Study On Preparation And Performance Of MOF Bulk Adsorbents With Photothermal Enhancement Of Uranium Extraction From Seawater

Extracting radioactive uranium from natural seawater is considered to be one of the most promising methods to solve the shortage of uranium resources.Metal organic framework material（MOF）has a high specific surface area and excellent ion selectivity,and has gradually become a potential uranium adsorbent.However,the current MOF crystal has a low uranium adsorption capacity and is difficult to recover,so its application to seawater extraction of uranium still has certain limitations.In this paper,Zr-MOF（UiO-66-NH₂）is used as the main body of the adsorbent.The thermodynamic behavior of uranium adsorption of UiO-66-NH₂ is first studied.The experiment found that the equilibrium adsorption capacity of uranium increases with the increase of temperature,indicating that the adsorption process belongs to The endothermic behavior is consistent with the uranium adsorption behavior of other types of MOF（such as UiO-66,HKUST-1,etc.）adsorbents.In this paper,suitable light-to-heat conversion substrates are screened to realize the compact growth of MOF in situ on its surface.The photothermal effect is used to increase the temperature of the adsorbent,thereby further improving the uranium adsorption performance.In addition,the MOF block structure with good mechanical properties is also conducive to the recovery and regeneration of the adsorbent.First,a lightweight carbonized sponge（CS）with high porosity and high light-to-heat conversion efficiency is used as the substrate material.UiO-66-NH₂ was grown in situ on the CS-PDA pre-coated with dopamine（PDA）to prepare the UiO-66-NH₂@CS-PDA bulk adsorbent with excellent light-to-heat conversion performance.The MOF can be recycled and reused.The experimental results show that UiO-66-NH₂ nanoparticles have good coverage and adhesion on CS,and exhibit a higher loading rate（>85wt%）than most MOF composites.The adsorption performance test under light field and dark field was carried out under simulated sea water environment.The results show that the surface temperature of the UiO-66-NH₂@CS-PDA adsorbent increases under the light field,which leads to a significant improvement in the selectivity,adsorption performance and cycle stability of the adsorbent.A 4-week uranium adsorption test was conducted in a natural seawater environment.Compared with the dark field conditions,the uranium adsorption capacity of the adsorbent under the light field increased by 32.37%to 5.52mg/g,which was mainly due to the uranium adsorption The endothermic behavior of the process.In addition,the elastic properties of CS are essential to slow down the impact of water flow and prevent material breakage.Although carbon sponge has good elasticity,its tensile strength is low.This paper further replaces CS with wood cellulose aerogel to improve the mechanical properties of UiO-66-NH₂ block adsorbent.By destroying the cell walls of lignin and hemicellulose in the wood,a cellulose aerogel（CNF）with high mechanical strength and high specific surface area is obtained.The abundant coordination functional groups（such as-OH）on the surface of the cellulose aerogel are used to realize the in-situ growth of UiO-66-NH₂,and the MOF loading rate is 45wt%.The MOF loading is further improved.By pre-coating the PDA layer on the MOF,the nucleation growth of the MOF is realized again,thereby obtaining a double-layer MOF adsorbent and structure,so that the MOF loading reaches 73wt%.On the one hand,PDA as a nucleation center increases the loading rate of MOF;on the other hand,PDA as a photothermal medium improves the photothermal conversion capacity of the adsorbent.After 50 days of natural seawater testing,the study found that compared with dark field,the uranium adsorption capacity of the adsorbent under the light field increased by 37%,reaching 6.23mg/g.In addition,the mechanical properties of the adsorbent are greatly improved compared to CS.The adsorbent can withstand a tensile strength of up to 35MPa in the axial direction of the fiber,showing good tensile strength.At the same time,cellulose aerogel also exhibits good compression resistance in water.Even after repeated compression/release 1000 times under a constant strain of 50%,its stress retention rate is still as high as 90%.The research in this paper provides a new solution for the application of MOF adsorbent in natural seawater extraction of uranium.