| Brominated flame retardants (BFRs) are widely used flame retardants in the world which have effectively reduced the risk of fire accidents. BFRs contain polybrominated diphenyl ethers (PBDEs), hexabromocyclododecanes (HBCDs), tetrabromobisphenol A (TBBPA) and polybrominated biphenyls (PBBs). BFRs have been detected in various samples including air, indoor dusts, water, soil, sediment and human tissues. BFRs can enter into the environment during the production, impregnation and after use (recycle or burn of end-of-life wastes). BFRs are commonly stable and persistent in the environment. They can undergo migration with the atmospheric transport process and become a kind of well-known persistent pollutant in the Earth. Commercial penta- and octa-brominated diphenyl ethers (PentaBDEs and OctaBDEs) and HBCDs are being phased out today because of their potential hazards to humans. Other BFRs including decabrominated diphenyl ether (DecaBDE), TBBPA and PBBs etc. are produced in high volumes. DecaBDE is evidenced to produce less brominated diphenyl ethers and many products containing PentaBDE, OctaBDE and HBCDs are still in use. These compounds have potential threat to the environment. Besides, novel BFRs have been produced as alternatives of PBDEs and HBCDs. These BFRs have also been found in the environment but their environmental fates are still vague.The degradations of BFRs include self-decomposition and free-radical reaction with other radicals (active hydrogen atom, hydroxyl radical, for example). These reactions are complex, easily influenced by many factors and difficult to be revealed experimentally. As an alternative, computational methods including quantum chemical methods and molecular simulation are supposed to be effective in exploring the detailed mechanism. This paper has investigated the transformation mechanism of PBDEs, HBCDs, TBBPA and PBBs in detail. Tris(2,3-dibromopropyl) isocyanurate (TDBIC) has also been studied as a representative of novel BFRs. Some important conclusions are drawn as follows.(1) The self-decomposition and bimolecular reactions with active hydrogen atom and hydroxyl radical of 2,2’,4,4’,5-pentabromodiphenyl ether (BDE99) have been explored at the level of MPWBlK/6-311++G(3df,2p)//MPWB1K/6-31+G(d,p). The self-decomposition of BDE99 starts with the departure of a bromine atom at high temperature. In a hydrogen-rich environment, such as combustion of materials laden with BFRs, considering reactions of hydrogen atom with BFRs are of great significance. Hydrogen atom prefers to abstract bromine atom to form HBr and substituted phenyl radicals. The intermediates with a single electron in ortho-C can undergo intramolecular reactions to form polybrominated dibenzofurans (PBDFs). Through the bimolecular reaction with oxygen molecule, these intermediates can transfer to polybrominated dibenzo-p-dioxins (PBDDs). Hydroxyl radical addition reactions mainly occur at the tribrominated phenyl rings of BDE99, which is definitely different with the results of less brominated diphenyl ethers. The main products are hydroxylated BDE99s (OH-BDE99s). Among these compounds, ortho-OH-BDE99s are precursors of PBDDs. Additionally, the reactions between BDE99 and hydroxyl radical in aqueous solution have been performed using hybrid quantum mechanics and molecular mechanics (QM/MM) methods. The results imply that hydroxyl radical initiated reactions of BDE99 in water are more feasible than that in the gas phase.(2) The reactions of HBCDs have been performed at the level of BB1K/6-311++G(3df,2p)//BBIK/6-31+G(d,p). (±)-α-HBCDs and (±)-γ-HBCDs are more stable than (±)-β-HBCDs. HBCDs can undergo isomerization and self-decomposition reactions by releasing HBr. Among the isomerization reactions, (+)-y-HBCD transferring to (+)-a-HBCD occupies the fastest rate constant. At low temperature, the rate constants of isomerization reaction are larger than that of the HBr-releasing reactions. At high temperature, the latter reactions are more important. The main products of HBCDs are cyclododecahexaene, ethene, bromoethene and 1,2-dibromoethene under reductive conditions. Hydroxylated HBCDs are main products of HBCDs with hydroxyl radical.(3) The unimolecular and bimolecular reactions of TBBPA have been investigated at the M06-2X/6-311++G(3df,2p)//M06-2X/6-31+G(d,p) level. The degradation of TBBPA initiates with the departure of methyl group. However, with catalysis of water molecule, hydrogen atom migration from hydroxyl to ortho-C is more important than the above reaction. The formed ketone compound can release bromine atom endothermically. The main products from self-decomposition of TBBPA and TBBPA+ H reactions are both 1,1-di(3,5-dibromo-4-hydroxyl)-ethene. Bimolecular reactions of TBBPA with hydroxyl radical will generate various products including 4-isopropyl-2,6-dibromophenol,2,6-dibromophenol, 4-(1-hydroxylpropyl)-2,6-dibromophenol and their derivatives.(4) The reactions of 2,2’,4,4’,6,6’-hexabromobiphenyls (PBB153),2,2’,4,4’,6,6’-hexabromobiphenyls (PBB155) and TDBIC have been calculated at the same level of TBBPA. PBB153 and PBB155 are congeners with six bromine atoms. Their dissociation of bromine atoms has to absorb similar energies. But the total rate constant of PBB155+ H is two times higher than PBB153+H. The removal of bromine atom from TDBIC is less endothermic than PBB153 and PBB155, implying that TDBIC will release bromine atom first under combustion condition.(5) Based on the rate constants at 298.15 K, the OH-determined atmospheric lifetime of these BFRs are obtained. Given that typical concentration of hydroxyl radical is 1.0×106 molecule cm-3 in the air, the atmospheric lifetime of BDE99, (-)-α-HBCD, (-)-γ-HBCD, TBBPA, PBB153, PBB155 and TDBIC are 442,20,8,23,857, 975 and 4 days. Apparently, BDE99, PBB153 and PBB155 stay in the air for a long time. Without considering other processes (reactions with other oxidants, for example), TDBIC has the shortest atmospheric lifetime, indicating that TDBIC has no potential hazard to environment. |
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