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Dynamic Tracking of Highly Toxic Intermediates in Photocatalytic Degradation of Pentachlorophenol by Continuous Flow Chemiluminescence Hai-Yan Ma, Lixia Zhao, Da-Bin Wang, Hui Zhang, and Liang-Hong Guo Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05518 • Publication Date (Web): 02 Feb 2018 Downloaded from http://pubs.acs.org on February 5, 2018

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Environmental Science & Technology

1

Dynamic Tracking of Highly Toxic Intermediates in Photocatalytic Degradation of

2

Pentachlorophenol by Continuous Flow Chemiluminescence

3

Hai-Yan Ma1,2, Lixia Zhao1,2*, Da-Bin Wang1,3, Hui Zhang1, Liang-Hong Guo1,2*

4

5

6

1

7

for Eco-environmental Sciences, Chinese Academy of Sciences, 18 Shuangqing Road,

8

P.O. Box 2871, Beijing 100085, China

9

2

University of Chinese Academy of Sciences, Beijing 100039, China

10

3

Tobacco Research Institute, Chinese Academy of Agricultural Sciences & Laboratory of

State Key Laboratory of Environmental Chemistry and Eco-toxicology, Research Center

11

Risk Assessment for Tobacco Products, 11 Keyuan Four Road, Qingdao, Shandong

12

266101, China

13

14

15

*To whom corresponding should be addressed: Dr.Lixia Zhao, Prof. Liang-Hong Guo,

16

State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center

17

for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, 18

18

Shuangqing

19

[email protected] (LHG); Tel: (86)10-62849338, Fax: (86)10-62849685

Road,

Beijing

100085,

China.

E-mail:

20

ACS Paragon Plus Environment

[email protected]

(LZ),


21

ABSTRACT

22

Photocatalytic degradation is a powerful technique for the decomposition of

23

pollutants. However, toxic intermediates might be generated which have become a great

24

concern recently. In present work, a continuous flow chemiluminescence (CFCL) method

25

was developed for dynamic monitoring of toxic intermediates generated in the

26

photocatalytic degradation of pentachlorophenol (PCP). Among the main intermediates,

27

tetrachloro-1,4-benzoquinone

28

(OH-TrCBQ) showed higher or similar toxicity to PCP. As both TCBQ and OH-TrCBQ

29

can produce CL in the presence of H2O2, a CFCL system was established for the dynamic

30

tracking of the two toxic intermediates. A PCP/TiO2 suspension was irradiated in a

31

photoreactor, then pumped continuously into a detection cell and mixed with H2O2 to

32

produce chemiluminescence (CL). The time-dependent CL response displayed two

33

distinctive peaks at pH 7, which were attributed to the generation of OH-TrCBQ and

34

TCBQ respectively by comparing with their changes measured by High Performance

35

Liquid Chromatography (HPLC). Furthermore, the CL response curve of PCP/TiO2

36

suspension showed a pattern very similar to their bacteria inhibition. Therefore, the

37

CFCL could be used as a simple and low-cost method for online monitoring of TCBQ

38

and OH-TrCBQ to ensure complete removal of not only PCP but also highly toxic

39

degradation intermediates.

(TCBQ)

and

trichlorohydroxy-1,4-benzoquinone

40 41


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KEYWORDS: Dynamic monitoring; Toxic intermediates; Pentachlorophenol;

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Photocatalytic degradation; Continuous-flow Chemiluminescence

44



45

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INTRODUCTION

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Advanced oxidation processes (AOPs) such as ozonation,1,2 Fenton,3,4 H2O2/UV,5,6

47

sonolysis,7 and photocatalysis have been widely and extensively explored to degrade

48

environmental contaminants such as pentachlorophenol (PCP).8-10 Heterogeneous

49

photocatalysis by TiO2 is of considerable interest in AOPs due to its low toxicity and

50

strong oxidizing power,11-13 which has been widely concerned as one of the most efficient

51

methods to eliminate chlorophenols from water.8,11-13 However, many reports focused

52

solely on the degradation efficiency and remove conditions of the target compound. It

53

should be worthy of noting that even after the complete removal of PCP, acute toxicity

