Photodegradation of 2,2′,4,4′-tetrabromodiphenyl ether in nonionic surfactant solutions

Chemosphere 73 (2008) 1594–1601

Contents lists available at ScienceDirect

Chemosphere journal homepage: www.elsevier.com/locate/chemosphere

Photodegradation of 2,20 ,4,40 -tetrabromodiphenyl ether in nonionic surfactant solutions Xue Li, Jun Huang, Lei Fang, Gang Yu *, Hui Lin, Lining Wang Department of Environmental Science and Engineering, POPs Research Center, Tsinghua University, Beijing 100084, PR China

a r t i c l e

i n f o

Article history: Received 21 July 2008 Received in revised form 21 August 2008 Accepted 25 August 2008 Available online 7 October 2008 Keywords: PBDEs e-Waste disposal sites Soil remediation Photodegradation Nonionic surfactants

a b s t r a c t Recently, polybrominated diphenyl ethers (PBDEs) contaminated soils in electronic waste (e-waste) disposal sites of China have been reported and aroused concern. Since surfactant-aided soil washing followed by photodestruction has been suggested as a promising technology for soil remediation, the photodegradation of 2,20 ,4,40 -terabromodiphenyl ether (BDE-47) in nonionic surfactant solutions under UV-irradiation at 253.7 m was studied in the present work. The investigation was carried out on the effect of the type and concentration of surfactants, initial pH, and the addition of cations and acetone on BDE-47 photodecay. BDE-47 photodegradation all well followed pseudo-first-order kinetics under various conditions, and the decay rate was 0.4–1.3 times greater in the presence of surfactants than that in water alone. It was found that BDE-47 photodegradation was hardly affected through varying initial pH levels and retarded due to the presence of cations and acetone. The primary decay pathways in all surfactant micelles were shown to be reductive photodebromination, in which tri- to mono-BDEs were formed through a consecutive loss of bromine atoms. 2,4,40 -tribromodiphenyl ether (BDE-28), 4,40 -dibromodiphenyl ether (BDE-15), and 4-monobromodiphenyl ether (BDE-3) appeared sequentially as the predominant photoproducts over reaction time. Polybrominated dibenzofurans (mono- to tri-) were tentatively identified as additional intermediates for BDE-47 photodegradation in surfactant micelles. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Polybrominated diphenyl ethers (PBDEs), a subgroup of brominated flame retardants, have been widely used in various commercial products such as plastics, textiles, surface coatings, foams, and man-made fibers to reduce the hazards of fire. Since PBDEs are physically added into the materials, they can easily leach into the environment during the production, application, as well as processing and recycling or combustion ( D’Silva, 2004; Kim et al., 2006). PBDEs have become a worldwide concern in recent years mostly due to their frequent detection and increasing concentration in the environment. These congeners also bear analogy to the long banned polychlorinated biphenyls (PCBs) in physicochemical properties. With regard to the environmental occurrence of PBDEs, soil may serve as a major reservoir and/or source, from which PBDEs may transfer into other organisms in terrestrial food webs and finally lead to human exposure (Sellström et al., 2005; Zou et al., 2007). Recently, high levels of PBDEs in soils, which were caused by recycling and disposal operations of electronic waste (e-waste), have been reported at Guiyu in China and aroused extensive concern (Wang et al., 2005; Leung et al., 2006, 2007; Liu et al., * Corresponding author. Tel.: +86 10 62787137; fax: +86 10 62794006. E-mail address: [email protected] (G. Yu). 0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.08.031

2008). In the place involved in acid leaching of shredded printed circuit boards, 2720–4250 lg kg1 dry weight (dw) of PBDEs (mono- to deca-) have been found, which were extremely higher than those from the control site (2.0–6.2 lg kg1 dw) (Leung et al., 2007). Moreover, high levels of PBDEs (mono- to hepta-) have also been mentioned for soil samples collected from the burnt plastic dump site (1140 lg kg1 dw) and print roller dump site (1169 lg kg1 dw) in the same region (Leung et al., 2006). Consequently, PBDEs-contaminated soils have become a site-specific environmental problems in the e-waste disposal sites, and since PBDEs have proved to be associated with developmental neurotoxicity and other adverse effect on living organisms (Costa and Giordano, 2007; Henrik et al., 2007; Hu et al., 2007), these PBDEs-contaminated soils may also pose potential negative effect on the local ecosystem and health of residents. Photodegradation has been regarded as the main degradation pathway for PBDEs environmental transformation. However, PBDEs showed much longer photodegradation half-lives when sorbed onto natural solid matrices, such as soil, sediment, or clay minerals (Palm et al., 2004; Söderström et al., 2004; Ahn et al., 2006). With contaminated samples prepared in the lab, the halflife of BDE-209 on soil under UV-irradiation (300–400 nm) was estimated to be 150–200 h, at least 600 times greater than that in toluene (Söderström et al., 2004); under the same experimental conditions, no photolytic breakdown of PBDEs (tetra- to deca-) was

