Cation-Intercalated Lamellar MoS₂ Adsorbent Enables Highly Selective Capture of Cesium

Highly selective capture of cesium (Cs⁺) from complex aqueous solutions has become increasingly important owing to its (¹³³Cs) indispensable role in some cutting-edge technologies and the environmental mobility of radioactive nuclide (¹³⁷Cs) from nuclear wastewater. Herein, we report the development of cation-intercalated lamellar MoS₂ as an effective Cs⁺ adsorbent with the advantages of facile synthesis and highly tunable layer spacing. Two types of cations, including Na⁺ and NH₄⁺, were employed for the intercalations between adjacent layers of MoS₂. The results demonstrated that the adsorption capacity of the NH₄⁺-intercalated material (M-NH₄⁺, 134 mg/g) for Cs⁺ clearly outperformed the others due to higher loading percentages of cations and larger layer spacing. The cesium partition coefficients for M-NH₄⁺ in the presence of 100-fold competing ions all exceed 1 × 10³ mL/g. A simulated complex aqueous solution containing 15.37 mg/L Cs⁺ and highly excess of competing ions Li⁺, Na⁺, K⁺, Mg²⁺, and Ca²⁺ (20-306 times higher) was introduced to prove the practical application potential using our best-performing M-NH₄⁺, showing a good to excellent partition ability of Cs⁺ among other cations, especially for Cs/K and Cs/Na with separation factors of 58 and 212, respectively. The adsorption and selectivity mechanisms were clearly elucidated using various advanced techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. These results revealed that the good selectivity for Cs⁺ can be ascribed to the differences in Lewis acidities, hydration energy, cation sizes, and in particular, the divergence of coordination modes which was successfully achieved after tuning the layer distance via the cation intercalation strategy. In addition, the material has fast kinetics (<30 min), wide range of pH tolerance (4-10), and good reusability. Overall, our studies point out that the tunable lamellar MoS₂-based materials are promising adsorbents for Cs⁺ capture and separation.