Environ. Sci. Technol. 2004, 38, 6710-6715
Characterizing the Metal Adsorption Capability of a Class F Coal Fly Ash J I A N M I N W A N G , * ,† X I N J U N T E N G , ‡ HAO WANG,‡ AND HENG BAN‡ Department of Civil, Architectural & Environmental Engineering, University of MissourisRolla, Rolla, Missouri 65409, and School of Engineering, University of Alabama at Birmingham, Birmingham, Alabama 35294
The surface physical-chemical characteristics of a class F coal fly ash were studied in an effort to establish a quantitative understanding of metal adsorption. The ash surface acidity (acid site density and acidity constant), surface electrical characteristics, and adsorption constants for selected heavy metal ions were determined using a batch titration method, an electrophoretic method, and a batch equilibrium metal adsorption method, respectively. Results showed that the fly ash has a pHzpc value of 6.2. Its surface contains three types of acid sites. The densities of these acid sites are 2.1 × 10-4, 1.8 × 10-5, and 5.3 × 10-5 mol/g, with acidity constants (pKH) of 2.7, 7.8, and 11.0, respectively. Metal adsorption results indicated that, of the three types of acid sites on surface, only the acid site with 7.8 pKH is responsible for metal adsorption. The adsorption constants (log KS) of Cd(II), Cr(III), Cu(II), Ni(II), and Pb(II) are 4.8, 7.0, 6.4, 4.9, and 8.6, respectively. Adsorption results indicated that the metal adsorption is in the linear range of the Langmuir isotherm if the total metal in the system is less than 10% of the total metal binding site. Results also showed that the presence of anionic metal ions does not affect the adsorption of cationic metal ions by the fly ash.
Introduction According to a survey conducted by the American Coal Ash Association (ACAA), U.S. electric utilities generated 128.7 million tons of coal combustion byproducts (CCBs) in 2002. Fly ash was the largest individual category with 2002 production estimated at 76.5 million tons (1). Approximately one-third of the fly ash was reused in cement and concrete, structural fill, waste stabilization, road base stabilization, etc., while the rest was disposed in landfills or impoundments. The American Society for Testing and Materials (ASTM) C618 specifies two types of fly ash, class F ash and class C ash, for use as a pozzolan or mineral admixture in concrete. Class F ash is normally produced from burning anthracite or bituminous coal. Class C ash is normally produced from burning lignite or subbituminous coal. Class F fly ash contains more silicon dioxide, aluminum oxide, and iron oxide (SiO2 + Al2O3 + Fe2O3) and less calcium than class C fly ash. Leaching of heavy metals from fly ash is a potential concern during ash disposal and beneficial use. Previous * Corresponding author phone: (573)341-7503; fax: (573)341-4729; e-mail:
[email protected]. † Department of Civil, Architectural & Environmental Engineering, University of Missouri-Rolla. ‡ School of Engineering, University of Alabama at Birmingham. 6710
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studies showed that trace elements and metals such as Sb, As, Ba, B, Cd, Cr, Co, Cu, Pb, Mg, Hg, Ni, Se, Ag, and Zn could be leached out from fly ash, resulting in potential surface water, groundwater, and soil contamination (2-5). It was also reported that As, Mn, and Mo concentrations in the leachate from an India subbituminous coal fly ash were higher than the World Health Organization (WHO) recommended value for drinking water; the concentrations of Fe, Mn, and As exceeded the maximum allowable concentrations prescribed by the United States Environmental Protection Agency (U.S. EPA) (6). However, the leaching of Cd, Co, Cr, Ni, Cu, and Pb from the same ash was insignificant (6). The pH significantly affects the leaching of heavy metals. Low pH favors the leaching of metal cations, including Ag, Cd, Cr, Hg, Ni, Pb, and Zn, from fly ashes (4, 7-11). For many coal fly ashes, cationic metal ions are released in very low quantities due to the alkaline nature of the ashes (12). At high pH, precipitation occurs resulting in complete removal of Cu(II), Ni(II), Zn(II), and Pb(II) (13). Under certain conditions, fly ash can be used as adsorbent to remove heavy metals, including Pb, Cu, Zn, Cd, Ni, and Ag, from wastewater (9, 14). Previous research also indicated that the effectiveness of fly ash as an adsorbent improved with an increase of calcium content (9, 15). Cr, As, and Hg can also be effectively adsorbed by some fly ashes under certain conditions (10, 16, 17). Carbon content in fly ash also plays a very important role on metal adsorption. Previous studies indicated that the carbon has significantly higher specific