Modes of occurrence of trace elements in coal from XAFS spectroscopy

International

ELSEZVIER

Journal

of

International Journal of Coal Geology 32 (1996) 31-53

Modes of occurrence of trace elements in coal from XAFS spectroscopy F.E. Huggins

*,

G.P. Huffman

Uniuersity of Kentucky, Lexington, KY 40506, USA

Received 7 June 1995; accepted 22 November 1995

Abstract Experimental aspects of X-ray absorption fine structure (XAFS) spectroscopy are described for determining the mode of occurrence of selected trace elements in coal. For elements between calcium and molybdenum in the periodic table, information relating to the mode of occurrence can be deduced from the XAFS spectrum provided the element’s concentration exceeds about 5 ppm. This spectroscopic method of determining elemental modes of occurrence complements electron microscope or microprobe methods because it provides information on element forms dispersed in the organic fraction of coal as well as on the mineralogical forms of the element. XAFS spectra for the lithophile elements, Ti, V, Cr and possibly Mn, indicate that these elements can be associated with both the minerals (principally illite) and the macerals in coals of rank up to high volatile bituminous. XAFS data also confirm that Mn, Zn, As and Br, can be largely organically associated in certain coals. XAFS spectra for As, and to a lesser extent, Se show that these elements will oxidize over time, once the coal has been exposed to air. Keywords:

mode of occurrence;

trace elements; XAFS spectroscopy; XANES; hazardous air pollutants

1. Introduction interest in trace elements in coal. Much of this interest issues and concerns regarding the effect on human health of various trace elements listed as potential hazardous air pollutants (HAPS) that may be released to the atmosphere as a result of coal combustion (CAAA, 1990; Clarke and Sloss, 1992). Similar concerns have been expressed about the possible deterioration of There

is currently

significant

is driven by environmental

* Corresponding author. Present Lexington, KY 40506-0043, USA. 0166-5162/96/$15.00 Copyright PZI SO166-5162(96)00029-8

address:

533 South Limestone

St., Suite 111, University

0 1996 Elsevier Science B.V. All rights reserved

of Kentucky.

32

F.E. Huggins, G.P. H@nan/Intemational

Journal of Coal Geology 32 (1996) 31-53

water quality arising from the leaching of trace elements present in coal cleaning wastes or in combustion ash products stored in refuse ponds or land fills. When addressing these environmental issues, one of the major gaps in our knowledge base regarding trace elements in coal is the lack of information available on elemental modes of occurrence and their role in determining the behavior of trace elements in utilization areas, such as coal cleaning, combustion, and coal-refuse and ash disposal. For, although there is an apparent wealth of published observations on elemental modes of occurrence in coal, as has been excellently summarized by Swaine (1990) it is not at all clear how much of this information has been derived from speculation about elemental modes of occurrence that has later become accepted as fact, or has been generated from coals in which the element is highly enriched and/or present in atypical forms. Hence, there is little in the way of general rules or trends that can be used to predict elemental modes of occurrence for a given coal that, for example, take into account such factors as rank or sulfur content. Although there is clearly a great need for mode-of-occurrence information, it is extremely difficult to determine how a specific element occurs in coal when its concentration is less than about 100 ppm because of the wide variation in elemental modes of occurrence in coal. The mode of occurrence may vary from highly dispersed forms, in which elements are covalently bound in the organic matrix of the macerals, to discrete forms, in which the elements are highly localized and concentrated in specific minerals. Various direct and indirect methods have been used to determine elemental modes of occurrence with varying success (see discussion in Huggins et al., 1994a). One of the best indirect methods is probably the method of organic affinity determinations originated by Zubovic (1966) at the U.S. Geological Survey and developed further by Gluskoter et al. (1977) at the Illinois Geological Survey. However, the values for organic affinity obtained with this method, which is based on the partitioning of trace elements between float and sink fractions in heavy-media separation tests, provide only a relative ranking of the organic association of the elements; the values themselves provide no information as to the actual modes of occurrence of the elements. More recent similar methods have concentrated on detailed chemical analysis of maceral convincingly separates (e.g. Lyons et al., 1989) and, although such measurements demonstrate that many inorganic elements can have a significant organic association, they too provide little information as to how such elements may actually be present in the macerals. Staged leaching methods and ashing methods (Finkelman et al., 1990) have also been used to determine elemental modes of occurrence indirectly. Direct methods have tended to fall into two complementary camps: microscopic methods that involve analysis of up to thousands of individual particles, and spectroscopic methods that attempt to obtain a single spectral signature that can then be interpreted in terms of the principal mode or modes of occurrence of the element. Microscopic methods are best suited for determinations of mineralogical modes of occurrence, in which the trace element is sufficiently concentrated to be determined in an electron microprobe (Minken et al., 1979; Ruppert et al., 1992) or a scanning electron microscope (SEM) equipped with an energy-dispersive X-ray (EDX) detector (Finkelman and Stanton, 1978; Finkelman, 1988) or a wavelength-dispersive X-ray (WDX) detector (Galbreath and Brekke, 1994). Normally, the element must be present in the

F.E. Huggins, G.P. Hufian/lntemational

Journal of Coal Geology 32 (1996) 31-53

33

host mineral at a level greater than 0.01 wt.% (microprobe or SEM with WDX) or 0.1 wt.% (SEM with EDX) to be detected by such methods. More sensitive microprobe methods have also been evaluated for direct determination of elemental modes of occurrence: e.g. ion microprobe mass analyzer (IMMA) (Finkelman et al., 19841, laser-ablation microprobe mass analyzer (LAMMA) (Lyons et al., 1987; Morelli et al., 19881, synchrotron radiation X-ray fluorescence microprobe (SRXFM) (White et al., 1989) and proton-induced X-ray emission microprobe (PIXEM) (Hickmott and Baldridge, 1991); however, such methods have not been widely applied. Before a spectroscopic method can be considered for determining modes of occurrence of trace elements in coal, it must meet the two following requirements: firstly, the method must be highly element-specific; that is, the obtained spectrum must arise only from the element of interest, without significant chance of interference; and, secondly, it must have sufficient sensitivity to obtain a useful signal-to-noise ratio from elements present in coal at trace concentration levels. Resonance spectroscopic methods, e.g. nuclear magnetic resonance, electron spin resonance and 57Fe Miissbauer spectroscopy, are capable of meeting the element specificity requirement and, although much useful information has been obtained about specific major elements in coal with these methods (especially 13C NMR and 57Fe Miissbauer spectroscopies), they are not sufficiently general or sensitive to be useful for investigations of trace elements in coal. Electron spectroscopic methods, such as X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy, are also highly element specific and are generally applicable to most elements in the periodic table, but they are also not sufficiently sensitive for trace-element analysis. Furthermore, such methods tend to be best suited for analysis of surfaces, rather than for bulk applications. One bulk spectroscopic technique that can meet both requirements of element specificity and sensitivity is X-ray absorption fine structure (XAFS) spectroscopy. This technique is, in theory, applicable to all elements, and is sensitive enough to obtain information on modes of occurrence of elements in the middle of the periodic table at concentration levels of L 5 ppm. Furthermore, it is equally good for both dispersed organic as well as mineralogical modes of occurrence and in this sense is complementary to the microprobe/microscope methods. There are, however, significant drawbacks with XAFS spectroscopy; the main ones are: (i) only a single spectrum is obtained that represents the weighted sum of all the modes of occurrence of an element and (ii) the need for a synchrotron as the source of X-rays. The application of this technique for the determination of information relevant to the modes of occurrence of specific trace elements in coal is the principal topic of this paper.

