Environ. Sci. Technol. 1997, 31, 1805-1810
Polychlorinated Dibenzo-p-dioxins and Dibenzofurans: Gas-Phase Hydroxyl Radical Reactions and Related Atmospheric Removal W. WAYNE BRUBAKER, JR. AND RONALD A. HITES* School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
Gas-phase reactions with the hydroxyl radical (OH) are expected to be an important removal pathway of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) in the atmosphere. Our laboratory recently developed a system to measure the rate constants of the gas-phase reactions of OH with semivolatile organic compounds using on-line mass spectrometry. We have now incorporated electron capture mass spectrometry (EC-MS) into this system to increase its sensitivity to PCDD/F, which tend to have low vapor pressures. OH reaction rate constants were determined in helium for 1,2,3,4-tetrachlorodibenzo-pdioxin at 373-432 K using a heated quartz reaction chamber. The photolysis of O3 in the presence of H2O and the photolysis of H2O2 (both at λ ) 254 nm) served as OH sources. An extrapolation using the Arrhenius equation gives a 1,2,3,4tetrachlorodibenzo-p-dioxin-OH reaction rate constant of 8.5 × 10-13 cm3 s-1 at 298 K, which is in excellent agreement with the value predicted by a structure-activity method. The predicted OH reaction rate constants for tetra- through octachlorodibenzo-p-dioxin and dibenzofuran isomers were used in a simple model of the atmospheric removal of PCDD/ F. The results of our model indicate that atmospheric removal is a combination of gas-phase removal processes of lower chlorinated dioxins and furans and particlephase removal processes of higher chlorinated ones.
Introduction Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/ F) are produced by numerous combustion processes and emitted into the atmosphere (1). These compounds can then be transported great distances before depositing to other environmental compartments, including soil, water surfaces (and the sediments beneath), and vegetation. One would expect the relative PCDD/F homologue concentrations in these sinks to be similar to those of the emissions; however, this is usually not the case. For example, Figure 1 shows a comparison of an environmental sink’s (Great Lakes sediments) homologue profile to that of a typical source (municipal incinerators; 2). Note the homologue distributions do not match. The PCDD/F profile in Great Lakes sediments (which is typical of most sinks) shows a strong predominance of the hepta- and octachlorodioxins and to some extent the heptachlorofurans. Since the only pathway to the sinks from combustion sources is through the atmosphere (2), this suggests that atmospheric processes preferentially remove the lower chlorinated dioxins and furans. * Corresponding author e-mail:
[email protected].
S0013-936X(96)00950-9 CCC: $14.00
1997 American Chemical Society
FIGURE 1. Emission (A) and deposition (B) homologue profiles representing sources and environmental sinks, respectively, for polychlorinated dibenzo-p-dioxins and dibenzofurans (2). The letters F and D designate furans and dioxins, respectively; the numbers indicate chlorination levels. PCDD/F have vapor pressures from 9 × 10-3 to 8 × 10-9 Torr at 298 K (3) and are distributed in the atmosphere between gas and particle phases. PCDD/F encounter similar types of processes in each of these phases: wet and dry deposition, photolysis, and chemical reactions (4-6). Investigations in our laboratory of both atmospheric wet and dry deposition of PCDD/F found distributions weighted toward the higher chlorinated homologues (7), but this is not the complete explanation for the differences shown in Figure 1. It is likely that the lower chlorinated dioxins and furans are more susceptible to photochemical processes in both the gas and particle phases. Recent work in our laboratory has partially refuted the latter part of this explanation: The photodegradation of particle-bound dioxins and furans was found to be negligible (8). Therefore, we now focus on PCDD/F gas-phase chemistry. Photochemical reactions of gas-phase organic compounds in the troposphere are dominated by OH, the hydroxyl radical (6, 9-11). Atmospheric PCDD/F with six or fewer chlorines exist largely in the gas phase (12, 13); thus, gas-phase OH reactions are likely to be an important PCDD/F removal pathway from the atmosphere (4). Unfortunately, kinetic studies of OH-PCDD/F reactions are experimentally difficult due to these compounds’ low vapor pressures at room temperature. In fact, experimentally determined OH reaction rate constants have only been determined for dibenzo-pdioxin, dibenzofuran (14), and 1-chlorodibenzo-p-dioxin (15). A system was recently developed in our laboratory for measuring OH reaction rate constants for gas-phase semivolatile organic compounds (16). The system consisted of a heated quartz reaction chamber, which is sampled by online electron impact ionization mass spectrometry (EI-MS). It was previously used to determine the experimental OH reaction rate constants for 16 polychlorinated biphenyls (17). Unfortunately, this system was not suitable for a similar investigation of PCDD/F because sensitivity decreases as the vapor pressure of the analytes decreases. The PCB study was limited to those congeners with vapor pressures greater than
