Environ. Sci. Technol, 1984, 18, 181-186
Registry No. Mercury, 7439-97-6.
(9) Kama, W.; Siegel, S. M. Org. Geochem. 1980,2, 99. (10) Siegel, S. M.; Okasako, J.; Kaalakea, P.; Siegel, B. 2.Org. Geochem. 1980,2, 139. (11) Siegel, S. M.; Siegel, B. Z.; Okasako, J. Water, Air, Soil Pollut. 1981, 15, 371. (12) Siegel, B. Z.; Siegel, S. M. Environ. Sci. Technol. 1978,12, 1036. (13) Siegel, B. Z.; Siegel, S. M. In “Biological Effects of Mount St. Helens Eruption”; Bilderback, J., Ed.; American Association for the Advancement of Science Publication: Washington, DC, 1983; in press. (14) Siegel, S. M.; Siegel, B. Z. Adv. Space Res. 1983,3,135-138.
Literature Cited Siegel, B. Z.; Siegel, S. M. Science (Washington,D.C.) (1981) 216, 292. Cannon, H. L. Science (Washington,D.C.) 1960,132,591. Cannon, H. L. Taxon 1971,20, 227. Cannon, H. L.; Shacklette, H. T.; Bastron, H. Geol. Surv. Bull. (U.S.) 1968, 1278-A. Danielsen, E. Science (Washington,D.C.) 1981,211,819. Siegel, S. M.; Siegel, B. Z. Water, Air., Soil Pollut. 1978, 9, 113. Casadevall, T.; Ewert, J.; Symonds, R., American Geophysics Union Annual Meeting, San Francisco, CA, Dec 7-15, 1982, Abstracts. Siegel, B. Z.; Siegel, S. M. In “Biogeochemistry of Mercury in the Environment”; Nriagu, J., Ed.; Elsevier/NorthHolland: Amsterdam, 1979; pp 131-159.
Received for review February 22,1983. Accepted August 29,1983. This work was supported in part by the British Columbia Science Council, the US.Department of Energy (Institutional Grant), and the Hawaii Natural Energy Institute.
Chemical Speciation of Elements in Stack-Collected, Respirable-Size, Coal Fly Asht Lee D. Hansen,” David Sllberman, Gerald L. Fisher,$and Delbert J. Eatough
Thermochemical Institute and Department of Chemistry, Brlgham Young University, Provo, Utah 84602, and Laboratory for Energy-Related Health Research, University of California, Davis, California 95616
rn The data reported in this paper effectively complete the description of the chemical speciation of the elements in a set of four stack-collected coal fly ash samples which have been used extensively in the determination of the biological effects of coal fly ash. The association of elements with the aluminosilicate glass or surface salts, the association of cations and anions on the surface of ash particles, and the oxidation states of nonmetal and transition metals are discussed. Introduction
Coal fly ash is being produced in the U.S.in ever increasing amounts as a byproduct of electric power generation by coal combustion. Approximately 2.5 million tons of coal fly ash is released into the atmosphere each year in the U.S.The respirable-size fraction of a particular set of coal fly ash samples collected from the stack breeching of a large, modern, power plant equipped with an electrostatic precipitator and burning low sulfur, high ash coal has been shown to contain at least two components which are mutagenic in the Ames test (1,2). Furthermore, acute animal inhalation studies (3) but not in vitro macrophage assays (4) indicate that this same coal fly ash may be as toxic to lung cells as a-quartz. The mutagenicity detected in the Ames test is due to organic compounds while another mutagen, detected in an assay using paramecium (5), appears to be inorganic (6). While the physical structure (7-10) and elemental composition (9, 11, 12) have been described for this set of size fractioned, stack-collected samples of coal fly ash, the actual inorganic compounds present have not yet been described. Since both the biological effects and chemical reactivity of many elements are highly dependent on the oxidation state and chemical
* Address correspondence to this author at the Thermochemical ~~
Institute, Brigham Young University, Provo, UT 84602. Thermochemical Institute Contribution No. 280. Present address: Battelle Columbus Laboratories, Toxicology Pharmacology Section, Columbus, OH 53201.
