Halogenation of Polycyclic Aromatic Hydrocarbons by ...

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Halogenation of Polycyclic Aromatic Hydrocarbons by Photochemical Reaction under Simulated Tidal Flat Conditions a

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Kenshi Sankoda , Kei Nomiyama , Tomonori Kuribayashi & Ryota Shinohara

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Graduate School of Environmental and Symbiotic Sciences , Prefectural University of Kumamoto , Kumamoto , Japan b

Center for Marine Environmental Studies (CMES), Ehime University , Matsuyama , Japan Published online: 30 Apr 2013.

To cite this article: Kenshi Sankoda , Kei Nomiyama , Tomonori Kuribayashi & Ryota Shinohara (2013): Halogenation of Polycyclic Aromatic Hydrocarbons by Photochemical Reaction under Simulated Tidal Flat Conditions, Polycyclic Aromatic Compounds, 33:3, 236-253 To link to this article: http://dx.doi.org/10.1080/10406638.2013.770406

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Polycyclic Aromatic Compounds, 33:236–253, 2013 C Taylor & Francis Group, LLC Copyright ISSN: 1040-6638 print / 1563-5333 online DOI: 10.1080/10406638.2013.770406

Halogenation of Polycyclic Aromatic Hydrocarbons by Photochemical Reaction under Simulated Tidal Flat Conditions Kenshi Sankoda,1 Kei Nomiyama,2 Tomonori Kuribayashi,1 and Ryota Shinohara1 1 Graduate School of Environmental and Symbiotic Sciences, Prefectural University of Kumamoto, Kumamoto, Japan 2 Center for Marine Environmental Studies (CMES), Ehime University, Matsuyama, Japan

Selected six kinds of polycyclic aromatic hydrocarbons (PAHs) (naphthalene: NA, phenanthrene: (PHE), anthracene (ANT), fluoranthene (FLR), pyrene (PYR), and benzo[a]pyrene (B[a]P)) were exposed to natural sunlight under simulated tidal flat conditions, to check possibilities for photochemical productions of halogenated derivatives in the coastal environment. Moreover, 11 molecular descriptors were derived based on PM6 Hamiltonian for each PAH. Photochemical reaction induced productions of various 13 kinds of halogenated derivatives, which include 7 unknown products. In the determined 6 halogenated derivatives, the highest production amount was 6-chlorobenzo[a]pyrene. Moreover, PYR showed the behavior that produced halogenated derivatives significantly higher than its structural isomer of FLR. After the determination of production amounts, correlation between the production amounts of halogenated derivatives and the molecular descriptors of individual PAHs were examined. The energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (EHOMO–LUMO ), which is a measure of chemical stability, showed the strongest significance (p < 0.01) and give enough explanation for differences in the production amounts among structural isomers. PAHs that have small EHOMO–LUMO showed high potential to produce halogenated derivatives. ANT, PYR and B[a]P would have potentially high capability to induce halogenated derivatives in the coastal environment. We further discussed the impact of photochemical reaction on environmental fate of PAHs and halogenated

Received 27 November 2012; accepted 23 January 2013. Address correspondence to Ryota Shinohara, Graduate School of Environmental and Symbiotic Sciences, Prefectural University of Kumamoto, 3-1-100, Tsukide, Higashiku, Kumamoto, 862-8502, Japan. E-mail: [email protected]

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Halogenation of PAHs by Photochemical Reaction PAHs. Congener profiles observed in our previous field surveys seemed to suggest contributions of photochemical reaction to occurrences of Cl-PAHs. Key Words: halogenated polycyclic aromatic hydrocarbons, photochemical reaction, polycyclic aromatic hydrocarbons, structure-property relationship


INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) have various emission sources such as asphalt, usage of fossil fuel, forest fires and oil spill accidents (1–3). Relatively high molecular PAHs have poor solubility, mobility, volatility, and stronger genotoxicity along with resistibility to biodegradation than those of low molecules (2, 4–6). Once emitted to the environment, PAHs are introduced to the aquatic environment through precipitation and runoff. Widespread contamination of PAHs was found in soil (6), sediments (7), airborne particles (8), water (1, 9), benthos (10), and fish as PAHs metabolites (11). Photochemical reaction is a key step for the elimination of PAHs in environments. However, it has also been recognized that exposure of light can greatly enhance the toxicity of some PAHs congeners on organisms such as fish (12), macroinvertebrates (13), and marine diatoms (14). The enhancement of toxicity mainly depends on two processes: photosensitization and photomodification (15–17). Photosensitization can increase production of highly reactive species of singlet oxygen through excitation of PAHs. In photomodification, photochemical products are derived, some of which are more toxic than the parent PAHs (18). Therefore, study of photochemical behavior, photochemical products, and the toxicity of PAHs are a critical issue. Several studies have investigated photochemical behavior and identified photochemical products of acenaphthylene (19), anthracene (ANT) (18, 20), benzo[a]pyrene (B[a]P) (4), benzo[e]pyrene (21), and naphthalene (NA) (22). These studies mainly focused on oxidized photochemical products. However, photochemical productions of halogenated PAHs have been rarely reported compared to those of oxidized products. Halogenated PAHs, which include chlorinated PAHs (Cl-PAHs) and brominated PAHs (Br-PAHs), recently have been detected soils (25, 26), mussels (27), and sediments (27, 28). Although concentrations of halogenated PAHs in environmental samples are generally lower than those of PAHs (26 27), it was shown that 1-monochloropyrene (1-Cl-PYR) and dichloropyrenes (di-ClPYRs) exhibit stronger mutagenicities than pyrene (PYR) in the presence of S9 (29). Moreover, halogenations of PAHs enhance aryl hydrocarbon receptormediated response in rat hepatome cell (H4IIE-luc) assay, depending on a combination of parent PAHs structures and the degree of halogenation (30). The literature emphasizes the importance of investigating distributions, fate, and

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sources of halogenated PAHs in the environments. Incineration and chloralkali processes, as well as automobiles exhaust, are considered to be anthropogenic emission sources of halogenated PAHs in the environment (23, 27). On the other hand, Sugiyama et al. found photochemical inductions of 1-Cl-PYR and di-Cl-PYR from PYR on the surface of metallic oxides containing chloride ion (31). The previous study dealt with only PYR. More recently, the authors found Cl-PAHs in tidal flat sediments from Japanese coast where low contamination was suggested, and also evinced that ANT can produce not only 9,10anthraquinone, but also chlorinated derivatives through photochemical reaction under simulated condition of tidal flats (32). Photochemical reaction to produce halogenated PAHs is likely in tidal flats because accumulated PAHs on the surface of sediments are exposed to strong natural sunlight in the presence of ambient seawater. Understanding environmental behavior of PAHs and halogenated PAHs in tidal flat ecosystems in which can provide rich marine resources are important. However, to the best of our knowledge, possibilities for halogenation of other PAHs by photochemical reaction under tidal flats are not elucidated other than ANT (32). The main purpose of this study is to identify photochemical products of selected PAHs under simulated tidal flat conditions, focusing especially on productions of their halogenated derivatives. Six kinds of PAHs congeners (naphthalene: (NA), phenanthrene (PHE), ANT, fluoranthene (FLR), PYR, and B[a]P) were selected in this study. These PAHs are all listed as priority pollutants by the U.S. EPA. In particular, PHE, FLR, and PYR are frequently detected in Japanese coastal sediments with predominance (10, 27, 32), and B[a]P has high toxicity for human beings and wild lives as International Agency for Research on Cancer (IARC) identified B[a]P as a carcinogen (33). On the other hand, ANT is a photoreactive compound, and can produce toxic oxidized derivatives by photochemical reaction in aqueous phase (18). In addition to identification of photochemical products, relationships between production amounts of halogenated PAHs and calculated molecular descriptors of parent PAHs were discussed in order to give explanations for differences in photochemical behavior among PAHs.

MATERIALS AND METHODS Chemicals and Instrument Sea sands (425–850 μm) which are composed of SiO2 , authentic standards of NA, PHE, ANT, FLR, PYR, and B[a]P were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). The sea sands had been washed by the regent supplier with methanol prior the purchase. The structures of target PAHs are shown in Figure 1. Certified artificial seawater salt was


Halogenation of PAHs by Photochemical Reaction

Figure 1: Structure or target PAHs in this study.

