Organic layer formation and sorption of U(vi) on acetamide diethylphosphonate-functionalized mesoporous silica

Abstract

Acetamide diethylphosphonate (AcPhos)-functionalized silica has been shown to have a high affinity for U(VI) in pH 2–3 nitric acid. Previous work with AcPhos-functionalized silica has focused on actinide and lanthanide extraction under various conditions, but has shown poor reproducibility in the functionalization process. For this work, four AcPhos-functionalized SBA-15 materials were synthesized and evaluated based on their U(VI) sorption capacity and their stability in nitric acid. Materials synthesized using pyridine as a basic catalyst were shown to form a greater fraction of polymeric structures at the silica surface, which correlated with higher structural integrity upon contact with acidic solutions. Single-pulse ³¹P and ¹H NMR spectra of these materials show evidence of phosphonic acid groups, as well as hydrogen-bonding interactions either between ligands or with the silica surface. Additionally, these materials were found to have significantly higher U(VI) sorption capacities and K_eq values than the materials synthesized without pyridine, most likely due to the ion-exchange properties of the phosphonic acid groups. The ³¹P–³¹P DQ-DRENAR NMR technique was used to compare the average strength of dipolar coupling interactions between phosphorus atoms for the four materials. Because the strength of dipolar coupling interactions depends on the number and proximity of neighboring spins, this technique provides information about the average density of ligands on the surface. The conventional functionalization procedure yielded materials with the lowest average surface ligand density, while those using extended reaction times and the pyridine base catalyst yielded materials with higher surface ligand densities.