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Published online 24 October 2007
Published in J Environ Qual 36:1784-1792 (2007)
DOI: 10.2134/jeq2007.0143
© 2007 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Heavy Metals in the Environment

The Leaching Characteristics of Selenium from Coal Fly Ashes

Tian Wanga, Jianmin Wanga,*,*, Joel G. Burken1, Heng Ban2 and Ken Ladwig3

a Dep. of Civil, Architectural & Environmental Engineering, Univ. of Missouri, Rolla, MO 65409
b Dep. of Mechanical & Aerospace Engineering, Utah State Univ., Logan, UT 84322
c Electric Power Research Inst. (EPRI), 3420 Hillview Ave., Palo Alto, CA 94304

* Corresponding author (wangjia{at}umr.edu).

Received for publication March 22, 2007.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
The leaching characteristics of selenium from several bituminous and subbituminous coal fly ashes under different pH conditions were investigated using batch methods. Results indicated that pH had a significant effect on selenium leaching from bituminous coal ash. The minimum selenium leaching occurred in the pH range between 3 and 4, while the maximum selenium leaching occurred at pH 12. The release of selenium from subbituminous coal ashes was very low for the entire experimental pH range, possibly due to the high content of calcium which can form hydration or precipitation products as a sink for selenium. The adsorption results for different selenium species indicated that Se(VI) was hardly adsorbable on either bituminous coal ashes or subbituminous coal ashes at any pH. However, Se(IV) was highly adsorbed by bituminous coal ashes under acidic pH conditions and was mostly removed by subbituminous coal ashes across the entire pH range. This result suggests that the majority of selenium released from the tested fly ashes was Se(IV). A speciation-based model was developed to simulate the adsorption of Se(IV) on bituminous coal fly ash, and the pH-independent adsorption constants of HSeO3– and SeO32– were determined. The modeling approach is useful for understanding and predicting the release process of selenium from fly ash.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
SELENIUM is an essential element for plant and animal nutrition at trace levels, but it can cause severe respiratory and neurological problems if uptake exceeds threshold levels (ATSDR, 2003). USEPA set both the maximum contaminant level (MCL) and the maximum contaminant level goal (MCLG) for selenium in drinking water at 50 µg/L (USEPA, 2002). Such contamination may originate from coal fly ash which contains selenium and various other trace elements. The content of selenium in coal fly ash can be as high as 200 mg/kg (Kim, 2002), although it is usually less than 50 mg/kg and is typically in the range of 10 to 20 mg/kg (EPRI, 1987). According to the American Coal Ash Association (ACAA), U.S. power facilities produced more than 71 million short tons (6.4 x 1010 kg) of coal fly ashes in 2005, and 41% were further utilized as concrete products, road bases, etc. (ACAA, 2006). Most of the remaining 59% were disposed of in landfills or impoundments. Therefore, the leaching potential of selenium from fly ash leading to possible contamination of ground and surface water is an environmental concern. Understanding the leaching behavior of selenium from coal fly ash is significant for assessing the potential environmental impact of fly ash.

Selenium leaching from coal fly ash has been investigated previously by various researchers (EPRI, 1987; van der Hoek and Comans, 1996; Jankowski et al., 2004; Iwashita et al., 2005; EPRI, 2006a, 2006b; USEPA, 2006). Most of these studies indicated that pH is a key factor affecting selenium leaching from fly ash. Selenium leaching tends to increase as pH of the aqueous phase is raised, although it may not always be the case. van der Hoek and Comans (1996) have recorded the least leaching of Se at pH 5 to 6 for an acidic ash, and Iwashita et al. (2005) reported a decrease of Se leaching as pH was increased from 8 to 12 for an alkaline ash with high Ca composition. Iwashita et al. (2005) also concluded that the leaching amount of selenium was essentially dependent on its concentration in fly ash, while other studies have found no correlation between the total content of selenium in the ash and the concentration in the leachate (EPRI, 1987; USEPA, 2006). In addition, it has been widely observed that selenate (Se(VI)) is less adsorbable than selenite (Se(IV)) by various minerals such as goethite, iron oxyhydroxide, and montmorillonitic soil (Merrill et al., 1986; Goldberg and Glaubig, 1988; EPRI, 1994, 2006a). Several previous studies have reported that selenium in fly ash and fly ash leachate exists predominantly as Se(IV) (Wadge and Hutton, 1987; Jackson and Miller, 1999; Narukawa et al., 2005).

