Environ. Sci. Technol. 2008, 42, 4404–4409
Photolysis Studies of Technical Decabromodiphenyl Ether (DecaBDE) and Ethane (DeBDethane) in Plastics under Natural Sunlight NATSUKO KAJIWARA,* YUKIO NOMA, AND HIDETAKA TAKIGAMI Research Center for Material Cycles and Waste Management, National Institute for Environmental Studies (NIES), 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, JAPAN
Received January 8, 2008. Revised manuscript received March 24, 2008. Accepted March 25, 2008.
Photodebromination of technical decabromodiphenyl ether (DecaBDE) incorporated into high-impact polystyrene (HIPS) and TV casings was compared under natural sunlight conditions with that of technical decabromodiphenyl ethane (DeBDethane). BDE 209 in pulverized HIPS+DecaBDE samples degraded with a half-life of 51 days. In contrast, no marked loss of DeBDethane occurred throughout the experimental period of 224 days. During BDE 209 photolysis in HIPS+DecaBDE samples, partly debromination to nona- and octa-BDE was observed, however, environmentally relevant polybrominated diphenyl ether (PBDE) congeners such as BDE 47, 99, and 100 were not formed. Formation of polybrominated dibenzofurans (PBDFs) was clearly apparent in the flame-retarded plastics that we investigated. In the HIPS+DecaBDE samples, the PBDF concentration increased by about 40 times after 1 week of exposure, with a concomitant decrease in BDE 209. In the TV casing, tetra- to octa-BDF congener concentrations increased continuously during the experiment. Although the concentrations of PBDFs found in the plastic matrices tested were 1 to 4 orders of magnitude lower than those of PBDEs, more attention should be paid to the fact that PBDFs are formed by sunlight exposure during normal use as well as disposal/recycling processes of flame-retarded consumer products.
Introduction Pollution by polybrominated diphenyl ethers (PBDEs), the popular brominated flame retardants (BFRs) incorporated into flammable polymers, is now a worldwide problem even in remote areas (1–3). PBDEs are proposed to be listed in the Stockholm Convention since they have been found to bioaccumulate, and their effects on the health of animals exposed to them are of concern. They also have potential endocrine-disrupting properties (4, 5). Interest in PBDEs has been growing as exponential as their apparent increase in occurrence in the environment over the past 20-25 years in various regions, in contrast to the declining trends observed for classical organochlorine contaminants. Another concern is that PBDEs have the potential to form polybrominated dibenzo-p-dioxins and furans (PBDD/Fs) in combustion processes and under thermal stress like extrusion, molding, or shredding (6), and the toxicity of these resultant com* Corresponding author phone: +81-29-850-2845; fax: +81-29850-2759; e-mail:
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pounds is estimated to be similar to that of chlorinated dioxins (7). Recently, PBDD/Fs were identified for possible future inclusion in the toxic equivalency factors (TEFs) and toxic equivalency (TEQ) concept proposed by World Health Organization (8). Three PBDE technical mixtures are commercially available, but the technical decabromodiphenyl ether (DecaBDE) formulation is the only one currently produced in large quantities worldwide (9). Since PBDEs are only blended with the polymer during the production, they can migrate from the flame-retarded products and contaminate the environment. The widely used DecaBDE product has also been hypothesized to be debrominated to some extent once released into the environment, resulting in a suite of less brominated congeners with enhanced toxicity and ability to bioaccumulate relative to the parent. Previous studies have reported photolytic decomposition of the fully brominated decabromodiphenyl ether (BDE 209), the dominant congener found in the commercial DecaBDE mixture, in various matrices such as solvents, sediment, soil, and house dust (10–13). However, to date no study has examined the potential for photodegradation of BFRs in the flame-retarded product itself. Risk assessment of plastic additives including BFRs is required not only for the sake of harmonious recycling as polymer resources but also from the viewpoint of possible human intake of BFRs transferred to air and dust from electrical appliances (14, 15). Furthermore, in developing countries, BFRs leached out from discarded electrical and electronic equipment (e-waste) dumped outside have became pivotal issues for the international community (16–18). Against such a background, determination of the extent of debromination that may occur in plastic materials under daily usage and also after disposal is important. Decabromodiphenyl ethane (DeBDethane, CAS No. 8485253-9) is another BFR; it has been used in polystyrene- and polyolefin-based thermoplastic formulations as an alternative to PBDEs since the early 1990s (19). In contrast to BDE 209, DeBDethane is a nondiphenyl ether based BFR that has been reported not to produce PBDD/Fs under pyrolysis conditions (20). In Europe and the U.S., DeBDethane has not been used as extensively as DecaBDE, but in Japan its consumption increased rapidly from 1993 and its annual consumption had already exceeded that of DecaBDE by the late 1990s (7). We performed a detailed study of the photolytic debromination of the major components of the technical DecaBDE and DeBDethane in plastics under natural sunlight to evaluate whether or not they were potential sources of the lower brominatedcongenersandPBDD/Fsfoundintheenvironment.
