Ruthenium Nanoparticles Supported on CeO2 for Catalytic

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Ruthenium Nanoparticles Supported on CeO2 for Catalytic Permanganate Oxidation of Butylparaben Jing Zhang, Bo Sun, Xiaohong Guan, Hui Wang, Hongliang Bao, Yuying Huang, Junlian Qiao, and Gongming Zhou Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 19 Oct 2013 Downloaded from http://pubs.acs.org on October 20, 2013

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Environmental Science & Technology

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Ruthenium Nanoparticles Supported on CeO2 for Catalytic Permanganate Oxidation of Butylparaben

4

Jing Zhang†‡, Bo Sun‡, Xiaohong Guan†‡*, Hui Wang§, Hongliang Bao£, Yuying

5

Huang£, Junlian Qiao†, Gongming Zhou†

1 2

†

6 7

Environmental Science and Engineering, Tongji University, Shanghai, P. R. China ‡

8

State Key Lab of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, P. R. China

9

§

10 11

State Key Laboratory of Pollution Control and Resources Reuse, College of

£

Waters Co., Shanghai, China

Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics,

12

Chinese Academy of Sciences, Shanghai, PR China

13

E-mail addresses: [email protected] (J. Zhang), [email protected] (B.

14

Sun), [email protected] (X.H. Guan), [email protected] (H. Wang),

15

[email protected] (H.L. Bao), [email protected] (Y.Y. Huang),

16

[email protected] (J.L. Qiao), [email protected] (G.M. Zhou)

17 18 19 20 21

*Author to whom correspondence should be addressed

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Xiaohong Guan, email: [email protected]; phone: +86-21-65980956

23

1

ACS Paragon Plus Environment


24

ABSTRACT

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This study developed a heterogeneous catalytic permanganate oxidation system with

26

ceria supported ruthenium, Ru/CeO2 (0.8‰ as Ru), as catalyst for the first time. The

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catalytic performance of Ru/CeO2 toward butylparaben (BP) oxidation by

28

permanganate was strongly dependent on its dosage, pH, permanganate concentration

29

and temperature. The presence of 1.0 g L-1 Ru/CeO2 increased the oxidation rate of

30

BP by permanganate at pH 4.0∼8.0 by 3∼96 times. The increase in Ru/CeO2 dosage

31

led to a progressive enhancement in the oxidation rate of BP by permanganate at

32

neutral pH. The XANES analysis revealed that 1) Ru was deposited on the surface of

33

CeO2 as RuIII; 2) RuIII was oxidized by permanganate to its higher oxidation state

34

RuVI and RuVII, which acted as the co-oxidants in BP oxidation; 3) RuVI and RuVII

35

were reduced by BP to its initial state of RuIII. Therefore, Ru/CeO2 acted as an

36

electron shuttle in catalytic permanganate oxidation process. LC-MS/MS analysis

37

implied that BP was initially attacked by permanganate or RuVI and RuVII at the

38

aromatic ring, leading to the formation of various hydroxyl-substituted and

39

ring-opening products. Ru/CeO2 could maintain its catalytic activity during the six

40

successive runs. In conclusion, catalyzing permanganate oxidation with Ru/CeO2 is a

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promising technology for degrading phenolic pollutants in water treatment.

42

Keywords Kinetics, XANES, catalyst, electron shuttle, reaction pathway

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2


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INTRODUCTION

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Parabens, esters of 4-hydroxybenzoic acid, are extensively employed as

46

preservatives not only in a wide range of personal care products (PCPs), such as tooth

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pastes, deodorants, beauty creams, bath gels and shampoos, but also in canned foods,

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beverages and pharmaceuticals.1,2 As in the case of many personal care chemicals,

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they are continuously released into the aquatic media through domestic wastewater.

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Some studies have confirmed the presence of parabens in sewage treatment plant

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influents,3,4 effluents,4 sludge,5 river water,6,7 and swimming pool water.8 Some

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parabens were detected in river water up to 0.15 µg L-1 in a river in SouthWales,9

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while the concentration is less than 0.017 µg L-1 in a Swiss river.10 However, the

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concentration would be higher in effluent-dominated streams and cause physiological

55

response in aquatic organisms, especially in a residential area without sewer system.11

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As a result, there has been growing interest in the environmental fate and drinking

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water treatment of parabens. Since butylparaben (BP) is one of the most commonly

58

used paraben species in various cosmetic products,12,13 it was chosen as the target

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contaminant in this study.

