Large-Pore 3D Cubic Mesoporous (KIT-6) Hybrid Bearing a Hard–Soft Donor Combined Ligand for Enhancing U(VI) Capture: An Experimental and Theoretical Investigation

A preorganized tetradentate phenanthrolineamide (DAPhen) ligand with hard and soft donors combined in the same molecule has been found to possess high extraction ability toward actinides over lanthanides from acidic aqueous solution in our previous work. Herein we grafted phenanthrolineamide groups onto a large-pore three-dimensional cubic silica support by the reaction of DAPhen siloxane with KIT-6 substrate to prepare a novel uranium-selective sorbent, KIT-6-DAPhen. The as-synthesized sorbent was well-characterized by scanning electron microscopy, high-resolution transmission electron microscopy, N₂ adsorption/desorption, X-ray diffraction, FT-IR, ¹³C cross-polarization magic-angle spinning NMR, and TGA techniques, which confirmed the consummation of the functionalization. Subsequently, the effects of contact time, solution pH, initial U­(VI) concentration, and the presence of competing metal ions on the U­(VI) sorption onto KIT-6-DAPhen sorbent were investigated in detail. It was found that KIT-6-DAPhen showed largely enhanced sorption capacity and excellent selectivity toward U­(VI). The maximum sorption capacity of KIT-6-DAPhen at pH 5.0 reaches 328 mg of U/g of sorbent, which is superior to most of functionalized mesoporous silica materials. Density functional theory coupled with quasi-relativistic small-core pseudopotentials was used to explore the sorption interaction between U­(VI) and KIT-6-DAPhen, which gives a sorption reaction of KIT-6-DAPhen + [UO₂(H₂O)₅]²⁺ + NO₃^– ⇄ [UO₂(KIT-6-DAPhen)­(NO₃)]⁺ + 5H₂O. The findings of the present work provide new clues for developing new actinide sorbents by combining new ligands with various mesoporous matrixes.