Environ. Sci. Technol. 2007, 41, 7016-7021
Modeled Direct Photolytic Decomposition of Polybrominated Diphenyl Ethers in the Baltic Sea and the Atlantic Ocean MIIKA KUIVIKKO, TAPIO KOTIAHO, AND KARI HARTONEN Laboratory of Analytical Chemistry, Department of Chemistry, University of Helsinki, A. I. Virtasen aukio 1, P.O. Box 55, FIN-00014 Helsinki, Finland AAPO TANSKANEN Finnish Meteorological Institute, UV Radiation Research, Helsinki, Erik Palmenin aukio 1, P.O. Box 503, FIN-00101 Helsinki, Finland A N S S I V . V A¨ H A¨ T A L O * Department of Biological and Environmental Sciences, University of Helsinki, Viikinkaari 1, P.O. Box 65, FIN-00014 Helsinki, Finland
In this study, tetra- (#47), penta- (#99), and decabrominated (#209) diphenyl ethers were exposed (in isooctane) to summer sunlight at 60° N, where their photochemical halflives ranged from 0.6 to 203 h. Apparent quantum yields, ranging from 0.16 to 0.28, were applied to optical models to calculate the rates of direct photochemical decomposition at the surface (depth of 0 m) and in the mixing layer of the ocean. The calculated photolytic half-lives were 4-100 times as long in the mixing layer of the Baltic Sea and the North Atlantic Ocean as at the surface of 0 m. Calculation of seasonal photochemical half-lives for the mixing layer of the North Atlantic Ocean from 0° N to 60° N showed that the solar photolysis effectively decomposes the congeners in the tropics. At mid- and high latitudes, where solar irradiances are lower outside summer, the photolysis rates for congeners #47 and #99 were often too low for their effective decomposition in the mixing layer. Although solar radiation can potentially decompose the congeners in the mixing layer of the ocean effectively, seasonal and latitudal variation in solar irradiance as well as optical and mixing properties of the ocean can make the direct photolytic decomposition ineffective at high latitude and the coastal ocean.
Introduction Large quantities of brominated flame retardants (BFRs) are produced and used in everyday consumer products (1, 2). BFRs prevent ignition or slow down burning and in this way save lives and property. Polybrominated biphenyl ethers (PBDE) are the most commonly used brominated flame * Corresponding author phone:
[email protected]. 7016
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+358 9 19157825; e-mail:
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retardants in Europe. Commercial PBDE products consist of three main groups: penta-, octa-, and decabrominated PBDEs. In the year 2003, 56 418 metric tons of decabrominated diphenyl ethers were produced globally (3). Unfortunately, PBDEs are ubiquitous in the environment, where they can accumulate in biota with endocrinic and neurotoxic effects (4, 5) being considered new persistent organic pollutants (POP). The oceans cover about 70% of the Earth’s surface. Oceans form a significant repository and reaction medium for anthropogenic contaminants. The deposition of contaminants is greatest near anthropogenic sources (i.e., in the coastal waters). In 2001, the deposition (gaseous+particle) of PBDEs into the Baltic Sea (257 300 km2) was 236 kg (6). Long-range atmospheric transport exports PBDEs and other POPs also to remote regions (7). PBDEs have been found, for example, in artic marine animals such as polar bears (8). PBDEs and similar chlorinated POPs resist microbial decomposition in the presence of oxygen but can decompose through microbial reductive dehalogenation in anaerobic environment (9-11). Photochemical reactions debrominate PBDEs in organic solvents (12-16), on clay minerals (17), on solid surfaces (18), and in water (19). Given the inefficiency of aerobic microbial decomposition of PBDEs, solar radiationinduced photochemical reactions can be expected to be the major pathway for the decomposition of PBDEs in aerobic water columns of the ocean. Photochemical decomposition of POPs can take place via direct and indirect photochemical reactions. In direct reactions, POPs absorb radiation themselves and decompose. Indirect photochemical reactions involve a sensitizer, which absorbs radiation and induces the decomposition of POPs. Little is known about the indirect photochemical decomposition of PBDEs, and the reported photolytic half-lives of PBDEs are for direct photolysis (14, 15, 19). The reported direct photolytic half-lives of 15 PBDEs under a UV lamp or solar radiation (15) range from 0.5 h for 2,2′,3,3′,4,4′,5,5′,6,6′-decabromodiphenyl ether (#209) to 290 h for 2,2′,4,4′-tetrabromodiphenyl ether (#47). These halflives indicate that solar radiation can decompose PBDEs effectively in a thin surface layer of the ocean (at depth of 0 m). When PBDEs deposit in the ocean, they quickly disperse into the mixing layer (Figure S1). There, natural particles and chromophoric