Fourier Transform Infrared Spectroscopy for Inorganic Compound Speciation R. Michael Gendreau", Robert J. Jakobsen, and William M. Henry Battelle Columbus Laboratories, 505 King Avenue, Columbus, Ohio 43201
Kenneth T. Knapp U.S. Environmental Protection Agency MD-46, Research Triangle Park, N.C. 277 11
Work at Battelle Columbus Laboratories has resulted in the development of a methodology that permits the identification of individual inorganic species, even in complex particulate pollutant mixtures. Development of this capability has permitted the identification of various sulfates and oxides obtained from both coal- and oil-fired fly ash samples. The analysis technique is based on the sensitivity, stability, and computerized data handling capabilities inherent to Fourier transform infrared systems (FT-IR). In our experience, FT-IR, when combined with proper sample handling, elemental analysis, the use of appropriate reference standards, and a simple separation step, has proven superior to other techniques in the identification of many inorganic compounds. In many cases, the infrared results were verified by also obtaining independent X-ray diffraction (XRD) data on the same sample. Inorganic compound speciation has long posed a problem to the analytical environmental chemist. Traditionally, there has been a lack of instrumental techniques that are capable of identifying discrete inorganic species in the midst of complex environmental samples. X-ray diffraction (XRD), electron microprobe, infrared, and Auger spectroscopy are some of the techniques that have been applied in the past, but each technique has been found to suffer from limitations. Because of a need for better methodologies, Battelle Columbus Laboratories in conjunction with the U S . Environmental Protection Agency has developed a Fourier transform infrared spectroscopic technique which, when backed by proper sample handling, elemental analysis, and an infrared reference library, is capable of identifying many discrete inorganic species in environmental samples such as coal fly ash and ambient air particulates. Experimental Sample Description. Samples employed in this study were obtained from fossil fuel emission sources of interest to the EPA. The amount of sample obtained ranged from 10 mg up to 10 g of material. S a m p l e Handling. Elemental cation and anion determinations were obtained on the samples as received, after which the samples were prepared for the infrared procedure described as follows. A portion of the total as-received sample was water extracted using a volume of 150 mL of distilled water. The water-insoluble fraction was recovered through filtration and drying. The water-soluble fraction was recovered by vacuum drying the filtrate. Weights were obtained on each of these fractions. In addition to the gravimetric data, elemental analysis (cation) was repeated on the water-soluble fraction to follow the distribution of the cations. Thus, after the water extraction there are three samples to be studied: (1)the remaining total, as-received sample, (2) the water-insoluble fraction, and (3) the water-soluble fraction. T o achieve a reproducible chemical and physical state in the inorganic components, portions of each of the three fractions were baked a t 250 O C under an argon atmosphere for 2 to 4 h. 990
Environmental Science 8. Technology
This heating tends to cause most hydrated species to assume a reproducible state of hydration, as will be shown later. I n f r a r e d P r e p a r a t i o n . The three unbaked and three baked portions were all handled in an identical manner: approximately 1 mg of sample was mixed with 500 mg of preground spectroscopic grade potassium bromide (KBr) powder and ground for several minutes. The KBr-sample mixture was then placed in a press under high pressure to make a KBrsample pellet, after which the pellet was placed in the spectrometer and scanned at 4-cm-' resolution over the range 4000-400 cm-*. Methods of KBr pellet preparation vary between laboratories; we have found that this simple grinding technique is quite satisfactory for our needs ( I ) . Other authors have sought to minimize physical grinding of the sample itself. D a t a Handling. The spectral system used was the Digilab FTS-14 Fourier transform infrared (FT-IR) system. The advantages of FT-IR have been discussed in the past ( 2 ) ,but the following two features have been helpful in inorganic compound identification. C o m p u t e r Data Handling. Our system has a dedicated minicomputer and mass data storage. In addition to spectral storage, the dedicated computer can also perform mathematical manipulations on the spectral data-this includes the ability to subtract two spectra directly, which will enhance the differences between two similar spectra. By obtaining a file of reference compounds as described below, an unknown sample can be subtracted against known standards previously stored in the computer library, thus helping in the identification and quantification of inorganic species present in the sample. Stability. FT-IR spectra are considerably more reproducible over periods of weeks and months than conventional instrumentation. The stability of the instrument is important, because this allows spectra to be directly compared and subtracted, even if they were run months apart. Reference Standards and Samples. As stated previously, a very important aspect of the development of this methodology is the preparation procedure used for both the samples and reference standards. Reference standards are a necessary part of any method based on infrared identifications, because the spectra of the reference standards are necessary to identify the components present in a sample-either by direct comparison of spectra or through subtraction routines. The reference compounds are handled exactly like samples, including baking and dissolution in water if the compound is water soluble. The spectra obtained are then added to the computer's library for future reference. During the preparation of a number of reference standards, it was noted that there is a wide range of behavior which an inorganic sulfate may display upon dissolution, drying, and heating. For example, CaS04.2H20 when heated will stabilize to form CaS04.0.5H20 under our procedure, regardless of whether or not it had been dissolved prior to heating ( I ) . Fe(S04).7H20, on the other hand, forms a complex compound intermediate, which will ultimately form what appears to be
0013-936X/80/0914-0990$01 .OO/O @ 1980 American Chemical Society
t h e monohydrate form. In addition, these types of changes appear t o be reproducible and characteristic of t h e Fe(SO1).7H20 species, and thus observation of the transition t o the monohydrate provides a useful clue for the identification of that species in a sample.
