Short communicationAn introductory TEM study of Fe-nanominerals within coal fly ash
Abstract
The investigation presented here was conducted during a wider experiment on the technical feasibility and environmental impacts of tire combustion in a Brazilian coal-fired power station. Nanometric-sized crystalline phases in fly ash were characterised using energy-dispersive X-ray spectrometer (EDS) and high-resolution transmission electron microscopy (HR-TEM) images. The nanoparticles, which register abundance peaks at 10 nm and 100 nm, include iron-rich oxide (e.g. hematite), Fe-sulphate (e.g., yavapaiite: KFe(SO4)2), and Fe-aluminumsilicate glass. Individual metalliferous nanoparticles have a heterogeneous microstructure in which elements such as iron, aluminum and silicon are not uniformly distributed. HR-TEM offers a powerful analytical technique in the study of fly ash nanoparticles, providing a better understanding of the detailed chemistry of this potentially strongly bioreactive component of atmospheric particulate matter.
Introduction
Coal combustion by electric utilities and power plants generates large quantities of fly ash that can have a significant impact on the environment, not least as a major source of inhalable atmospheric particulate matter (PM). Although PM1–10 (1–10 µm diameter) fly ash particles are readily identified using analytical electron microscopy, and are consequently well documented in the literature, there is relatively little information available on ultrafine (< 100 nm) particles, even though these are abundantly present in coal fly ash. In this short paper we build on recent work on fly ash micromineralogy (Chen et al., 2004, Giere et al., 2006, Hower et al., 2008) and demonstrate how high-resolution TEM (HR-TEM), in combination with energy-dispersive X-ray spectroscopy (EDS), can be used to investigate the elemental distribution, morphology, crystalline phases and electronic structure of individual coal fly ash particles, with emphasis on the ultrafine particles that may have the greatest impact on human health (Smith et al., 2005). We focus in particular on the detection and characterisation of Fe-nanominerals present in fly ash. Careful identification of these Fe-minerals is a necessary precursor to solving several technical and environmental problems associated with coal mining and coal fly ash (Bunt and Waanders, 2008, Bunt et al., 2008, Bunt and Waanders, 2009, Silva et al., In Press).
Section snippets
Analytical procedures
The samples for this study were collected over a five-day period in the principal Brazilian power plant (Santa Catarina, State), which uses coal and generates approximately 850 MW/h of electricity. The incineration temperature in the combustion chamber is ca. 1000–1500 °C, and about 97.5% of the fly ash is captured in the electrostatic precipitators. In 2008 around 90% of the fly ash produced by this power plant was utilized in the cement industry.
Samples for HR-TEM observation were prepared by
Results and discussion
Our energy-dispersive X-ray spectrometer data and high-resolution transmission electron microscopy (HR-TEM) images reveal the presence of fine crystalline phases, such as iron-rich oxide spinel and Fe-aluminumsilicate glass. The HR-TEM measurements demonstrate that Fe-nanoparticles show peaks at 10 nm and 100 nm. The fate of such nanominerals during coal combustion is determined by competitive processes by which different iron compounds are produced, and they typically exist either in iron
Overview and conclusion
The environmental fate and ecotoxicity of Fe-nanominerals from coal-fired power plant may be influenced by a number of properties, including nanoparticulate size and size distribution, solubility and state of aggregation, elemental composition, mass and number concentrations, shape and crystal structure, surface area, charge and chemistry and the presence of impurities. Heath effect studies, such as Gurgueira et al. (2002), have shown that harmful developments in the lung and heart that follow
Acknowledgements
The authors express their gratitude to the heartfelt help of FEHIDRO, Environmental Foundation of Santa Catarina State — FATMA and Ferrovia Teresa Cristina. The manuscript was much improved by the comments of Drs. Frans Waanders, Jim Hower and Wes Gibbons.
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