Tobacco Stem Ash as an Adsorbent for Removal of Methylene Blue from Aqueous Solution: Equilibrium, Kinetics, and Mechanism of Adsorption

Tobacco (Nicotiana tabacum L.) stem ash (TSA) was evaluated as an adsorbent for removal of methylene blue (MB) from aqueous solution by batch adsorption method. MB adsorption increased with increase in contact time, initial solution pH, and adsorbent dose. Contact time for adsorption equilibrium was 180 min. The MB adsorption per unit mass of adsorbent (in milligram per gram) increased with the increasing initial dye concentration. Adsorption of MB onto TSA followed the pseudo-second-order kinetic model with a rate constant (k2) of 0.017 g mg−1 min−1. The mechanism of adsorption was described with intra-particle diffusion model. It was found that the intra-particle diffusion was not a sole rate-controlling step. Equilibrium adsorption was investigated by the Freundlich, Langmuir, Temkin, and Jovanoic isotherms. On the basis of coefficient of determination, the order of isotherm fit was Langmuir (R2 = 0.974) > Freundlich (R2 = 0.957) = Temkin (R2 = 0.957) > Jovanoic (R2 = 0.764) isotherm. The maximum monolayer adsorption capacity of TSA was 35.7 mg g−1. The dimensionless separation factor (RL) was low (0.137), indicating favorable adsorption of MB onto TSA. The results clearly demonstrate the potential of TSA as a low-cost and an easily available adsorbent for sequestering MB from wastewater.