Results and Discussion T h e key t o the success of a program designed to identify inorganic species in fly ash samples rests on two factors: the sample must be handled in a fashion so that inorganic species are not lost or generated during the sample handling the sample components must be in physical and chemical (hydration and valence) states comparable to those of the reference compounds. T h e importance of the physical and chemical state of the sample is demonstrated in Figure 1.This figure shows spectra of CUSO&JH.,O(run as KBr pellets). The top spectrum is the sample as it existed when obtained from a freshly opened bottle, while the bottom spectrum is after the sample had been heated in the argon-flushed oven. T h e spectral differences observed most likely reflect changes in hydration state, but can also represent changes in crystalline structure. Of more importance, however, is the fact that these changes drastically alter the spectrum. If a sample contained CuSOJ6H20 in a condition such that it gave a spectrum similar to the top figure and the reference standard was in a state so that it gave the hottom figure, identification of CuSO4 in the sample would be difficult, if not impossible. However, if a reference library had each of these forms of CuS04.5H20 on file, these changes would be very useful in proving the CuS04.5HrO was or was not present.
Because of the obvious importance of obtaining reproducible samples, we studied the effects of baking the environmental samples and the effects of dissolving the samples in water and recwvering them. In addition, many pellets were prepared to determine the reproducibility of the KRr pellet preparation procedure. Figures 2 and 3 show representative results of these studies. T h e sample we worked with was a synthetic physical mixture which contained 13%NiSOcGH20,
Fr*(u.ncy,
cm-'
Figure 2. Infrared spectra of a mixture of NiS04.6H20 (13%), MgS04. 7 H 2 0 (41 YO), and VOSOl (46%):(top)after dissolving in H 2 0 and drying; (middle) actual physical mixture; (bottom)computer-generated spectrum of mixture (from components present in physical mix)
1
n
1
*
sha
O b
un-
1-
I*
0.m4.m m I W D B I y ( 9 )
Figure 1. Infrared spectra of CuSO4.5H2O: (top) freshly prepared sample; (bottom)baked sample
F r q u m y cm-'
Figure 3. Infrared spectra of a mixture of N iS 04.6H 20, MgS04.7H20, and VOS04 after heating for 4 h in argon at: (A) 350 ' C : (B) 120 'C; (C) 80 O C
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41% MgS04.7H.70, and 46% VOS04.XH20. In Figure 2 the spectrum labeled "physical mix" is a spectrum of the physical mixture as prepared; the spectrum labeled "computer mix" is a computer-generated spectrum prepared from previously stored standard reference spectra. The fact that these two spectra are not identical immediately indicated that an improved library would be required, since inorganic compounds can change physical or chemical state upon standing. The top figure is the spectrum of the physical mixture after being dissolved in water and recovered, without baking. The physical mix is quite different from the dissolved mix and would be impossible to identify as being of the same composition. The change in state due to dissolution results in not only the presence of new infrared bands, but also the strong S-0 vibration (1050-1200 cm-l) has broadened considerably. Thus, the physical state (as reflected by the infrared spectra) of the inorganic compounds has been considerably altered as a result of being dissolved in water. This is not always the case; it has been found that some inorganic compounds (such as CaS04) remain essentially unchanged after being dissolved in water and rerun by FT-IR. In either case, it is important to know how being dissolved in water affects the compound. Figure 3 is the result of taking the dissolved mixture shown in Figure 2 and baking a t 80 "C in air (Figure 3C), 120 "C in argon (Figure 3B), and 350 "C in argon (Figure 3A), each for 4 h. Note: The extreme similarity of these spectra demonstrates that baking causes a change of state which is evident when the dissolved mixture (Figure 2) is compared to any spectra in Figure 3 (baked). However, these changes (sharpening of the bands being the major change) quickly stabilize,
and no further changes take place when baked a t higher temperatures. Note that the similarity of these three samples also demonstrates that the KBr pellet preparation procedure is highly reproducible. These studies resulted in the establishment of the following procedure for handling fly ash samples: Obtain infrared spectra of the fly ash samples before and after heating in argon a t 250-350 "C for 2 to 4 h. Do a water extraction of the sample, separate the water solubles and the water insolubles, and dry each fraction. Obtain infrared spectra of the water solubles and the water insolubles before and after heating in argon a t 250-350 "C for 2 to 4 h. The above procedure was selected for the samples because separating the water solubles (mostly sulfates) and the water insolubles (oxides and silicates) aids in the interpretation of the infrared spectra. This separation aids the spectral interpretation by removing interfering absorption bands.
