Conjugated microporous polymers bearing phosphonate ligands as an efficient sorbent for potential uranium extraction from high-level liquid wastes

Abstract

Recovery of uranium from high-level liquid wastes (HLWs) is of vital importance for sustainable nuclear energy. However, inherent high acidity, strong radioactivity and massive nuclear fission products are the major challenges for efficient extraction of uranium from HLWs. In this work, a new approach is reported for potential uranium extraction from HLWs using conjugated microporous polymers (CMPs) bearing ethylphosphate ligands (CMP-EP) as sorbents. CMP-EP with a BET surface area of 58.2 m² g⁻¹ is prepared by the Suzuki coupling reaction of 1,3,5-tris(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzene and 2,7-dibromo-9,9′-bis(diethyl propylphosphonate)fluorene. HRTEM and SEM characterization shows that CMP-EP sheets possess an overlapping flaky morphology. The uranium sorption arises inside the pores of the material as confirmed by depth-profiling XPS of CMP-EP before and after adsorption. CMP-EP exhibits excellent binding selectivity for uranium(VI) ions with a high sorption capability of 73 mg U per g in 6 M HNO₃ solutions, and outstanding reusability for adsorption when regenerated with alkaline solutions. Thanks to its rigid π-conjugated skeleton, the CMP-EP after γ-ray irradiation of 1000 kGy in 6 M HNO₃ shows no dramatic changes in structure, sorption capacity and selectivity. The sorption mechanism is studied by XPS and FTIR measurements revealing that uranyl ions tend to form complexes such as UO₂(NO₃)₂ in 6 M HNO₃, and then bind to PO groups of phosphonate ligands. This work represents the first example for effective recovery of uranium from HLWs using functional CMPs as sorbents.