Article pubs.acs.org/est
Cite This: Environ. Sci. Technol. 2018, 52, 10532−10542
Elucidating the Variability in the Hexabromocyclododecane Diastereomer Profile in the Global Environment Li Li* and Frank Wania Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario Canada M1C 1A4
Environ. Sci. Technol. 2018.52:10532-10542. Downloaded from pubs.acs.org by UNIV OF BRITISH COLUMBIA on 01/03/19. For personal use only.
S Supporting Information *
ABSTRACT: Hexabromocyclododecane (HBCDD) is a hazardous flame retardant subject to international regulation. Whereas γ-HBCDD is a dominant component in the technical HBCDD mixture, the diastereomer profile in environmental samples shows substantial temporal and spatial variations, ranging from γ- to α-HBCDD-dominant. To explain such variability, we simulate the global emissions and fate of HBCDD diastereomers, using a dynamic substance flow analysis model (CiP-CAFE) coupled to a multimedia environmental fate model (BETR-Global). Our modeling results indicate that, as of 2015, 340−1000 tonnes of HBCDD have been emitted globally, with slightly more γ-HBCDD (50%−65%) than α-HBCDD (30%−50%). Emissions of γHBCDD primarily originate from production and other industrial processes, whereas those of α-HBCDD are mainly associated with the use and end-of-life disposal of HBCDDcontaining products. Presently, α-HBCDD dominates the contamination in the air of populated areas, while γ-HBCDD is more abundant in remote background areas and in regions with HCBDD production and processing facilities. Globally, the relative abundance of α-HBCDD is anticipated to increase after production of HBCDD is banned. Due to isomerization, α-HBCDD accumulates to a larger extent than γ-HBCDD in Arctic surface media. Since α-HBCDD is more persistent and bioaccumulative than other diastereomers, isomerization has bearing on the potential environmental and health impacts on a global scale.
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INTRODUCTION The past decades have witnessed extensive use of hexabromocyclododecane (HBCDD) as a brominated flame retardant in construction materials and household products, such as expanded (EPS) and extruded (XPS) polystyrene insulation boards, as well as upholstery textiles. The worldwide use and emissions of this compound, as well as subsequent long-range environmental transport, result in its ubiquity in a diversity of abiotic and biotic compartments around the world.1−3 Worldwide concern over HBCDD arose because it is persistent in the environment, bioaccumulative, and toxic to terrestrial and aquatic organisms.4 Whereas the Stockholm Convention has listed HBCDD as a persistent organic pollutant in 2013 and subjects this compound to international restrictions on its production, trade, and use, this does not imply that environmental issues surrounding this compound have ended: HBCDD is still in production in a few countries such as China under a “specific exemption” granted by the Convention, and the HBCDD residing in buildings in service or in demolition waste is anticipated to continue entering the environment in the next decades and even centuries.5,6 Technical (commercial) HBCDD is a mixture synthesized via bromination of cyclododeca-1,5,9-trienes and theoretically consists of 16 possible stereoisomers.7 Among them, γ© 2018 American Chemical Society
HBCDD is the most abundant (with a typical mass fraction of 81.6%), followed by α-HBCDD (11.8%) and β-HBCDD (5.8%).7 Such a γ-HBCDD-dominant diastereomer profile can explain dominance of γ-HBCDD observed in abiotic samples collected from industrial sites such as those in Weifang (China)8 and Tianjin (China).9 However, this γ-HBCDDdominant diastereomer profile has also been observed in remote background areas such as Ny-Ålesund (Norway)10 and Sleeping Bear Dunes (the United States)11 which are far from industrial activities. More often, α-HBCDD is identified as the dominant component in abiotic samples elsewhere. For example, α-HBCDD accounts for 59−68% of total HBCDD in the atmosphere in Guangzhou (China)12 and on average 49% (12−85%) in soils in Shanghai (China).13 Supporting Information (SI) Table S1 gives an overview of measured diastereomer profiles gathered from the literature. In addition to the divergent diastereomer profiles, temporal trends of contamination are also inconsistent between diastereomers. For instance, during the past decade, γ-HBCDD concentration Received: Revised: Accepted: Published: 10532
June 23, 2018 August 24, 2018 August 27, 2018 August 27, 2018 DOI: 10.1021/acs.est.8b03443 Environ. Sci. Technol. 2018, 52, 10532−10542
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Figure 1. Overview of the model approach used in this work: (a) CiP-CAFE and (b) BETR-Global. The numbers of regions in CiP-CAFE and of grid cells in BETR-Global that are referenced in this paper are indicated.
