Gerald P. Huffman

Ferrihydrite; surface structure and its effects on phase transformation

X-ray absorption fine structure (XAFS) spectra were collected on a series of ferrihydrite samples prepared over a range of precipitation and drying conditions. Analysis of the XAFS pre-edge structures shows clear evidence of the presence of lower coordination sites in the material. These sites, which are most likely tetrahedral, are believed to be at the surface and become coordination unsaturated (CUS) after dehydroxylation. With chemisorbed water molecules, the CUS sites become the crystal growth sites responsible for the phase transformation of ferrihydrite to hematite at low temperatures. On the other hand, when impurity anions such as SiO4−4 are present in the precipitation solution, the CUS sites may instead absorb the impurity anions, thereby blocking the crystal growth sites and inhibiting the formation of hematite.

Mineral behavior during coal combustion 1. Pyrite transformations

Abstract The physical and chemical transformation of excluded crystalline illite particles and of illite grains included within a carbon matrix were examined in a laboratory scale reactor. Scanning electron microscopy was used to determine the particle morphology, and energy dispersive X-ray analysis, Mossbauer spectroscopy, and XAFS expectroscopy were used to monitor the chemical changes. At temperatures above 1400 K, illite lost its crystalline structure and was transformed to a glass. Melting, pore generation, and cenosphere formation were observed. For both included and excluded illite particles, neither segregation of volatile components at the particle surface, nor vaporization of potassium species, was observed during combustion. Combustion of synthetic chars containing illite inclusions demonstrated coalescence of these inclusions to form larger ash agglomerates. Comparison of these results with ash particle compositional data obtained from the combustion of a bituminous coal containing illite showed intermediate compositions indicating interaction between the molten illite and quartz, kaolinite, and pyrite. Deposition experiments revealed a distinct temperature range above which the transformed illite particles had sufficiently low viscosity to deform and stick upon impaction.

Correlation between ash-fusion temperatures and ternary equilibrium phase diagrams

Experiments were conducted in which mixtures of selected coal ashes and Fe2O3, CaO, or K2CO3 were subjected to the ASTM standard test for ash fusibility (D1857). The measured ash-fusion temperatures all showed variations with mixture composition that correlated closely with liquidus temperatures for the appropriate Al2O3-SiO2-XO (X = Fe, Ca, or K2) phase diagram. The liquidus and ash-fusion temperatures generally showed parallel compositional trends, but were displaced from each other because of the influence of additional basic components in the coal ash. This displacement is of two forms: 1. (1) lowering of ash-fusion temperatures relative to the liquidus temperature; and 2. (2) shifting of pseudoeutectic compositions. Consequently, it was demonstrated that ash-fusion-temperature data for coal ashes define an approximate liquidus surface of the pseudoternary system, Al2O3-SiO2-base (base = FeO + CaO + MgO + Na20 + K20). These results support the view that the initial cone deformation in the ASTM test does not occur with first liquid formation, but requires extensive liquid formation.

Modes of occurrence of trace elements in coal from XAFS spectroscopy

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 elements 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.

Mode of occurrence of arsenic in four US coals

An integrated analytical approach has been used to determine the mode of occurrence of arsenic in samples of four widely used US coals: the Pittsburgh, Illinois #6, Elkhorn/Hazard, and Wyodak. Results from selective leaching, X-ray absorption fine structure (XAFS) spectroscopy, and electron microprobe analysis show that pyrite is the principal source of arsenic in the three bituminous coals, but the concentration of As in pyrite varies widely. The Wyodak sample contains very little pyrite; its arsenic appears to be primarily associated with organics, as As3+, or as arsenate. Significant (10–40%) fractions of arsenate, derived from pyrite oxidation, are also present in the three bituminous coal samples. This information is essential in developing predictive models for arsenic behavior during coal combustion and in other environmental settings.

XAFS characterization of mercury captured from combustion gases on sorbents at low temperatures

A review is presented of data obtained by X-ray absorption fine structure (XAFS) spectroscopy on the speciation of mercury captured on a variety of sorbent materials from simulated combustion flue gases at low temperatures (<200 °C). Data for other key elements (S, Cl) are also presented. Implications for bonding mechanisms for the capture of mercury are discussed, including the relative importance of chemisorption and physisorption processes. Systematics in the parameters derived from mercury X-ray absorption near-edge structure (XANES) spectra indicate that mercury can be captured by bonding to I, Cl, S or O anionic species on the surfaces of carbonaceous and other sorbents, but only as ionic Hg2+. None of the observations made by XAFS spectroscopy is consistent with the capture of mercury in the elemental state, i.e. physisorption.

