Magnetic properties of road dust from Visakhapatnam (India) - relationship to industrial pollution and road traffic

Magnetic methods provide a fast tool for delineation of industrial pollution. Mineral magnetic studies of anthropogenic magnetic phases in road dust from the industrial zone of Visakhapatnam city (India) reveal the presence of large anthropogenic spherules with diameters up to similar to300 mum. Different internal structures of the spherules and a wide variation in size of the spherules, as well as the presence of melt-like particles and irregular shaped grains containing heavy metals, point to multiple sources of pollution, including different industries and heavy vehicle traffic. Magnetic mineralogy of the samples is dominated by a magnetite-like phase. Hysteresis parameters measured for magnetic extracts and single grains, are typical for pseudo-single domain magnetite. This is in disagreement with the large grain size of the single particles. Scanning electron microscopy images reveals a complex internal structure, showing an agglomeration of smaller grains and in-part, an extreme porosity of the spherules, probably related to fast cooling. The chemical composition determined by energy dispersive X-ray analysis is strongly variable and can also be very heterogeneous within a particle. First order reversal curve analysis indicates a spectrum of single domain, pseudo-single domain and multidomain properties. Values of saturation magnetisation suggest that the particles either consist of mainly similar ferrimagnetic sub-grains, whereas others are a mixture of ferrimagnetic and non-magnetic phases. Several irregular grains, showing a ferrimagnetic behaviour, contain a large amount of chromium ( > 50 wt%). The variations of the magnetic susceptibility along the three major roads in the industrial zone of Visakhapatnam are interpreted in terms of the relative degree of pollution. (C) 2004 Elsevier Ltd. All rights reserved.