Metal-organic frameworks as a versatile platform for radionuclide management

Fuel fission products and fuel production byproducts contain radioactive nuclides such as ^129/131I, ¹²⁷Xe, ⁸⁵Kr, ²³⁵U, ¹³⁷Cs, ⁹⁰Sr, ⁹⁹Tc, and ⁷⁹Se that exist in gaseous, ionic, and other forms. Therefore, understanding the fundamental nature of each species is crucial for designing corresponding binding sites that offer high sorption capacity and selectivity over their competing species in nuclear waste. This review describes the use of (i) metal-organic frameworks (MOFs) as sorbents for radioactive species and (ii) actinide-based MOFs (An-MOFs) as crystalline alternatives for studying the fundamental properties of radioactive nuclides. To the former end, three different forms of radioactive species are discussed, namely, (1) gas-phase ^129/131I₂, organic iodides, and ¹²⁷Xe/⁸⁵Kr; (2) cationic ^235/238UO₂²⁺, ²³²Th⁴⁺, ¹³⁷Cs⁺, and ⁹⁰Sr²⁺; and (3) anionic ⁹⁹TcO₄⁻ (ReO₄⁻), ⁷⁹SeO₃²⁻, and ⁷⁹SeO₄²⁻. Certain MOFs can undergo single-crystal-to-single-crystal transformations during radionuclide capture, which facilitates the investigation of the binding modes and mechanisms of radioactive species by single-crystal X-ray diffractometry. Moreover, the customizable pore size and properties of MOFs endow them with exceptional sorption capacities and selectivities that have not been achieved in traditional sorbents. The acquired knowledge is beneficial for designing binding sites and optimizing the sorption performance of sorbent materials. Given that actinides have not been extensively studied because of their scarcity, An-MOFs provide a robust platform for investigating the chemical nature of these elements, which is critical for the effective management of the nuclear fuel cycle and nuclear waste.