Activation of peroxymonosulfate by BiVO4 under visible light for degradation of Rhodamine B

Chemical Physics Letters 653 (2016) 101–107

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Research paper

Activation of peroxymonosulfate by BiVO4 under visible light for degradation of Rhodamine B Yang Liu a, Hongguang Guo a,b,⇑, Yongli Zhang a, Weihong Tang a, Xin Cheng a, Hongwei Liu a a b

College of Architecture and Environment, Sichuan University, Chengdu 610065, China National Engineering Laboratory for Clean Technology of Leather Manufacture, Sichuan University, Chengdu 610065, China

a r t i c l e

i n f o

Article history: Received 11 March 2016 Revised 14 April 2016 In final form 19 April 2016 Available online 20 April 2016 Keywords: Photocatalysis Visible light Peroxymonosulfate BiVO4 RhB

a b s t r a c t A photocatalytic system involving visible light and BiVO4 (Vis/BiVO4) in the presence of peroxymonosulfate (PMS) has been developed to oxidize the target pollutant Rhodamine B (RhB) in aqueous solution. It was found that PMS could enhance the photocatalytic efficiency of BiVO4 and could be activated to promote the removal of RhB with sulfate radicals, hydroxyl radicals and superoxide radicals. Critical impacting factors in the Vis/BiVO4/PMS system were investigated concerning the influence of PMS concentration, solution pH, catalyst dosage, initial concentration of RhB and the presence of anions (Cl
and CO2
3 ). In addition, by using isopropanol, tert-butanol, 1,4-benzoquinone and ethylenediamine tetraacetic  acid disodium salt as probe compounds, the main active species were demonstrated including SO
4 , OH in the system, and a detail photocatalytic mechanism for the Vis/BiVO /PMS system was proand O
2 4 posed. Finally, up to 10 intermediate products of RhB were identified by GC/MS, included benzenoid organic compounds, organic acids and three nitrogenous organic compounds. This study provides a feasible way to degrade organic pollutants in wastewater using BiVO4 with PMS under visible light. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction With the rapid urbanization and industrialization, water pollutions have received extensive attention, which present a tough challenge to environmental governance. Furthermore, these pollutants are difficult to dispose of by traditional technologies [1,2]. Over 1 million tons of organic dyes are produced annually, of which 50,000 tons are left in effluents during application, 5–15% of which are discharged into the environment [3]. Many methods are available to remove organic dyes from wastewater, including physical technology [4], biological technology [5], electrochemical technology [6] and chemical technology [7]. Recently, advanced oxidation technologies (AOTs) have been generally recognized as the most effective methods for wastewater treatment, for the ability to completely degrade a wide variety of organic pollutants using active oxygen species. In recent years, the sulfate radical (SO
4 ) based-advanced oxidation process (SR-AOP) has raised considerable attention in emerging compounds decontamination [8,9]. In this process, SO
4 can be produced by peroxymonosulfate (PMS) or persulfate (PS) under photochemical [10], thermolysis [11], transition metal (ions) [12,13] and chemical conditions [14]. With ⇑ Corresponding author at: College of Architecture and Environment, Sichuan University, Chengdu 610065, China. E-mail address: [email protected] (H. Guo). http://dx.doi.org/10.1016/j.cplett.2016.04.069 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.

a highly oxidizing performance of free radicals, SO
4 can react with many organic compounds through a one-electron transfer mechanism, and promoting the decarboxylation of carboxylic acids [15,16]. Moreover, SO
4 demonstrates a higher standard reduction potential (2.6–3.1 V) than hydroxyl radicals (OH, 1.8–2.7 V), and is more selective in an oxidation process than OH (non-specific oxidant) [17]. More recently, activation of PMS to degrade contaminants by photocatalysis under visible light has caught the attraction of researchers. Chen et al. have determined the degradation pathways of AO7 in a dye-sensitization TiO2 photocatalysis system using visible light irradiation coupled with PMS [18]. In the study by Chi’s group, they found that the degradation rate of RhB increased greatly in the BiFeO3/PMS/Vis system [2]. Compared with transition metal ion catalysts, the photocatalyst has advantages of good chemical stability, environmental friendliness and reusability. Bismuth vanadate (BiVO4, bandgap 2.3–2.5 eV) is an excellent visible light photocatalyst widely used for organic compounds removal in wastewater treatment [19]. It has the additional advantages of a narrow bandgap for visible light absorption, abundant availability, low cost and good stability [20]. However, up to now studies on photocatalyst oxidation of BiVO4, with the presence of PMS, have not been reported. This approach represents the first study of the photocatalytic degradation of RhB using BiVO4 with PMS under visible light