2. Background on XAFS spectroscopy The fine structure associated with the absorption edge of an element was observed as long ago as the 1920s and the first attempts to provide a physical understanding of the phenomena were published by Kronig (1931). However, it was not until the late 1960s when laboratory XAFS experiments were analyzed according to the theory developed by Sayers et al. (1971) that the technique was given its correct physical interpretation and

34

F.E. Huggins, G.P. Hu@nan/International

Journal of Coal Geology 32 (1996) 31-53

the first insights were obtained as to the potential of XAFS spectroscopy for the determination of local structure and bonding around the absorbing atom. Within five years of the publication of this seminal paper, the first XAFS experiments were successfully carried out using the extremely powerful radiation emitted by a synchrotron, and the technique has grown greatly since, finding application in virtually all fields of science. A schematic of the basic XAFS experiment is shown in Fig. 1. Absorption of X-rays by the sample is measured as the ratio of the X-ray intensity of the monochromatic incident beam (I,> relative to either the transmitted beam (I,) or the fluorescent X-rays emitted in response to the absorption process (I,). The XAFS spectrum for a given element is obtained by measuring the change in X-ray absorption, either in transmission, [defined according to Fig. 1 as ln(Z,/Z,)], or in fluorescence geometry, (defined as Z/Z,>, as a function of energy across one of the characteristic absorption edges of the element. An intense source of monochromatic X-rays, the energy of which can be varied by rotating the crystals of the monochromator (Fig. l), is necessary to measure the variation in absorption coefficient or “fine structure” associated with the absorption edge. This absorption fine structure is determined by both the electronic structure (i.e. the valence state of the absorbing atom, the covalency of the absorbing atom - ligand bonding) and the local atomic structure (bond distances, coordination numbers, ligand types of the immediate atomic shells) around the absorbing atom. This combination of electronic and structural factors is typically unique for a given absorbing atom in a particular material and it is this uniqueness that enables the XAFS spectrum to be used to identify modes of occurrence of an element in coal or other materials. Ionization Chambers ..-..-..-.--.

(measures X-ray intensity) .---“1

controls rotation of monochromator and hence, energy of X-ray beam

Plotter

YicroVAX Computer controls experiment and records data Fig. 1. Schematic

illustration

of the typical XAFS experimental

layout.

F. E. Huggins, G.P. Huffman / Iniemational Journal of Coal Geology 32 (1996) 31-53

3s

It has become normal practice to subdivide XAFS spectroscopy into two distinct techniques, X-ray absorption near-edge structure (XANES) spectroscopy and extended X-ray absorption fine structure spectroscopy (EXAFS), in order to interpret the fine structure. As the names imply, these techniques deal with the fine structure that is in the vicinity of the edge itself, and further away from the edge, respectively (Fig. 2). Quite different analysis is done for each of these techniques. Theoretical interpretation of the XANES region is difficult and hence this region is typically used, without significant modification, as a “fingerprint’ ’ for the element in the material under investigation. Systematics in this region of the spectrum tend to reflect the formal oxidation state of the element in the material and the degree of covalent bonding involved in the immediate coordination shell around the absorbing atom or ion. For example, significant systematic differences in the XANES spectra are readily apparent when comparing metallic, sulfide and oxide/silicate materials of an element. The EXAFS region on the

0.7 0.65 0.6 0.55 0.5 5 'G g 0.45 :: a 0.4

I

‘\

I i

0.35

!

I

zn K-edge XAFS Spectrum

‘\!

i

s :\ :I (\

0.3

\ , I

Illinois #6 Coal

0.25 0.2

-I

9.5

I’

,

9.7

, I’

,

I I #’

‘\

\(

’ ’ \ \

10.3

10.1 9.9 \ Energy,keV ‘\ I, ‘\

: I I I

I

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I I

!

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\

\

I

,

I

0.1

g 'Z 9' P 8

E 00

-zoo.5 2

-20

F

EXAFS

XANES

E

-0.1

= 0 0

20 40 60 Energy,eV

60

100

Fig. 2. XAFS spectrum of zinc in Illinois #6 coal, showing XANES and EXAFS regions.

J 0

.

200

.

.

400 Energy,eV

how the spectrum

.

-

600

is subdivided

J

into separate

36

F.E. Huggins, G.P. Hufian/lntemational

Joumal of Coal Geology 32 (1996) 31-53

other hand reflects the short-range structure around the absorbing atom or ion. A physical interpretation in terms of interference phenomena involving scattering of photoelectron waves emanating from the absorbing atom by the surrounding atoms or ions was advanced (Sayers et al., 1971) to interpret this region of the spectrum that may extend as much as 1500 eV above the absorption edge. From such an analysis of the EXAFS spectral region, information may be obtained on local structural details regarding the nearest shells of atoms that surround the absorbing atom. Such information includes approximate coordination numbers, interatomic distances and even the identity of the element in these coordination shells. These analytical techniques are well described in reviews and textbooks on XAFS spectroscopy and the interested reader is referred to these articles for additional information (Eisenberger and Kincaid, 1978; Lee et al., 1981; Koningsberger and Prins, 1988). A very useful review of XAFS spectroscopy along with applications in Earth Sciences is the article by Brown et al. (1988).

