BDE-209: Kinetic Studies and Effect of Humic Substances on

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BDE-209: Kinetic Studies and Effect of Humic Substances on Photodegradation in Water J. F. Leal, V. I. Esteves, and E. B. H. Santos* Department of Chemistry and CESAM, Centre for Environmental and Marine Studies, University of Aveiro, 3810-193 Aveiro, Portugal S Supporting Information *

ABSTRACT: BDE-209 is a brominated flame retardant and a priority contaminant, which has been found in several environmental matrices, namely, in water. To date, there are no quantum yield data for BDE-209 photodegradation by sunlight in water, to allow predicting half-life times in aquatic systems. In this work, the kinetics of BDE-209 photodegradation in water was studied and the influence of different fractions of aquatic humic substances (HS) was evaluated. Aqueous solutions of BDE-209 exposed for different periods of time to simulated sunlight were analyzed by HPLC−UV after being concentrated using dispersive liquid−liquid microextraction (DLLME) or solid-phase extraction (SPE). The photodegradation of BDE-209 in aqueous solution followed pseudo-first-order kinetics. The average quantum yield obtained of 0.010 ± 0.001 (about 20-fold lower than the quantum yield determined in ethanol) allow to predict an outdoor half-life time of 3.5 h. The photodegradation percentage of BDE-209 was not significantly affected by the XAD-4 fraction of HS, but it decreased substantially in the presence of humic and fulvic acids. Light screening by the humic substances could not explain this delay, which is probably the result of the association of the compound with the hydrophobic sites of the humic material.

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L) was demonstrated for zebra fish (vertebrate model organism).12 Photodegradation of the environmental pollutants is especially relevant as a degradation pathway in surface water exposed to sunlight. Although the aerobic microbial decomposition of BDE-209 is also possible, this pathway is slow and inefficient whereas the photodegradation by sunlight may be the major pathway of BDE-209 decomposition in aerobic surface waters.13,14 The photodegradation of BDE-209 has been studied in organic solvents and under distinct irradiation conditions. It is known that the photodegradation can occur through a sequential debromination of the compound,15−17 giving rise to byproducts which are more toxic than the parent compound.18 To our knowledge, to date, there are no data in the literature for the BDE-209 photodegradation quantum yield in aqueous solution. Those data are essential to predict half-life times in surface waters. Kuivikko et al.14 predicted the photolytic half-life times in the mixing layer of the Baltic Sea and of the Atlantic Ocean using the photolytic quantum yield of BDE-209 in isooctane. However, it is known that the solvent can have a strong effect on the degradation kinetics.

INTRODUCTION Flame retardants (FR) are chemicals added to combustible materials, such as plastics, textiles, and wood, to improve the resistance to fire by chemical or physical mechanisms.1 There are two types of FR: reactive FR added during the polymerization process, becoming an integral part of the polymer, and additive FR, which are not covalently bound to the polymers.2 Polybrominated diphenyl ethers (PBDEs) are additive flame retardants3 easily blended and showing a great tendency for leaching compared to the reactive FR.4 Bis(pentabromophenyl) ether (IUPAC name) is also known as 2,2′,3,3′,4,4′,5,5′,6,6′-decabromodiphenyl ether, decaBDE, BDE-209, or decabromodiphenyl oxide, it is the most brominated flame retardant, it stands out among the brominated FR for its wide commercial use and it has been recently considered a priority contaminant.5 BDE-209 has been found in air, water, soil, and sediments, and there is a recent evidence of its presence in remote sites such as Canadian Lakes, North Pacific Ocean and Arctic.6,7 Despite the low levels found in surface waters,8,9 studies of BDE-209 in aquatic environments are fully justified due to the bioaccumulation characteristics of this compound which were confirmed by several studies that refer significant levels of BDE-209 in organisms of aquatic origin, for instance fish or freshwater birds.10,11 Furthermore, the toxicity associated with long-term chronic exposure to low doses of BDE-209 in solution (from 0.959 μg/ © 2013 American Chemical Society

Received: Revised: Accepted: Published: 14010

April 17, 2013 November 18, 2013 November 18, 2013 November 18, 2013 dx.doi.org/10.1021/es4035254 | Environ. Sci. Technol. 2013, 47, 14010−14017

