Preparation Of Porous Nanocomposite Adsorbents And Their Adsorption Properties For Uranium(?) Ions

During the process of nuclear fuel cycle production,large amounts of uranium-containing wastewater would be released arising from the mining of uranium?U????,preparation of nuclear fuel and reprocessing of spent fuel,which poses great potential threat to earth creatures and human environment due to the high chemical and radiological toxicity of U???.Hence,from the point of energetic and environmental security,enrichment and recovery of U???is of vital significance.Among various enrichment and recovery techniques,adsorption has attracted increasing interest owing to its simplicity,low-cost,versatility and so on.The key of adsorption is to select the highly effective adsorbent.Since inoganic/organic composite adsorbents could play a synergistic effect of each component,they have become the research trend of high efficiency adsorbents at present.However,composite adsorbents suffer from some disadvantages,including complicated preparation technology,pollution from toxic chemical solvent,long preparing period,high cost,etc.Recently,three dimensional?3D?porous monoliths,such as aerogel and hydrogel,have attracted extensive attention in the applications of adsorption of metal ions due to the tunable pore size,high specific surface areas,low density,easy recycling etc.However,research on the selective adsorption of radioactive U???by 3D functional porous nanocomposites is relative rare.In view of this,in this dissertation,we adopted one-dimensional?1D?attipulgite?AT?nanorods and two-dimensional?2D?graphene oxide?GO?nanosheets as the precursors,designed and prepared 3D porous nanocomposite aerogels,and studied their adsorption behavior for U???.Further,through the first hydrogen peroxide?H₂O₂?etching and the following hydrothermal self-assembly with phytic acid?PA?,we prepared highly porous phosphate-functionalized conductive graphene?HGP?hydrogel,and used it as electrode material to study the electrosorption behavior for U???.The main content of the dissertation is summarized as follows:?1?Preparation of AT@PDA/CS aerogel and its adsorption properties for U???ions1D AT nanoparticles were first modified by PDA,and then composited with chitosan?CS?to prepare 3D porous AT@PDA/CS aerogel.Its surface morphology and structure features were characterized by TEM?SEM?EDS?FTIR?XPS,etc.Its adsorption behavior towards U???was also investigated in detail.The results showed that the as-prepared aerogel possessed an interconnected 3D porous network structure,a loose and coarse surface due to the introduction of AT@PDA into CS matrix,and abundant homogeneous distributed nitrogen-and oxygen-containing functional groups.According to the adsorption experiments,the kinetic study suggested that its adsorption equilibrium could be reached within 40 min,and adsorption kinetics behavior fitted well with the pseudo-second-order kinetic model,which demonstrated that the adsorption process of U???onto AT@PDA/CS aerogel was mainly ruled by the chemisorption rather than the internal diffusion.Langmuir isotherm model could described its adsorption isotherms very well and the calculated maximum adsorption capacity was 175.1 mg g^-1 at 328K and pH 5.0.The thermodynamic analysis revealed that the adsorption was an endothermic and spontaneous process.Moreover,the removal ratio of AT@PDA/CS aerogel towards U???could be maintained 78%after five adsorption-desorption cycles,indicating its feasibility of reusability.?2?Preparation of GO@PDA/CS aerogel and its adsorption properties for U???ionsA 3D porous GO@PDA/CS aerogel was successfully prepared using 2D GO nanosheets as skeletons by a"sol-cryo"method.Its microstructure,thermal and mechanical stability,as well as the adsorption behavior towards U???were investigated in detail.The results showed that GO@PDA/CS aerogel possessed a 3D interconnected homogenous distributed and well-organized porous structure.Due to the modification by PDA,the enhanced active sites on the surface of GO were formed.According to the adsorption experiments,the maximum removal ratio of GO@PDA/CS towards U???appeared at pH 5.5-7.0,and its adsorption behavior was nearly not affected by ionic strength of solution,meaning that the inner-sphere surface complexation,rather than the outer-sphere surface complexation or ion exchange,dominated the adsorption of U???.The kinetics equilibrium of GO@PDA/CS toward U???was realized within 40 min,and the kinetic data of GO@PDA/CS fitted the pseudo-second-order kinetic model well.Its adsorption isotherms followed the Langmuir isotherm model and the maximum adsorption capacity was determined as 415.9 mg g^-1 at 298 K and pH6.0.The XPS result indicated that the coordination interaction were formed between oxygen-and nitrogen-containing groups of GO@PDA/CS and U???ions,and nitrogen-containing groups played a major role during the adsorption process.In addition,the removal ratio of GO@PDA/CS still reached⁹2%after six adsorption-desorption cycles,indicating its superior reusability.?3?Preparation of HGP hydrogel and its electrosorption properties for U???ionsA highly porous phosphate-functionalized graphene?HGP?hydrogel was prepared by a two step process,including the first hydrogen peroxide?H₂O₂?etching and the following hydrothermal self-assembly with phytic acid?PA?.Then,the as-prepared HGP hydrogel was used as binder free electrodes for U???electrosorption.The results showed that HGP possessed a large amount of pores,especially substantial mesopores and macropores,and abundant phosphate groups in its structure,which endowed it with higher specific surface area and pore volume,superior wettability as well as electrochemical properties.According to electrosorption experiments,the removal ratio of HGP towards U???progressively increased with the potential increasing,and came up to the maximum at 1.2 V.According to the kinetics results,the kinetics equilibrium of HGP electrode could be reached within 21min,and applied potential favored to enhance the adsorption rate effectively.The adsorption isotherms of HGP was fitted well with Langmuir model,and the calculated electrosorption capacity was as high as 545.7 mg g^-1.The XPS results suggested that physisorption,chemisorption as well as electrostatic adsorption were all contributed to its higher electrosorption capacity,and the latter two played a more important role during the electrsorption process.More importantly,the HGP electrode showed high selectivity towards U???due to the electrostatic interaction and complexation affinity between phosphate groups and U???in the presence of other competitive metal ions with different valence.In addition,a two step desorption method was successfully realized the regeneration of HGP electrode,and its removal ratio of U???only decreased 7.5%after six cycles.