( 5 ) Emerson, K.; Russo, R. C.; Lund, R. E.; Thurston, R. V. J.Fish.
below -20-50 mg/L total ammonia nitrogen, the effect of NH4+ is overridden by the effect of the much more toxic NH3 fraction, but at higher concentrations of total ammonia the toxic effect of NH4+ can be noticed. If this is the case, the data would indicate that NH3 is 300-400 times more toxic than NH4+ on a molecular comparison basis. Other factors, not noted here, may be operative. It has been demonstrated by this and the studies cited earlier that the toxicity to fishes of ammonia in terms of unionized ammonia alone does not remain constant over the pH range considered acceptable to freshwater aquatic life. It would therefore seem advisable that water quality criteria to protect aquatic life be based either on un-ionized ammonia in combination with pH or on total ammonia in addition to un-ionized ammonia.
Res. Board Can. 1975,32,2379-83. (6) Thurston, R. V.; Russo, R. C.; Emerson, K. “Aqueous Ammonia Equilibrium-Tabulation of Percent Un-ionized Ammonia”, EPA-600/3-79-091; Office of Research and Development, U S . Environmental Protection Agency: Duluth, MN, 1979. (7) European Inland Fisheries Advisory Commission. “Water Quality Criteria for European Freshwater Fish. Report on Ammonia and Inland Fisheries”; EIFAC Technical Paper No. 11,1970. (Also in Water Res. 1973,7,1011-22.) (8) US. Environmental Protection Agency. “Quality Criteria for Water”; Office of Water and Hazardous Materials, U.S. Environmental Protection Apencv: Washington. DC. 1977. (9) Armstrong, D. A.; Chippendale, D:;Knight, A. W.; Colt, J. E. Biol. Bull. (Woods Hole, Mass.) 1978,154, 15-31. (10) Robinson-Wilson, E. F.; Seim, W. K. Water Resour. Bull. 1975, 11.975-86. (11) Tabata, K. Bull. Tokai Reg. Fish. Res. Lab. 1962,34,67-74 (in English translation). (12) Thurston, R. V. in “Proceedings of the Third USA-USSR Symposium on the Effects of Pollutants Upon Aquatic Ecosystems. Theoretical Aspects of Aquatic Toxicology”, EPA-600/9-80-034; Office of Research and Development, U.S. Environmental Protection Agency: Duluth, MN, 1980; pp 118-37. (13) Tomasso, J. R.; Goudie, C. A.; Simco,B. A.;Davis,K. B. Trans. Am. Fish. SOC. 1980,109,229-34. (14) Mount, D. I.; Brungs, W. A. Water Res. 1967,1,21-9. (15) American Public Health Association, American Water Works Association, Water Pollution Control Federation. “Standard Methods for the Examination of Water and Wastewater”, 14th ed.; American Public Health Association: Washington, DC, 1976. (16) US. Environmental Protection Agency. “Methods for Chemical Analysis of Water and Wastes”, EPA-625/6-74-003; Methods Development and Quality Assurance Research Laboratory, U.S. Environmental Protection Agency: Cincinnati, OH, 1974. (17) Hamilton, M. A.; Russo, R. C.; Thurston, R. V. Enuiron. Sci. Technol. 1977,11,14-9; correction, ibid. 1978,12,417.
Acknowledgment
We thank Charles Chakoumakos and Kenneth Wang for contributions to the test apparatus design and for performing many of the chemical analyses. Victor T. Komov, Victoria E. Matai, and Boris A. Flerov provided helpful comments during the course of this research, and David J. Randall, William A. Spoor, and Kenneth E. F. Hokanson during preparation of the manuscript. Tanya Nezabilovskey provided a necessary link in communications at Sunoga Laboratory in Borok during discussions about the research. Literature Cited (1) Butler, J. N. “Ionic Equilibrium”; Addison-Wesley,Reading, MA, 1964. (2) Chipman, W. A,, Jr. Ph.D. Thesis, University of Missouri, Columbia, MO, 1934. (3) Wuhrmann, K.; Zehender, F.; Woker, H. Vierteljahrsschr. Naturforsch. Ges. Ziirich 1947,92,198-204 (in English translation). (4) Wuhrmann, K.; Woker, H. Schweiz. 2.Hydrol. 1948,11,210-44 (in English translation).
