Activation CuFe2O4 by Hydroxylamine for Oxidation of Antibiotic

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Activation CuFe2O4 by Hydroxylamine for Oxidation of Antibiotic Sulfametheoxazole Jianfei Yan, Jiali Peng, Leiduo Lai, Fangzhou Ji, Yunhong Zhang, Bo Lai, Qixuan Chen, Gang Yao, Xi Chen, and Liping Song Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03340 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 14, 2018

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Environmental Science & Technology

Activation CuFe2O4 by Hydroxylamine for Oxidation of Antibiotic

1 2

Sulfametheoxazole

3

Jianfei Yan†, Jiali Peng†, Leiduo Lai†, Fangzhou Ji†, Yunhong ZhangŠ, Bo Lai †,§,‡, ||,, Qixuan Chen†, Gang

4

Yao§, ||, Xi Chen£, Liping Song£

5

†Department

6

610065, China

7

§Sino-German

8

‡National

9

||Institute

of Environmental Science and Engineering, College of Architecture and Environment, Sichuan University, Chengdu

Centre for Water and Health Research, Sichuan University, Chengdu 610065, China

Engineering Research Center for Flue Gas Desulfurization, Sichuan University, Chengdu 610065, China

of Environmental Engineering, RWTH Aachen University, Germany

10

ŠBiogas

Institute of Ministry of Agriculture, Chengdu 610041, China

11

£SCIEX

Analytical Instrument Trading Co., Shanghai, 200335, P.R. China

12

ABSTRACT: A new potential oxidation process is provided by CuFe2O4/hydroxylamine (HA)

13

system for degradation of antibiotics in water. The CuFe2O4/HA system can generate reactive

14

oxygen species (ROS) for the degradation of sulfamethoxazole (SMX). The addition of radical

15

scavengers, including benzoquinone (BQ) and catalase (CAT), inhibited the oxidation of SMX in

16

CuFe2O4/HA system. Electron transfer in the CuFe2O4/HA system played a key function for the

17

generation of ROS and the degradation of SMX. The main ROS, was the superoxide radical (O2•-)

18

mainly generated from adsorbed oxygen (O2 (A)), which came from the oxidation of the lattice

19

oxygen (O2- (L)) in CuFe2O4. The CuFe2O4/HA system was effectively applicable for a broad pH

20

range (approximately 5−10). In addition, the activation mechanism for CuFe2O4/HA system was

21

studied with the target contaminant SMX. Finally, the degradation pathways of SMX were

22

proposed under the optimal conditions in CuFe2O4/HA system.

1

ACS Paragon Plus Environment


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23 24 25

TOC Art

INTRODUCTION

26

Due to the overuse of antibiotics, sulfamethoxazole (SMX) is frequently detected in related

27

aquatic environment.1-3 The concentration of SMX in surface water has been detected at the level

28

from 0.01 µg L-1 to 2.0 µg L-1 in different countries, which do not show directly side effects or

29

toxicity on human health.4,

30

bacteria and antibiotic resistant genes, reducing its the potential for healing human and animal

31

pathogens.6 When antibiotic resistant bacteria causing infections are no longer sensitive to

32

antibiotics treatment, which could pose a serious threat to mankind health. Thus, the increase of

33

antibiotic resistant bacteria caused by the residual SMX has caused serious public concern.7, 8

34

Therefore, it is urgent to exploit effective technologies to degrade or eliminate SMX in water.

5

Residual SMX leads to faster emergence of antibiotic-resistant

35

Cupric ion (Cu2+) can catalyze H2O2 to generate the reactive oxygen species (ROS).9 Ferrous

36

iron (Fe2+), due to the superiority of high activity, environmental-friendly and low cost nature, has

37

been widely used as a catalyst in Fenton and other Fenton-like processes.10,

38

homogeneous catalytic process has the problem of secondary pollution.3, 12 The heterogeneous

39

catalytic process is more efficient than the homogeneous catalytic process, due to its mild operated

40

conditions and little generated iron sludge.3, 13, 14 CuFe2O4, a type of magnetic spinel nano-particle,

41

has widely aroused people’s attention because of its good separation, remarkable catalysis and 2


11

However,

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desirable stability.14, 15 The catalyst has been widely applied for the heterogeneous catalysis of

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ozone, peroxydisulfate (PDS), H2O2 and peroxymonosulfate (PMS) to degrade organic

44

pollutants.14-17

45

In addition, the reduction of Fe3+ and Cu2+ to Fe2+ and Cu+ was difficult in advanced oxidation

46

processes (AOPs), which greatly limited the catalytic ability of catalyst.14, 18 Hydroxylamine (HA),

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an inorganic substance used to prepare oximes,19-21 has been used to active oxidants (H2O2, PDS

48

and PMS), showing good activation performance.19, 22 What is more, the addition of HA, has been

49

proved to be an effective way for accelerating the cycle of Fe3+ to Fe2+ or Cu2+ to Cu+ in

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homogeneous catalytic process.10, 18, 23 In addition, previous reports suggest the combination of

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HA with Cu2+ could produce O2•- and H2O2 by reducing dissolved oxygen, and Fe2+ could also

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generate O2•- by oxygen reduction in homogeneous catalytic process.18, 24 The following reactions

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describe the generation of ROS (reactions 1-16).9, 13, 18, 24-26 Recently, superoxide radical (O2•-)

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catches attention from the public for its potential to destroy highly toxic organic chemicals such

55

as pesticides, dioxins, chlorinated solvents and other carcinogenic chemicals.11, 13 O2•-, as a reactive

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oxygen species, was found in photocatalytic reactions under visible light irradiation, magnetite

57

nanoparticles activation persulfate process, reaction process of Fe2+ and O2, and Cu2+/HA system.