purchased from Marinetech Co., Ltd. (Tokyo, Japan) and dissolved in ultrapure water supplied by a Millipore water system (EMD Millipore corporation, MA, the USA). A mercury UV lamp B-100A that radiates mainly 365 nm was purchased from UVP, LLC (CA, the USA) and used in salt-controlled experiments. A UV intensity meter UV-340 was purchased from Lutron Electronics, Inc. (CA, USA). The standards of 1-Cl-PYR and 1-bromopyrene (1Br-PYR) were purchased from Frinton Laboratories, Inc. (NJ, USA) and Alfa Aesar (MA, USA), respectively. Standards of Cl-PAHs: 9-chloroanthracene (9Cl-ANT), 9,10-dichloroanthracene (9,10-di-Cl-ANT), 3-chlorofluoranthene (3Cl-FLR), 6-chlorobenzo[a]pyrene (6-Cl-B[a]P) were provided by Dr. Ohura, Meijo University, Japan. The standard of 1-chloronaphthalene was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Chrysene-d12 (CHRd12 ) was purchased from Kanto Chemical Co., Inc. (Tokyo, Japan.) In all experiments, PAHs and halogenated PAHs in samples were analyzed with a gas chromatograph HP 6890 (Agilent Technology, CA, the USA) equipped with a mass-spectrometer JMS-700 (JEOL, Tokyo, Japan). The column used was a DB-5 MS purchased from Agilent Technologies. The column length, diameter and thickness of liquid phase were 60 m, 0.25 mm and 0.25 μm, respectively. The electronic ionization source was set at 70 eV. The temperature of the GC/MS interface and the ion source was kept at 280◦ C. Instrumental detection limits for halogenated PAHs were determined from standard deviations of the lowest concentration in the calibration curve.

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Experimental Design


Sunlight Irradiation Experiment for PAHs under Mimic Conditions of Tidal Flats In this study, commercially available sea sands were used as model of adsorbent to simulate tidal flat conditions. Sea sands are supposed to have a certain degree of catalytic effect on the photochemical reaction. However, to discuss catalytic effects of solid phase on production amounts halogenated derivatives are not purpose in this study. We have confirmed that overall similar trends in production amounts of halogenated derivatives can be seen by photochemical reaction in seawater solution (i.e., without sea sands). The target PAHs congeners were dissolved in acetone at a concentration of 500 mg L−1. In order to mimic tidal flats conditions, sea sands (5 g) as model of an adsorbent was spread on petri dishes and PAHs isomers were spiked (25 μg), followed by the addition of artificial seawater (2.5 mL). Immediately, samples were irradiated with the sunlight for 3 h. After the irradiation, sea sands were transferred to centrifugation tubes. PAHs and their photochemical products were extracted with a mixture of dichloromethane and hexane (1:1, v/v) (20 mL) twice. The combined extracts were isolated from sea sands by centrifugation (3,000 rpm, 3 min) and filtration. The extracts were concentrated to 1 mL with a rotary evaporator. Finally, CHR-d12 as an internal standard was spiked to the samples and analyzed with the GC/MS. In the first step for analysis, samples were analyzed by scan mode to obtain mass spectra—an important key in identifying photochemical products. Again, samples were analyzed in selected ion monitoring (SIM) mode to determine the production amounts of fully identified halogenated derivatives and residual levels of PAHs. Since there were no available labeled standards of halogenated PAHs, the production amounts are reported without correction in this study. The monitored ions in SIM mode were shown in Table 1. The irradiation experiments were performed on fine days in August 2010, at a balcony in Prefectural University of Kumamoto, Japan. In this season, general UV intensity in the midday was 1,000–1,200 μW cm−2. In all experimental days, intensities of UV were classified as “very strong”, according to the estimated UV index by Japan Meteorological Agency (34). In fact, relative standard deviations (RSD) of production amounts of halogenated derivatives among repeated samples were below 20%. Therefore, the influences of daily variation of weather on the results are considered within an allowance. No precipitation was observed during the irradiation. The irradiations to NA/B[a]P, ANT/PHE, and FLR/PYR isomers were conducted in same days, respectively, to allow strict comparison between isomer pairs. Light-controlled experiments were conducted to determine losses of PAHs without sunlight and to confirm the absence of halogenated derivatives in the control samples. In the light

Halogenation of PAHs by Photochemical Reaction Table 1: Mass-to-charge ratio of detected halogenated derivatives and corresponding parent PAHs


Compounds (abbreviations) anthracene (ANT) 9-chloroanthracene (9-Cl-ANT) 9,10dichloroanthracene (9,10-di-Cl-ANT) fluoranthene (FLR) 3-chlorofluoranthene (3-Cl-FLR) pyrene (PYR) 1-chloropyrene (1-Cl-PYR) 1-bromopyrene (1-Br-PYR) two unknown dichloropyrene (di-Cl-PYRs) benzo[a]pyrene (B[a]P) two unknown bromobenzo[a]pyrene (Br-B[a]P) 6-chlorobenzo [a]pyrene (6-Cl-B[a]P) two unknown dichlorobenzo [a]pyrene (di-Cl-B[a]P) unknown bromochlorobenzo [a]pyrene (Br-Cl-B[a]P)