dissolved organic matter (CDOM) absorb most of the photolytic solar (ultraviolet) radiation and compete with PBDEs for the absorption of photons. Solar radiation can decompose PBDEs in the upper part of the mixing layer, called here the photolytic layer, but not below it (Figure S1). As long as PBDEs remain in the mixing layer, they will be exposed to photolytic solar radiation (at least periodically) and may become photodegraded. If sedimentation removes PBDEs below the mixing layer, the potential for photolysis is lost, and microbial decomposition in the aerobic water column is unlikely. The decomposition of PBDEs in the ocean must be understood, therefore, through study of the photolytic half-lives of PBDEs in the mixing layer. We determined the apparent quantum yields for direct photolysis of three PBDE congeners and calculated their photolytic half-lives at the depth of 0 m and in the mixing layer of the coastal and the open ocean. In addition, we calculated the photolytic half-lives of PBDEs in the mixing layer of the North Atlantic Ocean at latitudes 0-60° N in four seasons. 10.1021/es070422+ CCC: $37.00
2007 American Chemical Society Published on Web 09/13/2007
Materials and Methods Chemicals. 2,2′,4,4′-Tetrabromodiphenyl ether (#47), 2,2′,4,4′,5-pentabromodiphenylether(#99),and2,2′,3,3′,4,4′,5,5′,6,6′decabromodiphenyl ether (#209) (50 mg L-1 in isooctane, except #209 9:1 ratio isooctane:toluene) were purchased from Accustandard Inc. (New Haven, U.S.A.). Purities were 100% for #47 and #99 and >98.3% for #209 according to gas chromatography-mass spectrometry (GC-MS) analyses done by the manufacturer. HPLC-grade isooctane was from Fisher Scientific Ltd. (Loughborough, U.K.). Instruments. The concentrations of the congeners were measured with an Agilent 6890/5973N GC-MS instrument (Palo Alto, U.S.A.) equipped with a 3-5 m long retention gap (Agilent Technologies, i.d. 0.53 mm). A short analytical column was used for #209 (5-7 m long DB-5MS; J & W Scientific, Folsom, U.S.A.) and a longer column was used for #99 and #47 (13.9 m long HP-5MS; Agilent Technologies). The Supporting Information describes the GC-MS parameters in detail. Molar absorptivities of PBDEs (λ; mol-1 L cm-1 nm-1) were determined with a Cary 100 UV-vis spectrophotometer (Varian Inc., Palo Alto, U.S.A.) with use of a 10-mm quartz cuvette, a scanning rate of 30 nm min-1, and a slit width of 2 nm. The solubility of the studied congeners in water is too low for the determination of λ and the apparent quantum yields for decomposition (φPBDE). For the reliable determination of λ and φPBDE, the congeners were dissolved in isooctane. The absorptivities of congeners (50 mg L-1 in isooctane) decreased exponentially toward the visible range of the spectrum and fell below the level of quantitation (S/N < 10) at 313 nm (#47), 320 nm (#99), and 351 nm (#209). Instead of assuming zero absorptivities below the limit of quantitation, we used absorptivities extrapolated on the basis of exponential fit to the measured absorptivites close to the limit of quantitation. Decomposition Experiments. The congeners (250 µg L-1 in isooctane) were enclosed without headspace into custommade quartz (length 32 mm, outer diameter 12 mm; Laborexin, Finland) or into regular glass GC-autosampler vials cleaned by solvent and combustion (600 °C 2 h). The quartz vials, together with glass vials wrapped in aluminum foil (dark controls), were exposed to solar radiation in an outdoor pool on a roof in Helsinki (51 m above sea level, 60°32′ N 24°97′ E). The 5-6 cm deep pool was made of matte black tarpaulin and flushed with tap water (20-23 °C). The exposures lasted 60 min (#209, October 5 2:35 p.m. to 3:35 p.m.), 4 days (#99; June 13 1:00 p.m. to June 17 1:00 p.m.), and 12 days (#47; July 7 2:19 p.m. to July 19 2:19 p.m.). During the experiments, global radiation (W m-2) was measured at 1-min intervals with a pyranometer (Tartu, Estonia) located next to (20 m) the exposure pool. The concentrations of the congeners were determined 4-5 times from one dark control and three exposed samples at each measurement time. Since no decomposition was found in the dark controls, the amount of photolytically decomposed PBDE (∆CPBDE; mol L-1 exposure period-1) was calculated as the difference in the congener concentrations (mol L-1) between solar radiation exposure times T and T + 1 ()CPBDE,T - CPBDE,T+1). Calculation of Apparent Quantum Yields. Apparent quantum yields for the direct photolytic decomposition of congeners (φPBDE) were calculated as
φPBDE ) ∆PBDEQa-1
(1)
where ∆PBDE (mol decomposed PBDE exposure period-1) is the photolytically decomposed parent congener in the quartz vials (∆PBDE ) ∆CPBDEV; V is volume of vials) and Qa is the dose of photons absorbed by the congener during the exposure to solar radiation (Qa; mol photons exposure period-1).
