Lead Speciation and Association with Organic Matter in Various Particle-Size Fractions of Contaminated Soils

Lead (Pb) stabilization in polluted soils treated by a Pb immobilization technique may be dependent on the speciation of Pb present in specific particle-size fractions of the soil. However, the scale-dependency of Pb speciation in contaminated soils is still not clearly understood. In this study, the natures and amounts of Pb chemical forms were determined in five Pb-polluted soil samples from Klity Village, Thailand, and their particle-size fractions. This was achieved using multiple analytical tools, including bulk extended X-ray absorption fine structure (EXAFS) spectroscopy at the Pb LIII edge. Results suggested that cerussite, Pb sorbed to goethite, and Pb-humate were present in specific amounts in all bulk samples and their particle-size fractions. The highest amounts of Pb-humate were found in the smallest particles of the soil samples. This Pb form was present in the fine particles of a soil sample, but remained undetected when analyzing the bulk sample. Since Pb-SOM association may impede the formation of pyromorphite in soils, the results implied that the extent of Pb immobilization in a polluted soil treated by P may be less than predicted if Pb speciation is only characterized at the macroscopic scale from the bulk soil sample prior remediation