Efficient degradation of Orange G with persulfate activated by recyclable FeMoO4

Chemosphere 214 (2019) 642e650

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Efficient degradation of Orange G with persulfate activated by recyclable FeMoO4 Xueming Lin a, b, d, Yongwen Ma b, c, *, Jinquan Wan b, c, Yan Wang b, c, Yongtao Li a, d a

College of Natural Resources and Environment, South China Agricultural University, Guangzhou 510640, China School of Environment and Energy, South China University of Technology, Guangzhou 510006, China c Guangdong Plant Fiber High-Valued Cleaning Utilization Engineering Technology Research Center, Guangzhou 510006, China d Joint Institute for Environmental Research ＆ Education, South China Agricultural University, Guangzhou 510640, China b

h i g h l i g h t s
FeMoO4 can strongly catalyze persulfate for OG degradation.
FeMoO4 exhibited excellent sustained catalytic ability and reusability.

The contribution of SO4- was higher than that of HO
to OG degradation.
The possible mechanism of persulfate activation by FeMoO4 was elucidated.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 June 2018 Received in revised form 6 September 2018 Accepted 19 September 2018 Available online 28 September 2018

In this study, FeMoO4 was applied to activate persulfate (PS, S2O2 8 ) for azo dye Orange G (OG) degradation. The catalyst was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and nitrogen adsorption-desorption isotherms. FeMoO4 showed excellent efficiency in activating PS for OG removal. More than 95% could be removed after 40 min under reaction conditions of 4 mM PS, 0.3 g L1 FeMoO4 and 0.2 mM OG. The effect of different parameters (PS doses, FeMoO4 doses and pH) were evaluated. The results showed that acid condition provided higher efficiency and overdosing FeMoO4 and PS presented a scavenging effect. Major intermediates were identified and possible degradation pathway was proposed. Recycle tests presented that FeMoO4 had excellent recyclable stability in activating PS for OG removal. Sulfate radicals and hydroxyl radicals all occurred in the oxidation reactions and the former came first. The oxidation reaction was involved in the translation of Fe2þ/Fe3þ occurred on the surface layer. This study revealed that the FeMoO4/PS system is a very promising method for degrading organic contaminants in the environment. © 2018 Published by Elsevier Ltd.

Handling Editor: Jun Huang Keywords: FeMoO4 Persulfate Orange G Degradation Mechanism

1. Introduction Azo dyes are characterized by the existence of several benzene cycles and one or more azo bonds (-N]N-), which are often used in various industries, including textile, paper, leather, cosmetics and plastics industries. Many azo dyes are toxic, mutagenic, carcinogenic and non-biodegradable, which pose long-term risks to the ecosystem and cause human health problems (Gan et al., 2017; Aveiro et al., 2018). Orange G is one typical kind of azo dyes, whose

* Corresponding author. School of Environment and Energy, South China University of Technology, Guangzhou 510006, China. E-mail address: [email protected] (Y. Ma). https://doi.org/10.1016/j.chemosphere.2018.09.124 0045-6535/© 2018 Published by Elsevier Ltd.

structure was shown in Fig. S1. Since conventional treatment technology cannot effectively degrade and mineralize azo dyes in wastewater, advanced oxidation processes (AOPs) including Fenton, ozonation, photolysis and so on were applied to decompose toxic and non-biodegradable azo dyes (Li et al., 2016; BaenaNogueras et al., 2017; Larouk et al., 2017). However, these processes always needed additional energy or the oxidants such as H2O2 is not easily to be stored. Sodium peroxydisulfate used as
persulfate is more easily to be stored. In addition, compared to OH,
 SO4 have similar high oxidation potential of 2.8 V but longer life span of 30e40 ms (Yang et al., 2014; Lutze et al., 2015; Jawad et al.,
2018). Thus, AOPs based on sulfate radicals (SO4) are drawing more
 and more attentions. SO4 can be produced by different method

