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Oxygen Vacancy Promoted Heterogeneous Fentonlike Degradation of Ofloxacin at pH 3.2-9.0 by Cu Substituted Magnetic Fe3O4@FeOOH Nanocomposite Hang Jin, Xike Tian, Yulun Nie, Zhaoxin Zhou, Chao Yang, Yong Li, and Liqiang Lu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04503 • Publication Date (Web): 21 Sep 2017 Downloaded from http://pubs.acs.org on September 22, 2017

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Environmental Science & Technology

1

Oxygen Vacancy Promoted Heterogeneous Fenton-like Degradation of Ofloxacin

2

at pH 3.2-9.0 by Cu Substituted Magnetic Fe3O4@FeOOH Nanocomposite

3

Hang Jin, Xike Tian*, Yulun Nie, Zhaoxin Zhou, Chao Yang, Yong Li, Liqiang Lu

4 5

Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan

6

430074, P. R. China.

7 8

ABSTRACT: To develop an ultra-efficient and reusable heterogeneous Fenton-like

9

catalyst at a wide working pH range is a great challenge for its application in practical

10

water treatment. We report an oxygen vacancy promoted heterogeneous Fenton-like

11

reaction mechanism and an unprecedented ofloxacin (OFX) degradation efficiency of

12

Cu doped Fe3O4@FeOOH magnetic nanocomposite. Without the aid of external

13

energy, OFX was always completely removed within 30 min at pH 3.2-9.0. Compared

14

with Fe3O4@FeOOH, the pseudo-first-order reaction constant was enhanced by 10

15

times due to Cu substitution (9.04 /h vs. 0.94 /h). Based on the X-ray photoelectron

16

spectroscopy (XPS), Raman analysis, and the investigation of H2O2 decomposition,

17

•

18

formed oxygen vacancy from in-situ Fe substitution by Cu rather than promoted

19

Fe3+/Fe2+ cycle was responsible for the ultra-efficiency of Cu doped Fe3O4@FeOOH

20

at neutral and even alkaline pHs. Moreover, the catalyst had an excellent long-term

21

stability and could be easily recovered by magnetic separation, which would not cause

22

secondary pollution to treated water.

OH generation, pH effect on OFX removal and H2O2 utilization efficiency, the new

1

ACS Paragon Plus Environment


23

INTRODUCTION

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The worldwide usage of antibiotics such as ofloxacin (OFX) has posed an increasing

25

threat to the environment due to its poor biodegrability.1-4 Hence, to remove such

26

organic pollutants efficiently is a big challenge to the conventional water treatment

27

processes.5 Fenton reaction as one kind of advance oxidation technologies (AOTs) has

28

drawn much attention to the abatement of refractory organic pollutants due to the

29

formation of non-selective hydroxyl radicals.6, 7

30

At present, the heterogeneous Fenton process was developed to replace conventional

31

homogeneous Fenton reaction because of its advantages such as the wide working pH

32

range,8 reducing the need for large amounts of metal ions and the reusability of

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catalyst.9 Among the heterogeneous Fenton catalysts, unsupported and supported iron

34

based materials have been synthesized and exhibited excellent performance including

35

Fe3O4, Fe2O3, FeOOH,10,

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(α-FeOOH) as an ubiquitous natural mineral in soils, and sediments at the earth

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surface has been widely used as a heterogeneous Fenton catalyst due to its abundance,

38

availability, relative stability and low cost.14 However, its catalytic activity decreased

39

greatly at neutral or even alkaline condition,15, 16 and ultrasound or UV/visible light

40

irradiation has to be used for the acceleration of the reaction.17, 18 As reported, the low

41

performance is because only a small fraction of H2O2 is converted into •OH radicals.19

42

The requirement of ultrasound or UV/visible light irradiation also results in the need

43

for specific equipment at additional cost. On the other side, α-FeOOH is usually used

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in the form of fine powders, which makes solid/liquid separation and recovery

11

FeOOH@GO12 and Fe/SiO2.13 For example, goethite

2


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difficult. However, the easy separation and excellent reusability are the key

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parameters that determine the practical application of a heterogeneous catalyst.

