Selectivity, Kinetics, and Efficiency of Reversible An ion Exchange with TcO_4~- in a Supertetrahedral Cationic Framework

摘要

[ThB_5O_6(OH)_6][BO(OH)_2]·2.5H_2O (Notre Dame Thorium Borate-1, NDTB-1) is an inorganic supertetrahedral cattonic 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 mot~(-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.