Formulas

Characteristic ions (m/z )

C14 H10 C14 H9 Cl

178 (M+) 212 (M+) (35Cl)

C14 H8 Cl2

246 (M+) (35Cl, 35 Cl)

C16 H10 C16 H9 Cl

202 (M+) 236 (M+) (35Cl)

238 (M+ +2) (37Cl)

C16 H10 C16 H9 Cl

202 (M+) 236 (M+) (35Cl)

238 (M+ +2) (37Cl)

C16 H9 Br

280 (M+) (79Br)

282 (M+ +2) (81Br)

C16 H8 Cl2

270 (M+) (35Cl, 35 Cl)

272 (M+ +2) (35Cl, 37Cl)

C20 H12

252 (M+)

C20 H11 Br

330 (M+) (79Br)

332 (M+ +2) (81Br)

C20 H11 Cl

286 (M+) (35Cl)

288 (M+ +2) (37Cl)

C20 H10 Cl2

320 (M+) (35Cl, 35 Cl)

322 (M+ +2) (35Cl, 37Cl)

C20 H10 BrCl

364 (M+) (35Cl, 79Br)

366 (M+ +2) (35Cl, 81Br)

214 (M+ +2) (37Cl) 248 (M+ +2) (35Cl, 37Cl)

control test, PAHs were spiked on the sea sand and kept for 3 h under the dark condition. The mean recovery rates (± standard deviation) of NA, PHE, ANT, FLR, PYR, and B[a]P in the control tests were > 5%, 38% (±2.6), 82% (±6.2), 53% (±3.9), 61% (±5.0), and 73% (±5.4), respectively. Residual levels of PAHs in the irradiated samples were then corrected by the mean recovery rates in control experiments, and the corrected values are reported as survival rates (which mean relative residual concentrations of PAHs in irradiated samples to the initial spiked amounts) in this study. Moreover, salt-controlled experiments were performed to check the effects of seawater, using the UV lamp B-100A as a light source (1,200 μW cm−2) and ultrapure water. Halogenated derivatives were detected neither in the light-controlled nor the salt-controlled

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K. Sankoda et al. Table 2: Molecular descriptors of selected PAHs


Descriptor’s name: abbreviation (unit)

NA

PHE

ANT

FLR

PYR

B[a]P

168

227

249

321

257

328

1332

1849

1849

2094

2094

2611

6844

11206

11087

13491

13637

18866

5511

9357

9238

11397

11543

16255

167

210

213

229

224

267

165

219

220

245

241

294

8.88

8.77

8.37

8.79

8.35

8.18

0.40

0.53

0.96

0.99

1.00

1.20

8.48

8.24

7.41

7.80

7.34

6.98

EHOMO+LUMO (eV)

9.28

9.29

9.32

9.78

9.35

9.37

Molecular weight: MW (Da)

128

178

178

202

202

252

Heat of formation: Hf (kJ mol−1) Total energy: TE (−eV) Electronic energy: EE (−eV) Core-core repulsion: CCR (eV) Cosmo area: ˚ 2) CA(A Cosmo volume: ˚ 3) CV (A Energy of HOMO: EHOMO (−eV) Energy of LUMO:ELUMO (−eV) EHOMO−LUMO (eV)