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4, 13, 18, 24

59

heterogeneous generation of O2•- or other ROS.

Thus, the CuFe2O4/HA system might also be an effective way to degrade SMX by

60

Cu2 + + 1 2NH2OH→1 4N2O + 1 4H2O + Cu + + H +

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Cu2 + + NH2OH→1 2N2 + H2O + Cu + + H +

62

Cu + + O2→O2 + Cu2 +

63

Cu + + O.2- +2H + →Cu2 + + H2O2

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Cu + + H2O2→Cu2 + + HO + OH - or Cu3 + +2OH -

•-

(1) (2) (3) (4)

•

3


(5)


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Cu2 + + O•2 - →Cu + + O2

(6)

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2O•2 - + 2H + →O2 + H2O2

(7)

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Cu3 + + O•2 - →Cu2 + + O2

68

Cu2 + + H2O2→Cu + + O•2 - + 2H +

(9)

69

Cu3 + + H2O2→Cu2 + + O•2 - + 2H +

(10)

70

Fe2 + + O2→O•2 - + Fe3 +

71

Fe2 + + O•2 - + 2H + →Fe3 + + H2O2

72

O•2 - + H + ↔HO2

73

Fe2 + + H2O2→Fe3 + + HO

74

Fe3 + + H2O2→Fe2 + + HO2

75

HO2→O•2 - + H +

(8)

(11) (12)

•

(13) •

(14)

•

(15)

•

(16)

76

In this study, CuFe2O4 magnetic spinel nano-particles were synthesized with sol-gel

77

combustion method and coupled with HA to degrade SMX in aqueous solution. This study has the

78

following purposes: (i) evaluating the degradation potential of the CuFe2O4/HA system for SMX;

79

(ii) identifying the reactive oxidants generated from the CuFe2O4/HA system under the optimal

80

conditions; (iii) revealing the activation mechanism of the CuFe2O4/HA system; (iv) deducing the

81

SMX degradation pathways.

82

MATERIALS AND METHODS

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Reagents. Sources of chemicals are provided in the Supporting Information (SI) Text S1.

84

Synthesis of CuFe2O4 Magnetic Nano-particles. According to previously reported protocol,

85

CuFe2O4 particles were synthesized by using a sol-gel combustion method.

86

method is provided in SI Text S2.

87

Experimental Setup and Procedure. H2SO4 and NaOH solutions were used to adjust the initial 4


14

Details pertaining

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88

solution pH. The batch experiments were conducted in 500 mL glass beaker at 30 °C, 300 mL

89

stock SMX solution was added into the glass beaker and mixed by using a mechanical stirrer (300

90

rpm). The addition of CuFe2O4 and stock solution of HA would initiate reactions. 1 mL samples

91

were taken out and quenched with 50 µL sodium thiosulfate immediately at fixed time intervals,

92

then which were filtered with 0.45 µm filter membrane and stored at 4 °C. Quenching experiments

93

with tert-butyl alcohol (TBA), methanol (MeOH), catalase (CAT) and 1, 4-benzoquinone (BQ)

94

were performed by adding the desired quenchers into the reaction solution before the addition of

95

CuFe2O4 and HA. In order to investigate source of oxygen in O2•- of CuFe2O4/HA system, the air

96

was pumped into to the CuFe2O4/HA system during the reaction. What is more, the no dissolved

97

oxygen environment of CuFe2O4/HA system was obtained by pumping N2 in a sealed three-necked

98

flask. All experiments were done three times, and the error bars of standard deviations and the

99

average values are provided.

100

Analytical Methods. The SMX concentration was analyzed by high-performance liquid

101

chromatograph (HPLC) (Agilent USA) with an Eclipse XDB C-18 column. Acetonitrile and oxalic

102

acid solution (0.01 M) mixture was used as the mobile phase at a flow rate of 1 mL min−1, and the

103

detection wavelength was 264 nm. The samples were analyzed by HPLC within 6 hours, and the

104

quenched results of 50 µL sodium thiosulfate were provided in SI Figure S1. Total nitrogen (TN)

105

and residual HA concentration were determined by UV−vis spectrometer and the same HPLC

106

system respectively, the detailed procedures were shown in IS Text S3 and Text S4. 19, 27 The

107

concentration of NO3- and NO2- were analyzed by ion chromatograph (ICS-90). Dissolved N2O

108

was measured using a microsensor (UNISENSE). Total organic carbon (TOC) of degraded

109

samples were analyzed using TOC analyzer (Shimadzu, Japan). Dissolved oxygen was measured

110

with a DO Meter (2FD354, Germany). The method of potassium titanium (IV) oxalate was used 5



111

to determine the H2O2 concentration at 400 nm with a UV-vis spectrophotometer (UV-2550,

112

Shimadzu, Japan).28 Electron paramagnetic resonance (EPR) was used to detect O2•-, and detailed

113

experiments can be seen in SI Text S5. The SCIEX ExionLC™ AC -SCIEX X500R QTOF-

114

MS/MS (LC-QTOF-MS/MS) system was used to identify the intermediates of SMX, and detailed

115

information can be seen in SI Text S6. The characterization analytical methods of CuFe2O4 can be

116

seen in SI Text S7.

