Exploring the adsorption mechanisms of cationic and anionic dyes onto agricultural waste peels of banana, cucumber and potato: Adsorption kinetics and equilibrium isotherms as a tool

Abstract Banana (BP), cucumber (CP) and potato peels (PP) were used as biosorbents for the removal of both cationic (Methylene blue) and anionic (Orange G) dyes from wastewater. The biosorbents were characterized by ATR-FTIR, N2 Sorption, SEM, and XRD attempting to shed light on the adsorption mechanisms. The ATR-FTIR spectroscopy played a key-role for the chemical characterization of the different biosorbents in order to investigate their composition and in parallel for clarifying the nature of the adsorbent/dye interactions. The effect of pH, biosorbent dose, contact time and dye concentration were investigated. The ranking of adsorption capacity for CMB = 10–80 mg/L was BP > PP > CP whereas for CMB = 80–300 mg/L was BP > CP > PP. Regarding OG, the ranking was CP > PP > BP over the whole concentration range (10–300 mg/L). The biosorption of MB on BP and PP followed the Langmuir isotherm whereas biosorption on CP followed the Freundlich isotherm for low MB concentration values and the Langmuir isotherm for high MB concentrations. The biosorption of OG on BP, PP and CP was better described by the Langmuir isotherm. The maximum monolayer adsorption capacities for MB were found to be 211.9, 107.2 and 179.9 mg/g for BP, PP and CP (pH = 6) respectively, whereas the corresponding ones for OG were found to be 20.9, 23.6 and 40.5 mg/g (pH = 2). Adsorption kinetic models revealed that chemisorption was the dominant biosorption mechanism.