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Up to now, oxidative degradation of BP had been achieved by several methods,

61

including H2O2/UV14, O315, and photosensitized oxidation16 with rose bengal (RB)

62

and aluminium phthalocyaninechloride tetrasulfonic acid (PC) as sensitizers. However,

63

the high energy requirements and the low selectivity of •OH limits the application of

64

H2O2/UV

65

photosensitized oxidation process limits its application in drinking water treatment.

while

the

introduction

of

secondary

pollutants

3


(sensitizer)

in


66

On the other hand, ozonation bears the demerits of the possible generation of

67

bromate17 and the extremely low stability of ozone in real water and thus low

68

oxidative ability compared to permanganate in real water.18,19

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In recent years, the potential application of permanganate for the oxidative

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removal of emerging micropollutants containing electron-rich moieties during water

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and wastewater treatment has received great attention. Hu et al.17,20,21 reported that

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permanganate was fairly effective in treating antibiotics. Jiang et al.19,22,23 as well as

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Guan and her colleagues24-27 showed that permanganate could readily oxidize

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phenolic compounds. Jiang et al.19 reported that permanganate was much more

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effective for oxidative removal of phenolic endocrine disrupting chemicals (EDCs) in

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real water at pH 8.0 compared to ozone, chlorine and ferrate, mainly due to its

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relatively high stability as well as selectivity therein. Therefore, permanganate was

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expected to be a promising oxidant for BP removal from water. Considering the

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unpleasant color of permanganate, only very low inlet concentration was allowed to

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avoid the appearance of chromaticity in the treated water. Thus, catalyzing this

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process is becoming a necessity to achieve high BP removal with lower permanganate

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dosage.

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Among the catalysts which have potential for use in selective oxidations,

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ruthenium (Ru) takes a special position owing to its versatility. Ru can catalyze

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numerous oxidative transformations: the oxidation of alkanes, the cleavage of double

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bonds, the asymmetric epoxidation of alkenes, the oxidation of alcohols and ethers

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and the oxidation of amines and phenols.28-30 Nandibewoor and his colleagues had 4


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extensively investigated RuIII-catalyzed permanganate oxidation process either under

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strongly alkaline ([OH-] ≥ 0.05 M) or strongly acidic ([H+] ≥ 0.03 M) conditions.31-34

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RuIII was shown to be an excellent catalyst for permanganate oxidation of amino acids

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(L-leucine, L-isoleucine and L-arginine),31,32 atenolol33 and D-panthenol35 under

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strong alkaline conditions and of amitriptyline-A tricyclic antidepressant drug under

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strong acidic conditions.34 However, all previous studies employed soluble RuIII to

94

catalyze permanganate oxidation. Ru is rather expensive and the addition of RuIII into

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permanganate oxidation system is far from practical application since it is

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troublesome to remove or/and recover Ru from the effluents. Heterogeneous catalysts,

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especially solid oxide-based ones, can overcome those disadvantages of homogeneous

98

catalysts. Therefore, it is urgent to develop Ru-loaded solid catalyst, which can be

99

used repeatedly to catalyze permanganate oxidation. Ceria (CeO2), one of the most

100

reactive rare earth oxides and widely used as catalyst supports,36,37 was employed as

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the support of Ru in this study.

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Therefore, the objectives of this work were to 1) synthesize Ru/CeO2 catalyst and

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explore the role of Ru in Ru/CeO2 catalyzed permanganate oxidation process; 2)

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examine the performance of Ru/CeO2 for catalyzing permanganate oxidation of BP

105

under various conditions; 3) assess the stability of synthesized catalysts; 4) propose

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the degradation pathways of BP in Ru/CeO2 catalyzed permanganate oxidation.

107

MATERIALS AND METHODS

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Preparation of Catalyst. The Ru/CeO2 catalyst was prepared by immersing 10 g

109

commercial CeO2 with average particle size of 10~50 µm, purchased from Sinopharm 5



110

Chemical Reagent Co., China, in a 50 mL aqueous RuCl3 solution (0.25 M) at pH 2.0

111

with vigorous magnetic stirring for 4 h under ambient conditions. The solid was

112

separated by centrifugation, washed with deionized water for several times until pH of

113

the filtrate was ∼7.0, and then dried at 40 °C in a vacuum drying oven.

114

Experimental Procedure. In a typical experiment, brown glass bottles containing

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BP, Ru/CeO2 and 30 mM NaCl (as background electrolyte) were put in water bath and

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batch experiments were initiated after adding an aliquot of permanganate stock

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solution. At fixed time intervals, 10 mL of sample was immediately transferred to a

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small beaker containing 100 µL of NH2OH•HCl (0.10 M) to quench the residual

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permanganate and then filtrated with a membrane filter (pore size: 0.22 µm) before

120

subject to analysis with UPLC. No buffer was used at pH 4.0~7.0, while borate buffer

121

was used at pH 8.0. The pH values remained constant during the whole process by

122

adding HCl or NaOH if necessary. The background electrolyte NaCl had negligible

123

effects on BP degradation (data not shown). All experiments were performed in

124

duplicates or triplicates, and all points in the figures are the mean of the results and

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error bars show the standard deviation of the means. The details of the experiments to

126

assess the ability of Ru/CeO2 catalyzed permanganate for BP oxidation in real water,

127

bromate generation in this process, the leaching test of Ru/CeO2 and the stability of

128

Ru/CeO2 catalyst were presented in Text S1 in Supporting Information.

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Catalyst Characterization. The synthesized Ru/CeO2 catalyst was characterized

130

with XRD, SEM, EDAX, and TEM and the details of these analyses were presented

131

in Text S2 in Supporting Information. The Brunauer-Emmett-Teller (BET) specific 6


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surface area was measured by the N2 gas adsorption method on an ASAP analyzer

133

(Micromeritics, USA).

