Monitoring the species of arsenic, chromium and nickel in milled coal, bottom ash and fly ash from a pulverized coal-fired power plant in western Canada

Abstract

The concentration of As, Cr and Ni and their speciation (As^3+;5+, Cr^3+;6+ and Ni^0;2+) in milled coal, bottom ash and ash collected by electrostatic precipitator (ESP) from a coal fired-power plant in western Canada were determined using HGAAS, ICP-AES and XANES. The chemical fractionation of these elements was also determined by a sequential leaching procedure, using deionized water, NH₄OAC and HCl as extracting agents. The leachate was analyzed by ICP-AES. Arsenic in the milled coal is mostly associated with organic matter, and 67% of this arsenic is removed by ammonium acetate. This element is totally removed from milled coal after extraction with HCl. Arsenic occurs in both the As³⁺ and the As⁵⁺ oxidation states in the milled coal, while virtually all (>90%) of the arsenic in bottom ash and fly ash appears to be in the less toxic arsenate (As⁵⁺) form. Both Ni and Cr in the milled coal are extracted by HCl, indicating that water can mobilize Ni and Cr in an acidic environment. The chromium is leached by water from fly ash as a result of the high pH of the water, which is induced during the leaching. Ammonium acetate removes Ni from bottom ash through an ion exchange process. Chromium in milled coal is present entirely as Cr³⁺, which is an essential human trace nutrient. The Cr speciation in bottom ash is a more accentuated version of the milled coal and consists mostly of the Cr³⁺ species. Chromium in fly ash is mostly Cr³⁺, with significant contamination by stainless-steel from the installation itself.

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Introduction

Coal is a complex substance containing many elements that were concentrated under a generally acid and reductive environment of formation,^1,2 often with organic associations.³

The mobility of an element in the environment is related to its chemical nature and its association with minerals and organic matter in coal.⁴ The toxicity of an element is often related to its oxidation state. For elements such as selenium, only the concentration is important to its toxicity, however, for other elements, such as arsenic, chromium and nickel, both the concentration and the oxidation state determine the toxicological impact.⁵ An element can be beneficial to health and also toxic, depending on its valence speciation, for example, chromium is beneficial to human health as Cr³⁺, and is carcinogenic as Cr⁶⁺.⁵ The mobility of an element is a function of its chemical properties and many other factors, including particle size, physico-chemical form and its behavior under the range of pH conditions it encounters. Chromium is not soluble in water at normal pH levels for natural waters, but may become soluble at higher pH.⁶

This paper examines the toxicity (determined by oxidation state) and fractionation behaviour under neutral, high and low pH conditions, of the As, Cr and Ni found in milled coal, bottom ash and ESP fly ash collected from a coal-fired power plant in western Canada.

Toxicity of arsenic, chromium and nickel

The emission of As, Cr and Ni and their impact on the local environment is an important aspect of the environmental assessment of a coal-fired power plant.⁷ These elements are included on the list of toxic elements for which monitoring is recommended by Environment Canada. They are also at the top of the Electric Power Research Institute's (EPRI) list as carcinogenic health risks.⁷ However, the comprehensive study by the EPRI indicates that the risk factor associated with emission of these elements is low and for arsenic is less than 0.08 of a cancer occurrence, per year, for the entire population of the USA.

Arsenic has two oxidation states, depending on the conditions it encounters. Arsenic is most toxic in its As³⁺ oxidation state. The toxicity of arsenic decreases in the order: As³⁺ > As⁵⁺ > organo-arsenic.⁵ However, arsenic serves no known biological function.⁸ Whatever the form of As found in a power plant feed coal, burning the coal results in the formation of As₂O₃ in the stack gases, if there is not a flue gas desulfurizer in the power plant.⁹ The major As leachate in fly ash is in the form of arsenate, although some arsenite is also present.¹⁰ According to Jones¹¹ “thermodynamic considerations predict that water soluble As³⁺ oxides (As₂O₃) should be the predominant form of As on the surface of fly ash”. However, very recently, the presence of calcium and other arsenates has been demonstrated in fly ash and, consequently, it has been suggested¹² that the thermodynamic consideration should be revised to include the formation of arsenates.

