Visible-Light-Driven Photocatalytic Degradation of Organic Water

Subscriber access provided by Northwestern Univ. Library

Article

Visible-light-driven photocatalytic degradation of organic water pollutants promoted by sulfite addition Wei Deng, Huilei Zhao, Fuping Pan, Xuhui Feng, Bahngmi Jung, Ahmed Abdel-Wahab, Bill Batchelor, and Ying Li Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04206 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on November 4, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25

Environmental Science & Technology

1

Visible-light-driven photocatalytic degradation of organic water pollutants

2

promoted by sulfite addition

3

Wei Deng1, Huilei Zhao1, Fuping Pan1, Xuhui Feng1, Bahngmi Jung2, Ahmed Abdel-Wahab2,

4

Bill Batchelor3, and Ying Li1*

5

1

Department of Mechanical Engineering, Texas A&M University, College Station, Texas, USA

6

2

Chemical Engineering Program, Texas A&M University at Qatar, Doha, Qatar

7

3

Department of Civil Engineering, Texas A&M University, College Station, Texas, USA

8

*Corresponding Author: Ying Li, Tel.: +1-979-862-4465, E-mail: [email protected]

9

Abstract

10

Solar-driven heterogeneous photocatalysis has been widely studied as a promising technique for

11

degradation of organic pollutants in wastewater. Herein, we have developed a sulfite-enhanced

12

visible-light-driven photodegradation process using BiOBr/methyl orange (MO) as the model

13

photocatalyst/pollutant system. We found that the degradation rate of MO was greatly enhanced

14

by sulfite, and the enhancement increased with the concentration of sulfite. The degradation rate

15

constant was improved by twenty-nine times in the presence of 20 mM sulfite. Studies using hole

16

scavengers suggest that sulfite radicals generated by the reactions of sulfite (sulfite anions or

17

bisulfite anions) with holes or hydroxyl radicals are the active species for MO photodegradation

18

using BiOBr under visible light. In addition to the BiOBr/MO system, the sulfite-assisted

19

photocatalysis approach has been successfully demonstrated in BiOBr/rhodamine B (RhB),

20

BiOBr/phenol, BiOI/MO, and Bi2O3/MO systems under visible light irradiation, as well as in

21

TiO2/MO system under simulated sunlight irradiation. The developed method implies the

22

potential of introducing external active species to improve photodegradation of organic 1 ACS Paragon Plus Environment


23

pollutants and the beneficial use of air pollutants for the removal of water pollutants since sulfite

24

is a waste from flue gas desulfurization process.

25

Introduction

26

As an essential element for life from the perspectives of ecology and biology, water is

27

circulated through a natural hydrologic cycle with self-cleaning capability. However, tremendous

28

amounts of industrial and municipal wastewaters and landfill leachates make advanced water

29

treatment techniques indispensable to maintain not only considerable available water supply but

30

also a healthy large-scale hydrologic cycle. Removal of organic pollutants is one critical step in

31

water treatment because many organic pollutants cause severe threats to health. For example,

32

azo-dyes such as methyl orange (MO) are highly mutagenic and carcinogenic.1, 2 Currently well-

33

developed techniques for organic pollutants removal include adsorption,3 biodegradation,4

34

chlorination,5 ozonation,6 combined coagulation/flocculation using water treatment plant sludge,7

35

electrochemical oxidation,8 and reverse osmosis,9 etc. Apart from those techniques,

36

heterogeneous photocatalysis is a process for water treatment with the primary feature that it

37

utilizes free and inexhaustible solar energy, implying its great potential as a low-cost,

38

environmental friendly and sustainable treatment technology. As the model photocatalysis

39

system, TiO2/UV has been widely demonstrated to be capable of removing various organic

40

compounds in water.10 Nevertheless, UV light makes up only about 4% of the solar spectrum

41

while approximately 40% of solar energy is in the visible region.11 Therefore, application of

42

TiO2 in water treatment is hindered, because pristine TiO2 responds to only UV light due to its

43

wide band gap (3.2 eV for anatase). To address this problem, various modifications have been

44

applied to engineer TiO2, including doping, sensitization, forming heterostructures and coupling

2 ACS Paragon Plus Environment

Page 2 of 25

Page 3 of 25


45

with π-conjugated architectures. At the same time, research efforts have been made to explore

46

novel visible-light-active photocatalysts.12-18

47

Within the exploration of novel visible-light-active photocatalysts, bismuth-based

48

materials with low toxicity are appropriate candidates owing to the fact that Bi 6s in Bi(III) could

49

hybridize O 2p levels to push up the position of valence band (VB), thus narrowing the band gap

50

to harvest visible light.19 As layered ternary oxide semiconductors, bismuth oxyhalides (BiOXs,

51

X=Cl, Br, I) are one category of bismuth-based materials and are crystallized in a tetragonal

52

matlockite structure where [Bi2O2] slabs are interleaved with double halogen atom slabs along

53

[001] direction.20 Recent studies of BiOXs focus on designing alloyed BiOX compounds with

54

more than one type of halogen, like BiOCl/BiOBr and BiOBr/BiOI, and the ratio of each halogen

55

could be modified to optimize the performance.21-24 BiOXs with specifically exposed facets, like

56

(001) and (010) facets25, 26 and BiOX-based heterostructures, such as BN/BiOBr, BiOCl/Bi2O3,

57

Ag/AgBr/BiOBr and BiOBr/Bi2MoO6,27-30 have also been studied extensively. These modified

58

photocatalysts have shown exceptional photoactivity under visible light as measured by

59

degradation of organic pollutants; therefore, it is promising to further study BiOXs for improved

60

visible-light-driven photocatalysis.

61

The fundamental mechanism of photocatalysis is based on production of photo-induced

62

electron-hole pairs that react with oxygen and water to produce superoxide radicals and hydroxyl

63

radicals, which are the typical active species that cause photodegradation of organic pollutants.

