Elucidating the Variability in the Hexabromocyclododecane

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Elucidating the Variability in the Hexabromocyclododecane Diastereomer Profile in the Global Environment Li Li, and Frank Wania Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03443 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 27, 2018

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Environmental Science & Technology

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Elucidating the Variability in the Hexabromocyclododecane Diastereomer Profile in the Global

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Environment

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Li Li*, Frank Wania

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Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military

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Trail, Toronto, Ontario, Canada M1C 1A4

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* Corresponding Author: Li L.; E-mail: [email protected]. Phone: +1 (647)-601-4450.

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ORCID:

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Li Li: 0000-0002-5157-7366

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Frank Wania: 0000-0003-3836-0901

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TOC Art

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1

ACS Paragon Plus Environment


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Abstract

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Hexabromocyclododecane (HBCDD) is a hazardous flame retardant subject to international regulation.

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Whereas γ-HBCDD is a dominant component in the technical HBCDD mixture, the diastereomer profile

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in environmental samples shows substantial temporal and spatial variations, ranging from γ- to

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α-HBCDD-dominant. To explain such variability, we simulate the global emissions and fate of HBCDD

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diastereomers, using coupled dynamic substance flow analysis model (CiP-CAFE) and multimedia

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environmental fate model (BETR-Global). Our modelling results indicate that, as of 2015, 340–1000

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tonnes of HBCDD has been emitted globally, with slightly more γ-HBCDD (50%–65%) than α-HBCDD

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(30%–50%). Emissions of γ-HBCDD primarily originate from production and other industrial processes,

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whereas those of α-HBCDD are mainly associated with the use and end-of-life disposal of

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HBCDD-containing products. Presently, α-HBCDD dominates the contamination in the air of populated

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areas, while γ-HBCDD is more abundant in remote background areas and in regions with HCBDD

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production and processing facilities. Globally, the relative abundance of α-HBCDD is anticipated to

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increase after production of HBCDD is banned. Due to isomerization, α-HBCDD accumulates to a

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larger extent than γ-HBCDD in Arctic surface media. Since α-HBCDD is more persistent and

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bioaccumulative than other diastereomers, isomerization has bearing on the potential environmental and

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health impacts on a global scale.

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Introduction

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The past decades have witnessed extensive use of hexabromocyclododecane (HBCDD) as a brominated

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flame retardant in construction materials and household products, such as expanded (EPS) and extruded

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(XPS) polystyrene insulation boards, as well as upholstery textiles. The worldwide use and emissions of

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this compound, as well as subsequent long-range environmental transport, result in its ubiquity in a

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diversity of abiotic and biotic compartments around the world.1-3 Worldwide concern over HBCDD

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arose because it is persistent in the environment, bioaccumulative, and toxic to terrestrial and aquatic

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organisms.4 Whereas the Stockholm Convention has listed HBCDD as a persistent organic pollutant in

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2013 and subjects this compound to international restrictions on its production, trade and use, this does

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not imply that environmental issues surrounding this compound have ended: HBCDD is still in

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production in a few countries such as China under a “specific exemption” granted by the Convention,

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and the HBCDD residing in buildings in service or in demolition waste is anticipated to continue

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entering the environment in the next decades and even centuries.5, 6

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Technical (commercial) HBCDD is a mixture synthesized via bromination of cyclododeca-1,5,9-trienes

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and theoretically consist of 16 possible stereoisomers.7 Among them, γ-HBCDD is the most abundant

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(with a typical mass fraction of 81.6%), followed by α-HBCDD (11.8%) and β-HBCDD (5.8%).7 Such a

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γ-HBCDD-dominant diastereomer profile can explain dominance of γ-HBCDD observed in abiotic

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samples collected from industrial sites such as those in Weifang (China)8 and Tianjin (China).9 However,

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this γ-HBCDD-dominant diastereomer profile has also been observed in remote background areas such

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as Ny-Ålesund (Norway)10 and Sleeping Bear Dunes (the United States)11 which are far from industrial

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activities. More often, α-HBCDD is identified as the dominant component in abiotic samples elsewhere.

