Grantee Research Project Results
1997 Progress Report: Kinetics of Semi-volatile Organic Compounds and Hydroxyl Radical Reactions
EPA Grant Number: R825377Title: Kinetics of Semi-volatile Organic Compounds and Hydroxyl Radical Reactions
Investigators: Hites, Ronald A.
Institution: Indiana University - Bloomington
EPA Project Officer: Hahn, Intaek
Project Period: October 14, 1996 through October 13, 1999
Project Period Covered by this Report: October 14, 1996 through October 13, 1997
Project Amount: $356,212
RFA: Exploratory Research - Air Engineering (1996) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Land and Waste Management , Air , Safer Chemicals
Objective:
This project focuses on the atmospheric fates of polychlorinated dibenzo-p-dioxins and dibenzofurans and related compounds. Our goal is to obtain a detailed accounting of the behavior of these compounds while they are in the atmosphere. One of the largest gaps in our knowledge is rate constants of photodegradation in the gas-phase. Therefore, our specific goal is to measure second-order rate constants for the reactions of these compounds with the hydroxyl radical (OH) in the gas-phase.Progress Summary:
The main trick to making these measurement has been to actually get the dioxins (and related compounds) into the vapor phase. We have done this with a heated, glass reaction chamber monitored directly by a mass spectrometer operated in the electron capture, negative ionization mode. Thus, we gain sensitivity by this highly selective and sensitive ionization technique. However, it turns out that there is a limit to the elevated temperature approach: The sources of OH also tend to decompose thermally as the temperature increases. We have worked around this limitation by using both O3/H2O and H2O2 as the OH source. Some of this work is complete and a paper has been published in Environmental Science and Technology. A reprint of this paper is enclosed. Two other papers are ??in press??.
Accomplishments and Research Results:
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 semi-volatile 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-p-dioxin 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 #f = 254 nm) served as OH sources. An extrapolation using the Arrhenius equation gives a 1,2,3,4-tetrachlorodibenzo-p-dioxin-OH reaction rate constant of 8.5 H 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 particle-phase removal processes of higher chlorinated ones.Rate constants were also measured for gas-phase reactions of the hydroxyl radical (OH) with six polycyclic aromatic hydrocarbons (PAH) and four other polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F). These OH reaction rate constants were determined in helium at about 1 atm over various temperature ranges between 306-405 K. The experiments were carried out in a small, heated quartz reaction chamber sampled by on-line mass spectrometry, and OH was produced by the photolysis of O3 in the presence of H2O. Arrhenius regressions were performed with the rate constants of each compound, and the temperature dependencies were found to be slight to non-existent. The OH reaction rate constants of PCDD/F were in agreement with those predicted by a structure-reactivity method. The resulting rate constants at 298 K (in units of 10-12 cm3 s-1) were: naphthalene, 23; acenaphthene, 58; fluorene, 13; phenanthrene, 27; anthracene, 190; fluoranthene, 11; dibenzo-p-dioxin, 12; 2,7-dichlorodibenzo-p-dioxin, 4.4; dibenzofuran, 3.5; and 2,8-dichlorodibenzofuran, 2.2.
In addition, rate constants for the gas-phase reactions of the hydroxyl radical (OH) with #\- and #^-hexachlorocyclohexane (#\- and #^-HCH) and hexachlorobenzene (HCB) were measured over the temperature range 346-386 K. The experiments were carried out in He diluent gas at approximately 1 atm, in a 160-mL quartz chamber. OH was produced by the photolysis of ozone in the presence of H2O, and reactants in the chamber were monitored by on-line mass spectrometry. The rate constants measured at elevated temperatures were extrapolated by the Arrhenius equation, and OH reaction rate constants at 298 K were estimated for each compound. These rate constants at 298 K (in units of 10-13 cm3 s-1) were: #\-HCH, 1.4; #^-HCH, 1.9; and HCB, 0.27. Atmospheric lifetimes based on OH reactions (#nOH) were also estimated (in units of days): #\-HCH, 120; #^-HCH, 96; and HCB, 940. These relatively high atmospheric lifetimes indicate that these compounds can be transported great distances.
Publications Polychlorinated dibenzo-p-dioxins and dibenzofurans: Gas-phase hydroxyl radical reactions and related atmospheric removal, Environmental Science and Technology, 31, 1805-1810 (1997); with W. W. Brubaker.
