Zn0-CNTs-Fe3O4 catalytic in situ generation of H2O2 for heterogeneous Fenton degradation of 4-chlorophenol

Accepted Manuscript Zn0-CNTs-Fe3O4 catalytic in situ generation of H2O2 for heterogeneous Fenton degradation of 4-chlorophenol

Zhao Yang, Xiao-bo Gong, Lin Peng, Dan Yang, Yong Liu PII:

S0045-6535(18)31090-7

DOI:

10.1016/j.chemosphere.2018.06.016

Reference:

CHEM 21548

To appear in:

Chemosphere

Received Date:

15 March 2018

Accepted Date:

02 June 2018

Please cite this article as: Zhao Yang, Xiao-bo Gong, Lin Peng, Dan Yang, Yong Liu, Zn 0-CNTs-Fe3 O4 catalytic in situ generation of H2O2 for heterogeneous Fenton degradation of 4-chlorophenol,

Chemosphere (2018), doi: 10.1016/j.chemosphere.2018.06.016

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1

Zn0-CNTs-Fe3O4

catalytic

in

situ

generation

2

heterogeneous Fenton degradation of 4-chlorophenol

H2O2

for

Zhao Yang a, Xiao-bo Gong a, b, Lin Peng a, Dan Yang a, Yong Liu a, b, *

3 4

a

5

610066, China;

6

b

7

Education System, Sichuan, Chengdu 610066, China

8

*Corresponding

9

2886006795

10

of

College of Chemistry and Materials Science, Sichuan Normal University, Chengdu

Key Laboratory of Treatment for Special Wastewater of Sichuan Province Higher

author: Yong Liu: Email: [email protected], Tel: +86

Jingan Road 5#, Jinjiang District, Chengdu, Sichuan, 610066, China.

11 12

Abstract

13

A novel Zn0-CNTs-Fe3O4 composite was synthesized by the chemical co-

14

precipitation combined with high sintering process at nitrogen atmosphere. The as-

15

prepared composite was characterized by SEM, EDS, XRD, XPS, VSM and N2

16

adsorption/desorption experiments. A novel heterogeneous Fenton-like system,

17

composed of Zn0-CNTs-Fe3O4 composite and dissolved oxygen (O2) in solution,

18

which can in situ generate H2O2 and •OH, was used for the degradation of 4-

19

chlorophenol (4-CP). The influences of various operational parameters, including the

20

initial pH, dosage of Zn0-CNTs-Fe3O4 and initial concentration of 4-CP on the

21

removal of 4-CP were investigated. The removal efficiencies of 4-CP and total

22

organic carbon (TOC) were 99% and 57%, respectively, at the initial pH of 1.5, Zn01

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CNTs-Fe3O4 dosage of 2 g/L, 4-CP initial concentration of 50 mg/L and oxygen flow

24

rate of 400 mL/min. Based on the results of the radical scavenger effect study, the

25

hydroxyl radical was considered as the main reactive oxidants in Zn0-CNTs-Fe3O4/O2

26

system and a possible degradation pathway of 4-CP was proposed.

27 28

Keywords: In-situ generation H2O2; Zn0-CNTs-Fe3O4 composite; Heterogeneous

29

Fenton; 4-chlorophenol

30 31

1. Introduction

32

In recent years, advanced oxidation processes (AOPs) which can generate

33

hydroxyl radicals (•OH) with high standard potential of 2.80 V, have been widely

34

applied for the degradation of refractory, non-biodegradable and xenobiotic

35

contaminants, due to their advantages over the conventional methods (Prieto-

36

Rodríguez et al., 2013; He et al., 2015; Hou et al., 2017). Fenton process based on the

37

reaction of Fe2+ and hydrogen peroxide (H2O2) is one of the most frequently used

38

AOPs due to its high performance, simplicity and non-toxicity (Neyens and Baeyens,

39

2003; Wang and Xu, 2012; Rache et al., 2014). However, the classic homogeneous

40

Fenton reaction is limited with several disadvantages, such as high operating cost,

41

limited optimum pH range (2-4), high amounts of iron sludge after disposal and

42

difficulties in the recycling of homogeneous catalyst (Fe2+) (Diya'uddeen et al., 2012;

43

Asghar et al., 2015; Wang et al., 2016).

44

In order to overcome these disadvantages, the heterogeneous Fenton process that 2

ACCEPTED MANUSCRIPT 45

uses solid catalyst to replace Fe2+ has been developed (Wang et al., 2012; Duan et al.,

46

2014; Rache et al., 2014; Wang et al., 2016; Tang and Wang, 2018). Among the

47

heterogeneous Fenton-like reactions, the Fe3O4 has been proved to be a promising

48

catalyst, owing to its intrinsic peroxidase-like activity and stability, as well as easy

49

recycling and recovery (Costa et al., 2008; Hu et al., 2011; Niu et al., 2011; Xu and

50

Wang, 2012a; Sun et al., 2014; Wang et al., 2014; Yu et al., 2015). In order to

51

increase the catalytic activity of Fe3O4, Fe3O4 loaded carbon nanotubes (denoted as

52

CNTs-Fe3O4) has been developed and proved to have higher catalytic activity than

53

that of Fe3O4 alone in the degradation of methylene blue due to its large specific

54

surface and the higher H2O2-activating ability (Wang et al., 2014).

55

In the heterogeneous system, the catalysis process always occurs on the surface

56

of the catalyst (Xu and Wang, 2011, 2012a). The diffusion and adsorption processes

57

of H2O2 and other reactants to the surface of catalyst could be significant for the

58

catalysis process. The heterogeneous catalyst with high adsorption capabilities for the

59

contaminant is helpful for the degradation of the contaminant due to its good mass

60

transfer (Xu and Wang, 2012b). However, it is difficult to improve the mass transfer

61

of H2O2 from solution to the catalyst surface owing to the high hydrophilicity and

62

instability of H2O2. In the heterogeneous Fenton process, H2O2 is provided by bulk

63

feeding and the H2O2 does not yield a high efficiency due to poor mass transfer.

