The uranyl adsorption capacity and selectivity studies on organic/inorganic hybrid sol-gel sorbents

The synthesis and properties of microporous and mesoporous uranyl-imprinted organic/inorganic hybrid sol-gel sorbents are described. Both microporous and mesoporous imprinted sorbents exhibit (1) higher selectivity and (2) higher rebinding capacities than similar nonimprinted sorbents. Mesoporous uranyl-imprinted sorbents also exhibited fast diffusion of uranyl ions into the mesopores over nonimprinted sorbents. Porous imprinted sol-gel sorbents are prepared by molecular imprinting technologies.; To prepare organic/inorganic hybrid sol-gel sorbents that are able to strongly bind to hard Lewis acid, uranyl, hard Lewis bases such as ligands coordinating through nitrogens (APTS = 3-aminopropyltimethoxysilane = (CH ₃O)₃SiCH₂CH₂CH₂NH₂ , AEAPS = N-[2-aminoethyl-3-aminopropyl]trimethoxysilane = (CH ₃O)₃SiCH₂CH₂CH₂NHCH ₂CH₂NH₂, Schiff base 1 represents the bifunctional ligand prepared from AEAPS and 2,3-butanedione, Schiff base 2 represents the bifunctional ligand prepared from APTS and 2,6-diacetylpyridine and ethylenediamine), and ligand coordinating through phosphorus (DPES = diethylphosphatoethyltriethoxysilane = (C₂H₅O)₃SiCH₂CHP(=O) (OC ₂H₅)₂ have been employed.; Three different methods were employed to prepare sorbents exhibiting high uranyl affinities to uranyl: (a) The first method involved co-condensation of inorganic silyl precursors and organosilyl precursors. In these materials an organic moiety is covalently linked to a siloxane species that is hydrolyzed and grafted into the developing silicate network. Hybrid sorbents (25 mol % synthesis ratio) prepared with APTS, AEAPS, DPES, Schiff base 1 and Schiff base 2 showed higher uranyl binding capacities than the corresponding microporous; pure silicate sorbents toward the uranyl ion. However, uranyl-imprinted sorbents have not been successfully prepared because of the precipitation of uranyl-organic ligand complexes before gelation. (b) The second involved coating of organosilyl ligands on surface of preformed mesoporous silica. Coated, uranyl-imprinted sorbents prepared from ∼25 Å diameter mesoporous silica always exhibited higher surface ligand coverages and higher uranyl capacity than those of nonimprinted sorbents. Cu(II)-imprinted and uranyl-imprinted sorbents showed selective adsorption for Cu(II) and uranyl versus Zn(II)-containing solution mixtures. (c) Surface-grafting of bifunctional reagents on as-synthesized (base-catalyzed) mesoporous silica by ion-exchange reactions. Uranyl-imprinted sorbents prepared in DMSO (dimethylsulfoxide) exhibited higher surface ligand (AEAPS) coverages and higher uranyl binding capacities than those of the sorbents prepared by method (b).; Finally, new mesoporous titanium-based and phosphate-based sorbents have been prepared. Titanium-based sorbents showed higher uranyl capacity as Ti:Si ratio in mesoporous titanosilicate materials is increased. In phosphate-based sorbents prepared using a dodecylphosphate surfactant, phosphate based surface coatings remain after calcination, which have been studied with 31 P SSNMR using MAS (magic angle spinning). The phosphorus chemical shifts indicate that while no interaction between phosphate and silicon, strong interactions between phosphate and zirconium and titanium existed. These coated sorbents exhibited very strong binding (irreversible) toward uranyl. Uranyl preferentially binds in the order TiO₂ ≥ ZrO₂ > SiO ₂ in 10−3 M uranyl solution, which is parallel with the order of metal Lewis acidity Ti ≥ Zr > Si.