Iron Species in Argonne Premium Coal Samples: An Investigation

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Iron Species in Argonne Premium Coal Samples: An Investigation Using X-ray Absorption Spectroscopy Stephen R. Wasserman,* Randall E. Winans, and Robert McBeth Chemistry Division, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, Illinois 60439 Received July 31, 1995. Revised Manuscript Received November 10, 1995X

Iron K-edge X-ray absorption spectroscopy (XAS) has been used to examine the iron species that are present within the Argonne Premium Coal Samples. This technique was applied to both native coal samples and ones which had been extracted with concentrated hydrochloric acid. The near-edge absorption spectra (XANES) were analyzed using a deconvolution procedure to determine the relative amounts of oxo-ferrous, oxo-ferric, and pyritic environments in the coals. Three coals, Beulah-Zap, Wyodak-Anderson, and Lewiston-Stockton, apparently contain significant amounts of ferric species. Extended X-ray absorption fine structure (EXAFS) spectra were used to confirm the pyritic contents that were deduced from the near-edge spectra.

Introduction The future use of coal will depend, in part, on the elimination of inorganic species from native coals or the exploitation of these nonorganic materials for their catalytic properties in coal liquefaction. Consequently, a knowledge of both the elemental and chemical composition of the inorganic fraction of a coal is of primary importance. Such information will facilitate the development of new processes for the removal of these species from coal. It will also aid in the creation of new catalytic systems for the transformation of the organic constituents within coals. In this paper we describe the use of X-ray absorption spectroscopy (XAS) to characterize the nature of the iron species within the eight Argonne Premium Coal Samples (APCS).1 XAS provides two types of information about the absorbing atom.2,3 The X-ray absorption near-edge spectra (XANES) analyze the intensity of absorption of X-rays as a function of the energy of the incoming photon. During the absorption process, an inner shell electron is excited into valence orbitals. Therefore, these spectra provide information on the electronic structure of the absorbing atom. For iron, the absorption which results in excitation of a 1s electron, the K edge, begins in the neighborhood of 7112 eV. At energies greater than those used for near-edge spectroscopy, a free photoelectron is generated, which in turn is scattered by neighboring atoms. This scattering leads to small oscillations in the intensity of X-ray absorption, the extended X-ray absorption fine structure (EXAFS). The Fourier transform of an EXAFS spectrum represents the local distribution of atoms about the absorbing atom. Recently there have been several studies which have used X-ray absorption spectroscopy to examine the * Author to whom all correspondence should be addressed. Tel: 708252-3527. X Abstract published in Advance ACS Abstracts, January 15, 1996. (1) Vorres, K. S. Energy Fuels 1990, 4, 420-426. (2) Stern, E. A. In X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES; Koningsberger, D. C., Prins, R., Eds.; John Wiley and Sons: New York, 1988; Chapter 1. (3) Durham, P. J. In ref 2, Chapter 2.

0887-0624/96/2510-0392$12.00/0

chemical structure of coals.4 These investigations have probed the nitrogen5 and sulfur6,7 species that are contained within the coal matrix. There have also been related studies on petroleum asphaltenes.8,9 In addition, XAS has been used to speciate other elements in coals, including vanadium,10 potassium,11,12 chromium, arsenic, manganese, bromine,13 and chlorine.14 The iron content of the APCS coals has previously been investigated by several methods, including neutron activation analysis,15 atomic emission spectroscopy (AES),15 X-ray photoelectron spectroscopy (XPS),16,17 and Mo¨ssbauer spectroscopy.18 The first two methods were used to determine the total amount of iron in each coal. They are not, however, appropriate tools for examining the chemical nature of the iron. Both XPS and Mo¨ssbauer can overcome this deficiency. But these techniques have limitations. Mo¨ssbauer spectroscopy is not as generally applicable as XAS to a broad range of (4) Huffman, G. P.; Huggins, F. E.; Shah, N. In Advances in Coal Spectroscopy; Meuzelaar, H. L. C., Ed.; Plenum Press: New York, 1992; pp 29-47 and references contained therein. (5) Mitra-Kirtley, S.; Mullins, O. C.; van Elp, J.; Cramer, S. P. Fuel 1993, 72, 133-135. (6) Lytle, F. W.; Greegor, R. B.; Sandstrom, D. R.; Marques, E. C.; Wong, J.; Spiro, C. L.; Huffman, G. P.; Huggins, F. E. Nucl. Instrum. Methods Phys. Res. 1984, 226, 542-548. (7) Gorbaty, M. L.; George, G. N.; Kelemen, S. R. Fuel 1990, 69, 945-949. (8) George, G. N.; Gorbaty, M. L. J. Am. Chem. Soc. 1989, 111, 3182-3186. (9) Mitra-Kirtley, S.; Mullins, O. C.; van Elp, J.; George, S. J.; Chen, J.; Cramer, S. P. J. Am. Chem. Soc. 1993, 115, 252-258. (10) Maylotte, D. H.; Wong, J.; St. Peters, R. L.; Lytle, F. W.; Greegor, R. B. Science 1981, 214, 554-556. (11) Huffman, G. P; Huggins, F. E.; Shoenberger, R. W.; Walker, J. S.; Lytle, F. W.; Greegor, R. B. Fuel 1986, 65, 621-632. (12) Huggins, F. E.; Shah, N.; Huffman, G. P.; Lytle, F. W.; Greegor, R. B.; Jenkins, R. G. Fuel 1988, 67, 1662-1667. (13) Huggins, F. E.; Zhao, J.; Shah, N.; Huffman, G. P. Prepr. Pap.sAm. Chem. Soc. Div. Fuel Chem. 1994, 39, 504-508. (14) Huggins, F. E.; Huffman, G. P. Fuel 1995, 74, 556-569. (15) Palmer, C. A. In The Chemical Analysis of Argonne Premium Coal Samples; Palmer, C. A., Walthall, F. G., Eds.; U.S. Geological Survey, Open File Report 91-638, 1992; pp 86-98. (16) Kelemen, S. R.; Gorbaty, M. L.; George, G. N.; Kwiatek, P. J. Energy Fuels 1991, 5, 720-723. (17) Kelemen, S. R.; George, G. N.; Gorbaty, M. L. Fuel 1990, 69, 939-944. (18) Shah, N.; Keogh, R. A.; Huggins, F. E.; Huffman, G. P; Shah, A.; Ganguly, B.; Mitra, S. Prepr. Pap.sAm. Chem. Soc. Div. Fuel Chem. 1990, 35, 784-792.