A
1900
I la00
I
lem
I 1400
I 1Zoo
I loo0
I
800
I
em
J
ua
rrqurrn, om-'
Figure 4. Infrared spectra of coal fly ash (heated at 250 "C for 4 h): (A) total sample; (B)water-insoluble fraction; (C) water-soluble fraction
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1800
1300
loo0
700
1
400
Frequency, cm-
Figure 5. Infrared spectra of coal fly ash (heated at 250 O C for 4 h): (A) water-soluble fraction; (B)subtraction of total sample minus waterinsoluble fraction (Le., computer-generated spectrum of water-soluble fraction)
A further consideration involving the use of a water separation should be pointed out at this point. We were concerned that when water-soluble inorganics (sulfates, nitrates, and carbonates) are dissolved and recovered, we may be creating new species. For example, if iron and aluminum sulfate were both present in a coal fly ash sample and were dissolved in water together, it was felt that there would be a sulfate formed with both iron and aluminum placed in the lattice, a so-called mixed sulfate. Obviously, formation of these mixed sulfates would not be desirable. T h e computer system of the FT-IR proved the answer to this dilemma in the following manner. Spectra are obtained of the total sample as well as the water insolubles and the water solubles. The total sample contains both the sulfates and the insolubles, but the sulfates are generally in low enough concentrations (5-35%) in coal fly ash total samples that interpretation is obscured by the insoluble components. In order to study the portions of the infrared spectrum characteristic of sulfate, we simply mathematically subtract the spectrum of the insolubles from the total, giving a resultant (mathematically generated) solubles spectrum which can then be interpreted. The advantage of this approach is that a spectrum of the soluble components is obtained before exposure to water. This avoids the possibility that we are generating artifacts by the water extraction step. In practice, we compare the mathematically generated soluble spectrum to the actually obtained soluble sample. If they agree, we conclude that a significant amount of mixed sulfates has not formed. If they do not agree (which is not very often), we must use the computer-generated spectrum to do the analysis; otherwise, we prefer to use the actual sample spectrum because the quality of the actual spectrum is better. Figures 4 and 5 provide a real case in which this subtraction procedure was employed. Figure 4 is a coal fly ash sample obtained from a power plant. Figure 4A is the total sample
(baked), Figure 4B is the insoluble sample (baked), and Figure 4C is the water-soluble sample (baked). Figures 4B and 4C can be and were interpreted (see section on coal fly ash), but the question a t this point was whether Figure 4C truly represents the individual sulfate components. The computer was used to check this, and Figure 5B is the result of the subtraction of Figure 4B from 4A (4A - 4B). Figure 5B is, therefore, a computer-generated spectrum of the soluble-fraction compounds. For comparison purposes, Figure CIA is the actual baked soluble fraction. Clearly these two spectra are quite similar, and very little mixed sulfate formation, if any, has occurred. We have speculated that mixed sulfates only pose a problem when there are two or more major sulfate species in roughly equal concentrations. When there is one predominant species, as is usually the case, the formation of mixed sulfates upon dissolution in water seems to be of little importance. We have demonstrated t h a t use of the computer and the ability to do spectral subtractions are valuable for monitoring the formation of mixed sulfates, as well as for detecting species present in small quantities. By subtracting away major components and using scale expansion, minor components of the fly ash sample can be identified in many cases. Another additional use of spectral subtraction is shown in Figure 6. Figure 6 is a spectrum of the water-soluble fraction of a coal fly ash sample collected after passing through an ESP precipitator. This spectrum was interpreted as having CaC03 and CaS04 as the major components, and there is no question that CaCO:] is one component present in the soluble fraction of this fly ash sample. XRD analysis also confirmed the presence of CaC03 in this sample. This was of great concern because: (1)CaC03 is not water soluble a t neutral or basic pH and (2) initial elemental analysis of the total failed to show the presence of carbon or carbonate. The question then is where did the CaC03 come from? Spectral subtraction was utilized to answer this question by producing a spectrum of the soluble compo-
/ 0 F r l q u m . sm-1
Figure 6. Infrared spectra of coal fly ash sample collected downstream from ESP collector (soluble fraction, baked)
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993
Table 1. Infrared Resultsa for Oil Fly Ashes Oil
Yo
fly
H20
ash
method
sol.