Environmental Science & Technology

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and a frequency of 45 kHz was used to better dissolve the analyte. Quantitative analysis of BDE-209 was achieved by HPLCUV, using a device which consists of a degasser DGU-20A5, a bomb LC-20AD, an UV−vis detector SPD-20A and, a column oven CTO-10ASVP (T = 25 °C), all from Shimadzu. A New ACE C18 column-PFP (5 μm, 150 mm × 4.6 mm) and an injection loop of 20 μL were used. Cell temperature was maintained at 25 °C. The mobile phase was 100% acetonitrile and the flow rate was 1.000 mL/min. The detection was done at 230 nm wavelength, corresponding to the maximum absorptivity value (Figure S1, Supporting Information (SI)). Centrifugation was made using Mixtasel centrifuge, class 2.0 (J. P. Selecta, s.a., Spain). UV−vis spectra were obtained with a T90 + UV/vis Spectrophotometer (PG Instruments Ltd.) using a slit width of 2 nm. UV−vis spectra of BDE-209 solutions were made in rectangular quartz cuvettes of 10 cm path length and the spectra of aqueous solutions of the HS fractions were made in cuvettes of 1 cm. Dispersive Liquid−Liquid Microextraction (DLLME) Procedure. The DLLME procedure performed in this study is an adaptation of the procedure described by Li et al.24 Acetonitrile was used as dispersive solvent (1.6 mL) and carbon tetrachloride (45 μL) was used as extracting solvent for an aqueous sample volume of 8 mL. For each experiment three to five replicates were performed. The details of the procedure are described in the SI. Solid Phase Extraction (SPE) Procedure. The procedure used is adapted from others used for PBDEs in several matrices.25,26 After conditioning of the cartridge, the aqueous sample (90 mL) was loaded, the cartridge was dried and BDE209 was eluted with n-hexane. After evaporation of the solvent, BDE-209 was recovered in 100 μL of acetonitrile. Further details can be found in SI. Photodegradation Experiments. A 5 mg/L stock solution of BDE-209 in acetonitrile was prepared and completely solubilized after a good stirring and after a period of time (30 min) in the ultrasonic cleaning bath, at room temperature. To ensure that the ultrasonication process does not affect the stability of BDE-209, two solutions of BDE-209 in acetonitrile with 1% tetrahydrofuran and in 100% acetonitrile, were prepared with and without sonication. These experiments show that sonication did not cause the formation of any derivate of BDE-209 (more details in SI).From the stock solution of BDE-209 in acetonitrile, an aqueous working solution with a final concentration of 5 μg/L of BDE-209 was made. To guarantee that acetonitrile does not affect the photodegradation rates of BDE-209 in aqueous solution the concentration of auxiliary solubilizing agent (acetonitrile) in aqueous solution was maintained much lower (0.1%) than the value recommended by the OECD guideline TG316 (1%).27 The working solution (5 μg/L) was transferred into quartz tubes (45 mL in each tube). For each irradiation time, and each replicate, two tubes were introduced in the SolarBox: one was exposed to radiation and the other, the dark control, was wrapped in aluminum foil to protect from light. The dark controls were maintained inside the solarbox during the same time as the irradiated solutions and the degradation percentage was always calculated from the concentration difference between the exposed solution and respective dark control. To better evaluate the reproducibility of the results obtained using this procedure, the replicates of the experiments were non simultaneous and, in some cases (clarified in the

Thus, one of the main goals of this work was to study the kinetics of BDE-209 photodegradation in aqueous solution, under simulated sunlight radiation. Kinetic parameters such as rate constants, quantum yield and outdoor half-life times were determined. The other main goal was to study the influence of different fractions of humic substances (HS), isolated from river water, on BDE-209 photodegradation in aqueous solution. These fractions (humic acids, fulvic acids, and XAD-4 fraction) are operationally defined on the basis of an isolation procedure consisting on their adsorption onto two resin columns (XAD-8 and XAD-4) in tandem.19 Aquatic ecosystems are inevitably subject to the action of sunlight and it is known that the humic substances (HS) affect the photodegradation of several contaminants.20 HS are complex molecules that consist of aromatic cores highly substituted with functional groups and peripheral aliphatic units.21 They can both enhance or hamper the photodegradation of aquatic contaminants, depending on the contaminant and on the nature and concentration of HS.20,22 As there are structural differences between the three fractions of HS above referred, namely in what concerns elemental composition, aromaticity and functional groups contents (phenolic and carboxylic contents)19,23 a comparative study of their influence on photodegradation of BDE-209 was made. BDE-209 has a very low solubility in aqueous solution (20− 30 μg/L)4 requiring preconcentration methodologies to analyze the compound by HPLC−UV. DLLME revealed to be a good choice. However, the SPE procedure was also used to follow the photodegradation experiments of BDE-209 in the presence of HS since their precipitation occurred when using DLLME.

