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 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.

Point of Zero Charge: Role in Pyromorphite Formation and Bioaccessibility of Lead and Arsenic in Phosphate-Amended Soils

“…This study evaluated soils contaminated with Pb and As such as BO (Orchard, NC, USA), IKS (mining-impacted, Dewey-Humboldt, AZ, USA), and NW (smelter-impacted, Lincoln, NE, USA). BO and IKS soils have been evaluated in previous studies for Pb and As bioaccessibility [26] and NW soil provided an opportunity to investigate Pb immobilization at a Superfund site. BO was collected from Haywood country in North Carolina where an orchard was in use from 1908 to 1988; during this time, a lead-arsenate pesticide was heavily used and, accordingly, the site is highly contaminated with Pb and As.…”

“…More details on an initial mineralogy of IKS and BO can be referred to in R.R. Karna et al [26]. Lead-XAS data analysis in NW indicated Pb bound to humic acids (75%) as the primarily dominant mineral; other minor minerals identified were anglesite (14%) and Pb sorbed to ferrihydrite (11%).…”

“…Arsenic speciation in the original IKS was primarily associated with the weathering products of arsenic minerals, such as As_jarosite (52-70%), scorodite (hydrous ferrous arsenate) (31-38%), and As(V) ions sorbed on goethite (14-18%) [26]. Considering an initial As mineralogy, there were no significant changes in As-speciation upon different rates of P-amendment in IKS at pH PZC (pH of 4), >pH PZC (pH of 7), and <pH PZC (pH of 3).…”

“…Lead-XAS data analysis on post P-amended samples at different pHs indicated Pb adsorbed to ferrihydrite (27% to 37%) and Pb bound to organic matter (15% to 63%) as the primary dominant minerals [26] which are commonly observed in soil with high Fe oxides and high organic matter [48]. Adsorption of organic matter, especially humic substances, onto a mineral surface alters its physical and chemical properties, influencing the sorption and desorption processes of ions like phosphates and heavy metals.…”

Application of Synchrotron Radiation‐based Methods for Environmental Biogeochemistry: Introduction to the Special Section

“…The importance of considering bioavailability for the risk assessment of soil contaminants has gained much attention over the past couple decades. Karna et al (2017) provides an example of how a synchrotron‐based XAS technique can be used to validate in vitro bioaccessibility test results. Their research focused on two of the most common soil contaminants, Pb and As, and examined how speciation and bioaccessibility change as a function of soil particle size.…”