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-MnO2, 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.