Environ. Sci. Technol. 1992, 26, 991-998
Chamber Aging Studies on the Atmospheric Stability of Polybrominated Dibenzo-p -dioxins and Dibenzofurans Christopher C. Lutes, M. Judith Charles, Jay R. Odum, and Richard M. Kamens” Department of Environmental Sciences and Engineering, School of Public Health, University of North Carolina at Chapel Hill, Chapel HIII, North Carolina, 27599-7400
rn In this study dilute emissions from the combustion of
polyurethane foam containing polybrominated diphenyl ethers (PBDPEs) were introduced into 25-m3 outdoor Teflon film chambers and aged in the presence of sunlight. Concentrations of tetra- and pentabrominated dibenzop-dioxins and dibenzofurans were monitored over time by collecting and analyzing particulate- and vapor-phase samples. The results demonstrated that polybrominated dibenzo-p-dioxins (PBDDs) and polybrominated dibenzofurans (PBDFs) are stable or slowly degrade over periods of hours. The half-life of the reaction on actual soot particles agreed better with the predictions of solid surface experiments than with solution-phase experiments. Introduction Incineration is an attractive waste-disposal technology because land disposal of wastes can result in contaminated water supplies and because incineration drastically reduces the final waste volume and the land area used for waste disposal ( I ) . The latter is important because most landfills either have been or are being closed for environmental reasons, and greater than one-third of the remaining landfills will reach their design capacity by 1996 (1). Use of incineration is being opposed, however, due to health concerns about emissions of “toxic chemicals”, particularly the compound 2,3,7,8-tetrachlorinateddibenzo-p-dioxin. Risk assessment of the health hazards associated with incineration and ensuing regulatory policy are generally based on information about the toxicity of a compound in laboratory animals and an assessment of human exposure. The number derived for an acceptable emission level does not generally consider the atmospheric lifetime of compounds contained in the emissions. This lifetime and those of associated degradation products are of vital importance in assessing exposure since human exposure occurs through atmospheric transport and depositional processes. Past research has focused on incinerator emissions of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs and PCDFs). Recently, however, polybrominated dibenzo-p-dioxinsand dibenzofurans (PBDDs and PBDFs) have been detected in the pyrolysis products of flame-retardant materials [i.e., polybrominated diphenyl ethers (PBDPEs)] by numerous investigators (2-8). Other researchers have reported the presence of bromochlorinated dibenzo-p-dioxins and dibenzofurans in extracts of municipal incinerator fly ash (9-11). These compounds are important because data from animal studies have shown that the lethal dose of 2,3,7,8-tetrachlorinateddibenzofuran (2,3,7,8-TCDF)and 2,3,7,8-tetrabrominated dibenzofuran (2,3,7,8-TBDF) in guinea pigs is similar (12) and that 2,3,7,8-TBDFis a more potent inducer of arylhydrocarbon hydroxylase and 7-ethoxyresorufin in chick embryos (13). Furthermore, emissions of PBDDs and PBDFs are likely to increase due to increasing usage of brominated organic chemicals as flame-retardant materials. Results of laboratory studies indicate that photodegradation of brominated dibenzo-p-dioxinsand dibenzofurans 0013-936X/92/0926-099 1$03.00/0
can occur by debromination (14,15). The reaction is rapid in organic solution [for 2,3,7,8-tetrabrominated dibenzop-dioxin (2,3,7,8-TBDD) tl = 0.8 min] but much slower on quartz surfaces (2,3,7,8-&DD tlIz = 32 h)(14). Though halogens in the 2,3,7,8-positions are preferentially lost in solution for both the chlorinated and brominated species, the reverse appears to be the case on solid surfaces (14, 16,17). Accurate atmospheric degradation rates on real incinerator soot particles thus cannot be determined from laboratory data, and the paucity of experimental data under realistic conditions makes it impossible to derive degradation rates for these compounds in the ambient environment. The objective of this study was, therefore, to apply chamber methods previously used to study the atmospheric degradation of polycyclic aromatic hydrocarbons (18-20), to investigate the stability of polybrominated dibenzo-p-dioxins and dibenzofurans under realistic conditions. Accordingly, polybrominated diphenyl ethers, precursors of PBDDs and PBDFs, were combusted in a liquefied petroleum (LP) powered combustion unit (640-760 “C), and the emissions were added to 25-m3 outdoor Teflon film chambers. This “captured air parcel” was then exposed to real temperature, humidity, and sunlight regimes, and samples were taken periodically to determine the atmospheric stability of PBDDs and PBDFs. We describe herein