A Computer Simulation Study Of Uranyl Adsorbed On The Interfaces Between Clay Minerals And Water

Clay minerals are ubiquitous components in soils and sediments.Their unique physical and chemical properties,such as large specific surface area,swelling,cation exchange capacity and pH-depended adsorption ability,make them important adsorption materials.More and more information in atomic and molecular level is needed in both industry and laboratory.Although almost all of the available experimental techniques and some approximate theories have been employed in the research of clay minerals,many such aspects cannot be provided by modern experimental techniques unfortunately.Computer simulation can remedy these deficiencies by predicting these microscopic properties.In this thesis,by employing computer simulations,we investigate the adsorption of uranyl complexes on surfaces of clay minerals,which includes adsorption of uranyl and uranyl-carbonate complex on the clay minerals.First,we use molecular dynamics simulation to study the mechanisms involved in the adsorption of aqueous uranyl species and uranyl-carbonate complexes to the basal surfaces of clay minerals,including kaolinite,pyrophyllite and montmorillonite.The results show that uranyl ion can form outer-sphere complexes on siloxane surfaces only,but monocarbonato uranyl species can adsorb on both siloxane surface and gibbsite surface.Strong outer-sphere adsorptions of the dicarbonato and tricarbonato uranyl complexes on' gibbsite surface are observed.Outer-sphere adsorptions of uranyl and uranyl-carbonato species on siloxane surface and gibbsite surface occur through different mechanisms:electrostatic interaction and hydrogen bonds,respectively.Then,as a further study,we investigate the adsorption of these uranyl complexes on the?010?surface of phyllosilicates with first-principles molecular dynamics?FPMD?.In this study,we mainly focus on two possible adsorption sites on?010?surface:?Al?OH?₂ and ?SiO.The simulations show that uranyl and.uranyl-carboanto species can form bidentate complexes with the surface ligands,which can be expressed as ?Al?OH?₂-UO₂-?H₂O?3,?Al?OH?₂-UO₂-?CO₃??H₂O?,?Al?OH?₂-UO₂-?CO₃?₂,??SiO??AlOH?-UO₂-?H₂O?3,??SiO??AlOH?-UO₂-?CO₃??H₂O?and ?SiO-AlOH-UO₂-?CO₃?₂.When dicarbonato uranyl complex adsorbs on the ?SiO site,two coordinated carbonates bond with uranyl ion in different ways:one is monodentate and the other is bidentate,which makes the total coordination number of uranyl is 5 in this case.Through the study in this thesis,we explore the adsorption behavior of uranyl and uranyl-carbonate complexes on different surfaces of clay minerals.We reveal the adsorption forms.and microscopic structures of uranyl complexes on the basal surfaces and edge surfaces,respectively.