Hydroxyl radical based photocatalytic degradation of halogenated

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Environmental Processes

Hydroxyl radical based photocatalytic degradation of halogenated organic contaminants and paraffin on silica gel Ruijuan Qu, Chenguang Li, Jiaoqin Liu, Ruiyang Xiao, Xiaoxue Pan, Xiaolan Zeng, Zunyao Wang, and Jichun Wu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00499 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 11, 2018

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Environmental Science & Technology

1

Hydroxyl radical based photocatalytic degradation of halogenated

2

organic contaminants and paraffin on silica gel

3

Ruijuan Qu†, Chenguang Li†, Jiaoqin Liu†, Ruiyang Xiao‡, Xiaoxue Pan†, Xiaolan Zeng†, Zunyao

4

Wang†,*, Jichun Wu§,*

5

†

6

Nanjing University, Nanjing 210023, P. R. China

7

‡

8

University, Changsha 410083, P. R. China

9

§

10

State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment,

Institute of Environmental Engineering, School of Metallurgy and Environment, Central South

Key Laboratory of Surficial Geochemistry, School of Earth Sciences and Engineering, Nanjing

University, Nanjing 210023, P. R. China

11

*

Corresponding

author.

Tel:

+86-25-89680358;

Fax:

[email protected]; [email protected]. 1

ACS Paragon Plus Environment

+86-25-89680358.

E-mail:


12

Abstract

13

Photochemical materials are of scientific and practical importance in the field of

14

photocatalysis. In this study, the photochemistry of several organic contaminants, including

15

decabromodiphenyl ether (BDE-209), halogenated phenols (C6X5OH, X = F, Cl, Br) and paraffin,

16

on silica gel (SG) surface was investigated under simulated solar irradiation conditions. Photolysis

17

of these compounds at the solid/air interface proceeds with different rates yielding various

18

hydroxylation products, and hydroxyl radical was determined as the major reactive species.

19

According to density functional theory (DFT) calculations, the reaction of physically adsorbed

20

water with reactive silanone sites (>Si=O) on silica was indispensable for the generation of •OH

21

radical, where the required energy matches well with the irradiation energy of visible light. Then,

22

the BDE-209 was selected as a representative compound to evaluate the photocatalytic

23

performance of SG under different conditions. The SG material showed good stability in the

24

photodegradation process, and was able to effectively eliminate BDE-209 under natural sunlight.

25

These findings provide new insights into the potential application of silica gel as a solid surface

26

photocatalyst for contaminants removal.

27

2


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28

1. Introduction

29

Photocatalysis that makes use of a catalyst to accelerate a photoreaction is a promising

30

technique for the photocatalytic oxidation of organic pollutants. In this field, photochemical

31

materials have become scientifically and practically important, because they absorb light to

32

generate highly reactive species, particularly hydroxyl radicals, that are able to decompose

33

refractory organic substances via secondary reactions. The •OH radical (E = 2.8 V) is the second

34

most reactive species next to fluorine atom, and they attack a wide range of organic substrates

35

with rate constants usually in the order of 106−109 M−1s−1 1. Since the pioneer work of Fujishima

36

and Honda, many semiconductors such as TiO2 and ZnO have been tested and evaluated as

37

photocatalysts in ultraviolet (UV) light-activated processes2,3. By contrast, photocatalytic activity of

38

non-metal oxides is less well studied.

39

Silica is considered to be an interesting candidate material for future development of catalysts4.

40

Previous studies have shown that pure silica can promote some photocatalytic reactions under UV

41

irradiation, such as direct methane coupling5, photo-metathesis of propene6,7, and photo-epoxidation of

42

propene8. In recent years, the silica nanoparticles (SiO2 NPs) have received growing research

43

interests, due to its high efficiency towards catalytic degradation of water contaminants. The SiO2

44

NPs prepared from tetraethylorthosilicate and rice husk ash were found to be photocatalytically

45

active in the degradation of methyl red dye9,10. Anastasescu et al.11 revealed that tubular SiO2

46

could behave as an efficient photocatalyst for the oxidation of oxalic acid to carbon dioxide. Since

47

most of organic contaminants are slightly soluble or insoluble in water, it is of great significance

48

to develop potential methods for removal of hydrophobic compounds in contaminated

49

environments.

50

Silica gel (SG) is a typical manmade amorphous silica material, which lacks long-range order

51

and possesses a principally different siloxane framework architecture from crystalline silica12. It

52

has been widely used as catalyst supports, because of its large surface area, high optical

53

transparency in the UV/Vis region and excellent characteristics in adsorption of pollutants13-15.

