Porous Anionic Uranyl-Organic Networks for Highly Efficient Cs⁺ Adsorption and Investigation of the Mechanism

Exploitation of new materials for the removal of long-lived and highly radioactive actinides and their fission products produced in the nuclear fuel cycle is crucial for radionuclide management. Here, two rare porous anionic uranyl-organic frameworks (UOFs) have been successfully synthesized by a judicious combination of the tetratopic carboxylate ligand 1,3,6,8-tetrakis( p-benzoic acid)pyrene (H₄TBAPy) and D_{3 h}-symmetrical triangular [UO₂(COO)₃]^-. The resulting two compounds exhibit different architectures, albeit with similar coordination modes. Of interest is that they have excellent adsorption performance on Cs⁺ from aqueous solution. The high removal efficency would make them promising in applications of radioactive waste management. Notably, the framework of compound 2, [(CH₃)₂NH₂]₄[(UO₂)₄(TBAPy)₃]·22DMF·37H₂O is sufficiently robust to allow the accessibility of intriguing single crystals of a Cs⁺-adsorbed derivative, which helps to elucidate the adsorption mechanism. The structural, bonding, and spectroscopic properties of the above compounds are examined using relativistic density functional theory (DFT). It is found that the adsorption toward cesium on UOFs is energetically favored, which features largely ionic bonds and is dominated by electrostatic attraction.