Preparation Of Silicoaluminous Inorganic Adsorbent Based On Molecular Design And Their Adsorption Mechanism For Uranium Removal

Nuclear power is an important part of China's energy structure.The safe development of nuclear power is the main way to achieve the goal of carbon emission peak and carbon neutralization.However,the uranium containing waste water produced in the process of nuclear fuel cycle will result in security risks to the ecological environment and residents,which need to be properly treated and disposed.As the most potential and economical method,adsorption has attracted much attention.The key to the successful implementation of adsorption method is the development of adsorption material.Therefore,how to efficiently design and develop adsorption material from the perspective of molecular design and improve its adsorption capability are hot and difficult research topics.In order to solve the above problems,based on the concept of molecular design,used molecular simulation technology to establish molecular models of geopolymers and mesoporous silicon.After that molecular dynamics simulation was applied to simulate adsorption process of UO₂²⁺,NO₃^-,and H₂O in the channel of geopolymer and mesoporous silicon.Using this simulation results as a guide to prepare two type of inorganic silicon-aluminum adsorbents named metakaolin-based geopolymer(MKG)and hollow mesoporous silicon(HMS).The U(?)adsorption capability of MKG1,MKG2,HMS,MCM-41 and SBA-15 at different p H,dosage,adsorption time,initial concentration and temperature were comparatively studied.Combining simulation results with experimental characterization to clarify the adsorption and diffusion laws of UO₂²⁺,NO₃^-and H₂O in the channel of geopolymer and mesoporous silicon.The main contents and conclusions are as follows:(1)Using Na₂Si₂O₅ molecular model as the initial structure,the geopolymer models with different Si/Al ratios(3,4 and 5)and different channel widths(2.5,3.5 and 4.5 nm)were constructed by replacing Si in the Na₂Si₂O₅ molecular structure with Al.UO₂²⁺,NO₃^-and H₂O were added to the above models and molecular dynamics simulation was carried out under Clay FF force field.The effects of Si/Al ratio and channel size on the adsorption and diffusion of UO₂²⁺,NO₃^-and H₂O can be described by density distribution,radial distribution function(RDF)of UO₂²⁺-OH,NO₃^--OH and H₂O-OH,hydrogen bond network,mean square displacement(MSD)and diffusion coefficient.The results show that geopolymer has the strongest adsorption capacity for UO₂²⁺,followed by NO₃^-,and finally H₂O.They are distributed in layers on the surface of geopolymer to form electric double layer,however adsorption mechanism is completely different.In addition,with the increase of Si/Al ratio,the adsorption capability of ions and water molecules decreased,while the diffusion capability increased.Although the channel width has no greater influence on the adsorption of ions and water molecules than Si/Al ratio,it also has a great influence on the adsorption.UO₂²⁺,NO₃^-and H₂O are more likely to gather in the narrow channel of geopolymer.(2)Using SiO₂ glass molecular model as the initial structure,molecular models of mesoporous silicon with different channel sizes(2.5,3.5 and 4.5 nm)were constructed.Molecular dynamics simulation was carried out under Clay FF force field to reveal adsorption and diffusion of UO₂²⁺,NO₃^-and H₂O in mesoporous silicon with different channel sizes.The results of molecular dynamics simulation were described by density distribution,RDF of UO₂²⁺-OH,NO₃^--OH and H₂O-OH,average H bond number of H₂O,MSD and diffusion coefficient of ions and water molecules.The results indicate that mesoporous silicon exhibits different adsorption capability for UO₂²⁺,NO₃^-and H₂O,as well as its adsorption mechanism for ions and water molecules is different from geopolymer.In addition,with the increase of channel size,the adsorption capability of mesoporous silicon decreases,which result in the enhancement of diffusion ability of ions and water molecules.Therefore,UO₂²⁺,NO₃^-and H₂O are more easily adsorbed by mesoporous silicon with smaller channel size.(3)According to the results of molecular dynamics simulation,geopolymer adsorbents MKG1 and MKG2 with Si/Al=2 and different channel sizes were prepared.The adsorption behavior and mechanism of MKG1 and MKG2 for U(?)were studied under different p H,dosage,initial concentration,adsorption time and temperature.It can be found that MKG2 with smaller channel size has a better adsorption capability.The results show that both MKG1 and MKG2 adsorbents have the best adsorption performance at p H 3.Moreover,the adsorption kinetics conforms to pseudo-first-order kinetic model and adsorption isotherm conforms to Langmuir model.Molecular dynamics simulation and spectrum analysis show that the adsorption of U(VI)on MKG adsorbent is mainly physical adsorption,and chemical adsorption is the secondary.The diffusion of Na⁺in MKG molecular structure and ion exchange between Na⁺and H⁺could enhance electronegativity of MKG surface,which improve the electrostatic adsorption of MKG for U(?).This process is physical adsorption.When Na⁺diffuse into the solution,MKG molecular framework needs to absorb cations to maintain charge balance.At this time,ion exchange between U(?)ion and Na⁺occurs,it is chemical adsorption.After U(?)ions are adsorbed on the MKG surface due to electrostatic attraction and ion exchange,the-OH on the MKG surface will coordinate with U(?)ion,and it is chemical adsorption.(4)Based on molecular dynamics simulation,HMS with average channel size of 2.5 nm and short channel structure was synthesized.Compared its U(?)adsorption capability with adsorption performance of MCM-41 and SBA-15 whose channel size gradually increases,their adsorption behaviors and mechanisms for U(?)were studied at different p H,dosage,initial concentration,adsorption time and temperature.The results show that the optimal p H of HMS,MCM-41 and SBA-15 for U(?)adsorption is 6.Adsorption kinetics conforms to pseudo-second-order kinetic model and adsorption isotherm conforms to Langmuir model.Molecular dynamics simulation and spectrum analysis indicate that the adsorption of U(?)on mesoporous silicon is mainly chemical adsorption,physical adsorption is the secondary.When p H is 6,the deprotonation occurs on the surface of HMS,MCM-41 and SBA-15,which leads to the increase of surface electronegativity.At this time,the adsorption of U(?)on mesoporous silicon is physical action,and the narrower channel makes electrostatic attraction between interface and U(?)ions stronger.After U(?)ions are adsorbed on the surface of mesoporous silicon,they can coordinate with-OH on surface,it is chemical adsorption.Due to there are abundant hydroxyl groups on the surface of mesoporous silicon,chemical adsorption is dominant in the whole adsorption process.