X. Li et al. / Chemosphere 73 (2008) 1594–1601

observed on environmental soils after 21 d irradiation, and it was speculated to be the soil aging effect that may encapsulate and shield PBDEs from effective photodegradation (Alexander, 2000). UV destruction of halogenated organic contaminants in surfactant solutions has been suggested to be an important process with regard to the soil remediation (Chu and Jafvert, 1994; Chu, 1999; Chu and Choy, 2000; Chu and Kwan, 2002; Itoh et al., 2005; Jia and Chu, in press). The halogenated organics have been reported to be effectively mobilized from soils into surfactant solutions (Wu and Marshall, 2001; Ehsan et al., 2006) and successfully decomposed under UV-irradiation at 253.7 nm. In fact, the photodegradation of halogenated organics solubilized within surfactant micelles has several attractive advantages: (i) the hydrophobic organic contaminants can be separated from soils and thus become available for the subsequent treatment; (ii) the contaminants can be directly decomposed in aqueous solutions through which the surfactants may be recovered and/or reused; and (iii) Quantum yield of photodecomposition can be increased because surfactant micelles can act as effective hydrogen donors. Furthermore, from an engineered remediation perspective, nonionic surfactants are preferable to either anionic or cationic surfactants due to the following reasons: (i) they are more biodegradable and less toxic (Van der Meeren and Verstraete, 1996; Jin et al., 2007); (ii) they have lower critical micelle concentration (CMC) and thus form into micelles at lower surfactant concentration; and (iii) they are less adsorptive on natural soils (Edwards et al., 1991). Itoh et al. (2005) observed that photodecay of 1,3,5-trichlorobenzene in surfactant solutions irradiated by a 500-W high pressure mercury lamp was improved by various types of surfactants in the following order: nonionic > anionic > cationic; in addition, nonionic surfactants can also decrease the yield of more toxic byproducts as 1,2,3,5-tetrachlorobenzene and 2,30 ,4,50 -tetrachlorobiphenyl when compared with anionic and cationic surfactants. The photodecay of hexachlorobenzene (Chu and Jafvert, 1994) or 4,40 -dichlorobiphenyl (Chu and Kwan, 2002) irradiated by phosphor-coated low-pressure lamps at 253.7 nm was also found to be improved more significantly by nonionic surfactants than by anionic ones. As a proposed remediation strategy for PBDEs-contaminated soils, photodegradation of PBDEs in nonionic surfactant micelles has been rarely investigated. Thus, in the present study, PBDEs photodegradation was explored in three commonly used nonionic surfactants under various conditions. BDE-47 was selected as the probe compound due to its environmental abundance and recalcitrance to the natural degradation (Sellström et al., 2005; Cetin and Odabasi, 2007; Leung et al., 2007). With an investigation on the photokinetics and photoproducts of BDE-47, the photochemical characteristics and mechanisms of the reaction were discussed and proposed. 2. Materials and methods 2.1. Materials 50 mg L1 of BDE-47 in isooctane and 100 mg L1 of hexachlorobenzene (used as an internal standard) were purchased from AccuStandard Inc., USA. A stock solution of 39 PBDE congeners (BDE-AAP-A-15X) was also obtained from AccuStandard Inc., USA, which contained three mono-BDEs, seven di-BDEs, eight triBDEs, six tetra-BDEs, seven penta-BDEs, five hexa-BDEs, and three hepta-BDEs, with their concentrations ranging from 1.5 mg L1 to 3.75 mg L1. The organic solvents (hexane, methanol, and acetone) were HPLC grade, and provided by Dima Technology Inc. (USA), Fisher Scientific Inc. (USA), or J.T. Baker Inc. (USA). Nonionic surfactants of Brij 35, Brij 58, and Tween 80 were supplied by Amresco (USA), Sigma (USA), and Beijing Yili Fine Chemical Co., Ltd. (China), respectively. All three surfactants in the present work were com-

