Phosphate-induced lead immobilization from different lead minerals in soils under varying pH conditions

This study investigated phosphate-induced lead immobilization from different Pb minerals in soils under varying pHs. Four soils were used, including one Pb-contaminated soil (NC-Soil) and three soils spiked with litharge (PbO), cerrusite (PbCO3), or anglesite (PbSO4), referred to as PbO-soil, PbCO3-soil, and PbSO4-soil, respectively. The soils were equilibrated with KCl and Ca(H2PO4)2•H2O under pH of 3–7. At low pH (3 and 5), Pb solubility followed PbO-soil > PbCO3-soil > PbSO4-soil; while at pH = 7, it was PbSO4-soil > PbO-soil > PbCO3-soil. Phosphate decreased Pb dissolution time from >180 to <60 min and reduced soluble Pb by 67–100%. This was mostly via transformation of Pb minerals into chloropyromorphite [Pb5(PO4)3Cl]. Our results indicated that P addition can effectively transform various Pb minerals into insoluble chloropyromorphite in soils. This transformation was more significant at acidic condition (e.g., pH ≤ 5). Among the three Pb minerals tested, PbSO4 was the most effectively immobilized by P, followed by PbO and PbCO3. This study clearly demonstrated the importance of the form of Pb contamination and soil pH in determining the effectiveness of Pb immobilization in soils.