Chromium, as chromate (CrO₄; Cr⁶⁺), can cause lung cancer, when inhaled as dust,^13–15 whereas chromium in the trivalent form (Cr³⁺) is an essential nutrient element to mammals.¹⁶ The hexavalent form (Cr⁶⁺) converts to a trivalent form in mammals.¹⁷ Chromium is often associated with the clay content of soil¹⁸ and since clay minerals are one of the major mineral groups found in coal, it may therefore contain chromium in this form.¹⁹ It is critical to monitor chromium speciation during pulverized coal combustion, because Cr⁶⁺ compounds, which are toxic, might be present.²⁰ However, 95% of the chromium in most coals is in the Cr³⁺ valence state and remains as such in coal ash.¹⁴

Nickel, as a major component of stainless-steel, is widely used, the International Agency for Research on Cancer (IARC),²¹ which is part of World Health Organization, includes nickel and some of its compounds as probable human carcinogens.²² Two carcinogenic nickel compounds of possible concern in coal combustion are the carbonyl [Ni(CO)₄] and sub-sulfide (Ni_xS, where x ≥ 1) compounds.²² The oxidation states of these compounds are (0) and (II), respectively.

Fractionation of elements in milled coal and ashes during leaching

Selective sequential leaching is used for chemical fractionation of the elements in coal and coal ashes to examine their qualitative distribution and their mode of occurrence.^23,24 Leaching of elements from power plant ashes by water and ammonium acetate and various acids is carried out to determine the nature of the elements and their potential mobility during storage. There are numerous works on this subject.^10,25–28 There are some limitations of this procedure, however. For example, minerals encased by organic matter or by other minerals may not come into contact with the solvent and therefore will not be leached. Elemental interactions (i.e., precipitates, colloids, alkali halides (salts), or other water soluble minerals) may have a relatively large effect, especially during the water leaching.

The water soluble minerals are mostly alkali halides. Elements soluble in ammonium acetate are ion-exchangeable in nature, and are mostly associated with the organic matter in coal, such as salts of organic acids, and with clay minerals.²³ The elements removed by HCl are associated with acid soluble minerals such as carbonates, metasulfides and oxides, and also with organic matter functional groups such as carboxy groups. Hydrofluoric acid is the principal solvent of silicates, while disulfide minerals are mostly removed by nitric acid.

In the present study of coal and power plant ashes, the samples were leached by H₂O, NH₄OAc and HCl (see below). Any elements remaining after leaching by these three agents are associated with insoluble sulfide and silicate minerals.

Methodology

Sampling

Milled coal, bottom ash, ESP ash and stack ash were sampled following the recommendations of EPRI⁷ for a period of 3 days (one sample per day).

Analytical

Sulfur forms were determined for three samples (one for each day) using standard ASTM²⁴ methods for total, pyritic and sulfate sulfur (Table 1).

Table 1 Total sulfur and sulfur forms (wt.%) for the milled coal^a

Form of sulfur in coal	Day 1	Day 2	Day 3	s	Average
a Based on duplicate analyses.
Total sulfur	0.25	0.23	0.25	0.01	0.24
Sulfate sulfur	0.01	0.01	0.01	0.00	0.01
Pyritic sulfur	0.06	0.02	0.05	0.02	0.04
Organic sulfur	0.18	0.2	0.19	0.01	0.19

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The concentrations of arsenic, chromium and nickel in milled coal and power plant ashes were determined using HGAAS for arsenic and ICP-AES for chromium and nickel. Three samples (one for each day) were analysed in duplicate (Table 2). These methods are recommended by the EPRI⁷ for the study of heavy metal emissions from coal-fired power plants. The coal reference materials used for comparison were NIST 1633b and SARM 19 and 20. Analyses of duplicate samples and laboratory standards were used to monitor analytical accuracy and precision of all the above analytical methods. The reader is referred to EPRI,⁷ Stoeppler²⁹ and Sloss and Gardner³⁰ for further details on sample preparation, instrumentation, and detection limits of the applied methods.

Table 2 Concentration of arsenic, chromium and nickel (mg kg⁻¹) in all samples of milled coal, bottom ash and fly ash from a western Canadian coal-fired power plant

