UVA/B-induced formation of free radicals from decabromodiphenyl ether

Formation of environmentally persistent free radicals on microplastics under UV irradiations

Microplastics (MPs) are widely distributed in the environment due to breakdown of widespread plastic wastes through physicochemical and biological processes. Environmentally persistent free radicals (EPFRs) might be generated as intermediates when MPs are further fragmented and decomposed under ultraviolet (UV) radiation. Formation of EPFRs is highly depended upon the radiation energy level. This study was designed to establish the correlation between EPFRs concentrations and UV energy. Polystyrene (PS) and polyethylene (PE) were employed to investigate the generation of EPFRs under the irradiation of three ultraviolet light sources (long-wave UVA, medium-wave UVB and short-wave UVC). Electron paramagnetic resonance (EPR) spectroscopy revealed that free radical signals were detected on PS irradiated by UVC and UVB and PE irradiated by UVC, which may be due to the difference in the MPs structure and UV energy. The g-factor and ΔH_p-p of EPR suggested that three different types of EPFRs may be formed on PS while two types of EPFR may be formed on PE. Meanwhile, EPFRs were detected within shorter time under UVC radiation than UVB and UVA, indicating that UVC radiation could lead to faster generation of free radicals. Results of Fourier transform infrared spectroscopy (FTIR) and two-dimensional correlation spectroscopy revealed that tertiary alkyl radicals, peroxy radicals and tertiary alkoxy radicals were dominant in PS whereas alkoxy radicals and keto radicals for PE. The study provides insight to the mechanisms for EPFRs formation on ubiquitously found microplastic particles. Our finding is of great significance as EPFRs may not only play important roles in decomposition of MPs and abiotic reactions of MPs-bound pollutants, but also affect physicochemical properties of MPs and MPs toxicity to aquatic organisms, hence possessing broad impacts on MPs fate and transport in aquatic environmental systems.

Hydrogen-Donor-Controlled Polybrominated Dibenzofuran (PBDF) Formation from Polybrominated Diphenyl Ether (PBDE) Photolysis in Solutions: Competition Mechanisms of Radical-Based Cyclization and Hydrogen Abstraction Reactions

Polybrominated dibenzofurans (PBDFs) are characteristic dioxin-like products of polybrominated diphenyl ether (PBDE) photolysis. In this study, competition mechanisms of radical-based cyclization and hydrogen abstraction reactions are proposed in PBDF formation. Commonly, the ortho C-Br bond dissociation during photolysis generates aryl radicals, which undergo intramolecular cyclization to form PBDFs or hydrogen abstraction with hydrogen donors (such as organic solvents and water) to form lower brominated PBDEs. By using 2,4,4'-tribromodiphenyl ether (BDE-28) as the model reactant, the experimental PBDF formation ratios in various solutions are explained quantitatively by the calculated rate constants of cyclization and hydrogen abstraction reactions using the density functional theory (DFT) method. The solvent effect of pure and mixed solvents on PBDF formation is illustrated successfully. The structure-related hydrogen donation ability for hydrogen abstraction controls the bias of competition reactions and influences PBDF formation. Water resulted to be the most significant generation of PBDFs. Fulvic and humic acid display higher hydrogen donation ability than small-molecule organics due to the partitioning effect in aqueous solution. Quantitative structure-activity relationship (QSAR) models of the calculated rate constants for 512 cyclization and 319 hydrogen abstraction reactions using 189 PBDEs as the initial reactants in water are established, revealing the high risk of PBDF formation in aqueous solution.

Copper Cocatalyst Modulated Radical Generation for Selective Heterogeneous Photosynthesis of α-Haloketones

The α-haloketones are important precursors for synthetic chemistry and pharmaceutical applications; however, their production relies heavily on traditional synthetic methods via halogenation of ketones that are toxic and environmentally risky. Here, we report a heterogeneous photosynthetic strategy of α-haloketone production from aromatic olefins using copper-modified graphitic carbon nitride (Cu-C₃N₄) under mild reaction conditions. By employing NiX₂ (X = Cl, Br) as the halogen source, a series of α-haloketones can be synthesized using atmospheric air as the oxidant under visible-light irradiation. In comparison with pristine carbon nitride, the addition of Cu as a cocatalyst provides a moderate generation rate of halogen radicals and selective reduction of molecular oxygen into ^•OOH radicals, thus leading to a high selectivity to α-haloketones. The Cu-C₃N₄ also exhibits high stability and versatility, rendering it a promising candidate for solar-driven synthetic applications.

Dissolved Organic Matter Promotes the Aging Process of Polystyrene Microplastics under Dark and Ultraviolet Light Conditions: The Crucial Role of Reactive Oxygen Species

Microplastics (MPs) interact frequently with dissolved organic matter (DOM) commonly found in the environment, but information on the aging behavior of MPs under the participation of DOM is still lacking. Thus, the polystyrene microplastic (PSMP) aging process with DOM participation was systematically studied by electron paramagnetic resonance spectroscopy, high-performance liquid chromatography, Fourier transform infrared (FTIR) spectroscopy, and two-dimensional correlation spectroscopy analyses under dark and ultraviolet (UV) light conditions. DOM was found to promote electron transfer to generate reactive oxygen species (ROS) under dark conditions and the aging of PSMPs, while the process of DOM generating ROS under UV light was more susceptible to photoelectrons and accelerated the aging process of PSMPs. However, among the four DOM types, fulvic acid (FA) has a more significant promoting effect on the aging process of PSMPs than humic acid, which can be attributed to the stronger conversion ability of FA to semiquinone radicals. Density functional theory calculations are used to describe the difference in the aging process of different structures of plastics with the participation of DOM. This study provides a necessary theoretical basis for the study of the migration of MPs in groundwater and deep surface water.

