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

TLDR

Effective and selective removal of 99TcO4– from aqueous solution is highly desirable for waste partitioning and contamination remediation in the modern nuclear fuel cycle, yet remains a significant challenge. The sorption mechanism was elucidated by the first crystal structure of TcO4–‑incorporated SCU‑101, revealing a recognition site that accommodates TcO4– and is supported by DFT analysis, with no analogous site for other anions. SCU‑101, a hydrolytically stable and radiation‑resistant MOF, shows extremely fast kinetics, high capacity, and exceptional selectivity for TcO4– even against 6000‑fold excess SO42–, enabling effective separation of TcO4– from Hanford low‑level waste simulants.

Abstract

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 SO42–, as even 6000 times of SO42– 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.