54

Chromatography SG was shown to increase its photocatalytic activity for the CO oxidation

55

reaction with O2 under UV irradiation16. Some researchers have examined the photochemical

56

behavior of organic contaminants, especially polycyclic aromatic hydrocarbons (PAHs), at the

57

SG/air interface. Silica gel catalyzed the photochemical transformation of benzo[e]pyrene and 3



58

perylene, and the reaction was thought to occur through a radical cation-mediated process17,18. For

59

the photochemical oxidation of phenanthrene on SG, the addition of singlet molecular oxygen (1O2)

60

to the ground state molecules was determined as the initial reaction step19. Sotero and Arce20

61

reported that the irradiation of perylene on SG with 320−510 nm light induced its degradation, and

62

the participation of perylene radical cation and 1O2 was confirmed in the photodegradation process.

63

Photoirradiation of pyrene/SG samples was suggested to involve three primary intermediate

64

species, pyrene radical cation, superoxide radical anion (O2•–) and hydrogen radical (H•), among

65

which O2•– was generated from O2 via capture of trapped electrons on SG surface21 . The

66

decomposition of dibenzo-p-dioxin and 4-chlorophenol on the surface of SG occurs under UV-C

67

light, and the formation of biradical and carbine via excited state molecules was the primary

68

degradation step, respectively22,23. Under external force (e.g. grinding, heat), surface-associated

69

reactive free radicals (silicon-based radicals, O2•– radicals, oxyradicals, CO2• radicals and •OH

70

radicals) are generated on silica particles24,25. In a recent work, Romanias et al.26 found that

71

surface bound •OH radical was the dominant reactive species in pyrene oxidation on solid Al2O3

72

surface, leading to the formation of 28 gas/solid phase products. However, whether

73

photoirradiation can produce •OH radical for the catalytic degradation of organic contaminants on

74

the silica surface still remains unknown.

75

In this work, we examined the surface photochemical reactions of organic chemicals to

76

explore the photocatalytic behavior of SG. The commercial SG material was first characterized by

77

modern analytical techniques for its morphological structures and photoelectrochemical properties.

78

Then, decabromodiphenyl ether (BDE-209), halogenated phenols, and paraffin, as typical

79

environmental contaminants belonging to different categories, were selected as model compounds

80

to conduct a kinetics and product study. These compounds showed different photoreactivity at the

81

SG/air interface under simulated solar irradiation, and many hydroxylation products were detected

82

by mass spectrometry analysis. Mechanistic studies indicate the involvement of surface •OH

83

radical in the photooxidation process, and the generation of •OH radical was further rationalized

84

by density functional theory (DFT) calculations. Finally, the photocatalytic performance of SG

85

was evaluated under various conditions, with BDE-209 as a representative. To the best of our

86

knowledge, this is the first attempt to systematically elucidate the •OH radical initiated

87

photochemical reactions on SG surface. 4


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2. Materials and methods

89

2.1 Chemicals and reagents

90

Decabromodiphenyl ether (BDE-209, purity ≥ 98%) was provided by Tokyo Chemical

91

Industry Co., Ltd. Silica gel (SG, particle size: 200-300 mesh) was purchased from Qingdao

92

Haiyang Chemical Co., Ltd. The chromatographic grade methanol, tetrahydrofuran and n-hexane

93

were supplied by Merck Co., Ltd. Derivatization reagent bis(trimethylsilyl)trifluoroacetamide

94

(BSTFA)/trimethylchlorosilane (TMCS) (99:1, v/v), provided by AccuStandard, was diluted 100

95

times with n-hexane for use. Other chemicals, including halogenated phenols (C6X5OH, X = F, Cl,

96

Br) and solid paraffin were obtained from commercial sources and used as received.

97

Radical-trapping

98

2,2,6,6-tetramethyl-4-piperidinol (TEMP, 99%) was purchased from J&K Co., Ltd. and Tokyo

99

Chemical Industry Co., Ltd., respectively.

100

agent

5,5-dimethylpyrroline-N-oxide

(DMPO,

purity

≥

99%)

and

2.2 Characterization of silica gel

101

Field emission scanning electron microscopy (FESEM) images were obtained on a S-3400N

102

II scanning electron microscope (Hitachi, Japan) at an acceleration voltage of 20 kV. Transmission

103

electron microscopy (TEM) images were observed by a JEM-200CX electron microscope (JEOL,

104

Japan). X-ray diffraction (XRD) pattern was recorded on a X’TRA X-ray diffraction-meter (ARL,

105

Switzerland). Fourier transform infrared spectra (FT-IR) were analyzed in the range of 400−4000

106

cm-1 by a Nexus 470 FTIR spectrophotometer (ARL, Switzerland) using KBr pellet technique.