In Vitro Assessment Reveals the Effects of Environmentally Persistent Free Radicals on the Toxicity of Photoaged Tire Wear Particles

Tire wear particles (TWP) have been identified as one of the major sources of microplastics (MPs), and few studies have focused on their environmental behaviors and impacts. However, a thorough characteristic and toxicity assessment associated with environmentally persistent free radicals (EPFRs) on the photoaged TWP is missing. In this study, we investigated EPFRs in the process of TWP photoaging and evaluated their toxicity using in vitro bioassays. Our results showed that a total of around 1.0 × 10¹⁷ spins/g EPFRs (g-factors ranging 2.00308-2.00318) was formed on TWP with 60 days of light irradiation, which contained more than 29% of reactive EPFRs (r-EPFRs). Using macrophages as model cells for bioassays, TWP-associated EPFRs trigged endpoints, including the decrease of cell viability (27 to 45%) and the increase of oxidative stress response (46-93%) and inflammatory factor secretion. The enhancement of TWP toxicity with photoaging was confirmed to be attributed to the generated EPFRs combined with other TWP's chemical compositions (e.g., various metals and organics). Most importantly, the toxicity of photoaged TWP was closely correlated with the generated r-EPFRs, which induced reactive oxidant species (ROS) generation. This study provides direct evidence of toxicity on the photoaged TWP particles, revealing the potential contributions of EPFRs to the adverse effect on human health and highlighting the need for an improved understanding of the impacts of EPFRs on the risk assessment of TWP released into the environment.

CuO and TiO2 particles generated more stable and stronger EPFRs in dark than under UV-irradiation

Environmentally persistent free radicals (EFPRs) have recently attracted a great deal of research attention because of their significant toxicity and ubiquitous occurrence in the environment. The information is still very limited on how to estimate the intensity of EPFRs under ambient circumstances. This study is designed to specifically compare EPFRs generation during catechol degradation in dark and UV light irradiation. CuO and TiO₂ were selected as model metal oxides to coat on silica at 1% CuO has a large electron exchange capacity, which may mediate catechol degradation in dark, while TiO₂ possesses strong photocatalytic property and could accelerate catechol degradation under UV light. Under UV light irradiation, EPFRs were generated very quickly and reached the maximum value in 4 d, which was related to the photocatalytic property of the particle. However, these EPFRs dissipated quickly in 14 d. On the contrary, the intensities of EPFRs generated in dark were 2 times higher, and stabled for over 2 months. Therefore, the environmental impacts of EPFRs in dark may be widespread and long-lasting, which should be monitored more carefully. It should be noted that for CuO-coated silica, a significant amount of EPFRs (20% of the maximum) survived the UV-light irradiation and stabled during the experimental period (45 d). Stronger EPFRs were associated with more abundant dimer structures, suggesting the dimer structures were related to EPFRs formation during catechol degradation. Monitoring the generation of dimer structures in the degradation of organic chemicals may provide useful information to estimate EPFRs generation and risks.

Progress of Bromophenols in Marine Algae from 2011 to 2020: Structure, Bioactivities, and Applications

Marine algae contain various bromophenols that have been shown to possess a variety of biological activities, including antiradical, antimicrobial, anticancer, antidiabetic, anti-inflammatory effects, and so on. Here, we briefly review the recent progress of these marine algae biomaterials and their derivatives from 2011 to 2020, with respect to structure, bioactivities, and their potential application as pharmaceuticals.

Characterization of a Sequential UV Photolysis-Biodegradation Process for Treatment of Decabrominated Diphenyl Ethers in Sorbent/Water Systems

Decabrominated diphenyl ether (BDE-209) is a primary component of the brominated flame retardants used in a variety of industrial and domestic applications. BDE-209 bioaccumulates in aquatic organisms and has been identified as an emerging contaminant that threatens human and ecosystem health. Sequential photolysis-microbial biodegradation processes were utilized here to treat BDE-209 in clay- or soil-water slurries. The removal efficiency of BDE-209 in the clay-water slurries was high; i.e., 96.5%, while that in the soil-water slurries was minimal. In the clay-water slurries the first order rate constants for the UV photolysis and biodegradation of BDE-209 were 0.017 1/day and 0.026 1/day, respectively. UV wavelength and intensity strongly influenced the BDE-209 photolysis and the subsequent biodegradation of photolytic products. Facultative chemotrophic bacteria, including Acidovorax spp., Pseudomonas spp., Novosphingobium spp. and Sphingomonas spp., were the dominant members of the bacterial community (about 71%) at the beginning of the biodegradation; many of these organisms have previously been shown to biodegrade BDE-209 and other polybrominated diphenyl ether (PBDE) congeners. The Achromobacter sp. that were isolated (NH-2; NH-4; NH-6) were especially effective during the BDE-209 degradation. These results indicated the effectiveness of the sequential UV photolysis and biodegradation for treating certain BDE-209-contaminated solids; e.g., clays; in bioreactors containing such solids as aqueous slurries. Achieving a similar treatment effectiveness for more heterogeneous solids containing natural organic matter, e.g., surface solids, appears to be significantly more difficult. Further investigations are needed in order to understand the great difference between the clay-water or soil-water slurries.