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about 2 × 10-5 Torr at 298 K (3). The vapor pressures of the tetra- through octachlorinated dioxins and furans range from 2 × 10-6 to 8 × 10-9 Torr at 298 K (3), and they could not be detected in this reaction system. To overcome this problem, the system has been modified to use electron capture mass spectrometry (EC-MS). EC-MS is 100-1000 times more sensitive to electrophilic compounds than EI-MS (18), and the sensitivity of EC-MS increases with the degree of chlorination. As one investigates PCDD/F with higher chlorination, the decrease in sensitivity due to lower vapor pressures should be offset (at least partially) by the sensitivity gained with EC-MS. This modified reaction system was used to determine gasphase OH reaction rate constants for 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-D) over the temperature range of 373432 K. These experimental rate constants were extrapolated by the Arrhenius equation for comparison with the room temperature value predicted by Atkinson (4). Using Atkinson’s OH reaction rate constants predicted for each of the tetra- through octachlorinated dioxin and furan homologues (4), a simple model has been developed to evaluate the role of OH reactions in the removal of PCDD/F from the atmosphere.
Experimental Section The reaction system used in this investigation is similar to the one previously described (16, 17), with the important exception that the mass spectrometer operates in the EC-MS mode. All OH reaction rate experiments were carried out at atmospheric pressure in a quartz reaction chamber with a volume of 160 mL, which was mounted in the oven of a gas chromatograph (GC). The GC oven allows the use of elevated experimental temperatures and enhances the vapor pressure of analytes in the chamber. Helium (99.999%; Liquid Carbonic, Oak Brook, IL) served as the diluent gas. The use of EC-MS with this reaction system initially posed a challenge for sample introduction. Previously, the PCBs had been dissolved in a solution of CCl4 and injected directly into the chamber (16, 17). Although relatively non-reactive to OH (19), CCl4 is highly electrophilic. Thus, CCl4 would dominate the electron capture process in the mass spectrometer’s ion source and eliminate the detection of analytes. Most other solvents have known OH reaction rate constants greater than 10-13 cm3 s-1 and would significantly scavenge OH during an experiment. Our solution was a solvent-free method of introduction that used a heated sample probe. This quartz probe (approximately 10 cm × 9.5 mm o.d.) was inserted into a side arm of the reaction chamber using a vacuum-tight Cajon Ultra-Torr union (Swagelok, Macedonia, OH). Prior to each experiment, 20-90 µg of either 1,2,3-D, 1,2,3,4-D, 1,2,4,7,8-D, or 1,2,3,4,7,8-D (g98% purity; AccuStandard, New Haven, CT) dissolved in an acetone or 2-propanol solution was deposited onto the probe’s tip, and the solvent was allowed to evaporate. With the probe tip positioned inside the chamber during an experiment, an electric current was passed through a wire coil embedded within the probe tip, and resistive heating vaporized the sample. The probe was heated to about 70 °C above the reaction chamber’s temperature within a few seconds. OH reaction rate constants were measured by the widely accepted relative rate technique (19). The two reference compounds used in this study were hexafluorobenzene and 2,3,4,5,6-pentachlorobiphenyl (PCB 116). Both meet two very important prerequisites for use as reference compounds: (a) They have experimentally derived OH reaction rate constants in (or close to) our experimental temperature range (17, 20), and (b) they have at least four halogens, a general requirement for detection by EC-MS. When using PCB 116, 1-20 µg of the compound (g99% purity; AccuStandard) was deposited onto the sample probe tip along with the dioxin studied. Hexafluorobenzene, a volatile compound and a liquid at room
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temperature, was introduced neat into the reaction chamber by the split/splitless injection port on the GC oven, which was connected by a 30-50 cm length of 100 µm i.d. deactivated fused silica capillary (SGE, Austin, TX) to the quartz chamber. Helium was the carrier gas for the injection port. The approximate experimental concentrations of hexafluorobenzene (PCR Research, Gainesville, FL) in the chamber were between 1014 and 1016 cm-3, and those for PCB 116 ranged between 1013 and 2 × 1014 cm-3. OH radicals were produced by one of two methods. The first was the photolysis of O3 in the presence of H2O (16, 17):
