The brown marine algae “ Laminaria digitata ” for the outstanding biosorption of methylene blue and reactive blue 19 dyes: kinetics, equilibrium, thermodynamics, regeneration, and mechanism studies

In the present work, the brown alga Laminaria digitata ( L. Digita ) was used as a biosorbent material for the removal of Methylene Blue (MB) and Reactive Blue 19 (RB19) dyes. The L. Digita was characterized by Fourier transform infrared spectroscopy, Scanning electron microscopy coupled with Energy-dispersive X-ray spectroscopy (SEM–EDS), Elemental analysis, Thermogravimetric analysis, and pH of point zero charge. Preliminary tests showed that equilibrium was reached after 15 min of biosorption of the MB and RB19 dyes, and the highest efficiency achieved was 95% and 60%, respectively. The pseudo-second-order kinetic model was the most suitable to fit the experimental data of MB and RB19. The Langmuir model was the most suitable to describe the biosorption equilibrium of MB, with an outstanding maximum biosorption capacity that reached 2976.6 mg/g, while the Freundlich model described the biosorption of RB19. The thermodynamic parameters revealed that the biosorption of MB and RB19 was spontaneous and exothermic in nature. Finally, regeneration tests revealed that the L. Digita algae could be reused effectively four times for MB and twice for RB19. Given the overall obtained results, we can conclude that L. Digita can be used as an effective alternative for the treatment of aqueous solutions containing cationic and anionic dyes.