Synthesis of manganese dioxide with different morphologies for thallium removal from wastewater

Manganese dioxide (MnO₂) with different morphologies (tube-, wire-, rod-, and flower-like) was synthesized via hydrothermal method and then applied for thallium (Tl) removal from wastewater. During material synthesis, short reaction time (6 h) and low temperature (110 °C) were prone to form polycrystalline flower-like birnessite type MnO₂, while long reaction time (24 h) and high temperature (240 °C) were inclined to produce polycrystalline wire-like birnessite type MnO₂. Moderate reaction time (12 h) with low temperature at 120 °C/140 °C led to formation of mono-crystalline rod- and tube-like α-MnO_2, respectively. Wire-like MnO₂ was the most effective adsorbent for Tl(I) removal from both the synthetic and industrial wastewaters. The MnO₂ of four morphologies exhibited similarly high Tl(III) removal owing to the precipitation of Tl(III) as Tl₂O₃. Effective Tl(I)/Tl(III) removal (99%) was achieved with wire-like MnO₂ at an initial pH of 6 and an adsorbent dosage of 0.25 g/L. The Tl(I)/Tl(III) adsorption can be described with the pseudo-second-order kinetic. The Tl(I) removal was best fitted with the Freundlich model, with a maximum adsorption capacity of 450 mg/g. While the Tl(III) removal was best fitted with the Langmuir model, with an extremely high capacity of 6250 mg/g. Based on the results from XRD, SEM-EDS, FT-IR, and XPS analyses, the mechanisms of Tl removal using wire-like MnO₂ are primarily surface complexation and oxidative precipitation. Overall, wire-like MnO₂ is a highly effective adsorbent for Tl removal from both synthetic and actual wastewaters.