64

Moreover, bulk feeding H2O2 has potential safety hazards associated with the

65

instability of H2O2 (Asghar et al., 2015). To overcome these drawbacks, the

66

heterogeneous Fenton process with the in situ generation of H2O2 has been studied 3

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(Yalfani et al., 2011; Fang et al., 2013).

68

Recently, considerable research efforts have been devoted to in situ generation of

69

H2O2 through photochemical and electrochemical method that directly reduce oxygen

70

through a two-electron pathway (Teranishi et al., 2010; Liu et al., 2015; Luo et al.,

71

2015; Perez et al., 2017). However, these processes require strict operational

72

conditions of high temperature, UV radiation, high voltage or low generation

73

efficiency, which limit their full scale applications (Badellino et al., 2006; You et al.,

74

2010; Plakas et al., 2013; Dohyung et al., 2015). It has been reported that some zero-

75

valent metal, such as Zn0, Al0 and Fe0 can reduce O2 under mild condition to produce

76

H2O2, which is an alternative convenient process for in situ generation of H2O2 (Jiang

77

et al., 2008; Wen et al., 2014; Fan et al., 2015). However, the degradation efficiency

78

of organic contaminants is rather low due to the low concentration of H2O2 generated

79

in situ.

80

In our preliminary study, a novel material (Zn0-CNTs) was successfully prepared

81

through infiltration fusion method and the Zn0-CNTs/O2 system was established to

82

produce large amounts of H2O2 through forming numerous corrosion cells between

83

the particles of Zn0 and carbon nanotubes (CNTs) in aqueous solution (Gong et al.,

84

2018). The H2O2 generated in the Zn0-CNTs/O2 system was catalytically decomposed

85

by Fe2+ ions, ozone or Fe0/Fe2O3 in solution or on the surface of carbon nanotubes to

86

produce •OH radical, which was used to degrade some refractory contaminants such

87

as 4-chloropheenol and sulfamethoxazole (Liu et al., 2017a; Liu et al., 2017b; Liu et

88

al., 2017c; Wang and Bai, 2017; Liu et al., 2018). In the Zn0-CNTs/O2 system, 4

ACCEPTED MANUSCRIPT 89

although Zn0 was consumed continuously, CNTs could be reused as a catalyst for the

90

oxygen reduction and a carrier for Zn0. The separation of CNTs from aqueous

91

solution is difficult in Zn0-CNTs/O2/O3 system or in Zn0-CNTs/O2/Fe2+ system due to

92

fine particle size, and in Zn0-CNTs-Fe/O2/O3 system due to its weak magnetism,

93

which limited their further application for the removal of organic contaminants in

94

aqueous solution.

95

In Zn0-CNTs/O2 system, if Fe3O4 is used as Fenton-like catalyst and loaded on

96

the surface of CNTs, both the catalytic decomposition of H2O2 and the recovery of

97

CNTs can be obtained owing to its high catalytic activity and good magnetism.

98

Therefore, if both Fe3O4 and Zn0 are loaded on the CNTs, the H2O2 generated in situ

99

by the reaction between O2 and Zn0/CNTs corrosion cells and its utilization by Fe3O4

100

can be improved simultaneously. Therefore, the composites of Zn0, Fe3O4 and CNTs

101

could be a promising alternative as water treatment material for the degradation of

102

organic contaminants via the reaction with O2.

103

In this paper, Zn0-CNTs-Fe3O4 composites were synthesized and used for 4-

104

chlorophenol (4-CP) degradation, which is potentially carcinogenic and mutagenic to

105

mammalian as well as aquatic organisms, and has been listed as priority pollutant by

106

the US Environmental Protection Agency (EPA) (Palanisamy et al., 2013). The

107

physical and chemical properties of Zn0-CNTs-Fe3O4 composites were characterized

108

and the degradation performances were evaluated according to the effects of key

109

variables, such as initial 4-CP concentration, initial pH and Zn0-CNTs-Fe3O4 dosage.

110

The intermediate products were detected by HRLC–ToF-MS and IC, and the possible 5

ACCEPTED MANUSCRIPT 111

degradation pathway of 4-CP was proposed. In addition, the degradation mechanism

112

of the Zn0-CNTs-Fe3O4/O2 system was also studied.

113 114

2. Experimental

115

2.1. Materials and chemicals

116

The chemicals and reagents in this study were of analytical reagent grade or

117

better and used without further purification. FeCl3·6H2O, FeSO4·7H2O, H2SO4,

118

NaOH, NH3·H2O, zinc metal powder, polyethylene glycol 4000 and 4-chlorophenol

119

(4-CP) were obtained from the Kelong Chemical Reagent Co., Ltd (Chengdu, China).

120

Hydroxylated multi-walled carbon nanotubes were purchased from Chengdu Organic

121

Chemicals Co. Ltd. (Chengdu, China). Deionized water (DI) used in all experiments

122

was prepared by a Milli-Q system.

123

2.2. Synthesis and characterization of Zn0-CNTs-Fe3O4

124

The Zn0-CNTs-Fe3O4 composites were synthesized by the chemical co-

125

precipitation combined with high sintering process (Huang et al., 2012; Xu and Wang,

126

2012b). Briefly, the appropriate amounts of FeCl3·6H2O (0.7 g) and FeSO4·7H2O

127

(0.48 g) with a molar ratio of 1:2 were dissolved in 40 mL of deoxygenated water at

128

40℃ under vigorous stirring and N2 protection. Then 0.4 g of CNTs was added rapidly

129

and sequentially into the reaction solution. After 10 min stirring, the mixed solution

130

was sonicated at 22.5 kHz and 30 W for 30 min. Then 2.0 mol/L of ammonia solution

131

was drop-wised sequentially into the mixed solution until the pH value of the mixed

132

solution was higher than 10. The mixture was kept for 30 min under vigorous stirring 6

ACCEPTED MANUSCRIPT 133

and N2 protection and cooled naturally. The resulted precipitates were separated from

134

solution by a magnet and washed 3 times with DI water and alcohol alternately and

135

dehydrated in a vacuum drying oven at 60 ℃ for 14 h. The as-prepared magnetic

136

particles were marked as CNTs-Fe3O4. Then, the obtained CNTs-Fe3O4, zinc powder

137

and polyethylene glycol 4000 were mixed at a mass ratio of 1:2.5:1 at 60 ℃ under

138

vigorous stirring and N2 protection for 20 min. Finally, the mixture was sintered in the

139

Muffle furnace under N2 protection at 500℃ for 120 min to obtain Zn0-CNTs-Fe3O4.