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EXPERIMENTAL SECTION Chemicals. BDE-209 (pentabromophenyl ether) (98%) was provided by Sigma Aldrich. For the preparation of solutions, ultrapure water obtained from a milli-Q Millipore system (Milli-Q plus 185) and absolute ethanol PA (Panreac) were used. Carbon tetrachloride PA (99.9%, Panreac) and acetonitrile, HPLC grade (Lab-Scan, Analytical Sciences), were also used. Methanol (Fisher Scientific) and n-hexane 95% (Lab-Scan, Analytical Sciences), both HPLC grade, were used for SPE. Commercial Supelclean Envi-18 cartridges (Supelco) of 500 mg, 75 Å pore diameter, and 56 μm particle size were set up in a 12-place manifold from Phenomenex to perform the SPE experiments. The HS fractions, humic acids (HA), fulvic acids (FA), and XAD-4 fraction, were isolated from river Vouga water (Carvoeiro, Portugal) during September/October 1991, using two columns in tandem, one of Amberlit XAD-8 resin and the other of Amberlit XAD-4 resin.19,23 The relative abundance of each fraction in the sample was 10.3%, 69.4%, and 20.3% for HA, FA, and XAD-4 fraction, respectively.23 The humic substances were kept in a desiccator, protected from light. Instrumentation. Studies of photodegradation were performed using a simulator of solar radiation, the Solarbox 1500 (Co.fo.me.gra, Italy) equipped with a 1500 W arc xenon lamp and outdoor UV filters that restrict the transmission of light of wavelengths below 290 nm. To monitor irradiance and temperature of the experiments, a multimeter (Co.fo.me.gra, Italy), equipped with a UV 290−400 nm large band sensor and a black standard temperature sensor, was used. The device was refrigerated by an air cooled system. An ultrasonic cleaning bath (USC300TH, VWR) with a maximum output power of 160 W 14011

dx.doi.org/10.1021/es4035254 | Environ. Sci. Technol. 2013, 47, 14010−14017


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Results and Discussion), some replicates were performed one year later. For the kinetic studies the aqueous solutions were irradiated during 30, 60, 90, 120, and 150 min. After irradiation, 8.0 mL of the sample were placed into a glass test tube for DLLME procedure. For the study of the effect of HS on BDE-209 photodegradation in aqueous solution, solutions of BDE-209 (5 μg/ L) with HS fractions [(HA (8 mg/L), FA (8 mg/L and 16 mg/ L), and XAD-4 fraction (8 mg/L)] were irradiated during 60 min. These solutions were prepared, with milli-Q water, from the BDE-209 stock solution above referred and from stock solutions of each HS fraction. After irradiation the samples were submitted to the SPE procedure described above. For the photodegradation experiments in ethanolic solution a stock solution of BDE-209 in ethanol was prepared, with a concentration of 5 mg/L. The working solution in ethanol (2.0 mg/L) was distributed into quartz tubes which were irradiated during 2, 5, 8, 11, and 14 min. All irradiations were achieved in a sunlight simulator using an irradiance of 55 W/m2 (290−400 nm), corresponding to 550 W/m2 in all spectral range. Figure S2 in the Supporting Information (SI) presents the lamp spectrum at this irradiance level. Quartz tubes (internal diameter × height = 1.8 × 20 cm) were used for the irradiation experiments. Three nonsimultaneous replicates of the photodegradation experiments were done together with the respective dark controls, for each irradiation time.

Figure 1. Kinetics of BDE-209 photodegradation in aqueous solution and in ethanol (three independent experiments for each solvent). The curves of pseudo-first-order decay fitted to the data by nonlinear regression are also shown. 350

ka0 =

∑ 2.3 × z(λ) × εi(λ) (1)

295

where z(λ) corresponds to solar energy emitted by unit area and unit of time (millieinstein cm−2 s−1) for midday (noon) at sea level, at 40° N latitude, for a midseason clear summer day, over a given wavelength range Δλ. The z(λ) values were obtained from Schwarzenbach et al.30 The εi(λ) is the average molar extinction coefficient of BDE-209 within each Δλ range. The units of k0a are millieinstein.cm−3 dm3 mol−1 s−1 (= einstein mol−1 dm3 s−1). The near-surface first-order rate constant (k0p) was determined by multiplying the quantum yield (Φ) (see eq 2) by k0a . From k0p, the outdoor half-life time near aqueous surface was predicted to be 3.5 h. The diurnal fluctuations in sunlight intensity (24-h averaged z(λ)) were not considered because the outdoor half-life time calculated is lower than 24 h (one day). The values of half-life time and pseudo-first-order kinetics are totally related to the specific experimental conditions adopted, namely to the total irradiance of the lamp. Quantum yield (Φ) is a parameter of high relevance in photochemical studies that makes possible the comparison of results between different studies and may explain the differences in sunlight photodegradation rates of compounds with similar spectral overlap.31 According to IUPAC this parameter is defined as the ratio between the amount of reactant consumed or product formed and the amount of photons absorbed32 and can be calculated from eq 2.

