Direct Separation of UO₂ ²⁺ by Coordination Sieve Effect via Spherical Coordination Traps

Area of Science:

• Materials Science
• Separation Science
• Radiochemistry

Background:

• Traditional separation methods like adsorption-desorption face challenges such as co-adsorption and desorption difficulties.
• Molecule sieve effect (MSE) offers direct separation but requires specific framework designs.
• Developing advanced materials for selective ion separation is crucial for nuclear fuel reprocessing and waste management.

Purpose of the Study:

• To introduce and demonstrate the concept of coordination sieve effect (CSE) for direct separation of uranyl ions (UO2^2+).
• To develop a novel adsorbent material capable of selectively separating UO2^2+ from a mixture of other metal ions.
• To elucidate the separation mechanism at a molecular level.

Main Methods:

• Synthesis of a polyhedron-based hydrogen-bond framework (P-HOF-1) from a metal-organic framework (MOF) precursor via a two-step postmodification.
• Ion uptake and separation experiments using mixed solutions containing Cs+, Sr2+, Eu3+, Th4+, and UO2^2+ ions.
• Characterization using single crystal X-ray diffraction and density-functional theory (DFT) calculations to understand the separation mechanism.

Main Results:

• P-HOF-1 exhibited high uptake capacity for Cs+, Sr2+, Eu3+, and Th4+ ions while completely excluding UO2^2+ ions, demonstrating excellent CSE.
• Direct separation achieved >99.9% removal efficiency for Cs+, Sr2+, Eu3+, and Th4+ ions, with <1.2% removal for UO2^2+.
• A benchmark reverse selectivity (S_M/U) of >83 was obtained, leading to the direct generation of high-purity UO2^2+ (>99.9%).

Conclusions:

• The developed P-HOF-1 material effectively utilizes CSE for the direct and selective separation of UO2^2+ ions.
• The separation mechanism is attributed to the specific 'spherical coordination trap' within P-HOF-1, which accommodates spherical ions but excludes planar UO2^2+.
• This work presents a novel strategy for direct uranium separation, overcoming limitations of traditional methods and offering potential applications in nuclear chemistry.