Molecular Simulation Of Adsorption And Diffusion Behaviors Of Uranyl Ions In The Local Microenvironment Of Montmorillonite

The rapid development of nuclear power plants inevitably produces nuclear waste.Uranium is the most abundant element in high-level nuclear waste,which is leaked into the environment and is oxidized into uranyl ions(UO₂²⁺),which poses a serious threat to human safety and natural environment.Deep geological disposal repository is currently a widely accepted method for uranyl removal.Montmorillonite（MMT）,as the main component of the buffer backfill material in the geological disposal repository of nuclear waste,is the last defense line of the artificial barrier to block the migration of nuclides.Therefore,it is very crucial to investigate the adsorption mechanism and blocking behavior of uranyl in the interlayer of MMT for the safety evaluation and design of buffer backfilling materials in nuclear waste geological disposal repository.To date,the microelectronic mechanism for adsorption of UO₂²⁺in the interlayer of MMT and the surface chemical behavior,hydration state,migration and diffusion behavior of UO₂²⁺adsorption in various local micropores of MMT are still unclear.In this paper,Density Function Theory（DFT）and Molecular Dynamics methods were used to simulate the adsorption and diffusion behavior of UO₂²⁺in MMT.The electron transfer and bonding properties between UO₂²⁺and MMT were obtained.And its distribution,hydration state and diffusion behavior of UO₂²⁺in MMT local micropores（a,b,a-b,c axial pores and intergranular pores）were obtained.The main research contents and results are divided into the following parts:（1）Study on the optimum adsorption site of UO₂²⁺·x H₂O in the interlayer of MMT.The adsorption energy of UO₂²⁺·x H₂O（x=3,4,5）above the bottom oxygen atom（signed as T site）,the middle position of two bottom oxygen atoms（signed as B site）and the center of the six-membered ring（signed as C site）in the interlayer silicon-oxygen ring of MMT is study by using DFT+U method.It was found that UO₂²⁺·5H₂O and UO₂²⁺·3H₂O are more likely adsorbed at T site,while UO₂²⁺·4H₂O is preferentially adsorbed at B site.And the optimal C adsorption site did not exist,indicating that the hydroxyl group on the octahedron in MMT（in the center of the six-member ring）had the least effect on its adsorption.Dehydration phenomenon was found in the adsorption of UO₂²⁺·5H₂O and UO₂²⁺·4H₂O,and the dehydrated water molecule was near the tetrahedral replacement of MMT,which illustrates that tetrahedral replacement had the greatest effect on the adsorption of UO₂²⁺·x H₂O.In addition,during the adsorption of UO₂²⁺·x H₂O by MMT,∠O_U-U-O_U bents at the B1 site（167°）in the MMT-UO₂²⁺·4H₂O system and all sites in the MMT-UO₂²⁺·3H₂O system（about 170°）.This phenomenon indicates that with the decrease of water molecules in the UO₂²⁺·x H₂O system,the attraction of the inner surface of MMT to UO₂²⁺increases,resulting in the bending of the bond angle.（2）Study on the electronic structure of UO₂²⁺·x H₂O adsorbed in the interlayer of MMT.The electron structure changes of UO₂²⁺·x H₂O before and after adsorption in the interlayer of MMT was studied by using DFT+U.It can be seen from the energy band and state density diagram of the optimal adsorption configuration that the energy band and state density of the whole system moved to the lower energy level after MMT adsorption of UO₂²⁺·x H₂O,indicating that the optimal adsorption configuration is a stable structure.The electron density difference,Bader charge and Electron localization function（ELF）calculation results show that the O atom（O_M）on the MMT silico-oxygen six-member ring forms hydrogen bond with the H atom in the water ligand UO₂²⁺·x H₂O.As the number of water ligands around UO₂²⁺decreases,UO₂²⁺is directly exposed to the interlayer space of MMT,which results in the formation of weak chemical bonds between the U atom in UO₂²⁺and the O_M atom in MMT.The formation of hydrogen bonds and weak chemical bonds blocked the migration of UO₂²⁺.（3）The adsorption and diffusion behavior of UO₂²⁺in a-axial,b-axial,a-b axial,c-axial porosity and interparticle pore models of MMT were studied by molecular dynamics method.The surface chemical behavior and hydration state of UO₂²⁺and H₂O in various pores of MMT were obtained by analyzing the radial distribution function of UO₂²⁺-O_w,O_w-O_w and H_w-O_w.The results show that in various local micropores of MMT,two layers of hydration shells are formed around UO₂²⁺.Hydration of UO₂²⁺was weakest in interparticle pore.The effect of UO₂²⁺on water molecules is stronger in the c-axial pores.The hydration degree of H₂O in a-axis pore is the strongest,followed by b-axial,c-axial,a-b axial porosity and intergranular pore.In addition,the diffusion coefficients of UO₂²⁺and H₂O in a-axial,b-axial,c-axial,a-b axial porosity and interparticle pore of MMT were calculated by Einstein equation,and the migration and diffusion behaviors of UO₂²⁺and H₂O in the local micropore of MMT were obtained.The results show that diffusion coefficient of UO₂²⁺is the largest in the interparticle pore and the smallest in the b-axis pore.The diffusion coefficient of H₂O is the largest in the interparticle pore and the smallest in a-axis pore.It is observed from the concentration distribution diagram that UO₂²⁺is more distributed in the pores in the a-axis,b-axis and a-b axial porosity models,which may be due to the existence of broken bonds and more negative charges in the local micropores.The distribution of UO₂²⁺in the c-axis pore is mainly concentrated on the MMT surface.The reason for the phenomenon is that the MMT surface has negatively charged due to isomorphism substitution,which result in a strong absorption effect on UO₂²⁺.In the interparticle pore model,more UO₂²⁺is distributed near the surface of MMT,especially the junction of the two MMT models（the angle of a triangular interparticle pore）.In addition to concentrated distribution in the above places,UO₂²⁺is also distributed in the（010）end surface of MMT.In summary,the adsorption mechanism of UO₂²⁺in the interlayer of MMT and their distribution,hydration characteristics and diffusion behavior of in various local micropore of MMT were studied by using DFT+U method and MD method respectively.The theoretical simulation results of this work can provide theoretical guidance and basic data for future relevant experimental research.