Environ. Sci. Technol. 1984, 18, 765-768
Evaluation of Fly Ash Collection Methods for Short-Term Bioassay Studies of Fluidized-Bed Coal Combustion Judy Lee Mumford" and Joellen Lewtas
Genetic Toxicology Division (MD68), Health Effects Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, North Carollna 2771 1
w Coal fly ash samples from an experimental fluidized-bed combustor were evaluated for cytotoxicity and for mutagenicity. The samples were collected by cyclones, a fabric filter, and an electrostatic precipitator, at various temperatures. To assess cytotoxicity, fly ash particles were assayed in rabbit alveolar macrophages in vitro. To assess mutagenicity, dichloromethane extracts of particles were tested in the Ames Salmonella typhimurium assay. Sample collection parameters that may influence the cytotoxicity and mutagenicity of fly ash were examined. The cyclone samples, collected a t higher temperatures, contained larger particles and exhibited little cytotoxicity or mutagenicity. The fabric filter and electrostatic precipitator samples, collected downstream of the cyclones and at lower temperature, contained finer particles and showed higher cytotoxicity and mutagenicity. The electrostatic precipitator sample was the most mutagenic of the samples investigated. Introduction Several studies have shown that fly ash emitted from coal combustion sources is toxic and mutagenic in various bioassay systems (1-7);these reports of biological activity are sometimes conflicting. The biological activity of coal fly ash may differ for one or more of the following reasons: (A) the samples were obtained from different sources; (B) the sampling methods differed, and therefore, the samples had different chemical or physical properties; (C) the samples were prepared and bioassayed differently. These variables involved in coal fly ash studies are undoubtedly responsible for the diversity of biological effects observed. Previous studies of the sample preparation methods have shown that these methods affect the observed biological activity of coal fly ash (4). This study evaluated the influence of sample collection method on the biological activity of coal fly ash from a fluidized-bed combustion (FBC) miniplant. FBC has shown great potential as an alternative combustion technology for coal due to its advantages in reducing sulfur dioxide and nitrogen oxides emissions (8). Fly ash samples were collected from an FBC miniplant by using cyclones, a baghouse, and an electrostatic precipitator (ESP) (9,lO) at different temperatures. Alveolar macrophages, cells in the lung that are defensive against inhaled particles, were used to assess cytotoxicity (11). The Ames Salmonella typhimurium assay (12)was employed to evaluate mutagenicity. The Salmonella assay detects the mutation of frame shift and base-pair substitutions in histidine-requiring mutants that revert to prototrophy on exposure to a mutagen. This assay has been used as a preliminary screen to evaluate the potential carcinogenicity of pure compounds and complex mixtures (12). Experimental Section Sample Collection. Fly ash from the combustion of eastern bituminous coal (2% sulfur) was collected from a pressurized FBC miniplant (13).The internal diameter of the combustor was 33 cm, and the height of the vessel was 9.8 m. The plant had a coal-firing capacity of 1.8 M W .