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irradiation. Critical impacting factors on the degradation of RhB were investigated in the Vis/BiVO4/PMS system. The reaction intermediates and products of RhB were identified by GC/MS, and the reaction mechanism concerning major active species are proposed. This study demonstrated an environment-economical photocatalysis method under visible light irradiation using BiVO4 and PMS for organic dyes degradation. 2. Materials and methods 2.1. Materials Peroxymonosulfate (2KHSO5KHSO4K2SO4, AR), methanol (MA, HPLC) were acquired from Sigma–Aldrich (Shanghai, China). BiVO4 (99.9%) was acquired from Alfa Aesar China Chemical Co. Ltd. (Shanghai, China). RhB (C28H31CiN2O3, AR), islpropanol (IPA), tertbutanol (TBA) and ethylenediamine tetraacetic acid disodium salt (EDTA-2Na) was obtained from Kelong Chemical Reagent Co. Ltd. (Chengdu, China). All other reagents were of analytical grade quality and were used without further purification. The aqueous solutions were prepared by using Milli-Q water (18.2 mX cm). 2.2. Experimental procedures The photocatalytic activity of the catalysts was evaluated on the degradation of RhB under visible light irradiation. Reactions were conducted in a cooling jacket of a reactor. A 400 W metal halide lamp with a cutoff filter (k > 415 nm) was used as the visible light source. In all experiments, BiVO4 was added to 200 mL RhB aqueous solution (10 mg/L), and the suspension pH was adjusted to a set value by 1.0 mol/L HCl and 1.0 mol/L NaOH. Prior to irradiation, the suspensions were vigorously stirred in the dark for 30 min to ensure the adsorption equilibrium of RhB. Then, the suspensions were exposed to visible light irradiation to initiate the reaction, and maintained at 25 °C. 4.0 mL of the suspension was collected at predetermined times and filtrated with glass fiber filters (Waterman, 0.22 um) to remove the BiVO4 particles.

mance for the BiVO4 sample with an absorption edge at 550 nm, indicating that the sample is potentially valuable for sunlightdriven applications. Moreover, the band gap energy of the BiVO4 photocatalyst is approximately 2.33 eV, which is consistent with the previous studies concerning the monoclinic BiVO4. The catalyst performance of different systems on RhB degradation are shown in Fig. 1(a). Control experiments showed that the removal of RhB in the BiVO4/PMS without visible light system was negligible. The PMS removed about 25% of RhB in the solution within 60 min due to being a type of chemical oxidant under the visible light. The low removal rate of RhB was primarily due to the light source used in the current study: this light source mimics the spectrum of solar light, and a cutoff filter was used to remove wavelengths greater than 415 nm, indicating that PMS could not degrade the RhB directly and could not be activated by visible light irradiation. In addition, photo energy or pure BiVO4 power could not improve its performance. However, the degradation of RhB was increased to 92.4% in the Vis/BiVO4/PMS system, which provides evidence of the synergistic effect between BiVO4 and PMS under the visible light irradiation. Fig. 1(b) shows the temporal evolution of the UV–vis spectrum of the RhB solution in the Vis/BiVO4/PMS system. The intensity of the absorption peak of RhB at 554 nm decreases with time went by, which indicates the destruction of chromophore structure of RhB. In addition, the concentration of the oxidant also plays an important role with the degradation of RhB in the Vis/BiVO4/PMS system. Fig. 1(c) shows the photodegradation of RhB as a function of irradiation time, for different amounts of PMS under visible-light irradiation. The removal rates of RhB are 46.8%, 84.7%, 92.4% and 98.7% for concentrations of PMS 0.2 mM, 0.5 mM, 1.0 mM and 2.0 mM, respectively. The low concentration of PMS corresponding to a low removal efficiency of RhB, may be because of the lack of radicals produced by PMS. According to the results, the kinetic behavior of the photocatalytic degradation of RhB can be fitted by a pseudo-first-order model (Eq. (1)), as shown in Fig. 1(d).