64

Meanwhile, electrons and holes may react with other compounds in water to produce other active

65

species, so providing such compounds provides a way to enhance contaminant degradation.

66

Therefore, manipulating active species on carefully designed photocatalysts is an important

67

aspect of study. The sulfite radical is an example of an active species that possess great reducing 3 ACS Paragon Plus Environment


68

and oxidizing capabilities which can be applied to effectively degrade organics such as phenol,

69

chlorpromazine, olefin, and polyunsaturated fatty acid.31-35 Sulfite radicals are usually created

70

upon the photolysis of sulfite anions under middle UV light or via sulfite anions reacting with

71

transition metal ions and other radicals like hydroxyl radicals.36-39 Since sulfur dioxide is a major

72

air pollutant and sulfite is a product from flue-gas desulfurization process, it is an attractive idea

73

to convert sulfite waste into useful sulfite radicals through a photocatalytic process and to

74

enhance photodegradation of organic water pollutants.

75

In this paper, we have developed a novel and generalized approach of introducing sulfite

76

into photocatalyst/organic water pollutant systems and generating sulfite radicals to promote

77

degradation of organics under visible light. This new concept of sulfite-assisted photocatalysis

78

has been validated in BiOBr/MO, BiOBr/RhB, BiOBr/phenol, BiOI/MO, and Bi2O3/MO systems

79

under visible light, and TiO2/MO under simulated sunlight. A possible mechanism is that sulfite

80

anions react with photo-induced holes or hydroxyl radicals to produce sulfite radicals that attack

81

organic molecules. Considering that sulfur dioxide is a major air pollutant and sulfite is a waste

82

from flue-gas desulfurization process, the sulfite-enhanced photodegradation method may be

83

developed into a new and cost-effective technology of beneficial use of air pollutants for the

84

removal of water pollutants.

85

Materials and Methods

86

Materials

87

Bismuth nitrate pentahydrate (Bi(NO3)3•5H2O, ≥98.0%) and sodium sulfide nonahydrate

88

(Na2S•9H2O, ≥98.0%) were obtained from Aldrich-Sigma and potassium bromide (KBr) was

89

obtained from Beantown Chemical Inc (New Hampshire, US). Ethylenediaminetetraacetic acid


Page 4 of 25

Page 5 of 25


90

(EDTA, ≥99.4%), isopropyl alcohol (C3H8O, ≥99.5%), acetic acid (C2H4O2, ≥99.7%), potassium

91

iodide (KI, ≥99.0%), and hydrochloric acid (HCl, 37.5%) were purchased from BDH Middle

92

East LLC (Dubai, UAE). Sodium sulfite anhydrous (Na2SO3, ≥98.0%) was purchased from

93

Amresco (Ohio, US). Commercial TiO2 nanopowders, P25, were obtained from Evonik Corp

94

(Essen, Germany). All reagents were used directly for the experiments without any further

95

purification.

96

Preparation of photocatalyst materials

97

BiOBr microspheres were synthesized through a facile method at room temperature.40

98

Typically, bismuth nitrate (3 mmol, 1.47 g) was dissolved in a mixed solution of deionized water

99

(8.5 mL) and acetic acid (4.5 mL), followed by 15 min magnetic stirring to obtain a clear and

100

transparent solution. Then KBr solution (3 mmol, 0.357 g in 10 mL deionized water) was added

101

into the above solution under rigorous stirring and the mixture was stirred for another 30 min to

102

ensure complete precipitation. The precipitate was collected by centrifuging and washed

103

thoroughly with ethanol and deionized water for six times. The final product was dried at 80 °C

104

in a vacuum furnace overnight.

105

BiOI was synthesized using the same manner described above with KI as the iodine

106

source. Bi2O3 was obtained by annealing the synthesized BiOBr sample in air at 550 °C for 2 hr.

107

Materials characterization

108

X-ray diffraction (XRD) was performed on a D8 Advance diffractometer (Bruker-AXS,

109

Karlsruhe, Germany) using Cu Kα irradiation at 40 kV and 40 mA. XRD patterns from 10° to 80°

110

2θ were recorded at room temperature. The step increment was set as 0.05° 2θ and the counting



111

time per step was 1s. The surface morphology was obtained by an ultra-high resolution field-

112

emission scanning electron microscope (FE-SEM, JEOL JSM7500F, Japan) equipped with a

113

cold cathode UHV field emission conical anode gun. UV–vis diffuse reflectance spectra were

114

measured on a Hitachi U4100 UV–vis-NIR Spectrophotometer (Japan) with Praying Mantis

115

accessory. Micromeritics ASAP 2420 physisorption analyzer was used to determine the

116

Brunauer–Emmett–Teller (BET) surface area at liquid nitrogen temperature (77.3 K). Prior to

117

measurement, the BiOBr powder was degassed in vacuum at 100 °C for 12 hr.

118

Photocatalytic degradation experiments

119

Sulfite-enhanced visible-light-driven photocatalysis was evaluated by studying various

120

photocatalyst/organics systems with or without sulfite under visible light irradiation. The light

121

source was a 150 W solar simulator (Oriel® Sol1A, Newport) equipped with an UV cutoff filter

122

to provide visible light (λ ≥ 400 nm). The distance between the liquid surface and the lamp

123

housing exit was 18 cm. When BiOBr/MO was studied as the model system, 20 mg BiOBr was

124

added to 40 mL 10 ppm MO solution. A certain amount of Na2SO3 was then added to the

125

BiOBr/MO system along with HCl to keep the initial pH at 7.5. The same parameters were used

126

to evaluate BiOI and Bi2O3 for MO degradation. For the experiments of RhB and phenol

127

degradation, 0.5 g/L BiOBr was dispersed in a 20 ppm RhB solution and 1 g/L BiOBr in a 5 ppm