the results of three experiments, two performed during the winter under cold temperatures (7 to -5 “C) and low solar intensity, and one conducted in the spring under warm temperatures (23-36 “C) and high solar intensities. Results of a preliminary experiment conducted during the summer are also discussed. Experimental Section Description of Outdoor Chambers. The outdoor chambers used in this study are located in Pittsboro, NC, and have been previously described elsewhere (18-20). Two identical 25-m3 outdoor Teflon film chambers, referred to as the east and west chambers, were used to allow two experiments to be conducted under identical conditions of temperature and solar irradiation. The chambers were vented with rural ambient air for 2-3 h between experiments at a rate of approximately 50 ft3/min; thus several complete air exchanges were performed. The 5-mil Teflon FEP film used in the construction of these chambers has been shown to transmit greater than 70% of the incident light from 300 to 700 nm, measured normal to the sun. Transmission at 350 nm is greater than 50% between a 0 and 70” angle between the film plane and the sun and 77% normal to the sun (21). I t has been shown that particle suspension half-lives for combustion soot particles in these chambers are approximately 3-6 h. The major routes of particle loss include loss to chamber walls, sampling, and leakage (22). Ignition Vessel and Combustion of Starting Material. To burn precursors of PBDDs and PBDFs, a combustion vessel (5411. i.d., 15 in. high) was constructed of galvanized steel and insulated with kaolinite wool. All
0 1992 American Chemical Society
Environ. Sci. Technol., Vol. 26, No. 5, 1992 991
seams and joints were either mechanically sealed or welded to withstand high temperatures (at least 670 "C). Polybrominated dibenzo-p-dioxins and dibenzofurans were produced from the combustion of polyurethane foam (PUF) that contained an industrial flame-retardant mixture primarily consisting of tetra- and pentabrominated diphenyl ethers. The burner at the base of the vessel was 5 cm in diameter and was fueled with LP gas. A 28-cm tube and plunger assembly extended horizontally 8 cm above the burner and was used to quickly add polyurethane foam containing flame retardants (6.4% w/w DE-71, Great Lakes Chemical Co., West Lafayette, IN, in foam from Olympic Products, Greensboro, NC). The sample of PUF (0.5-1.2 g) entered the combustion zone and landed on a grid located 6 cm above the burner of the ignition vessel. Most of the sample was then combusted, over a period of less than 5 s, in a liquid petroleum flame generated from a burner built into the damper at the bottom of the vessel. The temperature (640-760 "C) of the flame was controlled manually by using a valve and was measured in the combustion zone by a nickel chromium vs nickel aluminum thermocouple (Omega, Stamford, CT, Type K). Chamber Aging Experiments. Emissions from the combustion vessel were introduced within a period of less than 15 s, through a 2-in.-diameter flexible steel transfer manifold to a 25-cm3outdoor Teflon chamber (18-20,22) and allowed to age within the chamber atmosphere. A slight negative chamber pressure was briefly applied during the introduction of emissions to draw the exhaust into the chamber. During the aging period, particulate samples were collected by passing chamber air through a sampling train that contained a 47-mm T60 A20 Teflon-impregnated glass fiber filter for collection of the particulate material (Pallflex, Putnam, CT), followed by a 4 in. X 1.5 in. PUF (Olympic Products) vapor trap. The PUF used for sample collection did not contain PBDPEs and was Soxhlet extracted in toluene and dried prior to use. Samples were taken for periods of 11-71 min, with the majority being collected for 20 min. Flow rates (35.3-67.2 L/min) were measured, and the mass of the particulate material collected on each filter was determined (Sartorious microbalance, Model 4503-MP6). Sample volumes ranged from 0.7 to 2.7 m3 and were typically between 1 and 2 m3. Particulate sample masses ranged from 0.3 to 3.6 mg. Samples on August 12, 1989, during the initial developmental phase of the project, were taken on 13.5-cm filters (Pallflex T60A20) followed by a 10 cm X 7.5 cm PUF vapor trap. Chamber ozone and nitrogen oxide were monitored with chemiluminescent monitors (Bendix Model 8101-B and 8002 analyzers, Lewisburg, WV). Dew point data were acquired with an EG&G (Model 800, Burlington, MA) dew point hydrometer. A total solar radiation sensor (an Eppley, Newport, RI, black and white pyranometer) was located on the floor in the east chamber. Particle size distribution data were taken with an Electrical Aerosol mobility analyzer, EAA, (TSI, Inc., St. Paul, MN). Total yields of PBDDs/PBDFs were calculated based on the sample weight, sampling flow rate, chamber volume, and amount of PBDPEs combusted. In these calculations we assume that all PBDDs and PBDFs produced were emitted into the chamber. Since it is likely that the ash remaining in the combustion vessel contains PBDDs and PBDFs, we believe that the actual yields are higher than the yields calculated in this study. Polybrominated Diphenyl Ether Photolysis Experiments. The rate and yield of PBDPE photolysis to 9Q2 Environ. Sci. Technol., Vol. 26, No. 5, 1992
