Overview
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ABSTRACT
The basics of the chemistry of actinides in aqueous solution is presented. Their electronic structure enables them to adopt several stable oxidation states, in the form of simple cations and actinyl molecular ions, which offers similarities and differences with lanthanides or transition metals. The speciation of actinides can be assessed following a thermodynamic approach based on species formation equilibria. After a brief presentation of thermodynamics, we focus in this article on the main chemical reactions for the actinides Th, U, Np, Pu, Am, Cm, in aqueous media: oxidation-reduction, hydrolysis, interactions with inorganic ligands, solubility, sorption onto mineral surfaces, and finally interactions with organic ligands.
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Thomas VERCOUTER: Research engineer, senior expert - French Atomic Energy and Alternative Energies Commission (CEA), Saclay, France - with contributions from Pierre Vitorge, retired CEA Research Director.
INTRODUCTION
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 ( 232 Th) and uranium ( 238 U and 235 U) 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 ( 232 Th), 4n + 2 ( 238 U) and 4n + 3 ( 235 U), which corresponds to the mass number of all the nuclides in the chain. The 4n + 1 family, whose parent nuclide is 237 Np, has now disappeared, as the radioactive half-life of 237 Np is around 2.1 million years. In addition, natural nuclear reactions have been involved in isotope formation. Naturally occurring isotopes include 234 U, 230 Th and 231 Pa. Artificial isotopes come from nuclear bombs and nuclear reactors. These mainly produce the isotopes 237 Np, 239 Pu formed from 238 U, 234 U and 236 U formed from 235 U, as well as 238 Pu, 240 Pu, 241 Pu, 242 Pu, 241 Am, 243 Am and 244 Cm.
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...
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KEYWORDS
solution chemistry | complexation | speciation | redox
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Actinide chemistry
Bibliography
Websites
IRSN radionuclide data sheets
https://www.irsn.fr/FR/Larecherche/publications-documentation/fiches-radionucleides
Nuclear Energy Agency's Thermochemical DataBase (TDB) project
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