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Therefore, it is of great importance to develop an ultra-efficient heterogeneous Fenton

48

catalyst that can be used at a wide pH range and easy separated without the aid of

49

extra energy input.

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Previous studies show that iron oxide doped with isomorphic cations such as

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mesoporous sulphur-modified Fe2O3 can extend the working pH range to 3.0-9.0 by

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changing the chemical environment of iron.20, 21 The introduction of a second metal

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into Fe containing materials such as Fe3O4/CeO2 also provides an alternative due to

54

theenhanced

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nanoparticle counterparts.22 Moreover, magnetic separation, as a quick and effective

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technique for the separation of magnetic particles,10 has drawn increasing attention in

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catalysis research. However, it is still urgent to solve the following two concerns if

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α-FeOOH was modified by the above strategies: (a) the modified α-FeOOH can not

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only be conveniently recovered by magnetic separation technology, but also retain

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desirable heterogeneous Fenton reaction efficiency; (b) to select a suitable isomorphic

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element that can greatly enhance the Fenton activity of α-FeOOH at a wide pH range

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and clarify the enhancing mechanism. 17, 23

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Herein, magnetic Cu doped Fe3O4@FeOOH was one-step prepared via hydrothermal

64

method, which exhibited unprecedented Fenton activity for OFX degradation at a pH

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range of 3.2 to 9.0 without ultrasound or UV/visible light irradiation. Moreover, it was

66

easy to recover the catalyst after reaction by magnetic separation. The mechanism

heterogeneous

catalytic

activity

compared

3


with

monometallic


67

investigation proved that the formed oxygen vacancy due to Cu substitution was

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responsible for the ultra-high Fenton activity of Cu doped Fe3O4@FeOOH. Different

69

from the traditional heterogeneous reaction mechanism, oxygen vacancy can elongate

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the O-O bond of H2O2 and change the electronic structure and chemical property of

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Cu doped Fe3O4@FeOOH, which favor the interfacial electron transfer and •OH and

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O2•- generation. Hence, the as-prepared catalyst provides a promising alternative for

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the application of heterogeneous Fenton reaction in practical water treatment because

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of the high reactivity, good stability and magnetic separation.

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EXPERIMENTAL SECTION

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Synthesis of Cu-Doped Fe3O4@FeOOH Fenton Catalyst. Cu-doped Fe3O4/FeOOH

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was one-step synthesized by a hydrothermal method. In a typical procedure, the

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desired amount of FeSO4⋅7H2O, Fe(NO3)3 and CuSO4⋅5H2O ([Cu]/[Cu+Fe]=1.0-10%

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in molar ratio), together with 1.5 g Na2S2O3 were added to 30 ml deionized water

80

under continuous stirring. Once a clear mixture formed, 5.6 g poly(ethylene glycol)

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6000 was added into the above mixture, which was stirred for 15 min to ensure the

82

total dissolution and form a viscous solution. Thirdly, 7.0 g NaOH was dissolved in 30

83

ml deionized water and added to the viscous solution dropwise. Finally, the solution

84

was transferred into a 100 ml Teflon lined stainless steel autoclave and heated at

85

120 ℃ for 20 h. After cooling, the solid was repetitively washed with deionized water

86

and anhydrous ethanol and Cu-doped Fe3O4@FeOOH was obtained after drying at

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60 ℃ overnight.

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For comparison, Fe3O4, α-FeOOH and Fe3O4@FeOOH were also prepared under the

4


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same conditions.

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Characterization. Powder X-ray diffraction (XRD) patterns were measured on a

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Rigaku D/MAXRC X-ray diffractrometer using Cu Kα radiation (γ=0.154 nm) as the

92

X-ray source. Transmission electron microscopy (TEM) images were collected on a

93

transmission electron microscope with field emission gun at 200 KV (JEOL 2000EX,

94

JEOL, Japan). Raman spectra were recorded on a RM-1000 Raman spectrometer

95

(Renishaw, England). X-ray photoelectron spectroscopy (XPS) was recorded on a

96

MULT1LAB2000 photoelectron spectroscopy. A JDM-13 magnetometer was used to

97

record at room temperature the magnetization (M) of samples as a function of the

98

magnetic field applied (H). ICP-MS analysis was carried out on a ICAPQ01890 to

99

measure the real amount of Cu in the as-prepared catalyst.

