Identifying the Recognition Site for Selective Trapping of ⁹⁹TcO₄^- in a Hydrolytically Stable and Radiation Resistant Cationic Metal-Organic Framework

Effective and selective removal of ⁹⁹TcO₄^- from aqueous solution is highly desirable for both waste partitioning and contamination remediation purposes in the modern nuclear fuel cycle, but is of significant challenge. We report here a hydrolytically stable and radiation-resistant cationic metal-organic framework (MOF), SCU-101, exhibiting extremely fast removal kinetics, exceptional distribution coefficient, and high sorption capacity toward TcO₄^-. More importantly, this material can selectively remove TcO₄^- in the presence of large excesses of NO₃^- and SO₄^2-, as even 6000 times of SO₄^2- in excess does not significantly affect the sorption of TcO₄^-. These superior features endow that SCU-101 is capable of effectively separating TcO₄^- from Hanford low-level waste melter off-gas scrubber simulant stream. The sorption mechanism is directly unraveled by the single crystal structure of TcO₄^--incorporated SCU-101, as the first reported crystal structure to display TcO₄^- trapped in a sorbent material. A recognition site for the accommodation of TcO₄^- is visualized and is consistent with the DFT analysis results, while no such site can be resolved for other anions.