Grantee Research Project Results

2001 Progress Report: Mechanistic Studies of the Transformation of Polychlorinated Dibenzo-p-Dioxins via Hydroxyl Radical Attack

EPA Grant Number: R828189
Title: 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, 2000 through September 30, 2001
Project Amount: $320,000
RFA: Combustion Emissions (1999) RFA Text |  Recipients Lists
Research Category: Air Quality and Air Toxics , Air

Objective:

Polychlorinated dibenzo-p-dioxins (PCDDs) 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 completely uncharacterized.

The overall goal of this research is to determine the rates and mechanisms of OH reactions with DD and selected 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.

Specific goals of this research include:

1. The first absolute rate measurements of the reaction of OH radicals with DD, 1-CDD, 2-CDD, 2,3-CDD, 2,7-CDD, 1,2,4-CDD, and 1,2,3,4-CDD over an extended temperature range with determination of accurate Arrhenius or modified Arrhenius parameters.

2. Based on product analyses and pressure-dependence studies and QRRK modeling, determination of the branching ratios for OH addition, OH addition followed by Cl elimination, and H-atom abstraction at temperatures between 295 and 1,000 K.

3. Development of thermochemical-based TST expressions describing H-atom abstraction from room temperature to combustion temperatures (> 1,000 K).

Progress Summary:

During the first year of this grant, we have focused on resolving the following experimental data collection issues prior to actual data acquisition: (1) design and construction of a smaller reactor with improved temperature control to transport low vapor pressure substrates; (2) increase LIF signal collection to allow measurements at lower initial OH concentrations; (3) incorporation of an online hydrogen flame ionization detector (FID) system to verify substrate concentration in reactor; and (4) optimization of nitrous acid (HONO) as OH radical precursor to minimize substrate photolysis. We made excellent progress in all four areas with one minor setback regarding issue one. The following paragraph summarizes this progress.

A significant effort was expended in testing a miniature optical reactor (see Figure 1) that was available from previous photochemical studies. This reactor had two beneficial aspects that warranted its investigation for these kinetic studies: (1) the ability to quantitatively introduce vapor phase samples from condensed samples via independent temperature control of the reactor and the sample inlet system; and (2) a large solid angle for collection of the LIF signal. Unfortunately, a detrimental aspect of this system was the collinear introduction of the pump and probe beams. Extensive efforts to minimize the overlap of these beams in the reaction zone using various focusing optics were unsuccessful. Overlap of the pump and probe beams resulted in rapid OH decay rates that were not suitable for kinetic measurements. A modified fused silica reactor that incorporates the beneficial aspects of this reactor and permits orthogonal interaction of the pump and probe beams has been designed and is under construction. A schematic of this reactor is shown in Figure 2.

Figure 1. Photograph of miniature optical reactor and sample inlet system (to the right)

Figure 2. Schematic of new optical reactor

While waiting for this new reactor to be constructed, we have investigated various methods for improving our S/N ratio, especially at elevated temperatures, using our existing fused silica reactor. We have explored the use of different interference filters to reduce the background signal from the probe laser (282 nm), with some success. Also, we have optimized the magnitude of the OH signal by measuring background OH decays for a wide range of precursor and total gas flow rates. We have discovered that larger OH signals at elevated temperatures can be obtained at slower total gas flow rates (between 500 and 1,000 cm³/min). We are examining the effect of these slower flow rates on experimental rate measurements for other substrates being investigated in a companion EPA grant study (R828187). To date, rate measurements at slower flow rates appear to be identical, within statistical uncertainties, to measurements at higher flow rates used in previous studies. The lack of photochemically induced secondary reactions is likely due to the use of the long wavelength, 351 nm, radiation. Under these conditions, the substrate and its reaction intermediates are opaque to the OH photodissociation source, thus permitting measurements at relatively slow total gas flow rates. A hydrogen FID was purchased and calibrated for online measurement of substrate concentrations in the reactor. This detector will be used to verify the substrate concentrations in the reactor that will be initially calculated based on vapor pressure data, sample inlet temperature, and total gas flow rate. The final optimization of this system will be completed upon installation of the new reactor. The HONO precursor system for OH generation is working well and has been optimized for kinetic studies in a companion EPA grant study on oxygenated hazardous air pollutants (R828187).

The proposed experimental approach for investigating gas-phase oxidative transformation of dioxins was presented at a recent American Flame Research Committee Meeting and at the 12th International Karasek conference on dioxin formation and control. At this latter meeting, Dr. Edward Roekens (VMM-Flemish Environment Agency) presented the results of his atmospheric dioxin deposition study, and Dr. Hermann Nordsieck (Biffa-Germany) presented the results of his dioxin source attribution study. The homolog and isomeric patterns of dioxin/furans obtained from atmospheric deposition study (conducted in Flanders region of Belgium) did not match with the established incinerator or smelting industry patterns. Similar findings have been reported in the United States. To explain these changes in isomeric/homolog patterns, we need to improve our understanding of the fate of dioxins in atmosphere. Similarly, dioxin atmospheric fate data also is needed for source attribution studies. Although the focus of our study is high temperature chemistry, our kinetic and modeling results under ambient conditions may be beneficial to this important atmospheric question regarding dioxin transformation.

Future Activities:

Our experimental goals for Year 2 include kinetic measurements for dibenzo-p-dioxin, 2-chlorodibenzo-p-dioxin, and 3-chlorodibenzo-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 octa-CDD) that cannot be evaluated experimentally because of the very low vapor pressures of these compounds.

Journal Articles:

No journal articles submitted with this report: View all 9 publications for this project

Supplemental Keywords:

combustion chemistry, environmental chemistry, 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 photolysis

Progress and Final Reports:

Original Abstract

2002 Progress Report

Final Report