Energy & Fuels 1993, 7, 482-489
402
Nondestructive Determination of Trace Element Speciation in Coal and Coal Ash by XAFS Spectroscopy Frank E. Huggins,' Naresh Shah, Jianmin Zhao, Fulong Lu, and G. P. Huffman 233 Mining and Mineral Resources Building, University of Kentucky, Lexington, Kentucky 40506 Received January 25, 1993. Revised Manuscript Received April 12, 1993
The environmental impact of specific trace element species in coal utilization and waste disposal depends not only on the abundance but also on the form(@of occurrence of the element present in coal and coal ash. While there are a number of analytical methods for determining the abundance of trace elements in coal and ash, there are very few methods available for determining the form of occurrence (speciation) of a trace element in such materials at abundances as low as 10 ppm. In this report, the potential of XAFS spectroscopy for trace element speciation is demonstrated by means of measurements on two environmentally important trace elements, arsenic and chromium, in coal and ash. Results indicate that arsenic in coal is most commonly present as arsenical pyrite (in which form it may readily oxidize to arsenate, AsO4%,even under ambient storage conditions), and that arsenic is present as arsenate in ash and slag. Conversely, chromium is present as Cr3+in coal and remains in that oxidation state in ash and slag.
Introduction
A number of trace elements that are of concern to air quality standards are included in the list of potentially hazardous substances in the 1990 Clean Air Act Amendmenta.' These elements, which include antimony, arsenic, beryllium, cadmium, chromium, cobalt, lead, manganese, mercury, nickel, and selenium, have become known collectively as the "air-toxics" group of elements. As a result of the large tonnages of coal combusted in the U.S.A., coal combustion is viewed as a major potential source of several of these elements in the environment, even though these elements are generally present in most US. coals in % addition, except for trace amounts (lo0 ppm), such as manganese and strontiuml5. To date, experiments with trace elements in coal have not been attempted at the abundance levels (5-20 ppm) more typical of the air-toxics elements in coal. In this report, the application of XAFS spectroscopy to the speciation of trace elements at this abundance level is demonstrated by a survey investigation of chromium and arsenic in coal and ash. Both of these elements have been identified as potentially hazardous to the environment in the 1990Amendments to the Clean Air Act.l In addition, some important new observations and results related to the forms of occurrence of these elements in coal and ash will be presented.
Experimental Section
(9) Minkin, J. A,; Chao, E. C. T.; Thompson, C. L.Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1979,24(1), 242-249. (10)Finkelman, R. B.; Simons, D. S.;Dulong, F. T.; Steel, E. B. Znt. J. Coal Geol. 1984,3, 279-289. (11)Morelli,J. J.;Hercules,D.M.;Lyons,P.C.;Palmer,C.A.;Fletcher, J. D. Mikrochim. Acta, 1988,111,105-118. (12) Finkelman, R. B.; Palmer, C. A.; Krasnow, M. R.; Aruscavage, P. J.; Sellars, G. A.; Dulong, F. T. Energy Fuels 1990,4,755-766. (13)Wong, J.; Spiro, C. L.; Maylotte, D. H.; Lytle,F. W.; Grwgor, R. B. In EXAFS and Near-Edge Structure ZII; Hodgson, K. O., Hedman, B., Penner-Hahn J. E., Eds.; Springer Proc. Phye. Springer: New York,
___
1984. _ _ _Vnl. ._ , _2I. rnn r 362-367.
(14) Silk, J. E.; Hansen, L. D.; Eatough, D. J.; Hill, M. W.; Mangeleon, N. F.; Lytle, F. W.; Greeaor, R. B. Physica E (Proc. XAFS V), 1989,158, 241-240. (15) Huggins, F. E.; Huffman, G. P.; Bauer, S.H. J. Coal Qual. 1989, 8(3,4), 119. See also: Bauer, S. H. Structural Environment of Metallic Constituents in Coal; Final Report, DOE/PC/90520-7, DOE Contract NO.DEFG22-86PC90520,1988. (16) Palmer, C. A. Energy Fuels 1990,4,436-439. (17) Robertson, J. D.; Wong, A. Personal communication, September 1992. (18) Helble, J. J. Personal communication, February 1992. (19) Glick, D. S. Personal communication, 1987. (20) Ruppert, L.F. Personal communication, March 1992. (21) Ruppert, L. F.; Minkin, J. A.; McGee, J. J.; Cecil, C. B. Energy Fuels 1992,6, 120-125.