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For example, α-HBCDD accounts for 59%–68% of total HBCDD in the atmosphere in Guangzhou

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(China)12 and on average 49% (12%–85%) in soils in Shanghai (China).13 Table S1 in the Supporting

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Information gives an overview of measured diastereomer profiles gathered from the literature. In

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addition to the divergent diastereomer profiles, temporal trends of contamination are also inconsistent

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between diastereomers. For instance, during the past decade, γ-HBCDD concentration in lake trout from

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Lake Ontario declined at an annual rate of ~20%, which is three times faster than that of α-HBCDD

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(6%).14 It is therefore of interest to establish what factors lead to the spatial and temporal variations in

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the relative abundance of HBCDD diastereomers in the environment. Obviously, such variations cannot

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be simply explained by stereo-isometric structural differences, because diastereomers differ marginally

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in physicochemical properties (e.g., partition tendency and degradability) and hence environmental fate

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(e.g., long-range transport potential).15

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Recent lab experiments have confirmed interconversion between diastereomers upon exposure to heat

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and radiation.16-19 Transformation from γ-HBCDD to α-HBCDD is predominant, resulting in an

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enrichment of α-HBCDD in products.20, 21 For instance, Ni et al.22 observed 12 times higher emission of

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α-HBCDD than γ-HBCDD during open burning of γ-HBCDD-rich polystyrene plastics. As such, the

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production and use of heat-treated HBCDD-containing products, instead of the technical mixture itself,

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can be hypothesized to explain the dominance of α-HBCDD observed in the environment. For example,

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in an investigation of diastereomer profiles in soils across 21 Chinese coastal cities, Zhang et al.8 linked

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the dominance of α-HBCDD in Cangzhou and Tianjin to these cities hosting the largest XPS-based

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boards/pipeline manufacturing and cutting site in China, and the largest e-waste recycling area in

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Northern China, respectively. Whereas these studies provide valuable information on how human

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activities determine diastereomer profile of HBCDD in the local or regional environment, it still remains

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to be explored to what extent isomerization influences the fate of HBCDD diastereomers globally, and in

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particular, how such small-scale processes interact with environmental factors to shape the big picture of

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HBCDD emissions and occurrence on a global scale.

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To address this issue, we model the emissions and fate of HBCDD diastereomers in the global

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environment, with a focus on how isomerization influences diastereomer profiles in emissions and in the

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environment. We employ a dynamic substance flow analysis model (CiP-CAFE) to calculate

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time-variant emissions of three HBCDD diastereomers, and a multimedia chemical fate model

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(BETR-Global) to calculate evolution of diastereomer-specific HBCDD contamination in the

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atmosphere around the world. The model’s performance is evaluated by comparing simulated and

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observed atmospheric HBCDD levels.

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Method and data

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CiP-CAFE modeling

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Figure 1 displays an overview of the modeling framework employed in this work. Starting with the

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global annual production of the technical HBCDD mixture, CiP-CAFE (ver. 1.2) simulates mass flow of

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HBCDD in the technosphere (i.e., the human socioeconomic system) and emissions into the

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environment (Figure 1a). The model has been described in detail in Li et al.23 Briefly, CiP-CAFE divides

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the global technosphere into seven interconnected regions, e.g., Region #1 represents mainland China,

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#5 Western Europe, and #6 North America. In each region, it describes the accumulation, transport and

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emissions of HBCDD in different lifecycle stages (Figure 1a): (i) production and industrial processes,

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including

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HBCDD-containing raw polystyrene materials (stage “formulation”), and expansion, extrusion, or

synthesis

of

the

technical

HBCDD

mixture

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(stage

“production”),

preparing

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molding (stage “processing”), (ii) use phase (in-use stock), comprising transient use and service life of

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HBCDD-containing products, and (iii) disposal of industrial and end-of-life waste (waste stock),

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including wastewater treatment, landfill, wastewater treatment, dumps and simple landfill, incineration,

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open burning of e-waste, as well as recycling and reuse of polystyrene material.

