residue reviews || physical and chemical properties of fly ash from coal-fired power plants with...

Physical and chemical properties of fly ash from coal-fired power plants

with reference to environmental impacts

By

A. L. PAGE, '" AHMED A. ELSEEWI, '" and I. R. STRAUGHAN'" '"

Contents

I. Introduction _______________________________________________________ 83 II. Physical properties of fly ash _______________________________________ 86

a) Particle size distribution _________________________________________ 86 b) Microscopic features ___________________________________________ 87 c) Mineralogy of fly ash ___________________________________________ 89

III. Chemical properties of fly ash ______________________________________ 89 a) Elemental composition __________________________________________ 89 b) Composition of sized fractions of fly ash __________________________ 95 c) Composition of fly ash extracts ___________________________________ 96

IV. Environmental impact assessment ____________________________________ 100 a) Atmospheric emission ___________________________________________ 100 b) Precipitator fly ash ________________ -'-____________________________ 104

1. Water-soluble constituents of fly ash-amended soils ______________ 105 2. Effect of fly ash on growth of plants ____________________________ 108 3. Effect of fly ash on mineral composition of plants ________________ 110

Summary and conclusions _______________________________________________ 115 Acknowledgment _______________________________________________________ 117 References ____________________________________________________________ 117

I. Introduction

Interest in coal residue research has recently been increased with anticipated increased dependence on coal as a source of energy. United States coal reserves are estimated at 3.6 trillion metric tons, from which 396 billion metric tons can economically be mined (SWANSON et al. 1976).

'" Department of Soil and Environmental Sciences, University of California, Riverside, CA 92521.

U Research and Development, Southern California Edison Co., 2244 Walnut Grove, Rosemead, CA 91770.

© 1979 by Springer-Verlag New York Inc.

F. A. Gunther et al. (eds.), Residue Reviews© Springer-Verlag New York Inc. 1979

84 A. L. PAGE, AHMED A. ELSEEWI, AND I. R. STRAUGHAN

United States electric power utilities consume from 60 to 65% of the amount of coal produced (BRACKE'IT 1973). In 1974, the power industry utilized approximately 400 million tons of coal (ASH AT WORK 1975). The magnitude of trace element mobilization into the environment from fossilfuels is substantial and is comparable to that originating from major sedimentary cycles such as river flows and natural sediments (BERTINE and GOLDBERG 1971). Consequently, increasing regulatory measures have been imposed on power industries by federal and local authorities restricting the amount of coal residue entering the atmosphere.

Emission control devices such as mechanical dust collectors, electrostatic precipitators, scrubbers, etc. are now widely used to reduce the amount of particulates discharged into the atmosphere. Bottom ash and boiler slag are residues collected through openings in the bottom of the fire box and fly ash (sometimes referred to as precipitator ash) refers to both the material cleaned from the flue gas and the particulate material that escapes to the atmosphere (Fig. 1). Production of the collected residues combined was estimated in 1974 at 59.5 million tons (ASH AT WORK 1975). Fly ash usually amounts to more than 70% of the total amount of coal ash produced in power plants. Residue from sulfur trapping scrubbers is known as flue gas desulfurization sludge (FGD-sludge). Production of this residue is estimated to reach 35 million tons by the year 1985 when a generating capacity of 42,636-megawatts (MW) will have FGD systems in operation (BERN 1976). An example of the typical amounts of coal ash and FGD-sludge produced at a 1,000-MW power plant equipped with lime/limestone desulfurization systems and burning approximately 2.6 million tons of coal/yr is shown in Table I.

Particulates emitted into the atmosphere are potentially hazardous due to their influence on human and animal health. Despite their relatively small quantities, they are composed of micron- and submicron-size particles and, as such, they are readily respired and may have far reaching effects on human health (NATUSCH and WALLACE 1974, LINTON et al.

Water

Refuse

Cleaned

coal

Power plant

Bottom ash (Boiler slag) (Sluice ash)

Lime/ limestone

slurry

Fly ash Flue gas (Precipita- Desulfur-

tor ash) ization sludge

Fig. 1. General con£guration of a modem coal-fired power plant.

Environmental impacts of fly ash

Table I. Typical coal-residue production in a l,OOD-MW power station controlled with lime/limestone flue gas desulfurization system. G

Residue

Coal ash (80 % solids) Limestone sludge (50% solids) Lime sludge (50% solids)

Total

Production,tons/yr

Wet

422,000 762,800 622,400

1,807,200

Dry

338,000 383,400 311,200

1,032,600

G Modified from BERN (1976). The station consumes 2,556,928 tons of coal annually. The coal contains 3% Sand 13% ash; the scrubber produces 85% SO. removal.

85

1976). Collected residues may also eventually adversely affect the environment since it is expected that huge tonnages will have to be disposed of onto land. Land disposal of these residues may mobilize their beneficial and! or hazardous constituents, thereby affecting, to varying degrees, the quality of surface and ground waters, soils, and vegetation which in turn affect accumulations of these constituents by animals and man.

Estimates made over the past decade indicate that although the electric power utilities produce 30 to 40 million tons of fly ash annually, less than ten percent of this amount is recycled (BRACKE'IT 1973). The published results of completed research demonstrate the potential for recycling or disposal of fly ash on land and point out possible beneficial effects as well as detrimental effects (MARTENS 1971, MARTENS et al. 1970, PLANK and MARTENS 1973, HIGGINS et al. 1976). Prior to 1974, most of the research on fly ash as well as dispersal of stack emissions has been carried out in the midwestern and eastern United States and abroad where coal burning was a significant source of energy. Since climate, soils, indigenous vegetation, and agricultural practices in the western United States are markedly different from those regions where most of the research has been completed, there is a definite need to obtain the necessary information to evaluate the impact of coal utilization in electric power generation in this region.

This review examines some of the existing knowledge on the physical and chemical characteristics of fly ash and evaluates some of their impacts on the environment. Frequently, reference will be made to studies conducted by the authors and others on fly ash derived from western United States coal sources. This fly ash was obtained from a 1,580-MW pulverized coal-fired power station (Mohave Generating Station, Clark County, Nevada). The coal at this station is relatively homogenized by transport in a 275 mile-long slurry pipeline from the mine at Black Mesa, Arizona. The coal is subbituminous and has average ash, S, and BTU Ilb content of eight percent, 0.5 percent, and 12,000, respectively ( SWANSON et al. 1976).


II. Physical properties of fly ash

a) Particle size distribution

Table II shows that about 68% of the fly ash mass is made up of particles with diameters <53 pm (silt and clay size). Wet sedimentation analysis also revealed that the mass fractions of particles with diameters equivalent to sand (>50 pm), silt (2 to 50 pm) and clay «two pm) were, respectively, 32.5, 63.2, and 4.3%. These results agree with those reported by TOWNSEND and HODGSON (1973) who reported a range of fine sand, silt, and clay fractions in four British coal ash samples at 26 to 51, 45 to 70, and 1 to 4%, respectively. REES and SIDRAK (1956) and COPE (1962) also pointed out that coal ash is predominantly made up of particles in the silt and clay size range. In a national survey of fly ash in 21 states conducted by FURR et al. (1977) it was found that about 60% of the samples were of floury consistency and 40% of fine granular texture.

Table II. Particle size distribution and physical indices of fly ash.

Particle-size distribution"

Size range (/Lm)

1-3

3-8 8-13

13-20 20-53 53-105

105-250 250-500 500-1,000

>1,000

Mass fraction (% )

3.5

19.8 15.5 12.7 16.2 16.2 7.2 1.8 2.6 0.1

Physical indices"

Moisture content @ Saturation 50.0% @ 20cb 50.8%

Bulk density, 1.01 g/cm8

Modules of rupture, 0.24 kg/em" Hydraulic conductivity, 2.4 em/ day

"Particles in the range 53 to 1,000 /Lm were separated by mechanical sieving. Those in the range 1 to 53 /Lm were separated by dry centrifugation (Bachomicroclassifier) and subsequently identified by light and electron microscopy.

• Except for saturation moisture, data are derived from CHANG et al. (1977).

The fine texture of fly ash reflects on a number of important physical parameters. For example, water permeability of fly ash was shown by BERN (1976) and TOWNSEND and HODGSON (1973) to be very low (0.01 to 70.5 cm/ day). Infiltration rates for dry and wet ash were reported by COPE (1962) at 76.8 and 12.0 em/day, respectively. COPE (1962) also indicated that lateral hydraulic conductivity in fly ash deposits is much higher than vertical hydraulic conductivity. Low permeability enhances

Environmental impacts of fly ash 87

surface run-off and creation of hydraulic pressures and unstable conditions in fill structures. Leaching of salts and trace metals will, however, be retarded and hence migration of fly ash constituents to ground waters would be limited.

When mixed with soil at rates > ten percent by volume, water retention by soils was increased and water release to plant roots was decreased (CHANG et al. 1977). The authors also pointed out that modules of rupture of soils (a measure of soil strength) was sharply reduced at very low rates of fly ash application to soils, thereby increasing the potential of erosion of fly ash-amended soils.

