Environ. Sci. Technol. 2006, 40, 3711-3716
Brominated Flame Retardants in Tree Bark from North America LINGYAN ZHU AND RONALD A. HITES* School of Public and Environmental Affairs, Indiana University, Bloomington, Indiana 47405
Brominated flame retardants were measured in 87 tree bark samples from 29 locations in North America. The concentrations of total polybrominated diphenyl ethers (PBDEs) ranged from 2.3 to 5700 ng/g lipid weight, with the highest concentrations found near or in Arkansas. A simple radial dilution model nicely describes the distribution of total PBDE concentrations in these samples and indicates the likely sources of these chemicals are emissions from the two brominated flame retardant manufacturing facilities, operated by Great Lakes Chemicals and Albemarle, located in southern Arkansas. The congener profiles of PBDEs are similar for all the tree bark samples and can be described by a mixture of the congener profiles of the penta- and deca-BDE commercial products combined in a ratio of 1:2. Two unusual BFRs, 1,2-bis(2,4,6-tribromophenoxy)ethane and decabromodiphenyl ethane, were also detected in tree bark samples collected in Arkansas, suggesting the two manufacturing facilities are the sources of these compounds as well. A polybrominated biphenyl congener (BB-153) was present in most tree bark samples at low levels relative to the PBDEs.
Introduction Brominated flame retardants (BFRs) have been used widely in commercial and household products for decades. BRFs save lives, but they are persistent in the environment, bioaccumulate in biota, and are atmospherically transported long distances from their sources (1-3). Polybrominated diphenyl ethers (PBDEs) are an important group of BFRs. The global market demand of PBDEs was about ∼70 000 tons/year in 2001 (4). PBDEs have been found in air (5-8), house dust (9, 10), aquatic sediment (11, 12), foods (13), biota (14-18), and people (19-21). Presumably because of their ubiquity, two of the three commercial PBDE mixtures (“penta-BDE” and “octa-BDE”) have been regulated by the European Union and by several U.S. states. Great Lakes Chemical Corporation, the major producer of these products in the United States, voluntarily phased-out their production in 2004 (22). Fish, birds, bird eggs, and other biota have been used to study the distribution of BFRs and other persistent organic pollutants in the environment, but sometimes it is difficult or expensive to sample these materials. On the other hand, tree bark is easy and inexpensive to sample, and it has been used as a passive sampler to monitor different inorganic and organic pollutants, such as ammonia, cadmium, and copper (23), polycyclic aromatic hydrocarbons (24), and polychlorinated dibenzo-p-dioxins (25). As a result of bark’s high lipid content and large surface area, bark is a good passive sampler for those persistent organic compounds with high Kow values, * Corresponding author e-mail:
[email protected]. 10.1021/es060225v CCC: $33.50 Published on Web 05/12/2006
2006 American Chemical Society
even when present at low atmospheric concentrations (26). Furthermore, tree bark usually stays on the tree for 3-5 years; thus, bark acts as an integrating sampler over this time period. For example, Simonich and Hites (27) used tree bark to investigate the global distillation of 22 potentially harmful organochlorine compounds, such as hexachlorobenzene and R- and γ-hexachlorocyclohexanes. Because BFRs have been used for decades and because they have relatively high Kow values, tree bark should be a useful media with which to investigate the long-range movement of these compounds through the atmosphere. The objective of this study is to investigate the transport and distribution of PBDEs and other BFRs in North America by measuring their concentrations in tree bark samples. The concentrations of 40 PBDE congeners and three other BFRs [2,2′,4,4′,5,5′-hexabromobiphenyl (BB-153), 1,2-bis(2,4,6-tribromophenoxy)ethane (TBE), and decabromodiphenyl ethane (DBDPE)] were determined, and a radial dilution model was used to localize the sources of these compounds.
