Mechanism of heavy metal stabilization by red mud-based geopolymer

It has been shown that the industrial wastes as the sources of silica and alumina can participate in a geopolymerization reaction. In this research, Bayer red mud and coal gangue is prepared as an geopolymer precursor (RG) through mechanochemistry-alkali activation, which is used for municipal solid waste incinerators fly ash (MSWIFA) solidification/stabilization (S/S) to achieve a win-win goal of waste recovery and pollution control. The TCLP tests and sequential extraction tests are used to evaluate the effectiveness and stability of the geopolymer S/S regent. The XRD, SEM-EDS and FTIR are conducted for characterization of the geopolymeric S/S solid samples. The results show that more than 99.6 percent of the heavy metal in geopolymeric S/S solid can be immobilized when the RG content exceeds 60%. The S/S effectiveness decreases in the order of Pb>Zn>Cr>Cd. According to the XRD results, MSWI fly ash can participate in the hydration process to generate C-S-H and zeolite phase which, together with the geopolymer, enhances the structural stability and improves the compressive strength of the geopolymeric S/S solid. In consequence, most heavy metals in the geopolymeric S/S solid are immobilized in the hydration phases and geopolymer structure, transform from the available fractions into the stable fractions. It is finally concluded that the Bayer red mud and coal gangue-bsed geoploymer i useful in reducing the availability and leaching of heavy metals in MSWIFA.