Received for review February 24, 1981. Accepted March 27,1981. This research was funded by the US.Environmental Protection Agency, Environmental Research Laboratory, Duluth, MN, under Research Grants R800861 and R803950.
NOTES
Long-Term Leachability of Selected Elements from Fly Ash Marvin J. Dudas Department of Soil Science, The University of Alberta, Edmonton, Alberta, Canada T6G 2E3
A long-term leaching experiment was conducted to determine the ion release and weathering characteristics of fly ash obtained from a power generating plant utilizing northern Great Plains coal. Fly ash was placed in small cylinders and continuously leached for up to 2 yr using a maximum of 421 L of distilled water. Leachates were monitored for levels of dissolved constituents, and leached ash residues were analyzed for total elemental composition and scanning electron microscopic (SEM) morphology. Results for leachate and residue composition indicate that initial ion release characteristics are largely controlled by dissolution of various simple discrete inorganic salts admixed with the glassy siliceous fly ash particles. After extended leaching and removal of the admixed salts, the slow dissolution of ash matrix with concomitant nearly constant and low levels of dissolved constituents becomes dominant. 840
Environmental Science & Technology
Introduction
Aqueous dissolution of fly ash has been studied by several investigators (1-7) to ascertain the ion release characteristics of this waste material which is produced in vast quantities by coal-fired power generating stations, Much of the interest in the dissolution of fly ash arises from concerns related to environmental degradation through the release of toxic levels of elements from ashes and from the potential beneficial properties of fly ash in soil amelioration (8) and in chemical restoration of certain lakes and acid mine wastes (6). Although a number of short-term leaching, extraction, and equilibration studies have demonstrated many of the initial dissolution characteristics of fly ash, the long-term weathering and ion release properties remain to be clarified. Since some major, minor, and trace elements in fly ash appear to be preferentially concentrated on the particle surfaces (7,9) in
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a more reactive chemical form than when locked within the silicate matrix (7), information obtained in short-term solubility experiments may not accurately represent the long-term dissolution behavior and concomitant environmental hazards or benefits of fly ash. Accordingly,this study was conducted to evaluate aqueous dissolution of fly ash under prolonged leaching conditions.
I
I
I
I
I
Experimental Section
Fly ash samples from electrostatic precipitator collection bins were provided by Calgary Power Ltd. f r o p their Sundance generating station located near Edmonton, Alberta. The plant utilizes a low-sulfurbituminous coal mined from Upper Cretaceous sedimentary deposits. For the leaching experiment, small lysimeters were prepared in triplicate by gently packing 250 g of fly ash into plastic cylinders of 6.9-cm internal diameter. From the determined value for particle density and the measured volume and amount of ash, each column contained 260 cm3of fly ash of 47%total porosity corresponding to a pore volume of 125 cm3. Samples were continuouslyleached with distilled water over a 2-yr period at a flow rate governed by the hydraulic conductivity of the fly ash cores. One of the triplicate samples, identified as column A, was leached with 124 L of water (equivalent to 993 pore volumes), the second sample, column B, with 245 L (1957 pore volumes), and the third sample, column C, with 421 L (3367 pore volumes). A total of 297 leachate samples were collected throughout the experiment and monitored for pH, aluminum by colorimetry, calcium, magnesium, sodium, and potassium by atomic absorption spectrophotometry, sulfate by a turbidimetric method, and hydroxide, carbonate, and bicarbonate by titration. On completion of leaching, the fly ash cores were removed from the cylinders, bisected horizontally into top and bottom halves, oven-dried at 105 "C, and gently pulverized to pass a 60-mesh sieve. A single aliquot of each of the dried ash samples was dissolved in concentrated HF-HCl according to the procedure of Pawluk (IO). Digests were then analyzed for major, minor, and trace elements by atomic absorption and inductively coupled plasma emission spectrophotometry. Scanning electron micrographs of fresh and weathered samples were obtained with a Cambridge Stereoscan 150.