Experimental Section Sample Information. Table 1 gives information about the tree bark samples analyzed in this study. Most of the tree bark samples were collected between June 2000 and October 2001. The locations were selected primarily not only for ease of accessibility but also for tree species. Very few of our samples were collected in cities or near industrial activities; most samples were from rural residential areas and were at least 10 m from the nearest roadway. Hardwood and coniferous trees with coarse bark were chosen, and trees with smooth bark, such as aspen or paper birch, were avoided (because smooth bark has less surface area to accumulate pollutants). Geographical coordinates for each sample were obtained using an Eagle Explorer (Catoosa, OK) global positioning system. At each sampling location, bark was chiseled from a 10 cm square spot from three different trees, all of which were within 50 m of each other, at a height of 1.5 m above ground level. The bark samples were wrapped in aluminum foil, sealed in plastic bags, placed in a cooler, and kept at ambient temperature until the samples were returned to the laboratory, where they were kept at -20 °C until extraction. Materials. An EPA method 1614 standard solution of 39 PBDE congeners from Accustandard (New Haven, CT) was used for the quantitation of the mono- through heptabrominated BDEs. Several octa- through deca-brominated BDE congeners were purchased individually. These were 2,2′3,3′,4,5,5′,6-OcBDE (BDE-198); 2,2′,3,4,4′,5,6,6′OcBDE (BDE-204); 2,3,3′,4,4′,5,5′,6-OcBDE (BDE-205); and 2,2′,3,3′,4,5,5′,6,6′-NoBDE (BDE-208) from Accustandard and 2,2′,3,4,4′,5,5′,6-OcBDE (BDE-203); 2,2′,3,3′,4,4′,5,5′,6-NoBDE (BDE-206); and 2,2′,3,3′,4,4′,5,6,6′-NoBDE (BDE-207) from Cambridge Isotope Laboratory (Andover, MA). The standard of decabromodiphenyl ether (BDE-209) was provided by the National Institute of Standard and Technology. In addition, we investigated the dominant polybrominated biphenyl congener, 2,2′,4,4′,5,5′-hexabromobiphenyl (BB-153), a standard of which was purchased from Ultra Scientific (North Kingstown, RI). The internal standard, 2,3′,4,4′,5-PeBDE (BDE-118), was donated to us by Cambridge Isotope Laboratories; decabromobiphenyl (BB-209) was purchased from Dr. Ehrenstorfer (GmbH, Augsburg, Germany). All the solvents used for the extraction and cleanup procedures were residue-analysis grade. Standards of TBE and DBDPE were purchased in nonane solution from Wellington Laboratories (Guelph, ON). VOL. 40, NO. 12, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Locations of Bark Samples and Their Total PBDE, TBE, and DBDPE Concentrations (in ng/g lipid)a tree species
latitude
longitude
total PBDE ( 1 SE
TBE ( 1 SE
DBDPE ( 1 SE
BB-153 ( 1 SE
elm oak elm pine pine pine pine pine pine pine poplar oak and elm pine pine pine pine pine pine oak oak pine oak oak oak pine pine oak and elm elm pine
42.9838 45.0407 48.2442 53.4857 62.4881 56.2315 52.1217 53.7975 50.9512 46.5708 39.6981 38.6132 33.3270 31.8669 30.4359 30.6696 32.3269 34.4165 38.1912 36.2240 32.4752 36.2420 34.2076 32.4203 30.2351 35.9096 34.0541 39.1716 43.8427
89.2377 92.8369 101.2894 117.4351 113.4735 117.3057 106.6568 77.6218 77.6460 75.7585 75.3973 88.9478 89.7524 90.4016 88.6228 81.4553 81.0514 79.4877 84.8530 84.0879 83.7513 89.7492 90.5464 91.1952 92.0069 92.6568 118.2417 86.5216 77.1536
19 ( 1 64 ( 7 12 ( 4 2.5 ( 1.4 63 ( 10 5.2 ( 2.1 2.3 ( 1.4 16 ( 4 6.7 ( 2.1 17 ( 9 110 ( 49 200 ( 45 600 ( 90 490 ( 150 80 ( 9 87 ( 26 42 ( 8 100 ( 2 83 ( 23 78 ( 12 160 ( 16 1600 ( 690 1500 ( 210 800 ( 330 480 ( 77 5700 ( 2700 190 ( 130 89 ( 50 61 ( 42
NDb 0.68 ( 0.13 NDb NDb NDb NDb NDb NDb 0.50 ( 0.29 NDb NDb NDb 1.9 ( 0.2 1.4 ( 0.5 NDb NDb NDb NDb NDb NDb NDb NDb 1.5 ( 0.3 NDb NDb 24 ( 5 NDb NDb NDb
NDb NDb NDb NDb NDb NDb NDb NDb NDb NDb NDb NDb 9.3 NDb NDb NDb NDb NDb NDb NDb NDb NDb NDb NDb NDb 100 ( 18 NDb NDb NDb
NDb 0.063 ( 0.012 0.027 ( 0.003 NDb NDb NDb NDb 0.003 ( 0.003 0.003 ( 0.003 0.003 ( 0.003 0.045 ( 0.045 0.030 ( 0.006 NDb 0.147 ( 0.041 0.007 ( 0.003 0.033 ( 0.007 0.020 ( 0.000 0.057 ( 0.007 0.077 ( 0.032 0.020 ( 0.000 0.020 ( 0.000 0.057 ( 0.003 0.107 ( 0.022 0.060 ( 0.012 0.067 ( 0.015 0.033 ( 0.018 0.205 ( 0.113 0.055 ( 0.015 0.015 ( 0.005
a
Each concentration is the average of three replicate samples collected at each site. b ND, not detected.
