Study On The Mechanism Of The Functional TiO₂ For The Photocatalytic Reduction Of Uranium

With the advantages of high energy density,nuclear energy is recognized as one of the promising alternative candidates to traditional fossil energy sources.The highly fluorinated-containing uranium wastewater,which is the most widely present in the process of uranium enrichment,uranium conversion,and nuclear fuel element manufacturing,is characterized by high fluoride ion concentration and uranium content.So,it is important for the efficient separation of uranium from highly fluorinated-containing uranium wastewater.In recent decades,a large amount of research work has been devoted to the development of uranium removal and recovery technologies.In particular,the reduction of the highly toxic and soluble U（VI）to the relatively less toxic and insoluble U（IV）is attracting increasing research interest and is considered a reasonable solution.Compared with the conventional uranium enrichment separation methods,photocatalysis is a feasible way to achieve efficient separation of uranium wastewater because of its selectivity for non-reducing metal interfering ions.Due to its large band gap and lack of uranyl ion coordination sites at the catalyst interface,conventional metal oxide photocatalysts lack visible light response,which makes it difficult to deal with the strong coordination of F-U in fluorinated-containing uranium wastewater and cannot achieve efficient photoreduction of U（VI）in fluorinated-containing uranium wastewater.Therefore,in this paper,the TiO₂-based photocatalytic materials with low band gap and abundant surface uranyl coordination sites were successfully prepared,and the ability of the prepared materials for U（VI）extraction in a complex simulated fluorine-containing uranium wastewater system was also investigated.The main conclusions are as follows:1.To address the problems of large band gaps and lack of coordination sites in conventional semiconductors,Density Function Theory（DFT）calculations are proposed as a conceptualization idea for material design.We prepared the restricted-site rich TiO₂ photocatalyst（Mn-TiO₂@PO₄）with ultrafast carrier separation by custom-coupled surface functional modification of the semiconductor energy band.The theoretical saturation catalytic capacity of the Mn-TiO₂@PO₄photocatalyst is up to 680 mg·g^-1.Specifically,the prepared Mn-TiO₂@PO₄ photocatalyst has a low band gap and is abundant.Furthermore,mechanistic studies showed that when Mn-TiO₂@PO₄ is exposed to light,photogenerated electrons are transferred from the conduction band of the semiconductor to the uranyl ion coordinated by the phosphate group.The photogenerated electrons directly reduce U（VI）while generating·O₂^-which further leads to a stable crystalline phase of（UO₂）O₂·2H₂O.Thus,this targeted semiconductor material design approach to synthesize Mn-TiO₂@PO₄ photocatalysts has good potential for photocatalytic UO₂²⁺from low concentrations of fluorine-containing uranium environments.2.To address the problem of limited phosphate groups in Mn-TiO2@PO4 for uranium extraction from highly fluorinated uranium,the construction of single-atom and organic phosphate double sites was proposed to solve the problems of large band gap and insufficient ligand sites.Mn-SAs/TiO₂@HEDP photocatalysts with dual-active locations were synthesized by building single atomic locations on the TiO₂ surface.Combined with X-ray absorption near edge structure（XANES）determinated that Mn-SAs are successfully confined to TiO₂surfaces with a stable Mn-O4-Ti4 structure,Mn-SAs together with surface organic phosphate sites from the TiO₂-surface dual point center was successfully prepared.The unique coordination structure of Mn-SAs/TiO₂@HEDP and uranyl ions ensures the effective capture of uranyl by the phosphate group in U（IV）solution with an F^-concentration of 5 g/L,and the removal efficiency of U（VI）remained as high as 88%.Correspondingly,Mn-SAs/TiO₂@HEDP also show significant uranium extraction ability in other complex environmental systems.In addition,the photocatalytic mechanism shows that Mn-SAs/TiO₂@HEDP has a unique electron transfer mechanism during the photocatalytic process with a U（IV）reduction ratio as high as 88%.Therefore,Mn-SAs/TiO₂@HEDP has good potential for photocatalytic U（VI）reduction in a high fluorine-containing uranium environment for the nuclear industry.By studying the energy band structure of TiO₂ and its surface phosphoric acid modification,the modified material with a low band gap and rich phosphoric acid groups on the surface has good potential for the photocatalytic reduction of U（VI）in a high fluorine-containing uranium environment.This work not only provides an effective photocatalyst for the photoreduction of U（VI）but also provides a new solution for the treatment of highly fluorinated-containing uranium wastewater in the nuclear industry.