Investigation of Adsorption Mechanism of Layered Double Hydroxides and Their Composites on Radioactive Uranium: A Review

With the rapid development of nuclear industry, nuclear energy, as a kind of low-carbon energy, has been widely used in the world. However, in the development and application of nuclear energy, a large amount of radionuclides, especially the radioactive uranium, have been inevitably discharged into the environment, causing serious environmental pollution and having great harm to human health. Layered double hydroxides (LDHs) have become the excellent adsorbents in environmental pollution treatments due to easy preparation, large specific surface area, the unique nanostructure and excellent ion exchange capacity. Hence, the preparation of layered double hydroxides and their composites for the efficient removal of radioactive uranium is one of the hot issues in the field of environmental science, which include coprecipitation, ion exchange, hydrothermal method, the urea hydrolysis method, aerogel, microwave-crystallization and separate nucleation/crystallization isolation method. Besides the aforementioned methods, other reported synthesis methods of LDHs include the secondary intercalation method (an intercalation method involving dissolution and the re-coprecipitation method), reconstruction method based on the memory effect, N-2 protection synthesis, mechanochemical synthesis, surface synthesis, template synthesis, and others. The modification methods of layered double hydroxides can be divided into calcination, intercalation and compounding method, which significantly increase the active sites and further improve the adsorption performance of the materials to radioactive uranium. In addition, the adsorption mechanism has been thoroughly investigated with spectroscopic analysis techniques such as Fourier transformed infrared spectroscopy (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Extended X-ray absorption fine structure (EXAFS). In conclusion, the review briefly discuss the application prospects of layered double hydroxides and their composites in the treatment of water pollution, which provide definitive reference values for the further research and practical application of environmental management in the future.