In nitric acid solutions, such as PUREX raffinates (where [HNO3] > 3 mol. L-1), the 4f lanthanide metallic cations (Ln) and the 5f americium and curium metallic cations (An) predominantly show the same oxidation state, +III, and many similar physical and chemical properties (Nash, 1993, 1994, Beitz, 1994, Morss, 1994, Marcus, 1997):

• They are considered as ‘hard acids’ in Pearson’s theory (Pearson, 1963).

• The 4f and 5f orbitals have a rather small radial extension and are more or less protected by the saturated lower electron orbitals, respectively

the 5s2-5p6 for the lanthanides and the 6s2-6p6 for the actinides. Thus, the nf electrons scarcely interact with electrons of neighbouring ligands and their electronic properties are only slightly affected by their environments.

• The ionic radius shortens along the 4f and 5f series as the atomic number increases. Thus, it is easy to predict the higher electrostatic reactivity (formalized by the ionic potential closely linked to the charge density) of an element of higher atomic number, Z, compared to that of an element of lower atomic number in the periodic table.

• Since Ln(III) and An(III) have the same positive charge (+3), their discrimination through solvent extraction involving ‘hard bases’ (e. g., ligands bearing oxygen donor atoms in their structures) will mainly be due to geometric and/or steric hindrance reasons: the better the fitting of a metallic cation radius with the cavity size of the complexing/extract — ing agent or its coordinating site, the better the discrimination. However, the separation of the two series of trivalent elements will not be com­plete because of the similarities in the ionic radii among 4f and 5f elements.

• Ln(III) and An(III) are highly hydrated in aqueous media: 8 to 9 water molecules can be numbered in their inner-coordination spheres, as com­pared to 4 to 5 only in the case of penta- and hexavalent actinides. It is, however, admitted, although difficult to demonstrate by a structural proof, that an outer-coordination sphere of water molecules exists and interacts with the water molecules present in the inner-coordination spheres of the metallic cations through hydrogen bonds.

• As for other metallic cations, hydration of the 4f and 5f trivalent ele­ments is of capital importance in their extraction mechanisms, since they can be partly or completely dehydrated while being extracted in organic solvents.

• The coordination numbers in complexes of trivalent lanthanides and actinides vary from 6 to 12, depending on the bonding chemical system involved.

However, a slight chemical behaviour difference does exist between the two series of trivalent elements: the 4f orbitals of the lanthanides are slightly more localized around their nuclei than the 5f orbitals of the actinides, which can consequently interact more easily with their electronic environ­ments than the corresponding lanthanides (Beitz, 1994, Morss, 1994). Unlike trivalent lanthanides, trivalent actinides create stronger chemical bonds with ligands bearing ‘softer’ donor atoms than oxygen, such as for instance sulphur or nitrogen (Musikas et al., 1983, Musikas, 1984). The drawback of hydrophilic and/or lipophilic compounds containing sulphur and/or nitro­gen atoms is their usually strong affinity for protons in acidic media.

Although more rational (considering the inventory of elements present), the direct and selective extraction of An(III) from PUREX raffinates has been the most challenging of the unresolved research topics radiochemists have addressed for the past 50 years throughout the world. This is why, except for specific single-step processes, such as SETFICS (Nakahara et al., 2007) or DIAMEX-SANEX (Madic et al., 2002) processes which will not be covered by this chapter, most of the strategies adopted to selectively recover trivalent minor actinides from PUREX raffinates show the same two-step process approach:

1. The co-extraction of the An(III) together with the Ln(III) in a front — head process, such as the TRUEX, DIAMEX, or TODGA processes, which make use of oxygen donor extractants (‘hard’ bases), such as carbamoyl phosphonate/phosphine oxide or diamide compounds, and specific scrubbings with hydrophilic masking agents to achieve the sep­aration of An(III) and Ln(III) from the rest of the fission products (FP).

2. The partition of An(III) from Ln(III) in a second cycle process, either through the selective stripping of the An(III) thanks to a hydrophilic highly selective ligand, or through the selective extraction of the An(III) thanks to a lipophilic highly selective extractant (both types of com­pounds bearing ‘soft base’ electron-donor atoms, the use of which is made possible by the lower acidity of the feeds coming from the front- head processes than that of the PUREX raffinates).