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such as ultraviolet, pyrolysis or chemical activation of persulfate 2 (PS, S2O2 8 ) or peroxymonosulfate (PMS, SO5 ) (Lin et al., 2017; Xia et al., 2017; Yan et al., 2017). In recent years, it has become a hot area to generate sulfate radical for pollutant degradation in heterogeneous PS activation (Lin et al., 2015; Oh et al., 2016; Jawad et al., 2018), which also falls into the scope of this journal. Iron elements was regarded as one of the most effective elements for PS activation (Anipsitakis and Dionysiou, 2004). Thus, different Fe-based materials thus as Fe0 (Zou et al., 2014),Fe/S modified carbon nanotubes (Cheng et al., 2016) and FeeC (Li et al., 2018) were developed for PS activation. For heterogeneous PS activation, deactivation is an important issue for practical use of catalysts. Developing recyclable persulfate activators is one of challenges for current and future research. Herein, FeMoO4 could be synthesized in acid aqueous solution and keep stable in acid or alkaline water. Furthermore, the produced Fe(III)materials Fe2(MoO4)3 after initial PS activation could also keep the ability for PS activation (Tian et al., 2013; Lu et al., 2015) for pollutant degradation. Thus, FeMoO4 may be a good recyclable material to activate persulfate activator for organic pollutants degradation. This study therefore aims to investigate FeMoO4 to activate PS for pollutant removal. To the best of our knowledge, it is the first time to study recyclable FeMoO4 material to activate PS for pollutant removal in aqueous solutions. Herein, we provided a recyclable material FeMoO4 to activate persulfate for pollutant removal. Material characterization methods and classis quenching tests were used to uncover the mechanism. It is expected that this study would provide an environmental and efficient technology for wastewater treatment. 2. Materials and methods 2.1. Chemicals Ultrapure water was produced by a Millipore milli-Q system. FeSO4 $7 H2O, CH2Cl2, Na2MoO4$2H2O and HCl were obtained from Sinopharm Chemical Reagent Co., Ltd (Beijing, China). 5, 5dimethyl-1-pyrrolidine N-oxide (DMPO), persulfate (PS) and Orange G (OG) were purchased from Aladdin Chemistry Co., Ltd (Shanghai, China). All reagents were analytical grade at least and used without any further purification. 2.2. Preparation of FeMoO4 FeMoO4 used in this report was synthesized by a solvothermal method according to the literature (Zhang et al., 2011) with certain modification to improve the production of FeMoO4 by one time. Briefly, 3 mmol of FeSO4 $7 H2O and 3 mmol of Na2MoO4$2H2O were dissolved in 51 mL HCl solution whose pH was 2.00. Then, the solution was put into a Teflon-lined autoclave of 100 mL capacity. The autoclave was soon transferred into an oven of 180  C. After heating for 4 h, the product FeMoO4 was washed by deionized water and ethanol several times.

