Simultaneous removal of anionic and cationic dyes from wastewater with biosorbents from banana peels

Untreated banana peel (BP) is able to adsorb efficiently cationic dyes like methylene blue (MB) but weakly anionic dyes like orange G (OG). One way to enhance the sorption capacity for both dyes is to convert BP into activated carbon (BPAC) after thermochemical treatment. Among the various BPACs tested, the highest sorption capacity for both dyes was achieved when BP was modified with NaOH and pyrolyzed at 700 degrees C (BPAC-NaOH-700). The pore structure of adsorbents was analyzed with scanning-electron-microscopy (SEM), nitrogen sorption isotherms, and mercury intrusion porosimetry (MIP), and the meso-and micro-pore size distribution of BPAC-NaOH-700 was the narrowest one with the smallest mean value and the highest specific surface area. Equilibrium tests in batch mode were fitted with the extended Langmuir and Freundlich isotherms and used to estimate thermodynamic properties. The MB and OG sorption dynamics onto BPAC-NaOH-700 was approximated with the multi-compartment model, and the external and internal mass transfer coefficients were estimated. The maximum sorption capacity of BPACNaOH-700 was found to be equal to 323 mg/g for MB and 76 mg/g for OG, both higher than the corresponding values for BP, and fully consistent with the high values of its surface area (S-BET = 530 m(2)/g) and total pore volume (V-P,V-N2 = 1.81 cm(3)/g). The synergistic interaction for the sorption of both dyes onto BPAC-NaOH-700 was associated with a push-pull mechanism, while the selective sorption of MB onto BPAC-NaOH-700 was attributed to the very slow rates of OG pore and surface diffusion in meso-and micro-porosity, respectively. The adsorption was exothermic for BP, and endothermic for OG.