Article pubs.acs.org/est
UV-Induced Formation of Bromophenols from Polybrominated Diphenyl Ethers Paul Bendig and Walter Vetter* Institute of Food Chemistry, University of Hohenheim, Garbenstrasse 28, D-70599 Stuttgart, Germany S Supporting Information *
ABSTRACT: Bromophenols (BPs) are both man-made industrial compounds and naturally produced secondary metabolites of algae and sponges. This study explored the formation of BPs by UV irradiation of polybrominated diphenyl ethers (PBDEs). Simulated sunlight (10−80 min) and natural sunlight irradiations (5 days) of BDE-153, BDE154, BDE-183, BDE-196, and technical octabromodiphenyl ether (DE-79) generated hydrodebrominated PBDEs along with up to 0.7−4 mass % BPs. UV absorption spectra were recorded to show that the para-substituted PBDEs and BPs are those predominately transformed because this structural feature causes a significant bathochromic shift of λmax to higher wavelength. A decrease of higher brominated BPs in favor of lower brominated BPs was observed with time. All possible substitution patterns on the BPs formed by the cleavage of the parent PBDEs and respective hydrodebromination products were observed. The main di- and tribromophenols detected were 2,4-diBP > 2,5-diBP and 2,4,6-triBP > 2,4,5-triBP on average. The irradiation conditions were similar to real-world scenarios and emphasized the environmental relevance of these photolysis products of PBDEs. The meta-substituted BPs can be used as markers to distinguish photolytic PBDE transformation products from naturally produced BPs, which exclusively feature bromosubstitutents in ortho- and para-positions.
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various abiotic matrices such as air, water, and sediment.11−14 However, in most of these studies BPs could be linked with natural sources. Marine algae and sponges also produce BPs as secondary metabolites.15,16 For instance, naturally produced BPs have been found to contribute significantly to the characteristic natural flavor of seafood.10 However, pentabromophenol (pentaBP) and tribromophenol (triBP) were and still are also industrially produced for use as reactive flame retardants as well as flame retardant intermediates.17 In addition, BPs, potentially originating from PBDEs, have been detected in human blood samples18 and as transformation products of the BFRs 2,3-dibromopropyl-2,4,6-tribromophenyl ether (DPTE) and 1,2-bis(2,4,6-tribromophenoxy)ethane (BTBPE).8,19,20 Hence, we explored the potential formation of BPs from different PBDE congeners by UV light using artificial and natural sunlight and PBDEs in methanolic or methanol/water solutions. Irradiations in methanol/water, which are considered to be a good compromise to simulate reactions in the aqueous phase,8,21 were chosen to document the environmental relevance of BP formation. We also wished to study the
INTRODUCTION Polybrominated diphenyl ethers (PBDEs) have been widely used as brominated flame retardants (BFRs) in fire protection.1 These large production volume additive flame retardants have been applied in textiles, electronic equipment, cars, etc. Due to the detrimental environmental properties of several PBDE congeners, the technical PentaBDE and OctaBDE mixtures were banned in 2009 under the Stockholm Convention on POPs.2 In contrast, BDE-209 (DecaBDE) is still in use, and therefore DecaBDE was the only PBDE produced in recent years.1 However, some governmental restrictions and a voluntary phase-out have been reported. PBDE residues in biota samples mainly consist of tetra- to hexabrominated congeners.1,3 UV-induced hydrodebromination has been identified as the major abiotic transformation pathway of highly brominated PBDEs.4 Rayne et al. postulated that bromophenols (BPs) could be a further class of possible UV transformation products of PBDEs.5 For instance, UV irradiation studies with diphenyl ether produced up to 4% phenol.6 Although UV irradiation of polychlorinated diphenyl ethers (PCDEs) did not generate chlorophenols,7 Christiansson et al. recently obtained traces of phenolic UV transformation products from BDE-209.8 BPs and bromoanisoles (biomethylation products of BPs) have repeatedly been detected in environmental samples, which included mussels, shrimps, marine fish, and seal blubber,9,10 and © 2013 American Chemical Society
