Bioaccumulation of Thallium and other heavy metals in Biscutella laevigata nearby a decommissioned Zn-Pb mine (Salafossa, Northeastern Italian Alps)

The mineral body cultivated in Salafossa (Eastern Dolomites) was one of the largest lead/zinc-containing mineral deposits in Europe. Both metals were mainly present as sulphides (sphalerite, ZnS and galena, PbS). Mining activity started around 1550 but it was only around 1960 that the richest veins of the minerals were discovered. The mine closed in 1985. The content of several heavy metals (Tl, Fe, Mn, Pb, Zn) was determined in soils and in plants (Biscutella laevigata) from twelve sites selected outside the mine. Biscutella laevigata is a “pseudometallophyta” species, and it often grows near mining areas, where soil metal contents are significantly higher than natural geochemical background levels. Total metal contents in biological (roots and leaves of B. laevigata) and inorganic (soil) samples were determined by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). In addition, metal bioavailability in the soil - including the B. laevigata root system (rhizo-soil) - was estimated by using an extracting solution (i.e. DTPA - Diethylene Triamine Penta-acetic Acid). Results showed that metals were present in a chemical form available for absorption by the plants roots. In fact, high concentrations of the metals were also found in the roots and leaves of B. laevigata, and these concentrations were higher than those in the corresponding rhizo-soil. Thus, B. laevigata has shown a marked ability to bioaccumulate heavy metals, especially Tl and, to a lesser extent, Zn, Pb, Fe, Mn, and it can influence metal mobility in the rhizo-soil. In particular, Tl has been recognized as one of the most toxic trace elements for the human health. When present in the aqueous solution, Tl may further be subject to adsorption on the surface of soil and sediment particles, for example in the presence of oxides and hydroxides of Fe and Mn. To assess absorption and translocation processes of heavy metals, resulting in their bioaccumulation, two different indices were calculated: the Enrichment Factor in roots (EFr), as the ratio between metal concentration in belowground biomass and in the respective rhizo-soil, and the Translocation Factor (TF), as the ratio between metal concentration in leaves and in the corresponding roots. For both indices, values higher than 1 denote an enrichment of the metal in the roots or its translocation to the upper tissues. The results showed that EFr and TF were considerably high only for Tl, reaching a maximum value of 60 for EFr and 11.6 for TF. Conversely, the other investigated metals did not show significant bioaccumulation (EFr < 1) and they showed TF > 1 only at a few sites. These evidences confirmed the ability of B. laevigata to absorb metals from the contaminated soil and to accumulate them in the roots and/or translocate them to the aboveground biomass, especially Tl, thus representing a good indicator of Tl bioavailability in the rhizo-soil of the study area.