Understanding Variation in Partition Coefficient, Kd, Values Volume II

5.2.3 Aqueous Speciation
Cadmium forms soluble complexes with inorganic and organic ligands resulting in an increase of
cadmium mobility in soils (McLean and Bledsoe, 1992). The distribution of cadmium aqueous species
was calculated using the water composition described in Table 5.1 and a concentration of 1 micro g/l
total dissolved cadmium (Table 5.2). Hem (1985, p. 142) lists this value as a median concentration of
dissolved cadmium based on the reconnaissance study of Duram et al. (1971) of metal concentrations
in surface waters in the United States. These MINTEQA2 calculations indicate that cadmium
speciation is relatively simple. In groundwaters of pH values less than 6, essentially all of the dissolved
cadmium is expected to exist as the uncomplexed Cd 2+ ion (Figure 5.1). The aqueous species included
in the MINTEQA2 calculations are listed in Table 5.3. As the pH increases between 6 and 8.2,
cadmium carbonate species [CdHCO 3 + and CdCO3 " (aq)] become increasingly important. At pH
values between 8.2 and 10, essentially all of the cadmium in solution is expected to exist as the neutral
complex CdCO 3 " (aq). The species CdSO4 " (aq), CdHCO3 + , CdCl + , and CdOH + are also present, but
at much lower concentrations. The species distribution illustrated in Figure 5.1 does not change if the
concentration of total dissolved cadmium is increased from 1 to 1,000 micro g/l.
Table 5.3. Cadmium aqueous species included
in the speciation calculations.
Aqueous Species
Cd 2+
CdOH + , Cd(OH) 2 " (aq), Cd(OH)3 - , Cd(OH)4 2- , Cd2OH 3+
CdHCO 3 + , CdCO3 " (aq), Cd(CO3) 3 4-
CdSO 4 " (aq), Cd(SO4) 2 2-
CdNO 3 +
CdCl + , CdCl 2 " (aq), CdCl3 - , CdOHCl " (aq)
CdF + , CdF 2 " (aq)
5.7