	Element	Day 1		Day 2		Day 3		Average	s(ppm)
		Sample 1	Sample 2	Sample 1	Sample 2	Sample 1	Sample 2
a HGAAS.b ICP-AES.
Milled coal	As^a	1.80	1.30	1.60	1.50	1.60	1.80	1.6	0.19
	Cr^b	13.00	10.00	10.00	11.00	10.00	16.00	11.7	2.42
	Ni^b	8.00	3.00	7.00	5.00	8.00	7.00	6.3	1.97
Bottom ash	As^a	1.60	1.60	1.30	1.90	1.50	1.60	1.6	0.19
	Cr^b	31.00	27.00	31.00	29.00	31.00	28.00	29.5	1.76
	Ni^b	25.00	24.00	22.00	22.00	23.00	24.00	23.3	1.21
ESP hopper fly ash	As^a	16.00	16.00	17.00	18.00	19.00	19.00	17.5	1.38
	Cr^b	51.00	49.00	50.00	54.00	52.00	51.00	51.2	1.72
	Ni^b	35.00	32.00	35.00	33.00	33.00	37.00	34.2	1.83

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Enrichment indices (Table 3) of the three elements were determined according to Meij et al.³¹

Table 3 Average elemental composition (ppm) and relative enrichment indices (RE) for milled coals and power plant ashes

Element	Milled coal	Bottom ash	RE	ESP hopper fly ash	RE
a HGAAS.b ICP-AES.
As^a	1.6	1.6	0.2	17.5	2.4
Cr^b	11.7	29.5	0.2	51.2	0.4
Ni^b	6.3	23.5	0.8	34.2	1.2

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Determination of the relative enrichment factor (RE)

The ash formed from the combustion of coal is enriched in the same elements as mineral matter in the milled/feed coal and this enrichment is related to the nature of the ash and element.^31–33 The term “relative enrichment factor” (RE) has been introduced and is defined as:The RE factor is equal to 1 if the concentration of an element is equal to its concentration in the coal multiplied by the ash content of the coal. The RE values of elements are used to distinguish the different classes of elements. The RE for class I, which do not volatilize during combustion (Al Ti, rare earths), is around ±0.3 and for class II, those which are redistributed in various ashes (such as As, Be, Cr and Ni), is <0.7 and for class III elements, elements that are mostly emitted as vapour (such as F, Hf and Cl) it is very low.³³

Leaching

The chemical fractionation of elements in coal and coal ashes is based on the differences in the solubilities of the coal constituents in solutions of deionized water, ammonium acetate (NH₄OAc) and HCl under standard conditions.³⁴ The leachates were analysed using ICP-AES (Table 4). Variation of the pH of the leachate water for milled coal, bottom ash and ESP fly ash were determined for one, three and seven day periods (Table 5).

Table 4 Leachability of elements in water, ammonium acetate and hydrochloric acid

Sample		Water		NH4OAc		HCl
	Untreated coal (ppm)	(ppm)	(%)	(ppm)	(%)	(ppm)	(%)	Total leachable (%)
As—
Milled coal	1.6	0.0	0.0	1.0	62.5	0.6	37.5	100.0
Bottom ash	1.6	0.0	0.0	0.0	0.0	0.6	37.5	37.5
Fly ash	17.5	0.0	0.0	0.0	0.0	11.0	62.9	62.9
Cr—
Milled coal	11.7	0.0	0.0	0.0	0.0	3.0	25.6	25.6
Bottom ash	29.5	0.0	0.0	1.2	4.1	28.3	95.9	100.0
Fly ash	51.2	10.0	19.5	3.3	6.4	16.5	32.2	58.1
Ni—
Milled coal	6.3	0.0	0.0	0.0	0.0	3.0	47.6	47.6
Bottom ash	23.0	0.0	0.0	5.9	25.7	17.0	73.9	99.6
Fly ash	34.2	0.0	0.0	0.0	0.0	4.9	14.3	14.3

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Table 5 Variation of leachate water pH for milled coal, bottom ash and fly ash from western Canada, with holding times

Sample	pH after 1 day	pH after 3 days	pH after 7 days, no stirring	pH after 7 days, stirred
Milled coal	6.20	7.20	6.90	6.40
Bottom ash	7.05	9.00	7.55	7.40
Fly ash	11.15	11.25	11.20	11.10
Fly ash	10.90	11.10	11.00	10.95
Fly ash	11.30	11.55	11.45	11.35

