Diffusion And Adsorption Of Re(Ⅶ)and Se(Ⅳ)in Tamusu Clay Rock

Clay rock in Tamusu Candidate area of Alxa league of Inner Mongolia autonomous region is one of the geological disposal sites for high radioactive waste in China.As an alternative surrounding rock for disposal site,clay rock has the advantages of low water conductivity and strong self-sealing ability.Many countries in the world have selected clay rock as surrounding rock for disposal site research.The safe disposal of high-level radioactive waste is one of the prominent problems in the development of nuclear energy and the utilization of nuclear technology.The weak adsorptive elements in high radioactive wastes have a fast migration rate in geological materials,which is the focus of the safety evaluation of the disposal repository.The migration mode of weak adsorptive elements in clay rock is mainly diffusion,whose diffusion behavior is affected by the characteristics of clay rock and groundwater（composition,ionic strength,pH value）.This paper select Re（Ⅶ）(instead of⁹⁹Tc（Ⅶ）)and Se（Ⅳ）(instead of ⁷⁹ Se（Ⅳ）)for representative of weak adsorption species,using static adsorption method and a variety of diffusion method,studied under the condition of different Re（Ⅶ）and Se（Ⅳ）in the diffusion behavior of lignin clay rock,this paper discusses Re（Ⅶ）and Se（Ⅳ）the diffusion law of diffusion mechanism is derived,for our country to alternate clay rock as the safety assessment of high-level radioactive waste repository surrounding rock provides important theoretical support.The main research contents of this paper are:（1）The adsorption properties of powder clay on Re（Ⅶ）and Se（Ⅳ）under different conditions were studied by batch test.The results show that the adsorption of Re（Ⅶ）and Se（Ⅳ）in clay under different conditions does not interfere with each other,all of them were weakly adsorbed.The adsorption equilibrium time of Re（Ⅶ）and Se（Ⅳ）was 12 hours and 18 hours respectively.The adsorption process of Se（Ⅳ）was in line with the quasi-second-order kinetic model,and was a nonlinear multi-layer adsorption.The q_e was 2.24 mg/g,which was close to the theoretical calculation value.When the pH of the solution was≥4,both q_tand K_d of Se（Ⅳ）in clay decreased with the increase of pH value,while q_tand K_d of Re（Ⅶ）decreased slightly.With the increase of solution ion strength,q_tand K_d of Se（Ⅳ）decreased significantly,while q_tand K_d of Re（Ⅶ）decreased slightly.With the increase of initial concentration of Se（Ⅳ）,q_tincreased,q_max was12.8 mg/g,and K_d decreased with the increase of C₀,which conforms to the Freundlich model and is non-uniform multi-layer adsorption.The D-R model shows that Se（Ⅳ）adsorption in clay is physical adsorption.The q_tand K_d decrease with the increase of solid-liquid ratio and increase with the increase of temperature.Se（Ⅳ）adsorption process is endothermic and spontaneous.In the presence of organic acids in the solution,the increase of q_tand K_d of Se（Ⅳ）in clay was obvious under acidic conditions,while the increase of q_tand K_d was not obvious under neutral and basic conditions.With the increase of organic acid concentration,q_tand K_d did not increase significantly.（2）The simultaneous diffusion behavior of Re（Ⅶ）and Se（Ⅳ）in Tamusu clay rock was studied by capillary method.The results show that the D_a and K_d of Re（Ⅶ）and Se（Ⅳ）in clay decreases with the increase of clay compaction density.With the increase of pH,D_aincreases,while K_d decreases.The D_a of Re（Ⅶ）and Se（Ⅳ）increased as the ionic strength of solution increased,while K_d decreased.The presence of humic acid increases the D_a and K_d of Re（Ⅶ）and Se（Ⅳ）in clay,but the D_a and K_d values obtained by diffusion did not change significantly with the increase of humic acid concentration.（3）The diffusion behavior of Se（Ⅳ）in Tamusu clay rock was studied by increasing the cross-sectional area of diffusion in claystone and using diffusion pool diffusion method.The results show that the D_a and K_d value of Se（Ⅳ）decreases with the increase of the compaction density of clay,and D_a increases with the increase of solution pH,while K_d value decreases.As the solution ion strength increases,D_a increases,while K_d value decreases.The larger the clay compaction density is,the smaller the pores between the clay particles are.The anionic diffusion channel is mainly free water layer,and the ionic strength of the solution will affect the width of free water layer.Under the influence of different conditions,the diffusion coefficient of Se（Ⅳ）in the diffusion pool method in the clay rock is close to that obtained by the capillary method,and the variation law is consistent,indicating that the diffusion effect obtained by the capillary method and the diffusion pool method is consistent,and the small-size experiment is also representative.（4）The simultaneous diffusion behavior of Re（Ⅶ）and Se（Ⅳ）in Tamusu clay rock was studied by means of through-diffusion method.The experimental results showed that the D_eof Re（Ⅶ）and Se（Ⅳ）in clay decreased with the increase of pH,while the K_d of Se（Ⅳ）decreased with the increase of pH,and K_d of Re（Ⅶ）was 0.It is mainly affected by the zero potential of claystone and the existence of nuclides in different pH solutions.With the increase of ion strength,the diffusion D_e increases,while the adsorption K_d of Se（Ⅳ）decreases.The increase of solution I makes the diffusion double layer narrow and the free water layer wide.At the same time,the concentration of anions and cations in the solution increases with the increase of solution I value,and the number of ions competing for adsorption increases,so that the adsorption K_d of Se（Ⅳ）decreases.After the addition of humic acid,the diffusion D_e values of Re（Ⅶ）and Se（Ⅳ）in clay increased in both non-contact and equilibrium tests,indicating that HA formed complex with nuclides,and the diffusion rate of complex was faster.The increase of theαvalue indicated that the presence of HA changed the clay surface properties,which was conducive to the adsorption and diffusion of nuclides.However,after 45 days of contact with HA,the D_e values of Se（Ⅳ）diffusion in clay all increased,indicating that it takes a certain amount of time for Se（Ⅳ）and HA to form complexes.By comparison,it was found that the diffusion of Re（Ⅶ）in the presence of HA was more affected by the HA affected clay characteristics,while the diffusion of Se（Ⅳ）was more affected by the Se-HA complex.Compared with other geological samples,the Tamusu clay rock has better retardation to Re（Ⅶ）and Se（Ⅳ）.Compared with granite,Tamusu clay rock and granite have the same retardation property of Re（Ⅶ）,while the retardation property of Se（Ⅳ）is better than that of granite.