Article pubs.acs.org/est
Tetradecabromodiphenoxybenzene Flame Retardant Undergoes Photolytic Debromination Da Chen,†,‡,§ Robert J. Letcher,*,†,‡ Lewis T. Gauthier,† and Shaogang Chu† †
Wildlife and Landscape Directorate, Science and Technology Branch, Environment Canada, National Wildlife Research Centre, Carleton University, Ottawa, ON, K1A 0H3, Canada ‡ Department of Chemistry, Carleton University, Ottawa, ON, K1S 5B6, Canada § Cooperative Wildlife Research Laboratory and Department of Zoology, Southern Illinois University Carbondale, Carbondale, Illinois 62901, United States S Supporting Information *
ABSTRACT: Highly brominated flame retardant compounds have relatively low bioavailability, but some of these compounds have been shown to be of environmental concern. Tetradecabromodiphenoxybenzene (TDBDPB) contains 14 bromine atoms and is the major component of commercial flame retardant mixtures such as the recently phased out SAYTEX 120. The chemical stability of TDBDPB has not been reported. We demonstrated that TDBDPB can photolytically undergo stepwise reductive debromination that follows first-order kinetic degradation models when exposed to UV or natural sunlight radiation and when dissolved in the solvents tetrahydrofuran, methanol, or n-hexane. Photolytic degradation half-lives of TDBDPB ranged from 98 to 169 min, 0.78 to 0.83 min, 1.0 to 1.8 min, and 4.9 to 7.4 min when exposed to UV-A, -B, and -C, and natural sunlight, respectively. However, the TDBDPB half-lives when exposed to UV-B and especially UV-C are likely underestimated since solutions were in borosilicate glass vials during irradiation resulting from increasingly lower % transmittance of λ < 300 nm. Neat technical TDBDPB powder exposed to UV-B and -C radiation also produced less brominated products, although the rate was much slower as compared to when in solution. Exposure of TDBDPB solutions to natural sunlight generated a number of polybrominated diphenoxybenzene (PBDPB) photolysis products, among which the Br4- to Br7-PBDPBs were the most frequently observed and estimated to be most concentrated. As evidenced by the TDBDPB half-lives and the degree of debrominated byproduct formation, the findings showed that the fraction of the absorbed irradiation that was of sufficient energy to break C−Br bonds of TDBDPB and lesser brominated PBDPBs increased from UV-B or -C to UV-A. Coincidentally, we recently reported on the presence of several Br4 to Br6 methoxylated PBDPBs in the Great Lakes herring gull eggs, which may be linked to a TDBDPB source via photolytic degradation to more bioavailable and persistent debromination products.
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INTRODUCTION
commercially manufactured mixtures: penta-, octa-, and decaBDE. Some highly brominated BFRs have relatively low bioavailability but are still identified as substances of environmental concern.1 This is partially due to their persistence in sediments, potential bioaccumulation, and/or persistence of their degradation/biotransformation byproducts. Chemicals with a log Kow (octanol−water partitioning coefficient) ≥ 6 are considered to be superhydrophobic. As log Kow increases, the bioconcentration factor (BCF) decreases, which is caused by the declining solubility in tissue lipids.4 Furthermore, large molecules, in terms of size and weight, increasingly are not able to permeate biological membranes and thus are not bioavailable. For example, decabromodiphenyl ethane (DBDPE, CAS# 8485253-9), listed as one of the top priority and emerging P&B
A recent review by Howard and Muir identified a priority list of 610 substances of potential persistence and bioaccumulation (P&B) relevance to the environment from a database of 22 263 commercial chemicals.1 The majority of these 610 potential P&B substances have not been considered in current North American, Great Lakes, or Arctic contaminant evaluation programs.1 Of these 610 substances, 13% (80 compounds) were brominated compounds, a majority of which are brominated flame retardants (BFRs).1 BFRs are a broad class of additive and reactive substances and technical mixtures used in commercial polymeric materials such as thermoplastics, thermosets, textiles, and coatings.2 As many BFRs are not chemically bound to the finished products, a fraction may escape throughout the life cycles (i.e., production, use, disposal, and recycling) of the finished products and ultimately enter into the environment.2,3 Thus far, the most studied BFR substances in the environment are polybrominated diphenyl ethers (PBDEs), including three © 2013 American Chemical Society
