Acid tolerant covalently functionalized graphene oxide for the selective extraction of Pd from high-level radioactive liquid wastes

The severely depleting reserves of highly demanded palladium (Pd) induce the growing pressure on its recycling. One of the potential secondary sources of Pd is a high-level liquid waste (HLLW) produced from spent nuclear fuel. HLLW contains several metal ions along with Pd in 3-4 M HNO ₃ medium. The preparation of highly acid tolerant and efficient functional materials is a challenging task in the solid phase recovery of noble metals. We have explored covalently functionalized graphene oxide (GO) to overcome this issue. GO was covalently functionalized with 1-(3-aminopropyl)imidazole (ImGO) (N-donor) and 2-methylthiophene (ThpGO) (S-donor) through amide or amine formation at different reactive sites. The prepared materials before and after Pd recovery were thoroughly characterized using a variety of state-of-the-art techniques. Solid-phase extraction experiments revealed that the Pd was recovered with remarkable selectivity from HLLW-like solutions in 3 M HNO ₃ using both the functionalized materials. The extraction efficiency was found to be higher than that of any of the solid-state materials reported to date. The sorption of Pd onto ImGO and ThpGO was best expressed by the Freundlich isotherm model. The advantages of the prepared materials are twofold: the first one is the high tolerance to harsh acidic conditions, and the second one is the higher selectivity and efficiency for Pd. The sorption of Pd onto ImGO was achieved by complete anion exchange with the nitrate ion, whereas ThpGO recovered Pd through a co-ordination mechanism. The results demonstrated that the developed covalently functionalized GO was the first choice material for the efficient and selective recovery of Pd.