54

was still observed,2,8,11,12,14 which might be caused by the generation of toxic PCP

55

byproducts

56

tetrachloro-1,4-benzoquinone (TCBQ),8-12,17,18 or other toxic intermediates. Therefore,

57

determination of the only pristine PCP concentration may not be sufficient for evaluating

58

an overall efficiency of wastewater treatment. The dynamic generation and control of

59

highly toxic intermediates must be studied before the practical application of PCP

60

degradation technology.

including

polychlorinated

dibenzodioxins/furans,15,16

61

Pentachlorophenol (PCP) has been known as an established priority pollutant, which

62

is toxic to plants, animals and humans.19-22 Owing to its toxic and carcinogenic nature,

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PCP was listed as a priority pollutant by the U.S. Environmental Protection Agency

64

(EPA), and classified in 1999 by the International Agency for Research on Cancer (IARC)

65

as a possible human carcinogen.23 As we know, PCP photocatalytic degradation process


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is complex and the degradation pathway is closely related to photocatalytic reaction

67

conditions. The toxicity changes during PCP photocatalytic degradation were determined

68

by the production and transformation of toxic intermediates. For example, M.

69

Antonopoulou et al. 11 found increased toxicity during the photocatalytic degradation of

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PCP by N-F-TiO2, which was empirically attributed to the formation of TCBQ

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intermediates. W. F. Jardim et al. 12 and German Mills et al. 17 reported the generation of

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more toxic intermediates through the attacking of hydroxyl radical (•OH) on the para

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position

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tetrachloro-1,4-hydroquinone (TCHQ), TCBQ and 2,3,5,6-tetrachlorophenol. However,

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the relationship between toxic intermediates generation and degradation conditions has

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not been studied in previous work. In addition, German Mills et al. 17 observed that the

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average number of Cl- ions released per PCP molecule was about 1.8 in the initial several

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minutes. That is to say, TCHQ/TCBQ were not the only dechlorination intermediates,

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unidentified intermediates carrying 2-3 chlorine atoms should be formed in the early

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stage of degradation.17 However, no report on them was published and the intrinsic

81

relation between the degradation process and toxicity changes during PCP photocatalytic

82

reaction is still unknown. The reason for this is that there may be lack of online methods

83

to detect degradation process especially the generation and transformation of highly toxic

84

intermediates. Therefore, how to achieve dynamic tracking of highly toxic intermediates

85

is the key issue.

86

of

the

PCP

ring,

and

the

principal

intermediates

were

To date, several analytical methods have been developed and employed for the



87

detection of intermediates generated during PCP degradation, such as Ultraviolet-Visible

88

(UV-Vis) absorbance,2 HPLC,1,8-12,15,24,25 and Gas Chromatography-Mass Spectrometer

89

(GC/MS) etc.2,3,24,25 These methods often require a separation step to remove the

90

photocatalyst. As such, they are cumbersome and time-consuming. Besides, these

91

methods need a relatively long duration from sampling for measurement, while, some

92

intermediates are probably reactive and unstable, then their composition and

93

concentration were probably changed during the photocatalytic degradation process.

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Therefore, these conventional methods cannot monitor the dynamic generation and

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transformation of some unstable intermediates.

96

Chemiluminescence (CL) is well-suited for the detection of unstable intermediates,

97

since the luminescence is instantly produced by a redox chemical reaction when CL

98

reactants including intermediates and an oxidant are mixed, and the light emission can be

99

detected immediately by a photomultiplier tube (PMT) with ultralow background.26-29

100

Therefore, CL is capable of following the dynamic process of unstable intermediates

101

generation and transformation in photocatalysis. In the previous work, a CFCL was

102

developed for the selective and online detection of O2• −, •OH, and H2O2 generated during

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UV irradiation of nano-TiO2 suspension with the advantages of high sensitivity, high

104

selectivity.30,31 However, no work was reported on tracking the dynamic process of

105

highly toxic intermediates generation during the PCP photocatalytic degradation using

106

CL.

