Use of BCR sequential extraction procedures for soils and plant metal transfer predictions in contaminated mine tailings in Sardinia

The total concentration of trace elements in soils does not always represent the fraction available for organisms and/or plants because they accumulate in several geochemical forms. The bioavailable concentration of Zn and Pb in mine tailing soils and its relation to the concentration in the leaves of plants growing there are analysed. The three-step sequential extraction BCR method was used to analyse samples from mine tailing and uncontaminated soils. The correlations of the Zn and Pb contents in the three BCR steps with the total concentrations of both trace elements in soils and plants and with the soil characteristics were determined. Three plant species were analysed: Dittrichia viscosa, Euphorbia pithyusa subsp. cupanii, Cistus salviifolius. The study area was situated in the Sulcis-Iglesiente biogeographic sector (SW Sardinia, Italy). The soils with the highest total concentration of trace elements also presented the greatest metal availability. In general, the ratios of Pb and Zn concentrations in the BCR steps and the respective total concentrations in the soils were similar. The mine tailing soils registered high values for total Zn and Pb and also showed the highest percentages of these trace elements in the first two BCR fractions. However, the uncontaminated soils gave the highest percentages in the residual fraction. The total Pb concentration in the soils correlated with the concentrations found in the first two BCR steps (BCR1 and BCR2) and in the residue. For Zn, positive correlations were found with the three BCR steps. The correlations between the element extractions in the BCR steps and the soils characteristics depended on the particular soil type. Plant characteristics determined the correlations between the Pb and Zn concentration in leaves and soils. Specifically, none of the plants studied showed significant correlations between the Pb concentration and that found in the BCR steps. The opposite occurred for Zn whose concentration in plants appeared to be positively correlated with the concentrations found in the three BCR steps. The Pb concentrations in plants and in the BCR steps were not indicative of the bioavailability of this element in the soil. On the contrary, the Zn concentration in the leaves showed the bioavailability of this element in the soil and the constituents to which it was related. The BCR method proved to be appropriate in case of Zn showing the existing correlations with the soil constituents and the plant contents.