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Heterogeneous Fenton like Degradation of OFX over Cu-Doped Fe3O4@FeOOH.

101

All the heterogeneous Fenton-like reactions were carried out at ambient conditions

102

without any extra energy input. In a typical experiment, 25 mg catalyst was dispersed

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in 100 ml 10 mg/L of OFX solution, which was stirred for 1 hour to establish the

104

adsorption/desorption equilibrium. Then, the desired amount of 30 wt.% H2O2 (the

105

molar concentration of 30 wt.% H2O2 was 9.7 mol/L) was added to the above

106

suspension under continuous magnetic stirring. The reaction solution was not buffered

107

and the pH changes during the reaction process were monitored by a pH meter. The

108

pH was adjusted by a diluted aqueous solution of NaOH or HCl, which remained the

109

same within 0.3 units at the end. At given time intervals, 2 ml sample was taken and

110

filtered immediately to remove the catalyst for analysis. Then 0.1 ml 0.04 mol/L

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Na2S2O4 was added to quench the residual •OH.24 The concentration of OFX in filtrate

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was determined by high performance liquid chromatography (HPLC) with an

113

UV-DAD detector. The chromatographic separation was performed by a reverse-phase

114

C18 column (250 mm×4.6 mm, 5 µm). The mobile phase composed of 15%

115

acetonitrile and 85% ultrapure water was acidified by 1% phosphoric acid with a flow

116

rate of 0.5 mL/min. The injection volume was 20 µL. Temperature of the column

117

chamber was maintained at 25 °C and the detection wavelength was 288 nm. The total

118

organic carbon (TOC) of OFX solution was analyzed using a TOC-VCPH analyzer

119

(Shimadzu). The measurements of nitrate (NO3-) and fluoride (F-) were conducted

120

using a Dionex model ICS 2000 ion chromatograph (IC) equipped with an IonPac

121

AS11-HC analytical column (4×250 mm) and using 40 mM KOH as an eluent. All the

122

experiments were repeated three times and the data represented the average of the

123

triplicates with a standard deviation less than 5%. Isopropanol and benzoquinone were

124

used as radical scavengers for •OH and O2•-, respectively. In addition, electron spin

125

resonance was also used to detect the involved radicals by a FA200 ESR spectrometer,

126

5,5-Dimethyl-1-pyrroline N-oxide (DMPO) was used as a nitrone spin trap which

127

mainly combined with •OH in water system and with O2•- in methanol system.25

128

Determination of hydroxyl radicals was performed with a photometric method

129

and the change of H2O2 concentration was determined by KMnO4 titration.28

130

RESULTS AND DISCUSSION

131

Characterization of Cu Doped Fe3O4@FeOOH. As shown in Figure S1 of the SI,

132

the XRD pattern can be assigned to a mixture of Fe3O4 (JCPDS no. 11-0614) and

6


26, 27

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α-FeOOH (JCPDS no. 29-0731) without the characteristic signals of Cu oxide. Fe2+

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and Fe3+ were identified since 710.54 eV, 724.04 eV for Fe2+, 712.76 eV and 726.26

135

eV for Fe3+ with a shake up satellite Fe 2p3/2 at 718.03 eV were found in Figure 1A.29,

136

30

137

observed in Cu2p region of XPS spectrum (Figure 1B). Hence, the framework iron

138

atom should be in-situ substituted by both Cu+ and Cu2+ species.31 Moreover, the

139

actual percentage of Cu, Fe3O4 and α-FeOOH in the 5% Cu doped Fe3O4@FeOOH

140

was 3%, 24.5% and 72.5%respectively, based the ICP-MS results. TEM images

141

further exhibited a significant rod-like structure of typical goethite (Figure 2A). Small

142

cubes or pseudocubic crystals, likely magnetite, attached to rod-like goethite were

143

observed, which agreed well with the detected magnetite in the as-prepared sample by

144

XRD analysis. Obviously, the length and width of goethite was 1 µm and 30 nm

145

respectively. As expected, Cu-doped Fe3O4@FeOOH had a Ms value of 73.93 emu g-1

146

at 15 kOe with an obvious hysteresis loop, which further confirmed the existence of

147

magnetite (Figure 2B).29 It indicated that this catalyst may be easily to be separated

148

from the treated water.

