Study On Modification And Composite Molding Of UiO-66 Series Metal-Organic Framework Materials And Their Uranium Adsorption Properties

Nuclear energy,as a high-density and pollution-free clean energy source,has become a new type of energy expected to replace traditional fossil fuels since the 21st century.Uranium,as an important nuclear fuel,is the key to solving the sustainable development of nuclear energy by ensuring the efficient extraction and utilization of uranium resources.Currently,uranium ore on land is the main source of uranium resources,but it is severely limited by its limited reserves.In contrast,there is a large amount of uranium in the ocean,approximately 1,000 times that on land.Therefore,developing efficient,convenient,and economical seawater uranium extraction technology is crucial for the utilization of nuclear energy.Adsorption is one of the widely researched methods in the field of seawater uranium extraction due to its high efficiency,convenience,and pollution-free characteristics.However,the low concentration of uranium ions in seawater（～3.3 ppb）and the coexistence of a large number of other ions greatly increase the difficulty of uranium extraction.Therefore,developing adsorbents with high adsorption capacity and selectivity is the key to solving the current problem.Metal-organic framework materials（MOFs）have advantages such as a large specific surface area,designability,and functionalization,making them have a wide range of applications in the field of adsorption.However,pure MOF materials lack corresponding active functional groups,which results in problems with adsorption capacity and selectivity.By modifying and grafting active functional groups,the adsorption capacity and selectivity of MOFs can be effectively enhanced.Furthermore,MOFs are usually in powder form and cause significant losses during water treatment processes.Therefore,loading MOFs onto aerogel materials can effectively reduce the loss of adsorbent materials and save costs.In this article,the UiO-66 series of metal-organic framework materials with good water resistance and corrosion resistance were selected to address the limited adsorption capacity,poor selectivity,and difficult recovery of materials for seawater uranium extraction.The following experiments were conducted:（1）UiO-66-L₂ and UiO-66-L₃,containing imidazole ring ligands with different nitrogen positions,were prepared by functional group modification.The uranium adsorption experiment results showed that their optimal pH values were both 9,partly because their zeta potential is positive under alkaline conditions,and U（Ⅵ）exists as the（UO₂）₃（OH）₇^-anion under alkaline conditions,which increases the adsorption capacity due to the electrostatic attraction.The adsorption capacities were 487.4 mg g^-1 and415.2 mg g^-1,respectively,and UiO-66-L₂ had a theoretical maximum adsorption capacity of up to 2526.3 mg g^-1.The XPS spectra of N 1s showed a shift after adsorption of uranyl ions,and a new O=U=O peak appeared,indicating that nitrogen positions have a significant effect on uranium adsorption performance.Thus,UiO-66-L₂with a higher nitrogen content had significantly higher uranium adsorption performance than UiO-66-L₃.（2）By functionalization using UiO-66-2CN as a precursor,UiO-66-2AO containing a bis-amidoxime group was successfully prepared.Uranium adsorption experiments showed that the optimal pH was 5 and the adsorption capacity was 299.6mg g^-1,nearly three times higher than UiO-66-AO containing a single amidoxime group(106 mg g^-1,pH=5.5).Additionally,UiO-66-2AO exhibited fast adsorption kinetics,reaching adsorption equilibrium in approximately 4 hours.Furthermore,due to the enhanced density of the amidoxime groups,UiO-66-2AO exhibited excellent adsorption selectivity towards U（Ⅵ）.（3）UiO-66-L₂ with the best adsorption performance was chosen for composite formation with carboxymethyl cellulose sodium to prepare UiO-66-L₂/cellulose composite aerogels.The spherical shape of the aerogels exhibited better physical properties,making them easier to store and transport.Radial channels were distributed inside the UiO-66-L₂/cellulose composite aerogel spheres,with UiO-66-L₂ distributed on the interior walls of the channels,allowing for easy capture of uranium（Ⅵ）ions.Mapping results showed that uranium（Ⅵ）ions were successfully captured inside the spheres.After composite formation,UiO-66-L₂/cellulose composite aerogels maintained good adsorption performance,reaching approximately 80%of the adsorption capacity of UiO-66-L₂ powder.Additionally,their cyclic adsorption performance was improved,with a removal rate decrease of only 5%after 5 cycles of adsorption and desorption.In dynamic adsorption experiments simulating seawater,uranium（Ⅵ）ions were completely removed in just 16 days.The UiO-66-L₂/cellulose composite aerogels have broad application prospects.This study successfully improved the adsorption capacity and selectivity of UiO-66series adsorbent materials through modification and composite formation,and significantly enhanced their recyclability and reusability by loading them onto cellulose aerogels.This provides a strategy for the design and formation of uranium adsorbent materials.