New insights into the uranium adsorption behavior of mesoporous SBA-15 silicas decorated with alkylphosphine oxide ligands

Alkylphosphine oxides functionalized mesoporous silicas (SBAVx-P(O)Pr-2) were prepared by a two-step process involving: (1) co-condensation synthesis of vinyl-containing mesoporous silica (SBAVx), and (2) addition reaction with secondary n-propylphosphine oxide to anchor the alkylphosphine oxide ligands. The functionalized mesoporous silicas exhibited ordered mesoporosity, high specific surface area, and narrow pore size distribution. Due to the strong binding ability of the alkylphosphine oxide ligands and the surface silanol groups, the mesoporous silicas showed effective adsorption toward uranyl ions in HNO3 solutions with a wide-range of concentrations. The existence of numerous ordered meso-pores in the silica matrix facilitated the mass transfer, resulting in fast adsorption kinetics with an equilibrium obtained in similar to 30 minutes. The relationship between structural parameters (ligand density, pore and network architecture) and uranyl adsorption performance was studied. Meanwhile, the adsorption mechanism associated with the variable role of the alkylphosphine oxide ligands and silanol groups in different acidic solutions was discussed. Besides, cycling experiments by column operation revealed that the mesoporous silicas had a reliable performance and could be conveniently recovered by elution with K2CO3 solutions. This paper highlights the potential application of mesoporous silicas functionalized with alkylphosphine oxide ligands as promising candidates for the preconcentration and adsorption of uranium from acidic or neutral aqueous solutions.