Understanding Variation in Partition Coefficient, Kd, Values Volume II

A third mechanism involves pyrite that may be present in soils or sediments and gets oxidized when
exposed to air. 1 The pyrite oxidizes to form FeSO 4, which generates high amounts of acidity when
reacted with water. The decrease in the pH results in the dissolution of cadmium minerals and increase
in the dissolved concentration of cadmium. This process is consistent with the study by Kargbo (1993)
of acid sulfate clays used as waste covers.
5.2.5 Sorption/Desorption
At high solution concentrations of cadmium (>10 mg/l), the adsorption of cadmium often correlates with
the CEC of the soil (John, 1971; Levi-Minzi et al., 1976; McBride et al., 1981; Navrot et al., 1978;
Petruzelli et al., 1978). During cation exchange, cadmium generally exchanges with adsorbed calcium
and magnesium (McBride et al., 1982). The ionic radius of Cd 2+ is comparable to that of Ca 2+ and, to
a lesser extent, Mg 2+ . At low solution concentrations of cadmium, surface complexation to calcite
(McBride, 1980) and hydrous oxides of aluminum and iron (Benjamin and Leckie, 1981) may be the
most important adsorption mechanism. Both Cd 2+ and possibly CdOH + may adsorb to aluminum- and
iron-oxide minerals (Balistrieri and Murray, 1981; Davis and Leckie, 1978).
As with other cationic metals, cadmium adsorption exhibits pH dependency. The effect of pH on
cadmium adsorption by soils (Huang et al., 1977), sediment (Reid and McDuffie, 1981), and iron
oxides (Balistrieri and Murray, 1982; Levy and Francis, 1976) is influenced by the solution
concentration of cadmium and the presence of competing cations or complexing ligands. At low
cadmium solution concentrations, sharp adsorption edges (the range of pH where solute adsorption
goes from ~0 to ~100 percent) suggests that specific adsorption (i.e., surface complexation via a strong
bond to the mineral surface) occurs. Under comparable experimental conditions, the adsorption edge
falls at pH values higher than those for lead, chromium, and zinc. Thus, in lower pH environments,
these metals, based on their propensity to adsorb, would rank as follows: Pb > Cr > Zn > Cd. This
order is inversely related to the pH at which hydrolysis of these metals occurs (Benjamin and Leckie,
1981).
Competition between cations for adsorption sites strongly influences the adsorption behavior of
cadmium. The presence of calcium, magnesium, and trace metal cations reduce cadmium adsorption
by soils (Cavallaro and McBride, 1978; Singh, 1979), iron oxides (Balistrieri and Murray, 1982),
manganese oxides (Gadde and Laitinen, 1974), and aluminum oxides (Benjamin and Leckie, 1980).
The extent of competition between cadmium and other ions depends on the relative energies of
interaction between the ions and the adsorbing surface, the concentrations of the competing ions, and
solution pH (Benjamin and Leckie, 1981; Sposito, 1984). The addition of copper or lead, which are
more strongly adsorbed, slightly reduces cadmium adsorption by iron and aluminum oxides, suggesting
that copper and lead are preferentially adsorbed by different surface sites (Benjamin and Leckie,
1 D. M. Kargbo (1998, personal communication).
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