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Mass exchange between regions occurs via trade flows of the technical HBCDD mixture, consumer

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products and waste. Input information to CiP-CAFE includes properties of HBCDD diastereomers

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(Table S2), annual regional production, use of HBCDD in different applications, international trade, as

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well as emission factors. Default values are used for other parameters not specified here. The model

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calculates annual emissions of HBCDD to lower air (i.e., the atmospheric boundary layer), freshwater

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and soil through 2100, and allocates them to 15° × 15° grid cells using the method in Li et al.23 (Figure

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1a). For example, within each region, emissions during the production stage are spatially allocated based

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on the locations of major producers (Table S3), and those occurring during the use phase are allocated

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based on the regional population density.

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Figure 1. Overview of the model approach used in this work: (a) CiP-CAFE and (b)

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BETR-Global. The numbers of regions in CiP-CAFE and of grid cells in BETR-Global that are

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referenced in this paper are indicated.

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Production of HBCDD. Production of the technical HBCDD mixture in individual regions as of 2015

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was gathered from different sources and is summarized in Table S3. The compiled information indicates

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that cumulatively 40% of production occurred in North America, followed by 27% in China and 25% in

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Western Europe (Table S3). Given that China is likely currently the only producer and the Stockholm

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Convention tentatively allows a five-year “specific exemption” for China with ongoing production and

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exclusive use in building insulation, we simply assume that production will linearly decrease after 2016

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and cease in 2021. This results in a cumulative production of 713 kilo tonnes (kt) worldwide for the

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entire modeled period. The production of three diastereomers is then estimated by multiplying weight

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fractions in the technical HBCDD mixture (i.e., 11.8% α-HBCDD, 5.8% β-HBCDD, and 81.6%

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γ-HBCDD7) with this total, yielding estimates of 84 kt α-HBCDD, 41 kt β-HBCDD and 582 kt

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γ-HBCDD. Given that the diastereomer composition was similar in technical HBCDD mixtures

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purchased from four major producers worldwide (9.5%-13.3% α-HBCDD, 0.5%-11.7% β-HBCDD, and

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72.3%-88.5% γ-HBCDD),18 we do not consider its variation between production regions and over time.

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The sum of the diasteromer-specific production amounts is not exactly equal to the production of the

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technical HBCDD mixture due to omission of minor stereoisomers, e.g., 0.5% δ-HBCDD and 0.3%

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ε-HBCDD. Figure 2 presents the temporal evolution of the global production of the three major HBCDD

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diasteromers.

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Figure 2. Production history of HBCDD diastereomers worldwide.

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Applications of HBCDD. While HBCDD appears in a vast diversity of products, it is predominantly

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used in five applications (APs): EPS-insulation boards (AP1) and XPS-insulation boards (AP2) in

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buildings, textile coating agent (AP3), EPS-package (AP4), high impact polystyrene (HIPS) in electrical

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and electronic equipment (AP5). The distribution of HBCDD consumption among the applications in

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individual regions, as compiled from the literature, is tabulated in Table S4. Note that, based on those

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data, CiP-CAFE calculates that insulation boards in buildings (AP1 and AP2) account for >97% of

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global HBCDD consumption.

