Environ. Sci. Technol. 2004, 38, 127-132
Photolytic Debromination of Decabromodiphenyl Ether (BDE 209) GUNILLA SO ¨ DERSTRO ¨ M,† ULLA SELLSTRO ¨ M , * ,‡ CYNTHIA A. DE WIT,‡ AND MATS TYSKLIND† Environmental Chemistry, Umea˚ University, SE-901 87 Umea˚, Sweden, and Institute of Applied Environment Research (ITM), Stockholm University, SE-106 91 Stockholm, Sweden
Polybrominated diphenyl ethers (PBDE) are commonly used flame retardants. During the past years, concerns have increased due to their occurrence in the environment and humans. In general, the concentrations of lower brominated (tetra-penta) diphenyl ethers in biota exceed those of the most heavily used product, decabromodiphenyl ether (DecaBDE). In this study, the photolytic debromination of DecaBDE has been investigated in order to study the formation of lower brominated diphenyl ethers. The time course of photolysis of DecaBDE was studied in toluene, on silica gel, sand, sediment and soil using artificial sunlight and on the natural matrices (sediment, soil, sand) also using natural sunlight. DecaBDE was photolytically labile and formed debromination products in all matrices studied. Nona- to tetraBDEs were formed as well as some PBDFs. The half-lives in toluene and on silica gel were less than 15 min, and half-lives on other matrices ranged between 40 and 200 h. No differences were seen in the debromination pattern of BDE congeners sequentially formed in the different matrices or under different light conditions. However, the debromination rates were strongly dependent on the matrix with longer half-lives on natural matrices than artificial ones.
Introduction During the 1990s, concern about the risks of brominated flame retardants, especially polybrominated diphenyl ethers (PBDEs), has increased (1, 2). This concern has led the European Union to conduct risk assessments of the three major PBDE technical products. Based on the risk assessments of the pentabromo- and octabromodiphenyl ether products (PentaBDE and OctaBDE), the EU decided in February 2003 to ban these, with the ban going into effect in August, 2004 (3). PBDEs are among the most commonly used additive brominated flame retardants and have been used in a range of applications including plastics, textiles and electronics. Annual worldwide production of Penta-, Octa- and DecaBDE technical products in 1990 was estimated to be 4000, 6000 and 30 000 metric tons respectively (4). The estimated world market demand for Penta-, Octa- and DecaBDE technical products in 2001 was 7500, 3790 and 56 100 metric tons (5). * Corresponding author phone: +46 8 674 71 81; e-mail: ulla.
[email protected]. † Umeå University. ‡ Stockholm University. 10.1021/es034682c CCC: $27.50 Published on Web 11/13/2003
2004 American Chemical Society
The different PBDE technical products are mixtures of diphenyl ethers with varying numbers of bromine atoms on the two rings. PentaBDE technical products such as Bromkal 70-5DE, DE-71 and FR-1205 contain primarily tetra-, pentaand some hexaBDEs (4, 6). In Europe, the use of higher brominated PBDE technical products, particularly DecaBDE has increased due to the voluntary discontinuation of PentaBDE and the impending ban in the EU. DecaBDE products such as Saytex 102E, DE-83 and FR-1210 contain primarily decaBDE (BDE 209) but may also contain low concentrations of nonaBDEs and octaBDEs. Since the PBDEs are used as additives, they can migrate from products and spread into the environment. The major PBDEs looked for and found in environmental samples have been 2,2′,4,4′-tetraBDE (BDE 47), 2,2′,4,4′,5-pentaBDE (BDE 99) and 2,2′,4,4′,6-pentaBDE (BDE 100), i.e., the lower brominated PBDEs associated with PentaBDE products. However, the major technical product