The Effect of Compost and Apatite on Pb Distribution in the Pb- Contaminated Soil

There are various techniques for rehabilitating soils contaminated by heavy metals, such as physical treatment, chemical treatment, and phytoremediation. Pb-contaminated soils are mostly treated with soil mixing, soil dressing, or soil washing. In this study, hydroxyapatite powder and organic compost of different mix ratios were added to two kinds of high-concentration synthesized Pb-contaminated soils (1000 mg/kg), which were processed with sequential extraction schemes after different incubation periods in order to examine the distribution of Pb forms in them. The form distributions analyzed included the exchangeable form Pb (PbE), Fe-Mn oxidation form Pb (PbFe-Mn), organic bonded form Pb (PbO), and residual form Pb (PbR). The findings indicated that after stabilization for one month, the alkaline Tk series soil was less susceptible to Pb contamination when no hydroxyapatite and compost was applied, which made it more difficult for Pb in the soil to interact with cations and trigger exchange reactions. Next, the PbR concentrations of the two soils tested were higher after hydroxyapatite was added. This result suggested that hydroxyapatite helped to stabilize the Pb in the soil samples and prevented it from precipitating, thereby controlling the Pb concentration at a certain level to avoid contamination. Lastly, when hydroxyapatite and compost of different mix ratios were applied to the two kinds of Pb-contaminated soils, we found that the PbE concentration of the Tk series soil declined as the concentration of compost increased, suggesting that increasing the compost concentration raised the pH value of the soil and enhanced its fertility. On the other hand, the PbE concentration of the Lp series soil increased as the compost concentration rose, indicating that an increase in compost concentration hampered plants from absorbing soluble nutrients in the soil, thereby making it easier for Pb to precipitate via cation exchange and resulting in Pb contamination.