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(13) Slanina,J.; Schoonebeek,C. A. M.; Klockow, D.; Niessner, R. Anal. Chem. 1985,57, 1955. (14) Newman, L. Atmos. Enuiron. 1978, 12, 113.
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Received for review November 12,1987. Accepted May 23,1988.
Photolysis of Polycyclic Aromatic Hydrocarbons Adsorbed on Fly Ash Thomas D. Behymer and Ronald A. Hites” School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
rn A rotary photoreactor was used to simulate the environmental conditions encountered by particle-bound polycyclic aromatic hydrocarbons (PAH) in the atmosphere. Eighteen PAH adsorbed on carbon black and 15 coal fly ash samples of varying physical and chemical compositions were photolyzed. Photolytic half-lives were found to be highly dependent on the substrate. On low-carbon fly ash samples, PAH showed a wide range of half-lives, indicating a relationship between PAH structure and photochemical reactivity. On fly ash samples with a carbon content greater than -590, PAH showed half-lives similat to one another. This indicates that the photolytic process is independent of PAH structure and dependent on the physical and chemical nature of the fly ash. Substrates that stabilize reactive PAH are black or gray in color; these dark substrates adsorb the most light and prevent the light from getting to the PAH. Introduction Polycyclic aromatic hydrocarbons (PAH), formed by the combustion of almost any fuel under oxygen-deficient conditions, are transported through the atmosphere in the vapor phase and adsorbed on particulate matter. Because some PAH are carcinogenic, there is considerable interest in their atmospheric fate (1-9). Currently, two schools of thought exist concerning the ultimate environmental fate of PAH. One school, which we will call the “Fast School”, says that PAH degrade quickly in the atmosphere. For example, some laboratory studies have found that PAH degrade with lifetimes as short as a few hours (20-22). The other school, which we will call the “Slow School”, says that PAH degrade slowly, if at all, in the atmosphere and eventually deposit on soil or water. This idea is supported by studies of marine and lacustrine sediments, the ultimate environmental sinks of PAH. These studies have shown that the relative abundances of PAH, even at the most remote locations, are similar to those in combustion sources (23-25); this suggests that PAH are stable in the atmosphere. To determine which school of thought is correct, it is useful to simulate the atmospheric degradation of PAH in the laboratory. To do so, one must simulate environmental photolytic conditions, and one must simulate the phase conditions of the reactants. The latter is tricky. PAH in the atmosphere are present in both the vapor phase and adsorbed to atmospheric particulates; the amount in each phase is a substrate-dependent equilibrium process. This paper addresses the photolytic degradation of PAH adsorbed on atmospheric particulates. We have selected experimental conditions to simulate the environment encountered by particle-bound PAH, and we have studied a wide range of PAH from acenaphthylene to coronene. 0013-936X/88/0922-1311$01.50/0
These PAH have been adsorbed onto several fly ash samples to elucidate relationships between PAH structure and photochemical reactivity. Dramatically different substrates such as silica gel and alumina were also studied to aid in determining the effect of the physical and chemical characteristics of a substrate on PAH stability. Because we have studied several PAH, we are able to make statistical comparisons between degradation half-lives and substrate composition. This experimental design tells us if fly ash can stabilize PAH and what properties of the fly ash are important. Experimental Section Eighteen common PAH have been chosen to represent combustion-generated PAH. These are acenaphthylene, acenaphthene, fluorene, dibenzothiophene, phenanthrene, anthracene, 4H-cyclopenta[deflphenanthrene,fluoranthene, pyrene, benz [a]anthracene, chrysene, benzo [e]pyrene, benzo[a]pyrene, indeno[l,2,3-~d]pyrene,benzo[ghi]perylene, anthanthrene, and coronene. Isomeric pairs were chosen, when possible, to study the effect of structure on reactivity. These PAH have all been adsorbed on silica gel, alumina, carbon black, and 15 different fly ash samples. We have used fluidized-bed and rotary photoreactors to simulate environmental conditions encountered by particle-bound PAH. The experimental procedures have been described elsewhere (26,27). In these studies, the rotary reactor was found to be better suited for long-term photolysis experiments because particulate matter could be sampled easily, small particles could be used, and the irradiation time could be lengthy. Although the rotary reactor does not simulate the environment as well as the fluidized-bed photoreactor, half-lives for PAH determined in both reactors under identical conditions were found to be equivalent. Additionally, half-lives were found to be reproducible in the rotary reactor. Thus, this photoreactor was used in our studies to simulate environmental processes. The light source was a 450-W medium-pressure mercury vapor lamp with a measured irradiance of 17.6 f 1.4 W/m2 at the photoreactor. Equivalent amounts of each substrate were used in all experiments. Although sticking problems occurred with substrates of very small particle sizes, continual monitoring and tapping of the reactor reduced this problem. To ensure experimental reproducibility, several replicates were done for each experiment. Along with PAH losses due to photodegradation, it was important to determine if reactions occurred in the absence of light. Therefore, dark experiments were carried out. Due to slight heating in the photoreactor from the light source and the rotary motor, volatilization of PAH adsorbed onto a substrate could have been possible. If this
0 1988 American Chemical Society
Environ. Sci. Technol., Vol. 22, No. 11, 1988
1311
Table I. Substrates Used in This Study substr
description
CB FA1 FA2 FA3 FA4 FA5 FA6 FA7 FA8 FA9 FAlO FA11 FA12 FA13 FA14 FA15
260-nm carbon black known to contain high PAH concentrations cyclone dust obtained from a fixed-bed, stirred gasifier using North Dakota lignite fly ash collected by electrostatic precipitation; coal type unknown cyclone dust obtained from a fixed-bed, stirred gasifier using Arkwright bituminous coal as feed cyclone dust obtained from a fixed-bed, stirred gasifier using Blacksville bituminous coal (in briquette form) as feed bottom ash from the same source as FA2; coal type unknown; 180-250-pm size fraction used spent bed material (bottom ash) from a fluidized-bed combustor; coal type unknown fly ash (baghouse) sample from a fluidized-bed combustor; coal type unknown fly ash sample collected in a primary cyclone from a fluidized-bed combustor; coal type unknown NBS standard reference material No. 1633a obtained by burning Pennsylvania and West Virginia coals Alpha Resources, Inc. reference material No. 4202; source and coal type unknown fly ash sample; source and coal type unknown Alpha Resources, Inc. reference material No. 4201; source and coal type unknown fly ash collected in electrostatic precipitator; coal type unknown fly ash collected in a tertiary cyclone from a fluidized combustion; same source as FA8; coal type unknown spent bed material (bottom ash) from a fluidized-bed combustor; same combustion source as FA8; coal type unknown silica gel (100-200 mesh; Fisher Scientific Co.);
