Study of the adsorption of methylene blue on graphene sheets obtained from rice husks: experimental and molecular dynamics simulation

The adsorption of methylene blue on different adsorbents has been widely studied due to the most common textile contaminant in waters. In this work, methylene blue adsorption onto graphene obtained from rice husks was studied experimentally and theoretically. Graphene was prepared by alkaline calcining solid mixture, in the absence of oxygen. Graphene was characterized by UV-visible and infrared absorption spectroscopy, atomic force microscopy, dynamic light scattering, and scanning electron microscopy. The adsorption isotherm was performed under standard temperature conditions. The experimental data indicated that graphene has good adsorbent qualities; the maximum adsorption capacity was 100 mg/g, the equilibrium constant of the process 1.44 +/- 0.03, and the Gibbs free energy of 92.3 +/- 0.2 kJ/mol, which shows that the adsorption is spontaneous and strongly favored. In addition, the surface active area of the material is 7826 +/- 3 m2/g. The experimental results were compared with theoretical using molecular dynamics (MD) to study the nature of different interactions involved in the adsorption of dye molecules onto graphene sheets. The computational simulation resulted in an adsorption energy of (- 132 +/- 13) KJ/mol. Equilibrium configurations obtained from MD simulations confirmed that the predominant interactions are pi-pi Lennard-Jones type. The results suggest that the methylene blue molecule acquires coplanar configurations with the graphene sheet and the analysis of the radial density function (RDF) revealed that the interaction is stronger in the aromatic portion of the dye molecule compared to the aliphatic groups.