Adsorption behavior and immobilization mechanism of typical heavy metals in synthetic C-A-S-H gels

Alkali-activated materials show excellent capture capacity on heavy metals, but the mechanisms to immobilize heavy metals remain inadequately explored. This work investigated the main hydration product C-A-S-H gels for encapsulation of heavy metals such as Pb(II), Zn(II) and Cd(II). Furthermore, the adsorption behavior and mechanisms of Pb(II) in pore solutions by C-A-S-H were examined. These findings reveal that aluminum is incorporated into the silicon chains of C-S-H in the form of aluminum-oxygen tetrahedra, thereby generating C-A-S-H. The bonding ratio, which depends on the Ca/Si ratio, reaches its peak value of 0.139 when the Ca/Si ratio is 0.9. In highly alkaline environments, amphoteric Pb(II) and Zn(II) form hydroxyl coordination polyhedra, enhancing silicon chain polymerization, whereas Cd(II) is immobilized via ion exchange with Ca(II), resulting in solid solutions. For C-A-S-H with a Ca/Si ratio of 0.6, Pb(II) adsorption occurs predominantly through physical interactions. However, as the Ca/Si molar ratio increases, the effect chemisorption and ion exchange become more significant along with the formation of Pb 8 Ca(Si 2 O 7 ) 3 . C-A-S-H gel with a Ca/Si ratio of 0.9 has the highest Pb(II) adsorption capacity, reaching 92.55 mg/g.