The range of specific gravity, bulk density, maximum dry density, and optimum moisture content of fly ash as reported by BERN (1976) was 2.1 to 2.6, 1.12 to 1.28 g/ cm3 , 1.19 to 1.49 g/ cm3, and 19.5 to 32%, respectively. TOWNSEND and HODGSON (1973) reported values for bulk density of British coal ashes ranging from 0.99 to 1.73 g/cm3 and COPE (1962) indicated that bulk density ranged from 0.56 to 1.13 g/cm3 • The low bulk density makes fly ash a good material for lightweight building blocks. However, it increases the potential of dust formation which creates problems in transportation and storage of dry fly ash.

b) Microscopic features

The most typical microscopic features of fly ash revealed by scanning electron microscopy of particles < 53 p'm are shown in Figure 2. As seen in this figure, the fly ash matrix is composed predominantly of spherical particles of various size. Large-size spheres have thick walls (Fig. 2a) and are hollow (Fig. 2b). Small size spheres are contained in the wall cavities of large spheres, attached to their surfaces or enclosed within their structure (Fig. 2c). Figure 2d shows evidence of submicron rodshaped crystalline structures attached to the surface of the superfine fly ash particles. Figure 2 also shows that particles of fly ash, regardless of their size, have a strong tendency for aggregation; this property makes it difficult to assess the effective particle diameter of fly ash, since clusters of particles seem to be continuously forming. However, ability to form aggregates appeared to decrease as the particle size was decreased.

The terms "cenospheres" and "plerospheres" have been used by previous investigators to describe the spherical shape of fly ash particles (PAULSON and RAMSDEN 1970, FISHER et al. 1976, and NATUSCH et al. 1975). The former term denotes the hollow spherical particles and the latter term refers to large spheres containing small-size spheres. Micrographs shown in Figure 2 confirmed the observation made by FISHER et al. (1976) that fly ashes derived from western United States coals have more plerospheres than cenospheres (Fig. 2c, upper). FISHER et al. (1976) also indicated that aging of fly ash increased growth of microcrystalline structures from the surface of the spheres, as four mon-old fly ash was shown to contain bundles of needle-shaped microcrystalline structures.

88 A. L. PAGE, AHMED A. ELSEEWI, AND 1. R. STRAUGHAN

(a)

(c)

10 ,urn

H

5,um

I---i

(b)

(d)

Fig. 2. Electron micrographs of fly ash particles of different sizes.

5,um

I---i

5,um

Microscopic examination of the present fly ash (Fig. 2) was performed approximately two yr after it was collected. Few microcrystalline structures were observed (Fig. 2d). The mechanism of cenosphere and plerosphere formation was elucidated by FISHER et al. (1976) and NATUSCH et al. (1975). Gas bubbling and temperature gradients between the outer and inner parts of the parent coal particle were thought to be responsible for the formation of small spheres within the large spheres.

Significance of fly ash particle size and shape in relation to precipitation efficiency in power generation plants was discussed by PAULSON and RAMSDEN (1970). They indicated that the larger the particle fraction > five ,urn in diameter and the more spherical the particle is, the greater the precipitation efficiency of the electrostatic precipitator and, hence,


the lesser the amount of fly ash particulates available for atmospheric emission. One of the most significant consequences of the spherical shape of fly particles is the large surface area available for trace metal bonding onto the surface of these particles. Surface area is inversely related to particle size. SWAINE (1977) indicated that surface area of bulk fly ash is in the order of one m2/ g. KAAIaNEN et al. (1975) showed that the surface area of precipitator ash varies from 3.06 m2/g for inlet ash to 4.76 m2/g for outlet ash. COPE (1962) reported a range of specific surface area of 0.20 to 0.45 m2/g. Accordingly, concentration of several trace elements would be expected to be also inversely related to particle size (see below). Surface predominance of trace elements, influence of particle size, and biological implications have been discussed by NATUSCH and WALLACE

(1974), NATUSCH et al. (1975), and LINTON et al. (1976).

c) Mineralogy of fly ash

SWAINE (1977) indicated that mineral matter in coal contains clay minerals and the minerals pyrite, siderite, calcite, and quartz. He further indicated that the primary products of breakdown of these minerals are, respectively, mullite (AlsSi20 1s ), ferric oxide, ferrous oxide, and calcium oxide, while quartz and some silicates remain unchanged. Quartz, mullite, and iron oxides have been identified in fly ashes from Australian coal sources. NATUSCH et al. (1975) examined the mineralogy of fly ashes from mid-western and western United States coal sources. Using X-ray powder diffraction techniques, they established the presence of a quartz (Si02 ),

mullite, hematite (Fe20s), and magnetite (Fes04) and small amounts of gypsum (CaS04' 2H20) were detected in few western fly ashes. The authors also indicated that fly ash contains a considerable fraction of amorphous material. The presence of large amounts of the amorphous material suggests that the fly ash matrix is predominantly a glass.

Preliminary investigations into the mineralogy of the fly ash used in this study using X-ray powder diffraction technique on bulk and various size fractions showed that the crystalline phase consisted primarily of quartz, gypsum, calcite, Fe and Al oxides, mullite, and chlorite. Mullite and quartz were concentrated in the coarse fraction of fly ash while gypsum and goethite were more concentrated in the fine fractions.

III. Chemical properties of fly ash

a) Elemental composition

The main source of the chemical elements in fly ash is, of course, the source coal. By virtue of its origin, coal contains every naturally occurring element. Substantial fractions of the amount of elements entering in coal could, however, be lost in the process of coal cleaning (see Fig. 1). FORD et al. (1976) found that up to 67, 63, 76, 68, and 61 % of the amounts of


As, Pb, Mn, Hg, and Se were removed in the coal cleaning process, respectively. Upon combustion, elements contained in coal are redistributed or partitioned between the various types of residues produced. Redistribution of elements is influenced by the power plant configuration and by the properties of the elements and their compounds. The power plant configuration determines the type and amount of the residue and elements and their compounds vary in their boiling, subliming, and volatilization points. For example, ash from cyclone-fired plants is composed of 80 to 85% boiler slag and 15 to 20% fly ash, wet bottom-pulverized coal furnaces produce ash composed of equal amounts of bottom and fly ash, and those of pulverized coal-dry bottom furnaces produce ashes that are 20 to 25% bottom ash and 75 to 80% fly ash (BERN 1976).

The coarse-textured bottom ash and boiler slag escape through openings located in the bottom of the fire box, whereas because of its relatively smaller particle size, fly ash leaves the fire box in the flue gas stream where its coarser particles are subsequently captured by electrostatic precipitators. Finer particles pass virtually uncaptured through precipitators and escape to the atmosphere through the stack, unless captured by additional scrubbing devices.

Elements and their compounds which volatilize either partially or completely at the prevailing temperature of combustion ("""1,500°C) recondense, as the temperature drops, presumably on the surface of particles in amounts which depend to a large extent upon the surface area available (DAVISON et al. 1974, NATUSCH et al. 1975). The smaller the particle size, the larger the surface area and, hence, the greater the concentration of the elements would be. Table III gives a list of some possible species boiling or subliming below and above 1,550°C and also gives the relative order of volatility of these elements and of their possible compounds. Elements that are largely volatile, e.g., Hg, Se, Sb, As, CI, F, and I, would be greatly enriched in fly ash and/or fine particulates emitted to the atmosphere. ANDERSON and SMITH (1977) estimated that 97 % of the amount of Hg in coal is lost to the atmosphere. Because of this enrichment behavior, BERTINE and GOLDBERG (1971) estimated that the amounts of As, Hg, Cd, Sn, Sb, Pb, Zn, TI, Ag, and Bi mobilized into the environment from fossil-fuel combustion are probably 20 times greater than would be predicted on the basis of the chemical composition of coal.

Studies on fly ash from Belgian coal sources indicated that CI, Zn, Cu, Ag, Se, Br, Sb, and I were enriched in the fly ash relative to the source coal (BLOCK and DAMS 1976). Progressive enrichment in Cu, Zn, As, Mo, Sb, Pb, Se, and Hg between coal, bottom ash, inlet precipitator ash, and outlet precipitator ash were reported by KAAIaNEN et al. (1975). The relative distribution of elements between bottom ash, fly ash, and particulates in the flue gas, obtained from a mass-balance study in a 350-MW power plant, is shown in Table IV. An element is enriched when the percentage of this element exceeds that of the residue itself. Table IV shows no enrichment in the bottom ash. Under this combustion configuration,


Table ID. Boiling points and relative order of volatility of possible inorganic species evolved during coal combustion.B

Species boiling or subliming, Species boiling or subliming, <I550°C >1550°C

As, As20" As2S. Ba

Bi Ca Cd, CdO, CdS Cr(CO)6, CrCL, CrS (155°C) K Mg Ni(CO). PbCI2, PbO, PbS Rb S

Se, SeO., SeO. Sb, Sb.S., Sb.O.

SnS Sr TI, TkO, TI.Oa Zn,ZnS

AI, AI.O. BaO BeO Bi20. C CaO Co, CoO, CoS Cr, Cr.O. Cu, CuO Fe, Fe.O., FeaO., FeO MgO, MgS Mn, MnO, MnO. Ni, NiO Pb (l620-1750°C) Si, SiO. Sn, Sn02 srO Ti, TiO., TiO U, UO.

ZnO

Relative order of volatility Oxides, sulfates, carbonates, silicates, and phosphates: As, Hg > Cd > Pb, Bi, TI > Ag, Zn > Cu, Ga > Sn> Li, Na, K, Rb, Cs

Elemental state: Hg > As > Cd > Zn > Sb "'" Bi > TI > Mn > Ag, Sn, Cu > Ga, Ge

Sulfides: As, Hg > Sn, Ge "'" Cd > Sb, Pb "'" Bi > Zn, Tl > Cu > Fe, Co, Ni,

Mn,Ag

BFrom DAVISON et al. (1974) and BERTINE and GOLDBERG (1971).