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Results
Leachate Samples. The changes in chemical characteristics of leachate samples collected periodically during the 2-yr leaching experiment are shown in Figure 1. Each 1000 pore volumes corresponds to 3342 cm of percolating water. Initial characteristics of leachates of the first 25 pore volumes include extremely alkaline pH values and maximum concentrations of dissolved calcium, sodium, sulfate, and hydroxide (Figure 1,a-c and f). Dissolved magnesium was not detected initially (Figure Id), lowest contents of dissolved aluminum occurred a t the onset of leaching (Figure le), and levels of potassium increased from nondetectable initial levels to a maximum concentration of 0.3 ppm in aliquots of the 50th pore volume (Figure IC). With leaching, pH values decreased in a somewhat stepwise fashion, and concentrations of dissolved cations each displayed unique solubility patterns. The trend in soluble levels of sulfate closely resembles that of calcium, while levels of hydroxide, carbonate, and bicarbonate reflect the declining pH values. Dissolved concentrations of sodium were nearly constant after leaching with 1000 pore volumes; however, nearly constant levels of dissolved calcium, magnesium, and aluminum were not obtained until after leaching with 2000-2500 pore volumes. Weathered Ash Residue. The chemical composition of fly ash before (fresh ash) and after leaching is shown in Table I.
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Leachate Volume (number of pore volumes) Figure 1. Chemical composition of leachates from fly ash leached with distilled water for up to 2 yr.
A number of elements, notably calcium, boron, strontium, and, to a lesser extent, vanadium, were preferentially leached so that their concentrations in the weathered ash residue are substantially lower than in fresh ash. This preferential removal leads to negative enrichment such that concentrations of other elements like aluminum, barium, iron, potassium, Volume 15, Number 7, July 1981 841
Table 1. Chemical Composltlon of Fresh and Weathered Fly Ash After Leaching with 993 (Column A), 1957 (Column B),and 3367 (Column C) Pore Volumes of Distilled Water
Composltlon in YO AI Ba
Ca Fe K
Ma Na
11.9 0.49 10.0 2.84 0.489 0.644 2.48
12.6 0.52 6.88 2.98 0.514 0.642 2.58
12.2 0.49 8.16 3.00 0.476 0.680 2.48
12.9 0.52 6.29 3.10 0.534 0.612 2.70
12.4 0.54 7.82 3.02 0.500 0.695 2.56
13.2 0.55 5.96 2.88 0.534 0.608 2.68
12.7 0.56 7.06 2.85 0.501 0.685 2.58
Composition in ppm B
cd Co Cr
cu
Ha
Mn Mo P Fb
Sr
v
Zn
480 2.0 14 37 45 0.22 480 52 890 100 1100 100 86
390 1.9 13 27 48 0.17 480 44 910 105 860 92 93
440 1.9 14 29 44 0.26 490 45 940 100 930 100 92
270 2.1 14 28 51 0.30 530 45 810 115 790 84 97
310 2.6 12 30 45 0.27 530 46 1000 105 920 100 95
110 1.9 12 26 55 0.24 500 43 770 125 770 80 98
170 2.1 14 27 47 0.23 520 45 950 115 860 94 93
Figure 2. Scanning electron micrographs 01 fresh fly ash (a) before leaching and (b) after leaching with 1957 pore volumes Of distilled water.