samples, implying that the effects of impurities in PAHs standards on results are ignorable. Relationship between Halogenation and Molecular Descriptors Basic molecular descriptors of individual parent PAHs were calculated by Molecular Orbital PACkage (MOPAC) 2009 and 2012. The PM6 Hamiltonian was calculated for obtaining the optimum geometries of parent PAHs structures. Eleven molecular descriptors (Table 2) were calculated for clarification of the relationship between geometries and production amounts of halogenated derivatives. An important provision on finding molecular descriptors related to toxicity or property of chemicals is not required special software and devices for their calculations. In view of this, the selected eleven descriptors can be easily derived by means of ordinary personal computers. Welch’s t test and spearman’s rank correlation test were performed with GraphPad Prism version 5 (GraphPad Software, Inc., California, USA) in order to examine. Spearman’s rank correlation test was applied because there were curved relationships between production amounts and molecular descriptors. Level of significance was defined as p values bromine (273 kcal/mol) > iodine (241 kcal/mol) (39). Although bromine radicals have lower reactivity than chlorine radicals, the low ionization energy makes their radicalization easier. On the other hand, production probability of chlorine radicals is low according to the relatively higher ionization energy. A significant difference in the production amounts was between 3-Cl-FLR and 1-Cl-PYR (p < 0.01), although their parent compounds (FLR and PYR) showed similar survival rates (Figure 2). In addition, FLR produced only one halogenated derivatives while four kinds of derivatives were produced from PYR. Similar results can be seen in another isomer pair of PHE and ANT. Two kinds of halogenated derivatives (9-Cl-ANT and 9,10-di-Cl-ANT) were produced from the ANT irradiated sample, while any halogenated derivatives were not detected in the PHE irradiated sample. These results also show that ANT has greater capability for production of halogenated derivatives than PHE. From these comparisons, it was shown that photochemical behaviors to produce halogenated derivatives are completely different among PAHs congeners, even among isomers.

Relationship between Molecular Descriptors Eleven molecular descriptors (Table 2) were calculated to find any correlations between the production amounts of halogenated derivatives and molecular descriptors. The production amounts of halogenated derivatives were added to obtain yields. The yields of halogenated derivatives under sunlight irradiation are presented in Figure 4. Only the determinable-halogenated derivatives were considered in the calculation of yields (i.e. unknown compounds were not included into the yields). The order of average yields was as follows: B[a]P (1.3%), PYR (0.4%), ANT (0.014%), and FLR (0.006%), respectively. For calculation, the yields of halogenated derivatives of NA and PHE were set to be one tenth of the yield of halogenated FLR. No significant correlations (spearman’s rank correlation: p > 0.05) were found between eight molecular descriptors of individual parent PAHs and the yields of halogenated derivatives (Figure 5). These descriptors were insufficient to explain differences in the yields of halogenated derivatives among structural isomers. In contrast, significant correlations with the yields were found in molecular descriptors of the energy of the highest occupied molecular

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Figure 4: Yields of halogenated derivatives.

orbital (EHOMO ) (p < 0.05), the lowest unoccupied molecular orbital (ELUMO ) (p < 0.05) and the energy gap between HOMO and LUMO (EHOMO−LUMO ) (p < 0.01) (Figure 6). In addition, the three molecular descriptors showed higher correlation factors with −0.899, 0.928, and −0.986, for EHOMO, ELUMO , and EHOMO−LUMO , respectively. In particular, EHOMO−LUMO showed the strongest significance and correlation factor in the molecular descriptors (Figure 6). The relationship between EHOMO−LUMO and the yields of halogenated derivatives described a trend that in photochemical reaction, PAHs that have low EHOMO−LUMO possess higher potential to produce halogenated derivatives than PAHs that have high EHOMO−LUMO .

Discussion In this study, six types of PAHs congeners were irradiated with sunlight under simulated conditions of tidal flats. ANT, FLR, PYR, and B[a]P produced detectable halogenated derivatives as a consequence of photochemical reaction (Table 1). In contrast, no halogenated derivatives were detected in NA and PHE irradiated samples. Since it is possible that any halogenated derivatives of NA had been produced and decomposed during photochemical reaction, such unstable intermediates would not remain in the environment. On the other hand, as for PHE that showed similar survival rate to ANT, considering that two kinds of halogenated derivatives were produced from ANT, PHE’s potential for conversion into halogenated derivatives can be concluded as low. Correlation between production amounts and eleven molecular descriptors of individual PAHs was investigated after determination of production amounts. Although molecular weight (MW) was not a significant descriptor, PAHs that have high MW tended to yield more abundant halogenated derivatives. The molecular descriptor that showed the strongest significance and correlation factor with the production amounts of halogenated derivatives was EHOMO−LUMO (Figure 6). Intrinsically, EHOMO−LUMO relates to the absolute hardness that is an important descriptor identifies stability of chemicals



Figure 5: Correlation between eight molecular descriptors and yields of halogenated derivatives.

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Figure 6: Correlations between energy of HOMO(EHOMO ), energy of LUMO(ELUMO ) and EHOMO−LUMO , and yields of halogenated derivatives. Yields of halogenated derivatives of NA and PHE were set to one-tenth of the yield of halogenated FLR for calculation.

