Study on the heavy metal immobilization mechanism in the alkali-activated red mud-ground granulated blast furnace slag-based geopolymer

This study presents an investigation into the challenge of alleviating heavy metal pollution while using red mud (RM) as an industrial byproduct, focusing on its application in the preparation of geopolymers. Synthesis RM-ground granulated blast furnace slag (GGBS)-based geopolymer (RMG) and studied with particular attention to optimizing compressive strength through modifying key parameters: RM content, Na2SiO3 modulus, and water-to-binder ratio. The immobilization of heavy metals, particularly lead (Pb) and copper (Cu), within geopolymer was thoroughly examined. Results indicate that optimal compressive strength was achieved at a 40 wt.% RM content, a Na2SiO3 modulus of 1.8, and a water-to-binder ratio of 0.65, with 28-day compressive strengths reaching 36.9 MPa. A 1% mass of heavy metals was observed to improve the mechanical characteristics of the geopolymer; however, beyond this threshold resulted in detrimental effects. The immobilization capabilities of RMG under various environmental conditions were robust, with immobilization efficiencies exceeding 97% for Pb and 94% for Cu. The immobilization mechanism was found to involve physical encapsulation, with Cu uniquely forming covalent bonds with non-bridging oxygens within the polymeric structure, creating stable Si-O-Cu bonds. This study highlights the potential of geopolymer as a viable technology for mitigating environmental impacts associated with RM disposal by effectively immobilizing heavy metals, thus facilitating safe and sustainable resource utilization. This work contributes to the field by demonstrating a novel approach to the valorization of industrial waste, offering a promising solution for the management of RM while addressing the critical issue of heavy metal pollution.