Environ. Sci. Technol. 1006, 2 0 , 175-180
Time-Resolved Solvent Extraction of Coal Fly Ash: Retention of Benzo[ a Ipyrene by Carbonaceous Components and Solvent Effects Pat A. SoItys,+Thad Mauney,* David F. S. Natusch,§ and Mark R. Schure*ll
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
rn Incomplete recovery of benzo[a]pyrene from artificially doped stack ash is observed for extractions with methylene chloride and cyclohexane. The carbonaceous fraction of the ash is suggested to be responsible for this retention. By use of a continuous flow extractor it is demonstrated that the removal of benzo[a]pyrene from the carbonaceous fraction of the ash is slow but essentially complete when benzene is used as the solvent. Fly ash samples from different sources are found to have different proportions of carbonaceous matter. The specific surface area and adsorptive affinity for benzo[a]pyrene of these samples are found to vary among carbonaceous components from different ash samples.
Introduction Polycyclic aromatic hydrocarbons (PAH) are among the most extensively studied classes of compounds pertinent to the analysis of airborne particulate matter (1,2).These compounds are of special interest because many of them, and in particular benzo[a]pyrene (B[a]p), are highly mutagenic or carcinogenic (1). The bulk of PAH emitted into the atmosphere as combustion byproducts is believed to exist in particulate form (1-4), and so emphasis has been placed on the analysis of PAH derived from airborne particulate matter rather than on gaseous emissions. In contrast to the considerable efforts expended on the development of techniques for the analysis of trace levels of PAH, little attention has been paid to the extraction behavior of PAH from environmental particulates or to establishing that quantitative recovery is obtained. One study (5) used dust particles that had been exposed to automobile exhaust gases as a specimen typical of PAH-containing material. To this sample was added a known amount of 14C-labeledB [alp dissolved in cyclohexane, and the solvent evaporated. Extraction with benzene for 6 h in a Soxhlet apparatus resulted in an 81% recovery of the labeled compound. Although incomplete recovery was shown to occur, this study did not examine the possible competition between previously adsorbed material and the tracer or the possible dependence of the doping level on recovery. Griest et al. (6),using an ultrasonic technique, found that the extraction of 14C-labeledB[a]p from coal fly ash was incomplete and that the unextracted tracer remained on the fly ash. It was suggested that the association of PAH with the fly ash surface might involve ?r complexes between the aromatic compounds and metals on the fly ash surface. These authors have concluded upon further study (7) that the association of aromatic species with Present address: Elars Bioresearch Laboratory, 225 Commerce Dr., Ft. Collins, CO 80523. *Present address:’ GeoResearch, Inc., 2815 Montana Avenue, Billings, MT 59101. $Present address: Liquid Fuels Trust Board, P.O. Box 17, Wellington 1, New Zealand. Present address: Digital Equipment Corporation, MR02-4/E33, One Iron Way, Marlborough, MA 01752. 0013-936X/86/0920-0175$01.50/0
carbonaceous particles is, in fact, the dominant interaction. Jager et al. (8),in a similar study, observed that the carbon content of the ash was the dominant factor controlling extraction efficiency. Both Griest and Jager noted the dependence of recovery on the PAH molecular size. Other studies (9-12) have demonstrated that the efficiency of PAH extraction is highly dependent on the solvent employed, the most common solvents being methanol, cyclohexane, and methylene chloride. Few of the studies mentioned have examined the kinetics of extraction. A rather complete set of literature references on the subject of the extraction of polycyclic aromatic hydrocarbons is given in ref 2. To test the efficiency and reproducibility of extraction, a sample must be prepared that contains an accurately known amount of PAH. This may be accomplished by adding (doping) known amounts of PAH to a sample, provided that the sample does not contain these compounds in advance. The sample must be chemically and physically representative of field samples, and so, because of the complexity of environmental samples, the only adequate model may be the field sample itself. In this paper we examine some of the aspects of solvent extraction of B[a]p from coal fly ash using three different solvents: methylene chloride, cyclohexane, and benzene. By use of a continuous flow extractor, the kinetics of extraction of B[a]p from doped fly ash is examined separately for the aluminosilicate and carbonaceous fractions of coal fly ash.
Experimental Section Samples. The three fly ash samples used in this study were collected from electrostatic precipitators of coal-fired electric power plants. The Corette ash is from a low-sulfur western subbituminous coal from the Rosebud mine in Montana. The Minnkota ash is derived from a mediumgrade coal, and the Niagra-Mohawk ash is derived from a low-grade coal. The samples were not size separated in this study; however, the carbonaceous fractions of the ash samples were separated from the bulk by density, using procedures described below. Carbon Content. Total carbon content was determined by the combustion of a weighed sample in pure oxygen at 1000 “C; the evolved carbon dioxide was measured with a carbon dioxide coulometer. The carbonate carbon was determined by carbon dioxide evolution upon acid leaching. The difference between the total carbon and the carbonate carbon measurements is referred to operationally as “graphitic” carbon content; as described below, no extractable organic species were found on the ash components. All of the carbon determinations were performed by Huffman Laboratories, Wheat Ridge, CO. Solid NMR Analysis. The solid nuclear magnetic resonance (NMR) spectra of the carbonaceous particles were obtained on an experimental instrument using the cross polarization-magic angle spinning technique (13,14) with a 13C resonance frequency of 15.007 MHz. Hexamethylbenzene was used as a reference material. Approximately 4000 scans were taken with a contact time
0 1986 American Chemlcal Society
Envlron. Sci. Technol., Vol. 20, No. 2, 1986
175
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Figure 1. Block diagram of the flow system used for adsorption and desorption rate studies.
(cross-polarization time) of 1ms and a delay time of 4 s. Further details of these experiments will be described in a forthcoming publication. Surface Area Measurement. Specific surface area measurements were performed by the BET method (15, 16) using nitrogen as the adsorbate at three partial pressures. All measurements were made with a Quantasorb Sorption System (Quantachrome Corp., Syosset, NY). Premixed nitrogen-helium mixtures of 0.1005,0.207, and 0.309 mole fraction nitrogen (UHPquality) were supplied by Scientific Gas Products (Denver, CO). Density Separation. A float-sink method (17)was used to separate the carbonaceous fraction of the ash from the mineral fraction using a mixture of carbon tetrachloride and diiodomethane. The carbonaceous fraction of the Corette and Minnkota samples was primarily contained in the fraction with density