128

phenol solution, respectively. In a separate experiment, 0.2 g/L P25 was loaded in a 40 ppm MO

129

solution to probe the sulfite-enhanced photocatalysis under simulated sunlight. Prior to

130

irradiation, the photocatalyst/organics suspension systems with or without sulfite were sealed

131

with a parafilm and stirred at 400 rpm in the dark for 1 hr to ensure adsorption/desorption

132

equilibrium. At given time intervals, a certain volume of the suspension sample was taken and


Page 6 of 25

Page 7 of 25


133

centrifuged for subsequent analysis. The concentrations of MO and RhB were analyzed on a

134

UV–Vis spectrophotometer (UV-2600, Shimadzu) at their characteristic wavelengths of 464 nm

135

and 554 nm, respectively. The concentration of phenol was measured by a high-pressure liquid

136

chromatography (HPLC-2030C, Shimadzu) equipped with a reversed phase C18 column from

137

Kinetex (00F-4601-E0). A mixture of acetonitrile and deionized water, flowing at a rate of 1.0

138

mL/min, was used as the mobile phase. The total organic carbon (TOC) level was measured by a

139

TOC analyzer (TOC-5000A, Shimadzu).

140

For degradation experiments under anaerobic condition, a 17 mL transparent cylindrical

141

cell (35-PX-10 from Starna Cells Inc.) was used. The BiOBr/MO suspension with or without

142

sulfite was firstly fed into the cell and degassed for 10 min. The suspension was then purged with

143

Argon (15 mL/min) for 20 min, sealed and stirred in dark for 30 min. After 30 min visible light

144

irradiation, the suspension was taken out for analysis.

145

Detection of active species

146

Relevant active species produced in the BiOBr/MO/sulfite system under visible light

147

were identified by adding different types of scavengers. EDTA and isopropyl alcohol (IPA) were

148

used as scavengers for holes or hydroxyl radicals, and sodium nitrate was used as an aqueous

149

electron scavenger.41 The degradation of MO was monitored to study the inhibition effect.

150

Results and Discussion

151

Structure, morphology, and optical properties

152

The XRD pattern of the prepared BiOBr sample is shown in Figure 1a, which indicates

153

high crystallinity of the sample with diffraction peaks well indexed to the tetragonal structure of 7 ACS Paragon Plus Environment


154

BiOBr (JCPDS File No. 73-2061). The morphology of the prepared BiOBr sample is shown in

155

both low (Figure 1c) and high-magnification (Figure 1d) SEM images. The BiOBr sample has a

156

3D hierarchical microspherical structure with an average diameter of 6 µm and it is constructed

157

by densely stacked thin microplates originating from the layered structure of the bismuth

158

oxyhalide. The microspheres are interconnected to form larger aggregates, which make them

159

easy to collect after water treatment due to their large sizes. The space between the microplates

160

within the microspheres can promote light trapping and thus enhance the photocatalytic

161

performance.42 In addition, the BET surface area of the BiOBr sample was measured to be 4.71

162

m2/g through N2 adsorption-desorption analysis (Figure S1).

163

Since photoabsorption and photocatalytic performance are closely related to the band

164

structure of photocatalysts, the optical properties of BiOBr microspheres were examined by UV-

165

vis diffuse reflectance spectra and results are shown in Figure 1 (a) XRD patterns, (b) UV-vis

166

diffuse reflectance (the inset gives the band gap that is 2.82eV) and (c, d) SEM images of the

167

synthesized BiOBr microspheres. . The absorption edge lies at 440 nm and covers part of the

168

visible light region. The optical absorption of a crystalline semiconductor near the band edge

169

follows the formula α hv = A(hv − Eg ) n /2 , where α , v , Eg and A are the absorption coefficient,

170

light frequency, band gap energy, and proportionality constant, respectively.43 The value of n is 4

171

for BiOBr because of its indirect transition characteristics. The band gap energy ( Eg ) of BiOBr

172

can be thus extracted from a plot of (αhv) versus hv and it is estimated to be 2.82 eV from the

173

inset in Figure 1 (a) XRD patterns, (b) UV-vis diffuse reflectance (the inset gives the band gap

174

that is 2.82eV) and (c, d) SEM images of the synthesized BiOBr microspheres. b. The flat-band

175

potential energy of BiOBr was calculated based on Mulliken electronegativity theory:

1/2


Page 8 of 25

Page 9 of 25


χ and Ee

176

EVB = χ − E e + 0.5Eg , where EVB ,

177

BiOBr and energy of free electrons on the hydrogen scale, respectively.40 The calcuated

178

conduction band and valence band positions are 0.27 V and 3.09 V vs NHE, respectively.

are the valence band enenrgy, electronegativity of

179 180

Figure 1 (a) XRD patterns, (b) UV-vis diffuse reflectance (the inset gives the band gap that is

181

2.82eV) and (c, d) SEM images of the synthesized BiOBr microspheres.

182

Photocatalytic degradation of organic pollutants assisted by sulfite

183

The photodegradation of MO under visible light was used as a probe to evaluate the

184

performance of BiOBr microspheres and the enhancement brought by sulfite (Figure 2). In the

185

absence of sulfite, only 25% of MO was degraded by BiOBr under visible light irradiation for 30

186

min, while 70% removal was achieved with the addition of 5 mM sulfite (Figure 2a). The



Page 10 of 25

187

enhancement was more evident at high sulfite concentrations, where 90% and 96% removals

188

were observed with 10 mM and 20 mM sulfite, respectively. Since 10 mM sulfite alone without

189

photocatalyst yielded negligible degradation of MO (Figure 2a), it is confirmed that sulfite

190

participated in the photocatalytic degradation process. The apparent pseudo-first-order rate

191

constants were calculated by regression using a linearized, first-order decay model

192

( − ln(C / C0 ) = kt , Figure 2b), where C0 is the initial concentration of MO, C is the

193

concentration after irradiation for a certain time t, and k is the rate constant. As displayed in

194

Figure 2c, the rate constant increased by about 10, 20, and 29 times upon the addition of 5, 10,

195

and 20 mM sulfite, respectively. The improvement greatly depended on the concentration of

196

sulfite, implying that the increase in the rate constant should be ascribed to sulfite. It was noted

197

in Figure 2c that the rate constant increased linearly with the sulfite concentration when sulfite

198

was less than 10 mM but the rate of improvement was less prominent at a higher sulfite

199

concentration, implying that the rate of production of photo-induced electrons and holes is the

200

rate limiting factor at high sulfite concentrations.