107

The Raman spectra was examined in the 300−1500 cm−1 region using an XploRA ONE Raman

108

spectrometer (Horiba, France) with 785 nm wavelength laser excitation. Specific surface area was

109

measured by the Brunauer−Emmett−Teller (BET) method, and nitrogen adsorption–desorption

110

isotherms were obtained at 77 K using an ASAP 2020 apparatus (Micromeritics, USA). Pore size

111

distribution

112

Barrett−Joyner−Halenda (BJH) model. The photoelectrochemical properties were measured in a

113

standard three-electrode system by a CHI760D electrochemical workstation (Shanghai, China).

114

Ag/AgCl electrode and platinum wire served as the reference and counter electrode, respectively.

115

The silica gel loaded onto ITO electrode (an active area of ca. 1 cm2) was used as the working

116

electrode, which was irradiated with light from a 500W Xenon lamp. 0.1 M Na2SO4 solution was

117

used as the electrolyte.

was

derived

from

the

desorption

branch

5


of

isotherms

using

the


118

2.3 Sample preparation and photolysis experiment

119

To prepare samples for irradiation, 0.2 mL of BDE-209 stock solution (25 mg/L) in

120

tetrahydrofuran was added to 0.1 g of SG in each cylindrical quartz tube. The dispersion was

121

magnetically stirred, and then evenly applied to the inner wall of the quartz tube for about half its

122

height from the base. After the solvent was removed by drying at 40 oC in an oven, the tubes were

123

cooled down to room temperature and sealed with glass stoppers for irradiation. According to the

124

weight of BDE-209 and SG, the surface coverage was calculated as 50 µg/g. The loading

125

concentration was selected according to previous studies on the photodegradation of BDE-209 in

126

solid phase27-29. Preloaded samples of halogenated phenols and paraffin on SG were achieved in a

127

similar manner, except for the stock solution that was prepared in methanol and n-hexane,

128

respectively.

129

The irradiation experiment was performed in an XPA-1 photochemical reactor (Nanjing

130

Xujiang Electromechanical Plant, China) equipped with a 500 W Xe lamp as the light source. The

131

lamp was vertically placed inside a quartz cooling water jacket to prevent it from overheating.

132

During irradiation, the tubes were rotated around the lamp by a merry-go-round apparatus for

133

uniform light exposure. The light intensity in the sample region was measured as 74.2 mW·cm-2

134

by a FZ-A Radiometer with a 400-1000 nm sensor (Photoelectric Instrument Factory of Beijing

135

Normal University, China). At designated time intervals, tubes were taken from the photoreactor.

136

The unreacted BDE-209 was extracted from the solid surface by adding 2.0 mL of tetrahydrofuran

137

to the tubes, followed by ultrasonic treatment for 10 min. The suspension was then centrifuged for

138

5 min to remove any suspended particles for GC analysis. The extraction efficiency of BDE-209

139

was about 98.1 %, and all kinetics experiments were performed in triplicate. To detect possible

140

products, 0.1 g 400 µg/g BDE-209/SG sample was irradiated for different time, and then

141

ultrasonically extracted with 2.0 mL of methanol for liquid chromatograph-time of flight-mass

142

spectrometry (LC-TOF-MS) analysis. In another batch of experiments, reaction samples of

143

BDE-209/SG were extracted with 2.0 mL of tetrahydrofuran for gas chromatography-mass

144

spectrometry (GC-MS) analysis. Sample pretreatment procedures of other compounds were

145

summarized in Table S1.

146

The degradation of BDE-209/SG was also evaluated under natural sunlight irradiation from

147

9:00 am to 3:00 pm for two successive days (December 1−2, 2016). The quartz tubes were placed 6


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148

on the roof top of our school building. The solar intensity was measured frequently by the FZ-A

149

radiometer (Figure S1).

150

2.4 Mass balance analysis

151

The actual surface coverage was measured by extracting the preloaded samples with organic

152

solvent. Then, the percentage of spiking was calculated from the ratio of the measured

153

concentration to the theoretical value. The un-extractable parts were estimated using adsorption

154

experiments. Adsorption was performed by adding 0.1g of SG into 10 mL of each compound

155

solution and shaking the samples for 24 h at 25 °C. Following centrifugation, the concentration of

156

target compound in the supernatant was analyzed.

