Environ. Sci. Technol. 1092, 2 6 , 502-507
Ac k n o w 1edgmen t s
Brierly, C. L.; Brierly, J. A. Technical Completion Report No. 140; Water Resources Research Institute: New Mexico,
We acknowledge the help of Dr. Robert Martin over the course of this work. Also, Michael Cain, Mark Groff, Juliana Morillo, Colleen Muir, and Linda Wicks worked on proton rates and Sr exchange. Registry No. H t , 12408-02-5; P b , 7439-92-1; Al, 7429-90-5; Sr, 7440-24-6; Cd, 7440-43-9; Ca, 7440-70-2.
Literature Cited (1) Sylva, R. N. Water Res. 1976, I O , 789-792. (2) Kuwabara, J. S.; Davis, J. A,; Chang, C. Limnol. Oceonogr. 1986, 31, 503-511. (3) Crist, R. H. Messiah College, Grantham, PA, unpublished work, 1987. (4) Becker. E. W. Water Res. 1983. 17. 459-466. (5) Darnall, D. W.; Greene, B.; Hosea; M.; McPherson, R.; Henzl, M.; Alexander, M. D. In Trace Metal Recouery from Aqueous Solution; Thompson, R. T., Ed.; Burlington House: London, U.K., 1986; pp 1-24. (6) Darnall, D. W.; Greene, B.; Henzl, M. T.; Hosea, J. M.; McPherson, R. A,; Sneddon, M. J.; Alexander, M. D. Enuiron. Sci. Technol. 1986, 20, 206-208.
1981. Gale, N. L.; Wixon, B. G. Control of Heavy Metals in Lead
Industry Effluents by Algae and Other Aquatic Vegetation; Process International Consultants Ltd.: Edinburg, U.K., 1979; pp 210-217. Folsom, B. R.; Popescu, N. A.; Wang, J. M. Enuiron. Sci. Technol. 1986,20,616-620. Crist, R. H.; Oberholser, K.; Swartz, D.; Marzoff, J.; Ryder, D. Enuiron. Sci. Technol. 1988, 22, 755-760. Crist, R. H.; Martin, R.; Guptill, P.; Eslinger, J.; Crist, D. R. Environ. Sci. Technol. 1990, 24, 337-341. Percival, E. Br. Phycol. J . 1979, 14, 103-117. Smidsrod, 0.;Haug, A. Acta Chem. Scand. 1972,26,70-88. Smidsrod, 0.; Haug, A. Acta Chem. Scand. 1968, 22, 1989-1997. (15) Percival, E.; Young, M. Phytochemistry 1971,10,807-812. (16) Dodson, J. R.; Aronson, J. M. Bot. Mar. 1978,21, 241-246. (17) Adey, W. H. U S . Patent 4333263, 1982.
Received for review March 27, 1991. Revised manuscript received August 6, 1991. Accepted September 23, 1991.
Photodegradation of Polychlorinated Dioxins and Dibenzofurans Adsorbed to Fly Ash Carolyn J. Koester and Ronald A. Hltes" School
of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
The photodegradation of polychlorinated dibenzo-pdioxins and polychlorinated dibenzofurans (PCDD/F) naturally adsorbed to fly ashes was investigated. Photodegradation experiments were performed in the solid phase using a rotary photoreactor and a medium-pressure, mercury vapor lamp. Although PCDD/F photodegradation occurred in solution and when PCDD/F were spiked on silica gel, no significant photodegradation was observed for the native PCDD F found on five different fly ashes, ranging in color from lack to gray to yellow. This suggests that photodegradation of PCDD/F bound to atmospheric particles is not a significant mechanism by which these compounds are removed from the environment.