lnðC t =C 0 Þ ¼ kapp t 2.3. Analytical methods The concentration of RhB in the system was analyzed by measuring the absorbance at 554 nm with a MAPADA UV-1800PC spectrophotometer. UV–vis diffuse reflectance spectrum of the BiVO4 power was measured at the range of 300–800 nm using a UV–vis spectrophotometer equipped with an integration sphere (UV8000, Yuanxi, China). The amount of total organic carbon of the solution was analyzed with Elementar liquid TOC II. The pH value was established by a Leici PHS-25 pH meter. The degradation intermediates and products were analyzed by a GC/MS-QP2010 Plus Shimadzu Gas Chromatograph Mass Spectrometer. For GC/MS analysis, a 20 mL sample was extracted using 10 mL dichloromethane three times under acidic (pH < 3.0), neutral (pH 7.0) and alkalified (pH > 12.0) conditions, respectively. Then, the organic phase was dried with anhydrous sodium sulfate and concentrated to 5 mL. Finally, the prepared organic phase was filtered with the 0.22 lm polytetrafluoroethylene membranes and stocked in brown sample bottles. The working conditions followed an initial temperature of 40 °C held for 5 min, and then ramped to 60 °C at 5 °C/min, ramped to 280 °C at 10 °C/min and held 280 °C for 10 min. 3. Results and discussion 3.1. Photocatalytic performance The diffuse reflectance UV–vis spectra of BiVO4 is shown in Fig. S1, which demonstrated a good visible light absorption perfor-

ð1Þ

where kapp is a pseudo-first-order rate constant (min
1); C0 and Ct are the concentrations of RhB at time 0 and t, respectively. The rate constants for 0.2 mM, 0.5 mM, 1.0 mM, 2.0 mM PMS dosage were estimated to be 0.01, 0.03, 0.04, 0.07 min
1, respectively. 3.2. Effects of the catalyst dosage and RhB concentration The effect of catalyst dosage on reactivity was studied in Fig. 2 (a). With the amounts of catalyst dosage increasing, the initial degradation rate of RhB was increased, showing a positive effect on the degradation. However, the removal rate of RhB only increased 4.6% (from 92.4% to 97.0%) at the end of reaction, when the catalyst dosage was increased four-time (from 0.5 g/L to 2.0 g/L). This is because the superfluous catalyst would make the reaction solution turbid, and it would diminish the penetration of light [20]. Since a certain catalyst dosage could have a high degradation rate for RhB, 0.5 g/L dosage of catalyst was maintained in this study. Fig. 2(b) shows the effect of RhB concentration on the degradation, ranging from 2 mg/L to 20 mg/L. With the initial concentration of RhB increasing, the degradation was inhibited, and the removal rate of RhB was decreased from 100% to 75.7% within 60 min. The reason that the degradation efficiency decreased may be attributed to the competition effect for reactive radicals caused by a large number of by-products, and the decrease of the solution permeability for light absorption effect, which could directly influence the photo-electron transition [21,22].

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Fig. 1. (a) Photocatalytic removal of RhB with BiVO4 in the presence of PMS; (b) UV–vis absorbance curves of degraded RhB solutions with the Vis/BiVO4/PMS system, (c) effect of PMS dosages on degradation of RhB with PMS activated by BiVO4 under vis-light irradiation, and (d) pseudo-first-order kinetic fitting (a, b reaction condition: [RhB] = 10 mg/L, initial pH = 3.0, [PMS] = 1.0 mM, [BiVO4] = 0.5 g/L; c, d reaction conditions: [RhB] = 10 mg/L, initial pH = 3.0, [BiVO4] = 0.5 g/L).

Fig. 2. Effect of catalyst and RhB dosages on degradation of RhB with the Vis/BiVO4/PMS system (a): catalyst dosages ([PMS] = 1.0 mM, initial pH = 3.0, [RhB] = 10 mg/L); and (b): RhB dosages ([BiVO4] = 0.5 g/L, initial pH = 3.0, [PMS] = 1.0 mM).

