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

ABSTRACT

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 µm. This size fraction was widely used in testing efforts for both in vivo and in vitro experiments. However, recent studies indicate the <150-µm 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 µm moving to the <150-µm fraction. The objectives of this study were to investigate <250-µm versus <150-µm 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 µm. Sieved versus ground subsamples of <250-µm and <150-µm 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 extractabletotal) 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-µm fraction will not undermine currently validated IVBA protocols in future bioavailability studies.