Samples. For this survey study, coal samples were obtained from a variety of sources. Samples of varying arsenic contents (10-70 ppm) and chromium contents (20-160 ppm) were obtained from the Argonne Premium Coal Sample (APCS) program, and from the coal sample bank administered by the Pennsylvania State University, including the Department of Energy Coal Sample (DECS) Bank. In addition, the Upper Freeport coal from the APCS program was used to generate the following specific gravity fractions: a 1.6 float sample, a 1.6 sink-2.85 float sample, and a 2.85 sink sample. These fractions were prepared by centrifugation using perchloroethylene (CeC4, sp. gr. 1.6) and bromoform (CHBra, sp. gr. 2.85) as the heavy liquid separation media. Both a freshly opened APCS vial of Upper Freeport coal and a similar sample from a vial opened for a few months and stored in the laboratory were prepared in thismanner. Samples of coals with unusually high arsenic contents were provided by Ms.Leslie Ruppert of the U.S. Geological Survey (Reston, VA). A list of analyzed samples and their arsenic and chromium contents is shown in Table I. Ash samples were generated from a Beulah lignite, a parent Illinois No. 6 coal, and a cleaned Illinois No. 6 coal, which had been prepared from the parent coal by the spherical oil agglomeration process (SOAP), in drop-tube combustion studies performed a t PSI Technology (PSIT), by Dr. J. Helble, and in larger scale combustion experiments performed at the University of Arizona (UAZ),under the direction of Prof. T. Peterson. Some of these samples were collected on an Anderson impactor and were subdivided into a coarse, intermediate, and f i e fiter fractions. Drop-tube ash samples were also generated a t PSI Technology from the Pittsburgh and Illinois No. 6 DECS coals and bottom ash samples were obtained from combustion of Kentucky No. 9 and Upper Freeport coals in the University of Arizona combustion unit. Except for the ash samples generated from the two DECS coals, the chromium and arsenic contents of the ash samples have not yet been determined. The ash samples are listed in Table I. XAFS Spectroscopy. X-ray absorption f i e structure (XAFS) spectroscopy was performed at beam-line X-19A at the National Synchrotron Light Source,Brookhaven National Laboratory, and at beam-line IV-3 a t the Stanford Synchrotron Radiation Laboratory, Stanford University. For chromium in coal and ash samples, the absorption of X-rays was measured over the spectral range from 5.9 to 6.2 keV; in the X-ray absorption near-edge structure (XANES)region (5.97-6.03 keV), absorption data were collected every 0.2 eV. For arsenic in coals and ash samples, the absorption of X-rays was measured over the spectral range from 11.75 keV to as high as 13.0 k e y in the XANES region (11.86 11.90 keV), absorption data were collected every 0.26 eV. Absorption of the X-rays was measured by means of a 13-element germanium array detector22 that detectad the fluorescent X-rays only in a specified tunable energy window that corresponded to the energy of the chromium or arsenic fluorescent Ka X-rays. In addition, the appropriate 6~ vanadium or germanium filter was usedto minimize background. Up to 10 scans were recorded and summed for the weakest absorbers. The spectral summations, the pulse-height windowing, and the low-energy filtering all contributed to enhance the signal-to-noise ratio of the trace element spectra. For reference standards, conventional ion chambers were used to measure both the fluorescent and absorption XAFS spectra in a single scan. Depending on the amount of arsenic or chromium in the standard, the standard was diluted in graphite to optimize the absorption spectrum. As is normally done,a* the chromium or arsenic XAFS spectra were divided intoseparate XANES and, where feasible, extended (22) Cramer,S. P.; Tench,O.;Yocum, N.; George,G. N.Nucl. Instrum. Meth. 1988, A266, 686-591. (23) Lee, P. A.; Citrin,P. H.; Eisenberger, P.; Kincaid, B. M.Reu. Mod. Phys. 1981,53, 769-806. (24) Konigsberger, D. C.; Prins, R. X-ray Absorption Spectroscopy; J. Wiley & Sons: New York, 1988.