201

As shown in Figure 2d, rapid decrease at the wavelength of 464 nm was achieved in the

202

UV-vis absorption band of MO solution under visible light in the presence of BiOBr and 10 mM

203

sulfite. Neither a shift of the absorption band nor an emergence of new absorption band was

204

observed, indicating that MO was degraded and no other chromophoric molecules were

205

produced. Besides high photoactivity, long term durability of the photocatalyst is also required

206

for practical water treatment applications. Figure S2 indicates that there was no significant

207

change in degradation efficiency after four cycles, demonstrating that BiOBr has high stability in

208

the developed sulfite-assisted photocatalytic process.


Page 11 of 25


209

210 211

Figure 2 (a) Photocatalytic activities of BiOBr for the degradation of MO under visible light

212

with/without Na2SO3, and (b) kinetic fitting. (c) The dependence of MO degradation rate

213

constant on sulfite concentration. (d) Time resolved UV-vis absorption of MO solution under

214

visible light with BiOBr and 10 mM Na2SO3. The initial concentration of MO was 10 ppm and

215

the loading of BiOBr was 0.5 g/L.

216

In addition to MO, RhB and phenol were also tested as target compounds to generalize

217

the effect of sulfite on photocatalysis by BiOBr. In the absence of sulfite, the degradation of RhB

218

was relatively slow (Figure 3a) and the gradual blue shifts of the absorption band were attributed



Page 12 of 25

219

to the removal of ethyl groups.44 After 30 min irradiation, RhB molecules were fully de-ethylated

220

to the core structure of rhodamine and a new absorption band at 506 nm appeared.44 Interestingly,

221

in the presence of 10 mM sulfite (Figure 3b), the absorption band of RhB at 554 nm decreased

222

without a blue shift, implying a direct destruction of the core rhodamine structure for faster

223

decolorization. However, the addition of sulfite did not improve the mineralization of RhB;

224

rather, the mineralization was slightly decreased in the presence of sulfite according to the TOC

225

removal result (Figure 3c). It is probably because sulfite scavenges holes and hydroxyl radicals

226

that are believed to be essential of mineralization.44 This implies that the developed approach of

227

sulfite-enhanced photocatalysis is as an efficient method for decolorization of wastewaters, while

228

further studies on this approach are necessary to offer improvement in terms of mineralization.

229

The photodegradation of phenol, a persistent water pollutant, was also examined to confirm the

230

function of sulfite. Figure S3 shows that without sulfite there was only 5% photodegradation of

231

phenol, while the addition of 20 mM sulfite resulted in 78% photodegradation after visible light

232

irradiation for 30 min. All these results on MO, RhB, and phenol degradation show that adding

233

sulfite is a powerful approach that can significantly enhance visible-light-driven photocatalysis

234

using BiOBr as the model catalyst.

235 236


Page 13 of 25


237 238

Figure 3. Time resolved UV-vis absorption of RhB solution with BiOBr under visible light

239

irradiation (a) in the absence and (b) presence of 10 mM Na2SO3; (c) TOC removal of 10 ppm

240

RhB after 2 h visible light irradiation with or without Na2SO3. The initial concentration of RhB

241

was 20 ppm and the loading of BiOBr was 0.5 g/L.

242

We have further demonstrated that sulfite-enhanced photocatalysis could be achieved

243

with several other photocatalysts besides BiOBr. The kinetics of MO degradation illustrated in

244

Figure S4 show that the photocatalytic activities of Bi2O3, BiOI and P25 in the presence of 10

245

mM sulfite far exceeded those in the absence of sulfite. Specifically, MO photodegradation by

246

Bi2O3 under 30 min visible light irradiation was increased from 16% without sulfite to 92% with

247

sulfite; the degradation by BiOI under 30 min visible light irradiation was increased from 40%

248

without sulfite to 86% with sulfite; and the degradation by P25 under 15 min simulated sunlight

249

irradiation was increased from 39% without sulfite to 92% with sulfite.

250

Moreover, we conducted experiments using other oxidants such as H2O2 and K2S2O8 to

251

evaluate if they would make similar contributions to enhanced photocatalysis as sulfite did. The

252

results are shown in Figure 4. H2O2 alone without BiOBr did not oxidize MO, while the addition



Page 14 of 25

253

of H2O2 together with BiOBr slightly increased the photodegradation of MO compared with that

254

without H2O2. This is because that H2O2 may react with photo-induced e-h pairs to produce more

255

reactive radicals (e.g. superoxide or hydroxyl radicals) 45 to promote photocatalysis. In contrast,

256

K2S2O8 as a strong oxidant46 directly oxidized MO, and the effect was the same with or without

257

BiOBr, indicating that K2S2O8 did not participate in any photo-induced reactions. More

258

significant enhancement was seen due to sulfite-enhanced photocatalysis when both NaSO3 and

259

BiOBr were present, resulting in more than 90% MO degradation, compared to 80% and 45%

260

degradation when K2S2O8 and H2O2 were added, respectively. This result clearly demonstrates

261

the unique advantage of using sulfite-enhance photocatalysis for removal of organic pollutants in

262

water.