1595

monly used for photodegradation of halogenated organics in previous studies (Chu and Jafvert, 1994; Chu and Choy, 2000; Jia and Chu, 2008). Some of their physical and chemical properties were listed in Table S1. Inorganic acid/base of analytical grade, including sulfuric acid and sodium hydroxide were used to adjust the initial pH of solutions to the expected levels. Other inorganic chemicals involved in the experiment were also of analytical grade. All chemicals were used without further purification. Surfactant solutions were prepared with purified water using a Milli-Q water purification system (with the resistivity greater than 18.2 MX cm). 2.2. Methods All photochemical experiments were carried out using an RPR-200 Rayonet photochemical reactor (Southern New England Ultraviolet, USA). In each experiment, 50 mL of aqueous solution containing BDE-47 were introduced into a quartz tube (29 mm i.d./32 mm o.d.  15 cm) subsequently irradiated with two lowpressure mercury lamps (253.7 nm dominant). The intensity of incident light was determined by a chemical actinometer, potassium ferrioxalate, contained in a quartz tube under UV-irradiation at 253.7 nm. The intensity of incident UV light was generally around 2.28  107 Einstein L1 s1. An aliquot was removed from the irradiated solution into a 15mL test tube at specified times, 4 mL of which was then quickly pippeted into a 16-mL brown vial sealed with a Teflon-lined cap after adding the internal standard. BDE-47 and its organic photoproducts were extracted by vigorously mixing the aqueous solution with 4 mL of hexane, and the mixtures were left in the dark for 4 h after adding sodium chloride as emulsion breaker. Sequentially, the organic phase at the top layer was transferred into a small tube, and the whole operation was repeated with another 2 mL of hexane. The two hexane volumes were combined together, and finally concentrated to the volume of 1 mL under a gentle stream of high purity nitrogen at 40 °C. The extraction efficiency by this method was greater than 80% for each surfactant. The dark control experiment was conducted in the surfactant solution of Brij 35, from which unexposed samples were collected over the same time periods. Photodegradation experiments were also conducted in purified water and hexane for comparison. Under all conditions studied, the initial concentration of BDE-47 did not exceed its solubility limit. Generally, BDE-47 and its photoproducts were analyzed on a 15 m DB-5 capillary column (i.d. = 0.25 mm, film thickness = 0.25 lm) installed on an Agilent 6890 GC equipped with a micro electron capture detector (lECD), and quantified by comparing retention times and peak areas with those of analytical standards. Table S2 contained the reaction times and some other information for those PBDE standards available for identification of BDE-47 and its possible PBDE photoproducts. The oven temperature was programmed from 60 °C (held for 2 min) to 110 °C (held for 5 min) at a rate of 30 °C min1, to 200 °C (held for 5 min) at a rate of 3 °C min1, and to a final temperature of 300 °C (held for 15 min) at a rate of 2 °C min1. The temperatures of the splitless injector and the detector were both set at 280 °C. PBDEs were also confirmed by GC–MS analysis with a Trace DSQ quadrupole mass spectrometry and Trace gas chromatography (Thermo Finnigan, USA) equipped with a 15 m DB-5MS capillary column (i.d. = 0.25 mm, film thickness = 0.25 lm). The oven temperature program was identical to that of GC–ECD. The ion source and transfer line were kept at 250 °C and 300 °C, respectively. Electron impact (EI) mass spectra were recorded in full scan (m/z = 60–500) and selective ion monitoring mode (SIM), both of which were compared to those obtained from the standards. The possible photoproducts as polybrominated dibenzofurans (PBDFs) were tentatively identified referring to the mass spectrograms of

1596

X. Li et al. / Chemosphere 73 (2008) 1594–1601

of BDE-47 in nonionic surfactant solutions of 4.17  104 M Brij 35, 4.45  104 M Brij 58 and 4.20  104 M Tween 80 was presented in Fig. 1a, and about 95% of BDE-47 has been degraded in all surfactant solutions after 75 min UV-irradiation. Since photodecay of BDE-47 in surfactant solutions all well followed pseudofirst-order kinetics over reaction time as shown in Fig. 1a, the photodecay rate constants were derived from the slope of a straight-line function resulted from regression of ln (Ct/C0) versus t. The quantum yield for BDE-47 photodegradation was calculated by Eq. (2), and found to be 0.6–1.3 times greater in the presence of surfactant micelles than that in water alone, in which 4.20  104 M Tween 80 showed the best enhancing performance (Table 1). As no additional hydrogen sources were added into the solutions, the possible hydrogen sources to accelerate the photodegradation reaction rate must be the surfactants themselves, such as the polyethylene oxide (PEO) chains and/or hydrocarbon moieties (Shi et al., 1997); in addition, regarding the different structures of surfactant hydrophobic chains, surfactants of different types will have dissimilar ability as the hydrogen sources and result in the different rate enhancement. Considering that the desorption of hydrophobic organic compounds from contaminated soils would be more effective by surfactant solutions at higher concentration (Edwards et al., 1991; Zhu and Feng, 2003; Zhou and Zhu, 2007), kinetics study of BDE47 photodecay was further conducted at higher surfactant concentration. Surfactant concentration was selected within the range that the micellar aggregation number of each surfactant remained unchanged (Rangel-Yagui et al., 2005). Similarly, the photodecay of BDE-47 in surfactant solutions at higher concentration also

mono- to tri-BDFs presented in previous studies (Watanabe and Tatsukawa, 1987; Sánchez-Prado et al., 2005). Pseudo-first-order decay of BDE-47 is expected at constant temperature, light intensity, and illumination wavelength

C t ¼ C 0 ekp t

ð1Þ

where Ct and C0 are the concentration of BDE-47 at t and time zero (M), respectively, and kp is the first-order decay rate constant (s1). The quantum yield for BDE-47 photodecay under the monochromatic light source is calculated by

Up ¼

kp 2:303I0;k ep ;k l

ð2Þ

where Up is the quantum yield, I0,k is intensity of the incident light at 253.7 nm (Einstein L1 S1), ep,k is the molar absorptivity at 253.7 nm (L mol1 cm1), and l is the cell path length (cm) (Chu and Jafvert, 1994). The molar absorptivity of BDE-47 in methanol is measured at 254 nm and applied to the quantum yield calculation.