91

bottom ash was depleted in Sb, As, B, Cd, CI, Co, Cu, F, Pb, Hg, Mo, Ni, Be, Se, Ag, and S. Consequently, Sb, As, B, Cd, Co, Cu, F, and Pb were enriched in the fly ash and Cr, I, Hg, Mo, Ni, Se, Ag, S, and V were enriched in particulates of the flue gas. Chloride, Hg, and S entering the station from coal are almost completely accumulated in the finer particles of the flue gas. Table IV also shows that AI, Cu, Fe, Mg, Mn, Ti, U, and Zn are distributed nearly equally between the residues, indicating neither enrichment nor depletion of these elements in any of the residues shown.

Total concentration of major, minor, and trace elements in coal, fly ash, and soil are given in Tables V and VI. Minor and trace element content of two mechanically-sieved size fractions (>250 and <53 /-tm) of fly ash are also shown in Table VI. Examination of data in Table V shows the fly ash matrix to be dominantly made up of AI, Si, Ca, Fe, Mg, Na,


Table IV. Fraction of elements entering with coal discharged in vari-ous coal residues.G

Fraction ( % )

Sluice ash Precipitator ash Flue gas Element (22.2% ) (77.1 %) (0.7% )

Aluminum (Al) 20.5 78.8 0.7 Antimony (Sb) 2.7 93.4 3.9 Arsenic (As) 0.8 99.1 0.05 Barium (Ba) 16.0 83.9 <0.09 Beryllium (Be) 16.9 81.0 <2.0 Boron (B) 12.1 83.2 4.7 Cadmium (Cd) <15.7 80.5 <3.8 Calcium (Ca) 18.5 80.7 0.8 Chlorine (CI) 16.0 3.8 80.2 Chromium (Cr) 13.9 73.7 12.4 Cobalt (Co) 15.6 82.9 1.5 Copper (Cu) 12.7 86.5 0.8 Fluorine (F) 1.1 91.3 7.6 Iron (Fe) 27.9 71.3 0.8 Lead (Pb) 10.3 82.2 7.5 Magnesium (Mg) 17.2 82:0 0.8 Manganese (Mn) 17.3 81.5 1.2 Mercury (Hg) 2.1 <0.1 97.9 Molybdenum (Mo) 12.8 77.8 9.4 Nickel (Ni) 13.6 68.2 18.2 Selenium (Se) 1.4 60.9 27.7 Silver (Ag) 3.2 95.5 1.3 Sulfur (S) 3.4 8.8 87.8 Titanium (Ti) 21.1 78.3 0.6 Uranium (U) 18.0 80.5 1.5 Vanadium (V) 15.3 82.3 2.4 Zinc (Zn) 29.4 68.0 2.6

G Data combined from EPA (1975). This is a 350-MW station burning subbituminous coal at the rate of 124 metric tons/hr and equipped with an electrostatic precipitator.

K, S, Ba, and Sr. These elements constitute about 50% of the dry weight of fly ash.

FISHER et al. (1976) indicated that the chemical composition of fly ash derived from western United States coal sources could be approximated by the formula, Sil.ooAI0,45Cao.o51Nao.o47Feo.oa9Mgo.o2oKo.o17Tio.ol1' This composition, the authors stated, is consistent with fly ash derived from coal with intrusions of clay minerals, particularly kaolinite and lesser amounts of quartz, and also with intrusions of CaCOa. The CaCOa content of the fly ash studied ranged from two to four percent and that of gypsum was about 3.1 %. Intrusions of CaCOa, gypsum, and pyrite in the source coal of the fly ash under study may explain the relatively high concentrations of Ca (10.0% ) and Fe (3.3%). Although the Ca concentration shown in


Table V. Maior elements in fly ash·, coal, and soil.

Total concentration (% )

Fly ash Coal" (Typical Soild

Element This study Lit. values' conc.) (Range)

Al 18.0 0.1-17.3 1.4 4--30 Si ::'".0 1.41-28.6 2.6 25-33 Ca 10.3 0.11-22.2 0.54 0.7-50 Fe 3.3 1-29 1.6 0.7-55 Mg 1.7 0.04-7.6 0.12 0.06-0.6 Na 1.2 0.01-2.03 0.06 0.04--3.0 K 0.54 0.15-3.5 0.18 0.04--3.0 S 0.4 0.1-1.5 2.0 0.01-2.0 P 0.04 0.04-0.8 0.05 0.005--0.2 N 0.02 1.1 0.01-1 Ba 0.37 0.011-1.0 0.015 0.01-0.3 Sr 0.18 0.006--0.39 0.010 0.05--0.4

• AI, Si, and Mg were determined spectrographically in a Na2CO.-fusion extract (BRADFORD et a1. 1978). S was measured turbidemetrically after BRADSLEY and LANCASTER (1960), P colorimetric ally in a N a2CO.-fusion extract, N with a semi-micro Kjeldahl method. Ca, Fe, Na, K, Ba, and Sr by neutron-activation analysis.

• Literature values were taken mainly from BERN (1976), BLOCK and DAMS (1975 and 1976), SWAINE (1977), SWANSON (1972), FURR et al. (1977), and NATUSCH et al. (1975).

C From SWANSEN et al. (1976). d Taken mainly from ALLAWAY (1968) and LISK (1972).

93

Table V is within the range commonly found in fly ash, it is only comparable with Ca-rich fly ash. The highest value of Ca concentration in seven fly ashes derived from western United States coal sources as reported by SWANSON (1972) was 12.7% in fly ash from the same power generation plant as this fly ash. Chemical composition of coal samples of various coal ranks in the United States shown by SWANSON et al. (1976) demonstrates that subbituminous and lignite coals contain considerably more Ca than anthracite or bituminous coals. The study of NATUSCH et al. (1975) also showed fly ashes derived from western lignite and subbituminous coals to be higher in Ca, Si, and trace elements and lower in S content than those derived from eastern bituminous and anthracite coals.

Because most of the S in coal is discharged in the flue gas (Table IV), concentration of S in the fly ash (0.4%) is considerably less than the average concentration in coal (2.0%) (Table V). Virtually all S in fly ash is in a soluble form as revealed by the NH40Ac analysis (BRADSLEY and LANCASTER 1960) which yielded a S04-S concentration of also 0.4%. The significance of this finding in terms of soil and vegetation enrichment of fly ash-derived S is discussed in a subsequent section of this review. Ba and Sr concentrations in fly ash are substantially greater than their


Table VI. Minor and trace elements in size fractions of fly ash and in coal and soil.

Total concentration (p.g/g)

FlyashB

Coalb

>250 <53 Lit. ( typical Soil Element I'm I'm Bulk valuesb cone.) (range)

As 10 17 14 2.3-6,300 15 0.1-40 B 148 300 237 10-618 50 2-100 Cd 0.7 1.5 1.4 0.7-130 1.3 0.01-7.0 Ce 84 112 108 22-320 7.7 50 Co 10 15 13 7-520 7 1-40 Cr 52 65 64 10-1,000 15 5-3,000 Cs 4.1 4.6 4.9 1.5-18 0.4 Cu 45 70 50 14-2,800 19 2-100 Eu 0.9 1.2 1.3 14.3 0.45 Ga 27 34 29 13-320 7 15-70 Hf 6 8.1 7.9 3.5-11 0.6 Hg 0.02-1.0 0.18 La 47 64 60 17-104 6.1 30 Lu 0.9 1.3 1.2 0.5-1.5 0.08 Mn 98 121 122 58-3,000 100 100-4,000 Mo 7 13 8.8 7-160 3 0.2-5.0 Nd 35 48 45 37 Ni 29 46 50 6.3-4,300 15 10-1,000 Pb 38 52 45 3.1-5,000 16 2-100 Rb 44 51 53 36-300 2.9 30-600 Sb 2.9 4.4 3.8 0.8-202 1.1 0.6-10 Sc 12 17 16 3.7-141 3 10-25 Se 16 22 19 0.2-134 4.1 0.1-2.0 Sm 6.8 9.2 8.9 5.4-24 0.4 Tb 0.8 1.2 1.0 1.6 0.1 Th 16 21 21 13-68 1.9 U 6.3 7.7 7.4 0.8-19 1.6 20-250 V 11.9 50-5,000 20 Yb 3.1 4.5 4.0 1.7-7.0 1 Zn 76 137 99 10-3,500 39 10-300 Zr 140 187 183 50-1,286 30 60-2,000

B B, Cd, Cu, Mn, and Pb were extracted in 4N HNO. and measured by atomic ab-sorption spectroscopy, except for B which was analyzed colorimetrically. The remain-der of the elements were measured by the neutron-activation technique.

b See Table V for references.

concentrations in coal (Table V). Enrichment of Sr in fly ash has enabled the monitoring of the extent of deposition of stack-emitted fly ash onto soils and vegetation in the vicinity of coal-fired power plants (BRADFORD

et al. 1978). Concentrations of major elements in fly ash shown in Table V are in

general comparable to their concentrations in soil. However, in situations where fly ash has been mixed with soils, enrichments in soluble Ca and


Sr (PHUNG et al. 1978) and S (ELSEEWI et al. 1978 a) were observed. Because Al in fly ash is mostly tied up in insoluble alumino-silicate structures, the solubility of this element in soil and hence its biological toxicity is limited.