*
0013-936X/84/0918-0181$01.50/0
combination of the element, it is important to know the actual compounds present. Also, since the biological effects on animals and man are dependent on particle size, it is of most importance to determine these parameters in the fine particle fraction of the ash that escapes particle emission control equipment and is dispersed into the atmosphere. This paper is concerned with the delineation of the chemical speciation of the elements in the stack-collected (post electrostatic precipitator), coal fly ash described in a previous series of papers (1-8,lO-13). The significance of this effort is specific to these particular samples and is of interest largely because the biological properties of these samples are better known than for any other coal fly ash sample. There are basically two kinds of chemical speciations which remain to be determined in this coal fly ash. First, the association among the elements must be determined, and second, the oxidation states of the transition elements and some of the nonmetals must be determined. Except for a few elements, data have already been reported on which to base a determination of element association and oxidation state (6, IO). This paper reports total concentration data on the remainder of the elements as a function of particle size and other data which are useful in determining the total speciation of the elements. A limitation of the present study is caused by the fact that the fly ash particles are morphologically and chemically heterogeneous even after size classification. This study cannot distinquish between a high concentration of a given component present in only a few particles or a low concentration present in all particles. The analysis of individual particles of this fly ash sample by scanning electron microscopy has been published elsewhere (14). The electron microscopy data give direct information concerning element association for the major elements but not for the trace elements. Electron spectroscopy for chemical analysis (ESCA), which might be used to obtain direct information on the oxidation states of elements, also is not sensitive enough to detect many trace elements of interest, e.g., Se and As (15),and therefore, the oxidation
@ 1984 American Chemical Society
Environ. Sci. Technol., Vol. 18, No. 3, 1984 181
states of the trace elements must be defined from their chemical behavior. Experimental Section
Materials. The ash samples used in this study were collected from a large southwestern US.power plant. The details of the collection method have been previously described (13). Briefly, size-classified samples (volume median diameters of the fractions 20, 6.3, 3.2, and 2.2 pm) were obtained from the stack breeching downstream from the electrostatic precipitator while the plant was burning low sulfur (0.5%) high ash (20%) coal. The samples were stored under air in sealed glass or paraffin-sealed plastic containers until use. Procedures. Total fluoride was determined in our laboratory by extracting sufficient ash with 0.08 M HN03 so that the final solution was between 0.02 and 0.07 mM in F.Such an extraction is well below the solubility limit of CaF,. The extract was injected into a Dionex Model 10 ion chromatograph equipped with a 400-mm anion separator column and NaHC03-Na2C03 eluent. Under these conditions fluoride gave a peak which was separated both from the initial sharp solvent peak and from chloride. Higher concentrations of HN03 could not be used because of excessively long tailing of the nitrate peak. This procedure assumes that F- is completely extracted from the ash by 0.08 M HN03 and that no interferents such as low molecular weight organic acids (e.g., formate and acetate) which have the same retention time as F- are present. Total F- has also been determined in these ash samples by direct potentiometry at the Norwegian Institute of Technology and by proton-induced y emission (PIGME) at the Australian Atomic Energy Commission Laboratories. Ion chromatography of aqueous, acidic (0.1 M HC1) extracts of the 2.2-pm