Several mechanisms have been proposed to interpret selenium leaching behavior from fly ash. Research indicates that the leaching of selenium from bituminous coal ashes is controlled primarily by iron hydroxide adsorption and that from subbituminous coal ash is controlled by calcium precipitation (van der Hoek et al., 1994; van der Hoek and Comans, 1996); the latter generally has a greater content of calcium oxide. Hassett et al. (1991) and Lecuyer et al. (1996) attributed the stabilization of selenium in subbituminous coal ash to the formation of ettringite (3CaO·Al2O3·3CaSO4·32H2O). Aluminum oxide may also contribute to the adsorption of selenite in fly ash (Rajan, 1979; Hansen and Fisher, 1980; van der Hoek and Comans, 1996). Surface complexation models considering surface electrostatic effects have been used to quantify the adsorption/desorption of selenium and other trace elements on various adsorbents (Goldberg, 1985; Goldberg and Glaubig, 1988; Dzombak and Morel, 1990) and proved to be successful in laboratory studies. However, in natural systems, not all the parameters necessary for the surface complexation model are known, and its application is limited for systems with multiple adsorbents and heterogeneous surface sites (Honeyman and Santschi, 1988). Other researchers used a simplified surface complexation approach without surface charge correction to model the sorption of arsenic and selenium on iron hydroxide, and obtained the apparent adsorption constants comparable with literature data (Belzile and Tessier, 1990; van der Hoek and Comans, 1996). Nonetheless, studies are lacking on how to determine the types and quantity of reactive surface sites in field samples for use in surface-complexation modeling.

The objectives of this study were to investigate the overall leaching behavior of selenium from both bituminous coal fly ash and subbituminous coal fly ash, determine the major factors affecting selenium leaching, and develop a simplified surface complexation approach without considering the electrostatic effect, to quantify the reactive surface sites on fly ash and the adsorption of selenium onto fly ash for better understanding the selenium leaching process.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Materials
A total of seven ash samples were used in this study. Ashes #1004, #33, #1008, and #1009 were all collected from one pulverized coal power plant (Plant ID 33106) burning eastern bituminous coal. The plant uses cold-side electrostatic precipitators (ESPs) to capture fly ash. Ashes #1004 and #1009 were collected from the same unit but at different times, when different eastern bituminous coals were being burned; #1008 and #1009 had the same coal source with a higher calcium content (Table 1 ), while #1008 was sampled when an ammonia-based selective non-catalytic reduction (SNCR) system was being tested. Ash #33 was collected from a separate unit burning the same coal as ash #1004, but with ammonia-based flue gas conditioning for the ESP. Ashes #1015, #1018, and #7 were collected from power plants burning primarily subbituminous coal. Ashes #1015 and #1018 came from a cyclone boiler power plant (Plant ID 25410) with cold-side ESPs and that was burning a blend of 80% subbituminous and 20% bituminous coal. Ash #1018 was sampled when a SNCR system was tested. Sample #7 came from a pulverized coal power plant (Plant ID 50213) with hot-side ESPs and that was burning 100% subbituminous coal.


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  		Table 1. Sample characterization.

