Sorption and reduction of hexavalent uranium by natural and modified silicate minerals: A review.

In the context of rising anthropogenic activities and world conflicts, there is an urgent need for energy supply, such as nuclear power from uranium minerals. Nonetheless, uranium is one of the most radioactive and hazardous metals due to its long half-life and chemical toxicity to living organisms, thus requiring advanced methods to clean uranium-contaminated waters and ecosystems. Silicate minerals, which constitute about 90% of the Earth's crust, have been widely studied as a means of removing radionuclides and heavy metals, including uranium, due to their abundance, low cost, and high efficiency. Here, we review uranium removal by silicate minerals, focusing on the sorption and reduction of uranium (VI) by silicate minerals with well-defined pore structures, including kaolinite, montmorillonite, silica, zeolites, and other natural or modified silicate minerals. The treatment process is influenced by the pH, uranium (VI) concentration, ionic strength, temperature, and the presence of carbon dioxide and organic ligands. In particular, the pore structure and surface charge of silicate minerals play critical roles in the sorption process, whereas the modification of silicate minerals and preparation of silicate mineral composites can substantially enhance the sorption of uranium (VI). The reduction of uranium (VI) produces sparingly soluble uranium (IV), which prevents the migration of uranium and facilitates the sorption process. Molecular dynamic simulations and density functional theory calculations can provide valuable insights into the mechanism of uranium (VI) sorption and reduction, especially when used in correlation with experimental studies. Future studies could focus on improving the alkali resistance and sorption selectivity of silica minerals for extracting uranium from seawater. [ABSTRACT FROM AUTHOR]