Catalytic degradation of ciprofloxacin by a visible-light-assisted peroxymonosulfate activation system: Performance and mechanism

Journal Pre-proof Catalytic degradation of ciprofloxacin by a visible-light-assisted peroxymonosulfate activation system: Performance and mechanism Fei Chen, Gui-Xiang Huang, Fu-Bing Yao, Qi Yang, Yu-Ming Zheng, Quan-Bao Zhao, Han-Qing Yu PII:

S0043-1354(20)30095-6

DOI:

https://doi.org/10.1016/j.watres.2020.115559

Reference:

WR 115559

To appear in:

Water Research

Received Date: 6 December 2019 Revised Date:

21 January 2020

Accepted Date: 26 January 2020

Please cite this article as: Chen, F., Huang, G.-X., Yao, F.-B., Yang, Q., Zheng, Y.-M., Zhao, Q.B., Yu, H.-Q., Catalytic degradation of ciprofloxacin by a visible-light-assisted peroxymonosulfate activation system: Performance and mechanism, Water Research (2020), doi: https://doi.org/10.1016/ j.watres.2020.115559. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Graphical Abstract

Catalytic degradation of ciprofloxacin by a visible-light-assisted peroxymonosulfate activation system: Performance and mechanism

Fei Chena, Gui-Xiang Huanga, Fu-Bing Yaob,c, Qi Yangb,c, Yu-Ming Zhengd, Quan-Bao Zhaod, Han-Qing Yua, d,* a

CAS Key Laboratory of Urban Pollutant Conversion, Department of Applied Chemistry, University of Science & Technology of China, Hefei, China b

c

College of Environmental Science and Engineering, Hunan University, Changsha 410082, China

Key Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of Education, Changsha 410082, China d

CAS Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China

*Corresponding author: Prof. Han-Qing Yu, Fax: 86-551-63607592, E-mail: [email protected]

1

1

Abstract

2

Peroxymonosulfate (PMS) is extensively used as an oxidant to develop the sulfate radical-based

3

advanced oxidation processes in the decontamination of organic pollutants and various PMS

4

activation methods have been explored. Visible-light-assisted PMS activation to construct a

5

Fenton-like process has shown a great potential for pollution control. In our work, BiVO4 nanosheets

6

were prepared using a hydrothermal process and used to activate PMS under visible light. A rapid

7

degradation of ciprofloxacin (CIP) was achieved by dosing PMS (0.96 g/L), BiVO4 (0.32 g/L) under

8

visible light with a reaction rate constant of 77.72-fold higher than that in the BiVO4/visible light

9

process. The electron spin resonance and free radical quenching experiments indicate that reactive

10

species of •O2-, h+, •OH and SO4•− all worked, where h+, •OH and SO4•− were found as the dominant

11

contributors to the CIP degradation. The spectroscopic analyses further demonstrate that the

12

photoinduced electrons were directly involved in the PMS activation process. The generated •O2- was

13

partially utilized to activate PMS and more •OH was produced because of the chain reactions

14

between SO4•− and H2O/OH-. In this process, PMS acted as an electron acceptor to transfer the

15

photo-induced charges from the conduction band of BiVO4 and PMS was successfully activated to

16

yield the high-powered oxidative species. From the degradation intermediates of CIP detected by a

17

liquid-chromatography-mass spectrometer, the possible degradation pathways were proposed. The

18

substantially decreased toxicity of CIP after the reaction was also observed. This work might provide

19

new insights into the visible-light-assisted PMS activation mechanisms and is useful to construct

20

environmentally-friendly catalytic processes for the efficient degradation of organic pollutants.

21 22

Key words: Peroxymonosulfate; visible light; BiVO4; catalytic degradation; water treatment;

23

mechanism.

2

24

1. Introduction

25 26

The ever-growing pollution caused by refractory organics in water has aroused widespread concerns

27

because of their high-toxicity, non-biodegradability and potential carcinogenic properties (Ding et al.

28

2018, Liu et al. 2017, Wei et al. 2019, Wu et al. 2019, Yan et al. 2018, Yao et al. 2019). Conventional

29

technologies such as adsorption (Sabio et al. 2006) and biodegradation (Zhou et al. 2015) suffer from

30

problems like secondary pollution, long periodicity and poor stability in the pollutant degradation.

31

Thus, more effective degradation methods are highly desired. Recently, sulfate radical (SO• )-based

32

advanced oxidation processes (SR-AOPs), which are involved with highly reactive oxygen species

33

generation have been successfully applied in the decontamination of organic pollutants in water and

34

wastewater (Ahmad et al. 2013, Ahn et al. 2016, Anipsitakis and Dionysiou 2003; 2004; Anipsitakis

35

et al. 2006, Chen et al. 2018a; 2019a). Compared with •OH in the conventional Fe2+/H2O2 Fenton

36

system, SO•

37

al. 2018b; 2019b). Besides, SO•

38

(vs 2-5 for •OH) (Darsinou et al. 2015, Du et al. 2018), higher oxidation selectivity of pollutants and

39

longer lifetime in reaction system (30-40 µs vs 10-3 µs for •OH) (Duan et al. 2015; 2018). SO•

40

usually generated through the activation of peroxymonosulfate (PMS) or persulfate (PS) with the

41

auxiliary methods of thermolysis, photochemical methods, transition metal ions or metallic oxidant

42

and carbon materials (Cheng et al. 2019, Duan et al. 2014, Lu et al. 2019, Qin et al. 2018). However,

43

higher energy consumption, metal ion release, secondary pollution and low degree of persulfate

44

activation stand for in these activation approaches.

has a higher standard reduction potential of 2.5-3.1 V (vs 1.8-2.7 V for •OH) (Chen et also possesses other advantages such as pH independence at 2-10

is

45

Among these approaches, PMS or PS activation assisted by photocatalyst under light irradiation

46

is sustainable and easy to operate without additional energy or extrinsic chemical consumption (Fang

47

et al. 2017, Ghauch et al. 2017). In fact, PMS itself possesses an asymmetric structure (HO-O-SO3-

48

vs

3OS-O-SO3

for PS(Li et al. 2018c)) and a longer superoxide O-O bond (IO-O=1.326 Å vs 3

49

IO-O=1.322 Å for PS (Zhu et al. 2018)). As a result, PMS is more easily activated and exhibits better

50

application potentials than PS. In conventional photocatalysis systems, the photoinduced

51

electron/hole pairs usually recombine rapidly, resulting in a lower efficiency for pollutant

52

degradation. PMS can serve as an electron acceptor and is activated to boost SO•

53

photogenerated electrons from photocatalysts could be utilized by the additive oxidant PMS, not only

54

the charge separation is drastically enhanced, but also the pollutant degradation performance could

55

be significantly improved. In this case, the synergistic effect will be achieved in the photo-assisted

56

PMS activation process.

and •OH. If the

57

In previous studies, photo-assisted PMS activation for pollutant degradation has been

58

demonstrated to be feasible. For example, Sr2CoFeO6 double perovskite oxide was prepared to

59

activate PMS under UV light and bisphenol F was completely degraded within 90 min (Hammouda

60

et al. 2018). PMS was efficiently activated by TiO2/FeIIFe2IIIO4@C under UV light and the removal

61

rate for different oxidants followed the order: PMS>PS>H2O2 (Jorfi et al. 2017). Nevertheless, the

62

photo-assisted PMS activation by Fe-based catalysts and UV light are limited by the metal ion

63

release and high energy consumption (Yang et al. 2017, Zhang et al. 2018b, Zhu et al. 2016). To

64

resolve these problems, chemically stable and visible-light-response materials should be considered.

65

Recently, BiVO4 with a narrow bandgap (~2.4 eV) has aroused intense concerns because of its low

66

costs, high stability, broad-spectrum light absorption, relatively efficient photo-induced electron-hole

67

pairs generation and separation under visible light (Chen et al. 2016; 2017, Li et al. 2017; 2018b).

