The removal of aqueous uranium by SBA-15 modified with phosphoramide: a combined experimental and DFT study

Abstract

Phosphoramide-modified ordered mesoporous silica (SBA-DEPA) materials were prepared via a two-step process involving: (1) the synthesis of phosphoramide via amidation of phosphoryl chloride with a primary amine and (2) modification of the phosphoramide onto SBA-15. The successful preparation was confirmed by FT-IR and NMR spectroscopy. As indicated by the ICP-AES and XRF analysis results, the phosphoramide group has a grafting ratio as high as 12.5%. The morphological information and N₂ adsorption–desorption technique proved the highly ordered structure and large specific area of the material, respectively. An excellent performance for uranium sorption was found with a loading maximum of 311.6 mg g⁻¹, a marked promotion relative to unmodified SBA-15. A strong pH and CO₂ dependence suggested that near neutral conditions favored the maximum uranium sorption. To our surprise, the sorption took only 5 min to reach a 90% capacity and 20 min for equilibrium, which is extraordinary when compared to many other sorbent materials. The ionic strength partially influences the sorption, which indicates outer and inner sphere sorption both play parts in the process. A sorption capacity of over 90% of the original indicates the excellent reproducibility of SBA-DEPA. DFT calculations based on cluster models suggested a ‘tri-dentate-like’ structure for the uranyl-surface binding site, which may explain the excellent sorption ability of this material.