(4) Li, N. N., Cahn, R. P., Shrier, A. L., U.S. Patent 3,617,546 (Nov. 2, 1971). (5) Li, N. N., Shrier, A. L., “Liquid Membrane Water Treating”, in “Recent Developments in Separation Science”, Vol I, p 163, CRC Press, West Palm Beach, Fla., 1972. (6) Cahn, R. P., Li, N. N., S e p . Sci., 9 (6), 505 (1974). ( 7 ) Matulevicius, E. S., Li, N. N., Sep. Purif. Methods, 4 ( l ) ,73 i1975).
(8) Kitagawa, T., Nishikawa, Y., Frankenfeld, J. W., Li, N. N., Enuiron. Sci. Technol., 11,602 (1977). (9) Frankenfeld, J. W., Li, N. N., “Waste Water Treatment by Liquid Ion Exchange in Liquid Membrane Systems”, in “Recent Developments in Separation Science”, Val 111, p 285, CRC Press, West Palm Beach, Fla., 1977. Received for review August 20, 1976. Accepted April 14, 1978.
Qualitative Model of Heterogeneous Equilibria in a Fly Ash Pond Robert W. Talbot“, Marc A. Anderson, and Anders W. Andren Water Chemistry Laboratory, 660 North Park Street, University of Wisconsin, Madison, Wis. 53706
w Laboratory experiments conducted with various fly ash samples collected a t the Columbia Energy Center near Portage, Wis., indicate that in ash ponds the dissolved concentrations of the major (Al, Fe, and Si) and minor (Ca, Mg, Na, and K) elements in fly ash are controlled by metastable equilibria with solid phases. Adsorption reactions have an important influence over the dissolved concentrations of the trace constituents cadmium and phosphorus. The most clearly defined isoelectric point ( ~ H I E Pof) the fly ash occurs a t pH -7.55 where essentially 100% of the aluminum and iron are incorporated into solid phases on the fly ash particle. The constant negatively charged ash surface a t low pH may result from isomorphic replacement of aluminum for silica in the Sios(,, matrix, whereas calcium and magnesium carbonates or hydroxides probably contribute to the negative surface charge a t high pH.
Recently, Bertine and Goldberg (12) emphasized the need for detailed studies of the sedimentary record in the Northern Hemisphere for those elements that are selectively volatilized as a result of fossil fuel burning. Yet, the largest quantity of trace elements involved in coal combustion remains associated with those fly ash particles that are deposited into ash ponds near power plant facilities. This study was designed to identify those heterogeneous reactions that control the chemical composition of ash pond waters. Extensive equilibrium leaching experiments were conducted to study dissolved and solid phase interactions that occur in ash ponds. These experiments permitted us to define the metastable equilibrium conditions that influence the observed elemental distribution. Using results obtained from the laboratory experiments, a qualitative equilibrium model for this complex heterogeneous system is described.
Plant Description The enriched presence of major, minor, and trace elements in fly ash produced from coal combustion processes has been well established (1-3). Short duration solubility studies conducted by Natusch et al. ( 4 ) have shown that certain toxic trace elements may become directly available to the external environment when fly ash is leached by waters draining into streams or groundwaters. Nevertheless, fly ash has been proposed for use in lake restoration projects (5,6) and in the neutralization of acidic mine wastes (5, 7 ) .Its excellent pozzolanic properties also make it a desirable material for concrete fabrication and highway construction (8-10). Appropriately, these applications have been designed to alleviate the solid waste disposal problem associated with our nation’s seventh most abundant raw material-coal (11).
The Columbia I coal-fired power plant located along the Wisconsin River near Portage, Wis., has a peak capacity of 527 MW. I t began operation in May 1975 and utilizes low-sulfur pulverized coal obtained from the Colstrip field (Rosebud seam), Montana. Operating a t peak capacity, this plant consumes 5 000 tons/day of coal with a typical ash content of 7-8%. High-efficiency electrostatic precipitators are employed for particulate pollution abatement. The fly ash collected by stages 11-IV of the precipitators is discharged into a 0.3 km2 ash pond adjacent to the plant.