157

2.5 Analysis methods

158

An Agilent 6890A gas chromatography equipped with an autosampler and an

159

electron-capture detector (ECD) was used for quantification of BDE-209. 2 µL of sample was

160

injected in splitless mode, and the separation was performed on a DB-XLB column (10 m × 0.25

161

mm, 0.25 µm, J&W Scientific, USA). The carrier gas was nitrogen (N2) at a constant flow rate of

162

3.0 mL/min. The injector and detector temperature was set at 280 °C and 320 °C, respectively. The

163

column temperature was programmed from 60 °C (2 min hold) to 290 °C at a rate of 20 ºC/min,

164

then from 290 °C to 310 ºC at 8 ºC/min and held isothermally at 310 ºC for 14 min. Analysis

165

methods of other compounds were listed in Table S1.

166

The LC-TOF-MS analysis was performed on an Agilent 1260 Infinity HPLC system

167

connected to a quadrupole time-of-flight Triple TOF 5600 mass spectrometer (AB SCIEX, USA).

168

For detailed information, please refer to Text S1.

169

A Thermo TRACE Ultra-TSQ Quantum GC-MS system (Thermo Scientific, USA) was used

170

to identify possible degradation products in BDE-209/SG reaction samples. Specific information

171

is detailed in Text S2.

172

Paraffin oxidation products were derivatized using BSTFA/TMCS and analyzed by a Thermo

173

Trace Ultra-DSQ II GC-MS system (Thermo Scientiﬁc, USA) to illustrate the generation of

174

alcohols. The detailed analytical procedure could be seen in Text S3.

175

2.6 Detection of free radicals

176

The electron paramagnetic resonance (EPR) spectra were recorded on a Bruker EMX-10/12

177

EPR spectrometer. Production of •OH and O2•− in the reaction mixtures was monitored using 7



178

DMPO as spin-trapping agent, while 1O2 was detected with TEMP as spin-trapping agent. 0.025 g

179

SG spiked with 0.05 mL of 100 mM DMPO/TEMP was irradiated for 25 min, and the reaction

180

samples were then diluted by 0.45 mL of water for •OH/1O2 detection. Due to the facile

181

disproportionation in water, formation of the O2•− radical was examined in ethanol solution. The

182

detailed instrumental parameters were set as follows: scanning frequency: 9.7 GHz; central field:

183

3480 G; sweep width: 200 G; microwave power: 20 mW; scanning temperature: 298 K.

184

2.7 Theoretical calculations

185

The density functional theory (DFT) calculations were performed at the B3LYP/LanL2DZ

186

level using the Gaussian 09 software package30. The structural geometry was optimized with the

187

keyword “Opt”, and vibrational frequencies of all stationary points were calculated to identify the

188

structures obtained as true minima or first-order saddle point. The Gibbs free energies (G) were

189

directly obtained from Gaussian output files.

190

3. Results and Discussion

191

3.1 Sample characterization

192

The nitrogen adsorption-desorption isotherms of SG are shown in Figure 1A. Type IV

193

isotherms with a hysteresis loop were observed, indicating that the material is mesoporous in

194

nature. From the pore size distributions (see the inset of Figure 1A), the sample presents a very

195

narrow distribution of pore diameter ranging from 3 to 15 nm. The BET specific surface area was

196

calculated to be 337.9 m2/g, and the average pore diameter was 8.36 nm.

197

Photoelectrochemical measurements are usually employed to qualitatively study the

198

separation and transfer of photogenerated charge carriers. In this work, the transient photocurrent

199

response of SG was recorded for several on-off cycles of light irradiation. As shown in Figure 1B,

200

the photocurrent increased sharply as soon as the lamp was switched on, while immediately

201

returned to its initial value as soon as the irradiation was terminated, suggesting that the generation

202

of photocurrent was completely attributed to the photoreactivity of the electrode. Moreover, the

203

photoelectrode displayed good reproducibility and stability as the photocurrent intensity did not

204

exhibit obvious changes under several on-off cycles. The steady transient photocurrent density of

205

SG was approximately 0.1 µA/cm2. Characterization results of FESEM, TEM, XRD, FT-IR and

206

Raman are provided in Text S4 and Figure S2.