117

RESULTS AND DISCUSSION

118

Characterization of CuFe2O4. The specific surface areas of the new CuFe2O4 and used CuFe2O4

119

were 22 and 24 m2/g, respectively. The saturation moments of the new CuFe2O4 and used CuFe2O4

120

were about 27.96 and 28.16 emu/g (Figure S2), respectively. These results suggest that the specific

121

surface area of used CuFe2O4 was increased and the saturation moment of CuFe2O4 had almost no

122

change. Comparing to the fresh CuFe2O4, no morphology change was found in the used CuFe2O4

123

by SEM (Figure S3) and TEM (Figure S4) images results. What is more, the SEM-EDS results

124

show that surface oxygen content of the reunion CuFe2O4 was increased and the atomic ratio of

125

Cu and Fe (closed to 1:2) had almost no change after reaction (Figure S5). The increased oxygen

126

content of the reunion CuFe2O4 and the increased specific surface area were presumed to be the

127

conversion of lattice oxygen to adsorbed oxygen according to the experimental results of X-ray

128

photoelectron spectroscopy (XPS) (Figure S11 (b)).29 XRD patterns of the fresh and used CuFe2O4

129

in CuFe2O4/HA system were illustrated in SI Figure S6. The characteristic peaks of the catalyst

130

were observed at 2θ= 29.9o, 35.1o, 43.8o, 57.8o and 62.2o, which could be indexed to the (112),

131

(221), (220), (321) and (224), respectively.14 The diffraction patterns were matched well with the

132

standard of the CuFe2O4 particles (JCPDS File No. 34-0425), suggesting the formation of CuFe2O4

133

with a considerable degree of crystallization. Similarly, according to the standard JCPDS File No. 6


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45-0937, the peak at about 2θ = 39° was CuO. Little change and no additional peaks were observed

135

in the used CuFe2O4, suggesting the component and structure of CuFe2O4 were not destroyed. The

136

result suggests that the structure of CuFe2O4 was stable in the CuFe2O4/HA system. In addition,

137

the average crystallite size of CuFe2O4 was 13.9 nm, calculating by Scherrer equation.

138

the solid particles size was smaller than 20 nm especially, which could cause quantum confinement

139

effects.13, 30 The O2•- solvation shell and the surface free energy might be altered by quantum

140

confinement, which might enhance the reactivity of reactants.

141

O2•- was lower than that of •OH and SO4•− in aqueous solution,13, 30 which would be enhanced when

142

the O2•- presented on the surface of CuFe2O4. 13, 28, 29

143

Parameters Optimization. To obtain optimal experimental conditions, the effects of different

144

CuFe2O4 dosages (0-7 g/L), HA dosages (0-1.2 mM) and initial pH (3-11) on oxidation SMX were

145

examined in CuFe2O4/HA system (Figure S7). The optimized CuFe2O4 dosage and HA dosage

146

were 6 g/L and 1.0 mM, and the CuFe2O4/HA system showed a wide range of pH applications

147

(approximately 5−10). Detail results and discussion can be seen in SI Text S8-S10.

148

Effects of Inorganic Anion. The presence of Cl- and Br- had a little adverse effect on CuFe2O4/HA

149

system (Figure S8 (a) and (b)), while the NO3-, HCO3- and H2PO4- anions obviously inhibited the

150

degradation of SMX in CuFe2O4/HA system (Figure S8 (c), (d) and (e)). Detail results and

151

discussion can be seen in SI Text S11.

152

Stability of the Spinel CuFe2O4. The CuFe2O4/HA system still exhibited favorable degradation

153

ability for SMX after using five times of CuFe2O4 (Figure S9). Detail results and discussion can

154

be seen in SI Text S12.

155

SMX Degradation in Different System.

156

13, 30, 31

14

When

Although the reactivity of

Six control systems (i.e., CuFe2O4 alone, HA alone, CuFe2O4/PMS, CuFe2O4/HA, 7



157

CuFe2O4/PDS and CuFe2O4/H2O2) were used to degrade SMX. The SMX removal rates were 0%,

158

0%, 4%, 65%, 86% and 100% within 300 s in CuFe2O4 alone, HA alone, CuFe2O4/H2O2,

159

CuFe2O4/HA and CuFe2O4/PMS systems, respectively (Figure 1). Neither CuFe2O4 or HA alone

160

could not degrade SMX, which might be the absence of reactive oxidants. When CuFe2O4 and HA

161

were simultaneously added to the SMX solution, the CuFe2O4/HA system exhibited a greater

162

oxidation capacity for SMX. All these evidences suggest that CuFe2O4 coupled with HA was an

163

effective system for the degradation of SMX from water. In addition, due to the excellent activation

164

ability of CuFe2O4 for PMS, the degradation of SMX in CuFe2O4/PMS system was faster than

165

other systems.

166

The TOC removal rate of SMX solution was measured to investigate the mineralization

167

capacity of the CuFe2O4/HA system. TOC removal of 0%, 0% and 37% were observed in CuFe2O4

168

alone, HA alone and CuFe2O4/HA systems in 300 s, respectively (Figure not shown). The result

169

suggests that the CuFe2O4/HA system possessed certain mineralization capacity for organic

170

pollutants and some organic intermediates were successfully mineralized to CO2. The major

171

transformation product of HA was N2, which suggests that the final fate of HA was friendly to

172

environment in the CuFe2O4/HA system (Figure S10). Detail results and discussion of HA

173

transformation products can be seen in SI Text S13.