263

Based on all the above experimental results of using various photocatalysts (BiOBr,

264

Bi2O3, BiOI, and P25) to degrade various organic pollutants (MO, RhB, and phenol) under

265

different illumination conditions (simulated sunlight and visible light), we believe that sulfite-

266

enhanced photocatalysis is applicable as a general approach to promote degradation of organic

267

water pollutants.

268 269


Page 15 of 25


270 271

Figure 4. Photodegradation of MO under visible light for comparison of 10 mM Na2SO3, H2O2

272

and K2S2O8. The initial concentration of MO was 10 ppm and the loading of BiOBr was 0.5 g/L.

273

Mechanism of sulfite-enhanced photocatalysis

274

Understanding the mechanism of sulfite-enhanced photocatalysis, including the roles of

275

sulfite, types of reactive species, and possible formation pathways of the reactive species will

276

help extend this novel approach to broader photocatalytic applications. The mechanism was

277

investigated using the BiOBr/MO/sulfite system by addressing the following three questions.

278

Does the addition of sulfite promote oxidization or reduction of MO? What are the reactive

279

species and how are they generated from sulfite and the photocatalyst? Are other active species

280

like oxygen important to sulfite-assisted photodegradation?

281

Na2SO3 and Na2S have been widely used as sacrificial agents for photoelectrochemical

282

hydrogen generation due to their hole-scavenging capability.47 Thus, if sulfite-enhanced

283

photocatalysis was mainly due to the effect of hole scavenging and promoting the availability of

284

electrons for reduction reactions, other scavengers like EDTA and Na2S should play the same



285

role. However, as indicated in Figure 5, both EDTA and Na2S almost completely inhibited the

286

photodegradation of MO by BiOBr. Therefore, this suggests that holes played an important role

287

in MO degradation and that the main function of sulfite is not to simply scavenge holes but

288

possibly to produce more active species.

Page 16 of 25

289

We believe that the active species responsible for increased MO degradation is the sulfite

290

•radical ( SO3 ), and its capability of reaction with organics has been reported in the literature.35

291

To exclude the contributions of other possible active species involved in the photodegradation

292

••process, scavenging experiments of aqueous electron, SO 4 / SO5 , and oxygen were also

293

conducted. The possibility of aqueous electrons reacting with MO was studied using nitrate as an

294

electron scavenger. Figure 5 shows that the addition of 20 mM nitrate to 10 mM sulfite solution

295

did not have significant effect on MO photodegradation compared to that without nitrate. This

296

result indicates that aqueous electrons did not make a substantial contribution to the sulfite-

297

assisted enhancement in photocatalysis. To probe other sulfite related active species, such as

298

SO•-4 and SO•-5 , that may be produced by reactions of SO•-3 with oxygen via a free radical chain

299

mechanism,48 experiments under anaerobic conditions in a sealed cell were conducted, and the

300

results in Figure 5 show that sulfite greatly improved MO degradation at about the same level in

301

the absence of oxygen as in the presence of oxygen. Therefore, sulfite radical itself, rather than

302

its reaction products with oxygen, was responsible for MO degradation.

303

•There are two possible pathways for the formation of SO3 . In the first pathway, sulfite

304

anions are oxidized by photo-induced holes directly. This is feasible, since the redox potential of


Page 17 of 25


305

SO•-3 / SO32- couple is 0.75 V (vs NHE, at pH=7),49 which is much lower than the VB position of

306

BiOBr (3.09 V):

307

+ hVB + SO32− → SO3•−

308

The second pathway to produce sulfite radicals is through reaction of sulfite anions with

309

hydroxyl radicals ( OH )50. This reaction is also feasible, because the redox potential of

310

(1)

•

•

OH/OH− (2.18 V vs NHE, at pH=7)51 is higher than that of SO•-3 / SO32- (0.75 V):

311 312

Figure 5. MO concentration percentages left after 30 min visible light irradiation in the absence

313

or presence of various scavengers under aerobic or anaerobic conditions. The initial

314

concentration of MO was 10 ppm and the loading of BiOBr was 0.5 g/L.

315 316

317

•

+ hVB +H2O → •OH+H+

(2)

OH + SO32− → SO3•− + OH−

(3)



Page 18 of 25

318

To confirm that photo-induced holes contributed to the formation of sulfite radicals via

319

the two pathways as illustrated in Eqs (1-3), active species trapping experiments were conducted

320

in the presence of 10 mM sulfite with isopropyl alcohol (IPA) or EDTA used as a scavenger for

321

holes and hydroxyl radicals (Figure 5). In the BiOBr/MO/sulfite suspension, the addition of IPA

322

significantly inhibited the degradation of MO, especially when the concentration of IPA was 330

323

mM. Likewise, the addition of 2 mM EDTA to sulfite almost completely inhibited MO

324

degradation. This supports the hypothesis that the sulfite radical is the active agent and is

325

produced by reaction with holes or hydroxyl radicals. Nevertheless, it is possible that the effect

326

•of IPA is not as a hydroxyl radical scavenger, but that it reacts with SO3 or with sulfite/bisulfite

327

and thus impeded the degradation of MO. To investigate this, the UV-vis absorption band of

328

SO32- (200 to 260 nm) was monitored in the BiOBr/MO system under visible light irradiation to

329

indicate the dependence of sulfite consumption rate upon IPA addition, and the results are shown

330

in Figure S5. In the absence of sulfite there was no absorbance band from 200 to 260 nm (Figure

331

S5a), while a clear absorption band appeared upon the addition of sulfite and the absorbance

332

decreased with time (Figure S5b), indicating the consumption of sulfite upon illumination. When

333

IPA was added at a concentration of 15 mM (Figure S5c), the rate of sulfite loss slowed down,

334

and this slowdown becomes very obvious at 330 mM IPA (Figure S5d). This result suggests that

335

IPA did not react with sulfite/bisulfite or sulfite radicals but rather competed with sulfite for

336

holes or hydroxyl radicals. It further supports the hypothesis that sulfite radicals were generated

337

by sulfite/bisulfite anions reacting with holes or hydroxyl radicals following Eqs. (1-3).

