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IN VITRO SOIL PB SOLUBILITY IN THE PRESENCE OF HYDROXYAPATITE
Citation:
Zhang**, P., J A. Ryan*, AND J. Yang. IN VITRO SOIL PB SOLUBILITY IN THE PRESENCE OF HYDROXYAPATITE. DOI: 10.1021/es97106, ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 32(18):2763-2768, (1998).
Impact/Purpose:
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Description:
The transformation of lead (Pb) in contaminated soils to pyromorphite, by the addition of phosphate minerals, may be an economic in-situ immobilization strategy which also results in a reduction of bioavailable Pb. To test this hypothesis, we conducted two sets of soil-solution experiments under constant (i.e., fixed) and 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 transformation was pH-dependent, and incomplete at relatively high pH (greater than/equal to 6). In the constant pH experiments, the soluble Pb concentration decreased with addition of apatite at pH 4 and above. However, the solubility of cerrusite (PbCO3), the major Pb mineral in this soil, still determined the solubility of soil Pb. In the hynamic pH experiments which simulated gastric pH conditions, i.e., pH variation from 2-7 within 25 or 45 minutes, both cerrusite and added apatite were dissolved at low pHs (Ph2 and 3), and chlorophyromorphite was rapidly precipitated from dissolved Pb and PO4 when the suspension pH was increased. Complete transformation of soil Pb to chlorophyromorphite occurred in the pH dynamic experiments within 25 minutes, 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. Further, 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 formation of chloropyromorphite, thus rendering ingested soil Pb non-bioavilable.