8


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400 300 200

2

0.20 0.15 0.10 0.05 0.00

0

10

20

30

40

Pore diameter (nm)

100 0 0.0

Control Silica gel

0.25

Photocurrent (µΑ /cm )

500

B 0.16 Pore volume (cm3/gnm)

Quantity adsorbed (cm3/g)

A 600

Adsorption Desorption

0.2

0.4

0.6

0.8

1.0

light on

0.12

0.08

0.04 light off

0.00 0

207

50

100

150

200

Irridiation time (s)

Relative pressure (P/P0)

208

Figure 1 (A) Nitrogen adsorption-desorption isotherms and (B) transient photocurrent response of

209

silica gel.

210

3.2 Mass balance analysis

211

With respect to total mass of the chemicals, (1) some of them were removed during solvent

212

evaporation, (2) some of them were strongly adsorbed onto SG, which are not possible to extract,

213

and (3) some of them were adsorbed onto SG, which are possible to extract. Thus, a mass balance

214

analysis was established to evaluate the reliability of experimental results. As shown in Table S2,

215

the percentage of spiking, indicative of the sample loss from solvent volatilization and the

216

un-extractable loss, ranges from 92.9±0.7% for BDE-209 to 98.5±3.4% for pentachlorophenol.

217

When adsorption equilibrium was achieved between solid particles and liquid phase, the

218

chemicals in solutions accounted for an overwhelming proportion of the total mass

219

(97.1%−99.5%). This suggests only very small parts is not possible to extract from preloaded

220

samples. Considering the negligible loss in sample preparation and extraction process, nominal

221

concentrations were used throughout the work.

222

3.3 Degradation products of BDE-209, halogenated phenols and paraffin

223

Foil covered dark control samples all exhibited no loss of target compound during the

224

reaction. In the Xe lamp irradiation system, BDE-209 was found to degrade with time, giving a

225

removal rate of 91.5% after 60 min (Figure 2A). Söderström and co-workers31 investigated the

226

photolytic debromination of BDE-209 on SG using mercury UV-lamp (1.6 mW/cm2, 300−400

227

nm), and they also observed a fast degradation of BDE-209 with a half-life time of

280

90% matching), this compound was assigned as Heptadecan-1-ol trimethylsilyl ether. Similarly,

281

the chromatographic peak at 24.26 min having a parent ion of m/z 356 was identified as

282

Nonadecan-1-ol trimethylsilyl ether. These results show that SG can also catalyze the degradation

283

of halogenated phenols and paraffin, resulting in the formation of hydroxylation products. Since

284

attack by •OH radical leads to hydroxylation products29,35,36, it is speculated that photodegradation

285

of organic contaminants on the surface of SG mainly involves •OH radical initiated oxidation

286

reaction. A

Cl

F OH C6H2F4O2

Cl

F

287

OH

OH F

F

OH Cl

Cl

Cl OH C6H2Cl4O2

HO

OH Cl

Cl

OH Cl

Br

OH Cl O Cl O C6HCl3O3 C6H3Cl3O3

Br

OH

OH Br

Br

Br Br

Br

Br HO O OH Br OH O Br C6HBr3O3 C6H2Br4O2 C6H3Br3O3

12


Page 13 of 23


288 289

Figure 4 (A) The representative degradation products of halogenated phenols identified by

290

LC-TOF-MS; (B) The GC-MS mass spectra of two derivatization products during photochemical

291

degradation of paraffin on silica gel.

292

3.4 Determination of the major reactive species

293

The EPR technology was used to characterize the radical species involved in the reaction

294

system. As shown in Figure 5, no peaks of free radicals (i.e., •OH, 1O2 and O2•−) were observed in

295

the reaction solutions in the dark. Four characteristic peaks of a 1:2:2:1 quartet pattern (aN = aH =

296

14.9 G) were detected under light irradiation (Figure 5A), indicating the generation of •OH radical.

297

TEMP could trap singlet oxygen, resulting in the spin adduct 2,2,6,6-tetramethylpiperidine-1-oxyl

298

(TEMPO). From Figure 5B, a 1:1:1 triplet signal characteristic of TEMPO radical (aN = 7.2 G, aH

299

= 4.1 G) were observed, demonstrating the existence of 1O2. Furthermore, the EPR spectra showed

300

six characteristic peaks of the DMPO-O2•− spin adducts (aN = 14 G, aH = 8 G) in the ethanol

301

solution (Figure 5C). After addition of SG, the intensity of the TEMPO and DMPO-O2•− peaks

13



Page 14 of 23

302

changed slightly, while the formation of DMPO-•OH spin adducts was significantly induced.

