Lead and Arsenic Bioaccessibility and Speciation as a Function of Soil Particle Size

Bioavailability research of soil metals has advanced considerably from default values to validated in vitro bioaccessibility (IVBA) assays for site-specific risk assessment. Previously, USEPA determined that the soil-size fraction representative of dermal adherence and consequent soil ingestion was <250 mu m. This size fraction was widely used in testing efforts for both in vivo and in vitro experiments. However, recent studies indicate the < 150-mu m size fraction better represents the particle size that adheres to skin for potential ingestion. At issue is the relevance of validated in vivo and in vitro methods developed with < 250 mu m moving to the < 150-mu m fraction. The objectives of this study were to investigate < 250-mu m versus < 150-mu m particle size and particle size groups for evaluating lead (Pb) and arsenic (As) IVBA and speciation. Soils with different properties were homogenized, oven dried, and sieved: < 250 to > 150, < 150 to > 75, < 75 to > 38, and < 38 mu m. Sieved versus ground subsamples of < 250-mu m and < 150-mu m bulk soils were also used for IVBA and synchrotron-based Pb and As speciation. Although we observed an increase in total and IVBA-extractable Pb and As with decreased soil particle size, changes in %IVBA of Pb and As (dependent on the ratio extractable: total) remained consistent in all of the tested soils. No significant changes in Pb and As speciation were observed across the soil fractions. The results suggest that using the more relevant < 150-mu m fraction will not undermine currently validated IVBA protocols in future bioavailability studies.