ash at 25 mg/mL failed to show the presence of any arsenate ion (16). However, extraction of the 2.2-pm ash with 0.2 M NaOH (40 mg/mL) at room temperature for several days produced an extract in which approximately two-thirds of the arsenic was present as arsenate. Oxidation of this extract with HzOz (16) gave an increased arsenate peak which was in agreement with the total amount of arsenic in the ash (9,11,12). An attempt as made to verify the oxidation state of the arsenic by selective arsine generation with NaBH4, first at a slightly basic pH and then in 1 M HC1 (17). The arsine was passed through cotton dampened with lead acetate, absorbed into 1mM I, solution, and determined by the heterpoly blue method (18). The results were not quantitative because recovery of total arsenic by this procedure was incomplete, i.e., 5&70%. However, most of the arsenic obtained as arsine was from the basic solution, thus indicating that approximately half of the arsenic is in the +3 oxidation state. Arsine generation from 1M HC1 suspension of the ash without pretreatment with the neutral buffer solution did give total arsenic ash concentrations in agreement with earlier published values (9,11, 12). The low recoveries could be due to insolubility of the arsenic compounds present or formed in the buffer or to the presence of some interferent in the reaction. Total phosphate was determined in an HF solution of the ash (12) by the chlorostannous-reduced molybdophosphoric blue colorimetric method with the addition of for elimination of fluoride interference (19). Boron was determined in an HF solution of the ash by use of a Perkin-Elmer Model 306 AA spectrophotometer. Serious difficulties with interference by both fluoride and silicon are present in this method. However, the concentrations of both interfering species were kept constant. Thus, there may be sizable systematic errors in the abso182
Environ. Scl. Technol., Vol. 18, No. 3, 1984
lute concentration of the B reported here, but we believe the relative B data between size cuts of the fly ash is accurate. Iron(I1) and iron(II1) were determined as described in a previous paper (6). Reducing agents, sulfate, and nitrite in 0.1 M HC1 and 2.5 mM FeC13extracts of ash were determined by microcalorimetric methods (20,21). Acids and bases in water extracts were characterized by a simultaneous pH and calorimeteric titration procedure (22). Water extracts (1and 4 mg of ash/mL, 20 min at room temperature in a low-power ultrasonic bath) were analyzed for anions and monovalent cations by ion chromatography using a Dionex Model 10 ion chromatograph. Two different anion eluents were used: 3 mM NaHC03-1.2 mM Na2C03at 30% pump rate in a 2 X 500 mm column and 3.0 mM Na2C03-2.5 mM NaOH at 15% pump rate in a 2 X 250 mm column. Elemental analyses of water extracts for Ca, Mg, Al, and Fe were made by atomic absorption spectrophometry (AAS) using a Perkin-Elmer Model 306 AA spectrophotometer. Water-soluble organic carbon analyses were done by extracting 250 mg of ash with 1mL of 0.16 M HC1 (which removes the interference from any carbonate present), centrifuging to separate undissolved ash, removing the supernatant, adding 2 drops of saturated BaClz(aq),centrifuging to remove BaS04, and injecting 10 pL of the supernatant into a Dohrmann-Envirotech Model DC-52D carbon analyzer. Reagent blanks were run in duplicate. The instrument was calibrated with aqueous potassium acid phthalate solution. In order to verify that the BaC12 did not precipitate any organics, single determinations were run on each fly ash sample without BaC1, addition. The results were the same. Results
