Application of CDs/SiO2 Composites for Photo-Induced Uranium Removal Under Air

Abstract

Uranium is the main fuel of nuclear power, and the removal of uranium from uranium-containing wastewater is important for environmental protection and resource recovery. Photo-induced uranyl ion removing is an economical and efficient method to remove U(Ⅵ) from uranium-containing wastewater. Many materials have been used for photo-induced reduction of U(Ⅵ). However, the practical application of most materials is still limited by many factors, such as the complexity of its synthesis process, the harsh conditions required, and the poor selectivity and reusability in the photocatalytic process. More importantly, most materials can only be photocatalyzed under the protection of an inert atmosphere. In the presence of oxygen, the photocatalytic product uranium dioxide(UO2) is unstable and easily oxidized resulting in reduced removal effect, so it becomes particularly vital to find an effective catalyst. Carbon dots(CDs) are a new type of carbon-based materials with excellent fluorescence performance, good water solubility and biocompatibility. However, due to the small size and hydrophilicity of the CDs, it is difficult to separate from water. It needs to be combined with other solid materials when applied. The application of CDs to the photocatalytic removal of uranyl ions in water has rarely been studied. Silica(SiO2) is an ideal loader due to its excellent mechanical strength and irradiation stability, as well as its large specific surface area and easy modification. In this paper, a blue carbon dot was prepared by solvothermal method and CDs/SiO2 composites were synthesized by sol-gel method. The morphology of the composite was characterized by scanning electron microscopy(SEM) and transmission electron microscopy(TEM) technologies, and their application in photocatalytic removal of uranium were studied. Under visible light irradiation, CDs/SiO2-b can remove more than 95% of uranium in 100 min. Experiments have found that the photo-precipitation rate of silica to uranium after the introduction of carbon dots is about 5.8 times of that of the pure silica, and uranium can be transformed into solid product of (UO2)O2•2H2O under air atmosphere, with a removal capacity of more than 800 mg/g. The morphology and structure of the photocatalytic products were characterized by SEM and XPS technology, and the solid products were easy to separate and remove. In order to evaluate the practical application of CDs/SiO2 light-induced reduction of uranium under visible light irradiation, pH and other influencing factors were systematically studied. Moreover, the removal efficiency of uranium is higher 85% after 5 cycles of recycling, showing excellent recycling performance. CDs/SiO2 composites are simple to prepare and can efficiently remove uranyl ions in water through photoinduction, which provides a new method and material for photoinduced removal of uranium.

Keywords

• carbon dots
• silica
• uranyl ions
• photocatalysis
• air atmosphere