140

The morphology of the obtained Zn0-CNTs-Fe3O4 composite was observed by

141

scanning electron microscope (SEM, SU8010, Hitachi, Japan). The spatial elemental

142

distributions were investigated by energy-dispersive spectrometry (EDS) elemental

143

mapping analysis. X-ray diffraction (XRD) patterns was investigated on a

144

diffractometer (Bruke D8 Adv., Germany) with a filtered Cu Kα radiation source (λ =

145

1.54178 Å) to analyze the crystalline structure of the obtained Zn0-CNTs-Fe3O4

146

before and after the removal of 4-CP. Nitrogen adsorption-desorption tests were

147

carried out (Quantachrome, US) to obtained the specific surface area and pores

148

distribution of the samples. The samples were degassed at 120°C for 5 h under

149

vacuum condition before the measurements. The X-ray photoelectron spectroscopy

150

(XPS) analysis was performed using an ESCALAB 250Xi spectrometer (Thermo

151

Fisher, USA) with Al Ka X-ray (1486.6 eV). Magnetization measurement was

152

obtained by vibrating sample magnetometer (VSM-Versalab, Qutumn Desig, USA).

153

2.3. Degradation experiments

154

Batch experiments for the degradation of 4-CP were carried out in a 250 mL glass 7

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bottles. The tests were initiated by turning on an shaker at 250 rpm immediately after

156

the additions of reactants into the bottles. The total volume of reaction solution was

157

150 mL. HCl (0.1 mol/L) and NaOH (0.1 mol/L) were used for adjusting initial pH of

158

the solution. The reaction solutions were not buffered against pH change to prevent

159

any potential interference. A series of batch experiments were conducted to evaluate

160

the effect of Zn0-CNTs-Fe3O4 dosage (0.5-4.0 g/L), initial pH (1-3) and 4-CP initial

161

concentration (25-200 mg/L) on 4-CP degradation. At the given time interval, suitable

162

volume of reaction solutions were sampled, and filtered immediately through a 0.22

163

μm membrane. Then the separated liquid phase was used for analysis. All

164

experiments were conducted in duplicate, and all results were expressed as a mean

165

value in triplicate.

166

2.4. Analyses

167

4-CP concentrations were measured by means of an Agilent 1290 Ultra

168

Performance Liquid Chromatography (UPLC). The mobile phase used for 4-CP was a

169

mixture of acetonitrile and 0.1% formic acid (60:40, v/v) at a flow rate of 0.3 mL/min

170

with a column temperature of 30 °C, and the analytical wavelength was 279 nm.

171

Total organic carbon (TOC) concentration was determined by total organic

172

carbon analyzer (Multi N/C 3000 TOC analyzer, Analytik Jena AG, Germany) after

173

filtration through a 0.22 μm membrane filter. The solution pH was measured with a

174

PHS-3C pH-meter (PHS-3C, REX Instruments, China). The concentration of chloride

175

ions (Cl−) and small molecule carboxylic acids were determined using an ion

176

chromatography (Dionex ICS 1100, Thermo Scientific, USA). The concentration of 8

ACCEPTED MANUSCRIPT 177

generated H2O2 was measured by spectrophotometry with potassium titanium oxalate

178

(K2TiO(C2O4)2) as color indicator using a UV-Vis spectrophotometer (Alpha-1500,

179

Shanghai, China) at 400 nm (Sellers, 1980). High-resolution liquid chromatography

180

combined with time-of-flight mass spectrometry (HRLC–ToF-MS) was performed on

181

a quadrupole orthogonal acceleration time-of-flight tandem mass spectrometer

182

(Waters MS Technologies, Manchester, UK) to determine the intermediate products.

183 184

3. Results and discussion

185

3.1. Characterization of Zn0-CNTs-Fe3O4

186

The SEM images of Zn0-CNTs-Fe3O4 (Fig. 1) showed that the tubular shape of

187

CNTs was not changed after the addition of Zn and Fe3O4. Some small particles

188

adhered on the surface of CNTs were likely to be Zn or Fe3O4 particles, suggesting

189

that Zn or Fe3O4 were successfully loaded on the CNTs. Fig. 1

190 191

The elemental distribution and relative element content of Zn0-CNTs-Fe3O4 were

192

confirmed by the EDS spectra and element mapping (Fig. 2). From the EDS

193

spectrum, C, O, Fe and Zn elements were recorded, which matched fairly well with

194

the calculated composition of the composites. From the results of EDS element

195

mapping of C, O, Fe and Zn, the uniform distribution of Fe and Zn on CNTs surface

196

was confirmed. The uneven distribution of O might be due to the part oxidation of Zn0

197

to ZnO during the high temperature heat-treatment process. The low distribution of O

198

might be owing to its low content. 9

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Fig. 2

200

Fig. 3 displayed the XRD patterns of fresh and used Zn0-CNTs-Fe3O4

201

composites. The broad peaks at 26.6° was corresponded to the (004) reflections of the

202

graphitic structure of CNTs (PDF #26-1080), indicating that the CNT structure was

203

not destroyed in the synthesis of Zn0-CNTs-Fe3O4. The diffraction peaks in the XRD