(40). This means that molecules of low EHOMO−LUMO are more reactive than the high EHOMO−LUMO molecules (40, 41), especially in radical reactions (42). Moreover, in general, compounds that have low EHOMO−LUMO are easily excited from ground state to excited states. Excitation of PAHs would be a key to produces PAHs cation radials that further react with bromide ion and chloride ion (32, 43). On the other hand, higher EHOMO−LUMO chemicals such as NA and PHE have UV absorption bands at short wavelength regions. It would make their excitation and eventually halogenation more difficult. These concepts on the EHOMO−LUMO support our obtained results in that: (1) higher yields of halogenated B[a]P and PYR; and (2) the absence of those in NA and PHE were observed. According to current organic chemistry, it is generally accepted that EHOMO−LUMO decrease with the expanse of the π-conjugated system of a molecule, as shown in Figure 7. Among U.S. EPA listed 16 PAHs, EHOMO−LUMO is the highest (8.481 eV) in NA, and B[a]P was the lowest (6.978 eV). Although previous studies have reported that EHOMO−LUMO have connections to



Figure 7: Correlation between molecular weight and EHOMO−LUMO of 16 PAHs.

photo-induced toxicity and half-life of PAHs under the sunlight irradiation (17, 44), relationship between halogenation and EHOMO−LUMO are reffered in this study for the first time. However, reasons for dramatic increase of the production amounts with EHOMO−LUMO is not known at the present time. The results suggested that potential to produce halogenated derivatives of ANT, PYR, and B[a]P were middle to high among the EPA listed PAHs congeners, while those of high EHOMO−LUMO and low MW congeners such as acenaphthylene (EHOMO−LUMO : 8.095 eV; MW: 152 Da) are relatively low. Here, we further discuss the impact of photochemical reaction on environmental fate of PAHs and halogenated PAHs. We have previously investigated residual levels of 15 PAHs and 20 Cl-PAHs in sediments collected from low contaminated tidal flats (32). The 2-monochloroanthracene (2-Cl-ANT), 9,10di-Cl-ANT, and 1-Cl-PYR were found at detectable levels in the sediments with the predominance of 9,10-Cl-ANT, while 6-Cl-B[a]P was not detected. The detections of 9,10-di-Cl-ANT and 1-Cl-PYR in tidal flats sediments partially consist with the result in the present study, suggesting that photochemical reaction contribute to Cl-PAHs profile as a natural source. However, given that the Cl-PAHs detected in sediments were produced by photochemical reaction, there is a mismatch in congener profile because PYR and B[a]P yielded higher halogenated derivatives than ANT in the present study. Chemical stability of photochemical products could be an explicable reason for this gap since sediments are exposed to sunlight continuously. The 6-Cl-B[a]P has been reported to undergo rapid degradation during light irradiation among Cl-PAH congeners (23). Indeed, EHOMO−LUMO of 6-Cl-B[a]P (6.839 eV) is relatively low, which supports its instability against chemical reactions. On the other hand, the predominance of 9,10-Cl-ANT in sediments cannot be well explained by alone EHOMO−LUMO because 1-Cl-PYR (EHOMO−LUMO : 7.215 eV) is suggested to

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be more stable than 9,10-di-Cl-ANT (EHOMO−LUMO : 7.05 eV). However, 1-ClPYR has shown to have higher degradability than other Cl-PAHs, as with 6-ClB[a]P (23, 45). Therefore, 9,10-di-Cl-ANT could be persistent than other photochemical products in tidal flats sediments. Moreover, various possibilities, including other unrecognized environmental change processes (e.g., microbial degradation) and anthropogenic sources that contribute to the predominance of 9,10-di-Cl-ANT should be considered in future studies. To conclude, the present study revealed that ANT, PRY and B[a]P have high potentials to produce halogenated derivatives through photochemical reaction under tidal flats condition. Moreover, this is the first study referred to the relationship between halogenation efficiency of photochemical reactions and EHOMO-LUMO of individual PAHs. Congener profile of Cl-PAHs in tidal flats suggests contribution of photochemical reaction to occurrences of Cl-PAHs. Halogenation of PAHs by photochemical reaction could be more serious when massive PAHs accumulations are caused by sudden oil spill accidents. In order to clarify some remaining issues, continued field and mechanistic surveys are needed.

ACKNOWLEDGMENTS The authors thank Dr. Ohura (Meijo University, Japan) for use of standard references of Cl-PAHs. The authors also thank Professor Jay Melton (Prefectural University of Kumamoto, Japan) for his review of English style and usage.

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