Minor and trace elements in fly ash (Table VI) exhibit concentrations that are, in most cases, considerably higher than their concentrations in coal. Table VI also shows that although substantial amounts of minor and trace elements could be mobilized into the environment from fly ash, their concentrations in fly ash and soil are, in general, comparable. Some exceptions, however, are B, Mo and Se. Concentrations of these biologically toxic elements (B is toxic to plants and Mo and Se are toxic to animals) in fly ash greatly exceed their concentrations in soil (Table VI). The availability of fly ash-derived Band Mo to plants has been demonstrated by ELSEEWI et al. (1978 a), PLANK and MARTENS (1974), and DORAN and MARTENS (1972) . Availability of fly ash-Se to plants and to animals feeding on plants cultivated in fly ash has been reported by STOEWSAND et al. (1978). Current investigations carried out by the authors indicate that plants grown on fly ash-amended soils are enriched in Se (see below).

b) Composition of sized fractions of fly ash

Except possibly for Cs, minor and trace elements shown in Table VI were increased in the fine fraction ( < 53 ttm) relative to the coarse fraction (> 250 ttm). Detailed fractionation of the < 53 ttm fraction showed greater dependence of element concentration on particle size (Table VII). Ca, Fe, S, Na, P, Zn, Mo, Cu, Ni, Cd, and Pb were markedly enriched in the one to three ttm fraction relative to the 20 to 53 ttm fraction. Concentration ratio in these two size fractions varied from 1.5 for K to 51.3 for Zn. Manganese, however, tended to show an opposite trend.

Dependence of concentration on particle size of fly ash has also been observed by DAVISON et al. (1974) for Pb, Tl, Sb, Cd, Se, As, Zn, Ni, Cr, and S, by LEE and VON LEHMDEN (1973) for B, Cd, Cr, Cu, Mn, Ni, Pb, and U, and by LEE et al. (1975) for Cr, Pb, Sb, Zn, and Se. Similar observations were made by KAAKINEN et al. (1975) for Cu, Zn, As, Mo, Sb, Pb, Se, and Hg. Using a specially designed aerodynamic sizing system, ONDOV et al. (1976) were able to obtain samples of in situ fly ash from a western power generation plant in the size range three to 15 ttm. As, Be, Cd, Co, Cr, Cu, Ga, Mn, Mo, Ni, Pb, Sb, Se, U, V, W, and Zn increased with decreasing particle size from > 15 to < three ttm. Their data, however, showed no enrichment in the small particles for AI, Ca, Fe, K, Na, Ni, Ti, Mg, Ce, Cs, Dy, Eu, Hf, La, Nd, Rb, Sc, Sm, Sr, Ta, Tb, Th, and Yb. NATUSCH et al. (1975) indicated that there is a general agreement that the elements As, Cd, Cu, Ga, Mo, Pb, S, Sb, Se, TI, and Zn tend to increase in concentration with decreasing particle size of fly ash. The elements AI, Ba, Ca, Ce, Co, Eu, Fe, Hf, La, Mg, Mn, Rb, Sc, Si, Sm, Sr,

96 A. L. PAGE, AHMED A. ELSEEVI'I, AND I. R. STRAUGHAN

Table VII. Trace elements in sized fractions of fly ash in the 1 to 53 I'm range."

Percentage

1-3 .>-8 8-13 13-20 20-53 Element (I'm) (I'm) (I'm) (I'm) (I'm)

Ca 9.5 11.8 12.5 7.7 4.4 Fe 3.0 2.6 2.4 2.7 1.9 Mg 1.5 1.5 1.3 1.3 1.1 S 1.3 0.7 0.6 0.3 0.2 K 1.2 0.9 0.8 0.8 Na 1.2 0.8 0.4 0.3 0.2

(I'g/g)

P 1540 1510 810 670 580 Zn 323 168 90 45 6.3 Mo 25.6 23.3 17.9 11.7 5.3 Cu 23.1 21.4 7.3 9.0 4.2 Ni 62.5 35.5 21.5 24.3 6.8 Cd 2.2 0.1 0.2 0.4 0.3 Pb 90.0 20.7 9.4 11.8 4.7 Mn 102 94 96 96 170

a Elements were extracted by Na2CO. fusion. S was measured turbi-dimetrically, P and Mo colorimetrically, K flame photometrically, and the remainder of the elements by atomic absorption spectroscopy.

Ta, Th, and Ti show little of this tendency and the elements Be, Cr, K, Na, Ni, Sc, U, and V exhibit behavior which is intermediate between the above two groups.

As mentioned earlier, mechanism of small particle enrichment is related to surface phenomenon. LINTON et al. (1976) have shown that Be, Ca, Cr, K, Li, Mn, Na, P, Pb, S, Tl, V, and Zn are preferentially concentrated on the surface of the fly ash particle as compared to a distance 500 A from the surface inside the particle. SWAINE (1977) indicated that the outermost layer on the surface of some fly ashes is chiefly composed of H 2S04 • This perhaps enables some trace elements to be enriched at the surface of the fly ash particle.

c) Composition of fly ash extracts

Knowledge of the amount of water-soluble constituents of fly ash is important for it gives an estimate of these constituents immediately available for the biological systems. Atmospheric emission and land disposal of fly ash subject the material to leaching by atmospheric moisture and by water used to pond the ash or to irrigate fly ash-amended soils and vegetation establishments on fly ash deposits. Composition of fly ash

Tab

le V

III.

Wat

er-s

olub

le c

onst

itue

nts

of

1:1

and

1:2

wat

er-f

ly a

sh e

xtra

cts

and

of

1:2

wat

er-f

ly a

sh e

xtra

ct a

cidi

fied

to

pH 6

.5.

Itg/

ml

Ext

ract

p

H

EC

(W

ater

:fly

ash)

(m

mh

o/c

m)

Ca

Mg

N

a K

O

H

CO

s C

l B

1:1

12.4

12

.3

1,00

0 0.

3 22

0 13

96

1 13

0 0

1:2

12.3

11

.8

660

trac

e 49

0 27

85

0 19

2 95

<

0.6

1:

2 a

cidi

fied

" 6.

5 38

,234

84

9 90

0 <

10

0

65

" F

rom

BR

AD

FO

RD

et

al.

(197

6).

The

wat

er-f

ly a

sh m

ixtu

re w

as a

cidi

fied

wit

h H

Cl

to p

H 6

.5.

til

::to Ji

C!

l 0

~ la

O=

C!l

C!l

0 0

0 p.

. =

..... ~

S· aa

a ~

""8'

~tIl

e.~=@'

~ .

...

0.. S

C!l

til

§ ti

l

-<: ~ o..~.

::::I:

t>C

Il&

t"'"'

<! ~

C!l

,.., ~

g, =

O

"'O

"'o

l:T

' Cl>

I

Cl>

Cl>

Cl>~~t5'

~ggg:

\l:>

I-!

I-!

!'O.

1tC

l>tI

lp

\l:>

~~g.

Cil~O"'@'

g.~ ~

§.

g: ..

.. \l:>

~.

'<! ~ ~ ()q

e. 0

g:.

~S

Cl>

S-~.

'0

C

l>::;

t ~

........

. 1=;

Cl>

'O

Cl>

§:

::I:

;-e:.

V~

0 s;

' §

~ e;

S'

'-

' Cl>

S

~ '0

S·

;:

I. I

-!

\l:>

g:. cP

I

t:o::1

t:l ~. i s· ~ s.. ~ ~ ~


Ca, Na, COs, and OH being the dominant ions. Concentrations of Mg and B are negligible and no HCOs was detected. Table VIII also shows that concentrations of water-soluble constituents are affected by water-fly ash ratio and by the acidity of the extract. Doubling the water ratio increased electrical conductivity, Ca, and OH and decreased Na, and acidifying the extract to pH 6.5 resulted in fin enormous increase in the concentration of Ca, Mg, Na, and B. The increase in electrical conductivity of the fly ash water extracts is probably related to increased production of OH ions upon dilution. SHANNON and FINE (1974) also noted an increase in Ca, Mg, and Fe with increased water-fly ash ratios of lignite fly ashes. They also noted that the rate of Na release was much less than that of these ions.

The high alkalinity of the extracts is undoubtedly attributed to the very high concentrations of OH ions (Table VIII). This high alkalinity also appears to be characteristic of most fly ashes, particularly those derived from western lignite and subbituminous coal sources. In the study of FURR et al. (1977), the pH of fly ashes from 21 states was measured, but only seven fly ashes had an acidic pH (4.2 to 5.9). Those were fly ashes from the state of Delaware, Kentucky, Maryland, New Hampshire, New York, Ohio, and South Carolina. The remaining 14 states produced fly ash with pH in the range of 8.2 to 11.8. THEIS and WmTH (1977) indicated that the relative amounts of lime and amorphous iron oxides on the surface of fly ash define the ultimate acidic or basic character of fly ash in solution. Their study indicated that a ratio of approximately three to one (Fe to Ca) is a rough delineation of the acid or basic nature of fly ash.

The electrical conductivity (EC) of water extracts from fly ash shown in Table VIII is also very high, reflecting high salinity of fly ash water solutions. This property is also characteristic of essentially all fly ash extracts. TOWNSEND and HODGSON (1973) reported a range of EC of fly ash extracts of eight to 13 mmhol cm. EfHuents from fly ash disposal sites will, thus, be enriched in soluble salts. Also application of fly ash to soil as an amendment will result in increasing soil salinity, perhaps to levels critical for salt-sensitive plants. The effect of fly ash on soil salinity is discussed in greater detail below.