204

spectrum of fresh Zn0-CNTs-Fe3O4 at 38.90°, 43.16°, 54.24°, 69.96°, 70.54°, 82.00

205

°and 86.45° was corresponded to the (100), (101), (102), (103), (110), (112) and (201)

206

crystal planes of Zn0 (PDF #87-0713), respectively. The peaks at 31.70°, 34.36°,

207

36.18°, 47.48°, 56.54°, 62.78°, 67.86°, 69.00° and 76.89° were assigned to the (100),

208

(002), (101), (102), (110), (103), (112), (201) and (202) reflections of ZnO (PDF #89-

209

0510), respectively. The existence of ZnO in the fresh Zn-CNTs sample suggested

210

that the part of zinc was oxidized during the preparation process, which was in

211

consistent with the results of EDS mapping analysis. The weak peaks located at

212

29.99° and 35.38° was attributed to the (220) and (311) reflections of Fe3O4 (PDF

213

#89-0688), respectively. Compared with fresh Zn0-CNTs-Fe3O4 sample, six new

214

diffraction peaks at 18.26°, 42.92°, 53.28°, 56.76°, 62.40° and 73.82° corresponding

215

to the (112), (400), (422), (511), (440) and (533) crystal planes of Fe3O4 (PDF #89-

216

0688), respectively, were found in the patterns of the used Zn0-CNTs-Fe3O4 sample,

217

and the characteristic diffraction peaks of Zn0 was obviously weakened or

218

disappeared. This indicated that the supported Zn0 was converted to other amorphous

219

matters and the covered Fe3O4 by Zn0 was exposed to the surface of Zn0-CNTs-Fe3O4

220

composite during the degradation of 4-CP. 10

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Fig. 3

222

To further verify the chemical compositions of Zn0-CNTs-Fe3O4, XPS analysis

223

was performed. Fig. 4a clearly illustrated the existence of Zn, O, Fe and C. The Fe 2p

224

spectra in Zn0-CNTs-Fe3O4 (Fig. 4b) showed that the Fe species mainly existed in the

225

form of Fe3O4 because the binding energies at 710.7, 712.2 and 724.6 eV assigned to

226

Fe 2p3/2 and Fe 2p1/2 were characteristic of Fe3O4 (Zhu et al., 2011; Xu and Wang,

227

2012b). This was in consistent with the XRD analysis in Fig. 3.

228

Fig. 4

229

The N2 adsorption-desorption isotherms and pore size distribution of Zn0-CNTs-

230

Fe3O4 were given in Fig. 4c. The specific surface area of Zn0-CNTs-Fe3O4 calculated

231

by the BET method was 22.7 m2/g. The sample exhibited typical type IV and

232

hysteresis loops type H3 according to the International Union of Pure and Applied

233

Chemistry (IUPAC) classification, which demonstrated that the Zn0-CNTs-Fe3O4

234

composite showed typical mesoporous structure (And and Jaroniec, 2001). The

235

average pore diameter of 36 nm was calculated by the Barrett-Joyner-Halenda (BJH)

236

method, and the corresponding pore size distribution curve (the inset) further

237

confirmed that the composite was mainly mesoporous.

238

The room temperature magnetization curves of the Zn0-CNTs-Fe3O4 composite

239

were displayed in Fig. 4d. It can be seen that almost no magnetic hysteresis loop

240

appeared, exhibiting the superparamagnetic properties of the synthesized composite.

241

The saturation magnetization (Ms) values of the composite before and after reaction

242

were found to be 4.4 emu/g and 9.9 emu/g, respectively. This might attribute to the 11

ACCEPTED MANUSCRIPT 243

dissolution of Zn0 in Zn0-CNTs-Fe3O4 composite after reaction, which increased the

244

relative content of Fe3O4 in Zn0-Fe3O4-CNTs composite. The superparamagnetism

245

shown in Fig. 4d (the inset) affirmed that the Zn0-CNTs-Fe3O4 composite could be

246

easily separated and recovered from solution by applying an external magnetic field,

247

which is an especially important advantage for composite materials.

248

3.2The synergetic effect of the Zn0-Fe3O4-CNTs/O2 system

249

The control experiments were performed to compare the removal efficiencies of

250

4-CP by various processes at pH 1.5 with initial 4-CP concentration of 50 mg/L. As

251

shown in Fig. 5a, the 4-CP removal efficiency was 99% within 10 min in the Zn0-

252

CNTs-Fe3O4/O2 system. The removal efficiencies of 4-CP were only 25% and 28% in

253

CNTs-Fe3O4/O2 system and Zn0-CNTs/O2 system, respectively, compared with the

254

Zn0-CNTs-Fe3O4/O2 system, suggesting that Fe3O4-CNTs and Zn0-CNTs had

255

synergistic effects for the removal of 4-CP. In addition, the degradation efficiency of

256

4-CP was 28% in the Zn0-CNTs-Fe3O4/N2 system, which was similar to the removal

257

efficiency of 4-CP in CNTs-Fe3O4/O2 system and Zn0-CNTs/O2 system, indicating

258

that the removal of 4-CP in Zn0-CNTs-Fe3O4/N2, CNTs-Fe3O4/O2 and Zn0-CNTs/O2

259

systems was primarily ascribed to the adsorption action.