484 Energy & Fuels, Vol. 7, No. 4, 1993
Huggins et al.
Table I. List of Coals and Ash Samples Investigated, Their Arsenic and Chromium Contents, and Their Source. As Cr coal sample source (ppm) (ppm) methodb UDoer FreeDort, PA APCS 17 20 INAA'B INAA16 P&ahontas-N0..3, VA APCS 10 INAA'S APCS Lewiston-Stockton, 36
wv
Illinois No. 6 Pittsburgh No. 8 Illinois No. 6 Illinois No. 6 Parent Illinois No. 6 SOAP Beulah Lignite, ND Ohio No. 5, Anthracite No. 2, PA American seam, AL (Pottsville fm.) Pratt seam, AL (Pottsville, fm.) Jefferson, AL Upper Freeport, PA Upper Freeport, PA 1.4 float Pittsburgh DECS-12 PSIT drop-tube ash Illinois No. 6 DECS-2 PSIT drop-tube ash Illinois No. 6 SOAP 11% 0 2 PSIT drop-tube ash Illinois No. 6 SOAP 21%
APCS DECS 12 DECS 2 PSIT PSIT PSIT PSOC 760 PSOC 629 USGS
-
2170
-
USGS
600
-
ref 20
USGS USGS USGS
434 670 25
-
ref 20 refs 20, 21 refs 20, 21
DECS 12
31
121
PEE17
DECS 2
21
260
PIXE"
PSIT
-
X
PSIT
-
X
69 10 (1) (1) (9)
-
33 10
30 16 16 3 100
138
INAA16 PIXE17 PIXE17 INAA18 INAAlS INAA'S
'_ee
0~19 0~19
ref 20
-20 0
02
PSIT drop-tube ash Illinois No. 6 Parent UAz. Coarse filter Illinois No. 6 Parent UAz. Interm. filter Illinois No. 6 Parent UAz. Fine filter Illinois No. 6 SOAP UAz. Coarse filter Illinois No. 6 SOAP UAz. Interm. filter Illinois No. 6 SOAP UAz. Fine filter Beulah Lignite, ND UAz. Coarse filter Beulah Lignite, ND UAz. Fine filter Kentucky No. 9 UAz. Bottom ash Upper Freeport (522) UAz. bottom ash
Erythrite
20 40 60 80 100 0 Energy, eV
2
4
Radius, A
UAz/PSIT
-
X
UAz/PSIT
-
X
UAz/PSIT
-
X
Figure 1. Arsenic XANES and RSF spectra of four standard arsenic compounds. The XANES spectra are shown on the left; the corresponding RSFs, derived from the EXAFS regions, are shown on the right.
UAz/PSIT
X
Results and Discussion
UAz/PSIT
X -
X
UAz/PSIT
X
X
UAz/PSIT
X
-
UAz/PSIT
X
-
UAz/PSIT
X
-
UAz/PSIT
X
-
Arsenic in Coal. As has been pointed out previously by other experimenter^^^, XAFS spectroscopy can readily distinguish among the different oxidation states of arsenic. The positions of both the major peak (white line) in the arsenic XANES spectrum and the first major peak in the radial structure function (RSF), obtained by Fourier transform of the isolated EXAFS oscillations, are reasonably diagnostic of the oxidation state of arsenic in standard compounds. This is especially so, if we consider only those standard compounds that contain arsenic surrounded by nearest-neighbor atoms (sulfur, oxygen, iron) that are likely to be found in geochemical occurrences of arsenic, such as coal. Examples of the XANES and RSF of arsenic in minerals and standard compounds are shown in Figure 1. Data on the positions of the major peaks in these spectra (Table 11) can be used to define a discrimination plot for all major geochemical occurrences of arsenic oxidation states (Figure 2). Also shown on t h i s plot are XANES and RSF peak position data for coals of high arsenic (As >50 ppm) abundance, for which reliable RSF data has been obtained.27 As can be seen from Figure 2, all of these peak positions fall in either the As-Fe(S) or the arsenate (As(V)-0) fields. Even without reliable RSF data, coals of lower arsenic abundance exhibit
0 XAFS spectra were obtained only from samples with elemental abundance8 not in parentheses and from samples indicated by an "X". b Superscript numbers are reference numbers.