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2.4. Experiment procedure and analysis Batch tests were carried out in glass conical flask with 100 mL aqueous solution and shaken in a rotary shaker (ZHWY-20102C, Shanghai, China) at 180 rpm and 25 ± 0.5  C. Orange G (OG) and persulfate (PS) stock solution were added to DI water, then catalysts were added to initiate the reaction. The initial pH could be adjusted by NaOH (0.1 mM) or H2SO4 (0.1 mM) solution if necessary. A certain amount of solution sample was withdrawn at specified time intervals, then immediately an equal amount of ethanol was added for detection at a maximum absorption wavelength of 478 nm by UVeVis spectrophotometry method (Thermo Fisher, Evolution 201, USA). PS concentration was analysis by a rapid spectrophotometric method provided by Liang et al. (2008). In addition, details of GCMS and Electron Paramagnetic Resonance (EPR) tests could be found in Supporting information. 3. Results and discussion 3.1. Characterization of FeMoO4 The XRD patterns of FeMoO4 activators are presented in Fig. 1 (A). Almost all peaks could be related to b- FeMoO4 (JCPDS 22e0628) and two weak peaks (marked with clover symbol) could be related to a- FeMoO4 (JCPDS 22e1115). The XRD presented that the samples mainly comprised b- FeMoO4 phase with a little aFeMoO4 impurity. The SEM morphology of FeMoO4 is shown in Fig. 1 (B). Based on the SEM images, the catalyst has a fine and uniform rodlike shape. The rods possessed a length of 300e600 nm and a diameter of 50e200 nm. TEM images of fresh samples are shown in Fig. 1(C) and (D). Fig. 1 (C) and the inset presented that the surface of fresh samples was similar to the inner part. Fig. 1 (D) presents the high-resolution TEM (HRTEM) image gotten from the same rod and the corresponding fast Fourier transform (FFT) image in the inset. About 0.33 nm of d-spacing could be known with the help of Digital Micrograph, which agreed with the result of similar literature (Zhang et al., 2015). BrunauereEmmeteTeller (BET) measurement shown in Fig. 1(E) presented that the samples possessed a BET surface area of about 12.1 m2 g1. The inset shows a mesoporous size distribution of about 35.78 nm in diameter which was calculated by Barrett-Joyner-Halenda method (BJH). XPS technology was applied to tell the surface element species and their oxidation states. The survey spectrum in Fig. 2 (A) confirms that the samples comprised Fe, Mo, and O. The Fe2p, Mo2p and O1s XPS spectra are shown in Fig. 2(B)e(D) respectively to present the oxidant states of the main elements. The peak Fe2p3/ 2 at about 711.0 eV and Fe2p1/3 at about 724.2 eV indicated the existence of Fe2þ oxidation state, which confirmed that FeMoO4 containing Fe2þ was synthetized successfully (Zhang et al., 2015). Mo 3d5/2 peak at binding energies of about 232.5 eV and Mo 3d3/2 peak 235.6 eV suggest the present of Mo6þ oxidation state. Fig. 2 (D) shows that the O1s spectrum contains two child components. The primary one at peaks at 530.4 eV corresponded to the lattice oxygen, and the minor one at 532.1 eV may be due to the chemisorbed oxygen. The XPS results confirm the successful synthesis of FeMoO4, which agreed with the XRD results.

2.3. Characterization 3.2. Catalytic activity of FeMoO4 Technologies such as X-ray diffraction (XRD) (Bruker, German), scanning electron microscopy (SEM) (Quanta 200, Holland), BrunauerEmmettTeller (BET) (ASAP 2020; USA), Transmission Electron Microscope (TEM) (FEI Tecnai G2 f20 s-twin 200Kv, USA) and X-ray photoelectron spectroscopy spectrometer (XPS) (Thermo Scientific ESCALAB 250, USA) were used to characterize the materials.

Tests based on different systems as comparison for degradation of OG were carried out to tell the catalytic activity of FeMoO4. As shown in Fig. 3 (A), it can be observed that FeMoO4 showed excellent efficiency in activating PS for OG removal. More than 95% could be removed after 40 min when the initial concentration of OG, PS and catalyst were 0.2 mM, 4 mM and 0.3 g L1 respectively.

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Fig. 1. Characteristics of FeMoO4 material (A) XRD of fresh and used, (B) SEM, (C) TEM, (D)HRTEM, (E) Nitrogen absorption/desorption isotherms and pore size distribution curve (inset).

However, little OG could be removed in control test with only FeMoO4 or PS. For comparison, Fe2(MoO4)3 prepared according to the literate (Tian et al., 2011) was also used to activate PS for OG removal. It can be observed that less than 10% of OG was removed

in Fe2(MoO4)3/PS system. Thus, the activity of Fe2(MoO4)3/PS was obviously worse than that of FeMoO4/PS system. To test the effect of MoO2 4 , Na2MoO4 was used to activate PS. However, no removal of OG could be found.

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Fig. 2. XPS spectrum before and after 12 cycles' reaction. (A) Survey (B) Fe2p (C) Mo3d. (D) O1s spectrum. Reaction conditions: [OG]0 ¼ 0.2 mM, [PS]0 ¼ 4 mM, [FeMoO4]0 ¼ 0.3 g L1, pH0 2.80, 25  C.

