In Vitro Soil Pb Solubility in the Presence of Hydroxyapatite

The transformation of lead (Pb) in contaminated soils to pyromorphite, by the addition of phosphate minerals, may be an economic in-situ immobilization strategy that results in a reduction of bioavailable Pb. To test this hypothesis, we conducted two sets of soil−solution experiments under constant (i.e., fixed) or dynamic (i.e., variable) pH conditions, as a function of time. In both sets of experiments, Pb-contaminated soil was reacted with synthetic hydroxyapatite in order to determine the transformation rate of soil Pb to pyromorphite and the soluble Pb level during the reaction period. In the constant pH system, the soluble Pb concentration decreased with the addition of apatite at pH 4 and above. However, the transformation was pH-dependent and incomplete at relatively high pH (≥6). The solubility of cerrusite (PbCO 3 ), the major Pb mineral in this soil, still exhibited a strong influence on the solubility of soil Pb. In the dynamic pH experiments, which simulated gastric pH conditions (i.e., pH variation from 2 to 7 within 25 or 45 min), both cerrusite and added apatite were dissolved at low pH values (pH 2 and pH 3), and chloropyromorphite was rapidly precipitated from dissolved Pb and PO 4 when the suspension pH was increased. Complete transformation of soil Pb to chloropyromorphite occurred in the pH dynamic experiments within 25 min, indicating rapid reaction kinetics of the formation of chloropyromorphite. Chloropyromorphite solubility controls the soluble Pb concentration during the entire duration of the pH dynamic experiments. This study demonstrates the importance of considering specific site conditions, such as pH, when considering evaluation of soil Pb bioavailability and in-situ immobilization of Pb in Pb-contaminated soils using phosphate amendment. Furthermore, this study demonstrates that the kinetics of conversion of soil Pb to chloropyromorphite in the presence of apatite is fast enough to occur during ingestion and that gastric pH conditions would favor the formation of chloropyromorphite, thus rendering ingested soil Pb nonbioavailable.