Rock magnetic, mineralogical and microstructural characterization of fly ashes from Bulgarian power plants and the nearby anthropogenic soils

The aim of the present study is to obtain a detailed characterization of fly ashes and nearby anthropogenic soils from different Bulgarian power plants, by using integrated rock magnetic and classical microscopic methods. These combined techniques, which enable a better understanding of the complexity of anthropogenic magnetic phases, impose the need for application of an interdisciplinary approach in environmental studies. The combined use of both single grain and bulk hysteresis measurements, combined with thermomagnetic analysis and microscopic (optical, scanning electron microscopy observations and energy dispersive X-ray analysis) reveal the major factors which control the magnetic properties of the fly ashes. Optical microscope observations on polished sections show that the complex internal structure (dendritic exsolutions) and in particular surface morphology of the fly ash particles, are the reasons for the observed significant discrepancy between the estimated magnetic grain size and the physical grain size. Bulk magnetic extracts show hysteresis parameters typical for pseudosingle domain and single domain magnetic states that are a result of superposition of the magnetic response from particles with widely varying grain size distributions. Magnetic mineralogy deduced from the thermomagnetic analysis is dominated by a magnetite-like phase, but in some samples there is a significant contribution of a phase with a lower blocking temperature of 500degreesC, probably indicating the presence of substituted ferrimagnetics. The magnetic mineralogy of the fly ashes is closely related to the type of coals used by different power plants. The results show a good correspondence between fly ash magnetic susceptibility and total Fe2O3 content which suggests the dominant role of strongly magnetic iron oxides. The magnetic behaviour of the anthropogenic soils is dominated by the magnetic signal from fly ash pollution, although contributions from pedogenic and lithogenic magnetite are also observed. (C) 2004 Elsevier Ltd. All rights reserved.