Ball-milling synthesized manganese dioxides for efficient thallium(i) removal: performance and mechanism

Manganese dioxide (MnO₂) has garnered significant attention as a green material for the removal of thallium(i) (Tl(i)) from aqueous environments. Developing simple and cost-effective synthesis strategies to produce MnO₂ with enhanced Tl(i) removal efficiency has become a key research objective. In this study, MnO₂ (denoted as bmMnO₂) has been synthesized via a ball milling method using potassium permanganate (KMnO₄) and manganese acetate tetrahydrate (Mn(CH₃COO)₂·4H₂O) as precursors, exhibiting efficient Tl(i) removal from aqueous solutions over a wide pH range with a maximum adsorption capacity (Q _max) of 408.14 mg g^-1, which was 136 times higher than that of multi-walled carbon nanotubes. bmMnO₂ was confirmed to consist primarily of the α-MnO₂ phase via XRD and XPS characterization, and to have a specific surface area of 309.65 m² g^-1 via nitrogen adsorption/desorption characterization. XPS characterization combined with DFT calculations demonstrated that Mn(iv) atoms served as the primary active sites for Tl(i) adsorption, wherein Tl(i) was oxidized to Tl(iii) by Mn(iv) and adsorbed within the 2 × 2 tunnel structure of bmMnO₂. Moreover, bmMnO₂ exhibited high Tl(i) removal efficiency in real water and good selectivity against common coexisting cations. bmMnO₂ retained over 60% of its initial Tl(i) adsorption capacity after four adsorption-desorption cycles. These results demonstrate that bmMnO₂ is a promising candidate for cost-efficient remediation of large-scale Tl(i)-contaminated waters.