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.