303

These spin-trap experiments revealed that •OH radical contributed mainly to the photochemical

304

transformation of halogenated organic contaminants and paraffin on SG. DMPO-OH

DMPO-25min

B

SG-0min

3450 3460 3470 3480 3490 3500 3510

305

SG-25min

C

TEMP-1O2

Intensity(a.u)

SG-25min

Intensity(a.u)

Intensity(a.u)

A

TEMP-25min

3440

3460

DMPO-O2¯

DMPO-25min

SG-0min

SG-0min

3480

3500

3520

Magnetic field (G)

Magnetic field (G)

SG-25min

3440

3460

3480

3500

3520

Magnetic field (G)

306

Figure 5 EPR spectra of (A) DMPO-•OH adduct, (B) TEMP-1O2 adduct, and (C) DMPO-O2•−

307

adduct formed in the silica gel (SG) system. Note that superoxide anion (O2•−) was measured in

308

ethanol solution.

309

In addition, the UV/H2O2 experiment that generates •OH radical as reactive species was

310

conducted to confirm the above conclusion. Due to the very strong hydrophobicity of BDE-209

311

and paraffin, the experiment was performed on halogenated phenols (see Text S5 for experimental

312

details). As shown in Figure S7, the typical hydroxylation products generated from photolysis of

313

halogenated phenols on SG were also detected in the UV/H2O2 oxidation process. Moreover,

314

experimental observations and quantum chemical investigations have revealed that atmospheric

315

photooxidation of polybrominated diphenyl ethers (PBDEs) by •OH, taking 4,4’-dibromodiphenyl

316

ether (BDE-15) as an example, leads to the formation of OH-PBDEs and brominated phenols as

317

reaction products37,38. These results provide reliable evidence for the role of •OH radical in

318

photodegradation of organic compounds on SG surface.

319

3.5 Detection of hydroxyl radical

320

As illustrated in Figure 6A, the DMPO-•OH adduct was clearly observed in the SG system

321

after Xe lamp irradiation for 25 min. The •OH signal intensity increased with irradiation time, due

322

to the accumulation of •OH generated from the surface of SG. Under the sunlight irradiation, the

323

•OH radical was also detected in SG reaction solution (Figure 6B). In presence of cut-off filters,

324

the signal intensity of •OH became lower than the control, and no radical was observed when 14


Page 15 of 23


325

orange filters were used (Figure 6C). Considering the light transmittance of different cut-off filters

326

(Figure 6D), we can conclude that short wavelength irradiation is more favorable to the generation

327

of •OH radical.

A

B

25 min

SG-sun

Intensity (a.u.)

Intensity (a.u.)

5 min

1.5 min

SG-Xe

0 min

3460

3470

3480

3490

3500

3510

3400

3440

Magnetic field (G)

C

3520

3560

D 100 Light transmittance (%)

orange yellow green

Intensity (a.u.)

3480

Magnetic field (G)

cyan purple uv no filter

100

80

80

60

60

40

40

20

20

0 200

400

600

Relative intensity (%)

3450

0 1000

800

Wavelength (nm) 3400

328

3440

3480

3520

red cyan

3560

Magnetic field (G)

orange purple

yellow uv

green lamp

329

Figure 6 (A) EPR spectra of the silica gel (SG) system after irradiation by the Xe lamp for

330

different time; (B) EPR spectra of the SG reaction mixture under Xe lamp and solar irradiation; (C)

331

EPR spectra of DMPO−•OH adduct in the SG system with different cut-off filters; (D) The

332

emission spectrum of the lamp and the light transmittance of different cut-off filters.

333

3.6 Theoretical analysis for the generation of •OH radical

15



57

Page 16 of 23

∆rG (kJ/mol)

OH + BDE-209 reaction

TS1 56.19

56 55 TS6 54.10 TS3 53.68 TS5 53.14

54 53

TS2 52.05

52

TS4 50.84

51 50 0

OH

0

OH generation

OH+BDE-209

F

Br

path

way

-100

-200

OH + Br

E pa th wa yA pa t hw ay C

th w ay

D ay

(434.29,435.35)(428.31)

-500 Si-O-Si

Br

Br Br +

HO Br

O (PR1)

OH

+ Br

Br

(540.19)

PR5 -152.97

Br4

O Br

-152

Br

Br

Br

PR2 -151.96

Br

Br

Br

PR3 -151.84

Br

Br

Br

3MRs H2O (389.32) Si10-OH Si10=O (418.05)

B

-150

-151

(211.51)

pa

ay pa

w th

pa th

w

-400

PR4 -150.25

Br 3 4 Br 5 Br

Br

Si10=O-H2O

-300

334

Br

Br 2 O1 6 BrBr

(PR2-PR6)

PR6 -159.75

-153 -160 -170

PR1 -178.82

-180

335

Figure 7 Calculated Gibbs free energies for possible generation of •OH radical and its subsequent

336

reaction with BDE-209 at the B3LYP/LANL2DZ level.

