Removal of Elemental Mercury in Flue Gas with H2O2 Solution

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Removal of Elemental Mercury in Flue Gas with H2O2 Solution Catalyzed by Zn-doped BiFeO3 Yi Zhao, Xiaoying Ma, and Peiyao Xu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00484 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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Energy & Fuels

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Removal of Elemental Mercury in Flue Gas with H2O2 Solution Catalyzed by Zn-doped BiFeO3 Yi Zhao*, Xiaoying Ma , Peiyao Xu

4 5

a

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University, Beijing 102206, P.R. China

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ABSTRACT:

School of Environmental Science and Engineering, North China Electric Power

8

Magnetic Zn-doped BiFeO3 catalyst was synthesized by the tartaric acid sol-gel

9

method and its characteristic was characterized by X-ray powder diffraction (XRD),

10

X-ray photoelectron spectroscopy (XPS), Brunauere Emmette Teller (BET) technique

11

and Vibration Sample Magnetometer (VSM). The catalytic activity of Zn-BiFeO3 was

12

evaluated for activating H2O2 to oxidize elemental mercury in a self-designed

13

bubbling reactor and the effects of the Zn doped ratio in catalyst, catalyst dosage,

14

H2O2 concentration, solution pH and reaction temperature on the removal of Hg0 were

15

also investigated systematically. The result indicated that the removal efficiency of

16

Hg0 was achieved 81% under the optimum conditions, in which the Zn doping ratio

17

was 0.2, H2O2 concentration was 0.15 mol L-1, catalyst dosage was 0.3g L-1, solution

18

pH was 6, reaction temperature was 50℃ and Hg0 concentration was 50µg/m3.

19

Moreover, it was demonstrated indirectly that •OH and HO2• were the active species

20

when Hg0 was oxidized to oxidation state mercury using H2O2 solution catalyzed by

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0.2 Zn doped BiFeO3, where tert-butyl alcohol and benzoquinone were used as

22

quenchers. The reaction mechanism was established eventually through the

23

characterizations of catalyst and reaction product.

24

Keywords: Hg0 removal; Zn-doped BiFeO3 catalyst; flue gas cleaning; reaction

25

mechanism 1

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1.Introduction

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Nowadays, mercury has attracted international attention because of its elusive

28

nature, high volatility, easy mobility, high bioaccumulation and potentially lasting

29

perniciousness. About 5000 t of mercury are discharged into air every year all around

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the world, and mercury from coal-fired power plants accounts for more than 30% 1 of

31

the 4000t produced by human beings. Generally, elemental mercury (Hg0), oxidation

32

state mercury (Hg2+) and particulate mercury (Hgp) are three main forms in typical flue

33

gas2. Among them, oxidation state mercury (Hg2+) is easily dissolved in water and

34

absorbed by existing wet desulphurization system, particulate mercury (Hgp)

35

absorbing on ashes can be removed by electrostatic precipitator or bag-type dust

36

collector. However, Hg0 is very difficult to remove from flue gas by existing air

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pollution control devices (APCD) because of its high volatility at room temperature

38

and low solubility in water. Thus, the synergetic removal Hg0 was widely attempted

39

by modifying the catalyst of selective catalytic reduction (SCR), in which the key is to

40

reduce NO by NH3 and oxidize Hg0 to the soluble Hg2+ simultaneously3-5, then the

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latter was absorbed by existing wet desulfurization system.

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To accommodate strict emission demands of Hg, developing more effective Hg0

43

control methods is currently an important area of research focus and need in the field

44

of energy and environmental protection6-7. In the past few decades, a number of Hg0

45

control technologies, such as traditional chemical oxidation methods and advanced

46

oxidation technologies were widely investigated. Among them, traditional chemical

47

oxidation methods including gaseous phase oxidizing technologies8-9, liquid phase

48

oxidizing technologies employing ferrate(VI) solution10-11 and gas-like phase

49

oxidizing technologies

50

technology that is usually used to treat waste water has also tried to remove Hg0 from

12-15

were commonly attempted. The advanced oxidation

2


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coal-fire flue gas using Fenton reagent since it is a kind of green oxidation system.

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For example, Dennis16 et al used it to study the removal of Hg0 in coal-fire flue gas,

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and the results indicated that Fenton reaction could oxidize Hg0 with an average

54

efficiency of 75% in the pH range of 1.0-3.0. Scale-up investigation about mercury

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removal in coal-fire flue gas was also carried out by Tan17 et al based on Fenton

56

reaction. However, it was found that the oxidation activity of reaction system

57

decreased due to the Fe3+ hydrolysis reaction in desulfurization slurry in the pH range

58

of 5.2 to 5.6. Besides, there were many problems associated, such as corrosion of

59

equipment 18 and flocculent precipitation of Fe species during the reaction, though the

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removal was efficient. Hence, more and more concerns are focused on the

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heterogeneous Fenton-like reaction to overcome disadvantages mentioned above 19-20.

