Understanding the risk to human health from toxic elements in soil by simplifying the black art of sequential extractions

The chemical, physical and biological processes that occur within soils govern whether the potentially harmful elements (PHE) are present in forms that could pose a threat to human health. Recently, there has been increasing use of in-vitro bioaccessibility tests which provide estimates of the amount of PHE that is available for absorption in the human gastrointestinal system. Although useful for risk assessment, this data does not provide information on the source of the bioaccessible fraction which is required to answer the question “What makes the PHE bioaccessible”. A well-adopted method to assess PHE element pools of differential relative lability in soils is the sequential extraction with phase targeted reagents of increasing dissolution strength. This study shows how a sequential extraction method which uses simple mineral acid extractants and a novel data processing methodology can be used in conjunction in-vitro bioaccessibility tests to provide process understanding of bioaccessibility. The approach is illustrated using NIST 2710 reference soil. So, for example, the As in NIST 2710 was mostly associated with four iron oxide components (68%), iron sulphide (20%) with smaller amounts associated with Mn oxides(7%) and a PbO component (3%). The bioaccessible fraction was found to be c. 50% of the total As which was shown to be made up from Mn, PbO, and the most labile iron oxide fractions.