3. Results and discussion 3.1. Selection of surfactants The presence of surfactant micelles has been reported to both increase the solubility and the decay rate of the probe compounds. As a result, a proper selection of surfactants is necessary for the application of this remediation technology. The photodegradation

10

0

0.04

4

-1

6

-2

4

Absorbance

8

kp×10 (s )

-1

ln (Ct/C0)

0.06

Dark control Water Brij 35 Brij 58 Tween 80

0.02

-3

0

20

40

60

80

0

100

2

4

6

Time (min)

8

10

12

pH

8

0.4

10

0.3

0.1

-1

0.3

6 0.2

4

4

4

k p × 10 (s )

0.2

-1

kp×10 (s )

6

Ab sorbance

8

4 0.1

2

2

0.0

+

0.02 M Na

+

0.05 M Na

Cation

0.00

+

0.05M K

0 0.000

Absorbance

-4

2

0.0 0.005

0.010

0.015

0.020

Concentration of acetone (M)

Fig. 1. (a) Pseudo-first-order photodecay of BDE-47 in each solution irradiated at 253.7 nm; (b) the trend of photodecay rate constants of BDE-47 and UV absorption of surfactant solutions at 253.7 nm at different initial pH levels; (c) the trend of photodecay rate constants of BDE-47 and UV absorption of surfactant solutions at 253.7 nm in the presence of 0.02 M Na+, 0.05 M Na+, and 0.05 M K+; (d) the trend of photodecay rate constants of BDE-47 and UV absorption of surfactant/acetone system at 253.7 nm at various doses of acetone.

1597

X. Li et al. / Chemosphere 73 (2008) 1594–1601 Table 1 Photodecay rate constants and quantum yields for BDE-47 photodegradation in aqueous solutions Solution Water Brij 35 Brij 58 Tween 80 a

Surfactant concentration (M) – 4.17  104 1.67  103 4.45  104 1.78  103 4.20  104 1.68  103

Micellar concentration (M)a – 6.45  106 3.77  105 5.21  106 2.43  105 6.38  106 2.81  105

BDE-47 initial concentration (M) 9

kp (s1)

ep,k

Up

r2

2130

0.16 0.25 0.26 0.24 0.23 0.37 0.08

0.988 0.999 0.998 0.999 0.999 0.998 0.968

4

9.80  10 1.50  108 7.58  108 1.44  108 5.82  108 1.30  108 4.75  108

5.12  10 7.97  104 8.33  104 7.92  104 7.33  104 1.19  103 2.44  104

The micellar concentration was calculated as follows: (surfactant concentration – CMC)/Naggr.

followed the pseudo-first-order kinetics, and the decay rate constants and quantum yields were summarized in Table 1. About 95% of the starting BDE-47 has been photodegraded after 70 min irradiation in Brij 35 and Brij 58, and the decay rate constants have not changed markedly when surfactant concentration was three times larger. One reason for such result might be that BDE-47 was solubilized into surfactant micelles rather than the aqueous phase, and therefore the reaction conditions were roughly unvaried even though the concentration of micelles has increased in aqueous solutions. In contrast, photodegradation of BDE-47 was significantly inhibited in the presence of higher concentration of Tween 80. Only 64% of BDE-47 has been converted after 70 min irradiation, and the quantum yield decreased from 0.37 to 0.08. Similar retardation effect on trichloroethylene photodegradation by the overdose of Brij 35 have also been reported in a recent study (Jia and Chu, in press), and the reason for this negative effect has been supposed to be resulted from the light attenuation or the viscosity increase by higher concentration of surfactant. As a result, both of the UV absorption and the viscosity were measured for surfactant solutions. From the UV absorbance provided in Fig. S1, Tween 80 exhibited a strong absorption at 253.7 nm. The UV absorbance of 1.68  103 M Tween 80 was 8.5 times higher than that of 4.20  104 M Tween 80; and on the other hand, the viscosity of surfactant solutions was comparable to that of purified water, indicating the retardation effect on BDE-47 photodegradation was mostly due to the light attenuation by higher concentration of Tween 80. As a result, in regard to the better performance of Brij 35 in a wide range of surfactant concentrations, it was thus selected exclusively for the following studies.

CMC of PEO nonionic surfactants can be decreased in the presence of cations. Thus, the increase in UV absorption of the solution at initial pH of 12.24 was supposed to be caused by the existence of the sodium cations at higher concentration. Therefore, experiments have been further conducted to confirm the potential effect of cations on BDE-47 photodegradation. As expected, the photodecay rate of BDE-47 in the presence of 5.00  102 M sodium cations and 5.00  102 M potassium cations slowed down to 67% and 47% of that in the presence of 1.74  102 M sodium cations, respectively; correspondingly, the UV absorption of individual solutions increased in the order: 0.05 M potassium cations > 0.05 M sodium cations > 0.02 M sodium cations, negatively related to the variation of photodecay rate constants (Fig. 1c). This phenomenon was reasonable since it has been mentioned that potassium cations were more effective than sodium cations in decreasing the CMC of PEO surfactants, and the aggregation number of the surfactant may be changed (Rosen, 2004; Rangel-Yagui et al., 2005). As a result, the existence of cations rather than the initial pH levels was noteworthy for PBDEs photodegradation during the treatment process. 3.3. Photodecay of BDE-47 in surfactant/acetone system The presence of organic solvents can enhance the photodegradation of halogenated organics in surfactant micelles by acting as a hydrogen donor or a photosensitizer. Acetone is a commonly studied additive and its triplet energy is 78 kJ mol1 (Yip et al., 1972; Chu and Jafvert, 1994; Chu and Kwan, 2002). Therefore, BDE-47 photodegradation in 1.67  103 M Brij 35 has also been investigated with the addition of various doses of acetone in the present work. Fig. 1d showed the trend of BDE-47 photodecay, in