surface area and Cu(II) adsorption capacity than the mineral fraction (8). Research has also indicated that the mercury content in unburned carbon is significantly higher than the mineral fractions, and the unburned carbons have equal or better adsorption capacity for elemental mercury as compared with some general purpose commercial activated carbons at low gasphase mercury concentration (18). It is speculated that the oxygen-containing functional groups may have an important role on mercury adsorption (19). The leaching (or extraction) of heavy metals from fly ash is mostly governed by adsorption/desorption and/or dissolution. Our previous leaching studies for raw ash indicated that the equilibrium concentrations of heavy metals including Cd(II), Cr(III), Cu(II), Ni(II), and Pb(II) in solution are far below the saturation concentration (20). Therefore, the leaching of these metal ions is mostly governed by the adsorption-desorption mechanism. The surface physicalchemical characteristics of fly ash (such as surface site densities, acidity constants, surface electrical characteristics, specific surface area, metal binding capacities, and metal binding strengths), in conjunction with pH condition, govern the metal partitioning in the fly ash. However, the metal adsorption-desorption fundamentals have not been studied sufficiently in the past for coal fly ash. Commonly the specific surface area (BET area) of fly ash is characterized, which ranges from less than 1 m2/g to dozens of m2/g, depending on the carbon content (8). Other surface characteristics relevant to metal adsorption, such as metal binding site densities and acidity constants, have not been quantified. The overall goal of this research was to establish a quantitative description of cationic metal ion adsorption by fly ash based on the surface characterization and metal adsorption studies. The specific objectives of this study were to determine the relevant surface properties and metal adsorption constants of a class F fly ash, and to examine and demonstrate the relationship between the surface characteristics and metal adsorption. Laboratory experiments and 10.1021/es049544h CCC: $27.50
2004 American Chemical Society Published on Web 11/16/2004
theoretical modeling were performed. The result of this study can be used for the fundamental understanding and quantitative description of metal partitioning in fly ash.
system (R) can be expressed as:
R)
Theoretical Aspects Ash Surface Acidity. Ash surface sites can be treated as weak monoprotonic acids. A batch equilibrium acidimetricalkalimetric titration method developed by Wang et al. (21) was used to determine the ash surface acidity, namely, acid site densities and acidity constants. If the surface only contains one type of acid site, the following equation can be used to express the relationship between the net acid/base consumption and the resulting solution pH (21):
∆VSS )
{
V0STKH 1 1 C [H+] + KH [H+]0 + KH
}
(1)
where ∆VSS is the net volume of stock acid/base (negative value for acid) solution consumed by surface sites (mL); V0 is the total volume of the ash solution (mL); ST is the total surface site concentration (M); KH is the acidity constant (M); C is the concentration of the acid/base stock solution (M); and [H+]0 is the hydrogen ion concentration of the control unit (without acid or base addition) (M). Note that the total surface site concentration ST ) Γm × SS, where Γm is the surface site density (mol/g-SS), and SS is the solids concentration (g/L). If the surface contains multiple (n) types of acid sites, the relationship between acid/base consumption and the solution pH can be expressed as:
∆VSS )
∑
{
V0STiKHi
i)1-n
C
1 +
[H ] + KHi
1
+
[H ]0 + KHi
}
(3)
where ∆Voverall is the total volume of acid or base added during titration (mL) and ∆Vwater is the volume of acid or base used for changing the pH of water for the same initial pH condition (mL). ∆Vwater for a certain pH is determined based on the ∆Vwater-pH curve, and the ∆Vwater-pH curve is determined by a titration experiment using a water sample that has the same ionic strength as that in ash leachate samples. Metal Partitioning. Assuming that only free metal ions are present in the system and that only one type of acid site is responsible for the metal adsorption, the modified Langmuir model that considers the pH effect on metal adsorption can be expressed as (22):
{SOM+} )
RHKSST[M2+] 1 + RHKS[M2+]
(4)
where {SOM+} is the adsorbed metal concentration (M); KS is the adsorption constant (M-1); RH is the ratio of free surface site concentration to the total nonmetal-complexed surface site concentration, RH ) KH/{[H+] + KH}; and [M2+] is the free metal ion concentration (M). The metal partitioning or the ratio of metal concentration in the solid phase to the total metal concentration in the
