Tangerine peel modified with sodium hydroxide for the removal of methylene blue: A calorimetric approach

Biosorbents have been studied as low-cost and efficient materials for removing dyes from wastewater. However, thermodynamic investigations into the interactions between these materials and dyes remain scarce, particularly concerning the effects of chemical modifications to the biosorbents. This article describes the biosorption of methylene blue (MB) by unmodified tangerine peel (BN) and tangerine peel modified with sodium hydroxide (BM). The materials were characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and by assessing the point of zero charge as well as the number of acidic and basic functional groups. The biosorption studies combined kinetic and equilibrium analyses with isothermal titration calorimetry (ITC) measurements to characterize the sorption process in detail. BM exhibited a sorption capacity (328 mg g−1) approximately three times higher than that of BN. The adsorption process was enthalpically driven and at least 3 kJ mol−1 more exothermic for the untreated material, indicating that treatment with NaOH was unfavorable for the interaction between the dye and biosorption sites but favored the process entropically. The enthalpy of sorption depended on the amount adsorbed, indicating a high heterogeneity of biosorption sites within the materials. The results suggest that the lower availability of biosorption sites in BN led to the sorption of MB dimers, whereas in BM, the sorption of monomers was the preferred mechanism at less hydrophilic sites. © 2024 Elsevier B.V.