Received: Revised: Accepted: Published: 3665
November 22, 2012 March 4, 2013 March 10, 2013 March 11, 2013 dx.doi.org/10.1021/es304785f | Environ. Sci. Technol. 2013, 47, 3665−3670
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transfer line and quadrupole temperature were kept at 280 and 150 °C, respectively. A 30 m (0.25 mm i.d., 0.25 μm df) HP5MS UI capillary column (Agilent/J&W Scientific, Folsom, CA, USA) was used in combination with a constant carrier gas flow of 1.2 mL/min helium (purity = 5.0, Westfalen, Münster, Germany). The initial oven temperature was set at 50 °C, which was held for 1 min, and then raised at 10 °C/min to 300 °C (14 min). The full scan mode covered m/z 50−800. In the selected ion monitoring mode, 10 ion traces were recorded in three time windows: time window 1 (8−16 min), m/z 79, 81 (isotope peaks of [Br]−), 172, 174 ([M]− of monoBPs), 249.9, 251.9, 253.9 ([M]− of diBPs), and 327.8, 329.8, 331.8 ([M]− of triBPs); time window 2 (16−22.5 min), m/z 79, 81 ([Br]−), 329.8, 331.8 ([M]− of triBPs and [M − HBr]− of tetraBPs), 409.7, 411.7 ([M]− of tetraBPs and [M − HBr]− of pentaBP), and 485.6, 487.6, 489.6 ([M]− of pentaBP); time window 3 (22.5−40 min), m/z 79, 81 ([Br]−), 159, 161 ([HBr2]−), 329.8, 331.8 ([M − HBr]− of tetraBPs and [M − Br2]− of pentaBP), 407.7, 409.7 ([M]− of tetraBPs and [M − HBr]− of pentaBP), and 434.7, 436.7 ([M − Br]−) of 6-MeO-BDE-47). Quantities of diBPs (when no reference standard was available) were calculated with the average response of diBPs. All triBPs as well as tetraBPs and pentaBP were determined with the response of 2,4,6-triBP. Structures were assigned to peaks by the reference standards available and according to the method of Crystale et al.25 GC/ECNI-MS sensitivity was insufficient to detect monoBPs (LOD > 100 pg). Mass spectra, peak assignment, and elution orders of BPs are shown in the Supporting Information. Quality Control. UV irradiation experiments were performed at least in duplicates, and the maximum standard deviation between replicate samples was generally 99%), toluene (>99%), and iso-octane (>99%) were from SigmaAldrich (Taufkirchen, Germany). The BP standards 2- and 4bromophenol, 2,4-, 2,6-, and 3,5-dibromophenol (99% each), and 2,4,6-tribromophenol were from Sigma-Aldrich, and 3bromophenol was from Alfa Aesar (Karlsruhe, Germany). 6MeO-BDE-47 (BC-3) was synthesized by Marsh et al.,22 technical OctaBDE (DE-79) was from Great Lakes Chemical Corp. (Indianapolis, IN, USA), and BDE-47 was prepared in our laboratory according to the method of Marsh et al.23 Methanolic solutions of BDE-154, BDE-153, BDE-183 and BDE-196 were obtained by HPLC isolation from DE-79.24 Irradiation Experiments and Sample Preparation. Irradiation experiments were conducted with a SOL 500 sunlight simulator (irradiation intensity = 400 W; Hönle, Gräfelfing, Germany) fitted with a cutoff UV filter WG 295, λ > 280 nm (Schott, Mainz, Germany) just as reported by Bendig et al.24 Methanolic PBDE solutions (400 μL) of 204 μg of DE79, 8.4 μg of BDE-183 (90% purity), 3.4 μg of BDE-153 (91%), 2.5 μg of BDE-196 (85%), and 0.28 μg of BDE-154 (95%) were irradiated for 10, 20, 40, 60, and 80 min. For this purpose, solutions (400 μL) were pipetted into 0.5 cm path length quartz glass cuvettes. These were placed in quartz glass beakers, which were kept at 20 °C by means of a water bath. After irradiation, 200 μL aliquots were taken, 21.6 ng of the internal standard BC-3 (IS) solution (20 μL) was added, and the samples were subjected to gas chromatography/mass spectrometry (GC/MS) analysis. Additional UV irradiation experiments were conducted with 4 μg of DE-79 in toluene or benzene (irradiated for 20 min), 160 μg of BDE-47 in methanol (irradiated