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[Display omitted] • ZJU-X6 is a 3-fold interlocking cationic MOF and could capture 99TcO 4. • ZJU-X6 exhibited superior sorption ability and reusability toward 99TcO 4. • PDF and crystal structure clearly elucidated the sorption mechanism. 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 (99TcO 4 -). Searching for sorbents that hold efficient and selective elimination of radioactive 99TcO 4 - 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 99TcO 4 -. Pair distribution functions (PDF) were applied to analyze the changes of atom distances in materials before and after capturing ReO 4 -, clearly showing that ReO 4 - entered into the body of ZJU-X6. More detailed sorption mechanism was atomically elucidated by obtaining the single crystal structure of 99TcO 4 --incorporated ZJU-X6. It is found that the water pairs were replaced by 99TcO 4 - and each Ni2+ coordinated with two 99TcO 4 - through oxygen atoms. In addition, each 99TcO 4 - engaged in multiple C–H...O hydrogen bonding with H atoms of aromatic rings that enhanced the trapping of 99TcO 4 -. [ABSTRACT FROM AUTHOR]