The results of the various analytical determinations are given in Tables I and 11. Several of these determinations have been repeated at different times since the ash was collected and placed in storage containers. In only one case (methyl sulfate; discussed later) has there been any significant change in the species determined with time. Plots of total concentrations of S, P, F, and B against the calculated surface areas (10) of the various ash fractions are shown in Figure 1. These plots show that all of the fluoride, 97% of the sulfur, and 83% of the phosphorus are on the surface of the 2.2-pm ash particles (10). The concentration of boron does not show a significant correlation with particle size, thus showing B to be associated with the aluminosilicate matrix material. The concentration of total fluoride found in the 2.2-pm ash by potentiometry and by ion chromatography appears to be anomalously high by about 0.07-0.1 w t % or 40-50 pmol/g based on a linear extrapolation of a line going through zero and the three data points for the three larger particle ashes. The results on water-soluble organic carbon are suggestive of the presence of an organic acid in the 2.2-pm ash which has the same retention time as fluoride in the ion chromatograph and may explain the high results obtained by this method. In any case, the conclusion that fluoride is totally on the surface of the ash particles is not affected. Thermometric titration of 0.1 M HC1-2.5 mM FeC1, extracts (20 min) of the ash with dichromate showed only very small heat effects. No end points were observed in these titrations. These results indicate the absence of S(1V) compounds and of readily soluble Fe(I1) compounds (20). If the ash was extracted with HCl(aq) for several days, Fe(I1) was seen in the dichromate titration, but no increase in sulfate was seen. The amount of Fe(I1) in-
Table I. Results of Analyses of Extracts of Size-Fractionated Coal Fly Ash pmol/g of ash for ash size fractionb of species analyzed s0,z-
total S Fc1OH- (free) OH- (total) Na+ K' NH,' Ca2+ Mgz+ ~ 1 3 +
extract HZO 0.125 M NaOH 0.100 M HCl, 2.5 mM FeCl, HZO HZO HZ0 HZ 0 HZO HZO HZO H2O HZO HZO HZO HZO HZO
methoda IC IC DIE from ref 11 IC IC PH TT IC IC IC AAS AAS AAS AAS from ref 10 and 1 l C Dohrmann
20pm 16 13 22
6.3pm 70 59 93
3.2pm 103 108 135
2.2 pm 168 173 21 2
31.5 3 1 158 260 6 0.6 1.4 117 0.2 12.8 0.06
94.8 22 3 8 135 32 0.4 1.0 102 1.7 21.2 0.05
133 44 4 0 0 39 0.4 1.0 88 4.4 1.5 0.00
222 127 4 0 0 50 2.2 1.4 102 7.7 46 0.73 19 52
Fe Si 3 5 3 water-solu ble organic carbon cation equiv/ HZO 0.95 1.02 0.91 0.89 anion equiv IC indicates ion chromatography. Other anions not detected and their approximate detection limits are the following: NO;, < 1.4; NO,-, < 0 . 4 ; PO,3-, < 2; AsO,,-, < 5 ; SO,z-, < 0 . 4 pmol/g. The accuracy in the determinations is i 10%. TT indicates thermometric titration with HClO,. The accuracy is again i 10%. DIE Is direct injection enthalpimetry, and TT is thermometric titration as described in ref 22. AAS is atomic absorption spectroscopy. Water-soluble organic carbon was Particle size is the volume median diadetermined with a Dohrmann-Envirotech carbon analyzer as described in the text. meter. Table I1 of ref 10 gives 0.2%of Si as soluble in H,O, and Table I1 of ref 11 gives the concentration of Si in fraction 4 as 26.8%. Table 11. Total Concentrations of Various Species in Coal Fly Ash pmol/g of ash for ash size fractiona of species determined 20 pm 6.3 pm 3.2 pm 2.2 pm 44.8 i 2.8 47.1 i 1.4 48.5 i 0 . 1 B 36.7 i 2.7 77.2 i 1.3 P 49.4 f 1 . 9 97.8 i 1.6 23.6 i 0.3 48 i 13 F-: by ion chromatography 188 i 18 89i 9 152 2 43 t 1 90r 2 161 i 5 21i 1 by potentiometryC 34f 1 1 1 5 i 11 by P-Yd 1 5 9 i 14 Fe( 11) in total ash 165 i 5 125 i 2 215 k 13 Fe(II1) in total ash 394 f 4 2 2 7 i 16 367 f 1 1 423 f 7 1 5 4 i 14 Fe( 11) in nonmagnetice 125 f 2 1 9 2 i 13 156 f 7 372 i 4 Fe( 111) in nonmagnetice 1 7 4 i 16 329 i 11 423 i 7 a Particle size is the volume median diameter. Uncertainties are the standard deviation of the mean of three determinations. Values determined by Gunhild K. Nagy, Sintef, Industrial Chemistry Division, Norwegian Institute of Technology, N 7034 Trondheim, Norway. Values determined by Eric Clayton, Lucas Heights Research Laboratories, Australian Atomic Energy Commission, New Illawarra Road, N.S.W., Australia. e Given as the percent of the total ash but, in this phase, calculated from data for the total ash given here and data on the magnetic fraction of the ash in ref 6.
