ReviewBrominated flame retardants and the formation of dioxins and furans in fires and combustion
Introduction
In order to protect the general public from accidental fires, flame retardants are widely used in numerous commodities and products, such as plastics, textiles, furniture, mattresses, electrical and electronic equipment (EEE) and other materials, reducing the flammability of combustible materials [1] and the likelihood of their ignition and propagation. There are more than 175 different types of flame-retardants in the market; at least 75 of them are brominated flame retardants (BFRs) [2]. A majority of studies on BFRs conducted to date have focused on three products or product classes: polybrominated diphenyl ethers (PBDEs), tetrabromobisphenol A (TBBP-A) and hexabromocyclododecane (HBCD) (Fig. 1) [3], [4], [5]. Once a common BFR, polybrominated biphenyls (PBBs) already in the 1970s drew public concern due to a poisoning accident in the US and were removed therefore from market supplies [6], [7].
More than 200,000 metric tons of BFRs are produced each year, of which 56% are consumed in Asia [2]. With this high production rate and concomitant expanding inventory, large amounts of BFRs can be emitted to the environment all along their lifetime, from cradle to grave: during the manufacture of masterbatches with BFRs, the compounding and processing of flame-retardant materials, the useful lifetime of products containing BFRs, their dismantling, as well as during recycling of waste containing BFR, during incineration, or in a fire [8], [9], [10]. Since their introduction, BFRs have developed into widespread global contaminants, drawing great concern because of the markedly increasing BFRs levels observed in the environment (air, soil, water and sediment) [11], [12], [13], in biota, human tissues, blood, or mother milk [14], [15], [16]. Another reason for concern is the reported harmful effects caused by some BFRs (e.g., PBDEs, TBBP-A, HBCD, etc.) on human health and that of other mammals [17], [18], [19], [20]. Therefore, some BFRs are listed in the Annex A of the Stockholm Convention and thus subjected to phase-out and prohibition [21].
Not only original BFRs, but also other groups of undesirable products are emitted into the environment, in particular during the processing, combustion and fires of materials containing BFRs. Several studies were devoted to the volatilisation of specific heavy metals in the presence of bromine or hydrogen bromide [22], [23]. Metal bromides and chlorides alike volatilise in the fire and de-sublimate from the vapor phase onto the finest particles preferentially, given their high surface to volume ratio; this creates catalytic sites that are probably very active in both the de novo route and the precursor pathways [24].
Yet, polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs) and mixed polybromochloro-dibenzo-p-dioxins and dibenzofurans (PXDD/Fs) are among the most toxic by-products. Having a chemical structure similar to those of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), PBDD/Fs (Fig. 1) refer to a group of 135 polybrominated dibenzofurans (PBDFs) and 75 dibenzo-p-dioxins (PBDDs), the same species as for their chlorinated analogous. Theoretically, there are 4600 mixed bromochlorodibenzo-p-dioxin or dibenzofuran congeners (mixed PXDD/Fs), consisting of 3050 polybromochloro dibenzofurans (PXDFs) and 1550 polybromochloro-dibenzo-p-dioxins (PXDDs) [25].
Several PCDD/Fs and polychlorinated biphenyls (PCBs) have been shown to cause toxic and biological effects similar to those caused by 2,3,7,8-tetrachlorodibenzo-p-dioxins (TCDD), which is the most potent congener within these groups of compounds. These toxic effects include dermal damage (chloracne), immunotoxicity, carcinogenicity, and adverse effects on reproduction, development, and endocrine functions [26]. The toxic equivalency factors (TEFs) of 17 2,3,7,8-substituted PCDD/Fs congeners and 12 dioxin-like PCBs, determined by the relative effect potency (REP) values of these toxic PCDD/Fs and PCBs compounds compared with 2,3,7,8-TCDD as a reference compound [27], have been developed to facilitate risk assessment of exposure to these PCDD/Fs and PCBs [28], [29]. PBDD/Fs and mixed PXDD/Fs are about comparable to PCDD/Fs in their persistence and toxicity [30]. Because toxic equivalency factors (TEFs) have not yet been determined for PBDD/Fs, the current TEFs of PCDD/Fs are generally applied for the corresponding congeners of PBDD/Fs [30], [31].
PBDD/Fs have physical–chemical properties similar to those of PCDD/Fs, albeit they could be less volatile and thus bound more to particulate matter. Besides, the analogous chemical properties of chlorine and bromine suggest that the formation mechanism of PBDD/Fs and PCDD/Fs could follow comparable pathways [32]. Formation mechanisms of PCDD/Fs have been investigated during the last decades, in laboratory studies, pilot-scale experiments and under real combustion conditions [33], [34], [35], [36]. However, PBDD/Fs and especially mixed PXDD/Fs are much less studied, due to the more advanced and complex techniques required for their GC/MS separation and quantification [37], [38]. The sheer number of PXDD/Fs congeners precludes systematic analysis of individual congeners. Considering the differences in content of chlorine and bromine in common combustibles, their diverse origin and speciation, and analogies and distinctions in properties between chlorine and bromine, the formation mechanisms of PBDD/Fs, as well as those of PXDD/Fs should be discussed distinctly.