© 1996 American Chemical Society

Iron Species in Argonne Premium Coal Samples

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Table 1. Estimates for the Types of Iron Species within the Argonne Premium Coals

minerals goethite (R-FeOOH) siderite (FeCO3) γ-Fe2O3 magnetite native coals Upper Freeport Wyodak-Anderson Illinois No. 6 Pittsburgh No. 8 Pocahontas No. 3 Blind Canyon Lewiston-Stockton Beulah-Zap acid-washed-coals Upper Freeport Wyodak-Anderson Illinois No. 6 Pittsburgh No. 8 Pocahontas No. 3 Blind Canyon Lewiston-Stockton Beulah-Zap

ferrous (%)

ferric (%)

pyritic (%)

χ2 a

9 77 29 46

95 30 62 34

-5 -9 9 17

0.0073 0.47 0.040 0.070

19 11 5 7 112 35 50 1

2 67 -2 -1 11 10 49 62

81 26 97 94 -21 59 4 41

0.0080 0.056 0.0028 0.0035 0.37 0.24 0.060 0.026

6 1 -2 0 23 7 26 0

7 34 3 0 29 17 59 8

86 66 98 100 44 75 20 93

0.011 0.057 0.0074 0.0 0.33 0.11 0.054 0.0068

a χ2 is the sum of the squared difference between each point in the original XANES spectrum and the best fit using the FeO, Fe2O3, and FeS2 standards.

elements, while XPS is sensitive only to surface species located within the first 30-50 Å of the sample.16,19 The study described here utilizes XANES and EXAFS spectra from both native and acid-washed coal samples. The latter samples were used to evaluate which iron species remain after removal of acid-soluble iron compounds from the coals. As part of our analytical protocol, we use a least-squares fitting method to reproduce the observed near-edge spectrum for each coal from those of model compounds. Our goal is to provide a semiquantitative analysis of the iron moieties within the Argonne Premium Coal Samples. In addition, we compare the results of this method with those from earlier studies and demonstrate that X-ray absorption spectroscopy can provide new insights into the speciation of iron in these coals. Experimental Section Argonne Premium Coal Samples (APCS). Each of the eight coals listed in Table 1 was obtained in sealed ampules from the Argonne Premium Coal Sample program. Samples were removed from their respective ampules and placed in aluminum holders 1-4 days prior to the acquisition of the XAS spectra. The front and rear windows of each holder consisted of Kapton tape 1 mil in thickness. The cross-sectional area of each sample was approximately 2 cm2. The Kapton windows minimized diffusion of air through each of the prepared coals. In addition, each Argonne Premium coal (5 g) was stirred in concentrated HCl (75 mL) for 5 days under N2. Each washed sample was then filtered, rinsed thoroughly with distilled water until free of inorganic acid, and dried under vacuum at 100 °C. These extracted coals were mounted as described for the fresh materials. X-ray Absorption Spectroscopy. Iron K edge XAS spectra of the APCS were acquired at beamlines X23A2 and X19A of the National Synchrotron Light Source. The beamlines were equipped with Si[311] (X23A2) and Si[220] (X19A) (19) Feldman, L. C.; Mayer, J. W. Fundamentals of Surface and Thin Film Analysis; North-Holland: New York, 1986; p 353.

double crystal monochromators. Harmonics were rejected on X19A by detuning the monochromator to approximately 50% of its maximum transmission. Transmission spectra were collected using ion chambers which were continually purged with N2. A Lytle detector6 with argon gas in the ionization chamber was utilized for the fluorescence data. A manganese filter, 3 absorption lengths in thickness, was placed between the sample and the ionization chamber of the Lytle detector. The energy calibration was maintained by simultaneously monitoring the maximum at 7112 eV in the derivative spectrum of an Fe foil (5 µm in thickness). The calibration is precise to 0.3 eV. Spectra were acquired for three samples of each coal. Each spectrum is an average of 2 or 3 individual scans. XAS spectra for ferrous oxide (FeO) and ferric oxide (RFe2O3) were obtained at the Stanford Synchrotron Radiation Laboratory (SSRL) on beamline 4-3. For these samples, spectra were acquired in transmission mode only using a Si[220] double-crystal monochromator. Harmonics were rejected by detuning of the monochromator. Energy calibration was monitored by taking the spectrum of iron foil immediately prior to or following the acquisition of the XAS spectrum of the oxide. The calibration for these two spectra is precise to 0.5 eV. Data analysis was performed using XAMath, a package for the reduction of XAS data which was developed at Argonne National Laboratory.20