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Perovskite-type mixed oxides (ABO3) is a kind of important heterogeneous

63

catalyst in industry including BiFeO3, it occurs the orbital hybridization between 6s

64

lone pair electrons of Bi3+ in BiFeO3 and its 6P unoccupied molecular orbit or O2-

65

orbit , which results in an asymmetric center distorting of the electron cloud,

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generating the ferroelectricity and antiferromagnetism and making for recycle under

67

the applied magnetic field. Moreover, Fe3+ in BiFeO3 can form a Fenton-like system

68

with H2O2 and generate •OH that is a high activity oxidative species. Hence, many

69

researchers used it to catalyze H2O2 and degrade organic materials. Luo et al.21

70

synthesized nanoscale BiFeO3 and used to decompose Rhodamine B. During the

71

reaction, Rhodamine B was absorbed on the surface of BiFeO3 strongly and the

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degradation efficiency of more than 90% was reached. But the ferromagnetism of

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BiFeO3 is weak at room temperature, so part of Bi3+ was usually substituted by La and

74

part of Fe3+ was also substituted by Co, Mn and Zn, especially Zn. By this way, the 3


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catalytic performance of catalyst is not only promoted, but also its magnetism is

76

enhanced, which is beneficial to the recycle of catalyst

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reagents costs and application prospect, the high prices of La(NO3)3.6H2O (40000 ¥/t),

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Co(NO3)2.6H2O(33600 ¥/t) and Mn(NO3)2.4H2O(30000 ¥/t) restrain their usages.

79

so authors think that Zn(NO3)2.6H2O(6000 ¥/t) is a suitable doping material with the

80

superiorities of lower cost that can be afford by industry.

22-25

. while in view of the

81

In recent years, a heterogeneous Fenton-like catalyst, Fe2-XCuXO4 with high

82

activity has been synthesized and used to remove Hg0 successfully by Zhou et al.26

83

and the results demonstrated that Fe2-XCuXO4 had a high stability and Hg0 removal

84

efficiency of more than 90% after utilized three times. In addition, Zhou et al.27 used

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Fe2.45Ti0.55O4/H2O2 advanced oxidation system to oxidize Hg0 in simulated flue gas,

86

and Hg0 of 93% was removed when an initial pH was 6.0, H2O2 was 0.5mol L-1,

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catalyst dosage was 0.6 g L-1 and temperature was 50℃. However, this kinds of

88

catalysts after reaction were recovered by centrifugation and filtration because of their

89

nonmagnetic, which increases complexity and operating costs of the process for the

90

future application. As far as we know, the utilization of Zn-BiFeO3 to catalyze H2O2

91

for the purpose of removing Hg0 from flue gas was rarely reported. In order to

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promote the availability of catalyst, Zn-doped BiFeO3 magnetic catalyst was

93

synthesized by tartaric acid sol-gel method and characteristics of it were determined

94

using X-ray diffraction (XRD), Brunauere Emmette Teller (BET) technique, X-ray

95

photoelectron spectroscopy (XPS, ESCALAB250 spectrometer) and vibration sample

96

magnetometer (VSM). The catalytic activity of Zn-BiFeO3 was evaluated for

97

activating H2O2 at a bubbling reactor and the main influencing factors on Hg0 removal

98

were examined and the optimal reaction condition was established. Meanwhile, the

4


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reaction mechanism was established initially through catalyst characterization and the determination of reaction products.

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In addition, compared with the oxidants used for removing Hg0 such as NaClO2

102

(5500 ¥/t, 82% w/w), K2FeO4 (26000 ¥/t), KMnO4 (15500 ¥/t) and K2Cr2O7 (16500

103

¥/t), H2O2 (500 ¥/t, 30% w/w) adopted in our work has an obvious advantages of low

104

cost and environmental friendly. Hence, the proposed method has well application

105

advance in industry.

106

2. Experimental section

107

2.1 Materials

108

Ferric nitrate (Fe(NO3)3·9H2O), bismuth nitrate (Bi(NO3)3·5H2O), and zinc

109

nitrate (Zn(NO3)2·6H2O) were purchased from Fuchen chemical reagent factory,

110

Tianjin; hydrogen peroxide (H2O2, 30%) was obtained from Huadong reagent factory,

111

Tianjin; Nitric acid (HNO3, 65%), tartaric acid (C4H6O6), sodium hydroxide (NaOH)

112

and potassium permanganate (KMnO4) were provided from Kermel chemical reagent,

113

Tianjin; concentrated sulfuric acid (H2SO4, 98%) was obtained from chemical reagent

114

Three Factory, Tianjin. All reagents were analytical grade (AR).

115

2.2 Preparation of Zn-doped BiFeO3

116

During the Preparation, Bi(NO3)3·5H2O, Fe(NO3)3·9H2O and Zn(NO3)2·6H2O

117

were weighted in proportion of 1：（1- χ ）： χ ( χ =0.1, 0.2, 0.3, 0.4), added into a

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beaker (100 mL) with HNO3 (20mL, 20%) and stirred for 30 min, then tartaric acid

119

as complex reagent (tartaric acid: positive ion was 1:1) was added in order to prevent

120

the volatilization of positive ion. Gelatinous materials were obtained after 1 hour-

121

stirring at 60℃ and then turned into luminous yellow xerogel after drying at 150 ℃.

