Environ. Sci. Technol. 1988, 22, 109-112
NOTES Leaching of Metal Ions from Fly Ash by Canine Serum Wesley R. Harris" and David Sllberman
Department of Chemistry, University of Idaho, Moscow, Idaho 83843,and Laboratory for Energy-Related Health Research, University of California, Davis, California 956 16
w Fly ashes from the combustion of a variety of fossil fuels have been leached with normal canine serum for 24 h, and the concentrations of selected elements in the serum leachate have been determined by atomic absorption spectroscopy. Results are compared with similar leaching experiments involving either 0.5 M HC1 or pH 7.4 tris(hydroxymethy1)aminomethane buffer. Chelation by serum ligands is the dominant process in serum leaching of all metal ions except manganese. Serum leaching efficiency, defined as the percent of acid-soluble material removed by serum, is consistently between 40 and 90% for Co, Ni, Cu, and Zn and is consistently below 30% for Al, Fe, V, and Pb. Serum leaching of vanadium is lower than had been expected, whereas a combination of high bulk concentration and effective serum leaching leads to extremely high nickel concentrations in some of the serum leachates. The combustion of fossil fuels for large-scale energy production leads to the formation of large amounts of ash from the inorganic elements in the original fuel. Although over 95% of this ash can usually be contained within the power plant, approximately 3 million tons of ash are released into the atmosphere each year from the combustion of pulverized coal in the U S . (1,2). The potential health effects from the inhalation of such material depend on several factors, including particle size, total concentration of heavy metals in the ash, the speciation of these metal ions, and their solubility in biological fluids. The particles that escape modern control equipment and enter the atmosphere tend to be in the smaller, respirable size range (3-6). Several trace elements are enriched on the surfaces of these smaller particles (7-11), and this surface material is accessible for leaching by biological fluids following inhalation. Although the dissolution of metal ions from fly ash has received considerable attention, most studies have used distilled water to mimic conditions in fly ash ponds and burial sites (12-18). The pH of these solutions varies from 2 to 11 depending on the acid-base properties of the ash, and metal ion concentrations are governed primarily by the solubility of the hydroxo, oxo, and carbonato complexes. Other studies involve leaching by strong acids, often to obtain an estimate of the fraction of trace elements on the surface of the particles (9,1942). Neither distilled water nor strong acid provides a realistic model of leaching under biological conditions, where the pH is buffered near 7 and a variety of chelating agents are present. Harris and Silberman have studied leaching of trace metals by solutions of individual chelating agents in pH 7.4 tris(hydroxymethy1)aminomethane (Tris) buffer *Author to whom correspondence should be addressed at the University of Idaho. 0013-936X/88/0922-0109$01.50/0
Table I. Sample Abbreviations and Particle Size Information sample abbreviation
fuel
collection point
VMD," pm
PCC-F OIL-B OIL-s FBC COM-F COM-C CWM
coal oil oil lignite coal-oil coal-oil coal-water
stack baghouse stack baghouse baghouse baghouse baghouse
2.2 ndb 22 2.0 2.6 12.8 1.5
Volume median diameter.
* nd = not determined.
(23). Strong ligands such as ethylenediaminetetraacetic acid (EDTA) and citrate remove significant quantities of trace metals, but they are much less effective than strong acid solutions. A 10 mM solution of the weaker ligand glycine does not significantly enhance leaching of trace elements over that observed with Tris buffer alone (23). This paper reports on the leaching of selected trace metals by canine serum, which was selected to mimic the biological fluids that ash particles would encounter following inhalation. A previous study on a single coal fly ash showed that serum was more effective than EDTA for leaching of copper, nickel, and zinc (23). This paper includes ashes from pulverized coal, coal-oil and coal-water mixtures, lignite, and oil. Results with serum are compared to leaching by Tris, a noncoordinatingbuffer, to determine the relative importance of chelation by serum ligands in the leaching process.
Experimental Section Fly Ashes. A total of seven ash samples was used. Sample abbreviations and particle size information are listed in Table I. One sample (PCC) was size-fractioned, stack-collected fly ash from the conventional combustion of pulverized western coal in a commercial power plant (3). Two samples were oil fly ashes from commercial power plants, one collected from a baghouse hopper (OIL-B) and the second from the course size fraction of stack-collected ash (OIL-S) (24). The other four samples were collected from small, experimental combustors. Two samples are fine (COM-F) and course (COM-C) fractions from sizeseparated hopper ash from the combustion of a coal-oil mixture (19),while a third sample (CWM) is a size-separated hopper ash from the combustion of a coal-water slurry. The final sample (FBC) is size-separated baghouse ash from the fluidized bed combustion of Beulah, ND, lignite (25). Two of the samples, COM-C and OIL-S, are outside the respirable size range. Metal Ion Analyses. Bulk elemental analysis and acid leaching data have been previously reported for the PCC (3, 9), OIL-S (241, COM (19),and FBC (20) ashes. Similar
0 1987 American Chemical Society
Environ. Sci. Technol., Vol. 22, No. 1, 1988
109
Table 11. Elemental Analysis of Fly Ash Particles PCC" matrix elements, % A1 Ca Fe Mg Si trace elements, r g / g As co Cr cu Mn Ni Pb V Zn Values from ref 3.
CWM
13.9 2.36 3.20 0.630 26.8 132 21.8 68 137 309 40 278 327 590
FBCb
OIL-B
2.56 20.9 2.55 7.69 2.49
0.24 2.70 6 for the oil-related ashes but are