creased with the length of extraction time for at least 2 weeks. No reducing agents were observed in extracts separated from the ash and allowed to stand for 2 weeks before analysis (23). No reaction of fresh HC1 extracta with sulfamic acid was observed by direct injection enthalpimetry, thus showing the absence of nitrite (21). However, a small peak was observed a t the same retention time as that of nitrite in the ion chromatogram of the water extract of the 2.2-pm ash. This peak diminished with time so that it was barely detectable in extracts of the same ash sample made 2 years later. We interpret these data to indicate the presence of traces of monomethyl sulfate. On the basis of these data, a maximum of 4 % of the sulfate in the 2.2-pm ash was in the form of monomethyl sulfate at the time the samples were collected. We have recently also reported the presence of monomethyl sulfate at 0.1% of the sulfate concentration in non-size-separated ash collected in the flue line downstream of the electrostatic precipitator of a different, large, western U.S.,coal-fired, power plant (23-25).
Simultaneous pH/thermometric titration of water extracts of ash at 5 mg/mL with perchloric acid and sodium hydroxide solutions (22) showed that hydroxide ion and the acid-base equilibria of Al(II1) are the major determinants of these properties of the extract. The total soluble hydroxide ion concentration [OHplus AI(OH)4plus CaOH+] was obtained from the titration curves and is given in Table I. Free hydroxide ion concentrations are also reported as calculated from the measured pH values which were 10.8, 9.5, 7.3, and 5.6 for H 2 0 extracts at 4 mg/mL. These data indicate that there are no acid salts such as bisulfate present in the ash. Also, no basic species with a pK, value near 7 were detected, thus showing the absence of soluble carbonates in the ash (22). The results of determinations of various other species in extracts are given in Table I. These data give the major water-soluble components of the ash. The ratio of cationic equivalents to anionic equivalents calculated from the data in Table I is near unity for all four ash fractions, showing that no major errors are present in the analyses. This ratio is probably a bit low in the cases of the 3.2- and 2.2-pm Environ. Sci. Technol., Vol. 18, No. 3, 1984
183
*
F by Potendometry
0282
y
024-
aQ
c
-40.0-
016-
0
0.25 0.5 0.75
1.0 1.25 6/D,P
1.5
1.75
2.0 2.25
6/D,P
Figure 1. Dependence of the concentration of F, P, 6,and SO,*-on fly ash particle size as given by Bl(D,p), where D v is the diameter of average volume and p is the density of the particles (see ref 10). is 3 times the atomic weight of Since the molecular weight of S, the data for total S from reference 11 are plotted as 3 times the weight percent of S in order to make the numbers directly comparable.
ash fractions because of complexation of fluoride with silicate. In these solutions, part of the soluble silicon should be counted as a cation [e.g., + F- Si(OH)3F+ OH-], and this would thus bring the ratios nearer to unity. Silicon is probably less soluble in the coarser ashes which also have less fluoride, both of which would diminish fluoride complexation.