122

The water and nitric acid in it were removed after 1 hour-heated at 300 ℃ in a muffle

123

furnace, and then it turned into bolarious solid 2 hours later at 600 ℃. The preparation 5


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of χ Zn- BiFeO3 was completed after grinded into powders.

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2.3 Characterizations of catalyst and product

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The structure and crystalline phase of catalyst were measured by X-ray

127

diffraction (XRD, D8 Advance, Bruker in Germany), with copper Ka1 target radiation,

128

graphite monochromator, 2θ ranged from 10º-90º, 0.15406 nm detection wave length

129

and 0.0001º measurement accuracy and a X-ray photoelectron spectroscopy (XPS,

130

ESCALAB250 spectrometer). The magnetism of catalyst at room temperature was

131

measured by MPMS magnetics measurement system (SQULD VSM, Quantum

132

Design). The adsorption and desorption isothermal curves of Zn-BiFeO3 at different

133

relative pressures were measured by full-automatic specific surface area and porosity

134

analysis meter (ASAP 2020, Mac in US) and the specific surface area was measured

135

by Brunauer Emmett Teller (BET) technique. The active species, such as •OH and

136

HO2• and O −2 • in Zn-BiFeO3/H2O2 system during the catalytic oxidation, were

137

speculated indirectly by the experiments of quenching free radicals with tert-butyl

138

alcohol (TBA) and benzoquinone (BQ) acting as quenchers, while the reaction

139

product, Hg2+ was measured by an atomic fluorescence spectrophotometer (AFS-933,

140

Jitian, Beijing).

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2.4 Recyclability of catalyst

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To examine the recyclability of 0.2 Zn-BiFeO3, the cyclic utilization experiments

143

of catalyst were carried out, in which, the fresh catalyst, first time and second time

144

recycling catalysts were used for catalyzing Hg0 oxidation under the determined

145

optimum conditions. For the recovery, the cyclic used catalysts were extracted from

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reaction solution with applied magnetic field, and impurity in it was washed out. Then

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the catalyst was put into vacuum drying oven at 100℃ to be dried to constant weight,

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from which, the recovery was calculated. 6


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3. Experimental apparatus and methods

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The experimental apparatus was consisted of four parts including flue gas

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simulation and flow control system, mercury generator system, catalytic oxidation

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system and mercury detecting system, as shown in Fig.1. Specifically, mercury vapor

153

was generated from a mercury generator (4) built in mercury permeation tube (50ng

154

min-1, VICI Metronics Co., USA) immersing in thermostat oil bath (5) (HH-S Wenhua

155

instrument limited company in Jintan, Changzhou) at 60 ℃, and carried with nitrogen

156

into mixed gas cylinder, where total flow was adjusted to 1L min-1 by nitrogen in

157

another branch. The catalytic oxidation reactor was a self-designed bubbling reactor

158

with volume of 250 mL and height of 15.5 cm. The temperature was adjusted and

159

controlled by thermostat water bath (8) (HH-ZK2, Yuhua Instrumental Company,

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Gongyi) and solution pH was adjusted by HNO3 and NaOH and tested by a pH meter

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(type PHS-3C, Leici, Shanghai, China). All pipelines were made up of Teflon and

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twined around with heater bands to maintain 110℃, in order to prevent condensation

163

of Hg0. The oxidation and absorption reactions happed when the mixed gas flowed

164

into catalytic reaction equipment. After reaction, the residual Hg0 was measured by

165

QM201H cold atom fluorescence mercury detector (Qing’an instrument company,

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Suzhou) and the spent gas was discharged into the air after absorbed by potassium

167

permanganate-sulfuric acid solution (12). In the determinations, the samples were

168

taken from the inlet and outlet of reactor with the sampling time of 1 min, and

169

determining time was 3 min. The Hg0 removal efficiency was calculated based on

170

different concentration via Eq. (1):

171

η=

172

where η is the removal efficiency of Hg0, %. C in is the inlet concentration of Hg0, µg

C in − C out × 100% C in

(1)

7


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m-3. C out is the outlet concentration of Hg0, µg m-3.

