Potentially toxic elements (PTEs) including nickel and chromium are often present in soils overlying basalt at concentrations above regulatory guidance values due to the presence of these elements in underlying geology. Oral bioaccessibility testing allows the risk posed by PTEs to human health to be assessed, however bioaccessibility is controlled by factors including mineralogy, particle size, solid phase speciation and encapsulation. X-ray diffraction (XRD) was used to characterise the mineralogy of 12 soil samples overlying Palaeogene basalt lavas in Northern Ireland and non specific sequential extraction coupled with chemometric analysis was used to determine the distribution of elements amongst soil components in 3 of these samples. The data obtained were related to total concentration and oral bioaccessible concentration to determine whether a relationship exists between the overall concentrations of PTEs, their bioaccessibility and the soils mineralogy and geochemistry.Gastric phase bioaccessible fraction (BAF%) ranged from 0.4% to 5.4% for chromium in soils overlying basalt and bioaccessible and total chromium concentrations are positively correlated. In contrast the range of gastric phase BAF for nickel was greater (1.4 to 43.8%), while no significant correlation was observed between bioaccessible and total nickel concentrations. However nickel BAF was inversely correlated with total concentration. Solid phase fractionation information showed that bioaccessible nickel was associated with calcium carbonate, aluminum oxide, iron oxide and clay related components, while bioaccessible chromium was associated with clay related components. This suggests that weathering significantly affects nickel bioaccessibility, but does not have the same effect on the bioaccessibility of chromium. Suggested keywordsoral bioaccessibility, nickel, chromium, basalt, sequential extraction, solid phase distribution 3

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