Natural Porphyrins Accelerating the ... - ACS Publications

School of Marine Sciences, Sun Yat-sen University, Guangzhou 510275, China. 10 ... carcinogens”) by various natural porphyrins under solar irradiati...
0 downloads 0 Views 465KB Size
Subscriber access provided by UNIV OF DURHAM

Article

Natural Porphyrins Accelerating the Phototransformation of Benzo[a]pyrene in Water Lijuan Luo, Zhengyu Xiao, Baowei Chen, Fengshan Cai, Ling Fang, Li Lin, and Tiangang Luan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05854 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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 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 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 32

Environmental Science & Technology

1

Natural Porphyrins Accelerating the

2

Phototransformation of Benzo[a]pyrene in Water

3

Lijuan Luo†,‡, Zhengyu Xiao§, Baowei Chen§, Fengshan Cai†, Ling Fang‖, Li Lin†,

4

Tiangang Luan†,*

5



6

Guangzhou 510275, China

7



8

Chinese Academy of Science, Guangzhou 510640, China

9

§

State Key Laboratory of Biocontrol, School of Life Sciences, Sun Yat-sen University,

State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry,

South China Sea Resource Exploitation and Protection Collaborative Innovation Center,

10

School of Marine Sciences, Sun Yat-sen University, Guangzhou 510275, China

11



12

510275, China

Instrumental Analysis and Research Center, Sun Yat-sen University, Guangzhou

1 Environment ACS Paragon Plus

Environmental Science & Technology

13 14

Abstract Phototransformation is one of the most important transformation pathways of organic

15

contaminants in the water environment. However, how active compounds enable and

16

accelerate the phototransformation of organic pollutants remains to be elucidated. In this

17

study, the phototransformation of benzo[a]pyrene (BaP, the first class “human

18

carcinogens”) by various natural porphyrins under solar irradiation was investigated,

19

including chlorophyll a, sodium copper chlorophyllin, hematin, cobalamin and

20

pheophorbide a. Transformation efficiency of BaP varied considerably with chemical

21

stabilities of the porphyrins. Porphyrins with a lower stability displayed higher BaP

22

transformation efficiencies. BaP transformation had a significant positive correlation with

23

the production of singlet oxygen. Identical phototransformation products of BaP were

24

observed for all investigated porphyrins, and the main products were identified as BaP-

25

quinones, including BaP-1,6-dione, BaP-3,6-dione and BaP-6,12-dione. The mechanism

26

of natural porphyrins accelerating the BaP phototransformation in water was proposed to

27

proceed via the photocatalytic generation of singlet oxygen resulting in the

28

transformation of BaP to quinones.

29

Keywords: Chlorophyll; Natural Porphyrins; Benzo[a]pyrene; Phototransformation;

30

Singlet oxygen

31

2 Environment ACS Paragon Plus

Page 2 of 32

Page 3 of 32

Environmental Science & Technology

32

Introduction

33

Benzo[a]pyrene (BaP), one of the polycyclic aromatic hydrocarbons(PAHs) containing

34

five fused benzene rings, is widespread in air, water and soil in significant amounts1-2 and

35

is included in the US-EPA priority pollutant list.3 On account of the potent mutagenic and

36

carcinogenic toxicities, BaP was ranked as the first class “human carcinogens” in the

37

World Health Organization (WHO) International Agency for Research on Cancer report.4

38

It is a well-studied member of the PAH family and serves as a model compound for

39

understanding the degradation of PAHs.5

40

BaP can be degraded through either direct or sensitized photochemical reactions.6-7 It

41

can absorb surface solar radiation, allowing for the possibility of direct

42

photodegradation.8 Moreover, photocatalysis is one of the most attractive methods for

43

BaP degradation from the viewpoint of solar energy utilization.9 Our previous study

44

found that chlorophyll in green microalgae cells can enhance the phototransformation of

45

BaP under simulated solar irradiation in water.10 Chlorophyll is the photosynthetic

46

pigment necessary for the photosynthesis in plants, algae and cyanobacteria, which is

47

abundant in environments.

48

The basic structure of the chlorophyll molecule contains a porphyrin ring coordinated

49

to a central metal atom. Many compounds have a similar structure to chlorophyll, such as

50

pheophorbide a, hematin, sodium copper chlorophyllin and colalamin, denoted as

3

ACS Paragon Plus Environment

Environmental Science & Technology

51

porphyrins. These compounds are different in the metal ions in the center, as shown in

52

Figure S1. They are considered natural porphyrins due to natural sources in the

53

environment, e.g., sodium copper chlorophyllin is derived from alkaline hydrolysis of

54

chlorophyll, where the magnesium atom is replaced with copper. Pheophorbide a, a

55

photoproduct of chlorophyll a, lacks the phytyl side chain and central magnesium atom.

56

Hematin is abundant in living animal’s blood. Cobalamin, also called vitamin B12, a

57

cobalt porphyrin, could be synthetized by algae and bacteria.11 These porphyrin

58

compounds exist in natural water because they enter into environmental systems with the

59

death of plants or animals. Herein, chlorophyll a is the most common porphyrin in the

60

aquatic environment, which is usually used to monitor water quality. The concentration

61

of chlorophyll a was up to be 434.3 mg m-3 during an algal bloom.12

62

Studies on the photocatalytic degradation of organic pollutants with porphyrins were

63

much reported,13-14 which were mainly focused on the modification and preparation of

64

porphyrins.15-16 However, the effect of natural porphyrins on the photocatalytic

65

degradation of PAHs in natural environments has not been deeply explored in the

66

literature. The fate of BaP in the natural environment remains to be understood, in

67

particular in the presence of porphyrins from natural sources. The main objective of this

68

study was to understand the roles of natural porphyrins in the phototransformation of BaP,

69

and elucidate the phototransformation mechanism of BaP induced by porphyrins under

4

ACS Paragon Plus Environment

Page 4 of 32

Page 5 of 32

Environmental Science & Technology

70

simulated solar irradiation in water. The effect of environmental parameters, such as pH

71

value, salinity, dissolved organic matters (DOM) on the BaP phototransformation process

72

containing porphyrins were also investigated.