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4. Results and Discussion

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4.1 Characterization of catalyst

176

4.1.1 XRD and XPS Analysis

177

XRD patterns of BiFeO3 and 0.2Zn-BiFeO3 are shown in Fig. 2(a). From the

178

spectrum of BiFeO3, it can be seen that stronger diffraction peaks appear at 22.49°,

179

31.81°, 32.14°, 39.51°,45.81°, 51.38° and 57.01°, and correspondent Prague crystal

180

planes are (101), (012), (110), (021) , (202), (113) and (122), which is in accordance

181

with standard BiFeO3 X-ray diffraction (JCPDS 20-0169). Since there is no Fe2O3,

182

Bi2O3 and unreacted precursor diffraction peaks in the spectrum, the sample shows a

183

high degree of crystallinity and pure perovskite structure. From a fact that the

184

diffraction peaks of 0.2Zn-BiFeO3 and BiFeO3 are all R3c points group, it can be

185

considered that crystalline shapes are not changed during the doping. However, Zn

186

doping can lead the excursion of (101) crystal plane, widening the half peak width of

187

diffraction peak, which may be because the radius of Zn2+ (0.074nm) is larger than

188

that of Fe3+ (0.064nm), as shown in Fig.2 (b). The results suggested that Zn had been

189

doped into BiFeO3 since there was a little Bi2Fe4O9 impure phase but no ZnO

190

diffraction peak, which was similar to Park J M’s experimental results 28. Fortunately,

191

it can be seen from the XPS spectra of 0.2 Zn-BiFeO3 displayed in Fig.3 that the XPS

192

signals of the Zn-2p have been observed. Meanwhile, the XPS photoelectron peaks

193

corresponding Bi, Fe and O elements are clearly observed. In order to verify the

194

function of Fe species in the activation of H2O2, the resolved XPS spectra of the Zn-

195

doped BiFeO3 before and after reaction were analyzed. As shown in Fig.4, two clearly

196

peaks at 710.8 eV and 712.8 eV represented Fe3+ in octahedral sites and tetrahedral

197

sites display in the XPS spectra of the Fe2p, respectively, moreover, the satellite peak 8


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of Fe3+ appears at 719.9 eV, and that of Fe2+ does not appear, which indicates that the

199

Fe valence state has not been changed after doping Zn. However, the peak at 710.8 eV

200

was migrated to 710.7 eV after reaction, this might seem like a Fe 3 + valence variation

201

slightly in octahedral sites.

202

4.1.2 BET Analysis

203

The N2 adsorption and desorption isothermal curves of BiFeO3 and 0.2Zn-

204

BiFeO3 belong to the typical type II according to the adsorption materials

205

classification of IUPA, as shown in Figure 5. For Fig.5(A), the adsorption capacity

206

increases slowly with an increase of partial pressure (P/P0) in the relative pressure

207

range of 0.1 and 0.9. However, the adsorption capacity increases sharply when the

208

relative pressure is between 0.9 and 1.0, which may be because that the pilled pores

209

are formed during the preparation of catalyst. In addition, it can be seen from the

210

diagram of pore size distribution that 0.2Zn-BiFeO3 is main macroporous structure. By

211

adopting BET model, the specific surface area of 0.2Zn-BiFeO3 was calculated as

212

2.169 m2, while that of BiFeO3 was 0.2498 m2 g-1, based on the Fig. 5(b), which

213

meant that Zn doping could promote the specific surface area of catalyst, making for

214

the activation of H2O2 and Hg0 oxidation.

215

4.1.3Analysis of magnetism

216

Fig.3 shows the hysteresis loops of BiFeO3 and 0.2Zn-BiFeO3 at room

217

temperature, in which, M-H curve of BiFeO3 appears linear, and the saturation

218

magnetization (Ms) is 0.08669 emug-1, residual magnetization (Mr) is 0.00783 emug-1

219

and coercivity (Hc) is 399Oe, which proves that BiFeO3 macroscopic magnetism was

220

weak. However, 0.2Zn-BiFeO3 magnetization intensity (6000Oe) in Fig.6 is not

221

saturated, so it is ferromagnetism, while the saturation magnetization (Ms) of that is 9


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2.631 emug-1, residual magnetization (Mr) is 0.7283 emug-1 and coercivity (Hc) is

223

700Oe. Compared with BiFeO3, 0.2Zn-BiFeO3 saturation magnetization and

224

coercivity are much stronger. Possibly because after doping Zn, the spinning

225

modulation structure of BiFeO3 crystal lattice was destroyed and the magnetism of

226

catalyst was strengthened, which was beneficial to recycle catalyst under the applied

227

magnetic field.

228

4.2 Removal of Hg0 from flue gas

229

4.2.1 Effect of Zn-doped amount on Hg0 removal efficiency

230

From Fig. 7 it is found that with the increase of doping ratio from 0 to 0.2, Hg0

231

removal efficiency increases from 65 to 85%. The possible reason was that the dopant

232

of Zn could promote the transformation of hydrone in absorption solution to •OH,

233

which improved the catalytic oxidation activity for Hg0

234

efficiency decreased when the doping ratio of Zn was more than 0.2, which might be

235

because Zn occupied more active sites (Fe)

236

appropriate Zn doping ratio, and expressed as 0.2Zn-BiFeO3.

