Environmental Photochemical Transformation Of Polybromodiphenyl Ethers And Their Hydroxylated Derivitives

Polybrominated diphenyl ethers (PBDEs), the most commonly used flame retardants, have been regarded as ubiquitous persistent pollutants. Hydroxylated polybrominated diphenyl ethers (HO-PBDEs) were also detected in various biotic samples and waters. The two kinds of pollutants attract considerable concern due to their large quantity existences, wide distributions, and endocrine disrupting activities. Knowledge on their environmental transformation is critical for their risk assessment. PBDEs and HO-PBDEs are proved to undergo photodegradation. However, it is still unclear about the mechanisms for the effects of environmental media on PBDEs photolysis. Photolytic kinetics, pathways and photoproducts of HO-PBDEs need to be clarified as well. Therefore, in the present study, BDE-209 was selected and 6 HO-PBDEs were synthesized as model compounds. The photodegradation kinetics, products and mechanisms of these compounds were investigated, and the effects of important environmental factors on the photodegradation were elucidated. As theoretical calculation shows great advantages on reaction pathway analysis, both experimental and theoretical calculation methods were adopted for the studies.BDE-209 distributes in various environmental media which are proposed to influence the photolysis of BDE-209 in multiple ways. In order to explore the effects of media on the BDE-209 photolysis, organic solvents were selected as the model media in which photochemical experiments were performed. The photolysis rate constants (k) of BDE-209 are noticeably different in the studied solvents. With the density functional theory (DFT) calculation, the average formal charge on Br and the vertical excitation energy of BDE-209 were found to be changed by the non-specific effect of solvent (such as dipole-dipole interaction). The two parameters have pronounced linear relationship with logk. Analysis about the relationship of solvent property and BDE-209 structure with logk indicates that solvents influence the photolytic kinetics via both specific effect as a hydrogen donor and non-specific effects. Light PBDEs were identified as the main photoproducts in the solvents with hydrogen donating ability, and the yields were influenced by solvents.Ionization of HO-PBDEs prevails in natural waters due to their pKa values ranging from 4.2 to 9.3. Differences of the photochemical properties between the netrual and anionic HO-PBDEs need to be unveiled. As a result, photolysis of HO-PBDEs at different pH levels was investigated. The light absorptions (A), quantum yields (Φ) and k for the two states were determined. The results show that HO-PBDE anions have higher A andΦthan the molecules, resulting higher k values. For the different HO-PBDEs congeners, bromination increases A and decreasesΦ. A series of equations related to structure and photolytic activity were developed. It was found that ionization decreased the energy gap of the frontier molecular obital (ΔEgap) and the strength of the ether bond, leading to higher A andΦvalues of the anions than the neutral molecules. A system for evaluating the direct photolysis half-lives(t1/2) of HO-PBDEs in different seasons and pH conditions was developed. The t1/2 values of studied HO-PBDEs in waters at 45°N latitude are 0.317-252 h. Photoproducts of 2’-HO-BDE-68 were analyzed with GC-MS.2,4-dibromophenol (2,4-DBP), dihydroxylated dibromobenzene,1,3,8-tribrominated dioxin (1,3,8-TBDD), and dihydroxylated tribromodiphenyl ether (di-HO-TBDE) were identified, and the proposed degradation pathways involve ether bond cleavage, photo-hydrolysis and photo-cyclization.Oxidation by the reactive oxygen species formed in sunlit waters is an important transformation pathway of many organic pollutants, but little is known for the photo-oxidation activity of HO-PBDEs. Therefore, the second order reaction rate constants of HO-PBDEs with 1O2 (k1O2) and·OH (kOH) at different pH conditions, as well as the values for the neutral molecules and anions were determined. The k102 values for anions are higher than those for the molecules, and increase with the increasing degree of bromination. DFT calculation and partial least squares analysis indicate that ionization increases the electron donating ability, and weakens the ether bond and thus favors the electrophilic reaction of 1O2. All the six HO-PBDEs are highly reactive toward OH, with kOH values of the anions range from 5.0±0.2 to 9.6±0.4 x 1010 M-1s1, which are about 7～27 times higher than the corresponding neutral molecules.2,4-DBP is the main product for both the 1O2 and·OH reactions, and no brominated dioxin is detected. Based on the results about direct photolysis rate and reactivities with 1O2 and·OH, direct photolysis is the dominant pathway, and reaction with 1O2 also plays an important role in the photochemical transformation of the studied HO-PBDEs in waters at 45°N latitude.Yields of 1,3,8-TBDD and di-HO-TBDE in direct photolysis of 2’-HO-BDE-68 depend on pH. To reveal the mechanisms, photo-cyclization and photo-hydrolysis of 2’-HO-BDE-68 were computed based on DFT. Photo-cyclization proceeds via two steps:rotation of the benzene ring and cyclization, with the latter to be the rate determining step. The cyclization activation energy (ΔEa) for anionic 2’-HO-BDE-68 at the first triplet state (T1 state) is lower than that of the molecular forms. Carbene is produced via the C-Br bond heterolysis, and it can be attacked by H2O or OH- to generate di-HO-TBDE. The reaction can be catalyzed by alkali, withΔEa to be 0.408 kcal/mol, which indicates that photo-hydrolysis in basic solution is an important photodegradation process.