Concentrations of Ca and OH in fly ash extracts tend to change with time. Equilibrating 1: 1 fly ash-water extracts over a period of 30 days resulted in a marked reduction in Ca and OH ions and increased concentrations of COs (Fig. 3). As a result, total salt content was reduced by about 30% at the end of the 30-day equilibration period. The increase in COs and the decrease in Ca and OH is apparently a result of an "OH + cot type reaction forming COs which reacts with Ca to form insoluble CaCOs.

TOWNSEND and HODGSON (1973) also noted the change of fly ashwater extracts over a period of nine hr. Their data indicate that in the first two to four hr pH and soluble salts rose sharply while B was sharply

5000

~ 1400 'C; :s. ~ 1200 o 'p III

~ 1000 ., " c: /3 800

600

400


pH

Na

200~

2 4 6

Equilibration time (days)

I 12.5 .,

(ij

12 " <II

I C.

99

Fig. 3. Total soluble salt, pH, and ion solubility of 1; 1 fly ash-water suspensions equilibrated for 30 days at room temperature in a closed system.

decreased. Ca and OH also increased while S04 was reduced. Mter four hr of equilibration, concentrations of most components leveled off.

Low solubility of a number of trace elements is expected based upon the high alkalinity of the ash extracts and the excessive OH concentration. Extractability of As, B, Be, Cd, Cr, Cu, F, Mo, Se, V, and Zn in water, HCI, HNOs, and citric acid was investigated by DREESEN et al. (1977). An increase in extractability was noticed for all elements as the acidity of the extract was increased. Be, Cr, Cu, V, and Zn showed considerably less extractability than other trace metals. The authors indicated that surface predominance of the element and ionic species are two important factors influencing the distribution of the trace elements in extracts and effiuents. They indicated that Mo, F, Se, As, and Cd have a surface predominance and, therefore, were readily extractable in acids. Elements which form anionic species such as B, Mo, F, Se, Cr, and V remain relatively soluble in alkaline environment while the more metallic cations would be precipitated. THEIS and WIRTH (1977) observed increased release of Cd, Cr, Cu, Pb, Ni, and Zn from fly ash as the pH of 1:5 extract


of fly ash was decreased from 12 to 3. Se, however, showed a substantial increase at pH 12 which was attributed to -educed concentrations of Fe and other trace metals which could precipitate Se at this pH.

The relative leachability of elements from fly ash was related to the position of the element in the fly ash matrix by NATUSCH et al. (1975). Their data indicated that matrix elements such as Fe, Si, Ba, Ca, and Mg exhibit every low extractability. Elements which are mown to be enhanced in the surface layer but which have most of their mass in the particle interior such as K and Na exhibit intermediate extractability, while elements which predominate in the surface layer, e.g., Cd, Co, Li, Mn, P, TI, and Zn, exhibit substantial extractability.

IV. Environmental impact assessment

The impact of fly ash on the environment comes either from the disposal of massive quantities captured or from fine particulates emitted to the atmosphere and eventually falling on the surrounding environs. Estimates of the latter quantities are difficult to obtain because of the obvious difficulties encountered in their measurements.

a) Atmospheric emission

An estimate of the amount of 33 elements discharged into the atmosphere which was obtained from a mass balance study of three southern United States coal-fired plants and three midwestern plants is illustrated in Table IX. This table also includes the estimated amounts of fly ash discharged daily into the atmosphere from the three midwestern plants. Among the factors influencing stack emission of elements are their relative distribution in coal, the power plant configuration, i.e., boiler configuration and type of emission control devices available, and properties of the elements and their compounds. Of the elements shown in Table IX, AI, Ca, CI, Fe, Mg, S, and Ti are relatively abundant in all coal ranks compared to minor and trace elements shown in the table. Amounts of the former elements released from stack emission are, therefore, substantially greater than the amounts of minor and trace elements released. Data of the midwestern plants show that the three stations combined emit approximately 172 tons of S daily into the atmosphere for every 1,000 MW of power produced. The data also show that stations equipped with electrostatic precipitors and/or S scrubbers (stations #1 and #2) emit substantially lesser quantities of elements and fly ash than those equipped with mechanical dust collectors (station # 3). The latter station emits about 120 tons/day of fly ash whereas the former stations emit four to five tons/ day. As shown earlier, the higher the volatility of the elements and its compounds, the more likely it will be almost completely discharged into the finer particles of coal residues.

Deposition of these quantities to ground level depends on particle


Table IX. Estimates of daily atrrwspheric emission of elements from coal-fired power plants.

Midwestern stations' Three southern (kg/day/1,000 MW)

stations combined" Element (kg/day) #1 #2 #3

AI 510 486 435 8,582 Ag 0.02 0.005 <0.1 As 3.4 0.62 om 13.7 B 19.9 12.4 783.4 Ba 5.1 12.2 <3.7 <87 Be 0.04 <0.06 0.35 Br 96.4 Ca 170 1,617 759 21,145 Cd 0.2 0.24 0.02 0.81 CI 535 411 510 Co 0.3 0.39 0.20 3.6 Cr 5.1 22.4 8.1 51 Cs 0.2 Cu 1.74 1.74 24.9 F 24.9 38.6 435.3 Fe 3,397 261.2 136.8 10,946 Hg 1.7 0.66 0.24 1.2 K 153 Mg 849 261.2 112 5,224 Mn 3.3 6.6 2.5 84.6 Mo 10.7 0.4 41.1 Na 68 Ni 3.0 4.1 37.3 Pb 3.3 1.5 0.8 4.4 Rb 1.2 S 150,000 34,827 33,584 103,239 Sb 3.3 0.05 0.04 2.23 Se 6.9 0.51 1.70 4.10 Th 0.2 Ti 68 23.6 29.9 236 U 0.3 0.17 0.04 1.74 V 6.8 12.2 3.6 34.8 Zn 33.9 7.5 1.2 41.1

fly ash 4,851 3,607 120,652

"Data modified from ANDREN et al. (1974) for three coal-fired steam plants with a combined daily consumption of coal of 25,000 tons.

• Data modified from EPA (1975): station # 1 is 330-MW, equipped with a venturi scrubber, bums subbituminous coal at the rate of 3,084 tons/ day; sta-tion #2 is 350-MW, equipped with electrostatic precipitators, bums subbitumi-nous coal at the rate of 2,997 tons/day; station #3 is 250-MW, has a cyclonic boiler equipped with mechanical cyclone particulate collector, bums lignite coal at the rate of 2,550 tons/ day. Amounts of elements and fly ash shown were normalized for 1,000-MW power capacity.


size, wind speed and direction, atmospheric moisture, and topography of the land, to mention a few. A computer modeling study conducted by JURINAK et al. (1977) pointed out that the maximum deposition rate of stack-derived By ash originating from a 3,OOO-MW coal-fired power plant located in a western semi-arid enviroJlment was 50 kg/km2/mo. ZOLLER et al. (1974) evaluated the amount of By ash emitted from two 355-MW units of an eastern coal burning power plant consuming 116 tons/ hr/unit of coal with 10 percent ash and one percent S at 1.2 tons/hr.

The impact of fine particulates of By ash on human health has been discussed by NATUSCH and WALLACE (1974) and LINTON et al. (1976). They indicated that Pb, Cd, Sb, Se, Ni, V, Zn, Co, Br, Mn, and S02, which are potentially toxic, predominate in small, lung depositing particles emitted from high temperature combustion sources. The dependence of lung deposition on particle size is illustrated in Figure 4. Particles with diameters Lone p'm are deposited almost exclusively in the lung.

Based upon maximum deposition rate found by JURINAK et al. (1977) and the chemical composition of the one-to-three p'm fraction of By ash reported by ONDOV et al. (1976) we have estimated potential contamination of soil and vegetation in regions adjacent to coal-fired power plants (Tables X and XI) . Except possibly for Se, short- and long-term trace

1.0 NISOPharynge~a,I ~~~

0.9

0.8

Particl_ diameter (I'm)

Fig. 4. Respiratory depOSition efficiencies as a function of particle size (from U.S. Department of Health, Education, and Welfare, 1969).


metal enrichment of soils and vegetation from stack-derived fly ash is negligible. Similar results were also reported by LYON (1977) who observed no enrichment in soil sampled a mile or more from a coal-fired steam plant. VAUGHN et al. (1975) using a different atmospheric deposition model also presents data which are quite comparable to those presented in Table X.

KLEIN and RUSSELL (1973) found that soils around a coal-burning plant (650 MW, 90% efficiency) were !10wever enriched in Ag, Cd, Co, Cr, Cu, Fe, Hg, Ni, Ti, and Zn. They also found that plant materials (native grass, maple leave.>, and pine needles) were enriched in Cd, Fe, Ni, and Zn. Soil enrichment was correlated with wind patterns and metal content of coal, except for Hg. BRADFORD et al. (1978) showed limited decrease in Ca, Mg, B, Ba, and Sr in soil-saturation extracts as the distance from the Mojave Generating Station was increased. They concluded, however, that four years of operation resulted in no measurable contamination of either soil or vegetation in the region surrounding the plant. They also concluded that Sr concentration in the soil-saturation extract may be used as an indication of the extent of fall-out from power plants.

Table X. Estimated maximum deposition of trace elements onto soil adjacent to a coal-fired power plant."

p.g element/ g soil

Amount deposited Common cone.

