How phase modifiers disrupt third-phase formation in solvent extraction solutions

• Published in: Solvent extraction and ion exchange Vol. 39; no. 2; pp. 204 - 232
• Main Authors: Berthon-Nigond, Laurence, Paquet, Amaury, Saint-Louis, Georges, Guilbaud, Philippe
• Format: Journal Article
• Language: English
• Published: Taylor & Francis 2021
• Subjects: Analytical chemistry; Chemical Sciences; Coordination chemistry
• ISSN: 0736-6299; 1532-2262
• DOI: 10.1080/07366299.2020.1831782

The structure of complex organic solutions based of N,N,N’N’-tetraoctyldiglycolamide (TODGA) used in actinides separation processes was characterized after water and neodymium extraction. The organic phases were characterized using FTIR spectroscopy, ESI-MS spectrometry, Small and Wide Angle X-ray Scattering (SWAXS) and molecular dynamic simulations. The role of the “phase modifier” like 1-octanol (used in the innovative SANEX process) or the “co-extractant” like DMDOHEMA (used, for example, in the EURO-GANEX process) added in the organic phase to avoid a third phase formation was investigated at molecular and supramolecular scale. Without metallic cation, in 0.2 mol.L-1 TODGA solution in n-heptane, TODGA molecules are organized in monomers and small aggregates containing mainly two or three TODGA molecules. After Nd(NO3)3 extraction, complexes or aggregates are formed where each neodymium cation is surrounded in average by 2 to 4 TODGA, 3 nitrate ions and 1 to 2 water molecules. Polymetallic species (aggregates) may also be formed. Adding 5%Vol 1-octanol or 0.5 mol.L-1 DMDOHEMA to 0.2 mol.L-1 TODGA solution in n-heptane leads to the formation of mixed aggregates containing TODGA and octanol molecules or TODGA and DMDOHEMA molecules. These molecules both interact directly with the extracted metallic cation in the organic phase, modifying the structure of the neodymium species, and improving their solubility in this organic phase. At least, one TODGA molecule remains in the cation first coordination sphere in both cases and preserves its significant role in the complexation/selectivity of the cations. But both 1-octanol and DMDOHEMA have a role in the speciation of the cations in the organic phase: 1-octanol replaces water molecule in the first coordination sphere of Nd, it acts as a co-surfactant; DMDOHEMA drives the organization, DMDOHEMA molecules replace partially TODGA in the first coordination sphere of Nd.