Fig. 3. (A) OG degradation with different systems, (B) TOC removal of OG in the FeMoO4/PS system. Reaction conditions: [OG]0 ¼ 0.2 mM, [PS]0 ¼ 4 mM, [catalyst]0 ¼ 0.3 g L1, pH0 2.80, 25  C.

To further investigate the efficiency of FeMoO4/PS system, TOC analysis was used to identify the mineralization result after

degradation process. Result got from TOC analysis was shown in Fig. 3 (B). It can be observed that the process of mineralization was

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slower than the degradation of OG and about 43% TOC removal could be realized after 12 h of reaction, which probably due to that the amount of PS was not enough for a complete mineralization of pollutants and decreased PS would slowed down the rate of mineralization (Xu and Li, 2010; Rodriguez et al., 2014). This indicated most OG was decomposed to small intermediates (Rodriguez et al., 2014; Matzek and Carter, 2016). The occurrence of degradation intermediates would also slow down the TOC degradation process. To explain this phenomenon better, tests for investigating PS consumption were carried out. As shown in Fig. S2, more and more PS was consumed in the process. Thus, the produced radicals could keep mineralizing OG. However, almost all PS was nearly consumed after 12 h. Though the amount of PS was not enough to mineralize all 0.2 mM of OG, FeMoO4/PS system was proved to be able to mineralize OG. 3.3. Effect of parameters on catalytic activity of FeMoO4 It is necessary to investigate the effect of factors in the process of degradation for better application of the activator. The main

1.0

FeMoO4

1.0

0.1g/L 0.2g/L 0.3g/L 0.4g/L 0.5g/L 0.6g/L

0.8

0.8

0.6

C/C0

0.6

C/C0

parameters studied in this report were mainly about FeMoO4 concentration, pH and PS concentration. Fig. 4(A) illustrates the OG degradation with time in FeMoO4/PS system at different amount of FeMoO4 in solution. It can be observed that increasing FeMoO4 concentration from 0.1 g L1 to 0.5 g L1 made the rate of OG removal increase. More than 90% of OG could be removed in 60 min at 0.1 g L1 of FeMoO4. The similar removal could be achieved within 40, 30, 25 and 20 min when the amount of FeMoO4 was 0.2, 0.3, 0.4 and 0.5 g L1, respectively. In addition, more than 95% removal could be achieved within 60, 40, 30 and 25 min at loading of 0.2, 0.3, 0.4 and 0.5 g L1, respectively. This fact was due to that increased catalyst offered more active sites for reaction with PS to produce more reactive radicals. However, no obvious enhancement of efficiency was found when the amount of FeMoO4 was more than 0.5 g L1. Perhaps the dose of PS became the main rate-limiting factor on this condition (Matzek and Carter, 2016; Wang et al., 2018). The influence of initial pH was studied at different value of 2.80, 3.00, 3.50, 4.00, 5.00, 7.00, 9.00 and 11.00. From Fig. 4(B), it can be observed that degradation rate decreased obviously when the pH

0.4

0.4

0.2

0.2

0.0

0.0

0

10

20

30

40

50

pH

2.80 3.00 3.50 4.00 5.00 7.00 9.00 11.00 0

60

10

20

30

Time/min

Time/min

(A)

(B)

40

50

60

1.1 1.0 0.9

PS

0.8 0.7

C/C0

0.6 0.5

0mM 2mM 4mM 6mM 8mM

0.4 0.3 0.2 0.1 0.0

-0.1 0

10

20

30

40

50

60

time/min

(C) Fig. 4. OG degradation with (A) different amount of FeMoO4, (B) different pH values, (C)different amount of PS. Reaction conditions: [OG]0 ¼ 0.2 mM, [FeMoO4]0 ¼ 0.3 g L1, [PS] ¼ 4 mM, pH0 2.80, 25  C.