68

However, the photocatalytic performance of single BiVO4 is not satisfactory yet due to the sluggish

69

exciton dissociation, rapid electron-hole pairs recombination and slow charge transfer kinetics (Zhu

70

et al. 2017). Inspired by above analyses, introducing PMS into the BiVO4/vis system might obtain

71

the mutual benefits. It is assumed that the photo-induced charges will be rapidly transferred and PMS

72

is activated to boost the high-oxidative SO•

73

pollutant degradation. However, in such a system, the internal electron transfer and intrinsic PMS

under visible light irradiation, further to accelerate the

4

74

activation mechanisms on photocatalytic oxidation of the BiVO4/PMS/vis system remain unclear.

75

Therefore, in this work, BiVO4 nanosheets were firstly prepared using a hydrothermal process

76

and then used to activate PMS under visible light. The catalytic performance of the BiVO4/PMS/vis

77

system was evaluated by degrading a target antibiotic ciprofloxacin (CIP). The impacts of operating

78

parameters such as catalyst dosage, PMS amount, initial pH value, CIP concentration and the

79

common coexisting anions on the CIP degradation were also examined. The possible PMS activation

80

mechanism was explored on the basis of photoelectric tests, radical quenching experiments and

81

electron spin resonance (ESR) measurements. In this way, a visible-light-assisted SR-AOP system

82

was constructed for efficient pollutant degradation.

83 84

2. Materials and methods

85 86

2.1 Chemicals

87

Ammonium metavanadate (NH4VO3, ≥99.0%), bismuth nitrate hydrate (Bi(NO3)3•5H2O,

88

≥99.0%), titanium dioxide, (anatase, TiO2, ≥99.0%), sodium persulfate (Na2S2O8, PS, ≥99.0%),

89

peroxymonosulfate (2KHSO5•KHSO4•K2SO4, PMS, ≥95.0%), sodium dodecyl benzene sulfonate

90

(C18H29NaO3S, SDBS, ≥88.0%), rhodamine B (RhB, 97.0%~99.0%), methyl blue (MB, ≥99.0%),

91

bisphenol A (BPA, ≥99.0%), tetracycline hydrochloride (TC, ≥99.0%), ciprofloxacin hydrochloride

92

(CIP, ≥98.0%), sodium hydroxide (NaOH, ≥99.0%), tert-butyl alcohol (C4H10O, TBA, ≥99.0%),

93

disodium ethylenediaminetetraacetate (EDTA-2Na, ≥99.0%), benzoquinone (BQ, ≥99.0%), methanol

94

(CH3OH, ≥99.0%), ethanol (CH3CH2OH, ≥99.7%) and nitric acid (HNO3, 65.0%~68.0%) were

95

purchased from Sinopharm Chemical Reagent Co., China or Shanghai Reagents Co., China. The

96

reagents were used as obtained without further treatment and all solutions were prepared with

97

Milli-Q water with a resistivity of 18.25 MΩ/cm.

98 5

99

2.2 Preparation of BiVO4 nanosheets

100

The BiVO4 nanosheets were synthesized according to a previous report with a slight

101

modification (Zhang et al. 2006). Typically, the same molar stoichiometric ratios of

102

Bi(NO3)3•5H2O/NH4VO3 (5 mmol/5 mmol) were respectively dissolved into HNO3 (4 M, 20 mL)

103

and NaOH (2 M, 10 mL). The two solutions were then mixed together and the pH value was adjusted

104

to 7.0 using 2 M HNO3 or 1 M NaOH. After constantly stirred for an additional 60 min at room

105

temperature, the resultant mixture was subsequently transferred into a Teflon-lined stainless steel

106

autoclave (100 mL) and put in an oven at 200 ºC for 0.5 h. Finally, the vivid BiVO4 product was

107

obtained by filtration, washing, and drying.

108 109

2.3 Characterizations

110

The X-ray powder diffraction spectrum of BiVO4 was recorded from a Rigaku D/max2500v/pc

111

X-ray diffractometer (XRD) with Cu Kα X-ray irradiation at 40 kV and 40 mA. The field emission

112

scanning electron microscope (FESEM, Hitachi S-4800) and transmission electron microscopy

113

(TEM) with a FEI Tecnai G20 were used to analyze the surface morphology. UV-vis diffuse

114

reflectance spectrum (UV-vis DRS) of BiVO4 nanosheets was measured by a UV-4100 spectrometer

115

at the range of 200-800 nm. The valence states of constituent elements were characterized by X-ray

116

photoelectron spectroscopy (XPS) using a Thermo ESCALAB 250XI spectrometer. The electron

117

spin resonance (ESR) was investigated by a Bruker ER200-SRC spectrometer. The total organic

118

carbon (TOC) measurement was carried out on a Shimadzu TOC-VCPH analyzer. A CHI660C

119

electrochemical workstation equipped with a standard three-electrode system (a saturated Ag/AgCl

120

electrode as reference electrode, a platinum wire as counter electrode and the BiVO4 sample coated

121

FTO glass as working electrode) was adopted to characterize the photoelectric properties of BiVO4.

122

The photoelectrochemical tests included the transient photocurrent responses and electrochemical

123

impedance spectrum (EIS) and Na2SO4 (0.25 M) and visible light (>420 nm) were respectively used 6

124

as the electrolyte and light source.

125 126

2.4 Photocatalytic activity tests

127

The catalytic oxidation of ciprofloxacin in the BiVO4/PMS/vis system was carried out in a 100

128

mL reactor containing 10 mg/L of CIP solution at (25 ºC) with circulating water system and a 300 W

129

Xe lamp (λ>420 nm) was served as the light source. First, a certain amount of prepared BiVO4 was

130

added into the 60-mL CIP solution and the mixed system was constantly stirred in dark to achieve the

131

adsorption-desorption equilibrium. The prepared PMS was dosed and the resultant mixture was

132

immediately exposed to visible light. At designed intervals, 1 mL aqueous sample was withdrawn,

133

rapidly mixed with 1.0 mL of ethanol to quench the reaction and centrifuged at 10000 rpm for 5 min.

134

The residual concentration of CIP was analyzed using a high-performance liquid chromatography,

135

(HPLC, 6460, Agilent Inc.) with a Kromasil C18 Column (4.6×250 mm, 5 µm). The mobile phase

136

consisted of acetonitrile and 0.1% formic acid with a ratio of 20: 80 and the flow rate was 1 mL/min.

137

The degradation products of CIP were identified by a liquid chromatography-mass spectrometry

138

(LC-MS) system equipped with a 6460 HPLC (Agilent Inc.) and an API 3000 mass analyzer. The

139

linear gradient elution of HPLC was changed from 93% A (0.03% (v: v) formic acid solvent) and 7%

140

B (acetonitrile) mobile phase to 50% A and 50% B within 25 min, then returned to the initial

141

conditions within additional 25 min. The pseudo-first-order model was utilized to calculate the

142

catalytic reaction rates as follows (Eqs. 1 and 2) (Zhu et al. 2018): r=− ln

d =− d



= −

= −

(1) (2)

143

where Ct is the residual CIP concentration at the reaction time of t min (mg·L-1), C0 is the initial CIP

144

concentration (mg·L-1) and kapp is the pseudo-first-order rate constant (min-1).

145 7

146

3. Results and discussion

147 148

3.1 Characteristics of BiVO4 nanosheets

149

As shown in Fig. 1a, all diffraction peaks in the XRD pattern of the BiVO4 sample were well

150

indexed to the monoclinic phase of BiVO4 (JCPDS card NO. 14-0688 (Gao et al. 2017, Li et al.