107

In the present study, an attempt was made to develop a CFCL system for dynamic


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monitoring of the highly toxic intermediates generated in PCP/TiO2 photocatalytic

109

reaction. Main intermediates and their acute toxicity were detected and evaluated using

110

electrospray ionization quadrupole time-of-flight mass spectrometry (ESI-Q-TOF-MS)

111

and luminescent bacterial test respectively. TCBQ and OH-TrCBQ were believed to be

112

highly toxic intermediates and played key roles in the toxicity changes during the

113

degradation process. According to their structures and properties, a CL system was

114

developed using H2O2 as the oxidant which can react with both TCBQ and OH-TrCBQ to

115

generate CL signals. Then coupled with the continuous flow method, the CFCL system

116

was constructed for dynamic tracking of the two highly toxic intermediates. As the

117

comparison of the dynamic production of TCBQ and OH-TrCBQ and their CL inhibition

118

of Vibrio Fischeri test during PCP degradation, the CL response curve showed a very

119

similar pattern with the toxicity changes. Therefore, the new developed CFCL system can

120

be used as a simple, rapid and low-cost method for not only on-line monitoring of TCBQ

121

and OH-TrCBQ to ensure complete removal of PCP and its highly toxic intermediates

122

but also real-time evaluating the acute toxicity risk during PCP photocatalytic

123

degradation process.

124 125

EXPERIMENTAL SECTION

126

Chemicals and Materials

127 128

PCP,

TCBQ

(also

called

p-chloranil),

TCHQ,

OH-DDBQ

(2,5-Dichloro-3,6-Dihydroxy-p-Quinone, also called Chloranilic Acid), Degussa P25



129

TiO2, Trifluoroacetic acid, HPLC-grade methanol, H2O2, KH2PO4, NaOH, HCl, Na2CO3,

130

NaHCO3 were purchased from Sigma−Aldrich (St. Louis, MO, USA). Degussa P25 TiO2

131

photocatalyst was used as naked particles (surface area, 49 m2g-1 by BET methods;

132

crystal structure, 80% anatase and 20% rutile by X-ray diffraction analysis).30

133

Tetrabutylammonium hydrogen sulfate (TBAHS) was purchased from Acros Organics

134

(New Jersey, USA). OH-TrCBQ was kindly supported by Ben-Zhan Zhu research group,

135

which was synthesized as already reported.32-34 Freeze-dried luminescence bacteria

136

Vibrio Fischeri, resuscitation solution, and adjustment solution (non-toxic 22% NaCl)

137

were obtained from Hamamatsu (Hamamatsu Photo Techniques Ltd., Hamamatsu,

138

Japan).

139 140

PCP Photocatalytic Degradation Experiments

141

All the irradiations were performed in a dark housing with a cooling system. The

142

sample was 250 mL aqueous solution containing typically 5×10-5 M PCP and 0.2 g/L

143

TiO2 powder. The initial pH of the PCP/TiO2 solution was adjusted by diluted NaOH or

144

HCl solution using a Ross Ultra Combination pH Electrode (Thermo Fisher Co. Ltd.,

145

New Jersey, USA). Irradiation was performed with a 500-W xenon lamp (Changtuo, Ltd.,

146

Beijing, China). A combination of 365 nm filter and 400 nm cut-off filter was employed

147

at the light-emitting window. The transmittance of the filters is shown in Figure S1A. By

148

the combination of the two filters, only about 70% of 365±13 nm light and about 0~20%

149

700~900 nm light was passed through the filters. The filters can also limit heating of the


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samples and prevent direct photolysis of the substrates. During the photocatalytic

151

experiments, the temperature of the solution was kept at 25 °C. The incident light at 365

152

nm on the sample was measured with a power meter (Photoelectric instrument factory of

153

Beijing Normal University, Beijing, China) and the intensity was 20 mW/cm2. The

154

suspension was kept under air-equilibrated conditions with constant stirring during

155

irradiation, and stirred in the dark for at least 30 minutes to establish an

156

adsorption/desorption equilibrium prior to irradiation. For all the reactions tested, 2 mL

157

PCP/TiO2 suspension was taken out at indicated time intervals, and was centrifuged for 5

158

minutes with 14000 r/min speed. Then the supernatant solutions were immediately

159

submitted to ESI-Q-TOF-MS analysis, HPLC analysis and acute toxicity assessment.