PBDDs and PBDFs were investigated by coating tetraand penta-PBDPEs onto Teflon-impregnated glass fiber filters and exposing the filters to sunlight in our outdoor chamber. These filters were selected as a test surface because degradation rates for benzo[a]pyrene on glass fiber filters not coated with Teflon have been found to be within a factor of 3 of the rate as observed on real soot particles (23, 24). Nine Teflon-impregnated glass fiber filters (Pallflex) were coated with a hexane solution (15.2 pg/ filter) containing the same commercial mixture of tetra-, penta-, and hexabrominated diphenyl ethers (DE-71, Great Lakes Chemical Co.) that is contained in the polyurethane foam we combusted. These filters were exposed to solar radiation on the floor of the smog chambers on August 30, 1990, in open glass Petri dishes. Triplicate samples were exposed for 0,2, and 6 h. Total solar radiation ranged from 0.3 to 1.2 langleys and temperatures ranged from 23 to 38 "C inside the chamber during the exposure. The samples were then Soxhlet extracted, and the extract was passed through a carbon gravity column prior to analysis by high-resolution gas chromatography/mass spectrometry. Extraction and Enrichment of PBDDs and PBDFs. The analytical protocol used to analyze PBDDs and PBDFs on filter and PUF samples were based on EPA method 8290 (25). Briefly, isotopically labeled l3CI2compounds [ 2,3,7,8-TBDD, 2,3,7,8-TBDF, 1,2,3,7,8-pentabrominated dibenzo-p-dioxin (1,2,3,7,8-PeBDD), 1,2,3,7,8-pentabrominated dibenzofuran (1,2,3,7,8-PeBDF); Cambridge Isotope Laboratory] were added to the samples as internal standards (12.5 ng/sample). The filter and PUF samples were Soxhlet extracted separately in 400 mL of toluene (Fisher Optima grade, Pittsburgh, PA). The extracts were then passed through acidic silica gel (Bio-si1 A, 100-200 mesh, BioRad Laboratories, Richmond CA), Florisil (60-100 mesh, Fisher), and carbon/Celite (7.9% carbon AX-21 by weight, Anderson Development Co., Adrian MI; Celite No. 545, Fisher) gravity columns in series to separate PBDDs and PBDFs from other components in the extract. All laboratory work was carried out under artificial lights, shielded against UV radiation, and samples were kept wrapped in foil to deter photodegradation of the compounds in the laboratory. Isotopically labeled 13C12 1,2,3,6,7,8-hexachlorinated dibenzo-p-dioxin was added as a recovery standard immediately prior to high-resolution gas chromatography/high-resolutionmass spectrometry (HRGC/ HRMS) analysis. High-Resolution Gas Chromatography/High-Resolution Mass Spectrometry. A Hewlett-Packard 8290 gas chromatograph interfaced to a VG 70-250SEQhybrid mass spectrometer was used to conduct HRGC/HRMS analysis. Gas chromatographic separation of the PBDDs and PBDFs was obtained by using a 30-m, 0.32-mm-i.d. DB-5 column (J&W, Folsom CA, 123-5032) and fused silica on column injection. Selected-ion monitoring of the A + 2 and A + 4 ions at m f 2 481.698 and 483.696 for tetrabrominated dioxins and m / z 493.738 and 495.736 for the tetrabrominated furans, and the A + 4 and A + 6 ions at m / z 577.601 and 579.599 for the pentabrominated dioxins and m / z 561.606 and 563.604 for the pentabrominated furans was performed at a resolving power of 10000 (10% valley definition). Additional ions were monitored at m / z 565.62, 643.53, and 721.441 (the A + 6 ion in each case) to determine the presence of fragment ions of penta-, hexa-, and heptabrominated diphenyl ether that appear at the same exact mass as tetra- and pentabrominated dibenzofurans and, thus, are potential interferences. The ratio between the halogen-containing fragments, the absence of interfering ions, and the relative retention time
Table I. Experimental Conditions in Chamber Studies"
August 12, 1989 east chamber chamber temperature, "C dew point, "C incinerator temperature, " C total solar radiation, cal cm-2 m i d NO concentration, ppm NOz concentration, ppm O3concentration, ppm injection time mass of polyurethane foam combusted, g
23-27 660
1:30 p.m. 0.56
December 20, 1989 east chamber
December 20, 1989 west chamber
March 14, 1990 west chamber
-5 to 5 -8 to -4 640 0.01-0.70 0.004-0.94 0.032-0.056 0.007-0.018 1:50 p.m. 0.59
-5 t o 7 -5 685 0.01-0.66 0.000-0.085 0.011-0.040 0.018-0.026 2:05 p.m. 1.17
23-36 12-14 760 0.22-1.0 0.001-0.056 0.005-0.058 0.076-0.186 10:12 a.m. 0.70
Ranges represent minimum and maximum values.
