Study On The Reduction Mineralization Of U(Ⅵ) Ions Based On Fe(Ⅱ)/Iron Oxides(Minerals)

The disposal of nuclear waste is a significant challenge in the peaceful use of nuclear energy.In deep geological repositories for nuclear waste,steel storage tanks may undergo anaerobic corrosion,leading to the release of radioactive nuclides and the formation of iron oxides(minerals),while also generating a large amount of Fe(Ⅱ)ions.The free Fe(Ⅱ)interacts with iron oxides(minerals),forming a coupled reaction system of Fe(Ⅱ)/iron oxides(minerals),which can cause reduction mineralization of U(Ⅵ)ions with higher migration.This thesis explored the impact of three forms of Fe(Ⅱ),namely free Fe(Ⅱ),structural Fe(Ⅱ),and adsorbed Fe(Ⅱ),on the reduction mineralization reaction of U(Ⅵ)ions by constructing a coupled reaction system between Fe(Ⅱ)and iron oxides(minerals),and also investigated the influence of Zn isomorphic substitution on the reduction of U(Ⅵ)ions by the Fe(Ⅱ)/iron oxides(minerals)system.The iron oxides(minerals)studied in this thesis are magnetite,maghemite with Fe(Ⅱ)oxidized in the structure,and franklinite with Fe(Ⅱ)replaced by Zn in the structure.Firstly,based on the coupled reaction system of Fe(Ⅱ)/iron oxide(mineral),the reduction performance of free Fe(Ⅱ),structural Fe(Ⅱ),and adsorbed Fe(Ⅱ)on U(Ⅵ)was studied.Then,the redox reactions mechanism of U on the surface of iron oxides(minerals)and the mineralization nucleation mechanism of U(Ⅵ)reduced on the surface of iron oxides(minerals)were studied by electrochemical methods.Finally,based on density functional theory(DFT),the reaction potential energy curve of Fe(Ⅱ)/U(Ⅵ)and iron oxides(minerals)particles was constructed to obtain the reaction kinetic process from diffusion collision to formation of outer-sphere complexation and inner-sphere complexation,as well as electron transfer process when Fe(Ⅱ)and U(Ⅵ)are co-adsorbed on the surface of iron oxides(mineral).The main research content and conclusions are as follow:The adsorption performance of Fe(Ⅱ)/iron oxides(minerals)systems on U(Ⅵ)was explored.Under anaerobic conditions,an increase in Fe(Ⅱ)concentration and prolonged interaction time can promote the transformation of maghemite and franklinite to magnetite,and the adsorption of Fe(Ⅱ)causes an increase in the surface potential of iron oxides(minerals).Thermodynamic adsorption experiments showed that iron oxides(minerals)mainly chemically adsorb U(Ⅵ),and the coupling effect of Fe(Ⅱ)promotes the adsorption of U(Ⅵ)by maghemite and franklinite,while inhibiting the adsorption of U(Ⅵ)by magnetite.The reduction performance of free Fe(Ⅱ),structural Fe(Ⅱ),and adsorbed Fe(Ⅱ)on U(Ⅵ)was studied.Different forms of Fe(Ⅱ)have different reduction performances on U(Ⅵ),it was found that the reduction performance of adsorbed Fe(Ⅱ)on U(Ⅵ)was stronger than that of structural Fe(Ⅱ),and both adsorbed and structural Fe(Ⅱ)as electron donors promoted the reduction of U(Ⅵ).The study also found that free and adsorbed Fe(Ⅱ)constituted a redox cycle of Fe(Ⅱ)/Fe(III),and the presence of free Fe(Ⅱ)promoted the reduction of U(Ⅵ).Based on the kinetic results of U(Ⅵ)adsorption and reduction on iron oxides(minerals),it was found that Fe(Ⅱ)/magnetite had the highest maximum reduction amount of U(Ⅵ)but the slowest reduction rate,while Fe(Ⅱ)/franklinite had a lower maximum reduction amount of U(Ⅵ)but the highest reduction rate due to its higher adsorption rate for U(Ⅵ).This is attributed to the fact that the adsorption rate of U(Ⅵ)on the coupling system of Fe(Ⅱ)with magnetite is greater than that on franklinite,indicating that the reduction of U(Ⅵ)on iron oxides(minerals)surfaces is limited by the adsorption process.Also,it is concluded that electron transfer occurs at mineral heterointerfaces while free Fe(Ⅱ)cannot directly reduce U(Ⅵ).The redox reactions mechanism of U on the surface of iron oxides(minerals)and the mineralization nucleation mechanism of U(Ⅵ)reduced on the surface of iron oxides(minerals)have been revealed.According to cyclic voltammetry curves,under cathodic scanning,U(Ⅵ)is first reduced to U(V),accompanied by a disproportionation reaction facilitated by H~+,partial U(V)is generated into U(Ⅵ)and U(IV).Under anodic scanning,U(V)is oxidized to U(Ⅵ),and the U(IV)generated by the disproportionation reaction is oxidized to U(Ⅵ)through an oxidation reaction.This indicates that U(Ⅵ)on iron oxides(minerals)surfaces is first reduced to U(V),then generated into U(IV)through a disproportionation reaction.According to chronoamperometry tests,the reduction-mineralization nucleation of U(Ⅵ)on three types of iron oxides(minerals)changes from instantaneous nucleation to continuous nucleation as the potential decreases.At lower potentials,the nucleation rate of U(Ⅵ)is mainly limited by the diffusion process of U(Ⅵ)towards the electrode surface.This study clarified the kinetics of Fe(Ⅱ)/U(Ⅵ)complexation on iron oxides(minerals)surfaces,as well as the electron transfer process between Fe(Ⅱ)and U(Ⅵ)during their co-adsorption on iron oxides(minerals)surfaces.The study showed that the rate of outer-sphere complex formation between Fe(Ⅱ)/U(Ⅵ)and iron oxides(minerals)was hardly influenced by the type of the iron oxides(minerals),and Fe(Ⅱ)forming outer-sphere complexes was faster than U(Ⅵ).Inner-sphere complexation between U(Ⅵ)and magnetite occurs faster than with maghemite and franklinite,while inner-sphere complexation between Fe(Ⅱ)and magnetite occurs slower than with maghemite and franklinite.This suggests that U(Ⅵ)readily forms inner-sphere complexes with magnetite,while Fe(Ⅱ)readily forms inner-sphere complexes with maghemite and franklinite.A redox reaction model was constructed for the co-adsorption of Fe(Ⅱ)and U(Ⅵ)on iron oxides(minerals)surfaces.It was found that in the absence of adsorbed Fe(Ⅱ),U(Ⅵ)only underwent reduction to U(V)on magnetite surfaces.However,when co-adsorbed with Fe(Ⅱ),U(Ⅵ)also underwent reduction on maghemite and franklinite surfaces,indicating that adsorbed Fe(Ⅱ)can transfer electrons to U(Ⅵ)on iron oxides(minerals)surfaces.Through the above research,the reaction mechanism for the reduction mineralization of U(Ⅵ)ions at iron oxide(mineral)interfaces has become clearer.In addition,by understanding and mastering the mechanism of magnetite’s reduction mineralization of U(Ⅵ),not only does it provide a theoretical basis for the application of magnetite nanomaterials in environmental governance and restoration,but it also provides inspiration for uranium extraction from seawater.This thesis contains 66 pictures,18 forms,and 205 pieces of refrences.