Received: Revised: Accepted: Published: 1373
October 16, 2012 December 29, 2012 January 4, 2013 January 4, 2013 dx.doi.org/10.1021/es3042252 | Environ. Sci. Technol. 2013, 47, 1373−1380
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substances, has a BCF of only 3.0 and a high log Kow of 13.64.1 In addition to the discovery of its presence in sediments and dust,5−8 studies also suggested it could degrade to less brominated, hence more bioaccumulative substances. 9 Similarly, the fully brominated PBDE congener 2,2′,3,3′,4,4′,5,5′,6,6′-decabromodiphenyl ether (BDE-209, CAS# 001163-19-5) has also been reported to degrade to less brominated, more bioavailable PBDE congeners via photolysis or metabolism pathways.10−13 Therefore, to comprehensively evaluate the environmental importance of anthropogenic compounds, research is needed to investigate their potential environmental degradation pathways and products. This is particularly true when evaluating highly halogenated compounds, which usually have high molecular weight and low bioavailability. Very recently, we identified a novel class of brominated substances, methoxylated tetra- to hexa-bromo-diphenoxybenzene (MeO−PBDPB) congeners, in Laurentian Great Lakes herring gull (Larus argentatus) eggs at concentrations of up to 100 ng/g wet weight (or 2200 ng/g lipid weight) depending on the colony from where the eggs were collected.14,15 These MeO−PBDPBs have the same brominated diphenoxybenzene base molecular structure as tetradecabromodiphenoxybenzene (TDBDPB, CAS# 58965-66-5 and 113588-14-0), which is also listed as a potential P&B chemical.1 TDBDPB was the major component of SAYTEX 120, a commercial flame retardant product finding its primary application in high performance polyamide and linear polyester engineering resins and alloys, as well as in polyolefins, wire and cable, and styrenic resins.16 Whereas Hardy et al.17 suggested that SAYTEX 120 has recently been discontinued, a number of suppliers from around the world (particularly in Asia) are still marketing the TDBDPB or TDBDPB-containing products.15,18 For example, TDBDPB is currently being produced in China and marketed by Haihang Industry Co. Ltd., Jinan Haohua Industry Co. Ltd. and Shanghai Biochempartner Co. Ltd.19 Also, in the U.S., TDBDPB is produced and sold by TCI America.20 To our knowledge, to date there are no reports in the scientific literature on the environmental presence of TDBDPB or with respect to its chemical stability in the environment. TDBDPB has 14 bromine atoms and a high molecular weight (approximately 1368 Da). Similar to DBDPE and BDE209, TDBDPB has a low BCF (= 3.0) and extremely high log Kow (= 16.89).1 These physicochemical properties greatly limit the bioavailability of TDBDPB. However, considering the highly similar molecular composition (e.g., phenyl carbon− bromine bonds and bond dissociation energies) of TDBDPB and BDE-209 or DBDPE, we hypothesized that TDBDPB undergoes chemical decomposition by photolysis to less brominated byproducts. The present study was hence conducted to (1) evaluate and compare TDBDPB photolysis as a solid and in solution, and when exposed to different energies and intensities of radiation including ultraviolet (UV) and natural sunlight; and, (2) understand the chemical degradation mechanisms and identify the decomposition products formed.
Figure 1. (A) LC-APPI(-)-Q-TOF-MS extracted ion mass chromatograms of tetradecabromodiphenoxybenzene (TDBDPB) and degradation products (Br10- to Br13-PBDPB) in tetrahydrofuran solution after natural sunlight irradiation. The chromatograms shown were from samples collected at time points of 0 and 2 h during the exposure, respectively. (B) LC-APPI(-)-Q-TOF-MS mass spectrum of TDBDPB (of the Br14 peak in (A)).