73

Materials and Methods

74

Chemicals. Standards of BaP (99.6%), acetone (99.5%), methanol (≥99.9%),

75

benzene (99.8%), phenol (98%) and furfuryl alcohol (FFA, 97.5%) were obtained from

76

Sigma-Aldrich (St. Louis, MO, USA). Benzo[a]pyrene D12 (BaP-D12, 99%) was

77

obtained from Dr. Ehrenstorfer (Germany). Chlorophyll a (98.1%) was purchased from

78

Wako Pure Chemical Industries, Ltd. (Japan). Cobalamin (>95.0%) was purchased from

79

Tokyo chemical industry Co., Ltd. (Japan). Hematin (97%) was purchased from Alfa

80

Aesar Chemicals Co., Ltd. (China). Sodium copper chlorophyllin was made in CNW,

81

China. Pheophorbide a was from J&K Scientific Ltd. (China). Six transformation

82

products of BaP, 1-hydroxybenzo[a]pyrene (1-OH-BaP, >96%), 3-

83

hydroxybenzo[a]pyrene (3-OH-BaP, >99%), benzo[a]pyrene-cis-4,5-dihydrodiol (BaP-

84

cis-4,5-diol, >99%), benzo[a]pyrene-1,6-dione (BaP-1,6-dione, >99%), benzo[a]pyrene-

85

3,6-dione (BaP-3,6-dione, >99%) and benzo[a]pyrene-6,12-dione (BaP-6,12-dione, >99%)

86

were supplied by Middlewest Research Institute (NCI Chemical Resource, Kansas, MO,

87

USA). Ethyl acetate (99.8%), sodium chloride and anhydrous sodium sulfate were

5

ACS Paragon Plus Environment

Environmental Science & Technology

88

provided by Farce Chemical Supplies (China). High-purity water was supplied by a Milli-

89

Q water system (Millipore, Eschborn, Germany).

90

The stock solutions of chlorophyll a and pheophorbide a were prepared by dissolving

91

the appropriate amounts of chlorophyll a and pheophorbide a in 90% ethanol (v/v) with

92

concentrations of 200 and 1000 mg L-1, respectively, and stored in the dark at 4 °C. For

93

sodium copper chlorophyllin and cobalamin, stock solutions were prepared by dissolving

94

appropriate amounts of solid powder in Milli-Q water at a concentration of 1000 mg L-1.

95

The hematin solution was prepared with 0.1 mol L-1 NaOH at a concentration of 1000 mg

96

L-1.

97

Photoreaction procedure. The photoreactions were performed under white light

98

irradiation at a light intensity of 50 µmol photons s-1 m-2, which was provided by a series

99

of cool white fluorescent lamps. The spectrum is from 310 to 750 nm in the wavelength,

100

resembling the solar spectrum. The pH value of the reaction system was adjusted using

101

0.1 mol L−1 NaOH and 0.1 mol L−1 HCl, and was determined using a pH meter (Sartorius

102

PB-10, Germany). The salinity of the solutions was adjusted with 5 mol L−1 NaCl. A

103

series of 250 mL conical flasks were prepared, and 100 mL of sterile water was added

104

into each flask. The stock solution of BaP was spiked, and the initial concentration was

105

1.0 mg L-1. The solutions of porphyrins were added at a concentration of 1.0 mg L-1.The

106

flasks without porphyrins were used as the control. The flasks were then shaken on a 6

ACS Paragon Plus Environment

Page 6 of 32

Page 7 of 32

Environmental Science & Technology

107

rotary shaker at 160 rpm at 22 ± 2 °C. Triplicate flasks from each of the treatments were

108

retrieved at different time intervals, and the residual amounts of BaP and the

109

transformation products were determined.

110

BaP extraction and analysis. BaP was extracted with ethyl acetate by liquid-

111

liquid extraction according to the methods described by Ke et al.17 BaP-D12, the

112

surrogate standard for the quantification of BaP, was added in the initial extraction. The

113

extracts were concentrated to nearly dry by rotary evaporation and then re-dissolved in

114

methanol. The final volume was adjusted to 2 mL and stored at 4 ◦C for further analysis.

115

The BaP samples were analyzed with an Agilent Technologies 7890 gas

116

chromatograph (GC) equipped with a 5975 mass spectrometer (MS). An HP-5MS fused

117

silica capillary column coated with 5% phenylmethyl polysiloxane (30 m length, 0.25

118

mm i.d., 0.25 µm film thickness; Wilmington, DE, USA) was used. An Agilent auto

119

liquid sampler was used for sample injection, and the injection volume was 1.0 µL.

120

Helium was the carrier gas with a constant flow rate of 1.0 mL min-1. The injection mode

121

was splitless, and the injector and detector temperatures were 280 °C. The GC column

122

temperature was programmed from 120 °C to 290 °C at a rate of 30 °C min-1, held for 1

123

min, then increased from 290 °C to 300 °C at the rate of 5 °C min-1, and held at 300 °C

124

for 6 min. The samples were analyzed in selected ion monitoring (SIM) mode. The limit

125

of detection (LOD), defined as a signal of three times the noise level, was 2.81 µg L-1. 7

ACS Paragon Plus Environment

Environmental Science & Technology

126

Photodegradation of porphyrins. As porphyrins would be photodegraded

127

under light irradiation, the concentrations of porphyrin were determined at Day 1, 4, 7

128

and 10. The chlorophyll a concentration was calculated according to the method

129

described by Huang and Cong.18 The method of alkaline hematin D-575 was used to

130

determine the concentration of hematin.19 For the other porphyrins, the absorbance of the

131

supernatant was measured at maximum absorption wavelength of 404, 667 and 356 nm

132

for sodium copper chlorophyllin, pheophorbide a and cobalamin respectively by a UV-vis

133

spectrophotometer. And then their concentration was analyzed with spectrophotometer

134

(Unico UV-2600).

135

Detection of •OH and 1O2. The photo production of •OH and 1O2 was

136

determined in the solutions of the porphyrins following the methods described by Luo et

137

al.10 In the determination of 1O2, the initial concentration of FFA in the aqueous solution

138

was changed to 500 µmol L-1.