151

2013)). From Fig. 1b, the absorption edge of BiVO4 was found at approximately 529 nm and the

152

band gap energy was estimated to be 2.34 eV based on the empirical equation (Eg=1240/λ, where λ is

153

the absorption edge and Eg is the band gap energy(Li et al. 2018a)). Thus, the BiVO4 nanosheets, as

154

an efficient visible-light-driven photocatalyst, could be excited to generate electrons and holes under

155

visible light irradiation. As illustrated in Fig.2a, the BiVO4 exhibited a typical nanosheet-like

156

structure with a size range of 20 nm to 1 µm. The TEM images in Fig. 2b and c show that the slices

157

were stacked on the top of each other. Besides, the TEM-based energy-dispersive X-ray spectroscopy

158

images (TEM-EDS mapping, Fig. 2d-g) also illustrate that Bi, O and V elements were uniformly

159

distributed in the entire BiVO4 sample. These results demonstrate that BiVO4 nanosheets were

160

successfully prepared as designed.

161 162

3.2 Overall CIP degradation in the BiVO4/PMS/vis system

163

Fig. 3a illustrates the variations of CIP concentration (Ct/C0) with the light irradiation time in

164

the PMS/dark, TiO2/PMS/dark, BiVO4/PMS/dark, PMS/vis, TiO2/vis, BiVO4/vis, TiO2/PMS/vis and

165

BiVO4/PMS/vis systems. The removal of CIP in all the catalytic systems without visible light was

166

negligible. Commercial TiO2 (a typical photocatalyst) was also chosen to examine whether it could

167

activate PMS. Under visible light, BiVO4 or TiO2 alone showed almost no CIP degradation capacity

168

and the rapid photo-induced electron-hole pairs recombination might be the main reason. After the

169

introduction of PMS, the constructed TiO2/PMS/vis and BiVO4/PMS/vis systems both displayed

170

significantly improved CIP removal capacities. Especially for the BiVO4/PMS/vis system, 94.38% of 8

171

CIP was decomposed, indicating that the stronger visible light absorption of BiVO4 was beneficial to

172

the PMS activation. The PMS/vis system also exhibited a CIP removal of 11.32%, suggesting that

173

PMS itself could be activated at a lower efficiency under visible light. Similar to PMS activation (Fig.

174

3b), dosing PS into the TiO2/vis and BiVO4/vis systems also considerably increased the CIP

175

degradation efficiency, in which the CIP removal efficiencies of 24.25% and 54.60% were

176

respectively achieved. These results indicate that either PMS or PS could be effectively activated in

177

the presence of BiVO4 nanosheets and visible light and the rapid recombination of photo-generated

178

charges in single photocatalysis was effectively reduced.

179

The correlation coefficients R2 in the all kinetic curves (Table S1) exceeded 98%, suggesting

180

that all the CIP degradation under visible light could be well described by the pseudo-first-order

181

model (Fig. 3c, d). The kapp values for CIP degradation in the BiVO4/PS/vis and BiVO4/PMS/vis

182

systems were 0.0208 and 0.0765 min-1, respectively, which was higher than that of the BiVO4 system

183

by 21.13- and 77.72-fold. Thus, the developed BiVO4/PMS/vis and BiVO4/PS/vis systems could

184

effectively degrade CIP. Furthermore, different targets such as RhB, MB, BPA, and TC were also

185

effectively degraded in the BiVO4/PMS/vis system and the relative results are provided in

186

Supplementary material (Figs. S1 and S2).

187 188

3.3 Effects of catalyst dosage, PMS concentration and initial reaction pH on CIP degradation

189

The effect of BiVO4 dosage on the BiVO4/PMS/vis was investigated (Fig. 4a). An increase in

190

catalyst dosage from 0.08 g/L to 0.32 g/L firstly resulted in a substantial elevation of CIP degradation

191

from 69.14% to 94.38%. This was attributed mainly to the increased photoinduced charges for PMS

192

activation to produce more reactive oxygen groups. However, after a further increase in BiVO4

193

dosage (>0.32 g/L), no obvious improvement was observed and even decreased slightly. This was

194

due to the increased turbidity of the reaction solution and the reduced light penetration in the

195

solution. 9

196

Generally, the amount of generated •OH and SO4•− directly relates to the PMS concentration. As

197

shown in Fig. 4b, the CIP decomposition rate was drastically accelerated when increasing the PMS

198

concentration from 0.08 to 0.96 g/L. However, when the PMS dosage was increased from 0.96 to

199

1.28 g/L, a slight decline in the CIP degradation after 40 min was noticed. This result might be

200

caused by two reasons: first, the appropriate PMS could serve as the electron acceptors to improve

201

the catalytic process while the photo-generated electrons might not be sufficient to activate the

202

redundant PMS. Secondly, the side reactions listed in Eq. 3 might also proceed between the

203

excessive PMS itself and SO•

204

which decreased the CIP degradation efficiency. Thus, considering the trade-off between costs and

205

CIP degradation performance, the BiVO4 nanosheets dosage of 0.32 g/L in the presence of 0.96 g/L

206

PMS addition was selected for the subsequent CIP degradation tests. SO• + HSO → SO• + SO

radical to boost the lower oxidative ability of SO5•−(Cai et al. 2016),

(3)

+ H /OH

207

Initial reaction pH is a crucial parameter in Fenton or Fenton-like reactions, thus, it is necessary

208

to investigate the effect of pH on the CIP degradation in the BiVO4/PMS/vis system. As shown in Fig.

209

4c, the CIP removal increased with the increased pH value from 3.56 to 11.38 and the differences

210

among the CIP degradation efficiencies in the pH range from 3.56 to 9.56 were not obvious.

211

According to previous studies, the dissociation constant (pKa) value of PMS was estimated to be 9.4,

212

indicating that HSO5- is the major form of PMS when the reaction solution pH is below 9.4. The

213

affinity between catalyst surface and PMS in acid solution was reinforced but the stabilization of H+

214

on HSO5- would also have an adverse effect (Huang et al. 2017). When the initial pH reached 11.38,

215

the CIP removal capacity was slightly improved, because the role of alkaline might work in the PMS

216

activation process. As a result, the high CIP removals of the BiVO4/PMS/vis system under acidic,

217

neutral and alkaline conditions were achieved, demonstrating its application potentials for

218

wastewater treatment in wide pH ranges.

219 10

220

3.4 Effects of initial CIP concentration and coexisting anions on CIP degradation

221

To evaluate the impact of CIP concentration on its degradation in the BiVO4/PMS/vis system, a

222

set of batch experiments with a CIP concentration of 10, 20, 30 and 40 mg/L was carried out (Fig.

223

4d). As the CIP concentration increased from 10 mg/L to 40 mg/L, the CIP degradation efficiency

224

declined from 94.38% to 60.91%. The enhanced competitive effect for the limited reactive oxygen

225

species between CIP molecules and the deuterogenic by-products might partially have adverse

226

effects on CIP degradation. Even so, the actual degradation rates were respectively estimated to be

227

0.001 (10 mg/L), 0.0018 (20 mg/L), 0.0023 (30 mg/L) and 0.0027 mmol/L/min (40 mg/L), which

228

increased with the increasing initial CIP concentration. These results indicate that the adsorptive or

229

active catalytic sites were efficiently utilized under high-pollutant-concentration conditions.

230

The inorganic anions are naturally present in actual wastewaters and have pronounced

231

influences on AOPs. The anions in the BiVO4/PMS/vis system might quickly react with the

232

generated radicals and accordingly affect the CIP degradation efficiency. Fig. S3 shows the effects of

233

four common anions Cl-, SO42-, NO3- and CO32- at 10 mM on the BiVO4/PMS/vis system. The

234

introduction of SO42- posed a negligible effect, while the inhibitory effects on the CIP degradation

235

were observed in the presence of the other three anions, which followed the order of SO42-
236

NO3-
237

reason, which resulted in the generation of the radicals with a lower oxidation capacity such as Cl▪

238

(Eqs. 4 and 5 (Zhang et al. 2018a)), ▪CO3- (Eqs. 6 and 7) (Wang et al. 2017) and NO3▪ (Eqs. 8 and

239

9(Lian et al. 2017, Zhao et al. 2017)) than SO• . SO• + Cl → SO

(4)

+ Cl•

(5)

• OH + Cl → ClOH • CO& + SO• → SO

(6)

+• CO&

• OH + CO& →• CO& + OH

(7)

SO• + NO& → NO•& + SO

(8) 11

• OH + NO& → NO&• + OH



(9)

240 241

3.5 Mineralization ability, CIP degradation pathways, and biotoxicity analysis

242

In our work, the results of three-dimensional excitation-emission matrix fluorescence

243

spectroscopy (3D EEMs), TOC changing profiles and the growth curves of E. coli were used to track

244

the CIP degradation in the BiVO4/PMS/vis system.

