Trace Elements in Fly Ash Dependence of Concentration on Particle Size Richard L. Davison, David F. S. Natusch,* and John R. Wallace S c h o o l of C h e m i c a l S c i e n c e s , U n i v e r s i t y of Illinois, U r b a n a , Ill. 6 1 8 0 1
Charles A. Evans, Jr. M a t e r i a l s R e s e a r c h L a b o r a t o r y , U n i v e r s i t y of I l l i n o i s , U r b a n a , 1 1 1 . 6 1 8 0 1
The concentrations of 25 elements in fly ash emitted from a coal-fired power plant have been measured as a function of particle size using spark source mass spectroscopy, optical emission spectrography, atomic absorption spectroscopy, and X-ray fluorescence spectroscopy. Of these elements, the concentrations of Pb, T1, Sb, Cd, Se, As, Zn, Ni, Cr, and S were found to increase markedly with decreasing particle size. A mechanism involving high-temperatwe volatilization of a species containing the trace element followed by preferential condensation or adsorption onto the smallest particles is proposed to account for the trace element concentration dependence on particle size. The environmental significance of the results is discussed. H
It is now well established that many high-temperature combustion and smelting operations emit particles containing toxic elements such as Be, Cd, As, Se, Pb, Sb, Hg, T1, and V into the atmosphere ( I ) . Many of these elements are enriched in ambient urban aerosols by as much as 100- to 1000.fold over their natural crustal abundance (2). Furthermore, most of their mass is concentrated in the particle sizch range 0.510.0 pm, which is inhaled and deposited in the human respiratory system. A number of workers (3-5) have shown that inhaled airborne particles are deposited in different regions of the body depending on their aerodynamic size. This behavior is illustrated fcr three compartments of the respiratory system ( 5 ) in Figure 1. From a toxicological standpoint, the smallest pai*ticles ( < 1 pm) which deposit in the pulmonary region of the respiratory tract are of greatest concern. This is because the efficiency of extraction of toxic species from particles deposited in the pulmonary region is high (60-80%) (1, 4, 6-8), whereas the extraction efficiency from the larger particles, which deposit in the nasopharyngeal and tracheobronchial regions and are removed to the pharynx by cilia1 action and swallowed, is low (5-1570). Consequently, toxic species, which predominate in submicrometer-sized particles, will have their entry to the bloodstream enhanced over those which predominate in larger particles. In fact. a nuinber of toxic elements including Pb, Se, Sb, Cd, Ni, V, Sin, and Zn in urban aerosols have been reported to have equivalent mass median diameters of the order of 1 pm or less, which is considerably less than those reported for common matrix elements such as Fe, Al, and Si. Mass median diameters of these elements lie in the range of 2.5-7.0 pm (9-12). It is therefore meaningful to determine whether certain toxic elements predominate in the smallest particles emitted from particulate sources or whether the mass median diameter differences in urban aerosols are simply due to mixing of particles characteristic of individual source emissions. The work reported here was designed to establish whether elements present in fly ash particles emitted from coal-fired power plants (essentially ubiquitous contribu-
tors to urban aerosols) exhibit a dependence of element concentration on particle size. A variety of analytical techniques was employed to choose the most reliable for the determination of individual elements in fly ash and to establish the data firmly. Experimental Sample Collection and Size Differentiation. Two types of samples are represented: (a) fly ash retained in the cyclonic precipitation system of a coal-fired power plant and (b) airborne fly ash collected in the ducting approximately 10 f t from the base of the stack. The retained material was collected in bulk and was size differentiated physically by sieving and aerodynamically in the laboratory with a Roller particle size analyzer (American Instrument Co.). Airborne fly ash samples were collected and size differentiated in situ using an Andersen stack sampler fabricated from stainless steel and designed to operate a t the stack temperature. Although results are reported for samples collected in a single plant, the trace element content of fly ash collected in this plant equipped with cyclonic precipitators and using southern Indiana coal was shown to be representative of that in eight U.S.power plants utilizing a variety of coal types. Particle size calibrations were based on the data supplied by the manufacturers of the analyzers employed. These data are established in terms of equivalence to the aerodynamic diameter of spherical particles of unit densit y (13, 14). Since fly ash particles are predominantly spherical, a rough check on the validity of the aerodynamic sizes can be obtained by determining the average physical size of particles in a given size fraction. For this pur-
10-2
10-1
100
101
102
PARTICLE DIAMETER (,urn)
Figure 1. Efficiency of particle deposition in the three respiratory system compartments (5) Volume8, Number 13, December
1974
1107