139

Determination of BaP transformation products. After GC-MS analysis, the

140

sample extracts were exposed to identify the possible BaP intermediates. An aliquot of

141

0.2 mL of the sample extract was diluted with methanol, and the volume was adjusted to

142

1 mL. High-performance liquid chromatography (HPLC) combined with atmospheric

143

pressure ionization mass spectrometry (APCI-MS) was selective and sensitive for the

8

ACS Paragon Plus Environment

Page 8 of 32

Page 9 of 32

Environmental Science & Technology

144

determination of the BaP transformation products. The Thermo Scientific LC system

145

consisted of an Accela 1250 pump and an Accela autosampler. The separation of BaP

146

transformation products were performed on a Hypeisil GOLD column (100 mm × 2.1

147

mm, i.d.; 1.9 µm particle size, Thermo Scientific). The column (temperature 35 °C) was

148

eluted for 8 min at a flow rate of 300 µL min-1 with a binary gradient of water: methanol

149

(25:75, v/v) for 4 min, followed by the same gradient at a ratio of 10: 90 for 2 min and

150

then returned to the initial gradient of 25: 75 for 2 min. The injection volume was 10.0

151

µL.

152

The mass spectrometric analyses were performed using a Thermo Scientific TSQ

153

Quantum Ultra mass spectrometer equipped with an APCI source. The measurements

154

were performed in positive ion mode at 400 °C vaporizer temperature, 350 °C capillary

155

temperature, 40 psig sheath gas pressure and 5 psig aux gas pressure. The discharge

156

current was set at 4.0 µA. The mass spectrometer was operated under select reaction

157

monitoring (SRM) mode. The identification of BaP intermediates was confirmed by

158

comparing their retention times and characteristic mass spectra with the standard

159

compounds. The retention times, qualitative and quantitative ions, and collision energies

160

for the standards of the BaP transformation products are shown in Table S1.

161 162

Effect of DOM on BaP phototransformation. In order to study the effect of DOM on BaP phototransformation in natural environment, the natural water was

9

ACS Paragon Plus Environment

Environmental Science & Technology

163

collected from the Pearl River (23°06′24.22″ N, 113°17′30.49″ E). After being filtered

164

with 0.45 µm filter, the water samples were stored in the dark at 4 °C. The TOC in these

165

samples was determined using TOC analyzer (Aurora 1030, OI Analytical Company).

166

Statistical analysis. The mean and standard deviation values of triplicates were

167

calculated. Kinetics of BaP transformation and porphyrins degradation were calculated

168

using first-order equation.20 The effect of pH and salinity on BaP transformation was

169

compared by one-way analysis of variance (ANOVA). If the ANOVA results were

170

significant at p ≤ 0.05, Turkey’s multiple comparisons as post-hoc tests were applied to

171

determine where the differences occurred. The relationship between the BaP

172

transformation percentages and the concentrations of 1O2 were tested by correlation

173

analysis and the Pearson coefficient was calculated. The statistical analyses were

174

performed using SPSS 16.0. Linear correlation between 1O2 concentrations and BaP

175

degradation percentages were also conducted using Origin 8.5. With respect to linear

176

regression, the data was checked to meet the normal distribution.

177

Results and Discussion

178

Effect of chlorophyll a on BaP phototransformation under dark and

179

light conditions. The effects of chlorophyll a on BaP phototransformation under dark

180

and light conditions are shown in Figure 1. Under dark conditions, chlorophyll a had no

10

ACS Paragon Plus Environment

Page 10 of 32

Page 11 of 32

Environmental Science & Technology

181

effect on the transformation of BaP during 7 days irradiation. The BaP transformation

182

percentages in the treatment of chlorophyll a and the control (sterile water without

183

chlorophyll a) were 9.4% and 10.1% at Day 7, respectively. Under light irradiation, the

184

concentration of BaP decreased dramatically during the first 4 days in the chlorophyll a

185

solution, and up to 95.8% of the BaP was transformed at Day 4. However, only 11.8% of

186

the BaP was phototransformed in the control flasks at Day 7, indicating that chlorophyll

187

accelerated the phototransformation of BaP observably under light irradiation and that

188

light was the essential element during the reaction.

189

Effect of porphyrins on BaP phototransformation. Chlorophyll has a

190

porphyrin structure as its molecular core and contains a metal atom at its center, which is

191

structurally similar to sodium copper chlorophyllin, hematin, pheophorbide a and

192

cobalamin (Figure S1). These four porphyrins were applied to compare with chlorophyll

193

a in the phototransformation of BaP under light irradiation (Figure 2). Among the

194

porphyrins, the removal of BaP, based on the residual amounts in the medium, increased

195

in the order of chlorophyll a > cobalamin > pheophorbide a > hematin > sodium copper

196

chlorophyllin. Chlorophyll a had the highest capability to transform BaP, and sodium

197

copper chlorophyllin had the least capacity to transform BaP. The BaP removal in the

198

solution of sodium copper chlorophyllin was even lower than that in the controls.

11

ACS Paragon Plus Environment

Environmental Science & Technology

199

The first-order kinetics of BaP transformation in the presence of different porphyrins

200

was calculated. Rate constants (k) and half-lives (t1/2) are shown in Table S2. Correlation

201

coefficients (R2) were in the range of 0.8827 to 0.9828 in four porphyrins except sodium

202

copper chlorophyllin, which demonstrated that BaP phototransformation in the solution

203

of chlorophyll a, hematin, pheophorbide a and cobalamin was in good accordance to the

204

first-order kinetic reaction. The first-order phototransformation rate of BaP ranged from

205

0.0061 to 0.395 d-1 and varied greatly with the types of porphyrins.

206

Photodegradation of porphyrins. During the light irradiation, porphyrins

207

bleached as time increased. As shown in Table S3, porphyrin photodegradation

208

conformed to the first-order kinetic reaction (R2 = 0.7459 ~ 0.9997). The

209

photodegradation rates of five porphyrins ranged from 0.050 to 0.798 d-1, with an order of

210

chlorophyll a > pheophorbide a > cobalamin > hematin > sodium copper chlorophyllin.