(5)
Integrating eqs 4 and 5, one gets:
R)
RHKSST
(6)
1 + RHKSST + RHKS[M2+]
Equation 6 is a general model that can be used to describe metal partitioning under different metal loading and different pH conditions. Under low metal loading conditions, the metal adsorption is in the linear range of the Langmuir isotherm. The metal partitioning equation can be simplified to
R)
RHKSST 1 + RHKSST
(7)
or
R)
KHKSΓmSS
(8)
KHKSΓmSS + [H+] + KH
Equation 8 shows that, under low metal loading conditions, metal partitioning is independent of the initial metal concentration or the total metal loading of the system. From eq 7, it is easy to derive the metal partitioning equation for a system that contains multiple surface sites provided that the metal adsorption is in the linear range of the Langmuir isotherm:
(2)
where STi is the total acid site concentration of species i (M) and KHi is the acidity constant (M). For a clean ash sample washed with distilled water five times, it is assumed that there are no carbon dioxide or other soluble acid- or base-consuming agents in the system. The net volume of stock acid/base solution consumed by surface sites under a certain pH condition, ∆VSS, can be calculated using the following equation:
∆VSS ) ∆Voverall - ∆Vwater
{SOM+} {SOM+} ) MT {SOM+} + [M2+]
∑R
HiKSiSTi
i)1-n
R)
1+
∑
(9) RHiKSiSTi
i)1-n
where RHi, KSi, and STi are respectively the ratio of the free surface sites to total sites, metal adsorption constant, and total site concentration for type i surface site.
Materials and Methods Ash Sample and Preparation. A class F fly ash generated from a full-scale power-generating unit burning medium sulfur eastern bituminous coal was used in the experiment. The ash was collected at the electrostatic precipitator (ESP). This plant injects ammonia as a flue gas conditioner to improve ESP performance, which resulted in an ash with an ammonia concentration of approximately 150 ppm (by weight). The ash has the BET surface area of 2.6 m2/g, carbon content of 3.6%, and loss on ignition (LOI) of 4.3%. Before performing adsorption experiments, soluble materials including metals were removed from the ash through a washing process using distilled and deionized water (DD water). The solids/liquid (S/L) ratio used for washing was 1:5. Air was used to mix the sample during the washing process. After 10 h of aeration, the air supply was stopped, and the sample was settled. After 2 h of settling, the supernatant was decanted. The procedure was repeated a total of five times. The sample was then heated at 105 °C in an oven until completely dried (usually 24 h). The dried sample was sieved with a 200-mesh sieve (75 µm openings), then mixed, and stored in an airtight container. Through this washing process a relatively clean solids surface was obtained. Batch Titration. Surface site densities and acidity constants of the fly ash were determined through a batch VOL. 38, NO. 24, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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acidimetric-alkalimetric titration method. Three S/L ratioss 1/20, 1/10, and 1/5swere used. The procedure was similar to that described in Wang et al. (21), except that the ash mass was weighed individually for each 125-mL polyethylene bottle. For example, for the set of experiments with S/L ratio of 1:10, 10.00 g of ash and 100.0 mL of water solution containing 0.01 M NaNO3 were added to each of the bottles. Different amounts of 1 M (or 10 M) standard acid or base stock solution were added to different bottles to obtain a desired pH distribution in the pH range from 2 to 12. One bottle was used as a control unit, and its pH was not adjusted. After 24 h of shaking under closed conditions, the final pH values of the mixtures were measured, and the relationship between the pH and the volume of acid/base addition was plotted. By subtracting the acid/base consumed by water solution with the same ionic strength for the same pH condition, the net titration curve (the relationship between the equilibrium pH and the net acid/base consumed by ash) was plotted. Equation 2 was fit to the net titration curve by assuming that the surface contains one or more types of acid sites. For the best fit scenario, the acid site concentrations and their corresponding acidity constants were recorded. Equilibrium Metal Adsorption Experiments. The metal adsorption behavior of the fly ash was investigated using batch equilibrium experiments. The experiment was conducted in both single- and multi-metal systems. The procedure was as follows: First, 10.00 g fly ash samples were