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RESULTS AND DISCUSSION DLLME−HPLC−UV Procedure. The recovery percentage of BDE-209 achieved for this method was 65.7 ± 3.5% (mean value ± standard deviation, n = 10), a value very similar to results obtained by other authors (69.6 ± 5.2)24 that used the tetrahydrofuran (THF) and CCl4 as dispersing and extracting agents, respectively. The calibration was performed with aqueous standards with concentrations between 1 and 5 μg/L which were submitted to the same concentration procedure. More calibration details are discussed in SI. Kinetics of Photodegradation. The degradation percentages for all irradiation times were calculated relatively to the dark controls (light protected solutions introduced inside the solar box together with the irradiated solutions); therefore, protected and exposed solutions were subjected to the same environmental conditions except exposure to radiation. The concentration of the dark controls exhibited random variations lower than 7% between replicates. The kinetic results of BDE-209 photodegradation in aqueous solution (5 μg/L) are presented in the figure 1, together with the curve C/C0 = e−kt, fitted to the data by nonlinear regression using GraphPad Prism 6 (demo version). In that equation, k is the rate constant, t is time, and C0 and C are the concentrations of BDE-209 when protected from light or exposed to it, respectively. The data for the variation of concentration of BDE-209 along irradiation time were well fitted by the abovementioned equation (R2 = 0.9687) showing that photodegradation of BDE-209 in aqueous solution follows a pseudo first-order kinetics with a rate constant of 0.0101 ± 0.0003 min−1. The half-life time of BDE-209, calculated as t1/2 = ln(2)/ k, was 69 ± 2 min. Assuming that the radiation used correctly simulates sunlight, the half-life time can be converted into outdoor half-life time.28−30 The near surface specific rate of light absorption (k0a ) was calculated, according to eq 1.30

Φav =

C0 × k 350 ∑290 I λ0i

× (1 − 10−ελi × l × C0) × Δλ

(2)

where C0 is the initial concentration of BDE-209 in solution (mol L−1), k is the apparent first order rate constant (s−1), ε is the molar absorptivity of BDE-209 at λi (L mol−1 cm−1), l is the path length inside the photoreactor (1.4 cm) and I0λi is the lamp irradiance at the wavelength λi (Ein m−2 s−1 nm−1), multiplied by the solution area exposed to light inside the container (m2) and divided by the volume of solution (units of I0λi = Ein L−1 s−1 nm−1). Δλ corresponds to the wavelength interval of acquisition of the spectral irradiance of the lamp and λi is the central wavelength of that interval. More details about the calculation of the quantum yield can be found in SI. The quantum yield determination was also made in ethanol, in the same irradiation conditions, for comparison with the literature values. In this study, the determination of the average quantum 14012



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aqueous solution, photodegradation experiments of BDE-209 in ethanolic solution were also performed and compared with literature data. As the solubility of BDE-209 in ethanol is much higher than in water, the analysis did not require any pretreatment of the sample (calibration details in SI). The kinetic results of BDE-209 photodegradation in ethanol are also presented in the Figure 1, together with the curve C/C0 = e−kt, fitted to the data by nonlinear regression, and one can observe a rapid photodegradation of the compound. Data on photodegradation of BDE-209 in ethanol were also well fitted by the equation of pseudo first-order kinetics (R2 = 0.9651), with a rate constant of 0.208 ± 0.008 min−1, corresponding to a half-life time (t1/2) of 3.3 ± 0.1 min. Average quantum yield (Φav) was also calculated for the BDE209 photodegradation in ethanol, by eq 2, for the same wavelength range (290−350 nm). The value obtained was 0.232 ± 0.005. This value is of the same order of magnitude but lower than 0.35, value obtained by Xie et al.17 in ethanol. Thus, we can conclude that the quantum yield for the photodegradation in water is at least 20 times lower than that for the photodegradation in ethanol. To the best of our knowledge there are no other data for the quantum yield in ethanol except that from Xie et al.,17 and no data were found for the quantum yield in water. Values from the literature for the quantum yield of the BDE-209 photodegradation in different solvents are presented in Table S1 of SI. As discussed above, Xie et al.17 and Eriksson et al.36 advocate that the capacity of the solvent to be “hydrogen donor” influences the rate constant and the quantum yields. Moreover, a highly polar solvent may quench the excitation state inducing a low quantum yield.17,38 The much lower quantum yield obtained in the present work for the photodegradation in water, compared with that in ethanol, is in agreement with the fact that water is more polar than ethanol and is not a good hydrogen donor. The presence of acetonitrile in solution (0.1%) and its possible effect on BDE-209 photodegradation should not be completely neglected. The amount of acetonitrile present in solution can contribute to change the lifetime of the excited state or act as extra hydrogen source, changing the rate constant of BDE-209 photodegradation. Thus, the calculated quantum yield in this work, although much more lower than the values reported in the literature, in organic solvents, may still be overestimated. Kuivikko et al.14 calculated the rates of direct photolytic decomposition of polybrominated diphenyl ethers in the Baltic Sea and the Atlantic Ocean, but they used quantum yields determined in isooctane. Since the quantum yield in isooctane, determined by the authors (0.28 ± 0.04), is similar to the quantum yield in ethanol, it is about 20 times higher than the quantum yield in water. Thus, the photolytic half-lives in the mixing layer of the Baltic Sea and in the Atlantic Ocean calculated by the authors are clearly underestimated (too short) and can be 20-fold higher. For example, the half-life of BDE209 in the mixing layer of the Baltic sea at 60°N during summer would be 36 days instead 1.8 days, while in the North Atlantic Ocean at 60°N, during winter it would be 660 days instead of 33 days. Influence of Different Fractions of Humic Substances (HS) on Photodegradation. The study of the influence of different fractions of HS on BDE-209 photodegradation in aqueous solution was tentatively done using DLLME as a preconcentration procedure. However, after centrifugation, the humic substances (HS) precipitated together with the extracting solvent, being not possible to remove all the organic