211

We found that BaP phototransformation rate was substantially related to chemical

212

stability of the porphyrin present in reaction solution. Generally, the lower stability of the

213

porphyrin, the higher BaP transformation rate.

214

Under light irradiation, chlorophyll a degraded fastest with the half-life of 0.87 d

215

(Table S2). To determine the effect of degraded chlorophyll a on the phototransformation

216

of BaP, chlorophyll a was irradiated under light for 4 days, and then BaP was spiked. As

217

shown in Figure S2, 97.75% of BaP was removed at Day 4. The rate constant was 0.658 12

ACS Paragon Plus Environment

Page 12 of 32

Page 13 of 32

Environmental Science & Technology

218

d-1, faster than that of the original chlorophyll a (Table S2). The result suggested that the

219

photodegradation products of chlorophyll a also accelerated the phototransformation of

220

BaP. According to literatures, Chlorophyll a first lost phytol, magnesium and

221

carbomethoxy groups forming pheophorbide a.21 and then the tetrapyrrole macrocyclic

222

ring cleaved, formed methyl vinyl maleimide, methyl ethyl maleimide, a C-E-ring

223

derivative and hematinic acid,22-23 the final products were low molecular weight acids,

224

such as glycerol, lactic, citric, succinic, malonic acids and alanine.24 To determine which

225

products played important roles in the BaP transformation, further research is needed.

226

Photoproduction of •OH and 1O2. Reactive oxygen species (ROS), in particular

227

hydroxyl radical (•OH) or singlet oxygen (1O2), play important roles in the

228

phototransformation of organic pollutants.25-28 Therefore, the formation of •OH and 1O2

229

was determined in our studies. The results are shown in Figures S3 and 3a. The

230

production of the •OH in the chlorophyll a treatments was lower than 2 × 10-2 µmol L-1

231

(Figure S3). The chlorophyll concentrations and exposure time had insignificant effects

232

on the production of the •OH. In our previous study, the phototransformation of BaP

233

increased with the chlorophyll concentration,10 which was inconsistent with the

234

production of the •OH. This result suggests that the •OH did not play an important role in

235

the process.

13

ACS Paragon Plus Environment

Environmental Science & Technology

236

The concentration of 1O2 was approximately four orders of magnitude higher than •OH

237

in the presence of porphyrins. The generation of 1O2 increased with the time, the

238

concentration of 1O2 increased in the order of chlorophyll a > cobalamin > pheophorbide

239

a > hematin > sodium copper chlorophyllin after 7 days of irradiation (Figure 3a). The

240

order was consistent with the BaP phototransformation efficiency among the five

241

porphyrins shown in Figure 2. The results indicate that the phototransformation of BaP

242

was closely related to the production of 1O2. According to the linear regression analysis,

243

BaP phototransformation percentages was positively related to 1O2 concentrations (r =

244

0.9261, p < 0.001, Figure 3b). The correlation analysis between the BaP transformation

245

percentage and the concentration of 1O2 in each porphyrin treatment was also tested, and

246

the results are listed in Table S3. The results show that BaP transformation had a

247

significant positive correlation with the production of 1O2 (p < 0.05), except in the control

248

treatment, suggesting that 1O2 played a major role in the phototransformation of BaP.

249

After absorbing light, the porphyrins reach triplet states, and energy is transferred to

250

ground state oxygen, resulting in the formation of 1O2 by a spin reversal process of one

251

electron in O2.29

252

BaP transformation products. To confirm the role of ROS in BaP

253

phototransformation and the transformation pathway, the BaP intermediates were

254

determined. Identical transformation products were observed for all investigated 14

ACS Paragon Plus Environment

Page 14 of 32

Page 15 of 32

Environmental Science & Technology

255

porphyrins and the main products were identified as BaP-quinones, including BaP-1,6-

256

dione, BaP-3,6-dione and BaP-6,12-dione (Figure 4). The concentration of the products

257

increased with the irradiation time, suggesting that the products accumulate in the

258

medium. Chlorophyll a had the highest BaP transformation production, and sodium

259

copper chlorophyllin had the least production, which was consistent with the BaP

260

phototransformation efficiency in Figure 2. A low amount of BaP-cis-4,5-diol was found

261

in the solutions with porphyrins; however, the signal to noise ratio was less than 3, which

262

was under the detection limit. Cis-dihydroxy BaP is known as the biodegradation

263

metabolite,10,30-31 which is rarely present in phototransformation process. Monohydroxy

264

BaP was also tested; however, none of 1-OH-BaP, 3-OH-BaP or their isomers were

265

detected. In this research, BaP-quinones were the major products of BaP, which is

266

different from human cells. BaP is metabolized to BaP-dihydrodiols, phenols and tetraols

267

via cytochrome P450 enzymes, one-electron oxidation, and dihydrodiol dehydrogenase in

268

human cells.32-33

269

Recently, density functional theory calculations was used to elucidate the degradation

270

mechanism of PAHs initiated by •OH in the presence of O2 and NOx,34-36 and the

271

dominant degradation products of BaP were OH-BaP, NO2-BaP, 7,10-BaP-dione, as well

272

as several ring-opened products.35 All the above mentioned products were not detected in

273

this study, suggesting that the phototransformation of BaP by porphyrins in the water was

15

ACS Paragon Plus Environment

Environmental Science & Technology

274

proceeded via other mechanisms rather than •OH. BaP-1,6-dione, BaP-3,6-dione and

275

BaP-6,12-dione were the main transformation production of BaP, indicating that 1O2

276

initiated the phototransformation of BaP. The mechanism of porphyrins catalyzing the

277

phototransformation of BaP was proposed as porphyrins photooxidized BaP to quinones

278

via the photocatalytic generation of 1O2.