X-ray absorption fine-structure (EXAFS) regions and standard XAFS analysis methods were employed to analyze the spectral regions. Unlike the XANES region, which is used in ita basic form ae a spectral fingerprint, the structure in the EXAFS region is extensively mathematically manipulated."* First, the periodic oscillations in the EXAFS region are isolated by subtraction of the edge-step, then they are converted to a reciprocal space representation, which is subjected to a Fourier transform to yield a 'radial structure function" (RSF) that describes the local structure around the absorbing atom. The arsenic K-edge XANES spectra shown in this report are calibrated with respect to a zero-energy point (11.867 keV) defined as the position of the white line in the spectrum of As208 that was run simultaneously with all arsenic spectra. The zero-energy point (5.989 keV) for chromium wae defined ae the first inflection point in the XAFS spectrum of a thin metallic chromium (310 stainless steel) foil.
(25) Brown, Jr., G. E.; Calas, G.; Waychunas, G. A.; Petiau, J. In Spectroscopic Methods in Mineralogy and Geology; Hawthorne, F. C . , Ed.; Rev. Mineral. Mineralogical Society of America: Washington, DC; 1988; Vol. 18, pp 431-512. (26) Wyckoff, R. W. G. Crystal Structures; Interscience: New York, 1968; Vol. 1-4, (27) Huggins, F.E.; Shah, N.; Zhao, J.; Lu, F.; Huffman, G. P. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1992,37(2), 1110-1116.
Trace Element Speciation in Coal
Energy & Fuels, Vol. 7, No. 4, 1993 485
Table 11. Peak Positions from XANES and RSF Spectra and Bond Distances for Arsenic Standard Compounds arsenic standard RSF peaks: A As-X bond distances,' A -3.0 2.02 (3.31) Arsenopyrite, FeAsS 2.30 lS,2.35 3Fe, (3.15 As) -2.6 Arsenical pyrite, FeSz 1.99 (2.64,3.04) 2.13 lS,2.26 3Fe, (3.076S,3.31 6S,3.43 3Fe) -1.6 1.83 (2.27,3.31) 2.23 25,(2.59 lAs,3.48 2As) Realgar, ASS -1.2 1.91 Orpiment, AszSa 2.26 35 -0.2 Enargite, Cu&S4 2.22 4s,(3.75 12Cu) 1.84 (3.37) 0.1 1.79 30,(3.02As) 1.46(2.93) h2Oa 3.2 1.29 Asl: 1.68 40,As2: 1.82 6 0 AszOsd 1.31 3.1 Arsenazo I11 Organic arsenate complex 1.34 (2.99) 3.5 Adamite, Zn&O4(OH) 1.65 4 0 (dist.) 3.2 Erythrite, (Co,Ni)a(As0&8H20 1.65 40,(3.30nCo) 1.33 (2.86) 4.2 1.34 Mimetite, Pb6(hO4,PO4)&1 1.76 4 0 (dist.) a Peak position of major white-line peak in XANES region in eV, relative to white-line peak position in As20~.b Peak position (uncorrected for phase shift) of major and (minor) peaks in radical structure function derived from Fourier transform of k2-weighted EXAFS region. Crystallographic data on interatomic distances, number, and identity of nearest neighbors around arsenic in standards obtained from data or references listed in Wyckoff.28 E.g., for arsenopyrite: 2.30 lS,2.36 3Fe indicates the nearesbneighbor shell to arsenic in this structure consists of a single sulfur atom at 2.30 A and three iron a t o m at 2.36 A, The nexbnearest-neighbor shells are given in parentheses. d T w o distinct arsenic sites in structure, one tetrahedral, the other octahedral. dist. = distorted tetrahedral coordination.