O3 + hν (λ e 315 nm) f O(1D) + O2
(1)
O(1D) + H2O f 2OH
(2)
A mercury, Pen-Ray, UV lamp (UVP, Upland, CA) provided UV radiation centered at wavelengths of 254 nm. The lamp was mounted outside the GC oven, and it illuminated the reaction chamber (approximately 7.5 cm away) through a quartz window. H2O was introduced into the chamber by bubbling the helium diluent gas though a bottle containing HPLC-grade water (EM Science, Gibbstown, NJ) at room temperature. O3 was generated by passing O2 (99.998% purity; Air Products, Allentown, PA) through a 12-kV discharge; the resulting O2/O3 gas flow was mixed with the He/H2O diluent flow just upstream from the chamber. The ozone concentration in the reaction chamber ranged between approximately 0.8 and 3 × 1016 cm-3 at room temperature for those experiments where O3/H2O served as the OH source. The second method of OH radical generation was the photolysis of H2O2 using the same radiation source described above:
H2O2 + hν (λ e 360 nm) f 2OH
(3)
For experiments using this method, we injected 10 µL of a 200:1 or 2000:1 H2O:H2O2 solution into the heated reaction chamber just prior to UV irradiation. The syringe needle was inserted directly into the chamber through a rubber septum. Once the analytes’ signals in each experiment (regardless of the OH source) were at relatively steady levels, the chamber was irradiated for 1-5 min with UV light. From the reaction chamber, analytes were introduced into the ion source of a Hewlett-Packard 5985B mass spectrometer (in the EC-MS mode) through a 100-µm i.d. deactivated fused silica capillary, 65-75 cm in length. This capillary was heated at 100-150 °C, and the ion source temperature was maintained at 150 °C. The pressure of the reagent gas for EC-MS, methane (Liquid Carbonic), was held at 0.43 Torr in the ion source. Operating in the selected ion monitoring (SIM) mode, the mass spectrometer measured m/z signals unique to each reactant in the chamber 20-35 times per minute. The m/z value monitored for a particular compound represented the most abundant ion in that compound’s EC-MS spectrum with one exception: PCB 116 was monitored by m/z 330 in experiments where it was the reference compound for 1,2,3,4D. The chlorine isotopic pattern of 1,2,3,4-D ranges from m/z 320 to 328 and interfered with the most abundant ion of PCB 116 (m/z 326). However, the isotopic pattern for PCB 116 (from m/z 324 to m/z 334) never interfered with the most abundant ion monitored for 1,2,3,4-D (m/z 322). Monitoring of m/z 330 for PCB 116 provided sufficient sensitivity under our experimental conditions, and m/z 330 exhibited the same response as m/z 326 in control experiments with only PCB 116 present in the reaction chamber. We monitored m/z 186, 286, 356, and 390 for the compounds hexafluorobenzene, 1,2,3-D, 1,2,4,7,8-D, and 1,2,3,4,7,8-D, respectively. An HP Series 1000 computer controlled the mass spectrometer and collected data files with a Real-Time Executive (RTE) operating system. After a file was transferred to and translated on a
TABLE 1. Experimental Conditions, Rate Constant Ratios, and Rate Constants the Reaction of 1,2,3,4-Tetrachlorodibenzop-dioxin with OH Radicals T (K)
ref compda
OH source
k1234-D/krefb
rate constant (×10-12 cm3 s-1)c
373 373 373 393d 393 403 403 424 430 431d 431 432
HFB HFB HFB HFB HFB HFB HFB PCB 116 PCB 116 PCB 116 PCB 116 PCB 116
O3/H2O O3/H2O O3/H2O O3/H2O O3/H2O O3/H2O O3/H2O H2O2 H2O2 H2O2 H2O2 H2O2
9.1 ( 0.5 8.1 ( 0.5 8.0 ( 0.3 9.4 ( 0.3 9.1 ( 0.3 12.5 ( 0.3 7.2 ( 0.3 1.39 ( 0.02 1.27 ( 0.07 1.23 ( 0.02 1.20 ( 0.08 1.24 ( 0.02
2.9 ( 0.9 2.6 ( 0.8 2.5 ( 0.8 3.4 ( 1.0 3.3 ( 1.0 4.8 ( 1.4 2.8 ( 0.8 5.1 ( 1.0 4.9 ( 1.0 4.8 ( 1.0 4.7 ( 0.9 4.9 ( 1.0
a HFB, hexafluorobenzene; PCB 116, 2,3,4,5,6-pentachlorobiphenyl. Stated uncertainties of the rate constant ratios represent 95% confidence limits. c Stated uncertainties of the experimental rate constants reflect the estimated overall uncertainty recommended for the reference rate constants: hexafluorobenzene (30% (20) and 2,3,4,5,6-pentachlorobiphenyl (20% (17). d Those experiments which are represented in the plots shown in Figure 2. b
personal computer, five-point smoothing and background subtraction were applied to the m/z signal intensities. Our relative rate experiments measured the losses of both the test compound and the reference compound in the chamber. Assuming OH reactions were the primary loss processes during UV irradiation of the chamber, the recorded signal intensities for the two compounds follow the equation:
ln
( ) ( ) ( [test]0 [test]t
)
)
ktest [reference]0 ln kref [reference]t
(4)
FIGURE 2. Typical relative rate plots using (A) hexafluorobenzene (HFB) and (B) PCB 116 as the reference compounds.
