Environ. Sci. Technol. 2005, 39, 7749-7756
Risk Ranking of Bioaccessible Metals from Fly Ash Dissolved in Simulated Lung and Gut Fluids J O H N T W I N I N G , * ,† P E T E R M C G L I N N , † ELAINE LOI,† KATH SMITH,† AND R E T O G I E R EÄ ‡ Australian Nuclear Science and Technology Organisation, PMB 1, Menai, 2234, Australia, and Mineralogisch-Geochemisches Institut, Albert-Ludwigs-Universita¨t, D-79104 Freiburg, Germany
Power plant fly ash from two fuels, coal and a mixture of coal and shredded tires, were evaluated for trace metal solubility in simulated human lung and gut fluids (SLF and SGF, respectively) to estimate bioaccessibility. The proportion of bioaccessible to total metal ranged from zero (V) to 80% (Zn) for coal-derived ash in SLF and from 2 (Th) to 100% (Cu) for tire-derived fly ash in SGF. The tirederived ash contained much more Zn. However, Zn ranked only 5th of the various toxic metals in SGF compared with international regulations for ingestion. On the basis of total concentrations, the metals closest to exceeding limits based on international regulations for inhalation were Cr, Pb, and Al. On dissolution in SLF, the most limiting metals were Pb, Cu, and Zn. For metals exposed to SGF there was no relative change in the top metal, Al, before and after dissolution but the second-ranked metal shifted from Pb to Ni. In most cases only a proportion of the total metal concentrations in either fly ash was soluble, and hence bioaccessible, in either biofluid. When considering the regulatory limits for inhalation of particulates, none of the metal concentrations measured were as hazardous as the fly ash particulates themselves. However, on the basis of the international ingestion regulations for Al, the maximum mass of fly ash that could be ingested is only 1 mg per day (10 mg based on bioaccessibility). It is possible that such a small mass could be consumed by exposed individuals or groups.
Introduction Ash is an inescapable byproduct of the use of coal and coalbased fuels for the production of heat and electricity in highefficiency power plants. Fly ash is produced in large quantitiessin Australia 8.6 Mt in total is generated annually (1.2 Mt fly ash per 10 Mt coal consumed), while in the United States approximately 63 Mt of fly ash is produced each year. It is typically a fine, powdered material comprising mostly aluminosilicate glass and oxides of iron. These ash constituents incorporate other elements in minor and trace quantities. Fly ash trapped in filters and bottom ash is either used for various applications (e.g., concrete) or stored in ponds near power plants. * Corresponding author phone: +61 2 9717 3060; fax: +61 2 9717 9260; e-mail:
[email protected]. † Australian Nuclear Science & Technology Organisation. ‡ Albert-Ludwigs-Universita ¨ t. 10.1021/es0502369 CCC: $30.25 Published on Web 08/30/2005
2005 American Chemical Society
During combustion and storage, ash has the potential to become mobilized in the atmosphere and hence become more widely distributed in the environment. It can be deposited on vegetation or washed into waterways and consequently be ingested by animals, including humans. In addition, a significant proportion of the material is of a size range that may be inhaled and deposited within lungs. Fly ash inevitably contains a number of toxic contaminants that are not volatilized during incineration or that condense during cooling and/or filtration processes. Hence, it is important to determine the potential risks that these contaminants in fly ash may pose to organisms in the broader environment, in particular humans, which become exposed to this material. To evaluate this potential, fly ash samples were collected from a power plant at Purdue University, West Lafayette, IN. The stoker-boiler unit and ash sampling ports at that plant are described in Giere´ et al. (1). The characteristics of the escaped fly ash (EFA) are reported by Giere´ et al. (2). This site was selected as it presented an opportunity to evaluate fly ashes from two types of fuelscoal alone and a mixture of coal (95 wt %) and discarded shredded vehicular tires (5 wt %). For the purposes of this paper these two ashes are hereafter designated CDF (coal-derived fly ash) and TDF (coal- plus tire-derived fly ash), respectively. A number of metals (some toxic) were found to exist in the two fly ashes (see results section). Often, the adverse impacts of exposure to these contaminants assume that the total concentration present in the material is able to produce toxic effects. The overall toxicity of many of the compounds discussed in this paper has often been determined as a result of exposure of animals to metals in readily bioavailable forms (e.g., ref 3). This is not typical of normal exposures. In most cases the material needs to become solubilized and impinge upon or cross biological membranes to have any adverse affect. The chemical speciation of the metals in question, as well as the chemical and physical nature of the bulk matrix in which the metals are carried, can substantially influence bioavailability. With respect to exposure pathways, the skin provides a large surface area but is fairly impervious to solids such as fly ash. Hence, dermal exposure to fly ash is considered of reasonably low risk. However, once the material enters the body via the respiratory and gastrointestinal tracts it mixes with biological fluids and hence has a greater opportunity to be dissolved and absorbed. In this study, we have approximated the latter exposure pathways to determine the solubility of various metals from fly ashes in simulated gut and lung fluids. Measurement of contaminant solubility in these fluids should provide a better estimate of the bioaccessible (and hence toxic) fraction of these contaminants in fly ash than the total concentrations. We acknowledge that some proportion of any material inhaled or ingested will result in exposure and consequent biotic response by means other than simple dissolution and absorption, for example by macrophage ingestion and translocation in lungs and particulate uptake into intestinal lymph nodes. However, for the purposes of this study, we are concentrating on solubility in the simulated fluids, thus neglecting the other pathways.
Methodology Characterization of the Fly Ashes. Detailed physical and chemical analyses of the various ashes derived from the two fuels are presented elsewhere (1, 2, 4, 5). Relative to the fuel, bottom ash, electrostatic precipitator ash, and EFA are VOL. 39, NO. 19, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
7749
progressively more enriched in metals, metalloids, toxic, and radioactive species. The experiments presented here were carried out with ash collected by an electrostatic precipitator rather than with EFA, because the latter was not available in sufficient quantity. The material used here is thus less enriched in problematic elements than the EFA. Detailed mineralogical characterization of this material is currently underway so as to compare it to the EFA. Giere´ et al. (1, 2) undertook detailed examination of EFA captured on filter papers at the top of the smoke stack using scanning and transmission electron microscopy. They found that EFA contains primarily very fine particles (
