Grantee Research Project Results
2002 Progress Report: Mechanistic Studies of the Transformation of Polychlorinated Dibenzo-p-Dioxins via Hydroxyl Radical Attack
EPA Grant Number: R828189Title: Mechanistic Studies of the Transformation of Polychlorinated Dibenzo-p-Dioxins via Hydroxyl Radical Attack
Investigators: Taylor, Philip H.
Institution: University of Dayton
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 2000 through September 30, 2003
Project Period Covered by this Report: October 1, 2001 through September 30, 2002
Project Amount: $320,000
RFA: Combustion Emissions (1999) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
Objective:
The objective of this research project is to determine the rates and mechanisms of OH reactions with dibenzo-p-dioxin (DD) and selected polychlorinated dibenzo-p-dioxins (PCDD) compounds over an extended temperature range. Mechanistic experiments will include studies of the effect of pressure on observed rate coefficients and product analyses. The mechanistic experiments will be used to guide and interpret quantum RRK modeling of the various reactions.
PCDD are considered among the most toxic organic chemicals associated with our industrial society. The gas-phase transformation of these chemicals under atmospheric conditions and high-temperature incineration (destruction) conditions is not well understood. Experimental and modeling studies have repeatedly shown that OH radical reactions are among the most important elementary steps under these reaction conditions. A review of the literature demonstrates that knowledge of the rate of reaction of OH with dibenzo-p-dioxin and PCDD is limited to three low temperature experimental studies, or inferred by estimates of room temperature reactivity. The mechanism of reaction is largely uncharacterized.
Progress Summary:
During the past year, we have focused our efforts on experimental measurements for dibenzo-p-dioxin and 2-chlorodibenzo-p-dioxin. Our experimental efforts represent a first attempt to perform absolute rate measurements for these compounds. Previous studies have utilized the relative rate technique. As a result, numerous studies were performed to assess the reliability of our experimental approach. The following paragraphs summarize these measurements and how they compare with previously published results. More detailed information regarding these rate measurements is available in the annual performance report.
Experimental Approach. The experimental procedures developed for PLP-PLIF studies
of the reaction of OH radicals with DD and 2-CDD were based on our recent studies
of chlorinated olefinic compounds. Two precursors, hydrogen peroxide, HOOH,
and nitrous acid, HONO, were employed to generate OH radicals. HOOH was used
as the primary OH precursor for these measurements. A pulsed, 10 Hz, KrF excimer
laser (Lamba Physik Compex Model 102) was used as the photodissociation source
for HOOH (Aldrich Chemical Co., 50 weight percent solution in water). Detection
of OH radicals was achieved by PLIF, exciting the (1,0) band of the OH (A2
- X2) system at about 282 nm using a pulsed (10 Hz) Nd:YAG pumped-dye laser
(Quanta Ray model DCR-1/PDL-2). The laser fluence at the entrance of the reactor
was about 200 µJ/pulse. Broadband fluorescence at 306 nm was collected
using a PMT/band-pass filter combination.
A modified fused silica optical reactor was designed and constructed for this
research. The objective of this reactor modification was to successfully transport
part per million concentrations of DD and 2-CDD in the gas phase from the point
of sample introduction to the reaction volume (intersection of pump and probe
laser beams). A fused silica sample inlet and inlet probe were located immediately
beneath the cylindrical reactor to provide a means for rapid and efficient introduction
of the substrate vapors to the reaction volume while preventing condensation
of the sample during transport.
