This study consisted of using deactivated thallus of the lichen species Parmotrema dilitatum as adsorbents in the treatment of water containing crystal violet (CV) and methyl red (MR), two toxic dyes. Scanning electron microscopy (SEM) and Fourier transform spectroscopy (FT-IR) were performed on the deactivated lichens before and after their interaction with the dyes. These analyses showed an affinity of the dyes for the adsorbent. The influence of physico-chemical parameters, related to the experimental environment and the adsorbent, on the removal of the dyes was evaluated. The study showed that the stirring speed for optimal removal of both dyes is 450 rpm. The equilibrium time of adsorption varies according to the nature of the dye. This time is 60 min (short) for the cationic dye (CV) and 100 min (long) for the anionic dye (MR). The amount of dye adsorbed increases as the initial concentration of the target dye increases. Conversely, an increase in lichen mass leads to a decrease in the adsorbed amounts of CV and MR. The high amounts of dyes removed were obtained with the granulometry [125-250 μm] of deactivated lichen powder. The thermodynamic study showed that the adsorption of CV is endothermic while that of MR is exothermic. The pH values of the reaction medium for optimal adsorption were observed at pH = 6 for CV and pH = 3 for MR. For all the physico-chemical parameters evaluated, the study showed that the amount of cationic dye removed is greater than that of the anionic dye.