for 20 min), and 4 μg of 2,4,6-triBP in methanol (irradiated for 0, 10, 20, 40, 60, and 80 min). A solution of 500 ng/μL of DE-79 in methanol in Teflon stopper-sealed quartz glass cuvettes (1 cm path length) was placed on a balcony in April 2012 in Stuttgart (Germany, central Europe) for 5 days. Furthermore, irradiations of BDE-183 in a methanol/water (80:20) system as employed in the studies of Eriksson et al. and Christiansson et al.8,21 and in a 50:50 methanol/water system were performed with simulated sunlight for 80 min. After the irradiations, 500 μL of toluene and 21.6 ng of the IS were added (20 μL). To enhance phase separation, 1 mL of saturated sodium chloride solution was added, and then an aliquot of the upper layer was removed for GC/MS analysis. Additionally, irradiations in an empty cuvette (solvent removed, in “air”) were performed in the same manner for 80 min. GC/MS. GC/MS in the electron capture negative ion (GC/ ECNI-MS) mode was performed with an Agilent 7890/5975C GC/MS system (Waldbronn, Germany). One microliter was injected with a PTV injector (CIS-4, Gerstel, Mülheim, Germany) programmed as follows: after 0.01 min at 80 °C, the temperature was ramped at 500 °C/min to 260 °C, which was held until the end of the run. The ion source temperature was set at 150 °C, and methane (putity = 5.5, Air Liquide, Bopfingen, Germany) was used as moderating gas. The ionization was performed with 175 eV and 150 μA. The
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RESULTS AND DISCUSSION BP Formation during UV Irradiation of PBDEs. UV irradiation of technical OctaBDE (DE-79) generated the widely described hydrodebromination products (i.e., lower brominated PBDEs)21,26−29 along with several abundant peaks originating from BPs (Figure 1a). BPs were also identified after UV irradiation of four individual hexa- to octaBDE congeners (Figure 2; Figure S3 in the Supporting Information). Although the BP amount increased with the irradiation time (peaking at 60−80 min), the abundance was dependent on the PBDE parent compound (Figure 2). For instance, UV irradiations of BDE-196 were accompanied with a linear increase of the BP 3666
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BDE-196 indicated that the highly brominated PBDEs were more efficiently transformed by hydrodebromination than by ring cleavage into bromophenols. Cleavage of PBDEs into BPs could also generate bromobenzenes. However, these counterparts of BPs were only detected at ultratraces (∼limit of detection). This feature was also in agreement with Ogata et al., who could not detect any benzene formed by irradiation of diphenyl ether.6 UV Absorption Spectra of Bromophenols and PBDEs. UV absorption spectra of BPs, BDE-47, and technical OctaBDE (DE-79) showed an absorption maximum (>290 nm) only for the para-substituted 2,4-diBP and 2,4,6-triBP moieties. By contrast, 2,6-diBP (Br only in ortho-positions) and 3,5-diBP (Br only in meta-position) did not show any maximum above 290 nm (Figure 3). The visible λmax of para-substituted BP originates from the k-band, which is bathochromically shifted by means of the electron-donating Br in para-position.30 Whereas a Br substituent in ortho- or meta-position shifts λmax only by ∼2 nm to higher wavelength compared to the unsubstituted backbone, this bathochromic effect amounts to ∼15 nm for the para-substitution.31 In accordance with that, λmax was also visible in the absorption spectrum of BDE-47 (two para-substituents) and technical OctaBDE (several congeners with para-substitution). For this reason, the absorption curves of BDE-47 and 2,4-diBP were virtually identical (Supporting Information, Figure S4). Natural sunlight shows the highest intensity in the range >300 nm. Accordingly, it is most harmful to (light-sensitive) compounds that can absorb light in the range of 300−500 nm. Although λmax of all para-substituted BPs and PBDEs was 300 nm. To estimate the approachability of the different compounds to transformation by sunlight, we determined the wavelength that