Results and Discussion XANES and EXAFS Spectra. The measured absorption edges (XANES) for the eight Argonne Premium Coal Samples are shown in Figure 1. The corresponding radial structure functions (RSF) from EXAFS are presented in Figure 2. For the following discussion we have divided the eight Argonne Premium Coals into two groups of four. The Upper Freeport, Illinois, Pittsburgh, and Blind Canyon coals comprise the first group. Previous elemental analyses have shown that, in these coals, more than 50% of the total iron is present as pyrite, FeS2.21,22 Although the edge spectrum for the Blind Canyon coal differs significantly from those of the other three members of this group, the radial functions indicate that all four coals contain species which have significant long-range order about the absorbing iron. Features in the radial structure functions can be discerned out to at least 8 Å. The second set of coals, whose edges and radial structure functions are also presented in Figures 1 and 2, includes the Wyodak-Anderson, Pocahontas, LewistonStockton, and Beulah-Zap coals. Within this subset, both the XANES and EXAFS spectra indicate that the iron is present in a variety of different forms. The EXAFS spectra suggest that, at least qualitatively, the other iron species in these four coals are not as well ordered as pyrite. There are no significant peaks in the (20) XAMath is available on the World Wide Web at http://xafsdb. iit.edu:80/exafs-database/programs/XAMath/. (21) Vorres, K. S. Users Handbook for the Argonne Premium Coal Samples; ANL/PCSP-93/1; Argonne National Laboratory: Argonne, IL, 1993; pp 23-27. (22) The percent of iron which is present as pyrite in each coal can be deduced from data contained in the Argonne Premium Coal Sample Handbook (ref 21). The analyzed amount of pyritic sulfur in the dried APCS measures a corresponding amount of pyritic iron. The conversion factor from pyritic sulfur to iron is 0.871, the ratio of the atomic weights of one iron atom and two sulfur atoms. The resultant quantities of pyritic iron were divided by the total iron in the coal, as determined by neutron activation analysis (ref 15), to estimate the fraction of iron that is in pyritic form. Similar results are obtained if the ferric oxide content of the high temperature ash is used as the measure of the total amount of iron in each coal.

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Figure 1. Normalized iron K edge XANES from the Argonne Premium Coal Samples: native (solid line) and after extraction with concentrated aqueous hydrochloric acid (dashed line).

radial structures past 5 Å. The Wyodak-Anderson and Beulah-Zap apparently contain similar iron species, since their edge and radial spectra are comparable. The lack of long-range features in the radial structure functions of the second set of coals indicates that the long range peaks in the high-pyrite coals are real. If these peaks were the result of nonrandom noise in the experiment, they would have appeared in each of the radial spectra, not just in those of the coals which have significant amounts of pyritic iron. Since these longrange peaks appear only in the first set of coals, pyrite is the most crystalline of the iron species within the Argonne Premium Coals. Pocahontas is, in some respects, unique among these samples. Its absorption edge appears at lower energy than any of the other low-pyrite coals. It also exhibits a small feature at approximately 2.6 Å in the radial distribution function which is not detected in the other seven coals. These observations suggest that there is an iron species in Pocahontas which is not found in the other coals. Model Compounds. Traditionally, the analysis of EXAFS spectra has relied on the determination of coordination number and bond distances about the absorbing atom. The use of such an analysis here is problematic. The number of degrees of freedom, dof, in a Fourier-filtered EXAFS spectrum is given by eq 1.

dof ) 2∆k∆r/π

(1)

In this relation, ∆k is the window for the Fourier transform used to obtain the radial distribution func-

Figure 2. Radial structure functions, F2(r′), for iron in the Argonne Premium Coal Samples: native (solid line) and after extraction with concentrated aqueous hydrochloric acid (dashed line). These distributions were obtained by the Fourier transform of the k2-weighted EXAFS data (∆k ) 2.3-13.4 Å-1).

tion, and ∆r is the window for the reverse transform back into the original k-space.23 In this study, the useful EXAFS data, ∆k, extend from 2.3-13.4 Å-1. The window for the back transform for the first coordination shells of the iron species was 0.7 to 2.3 Å. Therefore, the data contain only 11 degrees of freedom. A single coordination shell requires 4 degrees of freedom: the radial distance (r), the coordination number (n), the degree of disorder (σ), and the shift in energy of the edge (∆E). Consequently, the standard quantitation of the EXAFS data is restricted to at most two species. The use of three or more postulated compounds in the EXAFS analysis would be statistically invalid. It is possible to restrict the values for some of the parameters, in order to permit the use of a greater number of model compounds. Unfortunately, such an approach requires a prior detailed knowledge of the types of iron compounds that are present in coals. Because of these considerations, we have chosen an alternative method of analysis for this study. Our aim is to obtain a semiquantitative, chemically reasonable picture of the iron content for each of the coals being investigated. Our analysis consists of fitting the observed absorption edge from each coal to those of ferrous oxide (FeO), R-ferric oxide (R-Fe2O3), and pyrite (FeS2). Authentic samples were used for the former two compounds. The fluorescence spectrum of an acid-washed Pittsburgh coal serves as the standard for pyrite, since, after extraction, virtually all of the iron in this coal is present as FeS2. The shape of the spectrum for this (23) Teo, B. K. EXAFS: Basic Principles and Data Analysis; Springer-Verlag: Berlin, 1986; p 132.


