Thesis for the Degree of Doctor of Philosophy - DTU Orbit

1. Introduction
1.1 Iodine in nature
1.1.1 Stable iodine ( 127 I)
In 1811 the French chemist Bernard Courtois discovered iodine by a fortuitous accident when he sublimed
the element from seaweed ash with sulphuric acid. The name comes from a Greek word for "violet colored".
Due to the multiple oxidation states -1,0,+1,+3, +5 and +7 (Table 1) and an oxide IO2 with an oxidation
state of +4 its chemistry in aqueous solutions is quite complex (Figure 1). Except the oxide IO2 (+4), all
above forms are thermodynamically stable.
Fig. 1 Eh–pH diagram for iodine in water at 25 ◦C. This figure is reproduced from Liu & Gunten, 1988 and
Michel, et al., 2004.
In the earth’s surface environments such as oceans and seas, iodine concentration ranges between 40 and 65
µg/l (Buraglio, 2000a). In seawater iodine exists mainly as iodide (-1), iodate (+5) and to a lesser extent as
organic iodine (Wong and Zhang, 2003; Wong, 1991), Figure 2. In oxic marine waters iodate is the
thermodynamically stable form, while in anoxic seawater, such as the deep waters of the Black Sea and parts
of the Baltic Sea, iodide should constitute the major species of iodine (in Paper I; Tian and Nicolas, 1995).
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