OH reaction kinetics of polycyclic aromatic hydrocarbons and polychlorinated dibenzo-p-dioxins and dibenzofurans, Journal of Physical Chemistry, in press; with W. W. Brubaker.
OH reaction kinetics of gas-phase #\- and #^-hexachlorocyclohexane and hexachlorobenzene, Environmental Science and Technology, in press; with W. W. Brubaker.
Future Activities:
Products of Semi-Volatile Organic Compound and Hydroxyl Radical Reactions. Having determined the kinetics of gas-phase reactions of semi-volatile organic compounds (SOC) with the hydroxyl radical (OH), the next issue that must be addressed is the product identification of these reactions. That is, what are the products under atmospheric conditions, and are they more or less harmful than the parent SOC? If so, are the yields high enough in the atmosphere to pose a risk? Answers to these questions are scarce in the literature because product studies are much more difficult than kinetic studies. For example, a kinetics experiment can be carried out by monitoring minimal traces of the initial reactant in the gas-phase. OH reaction products however tend to be lower in abundance and volatility compared to the reactants. The products tend to partition more to the walls of reaction chambers and are much harder to collect and/or detect in the gas-phase. Because of its small size (~160 mL), our reaction chamber has an advantage in that it can be isolated and removed from the system. Products (and remaining reactants) can then be extracted from its walls while achieving a reasonable mass balance for the gas-phase reactions which took place. The present research goal is to extract, identify, and quantitate the products of the SOC-OH reactions, giving material balances indicative of chemical yields in the environment.
Methods are currently being developed by which gas-phase SOC-OH reaction products are extracted from our reaction system and analyzed by gas chromatographic mass spectrometry (GC/MS). After a SOC-OH reaction is carried out in the closed chamber, the chamber is removed and its walls are washed with solvent. Polar reaction products with hydroxyl functional groups and incompatible with gas chromatography are derivatized with diazomethane prior to GC/MS analysis. Our methods to date have reproduced results of a product identification study of the naphthalene-OH reaction (N. Bunce et al., Environ. Sci. Technol., 31, 2252-2259, 1997).
Our OH reaction product analyses are now focusing on biphenyl and its mono- and dichlorinated congeners. Preliminary results indicate that the gas-phase OH reactions of these compounds produce unchlorinated and chlorinated benzoic acids in yields of 10-50%. Future experiments will include polycyclic aromatic hydrocarbons and lower chlorinated dibenzo-p-dioxins and dibenzofurans once methods have been established for the more volatile SOC.
Direct Photolysis of Gas Phase Polychlorinated Dibenzo-p-dioxins and Furans. In addition to the OH reaction kinetics outlined above, we have decided that we should also be looking at direct photolysis. In general, PCDD/F have absorption maxima at about 325 nm, a wavelength available at the Earth ?-s surface. Thus, these reactions are possible, and they may be complementary or even faster than reactions with OH. Our recent, gas phase OH reaction rate constant for 1,2,3,4-TCDD (see above) was used in a simple model of the atmospheric removal of PCDD/F, and this improved the agreement between sink and source homologue profiles. However, there are still major discrepancies; for example, the predicted heptachlorodioxin and -furan homologue contributions are much higher in the sinks than observed values.
To further develop this atmospheric model for removal of PCDD/F, this study is determining the rate constants for the direct photolysis of gas phase PCDD/F. Outside of one brief report in the literature for 2378-TCDD (R. G. Orth, C. Ritchie, and F. Hileman, Chemosphere, 18, 1275-1282, 1989), there are no data on this subject available. Our study will use the on-line electron capture mass spectrometer fitted with a quartz reaction chamber in a gas chromatograph oven to monitor photolysis of gas phase PCDD/F when irradiated with a 450 W mercury vapor lamp. The rate constants will be incorporated into the existing model of atmospheric PCDD/F removal to further describe the fate of these important environmental contaminants.
Journal Articles:
No journal articles submitted with this report: View all 20 publications for this projectSupplemental Keywords:
atmosphere, measurement, anthropogenic, assessment, toxic, organic, vapor phase, PCB's, toxicology, RFA, Scientific Discipline, Air, Environmental Chemistry, tropospheric ozone, Engineering, Engineering, Chemistry, & Physics, fate, dioxin, hydroxyl radical, semi-volatile organic compounds, fate and transport, food chain, toxicology, PCBs, VOCs, chemical kinetics, furans, photodegradationProgress 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.