Experimental And Theoretical Study On Photocatalytic Reduction Of Radionuclide Uranium By Ti₃C₂ MXene And G-C₃N₄ Composites

With the rapid progress of nuclear energy technology,the risk of radioactive contamination from radionuclide leaks has continued to grow.Uranium,one of the key elements of the conventional nuclear fuel cycle,poses a serious threat to human health and ecological systems due to its high mobility and biological toxicity.Among various methods for removing radioactive uranium,photocatalytic reduction has attracted tremendous interest because of its environmental friendliness,low cost and efficiency.In recent years,as a popular photocatalytic material,g-C₃N₄ has been rapidly grown into a major area of photocatalytic degradation of pollutants.Unfortunately,for pristine g-C₃N₄ due to the rapid recombination of photogenerated charge carriers and the limited visible-light utilization have impeded its practical application.To solve such limitation,fabricating hierarchical nanostructures or heterogeneous element doping are alternative choices to improve the photocatalytic performance of g-C₃N₄.Two-dimensional layered MXene which has large specific surface area,abundant surface functional groups and excellent electrical conductivity,is suitable for the construction of composite with g-C₃N₄.In this thesis,g-C₃N₄ was combined with MXene to study its photocatalytic reduction of U（VI）in radioactive wastewater by a combination of photocatalytic experiments and density functional theory（DFT）calculations.The specific research contents are as follows:（1）MXene and g-C₃N₄ precursor dicyandiamide were mixed in various ratios by high-speed ball milling and a series of MXene/g-C₃N₄（MXCN）photocatalysts have been fabricated by in-situ thermal synthesis,including MXCN-1,MXCN-2,MXCN-3 and MXCN-4.The various techniques,such as scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray diffraction（XRD）and infrared spectroscopy（FT-IR）instruments were applied to investigate the morphology and microstructure of MXCN.The experimental results showed the MXCN had been successfully synthetized and retained the stacked nanosheet structure of g-C₃N₄.In addition,N2 adsorption-desorption,simultaneous thermal analysis（TGA）and photoelectrochemical experiments were carried out.The results demonstrated that compared to g-C₃N₄,the MXCN displayed superior properties,such as large specific surface area,great thermal stability,a broad photoresponse range,increased visible light adsorption and narrow bandgap.（2）Several experimental conditions,including pH,solid-to-liquid ratio concentration and competing ions,were employed in photocatalytic experiments to investigate the optimal catalytic conditions of MXCN materials for U（VI）.The results demonstrated that the optimal pH for MXCN photocatalytic reaction was at pH=6.While the excellent solid-liquid ratio concentration was 0.5 g/L,and the photocatalytic efficiency was little affected by the concentration of competing ions in the solution.At the optimum pH,all MXCN catalysts showed high photocatalytic activity,whereas the MXCN-2 material performed the best.Under the light irradiation,the MXCN-2 was able to achieve approximately 98.1%reduction of U（Ⅵ）within 90 minutes.Additionally,the photocatalytic reduction of U（Ⅵ）by MXCN-2 remained above 95%even after four cycles,indicating its photocatalytic stability.（3）Utilizing experimental spectra with theoretical calculations,the adsorption behavior and photocatalytic mechanism of MXCN were investigated.According to the XPS spectra,the U（Ⅵ）species have been successfully reduced to U（IV）.Radical capture tests and electron paramagnetic resonance characterisation（EPR）revealed the primary active species in the catalytic system were·O2-and·OH,whereas·O2-had a significant impact on the process.Meanwhile,DFT calculations were applied to explore the interaction between MXCN and uranyl.The adsorption process occurred essentially on the MXene side with an adsorption energy of-3.49 eV.It was determined that there was charge transfer of 1.97 e from MXene to g-C₃N₄ based on the results of the difference charge density and excited state analysis.The separation of HOMO and LUMO molecular orbitals on MXCN improved the separation and transfer of electron-hole pair,consequently playing a significant role in the photocatalytic reduction of U（Ⅵ）.This thesis elucidated the photocatalytic reduction mechanism of U（Ⅵ）by MXene/g-C₃N₄ from photocatalytic experiments and theoretical calculations,and support for the application of layered photocatalytic materials in the field of environmental remediation.