The buildup of reaction products was minimized by conducting experiments under slow flow conditions. Helium was the carrier gas for the HONO experiments while either helium or argon was used as the carrier gas for the HOOH experiments. All experiments were performed at a total pressure of 730 ± 10 torr. Samples of DD and 2-CDD were purchased from Ultra Scientific, 98 + percent purity, and used as received. Gas chromatography-mass spectrometry (GC/MS) analyses indicated that substrate purities exceeded 99.9 percent. The rate of disappearance of the OH may be represented as:
where k is the bimolecular rate constant, Ao the substrate concentration, and kd is the first-order rate coefficient for the reaction of OH with impurities and includes diffusion of OH out of the reaction volume. This relationship holds in the absence of any secondary reactions that may form or deplete OH. Solution of this equation yields [OH] = [OH]o exp(-k't), where k' = kbi[A0] + kd. For all experiments, reactive and diffusive OH radical decay profiles exhibited exponential behavior and were fitted by the following nonlinear expression:
where is the (constant) background signal level and t is the time delay between the pump and probe lasers. Pseudo-first order exponential OH decays were observed, confirming that the substrate concentration was in large excess of [OH] (typically a factor of 50-100 larger). OH decays were observed over two to three decay lifetimes over a time interval of 0.5-30.0 ms. The individual bimolecular rate constants were determined from the relation k' = kbi[substrate] + kd, where the bimolecular rate constant, kbi, is the slope of the least-squares fit of k' versus the A0.
Results and Discussion. Absolute rate measurements for DD and 2-CDD were recorded
over extended temperature ranges. 95 percent confidence intervals ranged from
8 to 19 percent and from 10 to 27 percent for DD and 2-CDD, respectively. Changes
in carrier gas (helium or argon) had no impact on the rate measurements. Potential
systematic errors (± 9 percent) dominated by uncertainties in substrate
concentration (± 8 percent) are not included in this analysis. Measurements
for DD were obtained between 344 and 874 K. Measurements for 2-CDD were obtained
between 346 and 629 K.
For our experiments, OH was generated using both HOOH (248 nm radiation) and
HONO (351 nm radiation). HONO was considered the ideal OH radical source for
these measurements due the unlikely impact of substrate photolysis. However,
thermal decomposition of HONO limits its effectiveness to relatively low temperatures
(< 700 K). HOOH is an effective OH precursor over a somewhat wider temperature
range (ambient to about 900 K). However, a significant concern using the 248
nm radiation was potential substrate photolysis. The HOOH measurements were
conducted at low laser fluences (=5 mJ/cm2). For both OH precursors, the rate
measurements at different temperatures were in agreement within statistical
uncertainties (±2). These results indicate that substrate photolysis
at 248 nm does not impact our absolute rate measurements.
An Arrhenius fit of our low temperature data for DD and 2-CDD yielded the following expressions (in units of cm3/molecule-s, error bars are 2):
Extrapolation of these Arrhenius fits to room temperature (296 K) results in the following rate coefficients (5.44 x 10-11 cm3/molecule-s and 6.19 x 10-11 cm3/molecule-s) for DD and 2-CDD, respectively. Arrhenius fits of our higher temperature data for DD yields the following expression (in units of cm3/molecule-s, error bars are 2s):
Limited high-temperature data for 2-CDD precluded determination of Arrhenius parameters above 500 K. The trend in the two high-temperature measurements for 2-CDD is roughly consistent with the Ea determined from the DD measurements (Ea 1 kcal/mol).
A comparison of our absolute rate measurements for DD with previous relative rate measurements was also performed. Our rate measurements for DD below 400 K are a factor of 2 to 3 higher than the previous measurements of Brubaker and Hites. Extrapolation of our low temperature data for DD to room temperature and comparison with the measurement of Kwok, et al. indicates a larger rate coefficient by a factor of 4. There are no previous measurements for 2-CDD. However, comparison of our low temperature data for 2-CDD with a single previous room temperature measurement by Kwok, et al. for 1-CDD (4.7 ± 1.6 x 10-12 cm3/molecule-s) indicates an order of magnitude higher rate coefficient.
