¹Center for Environmental Science and Technology and Department of Chemistry, University of Missouri-Rolla, Rolla, MO, 65401 and ²Environmental Affairs Division, Southern California Edison Company, Rosemead, CA, 91770
A recent report on health assessment of chlorinated dioxin has revealed a number of health-related issues of these controversial chemicals. The report states that dioxins, even at very low levels, pose health risks to humans. However, the report has given rise to new controversies on source and distribution of these chemicals in the environment. It indicated that all the known sources of dioxin account for only one-half of the estimated total environmental burden. The other half results from unknown sources including perhaps natural processes such as forest fires. While some of the contentions on sources may be true, one major reason for the discrepancy could be transformation, especially phototransformation in the atmosphere. This is especially relevant to the environmental concentration when expressed as toxic equivalents (TEQ). Since, on this scale, conversion of certain congeners to others can lead to significant changes in TEQ values; e.g., conversion of octachlorodibenzo-p-dioxin to congeners with lower chlorine substitution will lead to an increase in TEQ value. A study to examine this phenomena was undertaken. Phototransformation of chlorinated dioxins was examined in the vapor phase and on aerosol particles.
The gas phase studies were carried out with a two-dimensional gas chromatographic (GC) system. Studies on dioxin-bearing aerosol were carried out in a photoreaction chamber coupled to an electrostatic classifier and particle counter. These arrangements permitted isolation and irradiation of selected chlorinated dioxins in the photoreactor for varied periods and under different atmospheres. The irradiation experiments revealed that degradation rates in both the gas phase and on aerosol particles are dependent on dioxin structure; e.g. approximately 80% of 2,3,7-trichlorodibenzo-p-dioxin was transformed after a 20-minute irradiation while less than 30% of 2,3,7,8-tetrachlorodibenzo-p-dioxin was transformed over the same exposure period. Photodegradation rates decreased with an increase in the number of chlorines.
Degradation rates were also influenced by the position of chlorine substitutions. The results showed that, in contrast to solution phase studies, congeners with peri chlorines photodegrade more rapidly than congeners with laterally substituted chlorines. Addition of selected dopants to the photoreactor atmosphere effected changes in reaction kinetics and formation of hydrodehalogenation photoproducts. The addition of 100 ppm hexane to the photoreactor chamber enhanced formation of neutral hydrodehalogenation photoproducts. These photoproducts provided further evidence of peri position dehalogenation. Introduction of 100 ppm O2 or H2O to the photoreactor atmosphere increased the photodegradation rate significantly. The increase most likely results from the formation of highly reactive O3 and hydroxyl radicals. Surface area experiments indicate that the reactions occur primarily in the gas phase and not on the photoreactor walls. An increase in the photoreactor surface area did not yield a significant increase in the photodegradation rate. All of the above results indicate that in the gas phase more toxic (laterally substi-tuted) congeners are more persistent than less toxic (peri substituted) analogs and that less toxic congeners can be phototransformed into more toxic congeners by elimination of peri substituted positions, thus increasing the TEQ of irradiated samples.
phototransformation, polychlorinated dibenzo-p-dioxin, chlorinated dioxin, toxic equivalents
This paper is from the Proceedings of the 10th Annual Conference on Hazardous Waste Research 1995, published in hard copy and on the Web by the Great Plains/Rocky Mountain Hazardous Substance Research Center.