Figure 3. Standard compounds for the analysis of the iron K edge XANES of the Argonne Premium Coal Samples: FeS2 (pyrite, s), FeO (- - -), and Fe2O3 (-b-). The spectra for FeO and Fe2O3 are from transmission measurements. The fluorescence spectrum of the Pittsburgh coal after extraction with concentrated hydrochloric acid serves as the standard for pyrite.

standard matches that of pure pyrite.24 The spectra of the standards, which are clearly differentiated from one another, are shown in Figure 3. We follow standard practice and normalize each spectrum, both standards and samples, to the magnitude of the edge jump (µo). This normalization is based on an extrapolation of the EXAFS data. A Levenberg-Marquart least-squares algorithm25 was used to obtain a best fit to the normalized fluorescence XANES spectrum of each coal using linear combinations of the normalized spectra from the model compounds. The window for this fitting procedure extended from 7112 to 7142 eV. The lower bound of this region corresponds to the maximum in the derivative spectrum of iron foil and is the standard position for k ) 0 Å-1. The results of this fitting procedure indicate the relative amount of each type of iron compound that is present within each coal sample. We estimate that the errors in the percentages for each material are on the order of 10% absolute. Often a small amount (∼5%) of ferric ions are found within ferrous oxides and hydroxides.26,27 This contamination may be present in the ferrous oxide standard used here. If such an impurity is present, the deconvolution of the edge will slightly underestimate the ferric content of the sample, since some of the ferric species will be modeled by the small ferric content found within the FeO standard. These considerations will not, however, affect our general conclusions, since the magnitude of such errors is small. Generally, both ferric and ferrous species prefer octahedral coordination, although tetrahedral and other configurations are occasionally found for Fe(III) oxo species.27 For this reason we have chosen standards in which all the iron atoms have octahedral coordination. (24) Petiau, J.; Calas, G. In EXAFS and Near Edge Structure; Bianconi, A., Incoccia, L., Stipcich, S., Eds.; Springer-Verlag: Berlin, 1983; pp 144-146. The energy scales in this reference are approximately 21 eV below the correct values. (25) Press, W. H.; Teukolsky, S. A.; Vetterling, W. T.; Flannery, B. P. Numerical Recipes in C, 2nd ed.; Cambridge University Press: Cambridge, U.K., 1992; pp 683-688. (26) Evans, R. C. Crystal Chemistry; Cambridge University Press: Cambridge, U. K., 1964; pp 174-176. (27) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; John Wiley and Sons: New York, 1988; pp 711-720.


The choice of these standards represents the major simplification which we make in this analysis. In practice, iron-containing minerals exhibit a wide variety of absorption edges.24,28,29 Several minerals do incorporate ferric ions in tetrahedral as well as octahedral sites.26-28 In order to evaluate how our analytical protocol copes with both octahedral and tetrahedral ferric species, we have tested the model system with several known minerals that are composed of iron oxides. The results of these analyses are shown in Table 1. For goethite, R-FeOOH, a ferric oxyhydroxide in which each of the iron ions occupies an octahedral site,30 we assign the great majority of the iron content to ferric species. In the case of siderite, an octahedral ferrous carbonate,24 our analysis infers that the bulk of the sample is composed of ferrous species. This conclusion is qualitatively correct, although agreement between the deconvolution and the XANES spectrum of siderite is not high. This sample indicates that, even in cases where the quantitative conclusions are suspect, the qualitative inferences regarding the types of iron environments are reasonable. Two other test compounds, magnetite (Fe3O4) and γ-Fe2O3, contain some ferric ions in tetrahedral sites. For magnetite, which has an inverse spinel structure,26 one-third of the iron is in each of three environments: octahedral Fe(II), octahedral Fe(III), and tetrahedral Fe(III). In γ-Fe2O3, the iron, all of which is ferric, is distributed randomly between tetrahedral and octahedral sites. Since in this material there are two octahedral sites for every tetrahedral one, we expect that 67% of the Fe(III) in this mineral is situated within an octahedral environment. The remainder occupies tetrahedral locations. In these two cases the fits for each mineral are not particularly successful, although they do find approximately the correct amount of Fe(III). The analyses infer the presence of more ferrous species (both oxide and pyrite) than are actually in the samples. In part, these results reflect the empirical observation that the edge of γ-Fe2O3 occurs at a slightly lower energy than the corresponding octahedral material. The fitting method compensates for this shift by adding more of the species in which iron exists in the lower +2 oxidation state. These test compounds demonstrate a minor deficiency in our chosen standards. But, since for these two minerals there is a significant discrepancy between the spectrum and the deconvolution, these results suggest that we should at least suspect the presence of tetrahedral iron when the residuals from the fitting procedure are high. The deduced total percentages for iron in each of the minerals and coals range from 95 to 105%. These figures reflect the cumulative small errors in determining the edge jump, µo, for the three standard compounds and the sample. The error in µo for a single spectrum is less than 5%. XAS is used in this study as a tool for discriminating between coordinatively distinct environments. These experiments do not, in general, separate the iron species in the coals on a mineralogical basis. It will, however, (28) Waychunas, G. A.; Apted, M. J.; Brown, Jr. G. E. Phys. Chem. Minerals 1983, 10, 1-9. (29) Dra¨ger, G.; Frahm, R.; Materlik, G.; Bru¨mmer, O. Phys. Status Solidi B 1988, 146, 287-294. (30) Wyckoff, R. W. G. Crystal Structures, 2nd ed.; Interscience Publishers: New York, 1963; Vol. 1, p 290.

