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UNDERSTANDING VARIATION IN PARTITION COEFFICIENT, Kd ...

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eactions. Complexation enhances the solubility of lead-bearing solid phases. This enhancement<br />

in solubility is dependent on the strength of complexation [indicated by the magnitude of stability<br />

constant] and the total concentrations of complexing ligands. Also, as will be discussed shortly,<br />

adsorption of lead is affected by the type, charge, and the concentration of lead complexes present<br />

in solution. Cationic lead species, especially Pb 2+ and its hydrolysis species, adsorb more<br />

commonly than anionic lead complexes.<br />

5.5.4 Dissolution/Precipitation/Coprecipitation<br />

Lead solids in the environment may occur in a number of mineral forms (Rickard and Nriagu<br />

1978; Mattigod et al., 1986; Zimdahl and Hassett, 1977). However, these authors have identified<br />

a limited number of secondary lead minerals that may control the concentrations of dissolved lead<br />

in soil/water environments. If the concentration of dissolved lead in a pore water or groundwater<br />

exceeds the solubility of any of these phases, the lead-containing solid phase will precipitate and<br />

thus control the maximum concentration of lead that could occur in the aqueous phase.<br />

According to Rickard and Nriagu (1978), under oxidizing conditions, depending on pH and ligand<br />

concentrations, cerussite (PbCO 3), hydrocerussite [Pb 3(OH) 2(CO 3) 2], anglesite (PbSO 4), or<br />

chloropyromorphite [Pb 5(PO 4) 3Cl] may control aqueous lead concentrations. A review paper by<br />

McLean and Bledsoe (1992) included data which showed that lead concentrations in a calcareous<br />

soil was controlled by lead-phosphate compounds at lower pH and by mixed mineral phases at pH<br />

values exceeding 7.5. A study conducted by Mattigod et al. (1986) indicated that the mineral<br />

leadhillite [Pb 4SO 4(CO 3) 2(OH) 2] may be the solubility controlling solid for lead in a mine-waste<br />

contaminated soil.<br />

5.27

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