Solid-phase speciation of Zn in road dust sediment

X-ray absorption spectroscopy, scanning electron microscopy (SEM) and X-ray diffractometry (XRD) have been used to study the solid-phase speciation of Zn in urban road dust sediments (RDS) in Manchester, UK. X-ray absorption near-edge structure (XANES) analysis using linear combination modelling suggest that the soluble species Zn(NO3)2·6H2O and ZnCl2 represent 70—83%, and Zn-sorbed goethite 17—30%, of the Zn species present. The presence of goethite is not corroborated by extended X-ray absorption fine structure (EXAFS) modelled first shell scattering Zn—O distances of 2.01—2.03 Å, but this may be due to distortion of the Zn octahedra on the goethite surface, or the existence of Zn-sorbed species with other metal hydrous oxides, as inferred by the EXAFS-modelled second shell Fe and Al scatterers. Analysis by EXAFS also suggests that metallic Zn-Cu-Sn-Pb and Zn-silicate phases are present in the RDS, and this is corroborated by SEM and XRD. Other phases suggested by EXAFS include ZnO, franklinite, Zn-sorbed birnessite and zinc formate. Differences between the XANES and other results suggest that model compounds such as Zn-bearing phyllosilicates and metallic Zn phases may have been missing from the XANES fitting. Long-term low-level exposure to the RDS Zn phases identified may lead to an increased risk of cardiovascular or pulmonary diseases.