.
+ Standard
2 1.9
< na, u
Coal
-
1.7 1.8
1.6
v)
IL:
1.5
-
1.3 -
1.4
I.&
I
-3
-1
1 Peak, eV
4
i
Figure 2. Plot based on peak positions in arsenic XANES and RSF spectra of standards (Table 11) used to discriminate among different geochemical occurrences of arsenic from XAFS data.
XANES spectra that are diagnostic of one or both of these two occurrences. There is no evidence from the spectra of the coal samples examined for any significant presence of arsenic sulfide mineralsor As(III)-Ospecies in the coals. It is also essential for speciation studies of arsenic in coal to be able to distinguish between the discrete arsenic mineral, arsenopyrite (FeAsS),and arsenical pyrite (FeSz), in which arsenic substitutes for a minor amount of sulfur in the pyrite structure. These two forms-of-occurrence of arsenic have been postulated to be the likely major forms of arsenic in ~ 0 a l . ~ JThe 1 ~ arsenic K-edge XAFS spectra of arsenopyrite and arsenical pyrite are quite similar, but there are subtle differences that can be used to discriminate between these alternatives in both the XANES and EXAFS regions of the spectra. Figure 3 compares the XANES, EXAFS, and radial structure function (derived by Fourier transform of the EXAFS function, k 2 x )spectra from the two materials. There are more distinctive features, in both magnitude and structure, in the radial structure function, which is the most processed of these spectra. The nearest-neighbor peak is at asimilar distance and magnitude in both RSFs, which reflects the fact that the arsenic atoms in both the arsenopyrite and pyrite crystal structures is surrounded by three iron atoms a t approximately the same average distance. The more distant peaks, however, reflect differences in second and third nearest-neighbor shells that discriminate between the two crystal structures. Such differences in the RSF are quite significant. However, even in the XANES
spectra, there are sufficient differences in peak position and the presence of a minor peak a t 65-70 eV to discriminate between arsenopyrite and arsenical pyrite simply by inspection. Such distinctions can be seen in arsenic XANES spectra of coals with arsenic concentrations as low as 10 ppm. Figure 4 shows the normalized XANES spectra of arsenic in four coals that range in arsenic content from 10 to 434 ppm. Of these coals, the Pittsburgh seam DECS-12 coal is unique in that its XANES spectrum is closely similar to that of arsenopyrite, whereas the other coals have features consistent with arsenic in solid solution in pyrite. For the Pittsburgh and Jefferson coals, the two coals with the highest concentrations of arsenic,the signaVnoise ratio in the EXAFS region was sufficient to derive radial structure functions. These functions are shown in Figure 5, and, upon comparison with the RSFs shown in Figure 3,the RSF spectra confirm the inferences based on the XANES spectra. As indicated by the extra white-line peak in the XANES spectrum (Figure 4)and also by the peak at about 1.4 A in the RSF (Figure 5 ) , the Jefferson coal contains a significant fraction of its arsenic as arsenate. Even with this complication, it is clear that the remainder of the arsenic in this coal is present in substitutional form for sulfur in pyrite. The arsenic XANES spectrum for the Pocahontas No. 3 coal (Figure 4) shows that the form of occurrence of arsenic can be determined reliably at an abundance level of 10ppm with 10repeat scans. Clearly, some combination of more repeat scans, more channels in the germanium detector, and higher initial X-ray flux will make it possible to obtain similar quality data with coals containing significantly less arsenic. It should be noted that new developments are underway in these areas: for example, a 100-elementgermanium detector is being built a t NSLS, and the new national synchrotron facility, the Advanced Photon Source, currently under construction at Argonne National Laboratory, will have an X-ray flux some 3-4 orders of magnitude greater than that available at X-19A at NSLS. Arsenic K-edge XANES spectra of the Upper Freeport APCS coal and three float/sink fractions are shown in Figure 6. Except possibly for the 1.6 float sample, the spectra of the float/sink fractions are closely similar to that of the whole coal and indicate the presence of arsenical pyrite as the predominant arsenic mineral in the coal fractions. In all four spectra, there is a small peak on the high-energy side of the main peak that may arise from a
486 Energy & Fuels, Vol. 7, No. 4, 1993 3-
C
.-0
Huggins et al. 2
I
XANES
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-
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Radius, A Wave vector, 1iA Figure 3. Comparison of arsenic XANES, EXAFS, and RSF spectra for arsenical pyrite (top) and arsenopyrite (bottom).