237

4.2.2 Effect of catalyst dosage on Hg0 removal efficiency

29

. However, the removal

25

. In this paper, 0.2 was selected as the

238

As shown in Fig. 8, the Hg0 removal efficiency was 18% in the absence of

239

catalyst, although H2O2 standard redox potential (1.77V) was higher than Hg2+/Hg0 of

240

that (0.796V), while Hg0 removal efficiencies increase from 18 to 60% in the 0.2Zn-

241

BiFeO3 dosage range of 0 to 0.1g L-1 in the absorption solution. This phenomenon

242

could be explained by that30 the O-O in H2O2 was absorbed by the active point

243

locations (Fe) on surface of 0.2 Zn-BiFeO3, weakening the bonding interaction of O-

244

O, then producing •OH that could oxidize Hg0 in flue gas into water-soluble Hg2+. The

245

efficiencies increased from 60 to 85.23% when 0.2Zn-BiFeO3 dosage increased from

246

0.1 to 0.3g L-1. But the efficiency was basically unchanged, when 0.2Zn-BiFeO3 10


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dosage was more than 0.3 g L-1, which might be due to the "saturation" of active point

248

locations (Fe) on surface of 0.2Zn-BiFeO3 for the limited H2O2. Hence, the catalyst

249

dosage was determined as 0.3 g/L.

250

4.2.3 Effect of H2O2 concentration on the removal of Hg0

251

·OH was mainly produced from H2O2 whose concentration had a direct effect on

252

Hg0 removal efficiency. As shown in Fig. 9, the Hg0 removal efficiencies increase

253

obviously from 3 to 85.23% in the H2O2 concentration range of 0 to 0.15mol L-1.

254

Thereafter, the efficiencies decrease from 85.2 to % 56.1% with the H2O2

255

concentration increasing from 0.15 to 0.8 mol L-1. These experimental phenomena

256

mentioned above show the complexity of affecting Hg0 removal by H2O2

257

concentration.

258

Generally, the advanced oxidation of Hg0 included •OH production, •OH

259

diffusion and Hg0 oxidation by •OH. It was reported that when the diffusion speed of

260

•OH was 1010 M-1s-1, the oxidation speed of Hg0 could reached 2×109 M-1s-1 31, while

261

the •OH diffusion speed depended on its production that relied mainly on the H2O2

262

concentration when the catalyst was fixed. From an experimental phenomenon that

263

Hg0 removal efficiencies increase obviously from 3 to 85.23% in the H2O2

264

concentration range of 0 to 0.15mol L-1, it could be estimated that an increase of H2O2

265

concentration resulted in the •OH production increasing, promoting Hg0 removal

266

efficiency. But excessive H2O2 could react with •OH, as illustrated in Eqs. (2-6), and

267

the reaction speed was as high as 3.30 × 107 M-1S-1, which consumed existing •OH

268

and produced low-activity HO2• that was electrode potential of 1.50V, much lower

269

than that of •OH 33. Thus, the optimum H2O2 concentration was 0.15mol/L.

270

H 2 O 2 + ⋅OH → HO 2 ⋅ + H 2 O

(2)

271

HO 2 ⋅ + ⋅ OH → H 2 O + O 2

(3) 11


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(4)

272

⋅ OH + ⋅OH → H 2 O 2

273

HO 2 ⋅ + HO 2 ⋅ → H 2O 2 + O 2

274

⋅ OH + HO 2 ⋅ → H 2 O + O 2

275

4.2.4 Effect of reaction temperature on the removal of Hg0

(5) (6)

276

It can be seen from Fig.10 that when temperature is between 40℃ and 50℃, the

277

removal efficiencies of Hg0 increase from 78.25 to 85.23%. This was because the

278

reaction activation energy producing •OH was easier to overcame at higher reaction

279

temperature in the presence of 0.2Zn-BiFeO3. However, the removal efficiencies of

280

Hg0 decreased from 85.23 to 65.24% as reaction temperature increased from 50℃ to

281

70℃, which could be explained as follows: On the one hand, solubility coefficient of

282

Hg0 in water was 2.7×10-7mol·(Pa·L)-1 at 55℃, while it was 0.999×10-7mol·(Pa·L)-1 at

283

80℃34. That means, the higher temperature was, the lower solubility coefficient was.

284

On the other hand, H2O2 might be decomposed at high temperature. Therefore, The

285

optimal reaction temperature was 50℃.

286

4.2.5 Effect of initial pH on the removal of Hg0

287

The trend of Hg0 removal efficiency with variation of the initial pH is shown in

288

Fig. 11. Clearly, the removal efficiencies increase from 75% to a maximum of 85.23%

289

when solution pH enhance from 3 to 6. Here are the reasons: the lower the initial pH

290

is, the more dissolving-out metal ions (Fe, Zn) on 0.2Zn-BiFeO3 surface are 35 , which

291

would decrease the activity point locations on catalyst surface and Hg0 removal

292

efficiency at low solution pH. But increasing the solution pH could increase OH-

293

concentration and enhance the yield of HO2- that reacted with •OH to produce low-

294

activity O 2 ⋅ 36-38, which would consume ·OH when pH was greater than 6. Therefore,

295

Hg0 removal efficiency decreased (Eqs. 7, 8).

