A kinetics and thermodynamics study of methylene blue adsorption on wheat shells

The use of wheat shells (WHS) for the removal of methylene blue from aqueous solutions at different contact times, temperatures, pH, adsorbent doses and initial dye concentration was investigated. The adsorption process attains equilibrium within 60 min. The extent of dye removal decreased with increasing adsorbent dosage and also increased with increasing contact time, temperature, and in solution concentration. The adsorption isotherms are described by means of the Langmuir and Freundlich isotherms. It was found that the Langmuir equation fit better than the Freundlich equation. Maximum adsorption capacity (Qm) was calculated at different temperatures (303, 313, and 323 K) 16.56, 20.83, and 21.50 mg/g, respectively. The adsorption kinetics of methylene blue could be described by the pseudosecond- order reaction model. The activation energy of system (Ea) was calculated as 16.62 kJ/mol. Free energy of adsorption (ΔG0), enthalpy (ΔH0), and entropy (ΔS0) changes were calculated to predict the nature of adsorption. The estimated values for ΔG0 were −22.64, −24.11, and −26.35 kJ/mol at 303, 313, and 323 K, respectively, which are rather low, indicating that a spontaneous process occurred. The enthalpy changes and entropy of adsorption were 33.41 kJ/mol and 185 J/mol K, respectively. The obtained results are: (1) high levels of color removal (>95%) were achieved with low contact times adsorbent/dye (less than 90-min contact ); and (2) the whole WHS can be successfully used as adsorbent of methylene blue in aqueous solutions. WHS, an inexpensive and easily available material, can be an alternative for more costly adsorbents used for dye removal in wastewater treatment processes.