Ion-Exchange Behavior of One-Dimensional Linked Dodecaniobate Keggin Ion Materials

Ion-exchange media for cation or anion separations are employed in private industries such as mining, pharmaceutical, and chemical. Government organizations utilize ion-exchange processes for water treatment, environmental remediation and preservation, and nuclear waste treatment. Here, we present an investigation of the ion-exchange behavior of polyoxoniobate materials. Dodecaniobate Keggin ions, [TNb₁₂O₄₀]¹⁶⁻ (T = Si, Ge) linked by cationic dimer bridges, [Nb₂O₂]⁶⁺ or [Ti₂O₂]⁴⁺ form a one-dimensional anionic framework, charge-balanced by hydrated Na⁺ cations. The hydrated sodium resides in interchain tunnels, only loosely associated with the Keggin chains, a geometry that provides rapid exchange kinetics and high exchange capacity. The lattice water is extremely mobile and could not be located on fixed crystallographic positions. Rather, we characterized the water loading by both thermogravimetric studies and computational studies. The LeBail method was used to quantify unit cell change of Keggin chain materials in which the sodium was exchanged completely with Sr²⁺, Cd²⁺, and Yb³⁺. We located Sr²⁺ sites in the Sr-exchanged phase by Rietveld refinement. Ideal sites for Sr²⁺ in the interchain space were also predicted by computational studies, and the experimental and theoretical results were compared. Finally, we investigated the selectivity of the Keggin-chain phases for sorption of radionuclides Sr, Np, and Pu from high ionic strength, highly caustic simulants for Savannah River Site’s nuclear wastes.