Differential immobilization mechanisms of calcium silicate-based materials toward Pb(Ⅱ) and Cd(Ⅱ): From adsorption characteristics to long-term leaching stability

Calcium silicate-based materials hydrate to form calcium silicate hydrate (C-S-H), a reactive phase capable of immobilizing heavy metals. However, the mechanisms governing element-specific long-term stability remain unclear. In this study, dicalcium silicate (C 2 S) and tricalcium silicate (C 3 S) were used to compare the removal behavior, aging stability, and microscopic immobilization mechanisms of Pb(Ⅱ) and Cd(Ⅱ) in aqueous systems. Both minerals efficiently removed Pb(Ⅱ) and Cd(Ⅱ), while C 3 S showed stronger early Pb(Ⅱ) removal, possibly related to faster hydration-product formation under the tested conditions. Cd(Ⅱ) removal remained above 99% in both systems. Apparent isotherm and kinetic analyses indicated that metal removal was governed by a coupled apparent process involving interfacial adsorption, surface complexation, hydration-product formation, and precipitation, rather than by physical adsorption alone. Long-term aging and acidic leaching tests revealed contrasting stability: Cd-bearing products exhibited strong leaching resistance, whereas Pb-bearing products showed a potential risk of re-release after aging. XRD, XPS, TEM, and TEM-EDS analyses indicated that Pb and Cd were closely associated with C-S-H-related hydration products, suggesting possible Ca-associated binding or partial structural incorporation. The stronger acid-leaching resistance of Cd-bearing products implies more stable association with C-S-H-related phases than that of Pb-bearing products. These findings clarify the element-specific immobilization mechanisms of C 2 S/C 3 S and provide guidance for calcium silicate-based treatment of heavy-metal-contaminated water.