-
Discussion The most abundant compounds in the ash are the oxides of A1 and Si. Together, these constitute 84% of the ash (12). Therefore, the phases containing these two elements nekd to be discussed first. There are two crystalline phases in the ash containing these elements: mullite (3Al2O3. 2Si02) and a-quartz @io2) (6). These contain 4.3% and 10.2% of the Si and Al, respectively. A further 4.4% of the Si is soluble in dilute aqueous HC1 (IO) probably because of the presence of short-chain silicates and fluorosilicates in the ash. This leaves 91% of the Si present in an aluminosilicate glass. In the case of Al, 27% is soluble in aqueous HC1. Since high-temperature A1203does not dissolve in dilute HC1, we assume that this fraction of the A1 is mostly present as amorphous silicates in which the A13+ ion can exchange readily with H+ as illustrated in reaction 1. A1,(SiOJ3(s) + GH+(aq) 2A13+(aq)+ 3Si02.3H20(s)
-
(1) Some of the soluble Al is probably also present as fluorides, sulfates, and phosphates. This leaves 63% of the A1 tightly bound in the glassy aluminosilicate phase. 184
Environ. Sci. Technol., Vol. 18, No. 3, 1984
Iron is the next most abundant element in the ash. Only 0.6% of the iron is present in the crystalline magnetic iron oxides in the 2.2-pm ash (6). About one-third, 32%, of the iron is soluble in dilute HC1, thus suggesting the presence of this amount in the same phases as the soluble Al. However, since Fe tends to be more concentrated at the surface of the ash particles than A1 ( I O ) , a larger fraction of the soluble Fe will be present as sulfate, phosphate, and fluoride salts. Two-thirds, 68%, of the Fe is apparently contained within the glassy silicate phase. The rest of the metallic elements can be divided between the glassy silicate phase and those phases which are soluble in dilute HCl. A previous paper has shown that for most elements, solubility in dilute HCl is highly correlated with the concentration of the element on the surface of the particle (IO). All of the nonmetals except B and Si form volatile compounds under the conditions of coal combustion and as a result the volatile nonmetals are found exclusively on the surface of coal ash particles (26). In the ash samples used in this study, the major anions other than silicate, Le., fluoride, phosphate, and sulfate, are concentrated at the surface of the particles, and therefore, it follows that those cationic metals which are also concentrated at the surface (Le., Zn, Cd, Co, Cu, Cr, and possibly Mn, Be, and Sr) are present in the ash as salts of these anions. However, an examination of the probable mechanism by which fluoride, phosphate, and sulfate came to be concentrated on the surface of the ash particles clearly shows that a given cation is not necessarily associated with a given anion. Phosphorus was probably present in the coal as apatite or fluoroapatite which would undergo the following sequence of reactions during coal combustion: Ca3(P04),(s)+ 2Si02(1)+ 5C(s) Ca3Si2O7(l)+ 5CO(d + P2(d (2) 2p2(g) i502(g) = P401o(g) (3) P401o(g) + 6MO(s) = ~ M ~ ( P O A ( S ) (4) Reaction 2 occurs in the reducing, fuel-rich, part of the combustion process, reaction 3 in the oxidizing combustion zone, and reaction 4 at the surface of the ash particles. Reaction 4 will be largely nonselective and will occur with whatever basic metal oxides are present at the particle surface. Reaction 4 need not be viewed as a condensation of P4OlO(at T < 300 "C) followed by reaction but as a direct gas-solid reaction which can occur at any temperature below which the products are stable. It is unlikely that reaction 2 proceeds quantitatively, and unreacted, but melted apatite probably accounts for the small percentage of P that remains as matrix material (IO). Fluorine probably exists primarily as HF(g) following combustion. As evidence for this, HF(g) has been identified as a major component of the gases emitted by a coal-burning power plant (23). As the temperature of the flue gas drops below about 500 OC, HF(g) will react with silicates and basic metal oxides on the surface of ash particles to form fluorosilicates and fluoride salts. SO3formed from combustion of S compounds in the coal will react with basic metal oxides on the particle surfaces to form metal sulfates as soon as the flue gas temperature is low enough for sulfate salts to be stable. The data clearly show that essentially all of the sulfur in the ash is present as metal sulfates. The absence of acid-soluble reducing agents indicates that sulfites and sulfides are absent. The agreement between the total sulfur determination and the sulfate-specific determination in 0.1 M HC1 extracts also shows that sulfate is the major sulfur species. The slightly lower values for sulfate obtained in water and 0.1 M NaOH