sodium, manganese, lead, and zinc are higher in leached ash than in fresh ash. Regidue analysis also showed that chromium and molybdenum displayed rapid initial partial dissolution followed hy limited removal with subsequent leaching. Similarity in the contents of cadmium and cobalt in all leached ash samples and fresh ash indicated that these elements were sparingly diasolved and removed with leaching since immobile elements would display an increase in concentration through negative enrichment effects such as those shown by zinc. Abundances of the more readily dissolved elements, such as calcium and boron, are lower in the top portion of the leached fly ash cores than in the bottom section, while elements of limited leachability, such as aluminum and sodium, display an opposite trend. The apparent insoluble nature of sodium compared to that of calcium strongly suggests that some elements largely occur within the silicate matrix of the ash particles whereas others, like calcium, occur separately as discrete inorganic salts. Weathering of the interior glassy portion of fly ash particles was minimal since levels of highly ionic elements (Na, K) have not diminished in the leached samples. Scanning Electron M i c r m o p i c Morphology. Fresh fly ash consisted largely of spherical particles (Figure2a) classitied as solid spheres, plerwpheres, and cenospheres (12). Surfaces appeared smooth and partially covered with a fine dust and lath-shaped, platy-shaped, and acicular crystals similar to those described elsewhere (12). Leached fly ash particles displayed the typical spherical morphology but were 842 Environmental Scienca 8 Technology
encrusted with a porous, flake-appearing weathering product (Figure 2b). Acicular and lath shaped crystals were absent only in samples from column C, while ash from column C appeared to contain the largest amount of encrustations. Fractured plerospheres and cenwpheres with wall thickness approximately the same as those in fresh ash were observed in all leached samples which again suggested chemical alteration was restricted to surface reactions without substantial penetration into the glassy particles. X-ray diffraction, infrared. and differential thermal analyses failed to reveal the presence of crystalline weathering products in the leached ash samples. The observed surface encrustations likely consist of amorphous aluminum and possibly iron oxyhydroxides since only the presence of such compounds can account for the amphoteric properties and isoelectric point reported hv Talbot et al. ( 6 )for weathered fly ash. Discussion Results of this study can best he explained by viewing the chemical nature of the fresh fly ash as glassy spheres of nearly uniform composition but with surface deposits or physical admixtures of a variety of relatively simple and soluble inorganic salts. The change in leachate and residue composition then refleets both the rapid dissolution of inorganic salts which dominate compositional trends during the early stages of leaching and the slow dissolution of glassy ash particles which becomes partially evident only in the later stages of
leaching after most or all of the inorganic salts have dissolved. The presence of simple salts such as calcium oxide, anhydrite or gypsum, sodium and potassium sulfates, and magnesium and calcium carbonates is indicated by the composition of leachate samples. For example, the extreme alkalinity of leachates of the first few pore volumes (Figure la) is largely attributable to hydrolysis by calcium oxide. The relatively low concentrations of soluble sodium, potassium, and magnesium compared to calcium precludes salts of these elements as major causes of alkaline hydrolysis. Hydrolysis by most other salts likely to occur in the fresh ash would not generate such strong initial alkaline conditions. With leaching, the highly reactive calcium oxide dissolves and is largely removed by the percolating water after displacement of -20 pore volumes as evidenced by the 1-unit drop in pH values and the concomitant decline in soluble calcium levels. A portion of the calcium oxide upon contact with water may also be converted to calcium carbonate (6). The slow dissolution and hydrolytic reactions of carbonate compounds could then explain the progressive slow decline in pH values and the rise in soluble magnesium levels of leachates collected after the first 20 pore volumes. Similarly, the occurrence of sodium sulfate in ash could partially account for the initial high levels followed by a rapid decline in soluble sodium and sulfate levels in leachates of the first 200 pore volumes. The parallel trends displayed by soluble levels of sulfate and calcium with leaching from 500-2500 pore volumes indicate that gypsum (originally as anhydrite in fresh ash) is a likely common source for these two