Behaviors of exogenous Pb in P-based amended soil investigated with isotopic labeling method coupled with Tessier approach

Abstract

Lead (Pb) pollution in soil has attracted public attention because of its serious impacts on human health through food chain. However, the chemical and biological behaviors of exogenous Pb in soil remain unknown, specifically in the passivated soil with phosphate (P)-based materials. By spiking soil with stable enriched isotopes of 206Pb (>99%), the exogenous Pb2+ in soil was simulated in the present study; the distribution, transformation, and bioavailability of exogenous Pb in soil passivated with the P-based compound were quantitatively investigated using an isotopic labeling method coupled with a Tessier approach of sequential extraction procedures. The results showed that exogenous Pb in water-soluble fraction (F1) accounted for 0.58% of the total spiked isotopes, which was slightly higher than that of the intrinsic Pb (0.34%) in soil. Nevertheless, the exogenous Pb percentages in exchangeable fraction (F2), carbonate bound fraction (F3), Fe/Mn oxide fraction (F4), and organic matter bound fraction (F5) were about twice more than those of the intrinsic Pb in the four fractions. But exogenous Pb in residual fraction (F6) was only 45.6%, while intrinsic Pb was 70%. After the soil was passivated with P-based compound consisting of diammonium phosphate, bone meal, and hydroxyapatite, more than 90% of exogenous Pb in F2 could be immobilized, but small Pb percentage in F1 was immobilized. Additionally, Pb species in F2, F3, and F4 might be converted into different forms in F5 and F6, but the immobilized Pb in F5 and F6 was labile because more than 50% of the exogenous Pb in the two fractions was lost after planting. The results demonstrated that the isotopic labeling technique coupled with Tessier approach was a novel method that can evaluate the distribution, transformation, and bioavailability of exogenous Pb in soil. These results are helpful for further immobilization of exogenous Pb in agricultural soil.