 
The basic physical/chemical characteristics of these ashes, including BET (Brunauer, Emmett, and Teller) surface area (analyzed using Quantachrome Autosorb-1-C high performance surface area and pore size analyzer; Quantachrome Instruments, FL, USA), pHpzc (denoted as the pH at which the surface charge, surface potential, and {xi} potential are zero), loss on ignition (LOI) (determined using gravimetric methods), and total concentrations of selenium, calcium, and sulfur are shown in Table 1. {xi} potential of fly ash as a function of pH was analyzed using Zetasizer 3000 (Malvern Instruments, Worcestershire, UK) to determine pHpzc because the surface potential couldn't be directly measured. The total Se in fly ash was determined using microwave-assisted acid digestion (0.4 g fly ash + 10 mL HNO3 + 5 mL HF + 5 mL HCl) followed by graphite furnace atomic absorption (GFAA) measurement. The accuracy of the metal determination was demonstrated by using a certified reference material, NIST-1633a (National Institute of Standards and Technology, USA; certified Se = 10.3 ± 0.6 mg/kg, measured Se = 10.2 ± 0.7 mg/kg,). Total Ca and S concentration was determined using X-ray fluorescence spectroscopy (X-LAB 2000, SPECTRO Analytical Instruments GmbH & Co. KG).

Selenium(IV) and Se(VI) stock solutions were prepared from sodium selenite (MP Biomedicals, Inc.) and sodium selenate (Alfa Aesar). Sulfate stock solution was prepared from sodium sulfate (Fisher Scientific). All solutions were prepared using 18.2 mol/L {Omega} deionized (DI) water.

Batch Leaching of Raw Fly Ash
A batch leaching experiment was performed to determine the leaching behavior of selenium from raw fly ash (as obtained from the electrostatic precipitator in power plant) under different pH conditions. Raw ash was dried at 105°C for at least 24 h before use. Ten grams of ash and 100 mL of DI water were added to each of a series of 125 mL LDPE bottles to create a solid/solution ratio (S/L) of 1:10. Different volumes of 1 mol/L HNO3 or NaOH stock solution were added to these bottles to yield final pH values distributed in a range between 2 and 12, and pH was not adjusted during the leaching process and no replicates were made since many pH points were selected in the range. The bottles were sealed and shaken at 180 oscillation/min using an EBERBACH 6010 shaker for 24 h to achieve equilibrium (EPRI, 2005), then allowed to settle overnight. The supernatant was collected and acidified using concentrated HNO3 for selenium analysis using a GFAA spectrometer (AAnalyst 600, PerkinElmer Corp., Norwalk, CT). The final pH in the remaining slurry was measured using an Orion pH meter (perpHecT LoR model 370) equipped with an Orion PerpHecT Triode pH electrode (model 9207BN).

Equilibrium Fly Ash Titration and Selenium Adsorption Experiments
Batch equilibrium titration and Se adsorption experiments were conducted using washed ash. The washing process was used to obtain a relatively clean surface by removing soluble constituents, including selenium, from the fly ash. The washing was conducted using a 0.2 mol/L NaOH solution to remove readily soluble and adsorbed selenium from bituminous coal ashes (#1004, #33, and #1009). For subbituminous coal ashes (#1015, #1018, and #7), only DI water was used as a washing solution because natural pH of these ashes in DI water was already greater than 11. Washing was performed with a solid/liquid ratio of 1:5, and repeated five times. Each washing cycle lasted for 20 h. Air bubbling was used to agitate the ash-water mixture. At the end of each washing cycle, the mixture was allowed to settle for 2 to 3 h, and the supernatant was decanted. The washed ash was dried at 105°C for at least 24 h before use.

The procedures for fly ash titration and Se adsorption experiments were similar to the batch leaching experiments. Batch titration experiments were conducted with a liquid phase of 0.01 mol/L NaNO3 as a supporting electrolyte. The volume of acid or base used, and the corresponding final pH in each bottle were recorded to plot the overall titration curve. The 0.01 mol/L NaNO3 solution was also titrated as a blank. The net titration curve was obtained by subtracting the acid/base consumption by the blank (0.01 mol/L NaNO3 solution) from the overall titration curve for the same pH condition. For the adsorption experiments, the liquid phase contained pre-selected concentrations of selenium in 0.01 mol/L NaNO3 solution. For study on sulfate impact on selenium adsorption, pre-selected concentrations of sulfate were also added into the aqueous phase. All other conditions were the same as those of the batch leaching experiment (Wang et al., 2004).