were used as criteria to confirm the presence of PBDDs and PBDFs. The results for the tetra- and pentabrominated dibenzo-p-dioxins (TBDDs and PeBDDs), as well as tetra- and pentabrominated dibenzofuran (TBDFs and PeBDFs), are reported here primarily on a homologue basis. In each homologue group, a probable 2,3,7,8-isomer (2,3,7,8-TBDD, 2,3,7,8-TBDF, 1,2,3,7,8-PeBDD, and 1,2,3,7,8-PeBDF) was identified by reference to the retention times of authentic materials. Standards for the full complement of TBDD, PeBDD, TBDF, and PeBDF isomers are not available, and thus we do not know if the gas chromatographic conditions used adequately resolve the 2,3,7,8-isomers from the other isomers. High-Resolution Gas Chromatography/Low-Resolution Mass Spectrometry (HRGC/LRMS). Analyses of TBDFs (for the December 20 and March 14, west chamber aging experiments) were performed by using a H P 5890 Series I1 gas chromatograph interfaced to a Hewlett-Packard 5971a mass selective detector. Comparable results were obtained for this homologue group by LRMS and HRMS. Gas chromatographic conditions used were as previously described except that a 30-m, 0.25mm4.d. DB-5 column (J&W Scientific, 122-5032)was employed. This instrument was also used to characterize the composition of the flame-retardant material contained in the foam used as fuel in our combustion experiments (DE-71, Great Lakes Chemical Co. Lafayette, IN). This analysis was performed as above except that analysis was conducted by full scan and quantification of the PBDPEs present was done by integration of the total ion chromatogram. The response for all of the homologue classes of PBDPEs was assumed to be equivalent as adequate standards were not available. Quantification of the PBDDs and PBDFs: Quantification of PBDDs and PBDFs was achieved by using internal standardization. Response factors were calculated for each compound in a standard containing both native and isotopically labeled materials (sum of two native ions/sum of two internal standard ions). The standards contained the l3CI2compounds previously listed as internal standards as well as the following native materials: 2,3,7,8-TBDD, 2,3,7,8-TBDF, 1,2,3,7,8-PeBDD, and 1,2,3,7,8-PeBDF. Each congener class was quantified using a response factor calculated for one isomer within that congener class (for example, the TBDD congener class was quantified using a response factor calculated from data on 2,3,7,8-TBDDand [l3CI2]-2,3,7,8-TBDD).A standard was injected at the beginning and end of each analytical day, and a calibration curve was produced and shown to be linear for each set of analyses. Results Estimation of Analytical Variability. The experimental variability inherent in sample collection, transport,
cleanup, and analysis was estimated by plotting error bars at f 2 relative standard deviations calculated for a series of four samples collected at night in the December 20 east chamber experiment. We assumed that, under these conditions, no photodegradation occurs. The instrumental variability was also investigated by evaluating the percent relative standard deviation of five repetitive HRGC/ HRMS injections of the same sample. The analytical variability measured as the percent relative standard deviation of replicate night samples, for the four homologue classes, ranged from 6.2 to 12.7%. The instrumental relative standard deviation based on repetitive injection of a single sample ranged from 5.0 to 7.0%. A laboratory blank sample was analyzed in parallel with each set of field samples, and the concentration of PBDDs and PBDFs in these blanks as negligible (less than 1% of field sample particulate concentrations). Samples of chamber air taken before the introduction of combustion emissions for the December 20, 1989, east chamber experiment and the March 14,1990, west chamber experiment were found to contain less than 1% of the particulate concentrations of PBDDs and PBDFs observed during the aging of combustion emissions. Chamber Aging Environment. A summary of the experimental conditions from three chamber aging experiments involving PUF combustion emissions that were conducted on December 20,1989, and