149

Ultra-High Heterogeneous Fenton-like Activity of Cu Doped Fe3O4@FeOOH for

150

OFX Degradation. The catalytic activity of Cu doped Fe3O4@FeOOH was evaluated

151

by OFX degradation at neutral pH without any extra energy input. As shown in Figure

152

3, in the presence of H2O2, about 33% and 53% of OFX was removed at 60 min over

153

Fe3O4 and α-FeOOH (curve c and d). OFX removal efficiency was increased to 64%

154

over Fe3O4@FeOOH under the same conditions (curve e). In comparison, OFX was

The peaks at binding energies of 935.02 eV for Cu2+ and 933.50 eV for Cu+ were

7



155

completely degraded at only 20 min over Cu doped Fe3O4@FeOOH (curve f). OFX

156

degradation is also followed by pseudo-first-order kinetics and the reaction constants

157

for Fe3O4, α-FeOOH, Fe3O4@FeOOH and Cu doped Fe3O4@FeOOH was 0.32/h,

158

0.60 /h, 0.94 /h and 9.04 /h, respectively. However, the contribution of H2O2 oxidation

159

(curve a) and adsorption (curve b) to the OFX removal was not significant. Therefore,

160

OFX degradation mainly came from the contribution of heterogeneous Fenton

161

reaction and the introduction of Cu can greatly enhance the Fenton-like activity of

162

Fe3O4@FeOOH. As depicted in Figure S2 in the SI, Cu doped Fe3O4@FeOOH also

163

exhibited the highest OFX mineralization efficiency and 90% of TOC content was

164

removed after reaction for 150 min. At the same time, the original structure of OFX

165

was all destroyed since 96.7% of F and 97.0% of N in OFX was converted into F- and

166

NO3-. As shown in Figure 4A, the used catalyst can be easily recovered by magnetic

167

separation. The M-H curves of as-prepared catalyst before and after reaction were also

168

comparable with a Ms value of 73.93 emu g-1 and 67.36 emu g-1.32 Moreover, there

169

was no obvious deactivation of the catalyst when compared with the first cycle and

170

OFX can be always completely removed in six successive cycles (Figure 4B). Hence,

171

a novel heterogeneous Fenton-like catalyst with excellent Fenton activity and stability

172

was successfully prepared.

173

The optimum reaction conditions for OFX degradation were further investigated and

174

the results were shown in Figure S3 and Figure S4 of the SI. Figure S3 showed that

175

the catalytic activity of Cu doped Fe3O4@FeOOH increased with the Cu amount and

176

reached a steady status even when 10% of Cu was introduced. The effect of catalyst

8


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loading and H2O2 dosage on the OFX removal efficiency was also explored in Figure

178

S4. Obviously, the OFX degradation efficiency increased with the amount of catalyst

179

(0.15-0.3 g/L) and H2O2 (0.1-7.5 mL/L). Hence, 5.0% of Cu doped Fe3O4@FeOOH

180

(0.25 g/L) and 1.0 mL/L of H2O2 were chosen as the optimum experimental

181

conditions.

182

Based on the ESR results in Figure 5A, the 4-fold characteristic peak of DMPO-•OH

183

adducts with an intensity ratio of 1:2:2:1 and the characteristic 1:1:1 triplet assigned to

184

DMPO-HO2•/O2•- adducts were observed, which indicated the formation of •OH and

185

HO2•/O2•- in Cu doped Fe3O4@FeOOH-H2O2 system.33,

186

experiments were further conducted to investigate their separate contribution to OFX

187

degradation.35 As depicted in Figure 5B, the OFX removal efficiency decreased

188

significantly (65% at 20 min) due to the addition of benzoquinone for HO2•/O2•-.

189

However, only 18% of OFX was degraded in the presence of isopropanol (for •OH).