260

In order to further elucidate the ability of Zn0-CNTs-Fe3O4/O2 system for the

261

removal of 4-CP, the accumulation concentrations of H2O2 in different systems were

262

measured. As seen in Fig. 5b, the accumulation concentration of H2O2 at 10 min in

263

Zn0/O2 system and Zn0-CNTs/O2 system was 14.80 mg/L and 26.33 mg/L,

264

respectively, and then the concentration of H2O2 decreased slightly. 12

ACCEPTED MANUSCRIPT 265

The catalytic reactions in Zn0-CNTs-Fe3O4/O2 system include two process. One

266

is the reduction of oxygen into H2O2 by CNTs; and the other is the decomposition of

267

H2O2 into •OH by Fe3O4. The reduction of oxygen into H2O2 can be obtained by the

268

reaction of Zn0 and O2. If Zn0 was loaded on the CNTs, the formation of numerous

269

corrosion cells between the particles of Zn0 and CNTs made part of oxygen reduction

270

on the surface of CNTs. It was confirmed that H2O2 could be generated by the

271

reduction of oxygen on the surface of Zn0 (Eq. (1)) and the productivity of H2O2 could

272

be increased on the surface of Zn0-CNTs (Eq. (2), (3)). In Zn0-CNTs/O2 system, the

273

formation of numerous corrosion cells between the particles of Zn0 and CNTs made

274

part of oxygen reduction on the surface of CNTs. The accelerated corrosion rate of

275

Zn0 and the good catalytic performance of CNTs for the oxygen reduction based on

276

two-electron pathway were responsible for the high yield of H2O2 in Zn0-CNTs/O2

277

system. The slight decrease of the H2O2 concentration after 10 min might be due to

278

the reduction of H2O2 by Zn0 in Zn0-CNTs composite and the self-decomposition of

279

H2O2 (Eq. (4), (5)) (Gong et al., 2018).

280

Zn + O2 + 2H+ + 2e−→H2O2

(1)

281

Anode

Zn - 2e−→ Zn2+

(2)

282

Cathode

O2 + 2H+ + 2e−→H2O2

(3)

283

Zn + H2O2 → ZnO + H2O

(4)

284

Zn2+ + H2O2 + 2e−→Zn(OH)2

(5)

285

The reduction or self-decomposition of H2O2 could decrease the H2O2

286

concentration, high accumulation concentration of H2O2 in Zn0-CNTs/O2 system was 13

ACCEPTED MANUSCRIPT 287

due to that the generation rate of H2O2 was higher than its consumption rate.

288

Compared with the Zn0-CNT/O2 system, the accumulation concentration of H2O2

289

in the Zn0-CNTs-Fe3O4/O2 system was rather low (<12.5 mg/L), indicating that the

290

iron species in Zn0-CNTs-Fe3O4 composite accelerated the decomposition rate of

291

H2O2 (Eq. (6)-(18)) (Wang et al., 2016).

292

≡Fe2+·H2O + H2O2 → ≡Fe2+·H2O2 → ≡Fe3+ + •OHads + OH－

(6)

293

≡Fe3+ + H2O2 →≡Fe3+·H2O2 →≡Fe2+ + HOO•+ H+

(7)

294

≡Fe3+ + HOO• → ≡Fe2+ + O2 + H+

(8)

295

•OH

(9)

296

Fe2+ + H2O2→Fe3+ + •OHfree + OH－

(10)

297

Fe3+ + H2O2 →Fe2+ + HOO• + H+

(11)

298

Fe3+ + HOO• →Fe2+ + O2 + H+

(12)

299

•OH

+ Fe2+→OH－ + Fe3+

(13)

300

•OH

+ H2O2 →H2O + HOO•

(14)

301

HOO• + Fe2+ → HOO－ + Fe3+

(15)

302

HOO• + HOO• →H2O2 + O2

(16)

303

•OH

+ HOO•(O2•－) → O2 + H2O (+OH－)

304

•OH

+ •OH →H2O2

ads

+ H2O2 → HOO• + H2O

(17) (18)

305

The catalytic decomposition of H2O2 by Fe3O4 for the formation of oxidizing

306

species had been proved by many heterogeneous Fenton-like reactions (Wang et al.,

307

2016; Huang et al., 2017; Wan and Wang, 2017). Nearly no H2O2 was detected in the

308

CNTs-Fe3O4/O2 and Zn0-CNTs-Fe3O4/N2 system, demonstrating that the oxygen and 14

ACCEPTED MANUSCRIPT 309

Zn0 were necessary for the in situ generation of H2O2 in the Zn0-CNTs-Fe3O4/O2

310

system.

311

Based on above analysis, it could be concluded that: (1) the Zn0-CNTs-Fe3O4

312

composite had good adsorption capacity for 4-CP, which favored its degradation (Hu

313

et al., 2011); (2) the in situ H2O2 generation in the Zn0-CNTs-Fe3O4/O2 system was

314

mainly caused by the reaction between the Zn0-CNTs and O2; (3) the activation of

315

H2O2 generated in situ by Fe3O4 in Zn0-CNTs-Fe3O4 composite promoted the removal

316

of 4-CP. Fig. 5

317 318

3.3. Influence of the operating parameters on 4-CP degradation

319

3.3.1. Effect of initial concentration of 4-CP

320

The degradation experiment of 4-CP at different initial 4-CP concentrations was

321

investigated with initial pH of 2.0, Zn0-Fe3O4-CNTs dosage of 2 g/L and O2 flow rate

322

of 400 mL/min. It can be seen from Fig. 5c that the removal efficiency of 4-CP was

323

99%, 82%, 68% and 35%, respectively, when 4-CP concentration was 25, 50, 100 and

324

200 mg/L after 20 min, respectively. Lower degradation efficiency was observed at

325

higher 4-CP initial concentrations, because the contaminants could compete with

326

H2O2 for the active sites on the Zn0-Fe3O4-CNTs surface (Wan and Wang, 2017).

327

Similar results were also observed by other researchers (Xue et al., 2009; Xu and

328

Wang, 2011; Liu et al., 2017a).