Energy, eV
Upper Freeport
4 C
1.6 Sink, 2.85 Float
.-
Pittsburgh, PA
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z
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1 10 ppm As
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I
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Energy, eV
Figure 6. Arsenic XANES spectra for Upper Freeport coal and derived float/sink fractions. 80
40
s,= 5 0 2
80 40
0
Radius, A
Figure 5. Comparison of RSFs for arsenic in two high-arsenic coals. minor amount of arsenic present in the sample as arsenate. A similar feature is present in the arsenical pyrite standard (Figure 3); however, it too may arise from arsenate complexes. In the Upper Freeport float/sink fractions,
the relative height of this peak compared to the main peak is higher in the 1.6 sinkl2.85 float fraction than in the 2.85 sink fraction, indicating a proportionately higher fraction of arsenate species in the float fraction. Although this trend is consistent with specificgravity differencesbetween arsenate minerals and pyrite, other explanations involving the ease with which arsenical pyrite in this coal oxidizes may be responsible for this observation. In addition to obtaining the XAFS spectra of arsenic in a suite of float/sink fractions from newly opened vials of the Upper Freeport coal from the Argonne Premium Coal Sample bank, XAFS spectra were also obtained from similar float/sink fractions from samples in vials that had been opened a number of months earlier. Also, spectra were retaken of samples from the newly opened samples after a 3-month interval. After opening, samples were stored in glass vials with screw caps (not air-tight) at room temperature. The spectra of the Upper Freeport samples that were measured some months after the APCS vials
Trace Element Speciation in Coal
Energy & Fuels, Vol. 7, No. 4, 1993 487 4.5
I
4
' .
3.5
r
._
2
2 0
Cr (VI) Oxide
3
2.5
B 2 E
zb
0
w
-60
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-20
0
20
40
60
80
100
I
1
1.5
-20
0
20
40
60
80
120
100
120
Energy,eV
Figure 7. Arsenic XANES spectra for oxidized Upper Freeport 2.85 sink fraction taken 3 months apart, and for the whole coal sampletaken 3 months after the correspondingspectrum in Figure 6.
were opened are markedly different from those obtained from samples immediately after opening. The top two spectra in Figure 7 are from the same sample taken 3 months apart, and the lowest spectrum can be compared directly to the same fraction shown in Figure 6, obtained 3 months earlier. In both comparisons, the peak a t about 4.0eV, attributed to arsenate, is significantly more intense than the corresponding peak in the spectrum measured earlier. Clearly, the arsenate contents of these Upper Freeport coal fractions are significantly higher after 3 months' storage a t room temperature. For this particular coal, it is concluded that arsenical pyrite is extremely sensitive to oxidation. The peak a t 4.0 eV in the arsenic XANES spectrum that is indicative of arsenate has been observed in most coal samples examined. As reported elsewhere2' for high arsenic coals from Alabama, both the RSFs and the XANES spectra indicate arsenate, in addition to arsenic in pyrite, as a major, if not dominant, form of arsenic in these coals. Although the rapid change with time in arsenic forms has only been documented for the APCS Upper Freeport coal, it is likely that the arsenate species observed in most coals is derived from oxidation of arsenical pyrite during storage. As far as we know, this work constitutes the first direct observation of arsenic as arsenate in coals. Swaine,3in his excellent review of trace element occurrences, suggested that a possible minor occurrence of arsenic in coal might be in phosphate minerals, in which AsOr3- substitutes for Pods-. This work indicates that a much more likely source of arsenate in coals is oxidation of arsenic in pyrite. Indeed, all of the coals that had been pulverized and then stored for more than a few months before examination by arsenic XAFS techniques showed significant quantities of arsenate in their XAFS spectra. In a number of cases, the intensity of the white line for the arsenate form exceeded that for arsenical pyrite. Only samples obtained from freshly
Energy, eV
Figure 8. ChromiumXANES spectrafor two CrS+and two CrOds standards.
