Co-utilization of zinc contaminated soil and red mud for high-strength ceramsite: Preparation, zinc immobilization mechanism and environmental safety risks

In this research, high strength ceramsite was sintered by zinc contaminated soil and red mud. In addition, the influence of sintering temperature and red mud addition on ceramsite properties and Zn immobilization were investigated, and the Zn immobilization mechanisms were determined. The X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM-EDS), and BCR sequential extraction technique were used to determine the crystal phases, microstructures, and Zn immobilization. Under the optimum synthesis conditions (sintering temperature of 1150 °C and the red mud addition of 5%), the sintered ceramsite exhibited a Zn loss rate of 10.31%, Zn leaching concentration of 0.13 mg/L (performed by rotating vibration device), bulk density of 1052.6 kg/m3, 1 h water absorption of 1.06%, and cylindrical compressive strength of 27.06 MPa, implying that the ceramsite obtained under this conditions not only meets the Chinese National Standards and has no environmental risk. High temperature and the addition of red mud were beneficial to the immobilization of Zn in ceramsite. The main immobilization mechanism of Zn in ceramsite was the formation of ZnFexAl2−xO4 spinel and the encapsulation of glass phase. Thus, Zn can be immobilized effectively by ceramsite prepared from zinc contaminated soil and red mud at high temperature.