3.2. Effect of initial pH and cations 80 Total PBDEs BDE-47 BDE-28 BDE-17

60

Concentration (nM)

Since photodecomposition of halogenated organics in surfactant solutions can be influenced by initial pH, the photodecay of BDE-47 in Brij 35 (1.67  103 M) was investigated at different initial pH levels. The trend of BDE-47 decay rate was shown in Fig. 1b, in which the decay rate constants varied slightly when the initial pH value increased from 3.40 to 9.17; however, the rate constant decreased to 7.46  104 s1 when the initial pH of the solution continually increased to the value of 12.24. No obvious relationship was observed between the photodegradation rate and the initial pH levels. Additionally, it was found that the decline in BDE-47 photodecay rate was corresponding to an increase in UV absorption of the solution at initial pH level of 12.24. Therefore, this retardation effect on BDE-47 photodegradation at high initial pH level was resulted from the reduction in light availability. Furthermore, at initial pH level of 12.24, the concentration of sodium hydroxide was approximately 1.74  102 M, which was three orders of magnitude greater than that in the solution at initial pH level of 9.17. It has been reported that the existence of cations such as sodium can affect the physical and/or chemical properties of micelles in surfactant solutions (Rosen, 2004; Rangel-Yagui et al., 2005), e.g., the

40

BDE-15 BDE-8 BDE-7/4 BDE-3 BDE-1

20

0 0

20

40

60

80

Time (min) Fig. 2. Photodecay of BDE-47 and its decayed homologues (mono- to tri-BDEs) within 1.67  103 M Brij 35 at 253.7 nm (lines only indicate trends in the data).

1598

X. Li et al. / Chemosphere 73 (2008) 1594–1601

120

-Br2 248

7000

Relavtive abundance, 100%

100

(a) m/z: 402, 404

6000

BDE-28

Response

5000 4000

Tri-BDE

-COBr3 139

80 60

+

M

406

40 20 0 120 100

-COBr3 137

BDE-17

Tri-BDF

60

-COBr2 -Br -COBr -Br 218 246 2 297 326

20 0 100

2000

M 404

80 40

3000

+

Tri-BDF

150

200

250

300

350

400

450

m/z 1000 0 28

29

30

31

32

33

Retention time (min) +

120 100

M 328

Di-BDE

80

Relative abundance, %

60

12000

(b) m/z: 324, 326 10000

Response

8000

BDE-15

20 0

+

120

M 326

100 Di-BDF

80 -COBr2

138

60

-COBr 219

-Br2 164

40

6000

-COBr 221

-Br2 168

40

20

-Br 245

0 150

200

250

4000

300

350

m/z

BDE-8

Di-BDF

BDE-7/4 2000 0 20

21

22

23

24

25

Retention time (min) 120 100

+

-COBr 141

Mono-BDE

M 250

80 60

(c) m/z: 246, 248

Response

20000

15000

Relative abundance, %

25000

40 20 0 120 100

-COBr 139

80

+

M 246 Mono-BDF

60 40

10000

20

5000

0 100

BDE-3

Mono-BDF 150

200

250

m/z

0 12

13

14

15

16

Retention time (min) Fig. 3. Extracted ion chromatograms and mass spectrum obtained in full scan mode showing the photoproducts of PBDEs and PBDFs (straight line, 20 min sample; dash line, 60 min sample; dot line: 90 min sample).

1599

X. Li et al. / Chemosphere 73 (2008) 1594–1601

modiphenyl ether (BDE-7) and/or 2,20 -dibromodiphenyl ether (BDE-4). Mono-BDE photoproducts were identified as 4-monobromodiphenyl ether (BDE-3) and 2-monobromodiphenyl ether (BDE1), in which BDE-3 was the dominant one. From the total estimated concentration of all PBDE congeners, the sum PBDEs remained more than 90% of the starting BDE-47 in the samples collected before 15 min. Then, with the increasing formation of di-BDEs and mono-BDEs, the sum PBDEs kept decreasing and reduced to be less than 30% of the starting BDE47 in 90 min sample when nearly 90% conversion occurred for BDE-47. This result indicated that some unidentified products might be missed during the sample extraction, or some other degradation pathways might exist during BDE-47 photodegradation. In our study, tri-, di-, and mono-BDF have also been found in surfactant solutions as photoproducts of BDE-47, which were accompanied with the sequential occurrence of tri- to mono-BDEs as shown in Fig. 3. From GC–MS chromatograms in Fig. 3, the retention times of mono-, di-, and tri-BDF detected in the samples were identified as 15.75, 25.17, and 32.96 min, respectively. Photodegradation of PBDFs (tri- to mono-) was also observed over reaction time (Fig. 3), which was probably due to a consecutive loss of bromine atoms in a similar way to PBDEs (Watanabe and Tatsukawa, 1987).

