Cadmium (Cd), atomic number 48, is in Group IIb of the Periodic Table of the elements. It is greatest in chemical similarity to the other Group IIb elements, zinc (Zn) and mercury (Hg), with which it is commonly associated in natural geologic settings. The ground state electronic configuration of Cd can be listed in an abbreviated form as [Kr].4d 10.5s 2, where the [Kr] denotes the electronic ground state for krypton, the noble gas preceding Cd in the Periodic Table. The full ground state electron configuration for Cd is depicted graphically in Figure 2.1. Loss of the pair of 5s electrons corresponds to a total ionisation potential of 25.83 ev (Cotton and Wilkinson, 1966) a stable electronic structure of a filled outer d shell (4d 10) and a positive valence of 2. Thus Cd(II) is the most common valence of Cd in aquatic natural environments and virtually the only valence of Cd in aquatic systems (Baes and Mesmer, 1976).