190

Therefore, •OH as the primary reactive oxygen species was involved in the OFX

191

degradation process together with HO2•/O2•-.

192

α-FeOOH as Fenton catalyst was restricted to acidic conditions and a significant

193

decrease of catalytic activity was observed at neutral or even alkaline condition.19, 36

194

However, in this study as shown in Figure S5A in the SI, Cu doped Fe3O4@FeOOH

195

always exhibited a high catalytic activity over a wide pH range of 3.2 to 9.0 and OFX

196

was degraded by 86-100%. Moreover, OFX (3 mg/L and 10 mg/L) can be efficiently

197

degraded even in the real water solution. As depicted in Figure S5B and S5C in the SI,

198

three real water samples was collected from tap water, a local lake and pond, OFX

9


34

Radical trapping


199

was still completely removed. It meant that the effect of co-existing anions on the

200

Fenton-like activity of Cu doped Fe3O4@FeOOH was not significant.

201

Oxygen Vacancy for the Enhancement of Heterogeneous Fenton Activity of Cu

202

Doped Fe3O4@FeOOH. The effect of initial solution pH on the heterogeneous

203

Fenton catalytic activity of Fe3O4@FeOOH and Cu doped Fe3O4@FeOOH towards

204

OFX degradation was shown in Figure S6 in the SI. Only 62% and 36% of OFX were

205

removed at 60 min over Fe3O4@FeOOH when solution pH was 6.5 and 9.0

206

respectively. It agreed well with the published reports of a relatively slow and

207

inefficient process at circumneutral pH values.10, 11 In comparison, OFX was always

208

completely degraded over Cu doped Fe3O4@FeOOH at either pH 6.5 (20 min) or pH

209

9.0 (30 min). Based on the ICP-MS results, it was worthy to note that only 3.0% of

210

Cu was introduced in the as-prepared catalyst. Hence, the contribution of promoted

211

Fe(III)/Fe(II) cycle by Cu(I)/Cu(II) for •OH and HO2•/O2•- production towards OFX

212

abatement can be neglected.

213

Oxygen vacancy has been proposed as active sites for perovskite-type catalysts in

214

H2O2 activation, which was not significantly affected by solution pH.23, 37 Raman

215

spectrum and XPS analysis were then used to confirm the existence of oxygen

216

vacancy in Cu doped [email protected] As shown in Figure 6A, the characteristic

217

peaks of the typical α-FeOOH structure were found in Fe3O4@FeOOH, which was in

218

accordance with the majority of α-FeOOH (72.5%) in Fe3O4@FeOOH. Since Raman

219

scattering was sensitive to the defects and lattice disorder, the broadened peak and

220

decreased peak intensity for Cu doped Fe3O4@FeOOH may be attributed to the

10


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221

disordered Cu substitution and the formation of oxygen vacancy.22,

222

further compare the O1s spectra of Fe3O4@FeOOH and Cu doped Fe3O4@FeOOH.

223

The broad peak of O1s for Fe3O4@FeOOH could be deconvoluted into two peaks:

224

Fe-O at 529.80 eV and O-H at 531.31 eV.40 While a negative binding energy shift of

225

1.5 eV was observed and the corresponding B.E. for Fe-O and O-H was 528.4 eV and

226

529.7 eV, respectively for Cu doped Fe3O4@FeOOH. Lower binding energy of O1s

227

was due to the nonstoichiometric oxygen atoms within catalyst by maintaining the

228

overall charge balance in the lattice.41 Moreover, besides the Fe-O and O-H, the other

229

two peaks at 526.8 eV and 531.0 eV assigned to Cu-O and oxygen vacancy further

230

appeared in the FWHM of the O1s line. The replacement of Fe(II)/Fe(III) by

231

Cu(I)/Cu(II) could cause the missing of oxygen atoms, leading to a slight positive

232

charge in the lattice. It would attract the electron density of the neighboring oxygen

233

atoms, which resulted in a higher binding energy due to the electron ejection.42-44

234

Similar to the perovskite, in situ substitution of framework Fe ion by Cu with a lower

235

oxidation state could produce highly active oxygen vacancies, which should be

236

responsible to the ultra-high Fenton-like activity of Cu doped Fe3O4@FeOOH.