Photodecomposition properties of brominated flame retardants (BFRs)

This study investigates the geometric and electronic properties of selected BFRs in their ground (S₀) and first singlet excited (S₁) states deploying methods of the density functional theory (DFT) and the time-dependent density functional theory (TDDFT). We estimate the effect of the S₀→ S₁ transition on the elongations of the C-Br bond, identify the frontier molecular orbitals involved in the excitation process and compute partial atomic charges for the most photoreactive bromine atoms. The bromine atom attached to an ortho position in HBB (with regard to C-C bond; 2,2',4,4',6,6'-hexabromobiphenyl), TBBA (with respect to the hydroxyl group; 2,2',6,6'-tetrabromobisphenol A), HBDE and BTBPE (in reference to C-O linkage; 2,2',4,4',6,6'-hexabromodiphenylether and 1,2-bis(2,4,6-tribromophenoxy)ethane, respectively) bears the highest positive atomic charge. This suggests that, these positions undergo reductive debromination reactions to produce lower brominated molecules. Debromination reactions ensue primarily in the aromatic compounds substituted with the highest number of bromine atoms owing to the largest stretching of the C-Br bond in the first excited state. The analysis of the frontier molecular orbitals indicates that, excitations of BFRs proceed via π→π*, or π→σ* or n→σ* electronic transitions. The orbital analysis reveals that, the HOMO-LUMO energy gap (E^H-L) for all investigated bromine-substituted aromatic molecules falls lower (1.85-4.91 eV) than for their non-brominated analogues (3.39-8.07 eV), in both aqueous and gaseous media. The excitation energies correlate with the E^H-L values. The excitation energies and E^H-L values display a linear negative correlation with the number of bromine atoms attached to the molecule. Spectral analysis of the gaseous-phase systems reveals that, the highly brominated aromatics endure lower excitation energies and exhibit red shifts of their absorption bands in comparison to their lower brominated congeners. We attained a satisfactory agreement between the experimentally measured absorption peak (λ_max) and the theoretically predicted oscillator strength (λ_max) for the UV-Vis spectra. This study further confirms that, halogenated aromatics only absorb light in the UV spectral region and that effective photodegradation of these pollutants requires the presence of photocatalysts.

Photochemical reaction of tricresyl phosphate (TCP) in aqueous solution: Influencing factors and photolysis products

Organophosphate triesters (OPEs) have caused great concern as a class of emerging environmental contaminants due to their widespread use and their toxicity to organisms. However, the phototransformation behavior of OPE is still not fully understood, which is important for understanding their environmental fate. In the present study, the photodegradation of tricresyl phosphate (TCP), one of the most widely detected OPEs in aqueous environments, was investigated including the direct photolysis and in the presence of several natural water factors, NO₂^-, Fe³⁺ and humic acid. The degradation process followed the pseudo-first-order kinetics, with rate constant increasing slightly with increasing initial TCP concentration. The presence of NO₂^- and Fe³⁺ was observed to promote the photochemical loss of TCP, while humic acid played a negative role on TCP transformation. Electron spin resonance (EPR) analysis showed that carbon-centered radical was produced in the photolysis process of TCP, and hydroxyl radical contributed to the promotion of rate constant for Fe³⁺ and NO₂^-. Four photolysis products were tentatively identified by HPLC-LTQ-Orbitrap MS analysis, and the possible degradation pathways of TCP were proposed. These findings provide a meaningful reference for the fate and transformation of OPEs in natural water.

Formation of Environmentally Persistent Free Radicals on Microplastics under Light Irradiation

Microplastics (MPs) are presumed to be inert during aging under ambient conditions. In this study, four types of virgin MPs, including polystyrene (PS), phenol-formaldehyde resin (PF), polyethylene (PE), and polyvinyl chloride (PVC), were aged under simulated solar light irradiation. Surprisingly, several environmentally persistent free radicals (EPFRs), which are considered to be a type of emerging contaminant, were detected on the irradiated PS and PF, rather than PE and PVC, by electron paramagnetic resonance (EPR) spectroscopy. Depending on the photoaging duration time, the characteristic g-factors of the EPFRs produced on PS and PF were 2.0044-2.0049 and 2.0043-2.0044, respectively. The generated EPFRs on PS and PF decayed rapidly at the initial stage and then slowly disappeared with the elapsed aging time. Analyses by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR), and gel permeation chromatography (GPC) suggested that MPs might experience chemical chain scission, O₂/H₂O addition, and EPFR formation under the light irradiation. Accompanying with the formation of EPFRs, reactive oxygen species, such as O₂^•- and •OH, were also observed. The findings provide a novel insight to evaluate the potential hazards of MPs to organisms and ecosystems.