March 14,1990, is given in Table I. Results from a preliminary chamber experiment conducted on August 12,1989, are also presented (conditions listed in Table I). The concentrations of NO, NO2, and O3 measured do not exceed those compiled for "moderately polluted" atmospheres (26). The December 20 experiment was carried out from the midafternoon to the evening of a clear day with cold temperatures (-5 to 7 "C). During the course of the experiment, temperature and solar radiation were decreasing. The air in the west chamber was partially exchanged with ambient air during the first 27 min of the chamber aging period to reduce the particle concentration in the west chamber as compared to the east chamber. This resulted in a more rapid depletion of the combustion-generated NO and less production of NO2 in the west chamber than in the east chamber. The March 14 experiment began at 10:15 a.m. of a warm day (23-36 "C). Clear morning conditions became hazy about 12:30 p.m. Vapor/Particle and Homologue Class Distributions. In initial experiments, filter and polyurethane foam samples were analyzed to determine the distribution of PBDDs and PBDFs between the vapor and particulate phases. Values of the parameter a, the fraction of the total concentration associated with particles, were calculated (27, 28) and in all cases were greater than 0.95, as shown in Figure 1. Thus, we chose to focus this work on the analysis of the particulate phase. Results for particulate-phase Environ. Sci. Technol., Vol. 26, No. 5, 1992 993
Table 11. Comparison of Concentration of Homologue Groups in I n i t i a l Sample Particulate-Phase M a t e r i a l in Three Chamber Experiments"
December 20, 1989 east chamber
December 20, 1989 west chamber
11.6 1.55
104
140 0.651 3690 13.9
TBDD PeBDD TBDF PeBDF
March 14, 1990 west chamber
NDb 7350 873
20000 19.9
"Values are nanograms of analyte per milligram of particulate. kND,not detectable. December 20,1989, East Chamber 0
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Figure 3. Stability of TBDD and TBDF on particulate material under cold (-7 to 9 'C) temperatures and dim (0.01-0.7langley) sunlight conditions.
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Time (hrs.) Flgure 2. Stability of TBDD. TBDF. PeBDD. and PeBDF on particulate over 6 h under warm temperatures (30-39 OC) and bright (0.22-1.0 langleys) sunlight (March 14. 1990,experiment).
material are hereafter expressed as weight of the analyte per weight of particulate-phase material collected. QS4 Envlron. Sci. Technol.. Vol. 26. No. 5. 1992
The concentrations of tetra- and pentahomologues on the particles, presented in Table 11, varied by 3-4 orders of magnitude and followed the order TBDF > TBDD, PeBDF > PeBDD. The predominance of halogenated dibenzofurans over halogenated dibenzodioxins in combustion-generated samples has been previously reported in some cases (2,3,29,30) and might be expected in our system, since the formation of PBDFs from PBDPEs is most likely an intramolecular process of an additional carbon-carbon bond forming between the two benzene rings of the diphenyl ether to form a fivemembered furan ring, while the formation of PBDDs from PBDPEs must be an intermolecular process involving the addition of oxygen to form a six-membered ring. The preponderance of the tetrahomologues occurs because the starting material (e.g., the polyurethane foam) is primarily composed of tetra- and pentabrominated diphenyl ethers and these compounds are debrominated during thermolysis (2). Chamber Aging Results. Results of the chamber aging experiments for TBDD and TBDF suggest that these
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Time (hrs.) Flgure 4. Stablllty of PeBDD (a) and PeBDF (b, c) on particulate materlal over 3 h under cold (-7 to 9 "C) temperature and dim (0.01-0.7 langley) sunlight conditions.
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Time (hrs.) Figure 6. Stability of 2,3,7,8-TBDD, 2,3,7,8-TBDF, and 1,2,3,7,8PeBDD on particulate material over 3 h under cold (-7 to 9 "C) ternperature and dim (0.01-0.7 langley) sunlight condRions (December 20, 1989, east chamber experiment).
