Sustained Tl(I) removal by α-MnO₂: Dual role of tunnel structure incorporation and surface catalytic oxidation

Manganese oxide-based filtration technologies are considered cost-effective for thallium (Tl) removal in engineered systems. However, current gaps in understanding the heterogeneous adsorption and oxidation mechanisms of typical tunneled α-MnO₂ may lead to a serious underestimation of its long-term Tl removal potential. In this study, α-MnO₂ could continuously remove Tl(I) during the 584-h reaction, with its irreversible removal eventually increasing to 81 %-95 % in different anionic environments. The adsorbed low-loaded Tl(I) is preferentially oxidized, whereas the high-loaded Tl tends to be adsorbed in a nonoxidative pathway by α-MnO₂. The nonoxidized Tl(I) was gradually immobilized in the stable thalliomelane-like tunnel structure. More importantly, the synergism of surface Mn(III)-oxygen vacancies (O_v) on α-MnO₂ could catalyze the oxidation of Tl(I). Furthermore, the oxidized Tl(III) was bound to the tunnel surface via double edge-sharing and double corner-sharing. In addition, the phosphate anion occupied the surface active site and inhibited the oxidation of Tl(I), thereby reducing the binding strength of Tl. This study provides a new perspective on the effectiveness and stability of Tl(I) removal by MnO₂ and highlights the neglected mechanism of Mn(III)-O_v mediating Tl(I) oxidation, which expands our understanding of the removal and transformation fate of Tl in MnO₂-engineered systems.