in use today is DecaBDE. PBDEs have been found in sediments and sewage sludge (7, 8). Lower brominated BDEs have also been found globally in a wide variety of different organisms (1, 9-12), and since the 1970s, environmental concentrations of tetra- to hexaBDEs have increased including in human mothers milk (13). The lower brominated BDEs have been found to bioaccumulate in both aquatic and terrestrial ecosystems (14-16). Higher brominated BDEs (up to decaBDE) have recently been detected in peregrine falcon eggs (17, 18). Some BDEs have been shown to affect liver enzyme activity, negatively influence the regulation of the thyroid hormone system, induce immunotoxicity and affect neurological development at a sensitive period of brain growth (1, 19, 20). BDE 47 is the predominant BDE congener in environmental samples collected from areas affected by general pollution such as the Baltic Sea, whereas the congener pattern is more similar to the technical PentaBDE product in sewage sludge, sediments and fish from background sampling sites (1). The predominance of BDE 47 may be due to its higher potential for bioaccumulation (15) but may also be due to other sources of this congener in the environment than the PentaBDE product. PBDEs belong to the group of organobromine compounds that absorb light in the UV-A spectra. The energy supplied by UV-light often results in loss of bromine and thereby also a possibility for rearrangements. Photolytic degradation of organobromines is a well-known type of reaction in basic chemistry. Studies of photolytic debromination of DecaBDE have been published previously by Norris et al. (21) and Watanabe et al. (22). Norris et al. found that both DecaBDE and OctaBDE were degraded and identified PBDEs as degradation products down to hexaBDEs. Watanabe too found that DecaBDE was debrominated to lower brominated BDEs and also that brominated dibenzofurans (PBDFs) were formed. Recently, Eriksson et al. (23) performed photodecomposition experiments with PBDE dissolved in a methanol/ water solution. They found that PBDE congeners of different bromination degree seemed to have different photolytic stability since different degradation rates were noticed. They also found that PBDFs were formed from DecaBDE. Ohta et al. (24) studied the atmospheric composition of PBDEs as well as performing photolytic decomposition experiments in the laboratory. Their results indicated a difference in degradation products when using sunlight or UV-lamps, possibly due to differences in irradiance. The objective of this study was to perform a more detailed study of the photolytic debromination of BDE 209 to evaluate VOL. 38, NO. 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
127
TABLE 1. Matrices and Exposure Times (h) Used in the Experimentsa toluene silica gel sand sediment soil a
artificial UV-light (continuous)
sunlight (discontinuous)
0/0.25/0.5/1/2/4/8/16/32 0/0.25/0.5/1/2/4/8/16/32/64 0/0.25/0.5/1/2/4/8/16/32 0/0.25/0.5/1/2/4/8/16/32/121/244 0/0.25b/0.5b/1b/2b/4b/8b/16b/32b/121/244
0/4/24/48/72/96 0/4/24/48/72/96 0/4b/24b/48b/72b/96b
Control samples of each matrix kept in the dark were also included. b Samples not considered further due to inconsistent results.
if this was a potential source for BDE 47 and possibly other lower brominated BDEs in the environment. This included determining the time course of debromination, comparison of the debromination rates on both artificial (toluene, silica gel) and natural (sand, sediment, soil) matrices, under both artificial UV light and natural sunlight conditions. The degradation products formed were then compared to the BDE congeners found in environmental samples to determine if the photolysis of BDE 209 could be a source of these.