160

During the PCP direct photolysis experiments, the filters were taken off, and the

161

TiO2/PCP suspension was replaced with PCP solution. All the assays were repeated at

162

least three times.

163 164

Preliminary Analysis of Decomposition Products

165

Identification of the major reaction products by ESI-Q-TOF-MS during PCP/TiO2

166

photocatalytic process was performed on an Agilent-6540 mass spectrometer

167

(Agilent Technologies, Santa Clara, California, USA). The supernatant solution was

168

immediately introduced to MS instrument after centrifuged. MS spectra were acquired in

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negative-ion mode, using an ESI source. Capillary and nozzle voltages were 3.5 kV and 1

170

kV, respectively; drying gas and sheath gas temperatures were 300 °C and 350 °C,



171

respectively. Nitrogen was used as drying gas. The collision gas was argon at a pressure

172

of 35 psi, and the sheath gas flow was 11 L/min. Full-scan spectra were recorded in

173

profile mode. The range between m/z 50 and 1000 was recorded at a rate of 1 spectra/s.

174 175

HPLC Analysis

176

Agilent 1260 HPLC instrument (Agilent Technologies, Santa Clara, California, USA)

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and a separation column (Eclipse plus C18, 50 mm×3 mm, 2.7 µm, Agilent, USA) were

178

used to quantify PCP and the main intermediates. 2 mL PCP/TiO2 suspension was taken

179

out and centrifuged at a given time intervals. The supernatant solutions were directly

180

transferred into mini vials and immediately monitored by HPLC. For PCP, OH-TrCBQ

181

and OH-DDBQ, the mobile phase consists of solvent A (10 mM KH2PO4 at pH 6.8

182

containing 5 mM ion paring reagent TBAHs) and solvent B (methanol). TBAHS was

183

added in the KH2PO4 buffer to enhance retention time as the OH-TrCBQ is a strong polar

184

compound.35 Optimal separation was achieved at A:B=30:70 (v/v) for PCP (220 nm);

185

A:B=50:50 (v/v) for OH-TrCBQ (292 nm), and A:B=70:30 (v/v) for OH-DDBQ (320

186

nm). TCBQ and TCHQ were detected using a mobile phase consists of solvent A (water

187

plus 0.04 % trifluoroacetic acid (v:v)), and solvent B (methanol). Optimal separation was

188

achieved at A:B=40:60, and the UV detector at 254 nm. All the flow rates were set to 0.4

189

mL/min. The column temperatures were set to 35 °C. The retention times and absorption

190

maxima were compared with those of authentic standard compounds.

191


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Chemiluminescence Detection

193

The CL produced by PCP and intermediates was measured by a BPCL-2-TGC

194

ultra-weak CL analyzer (Institute of Biophysics, Chinese Academy of Sciences, Beijing,

195

China). The static injection CL method was carried out in a 3-mL glass cuvette contained

196

0.2 mL standard solution and started by the injection of 0.1 mL CL buffer which

197

contained 50 mM H2O2. The CFCL apparatus for the online monitoring of intermediates

198

generated from PCP degradation is shown in Figure 1. The apparatus mainly comprised

199

of a photoreactor, a computer-controlled ultra-weak CL analyzer, and two peristaltic

200

pumps (Longer Precision Pump Co., Baoding, Hebei, China). The photoreactor was a

201

cylindrical quartz container (250 mL) irradiated with a 500 W xenon light source with

202

365 nm filter and 400 nm cut-off filter (as described above). One glass container (200 mL)

203

was used for oxidant H2O2. The two containers are connected separately to the peristaltic

204

pumps through a tygon pump tubing (i.d. 1 mm). Fluids were pumped with 30 r/min flow

205

rate into a spiral detection cell in the CL analyzer, and CL intensity was measured with a

206

photomultiplier tube (PMT). The all CL data integration time was set at 0.01 s per

207

spectrum, and the work voltage of -900V was used for the CL detection. The CL signals

208

were recorded by the computer installed with a data-acquisition interface.