which the rate constant kept decreasing with the increase of acetone concentration; meanwhile, the UV absorption of individual mixtures of Brij 35/acetone solutions showed apparent increase with the addition of acetone. Therefore, acetone existing in Brij 35/acetone system can hardly act as a hydrogen donor or a photosensitizer for the photodegradation of BDE-47; on the contrary, the existence of acetone may virtually perform as a light barrier and retard BDE-47 photodegradation. 3.4. Photoproducts of BDE-47 In regard to the photoproducts as PBDEs occurred from BDE-47 photodegradation, only lower brominated PBDEs (mono- to tri-) were detected in irradiated samples, and no products derived from photohalogenation or photoisomeriztion were found for BDE-47 photodegradation in nonionic surfactant micelles. Since photodegradation of BDE-47 in each surfactant solution gave rise to an almost identical set of lower brominated PBDEs (mono- to tri-), a representative photodecay chronicle of BDE-47 and its debromination products was shown in Fig. 2. The sequential appearance of tri-, di-, and mono-BDEs indicated the main overall reaction occurring in surfactant micelles was photoreduction, resulting specifically in the photodebromination of PBDEs. As presented in Fig. 2, tri-BDE photoproducts as 2,4,40 -tribromodiphenyl ether (BDE-28) and 2,20 ,4-tribromodiphenyl ether (BDE-17) appeared initially in 5 min irradiated sample, in which BDE-28 was predominant over BDE-17. 4,40 -dibromodiphenyl ether (BDE-15) and 2,40 -dibromodiphenyl ether (BDE-8) were the most and the second most abundant di-BDE products; A third diBDE product has also been detected and supposed to be 2,4-dibro-

3.5. Photodegradation mechanisms and pathways of BDE-47 Since lower brominated PBDEs in surfactant micelles have been produced in a similar way to that in hexane, the photochemical mechanisms for PBDEs formation were therefore proposed to undergo comparable reactions as suggested for photochemical degradation of PBDEs in organic solvents (Eriksson et al., 2004;

Br O Br

Br

Br

BDE-47 Br

Br

O

Br

O

O Br

Br

BDE-28

Br

BDE-17

O

O

Br

Br

BDE-15

BDE-4

BDE-8

O

Br O

Brx

Br

Br

Br

Bry

Tri-BDF (x + y = 3)

Br

Br O

Brx

Bry

Di-BDF (x + y = 2)

BDE-7

O Br O

BDE-3

O Br

BDE-1

Fig. 4. Proposed photodegradation pathways of BDE-47 in nonionic surfactant micelles irradiated at 253.7 nm. ( possible pathway.)

Brx

Bry

Mono-BDF (x + y = 1) major pathway,

minor pathway,

1600

X. Li et al. / Chemosphere 73 (2008) 1594–1601

Söderström et al., 2004; Sanchez-Prado et al., 2006). Thus, the photodecay of BDE-47 in surfactant solutions may start with a homolytic cleavage of the aryl–Br bond, and result in the occurrence of an aryl radical, which sequentially abstracts an H atom from the surfactant micelles to complete the reductive debromination process. Because PEO chains and/or hydrogen carbon moieties of a surfactant can act as a better hydrogen donor than purified water, the photodegradation has thus been effectively improved in the presence of surfactant micelles as compared to that in purified water alone. Even though photodecay experiment has not been conducted for individual PBDEs, it still can be deduced from PBDE photoproducts that photodebromination of PBDEs in surfactant micelles readily occurred at ortho positions compared to that at para positions. This conclusion was in agreement with a recent study on the photolysis of six PBDE congeners in hexane irradiated by a 500-W mercury lamp (Fang et al., 2008). In regard to the reductive debromination of n-bromodiphenyl to (n1)-bromodiphenyl ethers, the higher reactivity of bromine atoms at ortho positions may be resulted from the elongated C–Br bond at ortho positions, as mentioned by Zhao et al. (2008) based on vertical electron affinities calculated by density functional theory for PBDE congeners. As a result, this preferential elimination of bromine atoms at ortho positions favored the formation of BDE-28, BDE-15, and BDE-3 during BDE-47 photodecay in surfactant micelles. Another reason for the predominance of BDE-28, BDE-15, and BDE-3 in the reaction was their lower heat of formation (Zeng et al., 2005). It has been suggested that n-bromodiphenyl ethers were debrominated to form the most stable (n1)-bromodiphenyl ethers with lower heat of formation (Zhao et al., 2008). The occurrence of PBDF photoproducts was probably due to a dibenzofuran-type ring closure process via an intramolecular elimination of HBr, similar to the case of PBDFs formation from PBDEs (tetra- to hexa-) photolysis in water under natural and artificial sunlight irradiation (Sanchez-Prado et al., 2006). The possible pathways of BDE-47 photodegradation in surfactant micelles were proposed and presented in Fig. 4. 4. Conclusion In the present study, photodegradation of BDE-47 irradiated at 253.7 nm has been improved in the presence of three nonionic surfactants. Brij 35 showed better performance on BDE-47 photodecay rate enhancement in a wide range of surfactant concentrations than Brij 58 and Tween 80. Photodegradation of BDE-47 was not affected through varying initial pH levels; however the decay rate was retarded due to the introduction of cations. As for the addition of acetone at various concentrations, acetone has proved to act as a light barrier and inhibit the photodecay. Photodebromination was shown to be the main decay way in which lower brominated PBDEs have been formed as intermediates. BDE-28, BDE-15, and BDE-3 appeared sequentially as the predominant photoproducts over reaction time. PBDFs (mono- to tri-) have also been found as additional intermediates through an intramolecular elimination of HBr. Acknowledgements This research was supported by the National Natural Science Foundation of PR China (Nos. 20507010 and 50538090) and the Distinguished Young Scholars Project (50625823). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.chemosphere.2008.08.031.

