Efficient sequestration of radioactive 99TcO4- by a rare 3-fold interlocking cationic metal-organic framework: A combined batch experiments, pair distribution function, and crystallographic investigation

Abstract

Technetium-99 is one of most problematic radionuclides with a long half-life, high environmental mobility and solubility, which exists mainly in the form of anionic pertechnetate (99TcO4-). Searching for sorbents that hold efficient and selective elimination of radioactive 99TcO4- from aqueous solution is still a big challenge and further visualizing it in the sorbents is extremely difficult. Herein, we utilized an elongated organic neutral pyridyl ligand linked with Ni2+ to yield a novel 3-fold interlocking cationic metal–organic framework (ZJU-X6). Unexpectedly, the structure of ZJU-X6 held unique displaceable water pairs on the Ni2+ node in comparison to cationic metal–organic framework constructed by Ni2+. Attainable Ni2+ nodes distinctly improved the positive charge density of the framework, affording high sorption capacity, excellent selectivity, and removal depth toward 99TcO4-. Pair distribution functions (PDF) were applied to analyze the changes of atom distances in materials before and after capturing ReO4-, clearly showing that ReO4- entered into the body of ZJU-X6. More detailed sorption mechanism was atomically elucidated by obtaining the single crystal structure of 99TcO4--incorporated ZJU-X6. It is found that the water pairs were replaced by 99TcO4- and each Ni2+ coordinated with two 99TcO4- through oxygen atoms. In addition, each 99TcO4- engaged in multiple C–H…O hydrogen bonding with H atoms of aromatic rings that enhanced the trapping of 99TcO4-.