279

The BaP transformation products were converted into BaP parent according to the

280

mass balance calculation and the result is shown in Figure 5. Nearly all the disappeared

281

BaP was transformed into quinones, and the recovery percentages were 93.6-105.4%,

282

indicating that the benzene ring of BaP cannot be broken and is just accumulated as

283

quinones in the medium. The concentrations of BaP photoproducts increased with BaP

284

degradation over the 7-days irradiation. This implied that these products were more stable

285

than parent BaP. BaP-1,6-dione was the major product among the three quinones in the

286

high transformation efficiencies treatments, such as chlorophyll a, pheophorbide a and

287

cobalamin (Figure S4), while BaP-3,6-dione maintain the similar relative percentages in

288

the all porphyrins solution during 7 days irradiation. This result was inconsistent with a

289

previous study, where BaP-3,6-dione was found as the dominant product of BaP under

290

different light irradiation conditions.31

291 292

PAHs require activation to electrophilic metabolites exert their mutagenic or carcinogenic effects in human cells.33 As a result, BaP-quinones are more toxic than the

16

ACS Paragon Plus Environment

Page 16 of 32

Page 17 of 32

Environmental Science & Technology

293

parent BaP.37-38 The environmental risk of BaP would be increase in the present of natural

294

porphyrins, which could not be ignored in the water environment. On the other hand, the

295

rate limiting steps of the high molecular weight PAH degradation is the introduction of

296

molecular oxygen into aromatic ring as demonstrated in previous studies.39-40 The

297

phototransformation of BaP by porphyrins could candidate as the first step for the initial

298

oxidization of BaP and thereafter transformation products were further degraded using

299

other degradation methods. It is believed that combined advantages of different

300

transformation or degradation processes could accelerate the photodegradation of BaP or

301

other PAHs.

302

Effect of pH, salinity and DOM on BaP phototransformation. In natural

303

water environments, the phototransformation of organic pollutants are influenced by

304

environmental parameters, such as pH, salinity and DOM. The effect of pH, salinity and

305

DOM on BaP phototransformation represented with porphyrins was conducted in this

306

research. Chlorophyll a was selected for the study, since it had the highest capability to

307

transform BaP and it is the most common porphyrin in the natural environments.

308

The BaP phototransformation efficiency increased as the pH value increased. The

309

transformation efficiency of BaP was highest at pH 12.0, and was low under strong acidic

310

condition (Figure 6a). It could be explained that the photostabilities of porphyrins were

311

dependent on the pH value, and the high pH value could enhance the photodegradation of 17

ACS Paragon Plus Environment

Environmental Science & Technology

312

porphyrins.41-42 As a result, it might led to high production of 1O2, and then the highest

313

BaP phototransformation efficiency was occurred at pH 12.0.

314

The range of salinity in this study was from 0 to 1.0 mol L-1, which covered the salinity

315

levels normally found in the aquatic environment. As shown in Figure S5, the salinity had

316

an insignificant effect on the phototransformation of BaP in chlorophyll a-

317

containingsolution (p > 0.05). The effect of chlorophyll a concentration on the BaP

318

phototransformation was investigated in our previous research.10 The results showed that

319

BaP phototransformation efficiency increased initially with an increase of chlorophyll a

320

concentration, but declined when the concentration reached 3.0 mg L-1.

321

The water samples from the Pearl River are rich in DOM, and the TOC was measured

322

as 23.5 ± 2.5 mg L-1. As shown in Figure 6b, the BaP phototransformation was inhibited

323

by DOM when compared with pure water in Figure 1. The half-life of BaP in the water of

324

the Pearl River was 57.8 d, which was much longer than that in pure water (38.9 d in

325

Table S2). The suppression of PAHs by DOM was attributed to light screening, reactive

326

species quenching, PAHs binding.43-45 With the addition of chlorophyll a into the water

327

of the Pearl River, the half-life of BaP was shortened to 15.2 d, indicating that

328

chlorophyll a was still able to accelerate the phototransformation of BaP in the natural

329

water containing the high level of DOM. The effect of DOM on the phototransformation

330

of organic pollutants was much reported.46-47 However, the effect of DOM on the natural

18

ACS Paragon Plus Environment

Page 18 of 32

Page 19 of 32

Environmental Science & Technology

331

porphyrin-involved transformation of organic pollutants was relatively few considered,

332

especially for the carcinogenic BaP.

333

Environmental Implication. Natural porphyrins, e.g., chlorophyll a, could be

334

decidedly resulted in the phototransformation of organic contaminants in the aquatic

335

environment under sunlight irradiation, BaP in our case. This transformation was initiated

336

by the formation of singlet oxygen from the photolysis of porphyrins. Our study

337

highlights the important roles of biologically-originated compounds in the transformation

338

of organic contaminants in the real aquatic environment. Understanding such a

339

transformation including determination of identities and toxicity of transformation

340

products and relative contribution to the total transformation could render a high

341

possibility to accurately assess the risks of organic pollutants to aquatic ecosystem and

342

the public.

343

Associated content

344

Supporting information

345

The supporting information is available free of charge on the ACS Publications website.

346

Table of retention time, qualitative and quantitative ions, and collision energy for the

347

standard of BaP transformation products, rate constants (k) and half-lives (t1/2) of BaP

348

phototransformation and porphyrins photodegaradation, correlation analysis between 19

ACS Paragon Plus Environment

Environmental Science & Technology

349

the BaP transformation percentage and the production of 1O2, figure of the structures

350

of five porphyrins, effect of degraded chlorophyll a on BaP phototransformation,

351

effect of chlorophyll a concentration on the production of •OH, relative percentages

352

of BaP transformation products in different porphyrins treatments during 7 days

353

irradiation, and effect of salinity on BaP phototransformation by chlorophyll a at Day

354

4 are available.

355

Author information

356

Corresponding author

357

*E-mail: [email protected], Telephone: +86-20-84112958; Fax: +86-20-84037549

358

Notes

359

The authors declare no competing financial interests.

360

Acknowledgements

361

This research was financially supported by the National Natural Science Foundation of

362

China (NSFC, No. 21707175, 21625703, 41473092, 21777198), State Key Laboratory of

363

Organic Geochemistry (OGL-201504).

