The effects of amidophosphonate ligand immobilization method on the uranium extraction efficiency of functionalized silica

• Grafted and impregnated silica supports investigated as uranium extractants. • Selective uranium extraction from sulfate acid solutions with competing cations. • Both materials show similar uranium extraction mechanism in low sulfates conditions. • Grafted functional groups compete poorly with high concentrated sulfates. • High mobility of impregnated groups improves efficiency and selectivity. The effects on the uranium extraction efficiency of the functionalization method of three kinds of functionalized silica materials were evaluated in low sulfate (pH = 2) and high sulfate (pH = 1) solutions, with compositions representative of typical effluents from uranium mines and leaching solutions from uranium ore treatment, respectively. Silica supports were functionalized with amidophosphonate moieties either by peptide grafting or by non-covalent impregnation. Prior to impregnation, the surface of the silica supports was modified with either alkyl chains or ionic liquid groups. The selectivity of the modified silica supports for uranium was determined in the presence of iron and molybdenum as competing cations. Our results show that both incorporation methods yield materials with good extraction efficiency and selectivity in low sulfate solutions. EXAFS data indicate that uranyl species have to first be desulfurized to be extracted by the phosphonate ligand. This process appears more energetically favorable for the impregnated ligands than for the grafted ones under high sulfate conditions; likely because the grafted ligands compete less efficiently with the bidentate sulfates coordination in uranyl coordination sphere. Impact of immobilization method of functional groups onto silica support to provide a promising uranium extraction capacity and a high selectivity versus iron and molybdenum.