ARTICLE pubs.acs.org/EF
Speciation of Selenium, Arsenic, and Zinc in Class C Fly Ash Yun Luo,†,‡,§ Daniel E. Giammar,‡ Brittany L. Huhmann,† and Jeffrey G. Catalano*,† †
Department of Earth and Planetary Sciences, and ‡Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, 1 Brookings Drive, St. Louis, Missouri 63130, United States
bS Supporting Information ABSTRACT: A major environmental concern associated with coal fly ash is the mobilization of trace elements that may contaminate water. To better evaluate proper use of fly ash, determine appropriate disposal methods, and monitor postdisposal conditions, it is important to understand the speciation of trace elements in fly ash and their possible environmental impact. The speciation of selenium, arsenic, and zinc was determined in five representative Class C fly ash samples from combustion of subbituminous Powder River Basin coal using synchrotron-based X-ray absorption spectroscopy to provide an improved understanding of the mechanisms of trace element association with the fly ash. Selenium in all fly ash samples occurs predominantly as Se(IV), with the exception of one sample, in which there was a minor amount of Se(0). Se(0) is likely associated with the high content of unburned coal in the sample. Arsenic exists in the fly ash as a single phase most consistent with calcium pyroarsenate. In contrast, zinc occurs as two distinct species in the silicate glass matrix of the fly ash. This work demonstrates that residual carbon in fly ash may reduce potential Se mobility in the environment by retaining it as less soluble elemental Se instead of Se(IV). Further, this work suggests that As and Zn in Class C fly ash will display substantially different release and mobilization behaviors in aquatic environments. While As release will primarily depend upon the dissolution and hydrolysis of calcium pyroarsenate, Zn release will be controlled by the dissolution of alkaline aluminosilicate glass in the ash.
1. INTRODUCTION Coal-fired power plants are the largest source of electricity generation globally. In 2009, approximately 63 million tons of fly ash were generated in the U.S. as a byproduct of coal combustion.1 About 40% of the fly ash was reused in industry, with the remainder mainly being stored in ash ponds or landfills. Fly ash has been exempted from being classified as a hazardous waste under the Resource Recovery and Conservation Act; however, in light of the ash spill in Tennessee on December 2008,2 new federal and state regulations have been proposed by the U.S. Environmental Protection Agency (EPA).3 To understand the potential environmental impacts of fly ash reuse and storage, a thorough understanding of the physical characteristics, chemical composition, and metal speciation of a given ash is required. This information provides a scientific basis for developing effective strategies to manage fly ash during its reuse and storage, which is critical to the sustained use of coal for energy production. The major physical and chemical characteristics of fly ash are dependent upon the source of the coal, conditions during combustion, and characteristics of the power plant operating system. Properties of interest include density, size, morphology, specific surface area, and chemical and mineralogical composition. Various classification schemes have been created to categorize fly ashes according to their chemical composition, potential industry use, and environmental impacts.4 6 The most often used classification for fly ash is that of the American Society for Testing and Materials (ASTM) International, which classifies ashes based on the major oxide content as either Class C or Class F. The Class C fly ashes are normally produced by burning lignite or sub-bituminous coal and must have at least 50% SiO2, Al2O3, and Fe2O3, whereas the Class F fly ashes are generated by r 2011 American Chemical Society
burning anthracite and bituminous coal and must contain a minimum of 70% SiO2, Al2O3, and Fe2O3. Class C fly ash has a greater calcium oxide content, typically more than 20%, and exhibits better cementitious properties compared to Class F fly ash. A major environmental concern associated with fly ash is the mobilization of toxic minor elements, which include As, Se, and Zn.7 Previous studies of the potential environmental impacts of toxic elements associated with fly ash have included aqueous extractions and spectroscopic characterization of trace metal speciation. Fly ash generated from different coal sources and combustion conditions have been investigated. The aqueous studies mainly involved leaching or sequential extraction methods to determine the speciation and mobility of trace metals in fly ash.8 13 Some studies have been enhanced by ion chromatography coupled with inductively coupled plasma mass spectrometry measurements to identify the oxidation state of selected trace elements in fly ash,10,14,15 an important control on their toxicity and mobility in the environment. Aqueous extraction procedures serve as indirect methods to determine metal speciation, whereas X-ray absorption fine structure (XAFS) spectroscopy provides direct information on molecular-scale metal speciation in fly ash without any sample pretreatment. Prior XAFS studies of trace elements in fly ash primarily determined the oxidation state(s) of metals and metalloids in ash samples using the X-ray absorption near-edge structure (XANES) region of the XAFS spectrum. The metals and metalloids investigated include arsenic, selenium, chromium, Received: April 11, 2011 Revised: May 31, 2011 Published: June 07, 2011 2980