338

All of these results support the hypothesis that the sulfite radical is the main active

339

species in degrading MO in the sulfite-promoted photodegradation process. These sulfite radicals

340

are derived from sulfite/bisulfite anions reacting with holes or hydroxyl radicals. The overall 18 ACS Paragon Plus Environment

Page 19 of 25


341

process is illustrated in Figure 6. It should be noted that although holes and hydroxyl radicals can

342

attack MO molecules directly,10 they make little contribution to the degradation of MO compared

343

to sulfite radicals when sulfite is present, since the degradation rate is slow without sulfite

344

whereas significant enhancement of degradation is obtained upon the addition of sulfite.

345

Furthermore, the concentration of sulfite (10 mM) was much higher than that of MO (0.03 mM),

346

so it is more likely for holes and hydroxyl radicals to react with sulfite anions than with MO.

347

Although sulfite-enhanced photocatalysis has been successfully demonstrated in this work, the

348

pathways how sulfite radicals react with organic molecules are not clear. Further investigations

349

on sulfite radical generation using time-resolved spectroscopy are recommended, as well as the

350

identification of reaction intermediates and products in sulfite-promoted photodegradation of

351

organics by using chromatography and mass spectroscopy techniques such as GC-MS and LC-

352

MS.46, 52, 53

353 354

Figure 6. The proposed pathways of sulfite radicals formation and MO photodegradation under

355

visible light using BiOBr.

356

Supporting Information 19 ACS Paragon Plus Environment


Page 20 of 25

357

Nitrogen adsorption–desorption isotherms of BiOBr; results of cycling tests for MO degradation

358

by BiOBr; photocatalytic activity of BiOBr for phenol degradation; photocatalytic activity of

359

Bi2O3 and BiOI for MO degradation MO; and UV-vis absorption band changes of sulfite and

360

bisulfite.

361

Acknowledgement

362

This study was made possible by a grant from the Qatar National Research Fund under its

363

National Priorities Research Program award number NPRP 8-1406-2-605. The paper’s contents

364

are solely the responsibility of the authors and do not necessarily represent the official views of

365

the Qatar National Research Fund. The use of the Texas A&M University Materials

366

Characterization Facility is also acknowledged.

367

368

References:

369 370

1. Brown, M. A.; De Vito, S. C., Predicting azo dye toxicity. Crit. Rev. Env. Sci. Technol. 1993, 23 (3), 249-324.

371 372

2. Puvaneswari, N.; Muthukrishnan, J.; Gunasekaran, P., Toxicity assessment and microbial degradation of azo dyes. Indian J Exp Biol. 2006, 44 (8), 618-626.

373 374

3. Ali, I.; Asim, M.; Khan, T. A., Low cost adsorbents for the removal of organic pollutants from wastewater. J. Environ. Manage. 2012, 113, 170-183.

375 376

4. Megharaj, M.; Ramakrishnan, B.; Venkateswarlu, K.; Sethunathan, N.; Naidu, R., Bioremediation approaches for organic pollutants: a critical perspective. Environ. Int. 2011, 37, (8), 1362-1375.

377 378

5. Slokar, Y. M.; Le Marechal, A. M., Methods of decoloration of textile wastewaters. Dyes Pigm. 1998, 37, (4), 335-356.

379 380

6. Ikehata, K.; Gamal El-Din, M.; Snyder, S. A., Ozonation and advanced oxidation treatment of emerging organic pollutants in water and wastewater. Ozone Sci. Eng. 2008, 30 (1), 21-26.


Page 21 of 25


381 382 383

7. Moghaddam, S. S.; Moghaddam, M. A.; Arami, M., Coagulation/flocculation process for dye removal using sludge from water treatment plant: optimization through response surface methodology. J. Hazard. Mater. 2010, 175 (1), 651-657.

384 385

8. Comninellis, C., Electrocatalysis in the electrochemical conversion/combustion of organic pollutants for waste water treatment. Electrochim. Acta. 1994, 39 (11-12), 1857-1862.

386 387

9. Kang, G.-d.; Cao, Y.-m., Development of antifouling reverse osmosis membranes for water treatment: a review. Water Res. 2012, 46 (3), 584-600.

388 389 390

10. Lachheb, H.; Puzenat, E.; Houas, A.; Ksibi, M.; Elaloui, E.; Guillard, C.; Herrmann, J.-M., Photocatalytic degradation of various types of dyes (Alizarin S, Crocein Orange G, Methyl Red, Congo Red, Methylene Blue) in water by UV-irradiated titania. Appl. Catal., A 2002, 39 (1), 75-90.

391 392 393

11. Li, Q.; Guo, B.; Yu, J.; Ran, J.; Zhang, B.; Yan, H.; Gong, J. R., Highly efficient visible-lightdriven photocatalytic hydrogen production of CdS-cluster-decorated graphene nanosheets. J. Am. Chem. Soc. 2011, 133 (28), 10878-10884.

394 395 396

12. Pelaez, M.; Nolan, N. T.; Pillai, S. C.; Seery, M. K.; Falaras, P.; Kontos, A. G.; Dunlop, P. S.; Hamilton, J. W.; Byrne, J. A.; O'shea, K., A review on the visible light active titanium dioxide photocatalysts for environmental applications. Appl. Catal., A 2012, 125, 331-349.

397 398 399

13. Dong, S.; Feng, J.; Fan, M.; Pi, Y.; Hu, L.; Han, X.; Liu, M.; Sun, J.; Sun, J., Recent developments in heterogeneous photocatalytic water treatment using visible light-responsive photocatalysts: a review. RSC Adv. 2015, 5 (19), 14610-14630.