245

Fig. S4 shows the 3D EEMs and TOCs of the five samples collected during the 40 min reaction

246

period. The samples of the initial CIP solution (Fig. S4a) and adsorption-desorption experiment (Fig.

247

S4b) exhibited almost the same fluorescence peaks, which were located at Ex/Em=250-300

248

nm/375-550 nm and Ex/Em=300-350 nm/400-500 nm, corresponding to the humic acid-like

249

region(Deng et al. 2017). This result indicates that no CIP degradation occurred in the dark

250

adsorption process. After 10-min light irradiation reaction (Fig. S4c), the fluorescence signals were

251

largely decreased; when the light reaction was extended to 20 min (Fig. S4d) or 40 min (Fig. S4e), no

252

characteristic peak was observed. This result indicates that CIP in the BiVO4/PMS/vis system was

253

effectively degraded into other smaller products or directly mineralized into CO2 and H2O.

254

As shown in Fig. S4f, the TOC removal by the pure BiVO4 photocatalysis was 0.51% only,

255

suggesting the poor mineralization ability of single BiVO4. The presence of oxidant PS or PMS

256

improved the CIP degradation. Similarly, the TOC removal was markedly improved and about 15.36%

257

and 61.24% of CIP were mineralized in the BiVO4/PS/vis and BiVO4/PMS/vis system. This result

258

also demonstrates the practical application potentials of the BiVO4/PMS/vis system.

259

To deeply understand the CIP degradation process in the BiVO4/PMS/vis system, LC-MS

260

results are provided in Table S2. The initial strong signal of mass/charge (m/z) at 332 belonged to the

261

CIP molecule with full structure. With the identified degradation products of CIP by LC-MS analysis,

262

three CIP degradation pathways were proposed and are shown in Fig. 5. Pathway I, the stepwise

263

oxidative degradation of the piperazine side ring was observed(An et al. 2010). The amide P1 (m/z 12

264

362) was the product of the ring-opening of CIP in the piperazine oxidative process. Followed by

265

the“-CO” group loss, the P2 (m/z 334) was generated and subsequently decomposed into the smaller

266

product of the P3 (m/z 291) by losing “CH2CH2NH2” group. The P3 decarbonylated to boost P4 (m/z

267

263) and further to form product P5 (m/z 219) with the loss of the “-COOH” group. Pathway II was

268

initiated by the substitution of fluorine by a hydroxyl group and the product P6 (m/z 330) was then

269

formed(Deng et al. 2017). The P6 was transformed into the P7 (m/z 285) through losing the

270

carboxylic group. The piperazine ring attacked by the oxidative species resulted in the product P8

271

(m/z 260). Pathway III was assigned to the hydroxylation process and one hydroxyl radical firstly

272

attacked the quinolone ring of CIP to produce the P9 (m/z 348). The P9 was then subjected to the

273

F/OH substitution and hydroxylation processes (Ji et al. 2014, Song et al. 2018). The P10 (m/z 362)

274

was produced and suffered the decarboxylation to produce the P11 (m/z 334). Finally, the above

275

intermediates could be mineralized into other smaller products or directly into CO2 and H2O.

276

To assess the toxicity of the CIP solution after the reaction, the growth of E. coli by the addition

277

of reactive solutions collected from different catalytic times in the BiVO4/vis and BiVO4/PMS/vis

278

systems were examined. Only an infinitesimal amount of E.coli was detected after 720-min

279

cultivation for the samples obtained from the dark adsorption process, suggesting that the

280

high-toxicity of CIP. Compared the result for the BiVO4/vis system (Fig. S5a), the cell concentration

281

in the BiVO4/PMS/vis system (Fig. S5b) increased with the prolonged light irradiation time. The

282

E.coli cell level obtained from the cultivation result (light reaction of 40 min) in the BiVO4/PMS/vis

283

system was about 120-fold higher than that in the BiVO4/vis system. This result indicates that

284

high-toxicity CIP was transformed into other low-toxicity products in the BiVO4/PMS/vis system

285

and this system could not only accelerate the CIP decomposition but also improve the CIP

286

mineralization and reduce the toxicity.

287 288

3.6 PMS activation mechanism in the BiVO4/PMS/vis system 13

289

To explore the role of active species for CIP degradation in the BiVO4/PMS/vis process, radical

290

quenching experiments were conducted through dosing various scavengers. Different quenchers, BQ

291

for superoxide radical (•O2- (Weon et al. 2018)), EDTA-2Na for holes (h+ (Liu et al. 2019)), TBA for

292

hydroxyl radical (TBA with a high reaction activity with •OH, k=3.8-7.6×108 M-1 s-1; whereas the

293

reaction activity of SO4•− is relative low, k=4.0-9.1×105 M-1 s-1) and MeOH with an effective

294

quenching reagent for both •OH (k=9.7×108 M-1 s-1) and SO4•− (k=2.5×107 M-1 s-1) species were

295

applied (Wang et al. 2018c, Wu et al. 2017). As shown in Fig. S6b, in the presence of 0.4 M TBA or

296

0.4 M MeOH, an obvious adverse effect on CIP degradation was observed and the inhibitory effect

297

of MeOH was more remarkable, indicating that both •OH and SO4•− were generated and worked in

298

the BiVO4/PMS/vis system. Dosing 10 mM BQ or 10 mM EDTA-2Na into the system reduced the

299

CIP degradation efficiency from 94.38% to 33.99% and 12.51%, respectively, suggesting that •O2-

300

and h+ were also the active species responsible for the CIP oxidation. These results demonstrate that

301

•O2-, h+, •OH and SO4•− all contributed to the CIP degradation.

302

To further explore the main oxidative species in the reactions, the ESR tests were conducted

303

with the help of a DMPO agent. Fig. 6a and c display the DMPO-•O2- and DMPO-•OH results in the

304

presence of pure BiVO4. No •O2- and •OH signals were observed in the dark but the correspondent

305

signals were observed under visible light irradiation. The relevant signal intensity increased along

306

with the irradiation time, suggesting that both •O2- and •OH were generated in the BiVO4/vis system.

307

After the introduction of PMS (Fig. 6b), the intensity of DMPO-•O2- signal at the same irradiation

308

time slightly declined in comparison with the pure BiVO4 system, implying that the generated •O2-

309

was involved in the PMS activation. Similar to •O2-, no characteristic peaks in the dark belonging to

310

DMPO spin adducts were observed (Fig. 6c, d). However, after the exposure to visible light, both

311

DMPO-SO4•− and DMPO-•OH signals appeared and rapidly increased with the increasing irradiation

312

time (Wang et al. 2018b), suggesting that PMS was activated by the BiVO4 nanosheets and visible

313

light. Furthermore, the signal intensity of DMPO-•OH in the BiVO4/PMS/vis system was 14

314

substantially higher than that in the BiVO4/vis system. These results demonstrate that the

315

introduction of PMS also increased the •OH amount and the produced SO4•− played a vital role in the

316

generation of more •OH via the reaction between SO4•− and H2O/OH- (Huang et al. 2017, Zhou et al.

317

2018).