added to each of several 125-mL bottles. There were five groups of bottles corresponding to the five initial metal concentrations to be tested. Each group had 14 sample bottles and one blank (no fly ash added). Then, 100.0 mL of water that contained 0.01 M of NaNO3 and a desired concentration of heavy metals were added to all bottles, including the blank (NaNO3 was used to adjust the ion strength). All bottles in the same group had the same initial metal concentration. The pH value in each group of bottles was adjusted with nitric acid (HNO3) or sodium hydroxide (NaOH) to a desired pH distribution in the pH range from 2 to 12. No acid or base was added to the blank. The bottles were tightly sealed and placed on a mechanical shaker for 24 h at 230 rpm. After being shaken, 20 mL of the solution was filtered through a 0.45-µm membrane syringe filter, and acidified. Dissolved metal concentrations were measured. The final pH values were measured on the samples left in the bottles. Chemical Analyses. An atomic absorption spectrometer (AAnalyst 800, Perkin-Elmer Corp., Norwalk, CT) was used to determine heavy metal concentrations in the solutions. An Orion Ross pH electrode was used to measure pH. A Zetasizer 3000 (Malvern Instruments, Worcestershire, UK) was used to determine the surface electric characteristics. Data Analyses. Kaleidagraph (Synergy Software, Reading, PA), a software that has the ability to perform nonlinear regression, was used to fit ash titration curves to determine ash surface acidity and to fit the metal adsorption data to determine metal adsorption constants.
Results and Discussion Surface Acidity. Figure 1 shows the net titration results for the fly ash under three S/L ratioss1/20, 1/10, and 1/5. By fitting the titration curves using eq 2, the total site densities and acidity constants of the fly ash were determined. The best curve fitting results were based on the three-site assumption, as indicated by smooth curves in Figure 1. Therefore, we conclude that the ash surface contains three types of acid sites, denoted as sites R, β, and γ. The concentrations of these acid sites and their acidity constants were also given by the model. Figure 2 plots the acid site concentration as a function of ash concentration for the three types of acid sites. The figure shows that the surface site concentration is linearly related to ash concentration. The 6712
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FIGURE 1. Net titration results for the fly ash under three S/L ratios. Experimental conditions: ionic strength ) 0.01 M (NaNO3), temperature ) 20-25 °C; equilibration time ) 24 h.
FIGURE 2. Relationship of total surface site concentration and solids concentration for the fly ash. slopes of the plots are densities of acid sites. For acid sites R, β, and γ, their densities are respectively 2.1 × 10-4, 1.8 × 10-5, and 5.3 × 10-5 mol/g. The average acidity constants (pKH) for these acid sites are respectively 2.7, 7.8, and 11.0. Commonly, the acidity constants (pKH) of metal oxides range from 6 to 10 (23). Since the class F ash is mainly composed of SiO2 and Al2O3, site β, which has the acidity constant (pKH) of 7.8, is likely to be the main metal adsorption site on ash surface. Since site γ has a very high acidity constant (pKH ) 11.0), it would not normally participate in the metal adsorption reaction since most cationic heavy metal ions form negatively charged metal-hydroxide complexes when pH is close to or greater than 11. The metal-hydroxide complexes are not adsorbable by negatively charged free surface sites. Surface Site Speciation. Figure 3 plots the ζ-potential as a function of pH for the fly ash. ζ-potential is the potential difference between the plane of shear (the layer in which ions move with the particle) and the bulk phase. It reflects the surface electrical characteristics of the fly ash. The pHzpc is the pH at which the surface charge, surface potential, and ζ-potential are zero. If the solution pH is less than pHzpc, the ash surface is positively charged. Otherwise the surface is negatively charged. Figure 3 shows that the pHzpc value of the fly ash sample is 6.2. When pH is greater than 6.2, the surface is negatively charged. As indicated earlier, the pKH for the site β is 7.8. This means that when the solution pH is 7.8, 50% of site β is deprotonated. Therefore, the pHzpc of 6.2 is a result of the deprotonation of the site β, and the deprotonated form of the site β is negatively charged.
TABLE 1. Adsorption Constants of Cd(II), Ni(II), Cu(II), and Pb(II) for the Fly Ash
FIGURE 3. ζ-potential of the fly ash. Ionic strength ) 0.01 M (NaCl).
FIGURE 4. Surface site speciation for a system containing 100 g/L of fly ash.