yield was performed by radiometry and was calculated using eq 2.31,34 As mentioned above, the BDE-209 solubility in water is very low and the concentration used in irradiation experiments was 5 μg/L. For such a low value of concentration, the determination of the molar absorptivity of the compound in aqueous solutions is very imprecise and presents a high associated error. To overcome this problem, solutions of BDE209 with different concentrations were prepared in ethanol and in ethanol/water (80:20 and 20:80) and analyzed by UV−vis spectrometry. The results of these analyses (Figure S3 and S4, SI) allow concluding that the values of molar absorptivity of the BDE-209 solutions do not significantly differ for the several proportions of these two solvents. Besides, the spectra and molar absorptivity values are similar to that presented by Bezares-Cruz et al.33 for BDE-209 in hexane. On the basis of these results, an average molar absorptivity of BDE-209 at each wavelength, ελi (Figure S4, SI) was used to calculate the rate of light absorption by BDE-209 at each wavelength (Iabs λi ), using eq 3.31,34 I λabs = I λ0i × (1 − 10−ελi × l × C0) i

(3)

where the symbols have the meaning previously described. In the range of the emission spectrum of the irradiation lamp (Figure S2, SI), BDE-209 absorbs more significantly for the wavelengths ranging between 290 and 350 nm. The summation in eq 2 was made for λi values between these limits (figure S5, SI) and a quantum yield of 0.010 ± 0.001 was obtained. In the literature, there are several studies of BDE-209 photodegradation under different light conditions and in different organic solvents.16,17,33,35 Those studies allow us to conclude that the photodegradation of BDE-209 in organic solvents occurs through a reductive dehalogenation and its rate is largely related to the capacity of the solvent to give a hydrogen atom to the aryl radical formed by homolytic cleavage of a C−Br bond after molecular excitation by radiation. Indeed, in general, a greater ability of the solvent to give hydrogen promotes a higher rate of the photolytic process.17,36 The studies performed by Eriksson et al.36 showed that the BDE209 photodegradation was slower in methanol/water (80:20) than in methanol, in agreement with the lower tendency of water to give a hydrogen atom; the H−O bond dissociation energy in water is 119 kcal/mol while the C−H bond dissociation energy in methanol is 96 kcal/mol.37 However, other reaction processes may occur. Xie et al.17 (and authors cited therein) refer that hydrogen donating capacity of the solvent did not play a decisive role on the photolytic rate of BDE-209 because they observed a fast photodegradation of BDE-209 in CCl4 with a half-life only three times higher than that in THF. Eriksson et al.36 made a tentative of determination of the kinetics of photodegradation in aqueous solutions containing 1% ethanol, but they refer that the results were rather poor and that “due to its extremely low water solubility, photolysis experiments of BDE-209 in the aqueous phase are practically impossible”. Eriksson et al.36 used a concentration of 50 nM (∼48 μg/L) which is higher than 20−30 μg/L, the solubility of BDE-209 in water at 25 °C, reported in the literature.4 In the present work, the use of a much lower concentration (5 μg/L) combined with a preconcentration by DLLME before analysis, allowed the determination of the quantum yield for photodegradation in water. Because of the absence of data in the literature for the photodegradation in 14013



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a higher concentration of FA were also performed in order to assess whether the effect of FA on BDE-209 photodegradation depended on their concentration. The results obtained are summarized in Figures 2A and 2B. In Figure 2A, the results obtained for the degradation percentage of BDE-209 in the absence of HS and for its degradation in the presence of FA 8 mg/L are separated into different groups, in order to evaluate the robustness of the results. The one-way ANOVA comparison of the three mean values for the degradation percentage of BDE-209 in the absence of HS allowed to conclude that they are not significantly different (p = 0.095). The two mean values of the degradation percentage of BDE209 in the presence of FA 8 mg/L, obtained with one year interval, are also not significantly different (p = 0.094). Thus, in Figure 2B the results obtained in the several replicates of the same experiment were combined, despite being obtained with one year interval. Comparing the photodegradation data a very significant inhibiting effect of humic and fulvic acids fractions on the BDE209 photodegradation (p < 0.00006 and p < 0.00003, respectively) was observed. XAD-4 fraction did not have any significant effect on BDE-209 photodegradation (p = 0.49). The analysis of Figure 2B also suggests that increasing of FA concentration (8−16 mg/L) does not increase the inhibition effect on BDE-209 photodegradation. The delay caused by HS on BDE-209 photodegradation can be explained by two mainly effects: (i) screening effect caused by the light absorption by HS and (ii) hydrophobic associations between BDE-209 and HS, promoting the quenching or deactivation of the excitedstate. To quantify the light attenuation caused by the three fractions of HS and their effect on BDE-209 direct photodegradation, the light-screening factors (Sλ) were calculated according to eq 4.31,39