318

As depicted in Fig. 7a, the transient photocurrent response of the pure BiVO4 was observed to

319

be about 6.5 ×10-5 mA under visible light and maintained stable after the five-cycle successive on-off

320

operation. After dosing PMS, a significant photocurrent enhancement was observed, indicating that

321

the direct electron transfer might firstly proceed from the visible-light-excited surface complex

322

(PMS/BiVO4) to the CB of BiVO4. Other PMS molecules readily acted as an electron acceptor to

323

undergo autoreductive conversions and accelerated the separation of electrons and holes pairs in the

324

photocatalytic system. The EIS (Peng et al. 2018) was also used to explore the photo-induced charge

325

transfer and separation (Fig. 7b). The arc radius in the EIS Nyquist plot of the BiVO4/PMS/vis

326

system after dosing PMS was smaller than that of the BiVO4/vis system, indicating that a lower

327

transfer resistance and more efficient charge separation were achieved in the presence of PMS.

328

Nitroblue tetrazolium (NBT) transformation and terephthalic acid photoluminescence (TA-PL)

329

probing techniques are efficient to quantify the generated •O2- and •OH in catalytic systems(Huang et

330

al. 2015). As shown in Fig. 7c and d, the NBT transformation efficiency after the introduction of

331

PMS was lower than that in the BiVO4/vis system. Thus, the reduction in the generated amount of

332

•O2- species was caused by PMS addition and the missed •O2- was used for PMS activation, which is

333

in agreement with the EPR result. A comparison between the fluorescence intensities of TA-PL in Fig.

334

S7a and S7b indicates that the signal strength after PMS addition was drastically enhanced, which is

335

also consistent with the EPR result. The PMS was activated to boost SO4•− and more •OH was

336

produced by the reactions between SO4•− and H2O or OH-.

337

The fresh and used BiVO4 nanosheets were collected and characterized by XRD and XPS. No

338

obvious changes were observed in the XRD patterns (Fig. 8a) and survey XPS spectra (Fig. 8b), 15

339

indicating that BiVO4 nanosheets remained unchanged in the recycling process. It is reported that the

340

redox reaction of the internal metal ions might also work in the PMS activation process, if the

341

valence states of metallic elements vary and it is simultaneously accompanied by the XPS spectra

342

shifting of the correspondent metal. Fig. 8c and d shows that no excursion was detected in Bi 4f and

343

V 2p, in which two main peaks located at 160.40 eV and 159.10 eV (Bi 4f spectrum) belonged to Bi

344

4f5/2 and Bi 4f7/2 and two distinct peaks with the binding energy of 524.40 eV and 516.80 eV (V 2p

345

spectrum) were attributed to V 2p1/2 and V2p3/2 respectively. These results demonstrate that the

346

electrons transfer by the possible redox reaction of Bi3+/Bi5+ or V5+/V3+ did not occur.

347

With the above results, the possible mechanism for the pollutant decomposition in the

348

BiVO4/PMS/vis system was proposed and is illustrated in Scheme 1. The enhanced catalytic

349

performance could be attributed to the fact that PMS was successfully activated by BiVO4 with the

350

aid of visible light, more oxidant species such as •OH and SO4•− were generated and the

351

high-efficient charge separation was achieved after dosing PMS (an electron acceptor). Under visible

352

light, BiVO4 with a relatively narrow bandgap of 2.34 eV was excited to produce electrons and holes

353

(Eq. 10) (Gao et al. 2017, Wei et al. 2018). The photo-induced electrons in the CB could absorb the

354

free oxygen in the solution to generate •O2- (Eq. 11). The accumulated holes in the VB of BiVO4 also

355

reacted with water molecules to form H+ and •OH (Eq. 12) because the VB position of BiVO4 (+2.85

356

eV) is more positive than the H2O/•OH potential (+2.72 eV vs. NHE) (Chen et al. 2016). Besides,

357

•OH might also be produced by the reaction between •O2- and water molecules (Eq. 13), further to

358

accelerate the CIP degradation process. In the BiVO4/PMS/vis system, the generated electrons (eCB-)

359

could be utilized by dissolved oxygen or directly trapped by the adsorbed PMS, where PMS was

360

activated to generate SO4•− (Eq. 14) (Chen et al. 2012). Through the above processes, the separation

361

of photo-generated electrons and holes was facilitated. The remaining holes in the VB could oxidize

362

pollutants directly due to its strong oxidation capacity. Furthermore, SO4•− might also bring out

363

radical interconversion reactions to yield •OH in aqueous solution by the reaction between SO4•− and 16

364

H2O/OH- (Eqs. 15 and 16), as confirmed by the EPR results. The formed •O2- was also partially

365

involved in the PMS activation to produce SO4•− (Eq. 17), as verified by the variations of

366

DMPO-•O2- signal before and after the introduction of PMS and NBT transformation comparisons.

367

PMS could also be slightly activated by direct visible light without BiVO4, as described in Eq. 18.

368

Unlike electrons, the photo-existed holes in the BiVO4/PMS/vis system played a crucial role in two

369

aspects. First, the photogenerated holes oxidized organic contaminants directly. Secondly, SO5•−

370

radicals with the lower oxidation capacity were also generated through the combination of PMS and

371

holes (Eq. 19) and the generated SO5•− slightly contributed to SO4•− production by its self-sacrificing

372

reactions (Eq. 20) (Wang et al. 2018a). In this way, more photoinduced carriers were efficiently

373

separated and more electrons were transferred to participate in the reactions, which favored the PMS

374

activation to boost SO4•−/•OH. As a result, main active species such as hVB+, •O2-, •OH, and SO4•−

375

contributed to the excellent catalytic activity in the developed BiVO4/PMS/vis system (Eq. 21) and

376

pollutants were readily degraded into smaller intermediates and finally into CO2 and H2O.

377

I.

378

The common reactions in the BiVO4/vis system:

BiVO + hν → BiVO (e23 + h43 )

(10)

e23 + O →• O

(11)

h43 + H O → H +• OH

(12)

• O + 2H O → 2OH + 2 • OH

(13)

II.

The special reactions in the BiVO4/PMS/vis system:

HSO + e23 → SO• + OH

(14)

SO• + H O → SO

+• OH + H

(15)

SO• + OH → SO

+• OH

(16)

HSO +• O → SO• + HO

(17)

HSO + hν → SO• +• OH

(18)

HSO + h43 → SO• + H

(19) 17

379

SO• → 2SO• + O

(20)

h43 /• O /• OH/SO• + organic pollutants → degradation products

(21)

4

Conclusions

380 381

In this work, BiVO4 was used as an effective visible light photocatalyst to activate PMS for the

382

enhanced degradation of CIP. About 94.38% of CIP was rapidly degraded within 40 min in such a

383

system, while the CIP removal efficiency for the BiVO4 system was 3.59% only. The BiVO4 dosage,

384

PMS content, reaction solution pH, initial CIP concentration and coexisting ions all affected the CIP

385

decomposition. The developed BiVO4/PMS/vis Fenton-like system exhibited excellent performance

386

and satisfactory reusability towards the degradation of dyes, phenols, and antibiotics. The high

387

degradation rate and mineralization efficiency were verified by 3D EEMs, TOC and the growth

388

curves of E. coli with the samples collected at different reaction times. The intermediates of CIP

389

degradation were identified and the possible degradation pathways were proposed. The results of

390

quenching experiments and ESR tests indicate that •O2-, h+, •OH and SO4•− were all generated in the

391

system. The enhanced photocurrent, reduced charge carriers transfer resistance and the reduced •O2-

392

production in the BiVO4/PMS/vis system demonstrate that the photo-generated electrons and •O2

393

were involved in the PMS activation process. PMS functioned as a photo-generated electron acceptor

394

to favorably enhance the transfer of charge carriers and boost the generation of more reactive oxygen

395

species to accelerate the CIP degradation process. This study demonstrates that the developed

396

BiVO4/PMS/vis Fenton-like system could be successfully applied as an effective process for the

397

degradation of organic pollutants, and also provides new insights into environmental-friendly PMS

398

activation processes.

399 400

Acknowledgments 18

401

The authors thank the National Key R&D Program of China (2018YFC0406303), the National

402

Natural Science Foundation of China (21590812, 51908528, 51538011 and 51821006), the

403

Postdoctoral Innovation Talent Support Program of China (BX20180290), the China Postdoctoral

404

Science Foundation (2018M640595) and the Fundamental Research Funds for the Central

405

Universities (WK2060120001) for supporting this work.