337

Previous studies have suggested the presence of some radical species on silica surface,

338

however, the generation mechanism of •OH radical still remains unclear39-41 . In this work,

339

theoretical calculations were employed to gain a deep insights into how •OH radical is produced

340

on the surface of SG. Figure 7 shows energy profiles for possible generation of •OH radical and its

341

subsequent reaction with BDE-209. Details of the six reaction pathways for •OH generation are

342

given in Figure S8. In the first case, homolytic cleavage of water molecules leads to the formation

343

of •OH and •H radicals (Figure S8A). The Gibbs free energy of reaction (∆rG) was estimated as

344

418.05 kJ/mol, corresponding to the irradiation energy of 286 nm UV light. Then, the [Si8O25H18]

345

model from the work of Narayanasamy and Kubicki42 was adopted for calculations. Surface ≡Si•

346

and ≡SiO• radicals are generated from homolytic cleavage of siloxane bonds (Si−O−Si) on the

347

silica surface, and the ≡SiO• radicals can easily react with H2O to produce •OH (Figure S8B).

348

Obviously, cleavage of the Si−O−Si bond is the energy-demanding step, which requires an energy

349

as high as 540.19 kJ/mol. Three-membered rings (3MRs), formed at high temperature and

350

“frozen-in” by rapid quenching in fumed silica, have been established as precursors of

351

oxyradicals40,41. Based on our calculations, the reaction of surface biradicals with water to 16


Page 17 of 23


352

generate •OH radical can occur spontaneously, but an additional energy of 389.32 kJ/mol is

353

needed for homolytic cleavage of 3MRs (Figure S8C).

354

When the crystal structure model of silica [Si10O28H16] was chosen, direct bond-breaking of

355

surface silanol groups would yield •OH radical, and the energy required for the two types of

356

Si−OH groups is very close, 435.35 kJ/mol and 434.29 kJ/mol, respectively (Figure S8D). Owing

357

to the presence of >Si=O sites on silica surface42, it is reasonable to slightly modify the previous

358

model [Si10O28H16] by replacing geminal SiOH group with the silanone >Si=O sites. As for the

359

newly constructed structure model [Si10O22H4], a similar energy of 428.31 kJ/mol should be

360

provided to create •OH radical by cleavage of silanol group (Figure S8E). Another possibility for

361

the generation of •OH could be the reaction of physically adsorbed water with reactive silanone

362

sites (>Si=O) on silica, which only consumes energy of 211.51 kJ/mol (Figure S8F). This energy

363

is equal to the irradiation of 566 nm visible light, largely consistent with above EPR analysis

364

results that the •OH radical was still detected until the use of orange cut-off filters transmitting

365

light of wavelength above 550 nm. Since the Xe lamp used in this work emits lights at

366

wavelength > 290 nm, cleavage of 3MRs via absorbing at least the energy of 307 nm photon may

367

also contribute slightly to the formation of •OH.

368

Next, the •OH radical would attack BDE-209, producing different OH-Nona-BDEs species

369

and a bromine atom (PR2-PR6), and pentabromophenol and phenoxyl radical (PR1) via the

370

transition states (see details in Text S6 and Figure S9). The fast combination of bromine

371

atom/phenoxyl radical and the silicon-based radical formed in the first step makes it available to

372

regenerate the [Si10O22H4] structure with associated hydrogen bromide/pentabromophenol. In this

373

sense, SG could catalyze the decomposition of H2O to generate •OH continuously. Furthermore,

374

the M06-2x functional is also used to test the robustness of the DFT results. As shown in Table S4,

375

the ∆rG values calculated at the M062X/LANL2DZ level are largely consistent with the data at the

376

B3LYP/LANL2DZ level, suggesting that it is reliable to interpret possible •OH generation

377

pathways by the calculation results.

