Study of the biosorption of two cationic dyes in aqueous media by heat-treated lichens (Xanthoria parietina)

This work is part of the valuation of biomaterials abundant in nature; lichens (Xanthoria parietina) are part of this family. The lichens were heat-treated at different temperatures (60.80, 100, and 120°C) and characterised by different techniques (FTIR, SEM, and BET). These biomaterials have been used as biosorbents for two cationic dyes which are methylene blue and gentian violet. After optimisation of the parameters, the biosorption of these dyes was carried out at an adsorbent dose of 1 g/L, pH = 6, and a contact time at the equilibrium of 1 h. The adsorption kinetics perfectly follows the pseudo-second-order model as well as that of intraparticle diffusion. The adsorption isotherms of these two cationic dyes were carried out at 25, 40 and 60°C. The isotherms are of type L, they are well described by the Redlich Peterson model (R2 > 0.99) and a low mean error (E% ≤ 10%). The physisorption of methylene blue is generally non-spontaneous (∆G° > 0), exothermic (∆H° < 0), and ordered (∆S° > 0). On the other hand, the physisorption of gentian violet is generally spontaneous (∆G° < 0), endothermic (∆H° > 0), and disordered (∆S° < 0). After adsorption of methylene blue or gentian violet by the heat-treated lichens, a decrease in the intensities of the infrared bands characteristic of our materials was observed. The attenuation of these bands could be attributed to the coverage of our materials by the cationic dyes.