Photodebromination behaviors of polybrominated diphenyl ethers in methanol/water systems: Mechanisms and predicting descriptors

Chemical reductive technologies for the debromination of polybrominated diphenyl ethers: A review

Polybrominated diphenyl ethers (PBDEs) are widely used as brominated flame retardants, which had attracted amounts of attention due to their harmful characteristics of high toxicity, environmental persistence and potential bioaccumulation. Many chemical reductive debromination technologies have been developed for the debromination of PBDEs, including photolysis, photocatalysis, electrocatalysis, zero-valent metal reduction, chemically catalytic reduction and mechanochemical method. This review aims to provide information about the degradation thermodynamics and kinetics of PBDEs and summarize the degradation mechanisms in various systems. According to the comparative analysis, the rapid debromination to generate bromine-free products in an electron-transfer process, of which photocatalysis is a representative one, is found to be relatively difficult, because the degradation rate of PBDEs depended on the Br-rich phenyl ring with the lowest unoccupied molecular orbital (LUMO) localization. On the contrary, the complete debromination occurs easily in other systems with active hydrogen atoms as the main reactive species, such as chemically catalytic reduction systems. The review provides the knowledge on the chemical reductive technique of PBDEs, which would greatly help not only clarify the degradation mechanism but also design the more efficient system for the rapid and deep debromination of PBDEs in the future.

Photoexcitation dynamics of bromodiphenyl ethers in acetonitrile-d3 studied by femtosecond time-resolved infrared spectroscopy

The efficient decomposition of polybrominated diphenyl ethers (PBDEs), onetime prevalent flame retardants, is central to the reduction of their harmful effects on human health. PBDE photodecomposition is a promising method, but its mechanism and products are not well understood. The photoexcitation dynamics of 3- and 4-bromodiphenyl ethers (BDE-2 and BDE-3) in CD₃CN were studied from 0.3 ps to 10 μs using time-resolved infrared spectroscopy. An excitation at 267 nm dissociated the Br atom from BDE-2 and BDE-3 within 0.3 ps and 14 ± 3 ps, respectively, producing a radical compound (R) and a Br atom. About 85% of R formed an intermediate (IM) that weakly interacted with the Br atom and the surrounding CD₃CN solvent in 7-12 ps. The remaining R separated from the dissociated Br and underwent slow geminate rebinding (GR) with Br within 35 to 54 ns. The IM competitively engaged in GR with the interacting Br in 40-60 ps or formed CD₃CN-bound radical compounds (RS) in 100-130 ps. The RS further degraded via either the dissociation of CD₃-producing a cyano-bound diphenyl ether (DE) in 150 or 550 ns-or the deuterium abstraction of CD₃CN in 180 or 430 ns-producing a deuterated DE. Overall, 33 ± 3 (22 ± 3)% of the photoexcited BDE-2 (BDE-3) decomposed in CD₃CN under 267 nm excitation. Efficient binding of the CD₃CN solvent to R deterred the yield-diminishing GR and slowed the rate of product formation. The observed photoexcitation dynamics of BDE suggest methods for the efficient decomposition of PBDE.

Study of the Photodegradation of PBDEs in Water by UV-LED Technology

Polybrominated diphenyl ethers (PBDEs) are persistent organic pollutants that can arrive to water bodies from their use as flame retardants in a wide range of applications, such as electric and electronic devices or textiles. In this study, the photodegradation of PBDEs in water samples when applying UV-LED radiation was studied. Irradiation was applied at three different wavelengths (255 nm, 265 nm and 285 nm) and different exposure times. The best degradation conditions for spiked purified water samples were at 285 nm and 240 min, resulting in degradations between 67% and 86%. The optimized methodology was applied to real water samples from different sources: river, marine, wastewater (effluent and influent of treatment plants) and greywater samples. Real water samples were spiked and exposed to 4 hours of irradiation at 285 nm. Successful photodegradation of PBDEs ranging from 51% to 97% was achieved for all PBDE congeners in the different water samples with the exception of the marine one, in which only a 31% of degradation was achieved.