Element Annual Lifetimeb Typical Range

As 0.00039 0.014 6 0.1-40 Cd 0.000014 0.0005 0.06 0.01-7 Ph 0.00082 0.029 15 2-200 Mo 0.00015 0.0052 2 0.2-5 Se 0.00059 0.021 0.2 0.01-2 U 0.000087 0.003 1 0.1-10 Zn 0.00163 0.057 50 10-300 Sh 0.000061 0.0021 6 2-10 Be 0.00003 0.0011 6 0.1-40 Cr 0.00019 0.0066 100 5-3,000 Co 0.000062 0.0022 8 1-40 Cu 0.00041 0.0144 20 2-100 Ga 0.00053 0.0182 30 0.4-300 Ni 0.00012 0.0042 40 10-1,000 Th 0.0009 0.0032 5 0.1-12 V 0.00097 0.034 100 20-500

a Derived from data of JURINAK et al. (1977) and ONDOV et al. (1976). 3,000-MW power plant, Western USA Coal, electrostatic precipitator efficiency 99%.

b Assuming 35-year lifetime.


Table XI. Estimated trace element contamination of vegetation arising from emission from a coal-fired

power plant.G

p.g element! g dry matter

Cone. in/on Element vegetationb Typical cone.

As 0.07 0.4 Cd 0.002 0.2 Cr 0.034 1.5 Cu 0.074 10 Ph 0.15 3 Mo 0.027 1 Se 0.107 0.2 Zn 0.30 25 Sh 0.011 0.06 Be 0.006 0.03 Co 0.011 <1.0 Ga 0.096 1.2 Ni 0.021 5 Th 0.016 0.05 U 0.016 0.04 V 0.18 1

G Derived from JURINAK et al. (1977) and ONDOV

et al. (1976). 3,000-MW power plant, Western USA Coal, electrostatic precipitator efficiency 99%.

b Assuming 100% canopy, yield 3,770 kg dry matter/ha, 4 mon exposure, particulates deposited are < three p.m, and all particulates deposited remain with harvested crop.

b) Precipitator fly ash

AI; mentioned earlier, the bulk of coal ash collected in modem power generating plants is made up of precipitator ash. Large tonnages of this material become available for disposal each year. They are collected in dry or slightly moist condition or are converted into an aqueous slurry and transported through pipes to disposal sites adjacent to power plants where they are lagooned. Only seven to 11 % of the amount of fly ash produced is used. The main uses as outlined by BRACKE'IT (1973), in order of decreasing tonnages, are: manufacturing cement, mine fire center, antiskid on winter roads, soil conditioner, fill material, airport pavement, fertilizer filler, rubber filler, and asphaltic water course aggregates.

The composition of water effiuents from fly ash derived from laboratory studies has been discussed in a previous section of this review. Long-term impacts on surface and ground waters resulting from water percolation through ash piles stored in the vicinity of coal-fired power


plants have not been evaluated, however. Composition of fly ash-amended soils has been investigated under field and greenhouse conditions.

1. Water-soluble constituents of fly ash-amended soils.--Greenhouse experiments with calcareous and acid soils amended with up to and including eight percent fly ash by weight and cropped to various crop species have demonstrated that addition of the waste material to soil tends to increase soil pH, soil salinity, soluble Ca, Mg, Na, S, and Band to decrease the concentrations of water soluble P in soil. Examples of these findings are illustrated in Figures 5 through 8. Data shown in these figures are from post-harvest analyses of the soil; therefore, they may reflect differences due to crop species as they also reflect inherent differences due to soil type.

12

11

10

J: 9 a.

'0 8 I/)

7

6

5

(A)

" ...

Calcareous soil, initial

~ _______ --o

.... "'''' Acid soil, initial ,0'"

4L--L~2--~3--~4--~5--6L-~7~~8

9 (B)

8

:J 7 " CT G> E 6

> .. 5 :~

iii :!!4 ..

G> :c 3 .. .. . ~ 2

I-

Fly ash in soil (% by wt)

234 5 678

Fig. 5. pH and titratable alkalinity of fly ash-amended soils. Final pH in A was measured after 12 months of cropping to alfalfa.

The increase in pH of the calcareous soil was diminished at the end of a 12-mon cropping period (Fig. 5A). Although the final pH of the acid soil was considerably less than the initial values, it remained well above the pH of the control soil, particularly at the four and eight percent fly ash treatments. Since most soils of arid and semi-arid environments of the western United States are calcareous, their high buffering capacity will resist changes in pH induced by fly ash additions. The increase in pH of the acid soil suggests that fly ash could be used as a liming material to raise low pH of acid soils in the humid regions. Laboratory studies with this fly ash showed its neutralizing capacity of acid soils to be 20 to 30% that of reagent grade CaCOa (PHUNG et al. 1978). PLANK et al. ( 1975) noted that two yr after addition of weathered fly ash with a pH


of 7.8 at the rate of 144 mt/ha (=<6.4% ), the pH of Groseclose soil increased from 5.8 to 6.8. In displaced soil solutions from this soil, the average Ca + Mg increased from 3.2 to 5.0 meq/L and B increased from 0.3 to 0.5 p,g/ml.

Soil salinity increased five- to sixfold by application of eight percent fly ash to soil (Fig. 6A). This increase was accompanied by substantial gains in water-soluble Ca and Mg (Fig. 6B). Sodium increased slightly and K was not significantly affected (data not shown). Data presented by MULFORD and MARTENS (1971) indicated that applications of about five percent fly ash to Tatum silt loam soil increased the EC of the soil saturation extract from about one to about four mmho/cm. Weathered fly ash should, however, cause little increase in the soluble salt content of soils. TOWNSEND and HODGSON (1973) indicated that the soluble salt content of fresh ash is considerably reduced during the lagooning process of the ash. They also indicated that two to three yr weathering in the field usually reduces the soluble salt content of the ash to harmless levels, i.e., to conductivity values less than four mmho/ cm. PAGE et al. (1977) have found that the amount of water required to reduce the salinity of leachates from two soils amended with five percent fly ash in the top three cm layer to background levels varied from 60 em for an acid soil to 116 em for a desert soil. They also found that approximately the same amounts of water were required to bring the B levels in the leachates to levels comparable to those of the background soils.

2,500 (A) 80 (B)

E 60 "Q ~ .=: .....

CT

II II>

iii E

In

'" II> ~ :c :l + '0 .. In U iii ... 0 I-

2 4 6 8 2 4 6 8


Fig. 6. Salinity and Ca + Mg in saturation extracts from By ash-amended soils.

B concentrations were elevated to nine and 11 p,g/ml in saturation extracts from the acid and the calcareous soils upon incorporation of


eight percent fly ash in the soil (Fig. 7 A). Such levels are undoubtedly detrimental to the growth of B-sensitive plants (EATON 1966). Figure 7B shows that B concentration in the fly ash-amended soils is related to the length of the cropping period, type of soil, and perhaps crop species. Concentrations of B in fly ash-amended soils were maximized after three to 5 mon in the acid soil and at about eight mon in the calcareous soil. Slightly more B was released from fly ash in the acid soil than in the calcareous soil.

12 (A) (8)

10 E ..... 5 8 m I .. U ~ 6 .. x ., c 0

4 " .. ~

" .. .. Ul 2

2 4 6 8 10 12 14

Fly ash in soil (% by wt) Cropping period (mon)

Fig. 7. Boron in saturation extracts from fly ash-amended soils.

Available B measured by the hot-water technique (JACKSON 1958) in the fly ash was 24 p.g/ g. Available B in British coal ashes ranged from three to 150 p.g/g with a mean of 43 p.g/g (TOWNSEND and HODGSON 1973). Hot water-soluble B in two fly ash samples from southeastern United States was 22 p.g/g for a fly ash with a pH of 4.8 and 50 p.g/g for a fly ash with a pH of 11.2 (PLANK and MARTENS 1974). The available B content of Tatum silt loam was increased from < one to 15 p.g/g upon incorporation of about 1.7% fly ash (618 ppm total B) in the soil (MULFORD and MARTENS 1971) .

One of the most noticeable and significant effects of fly ash on the chemical composition of soils is that of increased S content of the soil (Fig. 8). S in fly ash appears to be dominantly in an S04 form. When added to soil, fly ash increased the S04-S content in proportion to the amount added. At the eight percent addition rate, where plant uptake of S is negligible compared to the amount added, fly ash produced an S04-S concentration in the soil of about 300 p.g/g (Fig. 8). This concentration agrees well with concentrations predicted based upon the average S04-S content in fly ash of 0.4%. When four soils were treated with fly


-;:: l:

400

350

t 300 "0

'0 250 tn

'" .3 200 (J)

I 150 <t o (J) 100

50

2 4 6 8


Fig. 8. 50.-5 in NH.OAc extracts from fly ash-amended soils.

ash and gypsum at equal rates of S, the two materials produced similar concentrations of S04-S in the soil (ELSEEWI et al. 1978 b). TOWNSEND and HODGSON (1973) reported that the total S content of fly ash was 0.48%. The available S content ranged from 0.07 to 0.55% with a mean of 0.39% compared to a mean of available S in soil of 0.06%. Favorable response of plants to S in fly ash is discussed in a subsequent section in the report.

