Encapsulation of red mud with ultra-high performance concrete (UHPC) for immobilization of alkaline and heavy metals: Experiments and simulations

To alleviate the storage of red mud (RM) and the resulting pollution, this study presented an encapsulation-based strategy for the safe management of RM, and adopted ultra-high performance concrete (UHPC) as an encapsu-lating material due to its superior impermeability and chemisorption properties. It is noteworthy to find that encapsulated material suffered from a selective chemisorption and density functional theory (DFT) calculations were innovatively employed to investigate the selective chemisorption mechanism. The simulation results sug-gested that distinct bonding forms lead to the selective chemisorption of heavy metals, with Pb being adsorbed by Si-O-Pb chemical bonds and a strong Si-O-Pb-O-Si long chain, while As and Cr only form relatively weak As-O and Cr-O bonds through their attraction to O atoms. With the thickness of the encapsulating material increases, the synergistic effect of chemical solidification and physical package is provided to achieve a stronger stabili-zation effect. The leaching results indicated that the encapsulated RM exhibited exceptionally low concentrations of hazardous components (i.e., Na at 1.580 mg/L, As at 0.003 mg/L, Cr at 0.027 mg/L, and undetected levels of Pb in encapsulated RM with a thickness of 1.5 mm), far below the limits set by the United States Environmental Protection Agency (EPA). This study opening a new window for the sustainable and efficient management of hazardous solid waste.