Relative bioaccessibility of Pb-based paint in soil

The threat posed by lead (Pb) in soil for pediatric populations continues to be a public health issue. In long-established residential areas, a principal source of Pb in soil is likely to be old Pb-based paint originating from building surfaces. The health hazard posed by Pb from paint in soil will likely depend on quantity of paint incorporated, its Pb-mineral composition, whether the Pb is locked in some other material and the paint residence time in the soil (degree of aging). Here the relative bioavailability (RBA) of Pb in different types of Pb-bearing paint has been assessed. Tests were performed with individual paints, with paints mixed with a low-Pb soil, and with paints mixed with soil and the biogenic phosphate apatite II. Thirteen Pb-bearing paint samples were ground and passed through 250- and 100-µm screens. Samples nominally <100 µm from all the paints were analyzed, and six of the paints for which there was sufficient material in the 100- to 250-µm-size range were also tested. RBA extraction of Pb employed a simulated gastric fluid (SGF) of HCl and glycine adjusted to a pH of 1.5 in which samples were agitated (in an end-over-end rotator) for 2 h. Original paints were examined by SEM/EDX, and by XRD, residues collected after RBA extraction were examined by SEM/EDX. The concentration of Pb in the extraction fluid was measured by AAS. The quantity of Pb mobilized in each test batch was approximately an order of magnitude less in the paint–soil mix compared to the corresponding paint-only sample. The difference in the amount of Pb extracted from the paint–soil mix compared to the paint–soil–phosphate mix was minimal. However, in the post-RBA residues of the paint–soil mix, a PbCl precipitate was observed, and in the extraction residues of the paint–soil–apatite II mixes PbClP phases were recorded. Precipitation of these secondary phases obviously modified the amount of Pb in the extraction fluid, and this may need to be considered, i.e., under-reporting of extractable Pb, when this form of in vitro extraction is used to determine the RBA of Pb in environmental media.