Variation of Microbial Communities in Aquatic Sediments under Long-Term Exposure to Decabromodiphenyl Ether and UVA Irradiation

Abiotic components create different types of environmental stress on bacterial communities in aquatic ecosystems. In this study, the long-term exposure to various abiotic factors, namely a high-dose of the toxic chemical decabromodiphenyl ether (BDE-209), continuous UVA irradiation, and different types of sediment, were evaluated in order to assess their influence on the bacterial community. The dominant bacterial community in a single stress situation, i.e., exposure to BDE-209 include members of Comamonadaceae, members of Xanthomonadaceae, a Pseudomonas sp. and a Hydrogenophaga sp. Such bacteria are capable of biodegrading polybrominated diphenyl ethers (PBDEs). When multiple environmental stresses were present, Acidobacteria bacterium and a Terrimonas sp. were predominant, which equipped the population with multiple physiological characteristics that made it capable of both PBDE biodegradation and resistance to UVA irradiation. Methloversatilis sp. and Flavisolibacter sp. were identified as representative genera in this population that were radioresistant. In addition to the above, sediment heterogeneity is also able to alter bacterial community diversity. In total, seventeen species of bacteria were identified in the microcosms containing more clay particles and higher levels of soil organic matter (SOM). This means that these communities are more diverse than in microcosms that contained more sand particles and a lower SOM, which were found to have only twelve identifiable bacterial species. This is the first report to evaluate how changes in bacterial communities in aquatic sediment are affected by the presence of multiple variable environmental factors at the same time.

Phototransformation of 2,4,6-tribromophenol in aqueous solution: Kinetics and photolysis products

In this research, we studied the phototransformation of 2,4,6-tribromophenol (TBP) by UV irradiation in aqueous solution. Effects of typical environmental factors (i.e., pH and the initial concentration of TBP, Fe³⁺ and NO₂^-) were also investigated. Results showed that the transformation process followed pseudo-first-order kinetics. The transformation rate constant of TBP decreased with increasing initial concentration and increased with increasing Fe³⁺ and NO₂^- concentrations as well as increasing pH. Ten photoproducts were tentatively identified after irradiation of TBP, according to HPLC-LTQ-Orbitrap MS and GC/MS analysis. Among them, two dihydroxylated dibromobenzene compounds (di-OH-DBB) were found to be predominant over the reaction time, indicating that hydrodebromination of TBP could be the main phototransformation mechanism. In addition, the identification of eight OH-PBDEs and di-OH-PBBs suggested that photodimerization might also be a reaction pathway for TBP photochemical reactions.

Application of aluminum-supported Pd, Rh, and Rh-Pd nanoparticles in supercritical carbon dioxide system for hydrodebromination of polybrominated diphenyl ethers

Al-powder-supported Pd, Rh, and Rh-Pd catalysts were synthesized through a spontaneous redox reaction in aqueous solutions. These catalysts hydrodebrominated 4- and 4,4'-bromodiphenyl ethers in supercritical carbon dioxide at 200 atm CO2 containing 10 atm H2 and 80 °C in 1 h. Diphenyl ether was the major product of Pd/Al. Rh/Al and Rh-Pd/Al further hydrogenated two benzene rings of diphenyl ether to form dicyclohexyl ether. The hydrogenolysis of CO bonds on diphenyl ether over Rh/Al and Rh-Pd/Al was observed to generate cyclohexanol and cyclohexane (<1%). With respect to hydrodebromination efficiency and catalyst stability, Rh-Pd/Al among three catalysts is suggested to be used for ex situ degradation of polybrominated diphenyl ethers in supercritical carbon dioxide.

Exposure Assessment to Environmental Chemicals in Children from Ciudad Juarez, Chihuahua, Mexico

It has been demonstrated that the human biomonitoring of susceptible populations is a valuable method for the identification of critical contaminants. Therefore, the purpose of this study was to assess the exposure profile for arsenic (As), lead (Pb), mercury (Hg), 1-hydroxypyrene (1-OHP), 1,1-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT), 1,1-dichloro-2,2-bis(p-chlorophenyl) ethylene (DDE), polybrominated diphenyl ethers (PBDEs), and polychlorinated biphenyls (PCBs) in children living in Ciudad Juarez, Chihuahua, Mexico (a major manufacturing center in Mexico). In 2012, we evaluated a total of 135 healthy children living in Ciudad Juarez since birth. The total PBDEs levels ranged from nondetectable (< LOD) to 215 ng/g lipid, with a mean total PBDEs level of 29.5 ± 53.0 ng/g lipid (geometric mean ± standard deviation). The mean total PCBs level in the study participants was 29.0 ± 10.5 ng/g lipid (range 4.50-50.0 ng/g lipid). The mean concentration of total DDT (DDT + DDE) was 11.9 ± 6.70 ng/g lipid (range 3.00-26.0 ng/g lipid). The mean 1-OHP levels was 1.2 ± 1.1 µmol/mol creatinine (range <LOD to 3.90 µmol/mol creatinine). Regarding heavy metals levels, the mean urinary As levels was 19.5 ± 3.07 µg/g creatinine, for urinary mercury the levels ranged from <LOD to 11.5 µg/L, with a mean value of 2.10 µg/L, and finally, the mean blood lead level was 4.20 ± 3.80 µg/dL. In conclusion, our data indicate high exposure levels to chemicals analyzed in the children living in the study community. Therefore, a biomonitoring program for the surveillance of the child population in Ciudad Juarez is necessary.