FIGURE 5. Adsorption of Cd(II), Cr(III), Cu(II), Ni(II), and Pb(II) onto fly ash as a function of pH. Experimental conditions: initial metal concentrations ) 5 mg/L; S/L ) 1:10; ionic strength ) 0.01 M (NaNO3); temperature ) 20-25 °C; equilibration time ) 24 h. Since the surface is already positively charged when pH is less than 6.2, the site R, which has the pKH value of 2.7, could be in the positively charged form if it is protonated. The release of a proton results in a surface site that is neutrally charged. Since the deprotonated form of this acid site is not negatively charged, the R sites do not adsorb cationic metal ions. Figure 4 shows the surface site speciation for a system containing 100 g/L of the fly ash. Metal Adsorption Characteristics. Figure 5 shows the percentage of metal adsorption for Cu(II), Cd(II), Cr(III), Ni(II), and Pb(II) onto fly ash as a function of pH in singlemetal systems. The initial metal concentrations in this experiment are in the range of 5 mg/L. Results indicate that for all metal ions investigated, the metal adsorption increases with the increase of pH. Results also show that Pb(II) is most
metal
log KS
correlation coefficient
Cd(II) Cr(III) Cu(II) Ni(II) Pb(II)
4.8 7.0 6.1 4.9 8.6
0.998 0.996 0.996 0.944 0.983
easily adsorbed by fly ash. At pH 3, over 90% of Pb(II) is adsorbed while less than 10% of Cu(II) is adsorbed, and Ni(II) and Cd(II) are not adsorbed at all. Cd(II) and Ni(II) are adsorbed when pH is greater than 4 and are nearly completely removed from the solution at pH greater than 8. Since the removal of all metal ions is significantly dependent on the pH, it is unlikely that ion exchange with calcium plays a significant role in removing these metals. Both eq 8 (for single type of surface site) and eq 9 (for multiple types of surface sites) were used to fit the metal adsorption data. In neutral and low pH range, adsorption of the free metal ion is the only mechanism for metal adsorption because the formation of metal-hydroxide complexes is not significant. Site γ is not considered in modeling because it does not participate in the metal adsorption reaction in the experimental pH conditions due to its high pKH. Therefore, only site R and site β are considered during curve fitting using eq 9. Curve fitting results indicate that, for all metal ions studied, the adsorption constants for site R are negligible as compared to those for site β; the adsorption constants for site β determined using both eqs 8 and 9 are essentially the same. Based on this result and the surface site speciation information, it can be concluded that site β is the only type of surface site that contributes to metal adsorption for this fly ash. Table 1 shows the metal adsorption constants of these metal ions and the correlation coefficient for the curve fittings using eq 8. It shows that adsorption constants of these metals decreases in the following order: Pb(II) > Cr(III) > Cu(II) > Cd(II) ≈ Ni(II). It is apparent that surface site density and acidity constant are key parameters for metal adsorption modeling. The information on site density and acidity constant enables the determination of the type of surface site responsible for metal adsorption and the metal adsorption constants. In addition, the surface electrical characteristic data provide supplemental information in understanding the surface site speciation under certain pH conditions and in determining which site could possibly be involved in the metal adsorption reaction. Effect of Metal Loading on Adsorption. Effect of metal loading on adsorption was investigated using Ni(II). Batch equilibrium metal uptake experiments under different pH and metal loading conditions were conducted. Figure 6 shows experimental data (points) and model predicted results (smooth curves) for initial Ni(II) concentrations of 1.0, 5.0, 10, 50, and 100 mg/L. The model prediction was essentially based on eq 6, which was modified to reflect the metal partitioning as a function of total initial metal concentration rather than the dissolved metal concentration. The modified form of eq 6 can be written as:
1+ R)
MT 1 + ST RHKSST
x(
1+
)
MT 1 + ST RHKSST
2
-
4MT ST
2MT ST
(10)