3. Experimental considerations XAFS investigations of minor and trace elements in coal can only be conducted at a synchrotron radiation facility, where the typical X-ray flux is some 6-9 orders of magnitude greater than that available with a conventional laboratory X-ray tube. For the results described in this paper, experiments were carried out at both the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory, NY and at the Stanford Synchrotron Radiation Laboratory (SSRL), Stanford University, CA. At NSLS, a bending magnet beam-line (X-19A) was used exclusively for the measurements and the synchrotron was operated at 2.53 GeV with a current decaying from 250 to about 100 mA over the lifetime (16-24 h) of the beam fills, At SSRL, the 54-pole wiggler beam-lines VII-3 and IV-3 were used for XAFS measurements with the synchrotron operating at 3.0 GeV and a current that typically varied from 100 to 40 mA over the lifetime of the fill. At NSLS, a Si(ll1) monochromator was used for elements in the range P-Fe and a Si(220) or Si(3 11) monochromator for heavier elements; at SSRL, a Si(220) monochromator was used for elements heavier than vanadium. The detector also plays a major role in determining the detection limits for XAFS spectroscopy of trace elements. For concentrations less than about 0.1 wt.% (1000 ppm), the standard Stem-Heald or Lytle fluorescence detector (Stem and Heald, 1979; Lytle et al., 1984) is replaced by a more sensitive 13-element germanium fluorescence detector (Cramer et al., 1988) to maximize the signal/noise ratio. This detector uses energy-discrimination electronics to detect only the X-rays fluoresced by the element of interest and to exclude most of the X-rays fluoresced by other elements (Cramer et al., 1988). Further, the use of absorption filters and Soller slits in the path of the fluoresced radiation provides still further enhancement of the signal-to-noise ratio (Stem and Heald, 1979). Finally, for elements of the lowest concentration, the spectra are repeated a number of times and then added to improve the signal-to-noise ratio. By combining the number of elements in the Ge detector and the number of repetitive scans, the final spectrum of a given trace element in coal may be the summation of as many as 150 individual spectra and take as long as six hours to accumulate.

F. E. Huggins, G.P. Human / International

Journal of Coal Geology 32 (1996) 31-53

31

Under optimum conditions, the conventional detection limit for an element by XAFS spectroscopy is actually well under 1 ppm, but since we are interested in obtaining mode-of-occurrence information from the spectrum, rather than merely noting that the element is present in the sample, the detection limit for useful information is probably closer to 5 ppm. For example, Fig. 3 shows the XAFS spectrum of Zn in the Pocahontas #3 (VA) coal from the Argonne Premium Coal Sample program (Vorres, 1990). The zinc concentration in this coal is 6 ppm (Palmer, 1990). Despite this low concentration, even the first two or three broad EXAFS peaks can be discerned, in addition to the zinc K-edge XANES structure, from which mode of occurrence information may be deduced. Also present in this spectral range are very weak L edges for ytterbium and tungsten. Roth of these elements are present in the Pocahontas coal in sub-ppm concentrations: Yb: 0.55 ppm, W: 0.84 ppm (Palmer, 1990). However, no information relevant to the modes of occurrence of these elements can be deduced from these L edges that barely exceed the noise level of the spectrum. The XAFS spectra are normally recorded in much the same manner at both synchrotrons. The spectral range is divided into at least three energy intervals: the pre-edge region, the XANES region, and the EXAFS region. The pre-edge region, which does not contain any spectral information but has to be measured in order to determine the pre-edge slope, is measured relatively coarsely (1 or 2 eV/step) and rapidly from about - 100 eV to about - 20 eV before the edge. The edge or XANES region, which for trace elements often contains most of the information, is then measured very carefully and slowly (0.1-0.2 eV/step> from -20 eV below the edge to as much as 50 eV above the edge. The EXAFS region is then measured out to at least 250 eV above the edge up to as much as 1500 eV above the edge, depending on the strength of the EXAFS oscillations. Typically, the stepping across0 the EXAFS region is done in terms of reciprocal-space inverse distance (e.g. 0.05 A-‘/step) with the time per point

80 75 70 65 s ‘CI ,P :: 9

60 55 50

10.1 9.9 Energy,keV

10.3

Fig. 3. XAFS spectrum of zinc in Pocahontas #3, VA, coal. The zinc content in this coal is only 6 ppm, yet some EXAFS oscillations are clearly visible. The minor edges are from ytterbium (0.56 ppm) and tungsten (0.84 ppm).

38

F.E. Huggins, G.P. Huffman/lntemational

Journal of Coal Geology 32 (1996) 31-53

determined by the actual interval in real space. This procedure gives a reasonably uniform appearance to the chi spectra (isolated EXAFS spectrum converted to reciprocal space), resulting in better RSF spectra. It is, of course, possible to subdivide the spectrum into more than three regions. For example, another XANES region might be defined for detailed scanning over a second edge, should one occur near to the original edge. All XAFS spectra for a particular element are measured relative to a primary calibration standard. This standard is generally a relatively inert material that contains the element of interest. The material should be especially resistant to, or protected from, both oxidation and hydrolysis in moist air. Often, the first major peak in the first derivative of the XANES spectrum of the appropriate metallic foil is used to define the zero point of energy for a given element. However, for some elements, especially the alkali and alkaline earth metals and halogen gases, the elemental state is too reactive or not in a sufficiently convenient form to be a primary standard and another material must be chosen. Table 1 lists the primary standards used for calibration purposes for many of the elements we have investigated in coal. As indicated in Fig. 1, the spectrum of the calibration standard can be recorded at the same time as the unknown spectrum in a separate absorption spectrum (defined according to Fig. 1 as ln[I,/I,]) performed after the X-ray beam has been transmitted through the sample, provided there is sufficient flux. If not, the calibration spectrum must be run separately. Preparation of coal samples for XAFS spectroscopy basically consists of grinding a representative sampling of the coal to 70% - 200 mesh ( < 75 pm). As much as 1 g of the pulverized sample is suspended in the X-ray beam in ultrathin (6 km> polypropylene bags; however, the beam only samples a small fraction of the sample as the spot size varies from about 1 mm2 (focussed mode) to about 10 mm2 (unfocussed mode). The sample thickness is about 5 mm, but the actual penetration of the sample by the incident

Table 1 Primary calibration

standards

Element

Energy, eV

Primary standard

Comment

Sulfur Chlorine Potassium Calcium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Arsenic Selenium

2472.0 2825.0 3608.4 4038.1 4966.0 5465.0 5989.0 6539.0 7112.0 7709.0 8333.0 8979.0 9659.0 11867.0 12658.0 13474.0

Sulfur (sublimed) NaCl KC1 CaCO, Ti foil V foil Cr in stainless steel Mn in stainless steel Fe stainless steel Co foil Ni foil Cu foil Zinc oxide (ZnO)

White line peak 1st. derivative 1st. deriv. 1st. deriv. 1st. deriv. 1st. deriv. 1st. deriv. 1st. deriv. 1st. deriv. 1st. deriv. 1st. deriv. 1st. deriv. White line peak White line peak 1st. deriv. 1st. deriv.