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Energy & Fuels nickel, lead, and zinc.16 24 The extended X-ray absorption fine structure (EXAFS) portion of XAFS spectra, which is more sensitive to the local structural environment of an element, has been examined in only a few studies of trace elements in fly ash. These compared the spectra of samples to those of reference compounds and identified the first coordination shells of the target element.18,24 27 The main objectives of this study are to determine the speciation of selenium, arsenic, and zinc in a set of Class C fly ash samples produced by combustion of sub-bituminous Powder River Basin (PRB) coal. In support of metal speciation measurements, additional physical and chemical characterization was performed. Direct determination of selenium, arsenic, and zinc speciation was made by synchrotron-based XAFS spectroscopy to provide an improved understanding of the mechanisms of trace element association with the fly ash. Statistical analyses of the XAFS spectra coupled to fitting of the spectra to structural models were employed to determine the number of chemical forms of each element present and the dominant species that occur.
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Table 1. Physical Properties and Composition of the Fly Ash Samples 3
density (g/cm )
2.1. Fly Ash Samples. Five Class C fly ash samples were provided
2.2. Physical, Chemical, and Mineralogical Characterization. Powder X-ray diffraction (XRD) was used to characterize the mineralogical composition of the fly ash samples. The measurements were performed at room temperature on bulk samples using a Rigaku Geigerflex D-MAX/A diffractometer operating in a step scan mode with Cu KR radiation generated at 35 kV and 35 mA. XRD patterns were collected from 10° to 45° 2θ with a step spacing of 0.04° and a dwell time of 4 s per point, and the low-angle background was subtracted. Bulk fly ash samples were also sputter-coated with gold and then imaged by a JEOL JSM-7001FLV field emission scanning electron microscope (FE-SEM) to study the particle size and morphology. The surface area of all fly ash samples was measured by N2 adsorption using an Autosorb-1C instrument from Quantachrome, Inc. All samples were degassed at 120 °C for at least 2 h prior to the adsorption experiments. Multipoint Brunauer Emmett Teller (BET) measurements were made, and the BET surface area was obtained by applying the BET equation to the adsorption data. The pH of the fly ash samples was measured within 10 min of the initial mixing of the fly ash solid with deionized water using a 1:20 solid/solution ratio. 2.3. XAFS Data Collection and Analysis. Selenium, arsenic, and zinc K-edge XAFS measurements were conducted at beamline 20-BM-B (PNC/XSD) of the Advanced Photon Source, Argonne National Laboratory. Each fly ash sample was packed in a polycarbonate holder and sealed by Kapton tape. A Si (111) double-crystal monochromator was used to select the incident X-ray beam energy, with the second crystal detuned by 10% to reduce harmonics. The harmonic content was further reduced by insertion of a Rh-coated Si mirror 1 m before the sample. The incident X-ray beam was focused to an approximately
B-ash
C-ash D-ash E-ash
2.59
2.66
2.66
2.56
2.70
28.3 specific surface area (m2/g) fineness 325 sieve (wt % passing) 66.4
1.4 86.1
2.3 89.1
7.5 87.0
1.8 83.0
available alkali (% Na2O equiv)
1.2
1.6
1.9
1.2
1.1
pH
10.33
11.00
11.83
11.62 11.85
major oxides (wt %)
2. EXPERIMENTAL SECTION by a U.S. electric power company. These five fly ashes, denoted as A-ash, B-ash, C-ash, D-ash, and E-ash in this study, were produced at five different coal-fired power plants burning sub-bituminous PRB coal. The samples were collected from electrostatic precipitators by the company and stored dry until they were provided for this study. Total major elemental composition determined by X-ray fluorescence as well as physical properties, including density, fineness, and loss on ignition (LOI), were determined by the power company and provided with the ashes (Table 1). Selected minor element content (Table 1) was determined from sequential extraction of the fly ash samples (see the Supporting Information).