where MT is the total metal concentration (M). VOL. 38, NO. 24, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Ni(II) adsorption by the fly ash under five metal loading conditions. Experimental conditions: Ni concentrations ) 1, 5, 10, 50, and100 mg/L; ionic strength ) 0.05 M (NaClO4); temperature ) 20-25 °C; equilibration time ) 24 h. Previously determined parameters including ash metal binding site density, acidity constant, and Ni(II) adsorption constant were used in eq 10 for Ni(II) partitioning prediction. It can be seen that experimental results and model predictions are in good agreement with each other for all initial metal concentrations. Results also indicate that when the initial metal concentrations are less than 10 mg/L, the metal adsorption curves overlap, which indicates that the Ni(II) adsorption is in the linear range of Langmuir isotherm. It can be calculated that if all 10 mg/L Ni(II) is adsorbed by site β, the metal-surface complex occupies less than 10% of the total site β. Results also show that for higher initial Ni(II) concentrations (50 and 100 mg/L), the percentage of Ni(II) adsorption under the same pH condition decreases significantly with the increase of Ni(II) concentration; therefore, the “linear range” assumption is no longer valid. This research indicates that the metal adsorption constant determined under low metal loading conditions using the
simplified model (eq 8) is valid for the prediction of metal adsorption under high metal loading conditions as long as the general form of the metal partitioning equation (eq 10) is used for metal partitioning prediction. Therefore, the efforts needed to determine metal adsorption constant can be greatly simplified if experiments are conducted under low metal loading conditions. Using a similar approach, the pHindependent metal adsorption constants for other metal ions can be determined and used to predict the adsorption behavior of these metals under broader metal loading conditions. Effects of Other Metals on Metal Adsorption. Metal adsorption experiments were conducted under single-metal, 4-metal, and 7-metal systems to determine the impacts of other metals on metal adsorption. The 4-metal system contained 5 mg/L each of Cd(II), Cu(II), Ni(II), and Pb(II). The 7-metal system contained the same amount of Cd(II), Cu(II), Ni(II), and Pb(II) as the 4-metal system plus three anionic metals: 5 mg/L of Cr(VI), 5 mg/L of Se(VI), and 10 mg/L of As(V). Figure 7 shows the metal adsorption behavior for the fly ash in these systems. Figure 7a indicates that the presence of other metals does not affect Cu(II) adsorption. This is because other metals, with the exception of Pb(II), have smaller adsorption constants and could not compete with Cu(II) for adsorption sites. Although Pb(II) can compete with Cu(II) for surface sites, its concentration is so low (5 mg/L) that its adsorption is not significant enough to affect the Cu(II) adsorption capacity of surface sites. Therefore, the adsorption of Cu(II) is in the linear range of the Langmuir isotherm even after all Pb(II) is adsorbed. Figure 7b,c shows that the adsorption of Cd(II) and Ni(II) is affected by the presence of other metals. This is because the Pb(II) and Cu(II) in the metal mixtures are more easily adsorbed by the ash surface site, which reduces the adsorption capacity for Cd(II) and Ni(II). Moreover, Cd(II) and Ni(II) have similar adsorption behavior and compete with each other almost equally for surface sites. Figure 7 also shows that the two kinds of adsorption curves obtained under 4- and 7-metal systems for all cationic metals
FIGURE 7. Metal adsorption as a function of pH in single- and multi-metal systems: (a) Cu(II), (b) Cd(II), (c) Ni(II). 6714
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are the same, indicating that the anionic metals do not affect the adsorption of cationic metals.
Acknowledgments This work was partially supported by the Electric Power Research Institute (EPRI) (EPRI PID043352). Assistance from Ken Ladwig, Project Manager at EPRI, was greatly appreciated. The authors also thank Dr. C. P. Huang, Distinguished Professor of Environmental Engineering at the University of Delaware, for his assistance in determining the electrical characteristics of fly ashes. Conclusions and statements made in this paper are those of the authors, and in no way reflect the endorsement of the funding agency.
