Recognition and sensing of metal ions at the atomic level is a critical issue in many fields of sciences. In particular, selective adsorption of radioactive 90Sr2+ ions from nuclear waste has been of interest since the Fukushima Daiichi nuclear disaster. Here we present a combined experimental and computational study of KTiNbO5 (KTN) as a selective and durable adsorbent for Sr2+ ions. KTN grown from nitrate flux at 500-600 °C (KTNflux) has a zigzag layered gallery space. Structural analysis indicates that KTNflux crystals are platy with surface areas of 48-86 m2 g-1. These areas are ∼50 times larger than those of KTN prepared by solid-state reaction at 1100 °C (KTNSSR) as a result of efficient, anisotropic crystal growth. Sr2+ adsorption experiments indicate that the Sr2+ ion-exchange capacity of KTNflux is ∼1.04 mmol g-1, and most of the ion-exchange sites are homogeneous. Kinetic analysis shows that the Sr2+ ion-exchange rate on KTNflux is 1 order of magnitude higher than that on KTNSSR. The [Na+] concentration dependence of the distribution coefficient Kd for Sr2+ indicates that KTNflux shows high affinity for Sr2+ and remarkable durability, and Kd > 1.26 × 104 mL g-1 even at [Na+] = 0.1 mol L-1. The origin of the high selectivity for Sr2+ was studied by density functional theory (DFT). Our calculations indicate that the high preference for Sr2+ is due to confinement within subnanometer-sized pockets built from oxygen species of both the anionic metalate frameworks and intercalated water molecules, forming monocapped heptahedra or octahedra that resemble the active sites of enzymes.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films