209



210 211

Figure 1. CFCL experimental equipment for online monitoring of highly toxic

212

intermediates generated by TiO2/PCP. (1) xenon lamp, (2) filters, (3) magnetic stirrer, (4)

213

TiO2/PCP suspension, (5) chemiluminescence buffer (0.1 M carbonate buffer containing

214

50 mM H2O2), (6) peristaltic pumps, (7) detection cell, (8) computer.

215 216

Acute Toxicity Assessment

217

The toxicity of the treated samples was evaluated by monitoring inhibition in the

218

natural emission of marine luminescence bacteria Vibrio Fischeri, using the CL analyzer.

219

All chemicals were diluted and tested in 2% NaCl (isotonic solution for Vibrio Fischeri)

220

and the pH values were adjusted to 7.0. Before the tests, the purchased commercial

221

freeze-dried Vibrio Fischeri was removed from -20 °C and kept at room temperature for

222

10 min. Then 0.5 mL resuscitation solution was added and make sure the bacteria be fully

223

resuscitated at 25 °C for 15 min. 1 mL sample was added with 0.1 mL adjustment

224

solution and 20 µL Vibrio Fischeri solution. After thoroughly mixed, the mixture was

225

kept at 25 °C for 15 min. Then the mixture was injected into the luminescence detection


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cell and its luminescence intensity was measured. These tests determined toxicity based

227

on the inhibition of luminescence emitted by the bacteria Vibrio Fischeri. The inhibition

228

percentage of bacterial luminescence (INH%) caused by the addition of toxic chemicals

229

including the PCP/TiO2 photocatalytic suspension or its major degradation intermediates

230

was calculated as follows:36,37

231

INH%=100 − (

୍୘భఱ ୍୘బ ×୏୊

) × 100, with KF=

୍େభఱ ୍େబ

232

Where, IC0 and IT0 are the maximum values of luminescence of control and test

233

sample at t=0 min. IC15 and IT15 are corresponding luminescence values for control and

234

test samples measured after 15 min exposure time. KF is the correction factor which

235

usually characterizes the natural luminescence loss of the control (i.e., bacterial

236

suspension in 2% NaCl) because of the sample’s color or turbidity. In our experiment, the

237

value of KF was set to 1 because of the low turbidity or color of treated samples.

238 239

RESULTS AND DISCUSSION

240

PCP Photocatalytic Degradation and Their Toxicity Assessment



241 242

Figure 2 PCP concentration (■) and INH % of PCP/TiO2 solution (●) as function of

243

irradiation time during photocatalytic degradation of PCP. Experimental conditions:

244

[PCP]0 =50 µM, initial pH=7, [TiO2]=0.2 g/L.

245 246

Preliminary experiments were performed to determine the extent of PCP photocatalytic

247

degradation and its trend of toxicity changes in the presence of TiO2 as shown in Figure 2.

248

It was found that the concentration of PCP decreased with the illumination time and

249

complete abatement was achieved within 60 min. However, high acute toxicity could still

250

be observed from the INH% result and it continued for about 70 min, which was probably

251

caused by the generation of highly toxic intermediates. It’s worth noting that, even after

252

70 min irradiation of PCP/TiO2 when the PCP had been removed completely, there was

253

still high inhibition of bacteria luminescence observed. That means similarly or more

254

toxic intermediates than parent PCP must be generated and further research should be


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255


done to identify them.

256

The generation and pathway of intermediates were usually different according to

257

degradation conditions. In the present work, transient intermediates were identified

258

during PCP/TiO2 photocatalytic irradiation by ESI-Q-TOF-MS in the initial pH 7. As

259

shown in Figure S2A, we found that, besides TCBQ and TCHQ, which were previously

260

identified as oxidation products,1-3,11,17,38 OH-TrCBQ and OH-DDBQ were also the main

261

intermediates. The four-chlorine cluster evident at m/z 243.86, 245.86, 247.86, 249.86 is

262

the (M)-· ion of the TCBQ, the four-chlorine cluster evident at m/z 244.87, 246.87,

