Uranium stabilization in red mud by sintering: Mechanism and leachability

Uranium(VI)-bearing waste is inevitably produced in the process of aqueous U(VI) abatement by red mud (RM). Such U(VI)-bearing waste has been considered to be highly hazardous to the environment and human because of their easy mobility, high toxicity, and radioactive. The stabilization of uranium constitutes an urgent challenge that must be solved through an economic and feasible waste management strategy. Herein, a low thermal stabilization process was well-designed for stabilizing U(VI) in red mud into particular phase(s) such as geopolymer-like microstructures. Characterization and leaching experiments were further performed for U(VI)-bearing RM samples thermally stabilized at various temperatures, and the stabilization mechanism of uranium in the RM was explored. X-ray diffraction (XRD) results revealed that a geopolymer-like phase whose main components were Al, Si and O was formed when the U(VI)-bearing RM was treated at 600 °C; thus, uranium was successfully stabilized. When treated at higher temperature (i.e., 900 °C), the U(VI)-bearing RM formed feldspar minerals from the oxides of Si and Al, which originated from the broken geopolymers (a stable three-dimensional network), thus exposing uranium to the surface. The constant-pH leaching test evidenced that the sintering temperatures of 600–800 °C could facilitate the fixation of uranium by RM. The findings of this study offer us a promising strategy on the utilization of RM to eliminate and stabilize U(VI) from waste streams.