Investigation On The Retention Of Eu(Ⅲ) By Typical Phosphate/Phosphonate Materials And Its Interaction Mechanisms

The great development and utilization of nuclear energy have brought about great economic benefits to human being.However,a large amount of radioactive waste which is accompanied by the development of nuclear energy threatens the survival of human beings.In order to achieve sustainable nuclear energy development,it is necessary to ensure that radioactive waste generated in the nuclear fuel cycle process is properly disposed.For the treatment of high-level liquid waste,it is mainly to store the high-level waste liquid in the address repository deep underground.However,during a long-term geological change,the cladding material used to wrap high-level liquid waste may be corroded and cracked,leading to the leakage of radionuclides into the natural environment.Generally,the high-level radioactive waste contains a large number of trivalent minor atinides(e.g.,241Am and 244Cm)with long-live and high radioactivity.These trivalent minor actinides easily dissolve in the groundwater and migrate to environment,causing serious biosphere contamination.The migration and transformation of radionuclides in the environment is closely associated with the natural minerals.On the one hand,the clear insight into the macroscopic sorption/desorption behavior and the microscopic interaction mechanisms of radionuclides are conducive to predict the fate of radionuclides in environment.On the other hand,accordin g to the appearance of nuclide in nature,it is necessary to seek low-cost materials with high-efficiency pollution control for retarding the migration of these minor actinides,and to control the pollution of minor actinides in groundwater.Meantime,it is a good way to understand the rules of migration and transformation of radionuclides in environment system by investigating the underlying reactive mechanisms of radionuclides on materials.In addition,the microscopic mechanisms also provide the inspiration of design for the newly synthetic materials with high-efficiency.Phosphate/phosphonate-material is one of effective materials for the removal of heavy metal from solution and shows a broad application prospect in large numbers of studies.However,at present,the studies on removal of trivalent actinide by phosphate/phosphonate-material is not enough.Additionally,there is no clear explanation for the underlying reactive mechanisms of radionuclides by material.Europium(Eu(Ⅲ))is used as a chemical analogue of trivalent minor actinides(e.g.;241 Am(Ⅲ)and 244Cm(Ⅲ))due to their similar behavior in wet chemistry for the non-radioactive experiment.The macroscopic sorption behaviors and microscopic interaction mechanisms of trivalent minor actinides/Eu(Ⅲ)on different phosphate/phosphonate-materials in water environment were investigated in this thesis by using a combination of macroscopic batch experiments,spectrum characterization,equilibrium modeling by Visual MINTEQ software and XAS analysis.The main research findings are listed as followed:(1)The release of trivalent minor actinides into groundwater may lead to the migration of radionuclides to environment and the interactions of radionuclides with natural rocks and minerals.U(Ⅵ)is abundant in natural environment in the form of autunite on the phosphate-rich conditions.It is the first time to investigate the sorption behavior and underlying mechanisms of Eu(Ⅲ)on Ca-autunite by using Eu(Ⅲ)as a chemical analogue of Am(Ⅲ)and Cm(Ⅲ)in experiment.The focuses of this experiment is the affinity of phosphate towards the co-existing hexavalent actinides U(Ⅵ)and trivalent actinides/Eu(Ⅲ)on the solid surface containing phosphate.Under certain conditions,sorbed Eu(Ⅲ)exists in the form of an insoluble phosphate(EuPO4)after the reaction.The coordination environment of uranyl which was bonded with phosphate before reaction changes consequentially after Eu(Ⅲ)"catch" the phosphate from autunite.We tracked the changes of U(Ⅵ)chemical environment after the sorption of Eu(Ⅲ)by using spectroscopic technique and revealed the interrelation between sorbed Eu(Ⅲ)and the uranyl that change the chemical environment.The results of this study reveal the strong ability of Eu(Ⅲ)to "catch" phosphate to some extent,and provide an important reference for further study about the competition between trivalent actinide and hexavalent actinide for phosphate.(2)The removal efficiency and the retention mechanisms of Eu(Ⅲ)by two-dimensional phosphate minerals(e.g.;autunite)has been extended to the study on the removal of Eu(Ⅲ)in groundwater and its microscopic mechanisms by three-dimensional inorganic phosphate material,hydroxyapatite.Apatite is the most abound phosphate mineral in nature and has been successfully used to remediate the contamination of heavy metal in groundwater.However,the study focused on the removal of trivalent actinides/Eu(Ⅲ)by apatite is insufficient.Little work has fully explain the intrinsic interaction mechanisms between apatite and trivalent actinides/Eu(Ⅲ).Therefore,it is currently unclear that apatite can be used to remove the contamination of trivalent minor actinides from solution based on existing work.According to the conventional batch results,this work revealed that Eu(Ⅲ)forms a mass of insoluble europium phosphate(EuPO4 H2O)under certain experimental conditions,which could be removed from aqueous solution.Furthermore,according to the different dominate mechanisms under different conditions of Eu(Ⅲ),the possible mechanisms of various heavy metal ions/radionuclides on hydroxyapatite are summarized by analogy.On the basis of the experimental findings,the feasibility of using hydroxyapatite for remediation of polluted groundwater was further evaluated.(3)Based on the high affinity of phosphate functional group towards trivalent actinides/Eu(Ⅲ),the low-cost inorganic material,titanate nanotubes(TNTs),was decorated by phytic acid.The removal of Eu(Ⅲ)by PA/TNTs was investigated in this study and the results show that PA/TNTs exhibits high selectivity for Eu(Ⅲ)from a simulated wastewater containing multiple competing metal ions.According to the analysis of XPS results,the predominant removal mechanisms of PA/TNTs are inner-sphere surface complexation.(4)Based on the effectiveness of inorganic phosphate materials in the capturing Eu(Ⅲ),the study is extended to the application of organic phosphonate materials(SZ-4)in the remediation of Eu(Ⅲ).According to the Hard-Soft-Acid-Base,the soft organic amino between layers is easier to interact with soft cations than hard ones(e.g.;Eu3+,Am3+ and Cm3+).Therefore,Ca2+ was used to pre-treat the zirconium phosphonate(SZ-4)to reduce the content of organic ammonia between crystal layers.Additionally,the hydration ion radius of Ca2+ between inter-layer of Ca@SZ-4 is very similar to that of Eu3+,enabling Eu3+ to exchange ions directly on Ca@SZ-4.Ca@SZ-4 shows a great performance on Eu(Ⅲ)by comparison with the original SZ-4.This thesis shows the various sorption behavior and different underlying mechanisms of trivalent minor actinides/Eu(Ⅲ)by typical phosphate/phosphonate material under various hydrochemical conditions from a molecular-level perspective.The research findings in this thesis not only accurately reveal the physical and chemical behavior of trivalent minor actinides/Eu(Ⅲ)in water environment,but also have a meaningful theoretical reference value for the design and synthesis of advanced materials for the remediation of groundwater trivalent radioactive pollution.