Experimental Section Sample Preparation. High-impact polystyrene (HIPS), a composite material consisting of a polystyrene phase and a dispersed polybutadiene rubber phase, was selected as the target plastic matrix, since this polymer is widely used in flame-retarded electrical appliances, electronic instruments, and building materials. DecaBDE and DeBDethane are compatible with polystyrene and are frequently added to HIPS with antimony trioxide (Sb2O3) as the synergist. Three kinds of pulverized plastic samples were prepared: (1) HIPS compounded with the technical DecaBDE (HIPS+DecaBDE), (2) HIPS with the technical DeBDethane (HIPS+DeBDethane), and (3) TV casing. The TV was selected as a typical flameretarded consumer product familiar to the public. Nonflameretarded pure HIPS (AGI02), which was in the form of columnshaped solid (2.0-2.6 mm in diameter, 3 mm long), was purchased from A & M Styrene Co., Ltd., Japan (now PS Japan 10.1021/es800060j CCC: $40.75
2008 American Chemical Society
Published on Web 05/17/2008
Corporation). Commercially available technical DecaBDE (Wako Pure Chemical Industries, Ltd., Osaka, Japan) and DeBDethane (Weifang Yucheng Chemical Co., Ltd., Weifang, China) were used. The sample labeled “TV casing” was prepared from a composite of 50 used TV casings collected at a home appliance recycling plant in Japan. TV casings are made of flame-retarded HIPS. All the casings were crushed and homogenized into small pieces (40 times) on day 7 of exposure, with a concomitant decrease in BDE 209 concentration (Figure 2). Despite the absence of tri- to hexa-BDFs before the irradiation, they were detected in concentrations ranging from 36 to 3100 ng/g after 1 week of exposure. During the first week of the experiment the respective concentrations of hepta- and octa-BDFs increased to 25 and 10 times the initial levels. Intriguingly, the concentrations of photochemically generated PBDFs started to decrease gradually after 1 week of exposure (Figure 2B). This result demonstrates that tri- to octa-BDF congeners originating from the DecaBDE
were also photodegraded in a phased manner, as PBDFs are also photolytically unstable (25). Under our study conditions, the increased radiation energy available after the BDE 209 decomposition might have increased the further degradation of both PBDEs and PBDFs. From a mechanistic point of view, the formation of PBDFs from PBDEs requires the intramolecular elimination of Br2, HBr, or H2 for cyclization. For example, the elimination of two bromine substituents in the 2,2′ position of BDE 209 could result in the formation of octa-BDF. Watanabe and Tatsukawa (10) suggested that PBDFs were produced secondarily from debrominated PBDEs as photoproducts of BDE 209, but not directly from BDE 209, as was evident from the lack of agreement between the disappearance rate of BDE 209 and that of the formation of PBDFs. Hepta- and octaBDFs were not present in their samples. However, we could not draw a conclusion as to whether the octa-BDF that we found was formed directly from BDE 209 or from BDE 206 (nona-BDE) and/or BDE 194 (octa-BDE). Several investigations have also reported the formation of PBDFs as an alternative degradation pathway in the photolysis of BDE 209 (11, 26, 27). However, most of them have performed tentative analyses of PBDFs up to hexa-BDFs, and no investigations have reported the detection of hepta- and octaBDFs. As hepta- and octa-BDFs constituted approximately 40% of the total PBDFs in our irradiated samples, it is indispensable to include these homologues as potential BDE 209 photolytes. Recently, Hagberg et al. (27) used both chemical and bioassay analysis in a comprehensive approach of photolytically induced degradation products from BDE 209 in toluene, but unfortunately they focused on only monoto hexa-BDFs. More detailed data on PBDF formation by the photolysis of BDE 209 are needed if we are to better understand and describe the toxicity and environmental fate of these substances. In contrast to HIPS+DecaBDE samples, the TV casing samples showed no clear disappearance of BDE 209 and formation of less-brominated BDE congeners throughout the 224 days of sunlight irradiation (Figure 3A). There could be at least two explanations for the differences in degradation profiles between HIPS+DecaBDE (Figure 2A) and the TV casing samples. The first is the great difference in their initial concentrations of BDE 209. HIPS+DecaBDE and the TV casing samples contained approximately 0.1 and 10% BDE 209, respectively (Table 1). This hundredfold difference in the initial concentrations might have influenced their BDE 209 degradation rates. The second possibility is the effects of the other plastic additives in TV casings, such as coloring agents (pigments), UV absorbers, and stabilizers. These chemicals may have had marked impacts on the light penetration depth in each plastic sample by absorbing photoenergy. For example, our HIPS samples were creamy white, but the TV casing samples were black. Therefore, in the case of HIPS+DecaBDE samples, solar irradiation might have penetrated the plastic particles to degrade PBDEs and PBDFs efficiently. On the other hand, the TV casing samples contained complex UV absorbers other than the brominated compounds; these other compounds may have shielded the insides of the particles from light penetration, so that only the thin particle surface would have been targeted by the photoreaction. Hua et al. (28) demonstrated that the presence of humic acid slow down the photodegradation rate