A-ash
SiO2
27.85
35.31
33.50
39.43 36.02
CaO
18.69
24.11
24.39
21.48 25.67
Al2O3
14.38
20.24
19.74
19.07 18.15
Fe2O3 MgO
8.36 4.43
7.43 4.96
7.35 5.78
7.44 4.41
6.16 5.93
SO3
2.93
1.43
1.47
1.05
1.31
Na2O
1.85
1.78
2.05
1.39
1.43
P2O5
1.28
1.30
1.74
1.11
1.74
TiO2
1.08
1.43
1.36
1.37
1.39
K2O
0.73
0.56
0.57
0.68
0.42
BaO
0.59
0.73
0.88
0.65
0.88
SrO MnO
0.29 0.02
0.39 0.03
0.42 0.02
0.32 0.02
0.52 0.01
LOIa
17.52
0.32
0.73
1.57
0.37
total
100.00 100.00 100.00 99.99 100.00
selected minor elements (ppm) Se
15.0
7.4
14.4
10.1
13.5
As
22.7
24.1
26.9
25.1
21.4
Zn
230
218
333
45.0
34.2
a
LOI is loss on ignition, which represents the unburned carbon content in the fly ash samples.
700 700 μm size using a toroidal mirror coated with Pt and a 10 nm Al2O3 overcoat; the focusing mirror is located 2 m downstream of the monochromator. All spectra of fly ash samples were collected at room temperature in fluorescence mode at 90° to the incident beam using a 13-element energy-dispersive Ge solid-state detector, whereas the spectra of model compounds were collected in transmission mode. A metal foil was mounted between two ionization chambers downstream of the sample for energy calibration. The first inflection points of the absorption edges of standard metal foils were used to calibrate monochromator energy. Se and Zn foils were calibrated to 12 658 and 9659 eV, respectively; a Pt foil (LIII-edge of 11564 eV) was used for energy calibration near the As K-edge. Multiple scans were collected and averaged to obtain the raw spectrum for XAFS data analysis. XAFS data processing and analysis were performed using the Athena28 and SIXPack29 interfaces to the IFEFFIT XAFS analysis package.30 The XANES spectra were background-subtracted and normalized to an edge step of 1 using a linear pre-edge function and a second-order post-edge polynomial. Theoretical phases and backscattering amplitude functions for structural fitting of EXAFS spectra were calculated using FEFF 7.0231 from the crystal structure of mansfieldite (AlAsO4 3 2H2O),32 rhombohedral calcium arsenate [Ca3(AsO4)2],33 monoclinic calcium pyroarsenate (Ca2As2O7),34 and franklinite (ZnFe2O4). In the fitting of the As EXAFS spectra, multiple neighboring atoms occurring at interatomic distances separated by less than the resolution of the data (π/2Δk, in Å) were modeled as a single shell. Modeling these as multiple shells would cause their parameters to be highly correlated or require a large number of arbitrary constraints to be placed on the model. 2981
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Energy & Fuels Principal component analysis (PCA), a statistical method for assessing the variance in a data set, was conducted on the series of As and Zn XAFS spectra using SixPACK to determine the number of primary components or phases that are present.35 38 Spectral variance and reconstruction and the indicator function, which minimizes the approximate number of components in a set of spectra,39 were used to determine the number of primary components present in each series of XANES or EXAFS spectra. Target transformations40 of the spectra of model compounds were used to identify potential As or Zn phases occurring in the samples. Target transformation is a statistical method that provides an assessment of the similarity of the spectrum of a model compound to a component in a series of related