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
Selenium Leaching from Raw Ash
Figure 1 shows the batch leaching results of selenium for six ashes used in this research. For bituminous coal ashes (ashes #1004, #33, and #1009), the lowest release occurred in pH range 3 to 4. When pH was greater than 4, selenium leaching increased with the increase of pH, whereas at pH below 3, the leaching increased with the decrease of pH. The maximum release occurred at pH close to 12, with concentrations of 2500, 1700, and 2000 µg/L, corresponding to 55, 69, and 67% of total Se, for ashes #1004, #33, and #1009, respectively. By contrast, at their natural pH (4.4 for #1004, 4.5 for #33, and 6.0 for #1009), only 2, 2, and 25% of total Se were released from these ashes, respectively. Ash #1008 was not selected for batch leaching because of the limited amount of this sample.


Figure 1
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  		Fig. 1. Selenium leaching from bituminous and subbituminous coal fly ashes. Experimental conditions: S/L = 1:10; temperature = 20–25°C; equilibration time = 24 h.

 
Se(IV) was reported as the main selenium species in leachate from both bituminous and subbituminous fly ashes (Wadge and Hutton, 1987; Jackson and Miller, 1999; Narukawa et al., 2005), although Narukawa et al. found a higher fraction of Se(VI) in bituminous ash leachate. The pKa values of selenious acid (H2SeO3) are 2.64 and 8.36, respectively (Organisation for Economic Cooperation and Development, 2005). Therefore, when pH is less than 2.6, the neutral H2SeO3 species dominates in the system. Selenium leaching was increased with the decrease of pH below 2.6, indicating that the neutral selenium species may not be readily adsorbed by the ash surface; an alternative explanation is that the low pH might trigger dissolution of some oxidic surfaces, which can also result in the increase of selenium leaching. With the increase of pH, the total concentrations of anionic species HSeO3– and SeO32– would also increase. These anions can be adsorbed to ash surface sites. When pH was further increased, more and more surface species were deprotonated and hindered the adsorption of negatively charged selenium species for the same surface sites, resulting in the increase of selenium leaching.

Figure 1 also shows that the selenium leaching from subbituminous coal ashes #1015, #1018, and #7 was very low at all pH values compared with that from the bituminous coal ashes. The subbituminous coal ashes contained less selenium compared with the bituminous coal ashes (Table 1), but the releases do not appear to be proportional to the total selenium content. The subbituminous coal ashes contained significantly more calcium than bituminous coal ashes, which may reduce selenium leaching through the formation of ettringite under high pH conditions (Hassett et al., 1991; Lecuyer et al., 1996) or precipitation of calcium selenite (Isabel and Annette, 2003).

Impact of Selenium Speciation on Adsorption
Se(VI) Adsorption in Single Species System
Washed bituminous ash #1004 was selected for this experiment. Adsorption experiments were conducted using two Se(VI) additions, 1 mg/L and 2 mg/L, plus the background leaching (without Se(VI) addition). Results are shown in Fig. 2a . The background leaching curve of washed ash indicated that less than 500 µg/L (5 mg/kg ash) of selenium was leached at all pH levels; the reduced leaching was due to selenium elimination by washing process. Comparing soluble selenium concentrations from curves with and without (background) external Se(VI) addition at the same pHs, slight adsorption of Se(VI) was observed at acidic pHs. This behavior might be interpreted with the outer sphere (Hayes et al., 1987) or even inner sphere complexation (Fukushi and Sverjensky, 2007; Rietra et al., 2001) between selenate and surface oxides, which is positively charged at lower pHs. Nonetheless, the adsorption of Se(VI), compared with Se(IV) (See Fig. 2b), was not significant in the entire experimental pH range from 2 to 12. This result is consistent with previous studies on Se(IV) and Se(VI) adsorption on soils (Goldberg and Glaubig, 1988; EPRI, 1994) and goethite (Rietra et al., 2001).


