Understanding the immobilization mechanisms of hazardous heavy metal ions in the cage of sodalite at molecular level: A DFT study

The geopolymer containing the special porous structure unit sod (t-toc) cage, which can be made from fly ash, is a possible economic solution to reduce toxicity of hazardous heavy metals by its excellent immobilization potential. Herein, the zeolite sodalite which is composed of sod (t-toc) cage was selected to model the geopolymer to uncover the immobilization mechanisms for the hazardous heavy metal ions Cr³⁺, Pb²⁺, Zn²⁺, Cu²⁺, Hg⁺, Cd²⁺ and Ni²⁺ in this work. Based on experimental crystalline structures, exchange energy was introduced for the first time to quantitatively evaluate the immobilization long-term stability theoretically. The immobilization of hazardous heavy metal ions were discovered to be determined by the solvation from the crystalline H₂O and bonding from the sod (t-toc) cage. Based on the analysis of exchange energies, it was found that the immobilization for these heavy metals was correlated with the solvation radius: the larger the solvation radius was, the more favorable the immobilization was. Further detailed solvation effect analysis indicates that the immobilization of heavy metal ions is very sensitive to solvation effect based on the crystalline H₂O molecules. Due to the synergistic effect between the solvation effect and the adsorption of the sod (t-toc) cage, the immobilization of Zn²⁺ is always favorable inside sodalite whereas Cr³⁺, Cd²⁺, Cu²⁺, Ni²⁺ and Pb²⁺ could be unfavorable in the case of insufficient solvation. This study provides a fast, convenient and objective way to evaluate the immobilization of heavy metals.