134

Ru K-edge X-ray absorption near edge structure (XANES) spectra of the prepared

135

catalysts were collected at the BL14W1 beamline of Shanghai Synchrotron Radiation

136

Facility (Shanghai, China), with a Si(311) crystal monochromator. The storage ring

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was operated at 3.5 GeV with injection currents of 200 mA. Ru foil, RuCl3 and RuO2

138

were used as reference samples and their spectra were collected in the transmittance

139

mode while the spectra of the prepared catalysts were recorded in the fluorescence

140

mode. The XANES spectra were normalized to the absorption edge or the jump height.

141

The Ru K-edge inflection point was determined by measuring the maximum eV value

142

in the first derivative of the normalized spectra, which were then further normalized to

143

unity employing a Vitoreen polynomial function. A precise comparison of XANES

144

edge position requires careful energy calibration. In this study, the monochromator

145

was calibrated with Ru foil measurement before starting each sample analysis. The

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samples of freshly prepared Ru/CeO2 and Ru/CeO2 used after 10 successive runs in

147

their dry solid form were employed for XANES spectra collection. The spectra of

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Ru/CeO2+Mn(VII) and Ru/CeO2+Mn(VII)+BP were collected in an in-situ reactor at

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pH ~7.0, which was purged with N2 gas during the process of spectra collection. One

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gram Ru/CeO2 was dosed to 1 mL aqueous permanganate of 2 mM and reacted for

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about 5 min at room temperature before the XANES spectrum of Ru/CeO2+Mn(VII)

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was recorded. Afterwards, 1 mL BP of 1 mM was dosed into this mixture and the

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XANES spectrum of Ru/CeO2+Mn(VII)+BP was collected 2 minutes later. Even 7



154

though the calibration of the monochromator has been addressed, the reproducibility

155

of energy scale has to be ensured. Thus, parallel samples of Ru/CeO2, Ru/CeO2 used

156

after 10 cycles, Ru/CeO2+Mn(VII) and Ru/CeO2+Mn(VII)+BP were prepared and

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scanned respectively, and the results were reproducible.

158

Analytical Methods. The Ru contents of freshly synthesized Ru/CeO2 and

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Ru/CeO2 used after 10 successive experiments were determined by ICP-AES after

160

microwave digestion in nitric acid. The detection limit of this method was 0.01 mg L-1

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for Ru. The concentration of BP was quantified by UPLC (Waters Co.) with a

162

Symmetry C18 column and UV-visible detector set at 250 nm. The mobile phase,

163

water/acetonitrile (85/15, v/v), was run in an isocratic mode with a flow rate of 0.5

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mL min-1. UPLC together with electrospray-ionization quadrupole time-of-flight

165

tandem mass spectrometry was used to detect the byproducts of BP degradation. In

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this study, the mass spectrometer was operated in the m/z 50~500 range. The eluent

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was delivered at 0.5 ml min-1 by a gradient system (Table S1) with a C18 column 2.1

168

mm*100 mm, 1.7 µm. BP mineralization (TOC removal) was examined with a TOC II

169

analyzer (Elementar). In order to assure the accurate measurement of TOC, the

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concentrations of BP and permanganate were increased to 50 µM and 100 µM,

171

respectively. Bromide and bromate were analyzed using a reagent-free ion

172

chromatography system (ICS-3000, Dionex) coupled with a conductivity detector. A

173

high-capacity hydroxide-selective analytical column (AS19, 4×250 mm, Dionex) and

174

its respective guard column (AG19, 4×50 mm, Dionex) were used for separation. The

175

detection limits for bromate and bromide were 0.75 µg L-1 and 1.03 µg L-1, 8


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respectively, with an injection volume of 250 µL.38

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RESULTS AND DISCUSSION

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Characterization of Ru/CeO2 Catalyst. The content of Ru in the freshly

179

synthesized Ru/CeO2 catalyst was determined to be 0.80‰. The SEM-EDAX

180

elemental mappings of O(K), Ce(L) and Ru(L) of Ru/CeO2 catalyst were collected on

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a selected area (Figure 1a) and shown in Figure 1b, Figure 1c and Figure 1d,

182

respectively. The EDAX analysis revealed that Ru was uniformly dispersed on the

183

surface of CeO2. The TEM image of newly prepared Ru/CeO2 in Figure 1e showed

184

that Ru was present in clusters with a mean size of 2.5 nm on the CeO2 surface,

185

indicating that Ru was deposited on CeO2 surface as Ru hydroxides other than

186

physically adsorbed RuIII. The XRD patterns of CeO2 exhibited nine characteristic

187

peaks at 2θ around 28.6o, 33.1o, 47.5o, 56.4o, 59.3o, 69.6o, 76.8o, 79.2o and 88.6o, as

188

shown in Figure S1, indicating that it had fluorite-type structure.39 The XRD pattern

189

of Ru/CeO2 exhibited the diffraction peaks for the fluorite structure of CeO2 while no

190

peaks for Ru were identified, which may be due to its good dispersion and low

191

content. The BET measurements showed that the synthesized Ru/CeO2 has a specific

192

surface area of 4.87 m2 g-1, which is slightly higher than that of the CeO2 (4.53 m2

193

g-1).