dissolved constituents. The behavior of minor and trace elements may be considered in a similar fashion. For example, the apparent high leachability of boron likely arises from its presence in ash as an admixed borate salt of moderate solubility. Its relatively rapid loss from ash with leaching (Table I) without a proportionate parallel loss of sodium or other highly ionic major matrix element strongly suggests that most of the boron does not occur within the glassy ash particles. Other minor and trace elements, such as phosphorus and lead, are not leached from ash (Table I), which suggests that they occur within the silicate matrix or possibly as adsorbed constituents. The attainment of nearly constant levels of sodium and the simultaneous absence of detectable quantities of potassium after leaching with 1000 pore volumes (Figure IC)suggest that salts of these two elements in ash have completely dissolved. The subsequent soluble sodium concentration in leachates ( 4 . 4 5 ppm) probably represents dissolution of fly ash matrix. Although other elements also display nearly constant soluble concentrations after leaching with 2500 pore volumes, their levels in solution may not be solely related to dissolution of ash matrix since a disproportionate relationship exists for
some elemental ratios between leachate and residue composition. Differences in the content of relatively mobile constituents such as calcium, boron, and strontium in top and bottom halves of fly ash cores (Table I) further suggest that complete dissolution of admixed or surface adsorbed salts has not yet been reached. Results obtained in this leaching trial cannot be easily compared to other published studies on ion release characteristics of fly ash because of differences in experimental procedures. Other studies have usually involved leaching for extremely short time periods ranging from seconds ( 1 )to a few days ( 2 , 3 )or equilibrations or limited numbers of extractions in closed containers ( 5 - 7 , l l ) . In conclusion, results of this study indicate that short-term dissolution characteristics of fly ash are largely dominated by the nature and the quantity of admixed and surface adsorbed inorganic salts. As these salts are solubilized and removed from ash, concentrations of soluble constituents released from chemical alteration of the glassy matrix are maintained a t relatively low levels. Acknowledgment I express my appreciation to Lloyd Hodgins of the Alberta Soil and Feed Testing Laboratory for ICP analysis and to George Braybrook for assistance with SEM. Literature Cited (1) Cox, J. A.; Lundquist, G. L.; Przyjazny, A.; Schmulbach, C. D. Enuiron. Sci. Technol. 1978,12,722-3. (2) Eggett, J. M.; Thorpe, T. M. J. Enuiron. Sci. Health, Part A 1978, 13,295-313. (3) James, W. D.; Janghorbani, M.; Baxter, T. Anal. Chem, 1977,49,
,.
1994-7
(4) Phung, H. T.; Lund, L. J.; Page, A. L.; Bradford, G. R. J. Enuiron. Qual. 1979,8,171-5. ( 5 ) Shannon. D. G.: Fine. L. 0. Environ. Sci. Technol. 1974. 8. 1026-8. (6) Talbot, R. W.; Anderson, M. A.; Andren, A. W. Enuiron. Sci. Technol. 1978,12,1056-62. (7) Theis, T. L.; Wirth, J. L. Enuiron. Sci. Technol 1977, 11, 1096-100. ( 8 ) Plant, C. 0.; Martins, D. C. J. Soil Water Conseru. 1973, 4 , 177-9. (9) Linton, R. W.; Loh, A,; Natusch, D. F. S.; Evans, C. A.; Williams, P. Science 1976,191,852-4. (10) Pawluk, S. At. Absorpt. News. 1967,6,53-6. (11) Dressen, D. R.; Gladney, E. S.; Owens, J. W.; Perkins, B. L.; Wienke, C. L.; Wangen, L. E. Enuiron. Sci. Technol. 1977, 11, 1017-9. (12) Fisher, G. L.; Prentice, B. A.; Silberman, D.; Ondov, J. M.; Biermann, A. H.; Ragaini, R. C.; McFarland, A. R. Enuiron. Sci. Technol. 1978,12,447-51.
Received for review August 4,1980. Accepted March 19,1980. This project was funded by a grant from the Alberta Environmental Research Trust.
Evidence of the Unsuitability of Gravity Coring for Collecting Sediment in Pollution and Sedimentation Rate Studies Murdoch S. Baxter,* t John G. Farmer,* Ian G. McKinley,t David S. Swan,t and William Jacks University of Glasgow, Glasgow G12 8QQ, Scotland, United Kingdom
Introduction During the last decade there has been a rapid increase in the number of investigations which have examined records + Department of Chemistry. t Department of Forensic Medicine and Science. 8 Department of Natural Philosophy.
of (a) sedimentation, via radiometric dating techniques (l4), (b) metal (6-11) and organic (12, 13) pollution, and (c) reworking processes Of activity (l4-I6)9 from the measured vertical distributions of chemical species preserved in layers of coastal marine and lacustrine sediment. Of primary importance in such studies is the collection of undisturbed sediment, in particular the efficient sampling of top-
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Volume 15, Number 7, July 1981 843