phase with the microsyringe (step v of the DLLME procedure in SI). Therefore, for the study of BDE-209 photodegradation in the presence of HS, the concentration and cleanup of BDE209 solutions was done by SPE. The irradiation time of 60 min was chosen because it is the time closer to t1/2 of BDE-209 in aqueous solution. As before, the percentage of photodegradation was calculated relatively to the dark controls (light protected solutions inside the solar box). The percentage recoveries of BDE-209 from aqueous solution were somewhat lower than the recovery for the DLLME procedure and even lower in the presence of HS, as shown in Figure S6 (SI). However, those recoveries do not depend on the BDE-209 concentration and, thus, they do not influence the calculation of the degradation percentage, as shown in the discussion of Figure S6, SI. The degradation percentage in water, after 60 min of irradiation, in the absence of HS, obtained using this preconcentration procedure was also determined for comparison with the degradation percentage determined above, using DLLME. Furthermore, because of the problems in working with aqueous solutions of BDE-209 referred in the literature36 and to confirm the reproducibility of the results, some experiments of BDE-209 photodegradation were repeated one year later, as indicated in the legend of Figure 2. Since fulvic acids are the main fraction of dissolved HS in river waters, experiments with

Sλ =

(1 − 10−αλ × l) 2.303 × αλ × l

(4)

where αλ (cm−1) is the wavelength specific attenuation coefficient (considered as the absorbance of each HS fraction, for the selected wavelength, when the path length is 1 cm) and l (cm) is the path length of the irradiated quartz tubes (1.4 cm). Note that this equation is valid only in systems in which the attenuation due to the HS fractions is much higher than the attenuation of the contaminant,40 as is the case in the present work. A good estimate of the effect of light attenuation on photodegradation experiments in the presence of HS can be achieved calculating Sλ using the α value at the wavelength λm of the maximum specific rate of light absorption,30 that is, the wavelength (313 nm) corresponding to the maximum of the curve Iabs λi versus λ (Figure S5, SI). UV spectra of HS fractions in the range 290−350 nm are also presented in the Supporting Information (Figure S7). The values of S313 obtained were 0.81, 0.91, and 0.94 because of the presence of HA, FA, and XAD-4 fraction, respectively (S equals 1 when no attenuation of light occurs). On the basis of these results, there is a slight effect of light attenuation by HS, namely, by humic acids fraction. However, as these three fractions are together in the environment, a mixture S313 was calculated and the value obtained was 0.90. The mixture S313 was calculated using an αλ, in eq 4, which is a weighted average of the αλ (cm−1) of each HS fraction, taking into account the relative abundances of each fraction on the water sample (relative abundances are presented

Figure 2. Percentages of BDE-209 photodegradation (5 μg/L) in the absence of HS and in the presence of HS. n = number of replicates. Error bars = standard deviation (or range when n = 2). (A) Three mean values for the degradation percentage of BDE-209 (5 μg/L) in the absence of HS (two by SPE−HPLC−UV methodology, with one year of interval, and one by DLLME−HPLC−UV procedure) and two mean values of the degradation percentage of BDE-209 in the presence of FA 8 mg/L by SPE−HPLC−UV procedure, obtained with one year of interval, for the same irradiation time (60′). (B) Effect of different fractions of riverine HS on BDE-209 photodegradation (5 μg/L). 14014



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represent a very small fraction of marine HS.48 Considering only the screening effect caused by 8 mg/L of humic substances the photodegradation percentage, after 60 min, would be 30%, however the weighted average of photodegradation obtained was 13%. This can be explained assuming that hydrophobic association between BDE-209 and humic matter is predominant in the inhibition of photodegradation. With the results obtained in this work it is possible to determine the half-life time of the BDE-209 in water, under solar radiation, which can be considered a lower limit for the half-life in aquatic systems, since HS increase the half-life time. Besides, in natural waters, several other factors must be considered, such as: the nature of dissolved organic matter, the amount of suspended particulates, the adsorption of BDE-209 to solid surfaces and the depth. This means that the real half-life time could be considerably much higher. The degradation mechanism in water has not been studied. However, it must be noticed that several photodegradation products have been detected. In an aqueous sample of BDE-209, after 60 min of irradiation, photoproducts were detected by HPLC−UV (Figure 3) with retention times lower and higher than the

in Experimental Section). (The similarity between S313 values determined above and the integrated values of screening factor was confirmed as described in SI.) To estimate the experimental direct photolysis in the presence of HS, the eq 5 was used.39,41 kHS = S × k

