Removal of Methylene Blue and Eriochrome Black T from aqueous solutions by biosorption on Scolymus hispanicus L.: Kinetics, equilibrium and thermodynamics

The biosorption of Methylene Blue (MB) and Eriochrome Black T (EBT) from aqueous solutions was investigated using low-cost, natural and eco-friendly biosorbent, Scolymus hispanicus L., as an ideal alternative to the current expensive methods of removing dyes from wastewater. The biosorption studies were carried out under various parameters, such as average biosorbent particle size, pH, biosorbent dosage, contact time, initial dye concentration and temperature. The experimental results show that the percentage of biosorption increases with an increase in the biosorbent dosage and the decrease of particle size. The maximum biosorption occurred at the pH value of 6.8 for Methylene Blue and 3 for Eriochrome Black T. The equilibrium uptake was increased with an increase in the initial dye concentration in solution. Biosorption kinetic data were properly fitted with the pseudo-second-order kinetic model. The experimental isotherms data were analyzed using Langmuir, Freundlich, Toth and Tempkin isotherm equations. The best fit was obtained by the Toth model with high correlation coefficients (r2 > 0.99). The Langmuir model also yielded a good fit to experimental data (r2 around 0.98–0.99) with a maximum monolayer adsorption capacity of 263.92 mg/g for Methylene Blue and 165.77 mg/g for Eriochrome Black T. The adsorption of the dyes was exothermic in nature (ΔH° = −19.79 kJ/mol for MB and −8.03 kJ/mol for EBT). The reaction was accompanied by a decrease in entropy (ΔS° = −54.01 J/K mol for MB and −23.88 J/K mol for EBT). The Gibbs energy (ΔG°) increased from −4.04 to −1.82 kJ/mol and from −1.32 to −0.41 kJ/mol respectively for MB and EBT when the temperature was increased from 20 to 60 °C.