Theoretical Investigations Of Actinides In Aqueous Solution

The aqueous solution chemistry of actinides is an important part of actinide chemistry.Because of the likely existence of a variety of oxidation states,the study of actinides in aqueous solution is a complex task.In this dissertation,we focus on the theoretical study of the structures,natural charges,thermodynamics and kinetics properties of uranyl and neptunyl complexes.Even though the amount of uranium in seawater is about 4.5 billion tons,the concentration is extremely low.Therefore,it is of significant importance to efficiently extract uranium from seawater.However,the extraction of uranium from seawater currently faces many challenges,such as the low efficiency of extractants.Based on the model complexes [UO2?L??H2O?n]0/+,we studied the uranyl complexes with following ligands,acetate,bicarbonate,carbonate,acetamidoximate,and glutarimidedioximate ligands.The calculation results of the geometries,vibrational frequencies,natural charges,and bond orders show that the bind strength of amidoximate-based ligands is superior and uranyl-amidoximate complexes are more stable than the others.The thermodynamics and the linear transit energy calculations demonstrate that amidoximate-based ligands are preferred thermodynamically,but they are less efficient in terms of kinetics than carboxylic ligands.This part of our research gives a theoretical support for the design,analysis,and selection of high-performance ligands for uranium extraction from seawater.Uranium exists in a dominant form of uranyl carbonate complex in seawater.While quantum chemistry methods can give more accurate results,they often lack the consideration of the effects of explicit solvent molecules because of the limitation of the size of the system.In this dissertation,we use molecular dynamics?MD?simulation and the parameter-optimized AMBER force field to study the uranyl and neptunyl carbonate complex in aqueous solution.After radial distribution function?RDF?analysis,compared with the data of EXAFS and quantum chemistry,the geometry structures of actinyl carbonate complexes demonstrate that the MD simulation method and the force field used are reliable for the study of the uranyl and neptunyl complexes.In addition,the RDF analysis of the solvent water molecules gives the number of the secondcoordination sphere of uranyl carbonate complex,which provides a model for more accurate quantum chemistry calculations and ab initio MD?AIMD?calculations.Finally,the analysis of the total energy of system indicates the existence of counter ions around the uranyl carbonate anion,such as Ca²⁺,which gives a lower total energy of system.