Chemistry of Actinides. Complexes and Solids in Aqueous Medias

Actinides are f-block elements, like the lanthanides, and have a special position in Mendeleev's periodic table of elements, inserted in the 3rd column and 7th row. The actinide element series comprises 15 elements, starting with actinium (Z = 89) and ending with laurencium (Z = 103). They are radioelements in the sense that all their isotopes are unstable, and therefore radioactive. The lifetime of these isotopes is given by their half-life, which varies considerably from one isotope to another. Slightly radioactive isotopes of thorium (²³²Th) and uranium (²³⁸U and ²³⁵U) exist naturally, since their half-life is comparable to or greater than the age of the Earth, 14, 4.5, and 0.7 billion years respectively. They decay very slowly, giving rise to the formation of other actinide isotopes. These three chains of natural radioactive decay are noted by family 4n (²³²Th), 4n + 2 (²³⁸U) and 4n + 3 (²³⁵U), which corresponds to the mass number of all the nuclides in the chain. The 4n + 1 family, whose parent nuclide is ²³⁷Np, has now disappeared, as the radioactive half-life of ²³⁷Np is around 2.1 million years. In addition, natural nuclear reactions have been involved in isotope formation. Naturally occurring isotopes include ²³⁴U, ²³⁰Th and ²³¹Pa. Artificial isotopes come from nuclear bombs and nuclear reactors. These mainly produce the isotopes ²³⁷Np, ²³⁹Pu formed from ²³⁸U, ²³⁴U and ²³⁶U formed from ²³⁵U, as well as ²³⁸Pu, ²⁴⁰Pu, ²⁴¹Pu, ²⁴²Pu, ²⁴¹Am, ²⁴³Am and ²⁴⁴Cm.

The handling of actinides requires precautions to be taken with regard to their radioactivity (nature of radiation and activity), in line with safety and security requirements. Working with non-radioactive chemical analogues can be useful, so advice on analogues is given in this article. Some actinide isotopes, however, have very long half-lives, of several billion years (e.g. 4.47 billion years for ²³⁸U); they present more of a chemical hazard as heavy elements than a radiological one. Above atomic number Z = 104, the elements are called transactinides or superheavy elements, whose isotopes have lifetimes of a few seconds at most (with the exception of dubnium, some of whose isotopes have half-lives of several hours).

In industry, actinide chemistry (or geochemistry) is mainly involved in a variety of contexts or for a variety of purposes that follow the nuclear fuel cycle: prospecting for uranium or thorium deposits; ore processing; isotopic enrichment of uranium to increase fissile ²³⁵U content; fuel fabrication; behavior in high-temperature, high-pressure environments...