opened APCS vials appear not to exhibit peaks due to arsenate in their XANES spectra. Other freshly opened samples, such as the Illinois No. 6 (DECS-2) coal, showed appreciable amounts of arsenate. Chromium in Coal. It is a relatively easy matter to discriminate by means of XAFS spectroscopy between the two most common natural oxidation states of chromium: trivalent chromium, Cr3+, and hexavalent chromium or chromate, Cr0r2-. Chromium XAFS spectra of two chromic oxide and two chromate samples are shown in Figure 8. The chromate oxidation state is characterized by the presence of a large pre-edge feature at about 0 eV that is highly distinctive of first series transition metal 3d0species in tetrahedral coordination by oxygen anions. In addition, it should be noted that the maximum absorption in the chromate species occurs at about 40-50 eV, at which energy a minimum in the absorption occurs for Cr3+-oxygen compounds. These observations appear to be quite general and apply to all chromium standards so far examined. Figures 9 and 10show chromium XAFS spectra obtained for four different US. bituminous coals and an anthracite coal. The spectra for the bituminous coals are virtually identical in appearance and demonstrate that essentially all (>95% ) of the chromium is present in the C9+oxidation state. For the Upper Freeport APCS coal, there is little variation between these chromium XANES profiles for the raw and cleaned (1.6 float) coal samples. Similar observations were reportedn*28for the Illinois No. 6 parent and the coal fraction cleaned by the spherical oil agglomeration process (SOAP). These observations appear to indicate that there is only one significant form of occurrence of chromium in these coals. Furthermore, the chromium does not appear to be strongly associated with the easily separated mineral matter, as the chromium ~
~~~
(28) Helble, J. J.; Huggins, F. E.; Senior, C. L.; Srinivasachar, S.; Shah,
N.; Huffman, G. P. Proceedings, Ninth International Pittsburgh Coal Conference;University of Pittsburgh Pittsburgh, PA, 1992;pp 928-933.
Huggins et al.
488 Energy & Fuels, Vol. 7, No. 4, 1993 3 Lewiston-Slockton 36 ppm Cr
C
.o c
Bottom Ash
2
F
z D
a U
Fine Fly-Ash
Upper Freeport (1.6 Float)
N ._
m
E
P
Beulah Lignite
1
Coarse Fly-Ash
Upper Freeport 20 ppmi Cr
0
-20
0
20
40
60
80
100
120
Figure 9. Chromium XANES spectra for the Lewiston-Stockton and Upper Freeport APCS coals and for the cleaned (1.6 float) Upper Freeport coal.
0
20
40
60
0
20
40
60
80
100 120
Energy, eV
Energy, eV
-20
clc/
-60 -40 -20
80
100
120
Energy, eV
Figure 10. Chromium XANES spectra for two bituminous coals and an anthracite.
abundance does not drop significantly upon cleaning.2s All U.S.bituminous coals examined exhibit a chromium XANES spectrum that is similar to those shown in Figures 9 and 10. The spectrum of chromium in the anthracite sample (Figure lo), although similar to those of the bituminous coals, exhibits a somewhat sharper and more prominent white line peak, which may indicate the presence of a second chromium form. Although it is clear that the oxidation state of chromium in all samples examined is Cr3+ and that the likely nearestneighbor atom to chromium is oxygen, the actual form of occurrence has not yet been established. However, from comparison of the chromium XANES spectra of standards
Figure 11. Arsenic XANES spectra for a bottom ash sample and two fly-ash fractions collected during combustion runs on the Kentucky No. 9 and Beulah lignite coals in the University of Arizona combustion unit.
