Physical and chemical properties

PBDEs: The commercial products are waxy solids, have boiling points between 310°C and 425°C [8] and have low vapour pressures ranging from 3.9 to 13.3 Pa at 20-25°C [9]. Their solubility in water is very poor, especially that of the higher BDEs. The n-octanol/water partition coefficient (log Kow) ranges between 4.3 and 9.9 [9]. PBDEs are thermally labile and break down readily with heat, which enables them to act as effective FRs. Both aliphatic and aromatic carbon-bromine covalent bonds are relatively weak due to the large atomic size of bromine. As a result, polybrominated organics, including PBDEs, tend to break down often with the cleavage of an HBr leaving group [10]. This reactive leaving group 'quenches' the flame preventing further ignition. Higher BDEs, such as BDE 209 in particular, also can degrade upon exposure to UV light [11,12]. This can be particularly problematic during analysis if no proper measures are taken to prevent UV degradation.

PBBs: PBBs are similar in their structure, use, manufacture, contamination pathways, and toxicological impact as PBDEs [13]. PBBs have similar physico-chemical properties to PBDEs: PBBs are also solids with low volatility and water solubility that decreases with increasing bromine number [13]. These compounds are relatively stable and chemically unreactive. However, they are also thermally labile and susceptible to photo-degradation in the environment [14]. The convention regarding systematic numbering of PBB and PBDE congeners is analogous to that of polychlorinated biphenyls (PCBs; [15]) — there are 209 possible PBB and PBDE congeners. However, PBB formulations tend to contain a lower number of congeners compared to PCB formulations [13].

HBCDs: Aliphatic brominated compounds are less thermally stable than their aromatic analogues [10]. Technical HBCD is highly labile and readily decomposes at 230°C, just a little over its melting point (178-183°C) [1]. It is a yellowish white, waxy odourless solid (vapour pressure: 0.0002 Pa at 20°C) that is practically insoluble in water (0.002 g/100mL at 20°C), but readily soluble in organic solvents, and has a log Kow value of 7.6. HBCD has a half-life of 3 days in air and 2-25 days in water [16]. It exists predominantly as three different isomers (a, b, and g). However, the relative abundance of these isomers in the technical formulations is very different from that in environmental media. Research has revealed that environmental transformation and interconversion of the HBCD isomers occurs [17]. HBCD has been found in sediments that are several decades old, indicating it to be persistent [18].

TBBP-A: TBBP-A is a white crystalline powder, with a melting point of approximately 180°C and boiling point of 316°C. It is non-volatile with a vapour pressure much less than 1 mm Hg at 20°C. TBBP-A has a low solubility in water (log Kow = 4.5), but is very soluble in methanol and acetone [19]. UV light and bacteria can both degrade TBBP-A. The main UV photolysis product is 2,4,6-tribromophenol among a number of other decomposition products, including bromobenzenes and bisphenol-A [20]. Environmental photolytic degradation of TBBP-A has a half-life ranging from 7 to 81 days in water. Moreover, bacteria degrade TBBP-A in soils and sediments under both aerobic and anaerobic conditions with a half-life of approximately 2 months [16].

PBDD/PBDFs: Theoretically, there are a total of 210 PBDD/Fs congeners. Generally, PBDD/Fs resist chemical transformations, have a low biological degradation rate, and are lipophilic, chemically stable and persistent [21]. PBDD/ Fs in soil have been shown to decompose photolytically under daylight in laboratory conditions and degradation typically occurred via debromination [22]. Half-lives for some isomers were up to 160 days.

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