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Speciation

X-ray absorption fine structure (XAFS) spectroscopy^35,36 was used to determine the oxidation states of arsenic, chromium and nickel. XAFS is the name for the overall field of spectroscopy based on X-ray absorption. Usually XAFS spectroscopy is split into two distinct disciplines: XANES spectroscopy and EXAFS spectroscopy.^35–37 XANES (X-ray absorption near-edge structure) spectroscopy deals with the fine structure within ±50 eV of the absorption edge. EXAFS (extended X-ray absorption fine structure) spectroscopy deals with the fine-structure from about +50 eV above the edge to as much as +1500 eV above the edge. Whereas the XANES spectrum is used as a “fingerprint” spectrum for the element of interest in the material under examination, the EXAFS spectrum is mathematically manipulated by: (i) isolation of the fine structure from the general absorption; (ii) conversion to a reciprocal space representation (k-space); and (iii) application of a Fourier transform that converts the k-space EXAFS spectrum into a kind of radial distribution function, which is called a radial structure function (RSF). The RSF describes the local structure about the atom that absorbs the X-rays. Peaks in the RSF represent the coordination shells of atoms. For example, for As, in ash, we usually see a strong peak at about 1.3 Å in the RSF that represents the 4 oxygens around the As in arsenate complexes. The EXAFS spectra of As in coal is usually of good enough quality for useful conversion to the RSF. For Cr and Ni, the concentrations in coal and ash are generally too low to allow a useful conversion of the EXAFS spectrum into the RSF.

The XAFS spectroscopy for the present study was performed at Stanford Synchrotron Radiation Laboratory (SSRL), Stanford University, California. The XAFS spectra of arsenic, chromium and nickel of milled coal, bottom and ESP fly ash were obtained with minimal modification of their as-received state, by suspending them in the monochromatic X-ray beam in ultra thin (6 µm) polypropylene bags. The XAFS spectra were collected in fluorescence geometry using a 13-element germanium detector. The beam-line used at SSRL Stanford University, California, is an unfocused beam-line with a rectangular horizontal slit of approximately 15 mm × 1 mm defining the beam-size. Each spectrum consists of about 400–600 points collected at energies between about 100 eV below the arsenic, chromium, or nickel K absorption edges (at 11867 eV, 5989 eV, and 8333 eV, respectively) to about 300–500 eV above the edge. A thin sample of arsenic trioxide (As₂O₃) was used as the primary standard for arsenic, whereas thin metal foils of stainless-steel and metallic nickel were used as the primary standards for chromium and nickel. Where possible, the calibration spectra were acquired simultaneously with the data collection for the coal samples in absorption geometry after the sample experiment. Otherwise, separate calibration spectra were run periodically during the data collection. The spectra were collected and stored in a Micro VAX computer at SSRL. The spectra were then transferred electronically to a Micro VAX computer for analysis.

The raw XAFS data were first calibrated with respect to the primary standard, then normalized to the edge step, corrected for background slope above and below the edge, and finally divided into separate XANES), which is the structure within 20–50 eV of the absorption edge and EXAFS, which is the weaker and broader set of oscillations beginning at about 40–50 eV about the edge and extending out to several hundred eV. For all three elements, the XANES region was the principal region of interest that was used for the determination of forms of occurrence information, although some useful information could also be obtained from the EXAFS regions of the arsenic XAFS spectra of fly ash samples. Further details of the experimental set-up and analysis can be found in other publications.^38,39

Three sets of the primary standards were used for calibration, one each of arsenic, chromium and nickel (Figs. 1–3). The arsenic standard used for the XANES study consisted of coal with a high pyrite content (Fig. 1) and with two distinct forms of arsenic: As substituting for sulfur in pyrite (indicated by peaks marked “P” in Fig. 1), and As⁵⁺ in the form of arsenate (AsO₄³⁻, peaks marked “A” in Fig. 1).

Fig. 1 Arsenic K-edge XANES spectra of milled coal, bottom ash and fly ash from western Canadian coal and of a high pyrite content coal. The peak P is associated with As³⁺ and peak A is associated with As⁵⁺.

The standard for chromium consists of the two major Cr oxidation states that are found in nature, Cr³⁺ and CrO₄²⁻ (Fig. 2). The two oxidation states are easily distinguished by the height of the pre-edge peak in the XANES spectra.³⁸ The prominent pre-edge peak at about 2 eV is diagnostic of the hexavalent Cr oxidation state. The nickel XANES spectra of various standard materials is presented in Fig. 3. In general, the overall spectral shapes for the metallic and sulfide nickel compounds differ from those compounds in which nickel is coordinated by oxygen (Fig. 3).

Fig. 2 Chromium XANES spectra of Cr³⁺ and Cr⁶⁺.

Fig. 3 Nickel XANES spectra of various standard materials. Note how the overall spectral shapes for the metallic and sulfide nickel compounds differ from those for compounds in which nickel is coordinated by oxygen.