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CiP-CAFE requires information on the lifespan of products in each application. For AP1 and AP2 where

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HBCDD is embedded in insulation boards in buildings, we use region-specific building lifespan

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distributions (Figure S1). As shown in Figure S1, the lifespan varies substantially between regions,

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ranging from 23 years in mainland China24 to ~100 years in Western Europe.25, 26 We assume constant

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lifespans throughout the modeled period due to a lack of temporally-resolved information, although we

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recognize that the “average” lifespan may change with time, e.g., from ~50 years in 1997 to >60 years in

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2009 in the United States.27 For the remaining APs, we assign the same lifespans to all regions: a

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Weibull distribution with a shape parameter of 10 years and a shape parameter of 2.4 for textile products

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in AP3,28 97% of HBCDD is used in

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these two applications. However, since these estimates are associated with wide uncertainty ranges, it is

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difficult to conclude whether the cumulative emissions from the two “consumer” sources are larger than

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those from “industrial” sources, such as production and industrial activities (95–630 t; blue shades in

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Figure 5) and disposal of industrial waste (60–190 t; light blue shades in Figure 5). The relative 15



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importance of industrial and consumer sources differs between regions. For example, our models predict

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that consumer sources (280 t; range 110–450 t) contribute seven times more to the cumulative emissions

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in region #6 (North America) than industrial sources (40 t; range 10–70 t), whereas consumer (140 t;

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range 110–170 t) and industrial sources (100 t; range 10–200 t) contribute to a similar extent to

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cumulative emissions in region #1 (mainland China). This result for region #1 is quite close to an earlier

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estimate (industrial : consumer = 44% : 56%) using a China-specific model.5 The more notable

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consumer source contribution in North America is mainly due to the three times longer building lifespan

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in this region (Figure S1), because a longer lifespan delays the HBCDD mass flow entering waste stock

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and thus amplifies the size of in-use stock.

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Figure 5. Source-specific global emissions of α-HBCDD (panels a and d), β-HBCDD (panels b

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and e) and γ-HBCDD (panels c and f) under minimum (panels a to c) and maximum (panels d to

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f) scenarios.

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Occurrence of HBCDD diastereomers in the global environment

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Fed with global emission estimates, BETR-Global simulates the time-dependent atmospheric

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concentrations of three diastereomers in individual grid cells. Figure S2 compares the temporal trends in

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emissions and modeled atmospheric concentrations of selected cells, using α-HBCDD under the

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maximum scenario as an example. In general, atmospheric concentrations in cells with air emissions

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(e.g., cell #79) are responsive to the emissions within that cell (Figure S2a), whereas those in cells

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without emissions (e.g., cell #81) are strongly influenced by transport from adjacent cells (Figure S2b).

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In particular, the modeled contamination in cells representing the Arctic approximates emissions to the

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entire globe (mostly contributed from the Northern hemisphere) (Figure S2c).

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We limit the following discussion to the relative abundance of α-HBCDD and γ-HBCDD, as these two

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diastereomers are predominantly involved in isomerization whereas β-HBCDD is of less importance.

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Figure 6 visualizes geographic variation of the concentration ratio between α-HBCDD and γ-HBCDD in

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the lower atmosphere; an α/γ ratio greater than 1 indicates dominance of α-HBCDD. The prevalence of

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orange color (indicative of an α/γ ratio between 0.3 and 1) in Figure 6a indicates that γ-HBCDD is more

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abundant than α-HBCDD, but not much so, in most cells, in particular in remote areas without

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significant emissions such as the Arctic and above low- to mid-latitude oceans. This mirrors the slightly

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higher abundance of γ-HBCDD over α-HBCDD in the global cumulative emissions up to 2015 (Figure

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3). Our predicted α/γ ratio range aligns with earlier observations in these remote background areas, e.g.,

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Ny-Ålesund in Arctic Norway (0.3)10 and Sleeping Bear Dunes in the United States (0.8)11 (Table S1). In

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addition, dominance of γ-HBCDD is also observed in cells with significant HBCDD production and

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processing, e.g., cell #92 (white box in Figure 6a) encompassing Northern China which is the hub of

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Chinese HCBDD production.8 By contrast, α-HBCDD is dominant in the atmosphere of several