364

References

20

ACS Paragon Plus Environment

Page 20 of 32

Page 21 of 32

365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403

Environmental Science & Technology

1. Sun, J.-H.; Wang, G.-L.; Chai, Y.; Zhang, G.; Li, J.; Feng, J. Distribution of polycyclic aromatic hydrocarbons (PAHs) in Henan Reach of the Yellow River, Middle China. Ecotoxicol. Environ. Saf. 2009, 72 (5), 1614-1624. 2. Zhao, Z. Y.; Chu, Y. L.; Gu, J. D. Distribution and sources of polycyclic aromatic hydrocarbons in sediments of the Mai Po Inner Deep Bay Ramsar Site in Hong Kong. Ecotoxicology 2012, 21 (6), 1743-1752. 3. Keith, L. H. T., William A. Priority Pollutants: I. A Perspective View. Environ. Sci. Technol. 1979, 13, 416-423. 4. IARC. Monographs on the Evaluation of Carcinogenic Risks to Humans. http://monographs.iarc.fr/ENG/Classification/ (accessed July 2017). 5. Juhasz, A. L.; Naidu, R. Bioremediation of high molecular weight polycyclic aromatic hydrocarbons: a review of the microbial degradation of benzo a pyrene. Int. Biodeterior. Biodegrad. 2000, 45 (1-2), 57-88. 6. Mill, T.; Habey, W. R.; Lan, B. Y.; Baraze, A. Photolysis of polycyclic aromatic hydrocarbons in water. Chemosphere 1981, 10 (11/12), 1281-1290. 7. Fasnacht, M. P.; Blough, N. V. Mechanisms of the aqueous photodegradation of polycyclic aromatic hydrocarbons. Environ. Sci. Technol. 2003, 37, 5767-5772. 8. Miller, J. S.; Olejnik, D. Photolysis of polycyclic aromatic hydrocarbons in water. Water Res. 2001, 35 (1), 233-243. 9. Kou, J. H.; Li, Z. S.; Yuan, Y. U.; Zou, Z. G. Visible-light-induced photocatalytic oxidation of polycyclic aromatic hydrocarbons over tantalum oxynitride photocatalysts. Environ. Sci. Technol. 2009, 43, 2919–2924. 10. Luo, L.; Lai, X.; Chen, B.; Lin, L.; Fang, L.; Tam, N. F. Y.; Luan, T. Chlorophyll catalyse the photo-transformation of carcinogenic benzo a pyrene in water. Sci Rep-Uk 2015, 5, 12776. 11. Croft, M. T.; Lawrence, A. D.; Raux-Deery, E.; Warren, M. J.; Smith, A. G. Algae acquire vitamin B12 through a symbiotic relationship with bacteria. Nature 2005, 438 (7064), 90-93. 12. Choi, J.-K.; Min, J.-E.; Noh, J. H.; Han, T.-H.; Yoon, S.; Park, Y. J.; Moon, J.-E.; Ahn, J.-H.; Ahn, S. M.; Park, J.-H. Harmful algal bloom (HAB) in the East Sea identified by the Geostationary Ocean Color Imager (GOCI). Harmful Algae 2014, 39, 295-302. 13. Rebelo, S. L. H.; Melo, A.; Coimbra, R.; Azenha, M. E.; Pereira, M. M.; Burrows, H. D.; Sarakha, M. Photodegradation of atrazine and ametryn with visible light using water soluble porphyrins as sensitizers. Environ Chem Lett 2007, 5 (1), 29-33. 14. Liu, X.; Yu, M.; Zhang, Z.; Zhao, X.; Li, J. Solvothermal preparation of copper(II) porphyrin-sensitized mesoporous TiO2 composites: enhanced photocatalytic activity and stability in degradation of 4-nitrophenol. Res. Chem. Intermed. 2016, 42 (6), 5197-5208. 15. Luo, S. P.; Mejia, E.; Friedrich, A.; Pazidis, A.; Junge, H.; Surkus, A. E.; Jackstell, R.; Denurra, S.; Gladiali, S.; Lochbrunner, S.; Beller, M. Photocatalytic water reduction 21

ACS Paragon Plus Environment

Environmental Science & Technology

404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441

with copper-based photosensitizers: a noble-metal-free system. Angew Chem Int Edit 2013, 52 (1), 419-423. 16. Chen, X.; Lu, W.; Xu, T.; Li, N.; Qin, D.; Zhu, Z.; Wang, G.; Chen, W. A bioinspired strategy to enhance the photocatalytic performance of g-C3N4 under solar irradiation by axial coordination with hemin. Appl Catal B-Environ 2017, 201, 518-526. 17. Ke, L.; Luo, L.; Wang, P.; Luan, T.; Tam, N. F.-Y. Effects of metals on biosorption and biodegradation of mixed polycyclic aromatic hydrocarbons by a freshwater green alga Selenastrum capricornutum. Bioresour. Technol. 2010, 101 (18), 6950-6961. 18. Huang, T. L.; Cong, H. B. A new method for determination of chlorophylls in freshwater algae. Environ. Monit. Assess. 2007, 129 (1-3), 1-7. 19. Zander, R.; Lang, W.; Wolf, H. U. Alkaline haematin D-575, a new tool for the determination of haemoglobin as an alternative to the cyanhaemiglobin method. I. Description of the method. Clinica chimica acta; international journal of clinical chemistry 1984, 136 (1), 83-93. 20. Kot-Wasik, A.; Dabrowska, D.; Namiesnik, J. Photodegradation and biodegradation study of benzo[a]pyrene in different liquid media. Photochem. Photobiol. 2004, 168, 109-115. 21. Matile, P.; Hörtensteiner, S.; Thomas, H.; Kräutler, B. Chlorophyll Breakdown in Senescent Leaves. Plant Physiol. 1996, 112, 1403-1409. 22. Suzuki, Y.; Tanabe, K.; Shioi, Y. Determination of chemical oxidation products of chlorophyll and porphyrin by high-performance liquid chromatography. J. Chromatogr. A 1999, 839 (1-2), 85-91. 23. Suzuki, Y.; Shioi, Y. Detection of chlorophyll breakdown products in the senescent leaves of higher plants. Plant Cell Physiol. 1999, 40 (9), 909-915. 24. Llewellyn, C. A.; Mantoura, R. F. C.; Brereton, R. G. Products of chlorophyll photodegradation-2. Structural identification. Photochem. Photobiol. 1990, 52 (5), 10431047. 25. Chen, Z. F.; Ying, G. G.; Jiang, Y. X.; Yang, B.; Lai, H. J.; Liu, Y. S.; Pan, C. G.; Peng, F. Q. Photodegradation of the azole fungicide fluconazole in aqueous solution under UV-254: Kinetics, mechanistic investigations and toxicity evaluation. Water Res. 2014, 52, 83-91. 26. Xu, Z. H.; Jing, C. Y.; Li, F. S.; Meng, X. G. Mechanisms of photocatalytical degradation of monomethylarsonic and dimethylarsinic acids using nanocrystalline titanium dioxide. Environ. Sci. Technol. 2008, 42 (7), 2349-2354. 27. Chen, Y.; Liang, Q.; Zhou, D. N.; Wang, Z. P.; Tao, T.; Zuo, Y. G. Photodegradation kinetics, products and mechanism of timolol under simulated sunlight. J. Hazard. Mater. 2013, 252, 220-226. 22