ON, Canada). BDE-209 reference standard was purchased from Wellington Laboratories. Solvents used were HPLC grade (Caledon Laboratories, Georgetown, ON, Canada). Sample Preparation. Technical TDBDPB was dissolved to maximum extent possible into separate solutions of tetrahydrofuran (THF, 50 parts-per-million (ppm)), methanol (1% THF v/v) (5 ppm), methanol (10% THF v/v) (20 ppm), nhexane (1% THF v/v) (5 ppm), and n-hexane (10% THF v/v) (20 ppm). THF, methanol, and n-hexane were chosen due to their varying polarities and hydrogen-donating abilities. However, because of very limited solubility of TDBDPB in methanol or n-hexane, THF was added into these solvents to increase its solubility. BDE-209 was dissolved into n-hexane (1% THF, v/v) (5 ppm). Irradiation Experiments. The five TDBDPB stock solutions were transferred into borosilicate glass gas chromatography (GC) injection vials (1.5 mL, 12 × 32 mm; Chromatographic Specialties Inc., Brockville, ON, Canada) for UV-A (λ = 400 to 315 nm), −B (λ = 315 to 280 nm), and −C (λ = 280 to 100 nm) as well as natural sunlight irradiation. Borosilicate glass effectively transmits radiation from the infrared down to approximately λ = 300 nm. Thus, for the present irradiation experiments, the percent transmittance of these wavelength ranges through the borosilicate glass GC vials was 100% for UV-A, and on average of the wavelength range ∼50% for UV-B, and ∼10% for UV-C. Triplicates of each of the five TDBDPB stock solutions in GC vials were exposed to artificial UV-A (5 mW/cm2) emitted by a UVP 3UV Transilluminator (Model: LMS-26E; Upland, CA, USA). The percent relative standard deviations (%RSDs)
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EXPERIMENTAL SECTION Chemicals and Reagents. To our knowledge, there are currently no pure standards commercially available for TDBDPB (Figure 1 for the chemical structure) or its degradation products. Technical TDBDPB product was generously supplied by Wellington Laboratories (Guelph, 1374
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other parameters for MS were optimized as follows: gas temperature, 300 °C; vaporizer, 350 °C; drying gas, 10 L/min; neubilizer, 30 psi; fragmentor voltage, 200 V; skimmer voltage, 60 V. The TDBDPB and its degradation products (i.e., Br0 − Br13 polybrominated diphenoxybenzenes (PBDPBs)) were detected by monitoring [M−Br + O]−, except for Br0PBDPB (or diphenoxybenzene) for which the ion of [M + O]− was monitored. Table S1 of the Supporting Information lists the monitored ions for the parent and degradation compounds, as well as their corresponding m/z ratios. QA/QC and Data Analysis. Before the UV exposure experiments, triplicate aliquots (150 μL each) were transferred from each of the five TDBDPB stock solutions and kept as control samples. The controls were wrapped with aluminum foil and stored at −20 °C until analysis. They were analyzed immediately after the completion of UV exposure experiments along with the samples collected at various time points during exposure. With respect to LC-APPI(-)-Q-TOF-MS peak responses, no significant differences in the TDBDPB abundances were observed in the controls versus the t = 0 samples (the level of significance was set at α = 0.05). Solvent blanks, that is THF (n = 1), methanol (1% THF) (n = 1), methanol (10% THF) (n = 1), n-hexane (1% THF) (n = 1), and n-hexane (10% THF) (n = 1), were exposed and sampled in the same manner as the TDBDPB solutions during UV irradiation experiments. No TDBDPB or any PBDPB substances were detected in the solvent blanks. Control samples (n = 6, 50 ppm each in THF) were prepared before the natural light experiments, half of which were analyzed immediately along with an instrumental control standard (TDBDPB stock solution (50 ppm) in methanol (10% THF v/v)). The remaining three controls were wrapped in aluminum foil and kept in dark at −20 °C until analysis. They were analyzed after the completion of experiments, along with the samples collected at various time points and an instrumental control standard. The instrumental control standard was used to monitor and adjust the instrumental response changes between different analytical queues. The abundance ratios of TDBDPB in controls versus instrumental control standard were not significantly different between the controls analyzed before and after the experiments, and also between controls and the t = 0 samples. No TDBDPB or PBDPB compounds were observed in solvent blanks exposed to sunlight along with TDBDPB solutions. Given the lack of PBDPB standards, the integrated areas of the LC-APPI(-)-Q-TOF-MS peaks corresponding to TDBDPB or PBDPB substances were used to estimate the relative abundances or concentrations. The instrumental control standard (TDBDPB stock solution (50 ppm) in methanol (10% THF v/v)) was injected along with every 20 samples to monitor instrumental response changes. To calculate photolytic decomposition half-lives of TDBDPB, concentration (abundance) data for each analyte over time were fit to a first-order kinetics reaction model Ct = C0 e−kt, where Ct represents the concentration/abundance at a given time, C0 is the initial concentration, and k is the reaction rate constant.10
