Solid-Phase Speciation of Pb in Urban Road Dust Sediment: A XANES and EXAFS Study JUDITH E. S. BARRETT, † KEVIN G. TAYLOR,* ,† KAREN A. HUDSON-EDWARDS, ‡ AND JOHN M. CHARNOCK § Department of Environmental and Geographical Sciences, Manchester Metropolitan University, Manchester M15 6BH, U.K., Department of Earth and Planetary Sciences, Birkbeck, University of London, Malet St., London WC1E 7HX, U.K., and School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, M13 9PL, U.K. Received December 10, 2009. Revised manuscript received March 11, 2010. Accepted March 11, 2010. The quality of the urban environment is of growing concern as its human population continues to dramatically increase. X-ray absorption spectroscopy (XAS) and SEM have been used to study the solid-phase speciation of Pb in urban road dust sediments (RDS) in Manchester, UK. XANES analysis and linear combination modeling indicate that PbCrO 4 and Pb-sorbed goethite occur in 1000-500 µm, 250-125 µm, 63-38 µm, and <38 µm size fractions, collectively representing between 51-67% of the contributing Pb-phases. XANES analysis suggests that PbO, PbCl 2 , and Pb carbonates are also present. EXAFS modeling for all grain size fractions gives best fit models with a first shell of two oxygen atoms at 2.29-2.32 Å, which corroborate the possible presence of Pb-sorbed goethite, and also suggest the presence of Pb phosphates and Pb oxides. Second shell Pb-Fe and second and third shell Pb-Pb scattering distances confirm Pb-sorbed to Fe oxide, and PbCl 2 and PbCrO 4 , respectively. Many of the XAS models are corroborated by SEM observations. The Pb-phases may pose a risk to human health if inhaled or ingested, with insoluble phases such as PbCrO 4 potentially causing inflammation in the lungs, and soluble phases such as PbO potentially being the most bioaccessible in the digestive tract. Introduction Over 50% of the global population currently lives in urban areas (1), a figure set to increase considerably over the coming decades. Therefore, the quality of urban environments is a matter of global concern. Accumulations of particulates on road surfaces (referred to here as road dust sediment, RDS) are major sources of both water-borne and atmospheric particulates in urban environments and are commonly highly contaminated by metallic elements such as Pb (2). Lead is highly toxic to humans, with excessive intake causing anemia and diseases of the kidneys, heart, immune, nervous, reproductive, and gastrointestinal systems (3). Humans are exposed to Pb through natural and anthropogenic pathways that include RDS. Although RDS is often highly enriched in Pb (2, 4), few studies have been carried out to investigate its solid-phase speciation. Duggan and Williams (1977) used chemical extraction methods (dilute hydrochloric acid) to estimate the bioavailibility of Pb in RDS within Greater London (4). Biggins and Harrison (1980) used X-ray diffraction (XRD) to investigate RDS, and identified six crystalline Pb species, including PbSO 4 (5). They concluded, however, that as crystalline compounds only accounted for a small percentage of the total Pb there was a need to seek alternative techniques to more thoroughly understand the solid-phase Pb speciation. Such an understanding would also enable better estimates of the bioavailability of Pb in RDS to be made, since this is related to the stability and solubility of the Pb species. Numerous studies have demonstrated that a synergistic approach that combines many mineralogical and spectro- scopic techniques is highly effective for characterizing the solid-phase speciation of metallic elements in environmental materials such as soils, dredged sediment, and soils (6, 7). Techniques such as X-ray diffraction, transmission and scanning electron microscopy, and especially, X-ray absorp- tion near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies are used to give information about the composition, crystallinity, and bonding environments of the metallic elements and their hosts. In this paper, we use such an approach to characterize the solid-phase speciation of Pb in RDS from a well-used urban road in Manchester, a major city in NW England. Experimental Methods RDS was collected from Oxford Road, situated within the university area of the City of Manchester, in November 2004, four years after leaded-petrol was banned in the UK. It is an anthropogenically active site, with a residual population amplified daily and seasonally by commuters, students, and visitors. In the preliminary stages of this research, the chemical composition of RDS from the Oxford Road site over a transect of 550 m was characterized, and rigorous statistical analysis of these data reveal no significant compositional variability (Barrett, unpublished data). Hence, one statistically representative sample site (British national Grid Reference: SJ843973) was selected for the in-depth analysis reported here (limitations on synchrotron beamtime dictated this approach). A 10 kg RDS sample was collected from a traffic island site using a plastic dust pan and brush, as recom- mended by Charlesworth and Lees (8). Due to the large accumulation of RDS at this site, and the ability to sample coherent agglomerations, we believe that any loss of fines was minimal. The sample was air-dried in soil bags at an average temperature of 21 °C. Constant mass was attained after 14 days. Ten percent of the dried bulk sediment sample was retained as an archive. A meshed sieve was used to remove material g2000 µm from the remaining sediment sample. The e2000 µm sediment sample was then dry sieved into seven grain size fractions using standard sieve methods (see Supporting Information (SI) Table S1 for mass fractions by grainsize). Recovery of sediment was determined to be 99.5%. Storage and handling of all sediments samples was carried out in such a way as to minimize atmospheric contamination. Pseudototal Pb concentrations in each grain-size fraction were estimated using open-vessel digestion in aqua-regia at 85 °C for 2 h. To allow for the inhomogeneous character of RDS 6 replicas of 0.5 ( 0.02 g per grain size fraction were digested. Resulting solutions were analyzed for Pb and 18 * Corresponding author e-mail: k.g.taylor@mmu.ac.uk. † Manchester Metropolitan University. ‡ University of London. § University of Manchester. Environ. Sci. Technol. XXXX, xxx, 000–000 10.1021/es903737k  XXXX American Chemical Society VOL. xxx, NO. xx, XXXX / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 A