S a mp 1i ng Composite samples of fly ash used in the laboratory experiments include:
. SUBMERGED . WET DELTA
H0
DRY DELTA HOPPERS
10.0
PH 90
80
TIME
Figure 1. Change in pH with leaching time in a system open to atmosphere 1056
Environmental Science & Technology
0013-936X/78/0912-1056$01 .OO/O
@ 1978 American Chemical Society
1. Fly ash collected underwater off the end of the ash delta (submerged) 2. Freshly deposited fly ash on the ash delta (wet delta) 3. Fly ash deposited on the ash delt,a but dewatered (dry delta) 4. Fly ash from stages I1 and I11 of the electrostatic precipitator hopper bins (September 5, 1975). Samples I,2, and 3 were collected from the ash delta a t selected locations indicative of various stages of leaching by pond waters or rainfall. After collection, each composite sample of fly ash was air dried a t room temperature (20 "C) Standard #325 sieve. and subsequently sieved through a U.S. The fly ash fraction collected from sieving consisted of particles less than 44 wm in diameter. Drying of the sieved fly ash was completed in a desiccator where the samples were stored until use.
Experimental Duplicate equilibrium leaching experiments were conducted in large polyethylene containers. A solid-solution ratio of 1 g ash/L of doubly distilled water was utilized for each fly ash sample. These slurries were stirred for several months a t a constant rate using electric stirrers equipped with polyethylene blades. A t selected time intervals an aliquot was removed from each slurry and the p H measured with a lowjunction potential electrode (Sargent-Welch Model s30072-25). Each aliquot was then filtered through a prewashed 0.4-wm Nuclepore filter and the filtrate divided for analysis. Dissolved aluminum (131, dissolved reactive phosphorus (141, and dissolved reactive silica (15) were determined colorimetrically from one fraction. The remaining filtrate was acidified to 0.5% nitric acid for later analysis by atomic absorption spectroscopy (Perkin-Elmer Model 603) for Ca, Cd, Fe, K, Mg, and Na. To facilitate electrophoretic mobility measurements, large volumes of fly ash slurries were prepared using the same four fly ash samples. The slurries were stirred for one week to obtain stability with respect to pH. At this time, 12 aliquots from each ash slurry were dispensed into 250-mL linear polyethylene bottles. The p H was adjusted from 1.0 to 12.0, respectively, with perchloric acid or potassium hydroxide. These slurries were then shaken a t a constant temperature (20 O C ) for one additional week. The final pH was recorded and the electrophoretic mobility determined with a Lazer-Zee Meter (Pen-Kem Inc.). The remaining slurry was filtered and elemental analysis conducted on the filtrate as previously described. Additional electrophoretic mobility experiments were conducted in the same manner, but using hydrochloric acid and sodium or ammonium hydroxide for pH adjustment. These experiments were performed to determine if the excess Na+ or K+ ions affected the metastable equilibrium state of this system.