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temperatures and bright sunlight (Figures 2a,b and 3). In the December 20,1989, west chamber experiment, there appears to be a slight decreasing trend. Due to the absence of this trend in the other plots, especially those carried out under conditions of bright sunlight and high temperatures, we believe that this trend may arise from analytical variability, even though the absolute difference between the first and final measured concentrations falls slightly outside the range of our estimates (8.5% of observed concentrations beyond the estimated experimental variability). Similarly, the trends for photolysis of PeBDDs and PeBDFs (Figures 2c,d and 4) are questionable because few data points are outside of the range of experimental variability around the initial points. Further, analysis of data on the concentration of 2,3,7,8-TBDD and 2,3,7,8-TBDF demonstrates that these isomers are stable on soot particles under conditions of warm temperatures and high-intensity sunlight (Figure 5) and also under cold temperatures and dim sunlight (Figure 6). 1,2,3,7,8-PeBDD was also observed to be stable under cold conditions (Figure 6).
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Discussion In principle, the existence of a degradation trend could be further evaluated by testing the statistical significance of the slope of a regression line of the logarithm of the concentration vs time. Two difficulties, however, existed in fitting the data to a linear kinetic model that was based on the integrated form of the rate law. First, the reaction order must be known. Second, the conditions of the reaction must be constant. Ideally, the determination of the reaction order requires data in which at least 75% of the starting material is consumed (31). The reaction in our chamber aging ex-
2
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Time (hrs.) Flgure 5. Stablllty of 2,3,7,8-TBDD and 2,3,7,8-TBDF on particulate material over 6 h under warm (30-39 "C)temperatures and bright (0.22-1 langleys) sunlight (March 14, 1990, experiment).
compounds are stable on soot particles for 3 h (error bars are plotted at *2 relative standard deviations based on night data as discussed above) under cold temperatures and dim sunlight and on soot particles for 6 h under warm
Environ. Sci. Technoi., Voi. 26, No. 5, 1992 005
periments did not proceed to this extent. We replotted Buser’s brominated dioxin and furan data (14) derived from photolysis experiment in liquid solution and observed first-order kinetics for the reaction as the author had assumed. Photolytic PAH decay on wood soot particles has also been shown to be first order (19). Thus, it is reasonable to assume first-order kinetics. In our experiment, the incident solar radiation was constantly varying. Thus, the reaction conditions were not constant. This variability can be addressed by developing an analysis that extends the fundamental photolysis relationship for gas-phase reactants (26)to photolysis on a particle. This analysis relates the change in observed concentrations of analyte on the soot particles to the total amount of solar irradiation incident on this particulate material (TSR). If we assume that the primary site of reactivity is a surface layer on a solid particle and that the light absorbance on the surface is proportional to the number of molecules on the surface, then it can be shown that dX / dt = $AJAS~,X [XI
(1)
where [XI is a surface concentration expressed as mass of analyte per unit surface area (mol/cm2), S,,, is a surface adsorption coefficient, specific to the surface and to a given wavelength (cm2/mol),JA is the energy, at a given wavelength, incident on the surface layer [photons/(cm2/s)], and 4, is the surface primary quantum yield. Integration of this equation over time and all wavelengths capable of causing reactivity, assuming that Sf,, and have some constant average value if the solar spectral distribution remains reasonably constant during the experiment, results in eq 2.
If the integrated actinic flux over the range of interest is proportional to the measured TSR (which according to the manufacturer of the black and white pyranometer includes from X = 285 to X = 2800 nm), then (3) where p is a constant that includes a conversion factor from photons to calories in terms of Planck’s constant and Avogadro’s number [units of min photons/(s cal)]. Then
and thus
In a system in which the surface to volume ratio of the particles does not change considerably over the course of the reaction, moles of analyte per particle mass ([X’]) (mol/mg) may be used as a stand-in for moles of analyte/surface area. Thus In [X’], = S,,4$StTSR dt to
+ In [X’],
(6)
While there is some variation in the ratio of surface/volume with time in our chamber experiments, as determined from EAA data (21.5-25.6 pm-’ for December 20, east chamber, over 6 h; 17.5-21.8 pm-l for December 20, west chamber, over 2 h; and 17.2-31.7 pm-l for March 14, west 006
Environ. Sci. Technol., Vol. 26, No. 5, 1992
chamber, over 6 h), we do not believe this effect is strong enough to invalidate this assumption. The integral over TSR from to to t represents the total amounts of solar radiation incident on a unit area over the time period in question. It can be evaluated numerically (trapezoidal rule) and is linearly related to the logarithm of the concentration at time t (eq 6). If this relationship holds and there is evidence of significant reactivity, a plot of In [PBDD] or In [PBDF] vs STSR dt should be linear with a slope statistically different from zero. This significance was tested by a two-sided t test of the slope of the linear regression as outlined by Remington and Schork (32). The slope of a statistically significant degradation trend (S,,,+$) should be related to the amount of incident energy needed to degrade the concentration of PBDD or PBDF to half its original value (Ell2)as Ell2 = 0*693/St,A4$ (7) Such an Ellz value could then be used to estimate a half-life in time from known or assumed solar radiation fluxes. Alternately, if we assume the primary site of reactivity is a viscous liquid layer surrounding the soot particle (33), a similar expression can be developed.
