Platy KTiNbO₅ as a Selective Sr Ion Adsorbent: Crystal Growth, Adsorption Experiments, and DFT Calculations

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 (KTN_flux) has a zigzag layered gallery space. Structural analysis indicates that KTN_flux 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 (KTN_SSR) as a result of efficient, anisotropic crystal growth. Sr²⁺ adsorption experiments indicate that the Sr²⁺ ion-exchange capacity of KTN_flux 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 KTN_flux is 1 order of magnitude higher than that on KTN_SSR. The [Na⁺] concentration dependence of the distribution coefficient K_d for Sr²⁺ indicates that KTN_flux shows high affinity for Sr²⁺ and remarkable durability, and K_d > 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.