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Abstract

The implementation of actinide recycling processes is considered in several countries, aiming at the reduction of long-term radiotoxicity and heat load of used nuclear fuel. This requires the separation of the actinides from the fission and corrosion products. The separation of the trivalent actinides (An(III)) Am(III) and Cm(III), however, is complicated by the presence of the chemically similar fission lanthanides (Ln(III)). Hydrophilic N-donor ligands are employed as An(III) or Am(III) selective complexing agents in solvent extraction to strip An(III) or Am(III) from an organic phase loaded with An(III) and Ln(III). Though they exhibit excellent selectivity, the complexation chemistry of these ligands and the complexes formed during solvent extraction are not sufficiently characterized. In the present thesis the complexation of An(III) and Ln(III) with hydrophilic N-donor ligands is studied by time resolved laser fluorescence spectroscopy (TRLFS), UV/Vis, vibronic sideband spectroscopy and solvent extraction. TRLFS studies on the complexation of Cm(III) and Eu(III) with the Am(III) selective complexing agent SO3-Ph-BTBP (tetrasodium 3,3’,3’’,3’’’-([2,2’-bipyridine]-6,6’-diylbis(1,2,4-triazine-3,5,6-triyl))tetrabenzenesulfonate) revealed the formation of [M(SO3-Ph-BTBP)n](4n-3)- complexes (M = Cm(III), Eu(III); n = 1, 2). The conditional stability constants were determined in different media yielding two orders of magnitude larger 2-values for the Cm(III) complexes, independently from the applied medium. A strong impact of ionic strength on the stability and stoichiometry of the formed complexes was identified, resulting from the stabilization of the pentaanionic [M(SO3-Ph-BTBP)2]5- complex with increasing ionic strength. Thermodynamic studies of Cm(III)-SO3-Ph-BTBP complexation showed that the proton concentration of the applied medium impacts the hydration, resulting in more negative reaction enthalpies of the complexation in acidic media. Utilizing absorption spectroscopy, the formation of the [Am(SO3-Ph-BTBP)2]5- complex was quantified. The conditional stability constant of this complex is by log2 = 0.2 larger than of the analogous Cm(III) complex. This is in excellent agreement with the separation factor derived from solvent extraction (SFCm(III)/Am(III) = 1.6). Solvent extraction studies were performed in the SO3-Ph-BTBP/TODGA system (N,N,N’,N’-tetraoctyl-diglycolamide) at varied nitric acid concentration, SO3-Ph-BTBP concentration and temperature, demonstrating the separation of Am(III) from Cm(III) and the light Ln(III) with separation factors of SFCm(III)/Am(III) = 2.6-3.0 and SFEu(III)/Am(III) up to 1100. Furthermore, the VII selectivity of the SO3-Ph-BTBP/TODGA system is not affected by Eu(III) concentrations of 2 - 15 mmol/L or by hydrolysis after contact with nitric acid for several weeks. Radiolytic degradation occuring at  doses > 10 kGy, however, limits the recycling of SO3-Ph-BTBP. In case of SO3-Ph-BTPhen (tetrasodium 3,3',3'',3'''-((1,10-phenanthroline-2,9-diyl)bis(1,2,4-triazine-3,5,6-triyl))tetrabenzenesulfonate) the coordinating nitrogen atoms of the pyridine moieties of SO3-Ph-BTBP are fixed in cis position by the phenantroline moiety. This results in increased complex stability for lipophilic derivatives. However, TRLFS studies on SO3-Ph-BTPhen and SO3-Ph-BTBP in aqueous solution at pH 3 showed that the preorganization has no significant impact on the stability of the formed complexes. Due to the lower pKa of SO3-Ph-BTPhen the log2 values in acidic media are higher than for SO3-Ph-BTBP. The selectivity of the complexing agent, however, remains unaltered. The Cm(III) and Eu(III) 1:1 complexes formed with the decadentate Am(III) selective complexing agent H4TPAEN (N,N,N’N’-tetrakis[(6-carboxypyridin-2-yl)methyl]ethylenediamine) were investigated using TRLFS. The conditional stability constants of these complexes differ by 1.9 orders of magnitude. This is in excellent agreement with the separation factor derived from solvent extraction (SFEu(III)/Cm(III) ≈ 100). The complexation of M(III) with H4TPAEN is driven by the high positive reaction entropy. PTD (2,6-bis[1-(propan-1-ol)-1,2,3-triazol-4-yl]pyridine) is a charge neutral, CHON compatible complexing agent designed for the selective complexation of An(III). 1:1, 1:2 and 1:3 complexes with Cm(III) and Eu(III) were characterized in aqueous solution at pH 3 and in 0.44 mol/L HNO3. The difference of log3 = 4 for the Cm(III) complexes in the different media is primarily caused by the protonation of PTD. Ionic strength effects and the concurring complexation of Cm(III) by nitrate contribute to a lesser extent. In solvent extraction lower separation factors were observed than calculated from the difference of the log3 values of Cm(III) and Eu(III) (SF(experimental)Eu(III)/Cm(III) ≈ 200 vs. SF(calculated)Eu(III)/Cm(III) ≈ 1000). This was proven to be caused by the presence of lower coordinated species during solvent extraction. The 5D0 → 7F0 transition of more than 20 Eu(III)-BT(B)P/BTPhen complexes was studied. A correlation between the shift of the 7F0 emission band and the number of coordinating N-donors was established, allowing to easily identify the stoichiometry of Eu(III) N-donor complexes. Furthermore, the strong nephelauxetic effect observed is an excellent proof of a covalent share in the Eu(III)-N-donor bond. The results obtained in the present thesis represent a valuable contribution to the fundamental understanding of the complexation of trivalent actinides and lanthanides with hydrophilic N-donor ligands and, thus, to the development of new processes for the recycling of An(III).