406 References Ahmad, M., Teel, A.L., Watts, R.J., 2013. Mechanism of persulfate activation by phenols. Environ. Sci. Technol. 47(11), 5864-5871. Ahn, Y.Y., Yun, E.T., Seo, J.W., Lee, C., Kim, S.H., Kim, J.H., Lee, J., 2016. Activation of peroxymonosulfate by surface-loaded noble metal nanoparticles for oxidative degradation of organic compounds. Environ. Sci. Technol. 50(18), 10187-10197. An, T., Yang, H., Li, G., Song, W., Cooper, W.J., Nie, X., 2010. Kinetics and mechanism of advanced oxidation processes (AOPs) in degradation of ciprofloxacin in water. Appl. Catal. B: Environ. 94(3-4), 288-294. Anipsitakis, G.P., Dionysiou, D.D., 2003. Degradation of organic contaminants in water with sulfate radicals generated by the conjunction of peroxymonosulfate with cobalt. Environ. Sci. Technol. 37(20), 4790-4797. Anipsitakis, G.P., Dionysiou, D.D., 2004. Radical generation by the interaction of transition metals with common oxidants. Environ. Sci. Technol. 38(13), 3705-3712. Anipsitakis, G.P., Dionysiou, D.D., Gonzalez, M.A., 2006. Cobalt-mediated activation of peroxymonosulfate and sulfate radical attack on phenolic compounds. implications of chloride ions. Environ. Sci. Technol. 40(3), 1000-1007. Cai, C., Liu, J., Zhang, Z., Zheng, Y., Zhang, H., 2016. Visible light enhanced heterogeneous photo-degradation of Orange II by zinc ferrite (ZnFe2O4) catalyst with the assistance of persulfate. Sep. Purif. Technol. 165, 42-52. Chen, F., Yang, Q., Li, X.M., Zeng, G.M., Wang, D.B., Niu, C.G., Zhao, J.W., An, H.X., Xie, T., Deng, Y.C., 2017. Hierarchical assembly of graphene-bridged Ag3PO4/Ag/BiVO4 (040) Z-scheme photocatalyst: An efficient, sustainable and heterogeneous catalyst with enhanced visible-light photoactivity towards tetracycline degradation under visible light irradiation. Appl. Catal. B: Environ. 200, 330-342. 19

Chen, F., Yang, Q., Sun, J., Yao, F., Wang, S., Wang, Y., Wang, X., Li, X., Niu, C., Wang, D., Zeng, G., 2016. Enhanced photocatalytic degradation of tetracycline by AgI/BiVO4 heterojunction under visible-light irradiation: mineralization efficiency and mechanism. ACS Appl. Mater. Inter. 8(48), 32887-32900. Chen, J., Fang, C., Xia, W., Huang, T., Huang, C.H., 2018a. Selective transformation of beta-Lactam antibiotics by peroxymonosulfate: Reaction kinetics and nonradical mechanism. Environ. Sci. Technol. 52(3), 1461-1470. Chen, X., Oh, W.D., Lim, T.T. 2018b. Graphene- and CNTs-based carbocatalysts in persulfates activation: Material design and catalytic mechanisms. Chem. Eng. J. 354, 941-976. Chen, X., Wang, W., Xiao, H., Hong, C., Zhu, F., Yao, Y., Xue, Z., 2012. Accelerated TiO2 photocatalytic degradation of Acid Orange 7 under visible light mediated by peroxymonosulfate. Chem. Eng. J. 193-194, 290-295. Chen, Y., Zhang, G., Liu, H., Qu, J., 2019a. Confining free radicals in close vicinity to contaminants enables ultrafast Fenton-like processes in the interspacing of MoS2 membranes. Angew. Chem. Int. Ed. 58, 1-6. Chen, Z., Li, X., Zhang, S., Jin, J., Song, X., Wang, X., Tratnyek, P.G., 2019b. Overlooked role of peroxides as free radical precursors in advanced oxidation processes. Environ. Sci. Technol. 53(4), 2054-2062. Cheng, X., Guo, H., Zhang, Y., Korshin, G.V., Yang, B., 2019. Insights into the mechanism of nonradical reactions of persulfate activated by carbon nanotubes: Activation performance and structure-function relationship. Water Res. 157, 406-414. Darsinou, B., Frontistis, Z., Antonopoulou, M., Konstantinou, I., Mantzavinos, D., 2015. Sono-activated persulfate oxidation of bisphenol A: Kinetics, pathways and the controversial role of temperature. Chem. Eng. J. 280, 623-633. Deng, J., Ge, Y., Tan, C., Wang, H., Li, Q., Zhou, S., Zhang, K., 2017. Degradation of ciprofloxacin using α-MnO2 activated peroxymonosulfate process: Effect of water constituents, degradation intermediates and toxicity evaluation. Chem. Eng. J. 330, 1390-1400. Ding, R., Yan, W., Wu, Y., Xiao, Y., Gang, H., Wang, S., Chen, L., Zhao, F., 2018. Light-excited photoelectrons coupled with bio-photocatalysis enhanced the degradation efficiency of oxytetracycline. Water Res. 143, 589-598. Du, J., Bao, J., Liu, Y., Kim, S.H., Dionysiou, D.D., 2018. Facile preparation of porous Mn/Fe3O4 cubes as peroxymonosulfate activating catalyst for effective bisphenol A degradation. Chem. Eng. J. 376, 119193. Duan, X., Ao, Z., Sun, H., Indrawirawan, S., Wang, Y., Kang, J., Liang, F., Zhu, Z.H., Wang, S., 2015. 20

Nitrogen-doped graphene for generation and evolution of reactive radicals by metal-free catalysis. ACS Appl. Mater. Inter. 7(7), 4169-4178. Duan, X., Ao, Z., Zhang, H., Saunders, M., Sun, H., Shao, Z., Wang, S., 2018. Nanodiamonds in sp2/sp3 configuration for radical to nonradical oxidation: Core-shell layer dependence. Appl. Catal. B: Environ. 222, 176-181. Duan, X., Sun, H., Wang, Y., Kang, J., Wang, S., 2014. N-doping-induced nonradical reaction on single-walled carbon nanotubes for catalytic phenol oxidation. ACS Catal. 5(2), 553-559. Fang, G., Wu, W., Liu, C., Dionysiou, D.D., Deng, Y., Zhou, D., 2017. Activation of persulfate with vanadium species for PCBs degradation: A mechanistic study. Appl. Catal. B: Environ. 202, 1-11. Gao, S., Gu, B., Jiao, X., Sun, Y., Zu, X., Yang, F., Zhu, W., Wang, C., Feng, Z., Ye, B., Xie, Y., 2017. Highly efficient and exceptionally durable CO2 Photoreduction to methanol over freestanding defective single-unit-cell bismuth vanadate layers. J. Am. Chem. Soc. 139(9), 3438-3445. Ghauch, A., Baalbaki, A., Amasha, M., El Asmar, R., Tantawi, O., 2017. Contribution of persulfate in UV-254 nm activated systems for complete degradation of chloramphenicol antibiotic in water. Chem. Eng. J. 317, 1012-1025. Hammouda, S.B., Zhao, F., Safaei, Z., Ramasamy, D.L., Doshi, B., Sillanpaa, M., 2018. Sulfate radical-mediated degradation and mineralization of bisphenol F in neutral medium by the novel magnetic Sr2CoFeO6 double perovskite oxide catalyzed peroxymonosulfate: Influence of co-existing chemicals and UV irradiation. Appl. Catal. B: Environ. 233, 99-111. Huang, G.X., Wang, C.Y., Yang, C.W., Guo, P.C., Yu, H.Q., 2017. Degradation of bisphenol A by peroxymonosulfate catalytically activated with Mn1.8Fe1.2O4 nanospheres: Synergism between Mn and Fe. Environ. Sci. Technol. 51(21), 12611-12618. Huang, H., Han, X., Li, X., Wang, S., Chu, P.K., Zhang, Y., 2015. Fabrication of multiple heterojunctions with tunable visible-light-active photocatalytic reactivity in BiOBr-BiOI full-range composites based on microstructure modulation and band structures. ACS Appl. Mater. Inter. 7(1), 482-492. Ji, Y., Ferronato, C., Salvador, A., Yang, X., Chovelon, J.M., 2014. Degradation of ciprofloxacin and sulfamethoxazole by ferrous-activated persulfate: implications for remediation of groundwater contaminated by antibiotics. Sci. Total Environ. 472, 800-808. Jorfi, S., Kakavandi, B., Motlagh, H.R., Ahmadi, M., Jaafarzadeh, N., 2017. A novel combination of oxidative degradation for benzotriazole removal using TiO2 loaded on FeIIFe2IIIO4@C as an efficient activator of peroxymonosulfate. Appl. Catal. B: Environ. 219, 216-230. Li, H., Quan, X., Chen, S., Yu, H., 2017. Ferroelectric-enhanced Z-schematic electron transfer in 21

