Immobilization and leaching behavior of Cd²⁺ and Pb²⁺ heavy metal ions in Indonesian fly ash-based geopolymers

The immobilization of heavy metal ions in geopolymers is a promising solution to prevent environmental contamination. However, under acidic conditions, these ions can leach out. Our study focused on immobilizing Cd²⁺ and Pb²⁺ in fly ash-based geopolymers, assessing their effects on properties, and examining leaching kinetics at pH 5. We tested various ion concentrations (ranging from 1000 to 16000 ppm). Notably, the addition of 1000 ppm of Cd²⁺ and 4000 ppm of Pb²⁺ substantially improved compressive strength (24.28 and 35.56 MPa, respectively). Higher ion concentrations resulted in increased leaching. Post-leaching, we observed pore enlargement and surface cracks, indicating ion migration. The Shrinking Core Model best describes the leaching process, with diffusion being the primary mechanism, featuring reaction orders of 2.46 and 2.39 for Cd²⁺ and Pb²⁺, respectively. The model is suitable for predicting the release of immobilized heavy metal ions into the surrounding acidic environment.