Adsorption study of methylene blue dye removal with activated carbon derived from Leucaena leucocephala wastes prepared via H3PO4 activation

This study aims to valorize agricultural waste of Leucaena leucocephala pods (LP) as a low-cost precursor for synthesizing high-performance activated carbon (LP-AC) for the removal of methylene blue dye (MB). Phosphoric acid H₃PO₄ was employed as a chemical activator of the LP biomass with a mass ratio of phosphoric acid to the precursor (3/1) before being calcined at 500 °C for 55 min. Box Benken design was investigated to optimize the experimental parameters of initial concentration, adsorbent dose, and pH. Variable optimization indicated that the highest removal efficiency of MB dye, estimated as 99.99%, was noticed at the initial concentration of 300.87 mg L⁻¹, adsorbent dose of 0.049 g, and solution pH of 10.07. Isotherm study revealed that Temkin model shows the best agreement with the experimental data with a correlation coefficient of (R² = 0.990). The adsorption capacity of MB dye was determined as 584.32 mg g⁻¹. The kinetic study suggested that the pseudo-second-order model is the best-correlated model for data fitting with (R² > 0.997). The thermodynamic analysis indicated an enthalpy change (ΔH) of − 18.50 kJ/mol, confirming that the adsorption of MB dye onto LP-AC material is an exothermic process. SEM characterization of the surface showed that the LP-AC exhibits a heterogeneous structure. The BET analysis revealed a remarkable surface area of 1367.30 m² g⁻¹ for the produced carbon, including a blend of mesoporous and microporous structures. Furthermore, complementary analyses including EDS, TGA, and FTIR confirmed the presence of crucial properties, underscoring its potential effectiveness as an adsorbent for removing MB dye.