The magnitude of our rate coefficients for DD and 2-CDD compared to previous measurements suggest that systematic errors may be present in our low temperature measurements. Likely sources of error from our measurements include substrate impurities and substrate photolysis. We have previously demonstrated that substrate photolysis is unlikely. GC-MS analysis of our substrates did not indicate the presence of reactive impurities. Analysis of the reactor gas stream showed that HONO generation did not produce detectable concentrations of potentially interfering NO, NO2, or HCl (<1.2 x 1012 molecules cm-3). An additional potential source of error in our measurements is the substrate concentration in the reactor. Substrate concentrations were not measured directly. They were calculated based on the measured sample temperature and known vapor pressure at the point of introduction of the sample along with the measured sample carrier flow rate and measured total carrier flow rate. The very low vapor pressures of DD and 2-CDD precluded room temperature calibration measurements using this technique. As a result, verification was obtained by conducting room temperature rate measurements for a structurally similar, solid-phase sample with a higher vapor pressure, phenol. A rate of 2.1 ± 0.14 x 10-11 cm3/molecule-s was measured at 296 K, in excellent agreement with the previous measurements by Semadeni, et al. and Rinke and Zetzsch (2.32 ± 0.20 x 10-11 cm3/molecule-s and 2.81 ± 0.58 x 10-11 cm3/molecule-s, respectively).
As for other aromatic compounds, the gas-phase reactions of OH with PCDD likely proceed by initial addition of the OH radical to form a hydroxycyclohexadienyl-type radical, with the addition rate coefficient depending on the identity, number, and position of the chlorine substituent. In contrast to the measurements of Kwok, et al. and Brubaker and Hites, our rate measurements for DD and 2-CDD do not show a decrease in overall rate coefficient with Cl substitution on the adjacent aromatic ring. Our rate measurements do appear to be consistent with an OH addition pathway when one compares the measured rates for phenol, DD, and 2-CDD. Considering the simplistic model that OH addition is dominated by the number of carbon sites where OH addition can occur (5 for phenol, 8 for DD, and 7 for 2-CDD) and assuming OH addition to the ipso position(s) on phenol, DD, and 2-CDD is of minor importance, the larger extrapolated room temperature OH addition rates for DD and 2-CDD compared to phenol are plausible.
References:
Baker JI, Hites RA. Is combustion the major source of polychlorinated dibenzo-p-dioxins and dibenzofurans to the environment? A mass balance investigation. Environmental Science and Technology 2000;34(14):2879.
U. S. Environmental Protection Agency. The inventory of sources of dioxin in
the United States.
U. S. Environmental Protection Agency, Office of Research and Development, National
Center
for Environmental Assessment, Washington, DC, April 1998; External Review Draft,
EPA/600/P-98/002Aa.
Kociba RJ, Cabey O. Chemosphere 1985;14:649.
Senkan SM. In: Gardiner Jr WC, ed. Gas-Phase Combustion Chemistry. Springer-Verlag, New York, 2000, Chap. 4.
Finlayson-Pitts BJ, Pitts Jr. JN. Chemistry of the upper and lower atmosphere. San Diego, CA, 1999.
Kwok ESC, Arey J, Atkinson R. Gas-phase atmospheric chemistry of dibenzo-dioxin and dibenzofuran. Environmental Science and Technology 1994:28(3);528.
Kwok ESC, Atkinson R, Arey J. Rate constants for the gas-phase reactions of the OH radical with dichlorobipheyls, 1-chlorodibenzo-p-dioxin, 1,2-dimethooxybenzene, and diphenyl ether: estimation of OH radical reaction rate constants for PCBs, PCDDs, and PCDFs. Environmental Science and Technology 1995:29(6);1591.
Brubaker Jr WW, Hites RA. Polychlroinated dibenzo-p-dioxins and dibenzofurans: gas-phase hydroxyl radical reactions and related atmospheric removal. Environmental Science and Technology 1997:31(6);1805.
Brubaker Jr WW, Hites RA. OH reaction kinetics of polycyclic aromatic hydrocarbons and polychlorinated dibenzo-p-dioxins and dibenzofurans. Journal of Physical Chemistry 1998;A102(6):915.
Atkinson R. In: Hester RE, Harrison RM, eds. Issues in Environmental Science and Technology. The Royal Society of Chemistry: Cambridge, United Kingdom 1996;6:53.