Results and discussion

The content of sulfur in this milled coal is very low (Table 1). The low content of soluble sulfur is not surprising, since the sulfatic sulfur content is low (Table 1). Furthermore, most of the sulfur in western coals is organically associated in the form of S–H bonds³⁴ and this sulfur form is harder to remove by cleaning than the pyritic sulfur.

The content of As, Cr, and Ni for milled coal, bottom ash and ESP fly ash and their enrichment indices (RE) are presented in Table 3. The variation in concentration of elements for 3 days of sampling is low and within the normal variation found in milled coal and power plants ashes.³⁴

Arsenic

Arsenic in bituminous coal is generally thought to be associated with pyrite.^23,40–42 Arsenic XAFS of coal indicates that the arsenic is present in the pyrite structure in solid solution with sulfur.³⁸

The concentration of As in Canadian milled coals increases relative to the sulfur/pyrite content. Arsenic in western Canadian coal is mostly associated with the organic fraction and is held at the maceral (organic constituent) surface by polar oxygen functional groups.⁴³

Arsenic in milled coal, bottom ash and fly ash is not water soluble. However, 63% of the As in milled coal is leached by NH₄OAc (Table 4), indicating that it is associated with organic matter. Further examination of arsenic by XANES supports an organic association for the arsenic. Thirty-eight percent of the As in the milled coal was HCl soluble (Table 4). The HCl soluble elements in coal may be chelated or associated with HCl soluble minerals.²⁷ Very recently, it has been established⁴⁴ that arsenic species formed in coal by oxidation of other forms are also almost completely dissolved in HCl.

The presence of the arsenate form in US coals is attributed to the oxidation of the arsenical pyrite.³⁸ This point is confirmed by the recent study by Huggins et al.⁴³ for western Canadian sub-bituminous coals.

The bottom ash sample contains 1.6 ppm of As (Table 3), and contains the lowest amount of HCl leachable As. This is consistent with the classification of As as a class III element, which is vaporized during combustion and either concentrates on the surface of fly ash particles or is emitted from the stack.⁴³ In contrast to the bottom ash, ESP fly ash has the highest concentration of HCl soluble As (Table 4), and has the highest concentration of As (17.5 pm) for this suite of samples. Almost 62% of the As in fly ash is leached by HCl, indicating that arsenic deposited on the surface of the fly ash, is acid soluble and may be mobilised under acid conditions during the weathering process.

Most of the arsenic in bottom and fly ash samples is in the form of arsenate (As⁵⁺) (Fig. 1) in agreement with other studies,^45–47 indicating that arsenate (As⁵⁺) is the predominant species of arsenic in ash leachate. Oxidation of As³⁺ to As⁵⁺ may occur during leaching with water.¹¹ There may be as much as 10–15% As³⁺ in the bottom ash sample (Fig. 1). The smooth and relatively featureless profiles of the fly ash spectra (Fig. 1), points to the incorporation of the arsenate in the glass matrix of the ash samples, rather than the crystallization of specific arsenate phases. The XANES spectra (Fig. 1) for the fly ashes that have the most concentrated As are also consistent with >90% of the arsenic being in the As⁵⁺ form. The major peak positions in these spectra are closely similar for both ash samples, and consistent with an As–O bond distance of about 1.65–1.7 Å, as found in arsenate compounds. Such spectra are consistent with the previous work on As in ash and slag.^38,39

Chromium

The mode of occurrence of Cr in coal is less clear.⁴⁸ Chromium may be associated with organic matter as Cr³⁺.³⁸

Chromium in the milled coal is only leached by HCl (Table 4), indicating that it is either chelated or associated with HCl soluble minerals.²³ In agreement with the XANES spectral data of the milled coal (Fig. 4) this indicates that Cr is entirely Cr³⁺ and is mostly as Cr/illite.³⁸

Fig. 4 Comparison of simulated chromium XANES spectra for: a, milled-coal; b, bottom ash; c, ESP fly ash; d, the Cr in stainless-steel; and e, in a mixture of 60% Cr in stainless-steel and 40% Cr in milled coal (a).

Chromium in the bottom ash is leached by ammonium acetate and HCl (Table 4). Hydrochloric acid removes 96% of the chromium from the bottom ash (Table 4), indicating that chromium is highly mobile under acidic conditions. The chromium XANES spectrum of bottom ash is a more accentuated version of the coal spectrum; this is due to association of the chromium with an aluminosilicate (glass) matrix in the bottom ash (Fig. 4).