As203 Se

(gray form) KBr

F. E. Huggins, G. P. Huffman /International Journal of Coal Geology 32 (1996) 31-53

39

X-rays and hence the sample depth from which the fluorescent X-rays are emitted depends strongly on the X-ray energy. The XAFS spectra recorded at NSLS and SSRL are transferred to a dedicated computer at the University of Kentucky for analysis and archival storage. As is normally done (Eisenberger and Kincaid, 1978; Lee et al., 1981; Brown et al., 1988; Koningsberger and Prim, 19881, the raw spectra are first calibrated with respect to the zero-point of energy in the XAFS spectrum of the primary standard, then divided into separate XANES and EXAFS regions, and finally the radial structure function (RSF) is obtained from the EXAFS region by transformation to reciprocal space (chi spectrum), followed by application of a Fourier transform. Normally these three steps are sufficient for analysis of trace and minor elements in coal. However, additional steps can be taken, if the quality of the data warrants, to determine details of specific coordination shells that are represented by peaks in the RSF. These additional steps are described in detail elsewhere (Eisenberger and Kincaid, 1978; Lee et al., 1981; Brown et al., 1988; Koningsberger and Prins, 1988). However, for elements in coal, especially those with concentrations less than about 50 ppm, the EXAFS region is often too weak to be informative, and hence, reliance is made solely on the XANES region to interpret the elemental mode of occurrence in such situations.

4. Review of XAFS results on trace elements

in coal

To date, a number of the more abundant trace elements listed in the 1990 Clean Air Act Amendments (CAAA, 1990), have been investigated by XAFS spectroscopy. Interpretation of the mode of occurrence of an element in a coal from its XANES spectrum is a largely intuitive process that may make use of a number of interrelated exercises that are described below: (1) Usually of most value is a comparison of the coal spectra with XANES spectra of standard compounds and minerals containing the same element. This exercise will usually eliminate many possible chemical or mineral forms from consideration and may occasionally reveal the actual mode of occurrence. (2) Generally of next highest value is a comparison of the XANES spectrum of the trace element in coal with the XAFS spectra of chemically and geochemically similar major or minor elements in coal. Often, there are close correspondences between spectral profiles for elements found in a similar mode of occurrence. This is especially the case for elements with similar electron configurations, e.g. Cl and Br, but elements such as Ca and Mn*+ also give rise to similar spectra for similar modes of occurrence. (3) Subdivide the coal into different fractions, e.g. float-sink fractions, before and after chemical leaching, etc., and compare and contrast the XANES spectra of the element in the different fractions (Huggins et al., 1995). This exercise will generally indicate whether the mode of occurrence of the element in the coal consists of only one or of two (or more) major forms and hence may establish why the spectrum of the whole coal does not resemble any standard spectrum because it arises from a mixture of major forms.

F. E. Huggins, G.P. Huffman /International Journal of Coal Geology 32 (I 996) 31-53

40 Table 2 Summary

of selected trace-element

concentrations

Coal, rank, source, state

in the Argonne

Premium coal samples

Element concentration

in ppm (whole coal basis)

Cl

Ti

V

Cr

Mn

Ni

Zn

As

Se

Br

1500 < 100 500 650 1600 450
800 560 700 560 380 2600 200

26 14 32 15 11 44 4

20 620 31 14 9 40 380

41

14 5 21 10 9 15 1

20 11 220 9 6 14 6

17 4 5 8 10 6 3

1.9 1.6 4.3 1.5 2.5 6.2 0.6

66 3 7 16 50 17 1

Upper Freeport, mvb, Indiana Co., PA WyodakAnderson, sbb, Cambell Co., WY Illinois #6, hvb, St Clair Co., IL Pittsburgh, hvb, Greene Co., PA Pocahontas #3, lvb, Buchanan Co., VA Lewiston-Stockton, hvb, Kanawha Co., WV Beulah-Zap, lig, Mercer Co., ND

77 17 16 15

Sources of Data: Coal: Vorres (1990). Cl: Evans et al. (1990). by whole coal X-ray fluorescence. Ti-Mn: Doughten and Gillison (19901, by ICAF-AES on dissolved high-temperature ash. Ni-Br: Palmer (19901, by instrumental

neutron activation

analysis

on coal.

(4) Use derivative spectra for comparison. The derivative spectrum of the XANES is generally more diagnostic than the XANES itself for discriminating among different modes of occurrence. This exercise can be taken to the second or even third derivative of the XAFS spectrum, if the quality of the data is sufficient. (5) Simulation of the XANES or derivative XANES spectrum of an element in coal by weighted additions of XANES or derivative XANES spectra of standards. This exercise is valuable for establishing that the coal spectrum can be approximated by the addition of two or three standard spectra, representing different forms of the element in coal. Generally, such additions are only useful for approximately equal additions of two, possibly three distinct forms. Furthermore, there should be a good reason (e.g. a form identified independently of XAFS spectroscopy, or distinct spectra found for float-sink fractions of the coal) to expect that the unknown spectrum arises from a mixture of two or three different forms. Attempts to add four or more spectra to simulate an unknown usually have little merit, especially in the absence of independent information that establishes that the spectrum arises from a mixture of different forms. Examples of these analytical exercises will be found in the review of XAFS spectra of individual trace elements in coal that is presented below. Many of the examples shown in this section are XANES spectra of coal samples from the Argonne Premium Coal Sample (APCS) program (Vorres, 1990). A summary of the relevant trace-element concentrations for these coals is given in Table 2.

4. I. Chlorine

There is generally between 200 and 2000 ppm chlorine in coal. However, coals from specific areas can have much higher chlorine contents. For example, the chlorine concentration of certain coals from the Illinois basin and from various seams in the United Kingdom can exceed 8000 ppm. Hence, chlorine should be regarded as a minor rather than a trace element in coal. However, its inclusion in this review is principally

F.E. Huggins, G.P. Hu@nan/lntemational

Journal of Coal Geology 32 (1996) 3-53

41

motivated by its inclusion on the list of HAPS proposed by the 1990 Clean Air Act Amendments (CAAA, 1990). Recent XAFS investigations of chlorine in coal (Huggins and Huffman, 1991a,b,1995) indicate that there is one major chlorine mode of occurrence found in virtually all coals, regardless of rank and geographic location (Fig. 4). This mode of occurrence consists of chlorine present as chloride anions in the moisture associated with the macerals in coal, and not as specific mineral chlorides nor as organochlorine compounds. Furthermore, evidence obtained principally from in situ XAFS pyrolysis experiments (Huggins and Huffman, 1991a,1995) and more recently from Cl XAFS spectra of staged combustion tests (Chou et al., 1995) imply that there is a significant interaction between the hydrated chloride anions and the maceral surface. Except for very high chlorine coals (Cl > 4000 ppm), this interaction appears to be principally via polar nitrogen functional groups, most probably quaternary amines. A second mechanism may come into play for very high chlorine coals once all the polar nitrogen is associated with chloride; this mechanism involves the bonding of chloride anions via sodium cations (+ other soluble cations?) at polar oxygen functional groups. This two-step mechanism for interaction of chlorine with maceral surfaces is consistent with the observed correlation of sodium and chlorine noted for Illinois coals by Chou (1991).

2.8 2.6 2.4 2.2 2

Upper Freeport, PA

Silverdale Fines (U.K.)

