The kinetics and equilibrium sorption of methylene blue on plant residues in aqueous solution

The biosorption of methylene blue (MB) from aqueous solution by leaf residues of Thymus numidicus (RTN), Origanum glandulosum (ROG) and Sapindus mukorossi pericarp fibre (SMPF) was investigated in batch experiments. The effects of contact time, initial MB concentration, pH and biomass dosage on the biosorption capacity were investigated. Moreover, kinetic and equilibrium models were used to fit the experimental data. Equilibrium data were analysed using the Freundlich and Langmuir isotherms. Results show that the adsorption capacity increases until equilibrium is reached after 10 min of contact time. Furthermore, the amount of MB adsorbed per gram of biosorbent increases with pH in the pH range 2–5 but tends to decrease with pH in the pH range 7–10. Under optimal conditions, equilibrium sorption data are accurately represented by a pseudo-second-order kinetic model. The results show that the Langmuir model best describes the adsorption of methylene blue on the three biosorbents. At neutral pH and ambient temperature, the maximum adsorption capacities reached are 24.2, 33.3 and 41 mg of MB per gram of biosorbent, respectively, for SMPF, ROG and RTN. Thermodynamic studies showed that adsorption was spontaneous and endothermic with residues of ROG and SMPF but exothermic with RTN. Our results clearly reveal that RTN, ROG and SMPF act as effective adsorbents for the removal of methylene blue from aqueous solutions.