174

Identification of Reactive Oxidants Species in CuFe2O4/HA System. Previous literatures

175

reported that the degradation of organic pollutants was generally free radical mechanism or metal

176

oxo.18, 32 The H2O2 and O2•- could be generated from Cu2+/HA system,18 and the •OH could be

177

generated from CuFe2O4/H2O2 and Fe2+/H2O2 systems.16, 18, 33 Thus, O2•-, H2O2 and •OH probably

178

presented in the CuFe2O4/HA system. It is widely accepted in the literatures that tert-butyl alcohol

179

(TBA), 1, 4-benzoquinone (BQ) and catalase (CAT) are quenchers for •OH, O2•- and H2O2, 8


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respectively.10, 34-36 The addition of BQ and CAT obviously inhibited the SMX degradation in the

181

CuFe2O4/HA system, while the presence TBA weakly suppressed the degradation of SMX (Figure

182

2 (a)). With 500 mM TBA, the SMX degradation rate only decreased from 86% to 83% in 300 s

183

(Figure 2 (a)), which suggests that there was a small amount of produced •OH radicals, but they

184

were not the major oxidant species for the degradation of SMX. However, in the presence of 2

185

mM BQ and exceed 500 U/mL CAT, the SMX degradation efficiencies were notably decreased

186

from 86% to 24% and 64% in 300 s, respectively. The strong inhibitory effects of BQ and CAT

187

evidenced that the O2•- and H2O2 generated in the CuFe2O4/HA system and the degradation of

188

SMX was mostly induced by the O2•-. To further identify the production of O2•- in the CuFe2O4/HA

189

system, the EPR technique with 5, 5-Dimethyl-1-pyrrolidine N-oxide (DMPO) as a spin-trapping

190

agent was used (Figure 2 (b)). The hyperfine splitting constant of DMPO radical adducts were

191

aN=13.59 G and aH=7.74 G in the CuFe2O4/HA system. These were representative of O2•- radicals

192

added to DMPO (DMPO-O2), suggesting that the O2•- radicals presented in the CuFe2O4/HA

193

system. Quenching experiments results show that H2O2 presented in the CuFe2O4/HA system but

194

the produced •OH radicals were very limited, which suggests only a small part of H2O2 were

195

translated to •OH radicals. However, the concentration of the produced H2O2 in solution was not

196

measured in the CuFe2O4/HA system by the potassium titanium (IV) oxalate method. Thus, it was

197

deduced that the produced H2O2 presented on the surface of CuFe2O4, and the filtration affected

198

the concentration detection of H2O2. Due to the lower direction oxidation capacity of H2O2 for

199

SMX, the degradation of SMX by H2O2 was achieved by converting to superoxide radicals

200

(reactions 26, 27, 31 and 32). In a conclusion, the reactive oxidants species in the CuFe2O4/HA

201

system contained •OH, H2O2 and O2•-, in which the O2•- radicals were the major reactive oxidants

202

species. 9



203

Electron transfer in the CuFe2O4/HA system might be the reason of O2•- formation. As

204

potassium dichromate (K2Cr2O7) was able to remove electrons, it was determined to be used to

205

investigate the electron transfer in the CuFe2O4/HA system.35, 37 The degradation efficiencies of

206

SMX had an obvious decrease in the CuFe2O4/HA system after the addition of 5 mM K2Cr2O7

207

(Figure 2 (c)). According to previous data, the SMX degradation rate in 300 s was 86% in the

208

CuFe2O4/HA system. However, the removal rates in 300 s were only 3% and 4% in the

209

K2Cr2O7/CuFe2O4/HA system and the K2Cr2O7 alone system, respectively, and the little SMX

210

degradation caused by the oxidation of K2Cr2O7. These results suggest that the electron transfer

211

presented in the CuFe2O4/HA system played an irreplaceable function in the generation of O2•- for

212

the degradation of SMX.

213

The XPS experiments were conducted to detect chemical states of the fresh and used CuFe2O4,

214

which would further verify the electron transfer behavior of CuFe2O4 in the CuFe2O4/HA system

215

(Figure S11). Figure S 11 (a) shows that Cu (2p), Fe (2p) and O (1s) presented in CuFe2O4 particles.

216

The acquired spectra were calibrated by the carbon 1s signal at 284.8 eV. All XPS core level

217

spectra were fitted using linear or Shirley background. Figure S11 (c) shows that before catalytic

218

oxidation process, the peak at binding energies of 934.1 eV was attributed to Cu(II), the Cu 2p

219

peaks at 941.3 eV and 943.7 eV were attributed to Cu(II) oxide species.14 For the used CuFe2O4,

220

a new peak at 933.2 eV was found on the catalyst, which was attributed to Cu(I).14 In addition, the

221

effects of alcohols (Figure S12) and Br- (Figure S8 (b)) on CuFe2O4/HA system provided evidences

222

of the existence of Cu(III), detail results and discussion can be seen in SI Text S14. The three

223

peaks located at 710.9 eV and 713.0 eV for Fe 2p 3/2 and 724.4 eV for Fe2p 1/2, which were

224

attributed of Fe(III) in the new CuFe2O4 (Figure S11 (d)).14 A new Fe 2p peak at 713.9 eV was

225

found in the used CuFe2O4, which was attributed to Fe(II).38 These results indicate that reduction 10


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reactions of Fe(III)/Fe(II) and Cu(II)/Cu(I) were involved in the CuFe2O4/HA system. As shown

227

in Figure S11 (b), for the fresh CuFe2O4, the main O 1s peak at 530.0 eV was attributed to the

228

surface lattice oxygen of metal oxides (O2−, denoted as O2− (L)) and the peak at 531.2 eV was