Fly ash contains less leachable chromium than the bottom ash (Table 4). Nineteen percent of the Cr in the fly ash sample is water soluble (Table 4), which may be as a result of the high pH of the leachate water (11.1, Table 5) and mean that it is Cr⁶⁺. These results indicate chromium may be mobilised from the fly ash under basic conditions: Cr₂O₃ becomes soluble at a pH slightly below 13 to form CrO₄.⁶ Therefore, it is not surprising that a portion of the chromium is dissolved, as indicated by the present results (Table 4).

Chromium contamination. The spectra for the fly ash from western Canada are significantly different from the coal-ash spectrum that was measured previously for USA bituminous coal-ash samples.³⁸ The spectra for the fly ash from western Canada indicate the presence of a significant pre-edge feature at about 4 eV (Fig. 4). However, the shape of the peak is not consistent with the enhancement of the pre-edge feature due to Cr⁶⁺ (Fig. 2). Furthermore, it should be noted that the overall shape of spectra for the fly ash sample is inconsistent with all the chromium being coordinated by oxygen. The relatively featureless and flat spectra obtained are more consistent with metallic chromium (Fig. 4c).

There is no metallic chromium in milled coal (Fig. 4a). Therefore, the metallic chromium in fly ash is from another source and possibly a contaminant. To prove that the chromium in fly ash is metallic, a mixture of 60% metallic chromium (Cr in stainless-steel, Fig. 4d) and 40% milled coal under study (Fig. 4a) and also a sample stainless-steel were examined using XAFS (Fig. 4d). The XAFS spectra of the admixture of stainless-steel and coal developed a significant pre-edge feature at about 4 eV similar to stainless-steel (Fig. 4e). A characteristic feature of XAFS spectra for metallic Cr is the presence of peaks well removed from the absorption edge (Fig. 4d). These relatively strong peaks are present in the mixture of Cr from milled coal and stainless-steel (Fig. 4e). From these peaks one can also discriminate between pure Cr metal and Cr in stainless-steel. These strong peaks are also present in the fly ash sample (Fig. 4c) but are not present in the milled coal nor in the bottom ash (Fig. 4a,b). This indicates that significant metallic Cr contamination occurs in the fly ash samples. This contaminant metallic Cr, exceeds the original Cr content of the coal. The source of this chromium contamination is possibly the power plant installation itself. The unit that was sampled in this study was six month old and it is probable that the new unit contributed metallic chromium to the flue gas and therefore to the fly ash.

Nickel

Nickel in milled coal is part of same mineralogical assemblage as chromium.⁴⁹ It is often associated with Cr/Fe oxides.

Nickel was not was extracted by water from any of the samples. Ammonium acetate extracted 25% of the Ni from the bottom ash, and none from the coal and fly ash (Table 4). Hydrochloric acid extracted 48% of the Ni from the coal, 74% from the bottom ash, and 14% form the fly ash. The nickel spectra obtained from milled coal, bottom ash and fly ash are shown in Fig. 5. All samples exhibit a fairly strong peak at about 20 eV, and no observable pre-edge peak. In comparison with the standard compounds (Fig. 3), the spectra profiles are consistent with Ni²⁺ in coordination predominantly with oxygen, such as NiO (Fig. 4). The concentration of nickel in fly ash is greater than in the milled coal (Table 3). However, the solubility of fly ash nickel in HCl is less than that of the milled coal (Table 4). This indicates that the nickel in fly ash is mostly associated with a glassy matrix.⁵⁰ In contrast to milled coal and fly ash, nickel in bottom ash is extracted by both ammonium acetate and HCl and it is totally removed after extraction by HCl (Table 4), indicating that bottom ash nickel is mobile under acidic conditions.

Fig. 5 Nickel XANES spectra of milled coal, bottom ash and fly ash from western Canada.

The nickel XANES data for bottom ash and fly ash from western Canada appear quite similar to the published data⁵⁰ for nickel in aluminosilicate glasses. This indicates that there are no carbonyl and sulfide compounds of nickel (carcinogenic compounds) present in these two ashes.

Conclusions

The present results indicate that:

(i) Arsenic in milled coal is mostly associated with the organic fraction, and 67% of this arsenic is removed by ammonium acetate. The remainder is totally removed from milled coal after extraction with HCl and, based on the XANES data, this fraction is likely present in the coal as arsenate.