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Figure 4. Comparison of the iron K edge X-ray XANES (solid line) of Wyodak-Anderson with the best fit using FeO, Fe2O3, and FeS2 (pyrite) standards (dashed line). The deconvolution uses 11% ferrous oxide, 67% ferric oxide, and 26% pyrite.

become apparent in the following discussion that the three chosen standards are not sufficient to completely characterize the iron species in the Argonne Premium Coals. The iron content of these coals mirrors the complexity of the overall system. Determination of Iron Species. The results of our fitting procedure for each of the Argonne Premium Coals are listed in Table 1. The numbers presented are the actual output from the deconvolutions. A comparison of an original spectrum with the corresponding fit is presented in Figure 4. For six of the eight APCS, the fits are in good agreement with the observed spectra, as demonstrated by the residuals (Table 1). However, the deconvolutions for the Pocahontas and Blind Canyon coals do not correspond well with the original spectra. In particular, the deconvolution for Pocahontas is suspect, since it deduces a significant negative percentage of pyrite. This last result is consistent with the qualitative deduction from the XANES and EXAFS spectra that Pocahontas contains an iron species which is not present in the other coals. We note that the XANES spectra of both Pocahontas and Blind Canyon exhibit a local maximum at 7126.5 eV which our standards do not model well. This observation suggests that these two coals both contain an iron species which is not represented by the model compounds. It also implies that there is an additional distinct iron species in the Argonne Premium Coal Samples. Therefore, there are probably at least five iron environments within the eight APCS. The radial structure functions obtained from EXAFS provide an independent check on the pyrite content of the APCS. The peak in the radial distribution at 3.6 Å primarily arises from an Fe-Fe interaction in pyrite.31 Figure 5A plots the intensity of this feature for the APCS alongside the pyrite content as deduced by XANES. For this comparison, we have defined the intensity of the peak at 3.6 Å as the highest point in the structure function between 3.5 and 3.7 Å. These intensities are normalized to the value found in the acidwashed Illinois coal, which exhibited the greatest intensity in this region. The pyritic content of each coal estimated from the EXAFS radial distributions correlates well with that deduced from the near-edge (31) Montano, P. A.; Lee, Y. C.; Yeye-Odu, A.; Chien, C. H. In Mineral Matter and Ash in Coal; Vorres, K. S., Ed.; American Chemical Society: Washington, DC, 1986; Chapter 29.

Figure 5. Comparison of XANES (solid) and EXAFS (Shaded) results for the pyrite content of the Argonne Premium Coal Samples. The EXAFS data are the intensities of the Fe-Fe scattering peak at 3.6 Å in the radial structure functions, normalized to that of acid-washed Illinois No. 6.

spectra. Pocahontas is the one coal for which the correlation between the two techniques is poor. Since the XANES analysis resulted in a negative value for the pyrite content of Pocahontas, this result is expected. We recognize that the correlation between the EXAFS and XANES data is only semiquantitative, since other species besides pyrite may make minor contributions to the radial distributions in the region of interest. Nevertheless, the EXAFS data confirm the conclusions reached on the basis of XANES spectroscopy. The analysis of the near-edge spectra results in one noteworthy conclusion. Coals are generally considered to provide a reducing environment. Therefore, only reduced iron species, such as siderite, pyrite, and other ferrous materials, are traditionally assumed to appear within the coal matrix.18 We, however, conclude that three coals, Beulah-Zap, Wyodak-Anderson, and Lewiston-Stockton, contain significant amounts of ferric species. Thus it appears that iron can exist within a coal in its oxidized form. Since our XAS investigation did not rigorously exclude air from the samples, there is the possibility that the observed ferric species were formed by such oxidation. A previous study using X-ray photoelectron spectroscopy detected oxidation of iron species in coal, particularly of pyrite, after exposure to air for 2 days.17 However, XPS only examines the iron at the surface of the sample, while XAS probes the bulk iron species of the coals. In the former case, oxidation will have a much more pronouced effect on the acquired spectra. Since the XAS spectra of those coals in which pyrite is the predominant species, Illinois No. 6, Pittsburgh No. 8, and Upper Freeport, match that of au-