229

interpreted as oxygen in a single bond of the hydroxyl group adsorbed at the sample surface.14, 39

230

For the used CuFe2O4, the O 1s peaks at 531.8 eV and 533.0 eV were interpreted as adsorbed

231

oxygen (denoted as O2 (A)) or surface hydroxyl species and oxygen in the adsorbed H2O, in which

232

the O 1s peak with the observed position around 530.0 eV was attributed to the surface lattice

233

oxygen of metal oxides.37, 38 Previous research reported that lattice oxygen played a critical role

234

in oxidation reaction and was active oxygen species.29 The appearance of adsorbed oxygen on the

235

used CuFe2O4 was ascribed to lattice oxygen, which was initially oxidized to O2 and then adsorbed

236

on the catalyst surface.29 The decrease of lattice oxygen and the appearance of adsorbed oxygen

237

suggest that the lattice oxygen participated the reduction of Fe(III)/Fe(II) and Cu(II)/Cu(I), and the

238

electrons transfer presented in the CuFe2O4/HA system.29

239

Effects of Air and N2 on CuFe2O4/HA System. ROS, including O2•- and hydrogen peroxide

240

(H2O2), could generated from O2 by accepted electron from Fe(II) or Cu(I).9, 13, 18, 24, 40 To verify

241

the source of oxygen in superoxide radicals, the effects of air and N2 on the CuFe2O4/HA system

242

were done (Figure 3 (a)). The initial dissolved oxygen concentration ([O2]0) of the CuFe2O4/HA

243

system was 6.9 mg/L, without aeration during the reaction. When air was continuously pumped

244

into SMX solution during the reaction, the initial dissolved oxygen concentration was 7.1 mg/L,

245

and the SMX degradation rate decreased from 86% to 81% in 300 s. While the N2 presented in the

246

CuFe2O4/HA system (the dissolved oxygen concentration was 0 mg/L), the SMX degradation rate

247

increased from 86% to 94% in 300 s. The presence of air inhibited the SMX degradation while

248

without dissolved oxygen presented in CuFe2O4/HA system accelerated the SMX degradation. To

249

identify the reactive oxidants in the CuFe2O4/HA/N2 system, 2 mM BQ was used (Figure 3 (b)). 11



Page 12 of 29

250

After adding BQ, the SMX degradation rate decreased from 94% to 11%. The result suggests that

251

O2•- radicals were still produced when there was no dissolved oxygen in the CuFe2O4/HA system.

252

In addition, the O 1s peaks of the used CuFe2O4 in CuFe2O4/HA/N2 were presented in Figure S13,

253

only surface hydroxyl group (530.6 eV) and adsorbed oxygen (531.8 eV) were found on the used

254

CuFe2O4.10 What is more, no dissolved oxygen was found in the CuFe2O4/HA/N2 system (Date

255

not shown), which suggest that the disappeared the surface lattice oxygen completely converted to

256

surface hydroxyl group and adsorbed oxygen. These results suggest that the oxygen in superoxide

257

radicals came from the surface lattice oxygen of CuFe2O4 in the CuFe2O4/HA/N2 system. Previous

258

literature reported that the reactive oxidants could not be produced in the Cu(II)/HA/N2 system.18

259

Thus, it could be indicated that the oxygen in superoxide radicals mainly came from the O2 (A) in

260

CuFe2O4/HA system.

261

In PDS/MNPs (magnetite nanoparticles) system and photocatalyst process, the increasing

262

concentration of dissolved oxygen greatly accelerated the degradation of pollutants, which was

263

due to the fact that the increasing concentration of dissolved oxygen led to the generation of more

264

O2•- radicals.4, 13, 41 Furthermore, due to the lack of electronic acceptor (dissolved oxygen) in anoxic

265

environment, the degradation of pollutant was markedly inhibited in the photocatalysis process.41

266

However, in the CuFe2O4/HA system, the increasing concentration of dissolved oxygen reduced

267

the degradation of SMX (Figure 3 (a)). The reaction (reaction 17) of HA with the increase

268

dissolved oxygen reduced the HA amounts reacted with Cu(II) and Fe(III), which might cause the

269

decrease of SMX removal rate in the CuFe2O4/HA/air system.9

270

(17)

NH2OH + O2→N2 + H2O2 +2H2O

271

However, the reaction 17 was very slow, so which was not the major reason for the removal

272

rate decline in the CuFe2O4/HA/air system.18 Previous literature reported that, in Cu(II)/HA/O2

273

system, increasing concentration of O2 increased concentration of H2O2, but reduced the

274

concentration of Cu(I).9,

275

increased dissolved oxygen (reactions 3,11, 20 and 28), which would reduce the amount of Cu(I)

18

Thus, the concentration Cu(I) and Fe(II) might be reduced by the

12


Page 13 of 29


276

and Fe(II) in heterogeneous reactions. Heterogeneous reactions were mainly responsible for the

277

degradation of SMX (Figure S14 (b)), thus the reduction of Cu(I) and Fe(II) in heterogeneous

278

reactions might cause the decline of SMX removal rate. However, the Cu(I) might be oxidized to

279

Cu(III) (reactions 3, 4, 5, 20, 21 and 22) and a large amount of H2O2 presented in the CuFe2O4/HA

280

system according to the previous analysis, thus the generated Cu(III) could further produce radicals

281

(reactions 10 and 27), which means that the reduction of Cu(I) might not affect the SMX removal

282

rate and the reduction of Fe(II) was the major reason for the decrease in SMX removal rate. In

283

addition, the reactivity of O2•- would be enhanced when it presented in the solid surface.13

284

According to above analysis, the major amount of O2•- radicals generated from the O2 (A) and

285

presented on the surface of CuFe2O4, and the increase of dissolved oxygen would affect the amount

286

of O2•- (A) ( the O2•- adsorbed on the CuFe2O4 surface), which would also cause the degradation of

287

SMX removal rate.