Comprehensive exploration of the ultraviolet degradation of polychlorinated biphenyls in different media

As one of the most important natural transformation processes, photodegradation deserves more attention and research. In the current work, we comprehensively explored the photochemical behaviors of polychlorinated biphenyls (PCBs) in n-hexane (Hex), methanol/water, and silica gel under UV-irradiation. Photodegradation rates were found to be faster in methanol/water than in Hex. All of the three photochemical systems generated sigmatropic rearrangement products. The dominant photodegradation pathways were dechlorination, dechlorination/methoxylation/hydroxylation, and hydroxylation in Hex, methanol/water, and silica gel systems, respectively. Furthermore, some new photodegradation products, such as polychlorinated biphenyl ethers, polychlorinated dibenzofurans, polychlorinated biphenylenes, and methylated polychlorinated biphenyls, are reported for the first time. These findings would provide deeper insight into the phototransformation behaviors of PCBs.

Photocatalytic debromination of polybrominated diphenyl ethers (PBDEs) on metal doped TiO2 nanocomposites: Mechanisms and pathways

Nanoparticles of four noble metal doped titanium dioxide (i.e., Pd/TiO₂, Ag/TiO₂, Pt/TiO₂ and Cu/TiO₂) were synthesized and investigated for their effectiveness to degrade polybrominated diphenyl ethers (PBDEs) under UV light. All the investigated noble metal additives can greatly enhance the performance of TiO₂ to degrade 2,2',4',4'-tetrabromodiphenyl ether (BDE-47). However, the debromination pathways of BDE-47 in Ag/TiO₂ and Cu/TiO₂ systems are just contrary to those in Pd/TiO₂ and Pt/TiO₂ systems, and there was an induction period in the former systems but not in the latter systems. The hydrogenation experiment suggests a direct H-atom transfer mechanism in Pd/TiO₂ and Pt/TiO₂ systems, while in Ag/TiO₂ and Cu/TiO₂ systems, electron transfer is still the dominant mechanism. Electronic method was applied to explain why BDE-47 exhibit different debromination pathways based on different degradation mechanism. In addition, oxygen was proved to be able to capture both electrons and H atoms, and thus can greatly inhibit the degradation of PBDEs in all investigated systems. Finally, the merit and demerit of each metal doped TiO₂ were discussed in detail, including the reactivity, stability and the generation of byproducts. We proposed our study greatly enhance our understanding on the mechanisms of PBDE degradation in various metal doped TiO₂ systems.

In Situ Microbial Degradation of PBDEs in Sediments from an E-Waste Site as Revealed by Positive Matrix Factorization and Compound-Specific Stable Carbon Isotope Analysis

In the present study, positive matrix factorization (PMF) and compound-specific isotope analysis were used to investigate the in situ biodegradation of polybrominated diphenyl ethers (PBDEs) in sediment cores collected from a pond at an e-waste recycling site in South China. The potential microorganisms relevant to the degradation of PBDEs were also assessed to aid in the understanding of in situ biodegradation. The PMF results suggested that reductive debromination took place in the sediments. The debromination signal (ratio of the concentration of factor 5 (PMF result) to the total PBDE content) was positively correlated with the relative abundance of Dehalococcoidetes at different core depths. The clear ¹³C enrichment of five PBDE congeners (BDE 28, 47, 49, 99, and 153) with increasing core depth indicated that a measurable change in isotope fractionation might have occurred during PBDE biodegradation. The in situ biodegradation was further validated by the widespread detection of mono-BDE congeners (BDE 2, BDE 3) and diphenyl ether in the sediments. This study provides new evidence to enhance our understanding of the in situ biodegradation of PBDEs and suggests that the extensive removal of bromine from PBDEs was mediated by indigenous microorganisms at the e-waste site.

Effects of surfactant on the degradation of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) by nanoscale Ag/Fe particles: Kinetics, mechanisms and intermediates

Surfactants are known to enhance the degradation of halogenated organics by nanoscale zerovalent iron (n-ZVI) or n-ZVI-based bimetallic particles, but the mechanism of the promotion is not well understood. In this study, we used nanoscale Ag/Fe particles (n-Ag/Fe) to degrade 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) in different surfactant solutions. The results show that the nonionic surfactant TX-100 had the best promoting effect, which might be attributed to the decrease in particle agglomeration and improvement of mass transfer efficiency after the adsorption of TX-100 on n-Ag/Fe. The distribution analysis of BDE-47 in solid and liquid phases indicates that when the concentration of TX-100 in aqueous solution was above critical micelle concentration, BDE-47 started to dissolve in the liquid phase. Thus, TX-100 micelles can enhance the mass transfer efficiency of BDE-47. However, a too high concentration of TX-100 (above 1.0 mM) would influence the promotion effect of BDE-47 degration, which might be attributed to the excessive and thicker micelles of TX-100 hindering the contact between BDE-47 and n-Ag/Fe. We also studied the degradation pathway of BDE-47 and its products, and found that surfactants did not change the degradation pathway of BDE-47.