3.3. Effect of initial pH value and TOC Previous studies have confirmed that the initial pH strongly influences the activation of PMS [23]. Thus, it was necessary to investigate the effects of pH on the degradation rate of RhB in the Vis/BiVO4/PMS system. The influence of the initial pH on RhB degradation was studied at five different pHs from 3.0 to 11.0. The results are shown in Fig. 3(a), where the color removal of RhB was 90.9% at pH 3.0 within 60 min. It indicated that the degradation rate was significantly affected by the solution pH and the solution pH remained stable all through the react time. At the neutral condition, the degradation rate becomes very slow with a final removal efficiency of 29.8%.When the oxidation was carried out in strong alkaline conditions (pH 11.0), the degradation rate of RhB

was improved to 56.5% after 60 min. The increased degradation efficiency might be ascribed to other active radicals produced in the reaction [24]. It is obvious that the value of kapp decreases from 0.040 min
1 to 0.012 min
1 with solution pH increased from 3.0 to 11.0. In the strong acidic conditions, BiVO4 is positively charged, 2
while the negative charged anions including HSO
5 and SO5 contribute the major components of PMS and RhB [25]. The increase of solution pH could raise the amounts of negative charge on the catalyst surface. A strong lower pH could enhance the interfacial electron transfer to HSO
5 and the associated charge separation in BiVO4, eventually increase the reaction rate of photocatalytic degradation [26]. However, with the increase of solution pH, the static interactions between BiVO4 and RhB or PMS might be inhibited, leading to a slower rate of reactive species production and dye

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Fig. 3. Effect of pH on degradation of RhB with the Vis/BiVO4/PMS system (a) plots of pseudo first-order rate constants for RhB degradation; and (b) TOC removal of RhB in different pH systems ([RhB] = 10 mg/L, [PMS] = 1.0 mM, [BiVO4] = 0.5 g/L).

degradation. In addition, self-dissociation of PMS through nonradical pathways is strengthened with the solution pH increases, which would also reduce the degradation efficiency of RhB [27,28]. Fig. 3(b) shows a comparative performance of the pH on the TOC removal in the Vis/BiVO4/PMS system. The results clearly indicates that the pH value has a great effect on the removal efficiency of TOC. The final TOC removal decreased from 45.8% to 14.8% when the solution pH increased from 3.0 to 7.0, while it increased from 14.8% to 32.4% when the solution pH increased from 7.0 to 11.0. On the one hand, previous studies reported that a PMS solution in neutral and alkaline conditions becomes unstable, and is prone to decompose to sulfate and oxygen gas [23], resulting in a drastic drop in degradation efficiency and TOC removal. Moreover, it is known 
that the standard reduction potential of HSO
5 and SO4 in alkaline conditions are lower than that in acid condition [29]. However, Guan  et al. have confirmed that more than 10% SO
4 converted to OH at pH > 9.3 [23], showing that the OH radical was the main oxidant in the alkaline condition. On the other hand, PMS could selfdissociate mainly through non-radical pathway with the increase of pH, which would impair the oxidizing capacity of PMS toward the probe contaminant, and reduce the degradation efficiency of target contaminant [30]. Furthermore, the decreased degradation efficiency and the TOC removal decrease might be attributed to the decreasing of the adsorption ability of the catalyst for RhB, because the higher pH could reduce the electrostatic attraction between the anionic dye molecules and catalyst surface [21]. 3.4. Effects of anions In the dyeing process, large amounts of inorganics have been utilized as additives or mordants, which has a significant influence on wastewater treatment. To investigate the effect of the ions on the degradation rate of RhB, experiments were performed in the range of 1–10 mM for Cl
and CO2
3 by adding KCl and Na2CO3 to the solutions. Fig. 4 presents the degradation of RhB in the Vis/ BiVO4/PMS system under anion activation conditions. The Fig. 4(a) shows that Cl
was a promoter of degradation of RhB at a higher concentration condition. However, it can be seen that the photocatalytic degradation efficiency of RhB decreased at Cl
concentrations from 1 mM to 5 mM. Previous studies have suggested that PMS and sulfate radicals can oxidize chloride ions into active chlorine species HOCl/Cl2 or chlorine radicals, and chloride ions may be involved in PMS decomposition reaction, either by non-radical pathways or sulfate radical-based pathways (Eqs. (2)– (10)) [31,32]. The deceased degradation of RhB at a low Cl
concentration may be attributed to the chlorine radicals production, which reduced the amounts of SO
4 . The improving effect at a higher concentration condition may be attributed to the produc-

tion of free available chlorine species (such as HClO or Cl2) [33]. In addition, the reaction between positive holes (h+) and Cl
, would also result in the promotion of RhB removal [34]. As a hole scavþ enger, Cl
could inhibit the back recombination of hVB with e
CB left (Eq. (10)), leading to the continuous formation of sulfate radicals caused by e
CB .