Monohydroxylated Polybrominated Diphenyl Ethers (OH-PBDEs) and Dihydroxylated Polybrominated Biphenyls (Di-OH-PBBs): Novel Photoproducts of 2,6-Dibromophenol

Hydroxylated polybromodiphenyl ethers (OH-PBDEs) are emerging aquatic pollutants, but their origins in the environment are not fully understood. There is evidence that OH-PBDEs are formed from bromophenols, but the underlying transformation processes remain unknown. Here, we investigate if the photoformation of OH-PBDEs from 2,6-dibromophenol in aqueous solution involves 2,6-bromophenoxyl radicals. After the UV irradiation of an aqueous 2,6-dibromophenol solution, HPLC-LTQ-Orbitrap MS and GC-MS analysis revealed the formation of a OH-PBDE and a dihydroxylated polybrominated biphenyl (di-OH-PBB). Both dimeric photoproducts were tentatively identified as 4'-OH-BDE73 and 4,4'-di-OH-PBB80. In addition, three debromination products (4-OH-BDE34, 4'-OH-BDE27, and 4,4'-di-OH-PBBs) were observed. Electron paramagnetic resonance spectroscopy revealed the presence of a 2,6-dibromophenoxyl radical with a six-line spectrum (a(H) (2 meta) = 3.45 G, a(H) (1 para) = 1.04 G, g = 2.0046) during irradiation of a 2,6-dibromophenol solution in water. The 2,6-dibromophenoxyl radical had a relatively long half-life (122 ± 5 μs) according to laser flash photolysis experiments. The para-para C-C and O-para-C couplings of these 2,6-dibromophenoxyl radicals are consistent with the observed formation of both dimeric OH-PBDE and di-OH-PBB photoproducts. These findings show that bromophenoxyl radical-mediated phototransformation of bromophenols is a source of OH-PBDEs and di-OH-PBBs in aqueous environments that requires further attention.

Photochemistry of tetra- through hexa-brominated dioxins/furans, hydroxylated and native BDEs in different media

The aim of this study was to investigate (i) the behavior of native PBDEs during UV irradiation in different media, (ii) the possibility of their transformation into hydroxylated PBDEs in aqueous media, and (iii) the photochemistry/levels of brominated dioxins/furans formed from hydroxylated PBDEs. Debromination leading to the formation of a wide range of low-brominated congeners was the main path of photocatalyzed transformations of PBDEs. In organic solvents other than toluene, BDEs degraded in line with the pseudo first order kinetics (10-20 min half-life, depending on congener type and reaction medium). Irradiated BDE 209 congener behaved quite differently than lower-brominated BDEs: detectable amounts of various bromo-benzenes were found. That suggests that UV irradiation of BDE 209 leads to cleavage of the ether bound between the congener's aromatic rings. Formation of bromophenyl bromo-methyl-biphenyl ethers or benzyl-bromophenoxybenzenes was observed in irradiated PBDE toluene-based solutions. The total concentration of OH-BDEs found in the reaction medium did not exceed 0.2% of the initial precursor mass. Moreover, lower-brominated congeners detected in the reaction medium indicate subsequent debromination of OH-BDEs or hydro-debromination of the degraded congeners. Brominated dioxins and low levels of furans were observed in samples containing OH-BDEs. The total mass of dioxins did not exceed 3.5% of the initial precursor mass.

Characterization of the molecular degradation mechanism of diphenyl ethers by Cupriavidus sp. WS

Commonly used flame retardants, such as polybrominated diphenyl ethers, are extremely persistent in the environment, causing serious environmental risks. Certain strains of bacteria are able to degrade several low brominated congeners of PBDEs aerobically. However, the aerobic degradation pathway is not yet well understood, particularly at the genetic level. In this study, we isolated Cupriavidus sp. WS from the environment that could degrade diphenyl ether (DE), 4-bromodiphenyl ether, and 4,4'-bromodiphenyl ether. DE was completely degraded in 6 days without any detectable end-product. Using transposon mutagenesis, several DE degradation-deficient mutants were obtained. Knocking out bphA1, bphA2, and bphA3 eliminated the ability of the Cupriavidus sp. WS bacterium to degrade DE, indicating that the bph genes play a crucial role in DE degradation by this strain. The specific roles of bphA, bphB, and bphC were identified by systematically expressing these genes in Escherichia coli. The dihydrodiol product of BphA was dehydrogenated into 2,3-dihydroxydiphenyl ether by BphB. 2,3-Dihydroxydiphenyl ether was then decomposed into phenol and 2-pyrone-6-carboxylic acid by BphC. Thus, BphA, BphB, and BphC act sequentially in the aerobic degradation of DE, 4-bromodiphenyl ether, and 4,4'-dibromodiphenyl ether by the Cupriavidus sp. WS bacterium.

Photolysis of Low-Brominated Diphenyl Ethers and Their Reactive Oxygen Species-Related Reaction Mechanisms in an Aqueous System

To date, no report was concerned with participation of reactive oxygen species in waters during photolysis of low-brominated diphenyl ethers (LBDEs). Herein, we found that electron spin resonance (ESR) signals rapidly increased with increasing irradiation time in the solution of LBDEs and 4-oxo-TMP solutions. But this phenomenon did not occur in the presence of NaN₃ (¹O₂ quencher) demonstrating generation of ¹O₂ in process of LBDEs photolysis. The indirect photolytic contribution rate for BDE-47 and BDE-28 was 18.8% and 17.3% via ¹O₂, and 4.9% and 6.6% via ·OH, respectively. Both D₂O and NaN₃ experiments proved that the indirect photolysis of LBDEs was primarily attributable to ¹O₂. The bimolecular reaction rate constants of ¹O₂ with BDE-47 and BDE-28 were 3.12 and 3.64 × 10⁶ M^-1 s^-1, respectively. The rate constants for BDE-47 and BDE-28 (9.01 and 17.52 × 10⁻³ min^-1), added to isopropyl alcohol, were very close to those (9.65 and 18.42 × 10⁻³ min^-1) in water, proving the less indirect photolytic contribution of ·OH in water. This is the first comprehensive investigation examining the indirect photolysis of LBDEs in aqueous solution.

