Morphological and mineralogical forms of technogenic magnetic particles in industrial dusts
Highlights
► TMPs could be distinctive for pollution source identification. ► Magnetite or maghemite spherules are typical for hard coal ash. ► Hematite is more common in lignite ash decreasing their magnetic susceptibility. ► Metallic Fe and ferrimagnetic pyrrhotite are characteristic components of coke dust. ► Ca-ferrite is characteristic ferrimagnetic mineral for cement dust.
Introduction
Most of the magnetic particles occurring in urban and industrial dusts have a technogenic origin. With this term we define iron minerals that were produced in a wide variety of technological processes (metallurgy, fuel combustion, ceramics, cement production, coke production, etc.) in high temperatures and emitted into the atmosphere. They are mostly iron oxides with ferrimagnetic or antiferromagnetic properties (Thompson and Oldfield, 1986). This feature makes it possible to use them as tracers of industrial pollution because their presence, even in trace amounts, in dusts, soils, or sediments can be easily detected by magnetic measurements. Such physical properties also enable technogenic magnetic particles (TMPs) to be separated from urban and industrial dusts and fly ashes of different origin as well as from forest topsoil where the particles have accumulated as a result of atmospheric dust deposition. Such physical separation of TMPs facilitates their morphological, mineralogical, and grain size characterization and investigations of the opportunity for binding and transporting some trace elements by magnetic particles. The correlation between the magnetic phase of fly ashes and some elements was found by many authors (Hullet et al., 1980, Hansen et al., 1981, Vassilev et al., 2001, Vassilev et al., 2004). Also the existence of positive correlations between magnetic susceptibility measured in topsoil in urban and industrial regions and heavy metal concentration was observed in many cases (Strzyszcz, 1989, Strzyszcz, 1993, Strzyszcz et al., 1996, Georgeaud et al., 1997, Strzyszcz and Magiera, 1998, Petrovský et al., 2000, Hanesch and Scholger, 2002, Wang et al., 2005, Matýsek et al., 2008).
The sources of TMPs occurring in dusts are iron minerals present in raw materials, fuels, or additives which were transformed to magnetic particles during high temperature technological processes (Strzyszcz, 1991, Magiera and Strzyszcz, 2000, Shu et al., 2000). Such particles are usually observed in the form of highly magnetic iron oxides that have the tendency to bind some elements, and therefore more heavy metals present in fly ashes are connected with the magnetic than with the silicate phase (Hullet et al., 1980, Kukier et al., 2003). The crystal lattice of magnetite and different kinds of technogenic ferrites formed in high temperatures enables many elements to be incorporated within the structure that in the case of their high accumulation in a soil environment create a real threat for plants, animals, and people (Jabłońska and Smołka-Danielowska, 2008, Wójcik and Smołka-Danielowska, 2008, Giere and Querol, 2010, Grobety et al., 2010).
Geochemical study of fly ashes has proved that heavy metals are adsorbed on particles’ surfaces. Especially, fly ashes with the finest grain size, having a large active surface, were found to be enriched in such heavy metals as Cr, Mn, Pb, V, and Zn. If the elements are bound in a crystal lattice their potential mobility is low (Sørensen et al., 2000). The other situation is the case of elements adsorbed on the surface of iron oxides or connected with the amorphous phase of iron oxides or hydroxides. From such forms of binding the elements could be easily released, especially in the acid environment of forest topsoil where alkaline dusts are usually deposited. Progressive soil acidification as a result of SO2 emission and associated acid rains could be an accelerative agent in the process of metal release.
Due to the strongly magnetic properties, the presence of TMP could be easily detected by applying geophysical techniques. The parameter most quickly and easily measured is magnetic susceptibility. Its value is directly proportional to quantity and a mode of magnetic minerals present in the measured sample (in the case of laboratory measurement, described by mass specific susceptibility, χ) or in a volume of soil/sediment (in the case of field measurement, described by volume specific susceptibility, κ). The high magnetic susceptibility value explicitly indicates the high concentration of magnetic minerals (Maher, 1986, Thompson and Oldfield, 1986, Versoub and Roberts, 1995). This fact enables soil magnetometry (a method based on topsoil measurement of κ or χ values) to be applied for quantitative estimation of industrial and urban dust deposition into soil. In many cases such enhanced κ or χ values also indicate an increased content of heavy metals connected with TMPs (Strzyszcz and Magiera, 2000, Strzyszcz et al., 2006, Magiera et al., 2007, Jordanova et al., 2008, Fürst et al., 2009).
Diverse physico-chemical conditions associated with different technological processes in which TMPs arise allows their morphology and mineralogical composition to be discriminated. It also results in differentiation of their magnetic parameters (Lecoanet et al., 2003, Wang et al., 2009). These results suggest that some morphological and mineralogical forms may be typical and representative for particular emission sources produced by individual branches of industry as well as by traffic. The mineralogy, grain size, morphology and composition of vehicle-derived particles was analyzed during the earlier studies (Bućko et al., 2010, Bućko et al., 2011). The aim of the study was detailed morphological and mineralogical characterization of magnetic particles occurring in different kinds of industrial dusts and determination of their individual characters, which could be used as tracers of their sources of origin, which would be useful for assessment of pollution sources in urban and industrial areas.
Section snippets
Methods
The mass specific magnetic susceptibility (χ) was determined with the use of an MS2 "Bartington" laboratory magnetic susceptibility meter, equipped with a dual frequency MS2B sensor. Other magnetic parameters of fly ashes, presented in Table 1 such as: frequency dependence of magnetic susceptibility (χfd) isothermal magnetic magnetization (IRM), saturation magnetization (SIRM), coercivity of remanence - HCR, and S parameter (calculated as a ratio of IRM obtained in 100 mT to SIRM) were measured
Basic magnetic properties
The results of studies conducted in earlier years based on the magnetic parameters measurements showed that there were some differences in magnetic properties between fly ashes after hard coal and lignite combustion (Table 1). The average values of mass magnetic susceptibility (χ) of fly ashes from nine Polish hard coal burning heat- and power-generating plants exceeded 1000 × 10−8 m3 kg−1 in all cases, whereas fly ashes from lignite burning power plants had χ values between 400 and 800 × 10−8 m
Discussion
Morphological and mineralogical forms of TMPs as well as their magnetic properties are the result of the different technological processes from which they arise. Among the studied dusts, fly ashes after hard coal burning were relatively less mineralogically diverse. This was confirmed by the almost totally reversible thermomagnetic curve (Fig. 1a) with a clearly marked Curie temperature at 580 °C. However, as seen in detailed analysis of X-ray patterns, it is not pure stoichiometric magnetite.
Conclusions
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Particular technological processes from many branches of industry are sources of very characteristic morphological and mineralogical forms of technogenic magnetic particles. Their crystalline structure morphology and mineralogical composition determine different magnetic behaviours. These TMPs could be distinctive for pollution source identification and could serve as a tracer of dust origin and (if found in topsoil) in identification of soil pollution sources.
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Magnetic particles from coal
Acknowledgements
The study was granted by the Ministry of Sciences and Higher Education of Poland in the frame of Projects No. N523 074 32/2889 and N523 413035. We thank Ales Kapička from Institute of Geophysics, Academy of Sciences of the Czech Republic for help with thermomagnetic data interpretation and also Grażyna Bzowska for X-ray analysis and Józef Komraus for help with the interpretation of Mössbauer spectrum.
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