Cl þ HSO
5 ! SO2
4 þ HOCl


2Cl þ HSO
5 ! SO2
4 þ Cl2 þ H2 O 




 SO
4 þ Cl $ SO2
4 þ Cl 

ð2Þ ð3Þ ð4Þ



Cl þ Cl ! Cl2



Cl þ Cl !  Cl2



Cl2 þ  Cl2 ! Cl2 þ 2Cl

ð7Þ

Cl2 þ H2 O ! HClO þ HCl

ð8Þ







ð5Þ


ð6Þ










HClO ! Hþ þ ClO


2Cl þ

þ 2hVB

! Cl2

ð9Þ ð10Þ

A significantly inhibition on degradation of RhB was observed at dosages of 1–10 mM CO2
3 (shown in Fig. 4(b)). In the case of a system containing CO2
3 , the pH value of the solution remained at 11.0, and the sulfate radicals would decompose and transform to hydroxyl radicals rapidly (Eq. (11)). Thus, OH plays a major role in the Vis/BiVO4/PMS process in the alkaline solutions. According to published studies, CO2
is an effective sulfate radical (SO
3 4)  and hydroxyl radical ( OH) scavenger, which would have adverse effects on BiVO4/PMS under vis-light irradiation (Eqs. (12) and (13)) [35,36]. Also, the scavenging results indicated that reactive  radical species (SO
4 , OH) might dominate the RhB degradation. 

SO
4 þ OH
!  OH þ SO2
4



2

 SO
4 þ CO2
3 ! SO4 þ CO3



OH þ HCO
3 !  CO
3 þ H2 O

ð11Þ ð12Þ ð13Þ

3.5. Proposed photocatalytic reaction mechanism in the Vis/BiVO4/PMS system In order to determine the role of the active species for photocatalytic reaction in the Vis/BiVO4/PMS process, isopropanol (IPA), tert-butanol (TBA) 1,4-benzoquinone (BQ) and EDTA-2Na were  selected as probe chemicals. IPA can react with SO
4 and OH at significant rates [37,38], while TBA is an effective scavenger for only  OH [39,40]. As shown in Fig. 5, in the presence of 0.3 M IPA or TBA, clear inhibition of RhB degradation was observed, implying  that SO
4 and OH were both the main active species. When 3 mM BQ was added into the reaction solution, as an O
2 scavenger

Y. Liu et al. / Chemical Physics Letters 653 (2016) 101–107

105

Fig. 4. Effect of ion concentration on degradation of RhB with the Vis/BiVO4/PMS system (a) Cl
; (b) CO2
3 ([RhB] = 10 mg/L, [PMS] = 1 mM, [BiVO4] = 0.5 g/L, initial pH: a-3.0, b-11.0).

[21,41], a strong inhibiting effect was observed, which suggests that the O
2 is another main active specie controlling the oxidation reaction. The generated O
2 would further interact with H2O to produce more OH radicals. Otherwise, according to the previous þ study by Dalrymple, hVB existing in the photocatalytic reaction could also be oxidized to produce OH. In addition, PMS could react þ directly with the electron holes (hVB ) to produce  SO
5 , which would
increase the production of eCB , following the formation of SO
4 and 
O2 through the reactions. Therefore, in the presence of 3 mM EDTA-2Na (a kind of hole scavenger [41]), obvious inhibition of RhB degradation was observed. Based on these results, the activation mechanism of PMS by BiVO4 catalyst is proposed (Eqs. (14)– (23)). Based on the above results, the mechanism of the degradation of RhB concerning the interaction between BiVO4 and PMS under vis-light irradiation was proposed in Scheme 1. þ

BiVO4 þ hv ! hVB þ e
CB

ð14Þ


O2 

ð15Þ

4h þ 2H2 O
! 4Hþ þ  OH

ð17Þ

HSO
5 þ  O
2 !  SO
4 þ HO
2

ð18Þ

O2 þ 

e
CB

!

