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

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 ((TcO4)-Tc-99-). Searching for sorbents that hold efficient and selective elimination of radioactive (TcO4)-Tc-99- 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 (TcO4)-Tc-99-. 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 (TcO4)-Tc-99- incorporated ZJU-X6. It is found that the water pairs were replaced by (TcO4)-Tc-99- and each Ni2+ coordinated with two (TcO4)-Tc-99- through oxygen atoms. In addition, each (TcO4)-Tc-99- engaged in multiple C-H...O hydrogen bonding with H atoms of aromatic rings that enhanced the trapping of (TcO4)-Tc-99