BFRs are clearly the chief source of bromine in Br-containing dioxins and furans [13], [30]. Potentially, both PBDD/Fs and PXDD/Fs might be formed during the entire life cycle of BFRs materials and products, including their production [39], [40], recycling [41], pyrolysis/gasification [32], controlled incineration [42], and – in particular – any uncontrolled combustion and fires [43]. Due to the structural similarities between BFRs (e.g., PBDEs, PBBs and TBBP-A, etc.) and PBBD/Fs, BFRs can easily form PBDD/Fs even under only mild thermal stress, through precursor pathways. The largest amounts of PBDD/Fs are reportedly formed at the end-of-life stages of BFRs products during primitive or rudimental recycling of electrical and electronic waste [4], [44], [45]. In developing countries (China, India, Pakistan, Nigeria, etc.) often open burning is conducted to remove plastic coverings and recycle metals leading to serious contamination not only of the recycling sites, but also of surrounding cities [46], [47]. Well-controlled incinerators show high destruction efficiency for both BFRs and PBDD/Fs [48], inevitably still leading, however, to some de novo formation of PBDD/Fs and PXDD/Fs during flue gas cooling [49] so that eventual flue gas cleaning remains required. Moreover, halogen (chlorine–bromine) exchange reactions can change the bromine/chlorine ratio in the halogenated dioxins during their formation, resulting in different homologue patterns and fingerprints [50].
Given the widespread use and the rising inventory of BFRs, their considerable role in the formation and (possibly) emission of PBDD/Fs and the wide variety in combustion sources emitting PBDD/Fs, this review aims to (1) outline potential pathways and reaction steps in the formation of both PBDD/Fs and mixed PXDD/Fs starting from BFRs, (2) identify and analyse the effects of local factors of influence on their formation, and (3) summarise emission data on PBDD/Fs and PXDD/Fs arising from several combustion sources.
Section snippets
Presence and uses of Bromine
As a member of group VII (halogens) of the periodic table of elements, bromine is a highly reactive element. Consequently, it is mostly found in the form of inorganic salts of alkali and alkaline earth metals [5], [51]: bromine is widely spread in seawater (65–70 g Br/m3), yet in amounts roughly two orders of magnitude lower than chlorine. The production of bromine proceeds by the oxidation of bromide with chlorine, followed by absorption and purification [5]. The annual global production of
Formation pathways of PBDD/Fs and PXDD/Fs
As chief bromine donor during thermal treatment, recycling, and accidental fires, BFRs are regarded as the main contributor to PBDD/Fs emissions, at least at their end-of-life stage [4], [44], [45]. Bromine is not nearly as ubiquitous as chlorine in combustion and fires, yet it undergoes similar sets of reactions as chlorine and thus has the potential to form a class of compounds – PBDD/Fs – akin to PCDD/Fs [38], [62]. Mechanisms of PCDD/Fs formation have been investigated during the last
Enhanced formation of PCDD/Fs by addition of bromine
In order to evaluate whether the presence of bromine can impact emissions from waste incineration, particularly emissions of chlorinated species, a systematic series of experiment were performed by Lemieux and Ryan, on a pilot-scale rotary kiln incinerator simulator [96]. Liquid surrogate wastes consisting of a series of mixtures of methylene chloride (CH2Cl2) and methylene bromide (CH2Br2) were selected as fuel. During these combustion tests, 19 different volatile organic products of
Emissions of PBDD/Fs from combustion sources
Over the years, more and more brominated flame retardants have reached diverse waste treatment or metal recycling facilities, undergoing thermal treatment. Brominated flame retardants are very effective for preventing fires; conversely, they complicate any deliberate efforts to burn them adequately [95]. Incomplete destruction of BFRs not only leads to BFRs either volatising or remaining in residues, but also to PBDD/Fs formation and emission. Even in incinerators featuring sufficiently high
Conclusions
PBDD/Fs and mixed PXDD/Fs are environmental contaminants that are more or less similar to PCDD/Fs in their toxicity and formation mechanisms, yet they also thermal breakdown products of BFR-precursors. There are three distinct pathways towards PBDD/Fs (and also mixed PXDD/Fs) formation starting from BFRs: precursor formation, de novo formation and incomplete destruction of PBDD/Fs contained in BFRs as impurities. Acting as precursors, some BFRs are transformed into PBDD/Fs by simple
Acknowledgements
The authors are grateful to Dr. Jürgen Vehlow (Karlsruhe Institute of Technology, Karlsruhe, Germany) and to Prof. Bogdan Z. Dlugogorski and Mohammednoor Altarawneh (Murdoch University, Perth, Australia), who helped improving this paper by their constructive comments. The Program of Introducing Talents of Discipline to University (B08026) and Program 111 financed this study.
References (134)
An overview of brominated flame retardants in the environment
Chemosphere
(2002)- et al.
An overview of commercially used brominated flame retardants, their applications, their use patterns in different countries/regions and possible modes of release
Environ. Int.
(2003) - et al.
Export of toxic chemicals—a review of the case of uncontrolled electronic-waste recycling
Environ. Pollut.
(2007) - et al.
Environmental release and behavior of brominated flame retardants
Environ. Int.
(2003) - et al.
Certain organochlorine and organobromine contaminants in Swedish human milk in perspective of past 20–30 years
Chemosphere
(2000) Toxic effects of brominated flame retardants in man and in wildlife
Environ. Int.
(2003)- et al.
Exposure to tetrabromobisphenol A (TBBPA) in Wistar rats: neurobehavioral effects in offspring from a one-generation reproduction study
Toxicology
(2008) - et al.
Effects of the brominated flame retardant hexabromocyclododecane (HBCD) on dopamine-dependent behavior and brainstem auditory evoked potentials in a one-generation reproduction study in Wistar rats
Toxicol. Lett.
(2009) - et al.
Study on simultaneous recycling of EAF dust and plastic waste containing TBBPA
J. Hazard. Mater.
(2014) - et al.
Characterisation of chlorinated, brominated and mixed halogenated dioxins, furans and biphenyls as potent and as partial agonists of the Aryl hydrocarbon receptor
Environ. Int.
(2015)