showed 10% of the intensity of λmax (Supporting Information, Figure S4). This parameter was 302 nm for BDE-47 and 2,4diBP and 309 nm for 2,4,6-triBP (Supporting Information, Figure S4). Whereas λmax values of 2,4,6-triBP and OctaBDE were similar (297 vs 298 nm), the higher number of Br substituents in the technical product was accompanied with a slower attenuating curve. For this reason, the wavelength with 10% of the intensity of λmax was shifted to 320 nm (Supporting Information, Figure S4). Accordingly, OctaBDE can be much better transformed by sunlight than the other, lower brominated PBDEs and BPs. Although the absorption curves of triBPs and di-parasubstituted hexaBDEs were virtually identical, the half-life of 2,4,6-triBP (35.4 min, this study) was almost 4-fold higher than the one of BDE-154 (9.4 min24). One main reason for this difference is that there are two tribrominated rings in hexaBDEs which can be hydrodebrominated. Therefore, the statistical probability of hydrodebromination is twice as high in the PBDE as in the triBP. In addition, the resulting pentaBDE still features one tribrominated ring, which is preferably hydrodebrominated. Accordingly, BPs are less sensitive to UV irradiation than structurally related PBDEs. This higher accessibility to UV absorption of trisubstituted rings in PBDEs compared to disubstituted rings also explains why hydrodebromination reactions almost exclusively occur alternately on the two rings.32,33 The higher substituted ring absorbs at higher wavelength and thus is preferably hydrodebrominated. Solvent Dependency of the UV Transformation. UV irradiations of DE-79 in toluene and benzene decreased the hydrodebromination of PBDEs because polar solvents are
Figure 1. GC/ECNI-MS chromatogram solutions of (a) DE-79 irradiated for 80 min (full scan), (b) BDE-183 irradiated for 80 min (SIM, m/z 252), and (c) BDE-183 irradiated for 80 min (SIM, m/z 330).
Figure 2. Sum of bromophenols (in mass %) formed from single PBDE congeners with irradiation time.
amount (Figure 2), whereas the BP formation from the lower brominated PBDE congeners was delayed during the first 20 min but then could even surmount the BPs detected from BDE-196 (Figure 2). The total amount of BPs produced after 80 min ranged from 0.7 mass % of the initial amount of BDE153 to 4.1 mass % of the original amount of BDE-183 (Figure 2). These values were in the range of phenol formed by UV irradiation of diphenyl ether (0−4 mass % in ethanol or ethanol/diethyl ether), but a 300 W high-pressure Hg vapor lamp was used.6 In most cases, the diBPs and triBPs were the dominant BP homologues, whereas tetraBPs were detected in only small amounts in the UV-irradiated solutions of BDE-196 and BDE-183 (Table 1; Figure S3 in the Supporting Information). The low amounts of tetraBPs formed from 3667
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Table 1. Relative Contribution of Bromophenols to Its Isomer Group, Formed after 80 min of Irradiation of PBDE Congeners PBDE:
BDE-153
BDE-154
BDE-183
substitution:
2,4,5
2,4,5
2,4,5
2,4,6
bromophenol 2,3
−a
−
−
−
x
x
x
x
−
x
x
−
x
x
2,3,4,6
−
−
x
x
−
−
−
46%c −
x
−
x
−
x 0.9%
x
0.5% −
−
x
0.5%
3.1%
x 48%
x
x
−
x
x
48%c x
x 46%c
x
−
27%c
11% 3,5
−
52%
3.3% 3,4
2,3,4,5
48%c
67%
38% 2,6
xb
c
x
51% 2,5
BDE-196 2,4,5
diBPs 27%
2,4
2,3,4,6
x 4.4%
−
−
x 0.8%
triBPs 2,3,4
−
−
−
−
−
x
x
1.9% 2,3,5
−
−
−
−
−
−
x
−
−
−
x
−
−
4% 2,3,6
−
5.9% 15%
2,4,5
x
x
−
−
x
96% 2,4,6
−
−
−
−
−
34% −
x
−
39% −
−
−
x
43% x
x 16%
x
3.9% 96%
3,4,5
x 6.2%
x 38%
−
−
x 0.5%e
tetraBPs 2,3,4,5
−
−
−
−
−
−
−
x 100%d
2,3,4,6
−
−
−
−
−
x
−
2,3,5,6
−
−
−
−
−
x 100%d
100% −
−
−
−, bromophenols that cannot be formed by direct photolytic ether cleavage of a PBDE congener. bx, potentially resulting bromophenols by direct photolytic ether cleavage of a PBDE congener. ccoeluting pair 2,3- and 2,5-diBP given as sum of both congeners. dCoeluting pair 2,3,4,5- and 2,3,4,6tetraBP given as sum of both congeners. eCoeluting pair 3,4,5-triBP and 2,3,5,6-tetraBP given as sum of both congeners.