Doses of Photons Absorbed by PBDE. The doses absorbed by PBDEs during the exposure (Qa) were calculated according to Hu et al. (20) as
Qa )
∫ ∫ Tend
T0
λ,max
λ,min
Qd,v,λ[1 - exp(-aPBDE,λL)] dt dλ S (2)
where Qd,v,λ is the downward vector photon flux density above the irradiated vials at wavelength λ (mol photons m-2 d-1), aPBDE,λ is the Napierian absorption coefficient of PBDE at wavelength λ (m-1), L is the optical path length of the exposed sample (m), and S is the area of the exposed sample (127 × 10-6 m2). The integration over wavelengths at 1-nm band intervals (dλ) is from the shortest wavelength (λmin ) 300 nm) to the longest wavelength (λmax) where aPBDE,λ and Qd,v,λ overlapped. The integration over time at 1 min or 1 h intervals (dt) sums Qd,v,λ from the beginning (T0) to the end of the experiment (Tend). For further information, see the Supporting Information. Determination of Spectral UV Radiation at the Water Surface. The solar irradiances incident to the surface of the exposure pool were determined according to measured global radiation and radiative transfer calculations. Radiative transfer calculations were made with the DISORT2 code of the LibRadtran program package (21). The assumptions made in the radiative transfer model are summarized in Table S1. The radiative transfer model accounts for the scattering and the absorption of the extraterrestrial solar radiation by the atmosphere and gives the direct and diffuse components of the downward photon flux density, Qd,v,λ. Prior to calculation of the surface irradiance, the cloud optical depth at 550 nm required for calculation of the UV spectrum was determined iteratively by forcing the calculated global radiation to match the measured one. The irradiances were calculated at minute intervals in October and hourly intervals in June-July, with mid-hour solar zenith angle for the exposures. The seasonal mean solar irradiances at the surface of the North Atlantic Ocean along 30° W meridian were determined at four different latitudes (0, 20, 40, and 60° N) with the DISORT2 radiative transfer model (Table S1). The total column ozone data were obtained from the Earth Probe TOMS instrument, and the surface albedo was assumed to be 7%. The attenuation of the UV radiation by clouds was determined using the Lambertian equivalent reflectivity (LER) data from the Earth Probe TOMS instrument and the method of Eck et al. (22). The solar irradiances were calculated at 1-h intervals for the 15th day of each month and summed over the day (Qd,v,λ; mol photons m-2 d-1 nm-1). Seasonal irradiances were calculated as the mean of the 3-months’ irradiances (Dec-Feb ) winter, Mar-May ) spring, JunAug ) summer, Sep-Nov ) autumn). Model for the Direct Photochemical Decomposition of PBDEs in Surface Waters. The direct photochemical decomposition of PBDE at depth z (∆PBDEz; mol PBDE m-3 d-1) was expressed as
∆PBDEz )
∫
λ,max
λ,min
Qs,z,λaPBDE,λφPBDE dλ
(3)
where Qs,z,λ is the scalar photon flux density at depth z and wavelength λ (mol photons m-2 d-1 nm-1). The direct photochemical decomposition rate of PBDE in the whole water column (∆PBDE; mol PBDE m-2 d-1) was
∆PBDE )
∫
λ,max
λ,min
Qa,λaPBDE,λatot,λ-1 φPBDE dλ
(4)
where Qa,λ is the photon flux density absorbed by the water column (mol photons m-2 d-1), and the shading factor, aPBDE,λ atot,λ-1, expresses the fraction of photons absorbed by PBDE in relation to the total absorption (atot,λ; m-1). VOL. 41, NO. 20, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Molar absorptivities of congeners #47 (1), #99 (2), and #209 (3) and the mean solar irradiance at Helsinki, 0.30-1:30 p.m. on July 7, 2005, modeled with DISORT2. The photolytic half-lives of PBDEs in the mixing layer were calculated according to first-order kinetics. First, the decomposition rate (mol PBDE m-2 d-1, eq 4) was divided by the concentration in the mixing layer (mol PBDE m-2) to obtain the first-order rate constant k (d-1), and then the halflife was calculated from the rate constant (t1/2 ) ln2 k-1). Optical Characterization and Physical Structure of the Baltic Sea and the North Atlantic Ocean. Estimation of the photochemical decomposition of PBDEs in coastal and oceanic waters required information about the optical properties such as atot,λ of eq 4 and physical structure of the mixing layer. For that purpose, we collected water samples from the Baltic Sea (59°49′ N, 23°18′ E on Aug 31, 2005) and the North Atlantic Ocean (23°01′ N, 29°16′ W on Oct 4, 2004) and determined the total Napierian absorption coefficient of all natural optical components (atot,λ; m-1) as the sum of absorption coefficients of particles (apart,λ), chromophoric dissolved organic matter (aCDOM,λ), and water (aH2O,λ from refs 23 and 24, see Supporting Information, Figures S2 and S3). We estimated the seasonal optical properties and mixing layer depths in the North Atlantic Ocean along 30° W at latitudes 0, 20, 40, and 60° N according to the reported mixing layer depths ((25) Table S2) and the concentrations of CDOM and Chl-a ((26, 27) details in the Supporting Information, Table S2).