Combustion temperatures ranged form 870 to 900 OC; pressures ranged from 7 to 9 atm. Dolomite [CaMg(C03)2] was used as the bed sorbent during collection of cyclone and fabric filter samples. During collection of the ESP sample, the bed material was limestone (CaC03) and was regenerated. Figure 1is a simplified schematic of the sampling system at the FBC miniplant. The particulate effluent stream passed through three stages of high-efficiency conventional cyclones in the plant and then through an ESP or baghouse filter located outside the plant. The ESP and the baghouse were housed in two separate mobile vans. The fly ash investigated here included samples from the secondary and tertiary cyclones and from the ESP and the fabric filter. Ash collected by the primary cyclone was returned to the combustor for recycling. The operating temperature for the secondary cyclone was -830 "C; the temperature was -650 "C for the tertiary cyclone and -170 OC for both the ESP and fabric filter units. The cyclone and fabric filter samples were collected in the same run; the ESP sample was collected in a separate run. The fabric filter sample was taken from a partial gas stream (0.8 m3/min); the ESP sample was taken from the whole stream (13 m3/min) (14). Particle Sizing and Elemental Analysis. Light or scanning electron microscopy was used to obtain the particle size distribution of each sample. Particles were counted in each of several designated size range channels. Volume median diameter (VDM) was calculated according to the method of h a d e and Shendrikar (15).An average volume for each particle size range was calculated by using the midpoint diameter, D,: average volume = ?rDm3/6 The size distribution, weighted by particle count fraction, was converted to a volume distribution: volume in channel i = 7rD,i3/(6ni) The volume distribution was plotted on log-probability paper and the VDM for that sample taken as the size value a t 50% probability. Trace elements were measured by neutron activation according to the method of Weaver (16). Cytotoxicity Assay Using Rabbit Alveolar Macrophages (RAM) in Vitro. Cytotoxicity was assayed in rabbit alveolar macrophages exposed to fly ash particles in vitro by methods previously described (4,11). The secondary cyclone, tertiary cyclone, fabric filter, and ESP samples were each assayed at seven concentrations ranging from 10 to lo00 %/I& Macrophages were incubated with fly ash particles for 20 h at 37 O C . The parameters employed to assess toxicity were viability, viability index, and adenosine triphosphate (ATP). Viability index is defined as the combined effects of the test compound on cell viability and cell number. It is obtained by multiplying cell viability by cell number as a fraction of the control. The dose-response data were also used to estimate median effective concentration (EC,) values, which were defined as the sample concentrations required to reduce
Not subject to U.S. Copyright. Published 1984 by the American Chemical Society
Environ. Scl. Technol., Vol. 18, No. 10, 1984
785
PRESSURE
* n
7.9 atm TOTAL PRESSURE
&!!EL PRESSURE
9
-
EXHAUST
7 I
-
COAL SORBENT -----)
OR ESP
Flgure 1. Schematic dlagram of fly ash sampilng system at Exxon pressurized FBC miniplant.
cell viability or other cell functions to 50% of the control values. The data were mathematically transformed and then fitted to a straight line by the method of least squares. The ECWvalues were obtained from the simple regression line according to the method of Garrett et al. (11). Mutagenicity Assay Using Ames Salmonella typh imurium Plate IncorporationTest. Each fly ash sample was Soxhlet extracted with dichloromethane (DCM) for 24 h. The solvent was then removed by nitrogen evaporation. The weight of the extracted organics was determined and used to calculate the percentage of total organics extracted from the sample. The latter value was 0.02%, 0.03%, 0.08%, and 0.04% for the secondary cyclone, tertiary cyclone, filter, and ESP sample, respectively. Dimethyl sulfoxide was added, and the organic extracts were tested with and without metabolic activation in S. typhimurium tester strains TA98, TA100, TA1535, TA1537, and TA1538 according to the method of Ames et al. (12),with minor modification as described previously (4). A sample was considered to be mutagenic if the total revertants per plate was at least 2 times the spontaneous rate and the data showed dose response. The slope of the initial linear portion (R22 0.90) of each dose-response curve was calculated by least-squares linear regression. The slope from the linear regression analysis was reported as revertants per unit mass of extracted organics. Revertants per unit mass of fly ash was calculated by multiplying the revertants per unit mass of organics by the organic mass extracted per unit mass of fly ash.
Results Particle Size and Elemental Composition. The sample from the secondary cyclone contained the largest particles (VMD = 4.6 pm), followed by the samples from the tertiary cyclone (3.0 pm), ESP (1.7 pm), and filter (1.5 pm). The neutron activation results (Table I) showed that iron, aluminum, and calcium were the three major elementa in all four samples. The samples with smaller particle size (the ESP and filter samples) contained higher concentrations of trace elements, particularly selenium, chromium, cobalt, nickel, and vanadium. The filter sample 766
Environ. Sci. Technol., Vol. 18, No. 10, 1984
Table I. Elemental Composition of Fly Ash Samples by Neutron Activation Analysis 20 cyclone'
rglg 3O cyclone' ESP
109243 81187 90743 26695 17669