Experimental and theoretical investigations on debromination pathways of polybrominated biphenyls (PBBs) under ultraviolet light

Polybrominated biphenyls (PBBs) are brominated flame retardants that are widely used in textiles and electronic products. Recently, many researches have been devoted to determining their concentrations in food and in the environment. Yet, their degradation behavior has been less investigated and is not well understood. Here, we have investigated the debromination pathways of PBBs by (UV) light in the case of 2,4,5-tribrominated biphenyl (PBB-29). Our investigation indicates that para-bromine substituent on PBB-29 was preferentially removed. By means of density functional theory (DFT), we found that the energies of the debromination products, the CBr bond length in the excited state (S₁), the Mulliken charge of bromine in S₁, and the lowest unoccupied molecular orbital (LUMO) in S₁ correlated well with the debromination pathways of PBBs. Further, LUMO-based prediction of PBB debromination pathways in S₁ suggests that the bromine substituent on all brominated positions (i.e. ortho-, meta- and para-) can be preferentially removed, as the debromination sequence is not based on the brominated position but on the specific brominated arrangement pattern. In addition, reductive debromination preferentially occurs on the benzene ring that has the highest number of bromine substituents. This study provided useful descriptors to predict the debromination pathways of PBBs, and the theoretical result greatly improve our understanding of photolytic debromination of PBBs.

Debromination of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) by synthetic Pd/Fe0 and Cu/Fe0 in different protic solvents

Polybrominated diphenyl ethers (PBDEs) belong to a class of persistent organic pollutants (POPs), with potential toxicity to the liver, reproductive system, and development of mammals. The highly toxic and concentrated congener, 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), was chosen to investigate debromination mechanisms by the two synthetic iron-based bimetals (Pd/Fe⁰ and Cu/Fe⁰) in two protic solvents (water and ethanol). SEM, XPS, and BET analyses showed that the synthetic bimetals Pd/Fe⁰ and Cu/Fe⁰ were spherical with diameters of about 100 nm and loaded with ∼1% (wt%) of Pd and Cu, respectively. GC-MS was used for the analysis of degradation products and the chromatograms showed that both Pd/Fe⁰ and Cu/Fe⁰ bimetals had effective reducing properties in water solvent. In ethanol solvent, debromination of BDE-47 by Pd/Fe⁰ showed a similar high activity, but BDE-47 could be hardly degraded by Cu/Fe⁰. The dominant debromination products of BDE-47 by Pd/Fe⁰ and Cu/Fe⁰ were ortho-substituted and para-substituted BDEs, respectively. Active H-atomic transfer was found to play a key role in the debromination of BDE-47 by Pd/Fe⁰ in both, water and ethanol, with a preference for para-debromination along with the formation of dibenzo-p-furan (DF) as the by-product, mainly in water. In contrast, electron transfer with a preference for ortho-debromination was found to play a predominant role for Cu/Fe⁰ system in water. More importance should be provided to active H-atomic transfer for its high efficiency. In-depth study on the mechanism of formation of by-product DF would be significant for its higher toxicity, possibility of accumulation and migration in the environment.

Debromination of polybrominated diphenyl ethers (PBDEs) and their conversion to polybrominated dibenzofurans (PBDFs) by UV light: Mechanisms and pathways

Polybrominated diphenyl ethers (PBDEs) are typical flame retardant that have arose widely environmental concerns. Previous studies have found that PBDEs can generate lower BDEs and polybrominated dibenzofuran (PBDFs) under UV exposure, but these two processes were not well understood. In this study, we have investigated them through the case study of three BDE congeners (i.e. BDE-29, BDE-25 and BDE-21), which all have an ortho-, a meta- and a para-bromine substituents. The results shows that the vulnerability rank order of brominated position for these three BDE congeners are totally different, the bromine substituent at each position (ortho-, meta- or para-) can be preferentially removed, indicating it is not scientific to summarize the debromination pathways of PBDEs by comparing the brominated position. The lowest unoccupied molecular orbital (LUMO) of PBDEs in first excited state are well consistent with their actual debromination pathways, suggesting it is a good descriptor to predict the photodebromination pathways of PBDEs. In addition, the PBDEs with an ortho-bromine substituent can generate lower PBDFs, and the first step is to generate lower BDEs with an ortho-carbon radical, followed by ring closure reaction to generate PBDFs.