329

3.3.2. Effect of initial pH

330

The initial pH of the solution is an important factor that could affect the removal 15

ACCEPTED MANUSCRIPT 331

efficiency of 4-CP by the Zn0-CNTs-Fe3O4/O2 system, because it affected the in situ

332

generation of H2O2 from the reduction of O2 by Zn0-CNTs and the oxidation of 4-CP

333

by the •OH radicals from the reaction of H2O2 and Fe3O4. The initial pH was selected

334

in the range of 1–3 in this study, because lower initial pH values favor the removal of

335

organic contaminants by Fenton reaction (Masomboon et al., 2009) and the generation

336

of H2O2 from the reduction of O2 needed the participation of protons (Puértolas et al.,

337

2015). Fig. 5d showed that the removal of 4-CP was pH dependent obviously and the

338

removal efficiency of 4-CP increased with the decrease of initial pH. Only 18% of 4-

339

CP was removed after 20 min at initial pH 3.0, while the removal efficiency increased

340

to 81% when pH decreased to 2.0. The highest 4-CP removal efficiency (99%) was

341

found at initial pH 1.5. Nevertheless, when the initial pH was further decreased to 1,

342

the removal efficiency of 4-CP was not increased compared to the initial pH 1.5.

343

Therefore, the initial pH of 1.5 was chosen for the degradation of 4-CP by the Zn0-

344

CNTs-Fe3O4/O2 system.

345

To further illustrate the effect of initial pH on the removal of 4-CP, the variation

346

of pH in solution was determined. As shown in Fig. 5e, the pH value increased

347

gradually with reaction time. After 20 min of reaction, the final pH appeared to be

348

1.28, 6.25 and 6.26 when the initial pH was adjusted to 1.0, 2.0 and 3.0, respectively.

349

However, when the initial pH was 1.5, the final pH increased to 2.98 after 20 min,

350

which could explain why the highest removal efficiency of 4-CP was gained at initial

351

pH of 1.5 in Zn0-CNTs-Fe3O4/O2 system. Fenton oxidation of organic contaminants

352

could achieve better removal efficiency in the pH range of 2-3 (Daud and Hameed, 16

ACCEPTED MANUSCRIPT 353

2010; Xu and Wang, 2012a; Zhang et al., 2012).

354

In Zn0-CNTs-Fe3O4/O2 system, the consumption of H+ was resulted from the

355

oxygen reduction to H2O2 and the reduction of H+ to hydrogen, which lead to the

356

increase of pH value after reaction. In Zn0-CNTs-Fe3O4/O2 system, Zn0 might react

357

with H+ in acid solution to form hydrogen. The formation of H2O2 by the reaction of

358

H2 and O2 was feasible thermodynamically. However, compared with the reaction of

359

Zn0-CNTs and O2, the formation process of H2O2 by the reaction of H2 and O2 was

360

negligible due to its low reaction rate without catalyst. In Zn0-CNTs-Fe3O4/O2 system,

361

the amount of the H+ taking part in the generation of H2O2 was difficult to estimate

362

because the generation and consumption of H2O2 occurred simultaneously. The

363

reduction of H+ to hydrogen in Zn0-CNTs-Fe3O4/O2 system will further investigate.

364

The pH at potential of zero charge (pHPZC) of catalyst can be used to describe the

365

interaction among pollutant and catalyst surface area. In this study the pHpzc of the

366

Zn0-CNTs-Fe3O4 was determined to further understand the influence of pH on the

367

removal of 4-CP, which was about 11.5, indicating that the catalyst was positively

368

charged, there were substantial acid functional groups on its surface. Because 4-CP

369

was in negatively charged species in the solution, therefore the electrostatic attraction

370

between the catalyst and 4-CP was favorable for the adsorption of 4-CP onto the

371

catalyst, which would accelerate the removal of 4-CP in the Zn0-CNTs-Fe3O4/O2

372

system.

373

3.3.3. Effect of Zn0-CNTs-Fe3O4 dosage

374

The influence of Zn0-CNTs-Fe3O4 dosage on the removal efficiency against time 17

ACCEPTED MANUSCRIPT 375

was illustrated in Fig. 5f. The results indicated that the removal efficiency of 4-CP

376

increased with increasing dosage of Zn0-CNTs-Fe3O4 from 0.5 to 2 g/L, and the

377

highest removal efficiency of 99% was observed after 20 min at Zn0-CNTs-Fe3O4

378

dosage of 2.0 g/L. The higher removal efficiency at higher dosage was mainly

379

attributed to the higher amount of active site in the Zn0-CNTs-Fe3O4/O2 process

380

(Hassan and Hameed, 2011). Nevertheless, when the Zn0-CNTs-Fe3O4 composite

381

dosage further increased from 2.0 g/L to 4.0 g/L, the removal efficiency of 4-CP

382

slightly increased. This might be partly due to the reduction of •OH radical，cleared

383

by the adverse reaction with Fe(II) (Liu et al., 2017a). Thus, the optimum Zn0-CNTs-

384

Fe3O4 dosage was 2 g/L for the degradation of 4-CP in this experiment.

385

3.4. Involved active oxidation species

386

Free radical quenching studies are effective in identifying the actual reactive

387

species in the Fenton or Fenton-like systems (Chen et al., 2017; Huang et al., 2017).

388

Therefore, tertiary butanol (TBA) and p-benzoquinone (BQ) were used as strong

389

radical scavengers for •OH radical and O2•- radical respectively in Zn0-CNTs-

390

Fe3O4/O2 system. As shown in Fig. 6a, the removal efficiency of 4-CP decreased

391

from 99% to 19% at 20 min after adding 300 mmol/L tertiary-butanol, but 45% of 4-

392

CP was still removed within 20 min after adding 20 mmol/L p-benzoquinone, which

393

indicated that •OH radicals were the dominant reactive species for 4-CP degradation

394

in the Zn0-CNTs-Fe3O4/O2 system.

395

The variation of TOC concentration was observed to assess the mineralization

396

level of 4-CP in Zn0-CNTs-Fe3O4/O2 system (Fig. 6b). The removal efficiency of 18

ACCEPTED MANUSCRIPT 397

TOC (57%) after 20 min was lower than that of 4-CP (99%), indicating that some of

398

the intermediates derived from 4-CP decomposition remained in solution. This might

399

be due to the multi-step reactions of 4-CP mineralization (Huang et al., 2015; Wang et

400

al., 2016; Liu et al., 2017b; Shen et al., 2017). It was also shown that the synergetic

401

effect between Zn-CNTs and Fe(II) species in Zn0-CNTs-Fe3O4 composite played a

402

key role in the further mineralization of the intermediates, thus obtaining high

403

removal efficiency of TOC.

