Structural studies of calcium silicate hydrate modified with heavy metal cations.

[Display omitted] • C-S-H gels in the presence of heavy metal ions have been synthesized. • The effect of heavy metal ions on the structure of silicate gel has been investigated. • The vibrational spectra to describe the structure of C-S-H phase have been used. • XPS showed binding energies of metal ions, clarifying their role in immobilization. • We have attempted to evaluate the mechanism of immobilization of individual ions. This study investigates the immobilization mechanisms of heavy metal ions in the C-S-H phase. Synthetic C-S-H phases were prepared via the precipitation method, incorporating five different ions (Pb(II), Cd(II), Ni(II), Zn(II), and Cr(III)). Structural analysis of the obtained material was conducted using vibrational spectroscopy (both FT-IR and Raman), X-ray photoelectron spectroscopy, and X-ray diffraction. Spectroscopic methods were primarily employed to evaluate the structural effects and polymerization degree of the resulting C-S-H phase. Morphological changes were characterized using scanning and transmission electron microscopy (SEM and TEM, respectively). Our findings reveal several mechanisms for immobilizing heavy metal cations: precipitation of insoluble compounds (particularly notable for Ni(II) and Cr(III)), replacement of Ca(II) ions within the silicate structure (evident in the crystallization of Ca(OH) 2 in samples containing Cd(II), Ni(II), and Zn(II) in minimal quantities), and strong bonding of certain metals (such as Pb(II)) with the C-S-H phase structure. These insights contribute to understanding the potential applications of C-S-H phases in heavy metal immobilization. [ABSTRACT FROM AUTHOR]