Synthesis And Properties Of ZIFs-Based Porous Materials For Radioactive Iodide Anions Oxidation-Adsorption

Nuclear energy,due to its clean,high-efficiency and without excessive greenhouse gas emissions,is considered to be the most promising alternative to traditional fossil fuels.However,the application of nuclear power may cause radioactive pollution,which limits its development.Radioactive iodine,as one of the radionuclides,poses a serious threat to the biological and ecological environment when released into the environment.Thus,effective disposal of radioactive iodine is of great significance for the sustainable development of nuclear power.In this dissertation,a variety of low-cost and efficient porous materials were constructed to fill the gaps existing in silver-and copper-based adsorbents.The iodide anions（I^-）asorption performance and the removal mechanism of each material were investigated by using non-radioactive ¹²⁷I（Na I）as substitute which has the same chemical properties as radioactive iodine.The main contents and conclusions of this dissertation are described as follows:（1）Nano mixed-valence silver oxide modified zeolitic imidazolate framework-8（ZIF-8）nanocomposite（Ag₂O-Ag₂O₃@ZIF-8）was synthesized via a simple method.The morphology,microstructure and phase composition were characterized,and then the adsorption performance and removal mechanism for I^-were studied.The existence of Ag had no effect on the morphology of ZIF-8,and Ag mainly existed on the surface and channels of ZIF-8 in the form of Ag₂O₃ and Ag₂O.The adsorption capacity of Ag₂O-Ag₂O₃@ZIF-8 toward I^-was up to 232.1 mg g^-1,and the adsorption equilibrium could be reached within 2.5 min.In addition,the adsorption was barely affected by p H and competitive anions,even in the simulated salt-lake water.The removal mechanism of Ag₂O-Ag₂O₃@ZIF-8 could be summarized as three aspects:adsorption,oxidation,and re-adsorption,that was,the adsorption of Ag₂O for I^-to form Ag I,the oxidization of Ag₂O₃ for I^-to form I₂,and the re-adsorption of Ag I for I₂ to form Ag I₃.（2）Silver-decorated ZIF-8 derived Zn O（Ag@Zn O CNCs）was prepared by using ZIF-8 as precursor.The prepared material was characterized and applied to investigate its adsorption performance and the removal mechanism for I^-under visible light.The Zn O derived from ZIF-8 had concave cubic morphology.After modification,Ag mainly existed in the form of Ag and Ag₂O.Ag@Zn O CNCs exhibited high adsorption capacity(up to 129.3mg g^-1)and fast adsorption kinetics under visible light.The adsorption isothermal and kinetic analyses demonstrated that the adsorption process,with spontaneity and exothermic,followed the Langmuir isothermal model and pseudo-second-order kinetic model.Additionally,Ag@Zn O CNCs also had good adsorption selectivity,strong acid resistance and excellentγradiation resistance.The removal mechanism of Ag@Zn O CNCs for I^-could be summarized as the adsorption of Ag₂O for I^-and the photooxidation of Ag⁰@Zn O for I^-.DFT calculations further revealed that the photooxidation products of I^-could be captured by nearby Ag or Ag I sites yielding Ag I or Ag I₃.（3）In order to reduce the cost of I^-elimination,we tried to construct new copper-based material to achieve synergistic removal of I^-by using new method.For this,nanowhisker zero-valent copper（nw Cu⁰）decorated Zn O（nw Cu⁰@Zn O）were fabricated by two-step pyrolysis of bimetallic Cu-ZnZIFs precursors.The as-fabricated materials were extensively characterized and then applied to study its adsorption performance and removal mechanism toward iodide.The results showed that Cu⁰ nanoparticles were uniformly dispersed on the Zn O matrix,and the morphology and the amount of Cu⁰ could be tuned by adjusting the molar ratio of Cu/Zn in the bimetallic ZIFs precursors.The adsorption performance of nw Cu⁰@Zn O was related to the pyrolysis temperature,Cu⁰ content and oxygen concentration.When the pyrolysis temperature and Cu/Zn molar ratio were 800℃and 5:5 respectively,the adsorption capacity could reach 220.6 mg g^-1.Moreover,the adsorption capacity could be further increased to 270.8 mg g^-1 under the condition of sufficient oxygen.In addition,the material also exhibited high adsorption selectivity,strong acid resistance,and cycling adsorption performance.The excellent removal performance could be attributed to that nw Cu⁰induced molecular oxygen activation in water to release Cu⁺and H₂O₂,resulting in I^-being captured by Cu⁺and oxidized by H₂O₂.DFT calculations further revealed that Cu and Zn sites could be used as the capture centers for I^-and I₃^-,nevertheless Zn sites had stronger affinity for I₃^-than I^-,while Cu sites showed the opposite.（4）Cu-Zn@C nanocomposite was prepared by pyrolysis of bimetallic Cu-ZnZIFs precursors at N₂ atmosphere.After carbonization,the Cu-Zn@C maintained the cubic morphology,and the specific surface area was significantly higher than that of Cu-ZnZIFs precursors.The introduction of Cu was also beneficial to improve the adsorption capacity of Cu-Zn@C for I^-to a certain extent.But the adsorption capacity decreased with the increasing of Cu/Zn molar ratio.When the Cu/Zn molar ratio and the solution p H were 1:9 and 3respectively,the adsorption capacity could reach 235.5 mg g^-1.The removal mechanism could be concluded that iodide anions were immobilized to carbon atom in the form of covalent bond I-C.In addition,due to the occurrence of Fenton reaction of Cu⁰ within regenerated sample,Cu（I）,Cu（II）and reactive oxygen species（H₂O₂,·OH,etc.）were released,which resulted in a significant increase in the adsorption capacity of the regenerated adsorbent compared to the fresh one.Overall,four porous nanocomposites were developed by using ZIFs as the matrix and precursors,explores the law of its interaction with I^-,and confirms the synergistic and efficient removal mechanism of"adsorption-oxidation-re-adsorption"for I^-.The results obtained provides an important reference for the design of porous nanomaterials for subsequent synergistic and efficient removal of radioactive I^-.