spectra. It is evaluated using the SPOIL parameter,36,40 which is a non-negative dimensionless number that measures the increase in fit error in the target transformation that results from replacing an abstract principal component with the candidate spectrum. Smaller values of the SPOIL parameter indicate a greater likelihood that the model compound is present in the unknown samples. The value of the SPOIL parameter is affected by the noise in the sample spectra and is thus only applied for relative comparisons among a series of model compounds in the present work, as opposed to being used to assign descriptors of the quality of the match, as has performed previously.36 A number of model compounds were used as reference samples. Elemental selenium and reagent-grade sodium selenite (Na2SeO3, Alfa Aesar), sodium selenate (Na2SeO4, Alfa Aesar), sodium arsenite (NaAsO2, Fluka), and sodium arsenate (Na2HAsO4 3 7H2O, Sigma-Aldrich) were used as oxidation-state standards for Se and As XANES. Calcium arsenate [Ca3(AsO4)2, Alfa Aesar], scorodite (FeAsO4 3 2H2O), and mansfieldite (AlAsO4 3 2H2O) were used as reference compounds for arsenic in fly ash. Scorodite was synthesized using a method by Harvey et al.41 Mansfieldite was prepared using a modification of the scorodite synthesis by substituting ferric chloride with aluminum chloride. Gahnite (ZnAl2O4), willemite (Zn2SiO4), and hemimorphite [Zn4Si2O7(OH)2 3 H2O] were obtained from the Washington University in St. Louis mineral collection. A synthetic sample of Zn hydrotalcite [Zn6Al2(OH)16 3 xH2O] was prepared according to the method by Trainor et al.42 Reagent-grade zincite (ZnO, Alfa Aesar), franklinite (ZnFe2O4, Alfa Aesar), and smithsonite (ZnCO3, Alfa Aesar) were also used as zinc model compounds.
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Figure 1. Scanning electron micrographs of fly ash particles.
3. RESULTS 3.1. Physical Properties. Physical characteristics of the fly ash samples can be used to better understand their properties and behaviors in the environment. The five fly ash samples show similar density, and all have high lime (CaO) content (19 26%) which resulted in high pH values (Table 1). The specific surface area of fly ash is correlated with the LOI value, which represents unburned carbon in the fly ash.43,44 In this study, the specific surface area and the LOI (Table 1) of the fly ash samples have the same rank order of A-ash > D-ash > C-ash > E-ash > B-ash, which is consistent with the expected greater specific surface area for higher LOI. SEM demonstrates that the fly ash particles exhibit a similar spherical shape in all samples (Figure 1) and occur in the size range of hundreds of nanometers to several micrometers. 3.2. Chemical and Mineralogical Composition. The compositions (Table 1) demonstrate that all samples studied are Class C fly ashes, with the percentage of SiO2 + Al2O3 + Fe2O3 ranging from 51 to 66%. According to an alternative classification scheme,5 all samples fall into the calsialic category, which represents high concentrations of calcium and silicon. The selenium, arsenic, and zinc content of the fly ash samples range from 7.4 to 15.0 ppm, from 21.4 to 26.9 ppm, and from 34.2 to 332.7 ppm, respectively.
Figure 2. XRD patterns showing crystalline phases in fly ashes. Diagnostic lines for the dominant mineral phases present are shown.
