Identifying the Recognition Site for Selective Trapping of ~(99)TcO_4~- in a Hydrolytically Stable and Radiation Resistant Cationic Metal-Organic Framework

摘要

Effective and selective removal of ~(99)TcO_4~- 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 TcO_4~-. More importantly, this material can selectively remove TcO_4~- in the presence of large excesses of NO_3~- and SO_4~(2-), as even 6000 times of SO_4~(2-) in excess does not significantly affect the sorption of TcO_4~-. These superior features endow that SCU-101 is capable of effectively separating TcO_4~- from Hanford low-level waste melter off-gas scrubber simulant stream. The sorption mechanism is directly unraveled by the single crystal structure of TcO_4~--incorporated SCU-101, as the first reported crystal structure to display TcO_4~- trapped in a sorbent material. A recognition site for the accommodation of TcO_4~- is visualized and is consistent with the DFT analysis results, while no such site can be resolved for other anions.