S content of fine particles of fly ash emitted into the atmosphere is undoubtedly greater than that of plant-retained fly ash. The content of total S in bottom ash and fly ash collected from five southwestern United States power plants (SWANSON 1972) ranged from 0.02 to 0.12% with a mean of 0.07% for bottom ash and for fly ash the range was 0.06 to 0.41 % with a mean of 0.16%. The S03-S in these power plants ranged from 0.1 to 0.5% with a mean of 0.26% in the bottom ash and ranged from 0.4 to 1.7% with a mean of 0.63% in fly ash. These data indicate dependence of S concentration on particle size. S analyses of various size fractions of fly ash in the range of one to 53 {tm revealed increasing concentrations of total Sand SOrS and gypsum as particle size was decreased (Table XII). Also, atmospheric particles of fly ash contain various forms of oxidized S the impact of which on the environment is related to particle size and atmospheric conditions such as sunlight, humidity, temperature, and the presence of other catalytic substances (LESLIE et al. 1977).

2. Effect of fly ash on growth of plants.-Research in England dealing with the establishment of successful vegetative covers on fly ash deposits have indicated that plant growth is conditioned by the amount of total soluble salts, pH, and available B in the ash and by the physical charac-


Table XII. The content of sulfate-S and gypsum ( CaSO •• 2H.O) in size fractions of fly ash."

Percentage (dry wt) Size fraction

(}Lm) SO.-S Gypsum

1-3 1.05 2.33 3-8 0.83 3.10 8-13 0.54 2.98

13-20 0.31 3.07 20-53 0.18 1.01

"SO.-S was determined after BRADSLEY and LANCASTER (1960) and gypsum by the acetone method (RICHARDS 1954).

109

teristics of the ash deposits (HODGSON and TOWNSEND 1973). In general, tolerance of plants to excessive salinity and B and to pH-induced nutrient imbalances very much determines their tolerance to fiy ash. Surface crust formation induced by the pozzolanic characteristics of fiy ash and particle separation and subsequent layer stratification are among the unfavorable physical conditions which prevail in ash deposits. Minimum microbiological activities occur in ash deposits due to their sterility.

HODGSON and TOWNSEND (1973) presented an extensive list on growth of natural and crop species on fiy ash media. Of the natural species observed, Funaria hygrometrica and Atriplex hastata were among the first species to grow on fiy ash. As weathering of fiy ash proceeds and the content of soluble salts and B is reduced, additional species are introduced. Crop species were classified into tolerant, semi-tolerant, and sensitive species with respect to their ability to withstand fiy ash (HODGSON and HOLLIDAY 1966, HODGSON and TOWNSEND 1973\. The most tolerant crops belong to the families Leguminosa, Chenopodiaceae, and Germinae. Examples of these plants are sweet clover, alfalfa, rye, wheat, and red fescue. Lettuce, barley, potato, peas, and several types of beans were classified as sensitive while species such as turnip and radish were among the semi-tolerant species.

Based upon the available B content of fly ash, HODGSON and TOWNSEND (1973) proposed a guide to crop selection with respect to B toxicity in fiy ash. Available B contents of < four, four to ten, 11 to 20, 21 to 30, and > 30 p.gl g were considered as nontoxic, slightly toxic, moderately toxic, toxic, and highly toxic, respectively. A graph relating B content of barley in relation to available B from fiy ash suggested that ash content of 20 p.g Big fiy ash was likely to give tissue contents of 600 p.g BIg dry wt. Such a concentration in barley tissue was associated with both B-toxicity symptoms and yield reductions.

More recent investigations carried out in the United States have demonstrated positive growth response for a number of crop species to incor-

110 A. L. PAGE, AHMED A. ELSEEWI, AND I. R. SlRAUGHAN

poration of small to moderate amounts of fly ash in agricultural soil. Application of 144 metric tons of weathered Glen Lyn fly ash/ha (6.4% ) increased the yield of com on Woodstown soil by approximately 27% (PLANK et al. 1975). The increase in yield was attributed to increased water availability to plants. Application Gf 2.6 to 5.2% Muskingham River fly ash to Fredrick silt loam nearly doubled the yield of com as a result of increased availability of Zn (SCHNAPPINGER et al. 1975). Increased rates of application of Kanawha River and Crawford Edison fly ash to Groseclose silty loam increased the availability of Mo to alfalfa and markedly increased its yield (DORAN and MARTENS 1972).

Greenhouse experiments conducted with fly ash from a western United States coal source (Mohave fly ash) added to a desert calcareous and three acid soils in rates ranging up to eight percent by weight have demonstrated improved yields of dry matter of a number of crop species (PAGE et al. 1977). A summary of these results is shown in Table XIII. The increase in yield was attributed, in all cases, to increased availability of S from fly ash to plants (ELSEEWI et al. 1978 b), while reductions in lettuce yield were attributed to excessive salinity and B (ELSEEWI et al. 1978 a).

3. Effect of fly ash on mineral composition of plants.-The content of available trace elements in fly ash (2.5% acetic acid) and in soil as presented by TOWNSEND and BODGSON (1973) is shown in Table XIV. A vailable Ag, Pb, and Sn in fly ash is comparable to that in soil while that of AI, B, Co, Cr, Cu, Mn, Mo, Ni, Ti, V, and Zn is greater in the ash than in soil.

REES and SIDRAK (1956) noted that barley and spinach grown on fly ash accumulate excessive quantities of Al and Mn in their leaves and exhibit symptoms of toxicities of these metals. They further indicated, however, that Atriplex hastata var. deltoidae which grew vigorously on the ash had a high content of Al and Mn but did not show symptoms of toxicity. Trace element composition of barley grown on soil and on fly ash as reported by COPE (1962) is shown in Table XV. The content of AI, As, B, Cr, Cu, Mo, Ni, Ti, and V was higher in plants grown on fly ash compared to those grown on soil. Despite these higher concentrations, only B was in excess of critical concentrations associated with development of toxicity symptoms. The main cause of barley's sensitivity to fly ash is B, as was emphasized by HOLLIDAY et al. (1958), and not AI or Mn, as was previously suggested by REES and SIDRAK (1956).

The typical effect of fly ash incorporation into soils on the mineral composition of plants is illustrated in Table XVI for alfalfa grown on Arizo calcareous and Redding acid soils amended with up to eight percent fly ash (PAGE et al. 1977). The plants were adequately fertilized with N, P, and K from a mixed fertilizer. The data shown indicate that elements most affected by fly ash applications are P, S, Ca, Na, Zn, B, and Mn. The contents of P and Zn were generally reduced with fly ash application, while those of Ca, Na, and B were generally increased. In alfalfa (Table


Table XIII. Summary of dry matter yield of plants grown under greenhouse conditions on soils amended with variable rates of fly ash.G

Relative yield ( % )

Fly ash Ber-in soil muda White Let- Swiss Brittle Barley (% ) Alfalfa grass clover tuce chard bush' seeds

Arizo calcareous soil

0 100 100 100 100 100 100 100 1 240 182 185 69 117 125 800 2 315 172 276 39 114 133 1,100 4 343 183 210 20 122 142 1,100 8 306 156 150 35 87 102 1,200

Redding acid soil

0 100 100 100 100 100 100 100 1 184 144 133 74 116 118 600 2 259 155 89 162 130 800 4 261 141 27 68 127 120 850 8 274 153 47 14 89 107 850

White clover Turnip

Josephine acid soil

0 100 100 0.65 137 164 1.30 174 170 2.60 179 191

Laughlin acid soil

0 100 0.65 123 1.30 116 2.60 114

G From PAGE et al. (1977). b A native desert species common to the Mohave Desert.

XVI), as well as in several other plant species tested, Mn showed two distinctly different trends in plants grown on acid and on calcareous soils. Under acid soil conditions, the first application of fly ash sharply reduces the Mn content of plants, while under calcareous conditions plants show a moderate increase in Mn content up to a certain level of fly ash addition (two to four percent).

Reductions in P and Zn concentrations in the plant tissue were not, however, sufficient to induce deficiencies of these elements in the plants. Reduced availability of Zn with application of alkaline fly ash to soil was also observed by SCHNAPPINGER et al. (1975) on com grown on a slightly


acid soil (Fredrick silt loam) amended with fly ash at rates ranging from 0.8 to 13% by weight. Addition of acidic fly ash to the same soil increased Zn uptake and corrected the de£ciency in plants. Thus, although fly ash contains P and Zn, the high alkalinity of the material and of the soil-fly ash mixtures appears to curtail the availability of these two elements to plants.

The increase in S availability to plants (Table XVI) was associated with significant yield improvement in a number of plant species. Furthermore, the availability of fly ash-S compared well with that of H 2S04 ,

gypsum, and sewage sludge, as revealed by a series of greenhouse studies with alfalfa, bermuda grass, white clover, and turnips (ELSEEWI et al. 1978 band c). Application of these materials at the rate of 25, 50, and 100 mg S/kg soil increased the yield and S content of turnips grown on S-de£cient Josephine soil.

Table XIV. The content of available trace elements in fly ash from British coal sources and in soil.4

Available concentration (p.g/ g)

Element Fly ash" Soil

Ag 1 1 AI 144 58 B 43 2.5 Co 8.5 1.6 Cr 22 1.7 Cu 25 2.5 Mn 99 4.8 Mo 5.4 0.2 Ni 60 2.7 Pb 10 10 Se 2 Sn 10 10 Ti 15 10 V 6 1.3 Zn 2.1 1.5

4 From TOWNSEND and HODGSON (1973). The available concentrations of B, Cu, Mn, Mo, and Zn were, on the average, 18.2, 10.1, 11.7, 12.9, and 0.7% of the total concentrations of these elements in fly ash, respectively.

