Science Inventory

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

Citation:

Karna, R., M. Noerpel, A. Betts, AND K. Scheckel. Lead and Arsenic Bioaccessibility and Speciation as a Function of Soil Particle Size. JOURNAL OF ENVIRONMENTAL QUALITY. American Society of Agronomy, MADISON, WI, 46:1225-1235, (2017). https://doi.org/10.2134/jeq2016.10.0387

Impact/Purpose:

The EPA’s Technical Review Workgroup (TRW) Lead Committee initially recommended that Pb in <250 μm particle size fraction is more representative of ingestion exposure compared to unsieved soil (U.S. EPA, 2000). While this remains true, in July 2016, the TRW reviewed ten recent studies (Bergstrom et al., 2011; Choate et al., 2006a; Gong et al., 2013; Juhasz et al., 2011; Siciliano et al., 2009; U.S. EPA, 1995, 2000; Yamamoto et al., 2006;) reporting the relationship between dermal adherence of soil and dust as a function of particle size fractions (e.g., <150, <125, <40 μm) from a variety of samples, leading the TRW to recommend the <150 μm particle size for use in the assessment of human health risks for soil and dust exposures (USEPA, 2016). Reliable data on the particle size fraction and the metal enrichment that is most likely to adhere to children's hands can definitely improve the accuracy of exposure and risk calculations in risk assessments. At issue is the relevance of previous research with the <250µm soil fraction if new research focuses on the <150µm fraction pertaining to in vivo – in vitro correlations (IVIVC) predictive of relative bioavailability for site-specific risk assessment. To date, a tremendous amount of effort and expense has been put forth to develop and refine in vivo and in vitro methods utilizing the <250µm soil fraction. We hypothesized that the total and IVBA extractable Pb and As might increase with decrease in particle size fractions, still their percent bioaccessibility values may remain the same. Pulverization may enhance IVBA with increased reactive mineral surfaces, and greater dissolution thereby enhancing metal release due to increase in surface oxidation forming products that are more readily absorbed. The percentage of components for each Pb and As species may change, however overall speciation should remain similar in different soil fractions. The objectives of this study were: a) to investigate <250µm versus <150µm particle size for evaluating IVBA of Pb and As from soils with respect to other soil fractions on IVBA estimation, b) to measure any variability in IVBA in sieved versus pulverized soils, c) to determine any changes in Pb and As speciation in different soil fractions and sieved versus pulverized in order to provide more confidence in selecting the soil particle size for a particular soil type that best represents oral exposure to human, and help make it widely acceptable in decision making process. This information is of interest to Regional and Program Office decision makers, States, and local affected communities.

Description:

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 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 ration 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-µm fraction will not undermine currently validated IVBA protocols in future bioavailability studies.