Figure 2
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  		Fig. 2. Soluble selenium concentrations as a function of pH under different selenium addition conditions for 0.2 mol/L NaOH washed ash #1004: (a) Se(VI) and (b) Se(IV). Experimental conditions: S/L = 1:10; ionic strength = 0.01 mol/L NaNO3; temperature = 20–25°C; equilibration time = 24 h.

 
Se(IV) Adsorption in Single Species System
Washed ash #1004 was also used to determine the adsorption of Se(IV) on fly ash under two addition conditions, 1 mg/L and 2 mg/L. Results are shown in Fig. 2b, along with the background leaching results. Se(IV) was much more adsorbed than Se(VI) in the acidic pH range, with the maximum adsorption occurring at pH of approximately 4. Based on pKa values, when pH was less than 2.5 the dominant species of Se(IV) would have been the neutral species (H2SeO3). The decrease of adsorption in this pH range may be due to the poor adsorbability of H2SeO3 or dissolution of ash particles at very low pHs. When pH was greater than 6, the negatively charged different selenium species (HSeO3– and SeO32–) dominated in the system. Due to the decrease of protonated surface sites with the increase of pH, these Se species would have less sorption sites to bind to and thus less Se (IV) was adsorbed as the pH increased. Similar results were also observed for Se(IV) adsorption onto soils and minerals (Goldberg and Glaubig, 1988; EPRI, 1994; Rietra et al., 2001).

Selenium Adsorption in Mixed Species System
To determine the interactive effects of selenium species during adsorption, batch studies adding mixed species were performed using washed ash #1009 (bituminous coal ash) and ash #1018 (subbituminous coal ash). The test solution contained 2 mg/L Se(VI) and 2 mg/L Se(IV). For each ash, the adsorption in single species systems was also determined as a reference. Figure 3 shows the adsorption results plotted as the total soluble selenium concentration as a function of pH.


Figure 3
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  		Fig. 3. Selenium adsorption results in single and mixed species systems for different types of ashes: (a) 0.2 mol/L NaOH washed bituminous coal ash # 1009; and (b) DI water washed subbituminous coal ash #1018. Experimental conditions were same as Fig. 2.

 
The adsorption behavior of Se(IV) and Se(VI) for ash #1009 (Fig. 3a) was similar to that for ash #1004 (see Fig. 2 for comparison). Se(IV) adsorption was at the maximum at pH 3 to 4, and decreased with the increase of pH. Selenium(VI) adsorption was not significant across the entire experimental pH range. The background leaching curves for both raw ash and washed ash were also plotted in the same graph. Comparing all the soluble selenium concentration curves, selenium leaching from the raw ash followed the same trend as the Se(IV) added to washed ash, suggesting that the predominant selenium species of the released Se from fly ash was Se(IV), in agreement with conclusions from previous studies for other ashes (Wadge and Hutton, 1987). These data are also in agreement with field leachate data from bituminous coal ash ponds (EPRI, 2006b). The total selenium concentration in the mixed species system was approximately equal to the sum of selenium concentrations for two single species systems after subtracting the background leaching concentration. Therefore, the adsorption of one selenium species was not affected by the other in the mixed system under the experimental loading condition.