404

405

Fig. 6 3.5. The intermediates and possible pathways of 4-CP degradation

406

The variation of the concentrations of chloride, formic acid, acetic acid and oxalic

407

acid during the removal of 4-CP was illustrated in Fig. 6c. About 95% of the chlorine

408

was released from the aromatic ring at 20 min. The concentration of formic acid and

409

acetic acid both increased with the increase of reaction time and reached a peak at 5

410

min. Then the formic acid concentration remained almost unchanged and the

411

concentration of acetic acid decreased slightly after 5 min. Moreover, the

412

concentration of oxalic acid was very low (< 2 mg/L at 20 min).

413

The intermediate products of 4-CP degradation by the Zn0-Fe3O4-CNTs/O2

414

system were monitored by HRLC–ToF-MS analysis. The intermediates at m/z 143

415

and m/z 125 were found in the aqueous solution, suggesting that the •OH substitution

416

reaction and the •OH addition reaction were involved in 4-CP degradation in the Zn0-

417

Fe3O4-CNTs/O2 system. The presence of intermediates at m/z 117 and m/z 151

418

indicated the opening of the dehalogenation and aromatic ring in the degradation 19

ACCEPTED MANUSCRIPT 419

process. Because benzoquinone (BQ) was not detected in the Zn0-Fe3O4-CNTs/O2

420

system, the direct dechlorination of 4-CP by the reduction of Zn0 could be ignored.

421

Based on the above-mentioned analyses, the main degradation pathway of the

422

complete mineralization of 4-CP with •OH as the main oxidant was illustrated in Fig.

423

7a. As can be seen that 4-CP was converted to 4-chlorocatechol (D1) by the attack of

424

•OH

425

was attacked by the substitution of •OH radical to form intermediates D3. Moreover,

426

the C=C bond in the aromatic rings of 4-chlorocatechol was cleaved by the attack of

427

•OH

428

D4 and D5 were further oxidized to small molecule carboxylic acids, which were

429

further mineralized to CO2 and H2O.

430 431

radical at the ortho-position of the hydroxyl group. Afterward, 4-chlorocatechol

radical to form D2. Then D3 and D2 were rapidly transformed to D4 and D5.

Fig. 7 3.6. Proposed reaction mechanism of Zn0-CNTs-Fe3O4/O2 system

432

According to aforementioned analyses and the Fenton oxidation mechanism (de

433

la Plata et al., 2010b, a; Yuan et al., 2011; Liu et al., 2017b), the reaction mechanism

434

of the Zn0-CNTs-Fe3O4/O2 system were summarized and presented in Fig. 7b. The

435

Zn0-CNTs-Fe3O4/O2 system for the degradation of contaminants can be described by

436

three steps: (1) the in situ generation of H2O2: Based on the theory of corrosion

437

electrochemistry, zinc-carbon galvanic cells was formed in solution by the direct

438

contact of Zn0 with CNTs. When Zn0-Fe3O4-CNTs was in contact with dissolved O2

439

in solution, Zn0 released electrons and the produced electrons were transmitted to

440

CNTs, then transferred to dissolved O2 on the surface of CNTs to generate H2O2; (2) 20

ACCEPTED MANUSCRIPT 441

the in situ generation of •OH radicals by heterogeneous Fenton-like reaction: Fe(II)

442

species from Zn0-CNTs-Fe3O4, either in the form of solid or Fe2+ ions, could react

443

with H2O2 to generate •OH radicals in situ; (3) the degradation of contaminants by

444

•OH

445

contaminants or diffuse into the solution phase to oxidize non-adsorbed contaminants.

446

Small amount of Zn2+ might enter the solution of Zn0-CNTs-Fe3O4/O2 system due

447

to the oxidation of Zn0 and the hydration of formed Zn2+. Zn2+ was a common

448

contaminant and might decrease the soil microbial activity (Zhang et al., 2015).

449

However, Zn2+ in the solution could be converted to the low solubility Zn(OH)2 via its

450

combination with the OH− generated by the reduction of O2. The solubility product

451

constant (Ksp) of Zn(OH)2 at pH 7 and 25℃ was only 1.2 × 10−17. Therefore, the

452

secondary contamination of Zn2+ could be avoided by separation under neutral

453

condition. Although the formation of Zn(OH)2 may not happen when the solution pH

454

was kept lower than 3, the secondary contamination of Zn2+ could also be avoided by

455

increase the reaction time or adjust the pH value of solution to neutral after the

456

degradation of 4-CP finished.

radicals: The in situ generated •OH radicals could oxidize the adsorbed

457 458

4. Conclusions

459

Zn0-CNTs-Fe3O4 composites were successfully synthesized by the chemical co-

460

precipitation combined with high sintering process. A novel heterogeneous Fenton-

461

like system that could continuously generate H2O2 and •OH radicals by the reaction

462

between Zn0-CNTs-Fe3O4 and O2 was developed. The Zn0-CNTs-Fe3O4/O2 system for 21

ACCEPTED MANUSCRIPT 463

the oxidative degradation of contaminants included three progressive processes: 1) the

464

in situ generation of H2O2 by the reaction of Zn0-CNTs and O2, 2) the in situ

465

generation of •OH radicals through the decomposition of H2O2 by Fe3O4,

466

3) the degradation of contaminants by •OH radical. Under the conditions of initial pH

467

of 1.5, O2 flow rate of 400 mL/min, Zn0-CNTs-Fe3O4 dosage of 2 g/L and initial 4-CP

468

of 50 mg/L, the removal efficiencies of 4-CP and TOC were 99% and 57%,

469

respectively. This system could be used for the degradation and mineralization of

470

toxic organic contaminants in water without adding H2O2. It has to be mentioned that

471

zinc was consumed during the reaction, which will be further studied to improve the

472

durability of the composite. Even though, the novel Zn0-CNTs-Fe3O4/O2 system could

473

be a promising alternative strategy for wastewater treatment.

and then

474 475 476

Acknowledgement The research was financially supported by the Key Laboratory of Special

477

Wastewater

Treatment,

478

SWWT2015-1).