ACS Paragon Plus Environment

Page 22 of 32

Page 23 of 32

442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480

Environmental Science & Technology

28. Luo, X. Z.; Zheng, Z.; Greaves, J.; Cooper, W. J.; Song, W. H. Trimethoprim: Kinetic and mechanistic considerations in photochemical environmental fate and AOP treatment. Water Res. 2012, 46 (4), 1327-1336. 29. Hippeli, S.; Heiser, I.; Elstner, E. F. Activated oxygen and free oxygen radicals in pathology: New insights and analogies between animals and plants. Plant Physiol. Biochem. 1999, 37 (3), 167-178. 30. Warshawsky, D.; Keenan, T. H.; Reilman, R.; Cody, T. E.; Radike, M. J. Conjugation of benzo[a]pyrene metabolites by freshwater green alga Selanastrum capricornutum. . Chem. Biol. Interact. 1990, 74, 93-105. 31. Warshawsky, D.; Cody, T.; Radike, M.; Reilman, R.; Schumann, B.; LaDow, K.; Schneider, J. Biotransformation of benzo[a]pyrene and other polycyclic aromatic hydrocarbons and heterocyclic analogs by several green algae and other algal species under gold and white light. Chem. Biol. Interact. 1995, 97, 131-148. 32. Melikian, A. A.; Peng, S.; Prokopczyka, B.; El-Bayoumya, K.; Hoffmanna, D.; Wang, X.; Waggoner , S. Identification of benzo[a]pyrene metabolites in cervical mucus and DNA adducts in cervical tissues in humans by gas chromatography massspectrometry. Cancer Lett. 1999, 146, 127-134. 33. Xue, W. L.; Warshawsky, D. Metabolic activation of polycyclic and heterocyclic aromatic hydrocarbons and DNA damage: A review. Toxicol. Appl. Pharmacol. 2005, 206 (1), 73-93. 34. Zhang, Q. Z.; Gao, R.; Xu, F.; Zhou, Q.; Jiang, G. B.; Wang, T.; Chen, J. M.; Hu, J. T.; Jiang, W.; Wang, W. X. Role of Water Molecule in the Gas-Phase Formation Process of Nitrated Polycyclic Aromatic Hydrocarbons in the Atmosphere: A Computational Study. Environ. Sci. Technol. 2014, 48 (9), 5051-5057. 35. Dang, J.; Shi, X.; Hu, J.; Chen, J.; Zhang, Q.; Wang, W. Mechanistic and kinetic studies on OH-initiated atmospheric oxidation degradation of benzo[α]pyrene in the presence of O2 and NOx. Chemosphere 2015, 119 (Supplement C), 387-393. 36. Zhao, N.; Zhang, Q.; Wang, W. Atmospheric oxidation of phenanthrene initiated by OH radicals in the presence of O2 and NOx — A theoretical study. Sci. Total Environ. 2016, 563-564 (Supplement C), 1008-1015. 37. Cody, T. E.; Radike, M. J.; Warshawsky, D. The phototoxicity of benzo[a]pyrene in the green alga Selenastrum capricornutum. Environ. Res. 1984, 35 (1), 122-132. 38. Luo, Y. J.; Dai, J.; Zhong, R. G.; She, Y. B.; Liu, R.; Wei, H. C. Production of Polycyclic Aromatic Hydrocarbon Metabolites from a Peroxynitrite/Iron(Iii) Porphyrin Biomimetic Model and Their Mutagenicities. Environ. Toxicol. Chem. 2011, 30 (3), 723729. 39. Meulenberg, R.; Rijnaarts, H. H. M.; Doddema, H. J.; Field, J. A. Partially oxidized polycyclic aromatic hydrocarbons show an increased bioavailability and biodegradability. FEMS Microbiol. Lett. 1997, 152 (1), 45-49. 23

ACS Paragon Plus Environment

Environmental Science & Technology

481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505

40. Warshawsky, D.; Ladow, K.; Schneider, J. Enhanced degradation of benzo[a]pyrene by Mycobacterium sp. in conjunction with green alga. Chemosphere 2007, 69 (3), 500-506. 41. Rotomskis, R.; Bagdonas, S.; Streckyte, G. Spectroscopic studies of photobleaching and photoproduct formation of porphyrins used in tumour therapy. J Photochem Photobiol B Biol 1996, 33 (1), 61-67. 42. Menezes, P. F. C.; Melo, C. A. S.; Bagnato, V. S.; Imasato, H.; Perussi, J. R. Spectroscopic studies of photobleaching and photoproduct of the photosensitizer photogem during intense illumination. LaPhy 2004, 14 (9), 1214-1218. 43. Jacobs, L. E.; Weavers, L. K.; Chin, Y. P. Direct and indirect photolysis of polycyclic aromatic hydrocarbons in nitrate-rich surface waters. Environ. Toxicol. Chem. 2008, 27 (8), 1643-1648. 44. Clark, C. D.; De Bruyn, W. J.; Ting, J.; Scholle, W. Solution medium effects on the photochemical degradation of pyrene in water. J. Photochem. Photobiol. A: Chem. 2007, 186 (2-3), 342-348. 45. Shang, J.; Chen, J.; Shen, Z. Y.; Xiao, X. Z.; Yang, H. N.; Wang, Y.; Ruan, A. D. Photochemical degradation of PAHs in estuarine surface water: effects of DOM, salinity, and suspended particulate matter. Environ Sci Pollut R 2015, 22 (16), 12389-12398. 46. Guerard, J. J.; Chin, Y. P.; Mash, H.; Hadad, C. M. Photochemical Fate of Sulfadimethoxine in Aquaculture Waters. Environ. Sci. Technol. 2009, 43 (22), 85878592. 47. Tai, C.; Li, Y.; Yin, Y.; Scinto, L. J.; Jiang, G.; Cai, Y. Methylmercury photodegradation in surface water of the Florida everglades: importance of dissolved organic matter-methylmercury complexation. Environ. Sci. Technol. 2014, 48 (13), 73337340.

