Structural Evolution of Geopolymers Incorporated with Heavy Metals: Solidification Mechanism of Pb²⁺ and Cd²⁺

Geopolymers have emerged as a promising solution for solidifying heavy metals; however, the underlying mechanisms remain elusive. In this study, we synthesized geopolymers incorporating Pb²⁺ and Cd²⁺ to unravel their solidification mechanisms. Leaching analyses demonstrated superior solidification efficacy for Pb²⁺ (93.6%) compared to that for Cd²⁺ (65.2%). The dissimilarity in solidification mechanisms can be attributed to variances in ionic potential and covalent interactions between the cations. Furthermore, Cd²⁺ exhibited a higher ionic potential, facilitating its exchange with Na⁺ in the geopolymer matrix. Triple-quantum nuclear magnetic resonance experiments revealed a weak affinity of the geopolymer structure toward Cd²⁺. Density functional theory calculations affirmed stronger interactions between Pb²⁺ (−8.71 eV) and the matrix compared to those of Cd²⁺ (−5.83 eV). Consequently, Pb²⁺ demonstrated an enhanced covalent propensity within the geopolymer matrix, while Cd²⁺ primarily underwent solidification via ion exchange. This research provides novel insights and concepts regarding the mechanisms involved in solidifying heavy metals in geopolymers.