Labile lead in polluted soils measured by stable isotope dilution

It is well known that lead (Pb) is strongly immobilized in soil by adsorption or precipitation. However, the reversibility of these reactions is poorly documented. In this study, the isotopically exchangeable Pb concentration in soils (E-value) was measured using a stable isotope (²⁰⁸Pb). Soils were collected at three industrialized sites where historical Pb emissions have resulted in elevated Pb concentrations in the surrounding soil. Lead concentrations ranged from background values, in the control soils collected far from the emission source, to highly elevated concentrations (5460-14440 mg Pb kg⁻¹). The control soil of each site was amended in the laboratory with Pb(NO₃)₂ to the same total Pb concentrations as the field-contaminated soils. The %E values (E-value relative to total Pb content) were greater than 84% in the laboratory-amended soils, and ranged from 45% to 78% (mean 58%) in the field-contaminated soils. The relatively large labile fractions of Pb in the field-contaminated soils show that the majority of Pb is reversibly bound despite the fact that the binding strength is large. The Pb concentrations in soil solution were up to 3500-fold larger for the laboratory-amended soils than for field-contaminated soils at corresponding total Pb concentrations. These differences cannot be explained by differences in labile fractions of Pb but are attributed to the decrease in soil solution pH upon addition of Pb²⁺-salt.