Theoretical Exploration For Controlling Of U(Ⅵ) By Minerals:Adsorption By Montmorillonite And Orthoclase And Reduction Mechanisms Under Action Of Fe(Ⅱ) And Humic Acid

Uranium（U）is a common radioactive element and the most important nuclear fuel.The various anthropogenic activities associated with uranium,such as mining,milling and application,nuclear energy production,nuclear test explosions,the manufacture of nuclear weapons,and accidents of nuclear power plants,significantly aggravate the contamination to environment and endangerment to living creatures.Hexavalent（U（Ⅵ））is the main form of uranium in the natural environment with high solubility and mobility.Numerous studies have shown that soli minerals exhibit excellent adsorption properties for U（Ⅵ）due to their large specific surface area,abundant surface active groups and high negative charge density,and therefore can be used as good U（Ⅵ）adsorbents.Compared with adsorption,reduction of U（Ⅵ）to insoluble U（IV）species followed by precipitation is a superior way to control uranium contamination,significantly reducing uranium migration.It is of great significance to study the adsorption and reduction mechanism of U（Ⅵ）at the surfaces of soil minerals at a molecular level for a comprehensive understanding of the role of soil minerals in uranium pollution control.In this thesis,density functional theory（DFT）calculations have been conducted to study the adsorption behavior and reduction mechanisms of U（Ⅵ）at the surfaces of soil minerals,providing insightful information from a microscopic perspective.Firstly,the adsorption configuration and stability of U（Ⅵ）at the montmorillonite surface,as well as the effect of substitution ions have been elucidated.On this basis,the specific process and internal mechanism of U（Ⅵ）reduction by humic acid or/and structural Fe（Ⅱ）at the surface of montmorillonite are explored,revealing the influence of the coexistence of two reducing agents on U（Ⅵ）reduction.Finally,the reaction mechanism of U（Ⅵ）reduction by humic acid or/and adsorbed Fe（Ⅱ）at the orthoclase surface is investigated.The main results are shown below:（1）Using montmorillonite as a model,the adsorption properties of uranyl complexes at the more reactive edge surface are investigated in this paper,and it was found that the edge surface of montmorillonite exhibited a strong adsorption capacity for U（Ⅵ）with a minimum adsorption energy of-190.4 k J/mol.The unsaturated Mg（Ⅱ）site at the surface increases the diversity of U（Ⅵ）adsorption configurations,enabling both monodentate and bidentate complexes to exist stably:U（Ⅵ）acts at unsaturated Mg（Ⅱ）site via double-bonded oxygen atom or hydroxyl ligand oxygen atom;U center simultaneously bonds to surface Mg-H₂O and Si-OH;and U（Ⅵ）bonds to Si-OH with its double-bonded oxygen acts on Mg（Ⅱ）site.The hydrogen bonds formed upon U（Ⅵ）adsorption significantly enhance the thermodynamic stability of the adsorption configurations.In addition,negatively charged double-bonded oxygen,hydroxyl ligand oxygen,and hydrogen transfer reactions are also beneficial to the stabilization of the adsorption structure.When Mg（Ⅱ）is replaced by Fe（Ⅱ）,Fe（Ⅱ）does not affect the adsorption mode of U（Ⅵ）and does not cause U（Ⅵ）reduction.（2）Humic acid and ferrous ions,which are widely present in soil,are selected as U（Ⅵ）reducing agents to investigate the reduction mechanism of U（Ⅵ）at montmorillonite edge surface.When humic acid reduces U（Ⅵ）alone,it can be reduced to U（V）or U（IV）with moderate reaction energy barriers,and the specific reduction degree depends on the way U interacts with the surface.For example,U（IV）is the dominant product when U（Ⅵ）interacts with Mg（Ⅱ）via double bonded oxygen atom,while U（V）is the preferred product when it acts on the Mg（Ⅱ）site via the hydroxyl ligand oxygen atom.Fe（Ⅱ）reduces U（Ⅵ）to U（V）with a minimum energy barrier of only 8.7 k J/mol,and the reduction effect is better than that of humic acid.The coexistence of Fe（Ⅱ）with humic acid has a facilitating effect on U（Ⅵ）reduction.Fe（Ⅱ）is involved in charge transfer and significantly promotes the reduction of U（Ⅵ）by huminic acid.Vice versa,that is,humic acid also facilitates Fe（Ⅱ）to reduce U（Ⅵ）to U（V）,resulting in U（V）formation almost spontaneously.Both are involved in the reaction,humic acid and structural Fe（Ⅱ）promote each other,and U（IV）is produced spontaneously under their combined action.（3）The mechanism for U（Ⅵ）reduction by adsorbed Fe（Ⅱ）and impacts for coexistence of humic acid are addressed,using orthoclase as a model.Adsorbed Fe（Ⅱ）transforms U（Ⅵ）to U（IV）facilely,showing higher reactivity than humic acid.Its role is further examined by changing the Fe（Ⅱ）content（n）that decides the extent of U（Ⅵ）reduction:U（IV）,U（V）and U（Ⅵ）dominate respectively for n=2,1,and 0.Mechanisms with co-existence of humic acid are rather distinct.Only one of humic acid and Fe（Ⅱ）acts as reducing agent.Humic acid greatly alters Fe（Ⅱ）reactivity,and vice versa.Fe（Ⅱ）participates in charge transfers and significantly enhances humic acid reactivity,while replacement by Mg（Ⅱ）causes adverse effects.Both humic acid and Fe（Ⅱ）act as reducing agents,and their combined action results in not only U（IV）,but also U（ⅡI）and U（Ⅱ）of lower oxidation states,which recently become a focus of synthesis and hence U（Ⅵ）is turned into useful materials.The results of this thesis fully demonstrate that soil minerals can act as good sorbents for uranium contaminants.Humic acid and Fe（Ⅱ）at the surfaces of soil minerals can reduce U（Ⅵ）to the less soluble U（IV）,elucidating the specific reaction mechanism.In addition,detailed electronic structure analyses have been provided to guide experimental studies.These results will help to design more effective U（Ⅵ）adsorbents and reduction systems in the future,and also provide theoretical support for the prevention and control of uranium pollution in soil environment.