1
E
58
%
0
IR
2
E
3
IR E IR
4
E
23
0.1
2.2 v
VOS04
VOS04
0.8
9.0 v
0.2
E
VS04 VOS04 12.9 V VOS04 2.4 Mg
IR
Mgs0.1
72
E
98
IR
6
4.7 Mg
MgS04
IR
5
% indicated elements
"4
3.9 Na NanS04 1.2 Mg MgS04 Na2S04 5.0 Mg MgSOi
1.0 Ni
0.6 Ni
NiS04 CaSO, 1.1 Ni
NiS04 2.3 Ni NiS04 0.4 Fe
2.7 Mg MgS04 0.8 V VOSOa
E = elemental analysis, IR = infrared analysis. a Infrared analysis was only performed on water-soluble species in this series. vanadium sulfate other than voso4. For fly ash no. 4, a fifth unidentified sulfate has been detected.
nents. By comparing this computer-generated soluble-component spectrum (Le., those present in the unhandled sample to the actually derived soluble-component spectrum (Figure 6)), it was found that the unique CaCO:3bands a t 2520 and 1805 cm-l were absent in the mathematically generated spectra. Therefore, carbonate was not present in the original fly ash sample, and the CaC03 must have formed during the handling procedure. We now assume that the CaCO,3was the result of species such as CaO absorbing atmospheric C 0 2 to form CaCO:I during the sample handling procedure. Oil Fly Ash Analysis. Using the described fractionation procedure combined with elemental and infrared analysis, six oil-fired fly ash samples were studied. The result of this analysis is shown in Table I. As can be seen in Table I, a large percentage of each oil fly ash is water soluble, and this fraction is mainly composed of sulfates. In Table I, the percentage of water-soluble components, the percentage of NH4+,and the percentage of the four most abundant elements are listed for each fly ash. The infrared results for each fly ash are also listed in Table I, with the most abundant sulfate (based on estimations from infrared band intensities) listed first, the second most abundant next, etc. It can be seen in Table I that this infrared method identified several sulfates in each fly ash, and these identifications were, in general, supported by the elemental analysis. (Infrared analyses of the oil fly ashes were only performed on the water-soluble fractions; presumably elements which were present but not identified by FT-IR were present in the insoluble fractions or below detection limits of the FT-IR procedure.) Thus, FT-IR combined with the use of elemental analysis to guide the initial spectral subtraction has been successful in the identification of individual sulfate species, even in the face of a complex mixture of several sulfates. Coal Fly Ash Analysis. The water-soluble content of coal fly ash is, in general, much lower than that of oil fly ash samples. Thus, while the water solubles are still important, the water insolubles assume a much greater importance than for oil fly ash samples. Figure 7 shows the infrared spectra of the heated watersoluble fractions from two coal-fired power plants (samples A and B). Note that the spectra are remarkably similar, and even though there are several informative infrared bands in the 600-700-cm-' region, i t is difficult to identify individual sulfates from such spectra. Without having a n extensive in994
Environmental Science & Technology
0.6 Ca CaS04 0.3 Na
0.5 Na other 2.0 Na
0.3 Ni NiS04 CaS04
VSO4 is used to indicate a
organic infrared reference library available, all that can be concluded is that there are large amounts of CaS04 and/or Fe(S04) present. These two samples were obtained and run several months apart, yet the ability of the computer to recall spectra from memory and subtract them allows us to directly compare two spectra such as these. Therefore, the capability to subtract infrared spectra was useful in this case, especially since the two spectra were very similar. Figure 8B shows the subtracted spectrum of the fly ash samples A and B. T h e
,
F q u m i , m-'
Figure 7. Infrared spectra of (A) sample A; (B) sample B
water-soluble fraction of coal fly ashes:
differences are small, and a scale-expanded version of this subtraction (Figure 8A) was required t o observe the differences. T h e absorption band pointing downward (1210 cm-l) indicates t h a t while A12(S04)3 is present in sample A, there is more of it (relative to the other sulfates) in the sample B fly ash. Also, the hands in the 600-700-cm-' region indicate the presence of both CaS04 and Fe(S04),and since the hands in the 600-700-cm-' region are pointing upward, there must be more of these components in the A sample than in B. Thus, from the subtracted spectra, the amount of Al(S04):3(relative to the other sulfates) is higher in the B sample, and the relative amounts of CaS04 and Fe(S04) are higher in the A sample.
