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 chloropyromorphite [Pb5(PO4)3Cl] were monitored during the reaction of cerrusite (PbCO3), a highly bioavailable soil Pb species, and hydroxyapatite [Ca5(PO4)3OH] at various P/Pb molar ratios under constant and dynamic pH conditions. Under pH-constant systems at pH 4 and below, the dissolution rates of both cerrusite and apatite were rapid, and complete conversion of cerrusite to chloropyromorphite occurred within 60 min when the amount of phosphate added via apatite was stoichiometrically equal to that needed to transform all added Pb into chloropyromorphite. The concentration of soluble Pb depended upon the solubility 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 Pb level, therefore, was determined by the solubility of cerrusite. In the pH-dynamic system, which simulated the gastrointestinal tract (GI tract), a complete transformation of Pb from cerrusite to chloropyromorphite was achieved due to the complete dissolution of apatite and cerrusite at the initial low pH. In both the constant and dynamic pH systems XRD analysis indicated that chloropyromorphite was the exclusive reaction product. The differences in transformation rate and the Pb solubility between the constant and dynamic pH systems indicate the significance of kinetics in controlling the bioavailability of Pb and the potential for the reaction to occur during ingestion.