The role of soil geochemistry in the absorption of potentially toxic elements (PTEs) by edible hyperaccumulator plants: the case of Brassica rapa in volcanic and clay soils.

The hyperaccumulation behaviour of PTEs is observed in many edible plants. However, the role of soil geochemistry and the human health risks associated with the uptake of PTEs by hyperaccumulator plants remain poorly understood. This study analyses 10 topsoil and Brassica rapa samples collected from volcanic and clay soils, comparing the contents of As, Cd, Hg and Pb and assessing their relative health risks. To account for geochemical variations in volcanic and clay soils, samples were collected from two Italian regions (Sicily and Campania) characterized by different geological settings. The results indicate that volcanic soils exhibit higher concentrations of PTEs than clay soils, with Hg levels exceeding precautionary limits established by EU soil quality standards. Notably, Campania shows the highest concentrations of PTEs in soils, attributable to evolved magmatic products with tephritic-phonolitic composition. In clay soils, Sicilian samples reveal significant enrichment in Cd, while As, Hg and Pb are more concentrated in Campanian clays. Soil quality standards are not exceeded in clay soils. Regarding plant tissues, concentrations of Cd, Hg and Pb in edible organs (stems and leaves) exceed FAO-WHO standards in most samples from volcanic soils, with values up to 3, 8, and 14 times higher than the standards. Plants grown in clay soils show lower concentrations of PTEs than those grown in volcanic soils; only one Sicilian sample exhibits concentrations of Cd, Hg, and Pb 4, 1.5, and 6 times above FAO-WHO standards, respectively. The bioconcentration factor (BCF) and translocation factor (TF) confirm the hyperaccumulating behaviour of Brassica rapa, with concentrations of PTEs in roots and stems sometimes exceeding those present in the soil. Risk analysis revealed that total cancer risk and target hazard quotient are unacceptable for adults and children who consume Brassica rapa from volcanic soils. Both parameters generally show acceptable values in clay soils, with alarming levels only for high consumption rates. Finally, although Sicilian soils are generally impoverished in PTEs, Brassica rapa samples from this region exhibit higher levels than those from Campania. Therefore, while soil geochemistry is a crucial factor in metal absorption by Brassica rapa, other parameters (e.g., climatic, environmental, and biological) also play a significant role.