Results and Discussion Absorption Spectra of PDBEs. The absorptivity of congeners #99 and #47 reached the UVB part of the spectrum, while that of #209 reached the UVA part of the spectrum and was the largest among the studied congeners (Figure 1). The overlap between the absorption spectra of the congeners and the solar radiation spectrum indicates that the congeners can potentially photodegrade via direct photochemical reactions under solar radiation. Decomposition Kinetics and Apparent Quantum Yield. When the PBDE congeners were dissolved in isooctane and exposed to solar radiation, their concentrations decreased according to first-order kinetics (Figure 2). No decomposition was found in the dark control vials wrapped in aluminum foil (data not shown). These results show that PBDEs are decomposed via direct photochemical reactions, since the PBDEs were the only compounds capable of absorbing solar UV-radiation during the exposures (isooctane has a UV cutoff around 200 nm). The photodegradation rate increased with the degree of bromination (Figure 2, Table 1), primarily because the overlap between solar radiation and the absorption spectra increased too. The measured half-lives of 0.6 h (#209), 34 h (#99), and 204 h (#47) were similar to the halflives reported by Eriksson et al. (15) (0.5 h #209, 64 h #99, and 290 h #47) and So¨derstro¨m et al. (14) (95% of the mixing layer was below the photolytic layer. These calculations emphasize that the depth of the photolytic layer in relation to that of the mixing layer can vary between the coastal and the open ocean and greatly influences the photolytic half-lives of PBDEs. Seasonal Direct Photolytic Decomposition of PBDEs in the Atlantic. To demonstrate the impact that seasonal and latitudal differences in solar irradiance, depth of the mixing layer, and optical conditions have on the photolysis of the PBDEs, we calculated the photolytic half-lives of PBDEs (34 pg L-1) in the mixing layer of the North Atlantic Ocean along the 30° W meridian through latitudes 0-60° N during four 90-d-long seasons. Calculations were done using eq 4 and the seasonally appropriate solar radiation, the depth of the mixing layer, and water optical properties (Table S2, Figures S5 and S6). As an example of the variability in environmental conditions, the solar photon flux densities at 320 nm ranged from 0.001 mol m-2 d-1 to 0.022 mol m-2 d-1 (22-fold difference), while the total absorption of natural absorbing components at 320 nm ranged from atot,320 of 0.15 m-1 to 0.49 m-1 (3.3-fold difference) over the sites and seasons (Figure S6, Table S2). The depth of the mixing layer ranged from 30 to 180 m (6-fold difference), while the photolytic made up 7-87% of the mixing layer among the four sites and seasons (Table S2). The seasonal variation in environmental factors affecting photolysis was largest at latitudes 40° N and 60° N. As shown in Table 2 and Figure 4, the photolytic half-lives in the tropics (0-20° N) were generally shorter than the length of the seasons (90 d). Our results indicate the high potential for the photolysis of PBDEs in the mixing layer of tropical
FIGURE 3. Number of photons absorbed by PBDE congeners during the photolysis experiment (A #47, B #99, and C #209) consisting of 3-4 consecutive periods. oceans and for congener #209 at all latitudes and in all seasons (Table 2). At mid- and high latitudes (40-60° N), the photolytic half-lives varied among seasons up to 192-fold and were longest during winter. At 60° N, outside summer, the photolytic half-lives of #47 and #99 were longer than the length of the seasons (Table 2, Figure 4). These results indicate that solar-radiation-induced direct photochemical reactions decompose deposited #47 and #99 very ineffectively in the mixed layer of the ocean at high latitudes. Our results are alarming when considering the environmental fate of PBDEs or similar POPs (such as PCBs, halogenated dioxins or furans, DDT) in the coastal and the VOL. 41, NO. 20, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Photolytic Half-Lives (Days) of the PBDE Congeners in the North Atlantic Ocean from 0 to 60° N Latitude at Four Seasonsa latitude
spring
summer
autumn
0 20 40 60
36 121 1016 22244
#47 31 35 177 922
42 19 17 116
39 43 69 2967
0 20 40 60
9 32 202 2930
#99 7 13 43 188
12 5 5 27
11 12 17 585
0 20 40 60 a
winter
0.2 0.6 3 33
#209 0.2 0.3 0.7 3
0.2 0.1 0.1 0.4
0.2 0.2 0.3 8
PBDE concentration 3-4 pg L-1.