194

Role of Ru in Ru/CeO2 Catalyzed Permanganate Oxidation Process. The

195

application of 1.0 g L-1 Ru/CeO2 significantly enhanced the oxidation of BP by

196

permanganate over the pH range of 4.0~8.0, as demonstrated in Figure 2. Since

197

Ru/CeO2 particles exerted negligible BP adsorption and CeO2 had no catalytic effect 9



198

on BP oxidation by permanganate, as illustrated in Figure S2, Ru doped on the CeO2

199

surface was the active ingredient responsible for the improved BP oxidation by

200

permanganate in the presence of Ru/CeO2. Therefore, Ru K-edge XANES spectra

201

were employed to investigate the role of Ru in BP oxidation by Ru/CeO2 catalyzed

202

permanganate, as demonstrated in Figures 3. The XANES spectrum contains unique

203

information about the oxidation state40 and the mean Ru oxidation state in a sample

204

can be derived from the Ru K-edge position (EK) by comparing with the reference

205

materials. As the valence of Ru increased, the absorption edge energy tends to shift to

206

a higher region.37 The K-edges of standard compounds Ru0 foil, RuCl3 and RuO2

207

appear at 22117 (not shown), 22121 and 22124 eV (as shown in Figure 3(a)),

208

respectively, reconcilable with the reported K-edges of Ru0,41 RuCl3,41 RuO237 and

209

RuO437. Moreover, Choy et al. reported that the K-edge of RuV (Ba2YRuO6 and

210

La2LiRuO6) appeared at 22125.5 eV.42 There was a perfect linear correlation between

211

EK of the reference compounds and the average valence of Ru, as illustrated in Figure

212

3(b). The magnitude of the shift in the Ru K-edge with the average Ru valence

213

amounts to ∼1.5 eV per increase in oxidation state by one.43 By comparing with the

214

reference spectra, it was concluded that Ru was present in the prepared catalysts as

215

RuIII. The XANES spectra of freshly prepared Ru/CeO2 and Ru/CeO2 used for 10 runs

216

exhibited negligible difference in their K-edge energies, implying that the short-range

217

structures of Ru/CeO2 in 10 successive runs did not change and it was of great

218

stability.

219

The in situ XANES analysis showed that with the addition of permanganate, Ru 10


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K-edge position of Ru/CeO2 shifted to a higher region by 5 eV (EK 22127 eV), which

221

could be assigned to Rux+ (x=6.5) according to the linear relationship between the

222

average Ru valence and EK (Figure 3(b)). The K-edge position of Rux+ (x=6.5) may be

223

caused by the superposition of the spectra of RuVII (perruthenate) and RuVI (ruthenate)

224

due to the oxidation of RuIII by permanganate. To clarify this point, we bought

225

commercial KRuO4 and synthesized K2RuO4 in our lab following the method reported

226

by Greenwood and Earnshaw44, and then investigated the influences of RuVII and RuVI

227

on oxidation of BP by permanganate, as illustrated in Figure S3. The rate of BP

228

oxidation by RuIII catalyzed permanganate was similar to those by permanganate in

229

the presence of RuVII or RuVI. Our results also unraveled that RuVII was a much

230

stronger oxidant while the synthesized RuVI was rather unreactive toward BP, which

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may be ascribed to the instability of RuVI and its rapid transformation into Ru(OH)4 at

232

pH 7.0.43 Therefore, the major active species of Ru in higher oxidation state can be

233

safely assigned to RuVII in Ru/CeO2 catalyzed permanganate oxidation. Although RuVI

234

was unstable at neutral pH, the formation of trace mount of RuVI and its contribution

235

to BP removal could not be excluded in the process of Ru/CeO2 catalyzed

236

permanganate oxidation.45

237

After the mixture of Ru/CeO2 and permanganate reacted with BP for 2 minutes,

238

the average oxidation state of Ru was decreased from 6.5 to ∼4, as revealed by the

239

XANES spectra, which was higher than the oxidation state of Ru in the Ru/CeO2 used

240

after 10 runs. This may be because this spectrum was collected before the depletion of

241

permanganate and partial Ru was still present in high valence state and thus it was the 11



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superposition of the spectra of RuVII, RuVI and RuIII. On the other hand, the oxidation

243

state of Ru in the Ru/CeO2 used after 10 runs was 3, which may be associated with the

244

depletion of permanganate at the end of each run. Therefore, it was concluded that

245

Ru/CeO2 was oxidized by permanganate to its higher oxidation state RuVII and RuVI,

246

which acted as co-oxidants in BP oxidation and were reduced by BP to its initial state

247

of RuIII.