To address this issue, we model the emissions and fate of HBCDD diastereomers in the global environment, with a focus on how isomerization influences diastereomer profiles in emissions and in the environment. We employ a dynamic substance flow analysis model (CiP-CAFE) to calculate timevariant emissions of three HBCDD diastereomers, and a multimedia chemical fate model (BETR-Global) to calculate evolution of diastereomer-specific HBCDD contamination in the atmosphere around the world. The model’s performance is evaluated by comparing simulated and observed atmospheric HBCDD levels.
in lake trout from Lake Ontario declined at an annual rate of ∼20%, which is three times faster than that of α-HBCDD (6%).14 It is therefore of interest to establish what factors lead to the spatial and temporal variations in the relative abundance of HBCDD diastereomers in the environment. Obviously, such variations cannot be simply explained by stereoisometric structural differences, because diastereomers differ marginally in physicochemical properties (e.g., partition tendency and degradability) and hence environmental fate (e.g., long-range transport potential).15 Recent lab experiments have confirmed interconversion between diastereomers upon exposure to heat and radiation.16−19 Transformation from γ-HBCDD to α-HBCDD is predominant, resulting in an enrichment of α-HBCDD in products.20,21 For instance, Ni et al.22 observed 12 times higher emission of α-HBCDD than γ-HBCDD during open burning of γ-HBCDD-rich polystyrene plastics. As such, the production and use of heat-treated HBCDD-containing products, instead of the technical mixture itself, can be hypothesized to explain the dominance of α-HBCDD observed in the environment. For example, in an investigation of diastereomer profiles in soils across 21 Chinese coastal cities, Zhang et al.8 linked the dominance of α-HBCDD in Cangzhou and Tianjin to these cities hosting the largest XPS-based boards/pipeline manufacturing and cutting site in China, and the largest e-waste recycling area in Northern China, respectively. Whereas these studies provide valuable information on how human activities determine diastereomer profile of HBCDD in the local or regional environment, it still remains to be explored to what extent isomerization influences the fate of HBCDD diastereomers globally, and in particular, how such small-scale processes interact with environmental factors to shape the big picture of HBCDD emissions and occurrence on a global scale.
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METHOD AND DATA CiP-CAFE mModeling. Figure 1 displays an overview of the modeling framework employed in this work. Starting with the global annual production of the technical HBCDD mixture, CiP-CAFE (ver. 1.2) simulates mass flow of HBCDD in the technosphere (i.e., the human socioeconomic system) and emissions into the environment (Figure 1a). The model has been described in detail in Li et al.23 Briefly, CiP-CAFE divides the global technosphere into seven interconnected regions, for example, Region #1 represents mainland China, #5 Western Europe, and #6 North America. In each region, it describes the accumulation, transport and emissions of HBCDD in different lifecycle stages (Figure 1a): (i) production and industrial processes, including synthesis of the technical HBCDD mixture (stage “production”), preparing HBCDD-containing raw polystyrene materials (stage “formulation”), and expansion, extrusion, or molding (stage “processing”), (ii) use phase (in-use stock), comprising transient use and service life of HBCDD-containing products, and (iii) disposal of industrial and end-of-life waste (waste stock), including wastewater treatment, landfill (i.e., sanitary landfill sites), dumps and simple landfill (i.e., poorly equipped landfill sites), incineration, 10533
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Environmental Science & Technology open burning of e-waste, as well as recycling and reuse of polystyrene material. Mass exchange between regions occurs via trade flows of the technical HBCDD mixture, consumer products and waste. Input information to CiP-CAFE includes properties of HBCDD diastereomers (SI Table S2), annual regional production, use of HBCDD in different applications, international trade, as well as emission factors. Default values are used for other parameters not specified here. The model calculates annual emissions of HBCDD to lower air (i.e., the atmospheric boundary layer), freshwater and soil through 2100, and allocates them to 15° × 15° grid cells using the method in Li et al.23 (Figure 1a). For example, within each region, emissions during the production stage are spatially allocated based on the locations of major producers (SI Table S3), and those occurring during the use phase are allocated based on the regional population density. Production of HBCDD. Production of the technical HBCDD mixture in individual regions as of 2015 was gathered from different sources and is summarized in SI Table S3. The compiled information indicates that cumulatively 40% of