(a) SAXS profiles display the scattering intensity (I(Q)) as a function of the wave number (Q). In both graphs, the blue color refers to toluene solvent, and the red color refers to heptane solvent.

Solvent extraction process is essential for resource recycling supporting a sustainable future. It allows to selectively separate metal ions by using an extractant diluted in an organic solvent. Conventional approaches solely based on coordination chemistry were unable to explain unexpected solvent effect on selectivity and kinetics. However, the recent use of scattering techniques evidenced the strong contribution of extractant supramolecular assemblies on selectivity.

This study focused on the “supramolecular assemblies” formed during the separation of Palladium (Pd) and Neodymium (Nd) using malonamide in heptane or toluene solvent.

Small-angle X-ray scattering (SAXS) results revealed an assembly size recognition effect affecting selectivity (Fig. 1a). In toluene, the small assemblies (around 1.4 nm) only accept Pd in their core leading to high selectivity. In heptane, the large assemblies (around 3.5 nm) accept both metals leading to low selectivity. Neutron reflectometry (NR) results reveal an interfacial accumulation effect affecting extraction speed (Fig. 1b). At toluene interface, malonamide formed a thick and dilute layer leading to slow extraction. At heptane interface, malonamide formed a thin dense extractant layer leading to fast extraction.

To clarify this contribution, the international research team between France (CEA, CNRS) and Japan (JAEA, J-PARC, KEK, CROSS) used SAXS and NR. Going forward, we will use small-angle neutron scattering (SANS) at JRR-3 to develop an extraction system that can adapt its assembly size to control selectivity and extraction speed.

This work was supported by JSPS KAKENHI Grant Numbers (JP21K17911, JP22H02010, JP22K20487, JP23KK0096) and the JSPS, Japan-France Integrated Action Program (SAKURA Project) (JPJSBP120223211).

Micheau, C. et al., Organization of Malonamides from the Interface to the Organic Bulk Phase, Journal of Molecular Liquids, vol.401, 2024, 124372, 12p.

Paper URL: https://doi.org/10.1016/j.molliq.2024.124372

Press Release: A new perspective to evolve resource recycling technologies－Selectivity of rare metals and control of extraction rate by means of "supramolecular assemblies"－