thentic pyrite, we believe that reaction of ferrous species in the APCS with oxygen has little effect on the conclusions presented here. The iron species of the APCS have been studied previously by Mo¨ssbauer spectroscopy.18 In that study, the iron constituents were separated into clay, siderite, and pyrite components. A direct comparison between the present study and the Mo¨ssbauer one cannot be made, since the latter used two model compounds, siderite and clay, different from those employed here. An additional complication in such a comparison is that natural clays contain both ferric and ferrous species.32 For the four coals with high pyritic content, the XAS and Mo¨ssbauer results are similar. In the case of the Blind Canyon coal, the agreement between the two methods may be fortuitous, since this coal contains at least one species that is not well represented by our models. For the remaining four coals, Wyodak-Anderson, Pocahontas, Lewiston-Stockton, and Beulah-Zap, the two methods yield divergent conclusions. The question remains of which results, those from Mo¨ssbauer or XAS spectroscopies, best reflect the actual composition of the iron species within the APCS. While we cannot answer this question in every case, there is other evidence to support the results presented here. We conclude that approximately 50% of the iron in Beulah-Zap is pyritic, while the Mo¨ssbauer studies infer that only pyrite is present. Direct analysis of the pyritic sulfur in Beulah-Zap (see refs 15, 21 and 22) indicates that less than half the iron in this coal is present as pyrite. In addition, other workers have found a significant amount of iron, 0.2 wt %, can be extracted from Beulah-Zap by washing with 1 N HCl.33 Since the total iron content of this coal is 0.4-0.5 wt %,15 approximately half of the iron in Beulah-Zap is extractable. Since pyrite is not extracted by hydrochloric acid,34 only the results of the XAS analysis are consistent with the extraction results. We believe that the primary reason for the discrepancy between the XAS and Mo¨ssbauer results is that the Mo¨ssbauer study did not allow for the presence of ferric iron species in the Argonne Premium Coal Samples. Huggins and Huffman have observed that, at room temperature, it is not possible to separate the contributions to Mo¨ssbauer spectra of iron(III) oxyhydroxides and pyrite.35 This conclusion agrees with the results of this study. With the exception of Pocahontas, the sum of the ferric and pyritic species in each coal as determined by XAS approximately matches the pyritic content found by Mo¨ssbauer spectroscopy. Acid-Washed Coals. We have applied our analysis of the iron content of the APCS to acid-washed versions of each of the coals. The treatment with acid leaves the pyrite within the coal, while removing some, if not all, of the other iron species. Thus, this procedure results in coals whose pyrite contents have been enhanced (32) Newman, A. C. D.; Brown, G. In Chemistry of Clays and Clay Minerals; Newman, A. C. D., Ed.; Longman Scientific and Technical: Harlow, Essex, England, 1987; Chapter 1. (33) Botto, R. E.; Axelson, D. E. Prepr. Pap.sAm. Chem. Soc. Div. Fuel Chem 1988, 33, 50-57. (34) ASTM D2492, 1977 Annual Book of ASTM Standards, Part 26: Gaseous Fuels; Coal and Coke; Atmospheric Analysis; American Society for Testing and Materials; Philadelphia, 1977; pp 322-326. (35) Huggins, F. E.; Huffman, G. P. Mo¨ssbauer Analysis of IronBearing Phases in Coal, Coke, and Ash. In Analytical Methods for Coal and Coal Products; Karr, Jr., C., Ed.; Academic Press: New York, Vol. III, 1979; pp 371-423.


relative to the other iron species present. Therefore, the XAS spectra of acid-washed coals whose native forms contain iron species in addition to pyrite will appear closer to that of pure pyrite than did the untreated samples. The spectra for those coals which contain primarily pyrite should not change appreciably after washing with acid. The near-edge spectra and radial structure functions for the acid-washed coals are shown in Figures 1 and 2. As expected, many of these spectra more closely correspond to that of pure pyrite than do those of the original coals. Table 1 lists the results of fitting the XANES spectra for the acid-treated coals to the model compounds. In each coal which originally contained iron species in addition to pyrite, the amount of pyrite has increased relative to the other compounds. We have also compared the pyrite content as deduced from XANES with the intensity of the EXAFS feature at 3.6 Å in the same way as was done for the native coals (Figure 5B). Again, the correlation between the two techniques is strong. The spectra of the acid-washed coals provide further evidence that effects from oxidation are not significant in these samples. Each of these coals was exposed to air for the same duration as the native APCS. We therefore expect that, if oxidation were a problem, any conversion of pyrite to ferrous or ferric species would also have occurred in the acid-washed samples. However, each of the acid-treated coals contains a greater amount of pyrite relative to the other iron species than the corresponding native coal. This observation indicates that the majority of the species removed by the acid treatement was already present in the native coals, rather than products from reaction with air. We note one curious feature regarding the LewistonStockton coal. This coal is one of three, the others being Wyodak-Anderson and Beulah-Zap, for which we concluded that there is an appreciable ferric content in the native material. However, while the latter two coals show an appreciable change in their spectra upon extraction with hydrochloric acid, the changes in the XANES and EXAFS spectra for the Lewiston-Stockton coal are relatively minor. Similarly, the deconvolution of the edges for these acid-washed samples indicates that the ferric content of the Wyodak and Beulah-Zap has decreased while there is a relative increase in the ferric component of the Lewiston-Stockton. This result may suggest the presence of an additional iron species in the APCS. However, an analysis based on the amount of leachable iron suggests that a fraction of the ferric species within the Lewiston-Stockton coal may simply not be accessible to an acid wash (see below). Both the appearance of the XANES spectra and the edge deconvolution for the acid-washed Pocahontas coal indicate that there is an appreciable amount of pyrite in this sample. We did not reach a similar conclusion when analyzing the XANES spectrum of the native coal. Native Pocahontas does, however, exhibit a significant peak at 3.6 Å in its radial structure function, a fact which is consistent with the XANES analysis of the acidwashed coal. The observation of pyrite in the leached sample indicates that the edge deconvolution of the native Pocahontas does not correctly reflect the composition of the coal. Pyrite is, in fact, present in the native version of this material.