BiVO4-BiFeO3-CuInS2 for efficient photocatalytic pollutant degradation. Appl. Catal. B: Environ. 209, 591-599. Li, J., Wu, X., Pan, W., Zhang, G., Chen, H., 2018a. Vacancy-rich monolayer BiO2-x as a highly efficient UV, visible, and near-infrared responsive photocatalyst. Angew Chem. Int. Ed. 57(2), 491-495. Li, P., Chen, X., He, H., Zhou, X., Zhou, Y., Zou, Z., 2018b. Polyhedral 30-faceted BiVO4 microcrystals predominantly enclosed by high-index planes promoting photocatalytic water-splitting activity. Adv. Mater. 30(1), 1703119. Li, R.G., Zhang, F.X., Wang, D.G., Yang, J.X., Li, M.R., Zhu, J., Zhou, X., Han, H.X., Li, C., 2013. Spatial separation of photogenerated electrons and holes among {010} and {110} crystal facets of BiVO4. Nat. Commun. 4, 7. Li, X., Huang, X., Xi, S., Miao, S., Ding, J., Cai, W., Liu, S., Yang, X., Yang, H., Gao, J., Wang, J., Huang, Y., Zhang, T., Liu, B., 2018c. Single cobalt atoms anchored on porous N-doped graphene with dual reaction sites for efficient Fenton-like catalysis. J. Am. Chem. Soc. 140(39), 12469-12475. Lian, L., Yao, B., Hou, S., Fang, J., Yan, S., Song, W., 2017. Kinetic study of hydroxyl and sulfate radical-mediated oxidation of pharmaceuticals in wastewater effluents. Environ. Sci. Technol. 51(5), 2954-2962. Liu, S., Hu, Q., Qiu, J., Wang, F., Lin, W., Zhu, F., Wei, C., Zhou, N., Ouyang, G., 2017. Enhanced photocatalytic degradation of environmental pollutants under visible irradiation by a composite Coating. Environ. Sci. Technol. 51(9), 5137-5145. Liu, W., Li, Y., Liu, F., Jiang, W., Zhang, D., Liang, J., 2019. Visible-light-driven photocatalytic degradation of diclofenac by carbon quantum dots modified porous g-C3N4: Mechanisms, degradation pathway and DFT calculation. Water Res. 150, 431-441. Lu, X., Zhao, J., Wang, Q., Wang, D., Xu, H., Ma, J., Qiu, W., Hu, T., 2019. Sonolytic degradation of bisphenol S: Effect of dissolved oxygen and peroxydisulfate, oxidation products and acute toxicity. Water Res. 165, 114969. Peng, B., Tang, L., Zeng, G., Fang, S., Ouyang, X., Long, B., Zhou, Y., Deng, Y., Liu, Y., Wang, J., 2018. Self-powered photoelectrochemical aptasensor based on phosphorus doped porous ultrathin g-C3N4 nanosheets enhanced by surface plasmon resonance effect. Biosens. Bioelectron. 121, 19-26. Qin, Y., Li, G., Gao, Y., Zhang, L., Ok, Y.S., An, T., 2018. Persistent free radicals in carbon-based materials on transformation of refractory organic contaminants (ROCs) in water: A critical review. Water Res. 137, 130-143. 22

Sabio, E., Zamora, F., Ganan, J., Gonzalez-Garcia, C.M., Gonzalez, J.F., 2006. Adsorption of p-nitrophenol on activated carbon fixed-bed. Water Res. 40(16), 3053-3060. Song, S., Lu, C., Wu, X., Jiang, S., Sun, C., Le, Z., 2018. Strong base g-C3N4 with perfect structure for photocatalytically eliminating formaldehyde under visible-light irradiation. Appl. Catal. B: Environ. 227, 145-152. Wang, Y., Liu, C., Zhang, Y., Meng, W., Yu, B., Pu, S., Yuan, D., Qi, F., Xu, B., Chu, W., 2018a. Sulfate radical-based photo-Fenton reaction derived by CuBi2O4 and its composites with α-Bi2O3 under visible light irradiation: Catalyst fabrication, performance and reaction mechanism. Appl. Catal. B: Environ. 235, 264-273. Wang, Y., Zhao, S., Fan, W., Tian, Y., Zhao, X., 2018b. The synthesis of novel Co-Al2O3 nanofibrous membranes with efficient activation of peroxymonosulfate for bisphenol A degradation. Environ. Sci-Nano 5(8), 1933-1942. Wang, Y.B., Zhao, X., Cao, D., Wang, Y., Zhu, Y.F., 2017. Peroxymonosulfate enhanced visible light photocatalytic degradation bisphenol A by single-atom dispersed Ag mesoporous g-C3N4 hybrid. Appl. Cataly. B: Environ. 211, 79-88. Wang, Z., Jiang, J., Pang, S., Zhou, Y., Guan, C., Gao, Y., Li, J., Yang, Y., Qiu, W., Jiang, C., 2018c. Is sulfate radical really generated from peroxydisulfate activated by Iron(II) for environmental decontamination? Environ. Sci. Technol. 52(19), 11276-11284. Wei, T.C., Zhu, Y.N., Gu, Z.N., An, X.Q., Liu, L.M., Wu, Y.X., Liu, H.J., Tang, J.W., Qu, J.H., 2018. Multi-electric field modulation for photocatalytic oxygen evolution: Enhanced charge separation by coupling oxygen vacancies with faceted heterostructures. Nano Energy 51, 764-773. Wei, Y., Wei, S., Liu, C., Chen, T., Tang, Y., Ma, J., Yin, K., Luo, S., 2019. Efficient removal of arsenic from groundwater using iron oxide nanoneedle array-decorated biochar fibers with high Fe utilization and fast adsorption kinetics. Water Res. 167, 115107. Weon, S., Choi, E., Kim, H., Kim, J.Y., Park, H.J., Kim, S.M., Kim, W., Choi, W., 2018. Active {001} facet exposed TiO2 nanotubes photocatalyst filter for volatile organic compounds removal: From material development to commercial indoor air cleaner application. Environ. Sci. Technol. 52(16), 9330-9340. Wu, F., Huang, H., Xu, T., Lu, W., Li, N., Chen, W., 2017. Visible-light-assisted peroxymonosulfate activation and mechanism for the degradation of pharmaceuticals over pyridyl-functionalized graphitic carbon nitride coordinated with iron phthalocyanine. Appl. Catal. B: Environ. 218, 230-239. Wu, H., Xu, X., Shi, L., Yin, Y., Zhang, L.C., Wu, Z., Duan, X., Wang, S., Sun, H., 2019. Manganese 23