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populated areas such as East and Southeast Asia, Western Europe and North America, because these

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areas are strongly influenced by emissions from the use and disposal of HBCDD-containing products,

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which are dominated by α-HBCDD. For example, our model predicts an α/γ ratio of 3.5 for cell #116

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(white box in Figure 6a), which lies within the range of 2.2–3.6 observed in urban and suburban sites of

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Guangzhou (China)12 (Table S1). In fact, the dominance of α-HBCDD is also apparent in a number of

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indoor dust samples collected from, e.g., Stockholm (Sweden)53 and Antwerp (Belgium).54 This result

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also explains an earlier finding by Roosens et al.54 that α-HBCDD is the solely detected diastereomer in

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serum of urban residents, and its concentration is significantly correlated with human uptake via indoor

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dust (dominated by α-HBCDD) rather than dietary items originating from rural areas (dominated by

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γ-HBCDD).

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In 2030 when global HBCDD production is expected to have ended, α-HBCDD will be the dominant

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diastereomer across the global atmosphere (Figure 6b), because of the transition from “industrial” to 17



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“consumer” emissions occurring prior to 2030. Compared with the 2015 case, the α/γ ratio increases in

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almost all grid cells in 2030; the increase is more prominent in the Northern hemisphere than in the

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South because emissions of α-HBCDD primarily occur in the North. At that moment, the α/γ ratio in the

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Arctic will still reflect the relative importance of the two diastereomers in the global cumulative

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emissions, which is predicted to exceed 1. The α/γ ratio is higher in North America than in other

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populated regions such as East and Southeast Asia. In a word, our modeling results highlight the

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importance of α-HBCDD in future HBCDD contamination.

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Figure 6. Ratio of atmospheric concentrations of α-HBCDD to γ-HBCDD (medians of

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concentrations under the eight scenarios) in 288 BETR-Global grid cells in 2015 (panel a) and

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2030 (panel b). Cells #92 and #116 mentioned in the main text are indicated by white boxes.

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Global transport and enrichment of HBCDD diastereomers in the Arctic

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Having established the emissions and occurrence of HBCDD diastereomers in the global environment,

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we investigate next the difference in the long-range transport potential (LRTP) of the HBCDD 18


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diastereomers. We characterize the LRTP using the metric “Arctic Contamination Potential (ACP)”

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introduced by Wania.55 Since its original definition is based on the Globo-POP model, here we re-define

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the ACP as the mass of a HBCDD diastereomer present in surface media (i.e., all compartments except

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upper and lower atmosphere) within the Arctic Circle (i.e., BETR-Global grid cells #1 – 48) (MArctic)

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normalized by the cumulative global emissions during the preceding decade (MGlobal):

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ACP =

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The ACP is an “intensive” metric independent of the absolute global emissions. Figure 7 shows the

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calculated ACPs of α-HBCDD and γ-HBCDD for the period 2000–2030. For example, the ACP in the

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year 2000 represents the fraction of the actual cumulative global emissions between 1991 and 2000

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present in Arctic surface media in 2000. Overall, the ACPs of both diastereomers mostly decline with

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time during this period (Figure 7). This can be attributed to the transition from a dominance of

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“industrial” sources to that of “consumer” sources, because the former are distributed farther north than

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the latter. For example, all HBCDD producers (Table S3) are located at mid- and high-latitudes (north of

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30 °N), whereas >80% of consumption of HBCDD-containing products in the North Hemisphere

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(assumed to be identical to global population distribution in CiP-CAFE23) and all of the inappropriate

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disposal of e-waste occur at low latitudes35 (south of 30 °N). Wania55 found that, for particle-sorbed

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chemicals like HBCDD, the proximity of emission location to the Artic increases the magnitude of

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enrichment in the Arctic; for example, ACP decreases by a factor of 10 when an emission source is

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translocated from temperate to sub-tropical latitudes.55 Our model additionally predicts that, after 2023,

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the ACP of γ-HBCDD is anticipated to rebound moderately whereas that of α-HBCDD will level off

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(Figure 7). This is associated with a faster decline in the global emissions (i.e., the denominator in eq.8)

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of γ-HBCDD than α-HBCDD prior to 2020 as a result of shrinking worldwide HBCDD production

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during this period (Figure 4).