production occurred in North America, followed by 27% in China and 25% in Western Europe (SI Table S3). Given that China is likely currently the only producer and the Stockholm Convention tentatively allows a five-year “specific exemption” for China with ongoing production and exclusive use in building insulation, we simply assume that production will linearly decrease after 2016 and cease in 2021. This results in a cumulative production of 713 kilo tonnes (kt) worldwide for the entire modeled period. The production of three diastereomers is then estimated by multiplying weight fractions in the technical HBCDD mixture (i.e., 11.8% α-HBCDD, 5.8% β-HBCDD, and 81.6% γ-HBCDD7) with this total, yielding estimates of 84 kt α-HBCDD, 41 kt β-HBCDD and 582 kt γHBCDD. Given that the diastereomer composition was similar in technical HBCDD mixtures purchased from four major producers worldwide (9.5%−13.3% α-HBCDD, 0.5−11.7% βHBCDD, and 72.3−88.5% γ-HBCDD),18 we do not consider its variation between production regions and over time. The sum of the diasteromer-specific production amounts is not exactly equal to the production of the technical HBCDD mixture due to omission of minor stereoisomers, for example, 0.5% δ-HBCDD and 0.3% ε-HBCDD. Figure 2 presents the temporal evolution of the global production of the three major HBCDD diasteromers. Applications of HBCDD. While HBCDD appears in a vast diversity of products, it is predominantly used in five applications (APs): EPS-insulation boards (AP1) and XPSinsulation boards (AP2) in buildings, textile coating agent (AP3), EPS-package (AP4), high impact polystyrene (HIPS) in electrical and electronic equipment (AP5). The distribution of HBCDD consumption among the applications in individual regions, as compiled from the literature, is tabulated in SI Table S4. Note that, based on those data, CiP-CAFE calculates that insulation boards in buildings (AP1 and AP2) account for >97% of global HBCDD consumption. CiP-CAFE requires information on the lifespan of products in each application. For AP1 and AP2 where HBCDD is embedded in insulation boards in buildings, we use regionspecific building lifespan distributions (SI Figure S1). As shown in SI Figure S1, the lifespan varies substantially between regions, ranging from 23 years in mainland China24 to ∼100 years in Western Europe.25,26 We assume constant lifespans
Figure 2. Production history of HBCDD diastereomers worldwide.
throughout the modeled period due to a lack of temporally resolved information, although we recognize that the “average” lifespan may change with time, for example, from ∼50 years in 1997 to >60 years in 2009 in the United States.27 For the remaining APs, we assign the same lifespans to all regions: a Weibull distribution with a shape parameter of 10 years and a shape parameter of 2.4 for textile products in AP3,28 < 1 year for packaging materials in AP4,29 and a Weibull distribution with a shape parameter of 10 years and a shape parameter of 2.5 for products in AP5.30 Ignoring potential regional variation in lifespan is judged acceptable, because the three applications constitute only a small fraction of global HBCDD consumption (95% to emissions from the use phase because >97% of HBCDD is used in these two applications. However, since these estimates are associated with wide uncertainty ranges, it is difficult to conclude whether the cumulative emissions from the two “consumer” sources are larger than those from “industrial” sources, such as production and industrial activities (95−630 t; blue shades in Figure 5)
and disposal of industrial waste (60−190 t; light blue shades in Figure 5). The relative importance of industrial and consumer sources differs between regions. For example, our models predict that consumer sources (280 t; range 110−450 t) contribute seven times more to the cumulative emissions in region #6 (North America) than industrial sources (40 t; range 10−70 t), whereas consumer (140 t; range 110−170 t) and industrial sources (100 t; range 10−200 t) contribute to a similar extent to cumulative emissions in region #1 (mainland China). This result for region #1 is quite close to an earlier estimate (industrial: consumer = 44%: 56%) using a Chinaspecific model. 5 The more notable consumer source contribution in North America is mainly due to the three times longer building lifespan in this region (SI Figure S1), because a longer lifespan delays the HBCDD mass flow entering waste stock and thus amplifies the size of in-use stock. Occurrence of HBCDD Diastereomers in the Global Environment. Fed with global emission estimates, BETRGlobal simulates the time-dependent atmospheric concentrations of three diastereomers in individual grid cells. SI Figure S2 compares the temporal trends in emissions and modeled atmospheric concentrations of selected cells, using αHBCDD under the maximum scenario as an example. In general, atmospheric concentrations in cells with air emissions (e.g., cell #79) are responsive to the emissions within that cell (SI Figure S2a), whereas those in cells without emissions (e.g., cell #81) are strongly influenced by transport from adjacent cells (SI Figure S2b). In particular, the modeled contamination in cells representing the Arctic approximates emissions to the entire globe (mostly contributed from the Northern hemisphere) (SI Figure S2c). We limit the following discussion to the relative abundance of α-HBCDD and γ-HBCDD, as these two diastereomers are 10538