0.8 0.6 0.4 0.2 0 0

20

40

60

Energy in ev (NaCI standard) Fig. 4. XANES spectra of chlorine in three bituminous

coals from different geographical

locations.

42

F.E. Huggins, G.P. Huffman/Intemational

Journal of Coal Geology 32 (1996) 31-53

4.2. Titanium Titanium was one of the earliest elements in coal to be examined by XAFS spectroscopy because of a controversy regarding the behavior of titanium during coal liquefaction and its possible role in poisoning coal liquefaction catalysts. Early XAFS work by Sandstrom et al. (1982) and Wong et al. (1983) indicated the presence of two different titanium forms in coal, one mineralogical, the other associated with the organic matrix of the coal. The mineralogical forms were believed to be mainly titanium oxides, rutile or anatase, while the organic forms were believed to be similar to Ti alkoxides or phenoxides. More recent XAFS work has shown that significant titanium may be present in the clay mineral illite in coals (Huggins et al., 1995) (Fig. 5). Furthermore, application of gravity and column flotation to Kentucky #9 coal shows an obvious separation of mineral forms (Ti/illite + Ti oxides) into the tailings fractions and an organically associated Ti form in the float fraction (Huggins et al., 1995). The spectrum of this latter form is closely similar to that observed for low-rank coals and other cleaned fractions (Wang et al., 1983) and resembles published spectra of organo-Ti in “tyzor” compounds (Wong et al., 1983) or Ti alkoxides (Sandstrom et al., 1982).

3.4 3.2 3 2.8 2.6 2.4 c

.g g $ 2

Rutile:Ti/illite

2.2

50:50

2 1.8

8; ,N

1.6

g

1.4

z

1.2 1 0.8 0.6 0.4 0.2 0 -20

0

20 Energy,

40

60

80

eV

Fig. 5. XANJZS spectra of titanium in two Argonne Premium coals. Also shown are simulations from weighted addition of the XANES spectra of rutile, anatase and Ti in illite.

of the XANES

F. E. Huggins, G.P. Huffman /International Journal of Coal Geology 32 ( 19961 3 l-53

‘$3

4.3. Vanadium The first XAFS studies of vanadium in coal were conducted on a specific 15-cm thick bedding horizon at the top of the Kentucky #9 seam that contains up to 1800 ppm by weight of vanadium (Maylotte et al., 1981a,b; Wong et al., 1983). This coal sample was subjected to float-sink testing, as well as to direct coal liquefaction. Comparison of the spectra of the float and sink fractions with that of the raw coal indicated that there were two distinct forms of vanadium present: the spectrum of the sink fraction resembled the spectrum of roscoelite, a vanadium-rich muscovite [K(V3’, Al),AlSi30,0(OH)2]. whereas the spectrum of the float fraction was attributed to V4+ in a pyramidal or distorted octahedral structure inv$ving one short V-O bond at about 1.6 A and 4 or 5 longer V-O bonds at about 2.0 A. More recent vanadium XAFS investigations (Huggins et al., 1995) of whole seam samples of the Kentucky #9 coal have essentially confirmed the earlier findings of Maylotte et al. (1981a,b) and Wong et al. (1983), that there are two distinct forms of vanadium in this coal. However, it is our opinion that the mineral form that contains V3+ is probably illite, a structurally similar mineral to muscovite, as this mineral appears to be the “garbage pail” for most lithophile elements in Kentucky #9 coal (Huggins et al., 1995). 1.4. Chromium Despite the fact that the mean Cr content of raw U.S. coals is only about 15 ppm, (Bragg et al., 19941, chromium is one of the trace elements of most concern to environmental issues arising from coal utilization (Clarke and Sloss, 1992) because of the possible presence of the toxic and carcinogenic oxidation state of chromium, Cr(VI), in coal or coal products. Chromium in the trivalent oxidation state, on the other hand, poses no threat to human health at such low concentrations. To date, a significant number of coals have been examined by chromium XAFS spectroscopy, and none of them have shown any evidence for the presence of Cr@‘I) (Huggins et al., 1993a: Huffman et al., 1994). The detection limit for Cr(V1) in coal using XAFS spectroscopy is about 5% of the total chromium, or less than 1 ppm of the coal with the mean chromium content. Furthermore, there is really no reason to expect the presence of Cr(VI> in coal given the conditions of formation of coal in a typically anoxic environment. It should be noted that generally in fresh coal, there is no evidence for any significant amount of ferric iron, and iron is an element that can be oxidized to Fe3+ much more readily than chromium can be oxidized to Cr(V1). The chromium XANES spectra of coals tend to show relatively minor variations that are difficult to quantify. However, the first derivative of the XANES spectra usually shows significantly more variation and it is easier to identify different chromium forms in coal using the derivative spectrum (Fig. 6). As for titanium and vanadium, there appear to be two major forms of chromium present in high-volatile bituminous coals: one mineral, the other organic in association (Huggins et al., 1995). In a number of coals, chromium in illite can be identified as the major chromium form in tailings or sink fractions and sometimes in raw coal samples. Very rarely, other specific chromium

44

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Journal of Coal Geology 32 (1996) 31-53

01 CrOOH

-20

(amorph.)

1

I

I

0

I

I,

20

I

Energy,

oJ...,.......l -20

0

20

40 Energy,

60

60

I

40

I

60

I

I

60

I

10

eV

100

eV

Fig. 6. XANES and first derivative XANES spectra of three chromium standards and of the float and tailings fractions of a Kentucky #9 coal subjected to a Denver flotation cell. Note the similarity of the derivative XANES for the tailings fraction and for Cr in illite.

mineral forms can be recognized (e.g. chromite in an Indonesian lignite, Huggins et al., 1994b). However, in many coals and especially in cleaned fractions, chromium gives rise to a specific spectral signature that so far appears to be unique to coal. Various standard spectra are similar, but do not duplicate the observed chromium XANES spectra from coal. Closest are the amorphous chromium hydroxide and oxyhydroxide phases, prepared simply by precipitation in water (hydroxide), followed by drying in air at 150°C (oxyhydroxide). More crystalline oxyhydroxide and oxide minerals are not at all compatible. Hence, like both titanium and vanadium, chromium appears to exhibit a form that is associated with the organic macerals (Huggins et al., 1995). 4.5. Manganese Manganese in coal exhibits some similarities to calcium in its geochemical behavior. For example, for coals varying in rank from lignite to low volatile bituminous, both elements exhibit an evolution from being present in lignite in carboxyl form to being present in crystalline rhombohedral carbonate form [(Ca, M&O,) in high-rank coals (Huggins et al., 1983, 1993b,1994b; Huffman and Huggins, 1984). Fig. 7 shows the

Journal of Coal Geology 32 (I 996) 31-53

F. E. Huggins, G.P. Hujjimzn /International

35

Upper Freeport, PA

-20

0

20

40

60

80

100

120

Energy,eV Fig. 7. XANES spectra of manganese

in five Argonne Premium coals.

