Characterization of road dust collected in Traforo del San Bernardo highway tunnel: Fe and Mn speciation
Highlights
► We use XAS, XRD, TGA, SEM–EDS techniques to probe road dust of S. Bernardo tunnel. ► Fe and Mn is found in oxide forms and/or bound to chlorine or sulfur by XAS. ► XRD reveal silica, gypsum, calcite, phyllosilicates containing Si, Mg, Na, K, Al. ► Fe and Mn oxides, sulfides, and chlorides phases are below the XRD detection limit. ► TGA reveal a significant organic component and confirm the phyllosilicate presence.
Introduction
The study of elemental speciation in atmospheric Particulate Matter (PMx, where x indicates aerodynamic diameter in μm) and in road dust (RD), particulate on street surface is important under several aspects. For example it can be effective in identifying the origin of air pollution and in validating dynamic atmospheric simulations, using the various species as tracers. A detailed knowledge of the composition and speciation of RD can help evaluating the toxicity (Wang et al., 2006) of fine particles in road dust, which, in urban, suburban, and industrial areas contribute significantly to airborne particulate via resuspension (Taylor and Robertson, 2009), and is therefore crucial for the choice of pollution reduction policies. The present work focuses on the characterization of RD samples collected in the ventilation air shaft of Traforo del San Bernardo highway tunnel, and, in particular, on the speciation of iron and manganese by means of X-ray Absorption Spectroscopy (XAS), and aims at the identification of the processes that originate these particles. Preliminary XAS results were presented in a conference proceeding (Bardelli et al., 2009). XAS was demonstrated to be a powerful tool for the study of a wide range of environmental matrices (Qi et al., 2003, Ressler et al., 2000, Wang et al., 2006, Fittschen et al., 2008). X-Ray Diffraction (XRD), Scanning Electron Microscopy–Energy Dispersive Spectroscopy (SEM−EDS) and Thermo-Gravimetric Analysis (TGA) were used to determine the overall composition and the morphology of the samples.
The San Bernardo tunnel is an ideal sampling area for particulate emitted from the vehicular traffic, because the road dust collected inside it: i) represents the average emission from different vehicles burning different fuels; ii) comes almost solely from vehicular emission; iii) is not affected by important temperature fluctuations throughout the year; iv) is not subjected to photo-induced effects such as photo-chemical reactions. Heavy metal particles are emitted on the road surface from several sources, as part of brake abrasion, road paint, fuel exhaust particles, road construction materials, tire tread debris, and car catalyst materials. Nevertheless, the main sources of Fe particles are reported to be brake abrasion, automobile rust, and motorcar exhaust (Adachi and Tainosho, 2004). A few studies focused to the speciation of iron in RD are reported in literature, of which a few exploit advanced analytical tools such as Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Moreover, several papers report conflicting results about iron speciation in particulate matter collected at different sampling sites, emphasizing discrepancies due to the different methods used (Majestic et al., 2007). Iron in the reference material SRM-1648 (urban PM) is reported to consist mainly of magnetic oxides and hematite, with Fe(II) concentration less than 20% (Huggins et al., 2000). Measurements on particulate collected in industrialized cities of China, performed by EXAFS and Mössbauer spectroscopy, revealed Fe oxides and sulphate (Fe2O3, Fe3O4, Fe2(SO4)3), in proportions varying with both the sampling site and the particle sizes (Qi et al., 2003, Tong et al., 2001, Wang et al., 2006). Hoffman et al. (1996) showed that iron oxides are the main phases in the particulate collected in the air-conditioning system of buildings, and that the most part of the Fe(II)-compounds are present in grains with size larger than that of Fe(III) bearing compounds, which are less than 1 μm in size. In RD with particle size <2 μm collected in Manchester (UK) (Taylor and Robertson, 2009), different iron-containing particles (Fe oxide grains composed of both hematite (α-Fe2O3) and lepidocrocite (γ-FeOOH)) were found, arising from different sources. On the other hand, Fittschen et al. (2008) analyzed particulate matter from Hamburg using synchrotron radiation techniques and reported that, independently of the particle size, Fe(III) is the main oxidation state present.
Manganese, like iron, is an essential nutrient for the development of terrestrial and marine ecosystems. In urbanized and industrialized areas the main sources of Mn bearing particles are: i) Earth’s crust (Han et al., 2007, De Miguel et al., 1997, Yeung et al., 2003, Oliva and Espinosa, 2007, Manoli et al., 2002); ii) materials used for buildings construction (Chow et al., 2003); iii) industrial emissions (Funasaka et al., 2003, Wei et al., 2009, Sezgin et al., 2003); iv) burning of coal and oil for power generation (Han et al., 2007); and v) vehicular traffic (Karar and Gupta, 2006, Ressler et al., 2000). Concerning the latter source, Mn is emitted primarily by motor vehicles powered by unleaded gasoline. In this fuel, the additive tetraethyl lead (TEL), once used to increase the octanes number, is replaced by manganese Methyl-cyclopentadienyl tricarbonyl (MMT). This additive is also present in the gasoline sold in the European Union, where, starting from January 1st 2011, a directive of the European Parliament (Directive, 2009/30/EC, 2009) limited the maximum allowed content to 6 mg per liter of fuel. Even if TEL and MMT are organo-metallic compounds, the combustion of fuels containing these additives produces particulate air pollution characterized by the presence of inorganic compounds of the respective metals. Unlike iron, little research has been devoted to the speciation of Mn in RD samples and particulate air matter. Huggins et al. (2000) reported that in the standard of urban air particulate (SRM-1648) the Mn oxidation states are +2, +3, +4, while in the standard of particulates emitted by diesel vehicles (SRM-1650), Mn oxidation state is +2. Ressler et al. (2000), using XAS spectroscopy, identified the following main chemical species in the particulate air pollution emitted by vehicles powered by MMT-containing gasoline: Mn3O4, MnSO4⋅H2O, and Mn5(PO4)[PO3(OH)]2⋅4H2O. In this work we will try to find consistent results for the speciation of both Fe and Mn in RD samples originating from vehicular traffic only.
Section snippets
Materials and methods
The Traforo del San Bernardo is a 5.8 km long, two-lane roadway tunnel located at the Italian-Swiss border, at 1900 m above the sea level. In the period 2000–2007 the average flux of vehicles was 576,000 cars and 66,000 trucks per year. Six samples of RD were collected inside the ventilation air shaft, i.e., ventilation ducts placed in the vault of the tunnel, and in two vertical air ducts 354 and 169 m long. To prevent sample contamination, air particulate deposited on the floor of the ducts,
Results and discussion
The analysis of the particles <63 μm provides useful information about PM10 emitted from vehicular exhausts and generated by resuspension. In fact, particle size determination (not reported) show that most of their mass is made of particles with less than 10 μm equivalent spherical diameter. The study of PM10 is particularly relevant for human health, as demonstrated by several epidemiological studies (Zelm et al., 2008, Yi et al., 2010). Iron-containing air particulate was extensively studied
Conclusions
The composition and in particular the speciation of Fe and Mn in RD samples with different particle size were studied by XRD, TGA, SEM–EDS, and XAS. Samples were collected in the ventilation air shaft of Traforo del San Bernardo highway tunnel. Being the collection site unaffected by anthropic sources different from vehicular traffic, nor by important temperature fluctuations or photo-induced effects, a clear separation between natural and anthropic contributions was possible, pointing out the
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