Experimental Section The complete experimental setup was designed to provide debromination time trends and half-lives for BDE 209 on different matrices. Samples. The matrices were chosen to give results from artificial matrices causing few interactions as well as more complex, environmentally relevant matrices. Five different matrices were chosen to fulfill these requirements: toluene, silica gel, sand, soil and sediment. The toluene was glassdistilled quality from Fluka (Burdick and Jackson). The silica gel (70-230 mesh, Merck) and the lake-sand (Kebo) were thoroughly washed with toluene to remove any organic matter. The soil was an agricultural soil from Jyndevad, Denmark, selected as a typical Nordic soil (25), and the sediment was from Da¨ttern, a bay on Lake Va¨nern, Sweden, with low contaminant levels (26). Soil and sediment were not pretreated except for drying and homogenization. The matrices were portioned into Pyrex-tubes, 0.5 ((5%) gram per sample. DecaBDE (Dow FR-300 BA, Dow Chemical, Midland, MI, technical product), 10.5 ng/µL dissolved in toluene, was added, 100-200 µL per sample depending on the matrix. The toluene was then allowed to evaporate, while the samples were kept in the dark. Prior to the photolysis experiments, the sediment was reconstituted with 0.2 mL of water (Millipore-quality) to resemble more natural conditions. The samples were not further homogenized after the addition of DecaBDE, except for the continuous mixing caused by rolling the tubes during irradiation. The complete experimental design is shown in Table 1. Experimental Setup. UV-exposure experiments were performed both in the laboratory with artificial UV-light (all samples) and under natural conditions with outdoor sunlight (sand, soil, sediment). Each series consisted of solvent or matrix blanks, control samples kept in the dark, and the samples to be exposed and all experiments were performed in triplicate. However, all triplicates were not analyzed. The laboratory exposure light-source consisted of four mercury UV-lamps, Philips TLK 40W/09N, equipped with filters to give a spectra as close as possible to sunlight in the UV-A range (300-400 nm) (27). The irradiance intensity from the UV-lamps at the exposure spot was 1.6 mW/cm2. The Pyrex tubes with DecaBDE adsorbed to the matrices were placed on a RM5 “rocking/rolling action” apparatus (Assistent, Sondheim/Rho¨n, Germany). For the indoor experiments the setup was placed under mercury UV-lamps in a hood, and samples were removed at various time points between 0 and 32 h (Table 1). After a first evaluation of the experiments, the soil and sediment indoor series were extended with exposure times of 121 and 244 h. Sunlight exposure experiments were 128
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 1, 2004
FIGURE 1. UV-irradiation indoors vs outdoors. performed in July 1997 in Umeå, Sweden (63°51′ N, 20°17′ E, 20 m altitude above sea level), during the same period as the indoor experiments. For the outdoor experiments the setup was placed on a sunlit (no shadows) roof adjacent to the laboratory, and samples were removed at various time points between 0 and 96 h (Table 1). The weather conditions during the experiment were excellent with clear skies. Maximum UV-irradiance from the sun at midday was 2.3 mW/cm2. A comparison between indoor and outdoor irradiation can be seen in Figure 1. Temperature was measured regularly in both experimental setups and was never found to exceed 36 °C outdoors or 29 °C indoors. Irradiance from a 24-h cycle of sunlight was found to correspond to approximately 9 h of artificial UV-light in this experiment. All samples were stored in the dark before and after exposure. Analytical Procedure. An internal spike (IS), Dechlorane603 (Hooker Chemical), was added to the toluene samples, and more toluene was added to gain a suitable concentration for the GC/MS analysis. The silica gel samples were suspended in acetone and a small amount of water and then transferred to pasteur pipet columns with silanised glass wool plugs at the bottom. The samples were then extracted with 1 mL each of acetone, acetone/n-hexane 1:1, n-hexane, n-hexane/toluene 1:1, and finally 4 mL of toluene. The extracts were dried with anhydrous sodium sulfate and concentrated to a suitable volume, and the IS was added. All other samples were extracted directly in the Pyrex tubes used for UV exposure. For sand, soil and sediment, the method used was a scaled down version of the method described by Nylund et al. (28). In short, water was added to the samples, and after addition of IS, extraction was made first with acetone and then with a mixture of acetone and n-hexane. Sulfur was removed by use of tetrabutylammonium sulfite (TBA) reagent, and less persistent substances were removed with concentrated sulfuric acid. For the analyses, a Carlo Erba MEGA MFC 500 gas chromatograph equipped with a 15 m DB-5 (methyl + 5% phenyl) capillary column, 0.25 mm i.d. and a film thickness of 0.25 µm was used. The split-splitless injector was kept at a temperature of 280 °C, and the temperature program was as follows: Injection temperature 80 °C held for 2 min with the split valve closed for 1.5 min, followed by a rapid increase of temperature by 25 °C/minute up to 220 °C. Thereafter 5
degrees. The comparison between continuous (indoor) and discontinuous (outdoor) irradiation and also calculation of discontinuous half-lives was done by calculating “the area under the graphs” in Figure 1. This area corresponds to added energy (time × irradiation).