248

Influence of Reaction Conditions on BP Oxidation by Ru/CeO2 Catalyzed

249

Permanganate. The kinetics of BP degradation by permanganate and Ru/CeO2

250

catalyzed permanganate was determined as functions of pH, permanganate

251

concentration, catalyst dosage and temperature, as demonstrated in Figure 2, Figure

252

S4, Figure S5, and Figure S6, respectively. BP degradation may occur on the catalyst

253

surface (i.e., heterogeneous reaction) and/or in the bulk solution (i.e., homogeneous

254

reaction). A simplified kinetic model46,47 considering both homogeneous and

255

heterogeneous reactions was established as below to describe BP degradation in

256

catalytic permanganate oxidation:

257

-

258

u c c where k hom is the rate constant of uncatalyzed reaction while k hom and khet are BP

259

oxidation rates in homogeneous and heterogeneous phase in Ru/CeO2 catalyzed

260

permanganate oxidation, respectively; kT stands for the apparent second-order rate

261

constant of the overall reaction. Only 0.67∼1.45‰ of Ru was desorbed from the

262

Ru/CeO2 catalyst at the end of reaction at pH 4.0∼7.0 and leaching of Ru at pH 8.0

263

was undetectable, as summarized in Table S2. RuIII at the concentration equal to the

d [BP] u c c = {khom + (khom + khet [Ru/CeO 2 ])}[Mn(VII)][BP] = kT [Mn(VII)][BP] dt

12


(1)

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amount of Ru leached from the Ru/CeO2 catalyst listed in Table S2 was employed to

265

c roughly evaluate k hom , which was determined to be only 0.21∼1.66% of kT at pH

266

4.0∼6.0 and zero at pH 7.0∼8.0, as listed in Table S2. Since the concentrations of

267

leached Ru were quantified at the end of the reaction, the amount of leached Ru

268

c during the reaction and k hom should be smaller than those determined in this study.

269

c u Thus, k hom could be omitted from Eq. (1). The values of kT and khom were

270

determined by fitting the experimental data collected in the first 2 min of reaction,

271

during which permanganate concentration was assumed to be a constant, and

272

summarized in Table S3.

273

As shown in Figure 2, BP degradation by permanganate or Ru/CeO2 catalyzed

274

permanganate exhibited strong pH dependence. Without the presence of Ru/CeO2, the

275

u apparent second-order rate constant ( k hom ) was increased by ~7-fold as pH increased

276

from 5.0 to 8.0, which should be ascribed to the increased deprotonation of BP with

277

u increasing pH.26,27 On the other hand, k hom was elevated when pH decreased from 5.0

278

to 4.0, probably associated with the increasing oxidation potential of permanganate

279

with decreasing pH, as depicted in Figure S7. The presence of Ru/CeO2 greatly

280

improved BP removal at pH 4.0~8.0 and the oxidation rate of BP by Ru/CeO2

281

catalyzed permanganate dropped progressively from 90.4 to 16.8 M-1 s-1 as pH

282

increased from 4.0 to 8.0, which could be ascribed to the decreasing formation of Rux+

283

(x=6.5) with increasing pH. Since RuVII and RuVI were the major active oxidants in

284

the process of Ru/CeO2 catalyzed permanganate, as revealed by the XANES analysis,

285

the effective oxidation of RuIII to RuVII and RuVI was a critical step for rapid 13



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degradation of BP. The oxidation potential of permanganate increased with decreasing

287

pH over the pH range of 4.0∼8.0, which would result in more effective RuIII oxidation

288

to RuVII and RuVI and thus more rapid BP oxidation at lower pH.

289 290

291

u The ratio of kT to k hom , defined as γ and shown in Eq. (2), was employed to

estimate the contribution of Ru/CeO2 to BP degradation by permanganate.

γ =

kT u k hom

(2)

292

The degradation rate of BP degradation by permanganate was accelerated by 3.4∼95.7

293

times at pH 4.0∼8.0 with the most significant improvement occurred at pH 5.0, as

294

listed in Table S3. Therefore, BP degradation in the Ru/CeO2 catalyzed permanganate

295

process was predominantly contributed by the heterogeneous catalytic oxidation.

296

The catalytic performance of Ru/CeO2 toward BP oxidation by permanganate

297

was also dependent on the initial permanganate concentration at pH 7.0, as depicted in

298

Figure S4. The removal of BP increased significantly from 7.7% to 44.1% in the

299

absence of Ru/CeO2 with increasing initial permanganate concentration from 10 µM

300

to 100 µM. However, in the presence of Ru/CeO2, such an increase in permanganate

301

concentration only induced slight improvement in BP removal, e.g., from 74.3% to

302

92.0%. Since permanganate worked as both indirect oxidant (by oxidizing RuIII to

303

RuVII and RuVI) and direct oxidant for BP, the contribution from direct oxidation

304

would increase when permanganate was dosed in large excess. Thus less significant

305

enhancement in BP removal rate due to the presence of Ru/CeO2 catalyst at higher

306

permanganate concentration was observed. This property of Ru/CeO2 catalyzed

14


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permanganate oxidation process was beneficial to its application in real practice at

308

low concentration.

309

With the increase of Ru/CeO2 dosage from 0.13 to 2.0 g L-1, the rate constant of

310

BP degradation increased linearly from 6.7 to 74.7 M-1 s-1, as shown in the insert of

311

Figure S5, suggesting a first-order dependence of the reaction rate constants on

312

Ru/CeO2 dosage. The apparent rate constants for catalyzed heterogeneous reaction

313

c u ( khet ) and uncatalyzed homogeneous reaction ( k hom ) were determined to be 35.8 M-1

314

g-1 L s-1 and 3.2 M-1s-1, respectively, following Eq. (1). The theoretically determined

315

u u value was very close to the experimentally determined k hom value (2.3 M-1 s-1) k hom

316

under same conditions, verifying the correctness of kinetic model shown in Eq. (1).

