# Chemicals from Brominated Flame Retardants: Analytical Methods, Occurrence, Transport and Risks
## 4 Transport Mechanisms of BFRs
### Soil Sorption and Fate Processes
#### 4.3.1 BFRs Transformation in Soil  
BFRs in soil are transformed physically, chemically, and metabolically by biological activities, resulting in the production of hydrobrominated, hydroxylation, and methoxylation BFRs. The environment and public health may also be negatively impacted by these degradation products. Thus, the process of transformation in soil is crucial. The two primary transformation mechanisms for BFRs in soil are photolysis and microbial degradation [49]. In contrast to water systems that are homogeneous or heterogeneous, organic contaminants in surface soil photodegrade slowly. This primarily has to do with how earth blocks light but in contrast to typical ways that organic contaminants in soil change.  
Figure 4: Main microbial transformation mechanisms for \(\beta\)-TBCO and DPTE involve \(\beta\)-elimination (E2) and Sn2-nucleophilic substitution (Adapted from [PERSON] et al. [56]).  
BFRs have the ability to absorb ultraviolet rays in the UV-A range (315-400 nm). When exposed to irradiation, they undergo degradation or photolysis, leading to the loss of bromine atoms (debromination) and possible rearrangement by cyclization [64]. The debromination process, necessary for creating hydrobrominated products, involves the gradual removal of bromine atoms. As BFRs vary in the number of bromine atoms they contain, their effects on photolysis processes also vary. Generally, debromination processes tend to occur more readily in the benzene rings that are more heavily brominated, especially when there is a significant difference in the number of bromine substitutions between two rings [104]. For BFRs with fewer bromine atoms, ortho bromine is more photoreactive than para bromine. However, this difference is not statistically significant for BFRs with greater degrees of bromination [103; 104]. During the cyclization process, intramolecular cyclization occurs, leading to the detachment of HBr and the production of further hazardous brominated dioxins and furans. The discharge of these photolysis products increases the hazards to human health [105].  
BFRs can also undergo transformation in soil through the action of microorganisms. While there are numerous reports on the biodegradation of BFRs, current research on the impact of indigenous soil microorganisms on BFRs through debromination is limited, and specific studies on the microbial degradation of NBFRs by bacteria or fungi are scarce [55]. Most existing research has focused on traditional BFRs, particularly PBDEs.  
In soil, the biodegradation of BFRs primarily occurs through two pathways: anaerobic debromination and aerobic biodegradation [106]. Anaerobic debromination involves the removal of bromine atoms from BFRs in an oxygen-limited environment, while aerobic biodegradation occurs in the presence of oxygen. The microbial degradation of BFRs can also be influenced by the presence of plant in the soil. [PERSON] et al. [99] conducted a study to investigate the fate and metabolites of 14C-labeled tetarbonophenhenol A (14C-TBBPA) in a submerged soil system with an anoxic-oxide interface, both with and without the presence of rice (_Oryza sativa_) and reed (_Phargmites australis_) seedlings. In the unplanted soil, TBBPA dissipated with a half-life of 20.8 days, accompanied by mineralization (11.5% of initial TBBPA) and the significant formation of bound residues (60.8%). The study identified twelve metabolites, with ten found in unplanted soil and seven in planted soil. These metabolites were formed through four interconnected pathways: oxidative skeletal cleavage, O-methylation, type II ipso-substitution, and reductive debromination. Interestingly, the presence of seedlings significantly reduced the mineralization of 14C-TBBPA and the formation of bound residues while simultaneously stimulating debromination and O-methylation processes. These findings suggest a complex interplay between plant presence and microbial activity in the fate and transformation of TBBPA in soil environments.