Kinetic and thermodynamic study of methylene blue adsorption onto chitosan: insights about metachromasy occurrence on wastewater remediation

Methylene blue (MB) is a dye used in the textile industry and a potential wastewater pollutant. Adsorption of MB onto the chitosan is a promising method for MB removal. Upon contact with chitosan, MB molecules can aggregate leading to the appearance of additional bands in the MB absorption spectrum. This phenomenon is called metachromasy. In this paper the kinetics and thermodynamic parameters related to the removal of MB as single molecules (Sng-MB) and aggregates (Agg-MB) species by chitosan from simulated wastewater were studied. Additionally, it was investigated means to suppress metachromasy and improve the efficiency of the removal process. For both MB species, the adsorption process followed the pseudo-second-order adsorption kinetics model. For each temperature studied, Agg-MB always presented higher rate constant than Sng-MB, indicating faster adsorption of Agg-MB in comparison with Sng-MB, whereas the highest removal percentage (around 65%) for both species was obtained at 323 K. A lower activation energy was obtained for the adsorption of Agg-MB (28 kJ/mol) than for Sng-MB (65 kJ/mol), indicating Agg-MB adsorption is favored in relation to Sng-MB. The thermodynamic studies indicated the adsorption process for both MB species is endothermic and spontaneous from 298 to 348 K. The metachromasy could be suppressed by performing the experiments at pH 3, leading to a removal percentage around 70% in comparison with 20 and 60%, for experiments at pH 7 and 12, respectively, at 298 K. Analysis of chitosan before and after adsorption process, by using X-ray diffraction, infrared spectroscopy, scanning and transmission electron microscopies, indicated that no structural changes happened to chitosan throughout the process.