Complexation of actinides Am(III), Th(IV), Pu(IV) and U(VI) with poly-N-dentate ligands SO₃-Ph-BTP and SO₃-Ph-BTBP

The complexation of Th(IV), U(VI), Pu(IV) and Am(III) with the hydrophilic ligand SO₃-Ph-BTP^4-, and of Th(IV) and Pu(IV) with the hydrophilic SO₃-Ph-BTBP^4- ligand was studied. These new hydrophilic aromatic poly-N-dentate ligands are proposed, in the frame of recycling spent nuclear fuel, for a selective separation of actinides(III) from lanthanides(III) and from other fission products. The aim of this work was to compare the ability of the actinide ions to coordinate these N-dentate molecules. After some disappointing tests with classical spectroscopies, the method of liquid-liquid (solvent) extraction was applied to reach this goal. The extraction system consisted of two chelating ligands that competed for the actinide ions: a lipophilic tri-O-dentate neutral molecule of dioctyl-amide (TODGA) and a hydrophilic tri(or tetra)-N-dentate anion SO₃-Ph-BT(B)P^4-. The simple model we applied, well known in literature, considered chemical equilibria resulting in accumulation of the metal complexes with the lipophilic ligand in the organic phase, and those with the hydrophilic ligand - in the aqueous phase. With increasing concentration of the hydrophilic ligand (the concentration of the lipophilic ligand being constant) the equilibrium shifted towards the complexes with the hydrophilic ligand, and the distribution ratio of the metal decreased.The results have been interpreted in terms of the formation of 1:1 and 1:2 actinide complexes with tridentate SO₃-Ph-BTP^4- and only single 1:1 An(IV) complexes with tetradentate SO₃-Ph-BTBP^4- ligands in the two-phase systems studied. Two series of conditional stability constants of the complexes have been determined in our experiments: one set of the conditional stability constants, βL,i, related to 1 M nitrate media, whereas the other, βL,i, - to aqueous solutions of ionic strength I = 1 M, where the complexation by nitrates was taken into account. In the latter case, when the effect of the actinide complexation by nitrates was deducted, the conditional stability constants, βL,1, of the actinide complexes with SO₃-Ph-BTP^4- increase in the order UO₂²⁺ < Am³⁺ < Th⁴⁺ < Pu(IV), in accordance with the increasing z/r ratio (where z is the formal charge and r is the radius of the metal ion). The analysis of the β_L,i values suggests that the electrostatic effects play the major role in the formation of the complexes between the poly-N-dentate ligands and actinides ions. Concerning the complexation of Am³⁺ with the tri-N-dentate SO₃-Ph-BTP^4- ligand, if we compare our results with the literature values for the analogous Cm³⁺ complexes studied by a spectroscopic (TRLFS) technique, the stability constants of 1:1 and 1:2 complexes of Am³⁺ are much lower, and its 1:3 complex has not been found by the solvent extraction method. The stability constants of the SO₃-Ph-BTP and SO₃-Ph-BTBP complexes with the actinides(IV) have not been reported yet in literature, therefore such comparison was impossible in this case. However, the expected 1:3 complexes of Pu(IV) and Th(IV) with the SO₃-Ph-BTP^4- ligand have not been found in our solvent extraction experiments as well. Similarly, only 1:1 Pu and Th complexes with the tetra-N-dentate SO₃-Ph-BTBP^4- ligand have been found by solvent extraction, in spite of that the 1:2 complexes were also expected. These surprising results could be a result of oversimplification of the used model of extraction, and should be completed by further spectroscopic studies to identify all the complexes formed in the two-phase system studied. Nevertheless, the stability constants determined in the solvent extraction experiments ('practical' stability constants) allow us to correctly describe and to predict the behaviour of metal ions in such two-phase systems. (author)