Bioaccessible Lead (Pb) in Soil Source Attribution by Individual Particle Analysis (IPA): A Case Study of Six Soils from Mitrovica, Republic of Kosovo

Introduction: We conducted a detailed examination of the Pb particle content of surface media (soils, etc.) from six sites in Mitrovica, Kosovo. The samples were from a small region (approx. 4-km across), and despite the close proximity of the sites, the samples exhibited a range of Pb concentrations, and bioaccessibilities. Electron microscopy methods were used to determine the forms and solubility of the Pb in the samples at the individual particle level. The goal was to attribute the bioaccessible Pb in the soil to the responsible phases. Materials and Methods: Total Pb content was determined by HNO3 extraction and AAS analysis. Bulk sample Pb bioaccessibility was assessed by the Unified Bioaccessibility Method (UBM). Computer controlled scanning electron microscopy (SEM) and energy dispersive X-ray analysis was used to determine the composition of individual Pb particles. Using a hierarchical clustering technique, 18 Pb particle types (classes) were defined for the samples. Individual Pb particle solubility was assessed by Differential Individual Particle Analysis. This involved in situ immersion of Pb particles in a simulated gastric fluid for four sequential periods of 30 minutes, with the particles analyzed by SEM after each immersion. Results: The samples had a total Pb content of 625 – 47,000 mg/kg, and a bulk bioaccesible Pb content of 3 - 89%. Despite these differences, Pb isotope measurements suggested a similar origin for the Pb in the samples (mean [range]: 206Pb/204Pb 18.66 [0.05], 207Pb/204Pb 15.66 [0.13], 208Pb/204Pb 38.84 [0.47]). However, each sample contained different Pb particles. For example, samples were dominated by particles classified as high-Pb-content, PbMg,- PbFeZn,- and PbFeMn-bearing. Pb solubility at the individual particle level was a function of whether: (i) the Pb was in an isolated particle phase, (ii) the Pb was bound as a matrix element (with sparingly soluble elements, e.g., Fe), (iii) the Pb was in a separate phase locked within an insoluble particle. How the Pb was bound (and hence its solubility) differed radically between samples. Discussion: The solid Pb phases that control the bioaccessibility of Pb in the samples were site specific. Generalizations across the study area were difficult to make. This speaks to the heterogeneity of Pb speciation often found in soils