Characterization of road dust collected in Traforo del San Bernardo highway tunnel: Fe and Mn speciation

X-ray Absorption Spectroscopy (XAS), X-Ray Diffraction (XRD), Thermo-Gravimetric Analysis (TGA) and Scanning Electron Microscopy–Energy Dispersive Spectroscopy (SEM–EDS) techniques were used to determine the composition and the speciation of Fe and Mn in road dust samples collected in the Traforo del San Bernardo highway tunnel. Principal Component Analysis and Least Square Fitting on the XANES region of the absorption spectra and structural refinements on the EXAFS part of the spectra were applied to obtain complementary information on the speciation, average oxidation state, and local structure of Fe and Mn. XRD indicated the presence of silica, calcite, gypsum, and of various phyllosilicates. TGA analysis confirmed the presence of phyllosilicates and also detected a significant amount of organic phases. These findings indicate the co-presence of particles of natural origin along with the organic phases related to vehicular gas exhausts emission. On the other hand, XAS analysis showed that iron is mainly present in the Fe3O4 and FeCl3 forms, which can be considered to have both anthropic origin, i.e., exhaust emission and salt used to prevent ice formation, respectively.