Sichuan

Province

479

22

Higher

Education

System

(No.

ACCEPTED MANUSCRIPT 481

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Legends

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Fig. 1 SEM images of Zn0-CNTs-Fe3O4

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Fig. 2 SEM-EDS analysis of Zn0-CNTs-Fe3O4. (a) SEM image; (b) EDS spectrum; (c) EDS mapping pictures of C, O, Fe and Zn elements.

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Fig. 3 XRD patterns of fresh Zn0-CNTs-Fe3O4 (1) and used Zn0-CNTs-Fe3O4 (2).

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Fig. 4 (a) XPS full survey spectra of Zn0-CNTs-Fe3O4 composite and the highresolution scan of Fe 2p region (the inset); (b) Fe 2p spectra of Zn0-CNTs-Fe3O4 composite; (c) Nitrogen adsorption/desorption isotherms and corresponding pore size distribution curve (the inset) of Zn0-CNTs-Fe3O4 composite; (d) Magnetization curves of Zn0-Fe3O4-CNTs composite before and after reaction, and the photograph of the sample attracted by a magnet (the inset).

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Fig. 5 (a) The degradation of 4-CP in different systems; (b) Variation of H2O2 concentration with reaction time in different systems. Reaction conditions: Zn0-Fe3O4CNTs=2.0 g/L, CNTs-Fe3O4=0.57 g/L, Zn0-CNTs=1.71 g/L, Zn0=2.0 g/L, O2 flow rate=400 mL/min, 4-CP concentration=50 mg/L, initial pH=1.5, T=25 ℃. And, effect of operating parameters on 4-CP degradation in the Zn0-CNTs-Fe3O4/O2 system: (c) initial 4-CP concentration; (d) initial pH; (e) the variation of the solution pH; (f) Zn0CNTs-Fe3O4 dosage. Reaction conditions: O2 flow rate=400 mL/min, Zn0-Fe3O4CNTs=2.0 g/L, initial 4-CP concentration=50 mg/L, initial pH=1.5, T=25 ℃.

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Fig. 6 (a) Effect of radical scavengers on the degradation of 4-CP; (b) Temporal change in 4-CP and TOC removal; (c) Evolution of the concentration of chloride ion and small organic acids formed during the removal of 4-CP. Reaction conditions: O2 flow rate=400 mL/min, Zn0-Fe3O4-CNTs=2.0 g/L, initial 4-CP concentration=50 mg/L, initial pH=1.5, T=25 ℃.

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Fig. 7 (a) Proposed removal pathway of 4-CP by the Zn0-Fe3O4-CNTs/O2 system; (b) The reaction mechanism of the Zn0-CNTs-Fe3O4/O2 system.

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Fig. 1 SEM images of Zn0-CNTs-Fe3O4

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Fig. 2 SEM-EDS analysis of Zn0-CNTs-Fe3O4. (a) SEM image; (b) EDS spectrum; (c) EDS mapping pictures of C, O, Fe and Zn elements.

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Fig. 3 XRD patterns of fresh Zn0-CNTs-Fe3O4 (1) and used Zn0-CNTs-Fe3O4 (2).

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Fig. 4 (a) XPS full survey spectra of Zn0-CNTs-Fe3O4 composite and the high-resolution scan of Fe 2p region (the inset); (b) Fe 2p spectra of Zn0-CNTs-Fe3O4 composite; (c) Nitrogen adsorption/desorption isotherms and corresponding pore size distribution curve (the inset) of Zn0CNTs-Fe3O4 composite; (d) Magnetization curves of Zn0-Fe3O4-CNTs composite before and after reaction, and the photograph of the sample attracted by a magnet (the inset).

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Fig. 5 (a) The degradation of 4-CP in different systems; (b) Variation of H2O2 concentration with reaction time in different systems. Reaction conditions: Zn0-Fe3O4-CNTs=2.0 g/L, CNTsFe3O4=0.57 g/L, Zn0-CNTs=1.71 g/L, Zn0=2.0 g/L, O2 flow rate=400 mL/min, 4-CP concentration=50 mg/L, initial pH=1.5, T=25 ℃. And, effect of operating parameters on 4-CP degradation in the Zn0-CNTs-Fe3O4/O2 system: (c) initial 4-CP concentration; (d) initial pH; (e) the variation of the solution pH; (f) Zn0-CNTs-Fe3O4 dosage. Reaction conditions: O2 flow rate=400 mL/min, Zn0-Fe3O4-CNTs=2.0 g/L, initial 4-CP concentration=50 mg/L, initial pH=1.5, T=25 ℃. 32

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Fig. 6 (a) Effect of radical scavengers on the degradation of 4-CP; (b) Temporal change in 4CP and TOC removal; (c) Evolution of the concentration of chloride ion and small organic acids formed during the removal of 4-CP. Reaction conditions: O2 flow rate=400 mL/min, Zn0-Fe3O4CNTs=2.0 g/L, initial 4-CP concentration=50 mg/L, initial pH=1.5, T=25 ℃.

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Fig. 7 (a) Proposed removal pathway of 4-CP by the Zn0-Fe3O4-CNTs/O2 system; (b) The reaction mechanism of the Zn0-CNTs-Fe3O4/O2 system.

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A novel Zn0-CNTs-Fe3O4 composite was synthesized.

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Zn0-CNTs-Fe3O4 could react with O2 in solution to generate H2O2 and •OH in situ.

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The removal efficiencies of 4-CP and TOC could reach to 99% and 57%, respectively.

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Zn0-CNTs-Fe3O4 could be conveniently separated from solution.