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Environmental Science & Technology predominantly involved in isomerization whereas β-HBCDD is of less importance. Figure 6 visualizes geographic variation of
indoor dust samples collected from, for example, Stockholm (Sweden)53 and Antwerp (Belgium).54 This result also explains an earlier finding by Roosens et al.54 that α-HBCDD is the solely detected diastereomer in serum of urban residents, and its concentration is significantly correlated with human uptake via indoor dust (dominated by α-HBCDD) rather than dietary items originating from rural areas (dominated by γ-HBCDD). In 2030 when global HBCDD production is expected to have ended, α-HBCDD will be the dominant diastereomer across the global atmosphere (Figure 6b), because of the transition from “industrial” to “consumer” emissions occurring prior to 2030. Compared with the 2015 case, the α/γ ratio increases in almost all grid cells in 2030; the increase is more prominent in the Northern hemisphere than in the South because emissions of α-HBCDD primarily occur in the North. At that moment, the α/γ ratio in the Arctic will still reflect the relative importance of the two diastereomers in the global cumulative emissions, which is predicted to exceed 1. The α/γ ratio is higher in North America than in other populated regions such as East and Southeast Asia. In a word, our modeling results highlight the importance of α-HBCDD in future HBCDD contamination. Global Transport and Enrichment of HBCDD Diastereomers in Arctic Surface Media. Having established the emissions and occurrence of HBCDD diastereomers in the global atmosphere, we investigate next the difference in the long-range transport potential (LRTP) and enrichment in Arctic surface media. We characterize the LRTP using the metric “Arctic Contamination Potential (ACP)” introduced by Wania.55 Since its original definition is based on the GloboPOP model, here we redefine the ACP as the mass of a HBCDD diastereomer present in surface media (i.e., all compartments except upper and lower atmosphere) within the Arctic Circle (i.e., BETR-Global grid cells #1−48) (MArctic) normalized by the cumulative global emissions during the preceding decade (Mglobal):
Figure 6. Ratio of atmospheric concentrations of α-HBCDD to γHBCDD (medians of concentrations under the eight scenarios) in 288 BETR-Global grid cells in 2015 (panel a) and 2030 (panel b). Cells #92 and #116 mentioned in the main text are indicated by white boxes.
the concentration ratio between α-HBCDD and γ-HBCDD in the lower atmosphere; an α/γ ratio greater than 1 indicates dominance of α-HBCDD. The prevalence of orange color (indicative of an α/γ ratio between 0.3 and 1) in Figure 6a indicates that γ-HBCDD is more abundant than α-HBCDD, but not much so, in most cells, in particular in remote areas without significant emissions such as the Arctic and above lowto midlatitude oceans. This mirrors the slightly higher abundance of γ-HBCDD over α-HBCDD in the global cumulative emissions up to 2015 (Figure 3). Our predicted α/γ ratio range aligns with earlier observations in the remote background areas, for example, Ny-Ålesund in Arctic Norway (0.3)10 and Sleeping Bear Dunes in the United States (0.8)11 (SI Table S1). In addition, dominance of γ-HBCDD is also observed in cells with significant HBCDD production and processing, for example, cell #92 (white box in Figure 6a) encompassing Northern China which is the hub of Chinese HCBDD production.8 By contrast, α-HBCDD is dominant in the atmosphere of several populated areas such as East and Southeast Asia, Western Europe and North America, because these areas are strongly influenced by emissions from the use and disposal of HBCDD-containing products, which are dominated by α-HBCDD. For example, our model predicts an α/γ ratio of 3.5 for cell #116 (white box in Figure 6a), which lies within the range of 2.2−3.6 observed in urban and suburban sites of Guangzhou (China)12 (SI Table S1). In fact, the dominance of α-HBCDD is also apparent in a number of
ACP =
MArctic · 100% Mglobal
(8)
