Insight into adsorption mechanism of lead(II) from aqueous solution by chitosan loaded MnO2 nanoparticles

Manganese dioxide nanoparticles are loaded onto the surface of chitosan (CS) in order to enhance their adsorption properties. Various theoretical models and spectroscopic analytical methods are used to investigate the equilibrium isotherm, kinetics, and mechanisms of the removal of lead(II) from the aqueous solution. The kinetic studies based on a pseudo-second-order model show that the uptake of lead(II) occurs via three stages depending on the reaction time. The experimental data are fitted using four nonlinear isotherm models, namely the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models. The results obtained within the Langmuir model gave the highest R2, the smallest root-mean-squared error (RMSE), and the best nonlinear chi-square test (χ2) values, indicating that this model is the most appropriate approach for describing the adsorption of lead(II) onto the MnO2/CS nanomaterials. It is also found that the maximum monolayer adsorption capacity is 126.1 mg/g, which is higher than most of the values obtained from other materials. In addition, the results of the heat of the sorption process and the mean free energy calculated from the Fourier transform infrared spectroscopy (FTIR) suggested that the adsorption indeed follows a physical process. By using the Positron Lifetime Spectroscopy, which is used for the first time to investigate the “ions-holes” mechanism of the removal of lead(II) by CS loaded MnO2 nanoparticles, we found that the “ions-holes” mechanism in fact occurs at the micropores of MnO2 crystals in the MnO2/CS nanomaterials. This result is significant as it will be a reference for the use of the spectroscopic analytical methods for investigating the adsorption mechanism.