317

Figure S6 shows the effect of reaction temperature (4~30 oC) on BP removal by

318

permanganate in the presence and absence of Ru/CeO2. The rate constant of BP

319

oxidation by Ru/CeO2 catalyzed permanganate increased from 23.1 to 49.5 M-1 s-1

320

with temperature increasing from 4 oC to 30 oC while that of BP oxidation by

321

non-catalyzed permanganate was always ≤ 5.0 M-1 s-1 over this temperature range.

322

With the introduction of Ru/CeO2, the apparent activation energy of BP oxidation by

323

permanganate was decreased from 25.9 to 18.1 kJ mol-1, implying the lower

324

temperature dependence of Ru/CeO2 catalyzed permanganate oxidation than its

325

uncatalyzed counterpart.

326

The rate constants for the reaction of BP with permanganate in the presence and

327

absence of Ru/CeO2 were compared with selective oxidants (chlorine, singlet oxygen,

328

and O3) and nonselective oxidant (•OH), as listed in Table S4. Ru/CeO2 catalyzed 15



329

permanganate showed much higher activity than permanganate alone but was much

330

less reactive than other oxidants. However, BP degradations with other oxidants also

331

suffer some demerits. Chlorine (i.e. HOCl) could react with BP to generate chlorine

332

substituted BP, which might possess higher toxicity than its mother compound48 while

333

O3 could also react with Br- to generate carcinogenic bromate ion.49 Hydroxyl radicals,

334

susceptible to reacting with organic molecules indiscriminately, may be easily

335

consumed by various aqueous matrix components including humic acid and HCO3-

336

and thus may reduce the conversion efficiency of BP in real water.50 On the other

337

hand, Ru/CeO2 catalyzed permanganate may be advantageous in treating phenolic

338

compounds in real water since it will not form chlorinated or brominated byproducts

339

via reacting with natural organic matter, a ubiquitous component of the water matrix.

340

Moreover, bromate was not detected in the process of BP oxidation by Ru/CeO2

341

catalyzed permanganate when 0.5 mg L-1 Br- was spiked into the sample before the

342

reaction started. Ru/CeO2 catalyzed permanganate was selective and it was more

343

effective for oxidative removal of BP in real water than in deionized water at pH

344

6.0∼8.0, as shown in Figure S8. The improved BP degradation in real water might be

345

attributed to the contributions from the noncovalent interactions of humic acid with

346

BP through the formation of π-π interactions between the monoaromatic ring of BP

347

and humic acid aromatic components. The π-π interaction with humic acid could

348

enhance the density of the electron cloud of BP,24,25,27 which could result in the

349

enhancement of BP oxidation. Therefore, Ru/CeO2 catalyzed permanganate oxidation

350

is a promising process although this process degraded BP at a lower rate compared to 16


Page 16 of 35

Page 17 of 35


351

other oxidative processes.

352

Stability of Ru/CeO2 Catalyst. Since the stability and reusability of the catalyst

353

are critical in catalyzed reactions, especially for practical industrial applications, the

354

stability of the Ru/CeO2 catalyst was investigated by reusing catalyst in ten successive

355

experiments under same reaction conditions and the results are shown in Figure 4. BP

356

removal in the first six cycles remained almost constant while it decreased

357

progressively from 78.5% to 51.7% from the 6th run to 10th run. However, the initial

358

rate of BP decomposition experienced a gradual drop from the 1st run to 10th run. The

359

mass content of Ru in Ru/CeO2 decreased only slightly from 0.80‰ to 0.74‰ after

360

used for 10 cycles. Moreover, the XANES analysis had revealed that the short-range

361

structures of Ru/CeO2 in 10 successive runs kept unchanged. However, the mass

362

content of Mn in the catalyst was determined to be ~1‰ after 10 reaction cycles, due

363

to the deposition of MnO2, which was the reductive product of KMnO4 at pH 4.0∼8.0,

364

on the Ru/CeO2 catalyst surface. MnO2 could not catalyze BP oxidation by

365

permanganate and removal BP by adsorption at pH 7.0, as demonstrated in Figure S9.

366

Therefore, the depression in the catalytic performance of Ru/CeO2 may be mainly

367

associated with the masking of the active sites of Ru/CeO2 catalyst by the deposited

368

MnO2. The TEM image of spent Ru/CeO2 in Figure 1f confirmed that some Ru

369

hydroxides nanoparticles were masked by the adsorbed MnO2. The excellent stability

370

of Ru/CeO2 would favor its practical application in pilot or engineering practice but

371

efforts should be made to avoid the MnO2 deposition on Ru/CeO2 surface.