-
extracts are probably caused by the insolubility of BaS04 in these solvents. The differences are in reasonable agreement with the total concentrations of Ba in the ash fractions (11). In addition, ESCA spectra (15) of the ash samples do not indicate the presence of any sulfur species other than S(V1). Only trace amounts of organosulfur compounds or ammonium salts could exist in the ash. Therefore, we conclude that essentially all of the sulfur in the ash exists in the form of metal sulfates. The data in this paper do not eliminate the possibility that some of the sulfate is present on the particles as H2SO4 or HS04salts; however, considering the basic nature of the ash (see Table I) and that the ash was collected at a temperature above the dewpoint of H2S04,we think the presence of these acidic species is unlikely. Metal sulfites, which might be formed by reaction of SO2 with basic metal oxides, are not found in the ash. This is probably due to the much lower decomposition temperature of sulfites as compared to that of sulfates. Because of this fact, the ash particles become coated with a layer of sulfates which precludes any further reactivity after the ash cools to a temperature at which sulfites would be stable. The absence of carbonates, nitrates, and nitrites in the ash can be explained similarly. The presence of sulfites, nitrates, and nitrites in atmospheric particulate matter resulting from coal combustion (23) is not in disagreement with these conclusions since these species will be formed at ambient temperatures after water condenses on the ash particles. The major surface composition of fly ash particles in the stack of a coal-fired power plant is thus determined by three factors: (a) the concentration of basic metal oxides on the surface of the particle at the time it exits the combustion zone, (b) the concentrations of HF, SO3,P4OlO, and any other acidic gases present in the flue gas, and (c) the temperature and contact time between the gases and particles. If chlorine, the only other geologically abundant nonmetal not yet considered, were present in the coal in significant amounts, it would be expected to be present in the flue gases as HC1 and react to form metal chlorides as the flue temperature decreases. There was only 48 ppm of chlorine in the coal from which the ash used in this study came so no significant data on the behavior of chlorine were obtained. The specific salts present in coal fly ash are determined by the surface predominance of the cationic elements and not by their concentration in the total ash. When the surface concentration of cationic elements in the 2.2-pm ash as calculated from the particle size dependence of the concentration in an earlier paper (10) is used and when reasonable assumptions of the oxidation states of the metal ions are made, it is a simple matter to calculate the probable distribution of cations associated with sulfate, fluoride, and phosphate on the surface of the 2.2-pm ash particles. The results of this calculation, given in Table 111, indicate that the major cations associated with the surface anions are Fe, Na, Ca, Mg, AI, and Ba in decreasing order of significance. The same relative order of abundances should be found in aqueous extracts if no metathesis reactions occur during contact with water. The results given in the last column of Table I11 show that the assumption of no reactions occurring with water is a poor one for certain elements, namely, Fe, Ca, and AI. This conclusion is in disagreement with other workers, who have concluded that the composition of the insoluble portion of coal fly ash is not altered by extraction with water (27). Because water extracts of
Table 111. Cations Associated with Sulfate Ion in 2.2-pm Ash oxida% of % of nonmatrix soluble tion cation cation state element assumed equiva equivb 35 0.5 Fe 3 Na 1 22 12.1 Ca 2 13 49.4 2 11 3.7 Mg 33.4 A1 3 10 Ba 2 4 Zn 2 2 K 1 1 0.5 Ga 3 0.6 Mn 2 0.4 cu 2 0.3 Cr 3 0.2 Pb 2 0.2 a Calculated from data in Table I of ref 1 0 by the equation % equivalents = [ l O O ( C N , g of nonmatrix elementlg of ash)(oxidation state)/(atomic weight)]/[ Call elements(CN)(oxidation state)/(atomic weight)]. & Calculated from data in Table I.