Photochemical and microbial transformation of emerging flame retardants: cause for concern?

Among anthropogenic chemicals, flame retardants have attracted mounting environmental concerns. In recent years, an increasing number of studies have been conducted worldwide to investigate flame-retardant sources, environmental distribution, wildlife and human exposure, and toxicity. Data generated have demonstrated that some flame-retardant substances such as polybrominated diphenyl ethers (PBDE) are persistent, bioaccumulative, and toxic to exposed organisms. However, comparatively much less attention has been paid to the mechanisms and products of environmental transformation of flame retardants. This lack of information undermines our understanding of the environmental behavior and fate of flame retardants, as well as the associated risks to environmental and human health. Photochemical and microbial transformation of flame retardants in various matrices and environmental compartments can elevate the toxicological significance of flame retardant exposure, via the formation of, for example, lesser halogenated but more bioaccumulative degradation products and toxic radicals. Such pathways raise concerns related to the environmental safety of some alternative flame retardants that are presumably safe and used to replace PBDEs. To fully assess the environmental risks, more research is needed to investigate the environmental transformation potential of emerging flame retardants including polymeric flame retardants. Enhanced analytical efforts are needed to better characterize transformation products and transient radicals. Additional mesocosm and field studies are needed to elucidate transformation kinetics and consequences under environmentally relevant conditions.

Excited States and photodebromination of selected polybrominated diphenyl ethers: computational and quantitative structure--property relationship studies

This paper presents a density functional theory (DFT)/time-dependent DFT (TD-DFT) study on the lowest lying singlet and triplet excited states of 20 selected polybrominateddiphenyl ether (PBDE) congeners, with the solvation effect included in the calculations using the polarized continuum model (PCM). The results obtained showed that for most of the brominated diphenyl ether (BDE) congeners, the lowest singlet excited state was initiated by the electron transfer from HOMO to LUMO, involving a π–σ* excitation. In triplet excited states, structure of the BDE congeners differed notably from that of the BDE ground states with one of the specific C–Br bonds bending off the aromatic plane. In addition, the partial least squares regression (PLSR), principal component analysis-multiple linear regression analysis (PCA-MLR), and back propagation artificial neural network (BP-ANN) approaches were employed for a quantitative structure-property relationship (QSPR) study. Based on the previously reported kinetic data for the debromination by ultraviolet (UV) and sunlight, obtained QSPR models exhibited a reasonable evaluation of the photodebromination reactivity even when the BDE congeners had same degree of bromination, albeit different patterns of bromination.

Estimating stepwise debromination pathways of polybrominated diphenyl ethers with an analogue Markov Chain Monte Carlo algorithm

A stochastic process was developed to simulate the stepwise debromination pathways for polybrominated diphenyl ethers (PBDEs). The stochastic process uses an analogue Markov Chain Monte Carlo (AMCMC) algorithm to generate PBDE debromination profiles. The acceptance or rejection of the randomly drawn stepwise debromination reactions was determined by a maximum likelihood function. The experimental observations at certain time points were used as target profiles; therefore, the stochastic processes are capable of presenting the effects of reaction conditions on the selection of debromination pathways. The application of the model is illustrated by adopting the experimental results of decabromodiphenyl ether (BDE209) in hexane exposed to sunlight. Inferences that were not obvious from experimental data were suggested by model simulations. For example, BDE206 has much higher accumulation at the first 30 min of sunlight exposure. By contrast, model simulation suggests that, BDE206 and BDE207 had comparable yields from BDE209. The reason for the higher BDE206 level is that BDE207 has the highest depletion in producing octa products. Compared to a previous version of the stochastic model based on stochastic reaction sequences (SRS), the AMCMC approach was determined to be more efficient and robust. Due to the feature of only requiring experimental observations as input, the AMCMC model is expected to be applicable to a wide range of PBDE debromination processes, e.g. microbial, photolytic, or joint effects in natural environments.

Factors influencing on the bioaccessibility of polybrominated diphenyl ethers in size-specific dust from air conditioner filters

Size-specific concentrations and bioaccessibility of polybrominated diphenyl ethers (PBDEs) in dust from air conditioner filters were measured, and the factors influencing the PBDE bioaccessibility were determined. Generally, the PBDE concentrations increased with decreasing dust particle size, and BDE209 (deca-BDE) was generally the predominant congener. The bioaccessibility ranged from 20.3% to 50.8% for tri- to hepta-BDEs, and from 5.1% to 13.9% for BDE209 in dust fractions of varied particle size. The bioaccessibility of most PBDE congeners decreased with increasing dust particle size. The way of being of PBDE (adsorbed to dust surface or incorporated into polymers) in dust significantly influenced the bioaccessibility. There was a significant negative correlation between the tri- to hepta-BDE bioaccessibility and organic matter (OM) contents in dust. Furthermore, tri- to hepta-BDE bioaccessibility increased with increasing polarity of OMs, while with decreasing aromaticity of OMs. The tri- to hepta-BDE bioaccessibility significantly positively correlated with the surface areas and pore volumes of dust. Using multiple linear regression analysis, it was found that the OM contents and pore volumes of dust were the most important factors to influence the tri- to hepta-BDE bioaccessibility and they could be used to estimate the bioaccessibility of tri- to hepta-BDEs according to the following equation: bioaccessibility (%)=45.05-0.49 × OM%+1.79 × pore volume. However, BDE209 bioaccessibility did not correlate to any of these factors.