Literature Cited (1) American Coal Ash Association (ACAA). 2002 Coal Combustion Product (CCP) Production and Use Survey; 2003; http:// www.acaa-usa.org. (2) Mehnert, E.; Hensel, B. R. Coal combustion by-products and contaminant transport in groundwater. Proceedings, Coal Combustion By-Products Associated with Coal Mining Interactive Forum; 1996; Southern Illinois University at Carbondale; pp 161-171. (3) EPRI. Leaching of Inorganic Constituents From Coal Combustion By-product Under Field and Laboratory Conditions; EPRI Report TR-111773-V1: EPRI: Palo Alto, CA, 1998. (4) Kim, A. G. CCB Leaching summary: survey of methods and results. Proceedings of Technical Interactive Forum, Coal Combustion By-Products and Western Coal Mines; 2002; Golden, CO; pp 179-195; ISBN-885189-08-07. (5) Kim, A. G.; Kazonich, G. Release of trace metals from CCB: maximum extractable fraction. Proceedings, 14th International Symposium on Management and Use of Coal Combustion Products (CCPs); 2001; San Antonio, TX; pp 20-1-20-15. (6) Praharaj, T.; Powell, M. A.; Hart, B. R.; Tripathy, S. Leachability of elements from sub-bituminous coal fly ash from India. Environ. Int. 2002, 27 (8), 609-615. (7) Fleming, L. N.; Abinteh, H. N.; Inyang, H. I. Leachant pH effects on the leachability of metals from fly ash. J. Soil Contam. 1996, 5 (1), 53-59. (8) Lin, C. J.; Chang, J. Effect of fly ash characteristics on the removal of Cu(II) from aqueous solution. Chemosphere 2001, 44, 11851192. (9) Bayat, B. Combined removal of zinc(II) and cadmium(II) for aqueous solutions by adsorption onto high-calcium Turkish fly ash. Water, Air Soil Pollut. 2002, 136 (1-4), 69-92. (10) Rao, M.; Parwate, A. V.; Bhole, A. G. Removal of Cr6+, and Ni2+ from aqueous solution using bagasse and fly ash. Waste Manage.
2002, 22, 821-830. (11) EPRI. Chemical Characterization of Fossil Fuel Combustion Wastes; EPRI Report EA-5321; EPRI: Palo Alto, CA, 1987. (12) Drakonaki, S.; Diamadopoulos, E.; Vamvouka, D.; Lahanistis, M. Leaching behavior of lignite fly ash. J. Environ. Sci. Health, Part A: Toxic/Hazard. Subst. Environ. Eng. 1998, 33 (2), 237248. (13) Ricou, P.; Hequet, V.; Lecuyer, I.; Le Cloirec, P. Influence of operating conditions on heavy metal cation removal by fly ash in aqueous solutions. International Ash Utilization Symposium, University of Kentucky, Lexington, 1999. (14) Weng. C. H.; Huang, C. P. Treatment of metal industrial wastewater by fly ash and cement fixation. J. Environ. Eng. 1994, 120(6), 1470-1487. (15) Bayat, B. Comparative study of adsorption properties of Turkish fly ashes. I. The case of nickel(II), copper(II) and zinc(II). J. Hazard. Mater. 2002, 95 (3), 251-273. (16) Kapoor, A.; Viraaghavan, T. Adsorption of mercury from wastewater by fly ash. Adsorpt. Sci. Technol. 1992, 9 (3), 130147. (17) Pattanayak, J.; Mondal, K.; Mathew, S.; Lalvani, S. B. Parametric evaluation of the removal of As(V) and As(III) by carbon-based adsorbents. Carbon 2000, 38 (4), 589-596. (18) Hwang, J. Y.; Sun, X.; Li, Z. Unburned carbon from fly ash for mercury adsorption: I. Separation and characterization of unburned carbon. J. Miner. Mater. Charact. Eng. 2002, 1 (1), 39-60. (19) Li, Z.; Sun, X.; Luo, J.; Hwang, J. Y.; Crittenden, J. C. Unburned carbon from fly ash for mercury adsorption: II. Adsorption isotherms and mechanisms. J. Miner. Mater. Charact. Eng. 2002, 1 (2), 79-96. (20) Wang, H.; Teng, X.; Wang, J.; Ban, H.; Golden, D.; Ladwig, K. Environmental impact of metal leaching from ammoniated power plant fly ash. 2003 SWE (The Society of Women Engineers) Annual Conference, Birmingham, AL, October 2003. (21) Wang J.; Huang, C. P.; Allen, H. E. Surface physical-chemical characteristics of sludge particulates. Water Environ. Res. 2000, 72 (5), 545-553. (22) Wang, J.; Huang, C. P.; Allen, H. E. Modeling heavy metal uptake by sludge particulates in the presence of dissolved organic matter. Water Res. 2003, 37 (20), 4835-4842. (23) Anderson, M. A.; Rubin, A. J. Adsorption of Inorganics at SolidLiquid Interfaces; Ann Arbor Science: Ann Arbor, MI, 1981; 203 pp; ISBN 0250402262.
Received for review March 25, 2004. Revised manuscript received July 22, 2004. Accepted October 4, 2004. ES049544H
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