Sample Preparation. Approximately 15-25 g of each bark sample was cut into pieces of 80%, and the recoveries of 13C-BDE-209 were in the range of 3060%. For nona-BDE and TBE, the recoveries were determined by running spiked samples, and these recoveries were also >80%. Each batch of 8 samples included one method blank, which consisted of 60 g of precleaned Na2SO4 that had been spiked with the same amount of surrogate standards as the samples. BB-153, TBE, BDE-209, and DBDPE were not detected in the blank samples. The detection limits for BB153, TBE, and DBDPE were 0.01 ng, 0.05 ng, and 15 ng, respectively.
Results and Discussion PBDE Concentrations in Tree Bark Samples. The concentrations of the PBDEs (reported as the sum of the 47 congeners and notated as ΣPBDE) in the tree bark samples are given Table 1. The reported concentrations and standard errors were calculated from the three individual samples taken at each location. The concentrations were normalized to grams of bark lipids to account for differences in the tree species (27). Although the ΣPBDE concentrations vary over a large range, from 2.3 to 5700 ng/g lipid weight, these compounds were detected in all of the tree bark samples investigated, suggesting that PBDEs are ubiquitous in the North American environment. Given that PBDEs are not directly applied to trees, it seems likely that the PBDEs detected in these tree bark samples came from the accumulation of these compounds by way of atmospheric deposition. Figure 1 shows the sampling sites and the corresponding concentrations at each site. In general, the higher concentrations (>500 ng/g lipid wgt) were found around the borders of Arkansas, and one very high concentration was found in
Arkansas itself. This suggests that the source of PBDEs is in Arkansas, and, in fact, the largest PBDE manufacturing facilities, operated by Great Lakes Chemical (in El Dorado, AR) and by Albemarle (in Magnolia, AR), are located in southern Arkansas. Emissions from these two manufacturing plants may be sources of the high PBDE concentrations found in tree bark collected in and around Arkansas. Lower PBDE concentrations were found in the northern U.S. and in Canada. In these cases, long-range atmospheric transport may bring these compounds from the sources in Arkansas to these more remote locations. We can apply a previously developed model to these PBDE concentrations. McDonald and Hites studied the distribution of toxaphene concentrations in tree bark and created a simple radial dilution model to describe the variation of these concentrations as a function of distance from the source (28). There are a few assumptions in this model: First, it is assumed that there is a continuous point source at some location and that over long time periods the pollutant is carried by air movement in all directions more or less uniformly. Second, it is assumed that the average air velocity passing over the source is constant and, averaged over time, uniformly directional. Under these ideal conditions, McDonald and Hites (28) showed that the concentration of a pollutant (C) as a function of distance from its source (d) is given by
C(d) ) kd-2
(1)
where k is a fitted constant. This model is an inverse square dilution model. To apply this model, one needs to know or determine the location of the source and then to calculate the distance of each tree from that source using the spherical Euclidian distance, which is given by VOL. 40, NO. 12, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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di ) 6373 arccos{sin(lati)sin(latsor) + cos(lati)cos(latsor)cos(|loni - lonsor|)} (2) where di is the distance of the ith sampling site from the source location as measured on a sphere; lati and latsor are the latitudes of the sampling and source locations, respectively; loni and lonsor are the longitude of the sampling and source locations, respectively; and the factor of 6373 converts from radians to kilometers. To determine the location of the source, we let the values of latsor, lonsor, k, and the exponent in eq 1 vary such that the following function was minimized
ξd )
∑(kd - C ) j i
2
i
(3)
i
where Ci is the concentration of ΣPBDE in tree bark taken at location i and j is the exponent in eq 1, which should be close to -2. With the assistance of the Solver feature of Excel, the fitted variables for the ΣPBDE data set given in Table 1 were latsor ) 35.6°, lonsor ) 91.7°, k ) 13.5 × 106, and j ) -1.73. The fitted result is shown in Figure 2. This calculated source location (see the flag in Figure 1) is close to the location of the Great Lakes Chemical and Albemarle plants in southern Arkansas (at 33.13°, 92.40° and 33.16°, 93.14°, respectively), where PBDEs are manufactured. Note that the fitted value of j is close to the expected value of -2, suggesting that the inverse square dilution model is applicable to these data. Incidentally, using backward air trajectory analysis, Hoh et al. (8) also found that these two manufacturing facilities were responsible for significantly high PBDE atmospheric concentrations at some southern U.S. sites. It is impressive that this simple model works as well as it does. We have neglected all local effects such as human population densities, the locations of plastics manufacturing sites that use PBDEs, the elevation and average temperatures at the tree bark sampling sites, and distances of these sites from automobile and truck traffic. Even neglecting these other variables, the simple model used here explains 73% of the variation in the data set; see Figure 2. Congener Distributions. Generally, the PBDE congener profiles were consistent in tree bark samples from across the North America, suggesting these compounds share the same source. The average congener profile for all of the bark samples is shown in Figure 3, black bars. Generally, BDE-47, 99, 100, 183, 207, and 209 are the dominant congeners. These congeners are also the common ones found in other environmental matrices, such as Great Lakes sediment (12) and the U.S. atmosphere (8). The detection of these congeners suggests that tree bark may be a good passive sampling media with which to monitor the levels of PBDEs in the atmosphere without the complications or expense of actually sampling the atmosphere itself. The PBDE congener profile in tree bark differs from that of the three major commercial PBDE products. Thus, it is likely that the congener profiles we observed are due to the accumulation of PBDE congeners from all three of these commercial products. Given that the congener composition of the three commercial products is known (8), it is possible to fit the observed congener profile to a linear combination of the profiles of the commercial products. We did this using a least squares procedure implemented using the Solver feature of Excel, in which the following function was minimized
ξc )
∑[(f c
P i,P
+ fOci,O + fDci,D) - ci,obs]2
(4)
i
where fP is the fraction of the penta product in the tree bark, ci,P is the percent of congener i in the penta product, fO is the 3714
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FIGURE 2. Concentrations (in ng/g lipid weight) of total PBDEs in tree bark as a function of spherical Euclidian distance (in km) from 35.6°N, 91.7°N. The errors shown are one standard error. The central location and the line were fitted by minimizing ξd (see eq 3); this line has a slope (j) of -1.73.
