Pb, Zn, Cd, Cu, Cr and Ni Fractioning from a Soil Profile Collected in Old Deposit of Mining Waste - Ribeira Valley (Brazil)

The improper disposal of mining waste contributes greatly to the contamination of potentially toxic metals in soils. Adrianopolis (PR), located in the southeastern of Brazil, is an example of this reality, where from 1991 to 1995 mining wastes were placed directly on soil. Currently, this residue was covered with thin layer of uncompressed soil, which in some portions of the old deposit this material remains exposed and susceptible to leaching. Five soil samples were collected at different depths, in the most central part of the old mining deposit (collection depths: 0-20 cm, 20-40 cm, 40-55 cm, 55-88 cm and 88-100 cm). These samples were disintegrated with agate mortar and subsequently homogenized. The samples were analyzed for metal concentration by Atomic Absorption (AA), and the sequential extraction method was applied to determine the mobility of metals according to their chemical form (fractionation). It was noted that the concentrations of metals, Cr, Cu, Ni, Pb and Zn were higher in samples closer to the surface. Cd was found in higher concentrations when bound to carbonates and oxides and hydroxides; and Cr, when connected to oxides and hydroxides and organic matter (OM). Cu was found in higher concentrations when connected with OM in the most superficial samples and with Fe-Mn oxides and hydroxides in the deeper samples. Ni was more associated with Fe-Mn oxides, and secondarily to carbonates. Pb and Zn presented the highest concentrations; on the surface sample was obtained 518 mg.kg(-1) of Pb and 2.528 mg.kg(-1) of Zn, both linked to carbonates, which is considered as a mobile phase. Except for Cr and Cd, the other metals associated with OM showed a decreasing concentration with depth of the profile. According to the current legislation (CETESB and CONAMA - Brazil), the Cd concentrations along the profile are higher than the quality and intervention levels. In the sample 0-20 cm, the concentrations of all metals except for Cr were superior to the level of intervention. The metals analyzed, especially Cd, Pb and Zn, present considerable concentrations linked to carbonate fraction, which infers in the mobility of the metal. Thus, greater monitoring in this area is necessary, as these metals may be leaching and bioavailable to the biota.