Heavy metal contents and magnetic properties of street dust in two districts of different traffic density in Hong Kong

摘要

Vehicle emission and metal wear are considered an important source of heavy metal contamination in road dusts. In this study, we have examined the heavy metal contents and magnetic properties of road dust samples collected from a high-traffic and low-traffic district in Hong Kong. The objectives of the study are to determine the nature and differences of the magnetic carriers in the dust samples from the two districts and to evaluate the ability of using magnetic parameters to predict the presence of certain heavy metals. A total of 37 dust samples were collected from streets of Causeway Bay (17) and Sandy Bay (20). The former district is primarily a commercial-retail area, with 20000-30000 daily vehicle traffic, and the latter is a low-density residential and hospital zone, with daily traffic of < 5000 vehicle per day. The samples were subjected to magnetic measurements of NRM, ARM, low-frequency and high-frequency magnetic susceptibility, SIRM and IRM-300. Geochemical determinations for Zn, Cu, Cd, Cr, Pb, Mn and Fe abundances were also carried out. The magnetic results indicate the general presence of coarse-grained multidomain ferrimagnetic minerals in the street dust of Hong Kong. While both sets of samples have relatively high values of magnetic susceptibility, samples from Causeway Bay demonstrated markedly greater values of all of the magnetic parameters measured. A statistical analysis of the heavy metal concentration and various magnetic parameters reveals only weakly significant correlations between the two sets of parameters for only Zn, Cu and Fe. In fact, despite the generally greater magnetic remanent intensity measured for the Causeway Bay samples, the Sandy Bay samples actually yielded greater Fe contents. This finding has pointed to the insufficiency in the present understanding of the magnetic properties of street dust and cast some doubts on the viability of using magnetic methods as a proxy indicator of heavy metal contamination in urban sediment.