the ash are weakly buffered by silicate and aluminate at near neutral to somewhat basic pH values, metathesis reactions which exchange OH- with another anion are possible. Reaction 5 is an example of a reaction which Fe2(S04)3 60H2Fe(OHI3 + 3So4'(5)
+
-
would decrease the solubility of iron and which probably occurs during extraction of ash with water. The oxidation states of the elements which are on the surface of the ash particles will be determined by the redox potential of the gas phase in contact with the particles at the temperature of collection in so far as the reactions are rapid. The oxidation states of those elements contained within the particles matrix will be the most stable state of the element at the redox potential of the gases at the temperature at which the particle solidifies, unless electron transport or diffusion of gases within the solid particle equilibrates it to the varying redox potential in the cooling flue gases. Since the vast majority of the particles are glassy aluminosilicates which are nonconductors and are not permeable by gases, those elements which are distributed between the aluminosilicate and surface phases (e.g., Mn, Co, Cr, Cu, U, V, and Mo; see ref 10) may exist in different oxidation states in different phases. Because coal-fired power plants operate with a slight excess of air over that required for complete oxidation of the coal, the flue gases will have oxidizing properties although more than the equilibrium concentration of SOz and CO will be present at lower temperatures because of the rapid cooling of the gases after they leave the combustion zone. The most important gases in contact with the ash in descending order of concentration are thus HzO, COz, 02,and SO3. Another general fact is that lower positive oxidation states always become more stable with respect to higher ones with increasing temperature. For example, as the temperature increases, the position of equilibrium for SO3 SOz 02, 3Fe203 2Fe304+ 1/202, and COP CO + 1/z02 is shifted to the right. We thus expect to find an element in a somewhat lower oxidation state in the aluminosilicate matrix than in the surface phase. The data on Fe in the nonmagnetic (aluminosilicate) phase in Table I1 indicate this to be the case. The ratio of Fe(II)/Fe(III) increases as the particle size increases and hence as the ratio of Fe in the aluminosilicate to Fe on the surface decreases.
- -
+
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Environ. Sci. Technol., Vol. 18, No. 3, 1984
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The probable oxidation states of the other transition metals within the aluminosilicate matrix may be deduced by comparison of the redox potential of the various oxidation states to those for the Fe(I1)-Fe(II1) couple and by assuming that nothing can exist which is far removed from this in energy. On the basis of these premises, we predict the following oxidation states for those trace elements which are significant in the silicate matrix: V(IV), U(1V) and U(V), Cu(I), Cr(III), Co(II), and Mn(I1) and Mn(II1). The presence of a mixture of arsenic(V) and arsenic(II1) on the surface of the ash particles used in this study can be used to deduce the probable oxidation states of the other nonmetals and transition metals in the surface phases since the states exhibited must be compatible in energy. We thus predict the probable states of the surface associated trace elements to be the following: Mo(V1) as molybdates and Moo3, W(V1) as tungstates and W03, V as vanadate(V) and vanadyl(IV), U(V) and U(V1) as U30s and uranates, Se as SeO and selenites, and Cu(II), Cr(III), Co(II), Mn(II), and Mn(II1) as fluorides, sulfates, and phosphates. Acknowledgments
We thank T. Majors, B. E. Richter, and D. K. Rollins of Brigham Young University for assistance in the calorimetric and ion chromatographic work. Registry No. B, 7440-42-8; P, 7723-14-0; Fe, 7439-89-6; Na, 7440-23-5; Ca, 7440-70-2; Mg, 7439-95-4; Al, 7429-90-5; Ba, 7440-39-3; Zn, 7440-66-6; K, 7440-09-7; Ga, 7440-55-3; Mn, 7439-96-5; Cu, 7440-50-8; Cr, 7440-47-3; Pb, 7439-92-1; NH4+, 14798-03-9;mullite, 1302-93-8; a-quartz, 14808-60-7; silicon oxide, 7631-86-9; aluminum oxide, 1344-28-1; arsenic, 7440-38-2; monomethyl sulfate, 75-93-4.
(8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22)
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Received for review March 15, 1983. Accepted August 26, 1983. This work was supported in part by the U.S. Department of Energy and by the Electric Power Research Institute (RP 1639-2).
