MINERALOGICAL AND CHEMICAL CHARACTERIZATION OF ROAD DUST IN PHILADELPHIA, PA

Urban road dust is composed of particles accumulated on metropolitan roadways derived from natural (e.g., surrounding geologic materials, soils) and anthropogenic sources (e.g., road traffic, construction materials). Frequent particle re-suspension after settling leads to human interaction by a combination of inhalation, epidermal contact, and ingestion. Road dust can contain elevated amounts of heavy metals and other substances, making it a potential urban environmental health hazard for those facing regular exposure. We sampled road dust at 30 sites within the city of Philadelphia, PA to assess the effects of different average daily traffic counts, truck traffic percentages, proximity to industry, and different urban settings on its mineralogical and chemical composition. We applied the analytical techniques of X-ray diffraction (XRD), inductively coupled plasma optical emission spectrometry (ICP-OES), and ashing to determine mineral phases present, elemental composition, and organic content, respectively. Additionally, scanning electron microscope (SEM) images were used to determine particle shapes and sizes and to infer potential road-dust sources. XRD results demonstrate that the following five mineral phases are nearly ubiquitous: quartz (SiO₂), dolomite (CaMg(CO₃)₂), hematite (Fe₂O₃), anorthite (CaAl₂Si₂O₈), and magnetite (Fe₃O₄). In addition, more than 40 different minor phases were discovered only once at various sample sites, documenting that there is mineralogical variability in the studied road dust. ICP-OES results indicate that sites near industrial areas, as well as those with high average daily traffic, have higher concentrations of some heavy metals (e.g., Pb, Zn, V). In particular, the concentration of Pb at some sites is above the EPA safety threshold for soils children interact with (400 ppm). Potential Pb sources include Pb-based paint from buildings built before 1970, industrial activity, and smelting. The observed mineralogical and chemical variations in the studied samples can be attributed to natural and anthropogenic sources, both of which contribute to road dust accumulation in the Philadelphia metropolitan area.