where E,is the molar absorptivity at a given wavelength (L/(mol cm) and [X”] is the concentration in mass of analyte per volume of surface liquid layer (mol/L). If the ratios of the liquid layer mass to the particulate mass and the liquid layer concentration to the particulate concentration are constant, then we can use mole of analyte per unit particulate mass as a stand-in for mole of analyte per unit liquid layer volume. We may also relate total solar radiation to actinic flux as above. In [X”], = 4,c$AotTSR dt
+ In [X”l0
(9)
Thus, for both a particulate surrounded by a viscous layer and a solid particulate, the integral over TSR can be linearly related to the logarithm of the concentration at time t. This allows an identical significance test to be performed. The slope of a statistically significant curve could again be related to an EIl2value. = 0.693/(4xc$) (10) The slopes, r2’s,and levels of significance derived from such an analysis are given in Table 111. In 5 of 11cases, a significant negative slope is found, indicating that a degradation process may be occurring. The r2 in these cases ranges from 0.41 to 0.96. This suggests that a considerable degree of experimental variability exists and/ or that some other variables, assumed constant in this analysis, have a significant role. In the other six cases, a statistically significant negative slope is not found, thereby indicating atmospheric stability of PBDDs and PBDFs. The instances of an apparent significant degradation do not occur in a logical pattern. For instance, degradation is observed under cold conditions in four of seven cases and in only one of four cases under warm conditions. Thus, no consistent evidence exists in this data set that PBDDs and PBDFs are degrading on soot. This conclusion agrees with the conclusion obtained in the previous analysis of the data. If any of these individual experiments do show a genuine degradation process, then the Ellzvalue could be converted to half-lives based on the average solar radiation fluxes
Table 111. Summary of the Significance of the Slope of In (Concentration) vs Energy Plots
case TBDD PeBDD TBDF PeBDF TBDD TBDF PeBDF TBDD PeBDD TBDF PeBDF
east east east east west west west west west west west
slope 12/20/89 12/20/89 12/20/89 12120189 12/20/89 12/20/89 12/20/89 3/14/90 3/14/90 3/14/90 3/14/90
-0.0053 -0.0039 -0.0023 -0.0066 -0.0139 -0.0026 -0.0135 0.0009 -0.0016 -0.0006 0.0002
std dev of slope 0.0019 0.0035 0.0034 0.0018 0.0042 0.0088
0.0017 0.0004 0.0005 0.0006 0.0009
Table IV. Production of TBDF on Filters Coated With PBDPE Exposed to Sunlight"
hours of exposure
TBDF, pgffilter
0 2 6
1710 f 482 3241 f 1760 8663 f 186
aError stated at f l standard deviation based on triplicate samples. Nondetectable samples are assumed to be at the detection limit.