M Arctic ⋅100% M Global

(eq 8)

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Figure 7. Calculated Arctic Contamination Potential (ACP) of α-HBCDD and γ-HBCDD based

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on actual global emissions, and of technical mixture based on constant global emissions.

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When isomerization in the environment is considered, the ACP of α-HBCDD is almost double of that of

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γ-HBCDD (Figure 7). This is in contrast to an earlier calculation, in which α-HBCDD and γ-HBCDD

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share the same ACP in the absence of isomerization because their physicochemical properties are very

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close to each other.15 In other words, if the same amount of the two diastereomers were to be released

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worldwide, a higher level of α-HBCDD than γ-HBCDD would be detected in Arctic surface media. Such

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a relative enrichment can partially account for the consistently observed prevalence of α-HBCDD in

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tissues of Arctic organisms,1, 2, 4 although we cannot rule out the possibilities of, e.g., stereoselective

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biotic absorption and in vivo metabolism of HBCDD diastereomers. In addition, we perform a

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calculation for the technical HBCDD mixture, adopting the assumption in the original definition of the

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ACP55 of a constant global emission with its geographic distribution identical to the global population

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distribution. The calculated ACP (0.05%; the dashed line in Figure 7) is identical to an earlier one

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calculated using the Globo-POP model (the “default” scenario in Arnot et al.15), suggesting that results

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of the two models are consistent.

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Closing remarks

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Our work reveals that (i) the isomerization-induced generation of α-HBCDD in the technosphere and in

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the global environment is important, and (ii) as a result α-HBCDD will become the dominant HBCDD

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component when production is terminated. In other words, whereas γ-HBCDD dominates the technical 20


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mixture, α-HBCDD can be prominent in the environment, in particular in remote ecosystems and in the

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future. This finding intensifies the environmental concern about HBCDD, given that existing evidence

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suggests that α-HBCDD may be more problematic than γ-HBCDD. For instance, an in vitro assay finds

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that α-HBCDD is resistant to biotransformation by liver microsomes from marine mammals while

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β-HBCDD and γ-HBCDD are quickly metabolized within 90 mins.56 In zebrafish, α-HBCDD is three

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times more bioaccumulative (an average biomagnification factor of 29.7 vs. 7.61) and persistent (an

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average half-life of 17.3 vs. 6.08 days) than γ-HBCDD when the concentration in dosed food is low.57

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As such, isomerization and enrichment of α-HBCDD in technosphere and environment elevates the risk

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of exposure of biota and humans to HBCDD. Moreover, substantial bio-isomerization of γ-HBCDD, e.g.,

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96.2–98.6% of γ-HBCDD absorbed by mirror carp converts to α-HBCDD within 60 days,58 can further

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increase the abundance of α-HBCDD in biota, resulting in the prevalent dominance of α-HBCDD in

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biota samples from around the world.1, 2, 4 In this sense, environmental and health risks of HBCDD can

469

be underestimated if different diastereomers are evaluated as a whole or in the form of the technical

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mixture. Environmental risk assessment of HBCDD is therefore recommended to be conducted on a

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diastereomer-specific basis.

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Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI:xxx

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Model inputs and parameters including chemical properties, production, distribution of

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consumption among applications, international trades, emission and waste factors, waste disposal

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information, and isomerization factors; a table compiling measured relative abundance of three

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diastereomers around the world; and figures showing lifespan distribution of buildings in

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individual regions and response of modeled concentration to emissions.

479

References

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Elucidating the Variability in the Hexabromocyclododecane

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