Mineral magnetic characterization of anthropogenic magnetic phases in the Danube river sediments (Bulgarian part)

Magnetic methods have been widely used for delineation of environmental pollution of soils and sediments under certain conditions during the last decade. A number of structure-sensitive magnetic characteristics (coercive force, coercivity of remanence, remanent magnetization) were utilized for deduction of the grain size of the anthropogenic particulate matter which serves as a proxy for the evaluation of the distance from the pollution source, possible health effects, etc. However, up to now there is no detailed study on the influence of the morphology, internal structure and phase composition of anthropogenic particles on the corresponding magnetic characteristics. The aim of the present investigation is to obtain detailed microscopic, magnetic and microchemical data on single grains from polluted river sediments and thus to check the validity of the classical grain size-dependent trends of coercivity and remanence parameters obtained for natural and synthetic magnetites for the anthropogenic magnetic phases. Single anthropogenic particles from magnetic extracts have been separated from several sites along the Bulgarian part of the Danube river, which show the strongest magnetic enhancement. Results from the combined magnetic (thermomagnetic analysis and magnetic hysteresis), microscopic (optical and scanning electron microscopy (SEM)) and microchemical (electron microprobe analysis (EMPA)) study on single anthropogenic particles show that there is a general trend for significant underestimation of the physical size of the anthropogenic particles due to the following reasons: (1) in most cases the spherules possess a complex internal structure, in which the magnetic phase is presented as dendrites exsolved within a glassy non-magnetic aluminosilicate matrix; (2) the surface morphology is characterized by orange-peel or dendritic structures, or smaller strongly magnetic spherules are adhered to the surface. Magnetic mineralogy as deduced from single grain thermomagnetic K(T) analysis and further elucidated by microprobe data on the same particles shows that magnetite is the main ferromagnetic phase. In isolated cases significant A1 substitution leads to a decrease of T-c down to 500degreesC. Hysteresis parameters for all studied single grains of physical sizes varying from 39 mum up to 1.5 mm fall mainly in the pseudo single domain (PSD) range. On the basis of the results from single grain magnetic measurements, a new approach for semiquantitative estimation of the degree of anthropogenic pollution is proposed. (C) 2004 Elsevier B.V. All rights reserved.