a
TV casings when water was added.34 However, UV irradiation of DE-79 in the nonpolar toluene or benzene generated higher amounts of BPs (∼1 or 7 mass % of initial PBDE amount after 20 min of irradiation). The lack of a bathochromic shift of λmax in the nonpolar solvents30 was most likely responsible for the more favored ring cleavage compared to hydrodebromination. Accordingly, ring cleavage seemed to be less affected by solvent effects. To test this, we performed additional irradiation experiments with mixtures of 20 or 50% water in methanol and compared the results with irradiations in pure methanol. Eriksson et al. and Christiansson et al. showed that irradiations in methanol/water are a good compromise to simulate reactions in real world scenarios in water.8,21 In accordance with refs 8 and 21, the half-lives of BDE-183 increased from 9 min (pure methanol) over 24 min (20% water) to 44 min (50% water). Due to the lowered reactivity in aqueous methanol, the percentage of transformed BDE-183 in pure methanol after 20 min was comparable with 80 min in methanol/water (1:1). Likewise, methanol (20 min) generated 0.3% BPs, whereas methanol/water 80:20 and 50:50 generated 0.4 and 0.2%, respectively, after 80 min. The use of methanol/water instead of pure methanol lowered the velocity of the PBDE
Figure 3. UV absorbance spectra of BDE-47, DE-79, 4-bromophenol, 2,4-dibromophenol, 2,6-dibromophenol, 3,5-dibromophenol, and 2,4,6-tribromophenol, normalized to molar amounts.
known to cause a bathochromic shift of λmax of analytes,30 which in turn supports UV-induced hydrodebrominations. Such an effect was also observed by Kajiwara et al., who observed a faster transformation of PBDEs in high-impact polystyrene and 3668
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detected at the retention times of all these BPs (Figure 1b,c). However, 2,5-/2,3-diBP coeluted on the GC column used in this study (Figure 1b), and we could not establish if both isomers were present in the samples. In the case of polybrominated biphenyls (PBBs), the formation of 2,3substituted ring moieties was very unfavorable.32 Under the presumption that the hydrodebrominations of PBBs, PBDEs, and BPs followed similar pathways, the observed peak should predominately (if not exclusively) originate from 2,5-diBP (and not from 2,3-diBP). DiBPs from BDE-153, -154, -183, and -196 required both a ring cleavage and a hydrodebromination step. For all PBDEs, 2,4-diBP and the potential 2,5-diBP (see above) dominated the diBP pattern (Table 1). Moreover, the diBP pattern obtained from BDE-153 (2,4-diBP > 2,5-diBP ≫ 3,4-diBP; Table 1) reflected the intensities of the one-fold hydrodebrominated transformation products observed after irradiation of BDE-153, that is, pentaPDE BDE-99 (2,4,5-/2,4-substitution) ≥ BDE-101 (2,4,5-/2,5-substitution) > BDE-118 (2,4,5-/3,4-substitution).5 Interestingly, UV irradiation of BDE-183 in methanol/water provided a BP pattern different from irradiations in methanol. Under these more natural conditions, a higher proportion of 2,4,5-triBP was obtained (Supporting Information, Figure S5). Noteworthy, 2,4,5-triBP has been previously detected in human blood from the United States with levels exceeding those of 2,4,6-triBP.18 Qui et al. noted that 2,4,5-triBP could be a metabolite of PBDEs.18 This metabolism could both take place in humans18 but also photochemically, as shown in this study. Comparison of the BP Pattern from UV-Irradiated PBDEs with the Pattern of Naturally Produced BPs. Passive water samples from six