A semi-mechanistic aging model of Pb added to soil by a modified stable isotope dilution technique

The aging of Pb added to soils has not been studied by the isotopic technology because of difficulties in determination of isotopically exchangeable Pb in soil, so that a set of 10 typical agricultural soils in China and a one-year aging experiment with the addition of water-soluble Pb to the soils were carried out. A modified stable isotope dilution technique to determine isotopically exchangeable Pb in soil was developed where 0.2 mM EDTA (ethylenediaminetetraacetic acid) as the extractant. When water-soluble Pb was added to soil, the isotopically exchangeable Pb (E_add%, the percentage of isotopically exchangeable Pb to total Pb added to soil) initially decreased rapidly and gradually slowly. A semi-mechanistic aging model of Pb added to soils, including precipitation/nucleation (Y₁), micropore diffusion (Y₂), and organic matter encapsulation processes (Y₃) was developed with the root mean square error 8.3% where Y₁, Y₂, and Y₃ accounted for 0.02~26.9%, 1.4~21.8% and 3.8~11.3%, respectively, when the pH 4.0~8.0 and organic matter 2.0~6.0%. Soil pH was a vital factor affecting the aging rate. When the pH increased by 1 unit, the E_add value decreased by approximately 9%. The model could be used to scale ecotoxicological data of Pb in soil generated in different aging times.