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Speciation means the chemical forms of elements, such as compounds and complexes. Speciation analysis in turn means the methods to identify the forms. Speciation is essential in evaluation of the behavior of elements, including radionuclides, since different species of an element behave in a different manner, for example in their migration and retention processes in the environment. There are many factors affecting speciation, such as pH and redox potential.
Uranium has four oxidation states +III, +IV, +V and +VI, the most prevalent being +IV and +VI. In solutions, the oxidation state is governed by the redo potential, which is affected by a number of agents, such as dissolved oxygen. Elements with high oxidation states do not usually occur in aqueous solutions as free ions but form complexes with water or other completing agents. At its oxidation state +IV, uranium occurs as U4+
ion while at the oxidation state +VI it occurs as urinal ion, UO22+
. Both form strong complexes, typically with OH-
in natural waters. Most uranium minerals are readily soluble in toxic conditions and the dissolved uranium may be mobile, especially if present as urinal carbonate complexes. Accordingly, uranium is readily precipitated when conditions become reducing.