Selenium concentrations in batch solutions for the raw ash #1018 (primarily subbituminous coal ash) and those for the experiment with only Se(IV) addition were negligible compared to that with Se(VI) addition across the entire pH range (Fig. 3b). The leaching curve for the experiment with mixed selenium species addition and that with single Se(VI) species addition overlapped. Therefore, this ash acted as a sink for Se(IV), possibly due to high concentrations of calcium in the fly ash, which can trap Se(IV) through ettringite formation or precipitation (Hassett et al., 1991; Lecuyer et al., 1996; Isabel and Annette, 2003). However, as shown in Fig. 3b, almost all added Se(VI) stayed in the soluble phase in the entire pH range. Therefore, Se(VI) does not adsorb to this fly ash. In terms of adsorption, Se(IV) and Se(VI) did not affect each other during the experiment.

Field leachates collected from subbituminous coal ash landfills, including the landfill serving the power plant where ash #7 was collected, exhibit high selenium concentrations, with almost all of it present as Se(VI) species (EPRI, 2006b). Since the fresh subbituminous ash, including ash #7, exhibited low leaching potential consistent with Se(IV) in these lab studies, the field data may indicate conversion of Se(IV) to Se(VI) under landfill conditions.

Impact of Sulfate on Selenium Adsorption
Sulfate is a common component in coal fly ash, and was reported to compete with selenium for adsorption on several media including goethite, manganese dioxide, and soils (Balistrieri and Chao, 1987, 1990; Goh and Lim, 2004; EPRI, 2006a). Experiments were conducted to evaluate the sulfate impact on selenium adsorption on washed ashes #1009 and #1018 under different sulfate concentrations. All solutions contained 2 mg/L Se(IV), 2 mg/L Se(VI), and 0.01 mol/L NaNO3. For ash #1009, the sulfate concentrations added to the contacting solution were 0, 200, and 500 mg/L. For ash #1018, the sulfate concentrations added to the leaching solution were 0, 500, and 1000 mg/L. The experimental data plotted in Fig. 4 shows soluble selenium and sulfate concentrations as a function of pH for both ashes. No significant impact of sulfate on selenium adsorption was observed since all selenium concentration curves overlap, with the exception of one single point in Fig. 4a around pH 2, which could hardly affect the general conclusion. Apparently, ash #1018 had a higher soluble sulfate background after washing than ash #1009. For ash #1009, most sulfate added into the system remained in soluble phase in the entire experimental pH range. Sulfate does not appear to compete with selenium for adsorption on ash #1009 at the concentration levels studied in this research. For ash #1018, results indicated that the external sulfate tended to be trapped on the surface at lower pHs, which might be due to outer sphere adsorption. In spite of the adsorption potential of sulfate on ash #1018, selenium adsorption was not affected. This conclusion is at odds with previous studies which have found sulfate to influence Se adsorption onto other materials (such as goethite, soil, and manganese dioxide) (Goldberg, 1985; Goldberg and Glaubig, 1988; Balistrieri and Chao, 1990; Glasauer et al., 1995; Wu et al., 2002; EPRI, 2006a). However, fly ash has different properties and characteristics than these materials and this may explain the different outcome observed here.


Figure 4
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  		Fig. 4. Sulfate impact on selenium adsorption for different types of ashes: (a) 0.2 mol/L NaOH washed bituminous coal ash # 1009; and (b) DI water washed subbituminous coal ash #1018. Experimental conditions were same as Fig. 2.

 
Modeling Se(IV) Adsorption on Bituminous Coal Ash
Surface Site Characterization
The surface site density and acidity constant of fly ash are essential parameters for metal adsorption modeling. A previously developed titration method (Wang et al., 2004) was used to determine these parameters. Unlike the widely used surface complexation models (Dzombak and Morel, 1990; Stumm and Morgan, 1996), this method assumes that the solid surface contains more than one mono-protic weak acid site, with independent surface site densities and acidity constants. Based on the relationship between the mass of acid or base used and the corresponding pH in equilibrium, the surface site concentration and acidity constant for each site can be determined through modeling. The model is expressed as:

Formula 1[1]
where {Delta}VSS is the net volume of stock acid/base (negative value for acid) solution consumed by surface sites (mL); V0 is total volume of the ash mixture (mL); STi is the total acid site concentration of species i (M); KHi is the acidity constant of the species i (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 STi = {Gamma}i x SS, where {Gamma}i is the surface site density for species i (mol/g-SS) and SS is the solids concentration (g/L).