−

12


Page 13 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

（7）

−

296

H 2 O 2 → HO 2 + H +

297

⋅ OH + HO2 → H 2O + O 2 ⋅

298

−

（ 8）

−

4.2.6 Parallel tests

299

Parallel test was conducted under the optimum conditions where the doping ratio

300

of zinc was 0.2, H2O2 concentration was 0.15mol L-1, 0.2Zn-BiFeO3 dosage was 0.3g

301

L-1, reaction temperature was 50℃, pH was 6 and Hg0 concentration was 50µg m-3. As

302

shown in Table 1, the maximum of Hg0 removal efficiency was 89.24% and the

303

average of that was 85.23%. The experimental data indicates that the H2O2 solution

304

catalyzed by Zn-BiFeO3 system can provide a new method for removing Hg0 from

305

flue gas.

306

4.2.7 Cyclic utilization of catalyst

307

It can be seen from Table. 2 that the removal of mercury efficiencies are 85.32%,

308

85.01% and 83.52% for the fresh catalyst, first time and second time recycling

309

catalysts respectively, which may be resulting from a consequence partly lost of the

310

Fe. To investigate the feasibility of catalyst utilization, the recovery was calculated

311

according to the changes of catalyst mass before and after reaction. Generally, the

312

recycling of the 0.2Zn-doped BiFeO3 catalyst in Fenton-like solution is stable and the

313

recovery remain above 90% after 3 cycles.

314

4.3 Mechanism analysis of Hg0 removal

315

It was reported that as a lewis acid, iron ion on the surface of 0.2 Zn-BiFeO3

316

III could capture hydrone to form an active hydroxyl, (≡ Fe − OH) 39, and it reacted with

317

the H2O2 adsorbed on catalyst surface to generate coordination complex marked as

318

(≡ FeIII −OH) •(H2O2 )

by a hydrogen-bond interaction, as shown in eq.9.

13


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

319

Page 14 of 28

≡ FeIII − OH + H 2 O 2 → (≡ FeIII − OH) • (H 2 O 2 )

（9）

II ≡ Fe III in this coordination complex was converted as ≡ Fe by the electron

320

Then,

321

transfer within molecules, at same time, a peroxy radical(HO2•) with weak oxidability

322

was formed

323

catalyze H2O2 as hydroxyl radical (•OH) with strong oxidability (Eq. 11).

324

(≡ FeIII • H2O2 ) − OH →≡ FeII + H2O + HO2 •

325

≡ Fe II + H 2 O 2 →≡ Fe III − OH + • OH

40

( Eq. 10). For ≡ Fe

II

located on the active site of catalyst, it could

（10）（11）

326

It can be observed from Eq. 10 and Eq.11 that there is a circulation between

327

≡ Fe III and ≡ Fe II , which is similar to Hubble Veis Circulation and beneficial to

328

produce •OH continually. It was reported41 that the lower Zn doping would suppress

329

Fe3+ turning into Fe2+ due to the limitation of the movement of oxygen vacancies, but

330

more oxygen vacancies were introduced by higher Zn doping, which was beneficial to

331

the formation •OH (Eqs.12, 13).

332

Zn − BiFeO 3 → Zn − BiFeO 3 (e) + h +

(12)

333

h + + H 2 O → •OH + H +

(13)

334

To verify the existence of two free radicals mentioned above, the experiments

335

of quenching free radicals were carried out. As is well-known, the reaction rate

336

between tert butyl alcohol (TBA) and •OH is high as 3.8-7.6×108 m-1S-1. Hence, TBA

337

was used as quenching reagent of •OH

338

degradation of antibiotic flumequine by TiO2 43, benzoquinone (BQ) was used as the

339

quencher of HO2•. As shown in Fig. 12, when TBA and BQ concentrations are all

340

15mmol L-1 in the solutions, the mercury removal efficiency only decreases 5%; when

341

TBA is 75mmol L-1, it decreases from 85% to 45%; when BQ is also 75mmol L-1, the

42

.

Based on a study about photocatalytic

14


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Energy & Fuels

342

efficiencies decrease from 85% to 65%. The experimental results indirectly verified

343

that •OH and HO2• appeared in the catalytic reaction, while •OH was the main active

344

specie during the catalysis according to above data. For further speculating the

345

reaction mechanism, the removal products were determined and the average Hg2+

346

concentration of 5.14µg L-1 was found in the spent solutions, which proved that Hg0

347

was oxidized by •OH and HO2•.

348

Based on the characterizations of catalyst, experiments of quenching free radicals,

349

analyses of removal products and references, the Hg0 removal paths were discussed as

350

follows: 1)H2O2 was absorbed on the surface of 0.2Zn-BiFeO3 and then catalyzed into

351

•OH and HO2•; 2) Hg0 in the gas-liquid interface was diffused to the surface of 0.2Zn-

352

BiFeO3, and the absorbed Hg0 was oxidized by •OH and HO2• into Hg2+. The main

353

reactions are shown in Eqs. 14-17.