237

However, there still existed contradictory knowledge about the roles of oxygen

238

vacancy during the catalytic reaction process: (a) catalytic decomposition H2O2 into

239

O2 rather than •OH and HO2•/O2•-;43, 44 (b) to enhance the catalytic performance of

240

MnO2 in terms of benzene or CO oxidation and oxygen reduction reaction. It was then

241

necessary to investigate the real role of oxygen vacancy in Cu doped Fe3O4@FeOOH.

242

Firstly, the H2O2 decomposition over different catalysts was evaluated as provided in

11


39

Figure 6B


243

Figure S7 in the SI.28 Compared with 54.8% and 82% of H2O2 was decomposed over

244

Fe3O4 and α-FeOOH at 30 min, while Fe3O4@FeOOH had a relative lower H2O2

245

decomposition efficiency of 70%. However, their corresponding OFX removal

246

efficiency was 33%, 53% and 64% at 60 min, respectively (Figure 3). Only Cu doped

247

Fe3O4@FeOOH exhibited the highest activity for H2O2 decomposition (92%) and

248

OFX removal efficiency (100% at 20 min). Since •OH was the dominant radical for

249

OFX degradation, it was adopted to investigate the contribution of different

250

components of Cu doped Fe3O4@FeOOH to the •OH formation. As shown in Figure

251

7A and 7B, there was almost no •OH formation in aqueous CuO dispersion with 2%

252

of OFX removal. For separated Fe3O4 and α-FeOOH, the quantities of •OH increased

253

with the reaction time and reached a platform of 9.0 and 12.5 µM, which resulted in a

254

slightly higher OFX removal efficiency over α-FeOOH than Fe3O4. Especially, Cu

255

doped Fe3O4@FeOOH exhibited the highest •OH yield, where the maximum •OH

256

concentration was 45.7 µM at 10 min and then decreased and reached a certain value.

257

By calculating the H2O2 utilization efficiency, Cu doped Fe3O4@FeOOH also had a

258

much higher value of 56.7% than CuO, Fe3O4 and α-FeOOH of 1.1%, 19.2% and

259

30.84%. Hence, the formed oxygen vacancy by 3.0% Cu substituted Fe3O4@FeOOH

260

can not only promote •OH formation for OFX degradation but also increase the H2O2

261

utilization efficiency greatly.

262

Finally, the possible heterogeneous Fenton-like reaction mechanism of Cu doped

263

Fe3O4@FeOOH was proposed based on all the above experimental results. Similar to

264

MnO2,45 oxygen vacancy can change the electronic structures and chemical properties

12


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265

of Cu doped Fe3O4@FeOOH and the surface Fe orbitals move towards the low-energy

266

direction due to the interaction between surface Fe atoms around oxygen vacancy. As

267

shown in Figure 8, the presence of oxygen vacancies can result in a structure

268

distortion, and electron transfer from the oxygen vacancies to the surface Fe and O

269

atoms can cause the changes of electronic structure of Cu doped Fe3O4@FeOOH. On

270

one side, the oxygen vacancies benefit the electron transfer from Cu doped

271

Fe3O4@FeOOH to adsorbed H2O2.46 On the other side, the elongated O-O bond of

272

H2O2 due to moderate oxygen vacancies was much easier activated and decomposed

273

into •OH and O2•-.47 Hence, the ultra-high Fenton-like activity of Cu doped

274

Fe3O4@FeOOH was not directly dependent on the mixed valence such as Cu+/Cu2+

275

and Fe2+/Fe3+. The introduction of oxygen vacancies by Cu substitution due to

276

structural distortions should contribute greatly to the enhanced catalytic performance

277

for OFX oxidation.