oxide integrated catalytic ceramic membrane for degradation of organic pollutants using sulfate radicals. Water Res. 167, 115110. Yan, J., Peng, J., Lai, L., Ji, F., Zhang, Y., Lai, B., Chen, Q., Yao, G., Chen, X., Song, L., 2018. Activation CuFe2O4 by hydroxylamine for oxidation of antibiotic sulfamethoxazole. Environ. Sci. Technol. 52(24), 14302-14310. Yang, Y., Lu, X., Jiang, J., Ma, J., Liu, G., Cao, Y., Liu, W., Li, J., Pang, S., Kong, X., Luo, C., 2017. Degradation of sulfamethoxazole by UV, UV/H2O2 and UV/persulfate (PDS): Formation of oxidation products and effect of bicarbonate. Water Res. 118, 196-207. Yao, F., Yang, Q., Zhong, Y., Shu, X., Chen, F., Sun, J., Ma, Y., Fu, Z., Wang, D., Li, X., 2019. Indirect electrochemical reduction of nitrate in water using zero-valent titanium anode: Factors, kinetics, and mechanism. Water Res. 157, 191-200. Zhang, J., Zhao, X., Wang, Y., Gong, Y., Cao, D., Qiao, M., 2018a. Peroxymonosulfate-enhanced visible light photocatalytic degradation of bisphenol A by perylene imide-modified g-C3N4. Appl. Catal. B: Environ. 237, 976-985. Zhang, L., Chen, D., Jiao, X., 2006. Monoclinic structured BiVO4 nanosheets: hydrothermal preparation, formation mechanism, and coloristic and photocatalytic properties. J. Phys. Chem. B 110(6), 2668-2673. Zhang, W., Zhou, S., Sun, J., Meng, X., Luo, J., Zhou, D., Crittenden, J., 2018b. Impact of chloride ions on UV/H2O2 and UV/persulfate advanced oxidation processes. Environ. Sci. Technol. 52(13), 7380-7389. Zhao,

Z.,

Zhao,

J.,

Yang,

C.,

2017.

Efficient

removal

of

ciprofloxacin

by

peroxymonosulfate/Mn3O4-MnO2 catalytic oxidation system. Chem. Eng. J. 327, 481-489. Zhou, D., Xu, Z., Dong, S., Huo, M., Dong, S., Tian, X., Cui, B., Xiong, H., Li, T., Ma, D., 2015. Intimate coupling of photocatalysis and biodegradation for degrading phenol using different light types: visible light vs UV light. Environ. Sci. Technol. 49(13), 7776-7783. Zhou, Y., Wang, X., Zhu, C., Dionysiou, D.D., Zhao, G., Fang, G., Zhou, D., 2018. New insight into the mechanism of peroxymonosulfate activation by sulfur-containing minerals: Role of sulfur conversion in sulfate radical generation. Water Res. 142, 208-216. Zhu, K., Wang, J., Wang, Y., Jin, C., Ganeshraja, A.S., 2016. Visible-light-induced photocatalysis and peroxymonosulfate activation over ZnFe2O4 fine nanoparticles for degradation of Orange II. Catal. Sci. Technol. 6(7), 2296-2304. Zhu, M., Liu, Q., Chen, W., Yin, Y., Ge, L., Li, H., Wang, K., 2017. Boosting the visible-light photoactivity of BiOCl/BiVO4/N-GQD ternary heterojunctions based on internal Z-Scheme charge transfer of N-GQDs: simultaneous band gap narrowing and carrier lifetime prolonging. 24

ACS Appl. Mater. Inter. 9(44), 38832-38841. Zhu, S., Huang, X., Ma, F., Wang, L., Duan, X., Wang, S., 2018. Catalytic removal of aqueous contaminants on N-doped graphitic biochars: Inherent roles of adsorption and nonradical mechanisms. Environ. Sci. Technol. 52(15), 8649-8658.

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Figure captions Fig. 1. (a) XRD pattern and (b) UV-vis diffuse reflectance spectrum of BiVO4. Fig. 2. (a) SEM image of BiVO4 nanosheet, (b-c) TEM image of BiVO4 and (d-g) TEM-EDS mapping images of the elements (d) whole, (e) Bi, (f) V and (g) O. Fig. 3. (a, b) Removal variations of CIP degradation by different reaction systems and (c, d) the correspondent pseudo-first-order kinetic curves. Fig. 4. Effect of (a) catalysis dosage (Conditions: [CIP]0=10 mg/L, [PMS]=0.96 g/L, temperature 25ºC, visible light λ>420 nm); (b) PMS concentration (Conditions: [CIP]0=10 mg/L, [Catal.]=0.32 g/L, temperature 25ºC, visible light λ>420 nm); (c) solution pH (Conditions: [CIP]0=10 mg/L, [PMS]=0.96 g/L, [Catal.]=0.32 g/L, temperature 25ºC, visible light λ>420 nm) and (d) CIP concentration (Conditions: [Catal.]=0.32 g/L, [PMS]=0.96 g/L, temperature 25ºC, visible light λ>420 nm) on the removal efficiency of CIP. Fig. 5. Possible CIP degradation pathways in the BiVO4/PMS/vis system. Fig. 6. EPR spectra of BiVO4 as photocatalyst in the presence of PMS under visible light irradiation: (a, b) aqueous dispersion for DMPO-•O2- and (c, d) methanol dispersion for DMPO-•OH/SO4•− with or without the addition of PMS during the reaction. Fig. 7. Changes before and after the addition of PMS: (a) transient photocurrent, (b) EIS spectrum and (c-d) NBT transformation. Fig. 8. (a) XRD patterns, (b) survey XPS spectrum, (c) Bi 4f XPS spectrum and (d) O 1s+V 2p spectrum before or after reaction. Scheme 1 Proposed mechanism of CIP degradation in the BiVO4/PMS/vis system.

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Fig. 1. (a) XRD pattern and (b) UV-vis diffuse reflectance spectrum of BiVO4.

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Fig. 2. (a) SEM image of BiVO4 nanosheet, (b-c) TEM image of BiVO4 and (d-g) TEM-EDS mapping images of the elements (d) whole, (e) Bi, (f) V and (g) O.

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Fig. 3. (a, b) Removal variations of CIP degradation by different reaction systems and (c, d) the correspondent pseudo-first-order kinetic curves.

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Fig. 4. Effect of (a) catalysis dosage (Conditions: [CIP]0=10 mg/L, [PMS]=0.96 g/L, temperature 25ºC, visible light λ>420 nm); (b) PMS concentration (Conditions: [CIP]0=10 mg/L, [Catal.]=0.32 g/L, temperature 25ºC, visible light λ>420 nm); (c) solution pH (Conditions: [CIP]0=10 mg/L, [PMS]=0.96 g/L, [Catal.]=0.32 g/L, temperature 25ºC, visible light λ>420 nm) and (d) CIP concentration (Conditions: [Catal.]=0.32 g/L, [PMS]=0.96 g/L, temperature 25ºC, visible light λ>420 nm) on the removal efficiency of CIP.

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Fig. 5. Possible CIP degradation pathways in the BiVO4/PMS/vis system.

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Fig. 6. EPR spectra of BiVO4 as photocatalyst in the presence of PMS under visible light irradiation: (a, b) aqueous dispersion for DMPO-•O2- and (c, d) methanol dispersion for DMPO-•OH/SO4•− with or without the addition of PMS during the reaction.

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Fig. 7. Changes before and after the addition of PMS: (a) transient photocurrent, (b) EIS spectrum and (c-d) NBT transformation.

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Fig. 8. (a) XRD patterns, (b) survey XPS spectrum, (c) Bi 4f XPS spectrum and (d) O 1s+V 2p spectrum before or after reaction.

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Scheme 1 Proposed mechanism of CIP degradation in the BiVO4/PMS/vis system.

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Research highlights

PMS was effectively activated by BiVO4 nanosheets for water purification under visible light. Separation of electron/hole pairs and generation of oxidative species were enhanced. Visible-light-assisted PMS activation Fenton-like mechanism was elucidated. High mineralization and low biotoxicity validated the application potential of the system.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: