Divergence in immobilization pathways of Pb and Zn from zinc leaching residue in ceramic tiles: Selective crystallization and solid solution mechanisms via DFT

The valorization and environmentally sound management of heavy metal-containing zinc leaching residue (ZLR) remain persistent research challenges. This study systematically investigates the influence of Pb/Zn content on the physical properties and leaching safety of ZLR-based ceramic tiles, and elucidates their distinct crystallization behaviors and immobilization pathways during sintering. Results show that PbO exhibits a stronger fluxing effect than ZnO, leading to more pronounced changes in the physical properties and microstructure of the tiles. Pb 2+ is homogenously distributed via incorporation into anorthite solid solution and the glassy matrix, whereas Zn 2+ preferentially crystallizes as gahnite ferroan, resulting in localized enrichment within the matrix. Under 1170 °C, when the PbO content reaches 5–9 wt%, the leaching concentration of Pb increases markedly (0.39–0.92 mg/L), while that of Zn remains relatively stable (0.32–0.73 mg/L) across a ZnO content range of 1.1–9 wt%. Higher PbO content shifts Pb/Zn from crystalline to amorphous phases, whereas higher ZnO content maintains their crystalline-hosted forms. Density functional theory (DFT) calculations reveal that the substitution of Pb for Ca and Zn for Al/Ca in anorthite is thermodynamically spontaneous, reducing the total system energy and narrowing the bandgap. Mulliken population and electron localization function (ELF) analyses confirm that Pb adopts exclusive 4-fold coordination, while Zn exhibits both 3- and 4-fold coordination; moreover, Pb–O and Zn–O bonds show enhanced covalent character compared with the predominantly ionic Ca–O bond. • PbO exhibits a stronger fluxing effect than ZnO in ZLR-based ceramic tiles. • Pb disperses uniformly in matrix, whereas Zn concentrates locally via selective gahnite ferroan crystallization. • Pb@Ca-anorthite substitution exhibits the highest spontaneity, followed by Zn@Al- and Zn@Ca-anorthite. • Pb–O and Zn–O bonds in anorthite solid solutions show stronger covalency than the ionic Ca–O bond.