The XRD patterns for all fly ash samples (Figure 2) display a similar broad, low intensity feature between about 17° and 37° 2θ. This is consistent with the presence of a high content (>50 wt %) of calcium aluminosilicate glass.45 47 Quartz (SiO2) is the predominant crystalline phase, and the minerals anhydrite [Ca(SO4)], gehlenite [Ca2Al(AlSi)O7], merwinite [Ca3Mg(SiO4)2], periclase (MgO), and minor amounts of lime (CaO) are also present. The high concentration of anhydrite in A-ash and C-ash reflects 2982
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Energy & Fuels their high sulfur content and its interaction with CaO during combustion. 3.3. Metal Speciation. 3.3.1. Selenium. Selenium K-edge XANES spectra (Figure 3) indicate that Se(IV), selenite, is the dominant oxidation state in all of the fly ash samples analyzed in this study, except for the A-ash in which Se(0), elemental Se, was also observed. Evaluation of the selenium local structural environment and phase identification from the EXAFS spectra were hindered by the relatively low concentration of selenium in the fly ash samples and the interference from the Pb LIII-edge approximately 370 eV above the Se K-edge. 3.3.2. Arsenic. Both arsenic K-edge XANES and EXAFS spectra were collected for the five fly ash samples. A comparison of arsenic K-edge XANES spectra of the samples (Figure 4A) to spectra of oxidation-state standards demonstrates that arsenic is
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predominantly As(V) in all samples. The number of unique arsenic species present in the samples was evaluated by PCA of the EXAFS spectra. The XANES spectra of As in the fly ash samples were not used for PCA analysis because of the high intensity of the white line at 11 874 eV, which dominates the spectral variance and, thus, renders the XANES spectra less sensitive to As speciation beyond the oxidation state. The indicator function minimum obtained from PCA of the EXAFS spectra occurred for the first primary component, which indicates that there is only one major form of arsenic in the samples. This single component reconstructs the experimental spectra well, despite accounting for only 70% of the variance, which is likely due to the high noise level at high k (>10 Å 1). To assess the similarity of select model compounds to the single arsenate species in the series of fly ash samples, the spectra of calcium arsenate, mansfieldite, and scorodite were target-transformed using the one principal component obtained by PCA (Table 2). The SPOIL values suggest that calcium arsenate and mansfieldite are better candidates for representing As species present in the samples than scorodite. Visual comparison of the spectra (panels B and C of Figure 4) clearly demonstrate that scorodite does not occur in these samples. To further assess arsenic speciation, the EXAFS spectra were fit to a series of structural models. Three models were considered: mansfieldite (AlAsO4 3 2H2O),32 calcium arsenate [Ca3(AsO4)2],33 and calcium pyroarsenate (Ca2As2O7).34 This analysis was conducted in part because an experimental spectrum for calcium pyroarsenate was unavailable. The crystal structures of all phases were known, providing local structural models to refine versus Table 2. Results of the Target Transformation of the As K-Edge EXAFS Spectra of Potential Phases Using a OneComponent System
Figure 3. Se K-edge XANES spectra of the fly ash samples compared the white line positions of selenium in different oxidation states.
model
EXAFSa one-component SPOIL
Ca arsenate
1.8
mansfieldite
2.1
scorodite a
4.2
EXAFS spectra k range = 2.5 11 Å
1
and k weight = 3.
