Effects of Impregnated Amidophosphonate Ligand Concentration on the Uranium Extraction Behavior of Mesoporous Silica

Abstract

A series of solid-phase uranium extractants were prepared by post-synthesis impregnation of a mesoporous silica support previously functionalized with octyl chains by direct silanization. Five materials were synthesized with 0, 0.2, 0.3, 0.4 and 0.5 mmol of the amidophosphonate ligand DEHCEBP per gram of functionalized solid, and the effect of the ligand concentration on the uranium extraction efficiency and selectivity of the materials was investigated. Nitrogen adsorption–desorption data show that with increasing ligand loadings, the specific surface area and average pore volume decrease as the amidophosphonate ligand fills first the micropores and then the mesopores of the support. Acidic uranium solutions with a high sulfate content were used to replicate the conditions in ore treatment leaching solutions. Considering the extraction kinetics, the equilibration time was found to increase with the ligand concentration, which can be explained by the clogging of micropores and the multilayer arrangement of the DEHCEBP molecules in the materials with their highest ligand contents. The fact that the equilibrium ligand/uranium ratio is about 2 mol/mol regardless of the ligand concentration in the material suggests that all the ligand molecules remain accessible for extraction. The maximum uranium extraction capacities ranged from 30 mg∙g−1 at 0.2 mmol∙g−1 DEHCEBP to 54 mg∙g−1 in the material with 0.5 mmol∙g−1 DEHCEBP. These materials could therefore potentially be used as solid-phase uranium extractants in acidic solutions with high sulfate concentrations.

Functionalized graphene nanoplatelets: a promising adsorbent for solid-phase uranium extraction

“…However they had the same equilibrium ligand to uranium molar ratio (L/U B 2) whatever the ligand content was (0.2-0.5 mmol g À1 ), suggesting minimal hindrance of the silica support in the extractions mechanisms. 16 Here, the higher equilibrium L/U ratios (lower uranium extraction efficiency) at lower ligand concentrations (Fig. 11) point toward a possible chemical or physical interaction between the graphene-based support and the DEHCEBP molecules during extraction processes.…”

“…12 These studies and other results for strontium 13 and CO 2 adsorption 14,15 show that impregnated materials generally have higher adsorption capacities than grafted ones, presumably because of the absence of conformational constraints. More recently, 16 we observed in a series of silica-based uranium extractants impregnated with 10-20 wt% amidophosphonate ligand that a higher ligand concentration had a negative impact on the extraction kinetics.…”

“…This suggests that the organic ligand tends to fill the micropores before the mesopores in the support, as previously observed for silica-based impregnated materials. 16 Furthermore, the fact that the pores in samples Imp-1.0 and Imp-1.2@GNPs are almost completely filled by the ligand (Fig. 7) explains why they have a very low specific surface area (Table 1).…”

“…Following the same reasoning based on the Tanford formula (eqn ( 4)) as previously, 16 assuming that the movement of the impregnated ligands is not restricted and that a DEHCEBP molecule is about 2 nm long, each molecule occupies a circular area of about 3.3 nm 2 . The full saturation of the surface of the solid support with one layer of ligand molecules requires theoretically about 0.3 DEHCEBP nm À2 .…”

“…The selectivity of the DEHCEBP ligand impregnated on a solid support in high sulfate solutions has been demonstrated previously. 16 Fig. 6 Nitrogen adsorption-desorption isotherms of unmodified GNPs and of the impregnated materials with DEHCEBP concentrations of 0.3-1.2 mmol g À1 .…”