Environmental Photochemical Transformation Of Hydroxylated Polyhalodiphenyl Ethers And Novel Brominated Flame Retardants In Water

Hydroxylated polyhalodiphenyl ethers (HO-PXDEs) and novel brominated flame retardants (NBFRs) have been recognized as a group of widely detected emerging contaminants in the environment. It is of great significance to understand their environmental transformation behavior for the risk assessment of these pollutants. Previous studies proposed that photochemical transformation was an important transformation pathway for HO-PXDEs and their direct photolysis resulted in highly toxic dioxins. However, the detailed phototransformation pathways to form dioxins from HO-PXDEs are still unclear. Besides, photolytic kinetics, pathways and photoproducts of NBFRs are still unknown. Therefore, in the present study, the phototransformation pathways of HO-PXDEs and phototransformation kinetics and products of NBFRs were investigated, and the effects of dissolved organic matter (DOM) in natural water on the photolysis of HO-PXDEs and NBFRs were unveiled. The main research contents and results are as follows:(1) Triclosan and 2’-HO-BDE-28 were selected as model compounds of HO-PXDEs. Simulated sunlight experiments and density functional theory (DFT) calculations were performed to investigate the phototransformation pathways and the role of dissolved oxygen. Results show that the direct photolysis reactions of both triclosan and 2’-HO-BDE-28 occurred via their excited singlet states, leading to the formation of dioxins. In addition, dissolved oxygen promoted the phototransformation of the target compounds due to the generation of 1O2 and-OH. The DFT calculations indicated that 1O2 is generated in energy transfer reactions between excited triplet states and molecular O2. And OH is formed in electron transfer reactions of both excited singlet and triplet states with molecular O2.(2) The effects of Suwannee River natural organic matter (SRNOM) on the phototransformation kinetics, dioxin formation of triclosan and 2’-HO-BDE-28 were investigated in artificial estuarine water (AEW). Results show that although SRNOM induced indirect photolysis of triclosan and 2’-HO-BDE-28 due to the generation of excited triplet SRNOM, 102 and -OH., it decreased the observed photolytic rate constants due to light screening, static quenching and dynamic quenching effects. SRNOM increased the yields of 2,8-dichlorodibenzodioxin (2,8-DCDD) and 2,8-dibromodibenzodioxin (2,8-DBDD) that formed from triclosan and 2’-HO-BDE-28, respectively. It was also found that in AEW, SRNOM showed no obvious effects on the degradation of 2,8-DCDD due to the quenching of reactive species by high concentration halogen ions, and increased the degradation rate of 2,8-DBDD in AEW due to the coexistence with Cl-(3) Five tribromophenoxy flame retardants that synthetized with 2,4,6-tribromophenol [allyl-2,4,6-tribromophenyl ether (ATE),2-bromoallyl-2,4,6-tribromophenyl ether (BATE), 2,3-dibrornopropyl-2,4,6-tribrornophenyl ether (DPTE), 1,2-bis(2,4,6-tribromophenoxy) ethane (BTBPE). and 2,4,6-tris(2,4,6-tribrornophenoxy)-1,3,5-triazine (TTBP-TAZ)] were selected as model compounds. Photochemical transformation of these NBFRs in hexane and methanol was investigated and the phototransformation products of DPTE and BTBPE were identified with simulated sunlight experiments. Results show that all the five NBFRs can undergo photochemical transformation under simulated sunlight irradiation. The order of observed photolysis rate constant (kobs) is:BTBPE> ATE> BATE> TTBP-TAZ> DPTE. For ATE, BATE and DPTE that only have one phenyl and a hydrocarbon chain, theirkobs decreased with the increase of Br atoms on the hydrocarbon chain. Quantum yields (0) of the five NBFRs varied from 0.012 of TTBP-TAZ in hexane to 0.091 of BTBPE in methanol. Debromination on the phenyl is a main phototransformation pathway for DPTE, and both debromination and ether bond cleavage are main phototransformation pathways for BTBPE. 2,4,6-Tribromophenol and 2,4-dibromophenol were identified as the photoproducts of BTBPE. The formation of bromophenols enhances the potential risk of BTBPE in the environment.(4) The phototransformation kinetics and photoproducts of NBFRs were determined in natural water with DPTE as a model compound, which was widely detected in natural waters. The effects of four commercial freshwater DOM and extracted seawater DOM on the photolysis kinetics of DPTE were revealed. Results show that the degradation of DPTE in natural water is faster than in pure water, and in seawater is faster than in freshwater. The direct photolysis 0 of DPTE was determined to be 0.008 ± 0.001. Direct photolysis half-lives relevant with solar irradiation in surface waters at 40(?) latitude of DPTE were estimated to be 20.1-25.9 d in summer. Freshwater DOM can promote the phototransformation of DPTE due to the formation of 3D0M* and OH, and the coexistence of Cl- and DOM further increased the kobs of DPTE. The promotion effects of swawater DOM are more significant than the four freshwater DOM. Hydrodebromination, ether bond cleavage, and hydroxylated products were identified in the phototransformation of DPTE in freshwater and seawater samples. Chlorinated products were also identified in seawater, indicateing that Cl-is involved in the phototransformation of DPTE.This study revealed the roles of the excited states and dissolved oxygen in the photochemical transforamtion of HO-PXDEs, investigated the phototransformation behavior of NBFRs model compounds, and unveiled the effects of dissolved organic matter on their phototransformation. The results are important for elucidating the phototransformation mechanisms of halogenated organic pollutants, and are helpful for understanding the fate of these pollutants in the environment.