Figure 4. As K-edge (A) XANES and (B) EXAFS spectra and (C) the magnitudes of the Fourier transforms of the EXAFS spectra of the fly ash samples and arsenic standards. 2983
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the data. Detailed fitting parameters and results can be found in Table S1 and Figure S1 of the Supporting Information. On the basis of the comparison of the reduced χ2 values (Table 3), the calcium pyroarsenate model described the spectra significantly better than mansfieldite and calcium arsenate models for every sample. This suggests that calcium pyroarsenate is the dominant arsenic phase in the fly ash samples. The relatively small Debye Waller factors of both the first and the second shells further confirms that arsenic is in a crystalline phase, instead of a less ordered glass phase. 3.3.3. Zinc. Zinc K-edge XANES and EXAFS spectra were collected on the fly ash samples (Figure 5). PCA of both the XANES and EXAFS spectra were used to evaluate the number of zinc phases present in the fly ash samples. One component did not satisfactorily reconstruct the XANES spectra, especially for the A-ash and C-ash; inclusion of a second component substantially improved the reconstruction. Further, for the EXAFS spectra, the Table 3. Reduced χ2 Values for the Fitting of Different Structural Models to the As K-Edge EXAFS Spectra of the Fly Ash Samples
a
first primary component accounts for only 67% of the spectral variance; inclusion of a second component substantially improved overall spectral reconstruction and accounted for 77% of the variance. These statistical analyses of the XANES and EXAFS spectra demonstrate that the series of zinc samples contains two distinct forms of zinc. Target transformation of the XANES and EXAFS spectra of zinc model compounds were performed using the two principal components (Table 4). On the basis of the SPOIL values obtained for both the XANES and EXAFS spectra, willemite (Zn2SiO4) and hemimorphite [Zn4Si2O7(OH)2 3 H2O] are most similar to the zinc species in the samples. However, the SPOIL values do not suggest that these phases are actual components of the samples, and the spectra of these zinc silicates clearly do not match the data. The crystal structures of other anhydrous and Table 4. Results of the Target Transformation of the Zn K-Edge XANES and EXAFS Spectra of Potential Phases Using Two Components model
XANES SPOILa
EXAFS SPOILb
willemite
4.6
2.9
hemimorphite
5.6
3.1
mansfieldite
orthoarsenate
pyroarsenate
sample
(AlAsO4 3 2H2O)
[Ca3(AsO4)2]
(Ca2As2O7)
smithsonite
7.0
7.8
A-ash B-ash
3.6 5.5
3.3 5.8
2.2 4.7
zincite franklinite
4.4 4.9
7.9 12.0
C-ash
1.5
1.2
1.0
gahnite
27.3
8.5
D-ash
11.7
14.6a
9.5
Zn hydrotalcite
13.8
9.0
E-ash
9.3
8.1a
4.9
The fit resulted in some parameters with large uncertainties.
a
XANES spectra E range = 9650 9700 eV. b EXAFS spectra k range = 2.5 11 Å 1 and k weight = 3.
Figure 5. Zn K-edge (A) XANES and (B) EXAFS spectra and (C) the magnitudes of the Fourier transforms of the EXAFS spectra of the fly ash samples and zinc standards. 2984
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Table 5. Results of Fitting a Single Oxygen Shell to the Zn K-Edge EXAFS Spectra of the Fly Ash Samplesa sample
Nb
R (Å)c
σ2 (Å2)d
ΔE (eV) e
χv2 f
A-ash
4.2(4)g
1.957(7)
0.009(1)
0(1)
3.94
B-ash C-ash
3.9(4) 3.6(4)
1.967(8) 1.945(8)
0.008(1) 0.007(1)
2(1) 0(1)
8.01 4.63
D-ash
3.8(4)
1.955(9)
0.008(1)
0(1)
6.91
E-ash
4.5(6)
1.958(10)
0.010(1)
1(2)
4.51
All fitting was conducted using the same parameters: k range, 3.0 10.8 Å 1; R range, 1.1 2.5 Å; k weight, 3; and FT window, Hanning (dk = 1 Å 1). b Coordination number. c Interatomic distance. d Debye Waller factor. e Difference in the threshold energy between the experiment and theory. Constrained to be the same value for all shells fit to a sample spectrum. f Reduced χ2, a goodness-of-fit parameter. g The estimated standard deviations in parentheses represent the uncertainty in the last digit. a
hydrous zinc silicates48 52 have similar Zn coordination environments and, thus, similar spectral features, as willemite and hemimorphite. The spectra of these phases are also likely inconsistent with the fly ash spectra. These considerations suggest that, while zinc is associated with silicate phases in the fly ash, it is not present as a specific crystalline zinc silicate. Given the determination of two distinct zinc species in the fly ash samples, it is not feasible to conduct a full structural analysis of the EXAFS spectra. Only the first oxygen shell around Zn was modeled to establish the coordination state of zinc. Fitting of the EXAFS spectra (Table 5) determined first oxygen shell coordination numbers from 3.6 ( 0.4 to 4.5 ( 0.6 (average of 4.0 ( 0.2) and Zn O distances from 1.945 ( 0.008 to 1.967 ( 0.008 Å (average of 1.956 ( 0.004 Å), which indicate that zinc is tetrahedrally coordinated in all samples. Previous studies of zinc in silicate, aluminosilicate, and borosilicate glasses53,54 have found that the local coordination environment of Zn is always tetrahedral, with similar Zn O distances of 1.96 ( 0.02 Å. In addition, glass in fly ash has been found to be compositionally variable in a single sample,55 suggesting that multiple coordination environments should exist for metals substituting in this component of the ash. Given the spectral similarity to zinc silicate minerals and the tetrahedral coordination of zinc, we conclude that zinc likely exists in the silicate glass matrix of the fly ash samples. The two components identified by PCA indicate that there are two distinct zinc species that likely differ in their coordination to the tetrahedral silicate network of the glass.