506 507

24

ACS Paragon Plus Environment

Page 24 of 32

Page 25 of 32

Environmental Science & Technology

508

Figure captions

509 510 511

Figure 1 Effects of chlorophyll a on BaP phototransformation under dark and light conditions. C0 is the initial concentration of BaP, and C is the concentration of BaP at the sampling time.

512 513

Figure 2 Effects of porphyrins on BaP phototransformation under light irradiation

514 515 516 517

Figure 3 (a) Singlet oxygen production in the treatments of porphyrins under light irradiation, (b) Correlation between the singlet oxygen production and the BaP transformation percentage

518 519 520 521 522

Figure 4 Concentration of BaP transformation products in the treatments of (a) control, (b) chlorophyll a, (c) sodium copper chlorophyllin, (d) hematin, (e) pheophorbide a and (f) cobalamin under light irradiation. 1,6-dione, 3,6-dione and 6,12-dione represent BaP-1,6dione, BaP-3,6-dione and BaP-6,12-dione, respectively.

523 524 525

Figure 5 Percentage of BaP residual and its transformation products after treatments with porphyrins. The percentage of BaP-quinone was obtained by the equation:

526

pBaP - quinone (%)=

527 528

BaP-quinone (µg), MWBaP is the molecular weight of BaP (252), MWBaP-quinone is the molecular weight of BaP-quinone (282), and mBaP-initial is the initial mass of BaP (µg).

m BaP −quinone × MW BaP × 100% , where mBaP-quinone is the mass of MW BaP −quinone × m BaP -initial

529 530 531

Figure 6 (a) Effect of pH on BaP phototransformation in chlorophyll a solution after 4 days irradiation, (b) Effect of DOM on BaP phototransformation under light irradiation

532

25

ACS Paragon Plus Environment

Environmental Science & Technology

533

Page 26 of 32

Figure 1

Control-dark Chlorophyll a-dark Control-light Chlorophyll a-light

1.2

1.0

C/Co

0.8

0.6

0.4

0.2

0.0 0

1

2

3

4

5

Time (day) 534 535 536

26

ACS Paragon Plus Environment

6

7

Page 27 of 32

537

Environmental Science & Technology

Figure 2

538 1.2 Control Sodium copper chlorophyllin Hematin Pheophorbide a Cobalamin Chlorophyll a

1.0

C/Co

0.8

0.6

0.4

0.2

0.0 0

1

2

3

4

5

6

Time (day) 539 540 541

27

ACS Paragon Plus Environment

7

Environmental Science & Technology

542

Page 28 of 32

Figure 3

a

b

Control Sodium copper chlorophyllin Hematin Pheophorbide a Cobalamin Chlorophyll a

100

BaP degradation percentage (%)

500

Singlet oxygen (µmol/L)

400

300

200

100

0

y = -0.2378x + 0.1648 r = 0.9261

80

60

40

20

0 0

1

2

3

4

5

6

7

0

Time (day)

100

200

300

400

Singlet oxygen (µmol/L)

543 544

28

ACS Paragon Plus Environment

500

Page 29 of 32

545

Environmental Science & Technology

Figure 4

Concentration (µg/L)

(a) 600

Concentration (µg/L)

(b) 600

500

500

400

400

300

300

200

200

100

100

0

(c) 600

1,6-dione

3,6-dione

6,12-dione

Sodium copper chlorophyllin

0

(d) 600

500

500

400

400

300

300

200

200

100

100

0

(e) 600 Concentration (µg/L)

Day 1 Day 4 Day 7

Control

1,6-dione

3,6-dione

6,12-dione

0

(f) 600

Pheophorbide a

500

500

400

400

300

300

200

200

100

100

0

1,6-dione

3,6-dione

6,12-dione

0

Chlorophyll a

1,6-dione

3,6-dione

6,12-dione

3,6-dione

6,12-dione

3,6-dione

6,12-dione

Hematin

1,6-dione

Cobalamin

1,6-dione

546 547 548

29

ACS Paragon Plus Environment

Environmental Science & Technology

549

Page 30 of 32

Figure 5 BaP-6,12-dione BaP-3,6-dione BaP-1,6-dione BaP 100

Percentage (%)

80

60

40

20

0 D1 D4 D7

Control

D1 D4 D7

D1 D4 D7

Chlorophyll a

Sodium copper chlorophyllin

D1 D4 D7

D1 D4 D7

D1 D4 D7

Hematin

Pheophorbide a

Cobalamin

550 551 552

30

ACS Paragon Plus Environment

Page 31 of 32

Figure 6

b

1.0

1.0

0.8

0.8

0.6

0.6

C/C0

a

C/C0

553

Environmental Science & Technology

0.4

0.4

0.2

0.2

0.0

Pearl River water Pearl River water + chlorophyll a

0.0 2

4

6

8

10

12

0

1

pH

2

3

4

Time (day)

554 555

31

ACS Paragon Plus Environment

5

6

7

Environmental Science & Technology

TOC Art 152x101mm (120 x 120 DPI)

ACS Paragon Plus Environment

Page 32 of 32