(5)

where kHS is the experimental direct photolysis rate constant predicted in the presence of HS (min−1), S is the mixture screening factor (0.90), and k is the first-order rate constant in aqueous solution in the absence of HS fractions (min−1). The rate constant predicted for BDE-209 direct photodegradation in aqueous solution, in the presence of humic substances (kHS) is 0.0091 ± 0.0003 min−1. Similarly to what was done above in the absence of HS, the outdoor half-life time of BDE-209 in the presence of HS can be calculated assuming that the radiation used correctly simulates sunlight. Thus, by multiplying k0p by S, the kp is obtained.30 From kp, the outdoor half-life time predicted in the presence of HS was 3.9 h, which is only slightly higher than the half-life time predicted in the absence of HS (3.5 h). The absorbance of FA and XAD-4 fractions are more similar between them than the absorbance of FA and HA fractions (SI Figure S7), which is reflected in the values of the light screening factors obtained. However, one observes that HA and FA fractions inhibited the BDE-209 photodegradation in a similar way while XAD-4 fraction did not have any effect on its photodegradation (Figure 2B). These results suggest that the inhibiting effect of humic substances cannot be attributed only to a light filtering effect. Thus, effect ii, hydrophobic associations between BDE-209 and HS, promoting the quenching or deactivation of excited-state, seems to be dominant, while effect i, screening effect caused by the light absorption by HS, seems to have only a small contribution, although both effects contribute to the inhibition of BDE-209 photodegradation. The hypothesis of hydrophobic associations is supported by the quenching of HS fluorescence in presence of BDE-209 (experiments and results described in SI). In addition to these inhibiting effects, one should consider that HS can also produce reactive species that can induce the indirect photodegradation of BDE-209, and consequently, increase the degradation rate. Considering the results obtained, this effect is negligible or obscured by the inhibiting effects observed. Clark et al.42 observed a decrease of the photodegradation rate of pyrene in the presence of humic acids and attributed this effect to the protection of the compound from photodegradation because of the adsorption of pyrene within the complex humic acid matrix. It is known that humic substances can establish interactions with hydrophobic compounds43 and the sorption of other PBDEs to dissolved humic substances in aqueous solutions has been previously demonstrated.44 Furthermore, Akkanen et al.45 have demonstrated that PCBs, which have structural similarities to PBDEs, associate mainly with the hydrophobic fraction of DOM (dissolved organic matter). Some authors attributed the decrease of the photodegradation rate of organic contaminants, namely PAHs and PCBs, to the decrease of the lifetimes of their excited singlet or triplet states caused by binding to HS.46,47 This reduction of the photodegradation rate of BDE-209 by humic substances can have important consequences on the half-life of BDE-209 in natural waters. This effect will be probably more important in rivers than in the ocean, since in the ocean HS are less hydrophobic and humic acids (HA)

Figure 3. Chromatograms of the aqueous solution of BDE-209 after 60 min of irradiation.

retention time of BDE-209. The peaks I and II are also present in the dark controls. However, the chromatograms of the BDE209 solutions in ethanol (data not shown), after photodegradation, exhibit only peaks with retention times lower than that of BDE-209. According to several authors, these photoproducts with lower retention times are less brominated products, resulting from bromine substitution by hydrogen. However, other degradation products may give rise to peaks at retention times lower than BDE-209. For example, Eriksson et al.36 detected polybrominated dibenzofurans (PBDFs) in photodegradation experiments in methanol/water (80:20). In addition, the formation of hydroxylated species from BDE-209 photodegradation in aqueous solution was predicted by Hardy49 on the basis of the behavior of other halogenated aromatic compounds. In the literature, with other organic solvents, there is no reference to photoproducts that appear to higher retention times than BDE-209.To our knowledge and at these conditions, this is the first evidence of the formation of other degradation products in water (peaks a, b, c, and d). Further studies are under way to identify the degradation products formed. The study of photodegradation in water is of fundamental importance to understand the behavior of BDE-209 when it is discharged with the effluents of wastewater treatment plants into watercourses. Thus, the results obtained in the present 14015