manganese XANES spectra for some of the APCS program coals; the spectral profiles for the Beulah-Zap lignite (from carboxyl-bound Mn) and the Pocahontas #3 coal (from Mn in carbonate) are quite similar to those observed for calcium in corresponding coals. However, unlike calcium, other forms of manganese may also be present in coal. The spectral profiles observed for Mn in the Upper Freeport and Illinois #6 coals shown in Fig. 7 are quite different from that for carboxyl Mn or carbonate Mn. Furthermore, they can not be duplicated by weighted additions of the carboxyl and carbonate Mn spectra. Therefore, a third mode of occurrence for Mn in these bituminous coals is indicated. Based on very recent work on clay minerals (Huggins et al., 1995, and unpublished data), Mn in illite is a likely candidate. 4.6. Nickel

Little work has been carried out on nickel in coal. The combination of relatively low nickel concentrations coupled with a strong background in the signal because of fluorescence from the abundant iron in coal makes for relatively noisy spectra and ambiguous interpretation. XAFS data presented for nickel in Illinois #6 coal have been

46

F.E. Huggins, G.P. H$ftnan / International Journal of Coal Geology 32 (1996) 31-53

tentatively interpreted in terms of a nickel association with sulfides, possibly with nickel substituting for iron in pyrite (Huggins et al., 1993b,1994b). Similar findings for nickel in coals from the U.K. are indicated by results from SXRFM (Spears, 1991). However, other coals exhibit XANES spectra more consistent with Ni in an oxidic environment. 4.7. Zinc The abundance of zinc in coal is normally quite low (15-40 ppm), except for coals from the Illinois basin in which the content of zinc may reach as much as 500 ppm. These highly zinc-enriched coals from the Illinois basin appear to contain predominantly zinc sulfide (ZnS). The spectra of all three Illinois basin coals shown in Fig. 8 are virtually identical to the spectrum of ZnS. However, coals from other geographic locations exhibit significantly different spectra, in which zinc sulfide, if present, contributes only a relatively small fraction. The Wyodak coal shown in Fig. 8 exhibits a zinc XANES spectrum that may be attributed to carboxyl-bound zinc. The overall spectrum profile is similar to that noted for Mn in Beulah-Zap lignite (Fig. 7) and to those for Ca generally in low-rank coals (Huggins et al., 1983; Huffman and Huggins, 19841, in which these elements are predominantly carboxyl-bound.

Monterey,IL

o+ -20

,‘;

, , , , , , , , , , , 0

20

40

60

80

100

120

Energy,eV Fig. 8. XANES spectra of zinc in four Argonne Premium coal samples and a coal from Monterey,

Illinois.

F.E. Huggins, G.P. Hu#tnan/lntemational

Journal of Coal Geology 32 (1996) 31-53

41

4.8. Arsenic Arsenic is a trace element of major concern in coal combustion because of its toxicity and volatility (Clarke and Sloss, 1992). Arsenic in bituminous coals appears to be principally associated with pyrite. Evidence for this association comes both from electron microscope and microprobe investigations (Ruppert et al., 1992) and from XAFS spectroscopic studies (Huggins et al., 1993a; Huffman et al., 1994). Furthermore, the EXAFS spectra of high arsenic coals confirm that arsenic substitutes for sulfur in the pyrite structure (Huggins et al., 1993a), as would be anticipated from crystallochemical principles. Both XAFS and microscopic investigations indicate that the occurrence of the mineral arsenopyrite (FeAsS) in U.S. coals is rare. In addition, XAFS studies (Huggins et al., 1993a) have shown that arsenic in pyrite readily oxidizes upon exposure of crushed coal to air to form an arsenate species, most probably an iron arsenate phase (Fig. 9). Moreover, comparison of the relative amounts of pyritic iron (from Miissbauer spectroscopy) and pyritic arsenic (from XAFS spectroscopy) that oxidize over the same

II

-20

0

20

Upper Freeport, PA

Whole Coal

40

60

80

100

120

Energy,eV Fig. 9. XANES spectra of arsenic in different fractions of the Upper Freeport Argonne Premium coal: the whole coal immediately after opening the sealed vial; a 2.85 sink fraction from the same fresh coal sample; a sink fraction from a previously opened, air-exposed APCS vial; the same air-exposed fraction measured 3 months later. The peak at about - 2 eV arises from arsenical pyrite and is replaced over time as a result of oxidation by the peak at about 4 eV that arises from arsenate forms.

48

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Journal of Coal Geology 32 (1996) 31-53

time interval show that the fraction of oxidized arsenic increases much more rapidly than the fraction of oxidized pyritic iron, thus proving the conjecture that the substitutional mismatch of replacing sulfur by arsenic is a point of weakness in the pyrite structure at which oxidation can initiate and occur at a highly accelerated rate. With low-rank coals, quite different arsenic XANES spectra are obtained compared to those for bituminous coals (Huggins et al., 1994b). Arsenic in such low-rank coals is not associated with pyrite as the formal oxidation state of arsenic appears to be As3+ and not (AsS)~~ or As;- as in the pyrite structure. This difference in oxidation state is reflected in a 2.0 eV shift of the white-line peak in the arsenic XANES spectra. In addition, the spectrum obtained for low-rank coals is closely simulated by the spectrum from a lignite subjected to ion-exchange by arsenite species, in which the As oxidation state is As3+. This similarity suggests that the arsenic in low-rank coals is bound to the organic matrix by means of oxygen functional groups. Further details of the As XANES of low-rank coals will be given elsewhere (Huggins et al., 1996). 4.9. Selenium Some preliminary XAFS investigations of selenium in two of the APCS coals have been attempted. The Se XANES spectra are shown in Fig. 10. The spectra of the 4.5

4.0

3.5

s

Lewiston-Stockton,

WV

3.0

‘= : 9a

2.5

J+

xl Bg Z

2.0

1.5

Fresh

I

Illinois #6, IL Air-exposed

1 .o

0.5

0 -20

0

20

40

60

80

100

Energy, eV Fig. 10. XANES spectra of selenium in fresh and previously opened ( > 1 yr), air-exposed samples of the Lewiston-Stockton and Illinois #6 Argonne Premium coals. The relatively noisy spectra arise because the selenium concentrations in the two coals are only 5.5 ppm and 4.3 ppm, respectively.

F. E. Huggins, G.P. Huffman/ IntemarionalJournal of Coal Geology32 (1996) 31 -.U

49

2.5

Pocahontas #3, VA

Upper Freeport, PA

-20

0

20

40

60

80

100

Fig. 11. XANES spectra of bromine in the Pocahontas #3 and Upper Freeport Compare spectral profiles with those of chlorine in Fig. 4.