where the signal intensities were measured at time t ) 0 and at subsequent times t. Plots of ln ([test]0/[test]t) versus ln ([reference]0/[reference]t), obtained from the experimental data, gave slopes equal to the rate constant ratios, ktest/kref. OH reaction rate constants for the test compound (ktest) were then derived by multiplying this ratio by the known rate constants of the reference compound at the appropriate experimental temperatures. Values of kref were calculated from the temperature-dependent expressions given below for hexafluorobenzene (20) and PCB 116 (17):
k(hexafluorobenzene) ) +0.98 (3.88-0.79 ) × 10-12 e-(931(78K)/T cm3 s-1 (5) +4.6 k(PCB 116) ) (1.1-0.9 ) × 10-10 e-(1440(580K)/T cm3 s-1 (6)
The errors for eq 5 indicate two standard deviations for a least-squares analysis of available hexafluorobenzene-OH rate constants (20), and those for eq 6 represent standard errors for that of PCB 116 (17).
Results and Discussion PCDD-OH Reaction Rate Constants. The OH reaction rate constants for PCDD were difficult to measure; we were only successful with 1,2,3,4-D in the approximate temperature range 373-432 K. The experimental conditions and results for 1,2,3,4-D are summarized in Table 1. Note that the average 95% confidence limit is about 4% for the ratios (from eq 4) but about 26% for the rate constants; this reflects the uncertainty of the reference rate constants. This effect is also noticeable in Figure 2, which gives plots of ln ([test]0/ [test]t) versus ln ([reference]0/[reference]t) for two typical experiments in which hexafluorobenzene and PCB 116 served
FIGURE 3. Arrhenius plot for 1,2,3,4-tetrachlorodibenzo-p-dioxinOH reaction. Symbols: (b) hexafluorobenzene as the reference compound and O3/H2O as the OH source and (O) 2,3,4,5,6-pentachlorobiphenyl as the reference compound and H2O2 as the OH source. The dashed lines represent the 95% confidence limits of the fit. as reference compounds. Note the high correlation coefficients. The natural logarithm of the rate constants in Table 1 were then fit by a linear regression to the reciprocal of the experimental temperature (see Figure 3) giving us the experimentally derived expression: +2.3 k(1,2,3,4-D) ) (2.4-1.2 ) × 10-10 e-(1680(270K)/T cm3 s-1 (7)
where the indicated errors represent standard errors. This Arrhenius regression was significant at the 99% confidence level, and it allowed us to compare our OH reaction rate constant for 1,2,3,4-D with the value predicted by Atkinson at 298 K (4). Our estimated rate constant at 298 K is 8.5 ×
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FIGURE 4. Natural logarithm of the m/z 322 signal versus time for four separate experiments with 1,2,3,4-D, demonstrating the thermal decomposition of O3 (see text). The lag in response to UV irradiation at 373 K was due to insufficient heating of a portion of the transfer capillary. 10-13 cm3 s-1, which has 95% confidence limits of 4.7-16 × 10-13 cm-3 s-1. This rate constant is in excellent agreement with Atkinson’s predicted value of 9 × 10-13 cm3 s-1. We took great care in our data analysis to ensure the accuracy of our rate constant measurements. Regression lines were calculated for the signal intensities directly preceding UV irradiation to correct for processes other than OH reactions that could affect the loss rates of organic compounds in the chamber. Direct photolysis of an analyte was never evident in control experiments with O3 and H2O2 absent in the chamber. However, unlike the earlier PCB investigations (16, 17), there was evidence of possible “wall effects” in some of the experiments at the low end of our experimental temperature range (approximately 373 K). After 30-60 s of UV irradiation and the apparent initiation of OH reactions, the signal for the test compound would change and exhibit a slower rate of decrease. The signal would rise after the UV light was turned off (see Figure 4 at 373 K) and then level out again within 1 min. This change during UV irradiation and subsequent rise afterwards indicate that the test compound