After correction using the titration data for blanks, the net titration data for 0.2 mol/L NaOH washed ash #1004 with S/L ratio of 1:10 were plotted as the equilibrium pH as a function of the volume of acid (negative value) or base consumed by fly ash (mL), shown in Fig. 5a . A nonlinear regression program Kaleidagraph (Synergy Software, 2002) was employed for the curve fitting. Results showed that using three surface sites can best fit the experimental data. Table 2 lists the surface site density ({Gamma}) and acidity constant (pKH) for each site, {alpha}, ß, and {gamma}. Since the pHpzc of this ash was 6.4 (Table 1), which is between the pKHs of the site {alpha} and site ß (3.5 and 7, respectively), the protonated surface sites {alpha} is positively charged, denoted as S1OH2+, while the protonated species of the other two surface sites are in neutral form. Titration was also performed with another DI water washed ash #1008; results were displayed in Fig. 5b and Table 1.


Figure 5
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  		Fig. 5. Titration and curve fitting results for (a) 0.2 mol/L NaOH washed ash #1004 and (b) DI water washed ash #1008. Experimental conditions: S/L = 1:10; ionic strength = 0.01 mol/L (NaNO3); temperature = 20–25°C; equilibration time = 24 h.

 

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  		Table 2. Surface site density and acidity constants of 0.2 mol/L NaOH-washed ash #1004 and DI water washed ash #1008.

 
Modeling Se(IV) Adsorption
The protonated surface site {alpha} was hypothesized to be responsible for the adsorption of anionic Se(IV) species. The concentration of the protonated surface site {alpha} is expressed as:

Formula 2[2]
where ST is the total site (protonated and unprotonated) concentration, and {alpha}+ is the fraction of the protonated surface site:

Formula 3[3]
The updated acidity constants of selenious acid, pKa1 and pKa2, are 2.64 and 8.36, respectively (Organisation for Economic Cooperation and Development, 2005). Therefore,

Formula 4[4]

Formula 5[5]
where {alpha}1, and {alpha}2 are the fractions of Se(IV) as HSeO3– and SeO32–, respectively; {alpha}1=Formula 5, and {alpha}2=Formula 5; and [Se(IV)]D is the total dissolved Se(IV) concentration.

Assuming 1:1 stoichiometry between selenium species and the responsible surface sites, the adsorption reactions of selenium species are expressed as:

Formula 6[6]

Formula 7[7]
where KS1 and KS2 are adsorption constants of HSeO3– and SeO32– species, respectively.

The concentration of surface site {alpha} (obtained from batch titration) was 0.024 mol/L at S/L = 1:10 (100 g/L solids), whereas the 2 mg/L of Se(IV) (equivalent to 0.024 mmol/L) was only 0.1% of the total site concentration. It is reasonable to assume that the adsorption is in the linear range of the Langmuir isotherm; the concentrations of adsorbed Se(IV) species are expressed as:

Formula 8[8]

Formula 9[9]
Therefore, the adsorption ratio of Se(IV) is expressed as:

Formula 10[10]
where [Se(IV)]ads is total concentration of adsorbed Se(IV) species.

The Se(IV) adsorption ratio Rexp (experimental data) was calculated using the following equation:

Formula 11[11]
where and [Se(IV)]b represent concentrations of added Se(IV) and background Se(IV), respectively.