278

Moreover, the surface active Fe2+ and Cu+ centers involved a one-electron oxidation

279

and catalyzed H2O2 into •OH according to Haber-Weiss mechanism (reaction 1 and

280

2).48 At the same time, the chemisorbed H2O2 and Fe3+/Cu2+ formed Fe3+-H2O2 and

281

Cu2+-H2O2 complex, which generated O2•- and corresponding Fe2+/Cu+ (reaction 3 and

282

4). Of course, the reduction of Fe3+ by Cu+ (reaction 5) was also thermodynamically

283

favorable as shown by reaction 6 and 7, which also favored the generation of •OH and

284

HO2•/O2•- radicals. ≡ Fe 2+ + H 2 O 2 →≡ Fe 3+ + • OH + OH −

(1)

≡ Cu + + H 2 O 2 →≡ Cu 2+ + • OH + OH −

(2)

13



Page 14 of 33

≡ Fe3+ ---O-O---Fe3+ ≡ +2H 2 O 2 →≡ Fe 2+ ---O-O---Fe 2 + ≡ + HO •2 + 2H +

(3)

≡ Cu 2 + - O - O - Cu 2 + ≡ + 2 H 2 O 2 →≡ Cu + - O - O - Cu + ≡ + HO •2 + 2 H +

(4)

≡ Cu + + ≡ Fe 3 + →≡ Cu 2 + + ≡ Fe 2 +

(5)

≡ Cu 2+ + e − →≡ Cu +

E0=0.17 V

(6)

≡ Fe 3+ + e − →≡ Fe 2+

E0=0.77 V

(7)

OFX + • OH/O

•− 2

→ P roduct → CO 2 + H 2 O

(8)

285

Hence, the above results confirmed that oxygen vacancy played a more important role

286

than Fe(III)/Fe(II) and Cu(I)/Cu(II) for the ultra-efficient degradation of OFX over Cu

287

doped Fe3O4@FeOOH at neutral and even alkaline pH. Moreover, as shown in Figure

288

S8 in the SI, the amount of Cu and Fe leaching drops off by every cycle and become

289

stable after the third cycle in the reuse of Cu doped Fe3O4@FeOOH. The Fe and Cu

290

ions concentration was 2.15 mg/L and 0.33 mg/L. There was also no difference of

291

XRD patterns for Cu doped Fe3O4@FeOOH before and after Fenton-like reaction

292

(Figure S9 in the SI), which meant that Cu doped Fe3O4@FeOOH had an excellent

293

stability. After reaction, the catalyst was also easy to recover by magnetic separation

294

as depicted in Figure 5A. These findings will extend the scope of Fenton catalysts and

295

consolidate the fundamental theories of Fenton reactions for wide environmental

296

applications.

297

ASSOCIATED CONTENT

298

Supplementary information

299

Additional information includes XRD pattern, TOC removal and generation of nitrate

300

and fluoride, effect of Cu dosage on the Fenton-like activity of Cu doped

14


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301

Fe3O4@FeOOH, effects of catalyst amount and H2O2 dosage on the OFX removal

302

efficiency, effects of initial solution pH and water characteristics on the OFX

303

degradation efficiency in Cu doped Fe3O4@FeOOH/H2O2 system, OFX degradation

304

rate over Fe3O4@FeOOH and Cu doped Fe3O4@FeOOH at pH 6.5 and 9.0, H2O2

305

decomposition as a function of reaction time over different catalysts, and XRD

306

patterns of Cu doped Fe3O4@FeOOH before and after Fenton reaction. These

307

materials are available free of charge via the Internet at http://pubs.acs.org.

308

AUTHOR INFORMATION

309

Corresponding Author: Xike Tian, Tel.: +86-27-6788-4574, Fax: +86-27-6788-4574,

310

E-mail: [email protected] (X. K. Tian)

311

Notes

312

The authors declare no competing financial interest.

313

ACKNOWLEDGEMENTS

314

This work was supported by the National Natural Science Foundation of China (No.

315

41773126) and the Foundation for Innovative Research Groups of the National

316

Natural Science Foundation of China (No. 41521001) and the “Fundamental Research

317

Funds for the Central Universities”.

318

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Figure Captions

478 479

Figure 1. XPS spectrum of Cu doped Fe3O4@FeOOH: (A) Fe, (B) Cu.

480

Figure 2. (A) TEM images and (B) magnetization versus applied magnetic field for

481

Cu doped Fe3O4@FeOOH.

