The effect of particle size on oral bioavailability and bioaccessibility of soil Ni from different sources

The goal of the work was to contribute to a unified approach to assessing the risk to human health of soil ingestion, for contaminated sites with elevated [Ni]. Robust relationships between in vitro bioaccessibility and in vivo bioavailability of Ni in various soils, with mechanistic understanding, would enable site-specific assessments of human exposure through soil ingestion. Four soils (three ultramafic Brunisols with geogenic Ni and one Organic soil with anthropogenic Ni) were sieved into PS < 10 μm, 10-41 μm, 41-70 μm, 70-105 μm, 105-150 μm, and 150-250 μm, the [Ni]_T for which ranged from 560 to 103000 mg/kg. Mass fraction-adjusted [Ni]_IVBA (SBRC gastric) for each soil fraction was similar whether calculated for all particles <250 μm or <150 μm %Ni_IVBA ranged from 3% to 16% of [Ni]_T and %Ni_ABA (accumulated Ni in urine, kidneys, and small intestine of Sprague Dawley rats gavaged with a soil) ranged from 0% to 0.49%. The correlation between these two measurements was weak (R² = 0.06). Multiple linear dose response relationships attributing variation in %Ni_ABA to %Ni_IVBA plus soil physicochemical parameters known to influence trace element availability in soils were developed. As many soil properties measured in this study were highly correlated, ridge regression enabled a predictive relationship where the effect of each parameter was its true contribution to variation in %Ni_ABA. Using a ridge constant (k) of 0.012, %Ni_ABA could be predicted from %Ni_IVBA adjusted for soil absorptive entities (OrgC, and Fe oxides (negative coefficients)) and soil pH (positive coefficient). %Ni_ABA predicted from this relationship was very close to 1:1 with the observed %Ni_ABA except at the lowest observed values which were lower than predicted. This study shows that as the conditions increasingly favour soil Ni solubility, more of the Ni was bioavailable; this generalization was true regardless of particle size or soil origin.