4. DISCUSSION The results provide new insight into selenium, arsenic, and zinc speciation in Class C fly ash. The presence of selenium in the fly ash samples as selenite is consistent with previous studies.22,23,26 Huggins et al.26 also observed both elemental selenium and selenite in a fly ash derived from PRB coal and suggested that Se(0) originated from an organoselenium compound in Se-doped activated carbon that was used to enhance postcombustion mercury capture. However, the power plant producing the A-ash in our study did not inject activated carbon into the system. This high carbon content (indicated by the high LOI) likely results instead from incorporation of unburned coal into the sample. Past studies have observed that selenium exists in an elemental or organoselnium form in coal, consistent with the shoulder seen in the XANES spectrum of the A-ash.22,23,26 However, there is no correlation among our samples between
LOI and selenium content (R2 = 0.25), and fly ash is typically substantially enriched in selenium relative to coal.45 It is thus unlikely that the reduced selenium in the A-ash sample originates in the unburned coal because this should represent only a minor component of the selenium in the sample. Rather, the observation of Se(0) in the A-ash suggests that the carbon in the sample either reduced selenium from a more oxidized form (e.g., selenite) or prevented its oxidation to selenite upon condensation of the ash. A number of studies have investigated arsenic speciation in fly ash from different sources using XAFS spectroscopy.18,19,22 27 Those studies mainly focused on the oxidation state of arsenic, and the results showed that arsenic exists predominantly as As(V) in fly ash, with a small portion of As(III) present in fly ash generated from bituminous coal. Zielinski et al.27 concluded that arsenic occurs as a phase similar to calcium orthoarsenate in Class C fly ash. In contrast, Goodarzi and Huggins18 argued for the possible incorporation of arsenic into the glass matrix because of the relatively featureless profiles of the XANES spectra. The present study identifies calcium pyroarsenate as the likely arsenic host phase in the alkaline Class C fly ash samples. This identification is consistent with past kinetic and speciation studies of arsenic vapor captured by lime and calcium silicates.56,57 Less is known about Zn speciation in fly ash because few studies have examined this element. Shoji et al.24 reported that the dominant form of Zn in Class F fly ash is ZnFe2O4 but were unable to determine the Zn speciation for a Class C ash, which they attributed to the possible presence of multiple hosts. The present study demonstrates that Zn exists in two different forms in the Class C fly ash samples examined. Although the exact speciation cannot be determined, it is clear that two distinct Zn species exist, which are both tetrahedrally coordinated and associated with Si, strongly suggesting incorporation into the less ordered silicate glass matrixes in Class C fly ash. This speciation information also provides new insight into the potential release of these elements into aquatic environments. The predominant occurrence of selenite demonstrates that, once released, selenium migration will be strongly affected by adsorption, because Se(IV) adsorbs more strongly and over a greater pH range than Se(VI).58 In addition, the presence of substantial elemental selenium in fly ash containing unburned coal suggests that postcombustion addition of carbon may serve to limit selenium release by converting it to a less soluble form. This work also shows that As and Zn release will be primarily dependent upon the dissolution of solid phases in the ash. However, these two elements will likely display distinct release behaviors because As mobilization will depend upon the dissolution and hydrolysis of calcium pyroarsenate, whereas Zn release will depend upon the dissolution of calcium aluminosilicate glass. These solids are predicted to show solubilities and dissolution rates that have different dependencies on aqueous composition. Calcium pyroarsenate will have greater solubility under acidic conditions, while calcium aluminosilicate glass will be more soluble under alkaline conditions, where it undergoes pozzolanic reactions. Because of the speciation of these elements, desorption is not expected to be a primary control on their release from Class C fly ash.
