Ion-adsorption type rare earth tailings for preparation of alkali-based geopolymer with capacity for heavy metals immobilization

Ion-adsorption type rare earth tailings (RET), a type of solid waste that contains clay minerals, were used as a starting material to prepare alkali-based geopolymer. The effects on the compressive strength of RET-based geopolymer, including the modulus and concentration of the alkaline activator, curing temperature, and liquid/solid ratio, were investigated. Pb2+ and Cd2+ were added during the preparation of the RET-based geopolymer for evaluating the immobilization capacity of heavy metals in the as-obtained geopolymer. The results showed that geopolymer with a compact and dense microstructure was obtained through the optimization of the above factors, with the highest compressive strength being 28.5 MPa. The leaching test indicated that the RET-based geopolymer possessed a desirable capacity for immobilizing heavy metals, where the immobilization efficiency of Pb2+ was above 99% and that of Cd2+ ranged from 92% to 96%. Heavy metals were immobilized in the RET-based geopolymer matrix through the formation of chemical bonds (T (Si, Al)–O–M (Pb, Cd)) or through an electrostatic attraction between the metal cations and negatively charged [AlO4]−.