Results and Discussion
FIGURE 2. Mass balance of degradation and formation of PBDEs of different bromination degrees from the original DecaBDE for silica gel and sand, indoor exposure, normalized to BDE 209 at t0. °C/minute up to 315 °C and held there for 5 min. The GC was connected to a VG Trio-1000 mass spectrometer run in the chemical ionization mode, measuring the negative ions formed at chemical ionization (MS-ECNI). Ion source temperature was kept at 200 °C and electron energy was 70 eV. Ammonia was used as reaction gas at a pressure set to maximize sensitivity and stability for the system. The mass fragments monitored for the quantitative analysis were m/z -79 and -81 for PBDE and -237 and -239 for the IS. This technique is very sensitive for analyzing brominated organic compounds (29). Extraction and cleanup were performed avoiding exposure of samples to light with dark samples and blanks used as indicators of performance. For identification and quantification, technical PBDE products (Bromkal 70-5DE (PentaBDE) and Bromkal 79-8DE (OctaBDE) and DecaBDE) as well as single BDE congeners were used. The single congeners (BDE 17, 25, 28, 30, 32, 33, 35, 37, 47, 49, 51, 66, 71, 75, 77, 85, 99, 100, 105, 116, 119, 128, 138, 140, 153, 154, 155, 166, 181, 183 and 190) were kind gifts from E. Jakobsson and G. Marsh, Department of Environmental Chemistry, Stockholm University, Sweden. Five samples (toluene 0 h, silica gel 4 h, sand 96 h, sediment 244 h and soil 244 h) were further analyzed for PBDFs and PBDEs. A VG 250-S GC/MS (EI, resolution 7000) was used with the same type of GC-column and same GC-conditions as for the PBDE analysis described above. Retention times for PBDFs were determined using a mix of 10 PBDFs (2,3,7,8substituted congeners, Cambridge Isotope Laboratories) and a sample from combustion of brominated flame retardants (mixture of several congeners, unknown bromine substitution). In this analysis, screening for PBDFs was performed using three different mass channels for each homologue group of tetraBDFs, tetraBDEs, pentaBDFs and pentaBDEs. Soil 244 h and sand 96 h were also analyzed for hexaBDFs, hexaBDEs, heptaBDFs and heptaBDEs. Calculations For the mass balance calculations in Figure 2, average relative response factors were calculated for different homologue groups using known BDE isomers. These factors were then used on all peaks within the approximate retention time windows for the different BDE bromination
The results of these experiments clearly show that photolytic degradation of DecaBDE occurs under the experimental conditions tested. This is in agreement with previous studies (21-24). Reproducibility/System. The samples kept in the dark and the blanks were analyzed to establish that only the UVlight exposure affected the samples, not time or elevated temperatures. Some octa- and nonaBDEs were found to be present already from the beginning which might be due to their presence in the technical DecaBDE or as a result of degradation during storage. Reproducibility was also investigated by running the samples in triplicates, and all triplicates of the toluene, silica gel (indoors) and sand samples (indoors and outdoors) were analyzed. Results from these analyses indicated a fairly good reproducibility, and 90% of all chromatographic peaks in the toluene and silica gel samples and 57% of the peaks in the sand samples vary less than 20% between the replicates. In Table 2, the BDE 209 concentrations and variability between replicates at the