Mineralogical characterisation to improve understanding of oral bioaccessibility of Cr and Ni in basaltic soils in Northern Ireland

Geological soil parent material is often the source of elevated levels of potentially toxic elements (PTEs) in soils, and in some cases these are at concentrations which exceed regulatory guidelines for protection of human... [ view full abstract ]

Geological soil parent material is often the source of elevated levels of potentially toxic elements (PTEs) in soils, and in some cases these are at concentrations which exceed regulatory guidelines for protection of human health. However, geogenic contaminants are often bound tightly within the soil matrix and are not bioavailable to humans in significant concentrations. Therefore oral bioaccessibility testing is used to quantify the PTEs in soils that are accessible to humans.

A previous study investigated the distribution of Ni and Cr amongst soil components in 3 soils overlying Palaeogene basalt lavas in Northern Ireland using non specific sequential extraction coupled with chemometric analysis (CISED) and related this to oral bioaccessibility measured by the Unified Bioaccessibility Method (UBM). However, without data relating to soil mineralogy, interpretation of the role of mineralogy on the oral bioaccessibility of Ni and Cr was inconclusive. This paper presents the findings of elemental mapping using Electron Probe Microanalysis (EPMA) and mineralogical mapping undertaken using QEMSCAN®, an automated mineral/phase analysis system based on a scanning electron microscope. Results are related to oral bioaccessibility and CISED analyses to determine the effect of soil mineralogy on oral bioaccessibility of Ni and Cr.

Results indicate that Cr concentrations are principally related to recalcitrant chrome spinel, which explains the relatively low bioaccessibility of Cr. In contrast, Ni is more widely dispersed within the soils, with a proportion of total Ni found in weathered olivine, clay and carbonates, leading to the higher oral bioaccessibility measurements recorded for Ni than Cr.