Thesis presented April 30, 2024

Abstract:
Palladium (Pd) is considered a strategic metal due to its high industrial use combined with its risks of supply. Its recovery is therefore crucial for finding secondary resources, guaranteeing its sustainability, and securing its long-term supply. A secondary Pd resource could come from spent nuclear fuels since Pd is present in significant quantities as a fission product.
In this study, we use a solid/liquid extraction technique with graphene aerogels (GAs) as a solid support to achieve the separation of the Pd(II) from other fission products. GAs are three-dimensional monolithic and macroscopic structures resulting from disordered stacking of reduced graphene oxide (rGO) sheets. GAs exhibit a high specific surface area (500 m².g^-1) and a porous structure generating a high density of potential adsorption sites. Their remarkable chemical resistance makes them suitable for treating irradiated fuel dissolution solutions.
In the context of Pd extraction, the synthesis, functionalization, and characterization of GAs with extractant functions such as amines or carboxyl, chosen according to the speciation of Pd(II) in nitric media, have been accomplished. The adsorption capacities of the materials were determined in batch mode by adjusting the extraction parameters (acidity, nitrate concentration, residence time, and initial Pd concentration). This allowed us to establish a link between the structures of the GAs and their extraction performances. The study reveals the crucial role played by the primary amine functionalization of GAs on the metal extraction efficiency and selectivity in concentrated nitric media. Experimental results reveal that a GA functionalized with a shorter alkyl chain diamine and without secondary and/or tertiary amines (GAC-EDA) provides the highest Pd(II) recovery. GAC-EDA enables the selective separation of Pd(II) from a representative irradiated fuel dissolution solution with a recovery of up to 180 mg of Pd per gram, which is globally higher than that of the solid supports referenced in the literature. Furthermore, its remarkable selectivity allows GAC-EDA to be used for the exclusive recovery of Pd(II) from a dissolution solution of irradiated fuel containing around forty metal species. Finally, an extraction mechanism involving electrostatic interactions and anionic exchange between two -NH₃⁺ groups and an anionic complex [Pd(NO₃)₄]^2- is proposed.
GAC-EDA also opens up the possibility of converting Pd(II) into an electro-catalyst. Two routes, the Polyol and Bromide Anion Exchange routes, are studied to reduce adsorbed Pd(II) into Pd(0) nanoparticles (NPs) on the GAC-EDA surface. Our results show that the NPs formed on the surface of the reduced composite after extraction are well dispersed on the GAC-EDA and do not form agglomerates. This result seems to be explained by the homogeneous grafting of the extraction sites on GAC-EDA, enabling good distribution of the adsorbed Pd(II) complexes. The Polyol route seems to promote a more efficient reduction of Pd(II) and a higher electro-active surface area (30 m².g^-1) than the Bromide Anion Exchange route. Ensuing electro-oxidation tests on formic acid and electro-reduction of CO₂ show an interesting electro-activity of this reduced compound.
To conclude, this study shows that GAs functionalized with primary amines can be a new class of adsorbent materials for the selective recovery of Pd(II) from irradiated nuclear fuels. Unlike silicas or resins, this carbon-based solid support enables adsorbed Pd(II) to be recovered as an electro-active material, validating the combination of a selective extraction and reduction process for Pd valorization.


Keywords:
Selective extraction, Graphene, coordination chemistry