378

3.7 Degradation kinetics of BDE-209 under different conditions

379

The dependence of BDE-209 decomposition on the wavelength was investigated using

380

different cut-off filters (Figure 8A). Obviously, BDE-209 losses on SG followed the order of no

381

filter > purple > UV > cyan > green > yellow, while the red and orange light caused no 17



Page 18 of 23

382

degradation of BDE-209. This is related to the light transmittance of different cut-off filters and

383

the emission spectrum of the lamp (Figure 6D). Except for the UV, the removal efficiency

384

decreased with increasing wavelength of light. Although the photon energy of the UV light is

385

greater than that of the purple light, the emission intensity of the lamp is very low in the reaction

386

wavelength region, thus leading to a smaller reaction rate. When a cut-off filter of 550 nm (orange)

387

was used, the SG photocatalyst finally lost its activity. This agrees well with the EPR results that

388

the •OH radical was not observed in presence of orange filters, further confirming the dominant

389

•OH radical oxidation reaction mechanism on SG surface.

390

The reaction process was repeated five times to assess the stability and reusability of the SG

391

for BDE-209 degradation. The SG after reaction was collected, washed by tetrahydrofuran to

392

remove the residual reactant, and then dried in a vacuum oven for use. As seen in Figure 8B, the

393

removal rate of BDE-209 in the fifth cycling run was not remarkably reduced, still reaching 90.1%

394

after 6 h. Thus, the SG possessed adequate properties in catalyzing the degradation of BDE-209. When exposed to the natural sunlight, the SG can also induce the effective degradation of

395

BDE-209, and the removal efficiency was 94.2% after 11 h (Figure 8C). B

1.0

0.8

0.8

0.8

0.6

0.6

0.6 0.4 0.2 0.0 0

40

80

120

Time (min)

397

C 1.0

1.0

Ct/C0

Ct/C0

A

red cyan

orange uv

yellow purple

1st

2nd

3rd

4th

5th

Ct/C0

396

0.4 0.2 0.0

green 0 no filter

0.4 0.2

60

120

180

240

300

Time (min)

0.0

0

2

4

6

8

10

12

Time (h)

398 399

Figure 8 (A) Wavelength dependence of BDE-209 decomposition on silica gel (SG) under Xe

400

lamp irradiation; (B) The BDE-209 degradation kinetics in five recycling runs of SG; (C) The

401

degradation of BDE-209/SG under natural sunlight irradiation. Reaction conditions: sample mass

402

= 0.1 g, [BDE-209] = 50 µg/g.

403

Environmental Implications

404

Our results indicated that the reaction of physically adsorbed water with reactive silanone

405

sites (>Si=O) on photoactivated silica gel produced surface •OH radical to decompose various

406

organic contaminants, resulting in the formation of hydroxylation products. This sheds a new light

18


Page 19 of 23


407

on the application of SG as a simple and cost-effective photocatalyst for pollutants degradation.

408

The visible light responsive property of this material (active wavelength < 550 nm) is promising in

409

photocatalysis, as visible light covers the largest proportion of the solar irradiation beyond Earth’s

410

atmosphere. Because of its porous structure, large surface area and high surface energy, SG has

411

been widely used as adsorbents for the cleaning of polluted air. Further decomposition of air

412

pollutants by light irradiation, instead of just accumulating them, is expected to make SG

413

attractive in the field of air purification. Moreover, silicon dioxide is the most abundant

414

component of mineral dust that contributes to the largest mass emission rate of atmospheric

415

aerosol particles at a global scale43, and therefore photodegradation on mineral aerosols during

416

long range atmospheric transport may be an important fate process for hydrophobic organic

417

pollutants in the environment. The photochemical transformation into hydroxylation products

418

seems to be of interest in terms of health risk assessment due to the changed molecular polarity

419

and biological activity.

420 421

ASSOCIATED CONTENT

422

Supporting Information

423

Texts S1-S6, Figures S1-S9, and Tables S1-S4 can be found in the Supporting Information. This

424

material is available free of charge via the Internet at http://pubs.acs.org.

425

AUTHOR INFORMATION

426

Corresponding Author

427

* Phone: +86-25-89680358. Fax: +86-25-89680358.

428

E-mail: [email protected] (Zunyao Wang); [email protected] (Jichun Wu).

429

Notes

430

The authors declare no competing financial interest.

431

ACKNOWLEDGMENTS

432

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

433

21607073; 21577063) and the Natural Science Foundation of Jiangsu Province (No.

434

BK20160651).

435 436

References 19



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Hydroxyl radical based photocatalytic degradation of halogenated

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