Anomalous Heavy Metal Sequestration via MnO₂ Layer-to-Tunnel Transformation

Crystallization of poorly crystalline minerals generally remobilizes the adsorbed heavy metals. In contrast, we find that the conversion of poorly crystalline layered δ-MnO₂ to crystalline 2 × 2 tunneled α-MnO₂ significantly increases both the amount and stability of Pb²⁺ immobilization. The varied characterization analyses reveal that Pb²⁺ first exchanges surface K⁺ associated with δ-MnO₂ and is then sequestered into α-MnO₂ 2 × 2 tunnels via oriented attachment along the (001) plane. Without the crystallization process, Pb²⁺ cannot substitute for tunnel K⁺ in α-MnO₂ and can only weakly bind to the external (310) surface. Stability comparisons across outer-surface adsorption, wall substitution, and tunnel incorporation indicate that only tunnel-resident metals achieve strongly enhanced and persistent Pb sequestration. This study not only challenges the conventional understanding that mineral crystallization and layer-to-tunnel transformation lead to the release of adsorbed heavy metals but also suggests a potential strategy for selective metal trapping in engineered remediation materials and environmental matrices.