of the abundances (represented by integrated peak areas resulting from liquid chromatography − quadrupole-time-offlight mass spectrometry analysis) of TDBDPB and degradation products from the triplicate UV-A treatments were consistently ≤4% throughout the experiment. Therefore, only one treatment per solution (in THF, methanol (1% THF) or n-hexane (1% THF)) was subsequently exposed to UV-B (10 mW/cm2) or UV-C (8 mW/cm2) light. For the photolytic treatments of the TDBDPB solutions, at predetermined time points (0, 1, 3, 5, 10, 20, 30, 60, 120, and 180 min), a volume of 150 μL was removed from each of the exposed TDBDPB stock solutions, transferred to LC vials, wrapped in aluminum foil, and stored at −20 °C until analysis. All irradiation experiments were performed along with control samples and solvent blanks. Neat TDBDPB standard powder was ground to fine particles and homogenized using a solventrinsed mortar and pestle. This grinding and homogenization was carried out to maximize the exposure surface area. The homogenized neat TDBDPB powder was exposed noncontinuously to UV-A (5 mW/cm2) for 100 h and then UVB (10 mW/cm2) for 100 h. The irradiation of the TDBDPB neat standard was noncontinuous as we stopped the exposure during the night. An amount of approximately 20 mg of the homogenized TDBDPB powder was transferred and dissolved in 100 mL methanol (10% THF v/v) prior to and after the entire exposure experiment. Duplicates of each of the five TDBDPB stock solutions and also BDE-209 solution (in n-hexane, 1% THF v/v) were exposed to natural sunlight (May 22 to July 4, 2012). The coordinates for the experimental location were 45°25′15″N and 75°41′24″W. At predetermined time points (0, 5, 10, 20, 40, 70 min; 2, 4, 6, 24 h; and 2, 3, 4, 5, 6, 7, 8, 9, 15, 20, 33, and 42 days), a volume of 150 μL was removed from each of the stock solutions, transferred to LC vials, wrapped in aluminum foil, and stored at −20 °C until analysis. All irradiation experiments were performed along with control samples and solvent blanks. UV-A and -B radiation intensities at the site where the samples were located were recorded at mid-day (noon) on each collection day by the UVA 400C UV Meter and the UVB 500C UV Meter (National Biological Corporation, Beachwood, OH). Instrumental Analysis. Detection and characterization of TDBDPB and degradation products was performed on an Agilent 1200 liquid chromatographic system, which consists of a degasser, binary high-pressure gradient pump, autosampler, coupled to an Agilent 6250A quadrupole-time-of-flight-MS (QTOF) system (Agilent Technologies, Mississauga, ON, Canada). Liquid chromatography separation was carried out on a Xterra Phenyl column (2.1 mm ×100 mm, 3.5 μm particle size) (Waters, Mississauga, ON, Canada). The mobile phase (A, water; B, methanol) flow rate was 0.3 mL/min and the following gradient was employed: 5% B ramped to 100% B in 5 min (linear) and held for 20 min, followed by a change to 5% B and held for 15 min for next injection. Toluene was introduced into the Q-TOF at a flow rate of 0.02 mL/min by a Series 200 Micro pump (PerkinElmer, Woodbridge, ON, Canada) and via a T connector after the LC system. The Q-TOF instrument was tuned and calibrated with tuning calibration solution (G1969− 85000, Agilent Technologies). The TOF-MS was operated at resolution (R) > 20 000 at m/z 601.978977 and within 3 ppm mass error in mass range m/z 50−1700. Atmospheric pressure photoionization (APPI) was operated in the negative mode and the capillary voltage was 5.0 kV. Nitrogen was used as drying and nebulizing gas and helium was used as collision gas. The
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RESULTS AND DISCUSSION Instrumental Analysis of Technical TDBDPB Mixture. Employing the newly developed LC-APPI(-)-Q-TOF-MSbased method as detailed in the Experimental Section, we successfully detected the highly brominated TDBDPB dissolved in methanol (10% THF) (Figure 1). The method also 1375
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Table 1. Photolytic Degradation Half-Lives (t1/2, Min) of Tetradecabromodiphenoxybenzene (TDBDPB) and 2,2′,3,3′,4,4′,5,5′,6,6′-Decabromodiphenyl Ether (BDE-209) in Various Solutions upon UV or Natural Sunlight Irradiation; the Degradation Followed First-Order Reaction Kinetics Models in All Experiments TDBDPB
UV-Aa UV-B UV-C sunlightb
radiation intensity (mW/cm2)
THF
methanol (1% THF)
5 10 8 12c
98 0.78 1.7 6.2
141 0.83 1.8 5.0
methanol (10% THF) 169
4.9
BDE-209 n-hexane (1% THF) 110 0.82 1.0 5.0
n-hexane (10% THF)
n-hexane (1% THF)
122
7.4
5.3
a
Calculated from the mean values obtained from triplicate treatments. bCalculated from the mean values obtained from duplicate treatments. c Average UV-A + UV-B radiation intensity measured at mid-day (noon) on each collection day throughout the sunlight exposure experiment.
