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

Abstract

Abstract [ThB 5 O 6 (OH) 6 ][BO(OH) 2 ]·2.5H 2 O (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 − , NO 3 − , IO 3 − , ClO 4 − , MnO 4 − , and CrO 4 2− . More importantly, NDTB‐1 is selective for the removal of TcO 4 − from nuclear waste streams even though there are large excesses of competing anions such as Cl − , NO 3 − , 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 TcO 4 − in NDTB‐1 are second‐order with the rate constant k 2 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 × 10 4 mL g −1 . Finally, it is demonstrated that the exchange for TcO 4 − 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 PO 4 3− and SeO 4 2− , and therefore the material can be easily recycled and reused.