Transformation of Pb(II) from cerrusite to chloropyromorphite in the presence of hydroxyapatite under varying conditions of pH

The soluble Pb concentration and formation of chloro-pyromorphite [Pb-5(PO4 )(3)Cl] were monitored during the reaction of cerrusite (PbCO3), a highly b ioavailable soil Pb species, and hydroxyapatite [Ca-5(PO4)(3)OH] at various P/Pb molar ratios under constant and dynamic pH conditions. Under pH-const ant systems at pH 4 and below, the dissolution rates of both cerrusite and apatite were rapid, and complete conversion of cerrusite to chloropyromorph ite occurred within 60 min when the amount of phosphate added via apatite w as stoichiometrically equal to that needed to transform all added Pb into c hloropyromorphite. The concentration of soluble Pb depended upon the solubi lity of chloropyromorphite. The dissolution rates of apatite and cerrusite decreased with increasing pH, and the transformation was incomplete at pH 5 and above in the 60-min reaction period. The soluble Ph level, therefore, was determined by the solubility of cerrusite. In the pH-dynamic system, wh ich simulated the gastrointestinal tract (GI tract), a complete transformat ion of Pb from cerrusite to chloropyromorphite was achieved due to the comp lete dissolution of apatite and cerrusite at the initial law pH. In both th e constant and dynamic pH systems XRD analysis indicated that chloropyromor phite was the exclusive reaction product. The differences in transformation rate and the Pb solubility between the constant and dynamic pH systems ind icate the significance of kinetics in controlling the bioavailability of Pb and the potential for the reaction to occur during ingestion.