References Ahn, M.Y., Filley, T.R., Jafvert, C.T., Nies, L., Hua, I., Bezares-Cruz, J., 2006. Photodegradation of decabromodiphenyl ether adsorbed onto clay minerals, metal oxides, and sediment. Environ. Sci. Technol. 40, 215–220. Alexander, M., 2000. Aging, bioavailability, and overestimation of risk from environmental pollutants. Environ. Sci. Technol. 34, 4259–4265. Cetin, B., Odabasi, M., 2007. Particle-phase dry deposition and air–soil gas-exchange of polybrominated diphenyl ethers (PBDEs) in Izmir. Turkey. Environ. Sci. Technol. 41, 4986–4992. Chu, W., 1999. Photodechlorination mechanism of DDT in a UV/surfactant system. Environ. Sci. Technol. 33, 421–425. Chu, W., Choy, W.K., 2000. The study of lag phase and rate improvement of TCE decay in UV/surfactant systems. Chemosphere 41, 1199–1204. Chu, W., Jafvert, C.T., 1994. Photodechlorination of polychlorobenzene congeners in surfactant micelle solutions. Environ. Sci. Technol. 28, 2415–2422. Chu, W., Kwan, C.Y., 2002. The direct and indirect photolysis of 4,40 dichlorobiphenyl in various surfactant/solvent-aided systems. Water Res. 36, 2187–2194. Costa, L.G., Giordano, G., 2007. Developmental neurotoxicity of polybrominated diphenyl ether (PBDE) flame retardant. NeuroToxicology 28, 1047–1067. D’Silva, K., 2004. Brominated organic micropollutants-igniting the flame retardants issue. Crit. Rev. Env. Sci. Technol. 34, 141–207. Edwards, D.A., Luthy, R.G., Liu, Z.B., 1991. Solubilization of polycyclic aromatic hydrocarbons in micellar nonionic surfactant solutions. Environ. Sci. Technol. 25, 127–133. Ehsan, S., Prasher, S.O., Marshall, W.D., 2006. A washing procedure to mobilize mixed contaminants from soil: I. Polychlorinated biphenyl compounds. J. Environ. Qual. 35, 2146–2153. Eriksson, J., Green, N., Marsh, G., Bergman, Å., 2004. Photochemical decomposition of 15 polybrominated diphenyl ether congeners in methanol/water. Environ. Sci. Technol. 38, 3119–3125. Fang, L., Huang, J., Yu, G., Wang, L.N., 2008. Photochemical degradation of six polybrominated diphenyl ether congeners under ultraviolet irradiation in hexane. Chemosphere 71, 258–267. Henrik, V., Anders, F., Per, E., 2007. Changes in spontaneous behaviors and altered response to nicotine in the adult rat, after neonatal exposure to the brominated flame retardant, decabrominated diphenyl ether (PBDE 209). NeuroToxicology 28, 136–142. Hu, X.Z., Xu, Y., Hu, D.C., Hui, Y., Yang, F.X., 2007. Apoptosis induction on human hepatoma cells Hep G2 of decabrominated diphenyl ether (PBDE-209). Toxicol. Lett. 171, 19–28. Itoh, Y., Kaneko, T., Akahane, E., Shirai, H., Kojima, M., 2005. Photodegradation of 1,3,5-trichlorobenzene in aqueous surfactant solutions. Bull. Environ. Contam. Toxicol. 74, 365–372. Jia, J.C., Chu, W., in press. The photodegradation of trichloroethylene with or without the NAPL by UV irradiation in surfactant solutions. J. Hazard. Mater. doi:10.1016/j.jhazmat.2008.03072. Jin, D.Y., Jiang, X., Jing, X., Ou, Z.Q., 2007. Effects of concentration, head group, and structure of surfactants on the degradation of phenanthrene. J. Hazard. Mater. 144, 215–221. Kim, Y.J., Osako, M., Sakai, S.I., 2006. Leaching characteristics of polybrominated diphenyl ethers (PBDEs) from flame-retardant plastics. Chemosphere 65, 506– 513. Leung, A., Cai, Z.W., Wong, M.H., 2006. Environmental contamination from electronic waste recycling at Guiyu, southeast China. J. Mater. Cycles. Waste Manag. 8, 21–33. Leung, A.O.W., Luksemburg, W.J., Wong, A.S., Wong, M.H., 2007. Spatial distribution of polybrominated diphenyl ethers and polychlorinated dibenzo-p-dioxins and dibenzofurans in soil and combusted residue at Guiyu, an electronic waste recycling site in southeast China. Environ. Sci. Technol. 41, 2730–2737. Liu, H.X., Zhou, Q.F., Wang, Y.W., Zhang, Q.H., Cai, Z.W., Jiang, G.B., 2008. E-waste recycling induced polybrominated diphenyl ethers, polychlorinated biphenyls, polychlorinated dibenzo-p-dioxins and dibenzo-furans pollution in the ambient environment. Environ. Int. 34, 67–72. Palm, W.U., Kopetzky, R., Sossinka, W., Ruck, W., Zetzsch, C., 2004. Photochemical reactions of brominated diphenyl ethers in organic solvents and adsorbed on silicon dioxide in aqueous suspension. Organohalogen Comp. 66, 2269–2274. Rangel-Yagui, C.O., Pessoa Jr., A., Taveres, L.C., 2005. Micellar solubilization of drug. J. Pharm. Pharm. Sci. 8, 147–163. Rosen, M.J., 2004. Surfactants and Interfacial Phenomena, third ed. John Wiley and Sons, New York. pp. 145–146. Sánchez-Prado, L., Llompart, M., Lores, M., Garcia-Jares, C., Cela, R., 2005. Investigation of photodegradation products generated after UV-irradiation of five polybrominated diphenyl ethers using photo solid-phase microextraction. J. Chromatogr. A 1071, 85–92. Sanchez-Prado, L., Lores, M., Llompart, M., Garcia-Jares, C., Bayona, J.M., Cela, R., 2006. Natural sunlight and sun simulator photolysis studies of tetra- to hexabrominated diphenyl ethers in water using solid-phase microextraction. J. Chromatogr. A 1124, 157–166. Sellström, U., De Wit, C.A., Lundgren, N., Tysklind, M., 2005. Effect of sewage-sludge application on concentration of higher-brominated diphenyl ether in soils and earthworms. Environ. Sci. Technol. 39, 9064–9070. Shi, Z., Sigman, M.E., Ghosh, M.M., Dabestani, R., 1997. Photolysis of 2-chlorophenol dissolved in surfactant solutions. Environ. Sci. Technol. 31, 3581–3587.