354

Hg 0 + ⋅OH → Hg + + OH −

k1=2×109 M-1s-1

(14)

k2=1×1010

(15)

k2

355

Hg + + ⋅OH → Hg 2+ + OH −

356

Hg 0 + HO 2 • + H + → Hg 2

357

Hg 2

358

5. Conclusions

2+

2+

M-1s-1

+ H 2O 2

(16)

+ HO 2 • + H + → Hg 2 + + H 2 O 2

(17)

359

Zn-doped BiFeO3 catalyst was prepared by tartaric acid sol-gel sol-gel method

360

and the characterizations suggested that the catalyst had a high degree of crystallinity

361

and pure perovskite structure. By measuring the magnetic characteristic, the saturation

362

magnetization of the catalyst was 2.631emug-1, more than that of BiFeO3 catalyst,

363

which could provide the basis of recycling the spent catalyst from adsorption liquid.

364

0.2Zn-doped BiFeO3 catalyst firstly used to remove Hg0 from flue gas, and a new

365

process for the Hg0 removal was developed. Compared with the traditional method of 15


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

366

Hg removal, the disadvantages such as high operating cost and disposal of hazardous

367

discarded activated carbon could be overcame.

368

According to the relevant characterizations, product analysis and literature

369

references, the Hg0 removal mechanism was speculated, which was that in the

370

removal process, Hg0 was oxidized into Hg2+ by •OH and HO2• resulting from the

371

eactivated H2O2 by magnetic 0.2 Zn-doped BiFeO3 catalyst.

372

Acknowledgments

373

The authors appreciate the financial support by a grant from the National key

374

R&D Program of China (No. 2017YFC0210603, No. 2016YFC0203701, and No.

375

2016YFC0203705), National Science-technology Support Plan of China (No.

376

2014BAC23B04-06, Beijing Major Scientific and Technological Achievement

377

Transformation Project of China (No.Z151100002815012), Zhejiang Provincial

378

Engineering Research Center of Industrial Boiler & Furnace Flue Gas Pollution

379

Control, Hangzhou, 311202, P. R. China and Fundamental Research Funds for the

380

Central Universities (No. 2014ZD41).

381

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mercury by HCl over MnO2, catalyst: Insights from first principles. Chem. Eng. J.

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nanoparticles as efficient heterogeneous Fenton-like catalysts for degradation of

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organic pollutants with H2O2. J. Hazard. Mater. 2017,322,152-162.

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removal of organic pollutants with magnetic nanoscaled BiFeO3 as a reusable

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heterogeneous fenton-like catalyst. Environ. Sci. Technol. 2010, 44,1786.

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[22] An, J.; Zhu, L.; Zhang, Y.; Tang, H. Efficient visible light photo-fenton-like

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degradation of organic pollutants using in situ surface-modified BiFeO3 as a catalyst.

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J. Environ. Sci. 2013,25,1213-1225.

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[23] Antonov, V.; Georgieva, I.; Trendafilova, N.; Kovacheva, D.; Krezhov, K. First

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principles study of structure and properties of La- and Mn-modified BiFeO3. Solid.

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State. Sci. 2012, 14,782-788.

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[24] Qian, F.Z. Preparation and multiferroic properties of rare-earth doped BiFeO3

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nanoparticles. East China Normal University, 2010.

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[25] Zhong, Q.; Huang, M.; Wang, J.; Wei, Y.; Lin, J.; Jihuai, W.U. Synthesis and

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Photocatalytic Properties of Zn2+ Doped BiFeO3 Powders. Mater. Rev. 2013,27; 40-44.

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xCuxO4

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mercury removal. Chemosphere. 2015,125,16-24.

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Elemental mercury (Hg0 ) removal from containing SO 2 /NO flue gas by magnetically

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separable Fe2.45Ti0.55 O 4 /H 2O2 advanced oxidation processes. Chem. Eng. J. 2015,

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273,381-389.

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as highly active heterogeneous Fenton-like catalysts toward elemental

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Properties of Polycrystalline Sr-Substituted BiFeO3 Thin Films Prepared by Pulsed

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Laser Deposition. Curr. Appl. Phys. 2011,11, S270-S273.

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[29] Su, M.; He, C.; Sharma,;V.K.; Abou, A.M.; Xia, D.; Li, X.Z.; Deng, H.; Xiong,

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Y. Mesoporous zinc ferrite: synthesis, characterization, and photocatalytic activity

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with H2O2/visible light. J. Hazard. Mater. 2012, 211-212 , 95.

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persulfate activated by iron-modified diatomite. Appl. Catal. B-Environ. 2016,

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presence of iron oxides and ambient aerosol. Atmos. Environ. 1997, 3, 4125-4137.

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Fenton-like heterogeneous catalyst for degradation of 4-chlorophenol. Environ. Sci.

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leakage and ferroelectric properties of Zn-doped BiFeO3 thin films grown by sol-gel

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Page 22 of 28

508 509 510 511 512

Figures Fig. 1.