17



372

Mineralization of BP and Degradation Pathways. To further explore the

373

mechanisms of BP degradation in the process of Ru/CeO2 catalyzed permanganate

374

oxidation, LC-MS/MS was employed to analyze the reaction intermediates, based on

375

which BP degradation pathways were proposed, as illustrated in Figure 5. A number

376

of reaction products of BP were detected and structures for these products were

377

proposed upon the basis of (i) the masses of pseudo-molecular ions [M-H]-, (ii) the

378

major fragments of the MS/MS spectra, (iii) the relative chromatographic retention

379

times (tR) and (iv) previously well reported information on product formation during

380

the oxidative processes of ozonation (Table S5).16 Most of the byproducts had greater

381

molecular weights and earlier retention times than their parent molecules. It was very

382

hard to distinguish the retention times for the isomers, e.g. MW 210, 208, 242, 292,

383

etc, and thus the retention time for each byproduct was not offered. The proposed

384

structures of degradation products revealed that permanganate, RuVII and RuVI

385

initially attacked the aromatic ring, leading to the formation of various hydroxyl

386

substituted and aromatic ring-opening carboxylic acids, as shown in Figure 5. It was

387

found that a fraction of BP was hydrolyzed to generate p-hydroxybenzoic acid, which

388

was oxidized through the similar pathways as BP (Figure 5). However, the peak area

389

of p-hydroxybenzoic acid was only 0.16% of that of BP observed in the LC-MS/MS

390

spectra, indicating that the degradation of BP via hydrolysis and then further

391

decomposition was minor and thus it was not considered in constructing the kinetics

392

equation. Differing from permanganate, ozone was fairly active to butyl group,15 and

393

thus BP was attacked at both phenolic and butyl moieties during ozonation (Table S5). 18


Page 18 of 35

Page 19 of 35


394

Many studies have confirmed that the phenolic moiety is the essential functional

395

group responsible for the strong estrogenicity of estrogens, while the aromatic

396

ring-opening oxidation products retain much weaker estrogenic potency.19,27,51,52 With

397

the addition of the second -OH substituent to the benzene ring, the estrogenic potency

398

of mono-hydroxylated BP (OH-BP) would be increased. Based on the reaction

399

pathways proposed above, the estrogenic potency of BP in permanganate oxidation

400

might experience an enhancement before a decrease. Thus, it is significant to keep

401

long enough contact time to ensure the elimination of BP estrogenicity by Ru/CeO2

402

catalyzed permanganate oxidation.

403

The mineralization rate of BP was much slower than its disappearance rate in

404

either permanganate or Ru/CeO2 catalyzed permanganate oxidation process, as shown

405

in Figure S10. Only ∼80% of BP was mineralized by catalyzed permanganate

406

oxidation process in 17 h whereas only 19.5~65.6% of BP mineralization was

407

achieved in the absence of Ru/CeO2 over the pH range of 4.0∼8.0. The BP

408

mineralization was slow after reaction proceeded for 6 h, implying that some

409

degradation products of BP like organic acids were resistant to Ru/CeO2 catalyzed

410

permanganate oxidation. Because OH-BP was further decomposed and finally

411

transformed into small organic acids, water and CO2, Ru/CeO2 catalyzed

412

permanganate oxidation was effective not only for removing BP but also for

413

eliminating its estrogenic activity during water treatment. However, further studies are

414

needed to verify this expectation. The catalytic mechanisms of Ru/CeO2 in BP

19



415

oxidation by permanganate were proposed based on the XANES observations and the

416

LC-MS/MS results and shown in Figure 6.

417

Environmental Implications. A simple, nonhazardous, low energy input, and

418

high efficient oxidation process is always desirable for the degradation of

419

micropollutants in drinking water and wastewater treatment. For the first time

420

Ru/CeO2 was employed as heterogeneous catalyst to catalyze BP oxidation by

421

permanganate. This catalyst was very effective over wide ranges of pH, permanganate

422

concentration and temperature. Moreover, Ru/CeO2 was also effective for improving

423

BP mineralization over the pH range of 4.0∼8.0 and it possessed great stability and

424

could maintain its catalytic activity during six successive runs. Thus, catalyzing

425

permanganate oxidation with Ru/CeO2 is a promising technology for degrading

426

phenolic pollutants. But attention should be paid to the trace amount of leached Ru,

427

especially at pH ≤ 7.0. The broad-spectrum application of Ru/CeO2 catalyzed

428

permanganate oxidation process for degrading micropollutants with different moieties

429

should be examined in future study. In addition, the catalyst preparation conditions,

430

including the Ru loading as well as the selection of supports, should be optimized to

431

further improve the activity and the recyclability of Ru catalyst in permanganate

432

oxidation process.

433

ACKNOWLEDGMENT

434

This work was supported by Shanghai Rising-Star Program (12QA1403500), the

435

Foundation of the State Key Laboratory of Pollution Control and Resource Reuse,

436

China (PCRRY11001), the National Natural Science Foundation of China (21077029). 20


Page 20 of 35

Page 21 of 35


437

The authors would like to thank the anonymous reviewers and the editor for their

438

valuable comments and suggestions to improve the quality of this paper and thank

439

BL14W1 beamline (Shanghai Synchrotron Radiation Facility) for providing the beam

440

time.

441

ASSOCIATED CONTENT

442

Supporting Information. Two texts, ten figures and five tables are provided in the

443

Supporting Information. This material is available free of charge via the Internet at

444

http://pubs.acs.org.

