Adsorption of Uranium over NH₂-Functionalized Ordered Silica in Aqueous Solutions

The aim of this work was to obtain an in-depth understanding of the U(VI) adsorption mechanism over amino-functionalized mesoporous silica SBA-15 and highlights its high efficiency in aqueous media for U(VI) removal and preconcentration. The samples were synthesized and functionalized by both grafting and co-condensation methods, using different alkyl-substituted amine groups and were characterized using X-ray diffraction, N₂ physisorption, Fourier transform infrared spectroscopy, and elemental C-H-N-S analyses. The properties for U(VI) adsorption were evaluated under discontinuous conditions, with the determination of the effect of several parameters (initial pH, contact time, initial U(VI) concentration, functionalization method, and organic moiety composition). U(VI) adsorption over grafted materials reached equilibrium at around 30 min, with a maximum adsorption capacity of 573 mg_U·g_ads^-1 for the most efficient material at its optimal adsorption pH (equal to 6) at 20 °C. Functionalized materials by grafting exhibit better adsorption capacities than co-condensed samples because of higher function surface density and function availability. U(VI) adsorption mechanisms were also studied by measuring the electrophoretic mobilities of the particles, aqueous U(VI) speciation, in situ attenuated total reflection infrared and Raman spectroscopies, and transmission electron microscopy analysis. U(VI) adsorption occurred through the formation of an inner sphere complex. The localization of adsorbed U(VI) has also been determined inside of the mesopores, with the formation of several particles on the nanometer scale, in the size of U-hydroxy phases. Besides, the study of the reusability of amino-functionalized SBA-15 by applying adsorption-desorption cycles was also conducted. The adsorption capacity of the material remains stable for at least four adsorption-desorption cycles without any noticeable capacity decrease.