of BDE 209 absorbed to sand, suggesting that organic matter may attenuate the light intensity. Although substantial loss of BDE 209 was not observed in the TV casing samples, the concentrations of di- to octaBDF congeners showed a continuous increase during the experimental period (Figure 3B). Total PBDF concentrations in the TV casing samples after the 224-day exposure were more than 20 times the initial levels, suggesting that formation VOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Mass balance of PBDEs and PBDFs detected in (A) HIPS+DecaBDE samples and (B) TV casing samples. FIGURE 3. Degradation and formation of PBDEs and PBDFs of different bromination degrees from TV casing samples, normalized to [total PBDE] and [total PBDF] at t0, respectively. [A]: molar concentration of compound A. of PBDFs exceeded their photodegradation, unlike the case of HIPS+DecaBDE samples (Figure 2B). Among the congeners analyzed, the hexa-BDFs increased in concentration most dramaticallysby 70 timessduring the experiment (Supporting Information Table S2). Octa-BDF was the predominant congener throughout the exposure, constituting 60-80% of the total PBDFs. In the TV casing samples, the concentration of BDE 209sup to 10% by weightswas too high for us to observe any marginal decrease. However, these results suggest that BDE 209 added to the TV casing material indeed photodegraded and contributed to the synthesis of PBDFs. Since continuous PBDF formation will occur as long as BDE 209 remains, consumer products that are flameretarded by technical DecaBDE will keep producing PBDFs for a fairly long period, probably throughout the life cycle of the products. Thus, flame-retarded products under daily usage have the potential to be major sources of PBDD/F contamination in indoor air and dusts. We formulated a mass balance of PBDEs and PBDFs in the HIPS+DecaBDE and TV casing samples (Figure 4). Over the 224-day sunlight exposure, more than 90% of BDE 209 in the HIPS+DecaBDE samples was degraded, but the accumulation of di- to nona-BDEs and PBDF congeners accounted for only 1.1 and 0.24%, respectively, of the BDE 209 loss. Consequently, most of the BDE 209 in the HIPS+DecaBDE samples was lost to unknown pathways and products. A similar discontinued mass balance of PBDEs was reported by So¨derstro¨m et al. (11) and Stapleton and Dodder (13). So far, tetra- and pentabromobenzene and methoxylated BDFs have been found as minor photolytes from BDE 209 exposed in solvents to UV light (10, 26) . Identification of the major photoproducts of BDE 209 might be an important issue in the future, because the levels of these compounds 4408
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have the potential to rise in the environment. In the TV casing samples, the mass balance of PBDEs and PBDFs was more or less the same throughout the experiment, and the increment in PBDFs shown in Figure 4B was obscured by the overwhelmingly high levels of BDE 209. Limited information is available concerning PBDD/F levels in environmental samples, including sediments (3.8-590 pg/g dry wt) (29), house and office dusts (610-8800 pg/g) (30), and human adipose tissue (1.9-8.3 pg/g lipid wt) (31). Even though the concentrations of PBDFs formed in the plastic matrices investigated here were several orders of magnitude lower than those of PBDEs, the PBDF levels in the final samples reached 2.1 and 520 mg/kg in the HIPS+DecaBDE and TV casing samples, respectively (Supporting Information Tables S1 and S2)svalues far beyond those observed in the environmental media. Therefore, more attention should be paid to the fact that PBDFs are produced by sunlight exposure of flame-retarded plastics from the viewpoint of risk assessment during normal use as well as the processes of disposal and recycling of consumer products. Recently, interest was focused on the appropriate treatment and disposal of flame-retarded plastics owing to the increasing role of plastic waste recycling. In Japan, TV sets manufactured 10-20 years ago, when PBDEs were frequently used, are now being disposed of and are entering the waste stream. Furthermore, uncontrolled recycling of obsolete e-waste has become a serious problem in developing countries; improper processing releases a wide range of toxic chemicals, including PBDEs, chlorinated dioxins, and heavy metals, to the surrounding environment (16–18). Our results imply that BDE 209 photolysis in weathered e-waste might increase the environmental loads of PBDFs at certain dumping sites. It is imperative to gather information on the present status of waste flame-retarded products and the types of flame retardants used, since these waste products may be among the main sources of emission of BFRs and PBDD/Fs into the environment. Furthermore, in the frame of the
discussion of TEFs for PBDD/Fs also it should be considered to limit PBDD/F content in traded goods. This might be done on international level in the frame of the Basel and Stockholm guidelines.
Acknowledgments This research was supported by The Global Environment Research Fund (RF-064) of the Ministry of the Environment of Japan.
Supporting Information Available A description of QA/QC criteria, two tables of the detail concentrations of PBDE, PBDD/F, and DeBDethane in HIPS+DecaBDE and TV casing samples, and one additional figure of the daily accumulated UV irradiance during the experimental period. This material is available free of charge via the Internet at http://pubs.acs.org.
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