Debromination of polybrominated diphenyl ethers (PBDEs) by zero valent zinc: Mechanisms and predicting descriptors

Polybrominated diphenyl ethers (PBDEs) are a class of brominated flame retardants that are ubiquitous in the environment. The physical and chemical properties of PBDEs make them difficult to degrade, with the conventional remediation methods being relatively inefficient. In this study, the reactivity of zero valent zinc (ZVZ) toward 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) was evaluated under aqueous solution. First-order rate constants (k_obs) for BDE-47 disappearance increased with decreased pH, which is attributed to the dissolution of surface zinc oxides that promote the contact between the active site on zinc surface and BDE molecules. The k_obs of ten investigated PBDEs in ZVZ system are positively correlated with the energy of lowest unoccupied orbitals (E_LUMO) of PBDEs (R² = 0.902). The debromination pathways of BDE-47 in ZVZ system are: BDE-47 → BDE-28 → BDE-15 → BDE-3 → DE, which is the same to the debromination pathways of BDE-47 in zero valent iron (ZVI) in previous study. In addition, the singly occupied molecular orbitals (SOMOs) of the BDE anions can well reflect the actual debromination pathways of PBDEs by comparing the size of the CBr antibonding characterized lobes. Our results suggest that the debromination of PBDEs by ZVZ is based on the electron transfer mechanism, and the SOMOs of BDE anions can be used to predict the debromination pathways of untested PBDEs.

Assessment of exposure to polybrominated diphenyl ethers associated with consumption of market hens in Guangzhou

To evaluate contamination by polybrominated diphenyl ethers (PBDEs) in market hens and human PBDE exposure via hen consumption in Guangzhou, hens were collected and their muscle, liver, fat, blood, yolk, and ingluvies tissues were analyzed for 13 PBDE congeners. The median highest concentration of ∑PBDEs was found in the ingluvies (5.30 ng/g lw), followed by the muscle (2.53 ng/g lw), with the lowest located in the yolk (0.09 ng/g lw). The concentrations of PBDEs in the muscle tissue of market hens in Guangzhou were at medium levels compared to others reported around the world. BDE-47, -153, -99, and -183 were the predominant congeners. The daily intake concentrations of PBDEs from hen muscle were estimated to range from 0.08 to 0.31 ng/kg/day in this study, with a Hazard Quotient (HQ) below 1.0. These results suggest that the health risk of PBDEs for the general population, through the consumption of market hens in Guangzhou, was generally low. However, the intake of PBDEs via food consumption may be one major exposure pathway for the general population of Guangzhou.

Characterization and Photodegradation of Polybrominated Diphenyl Ethers in Car Seat Fabrics from End-of-Life Vehicles

In this study, we examined the photodegradation of decabromodiphenyl ether (BDE-209) on the surface of car seat covers from end-of-life vehicles (ELVs). Samples were collected at two car dismantling facilities in Sweden and cover car models from 1989 to 1998. The content of polybrominated diphenyl ethers (PBDEs) in nine real samples (fabric and polyurethane foam) was first characterized. Fabric samples that did not contain BDE-209 were then spiked with BDE-209 and irradiated in the laboratory and under sunlight. Photoproducts were identified using high performance liquid chromatography coupled to electrospray ionization mass spectrometer (HPLC-ESI-Orbitrap-MS), whereas volatile products were analyzed by gas chromatography-mass spectrometry (GC-MS). Similar photodegradation rates and oxidation products were observed in fabric samples irradiated in the laboratory and those collected from ELVs. Estimated half-life of BDE-209 on fabric inside vehicles ranged from 3 to 6 years. Thirteen major photoproducts were identified as lower brominated products, hydroxylated BDEs, brominated and hydroxylated dibenzofurans (PBDFs) and dioxins (PBDDs). Furthermore, several photoproducts were found to be transferable into water, particularly bromophenols and hydroxylated BDEs, and others into gas phase, such as bromomethanol and 1,2-dibromoethane. This should be taken into consideration for better estimating exposure to PBDEs and to develop strategies for ELV recycling.