Atomic-scale mechanisms of Pb immobilization via waste glass-assisted low temperature vitrification technology

Vitrification technology is an effective solution for rapid stabilization and resource utilization of heavy metal-containing hazardous waste. Waste soda-lime-silica (SLS) glass is an abundantly available vitrification flux and can contribute to the sustainable waste-treating-waste strategy. Understanding the interactions between the SLS glass network and heavy metals is vital for optimizing the vitrified composition systems. This study adopted molecular dynamics simulations and various experimental techniques to comprehensively elucidate the immobilization mechanism of the representative heavy metal ion of Pb 2+ in SLS glass. Our results indicated Pb ions were incorporated into the glass matrix primarily as three forms, namely, [PbO 3 ] triangles, [PbO 4 ] tetrahedra, and highly coordinated [PbO n ] ( n  > 4) polyhedra. With the increasing amount of Pb ions immobilized, the dominant role of Pb ions transformed from the network modifiers of highly coordinated [PbO n ] units to the network formers of [PbO 3 ] and [PbO 4 ] units. As a result, the degree of network polymerization first decreased and then increased with the turning point of 7.5 wt% PbO content. Different from regular [SiO 4 ] tetrahedra, the [PbO 3 ] and [PbO 4 ] units were less stable and prone to deformation, resulting in the small inter-polyhedral bond angle Pb-O-Si. The structural transformations significantly affected the macroscopic thermal and chemical stability of the vitrification products. Generally, the vitrification products with a PbO content of below 11 wt% were allowed to be recycled in construction materials according to the Chinese standard. This study has a high value for tuning vitrification chemistry and developing green Pb-containing vitrification materials.