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increased. When the pH was lower than 3.00, all or almost all OG could be removed in 60 min. When the pH was increased to 5.00 or 7.00, the degradation rate in 60 min was decreased to only 19% or 16%. In addition, little OG could be removed when the pH was increased to 9.00 or 11.00. This was similar to the related articles about Fe-based catalyst activating persulfate (Zhu et al., 2013; Li et al., 2015; Yuan et al., 2015), which presented that increasing pH value had adverse effect on activation rate of many kinds of Febased catalysts. This phenomenon could be due to followed reasons: (I) precipitation of iron ions might lead to the occurrence of electronic barriers on the catalyst surface and then disturb the process of PS activation; (II) when the pH is low enough, the surface of catalyst would present negative charge, which make that the electrical interaction between PS and the materials would facilitate the contact of PS and the material (Li et al., 2017); (III) when the pH is too high, the accompanying generation of plentiful SO2 4 would
reduce the reactivity of SO4  and produced HO
(Eq. (1)) (Li et al., 2017). SO4






þ OH/ SO4

2

þ HO


(1)

PS concentration could also make important effect on the degradation of the pollutants. Because persulfate was one kind of acid oxidant which can decrease pH, different amount of PS would lead to different initial pH value. As showed in Fig. 4(C), we can know that initial pH has high impact on the efficiency. Therefore, pH was all adapted to 2.80 (chosen as the optimal value) by adding different amount of PS. It can be observed from Fig. 4(C) that no PS led to no OG removal. When the dose of PS increased from 2 mM to 4 mM, the time of 95% OG removal reduced from 60 min to 40 min. This may be due to the increased radicals produced by increased persulfate. However, more PS did not increase the efficiency but even decreased the degradation rate when it rose to 6 mM or 8 mM. The phenomenon may be due to that redundant PS became inhibitor of radicals as the reaction Eqs. (2) and (3) showed (Monteagudo et al., 2015). Hence, the available produced sulfate radicals decreased for pollutant removal. SO4






þ SO4

SO4






þ S2O8






/ S2O8

2

/ SO4

2 2

(2) þ S2O8






(3)

3.4. Stability of FeMoO4 catalyst It is important to investigate the stability and reusability of catalyst for heterogeneously activating PS. From the preparation method of FeMoO4, it could be known that FeMoO4 was stable in acid solution whose pH was even only 2.00. As the amount of FeMoO4 and PS was 0.3 g L1 and 4 mM respectively, about 2.9 mg L1 of total iron ions (about 0.967% of FeMoO4) was leached to water, which was much less than the traditional Fe(II) system (Xu and Li, 2010; Rodriguez et al., 2014). Homogeneous activation test of PS by leaching solution at pH 2.80 was carried out to tell the contribution of homogeneous activation. As Fig. 3 (A) showed, OG degradation could be found in the homogenous tests which presented that the homogeneous effect also made some contribution to degradation process. However, the effect was much less than heterogeneous effect. To further evaluate the stability and reusability, catalyst was filtrated, washed and collected after reaction. Then fresh solution containing OG and PS was added to initiate new cycle of reaction. From Fig. 5, it can be observed that the degradation rate decreased slowly as the number of cycles increased. More than 85% removal

Fig. 5. OG degradation in multi-cycle batch experiments. Reaction conditions: [OG]0 ¼ 0.2 mM, [PS]0 ¼ 4 mM, [FeMoO4]0 ¼ 0.3 g L1, pH0 2.80, 25  C.

