Contrasting Lead Speciation in Forest and Tilled Soils Heavily Polluted by Lead Metallurgy

The concentration trends and chemical fractionation of Pb was studied in eight tilled and forest soil profiles heavily polluted by Pb metallurgy in the Pribram district, Czech Republic. The highest Pb concentrations were observed in surface and subsurface horizons attaining 35,300 mg kg-1 in forest soils and 1233 mg kg-1 in tilled soils. Total Pb concentrations were one order of magnitude lower in tilled soil due to intensive ploughing and annual crop off-take. The results of the Tessier sequential extraction procedure showed the preferential binding of Pb in forest soils to operationally-defined exchangeable positions and soil organic matter (oxidisable fraction). The Pb exchangeable fraction is thought to correspond to weak electrostatic binding on the functional groups of organic matter. In tilled soil, Pb is predominantly bound to operationally-defined Fe and Mn oxides (reducible fraction). A comparison with the background Pb concentration values showed a strong contamination even in mineral horizons IIC and confirmed a strong vertical mobility of Pb within the soil profiles. The calculated mobility factors (MF) showed that up to 72% of Pb is mobile and bioavailable in forest soils. In contrast, the bioavailability of Pb in tilled soils was significantly lower as the MF accounted for up to 30%. In the most polluted horizon of forest soil profile, the X-ray powder diffraction (XRPD) analysis confirmed the presence of anglesite (PbSO4), derived likely from the smelter emissions.