Efficient Capture Of Thorium And Uranyl Ion By β-Ray Irradiation Resistant Metal-Organic Frameworks

Effective radionuclide capture from nuclear wastewater is critical for the safe use of nuclear energy and resource recycling but still faces a large challenge.Due to large specific surface area,abundant active sites,and convenient structural designability,metal-organic frameworks（MOFs）exhibit great potential for radionuclide adsorption.However,with the rapid development of adsorption for nuclear wastewater treatment,adsorbents need to maintain structural stability under a radiation environment,especially inβ-ray radiated environment.Therefore,it is of great significance to screen and design MOF adsorbents with bothβ-ray irradiation stability and excellent adsorption capacity.In this paper,theβ-ray irradiation resistance of several common MOFs was investigated and the characteristics of stable structures were obtained.Besides,based on MOFs with ion trap andβ-ray irradiation resistance,the adsorption ability of thorium ion（Th（Ⅳ））and uranyl ion（U（Ⅵ））was investigated,while the microscopic interaction mechanism between the material and the target ion was deeply revealed.The detail contents are as follows:（1）The structural change behaviors of five MOF materials with different metal centers（UiO-66（Zr）,MIL-53（Al）,MIL-101（Cr）,ZIF-8（Zn）,and HKUST-1（Cu））underβ-ray irradiation were investigated.The irradiation experiments were conducted in a wide range of doses from 0 to 1000 kGy,and it was found that the five MOF materials was ranked as MIL-53（Al）>UiO-66（Zr）>MIL-101（Cr）>ZIF-8（Zn）>HKUST-1（Cu）,and the strength of the coordination bond formed by the metal center and ligand,the cross-sectional area of the radiation energy absorbed by the metal and the sensitivity to the redox reaction were the important factors affecting the stability of β-ray irradiation.In addition,the adsorption capacities of MIL-53（Al）and UiO-66（Zr）for Th（Ⅳ）and U（Ⅵ）remained essentially unchanged before and after irradiation,showing good potential for nuclear applications.（2）An aluminum-based metal-organic framework（MOF-303）with high-density ion traps was used to selectively capture Th（Ⅳ）from an aqueous solution.The results of the experimental study show that the features of the aluminum center endow this material with excellent resistance fromβ-ray irradiation even with a high dose of 1000 kGy.Further,due to the special chelation interaction from the ion trap,MOF-303 exhibits a large adsorption capacity for Th（Ⅳ）ion(461.7 mg g^-1)and excellent separation coefficients up to 97.6,97.3,and 81.3 for Th（Ⅳ）/Pr（Ⅲ）,Th（Ⅳ）/Eu（Ⅲ）and Th（Ⅳ）/Nd（Ⅲ）respectively.Besides,the regeneration of MOF-303 as well as the elution of the Th（Ⅳ）ion can be achieved via an acid-wash process.Further mechanistic studies based on quantum chemical theory calculations indicate that Th（Ⅳ）ion can be tightly anchored in the ion trap surrounded by six active atoms,via the chemical bonding mode.At the same time,the ion trap consisting of 6 active atoms（4 O and 2 N）has higher binding energy and is more stable than the ion trap consisting of 5 active atoms（3 O and 2 N）,and therefore the 6-atom trap is considered to be the most suitable adsorption site.（3）A titanium-based metal-organic framework（IEF-11）with high-density oxygen-rich ion trap was used to selectively capture Th（Ⅳ）and U（Ⅵ）from an aqueous solution.Due to the special arrangement of the coordination bond,the square acid ligand connectivity and lower metal node density,IEF-11 has excellent resistance toβ-ray irradiation,even under 1000kGy irradiation.Meanwhile,owing to the special chelating effect of the oxygen-rich ion traps,the maximum adsorption amounts of IEF-11 for Th（Ⅳ）（p H=3.0）and U（Ⅵ）（p H=5.0）ions were 305.9 mg g^-1and 240.7 mg g^-1,and the separation coefficients exceeded 200 for Th（Ⅳ）/Nd（Ⅲ）,Th（Ⅳ）/Sm（Ⅲ）and Th（Ⅳ）/Eu（Ⅲ）,and 100 for U（Ⅵ）/Eu（Ⅲ）,U（Ⅵ）/La（Ⅲ）and U（Ⅵ）/Sr（Ⅱ）.Moreover,IEF-11 has fast adsorption kinetics,and the adsorption equilibrium for both ions is reached after about 100 min.After four regeneration cycles,the adsorption amount of IEF-11 remained almost consistent with that of fresh samples.Finally,experimental and theoretical calculations indicate that Th（Ⅳ）and U（Ⅵ）ions are anchored in the ion trap by active O atoms in the form of chemical bonds.At the same time,due to the large atomic spacing of the square ion trap（classⅡtrap）,it is difficult to contribute to all atoms for the coordination of the target ion,while the active O atoms in the circular pore trap（class I trap）can be used for all the coordination and have uniform bond lengths.Thus,the circular ion trap（classⅠtrap）is considered to be the best adsorption site.