1
Introduction Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) are discharged daily into the atmosphere in car exhaust ( 1 ) and as byproducts of municipal and industrial waste incineration (2, 3). It is not known what happens to these highly toxic compounds once they enter the atmosphere. For example, are they susceptible to photodegradation? Atmospheric degradation is suggested by the observed differences between the PCDD/F homologue profiles of sources versus environmental sinks. Incineration, which is thought to be a major source of PCDD/F, produces homologue profiles with an almost uniform distribution of PCDD/F ( 4 ) . In contrast, homologue profiles found in the atmosphere and in lake sediments show a significant enhancement of octachlorodibenzo-p-dioxin relative to all other homologues ( 4 ) . The most likely explanation of this observation is degradation of the less chlorinated PCDD/F in the atmosphere. Degradation in the water column or in the sediment is not possible because the homologue profiles of atmospheric particles and sediments are virtually identical ( 4 ) . 502
Environ. Sci. Technol., Vol. 26, No. 3, 1992
The most probable route by which PCDD/F are lost from the atmosphere is photodegradation. In order for atmospheric PCDD/F to photodegrade, they must absorb light at wavelengths longer than 290 nm; shorter wavelengths are not transmitted through the ozone layer and are not available to excite tropospheric pollutants. Tetrathrough octachlorinated PCDD have absorption bands from 305 to 318 nm, depending on the particular congener ( 5 ) ;therefore, atmospheric PCDD might be highly susceptible to photodegradation. PCDF, which are structurally similar to PCDD, should also absorb light in this spectral range. Photodegradation of PCDD/F in solution has been well studied. It occurs in a variety of solvents such as hexane, benzene, methanol, acetonitrile, aqueous acetonitrile, and water (6-9). Typically, first-order kinetics have been observed, with half-lives on the order of hours (6). The most well characterized products are congeners with lower levels of chlorination (6, 10). However, these congeners may represent only a small fraction (perhaps 10%) of the products of PCDD/F photodegradation (12). Photodegradation of PCDD/F bound to surfaces has also been investigated, but the results are equivocal. PCDD/F adsorbed to silica, various soils, marble, and glass show little photodegradation (6,8,11-13). However, some degradation of PCDD/F on surfaces is observed if the PCDD/F are present as a component of a complex mixture (12, 14, 15).
The goal of this study is to understand the photolytic behavior of atmospheric PCDD/F. These compounds are present both as vapors and adsorbed to particles in the atmosphere, and they partition between these two phases in a process that is controlled by the individual congener's vapor pressure and the ambient temperature (16). Because most atmospheric PCDD/F are bound to particles (16), we chose to study the photodegradation of substrate-adsorbed PCDD/F.
0013-936X/92/0926-0502$03.00/0
0 1992 American Chemical Society
n
Table I. Comparison of Sunlight and the Mercury Vapor Lamp Used in This Study; Spectral Distributions and Intensities (in W/mz)
END UEW OF PHOTOIL*CTOR CELL
Figure 1. Diagram of the rotary photoreactor used in this laboratory (79).
Clearly, the results of the solution experiments discussed above cannot be directly applied to understanding the environmental behavior of substrate-adsorbed PCDD/F. Studies of PCDD/F adsorbed to various surfaces yield more relevant information; however, the matrices previously used were often cleaner than typical atmospheric particles, and the PCDD/F studied had been spiked, usually as single congeners, on the substrate. Thus, the atmospheric degradation of PCDD/F was not accurately simulated. We have chosen to use various fly ashes, containing native PCDD/F, as model atmospheric particles. Fly ash is easily obtained and, in many cases, contains high native concentrations (ppb) of PCDD/F. Fly ash contains many of the same elements (such as C, Al, Si, K, Ca, and Fe) as atmospheric particles. In addition, fly ash is not a clean substrate; it is covered by a variety of organic molecules, as are atmospheric particles. However, fly ash differs in size from urban atmospheric particles. Ninety percent of the mass of the fly ash we studied was between 45 and 125 pm. In contrast, particles between 0.01 and 1 pm are important in the atmospheric transport of urban particles carrying organic pollutants (17). Given the advantages of using fly ash to simulate atmospheric particles, we were willing to accept this size difference.
Experimental Section Photodegradation experiments were performed using a rotary photoreactor similar to the design of Korfmacher et al. (18) and of Behymer and Hites (19). The photoreactor (see Figure 1) consisted of a rotary evaporator motor (Valley Electromagnetics Corp., Spring Valley, IL) turning a custom-made Pyrex reaction cell at 70 rpm. The cylindrical, 4.5 cm diameter X 7.5 cm long cell had indentations in its side, which continually mixed the particles and provided them with uniform light exposure. A 450-W, medium-pressure, mercury vapor lamp (Hanovia, Inc., Neward, NJ) served as the light source. The lamp was surrounded by a water-cooled Pyrex jacket which filtered light with wavelengths of