288

Activation Mechanism of CuFe2O4/HA System. Fe2O3 and CuO were used to test the activity of

289

Cu(II) and Fe(III) in CuFe2O4 (Figure S14 (a)). The oxidative degradations of SMX in the CuO/HA,

290

Fe2O3/HA, CuO/Fe2O3/HA and CuFe2O4/HA systems were examined, respectively. Less than 12%

291

SMX was degraded by HA coupling with CuO or Fe2O3 or CuO and Fe2O3 in 300 s (Figure S14

292

(a)). A rapid decrease of the SMX concentration (86%) was observed for the HA/CuFe2O4 system.

293

The results suggest that Cu(II) or Fe(III) in a structure other than CuFe2O4 could not couple HA

294

to degrade effectively SMX. Due to the stable spinel structure, the leaching of copper ions from

295

CuFe2O4 were extremely few in CuFe2O4/PMS, CuFe2O4/PDS and CuFe2O4/O3 systems.14, 15, 17

296

The leaching of copper and iron were 12.5 mg/L and 0.068 mg/L in the CuFe2O4/HA system,

297

occupying the 0.8% and 0.002% of Cu and Fe quality in CuFe2O4 respectively. Previous studies

298

have also found that the dissolved copper concentration was higher than that of iron. 14 Due to the

299

fact that the formation of Fe(OH)3 was more easy than Cu(OH)2 around pH 5.2, which reduced 13



Page 14 of 29

300

the more concentration of Fe3+ comparing to Cu2+. The leaching of copper and iron coupled with

301

1 mM HA made 11% contribution for the SMX degradation (Figure S14 (b)). The heterogeneous

302

activation reactions made 75% contribution in the CuFe2O4/HA system. It can be concluded that

303

the heterogeneous activation reactions were the major reason of SMX degradation in the

304

CuFe2O4/HA system. In addition, the dissolved copper ions concentration was decreased to 0.46

305

mg/L by adding 3 g/L Fe0 to the effluent solution and stirring for 5 min. Then, the Fe/Cu bimetallic

306

particles in the effluent solution were magnetically separated from solution, which could be used

307

to activate PDS and O3 for the degradation of contaminant.42, 43 Thus, the dissolved copper ions of

308

final effluent solution were below maximum contaminant level (1.3 mg/L) set by U.S.

309

Environmental Protection Agency.18 According to above analysis and results, a likely catalytic

310

mechanism in the CuFe2O4/HA system was proposed (Figure 4). In heterogeneous activation

311

process, ≡ Cu(II) and ≡ Fe(III) in CuFe2O4 were first reduced to ≡ Cu(I) and ≡ Fe(II) by HA

312

( reactions 18, 19 and 33-36), and the O2−(L) was oxidized to O2 (A).9, 29, 33 HA and O2− (L) provided

313

electrons to CuFe2O4, which could speed up the transformation of ≡ Cu(II) to ≡ Cu(I) and ≡

314

Fe(III) to ≡ Fe(II). Second, reactive oxidants (O2•- and O2•- (A)) and H2O2 (A) (the H2O2 adsorbing

315

on the surface of CuFe2O4) were produced in CuFe2O4/HA system by electrons transfer processes.

316

The ≡ Cu(I) and ≡ Fe(II), generated by a series of single-electron transfer reactions (reactions

317

18, 19, and 33-36), reacted with the dissolved oxygen and O2

318

(reactions 20 and 28).9, 13, 18, 24 Previous literature also reported the adsorbed O2 on the Fe0 surface

319

could accept an electron and form O2•-.44 The concentration of H2O2 was not detected in solution,

320

thus the H2O2 came from the reactions of O2•- (A) with ≡ Cu(I) and ≡ Fe(II) (reactions 21 and 29)

321

and adsorbed on the catalyst surface.18 In addition, the reactions of between H2O2 (A) and ≡ Fe(III),

322

≡ Cu(II) and ≡ Cu(III) would also provide superoxide radicals (reactions 26, 27, 31 and 32).9, 11 14


(A)

to produce O2•- and O2•-

(A)

Page 15 of 29


323

These reactions were responsible for the O2•- and O2•- (A) generation in the CuFe2O4/HA system,

324

where the O2•-

325

heterogeneous activation process. In homogeneous activation process, the dissolved Fe3+ and Cu2+

326

were reduced to Fe2+ and Cu+, which then reacted with dissolved oxygen or O2 (A) to produce O2•-

327

and O2•- (A).9, 13, 18, 24 In addition, the Cu(III) presented in the CuFe2O4/HA system (reactions 5 and

328

22),18,

329

between Fe(III) and Cu(I) was thermodynamically favorable (reaction 37),45 where the product

330

Fe(II) (reaction 37) was beneficial to the generation O2•-

331

circulation of iron and copper might be accelerated in CuFe2O4/HA system, as shown in reaction

332

38.34 All in all, the O2•- (A) produced in the CuFe2O4/HA system played a particularly essential role

333

in the SMX degradation.