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Finally, we note one other feature in the XANES spectrum of Pocahontas after extraction with acid. The pre-edge feature at 7112 eV, which corresponds to a 1s3d transition,24,29 is quite well resolved. Although pyrite also has an intense pre-edge, its shape is different from that observed with Pocahontas. This pre-edge may suggest the presence of tetrahedral iron.24,29 Somewhat more speculatively, the acid-washed Blind Canyon appears to have a similar pre-edge. Such speculations may explain the reason for the large residuals when fitting the edge spectra of the native Pocahontas and the Blind Canyon coals. As discussed above, the model compounds do not include any tetrahedral iron species. It is possible that iron in tetrahedral coordination appears in significant quantities only in these two of the Argonne Premium Coal Samples. Principal Components. The modeling of the nearedge spectra for iron in the Premium Coals indicates that there are more than three distinct environments for this element among the eight coals. We have used some of the techniques of principal component analysis36-38 to verify this conclusion. This approach differs from that discussed above in that all analysis is performed on the spectra of the actual coals. The use of model compounds is not necessary. In principal component analysis, a set of orthogonal basis vectors are derived from the original spectra. These components are mathematical constructs. They do not necessarily correspond to the spectra of actual iron species within the Argonne Premium Coals. This technique can illuminate how many components are in the coals. In general, however, it does not determine what those components are. Figure 6 shows the results of the principal component analysis of the native Argonne Premium Coal Samples. These normalized components are derived from a set of spectra which were taken consecutively on a single beamline (NSLS X23A2). The use of these spectra minimizes variations which arise from changes in the beamline. In addition, all of the samples were prepared identically. The components are shown in Figure 6 in decreasing order of importance. The seventh and eighth components indicate the level of experimental noise in these constructions. Qualitatively, the first five components all contain features that are above the noise. Even the sixth and seventh components appear to include some peaks which are experimentally significant. This analysis demonstrates that there are at least five detectable iron species in the APCS. If only three iron species were present in the Argonne Premium Coal Samples, combinations of the first three principal components should reproduce each of the eight near-edge spectra to within experimental error. We performed a least-squares fitting analysis identical to that employed above except that the three primary principal components replaced the spectra of model compounds. Since the components are derived from the actual spectra of the Argonne Premium Coal Samples, we expect that fits using these components should (36) Aries, R. E.; Lidiard, D. P.; Spragg, R. A. Chem. Br. 1991, 27, 821-824. (37) Malinowski, E. R. Factor Analysis in Chemistry, 2nd ed.; Wiley: New York, 1991. (38) Auf der Heyde, T. P. E. J. Chem. Educ. 1990, 67, 461-469.

Wasserman et al.

Figure 6. Principal components from the iron K edge X-ray absorption spectra of the Argonne Premium Coal Samples. The components are shown in decreasing order of importance.

generally be better than those which utilize distinct model compounds.39 For five of the coal samples, the three most important components model the experimental spectra well. For two coals, Upper Freeport and Lewiston-Stockton, the agreement between the XANES spectra and the best fit using the components is marginal, given that the models originate from the actual spectra of the coals. Furthermore, the three principal components fail to reproduce the XANES spectrum from the Blind Canyon coal. These fitting results provide further confirmation that there are more than three iron species in the coal samples. Leachable Iron. In principle, X-ray absorption spectroscopy can be used as an alternative to wet chemical analysis to determine the amount of iron in each coal that is removed by extraction with hydrochloric acid. To do so, the intensity of the detected signal in XAS is compared before and after treatment with hydrochloric acid. This comparison requires a correction for the material that is removed from the coal during extraction. Unfortunately, such an approach is difficult to implement. Each of the samples must be very uniform in the amount of material in the X-ray beam, a property which is not easy to achieve with powdered materials. Any lack of uniformity will result in an apparent change in the concentration of iron. Figure 7 shows the intensity of X-ray fluorescence, µo, (39) When using the three primary principal components in the fitting procedure, the residuals, χ2, are Upper Freeport 0.017, WyodakAnderson 0.010, Illinois No. 6 0.0020, Pittsburgh No. 8 0.0032, Pocahontas No. 3 0.0063, Blind Canyon 0.098, Lewiston-Stockton 0.056, and Beulah-Zap 0.0067. With the exception of Upper Freeport, these residuals are less than those found when using FeO, Fe2O3, and pyrite as the model compounds.


Figure 7. Correlation of the magnitude of absorbance, µo, with the amount of iron in the Argonne Premium Coal Samples. The iron content of the coals is represented by the product of the weight percent of iron (as ferric oxide) found in the hightemperature ash and the weight percent of ash from each coal. See ref 40. The dashed line represents a linear fit to the data. Table 2. Percent of Iron Extracted by Concentrated Aqueous Hydrochloric Acid from the Argonne Premium Coals: Estimates from the X-ray Absorption Near-Edge Spectra coal

% of iron extracted

Upper Freeport Wyodak-Anderson Illinois No. 6 Pittsburgh No. 8 Pocahontas No. 3 Blind Canyon Lewiston-Stockton Beulah-Zap

6 61 1 6 147a 21 80 56

a The percent of extractable iron in Pocahontas is an artifact of the deduced negative pyrite content in the native coal. See text.

from the eight APCS as a function of the amount of ferric oxide in the high-temperature ash from each coal.40 While there is a reasonable correlation between the iron content of the coals and µo, the agreement between the two methods is not sufficient to quantitate the amount of iron in each coal. We have used an alternative procedure to determine the percent of leachable iron in each Argonne Premium Coal. Since pyrite is not susceptible to extraction by hydrochloric acid, it provides a standard for calculating how much non-pyritic iron is removed during the acid wash. Any relative increase in the amount of pyrite in the acid-washed coal reflects a corresponding decrease in the other ferrous and/or ferric species. The specific relation for the determination of the amount of leachable iron is given in eq 2.

