Selectivity, kinetics, and efficiency of reversible anion exchange with TcO₄^- in a supertetrahedral cationic framework

[ThB₅O₆(OH)₆][BO(OH)₂] ·2.5H₂O (Notre Dame Thorium Borate-1, NDTB-1) is an inorganic supertetrahedral cationic framework material that is derived from boric acid flux reactions. NDTB-1 exhibits facile single crystal to single crystal anion exchange with a variety of common anions such as Cl^-, Br^-, NO₃^-, IO₃^-, ClO₄ ^-, MnO₄^-, and CrO₄^2-. More importantly, NDTB-1 is selective for the removal of TcO₄ ^- from nuclear waste streams even though there are large excesses of competing anions such as Cl^-, NO₃^-, and NO ₂^-. Competing anion exchange experiments and magic-angle spinning (MAS)-NMR spectroscopy of anion-exchanged NDTB-1 demonstrate that this unprecedented selectivity originates from the ability of NDTB-1 to trap TcO ₄^- within cavities, whereas others remain mobile within channels in the material. The exchange kinetics of TcO₄^- in NDTB-1 are second-order with the rate constant k₂ of 0.059 s ^-1 M^-1. The anion exchange capacity of NDTB-1 for TcO ₄^- is 162.2 mg g^-1 (0.5421 mol mol ^-1) with a maximum distribution coefficient K_d of 1.0534 × 10⁴ mL g^-1. Finally, it is demonstrated that the exchange for TcO₄^- in NDTB-1 is reversible. TcO ₄^- trapped in NDTB-1 can be exchanged out using higher-charged anions with a similar size such as PO₄^3- and SeO₄^2-, and therefore the material can be easily recycled and reused.