"Extracted in 2.5% acetic acid.

Increased levels of N a and B in plants grown on fly ash-amended soils (Table XVI), although with no adverse effects on the growth of alfalfa, were partially responsible for yield reductions in lettuce (ELSEEWI

et al. 1978 a). The availability of B in fly ash to alfalfa was shown by M UL-


Table xv. Trace element composition of barley leaves grown on soil and fly ash.G

J.Lg element/ g dry wt

Grown on Grown on To show Element soil fly ash toxicity

Al 38 66 425 As 2 10 lOO B 18 1,200 600 Co 10 10 640 Cr 1 10 257 Cu 10 30 92 Fe 1,000 1,000 Mn 100 100 5,400 Mo 5 100 1,540 Ni 0.3 1.0 153 Se 10 10 137 Sn 100 100 Ti 30 100 V 3 10 Zn 57 40 200

G From COPE (1962).

FORD and MARTENS (1971) and PLANK and MARTENS (1974) to be essentially equal to that of sodium borate-B. The availability was, however, shown to decrease with time as indicated by results of a three yr field experiment with alfalfa (PLANK and MARTENS 1974).

Selected analysis of Mo in various plant species grown on calcareous and acid soils amended with fly ash is shown in Table XVII. Concentrations of Mo in tissue of forage crops in excess of ten ppm are considered hazardous with respect to animal nutrition. The Mo values shown in Table XVII reached as high as 22.0 and 44.4 p.g Mo/g (ppm) in ~lfalfa and white clover, respectively. The data, however, indicate that the availability of fly ash-Mo is inversely related to time. DORAN and MARTENS (1972) observed that the availability of fly ash-Mo to alfalfa was approximately equal to that of sodium-molybdate.

Analysis of Sr, Ba, Se, Co, Cs, Rb, Cr, W, Sb, Sc, Ce, Th, As, Sm, Eu, and Hg in fly ash-treated plants showed that only Sr, Ba, Se, Co, and Cs exhibited definite concentration trends in the plant tissue with increasing rates of fly ash application to soil up to eight percent by wt (Table XVIII). The increase in Sr and Ba concentrations again indicated that these two elements could serve as an index of fly ash deposition on soils and vegetation in areas adjacent to coal-fired power plants in the western United States. The increase in Se and Mo concentrations suggested potential animal nutritional problems when forage crops are grown on fly ash-amended soils.

.... ~

Tab

le X

VI.

Ana

lyse

s o

f top

s o

f al

falf

a gr

own

on fl

y as

h a

men

ded

soiZ

s.B

?- r'

P

erce

ntag

e p.

g/g

~ jl

Fly

ash

in s

oil

( % )

N

P K

S

Ca

Mg

Na

Fe

B

Zn

Cu

Mn

f A

rizo

cal

care

ous

soil

0 2.

2 0.

47

2.9

0.08

1.

4 0.

22

317

85

106

39

5.9

73

?-1

2.7

0.27

1.

9 0.

19

2.0

0.20

31

8 80

14

8 23

5.

3 14

4 t'1

2

2.8

0.27

1.

9 0.

11

2.4

0.24

55

0 85

18

3 25

6.

1 14

8 i

4 3.

0 0.

30

2.1

0.19

3.

2 0.

30

593

105

222

35

5.8

141

8 3.

0 0.

27

1.8

0.23

2.

5 0.

32

853

105

337

28

6.0

92

ft R

eddi

ng a

cid

soil

0 3.

4 0.

65

3.6

0.10

1.

4 0.

34

632

231

87

56

5.8

441

~ 1

4.0

0.60

3.

6 0.

24

1.7

0.32

69

7 25

2 90

50

6.

2 15

0 .....

. 2

4.1

0.55

3.

5 0.

33

2.0

0.32

75

3 22

2 93

45

5.

8 15

0 lJ:I

4

3.8

0.47

3.

7 0.

34

2.3

0.37

82

1 21

8 10

1 46

6.

5 13

3 8

4.3

0.40

3.

6 0.

31

2.4

0.43

67

2 20

7 13

7 39

6.

5 14

5 C

Il ~ B

A

naly

ses

show

n w

ere

perf

onne

d on

the

sec

ond

clip

ping

of

the

crop

and

fig

ures

are

mea

ns o

f fo

ur r

epli

cate

s of

eac

h fly

as

h tr

eatm

ent.

Con

cent

rati

ons

are

expr

esse

d on

dry

-wt

basi

s.

~


Table XVII. Molybdenum analysi8 of various plant species grown on Arizo calcareous and Redding acid soils amended

with fly ash.

Fly ash in soil (,ug Mo/g)

0 2 4 8 Soil-plant species (% ) (% ) (% ) (% )

Arizo calcareous soil Alfalfa

1st clipping 4.5 7.5 17.0 22.0 6th clipping 1.4 2.7 4.3 9.5 11th clipping 1.8 1.8 1.9 4.2

Bennudagrass 1st clipping 1.2 2.7 5.9 4.4

White clover 2nd clipping 2.5 1.7 10.0 17.5

Brittlebush leaf sample 1.1 3.4 4.5 8.1

Redding acid soil Alfalfa

1st clipping 0.9 5.2 7.8 17.2 6th clipping 2.0 3.4 7.5 9.5 11th clipping 3.2 3.6 5.1 5.7

Bennudagrass 1st clipping 0.5 4.7 5.2 5.5 4th clipping 0.9 2.8 4.3 4.5

White clover 2nd clipping 2.8 11.6 44.4 41.6

Brittlebush leaf sample 0.9 4.6 6.1 7.6

Summary and conclusions

Combustion of coal in power generating plants produces a number of residues (bottom ash, boiler slag, fly ash, flue gas desulfurization sludge, and noncaptured particulates). The relative amount of each residue depends on the power plant configuration and on emission control devices available.

Fly ash is collected by means of electrostatic precipitation. This industrial waste product is a fine-textured material with most of its particles in the silt and clay size range. It is characterized by low permeability, low bulk density, and high specific surface area. Microscopically, particles of fly ash are mostly spherical and "particles within particles" is a common feature. The fly ash matrix is predominantly amorphous with intrusions of lime, gypsum, and some clay minerals. Si, AI, Fe, and Ca are the major components of fly ash.

Trace elements originating from the source coal are redistributed amongst these various residue streams. Due to decreased particle size of residues, the concentrations of trace elements in residues are greatly increased relative to the original coal composition. In particular, elements


Table XVllI. Trace elements in plants grown on fly ash amended Arizo calcareous soil: elements showing definite concentration

trends.-

p.g/g (ppm) Fly ash (% ) Sr Ba Se Co Cs

Alfalfa 0 30 4.5 0.2 0.14 0.026 0.5 77 9.3 1.1 0.12 0.060 1.0 125 18.0 1.7 0.12 0.071 2.0 196 25.0 2.8 0.16 0.070 4.0 226 28.0 4.5 0.36 0.053 8.0 364 45.0 4.6 0.45 0.105

Lettuce 0 30 6.4 0.14 0.040 0.5 41 7.4 0.6 0.13 0.050 1.0 62 56.0 1.0 0.21 0.120 2.0 79 10.0 2.1 0.16 0.080 4.0 11.0 2.8 0.19 0.120 8.0 152 36.0 3.9 0.51 0.180

_ Results are from neutron-activation analysis.

which are largely volatile, e.g., Se, Mo, Hg, B, etc., exhibit considerably larger concentrations in the residues than in coal. Also, large quantities of S are emitted to the atmosphere from coal combustion. The use of additional scrubbing techniques has greatly reduced these amounts, however.

Compared to soil, fly ash is generally enriched in S, Ca, Sr, B, Mo, and Se. Water extracts from fly ash are commonly alkaline and contain excessive amounts of dissolved solids. High-S coals may, however, produce residues with acidic reactions. These are mainly found in the eastern part of the United States.

The intensity of the environmental impacts of coal residues varies between captured and noncaptured residues. Captured residues which represent about 90% of the total amounts of residues produced contain most of the trace element burden al.d exert their impacts only when they are discharged into the environment. As such, they only represent a potential long-term hazard with respect to their content of potentially harmful trace elements. Field and greenhouse experiments have, however, demonstrated some beneficial effects from these residues. They act as a source of some essential elements to plants such as S, Ca, Mo, B, Zn, and possibly Mn. By increasing soil pH, they also have the capacity of being used as acid soil amendment capable of improving soil conditions for proper plant growth under these conditions. Nevertheless, because of potential problems of salinity, B, Mo, and Se arising from application of coal ash to soils, these elements should be critically evaluated before large-scale disposal of the by-product on agricultural soils is recommended.


Noncaptured particulates emitted to the atmosphere have direct and immediate short-term impact on the environment. Although they are small in quantity their micron and submicron size makes them greatly enriched in many potentially harmful elements. Their contact with the biological systems is obviously determined by their deposition rate. In humid areas, high atmospheric moisture not only enhances their deposition but also is expected to alter their chemistry, thus producing "secondary particulates". The chemistry of these secondary particulates and their environmental relations is not, however, understood. In arid areas such as the southwest, most coal-burning power plants are situated in remote dry areas where this immediate environmental impact is minimal.

Acknowledgment

The authors would like to acknowledge the technical assistance by D. Thomason and J. W. Blair, Department of Soil and Environmental Sciences, University of California, Riverside, in the analysis of soil, plant, and fly ash samples.

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Manuscript received June 9,1978; accepted August 29,1978.

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