Speciation et biodisponibilite de metaux potentiellement toxiques dans le sol

The seminar reports the findings of a work aimed to evaluate the total amount and the geochemical forms of potentially toxic metals (PTMs) in mine tailings, soils and river sediments from an abandoned mine area of Morocco in order to assess the fate and transfer of metal contaminants.The High Moulouya Valley (Morocco) includes three former Pb-Zn mines (Aouli, Mibladen and Zaida); the main sources of contamination are the tailings stored in dumps without safety control or abandoned along fluvial banks. The chemical and mineralogical forms of PTMs are fundamental to define the mobility and the transfer of these metals. Tailings, river sediments and soils were sampled in and outside the mine sites and analysed for pH, EC, CaCO3, and grainsize; XRD analyses were performed for mineralogical characterization, while acid digestion was carried out for determining metal contents. Water samples were collected and analysed for pH, EC, CO2 volume, anions, cations and PTMs content.All the solid materials show an alkaline pH (7.9-9.3) and high CaCO3 content (72-604 g/kg). The main PTMs mineral forms are cerussite (PbCO3), barite (BaSO4) and anglesite (PbSO4).Mine tailings contain an abundance of Cd (1-3 mg/kg), Pb (2360-173000 mg/kg), and Zn (247-1063 mg/kg). The most contaminated soils are from the mine area of Mibladen (Cd: 0.4-1019 mg/kg; Pb: 34-31092 mg/kg; Zn: 46-190577 mg/kg) and Zaida (Pb: 8-1287 mg/kg). Sequential extraction was also undertaken for PTMs speciation : Cd, Pb and Zn are mainly present in form of carbonates (Cd: 8-53%; Pb: 6-80%; Zn: 7-52%) and bound to easily reducible Fe and Mn oxides (Cd: 17-29%; Pb: 8-68%; Zn: 6-60%).The high content and poor solubility of PTMs suggest a low degree of mobility by dissolution; the main transfer is likely due to wind and water erosion of the finest fractions.