400 401 402

14. Liu, L.; Jiang, Y.; Zhao, H.; Chen, J.; Cheng, J.; Yang, K.; Li, Y., Engineering coexposed {001} and {101} facets in oxygen-deficient TiO2 nanocrystals for enhanced CO2 photoreduction under visible light. ACS Catal. 2016, 6 (2), 1097-1108.

403 404 405

15. Rao, G.; Zhao, H.; Chen, J.; Deng, W.; Jung, B.; Abdel-Wahab, A.; Batchelor, B.; Li, Y., FeOOH and Fe2O3 co-grafted TiO2 photocatalysts for bisphenol A degradation in water. Catal. Commun. 2017, 97, 125-129.

406 407 408

16. Zhao, H.; Chen, J.; Rao, G.; Deng, W.; Li, Y., Enhancing photocatalytic CO2 reduction by coating an ultrathin Al2O3 layer on oxygen deficient TiO2 nanorods through atomic layer deposition. Appl. Surf. Sci. 2017, 404, 49-56.

409 410

17. Liu, L.; Gao, F.; Zhao, H.; Li, Y., Tailoring Cu valence and oxygen vacancy in Cu/TiO2 catalysts for enhanced CO2 photoreduction efficiency. Appl. Catal., A 2013, 134, 349-358.

411 412 413

18. Rao, G.; Zhang, Q.; Zhao, H.; Chen, J.; Li, Y., Novel titanium dioxide/iron (III) oxide/graphene oxide photocatalytic membrane for enhanced humic acid removal from water. Chem. Eng. J. 2016, 302, 633-640.

414 415 416

19. Ganose, A. M.; Cuff, M.; Butler, K. T.; Walsh, A.; Scanlon, D. O., Interplay of orbital and relativistic effects in bismuth oxyhalides: BiOF, BiOCl, BiOBr, and BiOI. Chem. Mater. 2016, 28 (7), 1980-1984.

417 418

20. Cheng, H.; Huang, B.; Dai, Y., Engineering BiOX (X= Cl, Br, I) nanostructures for highly efficient photocatalytic applications. Nanoscale 2014, 6 (4), 2009-2026. 21 ACS Paragon Plus Environment


Page 22 of 25

419 420 421

21. Jia, X.; Cao, J.; Lin, H.; Zhang, M.; Guo, X.; Chen, S., Transforming type-I to type-II heterostructure photocatalyst via energy band engineering: A case study of I-BiOCl/I-BiOBr. Appl. Catal., A 2017, 204, 505-514.

422 423 424

22. Dandapat, A.; Gnayem, H.; Sasson, Y., The fabrication of BiOClxBr1− x/alumina composite films with highly exposed {001} facets and their superior photocatalytic activities. Chem. Commun. 2016, 52 (10), 2161-2164.

425 426 427 428

23. Gao, S.; Guo, C.; Hou, S.; Wan, L.; Wang, Q.; Lv, J.; Zhang, Y.; Gao, J.; Meng, W.; Xu, J., Photocatalytic removal of tetrabromobisphenol A by magnetically separable flower-like BiOBr/BiOI/Fe3O4 hybrid nanocomposites under visible-light irradiation. J. Hazard. Mater. 2017, 331, 112.

429 430 431

24. Zhang, X.; Wang, C.-Y.; Wang, L.-W.; Huang, G.-X.; Wang, W.-K.; Yu, H.-Q., Fabrication of BiOBrxI1−x photocatalysts with tunable visible light catalytic activity by modulating band structures. Sci. rep. 2016, 6, 22800.

432 433

25. Jiang, J.; Zhao, K.; Xiao, X.; Zhang, L., Synthesis and facet-dependent photoreactivity of BiOCl single-crystalline nanosheets. J. Am. Chem. Soc. 2012, 134 (10), 4473-4476.

434 435 436

26. Li, H.; Hu, T.; Liu, J.; Song, S.; Du, N.; Zhang, R.; Hou, W., Thickness-dependent photocatalytic activity of bismuth oxybromide nanosheets with highly exposed (010) facets. Appl. Catal., A 2016, 182, 431-438.

437 438

27. Chai, S. Y.; Kim, Y. J.; Jung, M. H.; Chakraborty, A. K.; Jung, D.; Lee, W. I., Heterojunctioned BiOCl/Bi2O3, a new visible light photocatalyst. J. Catal. 2009, 262 (1), 144-149.

439 440 441

28. Cheng, H.; Huang, B.; Wang, P.; Wang, Z.; Lou, Z.; Wang, J.; Qin, X.; Zhang, X.; Dai, Y., In situ ion exchange synthesis of the novel Ag/AgBr/BiOBr hybrid with highly efficient decontamination of pollutants. Chem. Commun. 2011, 47 (25), 7054-7056.

442 443 444

29. Wang, S.; Yang, X.; Zhang, X.; Ding, X.; Yang, Z.; Dai, K.; Chen, H., A plate-on-plate sandwiched Z-scheme heterojunction photocatalyst: BiOBr-Bi2MoO6 with enhanced photocatalytic performance. Appl. Surf. Sci. 2017, 391, 194-201.

445 446 447

30. Di, J.; Xia, J.; Ji, M.; Wang, B.; Yin, S.; Zhang, Q.; Chen, Z.; Li, H., Advanced photocatalytic performance of graphene-like BN modified BiOBr flower-like materials for the removal of pollutants and mechanism insight. Appl. Catal., A 2016, 183, 254-262.

448 449

31. Huie, R. E.; Neta, P., Chemical behavior of sulfur trioxide (1-)(SO3-) and sulfur pentoxide (1)(SO5-) radicals in aqueous solutions. J. Phys. Chem. 1984, 88 (23), 5665-5669.

450 451

32. Norman, R.; Storey, P., Electron spin resonance studies. Part XXXI. The generation, and some reactions, of the radicals SO3–·, S2O3–·, S–·, and SH in aqueous solution. J. Chem. Soc. B 1971, 1009-1013.