high latitude ocean. Although solar radiation can effectively decompose congener #209, lower brominated PBDEs (#47 and #99 as well as similar photolytic decomposition products of #209) are ineffectively decomposed in high latitudes, and it is just these compounds that have the largest potential for long-range atmospheric transport and bioaccumulation (31). If solar irradiance cannot decompose the lower brominated PBDEs in the 30-180 m deep mixing layer of the North Atlantic Ocean, those PBDEs have high potential for bioaccumulation in the mixing layer or below it, in the dark deep aerobic water column of the ocean. The φPBDEs of this study were determined in isooctane and may not be directly applicable to water. The earlier
photochemical experiments of PBDEs in the mixture of methanol and water (80:20) give the best approximations of φPBDEs in water (15). The φ#47 is the same (0.22) in methanol/ water (15) and in isooctane (this study) suggesting that the photolytic half-lives for #47 hold in the ocean (Table 2). The φ#209 is 2-fold higher in isooctane than in methanol/water (15) suggesting that the photolytic half-lives of #209 in the ocean can be 2-fold longer than those given in Table 2. Even taking into account this possible increase in photolytic halflife, solar radiation decomposes much faster #209 than the lower brominated congeners (Table 2). Our photolytic half-lives of PBDEs may be too optimistic (i.e., short) for the overall decomposition of PBDEs in the water column, since we did not take into account the sedimentation of PBDEs from the mixing layer. We also did not take into account the indirect photochemical decomposition mediated by CDOM, and therefore the total ()direct + indirect) photochemical decomposition of PBDEs may be larger than estimated here. Although there are uncertainties in our results with regard to the ineffective photolytic decomposition of lower brominated PBDEs in the ocean, the results are in agreement with the demonstrated presence of these congeners in high latitude marine environments (32). Our findings indicate that the photolytic half-lives of PBDEs are longer in the coastal ocean than the open ocean. In the open ocean, the photolysis of PBDEs benefits from the optically clear water, where the photolytic layer makes up a large portion of the mixing layer. The coastal ocean frequently receives a substantial terrestrial load of CDOM and particles, which make the photolytic layer shallow in relation to the mixing layer. The shallow photolytic layer and the competition for light between CDOM and PBDEs cause a marked increase in the photolytic half-lives of PBDE in the mixing layer of the coastal ocean.
FIGURE 4. Photolytic half-lives of #99 in the North Atlantic Ocean along 30° W meridian at latitudes 0-60° N during four seasons. 7020
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In sum we have demonstrated that the natural absorbing components, the depth of the mixing layer, and the intensity of solar radiation greatly affect the photolytic half-lives of PBDEs in surface waters, and these factors need to be taken into account in any estimates of the photolytic decomposition of PBDEs and similar POPs.
Acknowledgments Financial support was received from the Maj and Tor Nessling Foundation and Academy of Finland. We thank the Division of Atmospheric Sciences (University of Helsinki) for the global radiation data. We are grateful to Jukka Seppa¨la¨, Pasi Ylo¨stalo, Gerhard Herndl, and the crew of RV Pelagia for their help in the field.
Supporting Information Available Additional information about the analytical techniques, optical data, and radiative transfer calculations. This material is available free of charge via the Internet at http:// pubs.acs.org.
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Received for review February 19, 2007. Revised manuscript received July 12, 2007. Accepted July 30, 2007. ES070422+
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