Anthropogenic magnetic particles and heavy metals in the road dust: Magnetic identification and its implications

Magnetic properties of road dusts in the East Lake area in Wuhan, China, were measured and compared with the results of heavy metal analyses in order to delineate the sources of pollutants. A total of ninety-seven dust samples were collected spatially from four segments with different traffic density and field settings from the roads encircling the lake. Thermomagnetic and hysteresis measurements revealed that the dominant magnetic carrier is coarse-grained magnetite. Correlations between magnetic parameters and element concentrations with traffic density and distances to the industrial region revealed that elements Cu, Ni and Fe mainly originate from vehicle traffic, which is also the major source of coarser magnetic particles (e.g., pseudo-single-domain/multi-domain (PSD/MD) grains), while element Pb and the smaller grains such as single-domain (SD) magnetic particles mainly originate from industrial emissions. The ratio between anhysteretic remanent magnetization and low-field magnetic susceptibility (ARM/χ_lf) can be employed as an indirect indicator for Cu, Fe and Ni emissions resulting from vehicle traffic. Due to the intermixture of elements from different sources, the element concentrations are not conclusive about the pollution source. A linear correlation between magnetic concentration-related parameters (e.g., ARM and saturation isothermal remanent magnetization, SIRM) and the concentrations of major elements (e.g., Cu, Co, Fe, Mn, Ni and V) suggests that they can be used as a proxy for heavy metal pollution. Road dusts in four segments show different magnetic characteristics, indicating various influxes of anthropogenic magnetic materials from vehicle traffic and industrial plants due to the different traffic loads and field settings. These results suggest that magnetic measurements can serve as an efficient complementary tool for the routinely employed geochemical methods to map the heavy metal pollution and trace the sources of pollutants in the road dust.