452 453

33. Ozawa, T.; Kwan, T., Esr evidence for the formation of new vinyl radicals in solution. J. Chem. Soc., Chem. Commun. 1983, 0 (2), 80-81.

454 455

34. Ozawa, T.; Kwan, T., Esr studies on the reactive character of the radical anions, so−2, so−3 and so−4 in aqueous solution. Polyhedron 1983, 2, (10), 1019-1023.


Page 23 of 25


456 457

35. Erben-Russ, M.; Bors, W.; Winter, R.; Saran, M., The reaction of sulfite anion radical (SO.-3) with polyunsaturated fatty acids. Int. J. Rad. Appl. Instrum. C 1986, 27 (6), 419-424.

458 459

36. Vellanki, B. P.; Batchelor, B.; Abdel-Wahab, A., Advanced reduction processes: a new class of treatment processes. Environ. Eng. Sci. 2013, 30 (5), 264-271.

460 461 462

37. Li, X.; Ma, J.; Liu, G.; Fang, J.; Yue, S.; Guan, Y.; Chen, L.; Liu, X., Efficient reductive dechlorination of monochloroacetic acid by sulfite/UV process. Environ. Sci. Technol. 2012, 46 (13), 7342-7349.

463 464 465

38. Liu, X.; Vellanki, B. P.; Batchelor, B.; Abdel-Wahab, A., Degradation of 1, 2-dichloroethane with advanced reduction processes (ARPs): Effects of process variables and mechanisms. Chem. Eng. J. 2014, 237, 300-307.

466 467

39. Jung, B.; Farzaneh, H.; Khodary, A.; Abdel-Wahab, A., Photochemical degradation of trichloroethylene by sulfite-mediated UV irradiation. J. Environ. Chem. Eng. 2015, 3 (3), 2194-2202.

468 469

40. Gnayem, H.; Sasson, Y., Hierarchical nanostructured 3D flowerlike BiOClx Br1–x semiconductors with exceptional visible light photocatalytic activity. ACS Catal. 2013, 3 (2), 186-191.

470 471 472

41. Serpone, N.; Texier, I.; Emeline, A.; Pichat, P.; Hidaka, H.; Zhao, J., Post-irradiation effect and reductive dechlorination of chlorophenols at oxygen-free TiO2/water interfaces in the presence of prominent hole scavengers. J. Photochem. Photobiol., A 2000, 136 (3), 145-155.

473 474

42. Zhang, D.; Wen, M.; Jiang, B.; Li, G.; Jimmy, C. Y., Ionothermal synthesis of hierarchical BiOBr microspheres for water treatment. J. Hazard. Mater. 2012, 211, 104-111.

475 476

43. Morales, A. E.; Mora, E. S.; Pal, U., Use of diffuse reflectance spectroscopy for optical characterization of un-supported nanostructures. Revista Mexicana de Fisica S 2007, 53 (5), 18.

477 478

44. Fu, H.; Pan, C.; Yao, W.; Zhu, Y., Visible-light-induced degradation of rhodamine B by nanosized Bi2WO6. J. Phys. Chem. B 2005, 109 (47), 22432-22439.

479 480

45. Hirakawa, T.; Nosaka, Y., Properties of O2•-and OH• formed in TiO2 aqueous suspensions by photocatalytic reaction and the influence of H2O2 and some ions. Langmuir 2002, 18 (8), 3247-3254.

481 482 483

46. Khataee, A.; Gholami, P.; Vahid, B.; Joo, S. W., Heterogeneous sono-Fenton process using pyrite nanorods prepared by non-thermal plasma for degradation of an anthraquinone dye. Ultrason. Sonochem. 2016, 32, 357-370.

484 485 486

47. Li, J.; Cushing, S. K.; Zheng, P.; Senty, T.; Meng, F.; Bristow, A. D.; Manivannan, A.; Wu, N., Solar hydrogen generation by a CdS-Au-TiO2 sandwich nanorod array enhanced with Au nanoparticle as electron relay and plasmonic photosensitizer. J. Am. Chem. Soc. 2014, 136 (23), 8438-8449.

487 488 489

48. Hayon, E.; Treinin, A.; Wilf, J., Electronic spectra, photochemistry, and autoxidation mechanism of the sulfite-bisulfite-pyrosulfite systems. SO2-, SO3-, SO4-, and SO5-radicals. J. Am. Chem. Soc. 1972, 94 (1), 47-57.

490 491

49. Das, T. N.; Huie, R. E.; Neta, P., Reduction potentials of SO3•-, SO5•-, and S4O6•3-radicals in aqueous solution. J. Phys. Chem. A 1999, 103 (18), 3581-3588.



Page 24 of 25

492

50.

Neta, P.; Huie, R. E., Free-radical chemistry of sulfite. Environ. Health Perspect. 1985, 64, 209.

493 494

51. Koppenol, W.; Butler, J., Energetics of interconversion reactions of oxyradicals. Adv. Free Radic. Biol. Med. 1985, 1 (1), 91-131.

495 496 497

52. Khataee, A.; Kayan, B.; Gholami, P.; Kalderis, D.; Akay, S.; Dinpazhoh, L., Sonocatalytic degradation of Reactive Yellow 39 using synthesized ZrO2 nanoparticles on biochar. Ultrason. Sonochem. 2017, 39, 540-549.

498 499 500

53. Khataee, A.; Gholami, P.; Vahid, B., Catalytic performance of hematite nanostructures prepared by N2 glow discharge plasma in heterogeneous Fenton-like process for acid red 17 degradation. J. Ind. Eng. Chem. 2017, 50, 86-95.

501


Page 25 of 25


84x47mm (220 x 220 DPI)

ACS Paragon Plus Environment

Visible-Light-Driven Photocatalytic Degradation of Organic Water

Recommend Documents