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 encapsulating 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 suggested 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 stabilization 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.