Based on Fig. 2b, the total background Se(IV) concentration of ash #1004 was estimated to be 0.50 mg/L. The adsorption ratio of Se(IV) under different selenium addition conditions was calculated using Eq. [11], shown as squares in Fig. 6a . Results indicated that the adsorption ratio curves for different Se(IV) additions overlap, indicating the adsorption was in the linear range of the Langmuir isotherm. The parameters for surface site {alpha}, including the site density and the acidity constant (Table 2), were substituted into Eq. [10] and KaleidaGraph was used to fit the experimental data. The solid line in Fig. 6a is the modeling results. The adsorption constants of HSeO3– and SeO32– (logKs1 and logKs2) on ash #1004 were determined and listed in Table 3 , together with the correlation coefficient R2. As a verification, another ash #1008 was also applied in the adsorption test, the curve fitting results and adsorption constants are shown in Fig. 6b and Table 3, respectively. The modeling results showed reasonable agreement with the experimental data, especially for ash #1008. The deviation between the experimental and modeling data in Fig. 6a may be due to the insufficient data points in certain pH range, or a larger experimental error. Nonetheless, the imperfection of the modeling might also be an indication of other mechanism involved in the adsorption; i.e., Hayes et al. (1987) showed with X-ray adsorption fine structure analysis (EXAFS) that selenite forms binuclear complex on goethite in aqueous suspension. Further study is desired to improve the accuracy of this model while maintaining its simplicity.


Figure 6
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  		Fig. 6. Se(IV) partitioning results–experimental data and modeling result for (a) 0.2 mol/L NaOH washed ash #1004 and (b) DI water washed ash #1008. Experimental conditions were same as Fig. 2.

 

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  		Table 3. Adsorption constants of HSeO3– and SeO32– for ash #1004 and #1008.

 
Since the soluble Se(IV) concentration curves for the washed bituminous coal ash with Se(IV) addition had the same trend as those for the raw ash with no Se(IV) addition, and the washed ash was successfully modeled using only an adsorption approach, it can be concluded that the leaching of Se(IV) from raw bituminous coal ash #1004 is mainly controlled by adsorption. On the other hand, the leaching of selenium from the subbituminous coal ash was very low and did not demonstrate any characteristics typically related to adsorption. It is hypothesized that the high concentrations of calcium in the subbituminous coal ash may control selenium leaching through the formation of ettringite or calcium selenite precipitate (Hassett et al., 1991; Lecuyer et al., 1996; Isabel and Annette, 2003).


   Conclusions
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This research demonstrates that pH is the most important factor affecting selenium leaching from bituminous coal fly ashes, with the lowest release occurring in the pH range 3 to 4. When pH increased above 4, selenium release increased concomitantly. As the pH approached 12, approximately 50 to 70% of the total selenium in the fly ash was released. Adsorption/desorption processes were found to be the main mechanisms controlling selenium leaching from these materials. For subbituminous coal ashes, very little selenium was leached, which may be due to the high calcium content in these ashes. Results from adsorption experiments suggest that Se(IV) was the predominant species in the released selenium from both types of ashes. In addition, Se(VI) was hardly adsorbed by either type of fly ash. Also, sulfate added in solution was found not to significantly impact the adsorption of selenium by either type of ash. A speciation-based adsorption model was capable of predicting Se(IV) adsorption by bituminous coal fly ash, and determining the adsorption constants (logKS) of HSeO3– and SeO32– This model is robust and simpler than other models reported in the literature for quantifying selenium adsorption.


   ACKNOWLEDGMENTS
 
This work was supported by the Electric Power Research Inst. (EPRI), the Missouri Water Resources Research Center (MWRRC), and the Environmental Research Center (ERC) for Emerging Contaminants at the Univ. of Missouri-Rolla (UMR). The authors also thank Dr. C.P. Huang and Mr. Minghua Li at the Univ. of Delaware for providing the zeta potential measurement, and Mr. Lenin Kasthuri for providing the adsorption data for ash #1008. Conclusions and statements made in this paper are those of the authors, and in no way reflect the endorsement of the aforementioned funding agencies.


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