FIGURE 3. Average congener profiles for all tree bark samples (black bars) and fitted congener profiles (white bars) based on a mixture of 31% penta-, 3% octa-, and 66% deca-BDE products. The error bars are standard errors. fraction of the octa product in the tree bark, ci,O is the percent of congener i in the octa product, fD is the fraction of the deca product in the tree bark, ci,D is the percent of congener i in the deca product, and ci,obs is the average observed percent of congener i in the tree bark. The results of the fit are fP ) 0.31, fO ) 0.03, and fD ) 0.66 and are shown in Figure 3, white bars. As Hoh and Hites have shown, the less brominated PBDEs tend to partition to the gas phase, and deca-BDE tends to partition to the particle phase (8). Our results suggest that tree bark accumulates the less brominated PBDEs (primarily BDE-47 and -99 found in the penta-BDE product) mostly from the vapor phase and the more brominated PBDEs (primarily BDE-209 found in the deca-BDE product) from the particle phase in a ratio of about 1:2. The less brominated compounds likely partition from the atmospheric vapor phase directly into the lipids of the tree bark, and the more brominated compounds likely accumulate by direct deposition of the particles onto the tree bark’s surface (29). Other BFRs. Hoh et al. (30) detected and identified TBE in the ambient atmosphere at various sites in the United States and in a sediment core from Lake Michigan. DBDPE was detected and identified by other workers in sewage sludge, sediment, and indoor air in Sweden (31). We report here (Table 1) the detection of these two compounds in tree
bark samples collected near the two manufacturing facilities in southern Arkansas. In fact, the highest concentrations of TBE and DBDPE were found in the sample with the highest ΣPBDE concentration; this was the sample collected in central Arkansas. Interestingly, Great Lakes Chemical is the only U.S. producer of TBE, which was first detected in particle-phase air samples around the facility in Arkansas as early as 1977 (32). The production of TBE will probably increase since Great Lakes Chemical has announced that they will market it (trade named FF-680) as an additive flame retardant to replace the discontinued octa-BDE product (33). DBDPE serves as a replacement for deca-BDE, now the most widely used PBDE flame retardant in the world. Albemarle is the only producer of DBDPE in the U.S., and they promote it because it produces no polybrominated dibenzo-p-dioxins and only a small amount of 2,3,7,8-tetrabromodibenzofuran when heated, thus meeting Germany’s strict dioxin ordinances (31). For most of the other bark samples, the concentrations of these two BFRs were under the detection limits. There are several reasons that may account for this observation: First, the production of these two compounds may be relatively low compared to the PBDEs, or TBE and DBDPE may be produced in such a way as to minimize their emissions from the manufacturing facilities. Second, especially for DBDPE, the analysis for these high molecular weight compounds is difficult. For example, a gas chromatogram (obtained by monitoring m/z 79 and 81 under electron capture negative ionization conditions) of a standard solution of DBDPE showed a tailing peak, and the peak height was only onetenth that of the same amount of BDE-209. To make more accurate DBDPE measurements, a shorter DB-5 column is recommended. Another factor that affects the analysis of DBDPE is its low solubility in most organic solvents. As a result, its extraction efficiency could be low, or it could come out of solution during N2 blowdown. Labeled DBDPE was not used in this study; therefore, the concentrations reported here should be considered only estimates. BB-153 was found in almost all of the tree bark samples at low concentrations (