was possibly volatilizing from the chamber walls to achieve an equilibrium. This phenomenon was particularly noticeable when a combination of relatively low experimental temperatures and compounds of lower volatility (especially 1,2,3,4,7,8-D) were used. Nevertheless, rate constant measurements for these experiments were still possible due to the rapid sampling provided by mass spectrometry. Each experiment provided at least 30 s of unaffected data that were more than sufficient to construct a plot according to eq 4. The experiments at approximately 424-432 K did not display this unusual behavior and produced rate constants that agreed with those in the lower temperature range. Given two sets of data that use two different OH sources and two different reference compounds (see Table 1) and still agree with one another, our OH reaction rate constants for 1,2,3,4-D must be reasonably accurate. H2O2 was investigated as an alternative OH source due to the apparent thermal decomposition of O3 at higher experimental temperatures. This is illustrated in Figure 4 where 1,2,3,4-D was introduced into the chamber at four different experimental temperatures under typical experimental conditions. In each experiment, everything except chamber temperature was held constant including the amount of 1,2,3,4-D deposited on the sample probe and the amount of O3 introduced into the chamber prior to heating from room temperature. Note that the natural logarithm of the signal displayed a lesser rate of decay during UV radiation as the temperature increased. There was little, if any, change at 433 K. Assuming that the amount of O3 (and thus OH) remained constant throughout the four experiments, one
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would expect the signal decay rate to actually increase since the rate constants for the reaction of OH with 1,2,3,4-D increase with temperature (see Table 1 and Figure 3). Clearly, ozone is thermally decomposing at the higher experimental temperatures. In fact, the half-life of O3 is 5.4 h at 363 K, the maximum experimental temperature used by Anderson and Hites (17), but this half-life drops to under 2 min at 413 K (21, 22). While some O3 decomposition can be tolerated, since the relative rate method does not require excess OH (19), there was clearly not enough OH at 413-433 K (see Figure 4) to allow the measurement of 1,2,3,4-D-OH reaction rate constants. This is unfortunate since 413-433 K is the approximate minimum experimental temperature range required for 1,2,4,7,8-D and 1,2,3,4,7,8-D volatilization. Hydroxyl reaction rate constants were obtained for 1,2,3,4-D in this higher temperature range using H2O2 as the OH source, but this alternative method was not successful for 1,2,3,4,7,8D. Signal changes could not be detected consistently for 1,2,3,4,7,8-D upon UV irradiation even in the presence of H2O2; thus, H2O2 appears to be an inadequate OH source for those compounds with OH reaction rates slower than that of 1,2,3,4-D. The problem was likely caused by the increasing thermal decomposition of H2O2 at the elevated temperatures needed to vaporize these larger dioxins. Also, H2O2 may have been less efficient as an OH source because H2O2 itself has an OH reaction rate constant of about 2.0 × 10-12 cm3 s-1 at 423 K (23), which suggests that unphotolyzed H2O2 likely scavenges OH. A possible solution to these problems was to increase the experimental amounts of O3 or H2O2. However, this approach failed in both cases. Most attempts to significantly increase the amount of O2/O3 in the diluent gas flow resulted in a severe loss of sensitivity for PCDD (at all m/z values) in the reaction chamber. It is possible that O3, which is highly electrophilic, was affecting the electron capture process in the mass spectrometer ion source. The O3 and PCDD were not reacting in the cell. While the gas-phase O3 reaction rate constants for some aromatics can increase by 2 orders of magnitude from 298 to 373 K (24), significant O3 reactions are unlikely since PCDD have O3 reaction rate constants of