Figure 2. Changes over time in the relative abundance of tetradecabromodiphenoxybenzene (TDBDPB) in various solutions as a consequence of exposure to UV-A, -B, or -C. The 1% or 10% represents the proportions of tetrahydrofuran (THF) in methanol or n-hexane (v/v), respectively. For UV-A irradiation, triplicate experiments were conducted for each solution and the mean values were presented (percent relative standard deviations (%RSDs) ≤ 4%).
TDBDPB solutions were exposed to the same type of radiation (Table 1). This suggested that the half-lives of the TDBDPB degradation were independent of the starting concentration. After the maximum 3 h exposure period to UV-A, the amount of TDBDPB present in the THF solution remained 22% of its initial abundance before exposure (Figure 2, Table S2 of the Supporting Information), and the first-order photolytic t1/2 of TDBDPB was estimated to be 98 min (Table 1). Previous studies have suggested a solvent effect during PBDE photolysis, for instance, BDE-207 was reported to degrade more rapidly in THF than in methanol.23,24 Also, it has been suggested that the hydrogen-donating ability of a solvent significantly affects the rate of PBDE photodegradation.25 Furthermore, UV-A and -B photodegradation studies and free (bromine) radical yields from BDE-209 irradiation in THF, dimethylformamide and toluene showed that radical yields were highest in THF, which is a good hydrogen donor and as measured by electron paramagnetic resonance (EPR), facilitates spin adduct formation more than the other solvents.26 The present study addressed the possible differences in photolytic susceptibility as a function of solvent interactions with TDBDPB in solution, and different solvents (methanol and n-hexane) where any appreciable TDBDPB dissolution was possible, which were used to assess and compare TDBDPB degradation rates. However, because of the extremely low
separated and identified a range of PBDPB compounds with 0− 13 bromine atoms based on the accurate mass determination. Part A of Figure 1 shows the extracted ion chromatograms of the technical TDBDPB solution and part B of Figure 1 presents the mass spectrum of TDBDPB under the instrumental conditions. Part A of Figure 1 shows that the technical product contains a minor Br13-PBDPB impurity, and the abundance of this impurity compound is approximately 5% of that of TDBDPB. The Br13-PBDPB may be present in the technical product as an impurity or as a degradation product that was generated during the synthesis of TDBDPB. It is noted that some nona-BDEs also exist as minor constituents in technical deca-BDE mixtures.21 TDBDPB Photolytic Degradation and Half-Lives. In the present study, it was a challenge to determine correlations between t and Ln(Ct), as many TDBDPB concentration measurements were zero due to rapid degradation. However, for the measurements for TDBDPB concentrations greater than zero, there were t versus Ln(Ct) linear correlations for each treatment, which fitted well into the first-order reaction kinetics model, that is Ln(Ct) = −kt + Ln(Co) (not shown). Furthermore, two different concentrations of TDBDPB (5 and 20 ppm) were assessed in methanol or in hexane exposed to UV-A or natural light. Significant variations were observed in degradation half-life (t 1/2 ) between experiments when 1376
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solubility of TDBDPB in methanol and n-hexane, it was necessary to spike a small quantity of THF to increase the TDBDPB solubility. A 1% and 10% portion of THF was spiked in methanol or n-hexane, and the potential influence on TDBDPB degradation rates was monitored as a function of increasing proportion of THF. The degradation rate of TDBDPB upon exposure to UV-A did not differ substantially in THF versus methanol (1% or 10% THF) or n-hexane (1% or 10% THF) (Figure 2, Table 1). For example, TDBDPB halflives were 110 and 121 min in methanol (1% and 10%, respectively) and very comparable to that in 100% THF. No substantial increase of the