locations at the Great Barrier Reef (Australia) generally contained 2,4-diBP, 2,6-diBP, and 2,4,6-triBP along with the related 2,4,6-tribromoanisole (TBA).14,15 However, no meta-substituted BPs and PBDEs12 could be detected (Figure S6 in the Supporting Information).12 Thus, whereas the naturally produced BPs were exclusively ortho-/para-substituted (2,4-diBP, 2,6-diBP, and 2,4,6-triBP), UV irradiations of PBDEs also generated relevant amounts of 2,5-diBP and 2,4,5-triBP (Table 1; Figure S5 in the Supporting Information). These meta-substituted BPs are thus suggested as indicator congeners for the identification of BPs originating from the transformation of PBDEs.
transformation. In addition, we noted a shift in the triBP patterns with a higher relevance of meta-substituted BPs (Figure S5 in the Supporting Information). The observations in methanol and water/methanol and the high amounts produced in nonpolar solvents suggested that the rate of BP formation from PBDEs will be higher in air (if considered as a nonpolar medium) than in water (polar medium). To test this assumption we performed irradiations of BDE-183 in an empty cuvette (i.e., in air). Under these conditions, the half-life of BDE-183 increased to t1/2 = 35 min, and 0.5 mass % BPs was produced. Accordingly, the half-life of BDE-183 was in the range of our methanol/water systems (t1/2 = 24−44 min), whereas more BPs (0.5 vs 0.4 mass %) were generated. Note, however, that the reaction rate in air is highly influenced in the presence of dust35 and aerosol particles, which is not considered in our experiments. We also exposed a methanolic solution of DE-79 to natural sunlight. For this purpose, irradiations with natural sunlight were performed on a balcony in Stuttgart in April 2012. After 5 days, ∼0.4% of the initial PBDE amount was transformed into BPs. This was ∼10−50% of the mass ratio obtained from individual PBDEs irradiated for 80 min with simulated sunlight. Without exception, these experiments illustrated that BP formation from PBDEs is to be considered as an issue of environmental concern. Especially during long-range atmospheric transport but also in the water (surface) phase, BP formation could be a significant competitive transformation reaction for highly brominated PBDEs. Accordingly, such BP formations may overlap with the occurrence of biogenic BPs in the marine air and water.11−14 Bromophenol Isomer and Congener Patterns Resulting from Irradiated PBDEs. Cleavage of the ether bond in PBDEs generated BPs with the substitution pattern of the native PBDEs (and those of their hydrodebromination products). BDE-183, for instance, can be cleaved into 2,3,4,6tetraBP and 2,4,5-triBP. Likewise, 2,3,4-, 2,3,5-, 2,3,6-, and 2,4,6-triBP can be formed by hydrodebromination of the 2,3,4,6-ring moiety (Figure 4). Furthermore, successive hydrodebromination of the tribrominated ring moieties can generate 2,3-, 2,4-, 2,5-, 2,6-, and 3,4-diBP (Figure 4). Peaks were
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ASSOCIATED CONTENT
S Supporting Information *
Assignment of BPs by means of GC/ECNI-MS and supporting Figures S1−S6, as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax: +49 711 459 24377; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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Figure 4. Possible formation of di- and tribromophenol by ring cleavage and/or hydrodebromination of BDE-183. 3669
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dx.doi.org/10.1021/es304785f | Environ. Sci. Technol. 2013, 47, 3665−3670
