Facile synthesis of α-MnO2 micronests composed of nanowires and their enhanced adsorption to Congo red

In this paper, α-MnO2 micronests composed of nanowires were fabricated via a hydrothermal reaction of MnSO4·H2O and K2S2O8 solutions. The α-MnO2 micronests were demonstrated to have a higher adsorption capacity than Γ-MnO2 microspheres due to their large specific surface area. The amount of Congo red adsorbed per unit weight of α-MnO2 micronests increased significantly from 114 to 282 mg·g−1 with concentration of Congo red solution increasing from 50 to 200 mg·L−1, but it had a little change with temperature. Kinetics, isotherms and thermodynamics for the adsorption of Congo red on α-MnO2 micronests were examined. The adsorption process followed the pseudo-second-order kinetics with good correlation. The experimental data were analyzed by Langmuir and Freundlich models, and equilibrium data fitted the Langmuir isotherm very well with maximum monolayer adsorption capacity of 625 mg·g−1 at 22 °C. The adsorption was spontaneous and endothermic according to thermodynamic studies. The experimental results indicate that α-MnO2 micronests possess a high adsorption capacity and could be employed as a replacement of traditional sorbents.