Fate of polybrominated diphenyl ethers during cooking of fish in a new model cooking apparatus and a household microwave

Fish is a major source of human exposure to polybrominated diphenyl ethers (PBDEs). Because fish is mainly consumed after cooking, this measure may alter the pattern and amounts of PBDEs that are finally consumed. To investigate this issue, we developed a model cooking apparatus consisting of a small glass bowl and a beaker glass with an exhaust fitted with a polyurethane foam filter connected to a water jet pump. In this model cooking apparatus, fish (1 g) and/or sunflower oil (0.2/0.4 g) spiked with three PBDE congeners was cooked for 30 min. Small amounts of the semi-volatile PBDEs were evaporated from the fish (BDE-47 < BDE-15), while the non-volatile BDE-209 was partly transformed. Additional experiments in a household microwave provided similar results, except that no transformation was observed for BDE-209. The model cooking apparatus proved to be well-suited to study the fate of polyhalogenated compounds in fish during cooking.

Photoreductive debromination of decabromodiphenyl ethers in the presence of carboxylates under visible light irradiation

Polybrominated diphenyl ethers (PBDEs) have aroused global environmental concerns because of their toxicity and ubiquitousness in the biological and environmental systems. It is important to find an efficient method for their decontamination and to understand their chemical transformation in the environment. Here, we report that decabromodiphenyl ether (BDE209) undergoes efficient reductive debromination reactions under visible-light irradiation (≥ 420 nm) in the presence of various carboxylate anions that are common in the environmental media. The debromination reactions occur in a stepwise manner, producing a series of lower brominated PBDE congeners. Solvent-derived radials are observed by spin-trapping electron spin resonance (ESR) experiments during the photoreaction. Further experiments by the UV-vis absorption and isothermal titration calorimetry (ITC), combined with theoretical calculations, reveal a new photochemical debromination pathway based on the halogen binding interaction. According to this pathway, the formation of halogen-binding-based complex between PBDE and carboxylate enables the visible-light absorption and debromination of PBDEs, although neither PBDEs nor carboxylates have visible-light absorption. The halogen-bond-based photochemical debromination could find its application for our better understanding of the transformation process of PBDEs in the environment.

Photolytic debromination pathway of polybrominated diphenyl ethers in hexane by sunlight

The objective of this work is to identify the photolytic debromination pathways of polybrominated diphenyl ethers (PBDEs). Thirteen PBDEs (BDEs 209, 208, 207, 206, 196, 183, 154, 153, 100, 99, 85, 47 and 28) in hexane were individually exposed to sunlight for up to 64 h. A total of 180 PBDEs were screened and 74 BDE debromination products were detected. The disappearance rate constant increased exponentially with increasing number of bromines. While no evident difference in debromination preference among ortho, meta and para bromines was found for heavier congeners, the vulnerability rank order was meta ≥ ortho > para for the lighter congeners (≤8 Br). The total molar mass of PBDEs continuously decreased during sunlight exposure, indicating PBDEs were transformed to non-PBDE compounds. A stochastic least squares debromination pathway model was developed to simulate the reactions and determine the yields to extend the results beyond the experimental observations.

Genotoxicity and development effects of brominated flame retardant PBDEs and UV-exposed PBDEs on grass shrimp (Palaemonetes pugio) embryo

To understand how polybrominated diphenyl ethers (PBDEs) affect the hatching sensitivity of grass shrimp embryos in different developmental stages, the hatching rates of embryos were measured after PBDE exposure on the early- and late-stage embryo of grass shrimp. PBDEs had no clear influence on the hatching rate. Also, to investigate on the reproductive and genetic toxicities of UV-exposed PBDEs, the hatching rate and DNA damage of stage 7 embryos were also measured. PBDEs had no significant impact on the hatching rate and DNA damage of grass shrimp in most experimental conditions regardless of embryo developmental stage or UV exposure, but in the case of UV-exposed BDE-47, the hatching rate was decreased, suggesting a potential reproductive toxicity. This research is meaningful because it is the first study to propose that PBDEs can cause reproductive toxicity in marine organism, after flowing into the ocean and undergoing photochemical reactions stimulated by UV.

Effects of excited-state structures and properties on photochemical degradation of polybrominated diphenyl ethers: a TDDFT study

This study presents new insight into the photochemical degradation of polybrominated diphenyl ethers (PBDEs), and it provides details about the structures and properties of 27 PBDE congeners in the electronically excited state using the time-dependent density functional theory method. Each PBDE congener exhibited remarkably different geometries in the ground state and the excited state. The significant lengthening of C-Br bond in each PBDE congener was observed in the excited state for the first time by theoretical calculation, which is directly involved in the photochemistry reductive debromination of n-BDE to (n-1)-BDE. Generally, the lengthening of C-Br bonds cannot occur at the para position. Furthermore, the calculated results demonstrated that the photoreactivity of PBDEs increased with an increase of bromination degree. It was also found that the pattern of Br substituents had an effect upon the photoreactivity of PBDEs. These findings suggest that the information obtained in the excited state is crucial to the mechanism explanation of the photochemical degradation of PBDEs.

