Boosting selective Cs+ uptake through the modulation of stacking modes in layered niobate-based perovskites

Abstract

Selective separation of ¹³⁷Cs is significant for the sustainable development of nuclear energy and environmental protection, due to its strong radioactivity and long half-life. However, selective capture of ¹³⁷Cs⁺ from radioactive liquid waste is challenging due to strong coulomb interactions between the adsorbents and high-valency metal ions. Herein, we propose a strategy to resolve this issue and achieve specific Cs⁺ ion recognition and separation by modulating the stacking modes of layered perovskites. We demonstrate that among niobate-based perovskites, ALaNb₂O₇ (A = Cs, H, K, and Li), HLaNb₂O₇ shows an outstanding selectivity for Cs⁺ even in the presence of a large amount of competing Mⁿ⁺ ions (Mⁿ⁺ = K⁺, Ca²⁺, Mg²⁺, Sr²⁺, Eu³⁺, and Zr⁴⁺) owing to its suitable void fraction and space shape, brought by the stacking mode of layers. The Cs⁺ capture mechanism is directly elucidated at molecular level by single-crystal structural analyses and density functional theory calculations. This work not only provides key insights in the design and property optimization of perovskite-type materials for radiocesium separation, but also paves the way for the development of efficient inorganic materials for radionuclides remediation.

Fig. 1. Schematic representation for the activation and ion exchange process of (ALaNb₂O₇ (A = Cs, H, K, and Li)).

a–c Schematic representation for the acid-activation process of CsLaNb₂O₇ by HNO₃ (3 M = 3 mol L⁻¹) and the Cs⁺ capture by HLaNb₂O₇. View of crystal structures of CsLaNb₂O₇ (a), HLaNb₂O₇ (b), HLaNb₂O₇-Cs (c), KLaNb₂O₇ (d), and LiLaNb₂O₇ (e). On the top are the powder and crystal images of CsLaNb₂O₇, HLaNb₂O₇, and HLaNb₂O₇-Cs (a–c). [NbO₈] octahedron: blue; La: lilac; O: red; Cs: orange; K: green; Li: modena.

Fig. 2. Cs⁺ removal performance of HLaNb₂O₇.

a The kinetics curve of HLaNb₂O₇ for Cs⁺ adsorption. b The adsorptions equilibrium data of HLaNb₂O₇ for Cs⁺ ions fitted with the Langmuir, Freundlich, and Langmuir-Freundlich isotherm models (Eqs. 5–7). c K_d^Cs and R^Cs values of HLaNb₂O₇ before and after irradiation with the Cs⁺ initial concentrations (C₀) of 30.80 mg L⁻¹ or 52.53 mg L⁻¹. d K_d^Cs values of HLaNb₂O₇ at various pH values (0.3 M = 0.3 mol L⁻¹). Source data are provided as a Source Data file.

Fig. 3. Selective removal of Cs⁺ by HLaNb₂O₇.

a K_d and R values of Cs⁺ and Mⁿ⁺ (Mⁿ⁺ = Na⁺, K⁺, Ca²⁺, Mg²⁺, Sr²⁺, and Eu³⁺) ions removed in neutral solution by HLaNb₂O₇ under different Na/Cs, K/Cs, Ca/Cs, Sr/Cs, Mg/Cs, and Eu/Cs molar ratios. Variations of K_d of Cs⁺ and Mⁿ⁺ (Mⁿ⁺ = Sr²⁺, Eu³⁺, and Zr⁴⁺ ions), and SF_Cs/M under various Sr/Cs (b), Eu/Cs (c), or Zr/Cs (d) molar ratios in neutral solution, respectively. Error bars present the standard deviation of the mean of three experiments. Source data are provided as a Source Data file.

Fig. 4. Selective removal of Cs⁺ by HLaNb₂O₇.

a The K_d and R of various metal ions by HLaNb₂O₇ in the coexistence of Cs⁺, Sr²⁺, K⁺, Na⁺, Ca²⁺, and Mg²⁺ under neutral condition and pH of 1.1. b The K_d and R of Cs⁺ and Mⁿ⁺ (Mⁿ⁺ = Sr²⁺, Eu⁺, and Zr⁴⁺) ions with equimolar rations of Cs⁺ and Mⁿ⁺ removed by HLaNb₂O₇ in neutral and acidic conditions (pH = 1.1). c The K_d and R of various ions by HLaNb₂O₇ in simulated nuclear waste liquid. d The K_d^Cs and R^Cs of HLaNb₂O₇ in Cs⁺-contained tap water (Fuzhou, Fujian), river water (Wulongjiang River, Fuzhou, Fujian) and lake water (Qishanhu lake, Fuzhou, Fujian), respectively. Error bars present the standard deviation of the mean of three experiments, and the upper and lower limits of the R^M and K_d^M value error bars were ± 6.22% and ± 0.12 × 10⁴mL g⁻¹. Source data are provided as a Source Data file.

Fig. 5. Results of density functional theory calculations.

a The optimized Cs⁺-substituted structure; b the optimized Sr²⁺-substituted structure; c the optimized Eu³⁺-substituted structure; d the optimized Zr⁴⁺-substituted structure. Bonds and the interlayer distance are in Å. DFT calculations of ESP (e) and DOS (f) of Cs⁺, HLaNb₂O₇, and HLaNb₂O₇-Cs. Partial images of the corresponding samples were enlarged to show the impotant peaks clearly.