PMS process. Conditions: [MoS2]0 = 0.3 g/L, [Fe2+]0 = 70 μM, [PMS]0 = 75 μM, [SMX]0 = 25 μM, initial pH 3.0, temperature 25 °C, stirring speed 400 rpm. The error bars represent the standard deviations from duplicate tests.

8

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Declaration of Competing Interest [21]

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

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Acknowledgements

[23]

The support by the Natural Science Foundation of China (Grant Nos. 51578258, 51608215 and 51878308) was appreciated. We also gratefully acknowledge the Analytical and Testing Center of HUST for related analysis.

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Appendix A. Supplementary material [27]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.116170.

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