Research On The Application Of Graphene Oxide Membranes In Radiochemical Separation

As a kind of clean energy,nuclear energy has become an important option for countries to solve the contradiction between rapid socio-economic development,energy demand and environmental protection.A large amount of radioactive waste were produced during the front-end production of nuclear fuel,nuclear power plant operation,spent fuel reprocessing and other nuclear fuel cycle process.The composition of these waste streams are complex and highly radioactive and may cause direct or potential threats to human health and ecological environment.Therefore,the separation process of radionuclides from radioactive liquid waste is essential for promoting the sustainable development of nuclear energy,which can reduce harm to the environment and facilitate further geological disposal.Thin laminates of a layered structure formed by single layer of graphene oxide?GO?nanosheets in close stacking have unique selective mass transfer characteristic and the GO membranes could precisely sieve and separate the molecular and ion based on the different size of their hydrated radius,where the permeation was driven by concentration gradients in aqueous solution.The main ions in the radioactive waste liquid could be separated by GO membranes because their hydrated radius are obviously different.However,the permeation mechanism of ions through the GO membranes is not clear.In order to study the feasibility of separating metal ions in radioactive waste liquid by using GO membranes,this thesis systematically explored the permeation behavior of the major ions in high level-liquid waste?HLLW?through the GO membranes.Firstly,GO was synthesized by improved Hummers method and was exfoliated into monolayer flakes by sonication in deionized water to obtain the stable GO suspension.Then,the 5 ?m-thick GO membranes were prepared by vacuum filtration method.The resulted GO membranes were characterized by atomic force microscopy?AFM?,fourier transform infrared spectroscopy?FT-IR?,X-ray diffraction?XRD?,Raman spectroscopy?Raman?,scanning electron microscopy?SEM?and transmission electron microscopy?TEM?.The permeation behavior of the major ions in HLLW through the GO membranes was investigated by using the homemade U-type device.The penetration and separation behavior of ions through GO membranes were studied as functions of the initial concentration of ions,acidity of the solution,complexing agent and the mixed ions.The results revealed that the permeation rates of ions through GO membranes markedly decreased with the increasing of hydrated ionic radii.Cs⁺,Sr²⁺,Cl-and NO₃-permeated through GO membranes quickly,whereas lanthanide ions and actinide ions infiltrated much more slowly.Moreover,the permeation rates of ions on GO membranes were proportional to the initial concentration of ions,and increased with the increase of ion concentrations.However,the increase of ion concentrations had different degree of influence on the ions with different hydrated ion radius,thus could improve the selectivity factor by changing the initial concentrations.The interlayer distance of GO membranes in the acidic solution was closely related to the acidity.When the acidity is more than 0.1 M,the interlayer distance decreased with increasing of acidity.Therefore,the permeability of the ions on the GO membranes decreased with increasing acidity,but the selectivity factor between the ions increased.Finally,the barrier separation tests indicated that GO membranes had promising ability for the separation of Cs⁺ and Sr²⁺ from lanthanide and actinide ions due to the difference of hydrated ion radius,and the separation factor would reach about 10 in single penetration.In addition,the permeation rate of H⁺ through GO membrane was higher than that of lanthanide and actinide metal ions by more than two orders of magnitude,which suggested that GO membranes could be used for the removal of acid from radioactive waste.The presented results demonstrate that the GO membrane is a promising candidate as an efficient and environmentally friendly materials for radiochemical separation.In addition,in order to remove 90 Sr with high heat-generating and long half-life from high level-liquid waste?HLLW?,an inorganic/organic composite ion exchanger polyantimony/polyacrylonitrile?PAA-PAN?with good chemical and adsorption stability under irradiation and acidic media was synthesized,and a series of batch adsorption and column separation experiments were carried out.The batch adsorption experiments indicated that the PAA-PAN composite adsorbent had good adsorption property to Sr²⁺.The adsorption thermodynamic parameters showed that the ion exchange was endothermic and spontaneous reaction,and the equilibrium adsorption data fit well with the Langmuir adsorption isotherms which was based on the theory of monolayer adsorption.PAA-PAN composite exhibited much higher adsorption affinity for Sr²⁺ within broad range of acidity than that for the other fission products?FPs?from simulated HLLW and the maximum uptake capacity of Sr²⁺ can reach upto 20.05 mg g^-1 on PAA-PAN composite in 3 mol L^-1 nitric acid.Furthermore,the interfering cations of Na⁺,Fe³⁺,Cs+,Ru³⁺,Rh²⁺,Zr⁴⁺,UO2²⁺,Th⁴⁺ and Ce³⁺ had little impact on the adsorption of Sr²⁺ on PAA-PAN composite.Through column operation,Sr²⁺ was retained on the PAA-PAN column and the other elements were quantitatively recovered.As a result,Sr²⁺ was selectively transferred onto the PAA-PAN resins from the simulated HLLW.PAA-PAN can be used for the selective removal of Sr²⁺ from HLLW,to reduce the radioactivity and heat releasing of HLLW.