Electron microbeam analysis of urban road-deposited sediment, Manchester, UK: Improved source discrimination and metal speciation assessment

Road-deposited sediment (RDS) is an accumulation of particulates upon street surfaces in urban centres. It is commonly highly contaminated and has major potential impacts upon surface water quality and human health, as well as becoming a waste material upon street sweeping. Although significant research has been undertaken upon the fine fraction within these systems, there is a lack of detailed, high-resolution grain-specific mineralogical and chemical data for the coarser fractions, which contain the bulk of the contamination by mass. The study reported here utilizes backscatter electron microscopic, electron microprobe and Raman spectroscopic analysis to provide novel data on RDS material from Manchester, UK, with significant implications for the improved understanding of source discrimination and contaminant metal speciation. The RDS samples studied are highly heterogeneous and the abundant nature of anthropogenic grains is particularly apparent. The RDS material has been found to be composed of a number of grain types: (i) silicate and alumino-silicate grains derived from a wide range of sources; (ii) iron oxide grains derived from the corrosion of galvanized steel; (iii) iron-rich glass grains derived as slag material from metal and waste processing activities and (iv) spherical Fe oxide and Fe-rich glass grains derived from high temperature combustion processes. Elemental metallic grains (Fe, Cu, Pb) are also present in minor amounts.