5. CONCLUSION The speciation of selenium, arsenic, and zinc was determined in five Class C fly ash samples using XAFS spectroscopy. Knowledge 2985
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Energy & Fuels of metal speciation is required to predict the environmental impacts of fly ash disposal and reuse. Selenium occurs as predominantly Se(IV), except when a substantial content of unburned coal is present, presumably because this acts as a reductant, forming elemental Se. Arsenic likely occurs as a solid calcium pyroarsenate phase, whereas zinc substitutes into the calcium aluminosilicate glass. This demonstrates that As and Zn in Class C fly ash will display distinct release and mobilization behaviors in aquatic systems that depend upon the composition of the aqueous phase.
’ ASSOCIATED CONTENT
bS
Supporting Information. Sequential extraction methods, As K-edge EXAFS fitting results for the fly ash samples using three different structural models (Table S1), expected local coordination environment of aluminum arsenate (AA), calcium orthoarsenate (COA), and calcium pyroarsenate (CPA) (Table S2), and EXAFS structural fitting results for fly ash samples (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Telephone: +1-314-935-6015. Fax: +1-314-935-7361. E-mail:
[email protected]. Present Addresses
§ Gemological Institute of America, Carlsbad, California 92008, United States.
’ ACKNOWLEDGMENT Financial support for this work was provided by the Consortium for Clean Coal Utilization, Washington University in St. Louis. Pacific Northwest Consortium/X-ray Science Division (PNC/XSD) facilities at the Advanced Photon Source and research at these facilities are supported by the Basic Energy Sciences, U.S. Department of Energy (DOE), a Major Resources Support Grant from the Natural Sciences and Engineering Research Council of Canada (NSERC), the University of Washington, Simon Fraser University, and the Advanced Photon Source. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. DOE Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract DE-AC02-06CH11357. Steve Heald and Dale Brewe are thanked for providing beamline support during data collection. ’ REFERENCES (1) American Coal Ash Association (ACAA). Corrected 2009 Coal Combustion Product (CCP) Production and Use Survey; http://acaa. affiniscape.com/associations/8003/files/2009_CCP_Production_Use_ Survey_Corrected_020811.pdf. (2) Tennessee Valley Authority (TVA). Corrective Action Plan for the TVA Kingston Fossil Plant Ash Release; TVA: Knoxville, TN, 2009. (3) United States Environmental Protection Agency (U.S. EPA). Fed. Regist. 2010, 75, 35128–35264. (4) Mattigod, S. V.; Rai, D.; Eary, L. E.; Ainsworth, C. C. J. Environ. Qual. 1990, 19, 188–201. (5) Roy, W. R.; Griffin, R. A. J. Environ. Qual. 1982, 11, 563–568. (6) Vassilev, S. V.; Vassileva, C. G. Fuel 2007, 86, 1490–1512.
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