Novel insights into the adsorption mechanism of methylene blue onto organo-bentonite: Adsorption isotherms modeling and molecular simulation

Abstract

The aim of this study is to investigate the suitability of raw and organo-bentonite for the selective adsorption of methylene blue (MB) from an aqueous solution at three temperatures. SEM characterization confirms the adsorption of MB since there is change in the morphology of the sample after adsorption process. The organo-bentonite allows a higher MB removal than raw-bentonite with a maximum adsorption capacity equal to 321 mg/g at 60 °C. This may be attributed to the carbonyl groups deriving from the rarasaponin used for the preparation of organo-bentonite. The adsorption process of MB is better described by monolayer model coupled to real gas (MMRG) rather than Langmuir and Freundlich models. It was found that the statistical physics approach covers the above-mentioned models. The organo-bentonite showed the highest adsorption energies (2.40; 2.73 and 21.3 kJ/mol) compared to the raw-bentonite (1.22; 2.44 and 15.2 kJ/mol) at 30°C, 45°C and 60°C, confirming the effect of carbonyl group from rarasaponin. MMRG model also allows the calculation of the Isosteric heat of adsorption. According to the retrieved Gibbs free energy values, the adsorption process was noticed spontaneous in nature. Molecular dynamics simulation is conducted to reinforce the results obtained from statistical physics approach. Starting from attraction energy results, it can be concluded that adsorption of MB is located on the bentonite part of the adsorbent, as well as on the rarasaponin part. The interactions that occurred during MB adsorption are identified by MD simulations as Van der Waals forces, which are repulsive for rarasaponin/MB with a stronger electrostatic interaction and attractive for bentonite/MB with zero electrostatic forces. Hydrogen bonding is found to be equal to zero for both systems.