33

(A)

was the main factor in the degradation and mineralization of SMX during

which was also capable of oxidizing organic compounds.18 Meanwhile, the reaction

(A)

(reaction 11). In addition, the

334

≡ Cu(II) + 1 2NH2OH→1 4N2O + 1 4H2O + ≡ Cu(I) + H +

335

≡ Cu(II) + NH2OH→1 2N2 + H2O + ≡ Cu(I) + H +

336

≡ Cu(I) + O2 O2 (A)→O•2 - O•2 -(A) + ≡ Cu(II)

337

≡ Cu(I) + O•2 -(A) +2H + → ≡ Cu(II) + H2O2 (A)

338

≡ Cu(I) + H2O2 (A)→ ≡ Cu(II) + HO + OH - or ≡ Cu(III) + 2OH -

339

≡ Cu(II) + O•2 - O•2 -(A)→Cu(I) + O2 O2 (A)

340

2O•2 -(A) + 2H + →O2 + H2O2 (A)

341

≡ Cu(III) + O•2 - O•2 -(A)→ ≡ Cu(II) + O2 O2 (A)

342

≡ Cu(II) + H2O2 (A)→ ≡ Cu(I) + O•2 -(A) + 2H +

343

≡ Cu(III) + H2O2 (A)→ ≡ Cu(II) + O•2 -(A) + 2H +

（27）

344

≡ Fe(II) + O2 O2 (A)→O•2 - O•2 -(A) + ≡ Fe(III)

(28)

345

≡ Fe(II) + O•2 -(A) + 2H + → ≡ Fe(III) + H2O2 (A)

(18) (19) (20) (21)

•

(22) (23) (24)

15


(25) (26)

(29)


•

Page 16 of 29

(30)

346

≡ Fe(II) + H2O2 (A)→ ≡ Fe(III) + HO

347

≡ Fe(III) + H2O2 (A)→ ≡ Fe(II) + HO2

348

HO2→O•2 -(A) + H +

349

≡ Fe(III)/Fe3 + + NH2OH→NH2O + ≡ Fe(II)/Fe2 + + H +

350

2NH2O →N2 + H2O

351

≡ Fe(III)/Fe3 + + NH2O →NHO + ≡ Fe(II)/Fe2 + + H +

352

2 NHO→N2O + H2O

353

≡ Cu(I)/Cu + + ≡ Fe(III)/Fe3 + → ≡ Cu(II)/Cu2 + + ≡ Fe(II)/Fe2 +

354

≡ Cu(III)/Cu3 + +Fe(II)/Fe2 + → ≡ Cu(II)/Cu2 + + ≡ Fe(III)/Fe3 +

•

(31)

•

(32) •

•

(33) (34)

•

(35) (36) (37) (38)

355

Degradation Pathways of SMX. The products produced from the SMX degradation by the

356

CuFe2O4/HA system were analyzed. The C2, C4, C6, S7, N8, O9, O10 and N11 of SMX were the

357

most favorable sites for free radicals attack by the results of fukui function calculations (Figure

358

5).7 These products and their fragment ions could be seen in SI Table S1 and Figure S15-S29.

359

Fourteen products including P-284, P-300, P-273, P-289, P-255, P-271, P-109, P-99, P-114, P-270

360

(RT: 4.8 min), P-286, P-270 (RT: 2.7 min), P-288 and P-272 were identified by LC-QTOF-MS/MS,

361

and six possible pathways of SMX oxidation were presented (Figure 5). Detail results and

362

discussion can be seen in SI Text S15.

363

Degradation of Different Pollutants in CuFe2O4/HA System. The CuFe2O4/HA system showed

364

good degradation abilities for four different contaminants (i.e., tetracycline, bisphenol A, SMX

365

and sulfathiazole) (Figure S30 (a)), detail results and discussion can be seen in SI Text S16.

366

Environmental Implications. This study demonstrated the CuFe2O4 activation by HA for

367

efficient degradation of SMX in water. Besides, the optimal operations (i.e., CuFe2O4 dosage of 6

368

g/L, HA dosage of 1.0 mM and initial pH of 6.2) were obtained. In addition, the CuFe2O4/HA 16


Page 17 of 29


369

system could effectively oxidize SMX in water at pH between 5 and 10. The •OH, O2•- and H2O2

370

(A)

371

surface of CuFe2O4 were mainly responsible for the degradation of SMX. Meanwhile, electron

372

capture experiments using K2Cr2O7 proved the electrons transfer presented in the CuFe2O4/HA

373

system. What’s more, the comparative XPS analysis of the fresh and used CuFe2O4 further

374

evidenced the electrons transfer and the change of oxygen species of CuFe2O4. Also, the

375

degradation of SMX was accelerated when without dissolved oxygen in the CuFe2O4/HA system.

376

SMX was degraded to fourteen products through six degradation pathways in the CuFe2O4/HA

377

system. CuFe2O4/HA showed stable excellent degradation performance and the CuFe2O4 showed

378

a good stability, and the major transformation product of HA was N2 in CuFe2O4/HA system.

