Resolving the Solid Phase Speciation of Pb-Bearing Particles that Control the Bioaccessibility of Pb in highly Polluted Soils

The importance of the association between lead (Pb) exposure and IQ deficits and behavior problems in pediatric populations has been well documented. The Pb exposure topology for children can be dominated by Pb in soil, where soil is inadvertently ingested through mouthing activities. Highly contaminated soils pose a particular risk, however the exposure threat can be modified by the relative bioaccessibility of the Pb. Lead, if present in a highly insoluble form in soil may, despite unintended ingestion, result in a minimally absorbed dose. Here we investigate a number of spatially juxtaposed soils with moderate to high Pb concentrations (from mining and industrial processes), and with disparate levels of Pb solubility. The samples, from the city/municipality of Mitrovica, in northern Kosovo, contained Pb levels ranging from approximately 625 – 47,000 mg/kg, and relative in vitro oral Pb bioaccessibiltities, of between 3 and 89%. The aim of this study was to evaluate which Pb phases in the soils controlled the overall soil Pb solubility. To this end, the Pb mineral phases were determined by XRD, and the element associations with Pb at the individual particle level by SEM and EDX. In the case of the latter statistically significant populations of particles were characterized by automated image analysis in the Computer Controlled SEM (CCSEM). Lead solubility was investigated at the bulk sample level by the Unified BARGE method (UBM), and by chemometric analysis of sequential extraction data (chemometric identification of substrates and element distribution, CISED), and at the individual particle level by Differential Individual Particle Analysis (DIPA). CCSEM characterization of soil particles (approximately 4,000 particles analyzed from each sample), identified specific Pb-particle associations (phases) in the samples. In situ immersion of Pb-particles in simulated gastric fluid (DIPA characterization of Pb-particles before and after fluid exposure) identified phases that appeared to be more sparingly soluble than others. Initial results suggest that across the suite of samples Pb-particles with Fe associations were less soluble than Pb-particles with Ca associations. Although, other Pb-particle associations that are sample specific are also important in controlling Pb solubility.