513 514

Fig. 1. Schematic diagram of experimental system.1.Nitrogen cylinder; 2.Pressure reducing valve;

515

3.Rotameter; 4.Mercury generator; 5.Thermostat oil bath; 6.Mixed gas cylinder; 7.Three-way

516

valve; 8.Thermostat water bath 9.Bubbling reactor; 10.Drying tower; 11. QM201H Cold atom

517

fluorescence mercury detector; 12. Potassium permanganate-sulfuric acid solution.

518 519

Fig. 2

520 521 522

(b) (a)

523

Fig. 2 (a) XRD patterns of BiFeO3 and 0.2Zn-BiFeO3 (b) expanded scan around (101) of BiFeO3

524

and 0.2Zn-BiFeO3

525

Fig. 3.

22


Zn2p

O1s

C1s

Bi4p Fe2p

Bi4d Bi4d

Intensity(a.u.)

Bi5d 0

200

400

600

800

1000

1200

1400

Binding Energy(eV)

526 527

Fig.3 XPS spectra of 0.2Zn-BiFeO3

528 529

Fig. 4.

530

8 . 4 2 7

Fe 2p before 8 . 0 1 7

Intensity

8 . 2 1 7

9 . 9 1 7

531 532 533 534 535

730

725

8 . 4 2 7

720

715

710

9 . 9 1 7

705

700

Fe 2p after

7 . 0 1 7

536 Intensity

8 . 2 1 7

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Bi4f

Page 23 of 28

537 538 539 540

730

725

720

715

710

705

541 542

Fig.4 XPS spectra of the 0.2Zn-doped BiFeO3 before and after reaction

543 544

700

Binding Energy (eV)

Fig. 5. 23


Energy & Fuels

9 0.014 0.012

7

3

Quantity Adsorbed (cm /g)

3

dv/dlog(D) Pore Volume (cm /g)

8

6 5 4

0.010 0.008 0.006 0.004 0.002 0.000

3

0

20

40

60

80

100

120

140

160

Pore Width (nm)

2 1

adsorption desorption

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure P/P0

545 546

Fig.5(A) N2 absorption and desorption curves of 0.2Zn-BiFeO3

3

4.0

dv/dlog(D) Pore Volume (cm /g)

547

Quantity Adsorbed (cm /g)

3.5 3.0

3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 28

2.5 2.0

0.0040 0.0035 0.0030 0.0025 0.0020 0.0015 0.0010 0.0005 0.0000 0

20

40

60

80

100

120

140

Pore Width (nm)

1.5 1.0

adsorption desorption

0.5 0.0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure P/P0

548 549

Fig.5(B) N2 adsorption and desorption curves of 0.2 BiFeO3

550 551

Fig. 6.

24


Page 25 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

552 553 554

Fig. 6.Magnetic hysteresis loops of BiFeO3 and 0.2Zn- BiFeO3 at room temperature Fig.7.

555 556

Fig. 7.Effect of 0.2 Zn-doped amount on Hg0 removal. H2O2 concentration, 0.15mol L-1;

557

solutionpH,6; catalyst dosage , 0.3g·L-1; reaction temperature, 50℃; total gas flow, 1L·min-1;

558

Hg0concentration, 50µg/m3

559 560

Fig.8

561

25


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

562

Fig. 8. Effect of catalyst dosage on Hg0 removal. H2O2 concentration, 0.15mol L-1; solution pH,

563

6; reaction temperature, 50℃; total gas flow, 1L·min-1; Hg0 concentration, 50µg m-3

564 565

Fig.9

566 567

Fig. 9 Effect of H2O2 concentration on Hg0 removal. Solution pH, 6; catalyst dosage, 0.3g·L-1;

568

reaction temperature, 50℃; total gas flow, 1L·min-1; Hg0 concentration,50µg m-3.

569

Fig.10

570 571

Fig.19 Effect of reaction temperature on Hg0 removal. H2O2 concentration, 0.15mol L-1; solution

572

pH,6; catalyst dosage ,0.3g·L-1; total gas flow, 1L·min-1; Hg0 concentration,50µg m-3.

573

Fig.11

26


Page 26 of 28

Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

574 575

Fig. 11 Effect of solution pH on Hg0 removal. H2O2 concentration, 0.15mol L-1; catalyst dosage,

576

0.3g·L-1; reaction temperature 50℃; total gas flow, 1L·min-1; Hg0 concentration,50µg m-3.

577

Fig.12

578 Fig. 12 Effects of quenching agents on Hg0 removal

579 580

Table

581

Table1. Parallel test results of mercury removal by 0.2Zn-BiFeO3 /H2O2 system

582 Number

1

2

3

4

5

Average

S2

Efficiency %

85.32

84.59

85.26

84.74

86.24

85.23

0.648

583 584

Table. 2 Cyclic utilization experiments of catalyst Re-use times Mass of 0.2Zn-BiFeO3

1

2

3

0.3g

0.276g

0.2484g

27


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Recovery % Mercury removal

Page 28 of 28

-

92%

90%

85.32%

85.01%

83.52%

efficiency %

585

28


Removal of Elemental Mercury in Flue Gas with H2O2 Solution

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