445

21



446

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599 28


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Page 29 of 35


600 601

Figure 1 (a) SEM image of newly prepared Ru/CeO2; (b) SEM-EDX elemental

602

mapping of O(K) on this area; (c) SEM-EDX elemental mapping of Ce(L) on this area;

603

(d) SEM-EDX elemental mapping of Ru(L) on this area; (e) TEM image of newly

604

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605

29



Page 30 of 35

606 1.0

pH=5.0

pH=4.0

pH=6.0

C/C0

.8 .6

[Ru/CeO2]=0 g L-1 [Ru/CeO2]=1.0 g L-1

.4 .2 0.0 0

10

20

30

40

50

1.0

60

0

10

20

30

40

50

pH=7.0

60

0

10

pH=8.0

20

30

40

50

60

70

100

-1

k (M s )

.6

-1

C/C0

.8

.4

kT

10

u khom

.2 1

0.0 0

607

10

20

30

40

Time (min)

50

60

0

10

20

30

40

50

60

70

Time (min)

4.0

5.0

6.0

7.0

8.0

pH

608

Figure 2 Influence of pH on uncatalyzed and Ru/CeO2 catalyzed permanganate

609

oxidation of BP. Reaction conditions: [BP]0=25 µM, [Mn(VII)]0=50 µM and T=20 oC.

610

30


Page 31 of 35


1.2

a

8

b

VIII

Ru O4

1.0 Ru average valence

Ru/CeO2+Mn(VII) Norm. χµ (E)

.8 .6 .4

6

RuIVO2 4

Ba2YRuVO6 or La2LiRuVO6

III

Ru Cl3 2

.2

Ru0 0

0.0 22110 22115 22120 22125 22130

22116

22118

E (eV)

611

22120

22122

22124

22126

22128

22130

Ru K edge position (eV)

612

Figure 3 (a) Ru K-edge XANES of RuCl3 (

, black dots), RuO2 (

613

dots), Ru/CeO2 (

614

Ru/CeO2 after dosing permanganate (

615

permanganate and BP (

616

of Ru K edge position with their average valence. The values of the K-edge positions

617

of RuIVO2/RuVIIIO4, Ru0/RuIIICl3, and Ba2YRuVO6/La2LiRuVO6 were from references

618

37, 41 and 42, respectively.

, black line), Ru/CeO2 used after 10 cycles (

, red , blue line),

, purple line), Ru/CeO2 after dosing

, dark green line with cross); (b) The linear relationship

619

31



100

60 BP removal Second order rate constant, kT

80

50

40

30 40

-1

-1

60

kT (M s )

BP Removal (%)

Page 32 of 35

20 20

10

0

0 1

2

3

4

5

6

7

8

9

10

620

Runs

621

Figure 4 BP removed by catalytic permanganate oxidation and the comparison of the

622

second order rate constant in ten consecutive batch experiments. Reaction conditions:

623

[BP]0=25 µM, [Mn(VII)]0=50 µM, [Ru/CeO2]=1 g L-1, pH=7.0 and T=20 oC.

624

32


Page 33 of 35


Hydrolysis (Side reaction)

O HO O

O HO OH

(MW 138)

(MW194)

(Main re action)

HO O

O

O O

O

O

OH

HOOC HOOC

O

(MW 292)

HO

O OH

(MW 290)

(MW 246)

HO

OH

(MW 290)

OH OH

OH

O

O HO

O HOOC HOOC

OH

OH

OH

OH

O

HOOC HOOC

O HOOC HOOC

OH OH

(MW 246)

OH

O

O

HO

OH

(MW 292)

HO

OH HOOC HOOC

HOOC HOOC

OH

OH

HOOC HOOC

OH

O

OH

HO

OH O

O

HO

HO

OH

OH OH

OH

OH

HO

O

O

O

OH

HOOC HOOC

HOOC HOOC

O HOOC HOOC

OH

HOOC HOOC

OH OH

OH

HO O

HO

OH

O HO

OH

O

HOOC HOOC

OH

O HOOC HOOC

(MW 186)

OH O

OH

HO

(MW 186)

OH

OH

HOOC HOOC

OH

(MW 242)

HOOC HOOC

O

OH

(MW 152) O

HOOC HOOC

O

O

HO

(MW 152)

O HOOC HOOC

HO

OH

HO

(MW 208)

O

HOOC HOOC

O OH O

O

(MW 242)

O

O O

O

O

(MW 154)

O

O

(MW 208)

HO

(MW 154)

O

O

O

OH OH

(MW 210)

O O

HO

HO

HO

(MW 210)

HOOC HOOC

O

HO

HO

HO

OH

(MW 244)

O HOOC HOOC

OH OH

(MW 244)

Organic acids, CO2 and H2 O

625 626

Figure 5 Proposed degradation pathways of BP in Ru/CeO2 catalyzed permanganate

627

oxidation.

628

33



629

630 631

Figure 6 Proposed mechanisms for Ru/CeO2 catalyzed permanganate oxidation of

632

BP.

633

34


Page 34 of 35

Page 35 of 35


634 635

TOC Art

636

35


Ruthenium Nanoparticles Supported on CeO2 for Catalytic

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