379

ASSOCIATED CONTENT

380

Supporting Information

381

Reagents (Text S1), synthesis of CuFe2O4 magnetic nano-particles (Text S2), total nitrogen (Text

382

S3), residual HA (Text S4), EPR studies (Text S5), LC-QTOF-MS/MS studies (Text S6),

383

characterization analytical methods of catalysts (Text S7), effects of key factors (Text S8-S10),

384

effects of inorganic anion (Text S11), stability of the spinel CuFe2O4 (Text S12), transformation

385

products of HA (Text S13), effect of different alcohols (Text S14), degradation pathways of SMX

386

(Text S15) degradation of different pollutants in CuFe2O4/HA system (Text S16), SMX products

387

detected by LC-QTOF-MS during CuFe2O4/HA system (Table S1), effect of different SMX

388

concentration detection time on experimental results (Figure S1), cyclic voltammograms (Figure

389

S2), SEM (Figure S3), TEM (Figure S4), SEM-EDS (Figure S5), XRD patterns (Figure S6),

390

effects of key factors (Figure S7), effects of inorganic anion (Figure S8), operational life of

391

CuFe2O4 (Figure S9), residual HA (a) and residual TN (b) (Figure S10), XPS spectra (Figure S11),

simultaneously presented in the CuFe2O4/HA system, but the O2•- radicals presented on the

17



392

effect of different alcohols (Figure S12), O 1s peaks in CuFe2O4/HA/N2 system (Figure S13), SMX

393

degradation in different catalyst/HA processes (Figure S14 (a)), homogeneous activation and

394

heterogeneous activation in CuFe2O4/HA system (Figure S14 (b)), SMX and its products (Figure

395

S15-29), degradation of different pollutants in CuFe2O4/HA system (a), apparent pseudo-second-

396

order reaction kinetics for different pollutants in CuFe2O4/HA system (b) (Figure S30).

397

AUTHOR INFORMATION

398



399

Address: No. 17, Section 3, Renmin South Road, Chengdu, Sichuan, China

400

Tel/fax: +86 15308172530

401

E-mail address: [email protected] (Bo Lai)

402

Acknowledgement. The authors would like to acknowledge the financial support from National

403

Natural Science Foundation of China (No. 51878423) and Fundamental Research Funds for the

404

Central Universities (No. 2015SCU04A09).

Corresponding authors.

405 406 407

18


Page 18 of 29

Page 19 of 29


408

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1.0 CuFe2O4 alone HA alone CuFe2O4/HA CuFe2O4/PMS CuFe2O4/PDS CuFe2O4/H2O2

[SMX]/[SMX]0

0.8 0.6 0.4 0.2 0.0 0

530

50

100

150

200

250

300

Elapsed Time (s)

531

Figure 1. SMX degradation in different processes: HA alone, CuFe2O4 alone, CuFe2O4/HA, CuFe2O4/PMS,

532

CuFe2O4/PDS and CuFe2O4/H2O2 systems. Experimental conditions: [SMX]0 = 39.5 µM, [CuFe2O4]0=6

533

g/L, [HA]0 = 1 mM, [PDS]=1 mM, [PMS]=1 mM, [H2O2]=1 mM, initial pH = 6.2, stirring speed = 300 rpm.

534 535 536 537 538 539 540 541 542 543

25



Page 26 of 29

544

(a)

Intensity (a.u.)

0.8

[SMX]/[SMX]0

(b)

BQ CAT TBA No scavenger

1.0

0.6

0.4

0.2

0.0

545

0

50

100

150

200

250

300

3460

3480

3500

3520

3540

Magnetic field (G)

Elapsed Time (s)

1.0

(c) 0.8

[SMX]/[SMX]0

K2Cr2O7/HA/CuFe2O4 K2Cr2O7 alone

0.6

HA/CuFe2O4

0.4

0.2

0.0

546

0

50

100

150

200

250

300

Elapsed Time (s)

547

Figure 2. Effects of scavengers on SMX degradation by CuFe2O4/HA system (a); EPR spectra in

548

CuFe2O4/HA system (b); effects of K2Cr2O7 in CuFe2O4/HA system (c). Experimental conditions: [SMX]0

549

= 39.5 µM, [CuFe2O4]0= 6 g/L, [HA]0 = 1 mM, initial pH = 6.2, stirring speed = 300 rpm, [BQ]=2 mM,

550

[TBA]=500 mM, [CAT]≥500 U/mL, [K2Cr2O7]=5 mM.

551 552 553 554 555 556 26


Page 27 of 29


557

(a)

1.0

(b)

1.0

Air sparging， [O2]0: 7.14 mg/L

0.8

Open to atmosphere, [O2]0 : 6.88 mg/L

0.8

[SMX]/[SMX]0

[SMX]/[SMX]0

N2 sparging, [O2]0 : 0.00 mg/L

0.6 0.4

CuFe2O4/HA/N2/2 mM BQ

0.4

0.2

0.2

0.0

0.0 0

558

50

100

150

200

250

CuFe2O4/HA/N2

0.6

0

300

50

100

150

200

250

300

Elapsed Time (s)

Elapsed Time (s)

559

Figure 3. Effects of air and N2 on CuFe2O4/HA system (a); effect of BQ in CuFe2O4/HA/N2 system (b).

560

Experimental conditions: [SMX]0 = 39.5 µM, [CuFe2O4]0= 6 g/L, [HA]0 = 1 mM, initial pH = 6.2, stirring

561

speed = 300 rpm.

562 563 564 565 566 567 568 569 570 571 572 573 574 27



575 576

Figure 4. Schematic illustration of activation mechanism in CuFe2O4/HA system. O2- (L) is surface lattice

577

oxygen, O2 (A) is the O2 adsorbed on the CuFe2O4 surface, H2O2 (A) is the H2O2 adsorbed on the CuFe2O4

578

surface.

579 580 581 582

28


Page 28 of 29

Page 29 of 29


583

584 585

Figure 5. Pathways of SMX oxidation by the CuFe2O4/HA system.

586 29


Activation CuFe2O4 by Hydroxylamine for Oxidation of Antibiotic

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