Molecular prediction of lanthanide cation transfer for liquid-liquid extraction

Liquid-liquid extraction is a key hydrometallurgical process used to selectively separate the lanthanide cations, such as in the SANEX process (Selective Actinide Extraction) developed by CEA for the recycling of spent nuclear fuels. In this process, the lanthanide cations are transferred from an acid-enriched aqueous phase to an organic phase containing malonamide extractants, like DMDOHEMA, diluted in a mixture of alkanes. This ion transfer between the two phases is governed by the Gibbs energy of transfer which is measured experimentally by the distribution coefficients of the ions. This work deals with the prediction of the Gibbs energies of transfer of lanthanide cations using Steered Molecular Dynamics (SMD), an out-of-equilibrium simulation technique for Molecular Dynamics. The SMD methodology uses a moving biasing harmonic potential to steer the lanthanide cations from the aqueous phase to the organic phase, allowing for accurate sampling of the free energy landscape of the interface. We simulated water octanol interfaces as a model system for liquid-liquid extraction. In nuclear hydrometallurgy, octanol (octan-1-ol) is used as a phase modifier to prevent the formation of a heavy organic third phase associated with criticality risks. Calculating the molecular orientation revealed that the octanol molecules at the interface organize themselves in a rigid bilayer structure, as previously observed (Fig. 1), preventing the water transfer toward octanol. SMD simulations will be used to determine the Gibbs energy barrier of this interface, allowing the calculation of the water solubility in octanol. (authors)