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 Pb and Cd to unravel their solidification mechanisms. Leaching analyses demonstrated superior solidification efficacy for Pb (93.6%) compared to that for Cd (65.2%). The dissimilarity in solidification mechanisms can be attributed to variances in ionic potential and covalent interactions between the cations. Furthermore, Cd 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 Cd. Density functional theory calculations affirmed stronger interactions between Pb (−8.71 eV) and the matrix compared to those of Cd (−5.83 eV). Consequently, Pb demonstrated an enhanced covalent propensity within the geopolymer matrix, while Cd primarily underwent solidification via ion exchange. This research provides novel insights and concepts regarding the mechanisms involved in solidifying heavy metals in geopolymers.