pose, particles collected on the third and fourth plates (4.6-7.1 pm and 3.0-4.6 pm equivalent aerodynamic diameter) of the Andersen stack sampler were examined using a Coulter counter (Coulter Electronics Inc., Hialeah, Fla.) in the timed analysis mode with a 100-pm aperture. Milligram portions of the fly ash were dispersed in a 50% mixture of methanol in water and ultrasonically agitated for 5 min before adding the suspension to the counter. When we assumed a particle density of 2.5 g/cm3 to convert volume median diameters to approximate aerodynamic diameters, values of 6.3 and 4.3 pm equivalent aerodynamic diameter were obtained. These indicate the general validity of the aerodynamic size calibration data. In this experiment there was no evidence of particle diameter changing with time due to particle solubility in the methanol-water mixture. Indeed, none was expected since the particle matrices consist predominantly of insoluble aluminum and silicon and iron oxides, and soluble species are relatively minor constituents. Analytical Procedures. The analytical methods employed fall into two classes, those which analyze the fly ash directly as the solid and those which analyze the sample in solution following wet digestion. The former methods retain sample integrity but involve calibration uncertainties; the latter allow easy calibration but are susceptible to possible formation of analytically intractable compounds during digestion. Sample digestion was achieved by heating 0.5 gram of fly ash, 3.5 ml of 3:l concentrated HCl/HN03 (aqua regia), 0.5 ml water, and 2.5 ml of an aqueous solution containing 48% H F for 2 hr a t 110°C in a 25-ml Teflonlined Parr pressure bomb (Parr Instrument Co., Moline, Ill.). After it cooled, 2.5 grams of boric acid were added to neutralize the HF. The small amount of black solid residue remaining was removed by centrifugation and was shown by spark sources mass spectrometry to contain mainly Ca, F, and A1 in addition to carbon. At least 95% extraction of the elements of interest was achieved. Atomic absorption analyses were performed by direct aspiration of dilutions of the original digest for Pb, T1, Cd, As, Ni, and Be. Air-acetylene flames were employed for all elements except Be for which nitrous oxide-acetylene was used. A Jarrel Ash 8-10 dual-beam double monochromator instrument was employed. Background corrections were achieved by monitoring a nonabsorbing wavelength within 40 A of the analytical wavelength. Se was determined by its atomic absorption after conversion to volatile H2Se according to the method of Schmidt and Royer (15). Standard addition calibrations were performed in all cases, and a precision of *lo% was achieved. The elements Pb, Be, Cr, Mn, Co, and Ni were determined by dc arc emission spectroscopy using a BairdAtomic 3-meter spectrograph. Samples coarser than 325 mesh (Tyler series) were ground to pass through a 325mesh sieve. One part by weight of fly ash was mixed with four parts of spectroscopic graphite for 1 min in a Wig-LBug mechanical shaker (Spex Industries). Spex mix A-7 pure graphite standards doped with 49 elements were used for comparative standards. Approximately 50 mg of graphite diluted sample were burned to completion in a cup electrode operating with a 4-mm gap and 10-amp current. Element concentrations were obtained with a precision of 3~30%. The fly ash matrix elements Fe, Ti, Al, Si, Ca, K, S, and Mg were determined using a vacuum-path, singlecrystal, Phillips X-ray spectrometer. All samples of nominal particle diameter >4 km were ground further so as to minimize surface sampling and inhomogeneity effects. 1108
Environmental Science & Technology
The powders were suspended in propanol and dispersed ultrasonically before deposition by filtration onto 0.4-prn millipore membrane filters (16). Mineral standards previously calibrated against NBS mineral standards were supplied by the Illinois State Geological Survey. K , radiation was monitored for all elements and a vacuum radiation path maintained for all elements except Fe and Ti. A lithium fluoride crystal was employed for detecting Fe and Ti, EDDT was used for Al, Si, Ca, K, and S, and ADP was used for Mg. For this method, precisions of &5% were achieved. An AEI model MS-7 spark source mass spectrometer was used for the qualitative determination of all elements of atomic number greater than 11 and for quantitative determination of Bi, Pb, T1, Sb, Sn, As, Zn, Cu, Ni, Fe, V, Ca, K, and Si. One part of fly ash was mixed by weight with two parts of spectroscopic graphite for 5 min in a Wig-L-Bug and the mixture pressed into an electrode. Electrodes were manually positioned and sparked using a 25-psec spark duration and a repetition rate of 300 sec-1 a t 10-6 torr source pressure. Mass spectra were recorded photographically. Internal standardization of the mass spectra was achieved by referencing line intensities both to the Pb in the sample and to 60 pg/g of solution-doped Au. The P b was determined independently by atomic absorption spectroscopy. The 197Au~ion was a t least two orders of magnitude more intense than 181Ta160+ from source contamination. Element concentrations were calculated from the expression by Farrar ( I 7).
