Kinetics and thermodynamics of Methylene Blue adsorption on Neem (Azadirachta indica) leaf powder

The dye, Methylene Blue, was adsorbed on an adsorbent prepared from mature leaves of the Neem tree (Azadirachta indica). A batch adsorption study was carried out with variable adsorbate concentration, adsorbent amount, pH, and temperature. Ninety-three percent of the dye could be removed by 2 g of the Neem leaf powder from 1 L of an aqueous solution containing 25 mg of the dye at 300 K. The adsorption followed pseudo first order kinetics with a mean rate constant of 3.73 × 10−3 min−1 and an intra-particle diffusion rate constant of 6.36 × 10−2 mg g−1 min−0.5. A possible mechanism of adsorption was suggested on the basis of concurrently operating surface adsorption and pore diffusion. The experimental data yielded excellent fits with Langmuir and Freundlich isotherm equations. The Langmuir monolayer capacity had a mean value of 8.76 mg g−1. The adsorption of the dye was endothermic in nature (ΔH: 4.62-16.74 kJ mol−1) and was accompanied by an increase in entropy (ΔS: 54.22-90.23 J mol−1 K−1) and a decrease in Gibbs energy (ΔG: −10.33 to −13.62 kJ mol−1 in the temperature range of 300-330 K). The results indicated that the dye, Methylene Blue, strongly interacts with a biomass-based adsorbent, the Neem leaf powder.