⁹⁹TcO₄^- Separation through Selective Crystallization Assisted by Polydentate Benzene-Aminoguanidinium Ligands

⁹⁹Tc is one of the most abundant radiotoxic isotopes in used nuclear fuel with a high fission yield and a long half-life. Effective removal of pertechnetate (TcO₄^-) from an aqueous solution is important for nuclear waste separation and remediation. Herein, we report a series of facilely obtained benzene-linked guanidiniums that could precipitate TcO₄^- and its nonradioactive surrogate ReO₄^- from a high-concentration acidic solution through self-assembly crystallization. The resulting perrhenate and pertechnetate solids exhibit exceptionally low aqueous solubility. The benzene-linked guanidiniums hold one of the highest TcO₄^- removal capacities (1279 mg g^-1) among previously reported materials and possess a removal percentage of 59% for ReO₄^- in the presence of Cl^- over 50 times. The crystallization mechanism was clearly illustrated by the single-crystal structures and density functional theory calculations, indicating that TcO₄^- is captured through a charge-assisted hydrogen bonding interaction and stabilized by π-π stacking layers. In addition, the removal process is easily recycled and no toxic organic reagents are introduced. This work provides a green approach to preliminarily separate TcO₄^- from high-level nuclear wastes.