Comparison of the Adsorption of Fe(III) on Alpha- and Gamma-MnO₂ Nanostructure

Abstract

Aqueous industrial wastes from heavy industry factories contain a large amount of Fe ions, which constitute a hazard for human life even at trace concentrations. Adsorption technology is a promising method for removing Fe(III) from aqueous solutions. In this report, the adsorption of the Fe(III) ion on γ- and α-MnO₂ nanostructures was compared. The results showed that the maximum adsorption was obtained at pH = 3.5 for both materials after 120 min for γ-MnO₂ and 80 min for α-MnO₂. Adsorption isotherm models, such as Langmuir, Freundlich, Sips, Tempkin, and Dubinin–Radushkevich were applied to determine adsorption capacity as well as the nature of the uptake. The highest R ², the smallest of root mean squared error (RMSE), and the nonlinear Chi-square test (χ²) values determined that the Sips model was the most appropriate equation to describe the adsorption of Fe(III) on γ- and α-MnO₂. The maximum monolayer adsorption capacity calculated from the Langmuir model and the maximum adsorption capacity calculated from the Sips model of γ-MnO₂ was more than four times that of α-MnO₂. The heat of the adsorption as well as the mean free energy estimated from Tempkin and Dubinin–Radushkevich was determined to be less than 8 kJ/mol, which showed that the adsorption on both materials followed a physical process. Kinetic studies showed that a pseudo-second-order model was accurately described on both samples with three stages.