(ii) Arsenic occurs in both As³⁺ and As⁵⁺ oxidation states in milled coal; virtually all (>90%) of the arsenic in bottom ash and fly ash appears to be in the form of arsenate (As⁵⁺).

(iii) Both Ni and Cr in the milled coal are extracted by HCl, indicating that these elements can be mobilised in an acidic environment.

(iv) Leaching of chromium from fly ash by water is due to the high pH of the water, induced during the leaching.

(v) Chromium in milled coal and bottom ash is present entirely as Cr³⁺, which is an essential trace nutrient. Chromium in fly ash is mostly Cr³⁺, with significant contamination by Cr possibly derived from stainless-steel supplied by the installation itself.

(vi) Ammonium acetate removes Ni in bottom ash through the ion exchange process.

(vii) The total leachability of the chromium and nickel (after leaching by HCl) is greater in bottom ash than in the fly ash, indicating greater mobility of these elements in the bottom ash.

(viii) Nickel appears to be present predominantly as Ni²⁺ in oxygen coordination in the milled coal, bottom ash and fly ash. The carcinogenic compounds of nickel, the carbonyl [Ni(CO)₄] [oxidation state (0)] and sub-sulfide (NiS) [oxidation state (II)] are not present in the coal ashes.

References

1. B. Mason and C. B. Moore, Principles of Geochemistry, Wiley, New York, 1982..
2. K. H. Wedepohl, Geochemistry, Holt Rinehart and Winston, New York, 1967..
3. G. D. Nicholls, in Coal and Coal-Bearing Strata, ed. D. G. Murchison and T. S. Westoll, Oliver and Boyd, Edinburgh, 1968, p. 269..
4. F. Goodarzi, Bull. Inst. Mineral. Met. Pet., 1994, 87, 47.
5. J. E. Fergusson, The Heavy Elements: Chemistry, Environmental Impact and Health Effects, Pergamon Press, Oxford, 1990..
6. D. G. Brookins, Eh-pH Diagrams for Geochemistry, Springer-Verlag, Berlin, 1987..
7. Electric Power Research Institute, Electric Utility Trace Substance Synthesis Report. 1994 1: Synthesis Report, EPRI TR-104614-VI, Project 3081, November, 1994..
8. J. R. Ashby and P. J. Craig, in Pollution, Causes, Effects and Control, ed. R. M. Harrison, The Royal Society of Chemistry, Cambridge, 1993, pp. 309–342..
9. R. Meij, KEMA, Sci. Tech. Rep., 1989, 7, 267.
10. E. E. van Der Hoek, P. A. Bonouvrie and R. N. J. Comans, Appl. Geochem., 1994, 9, 403.
11. D. R. Jones, in Environmental Aspects of Trace Elements in Coal, ed. D. J. Swaine and F. Goodarzi, Kluwer, Dordrecht, 1995, ch. 12..
12. M. E. Hirsch, R. O. Stirling, F. E. Huggins and J. J. Helble, Environ. Eng. Sci., 2000, in press..
13. E. Piperno, Adv. Chem. Ser., 1975, 141, 192.
14. L. Alessio and G. Bertelli, in Analytical Techniques for Heavy Metals in Biological Fluids, ed. S. Facchetti, Elsevier, Amsterdam, 1983, pp. 41–63..
15. N. P. Vela, L. K. Olson and J. A. Caruso, Anal. Chem., 1993, 65, 585.
16. B. H. Mahan, University Chemistry, Addison, Palo Alto, CA, 1965..
17. R. P. Beliles, in Toxicology—The Basic Science of Poisons, ed. L. J. Casarett and J. Doull, Macmillan, New York, 1975, pp. 585–597..
18. M. H. Martin and P. J. Coughtrey, Biological Monitoring of Heavy Metal Pollution, Applied Science, London, 1982..
19. M. E. Brownfield, R. H. Affolter, G. D. Stricker and R. T. Hildebrand, Int. J. Coal Geol., 1995, 27, 1553.
20. L. Lamarre, Electr. Power Res. Inst. J., 1995, 20, 7.
21. IARC, Working Group on the Evaluation of Carcinogenic Risks to Humans, IARC, Lyon, 1980..