Results pH. Figure 1 illustrates that a dramatic increase in pH occurred when each of the fly ash samples was dispersed in doubly distilled water. The most significant p H rise was caused by the hopper fly ash. The pH of all four fly ash slurries stabilized a t approximately the same value after one week of leaching. Similarly, Figure 2 shows the same pH increase with time when the experiment was conducted under a nitrogen atmosphere to exclude COn from the system. In this instance, however, several weeks were required for the pH to decline to lower values. Elemental Leaching with Time. The change in the dissolved concentrations of several major and minor elements in fly ash with time is shown in Figure 3a-i. The largest amount of leachable Al, Ca, Fe, Mg, Na, and Si was derived from the hopper fly ash. Nearly constant dissolved concen-
t OH
TIME
Figure 2. Change in pH with leaching time in a system closed to atmosphere
trations of Al, Ca, Cd, Fe, P, and Si are maintained for each fly ash sample after one month of equilibration, whereas K, Mg, and Na undergo a steady increase in solution following an initial rapid dissolution. Potassium, unlike other matrix elements, was leached from the submerged ash in the largest amount (Figure 3h). Figure 3e shows that dissolved reactive phosphorus concentrations increased slowly from the hopper fly ash and were lower than those derived from the other ash samples. Cadmium was quickly dissolved from the fly ash surface with the highest concentrations appearing from the hopper ash (Figure 3d). Mobility. Electrophoretic mobilities of the four fly ash suspensions are shown as a function of pH in Figure 4. A constant solid-solution concentration of 200 mg/L was used in all cases. Although the particles appear to experience two charge reversals, (pH -2.90 and 7.55), the most clearly defined isoelectric point ( ~ H I E Plies ) between pH 7.5 and 7.6. Electrophoretic mobility measurements below pH 3.0 were difficult to obtain owing to the extreme negative surface charge of the ash particle. Measurements at pH 1.0, therefore, are not reported. The change in the dissolved concentrations of several major and minor elements in fly ash with pH is shown in Figure 5a-h. Dissolution of the major matrix constituents of fly ash occurs below pH 5.0 with sharp increases in the dissolved Al, Fe. and Si concentrations (Figure Sa-c). Likewise, the fly ash particle dissolves gradually above pH 8.5 with increasing pH. Aluminum and iron exhibit the lowest dissolved concentrations in the pH regions where their hydrous-oxide phases are most stable. The concentration of dissolved silica depicted in Figure 5b remains constant except a t extreme pH values where the ash particle dissolves. Calcium and magnesium (Figure 5e and f ) exhibit identical behavior in the bulk solution below pH 10.0 and appear to be incorporated into the same solid phase. The pH dependence of dissolved reactive phosphorus and dissolved sodium is shown in Figure 5g and h. Maximum loss of phosphorus and sodium from solution occurs near pH 5.0. Further pH increases result in higher dissolved concentrations for both species. A t high pH, however, sodium is released into solution while phosphorus is removed from the dissolved phase. Cadmium remains fairly constant in solution between pH 4.5 and 8.0, but is slowly removed from the dissolved phase as the pH increases (Figure 5d). In fact, above pH 11.0 cadmium is almost completely removed from the bulk solution. Potassium was not measured over the p H range 1.0-12.0 in the same series of experiments shown here. Its behavior with pH was determined by additional experiments using sodium or ammonium hydroxide for pH adjustment. Potassium exhibits the same behavior in solution with pH as sodium (16). The dissolved concentrations of all other elements of interest in this study were monitored in these additional experiments, using sodium or ammonium hydroxide for pH adjustment, Volume 12, Number 9, September 1978
1057
3.01
- --LOG Ca
-LOG Fe.
-
--A--
J
[MI
[MI
Figure 3. Change in dissolved concentrations with leaching time
*
Submerged off delta, @ wet delta,
dry delta, t hoppers
and demonstrated the same behavior with p H as reported above. Discussion
pH Effects. The water-soluble layer surrounding a fly ash particle most likely contains various oxides of calcium, magnesium, potassium, and sodium ( 4 , 17). The precise chemical forms of these oxides on fly ash have not been firmly established. In addition, some conversion (e.g., Equation 1)to their respective carbonates may have occurred during particle formation.
+
M2O C02(g)= M2C03 (1) The dissolution of these oxides or carbonates from the fly ash surface should occur when fly ash is dispersed in doubly distilled water (16, 17). The highly electropositive character of potassium and sodium oxides permits them to be readily hydrolyzed by water as follows:
+
M20 HzO = 2M+ + 20H(2) Analogous reactions may also be written describing the hydrolysis of the carbonate forms of these elements. Likewise, the hydrolysis of soluble calcium oxides or carbonates takes place upon water contact, whereas most magnesium oxides are essentially inert and are not expected to be appreciably hydrolyzed (18).The magnesium that enters the bulk solution a t this time is most likely associated with soluble calcium compounds or magnesium compounds other than the oxide. 1058
Environmental Science i Technology
PH
Figure 4. Electrophoretic mobilities of Columbia fly ash suspensions as a function of pH
The high pH of water extracts collected during fly ash leaching has been attributed to concurrent hydrolytic reactions of calcium, magnesium, potassium, and sodium ( I 7 ) . Talbot (16) found that hydrolysis of potassium and sodium compounds is most important during the first few minutes of ash leaching. In addition, the continued pH increase after the
2 0
(e) a o
-LOG Ca, [M] -LOG A I . [MI 60
I
4 0
I
II
c
: : : : : : : : : : :
1 0
( 1 ) 4.0
(b)
-LOG Mg, [MI -LOG Si , [MI
5 0
(c)
5.0
-LOG Na, [MI
-LOG Fe , [ M]
6.0
5.0
(h) 8.0
-LOG
P.
