Adsorption Of Uranium On Clay Minerals And Colloids Irradiated By Gamma Rays

Nuclear energy is a safe and efficient clean energy.While nuclear energy brings huge economic benefits,it also produces a large amount of radioactive waste.Radioactive waste disposal is an urgent problem to be solved in the sustainable development and application of nuclear energy.Among them,the disposal of high-level radioactive waste is an important part of the common concern of the international community.At present,the internationally recognized high-level radioactive waste disposal scheme is deep geological disposal.Alxa area in Inner Mongolia is one of the preselection areas for high-level radioactive waste disposal repository in China（the surrounding rock of the repository is Alxa clay,and the preferred buffer material is Gaomiaozi bentonite（GMZ bentonite））.During deep geological disposal,high-level radioactive waste will release a large amount of radioactive rays for a long time and act on the repository.Long time irradiation will affect the stability of the repository to a certain extent,and then affect the adsorption and retardation of radionuclides.In addition,in the long-term disposal process,the groundwater around the high-level radioactive waste disposal repository will infiltrate along with the geological fissures and form colloids with the surrounding rock and buffer backfill materials of the repository.These colloids interact with radionuclides,which will lead to nuclide migration,thus affecting the ecological environment around the repository.However,after long-term irradiation,the nature and structure of the surrounding rock and backfill materials of the repository,as well as the nature of the colloid formed by their interaction with groundwater solution,need to be explored.At present,few scholars have conducted relevant research on Alxa clay,GMZ bentonite and colloids prepared with it before and after irradiation.Therefore,in this paper,for Alxa clay and GMZ bentonite,the changes of their structural stability after gamma ray irradiation and their adsorption behavior to U（Ⅵ）under different conditions were investigated in detail.At the same time,the changes of colloidal stability prepared by Alxa clay and GMZ bentonite after irradiation and the adsorption behavior of these two colloids to U（Ⅵ）were also explored.The research results provide a basis for further elucidating the environmental impact of Alxa clay and GMZ bentonite after irradiation and the interaction between colloid and nuclide formed after irradiation.The main results of this paper are as follows:（1）A variety of characterization methods were used to test and analyze the Alxa clay before and after irradiation.A variety of characterization methods were used to test and analyze the Alxa clay before and after irradiation.The results showed that the surface morphology of Alxa clay changed to some extent after irradiation.The partial dilapidation of tetrahedral or octahedral structure,the dehydroxylation and water radiolysis resulted in a change in the interlayer spacing of the sample,and a small change in thermal stability.The percentage of Fe²⁺in Alxa clay increased and the percentage of Fe³⁺decreased affected by irradiation and radiolysis products.The adsorption behavior of U（Ⅵ）on irradiated Alashan clay was studied by static adsorption batch experiment.The effects of pH value,groundwater,ion species and strength and temperature on the adsorption of U（Ⅵ）were discussed in detail.The results show that Langmuir model can better describe the adsorption process of U（Ⅵ）on Alxa clay.This adsorption process is a spontaneous endothermic reaction,and heating is conducive to adsorption.The Influence of pH is great on the adsorption of U（Ⅵ）on Alxa clay.The adsorption mechanism is mainly ion exchange and surface complexation.The groundwater solution at different pH all has a strong inhibitory effect on the adsorption of U（Ⅵ）on Alxa clay.Mg²⁺and HCO₃^-ions and high ion concentrations can effectively reduce the adsorption of U（Ⅵ）.Resolution rates are significantly affected by pH and ions.Unirradiated Alxa clay showed better adsorption performance for U（Ⅵ）in solution.（2）The characterization of the irradiated GMZ bentonite shows that the thermal stability of the irradiated bentonite is affected.and the bentonite lamella structure changes to some extent Because of lattice shrinkage due to skeleton fracture,hydrogen bond disruption and interlayer expansion due to dehydroxylation.XPS analysis proved that content of Fe²⁺and Fe³⁺in bentonite changed.The adsorption behavior of U（Ⅵ）on irradiated bentonite was studied by static adsorption batch experiments.The following conclusions were drawn:the adsorption is mainly controlled by ion exchange or outer-sphere complexation at low pH so that the adsorption capacity is low.With the increase of pH,the outer-sphere complexation gradually becomes the inner-sphere complexation,and the stability is enhanced,which increases the adsorption effect.Groundwater solution has a great influence on the adsorption of U（Ⅵ）on irradiated bentonite.The adsorption process can well conform to the pseudo-second-order kinetic equation,and the Langmuir model can better simulate the adsorption of U（Ⅵ）on bentonite.The adsorption process is a spontaneous endothermic reaction,and the temperature rise is beneficial to the adsorption.In the experiments of ions and ion concentrations,the order of inhibition effect was as follows:HCO₃^->Ca²⁺>Mg²⁺>SO₄^2->Na⁺.The irradiated bentonite increased the resolution rate of U（Ⅵ）.（3）Several characteristic diffraction peaks disappear after Alxa clay and GMZ bentonite form a colloid which indicate the types of minerals that make up the colloid decrease,and the interlayer spacing of the bentonite colloid increases.The percentages of O,Si,Al,Fe and other elements in the clay mineral colloid changed before and after irradiation.Simultaneously small angle scattering proved that the microstructure of bentonite colloids changed little after irradiation.The stability of colloid is greatly affected by pH value,storage time,ion and ion concentration and temperature.In addition,there are a lot of anions and cations in groundwater,which inhibit the formation of colloids.Overall the colloidal stability of unirradiated GMZ bentonite is higher than that of irradiated bentonite,while the colloidal stability of irradiated Alxa clay is better.（4）The adsorption of U（Ⅵ）on unirradiated and irradiated Alxa clay colloids is a fast kinetic equilibrium process,which is characterized by strong chemical adsorption.The adsorption is greatly affected by pH and ions,which is manifested by that the adsorption mechanism of colloid at low pH is mainly ion exchange and outer-sphere complexation for their poor stability and it turns to inner-sphere complexation at higher pH.In addition,high ion concentration tends to reduce the colloidal stability,resulting in a decrease in the adsorption partition coefficient for U（Ⅵ）.And humic acid and Alxa clay colloids are easy to form humic acid-clay colloid complexes,which can effectively improve the stability of the colloids and increase the adsorption of U（Ⅵ）.In general,the adsorption capacity of the irradiated Alxa clay colloid for U（Ⅵ）was larger,and it increased with the increase of the dose.（5）U（Ⅵ）on GMZ bentonite colloids before and after irradiation was studied by static adsorption batch experiments,and the results showed that the adsorption of U（Ⅵ）on bentonite colloids was a fast process,and its adsorption partition coefficient is greatly affected by pH.The percentage content of Si and Al atoms of GMZ bentonite colloid after irradiation decreased,and its adsorption percentage content of U（Ⅵ）also decreased.At the same time,the small angle scattering test proved that the colloidal structure after adsorption of U（Ⅵ）was basically stable.When humic acid exists,it will form a stable complex with bentonite and U（Ⅵ）,which is conducive to increasing the adsorption capacity of U（Ⅵ）.In conclusion,the non-irradiated GMZ bentonite colloid has better adsorption performance for U（Ⅵ）,and the adsorption capacity decreases with the increase of irradiation dose.