482

Figure 3. OFX degradation under different conditions (A) and the relative pseudo

483

first order kinetic analysis results (B): (a) only H2O2, (b) only Cu doped

484

Fe3O4@FeOOH, (c) Fe3O4 + H2O2, (d) α-FeOOH +H2O2, (e) Fe3O4@FeOOH + H2O2,

485

(f) Cu doped Fe3O4@FeOOH + H2O2. (Experimental conditions: 100 ml 10 mg/L

486

OFX, pH=6.5, catalyst amount 0.25 g/L and H2O2 dosage 1 mL/L)

487

Figure 4. (A) Magnetization versus applied magnetic field for fresh and used Cu

488

doped Fe3O4@FeOOH. (B) Stability of Cu doped Fe3O4@FeOOH in the multicycle

489

degradation of OFX in the presence of H2O2. (Experimental conditions: 100 ml 10

490

mg/L OFX, pH=6.5, catalyst amount 0.25 g/L and H2O2 dosage 1 mL/L).

491

Figure 5. (A) ESR spectra of (a) •OH and (b) O2•-; (B) Effects of radical scavengers

492

on the degradation of OFX over Cu doped Fe3O4@FeOOH.

493

Figure 6. Raman spectra (A) and O1s in XPS spectra (B) of Fe3O4@FeOOH (a) and

494

Cu doped Fe3O4@FeOOH (b).

495

Figure 7. The generation of •OH radicals (A) and the corresponding OFX removal

496

and H2O2 utilization efficiency (B) in different heterogeneous Fenton-like process.

497

Figure 8. The proposed oxygen vacancy involved heterogeneous Fenton-like reaction

498

mechanism of Cu doped Fe3O4@FeOOH.

499

24


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500 501 502 503 504 505

506 507

Figure 1. XPS spectrum of Cu doped Fe3O4@FeOOH: (A) Fe, (B) Cu.

508 509 510 511

25



512 513 514 515 516

517 518

Figure 2. (A) TEM images and (B) magnetization versus applied magnetic field for

519

Cu doped Fe3O4@FeOOH.

520 521 522 523

26


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524 525 526 527 528

529 530

Figure 3. OFX degradation under different conditions (A) and the relative pseudo

531

first order kinetic analysis results (B): (a) only H2O2, (b) only Cu doped

532

Fe3O4@FeOOH, (c) Fe3O4 + H2O2, (d) α-FeOOH +H2O2, (e) Fe3O4@FeOOH + H2O2,

533

(f) Cu doped Fe3O4@FeOOH + H2O2. (Experimental conditions: 100 ml 10 mg/L

534

OFX, pH=6.5, catalyst amount 0.25 g/L and H2O2 dosage 1 mL/L)

535 536 537

27



538 539 540 541 542 543

544 545

Figure 4. (A) Magnetization versus applied magnetic field for fresh and used Cu

546

doped Fe3O4@FeOOH. (B) Stability of Cu doped Fe3O4@FeOOH in the multicycle

547

degradation of OFX in the presence of H2O2. (Experimental conditions: 100 ml 10

548

mg/L OFX, pH=6.5, catalyst amount 0.25 g/L and H2O2 dosage 1 mL/L).

549 550 551 552

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553 554 555 556 557 558

559 560

Figure 5. (A) ESR spectra of (a) •OH and (b) O2•-; (B) Effects of radical scavengers

561

on the degradation of OFX over Cu doped Fe3O4@FeOOH.

562 563 564 565

29



566 567 568 569 570 571

572 573

Figure 6. Raman spectra (A) and O1s in XPS spectra (B) of Fe3O4@FeOOH (a) and

574

Cu doped Fe3O4@FeOOH (b).

575 576 577 578

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579 580 581 582 583 584

585 586

Figure 7. The generation of •OH radicals (A) and the corresponding OFX removal

587

and H2O2 utilization efficiency (B) in different heterogeneous Fenton-like process.

588 589 590 591

31



592 593 594 595 596

597 598

Figure 8. The proposed oxygen vacancy involved heterogeneous Fenton-like reaction

599

mechanism of Cu doped Fe3O4@FeOOH.

600 601 602 603

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