Eco-friendly neem leaf-based activated carbon for methylene blue removal from aqueous solution: adsorption kinetics, isotherms, thermodynamics and mechanism studies

Abstract

Using biomass materials for activated carbon preparation through a simple and expedient method is a highly appropriate approach for sustainable and green development. Here, for the first time the waste neem (Azadirachta Indica) leaf was used for the development of activated carbon in the microwave process assisted with K₂CO₃ as an activating agent, and its performance was checked for methylene blue dye (MB) removal. The neem leaves activated carbon (NLAC) was analyzed with sophisticated instrument, where the SEM, XRD and pH_IEP results showed that NLAC surface has highly active pores, is crystalline in nature and has a pH_IEP value of 6.9. The FTIR data indicated that a number of active functional groups (–OH, –COOH, etc.) were present on the NLAC surface. The BET data presented that material has a good surface area of 50.69 m²/g and a pore volume of 0.014 cm³/g. More than 96% of MB dye was removed in the obtained environments of 10 mg/L MB dye concentration, pH 7, contact time 100 min, adsorbent dose of 70 mg and temperature 80 °C, respectively. The removal data were suitably followed by Langmuir isotherm model (R² = 0.96) having an uptake capacity of 132.52 mg/g. Kinetics data for MB removal were matched with the pseudo-second-order model with a R² of 0.96. The adsorption process was spontaneous in nature, where $\Delta H$ and $\Delta S$ values were 0.029 kJ/mol and 0.095 kJ/Kmol. The adsorption mechanism was followed by three types of interaction, i.e., electrostatic attraction, hydrogen bonding and pi–pi interaction, respectively.