Selectivity, Kinetics, and Efficiency of Reversible Anion Exchange with TcO4- in a Supertetrahedral Cationic Framework

[ThB5O6(OH)6][BO(OH)2]center dot 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 CrO42-. 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 NO2-. 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 TcO4- 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 TcO4- is 162.2 mg g-1 (0.5421 mol mol-1) with a maximum distribution coefficient Kd of 1.0534 x 10(4) mL g-1. Finally, it is demonstrated that the exchange for TcO4- in NDTB-1 is reversible. TcO4- trapped in NDTB-1 can be exchanged out using higher-charged anions with a similar size such as PO43- and SeO42-, and therefore the material can be easily recycled and reused.