120 Argonne

Premium

coals.

air-exposed samples, especially for the Illinois #6 coal, are different to those obtained from freshly opened vials, indicating that selenium in coal is sensitive to oxidation. The main peaks in the fresh coal samples are slightly positive (by less than 1 eV) with respect to elemental selenium and consequently may arise from organo-selenium compounds, which are thought to constitute the most likely mode of occurrence of selenium in coal (Swaine, 1990; Finkelman, 1994). The second peak position at higher energy in the air-exposed Illinois #6 coal is indicative of the selenate (SeOf-> oxidation state. 4.10. Bromine XAFS spectra have been taken at the bromine K-edge of the APCS Pocahontas #3 and Upper Freeport coals (Fig. 11). These bromine spectra are similar in profile to the typical chlorine K-edge XAFS spectra of bituminous coals (Huggins and Huffman, 1991a,b,1995) and indicate that bromine has a similar mode of occurrence in coal to that of chlorine. It can therefore be concluded that bromine is ionically bound as Br- anions to the maceral/moisture interface surface and is essentially completely organically associated, as previous investigations of Br in coal (Gluskoter et al., 1977; Lyons et al., 1989) have also indicated. It would have been most unexpected, given the chemical and geochemical similarities of these two halogen elements, had their XAFS spectra and hence their modes of occurrence in coal been significantly different. 4. II. Other trace elements Attempts have been made to use XAFS spectroscopy to investigate other trace elements in coal, notably the very heavy elements, lead and mercury, at their L-edges

50

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around 12- 16 keV. However, such experiments have not been very successful. In the case of lead, weak but uninformative peaks have been detected for coals reported to contain 20 ppm of lead, whereas for mercury, no peaks were detected at all for coals that were reported to contain in excess of 1 ppm mercury. It is recognized that the fluorescence yield of L-edge X-rays is significantly less than that of K-edge X-rays from the same element; however, other factors must also contribute to the poor detection of these elements. Further work clearly needs to be carried out to investigate these heavy elements, as well as the radioactive elements, uranium and thorium, which are also listed in the 1990 Clean Air Act Amendments.

5. Conclusions In this review, the use of XAFS spectroscopy has been demonstrated for obtaining information on the modes of occurrence in coal of selected trace elements, some of which are listed as potential HAPS in the 1990 Clean Air Act Amendments. For elements such as chlorine, chromium and arsenic, with mean concentrations in excess of 10 ppm in U.S. coals and of major concern to the 1990 Clean Air Act Amendments, there is now a considerable database of XAFS spectra, which enable constraints to be placed on the mode of occurrence of these elements in coal. For other trace elements, particularly those with mean concentrations of less than 10 ppm in U.S. coals, new experimental developments are needed in either detector technology or X-ray flux enhancement or both before a corresponding database of sufficient spectra and quality can be generated. The new synchrotron at Argonne National Laboratory, the Advanced Photon Source, which is due to be commissioned in 1996, should provide the impetus to conduct investigations on a much broader range of trace elements in coal because of its much higher X-ray flux compared to those of existing synchrotron facilities in the U.S. Finally, it should be noted that a single XAFS spectrum from an element in coal by itself is not necessarily very informative. Only after a sufficient database of spectra on other coals, derived coal fractions and standards has been established do the pieces of the puzzle relating to an element’s mode of occurrence in coal start to fall into place. In large part, the collection of spectra that constitute the database are strongly influenced by results and conclusions from complementary techniques and geochemical reasoning. Hence, it should be emphasized that no one technique can supply all the information regarding the mode of occurrence, and a synthesis of data and results from all available methods will always be needed to determine an element’s mode of occurrence most completely.

Acknowledgements We are grateful to following personnel from the University of Kentucky who have assisted us with the collection of XAFS spectra at NSLS or SSRL from 1989 to present: Bhaswati Ganguly, Fulong Lu, Sudipa Mitra, Mohammad Najih, Anup Shah, Naresh Shah, Mehdi Taghiei, Shreeniwas Vaidya, and Jiamnin Zhao. In addition, we would like

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51

to acknowledge Drs. Farrel Lytle and Robert Greegor who introduced us to XAFS spectroscopy in the early 1980s and who have shared with us many insights and data over the years. This work has been supported by contracts with the U.S. Department of Energy and the Electric Power Research Institute. We also acknowledge the U.S. Department of Energy for its support of synchrotron radiation facilities at both NSLS and SSRL.

