Solenostemma argel Dead Leaves as an Eco‐Friendly and Low‐Cost Adsorbent for Effective Removal of Methylene Blue From Aqueous Media: Adsorption Performance and Mechanism

ABSTRACT Sustainable, low‐cost adsorbents derived from agricultural waste are needed for the removal of toxic dyes from contaminated water. Solenostemma argel dead leaves (SAL) were evaluated as a biosorbent for methylene blue (MB) removal from contaminated water. SAL was characterized by x‐ray diffraction (XRD), N 2 adsorption–desorption (BET), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). Batch adsorption experiments were followed to investigate the effects of adsorbent dosage, contact time, solution pH, initial MB concentration, and temperature. The obtained kinetic data were best described by the pseudo‐second‐order model, while equilibrium data were consistent with the Temkin isotherm, with a maximum adsorption capacity of 170.33 mg g −1 under the investigated conditions. Electrostatic attraction between the negatively charged SAL surface (above pH PZC = 2.9) and the cationic MB molecule is identified as the dominant supported mechanism; hydrogen bonding and π–π interactions are also proposed as secondary contributions inferred from the surface composition. The thermodynamic analysis confirmed that the adsorption process was spontaneous and exothermic, with Gibbs free energy (Δ G °) values ranging from −19.87 to −22.46 kJ mol − 1 and an enthalpy change (Δ H °) of −48.2 kJ mol − 1 . SAL also maintained effective adsorption performance over three adsorption–desorption cycles, demonstrating promising reusability.