March 3, 2026Open Access

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

Puntos clave

Leaching concentration of Pb increases from 0.39 to 0.92 mg/L with 5-9 wt% PbO, while Zn remains stable.
Observational analysis of Zn leaching residue used density functional theory to explore crystallization behaviors during sintering.
Results show that Pb forms a solid solution in anorthite, enhancing covalent character in Pb–O bonds compared to the ionic Ca–O bond.
Higher PbO content shifts immobilization from crystalline to amorphous phases, affecting the overall safety of ceramic tiles.

Resumen

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.

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