263

248.87, 250.87 is the (M-H)-· ion of the TCHQ (Figure S2B), the three-chlorine cluster

264

evident at m/z 224.89, 226.89, 228.88, 230.88 is the (M-H)-· ion of the OH-TrCBQ

265

(Figure S2C), and the two-chlorine cluster evident at m/z 206.92, 208.92, 210.91 is the

266

(M-H)-· ion of the OH-DDBQ (Figure S2D). To our knowledge, this was the first report

267

that TCBQ, TCHQ, OH-TrCBQ and OH-DDBQ coexisted during advanced oxidation of

268

PCP, and some unidentified intermediates carrying 2-3 chlorine atoms reported in the

269

previous studies were probably OH-TrCBQ or OH-DDBQ according to our MS

270

results.7,8,17,39 While, the generation of chlorophenols including tetrachlorophenols,

271

trichlorophenols, dichlorophenols or phenol has not been observed which was different

272

from some previously reported.25, 40 This was probably due to the large difference in the

273

photocatalytic conditions including photocatalysts and light source. Especially, in our

274

experiments, the combination of two filters was employed to strictly avoid the direct

275

photolysis of PCP which was probably inhibited the chlorophenols generation.17

276

Furthermore, HPLC method was also used to verify the intermediates, the results showed

277

that TCBQ, TCHQ, OH-TrCBQ, and OH-DDBQ were produced during the



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278

photocatalytic degradation of PCP, and no other intermediates were detected (Figure S3).

279

This was consistent with ESI-Q-TOF-MS results.

280

During the photo-degradation reaction, the intermediates described above may

281

potentially be harmful to the environment.41 Based on this, the acute toxicity of these

282

intermediates and the parent compound PCP were all evaluated using the luminescence

283

inhibition of the Vibrio Fischeri test, and the IC50 values were obtained (see Table 1). The

284

results showed that the IC50 of these five compounds were at micromole per liter level as

285

reported,34,39 and they were in the order of TCBQ < PCP ≈ OH-TrCBQ < TCHQ
5 conditions. However, TCBQ still appeared

411

with the increase of irradiation times and even its formation was lagged behind PCP

412

degradation, which was probably because the pH of the irradiated PCP/TiO2 solution

413

dropped significantly due to the formation of HCl as well as some organic acid

414

intermediates (Figure S11),7 and the hydrolyzation of TCBQ was very slow in the acidic

415

solution (Figure S10).34 Therefore, when the initial pH value greater than 7, produced

416

TCBQ was spontaneously hydrolyzed to form OH-TrCBQ, and OH-TrCBQ was the main

417

intermediate. When the initial pH value equal to 7, TCBQ and OH-TrCBQ were both

418

main intermediates because of the slower hydrolyzation rate. When the initial pH valve

419

less than 7, TCBQ would be the main intermediate. Of course, under high-intensity

420

irradiation, all the reaction intermediates were also attacked further by ROS in

421

illuminated PCP/TiO2 suspension to yield dechlorinated- or small-molecule ring-opening



422

products.17 Based on above, the dynamic generation and transformation process of TCBQ

423

and OH-TrCBQ was preliminarily concluded in Scheme 1.

424

425 426

Scheme 1. Proposed generation and transformation process of TCBQ and OH-TrCBQ

427

during photocatalytic degradation of PCP.

428 429

Comparison between Results of CFCL Method and Acute Toxicity Test

430


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431 432

Figure 5. Comparison of CFCL method (A) and Vibrio Fischeri test (B) for the

433

monitoring of the OH-TrCBQ/TCBQ and toxicity changes during PCP photocatalytic

434

degradation with different initial PCP concentration (0, 3, 10, 20, 30 µM). Experimental

435

conditions: initial pH=7, TiO2=0.2 g/L.

