UNDERSTANDING VARIATION IN PARTITION COEFFICIENT, Kd ...

Percent Distribution
100
80
60
40
20
0
o
PbSO4 (aq)
Pb 2+
+
PbHCO3
3 4 5 6 7 8 9 10
5.29
pH
o
PbCO3 (aq)
o
Pb(OH)2 (aq)
2+
Pb(CO3)2
PbOH +
Figure 5.2. Calculated distribution of lead aqueous species as a function of pH for the
water composition in Table 5.1. [The species distribution is based on a
concentration of 1 µg/l total dissolved lead.]
Lead may also exist in soils as solid-solution phases. Solid solutions are defined as solid phases in
which a minor element will substitute for a major element in the mineral structure. Depending on
the degree of substitution and the overall solubility of the solid-solution phase, the equilibrium
solubility of the minor element in the solid solution phase will be less than the solubility of the
solid phase containing only the minor element (pure phase). For instance, lead may occur as a
minor replacement in barite [Ba (1-x)Pb xSO 4], apatite [Ca (1-x)Pb x(PO 4) 3OH], calcite [Ca (1-x)Pb xCO 3],
and iron sulfides, [Fe (1-x)Pb xS] (Driesens, 1986; Goldschmidt, 1954; Nriagu and Moore, 1984;
Rickard and Nriagu, 1978). Consequently, the equilibrium solubility of lead controlled by these
phases will be less than the concentrations controlled by corresponding pure phases, namely
PbSO 4, Pb 5(PO 4) 3OH, PbCO 3, and PbS, respectively.