% leachable iron ) % pyrite (acid washed) 1× 100 (2) % pyrite (native)

(

)

Table 2 shows the amount of leachable iron in each of the APCS, as estimated using X-ray absorption spectroscopy. Those coals which in their native form contain large amounts of pyrite, Upper Freeport, Il(40) Reference 21, p 23. The amounts of ferric oxide were determined by multiplying the percent of ferric oxide in each ash by the total ash from each sample.


linois, and Pittsburgh, have only minor amounts of extractable iron species. The amount of extractable iron in Beulah-Zap, 56%, agrees with that found by previous workers.33 This analysis clearly overestimates the amount of extractable material in the Pocahontas coal, an artifact of the lack of a proper model compound for this sample. Nevertheless, Pocahontas does contain large amounts of leachable iron, as indicated by the dramatic change in the XANES spectrum after extraction. We also conclude that the Lewiston-Stockton coal loses significant amounts of iron. Because of the small amount of pyrite in both the native and acid-washed versions of this coal, this estimate is not as precise as that for the coals which contain greater quantities of FeS2. As noted above, the apparent change in pyrite content, combined with the similarity of the XANES spectra of the native and acid-washed coals, suggests that a small amount of the ferric iron in LewistonStockton is not accessible to the acid used in the extraction. Sulfur Species in the APCS. The objective of these experiments is to examine the iron constituents of the Argonne Premium Coals Samples. In addition, our results may have important implications for the elucidation of the sulfur species in these coals. Traditionally, the sulfur in coals is divided into three categories: sulfate, pyritic, and organic.21,34 In the ASTM standard, the quantity of sulfate in each coal is determined by extracting the sulfate into dilute hydrochloric acid, HCl. The residue from this extraction is then treated with dilute nitric acid, HNO3. This process oxidizes the pyrite to the equivalent of ferric sulfate, which is soluble in nitric acid. The nitric acid extract is analyzed for iron, all of which is assumed to originate from the pyrite contained within the original coal. This method deduces the amount of pyritic sulfur from the iron that is removed by the nitric acid. The difference between the total amount of sulfur in the coal and the quantities of sulfate and pyrite represents the organic sulfur. If there is an error in the quantitation of either the sulfate or pyritic sulfur, the results for organic sulfur will be affected. Since pyritic sulfur is actually analyzed through extraction of pyritic iron, the results of this study should be considered when analyzing for sulfur in coals. The extraction procedure used here is similar to that of the ASTM standard for the quantitation of sulfur in the sulfate form.34 We have used concentrated rather than dilute hydrochloric acid in the preparation of the acid-washed coals. The XANES spectra clearly indicate that the pyritic content of these coals is enhanced after treatment with concentrated HCl. Nevertheless, several coals still have significant amounts of non-pyritic iron after treatment with the acid. If any of this nonpyritic iron is extracted by the nitric acid, the amount of pyrite in the sample will be overestimated. This overestimate will result in the underestimation of the amount of organic sulfur. Since other studies have found a good correlation between the organic sulfur/ carbon ratios from elemental analysis and XPS studies,41 it is unclear to what degree these considerations affect the sulfur analysis of the Argonne Premium Coals. Further work to determine the types of non-pyritic iron, (41) George, G. N.; Gorbaty, M. L.; Keleman, S. R.; Sansone, M. Energy Fuels 1991, 5, 93-97.

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if any, left in the coals after treatment with dilute hydrochloric acid is required to address this issue. Conclusion. This study has demonstrated the utility of X-ray absorption spectroscopy for the analysis of the transition metal components of coal samples in general, and the Argonne Premium Coal Samples in particular. However, like all studies on complex systems such as coal, this type of analysis is quite complex and requires great care. These experiments suggest that even in analyzing iron, an element whose speciation is presumably well understood, the number of distinct moieties is probably greater than was previously assumed. The conclusion that ferric species exist in some of the Argonne Premium Coals is particularly notable. It is apparent that the models chosen for these experiments are not sufficient to completely describe the iron-containing components within these coals. While additional model compounds are clearly required, X-ray absorption spectroscopy does not have sufficient resolution to accurately determine which models are most suitable. Techniques such as principal component analysis can overcome part, but not all, of this deficiency. These conclusions have some important ramifications for the use of XAS to examine the trace elements within coals or petroleum. While XAS can detect minute (parts per million) levels of such trace

Wasserman et al.

elements, the work here clearly shows that chemical and mineralogical classification of these components will not be straightforward. Nevertheless, XANES and EXAFS spectroscopies can provide information which will be useful in the future development of coal as a fuel and an alternative source of organic starting materials. Acknowledgment. We thank Farrel W. Lytle and Frank E. Huggins for providing some of the spectra that were used in this study. Kathleen A. Carrado and Steven L. Yuchs aided in the acquisition of the X-ray absorption spectra. George D. Cody and Mark R. Antonio supplied many insights into the mineralogical aspects of coal research and the iron species found in coals. Mark R. Antonio, Clayton W. Williams, and Lynda C. Soderholm generated Mo¨ssbauer spectra which were used to confirm some of the XAS results. Dorothy Swain provided expertise in principal component analysis. This work was supported by the Office of Chemical Sciences, Basic Energy Sciences, U.S. Department of Energy under contract W31-109-ENG38. Research at the National Synchrotron Light Source is supported by the U.S. Department of Energy. EF950154D

Iron Species in Argonne Premium Coal Samples: An Investigation

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