Abstract
During this study, we investigated the mineralogical characterization of technogenic magnetic particles (TMPs) contained in alkaline industrial dust and fly ash emitted by coal burning power plants and cement plants. The reaction of tested dust samples varied between values of pH 8 and pH 12. Their magnetic properties were characterized by measurement of magnetic susceptibility (χ), frequency dependence of magnetic susceptibility (χfd), and temperature dependence of magnetic susceptibility. Mineralogical and geochemical analyses included scanning electron microscopy with energy dispersive spectroscopy, microprobe analysis and X-ray diffraction. The TMPs in fly ash from hard coal combustion have the form of typical magnetic spherules with a smooth or corrugated surface as well as a skeletal morphology, composed of iron oxides (magnetite, maghemite, and magnesioferrite) that occurred in the form of incrustation on the surface of mullite, amorphous silica, or aluminosilicate particles. The TMPs observed in fly ash from lignite combustion have a similar morphological form but a different mineralogical composition. Instead of magnetite and magnesioferrite, maghemite and hematite with lower χ values were the prevailing magnetic minerals, which explains the much lower magnetic susceptibility of this kind of ash in comparison with the ash from hard coal combustion, and probably results from the lower temperature of lignite combustion. Morphology and mineralogical composition of TMPs in cement dust is more diverse. The magnetic fraction of cement dust occurs mostly in the form of angular and octahedral grains of a significantly finer granulation (<20 μm); however, spherules are also present. A very characteristic magnetic form for cement dust is calcium ferrite (CaFe3O5). The greatest impact on the magnetic susceptibility of cement dust results from iron-bearing additives (often waste materials from other branches of industry), which should be considered the most dangerous to the environment. Stoichiometric analysis of micro-particles confirmed the presence of heavy metals such as Pb, Mn, Cd, and Zn connected with TMPs, which are carriers of magnetic signals in atmospheric dust. Therefore, in some cases, their presence in topsoil when detected by magnetic measurement can be treated as an indicator of inorganic soil contamination.
Highlights
The wide use of coal as an energy source, combined with the intensive development of many branches of industry, including chemical, ceramic, and cement production at the beginning of the twentieth century, has resulted in the emission into the atmosphere of extremely large amounts of industrial dust and fly ash
Compared with other types of analyzed industrial dust, fly ash originating from hard coal burning is characterized by the highest value of magnetic susceptibility
Such low values of cfd means an almost total absence of superparamagnetic grains within the magnetic fraction. This is a factor which allows ferrimagnetic grains of anthropogenic origin, originating from fly ash fallout that usually occurs as a multidomain size, to be distinguished from natural ferrimagnetic grains of biogenic origin, or from pedogenic grains formed as a result of precipitation of iron minerals from soil solutions
Summary
The wide use of coal as an energy source, combined with the intensive development of many branches of industry, including chemical, ceramic, and cement production at the beginning of the twentieth century, has resulted in the emission into the atmosphere of extremely large amounts of industrial dust and fly ash. This dust usually has an alkaline reaction (pH>7.2) and contains a significant amount of trace elements, constituting a potential threat to the natural environment and living organisms. It has been proven that pine bark, with a pH of between 3 and 4, can be substantially damaged during contact with aggressive alkaline dust of a pH higher than 10 (Farmer 1993; Migaszewski et al 2001)
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