degradation rate was observed by increasing the percentage of THF in methanol or n-hexane, which further indicated that a small percentage of THF in these solvents did not substantially affect or accelerate the photolytic reaction. Following first-order decomposition kinetics models, exposure of TDBDPB to UV-B or -C irradiation resulted in more rapid degradation than that in UV-A experiments, that is, with degradation t1/2 generally less than 2.0 min upon exposure to UV-B or -C radiation (Figure 2, Table 1). TDBDPB was completely absent and degraded within 10 min in all treatments upon exposure to UV-B (Table S3 of the Supporting Information). These results suggested that the electromagnetic energy necessary to photolytically rupture any molecular bond of TDBDPB is in the lower range of UV-B spectrum, that is, close to λ = 280 nm, which is approximately the same as the maximum absorption wavelength necessary for photolytic decomposition of BDE-209 according to Shih and Wang.24 In the present study, the TDBDPB half-lives when exposed to UVB and especially UV-C are likely underestimated because solutions were in borosilicate glass GC vials during irradiation, and where % transmittance of λ < 300 nm is increasingly lower. On average, over the UV-B wavelength range there is ∼50% transmittance and for UV-C ∼10% transmittance. Compared to continuous artificial UV exposure, the sunlight exposure represents a discontinuous and complex irradiation, and also being a natural environmental condition. The average UV-A and UV-B irradiation intensity measured at mid-day (noon) on each collection day throughout the sunlight exposure experiment was 12 and 0.2 mW/cm2, respectively. The degradation of TDBDPB upon discontinuous sunlight exposure also appeared to follow the first-order reaction kinetics models. The degradation half-lives in the various solvents ranged from an estimated 4.9 to 7.4 min, indicating rapid photolytic degradation upon sunlight exposure (Table 1). The half-lives were also very comparable to that of BDE-209 (5.3 min), which was irradiated in parallel to the TDBDPB solutions in the present study. Davis and Stapleton23 reported a similarly determined and comparable degradation half-life of BDE-209 (3.6 to 7.1 min in various solvents) resulting from sunlight exposure. They also reported in the same study that the half-lives of potential PBDE replacements 2-ethylhexyltetrabromobenzoate (TBB) and di(2-ethylhexyl)-tetrabromophthalate (TBPH) were 86 to 162 and 147 to 220 min, respectively, and much longer than that of TDBDPB. To completely exclude potential solvent effects on photolytic degradation, technical TDBDPB powder was also exposed to UV radiation. After a noncontinuous 100 h of UV-A irradiation followed by noncontinuous 100 h of UV-B irradiation, the relative percent abundance of Br13-PBDPB (the ratio of a given PBDPB integrated peak area to that of TDBDPB at t = 0) in exposed powder shifted from 5% at t = 0 to approximately 20%
at the end of irradiation period (Figure S1 of the Supporting Information). The Br12-PBDPB was also observed with a relative percent abundance changing from 0% at t = 0 to 14% at the end of exposure (Figure S1 of the Supporting Information). The results from the above treatments demonstrated that overall there is an energetic susceptibility of TDBDPB to photolytic degradation in solution or in solid phase, although the photodegradation was greatly catalyzed by the presence of solvents. Degradation Products and Discontinued Mass Balance. For PBDEs, the heat of formation, bond dissociation energy, and the energy of the highest occupied molecular orbital (EHOMO) and lowest unoccupied molecular orbital (ELUMO) have been used to predict PBDE degradation rates during irradiation.27 For PBDEs, several studies have concluded that reductive debromination is probably the main mechanism of photodegradation.24,25,28 Also, Suh et al.26 reported that for BDE-209 the activating wavelengths for observed photochemistry was in the UV-A to UV-B range (