436 437

According to the results above, the dynamic generation and degradation of TCBQ and

438

OH-TrCBQ were dominant factors for the toxicity changes during PCP photocatalytic

439

degradation process. To further investigate whether the CFCL system developed can be

440

used as a rapid and reliable method for tracking the toxicity changes, the relationship

441

between CFCL curves generated by PCP/TiO2 suspensions and the inhibition of bacterial

442

luminescence was studied. TiO2 suspensions with different PCP concentration were

443

irradiated, and the dynamic generation of TCBQ and OH-TrCBQ was monitored using

444

the CFCL method (Figure 5A). When PCP concentration increased, the CFCL intensity

445

of both OH-TrCBQ and TCBQ increased, indicating more OH-TrCBQ and TCBQ were

446

generated. The increase in the concentration of OH-TrCBQ and TCBQ led to the delay of



447

the time for CFCL intensity of TCBQ to reach the maximum. Figure 5B exhibits the

448

toxicity of irradiated PCP/TiO2 suspensions toward Vibrio Fischeri. According to INH%

449

results, PCP is a highly toxic compound toward Vibrio Fischeri, resulting in a high INH%

450

value at time = 0 min. Therefore, at the beginning of PCP photocatalytic degradation, the

451

INH% stayed high value and kept a plateau because OH-TrCBQ generated and PCP has

452

not been degraded completely. Then the CFCL intensity and INH% results plunged

453

indicating the concentrations of intermediates decreased and PCP was being degraded

454

completely. When the irradiation time prolonged, the CL intensity and INH% both

455

increased and then decreased again due to the generation and transformation of TCBQ.

456

Notably, the CFCL response curves of PCP/TiO2 suspension showed a pattern very

457

similar to their bacteria inhibition.

458

Under the same experimental conditions, the detection limit of PCP photocatalytic

459

degradation was studied. As shown in Figure 5, when the initial PCP concentration was 3

460

µM, CFCL signals were still observed compared with blank signal and the CFCL curve

461

was also comparable with INH%. But when the initial PCP concentration as low as 2 µM,

462

CFCL signals and INH% could not be distinguished clearly from 0 µM PCP signals. So,

463

the exact detection threshold was 3 µM. It was worth mentioning that, the CFCL signals

464

depended on the concentration of TCBQ and OH-TrCBQ generated during the PCP

465

degradation process. While, the generation of TCBQ and OH-TrCBQ was correlated with

466

the initial concentration of PCP, as well as the initial pH, the concentration of TiO2 and

467

the intensity of light et al. Therefore, the exact detection threshold was probably changed


Page 26 of 35

Page 27 of 35

468


under the different photocatalytic conditions.

469

To sum up, the developed CFCL method could not only be used for the dynamic

470

monitoring of production, conversion, and degradation of highly toxic intermediates

471

TCBQ and OH-TrCBQ, but also for tracking the toxicity changes process during PCP

472

photocatalytic degradation. Furthermore, this may provide a very convenient and

473

low-cost tool for screening the optimal photocatalytic conditions to remove PCP and their

474

highly toxic intermediates, control the generation of highly toxic intermediates, and

475

assessing their toxicological risk for the wastewater treatment. However, if the

476

chlorophenols intermediates generated during PCP photo-degradation under the certain

477

conditions, the application of this CFCL system was limited. Therefore, we will construct

478

a new CL system to dynamically track a wider variety of highly toxic intermediates such

479

as chlorophenols and quinones in the future work.

480 481

ASSOCIATED CONTENT

482

Supporting information

483 484

The Supporting Information is available free of charge via the internet at http://pubs.acs.org.

485 486

AUTHOR INFORMATION

487

Corresponding Authors

488

*Tel.: (86) 10-62849338. Fax: (86) 10-62849685. E-mail: [email protected] (L. Zhao).



489

*Tel.: (86) 10-62849338. Fax: (86) 10-62849685. E-mail: [email protected] (L.-H.

490

Guo).

491

Notes

492

The authors declare no competing financial interest.

493 494

ACKNOWLEDGEMENTS

495

This work was supported by the National Key Research and Development Program of

496

China (2016YFA0203102), the Chinese Academy of Sciences (XDB14040100), and the

497

National Natural Science Foundation of China (Nos. 21677152, 91543203, 21577156).

498

The authors would like to thank Dr. Ben-Zhan Zhu and Dr. Li Mao for supplying the

499

OH-TrCBQ regent.

500 501

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