Tichenor LB, Graham JL, Yamada T, Taylor PH, Peng J, Hu X, Marshall P. Kinetic and modeling studies of the reaction of hydroxyl radicals with tetrachloroethylene. Journal of Physical Chemistry 2000;104(8):1700 (abstract).
Yamada T, El-Sinawi A, Siraj M, Taylor PH, Peng J, Hu X, Marshall P. Rate coefficients and mechanistic analysis for the reaction of hydroxyl radicals with 1,1-dichloroethylene and trans-1,2-dichloroethylene over an extended temperature range. Journal of Physical Chemistry 2000;105(32):7588 (abstract).
Yamada T, Siraj M, Taylor PH, Peng J, Hu X, Marshall P. Rate coefficients and mechanistic analysis for reaction of OH with vinyl chloride between 293 and 730 K. Journal of Physical Chemistry 2000;105(41):9436 (abstract).
Shiu W, Ma K. Journal of Physical Chemistry Reference Data 2000;29:3.
Semadeni M, Stocker DW, Kerr JA. International Journal of Chemical Kinetics 1995;27:287.
Rinke M, Zetzsch C. Ber. Bunsenges. Phys. Chem. 1984;99:55.
Atkinson R. Journal of Physical Chemistry Reference Data, 1989, Monograph 1,1.
Future Activities:
There is a small change in experimental scope of this research based on the second year results. Initially, we proposed to study dibenzo-p-dioxin, 1- and 2-chlorodibenzo-p-dioxin, 2,3- and 2,7-dichlorobenzo-p-dioxin, 1,2,4-trichlorodibenzo-p-dioxin, and 1,2,3,4-tetrachlorodibenzo-p-dioxin. Our second year results indicate that reactivity is a complex function of chlorine substitution on each aromatic ring. As such, we currently see little value in studying both 1- and 2-chlorodibenzo-p-dioxin. We intend to study both 2,7- and 2,8-dichlorodibenzo-p-dioxin instead as we examine trends in reactivity when chlorine substitution is spread across both aromatic rings. Previous results for dibenzo-p-dioxin and 2-chlorodibenzo-p-dioxin and preliminary results for 2,3-dichlorodibenzo-p-dioxin and 1,2,3,4-tetrachlorodibenzo-p-dioxin suggest that OH reactivity is similar for all of these compounds where chlorine substitution is localized to one aromatic ring. As such, we currently see little value in studying 1,2,4-trichlorodibenzo-p-dioxin. Instead, attempts will be made to measure OH reaction rates with octachlorodibenzo-p-dioxin, a relatively low toxicity congener with strong relevance to combustion emissions.
Our experimental goals for year three include kinetic measurements for 2,3- and 2,7-, and 2,8-dichlorodibenzo-p-dioxin, 1,2,3,4-tetrachlorodibenzo-p-dioxin, and octachlorodibenzo-p-dioxin. Concurrently, we will perform mechanistic modeling of data using semi-empirical ab initio methods. The goal of this modeling is to predict the reactivity of heavier, more chlorinated PCDD (tetra- through hepta-CDD) that cannot be thoroughly evaluated experimentally because of their high toxicity.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 9 publications | 1 publications in selected types | All 1 journal articles |
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Taylor PH, Yamada T, Neuforth A. Kinetics of OH radical reactions with dibenzo-p-dioxin and selected chlorinated dibenzo-p-dioxins. Chemosphere 2005;58(3):243-252. |
R828189 (2002) R828189 (Final) |
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Supplemental Keywords:
combustion chemistry, environmental chemistry, oxidation, modeling, exposure, air pollution, hazardous air pollutants., RFA, Scientific Discipline, Toxics, Waste, Chemical Engineering, Environmental Chemistry, pesticides, Chemistry, Incineration/Combustion, Environmental Engineering, dioxin, gas-phase transformation, industrial waste, chemical contaminants, analytical chemistry, hydrocarbons, toxic organic chemicals, mechanistic study, fused silica test cell, incineration, combustion contaminants, laser photolysisProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.