Removal of methylene blue from aqueous solution using alkali-modified malted sorghum mash

The removal of methylene blue (MB) from aqueous solution by NaOH-modified malted sorghum mash (NSM) at different contact times, initial solution pH, adsorbent dose, temperature, and initial dye concentration was investigated. The adsorption of MB by NSM increased with increasing contact time and the process attained equilibrium within 18 min. The extent of dye removal decreased with increasing adsorbent dosage but increased with increase in temperature. The adsorption isotherm was found to follow the Langmuir model. The maximum monolayer adsorption capacity obtained was 357.1 mg/g at 306 K. The estimated adsorption free energy (ΔG ◦ ), enthalpy change (ΔH ◦ ), and entropy change (ΔS ◦ ) for the adsorption process were –20.85 kJ/mol, 38.31 kJ/mol, and 68.16 J/mol K, respectively, at 306 K. The adsorption process was feasible, endothermic, and spontaneous. The low value of ΔG ◦ suggests a physical adsorption process, while the positive value of ΔS ◦ reflects the affinity of NSM for MB.