Cationic Ratio-Regulated U(VI) Separation from Liquid Media by Zn-Al-LDH Nanocomposites: Sorption Mechanism and Performance Study

Uranium separation from aqueous media remains a critical challenge at the intersection of nuclear fuel cycle efficiency and environmental safety. Layered double hydroxides (LDHs) are promising adsorbents for U(VI) removal and serve as versatile inorganic matrices for the design of nanocomposite sorbents. However, the influence of their key structural parameter, Me2+/Me3+ cationic ratio, on sorption performance and composite functionality remains insufficiently explored. This study investigates how the Zn2+/Al3+ ratio governs the structure, sorption kinetics, and U(VI) uptake mechanisms of Zn-Al LDH. Characterization by XRD, TEM, XPS, XAFS, cryogenic laser-induced fluorescence spectroscopy, Raman, and positron annihilation spectroscopy revealed that increasing Al3+ content up to the optimal ratio of 2/1 enhances sorption capacity through increased positive charge density, reaching maximum static (47.2 mg/g) and dynamic (17.05 mg/g) capacities. EXAFS and fluorescence spectroscopy demonstrate that U(VI) adsorption at pH 4.0 proceeds via inner-sphere complexation involving mononuclear uranyl carbonate/hydroxyl species (UO2(CO3)22−, UO2(CO3)34−, UO2(OH)n(2−n)) and polynuclear complexes, predominantly (UO2)2(CO3)(OH)3−. Further increase to Zn/Al = 1/1 induced mesopore narrowing and inhibited diffusion, drastically decreasing dynamic performance. These findings establish the Zn/Al ratio as a key parameter governing LDH sorption efficiency and provide a mechanistic basis for the rational design of LDH-based composite sorbents.