[MI 7.0
-
D
O
'
PH
Figure 5. Change in dissolved concentrations with pH
*
Submerged off delta, @ wet delta,
initial leaching was ascribed to the hydrolysis of calcium compounds. The elemental depth profiles of fly ash reported by Linton e t al. ( 2 , 19) parallel these findings. The hopper ash, as expected, produced the highest p H values since it had not undergone previous leaching by pond waters or rainfall. The drop in pH after 1 h of leaching is believed t o result from atmospheric COn entering this highly alkaline system. When CO2 was excluded, the pH remained near 11.0 for several weeks before declining to lower values. Major M a t r i x Elements. When placed in water, solid phase dissolution reactions control the dissolved concentrations of the major elements in fly ash, i.e., aluminum, iron, and silica. The presence of a single, well-defined solid phase is questionable owing to the suite of elements composing fly ash and to its random formation upon leaving the combustion zone of the furnace. Instead, a complex aluminosilicate mixture has been postulated with condensed trace elements surrounding the ash particle (2, 3, 19). It is possible that A1203 and A14C:3 may serve as additional precursors to hydrous aluminum oxides as shown in Equations 3 and 4.
+ 3H20 = 2Al(OH)3(,) A14C3+ 12H20 = 4Al(OH)s(,) + 3CH4(,, A1203
(3) (4)
Iron may initially be present on the fly ash as complex oxides ( 2 0 )and possibly carbonates, sulfates, or carbides (e.g., Fe4C3). Brimblecombe and Spedding (20) suggest that iron originally combined with calcium and aluminum oxides on the ash
dry delta,
*
hoppers
surface should be readily released to the bulk solution. The presence of iron sulfates may also enhance its dissolution rate ( 1 9 ) . Results from this work indicate that these various components of the fly ash are dissolved by hydrolysis reactions and ultimately form hydrous iron oxides which are best described by Fe203.nH20. Figure 5a-c indicates that incongruent dissolution of the fly ash occurs between pH 4.0 and 9.0 in an aqueous system. Aluminum, iron, and silica are released as dissolved components, while amorphous or microcrystalline Al(OH)3 was formed concurrently as a surface precipitate (16). In addition, direct precipitation of aluminum and iron hydrous oxide phases from the bulk solution is expected within this pH region. As evidenced by Figure 5a, aluminum responds with pH (4.5-8.5) in a manner similar to that reported by Parks ( 2 1 ) for aluminum hydroxide systems. Although the solubility of iron is also controlled by its hydrous oxide phase (16),the bulk solution values below pH 3.0 are lower than expected with respect to Fe(OH)3 equilibria. Nevertheless, the solubility of iron shown in Figure 5c is in good agreement with that observed by Byrne and Kester (22)and Schindler (23)for freshly precipitated hydrous ferric oxide. Figure 6 shows that a t the p H r ~ pessentially 100%of the aluminum and iron is associated with various solid phases on the fly ash particle. This would suggest that the presence of hydrous iron and aluminum oxides, along with the aluminosilicate matrix, control the surface characteristics of the fly ash particle over the pH range 4.0-9.0. This heterogeneous Volume 12, Number 9, September 1978
1059
Table 1. Total Elemental Concentrationsa in Columbia Fly Ash Expressed as YO Dry Weight element
AI Ca Cd Fe
K A A1
Mg Na P Si
SI 0
Fc
/
submerged
wet delta
6.3 11.4
