Kinetic and Thermodynamic Study of Methylene Blue Adsorption onto Chitosan: Insights about Metachromasy Occurrence on Wastewater Remediation

Methylene Blue (MB) is a dye frequently used in the textile industry and a potential wastewater pollutant. MB adsorption onto the surface of solid materials is a promising method for wastewater remediation. The biopolymer chitosan is one of the promising candidates as an adsorbent for MB removal from wastewater. However, the contact between MB and chitosan induces the aggregation of MB molecules. The formation of MB aggregates can lead to an appearance of additional bands in the MB visible absorption spectrum. This phenomenon is called metachromasy. In this paper, chitosan was used as adsorbent to study the kinetics and thermodynamic parameters related to the elimination of MB as single molecules (Sng-MB) and aggregates (Agg-MB) species from simulated wastewater. It was found that, for MB single molecules or aggregates, the adsorption process follows the pseudo second order adsorption kinetics model. For each temperature studied, Agg-MB always presented higher rate constant than Sng-MB, indicating a faster adsorption of Agg-MB in comparison to Sng-MB. Additionally, the adsorption process presented a lower activation energy for Agg-MB, 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 adsorption isotherms could be properly fitted and interpreted according to Temkin and Dubinin-Radushkevich models. Studies varying the initial pH of MB solution indicated that metachromasy could be suppressed in acidic pH values, whereas alkaline pH values led to an increase in the removal percentage of Sng-MB and Agg-MB in relation to neutral conditions. The analysis of chitosan powder before and after adsorption indicated the adsorption process does not cause structural changes to the chitosan. The kinetic and thermodynamic study of the relative adsorption of MB single molecules and aggregates onto chitosan can provide fundamental information for MB wastewater remediation.