# Chemicals from Brominated Flame Retardants: Analytical Methods, Occurrence, Transport and Risks
## 4 Transport Mechanisms of BFRs
### Water Transport and Fate Processes  
The surface or groundwater transport of BFRs is primarily influenced by their hydrophobic nature and tendency to partition to organic matter [52]. BFRs can enter surface waters through direct discharge from industrial facilities, runoff from treated surfaces, and atmospheric deposition (Figure 1). Once in surface waters, BFRs can undergo various fate processes, including sorption, transformation, transport, and volatilization [55]. Sorption is a key process that controls the fate of BFRs in surface waters. BFRs tend to sorb to suspended particles including microplastics and sediment in water bodies, leading to their accumulation in these matrices and then their consumption by aquatic biota [41; 55]. [PERSON] et al. [52] reported a high bioaccumulation factor (BAF) of (\(>\)5000) in aquatic biota (invertebrates, fish, and reptiles), which was due to the lipophilic nature of BFRs as they are easily sequestered in lipids [54; 100]. These can also influence the transport of BFRs in surface waters, as sorbed or ingested BFRs can be transported with the aquatic biota serving as a vector in the aquatic environment [90]. Transformation processes, such as photodegradation and biodegradation, can alter the fate of BFRs in surface waters. Photodegradation breaks down BFRs using sunlight, while biodegradation breaks them down using microorganisms [101]. These processes can create transformation products with different environmental properties and toxicity than the original compounds. [PERSON] et al. [101] reviewed the transformation of BFRs, and they found that in biota, BFRs undergo degradation pathways such as reductive debromination and Phase I oxidative metabolism. For PBDEs, reductive debromination converts hepta- to deca-BDE congeners into tetra- to hexa-PBDEs, while Phase I metabolism produces hydroxylated compounds (e.g., HO-PBDEs). Phase II conjugation (glucuronidation or sulfation) may also occur. HBCD undergoes in vivo metabolism producing Phase I metabolism products (HO-compounds) through oxidative metabolism, dehydrobromination or debromination, and stereospecific metabolism with isomer interconversion. Studies on rats, aquatic organisms, and/or microorganisms have shown that debromination and Phase II conjugation are major pathways for TBBPA metabolism [101]. Abiotic factors in aquatic environments can significantly impact the phototransformation of BFRs. Recently, [PERSON] et al. [65] conducted a study on the phototransformation of TBBPA in alkaline saline water under simulated sunlight irradiation. They investigated the effects of various abiotic factors, including the initial concentration of TBBPA, chloride ion concentration, solution pH, inorganic anions and cations, and dissolved organic matter (DOM). The results revealed that the rate of TBBPA phototransformation increased with a decrease in the initial TBBPA concentration, increasing chloride ion concentration, and higher solution pH. Scavenging experiments indicated that hydroxyl radicals (\(\bullet\)OH), singlet oxygen (\({}^{1}\)O\({}_{2}\)), superoxide radicals (O\({}_{2}\)\(\bullet\)-), and triplet excited state TBBPA (STBBPA*) all played roles in the phototransformation of TBBPA. Additionally, the presence of nitrate (NO\({}_{3}\)-), carbonate (CO\({}_{3}\)\({}^{2-}\)), sulfate (SO\({}_{4}\)\({}^{2-}\)), magnesium ions (Mg\({}^{2+}\)), calcium ions (Ca\({}^{2+}\)), ferric ions (Fe\({}^{3+}\)), and fulvic acid (FA) inhibited the phototransformation of TBBPA in their study. [PERSON] et al. [56] conducted a study evaluating the anaerobic biotransformation of two non-brominated flame retardants (NBFRs), specifically 1,2,5,6-tetrabromocyclooctane (\(\beta\)-TBCO) and 2,3-dibromopropyl-2,4,6-tribromophenyl ether (DPTE), using the Dehalococcoides mccartyi strain CG1. Their findings indicated that both \(\beta\)-TBCO and DPTE were readily transformed by D. mccartyi CG1, with observed rate constants (k\({}_{\text{obs}}\) values) of 0.0218 \(\pm\) 0.0015 h\({}^{-1}\) and 0.0089 \(\pm\) 0.0003 h\({}^{-1}\), respectively.