different time points, normalized to time t ) 0, are shown. The toluene, silica gel and sand series show straightforward results with a gradual disappearance of BDE 209 when exposed to UV-light. No degradation was seen in the unexposed samples. The first indoor and the outdoor sediment and soil series showed some irregular DecaBDE degradation, which make these results more uncertain. The results from one sediment indoor series and the outdoor results are shown in Table 2, but for soil only the prolonged time-points (121 and 244 h as well as the time zero belonging to them) gave reliable results. Half-Lives. These experiments show that the photolytic half-life of BDE 209 is dependent upon the matrix it is adsorbed to, as summarized in Table 3. The half-lives were much shorter in toluene and on silica gel (less than 0.25 h for continuous exposure) compared to natural matrices (about 53 h for exposure on sediment, between 150 and 200 h estimated for soil). However, the results of photolysis of DecaBDE on the different matrices showed the same debromination pattern (Figure 3). The matrices used here differ both in surface structure and chemical composition. The sand and silica gel have rather smooth surfaces where the DecaBDE adsorbs but does not migrate much into the particle. The shorter half-lives of DecaBDE on these matrices indicates that exposure to UVlight is quite high, and debromination is rapid. In contrast, soil and sediment particles are porous, which enables the DecaBDE to be absorbed into the particle where it is more shielded from UV-radiation. Soil and sediment also contain higher amounts of organic carbon that can noncovalently bind planar organic compounds. These qualities could possibly increase the half-lives both due to the UV-shielding and stabilizing effect of chemical binding. Hua et al. found recently that humic acid decreased degradation rates for UVirradiated DecaBDE (30). There is also a possibility for the chemical composition of soil and sediment to enhance decomposition of halogenated organic compounds if compounds (i.e. metals) are present which can act as catalysts. In the simplest system using toluene, only slight radiation shielding is possible, exposure is even and the half-lives were expected and found to be very short. Further, no shielding or scattering effect could be observed using silica in the experimental set up although a slight effect was expected. VOL. 38, NO. 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
129
TABLE 2. Relative Amount of BDE 209 Present, Normalized to Time Zero (t0 ) 100%) at Different Exposure Times artificial UV-light
toluene n)3
silica gel n)3
dark 0h 0.25 h 0.5 h 1h 2h 4h 8h 16 h 32 h 64 h
96 ( 10% 100 ( 7%c,d 38 ( 1% 19 ( 2% 8 ( 7% 4 ( 10%c,d 2 ( 6% 1 ( 18%
93 ( 14%b,d 100 ( 9% 43 ( 15% 21 ( 5% 8 ( 6% 5 ( 9% 3 ( 5%b,d 1 ( 8%c,d
sand n)3
Series 1
102 ( 8% 100 ( 6% 97 ( 4% 100 ( 8% 88 ( 8% 93 ( 8% 75 ( 17% 62 ( 21% 39 ( 18% 21 ( 44%
Series 2 dark 121 h 244 h
a
sediment n)2
soil n)2
67a 100a 33a 87a 102a 81a 89a 86a 86a 57a
100 ( 7% 34 ( 3% 26 ( 16%
100 ( 9% 64 ( 17% 38 ( 34%
sunlight
sand n)3
sediment n)2
dark 0h 4h 24 h 48 h 72 h 96 h
92 ( 6% 100 ( 2% 74 ( 1% 59 ( 3% 45 ( 10% 37 ( 7% 36 ( 12%
73 ( 48% 100 ( 14% 100 ( 4% 90 ( 1% 68 ( 7% 33 ( 86% 43 ( 42%
n ) 1. b n) 4. c n) 5. d Average from two separate series with the same exposure time.
TABLE 3. Half-Lives (h) for BDE 209 on Different Matrices, Indoors and Outdoors artificial UV-light “continuous” toluene silica gel sand sediment soil a
