Fe3O4 Nanoparticles Loaded Bentonite/Sawdust Interface for the Removal of Methylene Blue: Insights into Adsorption Performance and Mechanism via Experiments and Theoretical Calculations

Abstract

Herein, magnetite nanoparticles (MNPs) synthesized from altered basalt were used to develop a composite with H2O2–activated bentonite (BE) and fibrous sawdust (SD). The as-prepared BE/SD–MNPs were characterized by FTIR, FESEM, TEM, TGA, DSC, and Zeta potential techniques and utilized as an effective multifunctional composite for removing methylene blue (MB). The adsorption isotherms of MB at 25–55 °C were analyzed via kinetics, classical, and advanced statistical physics models. Theoretically, the pseudo-second-order of kinetics and the Freundlich isotherms model fit the experimental data well without microscopically clarifying the adsorption mechanism. Studying a multilayer model’s steric and energetic parameters was a reliable approach to understanding the MB uptake mechanism at the molecular scale. Sterically, the removed MB molecules offered a combination of horizontal and vertical geometry (i.e., mixed orientation). The MB adsorption capacity at saturation (Qsat) increased from 829 to 849 mg/g with temperature, suggesting endothermic interactions. Energetically, the MB uptake by BE/SD–MNPs was controlled by physical interactions (i.e., adsorption energy < 20 kJ/mol). The BE/SD–MNPs retained more than 85% of the MB uptake after five adsorption-desorption rounds. Overall, this study aimed to understand the MB adsorption mechanism using a magnetic clays/lignocellulosic interface such as the utilized BE/SD–MNPs composite as a promising strategy in wastewater remediation.