Comparison of the adsorption of Zn(II) on a- and g-MnO2 nanostructures

Abstract

In this report, the adsorption of Zn(II) ion on g- and a-MnO₂ nanostructures was compared. The results showed that the maximum adsorption was obtained at pH = 4.0 for both materials and after 60 minutes for g-MnO₂ and 80 minutes for a-MnO₂. Adsorption isotherm models demonstrated that the Langmuir was the best model to describe the adsorption of Zinc(II) on g- and a-MnO₂ because of the highest correlation coefficient (R2), the smallest root mean square error (RMSE), and the nonlinear chi-squared test (c2) values. The maximum adsorption capacity of g-MnO₂ calculated from Langmuir model was 55.23 mg/g, which was roughly double a-MnO₂. The lower 1/n value from Freundlich model for a-MnO₂ revealed that it was not as favourable as g-MnO2. The heat of the adsorption as well as the mean free energy estimated from Temkin and Dubinin - Radushkevich models to be less than 8 kJ/mol indicated that the adsorption on both materials followed a physical process. Kinetic studies showed that pseudo-second-order model was accurate to describe both materials in three stages.