could be still achieved in the tenth cycle. The decline of activity may be due to the loss and inactivation of activator in the cycle tests. Even though, the recyclability of FeMoO4 could be still at the head compared to other similar PS activators (Li et al., 2017; Liu et al., 2017). 3.5. Possible degradation pathway To understand the possible degradation pathway, GC-MS technique was applied to investigate the degradation intermediates. About 7 intermediates were found and listed in Table S1, including phenol, 8-Hydroxy-7-methoxycoumarin, fraxidin, dimethyl phthalate, diphenyl ether, 7-Hydroxucoumarin-3-carboxylic acid, 3-Hydroxycoumarin. Based on the detected intermediates, a plausible degradation pathway could be proposed in Fig. S3. The cleavage of azo bond led to the production of 1,3Naphtalenedisulfonicacid, 8-amino-7 hydroxy and aniline. However, they were not detected because they are highly soluble in water or too sensitive to radicals (Lin et al., 2014). Then, aniline could be transformed to phenol. Interestingly, diphenyl ether, whose molecular weight was more than phenol, was detected. To the best of our knowledge, it is the first time to detect diphenyl ether or its derivative in the process of azo dyes degradation by radicals. It indicated that phenoxyl radicals was produced to form diphenyl ether with the help of HO$ or SO$4 (Olmez-Hanci and Arslan-Alaton, 2013). The other initial intermediate 1,3Naphtalenedisulfonicacid, 8-amino-7 -hydroxy could degraded into naphthalene derivatives. In the meantime, some kinds of benzopyran derivatives were produced. Subsequent production of phthalic acid derivatives and other benzene-type intermediates may occur. Then rings could be open to produce short chain acids (such as oxalic, fumaric or maleic acid). Finally, mineralization could occur to produce CO2 and H2O. 3.6. Activation mechanism of PS on FeMoO4 It is necessary to discuss whether radicals were produced and what main radicals were produced in the process. Therefore, the EPR tests were carried out to detect radicals generated (Wang et al., 2015; Liu et al., 2017). As showed from Fig. 6 (A), the result clearly
showed the existence of both SO4 and HO
radicals in PS/FeMoO4
system compared to only PS. However, the signal of DMPO- SO4

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Fig. 6. (A) EPR spectra (B) Inhibiting tests of EtOH and TBA. Reaction conditions: [OG]0 ¼ 0.2 mM, [PS]0 ¼ 4 mM, [FeMoO4]0 ¼ 0.3 g L1, pH0 2.80, 25  C.

was much weaker than DMPO- HO
. It was caused by the trans
formation from not only SO4 to HO
but also DMPO-SO4 adduct to DMPO-OH adduct via nucleophilic substitution during the determination process as shown below (Timmins et al., 1999). 2 þ
SO$ 4 þ H2O / SO4 þ OH þ H

(6)

(4)

(5) To verify this result further and discuss what main radicals were produced in the process, quenching tests were carried out using ethyl alcohol (EtOH) and tert butyl alcohol as radical scavenger. The reasons of using them as radical scavenger have been reported in previous article (Lin et al., 2015). In short, EtOH is an effective
quencher for both SO4 (1.6e7.7$108 M1s1) and $OH 9 1 1 (1.2e2.8$10 M s ), while TBA is an effective quencher for $OH
(3.8e7.6$108 M1s1) but not for SO4 (4e9.1$105 M1s1). From Fig. 6 (B), it can be observed that the addition of TBA (0.1 M) led to 90% removal of OG which was only a little less than the removal rate of the control test without quenching agents. However, same amount of EtOH (0.1 M) led to a significantly lower degradation of
OG to only 21.5%. It indicated that both SO4 and HO
existed and sulfate radicals made main contribution to OG removal in the oxidation processes. The result above was similar to the previous reports (Han et al., 2014; Lin et al., 2015), which concluded that sulfate radicals were the primary species in the process of PS activation. To investigate the mechanism of FeMoO4/PS system better, technologies including XRD and XPS were applied to study the samples after reaction. It can be observed from Fig. 1 (A) that the positions of peaks had little change after three or twelve cycles though the peaks became a little weaker than the fresh ones. The difference among the three periods of reaction time indicated that the primary contents of materials had no changes. The stability of catalyst may be also due to the cycle of Fe3þ/Fe2þ on the surface by that Fe2þ could be generated from the reduction of Fe3þ by PS as the reaction (6) and (7) showed. However, as the trend of transition from Fe2þ to Fe3þ was certainly more obvious than the inverse trend, the activity of catalyst became weaker after excess cycles.

