Azo-Functionalized Ultrathin 2D MOF Nanosheets with Cage-Like Cavities: Synergistic Effect Enables High-Capacity Capture and Trace Uranium(VI) Extraction

Efficient and complete uranium removal from aqueous solution holds immense importance in protecting humans from harmful effects of uranium contamination. Herein, an ultrathin two-dimensional MOF nanosheet with cavity structures was elaborately constructed for uranium removal. The cavity, decorated with extensive adsorption sites (azo groups) inside, can fully utilize the structural advantages of MOF nanosheets in pollutant removal, including significantly exposed surfaces and readily accessible sites. Such features enhanced interactions with uranium, ultimately leading to an exceptionally high adsorption capacity (482.77 mg/g). Moreover, the MOF nanosheets were able to preconcentrate and extract uranium from aqueous solution, owing to the functionalized cavity structure, which achieved ultrahigh removal efficiency (>97%) in the low concentration range of 145-1169 ppb. Even for the ultralow concentration of 21 ppb, 94.29% uranium still can be removed, with an ultralow residual concentration of 1.2 ppb. The MOF nanosheets also exhibited high anti-interference ability, which could efficiently remove low-level uranium from various real samples, with residual uranium well below the WHO limit (30 ppb). Investigations on adsorption mechanism revealed that the synergistic effect between the cage-like cavities and the internal sufficient azo groups with uranyl ions can be responsible for the excellent uranium extraction performance.