Adsorption of methylene blue and bright blue dyes on bayleaf capertree pods powder: Understanding the adsorption mechanism by a theoretical study

A theoretical analysis of the adsorption of two dyes (methylene blue: MB, and bright blue: BB) on a bayleaf capertree pods powder (BCP) is reported in this paper. The adsorption mechanism of both the dyes is discussed in light of a model that suggests the adsorption of these compounds is a multi-layer process, specifically characterized by a double layer mechanism, due to the specific properties of the adsorbent. The application of the model on the data set of both the dyes also indicated that an aggregation phenomenon occurs upon adsorption, but with a low degree. The molecule orientations on the BCP adsorbent are analyzed at different temperatures. Overall, this study demonstrates that the BB dye is adsorbed at higher extent as compared to the MB dye, but the difference in adsorption capacity is not very significant. The increase of temperature determines an increase in the maximum adsorption capacity,suggesting that the adsorption of both the dyes is endothermic. To attribute new insights to the adsorption mechanism and clarify the dynamics of adsorbent-adsorbate interactions, adsorption energies are calculated,corroborating the evidence that dyes adsorption is endothermic and physical interactions are involved during the adsorption of both MB and BB.