X. Li et al. / Chemosphere 73 (2008) 1594–1601 Söderström, G., Sellström, U., De Wit, C.A., Tysklind, M., 2004. Photolytic debromination of decabromodiphenyl ether (BDE-209). Environ. Sci. Technol. 38, 127–132. Van der Meeren, P., Verstraete, W., 1996. Surfactants in relation to bioremediation and wastewater treatment. Curr. Opin. Colloid Intref. Sci. 1, 624–634. Wang, D.L., Cai, Z.W., Jiang, G.B., Leung, A., Wong, M.H., Wong, W.K., 2005. Determination of polybrominated diphenyl ethers in soil and sediment from an electronic waste recycling facility. Chemosphere 60, 810–816. Watanabe, I., Tatsukawa, R., 1987. Formation of brominated dibenzofuran from the photolysis of flame retardant decabromobiphenyl ether in hexane solution by UV and sun light. Bull. Environ. Contam. Toxicol. 39, 953–959. Wu, Q.X., Marshall, W.D., 2001. Extractions of polychlorinated biphenyl (PCB) compounds from surfactant suspension/soil extracts with dechlorination online. J. Environ. Monitor. 3, 499–504.

1601

Yip, R.W., Loutfy, R.O., Chow, Y.L., Magdzinski, L.K., 1972. The triplet state of ketones in solutions: quenching of triplet acetone by amines. Can. J. Chem. 50, 3426– 3431. Zeng, X., Freeman, P.K., Vasil’ev, Y.V., Voinov, V.G., Simonich, S.L., Barofsky, D.F., 2005. Theoretical calculation of thermodynamic properties of polybrominated diphenyl ethers. J. Chem. Eng. Data 50, 1548–1556. Zhao, Y.Y., Tao, F.M., Zeng, E.Y., 2008. Theoretical study on the chemical properties of polybrominated diphenyl ethers. Chemosphere 70, 901–907. Zhou, W.J., Zhu, L.Z., 2007. Enhanced desorption of phenanthrene from contaminated soil using anionic/anionic mixed surfactant. Environ. Pollut. 147, 350–357. Zhu, L.Z., Feng, S.L., 2003. Synergistic solubilization of polycyclic aromatic hydrocarbons by mixed anionic-surfactants. Chemosphere 53, 459–467. Zou, M.Y., Ran, Y., Gong, J., Mai, B.X., Zeng, E.Y., 2007. Polybrominated diphenyl ethers in watershed soils of the Pearl River delta, China: occurrence, inventory, and fate. Environ. Sci. Technol. 41, 8262–8267.