O
2 þ H2 O ! OH þ OH
þ

ð16Þ

HSO
5 þ e
CB !  SO
4 þ OH


ð19Þ

 HSO
5 þ e
CB ! SO2
4 þ OH þ HSO
5 þ hVB !  SO
5 þ Hþ 2 SO
5 ! 2 SO
4 þ O2  SO
4 ð O
2 ;  OHÞ þ Dye ! Products

ð20Þ ð21Þ ð22Þ ð23Þ

Fig. 5. Inhibiting effect of quenching agents on RhB degradation with the Vis/ BiVO4/PMS system ([BiVO4] = 0.5 g/L, [PMS] = 1.0 mM, [RhB] = 10 mg/L, pH = 3.0; [IPA] = 0.3 M, [TBA] = 0.3 M, [BQ] = 3 mM, [EDTA-2Na] = 3 mM).

Scheme 1. Mechanism for RhB in the Vis/BiVO4/PMS system.

3.6. Degradation products analysis of RhB To determine the degradation products, GC/MS analysis of the sample has been performed at 30 min and the end of complete degradation of RhB (60 min). The reaction intermediates and products were identified with the results presented in Table 1. RhB was found to be stable in the absence of BiVO4 under visible light irradiation, but degradation easily occurs in the presence of 
photo-generating active species (OH, O
2 and SO4 ), which could attack the central carbon of RhB to decolorize the dye solutions and further degrade it via N-de-ethylation [42]. After 30 min of irradiation, formamid, methylbenzene, M-xylene ethylbenzene and Methyl 4,4-bis (4-hydroxyphenyl) pentanoate were detected by GC/MS. After 60 min of irradiation, more products including hydroxymalonic acid, butanedioic acid and three nitrogenous organic compounds (N-methylmaleimide, N-ethylmaleimide, N, N-diethylaniline) were identified. Some of the intermediates were in accordance with other studies for RhB oxidation [43–45]. It was easily found that the amounts of benzenoid intermediates were decreased after 30 min later, leading to the small molecular intermediates formation through the cleavage of the xanthene ring in maternal RhB and N-de-ethylated intermediates. Similar to the published papers, in this work, both the destruction of the conjugated p structure and de-ethylation were observed [21,43]. The oxidized products would be converted into smaller organic species + and ultimately be mineralized to H2O, CO2, NO
3 and NH4.

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Y. Liu et al. / Chemical Physics Letters 653 (2016) 101–107

Table 1 Reaction intermediates for the degradation of RhB in the Vis/BiVO4/PMS system identified by GC/MS analysis. Reaction (time/min) 30

60

Number

Retention (time/min)

Intermediates identified by MS

1

5.02

Formamid

2

5.48

Methylbenzene

3

10.40

M-xylene

4

22.45

Ethylbenzene

5

34.32

Methyl 4,4-bis (4-hydroxyphenyl) pentanoate

6

33.45

Hydroxymalonic acid

7

35.342

Butanedioic acid

8

37.20

N-methylmaleimide

9

38.31

N-ethylmaleimide

10

39.54

N,N-diethylaniline

Therefore, it was demonstrated that chromophore cleavage, ring opening and mineralization were the main degradation pathways for RhB in Vis/BiVO4/PMS system.

4. Conclusions In this study, effective photocatalysis process was proposed using BiVO4 and PMS under visible light irradiation for the degradation of RhB. It was confirmed that PMS could be used as an efficient oxidant to accelerate the Vis/BiVO4 system. The results revealed that the higher PMS concentration and BiVO4 dosage could accelerate the reaction rate and the pH 3.0 was found to be most suitable for PMS activation. However, the reaction rate would decrease with the RhB concentration increased. Furthermore, Cl
with a high concentration ([Cl
] > 10 mM) was proved as a promoter, while inhibition was observed for lower concentration ([Cl
] < 10 mM). Despite that, bicarbonate was an inhibitor on degradation of RhB in the Vis/BiVO4/PMS system. It was demonstrated that the main active species in the Vis/BiVO4/PMS system  
included SO
4 , OH and O2 . In addition, with 10 products identified, the detail degradation mechanism of RhB was proposed. The feasibility of RhB degradation using Vis/BiVO4/PMS system has been demonstrated, while the material optimization of m-BiVO4 and the toxicity for the degradation intermediates could be further studied in the future.

Structural formula

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NO. 51508354) and the Sichuan Provincial Environmental Protection Office (NO. 2013HB08). The authors are thankful to all the anonymous reviewers for their insightful comments and suggestions.

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