experienced during the course of that experiment (see Table 111). These calculated half-lives range from 2.9 to 10.7 h. In 6 of 11 cases stability was observed. In two additional cases the calculated half-life exceeded 6 h. Formation of TBDF from PBDPE Photolysis under Chamber Conditions. The photolysis of polybrominated diphenyl ethers on solid surfaces was investigated because ultraviolet or sunlight irradiation of decabrominated diphenyl ether has been shown to produce PBDFs with a 10-20% yield in an organic solution containing acetone (34). If the PBDPEs were not fully consumed in the combustion unit, they might be present in the chamber atmosphere. Photolysis of penta- and hexabrominated diphenyl ethers (PeBDPEs and HxBDPEs) to tetrabrominated dibenzofurans under the experimental conditions of this study could conceivably affect the concentrations of TBDFs observed in our chamber aging experiments. This occurrence would make it difficult to deconvolute the kinetics of the photodegradation of thermolytically formed TBDFs from the kinetics of photolytic formation of TBDFs from PBDPEs. The yields of tetrabrominated dibenzofuran, produced from the 6-h photolysis of the polybrominated diphenyl ethers on filter surfaces (0.095% expressed as mass of TBDF produced/mass of PeBDBE and HxBDPE photolyzed), (Table IV) were much lower than those observed in previous solution-phase experiments (34). The importance of this reaction in our system was estimated by calculating the concentration of TBDF that could be produced by photolysis of PBDPEs as follows. The amount of PeBDPE and HxBDPE introduced into the combustion unit was calculated from manufacturers information (Olympic Products) and our analysis of the composition of flame-retardant mixture DE-71 (57% PeBDPE and 3% HxBDPE). The amount of PeBDPE and HxBDPE actually combusted was then calculated, based on the observed yield of PBDDs and PBDFs in the chamber (ranging for various experiments from 0.25 to 5%, expressed as total PBDD and PBDF produced/PBDPE combusted) and the distribution between PBDDs and PBDFs and other products such as polybrominated benzenes and phenols reported by Buser (2). The differ-
r2 0.57
0.18 0.07 0.69 0.79 0.04 0.96 0.41 0.57 0.15 0.01
two-sided level of significance of slope
cal/cm2 E1121
95 NS NS 98 95 NS 99 90 98 NS NS
131.38 stable stable 104.38 49.97 stable 51.42 stable 430.58 stable stable
derived tip, min
400 stable stable 318 175 stable 180 stable 639 stable stable
ence between the amount of PeBDPE and HxBDPE introduced into the combustion unit and the amount actually combusted is the residual PeBDPE and HxBDPE. All of this residual was assumed to be released into the chamber and photolyzed for 4 h. The yield of this photolysis reaction was assumed to be the same as that obtained in our filter-coating experiments. The TBDF thus produced photolytically was thus calculated to be equal to 4.8-15.8% (depending on experiment) of the thermolytically produced TBDF. In comparison to the analytical variability of our chamber kinetics data, discussed earlier, this contribution is not significant. In addition, this calculation represents a worst case and the true contribution is probably lower. Conclusions
Analysis of data derived from chamber aging experiments demonstrates that PBDD and PBDF homologue classes and individual 2,3,7,8-isomers are stable on particulates for 3-6 h under winter and spring temperatures and sunlight regimes. Although the photolytic production of TBDFs from PeBDPEs was observed, the concentration of TBDFs produced by this process did not affect observed concentrations of TBDF in our chamber aging experiments. The conclusion that the tetra- and pentahalogenated dibenzo-p-dioxin and dibenzofuran homologues are stable on soot particles was supported by an examination of the individual data points and their experimental variability, and a statistical analysis of the change in concentration of analyk as a function of incident solar radiation. Therefore, we believe that a photolytic degradation process for these compounds, if occurring on realistic soot particle surfaces, has a half-life of at least 3 h and probably much, much longer. The stability of PBDDs and PBDFs over the course of hours, observed in this work, is in better agreement with laboratory experiments of compounds sorbed on surfaces than in solution (14). Thus, lifetimes for substances on atmospheric particulates, predicted by using laboratory data derived from solution-phase experiments, can underestimate actual half-lives. Acknowledgments
We thank R. Goodman for help in designing and constructing the pyrolysis vessel, G. D. Marbury for assistance in mass spectrometry analyses, S. McDow for helpful suggestions regarding data analysis, A. J. Rabideau for methods development and laboratory setup work, and Parag Birla and Tom Merz for conducting some of the confirmation experiments. Registry No. TBDD, 103456-39-9; PeBDD, 103456-36-6; TBDF, 106340-44-7;PeBDF, 68795-14-2; 2,3,7,8-TBDD, 50585Envlron. Sci. Technol., Vol. 26, No. 5, 1992 997
41-6; 2,3,7,8-TBDF, 67733-57-7; 1,2,3,7,8-PeBDD, 136471-65-3.
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Received for review June 6, 1991. Revised manuscript received January 22,1992. Accepted manuscript January 28,1992. Initial funding for the project was received from The EPA Research Center for Waste Minimization and Management, N.C. State University, M. Overcash project officer. Further support was provided by a grant (R818534)from the Officeof Exploratory Research, U.S. EPA, Deran Pashayan, project officer to the University of North Carolina. The Brominated Flame Retardant Industry Panel provided funds for chemical standards. Olympic Products from Greensboro,NC, provided the brominated fire retardant PUF. The GCIMSD instrument was purchased from a Ford Motor Co. gift to U.N.C.