Speciation of elements in NIST particulate matter SRMs 1648 and 1650

X-ray absorption fine structure (XAFS) spectra for S, Cl, V, Cr, Mn, Cu, Zn, As, Br, Cd and Pb and Mossbauer spectra for Fe have been obtained for two National Institute of Standards and Technology (NIST) particulate matter (PM) standard reference materials (SRMs): urban PM (SRM 1648) and diesel PM (SRM 1650). The spectral data, complemented by information on elemental concentrations from proton-induced X-ray-emission (PIXE) spectroscopy, were used to interpret the speciation of these elements in these complex materials. It appears that all the metallic elements investigated occur in oxidized forms, principally as sulfates in the diesel PM SRM and as sulfates, oxides, and possibly other forms (e.g. clays?) in the urban PM. A minor fraction of the sulfur and major fractions of the halogens, Cl and Br, occur as organosulfide (thiophene) and organohalide occurrences, respectively, that must be associated with the abundant carbonaceous matter that constitutes the major component of the two PM SRMs. Most of the sulfur, however, occurs as sulfate in the urban PM and as bisulfate in the diesel PM. In addition, elemental oxidation states have been determined directly by the spectroscopic techniques. Such information is often the key parameter in determining the toxicity and solubility of specific elements in PM, both of which are important in understanding the threat that such elements may pose to human health. For the two HAP elements, Cr and As, for which the toxicity depends greatly on oxidation state, the XAFS data showed that both elements are present in both SRMs predominantly in the less toxic oxidation states, Cr(III) and As(V). The potential of the XAFS spectra for use as source apportionment signatures is illustrated by reference to chromium, which exists in these two PM SRMs in very different forms.

Speciation of arsenic and chromium in coal and combustion ash by XAFS spectroscopy

Abstract While there are a variety of methods to determine the concentration of trace elements in coal and ash, there have been few attempts to determine the speciation of these elements. In this paper, it is demonstrated that X-ray absorption fine structure (XAFS) spectroscopy is capable of providing speciation information at realistic concentration levels of 10–100 ppm, provided a solid-state multielement germanium detector is used. The initial studies have concentrated on arsenic and chromium. For arsenic, two principal forms of occurrence are observed in coal: As contained in pyrite and As(V) in arsenate (AsO4−3). In one coal (Pittsburgh #8, DECS-12), the As is present as arsenopyrite. The As in pyrite is readily oxidized under ambient conditions to the arsenate form. In combustion ashes, all arsenic is in the form of arsenate, with at least two forms of arsenate present; As in aluminosilicate slag and calcium orthoarsenate are possibilities. Chromium in coal and in ash is observed to be present predominantly (> 95%) in the Cr+3 state. Chromium oxyhydroxide is the standard compound whose XAFS spectrum most closely resembles that of the chromium in coal, while the chromium in ash may be incorporated into the aluminosilicate slag phase.

Chlorine in coal: an XAFS spectroscopic investigation

K-edge XAFS spectroscopy has been used to examine directly the occurrence of chlorine in a variety of coals worldwide. Chlorine XAFS spectra were also obtained for Illinois No. 6 coal before and after aqueous leaching and conventional coal cleaning and during low-temperature (<350°C) slow pyrolysis under helium, and for coals treated with chlorinated reagents. The chlorine XANES spectra for coals of rank higher than subbituminous are closely similar, regardless of chlorine content, rank and geographic origin, and indicate that there is but one major, universal mode of occurrence of chlorine in coal; chloride anions in moisture anchored to the surface of micropores in coal macerals by organic ionic complexes, such as quaternary amine groups and alkali carboxyl complexes. However, except in an Australian brown coal with 60% moisture, the chloride anions are not in true aqueous solution, because the interaction between the maceral surface and the chloride anion is relatively strong. No evidence was gained for any organic chlorine in any coal. Crystalline sodium chloride was observed in minor amounts in only a few coals. Moreover, it appears that the observation of NaCl and other inorganic chlorides in coals is an artefact due to precipitation from the major chlorine form upon release of moisture from the coal during sample size reduction and subsequent storage. Preliminary bromine XAFS data for two US bituminous coals indicate that bromine is found in coal in a mode of occurrence closely similar to that of chlorine.

Distribution of trace elements in selected pulverized coals as a function of particle size and density

Abstract Trace elements in coal have diverse modes of occurrence that will greatly influence their behavior in many coal utilization processes. Mode of occurrence is important in determining the partitioning during coal cleaning by conventional processes, the susceptibility to oxidation upon exposure to air, as well as the changes in physical properties upon heating. In this study, three complementary methods were used to determine the concentrations and chemical states of trace elements in pulverized samples of four US coals: Pittsburgh, Illinois No. 6, Elkhorn and Hazard, and Wyodak coals. Neutron Activation Analysis (NAA) was used to measure the absolute concentration of elements in the parent coals and in the size- and density-fractionated samples. Chemical leaching and X-ray absorption fine structure (XAFS) spectroscopy were used to provide information on the form of occurrence of an element in the parent coals. The composition differences between size-segregated coal samples of different density mainly reflect the large density difference between minerals, especially pyrite, and the organic portion of the coal. The heavy density fractions are therefore enriched in pyrite and the elements associated with pyrite, as also shown by the leaching and XAFS methods. Nearly all the As is associated with pyrite in the three bituminous coals studied. The sub-bituminous coal has a very low content of pyrite and arsenic; in this coal arsenic appears to be primarily organically associated. Selenium is mainly associated with pyrite in the bituminous coal samples. In two bituminous coal samples, zinc is mostly in the form of ZnS or associated with pyrite, whereas it appears to be associated with other minerals in the other two coals. Zinc is also the only trace element studied that is significantly more concentrated in the smaller (45 to 63 μm) coal particles.