with those of the coals, a number of postulated occurrences for chromium can be eliminated. By comparison of Figures 8 with Figures 9 and 10, the chromium oxide minerals, such as chromite (FeCrO4) and eskolaite (CrzOd, do not contribute significantly (i.e., no more than -20%) to the spectral profiles exhibited by bituminous coals. Nor, by comparison with published spectra of chromium silicates,% is it likely that any chromium-bearing silicate examined to date by XAFS spectroscopy contributes significantly to these spectral profiles. The published chromium XANES spectrum that comes closest to the spectral profile for chromium in coal was obtained from synthetic Cr(OH)329,suggesting that chromium may exist in coal as a chromium hydroxide or oxyhydroxide. Experiments are currently being conducted to test this possibility. Note Added in Proof: Recent (May 1993) chromium XAFS experiments a t beam-line VII-3 at the Stanford Synchrotron Radiation Laboratory have confirmed that the prevalent chromium XANES spectral profile for coals, shown in Figures 9 and 10, does arise from chromium in an oxyhydroxide structure. Arsenic and Chromium i n Ash. The transformation of arsenic and chromium forms in coal during combustion has been investigated by XAFS examination of the occurrence of these elements in ash samples derived from a drop-tube furnace at PSI Technology Co. and from a larger scale combustion unit at the University of Arizona. All the coal ash samples examined by XAFS spectroscopy contain arsenic predominantly, if not entirely, in the form of arsenate ( A ~ 0 4complexes. ~) Such complexes are readily recognized by the strong white line absorption at about 4 eV and the broad low-intensity peak that is a maximum in the range 80-90 eV in the arsenic XANES spectrum, and by the major peak at about 1.3-1.4A in the RSF of samples rich in arsenic. Figure 11showsthe arsenic (29) Manceau, A.; Charlet, L.J.Colloid Interface Sci. 1992,148,425442.
Trace Element Speciation in Coal
Energy & Fuels, Vol. 7,No. 4, 1993 489
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based on the spectra of ash samples examined,27*28 it is concluded that chromium is predominantly (>95 % ) , if not exclusively,present as Cr3+in ash samples, even in the finest fly-ash fractions.
Conclusions
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Figure 12. ChromiumXANESspectra for three fly-ashfractions collected during combustion of the Illinois No. 6 SOAP cleaned coal in the University of Arizona combustion unit.
XANES spectra for a bottom-ash sample and two fly-ash samples collected in the University of Arizona combustion unit. Chromium XANES spectra of the ash samples indicate that the oxidation state of chromium is unaltered from that in the coal, although the spectral shape does differ somewhat from that of the coal samples. Figure 12 shows chromium XANES spectra of three size-segregated flyash samples collected on an Anderson impactor in the University of Arizona combustion unit. The pre-edge peak is a little more intense than in the coals, but rather than attribute this difference to a very small percentage of chromate, it is much more probable that it reflects the more distorted crystallographic environments likely to be found for chromium in ash and slag. The white-line peak at about 20 eV above the chromium edge is significantly more pronounced and sharper in the ash spectra than that in the coal spectra. In this respect, the chromium XANES spectra of the ash begin to resemble spectra of Cr3+ in aluminosilicate mineral~,~5 although the minor details of the mineral spectra are lacking in the ash spectra. Hence,
This survey demonstrates that XAFS spectroscopy is capable of obtaining significant information on the oxidation state and form of occurrence of trace elements that exist in coal at about the 10 ppm level. This information complements conventional analytical data on trace element abundances and provides additional information for evaluation of the potential environmental hazard of specific elements in coal combustion. For example, of the two oxidation states of chromium considered here, the CrOr” oxidation state is both toxic and carcinogenic, whereas the Cr3+oxidation state a t such low concentration levels (