Levels of polybrominated diphenyl ethers (PBDEs) in dust from personal automobiles in conjunction with studies on the photochemical degradation of decabromodiphenyl ether (BDE-209)

The levels of 21 PBDE congeners were detemined in the dust sampled from 66 personal automobiles. The dominant congener in automobile dust was BDE-209 with a median level of 8.12 μg g⁻¹. Personal vehicle dust samples contained the characteristic profile of the PBDE congeners that comprise the PentaBDE and DecaBDE commercial formulations. Levels of PBDEs in personal automobiles are generally reduced in comparison to our previously reported levels in resale vehicles on dealership lots presumably due to a dilution effect introduced by dust or debris that does not originate from the vehicle. Laboratory photochemical studies were conducted on both automobile dust collected from personal vehicles as well as BDE-209 adsorbed to sodium sulfate. No significant degradation occurred in the personal vehicle dust after 56 days of constant UVA irradiation while significant degradation did occur with BDE-209 adsorbed to sodium sulfate. PBDEs from the degradation of BDE-209 were identified and potential degradation pathways elucidated. Human exposure potential to PBDEs from automobile dust ingestion remains a serious concern in the U.S. population.

Degradation of gas phase decabromodiphenyl ether by resonant interaction with low-energy electrons

Resonance electron attachment in a series of brominated phenyl ethers, including decabromodiphenyl ether (DBDE), was investigated in the gas phase by means of electron transmission spectroscopy (ETS) and dissociative electron attachment spectroscopy (DEAS). Attachment of thermal electrons to DBDE leads to various dissociative decay channels of the temporary molecular anion. In contrast to other bromophenyl ethers, the bromide anion is not the most intense negative fragment. The neutral counterparts of the observed [Br(2)](-) and [C(6)Br(4)O](-) anion fragments are ascribed to the closed-shell species octabromodibenzofuran and hexabromobenzene, respectively, although their formation implies complex atomic rearrangements. Density functional theory calculations are employed to evaluate electron affinities, thermodynamic energy thresholds for production of the anion fragments observed in the DEA spectra and the proton affinities of the corresponding neutral radicals. Since DBDE is one of the most widespread organic pollutants, the present gas-phase DEA study can provide indications on the reaction mechanisms which occur in vivo and cause injuries to living cells.

Effect of decabromodiphenyl ether (BDE 209) and dibromodiphenyl ether (BDE 15) on soil microbial activity and bacterial community composition

There is now increasing concern regarding the effect of polybrominated diphenyl ethers (PBDEs) on the environment. These compounds are widely used as fire retardants and by the electronic industry. Our study examined the effects of adding different doses of BDE 15 and BDE 209 on the soil microbial activities and function by using denaturing gradient gel electrophoresis (DGGE), fluorescence in situ hybridization (FISH) and soil enzymatic activity analyses. Soils were spiked with 1, 10, and 100 mg kg(-1) BDE 209 and BDE 15, respectively, and incubated for up to 180 days. No degradation of BDE 209 was observed; however, about 40% of the added BDE 15 underwent declining extractable concentration. Bacterial counts were significantly higher in the microcosms amended with BDE 15, while the suppression effect increased as the BDE 209 concentration increased. Pseudomonas, Bacillus and uncultured bacteria dominated the bacterial communities in all soil treatments, and PCA analysis showed that high doses of BDE 209 and BDE 15 altered the soil microbial community structure. This study provides new information on the effect of higher and lower PBDEs on the soil microbial community in an aerobic environment.

Is decabromodiphenyl ether (BDE-209) a developmental neurotoxicant?

Polybrominated diphenyl ether (PBDE) flame retardants have become ubiquitous environmental pollutants. The relatively higher body burden in toddlers and children has raised concern for their potential developmental neurotoxicity, which has been suggested by animal studies, in vitro experiments, and recent human epidemiological evidence. While lower brominated PBDEs have been banned in several countries, the fully brominated decaBDE (BDE-209) is still utilized, though manufacturers will discontinue production in the U.S.A. in 2013. The recent decision by the U.S. Environmental Protection Agency to base the reference dose (RfD) for BDE-209 on a developmental neurotoxicity study has generated some controversy. Because of its bulky configuration, BDE-209 is poorly absorbed and does not easily penetrate the cell wall. Its acute and chronic toxicities are relatively low, with the liver and the thyroid as the primary targets, though there is some evidence of carcinogenicity. A few animal studies have indicated that BDE-209 may cause developmental neurotoxicity, affecting motor and cognitive domains, as seen for other PBDEs. Limited in vivo and in vitro studies have also evidenced effects of BDE-209 on thyroid hormone homeostasis and direct effects on nervous cells, again similar to what found with other lower brominated PBDEs. In contrast, a recent developmental neurotoxicity study, carried out according to international guidelines, has provided no evidence of adverse effects on neurodevelopment, and this should be considered in a future re-evaluation of BDE-209. While estimated exposure to BDE-209 in children is believed to be several orders of magnitude below the most conservative RfD proposed by the USEPA, questions remain on the extent and relevance of BDE-209 metabolism to lower brominated PBDEs in the environment and in humans.