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(9) Labadie, P.; Tlili, K.; Alliot, F.; Bourges, C.; Desportes, A.; Chevreuil, M. Development of analytical procedures for trace-level determination of polybrominated diphenyl ethers and tetrabromobisphenol A in river water and sediment. Anal. Bioanal. Chem. 2010, 396 (2), 865−875. (10) Stapleton, H. M.; Alaee, M.; Letcher, R. J.; Baker, J. E. Debromination of the flame retardant decabromodiphenyl ether by juvenile carp (Cyprinus carpio) following dietary exposure. Environ. Sci. Technol. 2004, 38 (1), 112−119. (11) Elliott, J. E.; Wilson, L. K.; Wakeford, B. Polybrominated diphenyl ether trends in eggs of marine and freshwater birds from British Columbia, Canada, 1979−2002. Environ. Sci. Technol. 2005, 39 (15), 5584−5591. (12) He, J.; Yang, D.; Wang, C.; Liu, W.; Liao, J.; Xu, T.; Bai, C.; Chen, J.; Lin, K.; Huang, C.; Dong, Q. Chronic zebrafish low dose decabrominated diphenyl ether (BDE-209) exposure affected parental gonad development and locomotion in F1 offspring. Ecotoxicology 2011, 20, 1813−1822. (13) He, J. Z.; Robrock, K. R.; Alvarez-Cohen, L. Microbial reductive debromination of polybrominated diphenyl ethers (PBDEs). Environ. Sci. Technol. 2006, 40 (14), 4429−4434. (14) Kuivikko, M.; Kotiaho, T.; Hartonen, K.; Tanskanen, A.; Vahatalo, A. V. Modeled direct photolytic decomposition of Polybrominated diphenyl ethers in the Baltic sea and the Atlantic ocean. Environ. Sci. Technol. 2007, 41 (20), 7016−7021. (15) Christiansson, A.; Eriksson, J.; Teclechiel, D.; Bergman, A. Identification and quantification of products formed via photolysis of decabromodiphenyl ether. Environ. Sci. Pollut. Res. 2009, 16 (3), 312− 321. (16) Shih, Y.-h.; Wang, C.-K. Photolytical degradation of polybromodiphenyl ethers under UV-lamp and solar irradiations. J. Hazard. Mater. 2009, 165, 34−38. (17) Xie, Q.; Chen, J.; Shao, J.; Chen, C. e. Important role of reaction field in photodegradation of deca-bromodiphenyl ether: Theoretical and experimental investigations of solvent effects. Chemosphere 2009, 76, 1486−1490. (18) Gandhi, N.; Bhavsar, S. P.; Gewurtz, S. B.; Tomy, G. T. Can biotransformation of BDE-209 in lake trout cause bioaccumulation of more toxic, lower-brominated PBDEs (BDE-47,-99) over the long term? Environ. Int. 2011, 37 (1), 170−177. (19) Esteves, V. I.; Cordeiro, N. M. A.; Duarte, A. D. Variation on the adsorption efficiency of Humic Substances from Estuarine waters using XAD resins. Mar. Chem. 1995, 51 (1), 61−66. (20) Okamura, H.; Sugiyama, Y. Photosensitized degradation of Irgarol 1051 in water. Chemosphere 2004, 57, 739−743. (21) Aleksandrova, O. N.; Schulz, M.; Matthies, M. A quantum statistical approach to remediation effect of humic substances. Water Air Soil Pollut. 2011, 221 (1−4), 203−214. (22) Canonica, S.; Laubscher, H.-U. Inhibitory effect of dissolved organic matter on triplet-induced oxidation of aquatic contaminants. Photochem. Photobiol. Sci. 2008, 7, 547−551. (23) Santos, M. E. B. H. Extracçaõ , caracterizaçaõ e comportamento ácido-base de substâncias húmicas aquáticas (Extraction, characterization and acid-base behavior of aquatic humic substances). Ph.D. dissertation, University of Aveiro, Aveiro, Portugal, 1994. (24) Li, Y.; Wei, G.; Hu, J.; Liu, X.; Zhao, X.; Wang, X. Dispersive liquid−liquid microextraction followed by reversed phase-high performance liquid chromatography for the determination of polybrominated diphenyl ethers at trace levels in landfill leachate and environmental water samples. Anal. Chim. Acta 2008, 615, 96− 103. (25) Medina, C. M.; Pitarch, E.; Portolés, T.; López, F. J.; Hernández, F. GC-MS/MS multi-residue method for the determination of organochlrine pesticides, polychlorinated biphenyls and polybrominated diphenyl ethers in human breast tissues. J. Sep. Sci. 2009, 32, 2090−2102. (26) Nácher-Mestre, J.; Serrano, R.; Hernández, F.; Benedito-Palos, L.; Pérez-Sánchez, J. Gas chromatography-mass spectrometric determination of ploybrominated diphenyl ethers in complex fatty

work are important for a better understanding of the behavior and the interactions of BDE-209 in the aquatic environment.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed description of experimental procedures (DLLME and SPE), solubilization of BDE-209 stock solution in acetonitrile, molar absorptivity of BDE-209 in acetonitrile, spectral irradiance of the 1500 W arc xenon lamp; calibration details of DLLME−HPLC−UV procedure, details of quantum yield determination, variation of molar absorptivity of BDE-209, for the different proportions of water and ethanol, average molar absorptivity of BDE-209 at each wavelength (290−350 nm), rate of light absorption of BDE-209 for each wavelength (290− 350 nm), calibration details for BDE-209 determination in ethanol, a table with values from the literature for the quantum yield of the BDE-209 photodegradation in different solvents, recovery percentages of BDE-209 in the absence and in the presence of FA, UV−vis spectra of HS (8 mg/L) in aqueous solution, and light-screening effect calculation; hydrophobic interactions between BDE-209 and HS. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +351-234-370-725. Fax: +351-234-370-084. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors would like to acknowledge funding from Fundaçaõ para a Ciência e TecnologiaPortugal (FCT) to Centre for Environmental and Marine Studies (CESAM). J.F.L. thanks FCT for her PhD grant (SFRH/BD/88572/2012).

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REFERENCES

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BDE-209: Kinetic Studies and Effect of Humic Substances on

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