(7) To study the reaction mechanism of PS on catalyst surface, XPS analysis was used to investigate the surface element status. As the structure have no apparent change even after twelve cycles according to XRD spectrogram image, samples after reaction of 12 cycles was used for XPS analysis. As Fig. 2 (A) showed, main elements remain as same as fresh ones. XPS spectrogram of main elements including Fe, Mo and O was analyzed as Fig. 2(B)e(D) showed. It can be observed from Fig. 2 (B) that the Fe 2p3/2 and Fe 2p1/2 were shifted to 711.7 eV and 725.1 eV respectively, which indicated the formation of Fe3þ (Feng et al., 2016). Mo 3d and O 1s of used ones observed in Fig. 2(C) and (D) have little shift about the position of child components, which indicated the present of the primary oxidation state of Mo6þ and O2 element in MoO2 4 component. It confirmed that MoO2 4 component made no contribution to pollutant removal. According to the above analysis, it can be concluded that the mechanism of PS activation was involved in the conversion of Fe2þ/Fe3þ on the surface layer of samples, which was shown in Scheme 1 and explained by Eqs. (3), (4), (6)e(8):

(8)

4. Conclusions FeMoO4 was applied for the first time to activate PS for organic pollutant removal. This research aimed to know the basic performance of FeMoO4/PS system by laboratory experiment. The application of this system in the real world will be also our next plan. It is expected that FeMoO4/PS system could effectively applied in wastewater treatment, which will make contribution to cleaner production of water. In this research, complete or nearly complete removal of OG could be realized in 1 h by FeMoO4/PS system, which showed obvious better performance than other systems. Main

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Scheme 1. Proposed pathways for persulfate activation on FeMoO4 surface for radicals generation and pollutant degradation.

parameters including dose of FeMoO4, pH and PS were studied to get the optimal reaction condition. Increasing amount of FeMoO4 and PS in proper range would promote the removal rate, otherwise excess amount of them would not enhance but even decrease the degradation rate. In addition, increasing pH value would decrease the degradation rate obviously. Recycle tests presented that the catalyst possessed excellent stable performance even after ten cycles. GC-MS analysis was applied to detect the reaction intermediates and presented the possible degradation pathway. The Sulfate radicals and hydroxyl radicals all occurred and sulfate radicals were the primary ones in the process of activation by quenching studies and EPR result. XRD, SEM, TEM and XPS technologies were all applied to study the catalyst and implied that the activation mechanism was involved in the Fe2þ/Fe3þ cycle on the surface layer of FeMoO4. Acknowledgements This study was funded by National Science Foundation of China (Nos.31570568, 31670585), China Postdoctoral Science Foundation (2017M622711), Guangdong High Level Talent Project (No.201339), Science and Technology Planning Project of Guangzhou City, China (No. 201607010079, 201607020007), Science and Technology Planning Project of Guangdong Province, China (No. 2016A020221005). The authors are thankful to all the anonymous reviewers for their insightful comments and suggestions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2018.09.124. References Anipsitakis, G.P., Dionysiou, D.D., 2004. Radical generation by the interaction of transition metals with common oxidants. Environ. Sci. Technol. 38, 3705e3712. Aveiro, L.R., Da Silva, A.G.M., Candido, E.G., Antonin, V.S., Parreira, L.S., Papai, R., Gaubeur, I., Silva, F.L., Lanza, M.R.V., Camargo, P.H.C., Santos, M.C., 2018. Application and stability of cathodes with manganese dioxide nanoflowers supported on Vulcan by Fenton systems for the degradation of RB5 azo dye. Chemosphere 208, 131e138. Baena-Nogueras, R.M., Gonz alez-Mazo, E., Lara-Martín, P.A., 2017. Degradation kinetics of pharmaceuticals and personal care products in surface waters: photolysis vs biodegradation. Sci. Total Environ. 590e591, 643e654. Cheng, X., Guo, H., Zhang, Y., Liu, Y., Liu, H., Yang, Y., 2016. Oxidation of 2,4dichlorophenol by non-radical mechanism using persulfate activated by Fe/S modified carbon nanotubes. J. Colloid Interface Sci. 469, 277e286. Feng, Y., Wu, D., Deng, Y., Zhang, T., Shih, K., 2016. Sulfate radical-mediated degradation of sulfadiazine by CuFeO2 rhombohedral crystal-catalyzed

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