The ACP is an “intensive” metric independent of the absolute global emissions. Figure 7 shows the calculated ACPs of α-HBCDD and γ-HBCDD for the period 2000−2030. For example, the ACP in the year 2000 represents the fraction of the actual cumulative global emissions between 1991 and 2000 present in Arctic surface media in 2000. Overall, the ACPs of both diastereomers mostly decline with time during this period (Figure 7). This can be attributed to the transition from a dominance of “industrial” sources to that of “consumer” sources, because the former are distributed farther north than the latter. For example, all HBCDD producers (SI Table S3) are located at mid- and high-latitudes (north of 30°N), whereas >80% of consumption of HBCDD-containing products in the North Hemisphere (assumed to be identical to global population distribution in CiP-CAFE23) and all of the inappropriate disposal of e-waste occur at low latitudes35 (south of 30°N). Wania55 found that, for particle-sorbed chemicals like HBCDD, the proximity of emission location to the Artic increases the magnitude of enrichment in the Arctic; for example, ACP decreases by a factor of 10 when an emission source is translocated from temperate to subtropical latitudes.55 Our model additionally predicts that, after 2023, the ACP of γ-HBCDD is anticipated to rebound moderately 10539
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quickly metabolized within 90 min.56 In zebrafish, α-HBCDD is three times more bioaccumulative (an average biomagnification factor of 29.7 vs 7.61) and persistent (an average halflife of 17.3 vs 6.08 days) than γ-HBCDD when the concentration in dosed food is low.57 As such, isomerization and enrichment of α-HBCDD in technosphere and environment elevates the risk of exposure of biota and humans to HBCDD. Moreover, substantial bioisomerization of γHBCDD, for example, 96.2−98.6% of γ-HBCDD absorbed by mirror carp converts to α-HBCDD within 60 days,58 can further increase the abundance of α-HBCDD in biota, resulting in the prevalent dominance of α-HBCDD in biota samples from around the world.1,2,4 In this sense, environmental and health risks of HBCDD can be underestimated if different diastereomers are evaluated as a whole or in the form of the technical mixture. Environmental risk assessment of HBCDD is therefore recommended to be conducted on a diastereomerspecific basis.
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Figure 7. Calculated Arctic Contamination Potential (ACP) of αHBCDD and γ-HBCDD based on actual global emissions, and of the technical mixture based on constant global emissions.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.8b03443.
whereas that of α-HBCDD will level off (Figure 7). This is associated with a faster decline in the global emissions (i.e., the denominator in eq 8) of γ-HBCDD than α-HBCDD prior to 2020 as a result of shrinking worldwide HBCDD production during this period (Figure 4). When isomerization in the environment is considered, the ACP of α-HBCDD is almost double of that of γ-HBCDD (Figure 7). This is in contrast to an earlier calculation, in which α-HBCDD and γ-HBCDD share the same ACP in the absence of isomerization because their physicochemical properties are very close to each other.15 In other words, if the same amount of the two diastereomers were to be released worldwide, a higher level of α-HBCDD than γ-HBCDD would be detected in Arctic surface media. Such a relative enrichment can partially account for the consistently observed prevalence of αHBCDD in tissues of Arctic organisms,1,2,4 although we cannot rule out the possibilities of, for example, stereoselective biotic absorption and in vivo metabolism of HBCDD diastereomers. In addition, we perform a calculation for the technical HBCDD mixture, adopting the assumption in the original definition of the ACP55 of a constant global emission with its geographic distribution identical to the global population distribution. The calculated ACP (0.05%; the dashed line in Figure 7) is identical to an earlier one calculated using the Globo-POP model (the “default” scenario in Arnot et al.15), suggesting that results of the two models are consistent. Closing Remarks. Our work reveals that (i) the isomerization-induced generation of α-HBCDD in the technosphere and in the global environment is important, and (ii) as a result α-HBCDD will become the dominant HBCDD component when production is terminated. In other words, whereas γHBCDD dominates the technical mixture, α-HBCDD can be prominent in the environment, in particular in remote ecosystems and in the future. This finding intensifies the environmental concern about HBCDD, given that existing evidence suggests that α-HBCDD may be more problematic than γ-HBCDD. For instance, an in vitro assay finds that αHBCDD is resistant to biotransformation by liver microsomes from marine mammals, whereas β-HBCDD and γ-HBCDD are
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Model inputs and parameters including chemical properties, production, distribution of consumption among applications, international trades, emission and waste factors, waste disposal information, and isomerization factors; a table compiling measured relative abundance of three diastereomers around the world; and figures showing lifespan distribution of buildings in individual regions and response of modeled concentration to emissions (PDF)
AUTHOR INFORMATION
Corresponding Author
*Phone: +1 (647)-601-4450; e-mail: environ.li@mail. utoronto.ca. ORCID
Li Li: 0000-0002-5157-7366 Frank Wania: 0000-0003-3836-0901 Notes
The authors declare no competing financial interest.
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