These conclusions were substantiated by the elemental analysis performed on these two samples. T h e spectrum of the water-insoluble fraction of the €3 coal fly ash sample was shown in Figure 4B. T h e spectrum of the sample A insoluble fraction is almost identical with the Figure TB fraction, except that the Figure 7A spectrum shows a greatly reduced 560-cm-' iron oxide infrared band. These spectra of the insoluble fractions are representative of primarily S O l , and iron oxide (with more iron oxide in the B sample). No aluminum oxide was detected in either insoluble fraction in spite of the fact that relatively large (about 10%) amounts of elemental A1 were found in each sample on spark source emission analysis. This probably indicates that the Si02 component was in fact part of a n aluminum-iron-silicate complex glass. We currently do not have many reference spectra of silicates; thus such identifications are not possible a t this time.
Summary T h e results obtained from the use of the methodology described in this paper demonstrate t h a t FT-IR, when coupled with careful sample preparation, appropriate reference spectra, and spectral subtraction, and when guided by elemental analysis, can provide unique information on inorganic compound speciation. This information can he obtained on amorphous as well as crystalline samples. Relative quantification of the compounds identified is already possible (and being done) but absolute quantification will require obtaining accurate extinction coefficients for all needed reference standards under a variety of conditions. Many factors have been identified that influence the infrared spectrum of a fly ash sample, but each factor can he taken into account, and a realistic inorganic speciation performed. Future work will involve the compilation of a large library of known reference compounds in as many states as possible. In addition to this, far-infrared spectra will also be obtained. Far infrared will assist in the identification of many of the oxide species whose vibrations fall mainly in the far-infrared region, thus improving our capabilities with respect to oxide identifications.
400
Literature Cited (11 Gendreau, R. M., Burton, R., A p p l . Spectrosc., 33,6 (1979). (21 Griffiths, P. R., "Chemical Infrared Fourier Transform Spectroscopy", Wiley-Interscience, New York, 1975.
Figure 8. Subtracted infrared spectra (water-solublefractions of sample A minus sample B): (A) scale expanded; (B) no scale expansion
Receiued f o r reuieu' September 20, 1979. Accepted M a y 2, 1980. This u,ork u,as supported under Contract 68-02-2296 from the l ' . S . En[:ironmentalProtection A g e n c y
ls00
1200 F-~w,
8W
cm-1
Factors Influencing the Release of Boron from Coal Ash Materials Anne S. Halligan and Gordon K. Pagenkopf Department of Chemistry, Montana State University, Bozeman, Mont. 597 17
Five coal ash materials have been leached with distilled and natural waters. T h e amount of boron released is dependent upon the contact time, the ratio of ash to leachate water, and ash particle size. When the ratio of ash to water is greater than 1 g/L, the ash is capable of retaining a sizable fraction of the water-available boron. For example, a n upper stack ash released 520 p g of boronlg when it was leached a t a rate of 1 g of ash/L. When 'the leaching rate was increased t o 50 g of ash/L, 318 pg of boronlg was released.
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T h e increasing utilization of coal for hydrothermal generation of electricity is correspondingly going to increase the amount of coal ash. T h e ash has been disposed of in a variety of ways with varying degrees of success. One of these processes involves sluicing in large lagoons. Ash generated in plants utilizing Western coals often contains sizable quantities of alkali and alkaline earth oxides, which upon contact with water provide basic solutions. Another component of the coal, boron, is present in higher concentrations in the ash than it was in the coal ( I ) . A sizable fraction of the boron present in
@ 1980 American Chemical Society
Volume 14, Number 8, August 1980
995
