Unveiling the crystal phase effect of MnO2 in deep thallium(I) removal: Structure-activity relationships and mechanisms.

The deep removal of monovalent thallium (Tl+) from water is widely recognized as a global technical challenge. This study combined experiments, characterizations and analog computation, resolved the deep structure-activity relationship of three typical crystal forms (α, γ, δ) of MnO2 capturing Tl+. The results show: δ-MnO2 with a two-dimensional interlayer domain breaks the steric hindrance, showing extremely fast adsorption kinetics and the highest theoretical capacity; while α-MnO2 with a [2 × 2] one-dimensional wide tunnel relies on the spatial confinement effect, showing excellent selectivity. Multi-dimensional analysis deeply revealed the "surface complexation-heterogeneous oxidation" synergistic thallium removal mechanism driven by crystal phase: after Tl+ complexed with surface active hydroxyl groups, via Mn4+ mediated direct interfacial electron transfer, it was oxidized in situ to Tl3+, accompanied by the generation of oxygen vacancies and hydroxyl groups. The microscopic reaction path highly depends on the crystal structure: long-range ordered δ and α phases can efficiently drive the above direct electron transfer, while the structurally disordered γ phase highly depends on the free radical oxidation pathway. This study elucidated the microscopic reaction path differentiation law dominated by crystal spatial configuration, providing core theoretical support for the design of efficient heavy metal targeted adsorption materials.