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Gluskoter, H.J., Ruth, R.R., Miller, W.G., Cahill, R.A., Dreher, G.B. and Kuhn, J.K., 1977. Trace Elements in Coal: Occurrence and Distribution. Illinois State Geol. Surv. Circ., 499. Illinois State Geol. Surv., Urbana, IL, 154 pp. Hickmott, D.D. and Baldridge, W.S., 1991. Trace element and S isotopic microanalysis of coal - a combined PIXE and SIMS approach. In: Proc. 8th Annu. Int. Pittsburgh Coal Conf., Univ. Pittsburgh, pp. 148-152. Huffman, G.P. and Huggins, F.E., 1984. Analysis of the inorganic constituents in low-rank coals. In: H.H. Schobert (Editor), Chemistry of Low-Rank Coals. ACS Symposium Series, Vol. 264. Am. Chem. Sot., Washington, DC, pp. 159-174. Huffman, G.P., Huggins, F.E., Shah, N. and Zhao, J., 1994. Speciation of arsenic and chromium in coal and combustion ash by XAFS spectroscopy. In: S.A. Benson, E.N. Steadman, A. Mehta, and C.E. Schmidt (Editors), Trace Element Transformations in Coal-Fired Power Systems. Fuel Proc. Technol., 39: 47-62. Huggins, F.E. and Huffman, G.P., 1991a. Chlorine in coal: an XAFS spectroscopic investigation. In: IEA Coal Research (Editors), Coal Science Proceedings (1991 Int. Conf. Newcastle, England). Butterworth-Heinemann, Oxford, pp. 981-984. Huggins, F.E. and Huffman, G.P., 1991b. An XAFS investigation of the form-of-occurrence of chlorine in U.S. coals. In: J. Stringer and D. D. Banerjee (Editors), Chlorine in Coal. Coal Science and Technology, 17. Elsevier, Amsterdam, pp. 43-58. Huggins, F.E. and Huffman, G.P., 1995. Chlorine in coal: an XAFS spectroscopic investigation. Fuel, 74: 556-569. Huggins, F.E., Huffman, G.P., Lytle, F.W. and Greegor, R.B., 1983. An EXAFS investigation of calcium in coal. In: Proc. 1983 Int. Conf. Coal Science, (Pittsburgh, PA). Int. Energy Agency, pp. 679-682. Huggins, F.E., Shah, N., Zhao, J., Lu, F. and Huffman, G.P., 1993a. Nondestructive determination of trace element speciation in coal and ash by XAFS spectroscopy. Energy Fuels, 7: 482-489. Huggins, F.E., Zhao, J., Shah, N. and Huffman, G.P., 1993b. Speciation of trace elements in coal from XAFS spectroscopy. In: K. H. Michaelian (Editor), Proc. 7th. Int. Conf. Coal Science (Banff, Canada). Vol. 1, pp. 660-663. Huggins, F.E., Huffman, G.P. and Helble, J.J., 1994a. Determination of the form of occurrence of hazardous trace elements in coal and ash. In: W. Chow and L. Levin (Editors), Proc. 2nd Int. Conf. Managing Hazardous Air Pollutants, 1993, EPRI Rep., TR-104295. Electric Power Res. Inst., Palo Alto, CA, pp. 11-113-11-133. Huggins, F.E., Zhao, J., Shah, N. and Huffman, G.P., 1994b. Modes of occurrence of trace elements in coal from XAFS spectroscopy. Preprints, Am. Chem. Sot. Div. Fuel Chem., 39(2): 504-510. Huggins, F.E., Parekh, B.K., Robertson, J.D. and Huffman, G.P., 1995. Modes of occurrence of trace elements in coal: geochemical constraints from XAFS and PIXE spectroscopic analysis of advanced coal cleaning tests. In: J. A. Pajares and J. M. D. Tascdn (Editors), Coal Science (Proc. 8th. Int. Conf. Coal Science, Oviedo, Spain). Coal Science and Technology, 24. Elsevier, Amsterdam, Vol. 1, pp. 175-178. Huggins, F.E., Goodarzie, F. and Lafferty, C.J., 1996. Mode of occurrence of arsenic in subbituminous coals. Energy Fuels, 10: 1001-1004. Koningsberger D.C. and Prins, R., 1988. X-ray Absorption. Principles, Applications, Techniques of EXAFS, SEXAFS, and XANES. Wiley, New York, 673 pp. Kronig, R. de L., 1931. Zur Theorie der Feinstruktur in den Rontgenabsorptionsspektren. Z. Phys., 70: 317-323. Lee, P.A., Citrin, P.H., Eisenberger, P.A. and Kincaid, B.M., 1981. Extended X-ray absorption fine structure - its strengths and limitations as a structural tool. Rev. Mod. Phys., 53: 769-808. Lyons, P.C., Hercules, D.M., Morelli, J.J., Sellers, G.A., Mattern, D., Thompson-Rizer, CL., Brown, F.W. and Millay, M.A., 1987. Application of laser microprobe (LAMMA 1000) to “fingerprinting” of coal constituents in bituminous coal. Int. J. Coal Geol., 7: 185-194. Lyons, P.C., Palmer, C.A., Bostick, N.H., Fletcher, J.D., Dulong, F.T., Brown, F.W., Brown, Z.A., Krasnow, M.R. and Romankiw, L.A., 1989. Chemistry and origin of minor and trace elements in vitrinite concentrates from a rank series from the eastern United States, England, and Australia. Int. J. Coal Geol., 13: 481-527. Lytle, F.W., Greegor, R.B., Sandstrom, D.R., Marques, EC., Wong, J., Spiro, C.L., Huffman, G.P. and Huggins, F.E., 1984. Measurement of soft X-ray absorption spectra with a fluorescent ion chamber detector. Nucl. Instrum. Methods. 226: 542-548.

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Maylotte, D.H., Wong, J., St. Peters, R.L., Lytle, F.W. and Greegor, R.B., 1981a. Trace vanadium in coal: An EXAFS study. In: International Energy Agency (Editors), Proc. Int. Conf. Coal Science (Diisseldorf. Germany). Gliickauf, Essen, Germany, pp. 756-761. Maylotte, D.H., Wong, .I., St. Peters, R.L., Lytle, F.W. and Greegor, R.B., 1981b. X-ray absorption spectroscopic investigation of trace vanadium sites in coal. Science. 214: 554-556. Minken, J.A., Chao, E.C.T. and Thompson, CL., 1979. Distribution of elements in coal macerals and minerals: determination by electron microprobe. Preprints, Am. Chem. Sot. Div. Fuel Chem.. 24( 1): 242-249. Morelli, J.J.. Hercules, D.M., Lyons, P.C., Palmer, C.A. and Fletcher, J.D., 1988. Using laser micro mass spectrometry with the LAMMA-IOOO for monitoring relative elemental concentrations in vitrinite. Mikrochim. Acta (Wien), 111: 105-l 18. Palmer. CA., 1990. Determination of twenty-nine elements in eight Argonne Premium Coal sample\ by instrumental neutron activation analysis. Energy Fuels, 4: 436-439. Ruppert, L.F., Minken, J.A., McGee, J.J. and Cecil, C.B., 1992. An unusual occurrence of arsenic-bearing pyrite in the Upper Freeport coal bed, West-central Pennsylvania. Energy Fuels, 6: 120- 125. Sandstrom. D.R., Filby, R.H., Lytle, F.W. and Greegor. R.B., 1982. Study of TI in solvent-refined coal bq X-ray absorption spectroscopy. Fuel, 61: 195-197. Sayers. D.E., Stem, E.A. and Lytle, F.W., 1971. New technique for investigating noncrystalline structures: Fourier analysis of the extended X-ray absorption fine structure. Phys. Rev. Lett., 27: 1204- 1207. Spears, D.A.. 1991. Pyrite in some U.K. coals. In: P.R. Dugan, D.R. Quigley, and Y.A. Attia (Editors). Processing and Utilization of High Sulfur Coals IV. Coal Science and Technology, 18. Elsevier. Amsterdam, pp. 85-93. Stem, E.A. and Heald, S.M., 1979. X-ray filter assembly for fluorescence measurements of x-ray absorption fine structure. Rev. Sci. Instrum., 50: 1579-1582. Swaine, D.J., 1990. Trace Elements in Coal. Butterworths, London, 278 pp. Vorres, K., 1990. The Argonne Premium Coal Sample Program. Energy Fuels, 4: 420-426. White, R.N., Smith, J.V., Spears, D.A., Rivers, M.L. and Sutton, S.R., 1989. Analysis of iron sulphides from U.K. coal by synchrotron radiation X-ray fluorescence. Fuel, 68: 1480-1486. Wong. J., Maylotte, D.H., Lytle, F.W., Greegor, R.B. and St. Peters. R.L., 1983. EXAFS and XANES investigations of trace V and Ti in coal. In: A. Bianconi, L. Incoccia, and S. Stipcich (Editors), EXAFS and Near Edge Structure. Springer Series in Chemical Physics, Vol. 27. Springer, Berlin, pp. 206-209. Zubovic, P., 1966. Physicochemical properties of certain minor elements as controlling factors in their distribution in coal. In: R.F. Gould (Editor), Coal Science. Advances in Chemistry Series, Vol. 55. Am. Chem. Sot.. Washington, DC, pp. 221-231.