The Adsorption Research Of Porous Organic Frameworks For Uranium And Rhenium

As a clean energy with high energy density,nuclear energy can meet the human demand for low-carbon energy.The ocean contains about 4.5 billion tons of uranium and uranium extraction from sea water is an effective way to solve the shortage of terrestrial uranium resources.However,the complex marine environment and extremely low uranium concentration make extracting uranium from actual seawater a serious challenge.In addition,Tc O₄^-,as one of the most difficult radiation pollutants,mainly originates from the nuclear fission process of ²³⁵U and ²³⁹Pu in nuclear power plants.Due to the high mobility,good water solubility,and low complexing ability of Tc O₄^-,most materials are difficult to selectively remove Tc O₄^-from nuclear waste liquids.Therefore,the design of novel materials with excellent performance is very important for the sustainable development of the ecological environment.Covalent organic frameworks（COFs）,a class of porous crystalline materials linked by covalent bonds,possess the advantages of precisely designable structures,regular internal channels,and excellent stability.Based on diverse topologies and abundant building units,we can pre-engineer COFs with specific structures and functions,enabling efficient and rapid ion extraction in complex environments.Therefore,two different types of COFs were designed and synthesized for the extraction of radionuclides in this paper.The main research contents are as follows:1.The selective reduction of highly soluble U（VI）to insoluble U（IV）by photocatalysis is the most ideal method for uranium extraction.Therefore,it is very necessary to design and synthesize porous adsorbents with high photocatalytic activity,excellent stability,and easy regeneration.Herein,a highly conjugated sp²-carbon covalent organic framework（Tp-TMT）was synthesized and captured uranium through three coordinated mechanisms of selective chelation,chemical reduction,and photocatalytic reduction.Tp-TMT exhibits an excellent uranium adsorption capacity(2362.4 mg g^-1),which can be attributed to:（i）Tp-TMT has a large specific surface area and regular porous channel structure,which is conducive to the rapid diffusion and transport of uranyl ions.At the same time,more specific binding sites are exposed to realize the selective adsorption of uranium;（ii）The abundant hydroxyl groups in the skeleton can chemically reduce the adsorbed U（VI）moiety to U（IV）,thereby releasing binding sites for additional adsorption.（iii）The synergistic effect of triazine unit and hydroxyl group can significantly reduce the energy level band gap of Tp-TMT,which greatly improves the photocatalytic performance of U（VI）to U（IV）conversion.As an efficient photocatalytic reduction platform,Tp-TMT is a promising adsorbent for enrichment and separation of uranium from nuclear wastewater.2.With the rapid development of nuclear energy industry,the treatment of Tc O₄^-in nuclear waste liquid is still a huge challenge.However,it remains a challenge to synthesize Tc O₄^-removal materials with excellent radiation resistance,good selectivity,high ion exchange capacity,fast kinetics,and high stability.In view of the similar chemical and physical properties of Re O₄^-and Tc O₄^-,laboratories usually use Re O₄^-as a non-radioactive substitute for Tc O₄^-to evaluate ion exchange performance.Herein,imine-bonded guanidine-based cationic covalent organic frameworks（TBT-TGC）were synthesized for selective removal of Re O₄^-by ion exchange and electrostatic attraction.The regular porous hydrophobic channels in the TBT-TGC framework enhance the diffusion and exchange rates of Re O₄^-and Cl^-.The guanidine-based units in the backbone generates ion binding sites that can efficiently capture Re O₄^-through strong Coulomb interactions.Meanwhile,the chlorine atom as an electron withdrawing group is beneficial to enhance the positive charge density of the guanidine group,and further improving the selectivity and adsorption performance of the material for Re O₄^-.Benefiting from the unique structural characteristics of TBT-TGC,the material exhibits fast removal efficiency and adsorption capacity up to 542.8 mg g^-1.Therefore,the i COFs（TBT-TGC）prepared in this study can effectively remove Re O₄^-and show good potential for application in environmental remediation.