where
I = peak intensity of ion beam @ = isotopic abundance
M = mass C = concentration k = sensitivity factor for a given element relative to the standard S t and X = internal standard and analyte quantities, respectively This expression assumes that the line width on the photographic plate is proportional to MI 2 (18). Values of k were determined by doping increasing amounts of Pb, T1, Sb, Sn, As, and Ni into the graphite before forming a series of electrodes with fly ash. For these elements, values of k ranged from 1.0-1.8. For the remaining elements, k was set equal to unity, an assumption usually valid within a factor of three (17). Precisions of &20% were achieved. Carbon present as S i c , FeC, and free C was determined as C02 after combustion with 0 2 on a VzO5 catalyst (29).
Results Results of the fly ash analyses are listed in Tables 1-111 for the technique considered most reliable for each element. Sieved fly ash fractions are listed with physical diameters, but all other fractions are represented in terms of equivalent aerodynamic diameters. Fly ash particles larger than 74 pm (200 mesh, Tyler series) exhibited no dependence of element concentration on particle size so that the concentrations listed for this fraction are averages over all larger fractions. The 25 elements are classified into three groups. In Table I are listed those elements exhibiting concentration increases with decreasing particle diameter. These concentration increases, which were well above experimental error and confirmed by a t least two analytical techniques,
were consistently observed in a range of samples and were present in the airborne fly ash collected from the ducting. Table I1 contains elements which showed concentration trends only in the retained or in the airborne particle size fractions or which, like V, Mn, and Be, exhibited nonuniform dependence on particle size. Table I11 contains ele-
ments which showed no convincing trends within our experimental errors, It should be noted that some of the values listed in Tables I-III show considerable deviation from the apparent trends. Repeated analyses of duplicate samples indicate that such deviations are essentially random and are thus
Table I. Elements Showing Pronounced Concentration Trends Pb
TI
Sb
Cd
Se
Particle diam, pm
As
Ni
Cr
Zn
s, wt %
rg/g
Mass fraction
%
A. Fly A s h Retained in Plant
>74 44-74
140 160
7 9
>40 30-40 20-30 15-20 10-15 5-10 2 160 55 >2 >2 220 50 >2 >2 220 55 0.66 >2 >2 390 46 1.09 >2 >2 490 54 B. Airborne Material >1.7 7 270 60 1.12 >3.5 11 390 85 . >4.0 18 380 90 0.92 ... 95 >4.8 19 >4.5 16 330 90 1.59 >4.4 18 300 130 , ., 1.08 Analytical m e t h o d
...
...
>11.3 7.3-11.3 4.7-7.3 3.3-4.7 2.06-3.3 1.06-2.06 0.65-1.06
a
...
...
...
0
a
b
.. ... .. .
Ca, wt%
...
K, wt %
...
5.4
1.2
2.5 6.3
2.54 6.26
4.5 4.0
4.46 4.04
4.9
4.9
4.2
4.2
5.0
5.0
2.6
2.6
.. .
... ...
., .
...
.. .
c
., .
... ...
., . ... ...
c
c
a Spark source mass spectrometry. b Dc arc emission spectrometry.
c
X-ray fluorescence spectrometry.
Table IV. Boiling Points of Possible Inorganic Species Evolved During Coal Combustion Species boilin or subliming,