22. W. L. Linton, in Pollution. Cause, Effects and Control, ed. R. M. Harrison, Royal Society of Chemistry, Cambridge, 1993, pp. 297–308..
23. R. B Finkelman, in Environmental Aspects of trace elements in Coal, ed. D. J. Swaine and F. Goodarzi, Kluwer, Dordrecht, 1995, ch. 2..
24. Annual Book of ASTM Standards, Forms of Sulfur in Coal, D2492-77, ASTM, Philadelphia, PA, 1978, part 26, p. 332..
25. R. R. Frost and R. A. Griffin, Soil Sci. Soc. Am. J., 1977, 41, 53.
26. G. J. de Groot, J. Wjikstra, D. Hoede and H. A. van der Sloot, in Environmental aspects of stabilization and solidification of hazardous and radioactive wastes, ed. P. Cote and M. Gilliam, Am. Soc. Mater., Philadelphia, PA, 1989, ASTM STP 1033, pp. 170–183..
27. R. B. Finkelman, C. A. Palmer, M. R. Krasnow, P. J. Aruscavage, G. A. Sellers and F. T. Dulong, Energy Fuels, 1990, 4, 755.
28. S. K. Falcone and H. H. Schobert, ACS Symp. Ser., 1984, 301, 114.
29. M. Stoeppler, Hazardous metals in the environment, Elsevier, Amsterdam, 1992, pp. 97–122 and 157–164..
30. L. L. Sloss and C. A. Gardner, Sampling and analysis of trace emissions from coal-fired power stations, IEA Coal Res. Publ., London, 1995..
31. R. Meij, J. Kooij, J. L. G. van der Sluys, F. G. C. van der Siepman and H. A. van der Sloot, KEMA, Sci. Tech. Rep., 1984, 1, 1.
32. R. Meij, J. Kooij, J. L. G. van der Sluys, F. G. C. van der Siepman and H. A. Sloot, Proc. VIth World Congress Air Qual., IUAPPA, Paris, May 1983, vol. IV, pp. 317–342..
33. R. Meij, KEMA Rep. 32561-MOC 92-3641, N. V. Kema, Arnhem, 1992..
34. F. Goodarzi, J. Brown, J. P. Charland, F. Huggins, D. Desphandi and S. M. Pollock, Geol. Surv. Can., Bull., in press..
35. P. A. Lee, P. H. Citrin, P. Eisenberger and B. M. Kincaid, Rev. Phys., 1981, 53, 769.
36. D. C. Koningsberger and P. Prins, X-ray Absorption Spectroscopy, I. Wiley, New York, 1988..
37. D. E. Sayers, F. E. Lytle and E. A. Stern, Phys. Rev. Lett., 1971, 27, 1204.
38. F. E. Huggins, N. Shah, J. Zhao, L. Fulong and G. P. Huffman, Energy Fuels, 1993, 7, 482.
39. G. P. Huffman, F. E. Huggins, N. Shah and J. Zhao, Fuel Process. Technol., 1994, 39, 47.
40. D. L. Swaine, Trace elements in coal, Butterworths, London, 1990..
41. F. Goodarzi and D. L. Swaine, Int. J. Coal Geol., 1993, 24, 281.
42. D. Birk, Can. J. Earth Sci., 1990, 27, 163.
43. F. E. Huggins, F. Goodarzi and C. J. Lafferty, Energy Fuels, 1996, 10, 1001.
44. F. E. Huggins, G. P. Huffman, A. Kolker, S. Mroczkowski, C. A. Palmer and R. B. Finkelman, Direct comparison of XRFS spectroscopy and sequential extraction for arsenic speciation in coal, Preprint, ACS Div. Fuel Chem., 2000, 45, in press..
45. K. Sato, Report, Et 86001, EPRI, Palo Alto, CA, 1986, vol.1..
46. A. Wadge and M. Hutton, Environ. Pollut., 1987, 48, 85.
47. C. J. Warren and M. J. Dudas, Report A-4747, EPRI, Palo Alto, CA, 1986..
48. R. B. Finkelman, US Geol. Survey Open File Rep. 81-99, US Geological Survey, Denver, CO, 1981..
49. S. M. Pollock, F. Goodarzi and C. Riediger, Int. J. Coal. Geol., 2000, 43, 259.
50. Y. Tabira, N. Ishizawa and F. Marumo, Mine J. (Jpn.), 1993, 16, 345.

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Footnotes

† Presented at the Whistler 2000 Speciation Symposium, Whistler Resort, BC, Canada, June 25–July 1, 2000.

‡ Geological Survey of Canada (GSC) Contribution No. 2000113.

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